Drosophila gene families: Taste receptors

The Interactive Fly

Zygotically transcribed genes

Taste receptors and taste processing: Antenna, The labial palps, labellum, legs and subesophageal ganglion and central brain

Neuronal basis of taste reception
Function of specific Taste Receptors
Salt Taste
Sweet Taste
Acid taste sensation in Drosophila
Ionotropic Receptors
Sensing Bacteria
Taste Circuits
Gustatory Receptors

A map of sensilla and neurons in the taste system of Drosophila larvae

In Drosophila melanogaster larvae, the prime site of external taste reception is the terminal organ (TO). Though investigation on the TO's implications in taste perception has been expanding rapidly, the sensilla of the TO have been essentially unexplored. This study consisted of a systematic anatomical and molecular analysis of the TO. Morphological types of TO sensilla were precisely defined taking advantage of volume electron microscopy and 3D image analysis. The presence of five external types of sensilla were corroborated: papilla, pit, spot, knob, and modified papilla. Detailed 3D analysis of their structural organization allowed a finer discrimination into subtypes: three subtypes of papilla and pit sensilla, respectively, and two subtypes of knob sensilla. Further, the repertoire of receptor genes for each sensillum was determined by analyzing GAL4 driver lines of Ir, Gr, Ppk, and Trp receptor genes. A map of the TO was constructed, in which the receptor genes are mapped to neurons of individual sensilla. While modified papillum and spot sensilla are not labeled by any GAL4 driver, neurons of the pit, papilla, and knob type are labeled by partially overlapping but different subsets of GAL4 driver lines of the Ir, Gr, and Ppk gene family. The results suggest that pit, papilla and knob sensilla act in contact chemosensation. However, they likely do these employing different stimulus transduction mechanisms to sense the diverse chemicals of their environment (Rist, 2017).

A major challenge in the field of neurobiology is to find out how animals detect, discriminate, and respond to the huge variety of sensory stimuli in the environment. Reception of those stimuli starts in the periphery, where specialized receptor proteins recognize certain stimuli and translate these into neuronal activity patterns that are then reported to the central nervous system. In insects, the first structures that come into contact with external stimuli, like tastants, are the sensilla on the body surface. These small organs are formed by cuticle structures associated with sensory neurons for the reception of chemical or mechanical stimuli. Sensilla might form long or short hairs, small pegs or shallow domes. Despite variations in form and shape, across insect species structural properties of sensilla were identified that are required for a particular sensory modality. The reception of chemical molecules requires contact between the molecule and the specific receptors on the dendritic membrane. Therefore, the cuticle wall of olfactory sensilla invariably is perforated by multiple pores that enable olfactory molecules to enter. Inside the sensillum, the dendrites of olfactory receptor neurons most often are highly branched but might also be unbranched. In contrast, reception of taste molecules seems to require only a single but larger pore opening that is located at the tip of taste sensilla (terminal pore). The taste-sensitive neurons do not branch and most often are accompanied by one mechanosensitive neuron (Rist, 2017).

The major taste sensilla of cyclorrhaphan fly larvae are supposed to cluster in the terminal organ (TO) at the tip of the cephalic lobes. A detailed description of TO ultrastructure has been provided for Musca domestica larvae. Hence, the TO consists of 14 external sensilla grouped into five types which divide into a dorso-lateral group and a distal group. Gustatory function was inferred for most of the sensilla due to relevant structural properties. Investigations on the ultrastructure of the TO of Drosophila melanogaster larvae, however, have been rare. This paucity of information might derive from the technical difficulty to examine sensillar ultrastructure in three dimensions using transmission electron microscopy (TEM). Only recently a number of advanced techniques to effectively acquire volume electron microscopy data have been introduced to the field of biology (Rist, 2017).

Nevertheless, in Drosophila larvae, a function in taste reception of the TO was confirmed by behavioral, physiological and anatomical studies. Importantly, a receptor-to-neuron map of gustatory receptors (Grs) and neurons of the TO was constructed using the GAL4/UAS system. In this map, eight neurons of the TO are defined by their combinatorial expression of 28 Gr-GAL4 drivers. Subsequent studies assigned a role to mapped TO neurons in the reception of bitter compounds and also of other tastants. In addition to Grs, neurons of the TO might express Ionotropic receptors (Irs) and receptors of the Pickpocket receptor (Ppk) gene family (Rist, 2017).

Despite the increasing number of investigations on the neuronal and molecular basis of larval taste reception, the functional organization of the TO is still unclear. In particular, the specific stimuli for most neurons of the TO remain unknown. Previous work has not defined which receptor neurons are housed in individual sensilla nor in which sensilla individual receptor genes are expressed. Thus, it has been difficult to carry out well-designed experiments on the function of sensory neurons and sensilla (Rist, 2017).

This study define morphological types of TO sensilla based on their ultrastructure taking advantage of focused-ion-beam scanning-electron microscopy (FIB-SEM) that allows for automated acquisition of serial sections with highest resolution. Further, receptor genes of the Ir, Ppk and Gr family were mapped to sensilla using the GAL4/UAS system. A neuron-to-sensillum map was created that lays a foundation for subsequent investigations on how Drosophila larvae use sensory information to perceive their environment (Rist, 2017).

The results clarify and alter several of the previous findings by light and electron microscopy. The TO consists of two groups of sensilla, the distal and the dorso-lateral group. In addition to the external sensilla of both groups, mechanosensitive scolopidia sensilla lie internally below the cuticle surface of the TO. Neurons of the TO-distal group cluster in the TO-ganglion. Neurons of the dorso-lateral group lie in the DO-ganglion, in the TO-ganglion or independently of both ganglia. The number of neurons belonging to the TO, and DO has been estimated in a previous study according to anti-Elav-stainings of neuronal cell nuclei. A total of 32-33 anti-Elav-stained neurons was in the ganglion of the TO. Of these, 30 neurons were assumed to be associated with the TO-distal group including 26 neurons that belong to the external sensilla and four neurons that belong to internal scolopidia sensilla. The FIB-SEM analysis in the present study is restricted to the external sensilla of the TO. It was found that the external sensilla of the TO-distal group comprise in total 28 neurons by counting the dendrites in the sensilla. This number is slightly different from the previous study but consistent with another study that also examined the ultrastructure of external sensilla of the TO of Drosophila larvae. The external sensilla of the dorso-lateral group comprise five neurons in agreement with all previous studies (Rist, 2017).

Twenty-six neurons of the TO-distal and dorso-lateral group were assumed to be gustatory according to interpretations on the fine structure of associated sensilla. This study considered 25 neurons of the TO to be gustatory based on their structural and molecular characteristics. Despite the similar number, in contrast to the previous studies this study include the neurons of the knob sensilla and exclude the neurons of Pmodand the spot sensilla. Pmod and spot sensilla most likely are not gustatory because they lack fine structural characteristics of gustatory sensilla and also lack chemosensory receptor gene expression. This study waw able to define 19 of the 25 putative chemosensory neurons by GAL4 driver expression thereby expanding the established receptor-to-neuron map by 11 neurons. Six neurons remain 'orphan'. These neurons might be labeled by one of the GAL4 drivers which could not be mapped to a single neuron but to the sensillum, or they might express receptor genes not covered by the screened GAL4 lines (Rist, 2017).

Characterization of Drosophila taste receptors

Candidate taste receptors in Drosophila

A computer algorithm that seeks proteins with particular structural properties, as opposed to proteins with particular sequences, has identified a large family of candidate odorant receptors from the Drosophila genomic database. Further analysis of genes identified by this algorithm has revealed one gene that defines a distinct large family of membrane proteins; 43 members of this family have been identified in the first 60% of the Drosophila genome that has been sequenced thus far. If the sequenced part of the genome is representative, then extrapolation suggests that the entire genome would encode on the order of 75 proteins, a figure comparable to the estimated number of odorant receptors. The previously unidentified family of proteins shows no sequence similarities to any known odorant receptors or to any other known proteins. This family has been tentatively named the gustatory receptor (GR) family, with each individual gene named according to its cytogenetic location in the genome. Thus, the GR59D.1 and GR59D.2 genes, which are abbreviated here as 59D.1 and 59D.2, refer to two family members located in cytogenetic region 59D on the second chromosome (Clyne, 2000).

The amino acid sequences of 19 members of the GR family indicate the high degree of sequence divergence. Sequence alignment reveals only one residue conserved among a select group of the family and only 24 residues conserved among more than half of the genes in this group. Fifteen of these conserved residues lie in the vicinity of the COOH-terminus. Amino acid identity between individual genes ranges from a maximum of 34% to <10%. By contrast, other features of the gene family show substantial conservation. The positions of a number of introns are conserved, suggesting that the family originated from a common ancestral gene. Overall sequence length, ~380 amino acids, is another common feature. All of the genes encode approximately seven predicted transmembrane domains, a feature characteristic of G protein-coupled receptors (GPCRs). All 43 of the predicted GR gene products have been identified as GPCRs by an algorithm trained to distinguish between GPCRs and other multitransmembrane proteins (Clyne, 2000).

The genes are widely dispersed in the genome, but at the same time, many are found in clusters. The two largest clusters each contain four genes; there are also several clusters of two or three genes. Genes within these clusters are closely spaced, with intergenic distances ranging from 150 to 450 base pairs (bp) in all cases for which the data are currently available. There is no rule specifying the orientation of genes within clusters, unlike the case of the Drosophila odorant receptors, in which genes within a cluster are in the same orientation in all clusters examined (Clyne, 2000).

An unusual form of alternative splicing occurs in at least two chromosomal locations. Four large exons in cytogenetic region 39D each contain sequences specifying six predicted transmembrane domains, followed by three small exons that together specify a putative seventh transmembrane domain and the COOH-terminus. Reverse transcription-polymerase chain reaction (RT-PCR) analysis reveals that each of the four large exons is spliced to the smaller exons, thereby generating four predicted seven transmembrane domain proteins. These four proteins are thus distinct through the first six transmembrane domains and identical in the seventh and in the COOH-terminal sequences. Likewise, in cytogenetic region 23A, there are two large exons, each of which specifies six transmembrane domains and each is spliced to two small exons that together encode a seventh transmembrane domain and the COOH-terminus. Thus, the gene in region 23A encodes two related proteins. This pattern of splicing, in which alternative large 5' exons encoding most of the protein are joined to common short 3' exons encoding only a small portion of the protein, is unusual among genes encoding GPCRs and proteins in general. This pattern of splicing provides a mechanism at a single locus for generating products that exhibit a pattern observed for this family in general: extreme diversity among all sequences of the proteins except in a small region in the vicinity of the COOH-terminus (Clyne, 2000).

To assess the tissue specificity of expression, RT-PCR was performed with primers that span introns in the coding regions. Of the 19 transcripts tested, 18 are expressed in the labellum, the major gustatory organ of the fly. Moreover, for most of these genes, expression is labellum-specific: only 1 of the 19 yielded amplification products from heads depleted of taste organs and only 2 showed expression in the thorax, which contains the thoracic nervous system but no characterized taste sensilla. Likewise, expression in several other tissues, including the abdomen, wings, and legs, is limited to a small fraction of genes (Clyne, 2000).

To further analyze gene expression by in situ hybridization, 12 GR transcripts were used as probes. Each probe was used individually and in mixtures of multiple probes. Sequences encompassing all, or nearly all, of each transcript were used, and several diverse methods of signal amplification and detection were used, with a variety of experimental conditions. None of the genes shows detectable expression in any tissue, including the taste organs. As positive controls, the pheromone-binding protein-related protein-2 gene (pbprp-2), which may encode a carrier of hydrophobic molecules, shows hybridization in taste sensilla on the labellum, and the Drosophila olfactory receptor gene 22A.2 (DOR22A.2) hybridizes to olfactory sensilla on the antenna. The simplest interpretation of these results is that expression levels of the GR genes are exceedingly low. Consistent with this interpretation is the fact that no expressed sequence tags have been identified for any of the 43 GR transcripts (Clyne, 2000).

To further analyze the tissue specificity of GR expression, a microdissection experiment was performed in which the labral sense organ (LSO), a small taste organ that lines the pharynx, was surgically excised from each of 50 animals. The LSO consists of a very limited number of cells and is highly enriched in taste neurons; it does not, for example, contain muscle cells. By RT-PCR amplification, the expression of seven GR transcripts in this taste organ was detected. These results indicate that expression of the GR family extends to include at least one additional taste organ besides the labellum. The data are also fully consistent with the notion that the GR genes are expressed in taste neurons (Clyne, 2000).

To confirm the gene expression in taste receptor neurons, a Drosophila mutant, pox-neuro70 (poxn70), in which chemosensory bristles are transformed into mechanosensory bristles, was used. Specifically, in poxn70, which behaves as a null mutation with respect to adult chemosensory organs, chemosensory bristles are transformed into mechanosensory bristles with respect to various morphological and developmental criteria. In particular, most chemosensory bristles in wild-type Drosophila are innervated by five neurons: four chemosensory neurons and one mechanosensory neuron. In contrast, wild-type mechanosensory bristles contain a single mechanosensory neuron. In chemosensory bristles transformed to mechanosensory bristles by poxn70, the number of neurons is reduced from five to one. It was predicted that if the GR family is in fact expressed in the chemosensory neurons of taste sensilla, their expression would likely be eliminated in the poxn70 mutant. Consistent with this prediction, 18 of 19 GR transcripts examined were not expressed in the labellum of the poxn70 mutant. These results indicate that the GR gene family is expressed in labellar chemosensory neurons (Clyne, 2000).

The large size of this protein family likely reflects the diversity of compounds that flies can detect. The labellar hairs of larger flies are not only sensitive to a variety of simple and compound sugars, but also to a wide variety of other molecules, such as amino acids. Behavioral studies have shown that Drosophila is sensitive to quinine, which is perceived by humans as bitter, and other insects have been shown to be sensitive to an array of structurally diverse bitter compounds. Moreover, an individual insect taste receptor cell can respond to a broad range of structurally heterogeneous alkaloids and other bitter molecules. The extreme diversity of these receptors may not only reflect diversity among the ligands that they bind, but also diversity in the signal transduction components with which they interact. For example, the lack of conserved intracellular regions suggests the possibility that, during the evolution of this sensory modality, multiple G proteins arose, each interacting with a different subset of receptors. Finally, it seems likely that the Drosophila genome encodes taste receptors in addition to those of the GR family. Although expression in the labellum and the LSO has been detected, few if any family members are expressed in the leg or wing chemosensory hairs, some of which are morphologically similar to labellar taste hairs. The Drosophila olfactory system also contains two sense organs: the antenna and the maxillary palp. Both olfactory organs respond to all, or nearly all, of the same odorants and both are derived from the same imaginal discs. However, most individual members of the DOR gene family are expressed in one or the other but not in both olfactory organs. Perhaps the distinction among taste receptor genes is even more extreme in the gustatory system, whose organs derive from different imaginal discs. For example, the legs may express a completely distinct family of genes or a subfamily whose similarities to the present family are sufficiently tenuous as to place it slightly beyond the boundaries that define the GR family (Clyne, 2000).

A chemosensory gene family encoding candidate gustatory and olfactory receptors in Drosophila

Gustatory (taste) neurons sense soluble chemical cues that elicit feeding behaviors. In insects, taste neurons also initiate innate sexual and reproductive responses. In Drosophila, for example, sweet compounds are recognized by chemosensory hairs on the proboscis and legs that activate proboscis extension and feeding. Sexually dimorphic chemosensory bristles on the foreleg of males recognize cues from receptive females that are thought to elicit the embrace of mating. Females have yet a third set of specialized bristles on their genitalia that may cause oviposition in response to nutrients. In this manner, gravid females will preferentially deposit their eggs on a rich environment that enhances survival of their offspring. These robust and innate gustatory responses provide the opportunity to understand how chemosensory information is recognized in the periphery and ultimately translated into specific behaviors (Scott, 2001 and references therein).

Taste in Drosophila is mediated by sensory bristles that reside on the proboscis, legs, wing, and genitalia. Most chemosensory bristles are innervated by four bipolar gustatory neurons and a single mechanoreceptor cell. The dendrites of gustatory neurons extend into the shaft of the bristle and are the site of taste recognition that translates the binding of tastants into alterations in membrane potential. The sensory axons from the proboscis project to the brain where they synapse on projection neurons within the subesophageal ganglion (SOG), the first relay station for gustatory information in the fly brain. Sensory axons from taste neurons at other sites along the body project locally to peripheral ganglia. Drosophila larvae, whose predominant activity is eating, sense their chemical environment with gustatory neurons that reside in chemosensory organs on the head and are also distributed along the body surface. What remains unknown is the pattern of projection of functionally distinct classes of taste cells and therefore the nature of the representation of gustatory information in the Drosophila brain (Scott, 2001 and references therein).

The identification of the genes encoding taste receptors and the analysis of the patterns of receptor expression may provide insight into the logic of taste discrimination in the fly. In Drosophila, the recognition of odorants is thought to be accomplished by about 60 seven-transmembrane domain proteins encoded by the Drosophila odorant receptor (DOR) gene family. Recently, a large family of putative G protein-coupled receptors was identified by searching the Drosophila genome with an algorithm designed to detect seven-transmembrane domain proteins (Clyne, 2000). These genes were suggested to encode gustatory receptors (GRs) because members of this gene family were detected in the proboscis by RT-PCR experiments (Scott, 2001).

This study has characterized and extended the family of putative G protein-coupled receptors originally identified by Clyne (2000) and provides evidence that they encode both olfactory and gustatory receptors. In situ hybridization, along with transgene experiments, reveals that some receptors are expressed in topographically restricted sets of neurons in the proboscis, whereas other members are expressed in spatially fixed olfactory neurons in the antenna. Members of this gene family are also expressed in chemosensory bristles on the leg and in larval chemosensory organs. The projections of different subsets of larval chemosensory neurons have been traced to the subesophageal ganglion and the antennal lobe. These data provide insight into the diversity of chemosensory recognition in the periphery and afford an initial view of the representation of gustatory information in the fly brain (Scott, 2001).

The GR gene family identified by Clyne (2000), has been extended by analyzing the recently completed euchromatic genome sequence of Drosophila using reiterative BLAST searches, transmembrane domain prediction programs, and hidden Markov model (HMM) analyses . These searches have identified a total of 56 candidate GR genes in the Drosophila genome, including 23 GRs not previously described. Gene sequences are available at the Columbia University site titled Newly identified GRs. The family as a whole is extremely divergent and reveals an overall sequence identity ranging from 7%-50%. However, all genes share significant sequence similarity within a 33 amino signature motif in the putative seventh transmembrane domain in the C terminus. Analysis of the sequence of the 56 genes reveals the existence of four discrete subfamilies (containing ten, six, four, and three genes) whose members exhibit greater overall sequence identity ranging from 30%-50%. Twenty-two of the GR genes reside as individual sequences distributed throughout each of the Drosophila chromosomes, whereas the remaining genes are linked in the genome in small tandem arrays of two to five genes (Scott, 2001).

The GR family shares little sequence similarity outside of the conserved C-terminal signature in the putative seventh transmembrane domain and therefore the searches of the genome database described here are unlikely to be exhaustive. Thus, this family of candidate gustatory receptors consists of a minimum of 56 genes. Moreover, this analysis would not detect alternatively spliced transcripts, a feature previously reported for some members of this gene family (Clyne, 2000). cDNAs or RT PCR products could be detected from only six genes and verification of the gene predictions therefore awaits the isolation and sequencing of additional cDNAs (Scott, 2001).

Interestingly, the 33 amino acid signature motif characteristic of the GR genes is present but somewhat diverged in 33 of the 60 members of the family of Drosophila odorant receptor (DOR) genes. However, the DOR genes possess additional conserved motifs not present in the GR genes and these motifs define a distinct family. These observations suggest that the putative gustatory and olfactory receptor gene families may have evolved from a common ancestral gene (Scott, 2001).

Insight into the specific problem of the function of these candidate receptor genes and the more general question as to how tastants are recognized and discriminated by the fly brain initially requires an analysis of the patterns of expression of the individual GR genes in chemosensory cells. In situ hybridization was performed on sagittal sections of the adult fly head with RNA probes obtained from all 56 family members. Six of the genes are expressed in discrete, topographically restricted subpopulations of neurons within the proboscis. Three of the genes reveal no hybridization to the proboscis but are expressed in spatially-defined sets of neurons within the third antennal segment, the major olfactory organ of the adult fly. The remaining genes show no hybridization to adult head tissues (Scott, 2001).

This analysis of the pattern of GR gene expression by in situ hybridization demonstrates that a small number of GR genes is transcribed in either the proboscis or the antenna, suggesting that this family encodes chemosensory receptors involved in smell as well as taste. However, expression of over 80% of the family members could not be detected using the available in situ hybridization conditions. The sequence of these GR genes does not reveal nonsense or frameshift mutations that characterize pseudogenes. The inability to detect transcripts from the majority of the GR genes by in situ hybridization might result from low levels of expression of GR genes, expression in populations of chemosensory cells not amenable to analysis by in situ hybridization (e.g., leg, wing, or vulva), or expression at other developmental stages (Scott, 2001).

Lines of flies expressing GR promoter transgenes were generated to visualize the expression in a wider range of cell types with higher sensitivity. Transgenes were constructed in which putative GR promoter sequences (0.5-9.5 kb of DNA immediately upstream of the translational start) were fused to the Gal4 coding sequence. Flies bearing GR transgenes were mated to transgenic flies that contain either ß-galactosidase (lacZ) or green fluorescent protein (GFP) under the control of the Gal4-responsive promoter, UAS. GR promoter-Gal4 lines were constructed with upstream sequences from 15 chemoreceptor genes and transgene expression was detected for 7 lines. Five of the genes that were expressed by transgene analyses were also detected by in situ hybridization (Scott, 2001).

Expression of the GR transgenes in the proboscis was initially visualized using the UAS-lacZ reporter. The labellum of the proboscis is formed from the fusion of two labial palps, each containing 31-36 bilaterally symmetric chemosensory bristles arranged in four rows. The sensilla of the first three columns contains four chemosensory neurons and a single mechanoreceptor cell whereas the sensilla in the most peripheral row are composed of only two chemosensory neurons and one mechanoreceptor. Each labial palp therefore contains approximately 120 chemosensory neurons (Scott, 2001 and references therein).

The GR promoter-Gal4 lines were crossed to UAS-lacZ flies and the progeny were examined for lacZ expression by staining of whole-mount preparations of the labial palp. Five transgenic lines exhibit lacZ expression in sensory neurons of the labial sensilla. The expression of each transgene is restricted to a single row of chemosensory bristles. Gr47A1, for example, is expressed in sensilla innervating the most peripheral row of bristles, whereas Gr66C1 is expressed in sensilla that occupy a medial column. Flies bearing a GR promoter-Gal4 gene were also crossed with UAS-GFP stocks. The expression of GFP allows greater cellular definition and reveals that each receptor is expressed in a single neuron within a sensillum. The pattern of GR gene expression determined by GR promoter transgenes resembles that seen by in situ hybridization. However, coexpression of the transgene reporter and the endogenous gene could not be directly demonstrated by dual label in situ hybridization due to low levels of GR gene expression. Nevertheless, this pattern of expression, in which a receptor is expressed in only one neuron in a sensillum and in one sensillar row, is maintained in over 50 individuals examined for each transgenic line and is also maintained in independent transformed lines for each GR transgene (Scott, 2001).

Chemosensory bristles reside at multiple anatomic sites in the fly including the taste organs in the mouth, the legs and wings, as well as in the female genitalia. Three sensory organs reside deep in the mouth: the labral sense organ (comprised of 10 chemosensory neurons) and the ventral and dorsal cibarial organs of the mouth (each containing six chemosensory neurons). The function of these specialized sensory organs is unknown, but their anatomic position and CNS projection pattern suggests that they participate in taste recognition. Three of the five GR promoter-Gal4 lines that are expressed in the proboscis are also expressed in the cibarial organs. One gene, Gr2B1, is expressed solely in the labral sense organ and is not detected in the proboscis labellum or in the cibarial organs (Scott, 2001).

Chemosensory bristles also decorate both the legs and wings of Drosophila with about 40 chemosensory hairs on each structure. One gene, Gr32D1, expressed both in the proboscis and cibarial organ, is also expressed in two to three neurons in the most distal tarsal segments of all legs. These results are consistent with the observation that exposure of the legs to tastants results in proboscis extension and feeding behavior. The observation that members of this gene family are expressed in the proboscis and in chemosensory cells of the internal mouth organs and leg suggests that this gene family encodes gustatory receptors (Scott, 2001).

GR transgene expression has also been examined in larvae. The detection of food in larvae is mediated by chemosensors that reside largely in the antennal-maxillary complex, a bilaterally symmetric anterior structure composed of the dorsal and terminal organs. Each of the two larval chemosensory organs comprises about 40 neurons. Neurons of the dorsal organ primarily detect volatile odorants, whereas the terminal organ is thought to detect both soluble and volatile chemical cues (Scott, 2001 and references therein).

It was asked whether members of the GR family are expressed in larval chemosensory cells by examining the larval progeny that result from crosses between GR promoter-Gal4 and UAS-GFP flies. Examination of live larvae by direct fluorescent microscopy reveals that five of the seven GRs expressed in the adult are expressed in single neurons within the terminal organ. GR-promoter fusions from each of the 5 genes show bilateral expression of GFP both in the neuronal cell body and in the dendrite. The dendrites extend anteriorly to terminate in the terminal organ, a dome-shaped structure that opens to the environment. In about 5% of the larvae, a second positive cell is observed in each of the lines (Scott, 2001).

Gr2B1 is expressed in only a single neuron in the labral sense organ of the adult, but is expressed in an extensive population of chemosensory cells in larvae. This gene is expressed in two neurons innervating the dorsal organ, one neuron innervating the terminal organ, and a single bilaterally symmetric neuron innervating the ventral pit in each thoracic hemisegment. The ventral pit contains a single sensory neuron that may be involved in contact chemosensation. The GR genes are therefore likely to play a significant role in chemosensory recognition in larvae as well as adults (Scott, 2001).

Olfactory neurons of mammals as well as Drosophila express a single odorant receptor such that the brain can discriminate odor by determining which neurons have been activated. In contrast, nematode olfactory neurons and mammalian gustatory cells coexpress multiple receptor genes. Therefore the diversity of GR gene expression has been examined in individual larval taste neurons. In larvae, most receptors are expressed in only one neuron in the terminal organ. Crosses between five GR promoter-Gal4 lines and flies bearing UAS-GFP reveal a single intensely stained neuron within each terminal organ. Seven lines were generated bearing two different GR promoter-Gal4 transgenes along with the UAS-GFP reporter. In every line bearing two GR promoter-Gal4 fusions, two GFP positive cells per terminal organ were identified. These experiments demonstrate that individual gustatory neurons of larvae express different complements of receptors and are likely to respond to different chemosensory cues (Scott, 2001).

In other sensory systems, a spatial map of receptor activation in the periphery is maintained in the brain such that the quality of a sensory stimulus may be encoded in spatially defined patterns of neural activity. GR promoter-Gal4 transgenes have been used to drive the expression of UAS-nSyb-GFP to visualize the projections of sensory neurons expressing different GR genes. nSyb-GFP is a C-terminal fusion of green fluorescent protein to neuronal synaptobrevin that selectively labels synaptic vesicles, allowing the visualization of terminal axonal projections. Whole-mount brain preparations from transgenic flies were examined by immunofluorescence with an antibody against GFP and a monoclonal antibody, nc82, which labels neuropil and identifies the individual glomeruli in the antennal lobe. These experiments were initially performed with larvae because of the relative simplicity of the larval brain and the observation that a given GR is expressed in only a small number of gustatory neurons (Scott, 2001).

The Drosophila larval brain is composed of two dorsal brain hemispheres fused to the ventral hindbrain. The brain hemispheres and the hindbrain contain an outer shell of neuronal cell bodies and a central fibrous neuropil. Determination of the number of neuroblasts and the number of cell divisions suggest that there are ~10,000-15,000 neurons in the larval brain, a value 10- to 20-fold lower than in the adult. Chemosensory neurons send axonal projections to two distinct regions of the larval brain, the antennal lobe and the subesophageal ganglion (SOG). The antennal lobe is a small neuropil in the medial aspect of the deuterocerebrum within each brain hemisphere. The antennal lobe receives input from neurons of the dorsal and terminal organ and presumably participates in processing olfactory information. The SOG resides in the most anterior aspect of the hindbrain, at the juncture of the hindbrain with the brain hemispheres. The SOG receives input from the terminal organ and mouthparts and is thought to process gustatory information. Whereas the projections of populations of chemosensory cells have been traced to the antennal lobe and the SOG, the patterns of axonal projections for individual sensory cells have not been described. Moreover, the connections of chemosensory axons with second order brain neurons is unknown for the larval brain (Scott, 2001).

Gr32D1-Gal4 is expressed in multiple neurons in the proboscis of the adult, but it is expressed in only a single neuron in the terminal organ of larvae. In larvae containing the Gr32D1-Gal4 and UAS-nSyb-GFP transgenes, it is possible to visualize the axons of Gr32D1-expressing cells as they course posteriorly to enter the subesophageal ganglion. The axons then turn dorsally and intensely stained fibers terminate in the medial aspect of the SOG. A similar pattern is observed for neurons expressing Gr66C1, a gene expressed in the adult proboscis and in a single neuron in the larval terminal organ and two neurons in the larval mouth. However, the terminal arbors of Gr66C1 neurons are consistently thicker than those observed for Gr32D1, perhaps reflecting the increased number of Gr66C1-bearing neurons. The reporter nSyb-GFP stains axons only weakly but shows intense staining of what are likely to be terminal projections of sensory neurons that synapse on second order neurons in the neuropil of the SOG. This terminal arbor extends for about 40 µm and reveals a looser, more distributed pattern that the tight neuropil of the olfactory glomerulus. The position and pattern of the terminal projections from individual chemosensory cells in the terminal organ show bilateral symmetry and are maintained in over 20 larvae examined (Scott, 2001).

A more complex pattern of projections is observed for Gr2B1, a gene expressed in one neuron in the terminal organ, two in the dorsal organ, and a single bilaterally symmetric neuron in each thoracic hemisegment. One set of fibers appears to terminate in the antennal lobe. A second more posterior set of fibers can be traced from the thorax into the hindbrain, with fibers terminating posterior to the antennal lobe. This pattern of projections is of interest for it implies that neurons in different locations in larvae that express the same receptor project to discrete locations in the larval brain, suggesting the possibility that the same chemosensory stimulus can elicit distinct behavioral outputs (Scott, 2001).

Attempts have been made to determine whether neurons in the terminal organ that express different GRs project to discrete loci within the SOG. Therefore larvae that express two promoter fusions, Gr66C1-Gal4 and Gr32D1-Gal4, along with a UAS-nSyb-GFP transgene were generated. The projections in these flies are broadened, suggesting that these sets of neurons terminate in overlapping but nonidentical regions of the SOG. More definitive data to support the existence of a topographic map of taste quality will require two-color labeling of the different fibers to discern whether the projections from neurons expressing different GRs are spatially segregated in the SOG (Scott, 2001).

Are GRs also odorant receptors? A large family of presumed olfactory receptor genes in Drosophila (the DOR genes) has been identified that is distinct from the GR gene family. Expression of the DOR genes is only observed in olfactory sensory neurons within the antenna and maxillary palp, where a given DOR gene is expressed in a spatially invariant subpopulation of cells. In situ hybridization experiments demonstrate that three members of the GR gene family are also expressed in subpopulations of antennal neurons. These observations suggest either that the odorant receptors in Drosophila are encoded by at least two different gene families or that previously unidentified taste responsive neurons reside within the antenna (Scott, 2001).

In Drosophila, olfactory information is transmitted to the antennal lobe, whereas gustatory neurons in the proboscis and mouth relay sensory information to the subesophageal ganglion. Therefore the spatial pattern of expression of GRs in the antenna was examined and the pattern of projections of their sensory axons in the brain. In situ hybridization with the three GR genes reveals that each gene is expressed in about 20-30 cells/gene in the antenna. Similar results are obtained in a cross between an antennal GR promoter-Gal4 line, Gr21D1-Gal4, and UAS-LacZ or UAS-GFP lines. This pattern of GR gene expression is maintained in over 50 antennae analyzed. The GR-positive cells occupy regions of the antenna that do not express identified members of the DOR gene family, suggesting that there is spatial segregation of these two receptor families (Scott, 2001).

It was then asked whether antennal neurons expressing a GR gene project to the antennal lobe in a manner analogous to that observed for cells expressing the DOR genes. Transgenic flies expressing a Gr21D1 promoter-Gal4 fusion were crossed to animals bearing the UAS-nSyb-GFP transgene. These studies demonstrate that neurons expressing the Gr21D1 transgene project to a single, bilaterally symmetric glomerulus in the ventral-most region of the antennal lobe (the V glomerulus) and do not project to the SOG. Thus, as in the case of the family of DOR genes, neurons expressing the same receptor project to a single spatially invariant glomerulus (Scott, 2001).

Gr21D1 is also expressed in one cell of the terminal organ of larvae. Therefore, the projections of Gr21D1-bearing neurons to the larval brain have been traced. Gr21D1 axons enter the larval brain and terminate in the antennal lobe rather than the SOG. The segregation of projections from presumed olfactory and gustatory neurons is apparent in larvae that contain Gr21D1-Gal4 and Gr66C1-Gal4 along with UAS-nSyb-GFP. In these transgenic flies, two distinct sets of termini are observed, one entering the SOG, and a second entering the antennal lobe (Scott, 2001).

Thus, a member of the GR gene family is expressed in sensory neurons of the antenna and the terminal organ of larvae, and GR-bearing neurons project to the antennal lobe. These data suggest that at least two independent gene families, the DORs and the GRs, recognize olfactory information. The GR gene family is therefore likely to encode both olfactory and gustatory receptors, and neurons expressing distinct classes of GR receptors project to different regions of the fly brain (Scott, 2001).

A common gene family encoding both olfactory and taste receptors is not present in vertebrates, where the main olfactory epithelium, the vomeronasal organ, and the tongue express receptors encoded by independent gene families. The observations presented here are more reminiscent of the chemosensory receptor families in C. elegans that encode odorant receptors expressed in the amphid neurons and taste receptors in sensory neurons responsive to soluble chemicals (Scott, 2001).

The size of the family of candidate taste receptors and the pattern of expression in chemosensory cells provides insight into the problem of the recognition and discrimination of gustatory cues. On average, each GR is expressed in 5% of the cells in the proboscis labellum, suggesting that the proboscis alone will contain at least 20 distinct taste cells expressing about 20 different GR receptors. Moreover, a given receptor is expressed in one of the four rows of sensilla such that the sensilla in different rows are likely to be functionally distinct. Electrophysiologic studies have suggested that all sensilla are identical and contain four distinct cells, each responsive to a different category of taste. The data presented here are not consistent with these conclusions and argue that different rows of sensilla are likely to contain cells with different taste specificities (Scott, 2001).

At present, the nature of the ligands recognized by these GR receptors are not known, nor is it known whether all taste modalities are recognized by this gene family. In mammals, gustatory cues have classically been grouped into five categories: sweet, bitter, salt, sour, and glutamate (umami). Sugar and bitter taste are likely to be mediated by G protein-coupled receptors since these modalities require the function of a taste cell-specific Ga subunit, gustducin. Recently, two novel families of seven transmembrane proteins (the T1Rs and T2Rs) were shown to be selectively expressed in taste cells in the tongue and palate epithelium. Genetic experiments have implicated members of the T2R family in the recognition of bitter tastants and functional studies have directly demonstrated that members of the T2R family serve as gustducin-linked bitter taste receptors. A large number of candidate genes have been suggested to encode receptors for other taste modalities, but in only a few instances have functional data and expression patterns supported these assumptions. In mammals, an amiloride-sensitive sodium channel has been suggested as the salt receptor, a degenerin homolog (MDEG-1) and a potassium channel as sour or pH sensors, and a rare splice form of the metabotropic glutamate receptor as the umami sensor. In Drosophila, genetic analysis of mutant flies defective in the recognition of the sugar trehalose has led to the identification of a transmembrane receptor distinct from GRs that reduces the sensitivity to one class of sugars. The interpretation of the role of these putative taste receptors in taste perception awaits a more definitive association between tastant and gene product (Scott, 2001).

How does the fly discriminate among multiple tastants? One mechanism of chemosensory discrimination, thought to operate in the olfactory system of insects and vertebrates, requires that individual sensory neurons express only one of multiple receptor genes. Neurons expressing a given receptor project axons that converge on topographically invariant glomeruli such that different odors elicit different patterns of spatial activity in the brain. The nematode C. elegans uses a rather different logic, in which a given sensory neuron dictates a specific behavior but expresses multiple receptors. In the worm olfactory system, discrimination is necessarily more limited and exploits mechanisms to diversify the limited number of sensory cells. A similar logic has been suggested for mammalian taste. Several members of the T2R family of about 50 receptor genes, each thought to encode bitter sensors, are coexpressed in sensory cells within the tongue. This organization allows the organism to recognize a diverse repertoire of aversive tastants but limits the ability to discriminate among them (Scott, 2001).

What can be discerned about the logic of taste discrimination from the pattern of GR gene expression in Drosophila? First, the number of GR genes, 56, approximates the number of DOR genes, suggesting that the fly recognizes diverse repertoires of both soluble and volatile chemical cues. Moreover, the data argue that individual sensory neurons differ with respect to receptor gene expression and are therefore functionally distinct. Experiments with Drosophila larvae demonstrate that a given GR gene is expressed in one neuron in the larval terminal organ. Strains bearing two different GR-promoter fusions reveal twice the number of expressing cells. Similar results are obtained in adult gustatory organs. More definitive experiments to examine the diversity of receptor expression in a single neuron, employed successfully in the olfactory system, have been difficult since the levels of GR RNA are 10- to 20-fold lower than odorant receptor RNA levels. Nevertheless, these experiments demonstrate that different gustatory neurons express different complements of GR genes and at the extreme are consistent with a model in which gustatory neurons express only a single receptor gene (Scott, 2001).

How does the brain discern which of the different gustatory neurons is activated by a given tastant? As in other sensory systems, it is possible that axons from different taste neurons segregate to spatially distinct loci in the subesophageal ganglion. In such a model, taste quality would be represented by different spatial patterns of activity in the brain. Preliminary experiments suggest that neurons expressing different GRs project to spatially segregated loci within the brain. Clear segregation of axonal termini is observed for presumed taste neurons that project to the SOG and olfactory neurons that project to the antennal lobe. A second interesting pattern of projections is observed for the presumed gustatory receptor Gr2B1, a gene expressed in neurons in the terminal and dorsal organs and in a single neuron in the ventral pit present bilaterally in each thoracic segment. At least two spatially segregated targets are observed for these neurons in the larval brain: one set of fibers terminates in glomeruli of the antennal lobe and a second set of fibers (from the ventral pits) project to the SOG. Thus, neurons expressing the same receptor in different chemosensory organs project to distinct brain regions. In this manner, the same chemosensory cue could elicit distinct behaviors depending upon the cell it activates. Sucrose, for example, could elicit chemoattraction upon exposure to the thoracic neurons and eating behavior upon activation of neurons in the terminal and dorsal organ (Scott, 2001).

These data establish that presumed olfactory neurons and gustatory neurons expressing GR genes project to different regions of the larval brain. Taste neurons expressing different GR genes, however, all project to the SOG. The current data do not permit a discernment of whether axons from neurons expressing different GR genes project to spatially distinct loci within the SOG. The axon termini of gustatory neurons terminate in more diffuse, elongated structures than the tightly compacted glomeruli formed by olfactory sensory axons, rendering it difficult at present to discern a topographic map of gustatory projections in the larval brain (Scott, 2001).

Spatially restricted expression of candidate taste receptors in the Drosophila gustatory system

An additional study (Dunipace, 2001) provides evidence for spatially restricted expression of candidate taste receptors in the Drosophila gustatory system. BLAST searches with the predicted amino acid sequences of 7-transmembrane-receptor genes of unknown function and 20 previously identified, putative gustatory receptor genes led to the identification of a large gene family comprising at least 54 genes. The sequences of all genes are deposited at the Amrein web-site at Duke University. Expression of eight of these genes was examined by using a Gal4 reporter gene assay; five of them are expressed in the gustatory system of the fly. Four genes are expressed in 1%-4% of taste sensilla, located in well-defined regions of either the proboscis, the legs, or both. The fifth gene is expressed in about 20% of taste sensilla in all major gustatory organs, including the taste bristles on the anterior wing margin. Axon-tracing experiments have demonstrated that neurons expressing a given Gr gene project their axons to a spatially restricted domain of the subesophageal ganglion in the fly brain. These findings suggest that each taste sensillum represents a discrete, functional unit expressing at least one Gr receptor and that most Gr genes are expressed in spatially restricted domains of the gustatory system. These observations imply the potential for high taste discrimination of the Drosophila brain (Dunipace, 2001).

The genomic organization of the Gr genes was investigated. Thirty-six Gr genes are arranged in clusters of two to six members. Genes within a cluster are more conserved (up to 50% identity; 70% similarity). The intergenic distance between clustered genes is very short, in many cases between 150-300 base pairs from the end of one open reading frame to the beginning of the next. Two genes appear to be pseudogenes (Gr22b and Gr22d) since their coding sequence is interrupted by a stop codon and a frame shift mutation, respectively. The number of introns varies widely because some genes have five or more introns and others have none. About half of the genes have a single conserved intron near the carboxy terminus. There are potentially alternative spliced genes at two loci, but in principle all genes that are arranged head to tail within a cluster and have conserved introns might be subjected to alternative splicing (Dunipace, 2001).

Because Gr gene expression could not be detected by in situ hybridization, a transgenic approach was used. Putative promoter fragments of the Gr22a, Gr22c, Gr22e, and Gr22f genes were cloned from genomic DNA by PCR and inserted into the expression vector SM1 in front of the GAL4 gene. These 'drivers', in combination with a UAS-lacZ reporter, allow visualization of transgene expression in vivo. At least three independent transgenic lines of flies homozygous for each of these drivers were crossed to flies homozygous for a UAS-lacZ reporter, and the double-hemizygous progeny were analyzed by whole-mount staining to monitor the activity of the promoter fragments. No ß-gal activity was observed in the main body parts, such as the head, thorax, and abdomen. Analysis of the appendages of flies with the driver P [22e]-Gal4, however, demonstrates that this gene is expressed in many cells in the antenna, maxillary palps, proboscis, legs, and wings. Specifically, P [22e]-Gal4 is expressed in cells at the base of many chemosensory bristles of the labial palps and in cells located in the dorsal and ventral cibarial sense organs. It is also expressed along the tibiae and tarsi of all legs and the anterior wing margin in cells that are located at the base of chemosensory bristles. This highly specific, localized expression is particularly evident in the anterior wing margin, which contains two morphologically distinct types of bristles. Thinner and slightly bent chemosensory bristles occasionally interrupt thick and straight mechanosensory bristles. Only cells at the base of chemosensory bristles express ß-Gal. Finally, a number of cells in the third antennal segment and the maxillary palp also express Gr22e. Their expression pattern in olfactory organs is similar to the one observed for most Or genes, but it appears to cover a somewhat larger area (Dunipace, 2001).

Expression of the driver P [22c]-Gal4 is restricted to the tarsi of the foreleg at the base of bristles, which are gustatory, based on morphological criteria. No transgene expression is observed in the labial palps, the labral and cibarial sense organs, or the anterior wing margin. Yet another expression pattern was observed with the driver P [22f]-Gal4, for which only four to eight positive cells are found on the labial palps in chemosensory sensilla. The cell bodies of these neurons are farther from the epithelial surface than are those of neurons in the wing margin, and it is therefore not always possible to correlate a LacZ-positive cell with a particular bristle. LacZ staining can be seen within the taste bristles themselves. Only chemosensory, not mechanosensory, neurons extend their dendrite into the bristle cavity, and this observation suggests that these LacZ-positive cells are gustatory neurons (Dunipace, 2001).

These studies have shown that three Gr genes located at 22B are excellent candidates for encoding taste receptors. To generalize these findings for the entire Gr gene family, four additional transgenes were created by cloning the promoters of Gr10a, Gr59b, Gr63a, and Gr66a into the GAL4 expression vector. Using these constructs, three independent lines of transgenic flies were generated and analyzed. Two of these drivers were expressed exclusively in the gustatory neurons of the adult. The driver P [66a]-Gal4 is expressed in sensilla of the foreleg, the labial palps, and the labral and ventral cibarial sense organs but not in the mid leg, hind leg, or wing. In the labial palps, the expression of Gr66a is not restricted to a single row of bristles, as is Gr22f expression, but extends more laterally through two rows of sensory bristles. The driver P [59b]-Gal4 is expressed only in the labial palps and at weaker levels than either Gr22f or Gr66a (Dunipace, 2001).

Taken together, these analyses show that five of eight analyzed genes (62.5%) are expressed in distinct gustatory sensilla in all major taste organs of Drosophila. Most of the genes are expressed in a very small fraction (1%-4%) of gustatory sensilla in a spatially restricted region of the fly, whereas one gene is expressed in about 20% of sensilla distributed all over the fly. Thus, they are excellent candidates for taste receptor genes (Dunipace, 2001).

To establish the neuronal identity of the Gr-expressing cells, immunolocalization experiments were performed. Each sensillum contains not only gustatory neurons (and in most cases one mechanosensory neuron) but also three non-neuronal cells that form the hair, socket, and sheet surrounding the dendrites of the neurons. Confocal microscopy was performed and all neurons were visualized with an anti-Elav antibody. The Gr-expressing cell was visualized with an anti-ß-Gal antibody. Both drivers, P [22e]-Gal4 and P [22f]-Gal4, stain neurons that project a dendrite toward the epithelium and an axon toward the brain. Interestingly, none of the more than 50 sensilla analyzed contained more than one ß-Gal-positive cell (Dunipace, 2001).

The projection patterns of neurons expressing the putative gustatory receptors were investigated. The gustatory neurons in the labial palps project their axons through the labial nerve to the subesophageal ganglion (SOG) in the brain. It was asked whether axons expressing a given receptor converge to a specific region within the SOG. Flies with the drivers P [22e]-Gal4 and P [66a]-Gal4 were crossed to a UAS-lacZ or UAS-Tau-lacZ line. Tau-LacZ is a Tau-ß-Gal fusion protein that is preferentially localized in axons and dendrites. Flies with the driver P [66a]-Gal4 show distinct staining of the labial and accessory pharyngeal nerves, and this staining reflects the expression of this gene in neurons of the labial palps and the labral/cibarial sense organs. As the nerves enter the brain, the axons terminate in two distinct regions, occupying only a fraction of the SOG. Axon projections of the driver P [22e]-Gal4 were visualized with confocal microscopy. These axons converge to a somewhat larger domain within the SOG, and this finding presumably reflects the wider expression domain of Gr22e when compared to Gr66a. Nevertheless, the ß-Gal-positive region occupies only a part of the SOG. Axonal convergence of gustatory receptor neurons, however, is not as defined as in the olfactory system, where axons expressing an individual Or gene project to one glomerulus in the antennal lobe (Dunipace, 2001).

A genetic approach was used to show that the neurons expressing the Gr genes are indeed chemosensory neurons. Pox-n is a paired-box-containing transcription factor involved in several steps of neuron specification in all developmental stages. Some pox-n alleles do not interfere with the early steps in neurogenesis but affect the determination of chemosensory neurons in the adult. For example, flies carrying the poxn70-23 allele are viable but show complete transformation of gustatory neurons into mechanosensory neurons. pox-n mutant flies were generated carrying three Gal4 drivers and a single copy of a UAS-lacZ reporter and reported expression was analyzed. None of the drivers was expressed in the gustatory organs in these flies, whereas siblings that carried one wild-type pox-n allele expressed each driver in a normal pattern. Antenna and maxillary palp expression of Gr22e remained normal in pox-n flies (Dunipace, 2001).

Thus, by several different criteria, the Gr genes are expressed in gustatory neurons of the adult. Furthermore, these experiments also demonstrate that neurons in the labial palps and the labral/cibarial sense organs expressing individual taste receptors project their axon to a distinct region in the SOG of the brain (Dunipace, 2001).

The location of neurons expressing a given receptor is conserved between individuals. This was determined by comparing the location and number of LacZ-positive sensilla of flies from three independent lines containing either the P [22f]-Gal4 or the P [22c]-Gal4 driver. The expression patterns of both transgenes were highly reproducible, but small variations were observed. For example, most flies with the P [22c]-Gal4 driver had two positive cells at the tip of the foreleg; however, occasionally only one LacZ-positive cell was found at that location or one additional cell at a more proximal location in the foreleg or on the tarsi of the second and third leg. No positive cells were recorded on the labial palps, the labral and cibarial sense organs, or the wing. Flies with the P [22f]-Gal4 driver had between two and four ß-Gal-positive cells in a discrete row of taste bristles on the labial palps; again, this pattern is very restricted, since no ß-Gal-positive cells were encountered in the legs or wings (Dunipace, 2001).

Variations in receptor gene expression might reflect a stochastic mechanism underlying the transcriptional control of Gr gene expression. For example, a neuron within a given sensillum might have a certain probability of expressing Gr X, a lower probability of expressing Gr Y, and virtually no probability of expressing any of the remaining Gr genes. The possibility that these differences are transgene-dependent effects and that the endogenous gene is precisely expressed in the same cells in each animal cannot be excluded. Whatever the reason for these modest variations, these experiments demonstrate that there exists a relatively precise topographic map for individual receptor gene expression. It should be noted that Gr22e is expressed in many more neurons than any other gene analyzed and that such a topographic map might not apply to this gene (Dunipace, 2001).

What is the neuron-to-receptor ratio in the Drosophila gustatory system? Based on analysis of about 15% (8/54) of all Gr genes, a global expression profile of this gene family can be predicted. The ratio of expressed genes to total number of genes appears to be similar in the gustatory (62.5%) and olfactory systems (66.7%). Thus, about 30-35 Gr genes might be expressed in the gustatory system. These studies reveal two distinct expression profiles. One gene, Gr22e, is expressed in about 20% of the taste sensilla throughout the gustatory system, whereas the remaining genes are expressed in more defined regions that occupy only about 1%-4% of all taste sensilla. If the remaining Gr genes are expressed in a similar profile, about 6-7 genes would be found to be expressed in 20% of sensilla and 27-28 genes would be found to be expressed in 1%-4% of sensilla (about 20 Gr genes would not be expressed at all). Such a breakdown would ultimately require that a given sensilla express about two Gr genes. However, since each sensillum contains on average of three gustatory neurons, the one-neuron-to-one-Gr-gene rule would still apply. In fact, the remaining neuron in each sensillum might express another type of receptor, such as the trehalose receptor. Therefore, these data are consistent with a model found in the olfactory system of both Drosophila and mammals in which each sensory neuron expresses only one receptor gene. Convergence of axons expressing a specific Gr gene to a specific domain within the SOG is also found. This situation is similar to that found in the olfactory system in which neurons expressing a specific Or project their axons to a single glomerulus in the antennal lobe (Dunipace, 2001).

Molecular identification of a taste receptor gene for trehalose in Drosophila

An independent study (Ishimoto, 2000) has identified a Trehalose-sensitivity gene that is unrelated to the GR family discussed above. In Drosophila, taste sensilla are present on the labellum, tarsi, and wing margins. In a typical chemosensillum on the labellum, there are four taste sensory cells, each of which responds to either water, salt, or sugar. The Trehalose-sensitivity (Tre) gene was identified through studies on natural variants. The Tre gene has been cytologically mapped to the region between 5A10 and 5B1-3 on the X chromosome. Because the Tre gene controls taste sensitivity to trehalose without affecting the responses to other sugars, the gene product of Tre should function in sugar receptor cells. Disruption of the Tre gene lowers the taste sensitivity to trehalose, whereas sensitivities to other sugars are unaltered. Overexpression of the Tre gene restores the taste sensitivity to trehalose in the Tre deletion mutant (Ishimoto, 2000).

Although several conserved regions are found between Tre1 and other GPCRs, the structures of the third and fourth cytoplasmic domains may be unique, because they are longer than the corresponding domains of typical GPCRs. The Tre1 gene most closely resembles two other orphan receptors of Drosophila: EG:22E5.11 and EG:22E5.10. It is suggested that the Tre gene may represent a new subclass of taste receptors (Ishimoto, 2000)

Taste perception and coding in Drosophila

Discrimination between edible and contaminated foods is crucial for the survival of animals. In Drosophila, a family of gustatory receptors (GRs) expressed in taste neurons is thought to mediate the recognition of sugars and bitter compounds, thereby controlling feeding behavior. The expression of eight Gr genes in the labial palps, the fly's main taste organ, has been characterized in detail. These genes fall into two distinct groups: seven of them, including Gr66a, are expressed in 22 or fewer taste neurons in each labial palp. Additional experiments show that many of these genes are coexpressed in partially overlapping sets of neurons. In contrast, Gr5a, which encodes a receptor for trehalose, is expressed in a distinct and larger set of taste neurons associated with most chemosensory sensilla, including taste pegs. Mapping the axonal targets of cells expressing Gr66a and Gr5a reveals distinct projection patterns for these two groups of neurons in the brain. Moreover, tetanus toxin-mediated inactivation of Gr66a- or Gr5a-expressing cells shows that these two sets of neurons mediate distinct taste modalities -- the perception of bitter (caffeine) and sweet (trehalose) taste, respectively. It is concluded that iscrimination between two taste modalities -- sweet and bitter -- requires specific sets of gustatory receptor neurons that express different Gr genes. Unlike the Drosophila olfactory system, where each neuron expresses a single olfactory receptor gene, taste neurons can express multiple receptors and do so in a complex Gr gene code that is unique for small sets of neurons (Thorne, 2004).

The labellum, considered to be the main taste organ in Drosophila, has approximately 62 chemosensory bristles (sensilla) that are arranged in a stereotyped pattern. These sensilla are morphologically identified as short (S), intermediate (I), and long (L). S and L bristles house dendrites of four chemosensory neurons, whereas I bristles are associated with two chemosensory neurons. To determine expression of Gr genes in these chemosensory neurons, the Gal4/UAS system has been used. This indirect method of expression analysis has proven far superior to RNA in situ hybridization due to low levels of Gr transcripts per cell and the wide distribution of taste neurons in tissues not amenable to sectioning procedures. The Gal4/UAS analyses revealed that a given Gr gene is expressed in a small number of chemosensory neurons per labial palp and, in each case, in only one neuron per chemosensory bristle. Hiroi (2002) demonstrated an association of specific Gr genes with certain bristles of the labellum. The majority of receptors examined are expressed in one of the four neurons of S type sensilla. For example, several Gr genes are strongly expressed in a single neuron associated with three S type sensilla (S1, S3, and S6) (Thorne, 2004).

Several issues with broad implications for taste coding remain to be elucidated. For example, it is still not known whether some Gr genes are coexpressed in the same neurons and, if so, to what extent. Similarly, it is not known what kind of taste properties are mediated by GRNs expressing these receptors. Finally, experiments to visualize axonal targets in the CNS of neurons expressing individual Gr genes have not been performed in any detail. To further advance understanding of Drosophila taste perception, these questions were addressed: the number of neurons expressing novel and previously characterized Gr genes was addressed, their extent of coexpression was investigated, the projection patterns of GRNs expressing these genes was visualized, and taste perception of flies lacking specific sets of GRNs was determined (Thorne, 2004).

Gal4 drivers (p[Gr]-Gal4) for eight Gr genes, Gr5a, Gr22b, Gr22e, Gr22f, Gr28be, Gr32a, Gr59b, and Gr66a, were combined with a UAS-nucGFP reporter gene encoding a green fluorescent protein tagged with a nuclear localization signal and images of optical sections through the entire labellum were collected by using confocal microscopy after anti-GFP antibody staining. By using the map generated by Hiroi (2002) as a guide, detailed analysis of confocal stacks allowed the organization and number of neurons expressing each of these genes to be more accurately determined. The expression patterns fell into two broad groups: Gr5a (representing the first group) was expressed in a large number of neurons throughout the entire labial palp, whereas the other Gr genes had restricted expression to relatively few neurons (Thorne, 2004).

Of the second group, Gr66a was expressed in the largest number (n = 22 ± 1)of cells per palp. Significantly, only a single neuron per S and I type sensillum stained positive for this driver. The neurons associated with S type bristles, which are located more medially, appeared larger in size compared to more laterally located neurons of I type sensilla. Gr22b, Gr22e, Gr22f, Gr28be, Gr32a, and Gr59b were expressed in fewer neurons than Gr66a. Expression of these receptors appears more restricted to larger neurons associated mostly with S type bristles. These expression studies provided the groundwork necessary to determine whether two or more Gr genes are actually coexpressed in the same neuron associated with an S type bristle (Thorne, 2004).

Ideally, coexpression of Gr genes may be addressed by labeling individual Gr gene-specific probes with different markers. However, expression levels of these genes are too low for reliable detection of transcripts by RNA in situ hybridization. Attempts were made to use the Gal4/UAS system along with a second reporter system, the tetracycline transactivator/tet-O reporter system. The sensitivity of this system, however, was too low to obtain reliable cell staining in taste neurons. Therefore, the issue of coexpression was addressed by quantification of labeled cells using the Gal4/UAS system, an approach that seemed feasible given the relatively low number of cells in which each receptor is expressed. Transgenic fly lines were made expressing UAS-nucGFP under the control of two different Gal4 drivers and then the number of labeled neurons was counted and compared to that of flies containing each driver alone. Surprisingly, in all cases where such double-driver experiments were carried out, the number of labeled cells expressing two drivers was close or equal to the number of labeled cells of flies containing the single driver with the higher cell count. For example, in flies that express either the p[Gr66a]-Gal4 or p[Gr22e]-Gal4 driver, an average of 22 and 14 neurons/labial palp are labeled, respectively. In flies that express both drivers, again approximately 22 neurons are detected per palp, which indicates that most if not all cells that express Gr22e also express Gr66a (Thorne, 2004).

A crucial determinant for discerning chemical cues present in the environment is embedded in the peripheral expression pattern of cell surface receptors in sensory epithelia. In the olfactory systems of Drosophila and mice, each olfactory receptor neuron expresses only one of 60 or one of approximately 1000 Or genes, respectively, enabling these animals to discriminate between hundreds or thousands of different odors. In contrast, taste cells of the tongue allow mammals to distinguish only a few taste qualities: bitter, sweet, umami, salty, and acidic taste. Lack of discrimination between the hundreds of diverse chemical compounds -- all perceived as bitter -- is thought to be caused by coexpression of the approximately 40 T2R receptors in a single set of taste cells (Chandrashekar, 2000; Zhang, 2003). Therefore, activation of the bitter taste cells by any one of the T2Rs is likely to generate a single activation pattern in taste centers of the brain, leading to a similar, repulsive behavioral output. Associating primary taste centers in the mammalian brain with specific taste modalities has, as of yet, proved challenging (Thorne, 2004).

Insect taste is still rather poorly understood, especially at the molecular level. Drosophila, which exhibits remarkably similar taste preferences with humans, is the only insect for which candidate receptors have been characterized experimentally. The investigations presented here provide significant new insights into insect taste perception (Thorne, 2004).

Initial expression studies suggested that the fly gustatory receptors are not simply coexpressed in three sets of cells dedicated to bitter, sweet, and umami taste like the T2Rs, T1R2/T1R3, and T1R1/T1R3 receptors of mammals. Instead,these experiments suggested that they either are expressed according to the one receptor (the 'one neuron' hypothesis well established for insect and mammalian olfactory systems) or they are expressed in partially overlapping sets of neurons. The current analysis supports the latter of these possibilities. Most labellar Gr genes (seven out of eight) are expressed in a single neuron of mostly S and some I type bristles. Most interestingly, coexpression studies provide evidence that individual neurons express anywhere from one to six receptors. In this way, S bristle-associated neurons are defined by unique receptor gene codes, thereby outfitting the labellum with an array of sensory assemblies that may exhibit distinct, albeit overlapping, ligand specificities (Thorne, 2004).

The functional implications of distinct neuronal receptor codes on taste perception are currently unclear and will require analysis of mutations of individual Gr genes. However, a general role for these neurons in feeding inhibition ('avoidance neurons') can be inferred from experiments presented in this study and supported through analogy with the mammalian taste system/receptors. (1) Avoidance neurons express the majority of analyzed Gr genes -- and by extension -- the majority of the genes in the entire Gr gene family. In mammals, bitter taste receptors far outnumber the sweet taste receptors (40:3). (2) Avoidance neurons associated with S type bristles do not express the receptors for the sugar trehalose encoded by the Gr5a gene. In fact, avoidance neurons associated with S type bristles have a distinct appearance compared to neurons expressing the Gr5a gene. In mammals, the sweet/umami taste receptors and the bitter taste receptors are expressed in distinct group of cells (Chandrashekar, 2000; Zhang, 2003). (3) Inactivation of avoidance neurons has no effect on sucrose or trehalose sensitivity in flies but significantly reduces their sensitivity to caffeine. (4) Avoidance neurons and Gr5a-expressing neurons have distinct targets in the subesophageal ganglion (SOG), a feature consistent with the detection of different taste qualities by these neurons (Thorne, 2004).

If the avoidance neurons have a general function in the detection of toxic or otherwise undesirable chemicals, what is the rationale for a complex and distinct Gr gene code among different groups of such neurons? It is proposed that the receptor code allows a fly to discriminate among different chemicals, which are in general avoided but might have distinct consequences on their health if ingested. According to such a proposal, a fly encountering a food source rich in nutrients (sugars) but contaminated with toxic chemicals may choose between feeding and avoidance, depending on the impact the particular toxic compound may have on its health. There is indirect evidence from feeding studies in Maduca sexta larvae that discrimination between the bitter substrates caffeine and aristolochic acid does occur in insects, even though actual taste preference, adaptation, or both may contribute to this phenomenon (Glendinning, 2001). Thus, discrimination among toxic/bitter-tasting compounds might be possible in insects including Drosophila (Thorne, 2004).

It was somewhat surprising that the sensitivity to other compounds known to be avoided by insects -- denatonium benzoate, quinine hydrochloride, and berberine -- was not affected in animals lacking Gr66a-expressing neurons. This may simply be explained by the presence of additional neurons expressing receptors that recognize these particular substrates. Alternatively, one or a few neurons coexpressing Gr66a along with a receptor for one (of these) ligand(s) might not have been completely inactivated by TNT. Finally, studies in rodents indicate that caffeine may directly affect neurons in the brain, circumventing activation of taste cells altogether. This is not likely to be the case in the current experiments, because none of the Gr genes examined is expressed in the CNS (Thorne, 2004).

Relatively few studies have investigated bitter taste sensitivity in insects, particularly Drosophila. Electrophysiological studies have identified bristles in the legs, but not the labellum of Drosophila, that respond to bitter-tasting chemicals (Meunier, 2003). However, S type sensilla are notoriously difficult to record from, because their bristles are extremely difficult to access for this type of experiment (Thorne, 2004).

Gr5a-expressing neurons represent more than half of chemosensory cells in the labellum and appear to be associated with all sensilla types, including the taste pegs. In fact, association of Gr5a with taste pegs provides the best evidence yet that these sensilla have a specific chemosensory function in the detection of trehalose. Significantly, Gr5a-expressing neurons define a largely distinct set of neurons from the avoidance neurons. This observation is consistent with the results from behavioral investigations of flies lacking the function of specific sets of neurons. Specifically, inactivation of Gr5a-expressing neurons leads to a reduction in trehalose sensitivity, but the sensitivity to any bitter substrate tested was unaffected. These flies did not exhibit reduced sucrose sensitivity, another nutrient-relevant sugar for Drosophila. This result is somewhat unexpected, since electrophysiological investigations have led to the proposal that a single neuron in L, I, and S bristles is responsive to several sugars including trehalose and sucrose (the 'sugar' neuron) (Dahanukar, 2001; Hiroi, 2002; Rodrigues, 1981; Dethier, 1976). According to these studies, sugar neurons may express a single, broadly tuned sugar receptor, or more likely, they may coexpress several distinct sugar receptors, each of which recognizes a specific sugar (i.e., sucrose, trehalose, fructose, etc.). This latter possibility is favored from genetic studies, which have shown that mutation in the Gr5a gene reduces the sensitivity of flies to trehalose, but not to sucrose. However, the proposition of a single sugar neuron per bristle is also not consistent with expression studies, which show that two to three neurons within a bristle can express Gr5a. The possibility that the p[Gr5a]-Gal4 drivers do not represent endogenous Gr5a expression cannot be excluded, but this is unlikely to be the case for two reasons: (1) several lines with p[Gr5a]-Gal4 show the same expression, and (2) the p[Gr5a_C]-Gal4 driver containing a much larger promoter fragment produces a similar expression profile, with many clusters of Gr5a-expressing neurons associated with the same bristle (Thorne, 2004).

In order to realign the electrophysiological data with expression analysis, another explanation is proposed: the 'sugar neuron' identified in electrophysiological studies expresses many (possibly all) distinct sugar receptors, including GR5a. However, one or two additional neurons per bristle express only a fraction, or possibly just one, of the sugar receptors present in the sugar neuron. Worth noting in this context is the fact that electrophysiological recordings are carried out at significantly higher substrate concentrations (up to 100 mM for sucrose and trehalose) than behavioral experiments (2 mM for sucrose and 25 mM for trehalose). The model is also more consistent with recent experiments (Hiroi, 2002) that noted different electrophysiological sugar responses among labellar sensilla (Thorne, 2004).

Approximately 45 labellar neurons have not yet been associated with any Gr gene, and some of these neurons might express putative candidate receptors for sucrose or additional sugars. These genes are likely to be encoded by members of the Gr64 gene cluster, which share much higher sequence similarity with Gr5a than any other Gr genes. Gal4 drivers for two of these genes (Gr64a and Gr64e) were analyzed and found to be expressed in the pharyngeal taste organs, but not in the labellum. Whether these two receptors are indeed involved in sugar detection remains to be seen, but it is predicted that other Gr genes for sugars like sucrose and fructose are be expressed more broadly and in taste neurons of labellar bristles and pegs (Thorne, 2004).

In summary, expression and behavioral studies suggest two fundamentally different roles for neurons expressing nonoverlapping groups of Gr genes in the detection of substrates that lead to feeding or avoidance behavior. According to this new model, S and I bristles on the labial palps contain one avoidance neuron and one or more feeding neurons (depending on the number of neurons associated with the particular bristle). The avoidance neuron expresses multiple Gr genes, and avoidance neurons of different bristles express these Gr genes in different combinations (the Gr gene code). The feeding neurons, which appear morphologically smaller than the avoidance neurons, express an entirely different set of receptors that includes Gr5a and possibly Gr genes encoding receptors for other sugars, amino acids, and peptides (Thorne, 2004).

The different functions for GRNs expressing Gr66a and Gr5a are also supported by their different projection patterns in the brain. Neurons expressing Gr66a or any of the partially coexpressed receptors target similar regions in the SOG/tritocerebrum, though the number of termini differs significantly depending on the number of peripheral sensory neurons the Gr is expressed in. For example, Gr66a-expressing neurons show a robust array of termini in the SOG/tritocerebrum, whereas the termini of Gr59b- and Gr22f-expressing neurons are significantly less numerous. In all cases, dense, contralaterally projecting fibers provide extensive innervation of both halves of the SOG by labellar neurons, as demonstrated by labial palp ablation experiments (Thorne, 2004).

An entirely different projection pattern is observed for feeding neurons that express Gr5a. Most strikingly, the axon termini of these neurons are distributed over a very large area of the SOG and extend into regions not innervated by avoidance neurons. A second striking difference is the poorly established contralateral connective between the two halves of the SOG, suggesting that neurons located in the right labial palp preferentially terminate in the right half of the SOG. This idea was tested and confirmed through ablation studies and has interesting implications, namely that spatially restricted activation of neurons in one palp will preferentially stimulate the same side of the SOG; this could potentially allow for spatial discrimination of taste input in the brain. This feature might allow the fly to orient its labellum in the direction of a food source, identifying regions with high concentrations of trehalose or other sugars (Thorne, 2004).

The distinct pattern of axon termini in the SOG of neurons required for feeding and avoidance suggests that these behaviors are mediated through different neuronal pathways. Anatomical studies in honeybees have identified second-order neurons that mediate synaptic activity of primary taste neurons to higher brain centers. It will be interesting to see whether second-order neurons contacting synapses of avoidance and feeding neurons define different target regions in these higher brain centers (Thorne, 2004).

Taste is an ancient sense, which exists in bacteria in the form of chemotaxis. Neuroanatomical and molecular comparison of taste systems between mammals and insects imply that this sense has evolved independently in these phyla. In mammals, for example, taste ligands are perceived through sensory epithelial cells in the lingual epithelium of the tongue. These cells then activate secondary neurons that innervate taste centers in the brain. In insects, tastants are detected by primary sensory neurons that directly innervate the CNS. Moreover, insects have multiple taste organs (legs, wings, and in some cases, the female genitalia) for which no counterparts exist in mammals. Finally, sequence comparison of the Gr and T1R/T2R genes has failed to reveal any direct kinship between mammalian and insect taste receptors (Thorne, 2004).

However, a remarkable convergence of anatomical as well as molecular features of gustatory systems between mammals and insects (Drosophila) appears to emerge from these studies. The functional taste units, the taste buds in the tongue and the taste bristles of the labellum, are composed of 30 to 100 taste cells and two to four chemosensory neurons, respectively. Individual taste cells in each taste bud are dedicated to the perception of sweet, umami, or bitter taste sensation based on the T1R or T2R receptors they express. Similarly, the data indicate that taste bristles of the labellum contain neurons that either respond to repulsive or attractive stimuli, properties that are likely determined by the specific (set of) taste receptors they express (Thorne, 2004).

Despite the sequence divergence of mammalian and insect taste receptors, it is believed there are intriguing similarities at the molecular level as well. The number of taste receptors in mammals and Drosophila is very similar. The eight genes described in this study probably encode a significant number of the functional labellar taste receptors. Some of the 60 Gr genes are likely to encode taste receptors only expressed in the pharynx, legs, and wings or might only be expressed in the larva. Other Gr genes are likely to function as pheromone receptors, or might recognize internal ligands based on their restricted expression in the CNS. Considering these alternative functions for some Gr genes, it is estimated that the fly has about 30 to 45 labellar taste receptors, a number close to the total number of T1Rs and T2Rs (30 in humans and 45 in mice) (Thorne, 2004).

In addition to the similar size of the Gr and T1R/T2R gene families, taste receptors of mammals and Drosophila fall into similar functional groups. Only three mammalian T1R receptors are thought to be dedicated to the detection of attractive stimuli (sugars and amino acids/proteins), whereas the large majority -- the T2Rs -- are thought to be exclusively involved in the detection of repulsive (bitter) ligands. If the current expression analysis is more or less representative of the entire Gr gene family, it might be expected that 25 to 40 Gr genes will be expressed in the avoidance neurons, whereas just three to six are expected to be expressed in feeding neurons. Identification and analysis of Gr genes encoding receptors for known ligands, combined with biochemical analyses, should reveal whether additional molecular features are shared between the GRs and T1Rs and T2Rs, such as whether Drosophila also possess a specific receptor for amino acids and whether some receptors also function as multimers, as is proposed for mammalian T1Rs (Thorne, 2004).

Behavioral analysis of bitter taste perception in Drosophila larvae

Insect larvae, which recognize food sources through chemosensory cues, are a major source of global agricultural loss. Gustation is an important factor that determines feeding behavior, and the gustatory receptors (Grs) act as molecular receptors that recognize diverse chemicals in gustatory receptor neurons (GRNs). The behavior of Drosophila larvae is relatively simpler than the adult fly, and a gustatory receptor-to-neuron map was established in a previous study of the major external larval head sensory organs. This study extensively examined The bitter taste responses of larvae using 2-choice behavioral assays. First, a panel of 23 candidate bitter compounds was tested to compare the behavioral responses of larvae and adults. Nine bitter compounds were tested, which elicit aversive behavior in a dose-dependent manner. A functional map of the larval GRNs was constructed with the use of Gr-GAL4 lines that drive expression of UAS-tetanus toxin and UAS-VR1 in specific gustatory neurons to identify bitter tastants-GRN combinations by suppressing and activating discrete subsets of taste neurons, respectively. The results suggest that many gustatory neurons act cooperatively in larval bitter sensing, and that these neurons have different degrees of responsiveness to different bitter compounds (Kim, 2015).

Gustatory receptor 21a and Gustatory receptor 63a confer CO2-chemosensation in Drosophila

Blood-feeding insects, including the malaria mosquito Anopheles gambiae, use highly specialized and sensitive olfactory systems to locate their hosts. This is accomplished by detecting and following plumes of volatile host emissions, which include carbon dioxide (CO2). CO2 is sensed by a population of olfactory sensory neurons in the maxillary palps of mosquitoes and in the antennae of Drosophila. The molecular identity of the chemosensory CO2 receptor, however, remains unknown. This study reports that CO2-responsive neurons in Drosophila co-express a pair of chemosensory receptors, Gr21a and Gr63a, at both larval and adult life stages. Mosquito homologues of Gr21a and Gr63a, GPRGR22 and GPRGR24 have been identified; these are co-expressed in A. gambiae maxillary palps. Gr21a and Gr63a together are sufficient for olfactory CO2-chemosensation in Drosophila. Ectopic expression of Gr21a and Gr63a together confers CO2 sensitivity on CO2- insensitive olfactory neurons, but neither gustatory receptor alone has this function. Mutant flies lacking Gr63a lose both electrophysiological and behavioural responses to CO2. Knowledge of the molecular identity of the insect olfactory CO2 receptors may spur the development of novel mosquito control strategies designed to take advantage of this unique and critical olfactory pathway. This in turn could bolster the worldwide fight against malaria and other insect-borne diseases (Jones, 2007).

CO2 elicits a response from many insects, including mosquito vectors of diseases such as malaria and yellow fever, but the molecular basis of CO2 detection is unknown in insects or other higher eukaryotes. Gr21a and Gr63a, members of a large family of Drosophila seven-transmembrane-domain chemoreceptor genes, are coexpressed in chemosensory neurons of both the larva and the adult. The two genes confer CO2 response when coexpressed in an in vivo expression system, the 'empty neuron system.' The response is highly specific for CO2 and dependent on CO2 concentration. The response shows an equivalent dependence on the dose of Gr21a and Gr63a. None of 39 other chemosensory receptors confers a comparable response to CO2. The identification of these receptors may now allow the identification of agents that block or activate them. Such agents could affect the responses of insect pests to the humans they seek (Kwon, 2007; full text of article).

Modification of CO2 avoidance behaviour in Drosophila by inhibitory odorants

The fruitfly Drosophila melanogaster exhibits a robust and innate olfactory-based avoidance behaviour to CO2, a component of odour emitted from stressed flies. Specialized neurons in the antenna and a dedicated neuronal circuit in the higher olfactory system mediate CO2 detection and avoidance. However, fruitflies need to overcome this avoidance response in some environments that contain CO2 such as ripening fruits and fermenting yeast, which are essential food sources. Very little is known about the molecular and neuronal basis of this unique, context-dependent modification of innate olfactory avoidance behaviour. This study identified a new class of odorants present in food that directly inhibit CO2-sensitive neurons in the antenna. Using an in vivo expression system it was established that the odorants act on the Gr21a/Gr63a CO2 receptor. The presence of these odorants significantly and specifically reduces CO2-mediated avoidance behaviour, as well as avoidance mediated by 'Drosophila stress odour'. A model is proposed in which behavioural avoidance to CO2 is directly influenced by inhibitory interactions of the novel odours with CO2 receptors. Furthermore, differences were observed in the temporal dynamics of inhibition: the effect of one of these odorants lasts several minutes beyond the initial exposure. Notably, animals that have been briefly pre-exposed to this odorant do not respond to the CO2 avoidance cue even after the odorant is no longer present. Related odorants were shown to be effective inhibitors of the CO2 response in Culex mosquitoes that transmit West Nile fever and filariasis. These findings have broader implications in highlighting the important role of inhibitory odorants in olfactory coding, and in their potential to disrupt CO2-mediated host-seeking behaviour in disease-carrying insects like mosquitoes (Turner, 2009).

CO2 is an important sensory cue for many animals, including insects, in a variety of behavioural contexts. In Drosophila, CO2 is exclusively detected by a unique heteromeric receptor encoded by Gr21a and Gr63a that is expressed in a single population of antennal olfactory receptor neurons (ORNs), called ab1C, which innervate the ab1 class of large basiconic sensilla. These neurons send stereotypical axonal projections to the V glomerulus, and activation of this dedicated uni-glomerular circuit leads to an innate avoidance of CO2 (Turner, 2009).

In fact, CO2 is a major component of Drosophila stress odour (dSO), which is emitted by flies subjected to vigorous shaking or electric shock, and which elicits an immediate escape response in naive flies (Suh, 2004). However, CO2 is also present in significant quantities in several important food sources that elicit behavioural attraction of Drosophila. Fruits and plants emit CO2 as a by-product of respiration, as do fruits undergoing fermentation by microorganisms and yeasts. Flies are attracted to headspace odours containing CO2 collected from over-ripe fruits, fermenting yeast and beer when presented with a choice between two tubes in a T-maze assay, one containing air and the other containing headspace odours. However, flies avoid headspace odours collected from green fruits, which also emit CO2. A subset of specialized gustatory neurons mediate a small degree of attraction to carbonated water upon contact (Fischler, 2007); however, they do not respond to CO2 in the gas phase and are not likely to contribute to long-range or short-range behavioural attraction towards a food source. Therefore, olfactory avoidance to CO2 may be modified by context for some CO2-rich sources such as over-ripe fruit, yeast and beer (Turner, 2009).

Little is known about the molecular and neuronal mechanisms that lead to such a dramatic modification of innate avoidance behaviour. Two alternative models, although not mutually exclusive, may be evoked to explain this phenomenon. In the first model, avoidance to CO2 is overcome simply by detection of attractive odorants emitted by the same food sources. In the second model, some components of food volatiles may also directly inhibit the CO2-responsive circuit, and thereby suppress avoidance behaviour to CO2 (Turner, 2009).

To test whether odorants present in fruits and other natural environments of Drosophila can directly inhibit CO2-sensitive ab1C neurons, a simple electrophysiology screen was performed. Several individual odorants were tested for their ability to inhibit the baseline activity of the ab1C neuron (to about 0.03% CO2 present in room air) using single-sensillum electrophysiology. These experiments were performed using Or83b2 mutant flies in which the ab1C neuron remains the sole functional neuron in the ab1 sensillum. In a screen with 46 odorants, two, 1-hexanol and 2,3-butanedione, were identified that strongly inhibit the baseline activity of ab1C neurons. Both of these compounds are present in Drosophila food sources including various types of fruit. More interestingly, the abundance of both these compounds is greatly increased during the ripening process of fruits: for example, in banana, 1-hexanol increases by 777% and 2,3-butanedione by 14,900%. 1-Hexanol is formed during ripening by lipid oxidation of unsaturated fatty acids, whereas 2,3-butanedione is a natural by-product of fermentation of carbohydrates through pyruvate by yeasts and bacteria and is thus also present in fermenting fruit, wine (Turner, 2009).

Both 1-hexanol and 2,3-butanedione inhibit CO2 response in a dose-dependent manner, irrespective of whether their application is initiated before, or after, the presentation of the CO2 stimulus at relatively low, physiologically relevant concentrations (Turner, 2009).

A fly approaching an odour source from a distance likely contacts plumes of CO2, which will vary widely in concentration over baseline atmospheric levels. When several concentrations of CO2 were tested, it was found that the presence of 2,3-butanedione (10-1 dilution) completely inhibits responses up to 3.2% CO2; 1-hexanol (10-1 dilution) also causes a significant reduction of CO2 response across most tested concentrations, but complete inhibition occurs only at 0.1% CO2 (Turner, 2009).

To understand odorant structural features that might have a role in inhibition, a rationally designed panel of odorants was tested that varied in the number of carbon atoms and in the nature of the functional group. On the basis of this analysis, additional odorants were identified that also inhibit CO2 response. The inhibitory effects of each of the compounds identified so far are specific to the CO2-sensitive neuron; previous studies have shown that all of them can excite other classes of Drosophila ORNs, which suggests that they are not general inhibitors of ORN function. Surprisingly, these compounds are structurally quite different from CO2, thus raising the possibility that they may act through allosteric binding sites within the Gr21a/Gr63a receptor, or on other components of the CO2 detection pathway such as factors present in the sensillar lymph or in ab1C neurons (Turner, 2009).

To investigate whether the inhibitors act directly on the CO2 receptor, Gr21a and Gr63a were expressed in an in vivo decoder system called the 'empty neuron'. It was found that expression of Gr21a and Gr63a in the empty ab3A neuron is sufficient to impart a robust and reproducible dose-dependent CO2 response, comparable to the levels reported previously. Upon application of each of the four inhibitory odorants along with CO2, dose-dependent inhibition was observed of CO2 response of the ab3A neuron in a Gr21a/Gr63a-dependent manner. The simplest interpretation of these results is that the odorants that were identified inhibit CO2 response by direct interaction with the CO2 receptor, Gr21a/Gr63a. However, the inhibitory effect appears shorter in duration than observed in the endogenous ab1C neurons, suggesting that additional neuron- or sensillum-specific factors may also influence the temporal aspects of the inhibition (Turner, 2009).

Next it was asked whether the inhibitory odorants identified using electrophysiology could disrupt avoidance behaviour of Drosophila to CO2. Using a T-maze choice assay, it was found that wild-type Drosophila show a robust avoidance behaviour to 0.67% CO2. Inclusion of 2,3-butanedione with CO2 completely abolishes avoidance to CO2. Importantly, 2,3-butanedione by itself does not elicit any significant attraction or avoidance behaviour. In wild-type Drosophila, however, several other ORN classes are activated by 2,3-butanedione, raising the possibility that behavioural avoidance to CO2 may be overcome by activation of these other classes of ORN, and not solely by inhibition of CO2-responsive neurons (Turner, 2009).

To distinguish between these possibilities, the behaviour of Or83b2 mutant flies was tested under conditions in which most ORNs are non-functional, but electrophysiological responses to CO2 are not affected. Consistent with the electrophysiological analysis, flies lacking Or83b have a robust avoidance response to CO2, which is absent when 2,3-butanedione is included with CO2 or is presented alone. Similar results, albeit with weaker effects, are obtained using 1-hexanol. Taken together, these results show that inhibitory odorants can effectively block CO2-mediated innate avoidance behaviour (Turner, 2009).

CO2 is one of the main components of dSO, which is emitted by stressed flies, and which triggers a robust avoidance behaviour in naive flies. Therefore, whether 2,3-butanedione can disrupt avoidance to dSO was tested. It was found that naive flies avoid odour collected from a tube of vortexed flies (dSO), but not that collected from a tube of untreated flies (mock), in a T-maze assay. Remarkably, addition of 2,3-butanedione to dSO effectively abolishes avoidance behaviour (Turner, 2009).

Interestingly, it was observed that with increasing concentrations of 2,3-butanedione, the CO2 neuron is silenced well beyond the period of application. This effect is specific to 2,3-butanedione and is not observed for 1-hexanol. To investigate this further, the fly was exposed to a 3-s stimulus of 2,3-butanedione (10-1 dilution) and subsequently tests were performed for the recovery of ab1C neuron responsiveness by applying a 0.5-s stimulus of 0.3% CO2 every 30 s, over a period of 10 min. Surprisingly, the inhibitory effect of the initial exposure to 2,3-butanedione persisted for an extended period (Turner, 2009).

It was of interest to test whether behaviour was also affected in a similar manner. Flies were exposed for 1 min to 2,3-butanedione and then transferred them to clean air for 2 min before testing for CO2-mediated avoidance behaviour. Remarkably, CO2 avoidance is almost abolished in pre-treated flies. Prior exposure to another odorant 2-methyl phenol, which does not inhibit the CO2 response, does not have any effect on behaviour. Moreover, pre-exposure to 2,3-butandione does not have a significant effect on behavioural attraction towards a different odorant, ethyl acetate. Taken together, these observations show that exposure to a long-term CO2 response inhibitor can exert a profound and specific effect on the behaviour of the animal, even after it is no longer present in the environment. Similar observations were made with Or83b mutant flies (Turner, 2009).

To demonstrate unambiguously that 2,3-butanedione causes behaviour modification primarily by inhibiting CO2 responsiveness of ab1C neurons and not by other peripheral or central mechanisms, the following experiment was performed. The ab1C neuron was activated in a manner that is not inhibited by 2,3-butanedione, and it was asked whether 2,3-butanedione inhibits avoidance behaviour in this context. The odorant, butanone, which activates ab1C neurons strongly at 10-1 dilution in a Gr63a-dependent manner, was identified. It was found that Or83b mutant flies strongly avoid butanone (10-1 dilution) whereas flies lacking both Or83b and Gr63a do not, as predicted from the electrophysiology data. However, electrophysiological response to butanone is not affected by pre-exposure to, or the presence of, 2,3-butanedione, unlike what was observed for CO2. In a T-maze behaviour assay, 2,3-butanedione has no effect on behavioural avoidance of Or83b mutant flies to butanone, regardless of whether it is used to pre-treat the flies as described above or is included in a mixture with butanone. These results demonstrate that 2,3-butanedione disrupts CO2 avoidance behaviour by directly inhibiting the CO2 responsiveness of ab1C neurons, rather than by other indirect mechanisms (Turner, 2009).

CO2 emitted in human breath is a critical component of odour blends used as host-seeking cues by many vector insect species that carry deadly diseases, including Culex quinquefasciatus mosquitoes that transmit filarial parasites in tropical countries, and West Nile virus in the USA and various parts of the world. Culex mosquitoes have three conserved proteins that are closely related to the Drosophila CO2 receptors, Gr21a and Gr63a. To test whether odorants that inhibit Drosophila CO2 receptors can also inhibit CO2 response in Culex, CO2-sensitive A neurons in peg sensilla on the surface of the maxillary palps of Culex mosquitoes were tested using a panel of structurally related odours. It was found found that electrophysiological response to CO2 is not inhibited by 2,3-butanedione, but is strongly inhibited by 1-butanal and 1-hexanol. These odours are the first reported inhibitors of CO2-sensitive neurons in mosquitoes and may provide a valuable resource for the identification of economical, environmentally safe, volatile compounds that may reduce mosquito-human contact by blocking responsiveness to CO2 (Turner, 2009).

A heuristic underlies the search for relief in Drosophila melanogaster

Humans rely on multiple types of sensory information to make decisions, and strategies that shorten decision-making time by taking into account fewer but essential elements of information are preferred to strategies that require complex analyses. Such shortcuts to decision making are known as heuristics. The identification of heuristic principles in species phylogenetically distant to humans would shed light on the evolutionary origin of speed-accuracy trade-offs and offer the possibility for investigating the brain representations of such trade-offs, urgency and uncertainty. By performing experiments on spatial learning in the invertebrate Drosophila melanogaster, this study showed that the fly's search strategies conform to a spatial heuristic-the nearest neighbor rule-to avoid bitter taste (a negative stimulation). That is, Drosophila visits a salient location closest to its current position to stop the negative stimulation; only if this strategy proves unsuccessful does the fly use other learned associations to avoid bitter taste. Characterizing a heuristic in D. melanogaster supports the view that invertebrates can, when making choices, operate on economic principles, as well as the conclusion that heuristic decision making dates to at least 600 million years ago (Meda, 2022).

Two Gr genes underlie sugar reception in Drosophila

The molecular basis of sugar reception in Drosophila has been analyzed. The response spectrum, concentration dependence, and temporal dynamics of sugar-sensing neurons has been defined. Using in situ hybridization and reporter gene expression, members of the Gr5a-related taste receptor subfamily were identified that are coexpressed in sugar neurons. Neurons expressing reporters of different Gr5a-related genes send overlapping but distinct projections to the brain and thoracic ganglia. Genetic analysis of receptor genes shows that Gr5a is required for response to one subset of sugars and Gr64a for response to a complementary subset. A Gr5a;Gr64a double mutant shows no physiological or behavioral responses to any tested sugar. The simplest interpretation of these results is that Gr5a and Gr64a are each capable of functioning independently of each other within individual sugar neurons and that they are the primary receptors used in the labellum to detect sugars (Dahanukar, 2007).

A major problem in neurobiology is how an animal decides what to eat. The fruit fly evaluates gustatory input to assess the nutritive value of a potential food source. In particular, the detection of sugars is a crucial factor in determining whether a food source is accepted. Despite its critical importance to the survival of the species, little is known about the molecular basis of sugar perception in the fly. A central goal in the field has been to define the receptors that mediate sugar detection (Dahanukar, 2007).

Sugars, salts, bitter compounds, and certain other molecules are detected by gustatory neurons, which are widely distributed in the body of the fly. Neurons that influence feeding behavior are present in the labellum as well as the tarsal segments of each of the legs. Activation of either labellar or tarsal gustatory neurons with a sugar solution results in proboscis extension, which is a component of feeding behavior (Dahanukar, 2007).

Gustatory neurons are housed in sensory hairs called sensilla. Each half of the labellum is covered with ~31 prominent taste hairs, arranged in a stereotypical pattern, and a number of smaller structures called taste pegs. Each of the 31 sensilla is typically innervated by four gustatory neurons and a single mechanosensory neuron. Physiological analysis has shown that one of the chemosensory neurons is activated by sucrose and other sugars, and has been referred to as the 'sugar' neuron. Another neuron is activated by salts and has been named the 'salt' neuron. A third neuron is activated by pure water but not by solutions of high osmolarity; it has been named the 'water' neuron. The fourth chemosensory neuron responds to aversive compounds such as caffeine, and has been named the (Dahanukar, 2007 and references therein).

In Drosophila, a large, highly diverse family of gustatory receptor (Gr) genes was identified by genomic analysis. The family consists of 60 genes encoding 68 predicted seven-transmembrane-domain receptors. In previous studies, Gr5a was identified as a receptor for trehalose, a disaccharide sugar (Chyb, 2003). Gr5a is expressed in a large number of gustatory neurons in the labellum (Chyb, 2003), and recent studies have shown that Gr5a serves as a marker for the sugar neuron in each sensillum. Bitter neurons express Gr66a, also a member of the Gr gene family, which is required for physiological and behavioral responses to caffeine (Moon, 2006). Promoter expression analysis of several other gustatory receptor genes in the labellum suggested that all of those tested were coexpressed with Gr66a in subsets of bitter neurons (Thorne, 2004; Wang, 2004; Dahanukar, 2007 and references therein).

Axonal projections of Gr5a-positive and Gr66a-positive neurons have been mapped to the subesophageal ganglion (SOG) of the brain (Thorne, 2004; Wang, 2004). The two classes of neurons project to nonoverlapping regions in the SOG, suggesting that at the first level of processing, attractive and aversive inputs may be segregated. Evidence that Gr5a neurons mediate attractive signals and Gr66a neurons mediate aversive signals was provided by expression of a capsaicin receptor in each of these classes of neurons (Marella, 2006). In the first instance, flies showed behavioral attraction to capsaicin, and in the second instance they were repelled by it (Dahanukar, 2007).

Gr5a-labeled neurons are responsive not only to trehalose, but to sucrose and other sugars (Wang, 2004; Marella, 2006). Physiological and behavioral analysis showed that sucrose response is not affected in flies lacking Gr5a, suggesting that these neurons express at least one other receptor; however, other receptors in sugar neurons were not identified (Dahanukar, 2007).

This study examined the responses of sugar neurons in the largest sensilla of the labellum, the 'L' sensilla. Of 50 compounds tested, including 34 diverse sugars, a small number were identified, primarily disaccharides and oligosaccharides, which elicit robust electrophysiological responses in sugar neurons. In situ hybridization and reporter gene expression determined that two other Gr genes, both phylogenetically related to Gr5a, are coexpressed with Gr5a in sugar neurons. Neurons expressing reporters of each receptor gene show distinct projection patterns, providing a mechanism by which information from different subpopulations of sugar cells in the periphery could be spatially represented in the brain (Dahanukar, 2007).

Having found coexpression of Gr5a-related genes in sugar neurons, mutants of Gr5a and two related genes were examined by electrophysiology and behavioral analysis. Gr5a was found to be required for detection of a small subset of sugars including trehalose. Deletion mutants lacking Gr64a shows that it is required for response to a complementary subset of sugars. Strikingly, flies lacking both Gr5a and Gr64a do not show electrophysiological or behavioral responses to any tested sugar. These results demonstrate that the sugars divide into two classes that are dependent either on Gr5a or on Gr64a for their responses. The simplest interpretation of these results is that these two receptors are capable of operating independently of each other in an individual sugar neuron, and that they constitute the primary basis of sugar reception in the fly (Dahanukar, 2007).

Sucrose generated the strongest responses among a panel of 50 compounds tested at 100 mM. Sucrose is present at comparable concentrations in many fruits, including citrus, peaches, and pineapples. Turanose, palatinose, and leucrose are all isomers of sucrose and also elicit responses of various strengths. Many of the sugars that evoke responses, including glucose and trehalose, are found in fruits and vegetables or in yeasts and may thus be encountered by the fly in its natural environment (Dahanukar, 2007).

The responses depend on sugar concentration as well as identity. The neurons are sensitive to a number of sugars over concentrations that span three orders of magnitude. The dose-response curves of different sugars, however, are distinct: they differ in threshold, slope, and maximal firing rate observed. Many of these sugars are present in fruits at concentrations of 100-300 mM, and at these concentrations the responses lie well within the dynamic range of the neurons. Surprisingly, responses to fructose and glucose, which are particularly abundant in fruits, are much weaker than those of sucrose, even when compared at concentrations that have equal caloric values. However, the concentrations of both fructose and glucose are typically higher than that of sucrose in fruits such as apples, bananas, and grapes, suggesting that sugar neurons may be most sensitive to changes in sugar concentrations over a range that is ecologically relevant (Dahanukar, 2007).

Molecular analysis has revealed coexpression of Gr61a and Gr64f with Gr5a, and genetic analysis of a double mutant has provided evidence for coexpression of Gr64a with Gr5a in sugar neurons. These results suggest that at least some labellar sugar neurons, including those of L-type sensilla, coexpress four receptors of the Gr5a subfamily (Dahanukar, 2007).

Molecular and genetic evidence indicates that Gr5a is expressed in essentially all labellar sensilla. Molecular analysis has provided evidence that Gr64f is also broadly expressed, and functional evidence suggests that Gr64a is as well. Specifically, an electrophysiological survey showed that all labellar sensilla in wild-type flies respond to sucrose, a sugar that acts via Gr64a. In a Gr5a;Gr64a mutant all morphological types of sensillum (L, M, S, I, P) showed no activity in response to sucrose; moreover, nearly all of the L, M, I, and P sensilla were tested, suggesting that Gr64a acts in all, or almost all, of the 31 sensilla on the labellum. Furthermore, the double mutants are also behaviorally unresponsive to sugars. Thus Gr5a and Gr64a seem likely to be expressed in all or almost all sugar neurons in the labellum, and perhaps Gr64f is as well (Dahanukar, 2007).

Gr61a, however, appeared to be restricted in its expression among labellar sensilla, both by in situ hybridization and by analysis of a Gr61a-GAL4 driver. These results suggest a subdivision of labellar sugar neurons into two classes based on the presence or absence of Gr61a. No function was defined for Gr61a; however, mutational analysis suggests that it does not play a role in responses to any of the sugars in the panel. It is possible that Gr61a is required for response to other sugars or sugar derivatives that have not yet been tested or for responses to another class of behaviorally attractive compounds. Further electrophysiological analysis with an expanded panel of tastants may provide insight into whether there are functional differences among sugar-sensing neurons and whether these differences correlate with the expression of Gr61a (Dahanukar, 2007).

Gr5a and Gr64a are both required for normal responses of sugar neurons, but for different subsets of sugars. Flies lacking Gr5a are severely defective in physiological and behavioral responses to one subset of sugars, including trehalose; flies lacking Gr64a are severely defective in responses to a complementary subset of sugars, including sucrose. All tested sugars fall into one of these two subsets. These results suggest that Gr5a and Gr64a function as distinct receptors in the same neurons, rather than as obligate heterodimeric coreceptors, as in the mammalian sugar receptor T1R2+T1R3 (Dahanukar, 2007).

It is possible that Gr5a and Gr64a function as heterodimeric receptors with other members of the Gr family, such as Gr64f. Two recent studies report deletions of part or all of the Gr64 cluster that result in reduced behavioral responses to trehalose; the phenotype is rescued by supplying a transgene containing five of the six receptors encoded by this cluster (Slone, 2007), but not by Gr64a alone (Jiao, 2007). These data support the idea that one of the receptors in this cluster other than Gr64a may function in concert with Gr5a to mediate trehalose response. There is precedent for such interactions from Or proteins, which dimerize with the noncanonical receptor Or83b (Dahanukar, 2007).

The neat subdivision of sugars into those dependent on Gr5a and those dependent on Gr64a was surprising. A simple structural criterion to distinguish the two classes of sugars is not immediately evident upon inspection. The Gr64a-dependent sugars are remarkably diverse in structure, with some containing glucose units and some containing fructose subunits; they ranged in size from one to four subunits. Gr5a-dependent sugars also vary in size, subunit composition, and linkage types (Dahanukar, 2007).

In Gr5a mutants, there are some weak residual responses to the affected subset of sugars; likewise, in Gr64a mutants, some of the affected sugars continue to elicit some response. Since there is no residual response in the Gr5a;Gr64a double mutant, the simplest interpretation of these results is that each receptor provides the residual function observed when the other is eliminated, i.e., the two receptors exhibit some limited redundancy (Dahanukar, 2007).

Gr5a and Gr64a share 28% amino acid identity and 47% amino acid similarity. Both receptors are evolutionary conserved and are found in all of the 12 Drosophila species for which genome sequences are available, with the exception that D. pseudoobscura appears to have lost Gr5a. The receptor most closely related to Gr5a is Gr64f (40% amino acid identity), and the receptor most closely related to Gr64a is Gr61a (36% amino acid identity). Although evidence was found that Gr64f and Gr61a are both expressed in sugar neurons, no functions have been identifed for them. The possibility cannot be excluded of a role for Gr61a or Gr64f in response to compounds not tested, such as glycoproteins or glycolipids, or in neurons whose responses have not been measured, such as those of internal chemosensory cells. It is noted that in mammals, an amino acid receptor (T1R1+T1R3) comprises a subunit, T1R3, of the heterodimeric sugar receptor (T1R2+T1R3). However, L-type sensilla did not respond to any of 18 amino acids tested, making it unlikely that either Gr61a or Gr64f mediates responses to this class of compounds (Dahanukar, 2007).

Classic physiological and biochemical studies led to the proposal of a 'fructose' site in sugar-sensing neurons. The current studies provide a molecular and genetic identity to this site: fructose response is completely abolished by loss of Gr64a and is completely restored by the addition of a Gr64a transgene. These results also provide a molecular explanation for the earlier finding that sucrose responses were not affected in a Gr5a mutant. These results suggested the presence of another receptor within the sugar neuron, a receptor that has now been identified as Gr64a (Dahanukar, 2007).

It is noted that two recent studies have identified a role for members of the Gr64 cluster in mediating sugar responses (Jiao, 2007; Slone, 2007), particularly that of Gr64a in response to sugars including sucrose, maltose, and glucose (Jiao, 2007). Consistent with the observations, physiological and behavioral responses to sucrose were restored to wild-type levels in transgenic rescue experiments; no role was observed for Gr64a in glucose response. One of these studies (Jiao, 2007) also provided biochemical evidence that Gr5a-related receptors are expressed in sugar-sensitive neurons (Dahanukar, 2007).

In summary, the simplest interpretation of the results is that Gr5a and Gr64a are the primary sugar receptors in the labellum of the adult fly. Each is capable of mediating response to a subset of sugars independently of the other, and together they are able to identify the food sources that are sufficiently rich in caloric value as to sustain the life of the fly (Dahanukar, 2007).

Which Sugar to Take and How Much to Take? Two Distinct Decisions Mediated by Separate Sensory Channels

In Drosophila melanogaster, gustatory receptor neurons (GRNs) for sugar taste coexpress various combinations of gustatory receptor (Gr) genes and are found in multiple sites in the body. To determine whether diverse sugar GRNs expressing different combinations of Grs have distinct behavioral roles, this study examined the effects on feeding behavior of genetic manipulations which promote or suppress functions of GRNs that express either or both of the sugar receptor genes Gr5a (Gr5a+ GRNs) and Gr61a (Gr61a+ GRNs). Cell-population-specific overexpression of the wild-type form of Gr5a (Gr5a(+)) in the Gr5a mutant background revealed that Gr61a+ GRNs localized on the legs and internal mouthpart critically contribute to food choice but not to meal size decisions, while Gr5a+ GRNs, which are broadly expressed in many sugar-responsive cells across the body with an enrichment in the labella, are involved in both food choice and meal size decisions. The legs harbor two classes of Gr61a expressing GRNs, one with Gr5a expression (Gr5a+/Gr61a+ GRNs) and the other without Gr5a expression (Gr5a-/Gr61a+ GRNs). Blocking the Gr5a+ class in the entire body reduced the preference for trehalose and blocking the Gr5a- class reduced the preference for fructose. These two subsets of GRNs are also different in their central projections: axons of tarsal Gr5a+/Gr61a+ GRNs terminate exclusively in the ventral nerve cord, while some axons of tarsal Gr5a-/Gr61a+ GRNs ascend through the cervical connectives to terminate in the subesophageal ganglion. It is proposed that tarsal Gr5a+/Gr61a+ GRNs and Gr5a-/Gr61a+ GRNs represent functionally distinct sensory pathways that function differently in food preference and meal-size decisions (Kohatsu, 2022).

Exposure to high-sugar diet induces transgenerational changes in sweet sensitivity and feeding behavior via H3K27me3 reprogramming

Human health is facing a host of new threats linked to unbalanced diets, including high-sugar diet (HSD), which contributes to the development of both metabolic and behavioral disorders. Studies have shown that diet-induced metabolic dysfunctions can be transmitted to multiple generations of offspring and exert long-lasting health burden. Meanwhile, whether and how diet-induced behavioral abnormalities can be transmitted to the offspring remains largely unclear. This study showed that ancestral HSD exposure suppressed sweet sensitivity and feeding behavior in the offspring in Drosophila. These behavioral deficits were transmitted through the maternal germline and companied by the enhancement of H3K27me3 modifications. PCL-PRC2 complex, a major driver of H3K27 trimethylation, was upregulated by ancestral HSD exposure, and disrupting its activity eliminated the transgenerational inheritance of sweet sensitivity and feeding behavior deficits. Elevated H3K27me3 inhibited the expression of a transcriptional factor Cad and suppressed sweet sensitivity of the sweet-sensing gustatory neurons, reshaping the sweet perception and feeding behavior of the offspring. Taken together, this study uncovered a novel molecular mechanism underlying behavioral abnormalities spanning multiple generations of offspring upon ancestral HSD exposure, which would contribute to the further understanding of long-term health risk of unbalanced diet (Yang, 2023).

Transgenerational behavioral change is present in many animal species. In C. elegans, exposure to pathogenic threats induces avoidance memories which can transmit for four generations via sRNA signaling. In fruit flies, exposure to predatory wasps leads to the inheritance of ethanol preference for five generations via maternal NPF repression (Bozler, 2019). Such a 'behavior memory' may be evolutionarily beneficial in a sense to pre-adapt offspring for changing environmental conditions. Nevertheless, it may also lead to devastating effects in human health. Individuals who were exposed to the Dutch Famine in early gestation exhibited deficits in metabolism, cardiovascular health, and mental health. Similarly, mice exposed to traumatic experiences and drugs induce depressive-like or autism-like behaviors in their progeny (Yang, 2023).

Globally, HSD has become a routine of modern lifestyle, which is linked to various human diseases, including obesity, type 2 diabetes, and neurobiological diseases. HSD also induces many behavioral disorders in animal models such as feeding abnormalities and addiction-like behaviors. This present study uncovers that HSD not only affects flies' overall physiology and metabolism within the same generation, but also affects their sweet sensitivity and feeding behavior in a manner spanning multiple generations of offspring. If similar observations hold true in human society, HSD exposure may lead to an additional layer of health risk that needs to be recognized and addressed (Yang, 2023).

Mechanistically, the data indicate that elevated H3K27me3 modifications upon ancestral HSD exposure are the key epigenetic factors underlying the transgenerational regulations of sweet sensitivity and feeding behavior. HSD exposure enhances genome-wide H3K27me3 but not H3K27ac or H3K9me2/3 modifications in early embryos. E(z) and Pcl, the key components of PCL-PRC2 complex, play a crucial role in catalyzing trimethylation of the repressive chromatin marker histone H3 lysine 27, thus maintaining this imprinting during fly development. Perturbation of PCL-PRC2 complex, via both genetic and pharmacological approaches, blocks the transmission of such repressive histone imprinting to offspring and eliminates the transgenerational modulation of sweet sensitivity and feeding behavior. Such epigenetic modulations are transmitted via the female germline, and exert a long-lasting effect on the expression of Cad (and possibly other transcriptional regulators) in the offspring. Cad, a transcription factor belonging to the Hox family, regulates the sensitivity of sweet-sensing gustatory neurons and plays a role in modulating PER responses to sucrose (Yang, 2023).

Feeding behavior is tightly regulated by various factors, such as internal nutritional needs, overall physiological status, and environmental cues. However, most of these regulations are quite dynamic in nature and do not last for long. For example, in fruit flies energy shortage can trigger foraging and feeding behavior in the timescale of hours, which can be rapidly suppressed upon acquisition of desirable food sources. These regulations are often mediated by rapid-acting molecular and cellular mechanisms such as ion channels, hormones, and neuropeptides. HSD can also modulate feeding behavior in a dynamic manner. Moreover, this mechanism is reversible within the same generation. For example, the number of PLCβ2+ taste bud cells in the fungiform papilla decreased within 4 weeks of HSD exposure in mice and was completely restored within 4 weeks upon the removal of HSD. However, the present study reveals a highly persistent and heritable mechanism of transgenerational modulation of sweet taste perception and feeding behavior that can transmit through multiple generations without re-exposure to the original HSD treatment. These findings suggest that feeding behavior can be modulated by different regulatory mechanisms with distinct timescales in response to various types of environmental and internal state changes. The relationship between these different mechanisms will be of great interest to further study. For example, it remains unclear whether and how hormonal and neurotransmitter changes that occur upon HSD exposure in flies within the same generation contribute to the reprogramming of H3K27me3 that can last for multiple generations (Yang, 2023).

Based on the present study, several important questions remain to be answered. The first question is how HSD evaluates H3K27me3 modifications. Previous work reported that some dietary bioactive compounds could regulate histone modifying enzymes. Thus, HSD may directly regulate the activity of histone methyltransferases or histone demethylases. Alternatively, HSD may affect H3K27me3 modifications via certain nutrient-sensing mechanisms such as O-GlyNAc transferase, which is known to facilitate H3K27me3 formation with PRC2. The second question lies in how elevated H3K27me3 modifications in specific gene loci are retained during gamete formation and embryo development, and how such epigenetic imprinting is removed after four to five generations on ND. Previous work reported that long-term memory of H3K27me3 depends on efficient copying of this mark after each DNA replication cycle in a PRC2-dependent manner (Coleman, 2017). It is therefore possible that PRC2-mediated maintenance of H3K27me3 imprinting can be regulated by dietary exposure. The third question is how H3K27me3 imprinting affects specific neurons in the adult via Cad signaling. This study showed that upregulated H3K27me3 modifications decreased Cad expression. Future studies are needed to examine how Cad signaling impacts the development and function of Gr5a+ gustatory neurons, and whether other transcription factors may also be involved. Previous work has reported that Cad regulates a network containing 119 candidate genes that were implicated in sensory perception of chemical stimulus, neuropeptide signaling pathways, signal transduction, and transcription factor activity, which could impact the sweet gustatory neurons (Vaziri, 2020) and play a role in modulating PER responses to sucrose (Yang, 2023).

This study focused on the function of histone modifications on transgenerational behavioral inheritance. It will also be of interest to explore other mechanisms that may also participate in the transgenerational inheritance of sweet perception. Especially, as the Piwi protein negatively regulates H3K27 trimethylation, future studies are needed to understand whether piRNAs, which have been reported to direct transient heterochromatin formation and stabilize maternal mRNAs during embryogenesis, are involved in the transgenerational inheritance identified in this study. Given HSD exposure imposes a systemic influence on flies' physiology and metabolism, other non-epigenetic factors, such as maternal nutritional conditions and their potential influences on the development and maturation of oocytes, may also play a role in the transgenerational inheritance of sweet perception upon ancestral HSD exposure. Furthermore, the identification of HSD exposure-induced transgenerational changes in sweet sensitivity and feeding raises the question of whether similar phenomena and mechanism can be extended to other behaviors. These findings highlight a novel and pivotal role of epigenetic modifications in preparing animals for the dynamic environment, which opens a new avenue of research to further uncover the interactions among prior experience, epigenetics, and behavioral modulations across generations (Yang, 2023).

Persistent epigenetic reprogramming of sweet taste by diet

Diets rich in sugar, salt, and fat alter taste perception and food preference, contributing to obesity and metabolic disorders, but the molecular mechanisms through which this occurs are unknown. This study shows that in response to a high sugar diet, the epigenetic regulator Polycomb Repressive Complex 2.1 (PRC2.1) persistently reprograms the sensory neurons of Drosophila melanogaster flies to reduce sweet sensation and promote obesity. In animals fed high sugar, the binding of PRC2.1 to the chromatin of the sweet gustatory neurons is redistributed to repress a developmental transcriptional network that modulates the responsiveness of these cells to sweet stimuli, reducing sweet sensation. Half of these transcriptional changes persist despite returning the animals to a control diet, causing a permanent decrease in sweet taste. These results uncover a new epigenetic mechanism that, in response to the dietary environment, regulates neural plasticity and feeding behavior to promote obesity (Vaziri, 2020).

In this study set out to understand how dietary experience alters the gustatory system to promote food intake and weight gain. Specifically, advantage was taken of the simple sensory system of D. melanogaster and its exquisite genetic and neural tools to identify the molecular mechanisms through which diet composition changes neural physiology and behavior. Previous work has shown high dietary sugar decreased the responsiveness of the sensory neurons to sugar stimuli, leading to a dulled sense of sweet taste, independently of fat accumulation. This study show sthat the decrease in sweet taste sensation that flies experience after chronic exposure to a high sugar diet is caused by the cell-autonomous action of the PRC2.1 in the sweet gustatory neurons. Mutations and pharmacological inhibition of PRC2.1 blocked the effects of the food environment on neural activity, behavior, and obesity. While the possibility that PRC1 and Pho RC may also be involved, this study found that mutations or knockdown in these complexes had no effect on taste. To this point, it was observed that in Pcl mutants, even if the neural responses to sucrose were identical in control and sugar diet fed flies, they were of lower in magnitude than those of control flies, suggesting that PRC2.1 may modulate plasticity bidirectionally in response to the nutrient environment. This also suggests that, within limits, it is the relative rather than the absolute output of the sensory neurons that is important for taste sensation and diet-induced obesity (Vaziri, 2020).

In the high sugar food environment, PRC2.1 chromatin occupancy was redistributed, leading to the repression of transcription factors, neural, signaling, and metabolic genes that decreased the responsiveness of the Gr5a+ neurons and the fly's sensory experience of sweetness. However, it was found that PRC2.1 did not directly bind to neuronal genes in these cells and that, instead, it targeted transcription factors involved in processes such as sensory neuron development, synaptic function, and axon targeting. Specifically, on a high sugar diet Pcl binding was increased at the loci of transcription factors cad, GATAe, nub/pdm, Ptx1 and decreased at the scro locus and lead to corresponding changes in the mRNA levels of these genes (Vaziri, 2020).

Computational analysis revealed that in the Gr5a+ cells, these five transcriptional factors regulate a network of ~658 candidate target genes implicated in synaptic function, signal transduction, and metabolism. Changes in the levels of the five transcription factors on a high sugar diet resulted in the overall repression of their target genes, providing a possible explanation for the alterations in the responsiveness of the Gr5a+ cells that were observed. Several positive and negative regulatory loops were predicted among the five transcription factors, suggesting that they could form a regulatory hub that is responsive to changes in the dietary environment. Knockdown of the four activators and a few of their targets and overexpression of scro resulted in a decrease in sweet taste on a control diet. However, overexpression of cad, Ptx1, and nub alone and knockdown of scro did not rescue taste deficiencies in animals fed a high sugar diet. Since there is (i) overlap in the predicted targets between the repressor scro and each of the four activators and (ii) overlap among the targets of the four activators, one possibility is that, as long as scro levels are higher because of the sugar diet, the repressive drive is so strong that overcoming it requires collaborative binding among the activators. Together, these findings suggest that this transcriptional hub and the gene battery it controls are necessary for sweet taste and reshaped by high dietary sugar (Vaziri, 2020).

How do these transcription factors and their targets modulate the physiology of these gustatory neurons? Several of these transcription factors (Ptx1, scro, and nub/pdm) control the proper branching, synaptic connectivity, and activity of sensory neurons, while others (cad and nub/pdm) play a role in neuroblast development; PRC2 also functions as a competence factor in neural proliferation, differentiation, and sensory neurons. It is proposed that the gene battery of ~658 genes controlled by this transcriptional hub may define the intrinsic properties of the sweet sensing neurons. It was observed that the four activators that are repressed by Pcl under the high sugar condition are enriched in the Gr5a+ cells while scro is depleted. Further, many of the target genes are involved in signaling, synaptic function, and cell adhesion, including the kinase haspin, the adenylate cyclase ACXD, syt-alpha, Arc1, and the tetraspanin, jonan, and innexin proteins among others. These genes were part of a highly interconnected network, which could affect the responsiveness and wiring of the sweet gustatory neurons. Since no change in the expression of the sweet taste receptors, or the misexpression of other taste receptors was detected, the data are not consistent with a complete 'loss' of identity of the Gr5a+ neurons with a high sugar diet. Instead, it is hypothesized that PRC2.1 tunes these sensory neurons to the dietary environment by altering a developmental transcriptional network that controls the intrinsic properties of the Gr5a+ cells, particularly those involved in signal transduction, connectivity, synaptic function, and metabolism. Studies that test the effects of this network on the wiring, morphology, and signal transduction of the sweet sensory neurons will shed light on how exactly the transcriptional remodeling caused by PRC2.1 found in this study affects the physiology of Gr5a+ cells (Vaziri, 2020).

While the experiments show that PRC2.1 chromatin occupancy shifts with the dietary environment, this study did not define the signaling mechanisms through which this change in binding occurs. Thus, the question of how exactly PRC2.1 binding is altered in response to the food environment remains open. Recent studies suggest that the activity of Polycomb Group Proteins is directly and indirectly linked to cellular metabolism, including kinase signaling cascades, the post-translational modification O-linked-N-acetylglucosaminylation (GlcNAcylation), and the availability of cofactors for histone modifications. Previous work showed that the hexosamine biosynthesis pathway enzyme O-GlcNAc transferase (OGT) acts in the Gr5a+ neurons to mediate the effects of high dietary sugar on sweet taste; whether the interaction between OGT and PRC2 is what promotes the repressive activity of the latter in these sensory neurons is a question worth investigating. Notably, the dysregulation of Polycomb-associated chromatin has been found in mice and humans with diet-induced obesity, suggesting that the mechanisms found in this study could also underlie the chemosensory alterations reported in mammals (Vaziri, 2020).

More broadly, this work opens up the exciting possibility that PRC2 may modulate neural plasticity in response to environmental conditions by reengaging developmental programs. Despite its central role in development and maintenance of neural identity, studies have not directly linked PRC2.1 to neural plasticity. However, in other postmitotic cells such as muscle, Polycomb Group Proteins are known to reshape transcriptional programs according to environmental stressors, such as oxidative stress, injury, temperature, and light. These findings advance the conceptual understanding of the role of Polycomb Group Proteins in the nervous system and suggest that they could also modulate 'neural states' and metaplasticity in response to environment stimuli. Using neuroepigenetic mechanisms such as those used by Polycomb Group Proteins to tune neural states to external conditions could provide several advantages compared to the medley of other cellular-, receptor-, or synaptic plasticity-based mechanisms. Specifically, it would allow cells to (i) orchestrate a coordinated response, (ii) create a memory of the environment, and (iii) buffer small fluctuations until a substantial challenge is perceived. It is particularly fascinating to think about the molecular mechanisms through which these neural states may be established. The need of neurons to constantly maintain their identity may mean that environmental signals such as the extent of sensory stimulation could alter the expression of developmental gene batteries and affect neural physiology. It has been speculated that some forms of plasticity may reengage developmental programs that specify the intrinsic properties of neurons. In this study it was observed that the regulators of the transcriptional network that was uncovered functions in sensory neuron development and are enriched in the Gr5a+ cells. Thus, it could be a hallmark of neuroepigenetic plasticity to exploit developmental programs, linking the known role of PRC2 in establishing cell fates with this newly found function in modulating cell states (Vaziri, 2020).

Incidentally, reengaging developmental programs could be the reason why some environments and experiences leave a memory that leads to the persistent expression of the phenotype beyond the presence of the triggering stimulus, as these could target neural connectivity and set synaptic weight thresholds. This study found that the changes in taste sensation and half of the sugar diet neural state set by PRC2.1 remained even after animals were moved back to the control diet for up to 20 days. A limitation of this study is that because of the small number of Gr5a+ neurons and their anatomically inaccessible location, it was not possible to measure the identity of the molecular memory in these cells alone. However, it was seen that the phenotypic memory of the high sugar food environment was dependent on the constitutive action of PRC2.1. Thus, on the basis of other studies showing that the H3K27 methyl mark acts as a molecular memory during development, it is speculated that this is likely to be the memory signal in the Gr5a+ cells too. Stable maintenance of the memory requires active recruitment of PRC2; while this study did not measure Pcl occupancy and chromatin accessibility at PREs in the recovery diet with and without the inhibitor, the findings that PRC1.2 is actively required for the maintenance of the taste phenotype and that 47% of its indirect targets are still repressed indicate that PRC2.1 may be stably recruited to the transcription factors loci. Perhaps, conditions that lead to metabolic remodeling such as prolonged fasting could reset its binding. Last, it is not known whether the diet-induced chemosensory plasticity observed in humans and rodents is persistent or reversible. Unlike in D. melanogaster, mammalian taste cells are not postmitotic neurons, and so, they regenerate every few weeks. Thus, the persistence of chemosensory plasticity in mammals, if it exists, may involve different mechanisms in the taste cells, such as a decrease in their renewal or changes in their wiring to sensory neurons. However, a decrease in the responses of the chorda tympani to sweetness has been observed in rats fed a 30% sucrose diet, and thus, the current findings may be applicable to the sensory nerves (Vaziri, 2020).

In conclusion, this study shows that PRC2.1 mediates the effects of high dietary sugar on sweet taste by establishing persistent alterations in the taste neurons that remain as a phenotypic and transcriptional memory of the previous food environment. It is speculated that this memory may lock animals into patterns of feeding behavior that become maladaptive and promote obesity. Thus, dietary experience, in ways like trauma, can induce lasting molecular alterations that restrict the behavioral plasticity of animals and affect disease risk. Since the content of sugar in processed foods is similar to that fed flies in this study and the function of Polycomb Group Proteins is conserved from plants to humans, this work is broadly relevant to understanding the effects of processed food on the mammalian taste system and its impact on food intake and a whole range of diet-related conditions and diseases that affect billions of people around the globe (Vaziri, 2020).

Molecular and cellular basis of acid taste sensation in Drosophila

Acid taste, evoked mainly by protons (H(+)), is a core taste modality for many organisms. The hedonic valence of acid taste is bidirectional: animals prefer slightly but avoid highly acidic foods. However, how animals discriminate low from high acidity remains poorly understood. To explore the taste perception of acid, the fruit fly was used as a model organism. Flies employ two competing taste sensory pathways to detect low and high acidity, and the relative degree of activation of each determines either attractive or aversive responses. Moreover, one member of the fly Otopetrin family, Otopetrin-like a (OtopLa), was established as a proton channel dedicated to the gustatory detection of acid. OtopLa defines a unique subset of gustatory receptor neurons and is selectively required for attractive rather than aversive taste responses. Loss of otopla causes flies to reject normally attractive low-acid foods. Therefore, the identification of OtopLa as a low-acid sensor firmly supports a competition model of acid taste sensation. Altogether, this study has discovered a binary acid-sensing mechanism that may be evolutionarily conserved between insects and mammals (Mi, 2021).

Sour taste, like sweet, bitter, salty, and umami tastes, represents a fundamental taste modality across many species ranging from insects to mammals. Typically, humans like slightly acidic foods such as lemon juice, which potentially indicates the presence of nutrients. In contrast, humans dislike highly acidic foods, which can cause digestive tract tissue injuries. The bivalent taste response to acid is also documented in rodents. Similar to mammals, the fruit fly, Drosophila melanogaster, prefers low levels of acid, which stimulate feeding and reproduction, and avoids high acid concentrations. Therefore, although flies and humans appear drastically different, the hedonic valence of their acid-taste response is similar: it can be either attractive or aversive, depending on the acid concentration of food. It is proposed that the bidirectional characteristic of acid perception constitutes an evolutionary fitness that enables animals to choose nutritious and reject unhealthy food sources. How do animals make this seemingly challenging decision? It is hypothesized that a taste-coding mechanism underlies the opposing feeding behavior triggered by low and high levels of acids. However, the molecular and cellular nature of the acid-taste coding has remained unclear (Mi, 2021).

Several lines of research demonstrate that type III taste receptor cells (TRCs) are responsible for acid sensing in mice. Nevertheless, the type III TRC population may be heterogeneous and contain different cell subtypes. Due to the lack of molecular markers and genetic tools to manipulate different subsets of type III TRCs, the question of how type III TRCs differentially respond to different concentrations of acid appears to be difficult to address in mammals. Moreover, other than eliciting taste sensation, acid also activates trigeminal nerves in the oral cavity of mammals, leading to burning or pain sensation. This side effect further confounds the investigation of sour-taste coding and sour-taste-triggered behavior in mammals. In contrast, flies exhibit much more pronounced and distinct taste responses to varying concentrations of acid than do mammals. Therefore, the fly serves as an excellent animal model to elucidate the taste coding of acid. This study reports that the fly mainly uses two different subsets of gustatory receptor neurons (GRNs) to selectively sense low or high concentrations of acids. The taste transduction pathways orchestrated by low- and high-acid GRNs antagonize each other, and the net behavioral response to a particular concentration of acid is predominantly determined by the relative activities of low- vs. high-acid GRNs (Mi, 2021).

Animals take advantage of highly diversified taste receptors and TRCs to detect varying taste substances, including sugar, salt, acid, and bitter compounds. In mammals, in contrast to the well-characterized sweet and bitter receptors, the molecular identity of sour-taste receptor had not been determined until a recent discovery showing that the Otopetrin (Otop) protein family functions as proton channels. In mice, one of the Otop family members, Otop1, is essential for sour-taste transduction. Despite this significant finding, the exact role played by Otop1 in discriminating low- from high-acid foods remained unclear. As the Otop family is fairly conserved between mammals and insects, it was of interest to see if the Otop family is also required for taste sensation of acids in Drosophila, given that no bona fide sour-taste receptors had been established in insects. One of the fly Otop orthologues, Otopetrin-like a (OtopLa), was shown to act as a proton channel and is selectively required for attractive taste sensation of acids in Drosophila. The fly OtopLa protein is localized at the tip of the GRN dendrite, the forefront site of taste sensory cells that is responsible for directly sensing tastant stimuli. Further, OtopLa defines a novel class of GRNs, which are largely distinct from other groups of GRNs responding to sugar, salt, or bitter tastants. Furthermore, genetic analysis showed that loss of otopetrin-like a (otopla) selectively abolishes the attractive acid-taste pathway, leaving the aversive pathway intact. Notably, the otopla mutant flies became abnormally averse to low concentrations of acid. Thus, this study provides strong genetic evidence to establish not only that the attractive and aversive taste pathways responsible for acid sensation exist but also that they are genetically segregated. Finally, by establishing OtopLa as a bona fide taste receptor for acid in flies, this work overturns the long-standing view that insects and mammals use fundamentally different gustatory receptors (Mi, 2021).

According to our behavioral assays, the wild-type fly displays opposing taste responses to low and high concentrations of acid: low concentrations are attractive, whereas high concentrations are aversive. Thus, the hedonic valence of acid taste is closely associated with the concentration of acid that the animals detect. The bidirectional valence of sour taste is reminiscent of salty taste, which is also dependent on salt concentrations. Given these findings, a key question arises as to how the animals discern low- from high-acid foods. Rlectrophysiology analyses of different groups of taste sensilla provide an important clue to this question. Flies were found to mainly use L- and S-type sensilla to perceive low and high concentrations of acid, respectively. The L-type sensilla mediate an attractive pathway, whereas the S-type sensilla operate an aversive pathway for the response to acids. It is postulated that acid-taste signals relayed by low- and high-acid GRNs antagonize each other in the brain, where feeding decisions are made. When the fly encounters low-acid foods, the attractive pathway mediated by low-acid GRNs dominates the aversive pathway mediated by the high-acid GRNs, driving the animal to choose the low-acid food. Conversely, when the animal encounters high-acid foods, the aversive pathway dominates the attractive pathway, leading to the avoidance of high-acid foods (Mi, 2021).

In support of this hypothesis, genetic analysis reveals that otopla is specifically required for the attractive rather than aversive acid-taste response. Further, we found that otopla is mostly expressed in the GRNs housed within the L-type rather than the S-type sensilla. In addition, recent work shows that a member of the ionotropic receptor (IR) family, Ir7a, is selectively required for the repulsive response to high concentrations of acetic acid in Drosophila. However, flies lacking Ir7a show normal attractive feeding responses to low concentrations of acetic acid. That study, combined with the present study on otopla, provides substantial evidence to support the model that the attractive and aversive pathways for acid sensation are segregated in the peripheral taste organ (Mi, 2021).

These findings lay the foundation for a more detailed analysis of the genetic program and neural circuit involved in sour-taste perception. In mammals, type III TRCs are mainly responsible for sour-taste sensation. Nevertheless, whether distinct subgroups of type III TRCs in the taste bud selectively respond to low or high acid remains an open question. Given the conservation of acid-taste sensation between flies and mammals, the acid-taste coding mechanism identified in the fly will inform the investigation of sour-taste coding in mammals, including humans (Mi, 2021).

Using multiple lines of evidence, these studies lead to the identification of OtopLa as a long-sought taste receptor for acid in Drosophila. First, genetic analyses show that OtopLa is both necessary and sufficient to orchestrate the attractive response to foods containing low concentrations of acid. In addition, loss of otopla has no effect on sweet, bitter, or salty tastes. Second, cell biological studies reveal that OtopLa is expressed in a group of GRNs different from sweet, bitter, and salty GRNs. Moreover, OtopLa proteins selectively reside in the distal portion of the dendrite, the forefront of the GRN responsible for detecting taste substances presented from the food environment. Last but not least, patch-clamp recordings reveal that OtopLa functions as a proton channel and can be directly activated by protons. Collectively, this work establishes OtopLa as a bona fide receptor that is dedicated to the attractive taste sensation of acids in Drosophila. Recent studies in mice show that the Otop1 proton channel is both necessary and sufficient for sour-taste transduction. In light of these discoveries in flies and mice, it is concluded that the Otop family is an evolutionarily conserved proton channel dedicated to taste the sensation of acids in both insects and mammals. Over the past two decades, various types of taste receptors, including sweet, bitter, and salty taste receptors, have been identified and functionally characterized in both invertebrates and vertebrates. Although insects and mammals exhibit a striking homology in taste responses, the molecular identities of their sweet, bitter, and salty taste receptors appear to be distantly related to each other. Consequently, there is a long-held view in the chemoreception field that taste receptors for insects and mammals are evolutionarily distant from each other. In this study, the discovery in the fly acid-taste sensation has overturned this notion. The Otop family represents the first class of taste receptors that is functionally conserved between insects and mammals. From an evolutionary perspective, it is proposed that Otop is a well-conserved proton channel family involved in acid-taste sensation throughout the animal kingdom. Thus, further research is needed to explore the gustatory role of the Otop family in other animal species, including humans (Mi, 2021).

otopla is necessary for the attractive taste sensation of both the strong acid HCl and the weak acids citric acid and malic acid. Psychophysical studies in human subjects report that weak acid usually tastes more sour than strong acid at the same pH40, implying that, in addition to protons, the undissociated weak acid molecules may also elicit sourness. These studies demonstrate that the proton channel OtopLa is broadly required for the taste sensation of both strong and weak acids. Therefore, it is proposed that the taste response to acids orchestrated by OtopLa mainly results from the gustatory stimuli of protons that are dissociated from either strong or weak acids. In addition, several members of the fly IR family are involved in taste responses to carbonation and acetic acid. As there has been no evidence showing that the IRs form a proton channel, the IRs are likely to be narrowly tuned to the specific structures of weak acids rather than to protons. Collectively, the fly may use different taste transduction pathways to perceive various acid molecules present in the environment (Mi, 2021).

In conclusion, given the significant conservation of taste receptors for acid between flies and mammals, the fly model will significantly advance understanding of the acid-taste sensation in other animals, including humans (Mi, 2021).

Requirement for an Otopetrin-like protein for acid taste in Drosophila

Receptors for bitter, sugar, and other tastes have been identified in the fruit fly Drosophila melanogaster, while a broadly tuned receptor for the taste of acid has been elusive. Previous work showed that such a receptor was unlikely to be encoded by a gene within one of the two major families of taste receptors in Drosophila, the ‘gustatory receptors' and ‘ionotropic receptors.' To identify the acid taste receptor, this study tested the contributions of genes encoding proteins distantly related to the mammalian Otopertrin1 (OTOP1) proton channel that functions as a sour receptor in mice. RNA interference (RNAi) knockdown or mutation by CRISPR/Cas9 of one of the genes, Otopetrin-Like A (OtopLA), but not of the others (OtopLB or OtopLC) severely impaired the behavioral rejection to a sweet solution laced with high levels of HCl or carboxylic acids and greatly reduced acid-induced action potentials measured from taste hairs. An isoform of OtopLA that was isolated from the proboscis was sufficient to restore behavioral sensitivity and acid-induced action potential firing in OtopLA mutant flies. At lower concentrations, HCl was attractive to the flies, and this attraction was abolished in the OtopLA mutant. Cell type-specific rescue experiments showed that OtopLA functions in distinct subsets of gustatory receptor neurons for repulsion and attraction to high and low levels of protons, respectively. This work highlights a functional conservation of a sensory receptor in flies and mammals and shows that the same receptor can function in both appetitive and repulsive behaviors (Ganguly, 2021).

The functional conservation of the Otop channels for acid taste in flies is striking given that chemosensory receptors tend to vary greatly in flies and mammals, which diverged ∼800 million y ago. In contrast to the Otop channels, the two major families of fly receptors (GRs and IRs), which function in tasting sugars, bitter compounds, acetic acid, amino acids, polyamines, N, N-diethyl-meta-toluamide (DEET), CO2, and other tastants are not present in mammals. The retention of Otop channels for acid taste in flies and mice is remarkable since the gross anatomies of the gustatory systems are very different. In addition, the taste receptor cells in flies are neurons, while they are modified epithelial cells in mammals (Ganguly, 2021).

The conserved role for Otop proteins for acid taste in flies and mammals cannot be explained by greater selective pressure for maintaining a receptor for a mineral (e.g., H+) versus organic molecules since other minerals (Ca2+ and Na+) are sensed in flies through IRs, which are not present in mammals. Thus, the retention of Otop channels for acid taste in flies and mammals underscores the very strong selection for this acid sensor for animal survival. Otop-related proteins are encoded in many distantly related terrestrial and aquatic vertebrates ranging from the platypus to frogs and pufferfish, as well as ancient invertebrates such as worms and insect disease vectors, including Aedes aegypti. Thus, despite the considerable diversity of most chemosensory receptors, it is plausible that Otop channels endow a large proportion of the animal kingdom with acid taste (Ganguly, 2021).

A question concerns the cellular mechanism through which the sensation of protons is detected. OtopLA is expressed in the four classes of GRNs in taste hairs (A to D). The B and D GRNs respond to aversive tastants (B, bitter, high Na+ etc; D, Ca2+, high Na+, K+), while the A and C GRNs are activated by chemicals that stimulate consumption (A, sugars, low Na+ fatty acids, etc; C, water). The data indicate that both B and D GRNs contribute to acid repulsion but that B GRNs comprise the major class required for acid repulsion, while D GRNs are the minor class. In support of this conclusion, RNAi knockdown of B but not D GRNs impaired acid repulsion. In addition, we fully rescued the OtopLA1 mutant phenotype by expression of the OtopLAp transgene in B GRNs but only partially rescued the deficit by expression of OtopLAp in D GRNs. In addition, our data indicate that both A and C GRNs contribute to the modest attraction to 0.01 HCl in wild-type flies. This attraction is eliminated in the OtopLA1 mutant. The C GRNs may be more important, as expression of the OtopLAp transgene in A GRNs reduced the impairment in the mutant, but the suppression of the phenotype fell below the threshold for statistical significance (Ganguly, 2021).

It has been reported that acids cause repulsion of sugary foods by direct activation of B GRNs and suppression of sugar-induced activation of A GRNs. This previous study focused on behavioral responses to carboxylic acids, and this study repeated this finding for citric acid. However, at the cellular level, when the pH of sucrose was decreased, no reduced sucrose-induced action potentials were induced. Thus, it is concluded that protons do not suppress A GRNs. Rather, it is suggest that A GRNs are inhibited by certain organic anion moieties of carboxylic acids. A mechanism by which the activities of both A GRNs and B GRNs are affected by carboxylic acids, but only B GRNs by protons could provide a coding mechanism for differentiating between protons and carboxylic acids (Ganguly, 2021).

Following submission of the initial version of this manuscript, another group also reported a role for OtopLA in acid taste in Drosophila (Mi, 2021). These researchers found that OtopLA is required for attractive responses to low concentrations of acids, as did this study, but not for aversive responses to higher concentrations of acids. However, even in wild-type controls, they did not observe significant repulsion until the pH was reduced to high levels (≤2) that may be damaging to cells. At these very low pHs, the nociceptive response is likely to have a major contribution to avoidance. Mi (2021). also reported that the flies exhibited a much higher level of attraction to acids than what was observed in the current study. The differences in levels of attraction and repulsion might be due to variations in fly food between laboratories, the precise ages of the flies, hours of starvation, or a combination of these factors. Nevertheless, although the level of acid attraction differs, both studies find that the deficit in attraction in OtopLA mutants can be suppressed by expression of a wild-type transgene in A GRNs. In addition, this study found that this phenotype is suppressed by expression of the OtopLA rescue transgene in C GRN (Ganguly, 2021).

Another difference between the current report and that of Mi (2021) is that they reported that expression of OtopLAa in human embryonic kidney 293 (HEK293) cells led to the appearance of inward currents in response to acid stimuli (pH range 6 to 3). It has been previously demonstrated that both vertebrate and invertebrate Otop proteins form proton channels. However, this study did not observe any acid-induced currents using stimuli as low as pH 3.0 for either OtopLAp or OtopLAa (FBgn0259994) expressed in either HEK293 cells or Xenopus oocytes, even though surface expression was detected when the channels were tagged with GFP. There are several possible reasons why the Drosophila OtopLA channel did not generate functional currents in either cell type. One possibility is that the native system provides factors or binding partners necessary to gate the channels. It is noted that OtopLA is the only one of the Drosophila Otop channels to have a large extracellular domain between transmembrane domains 5 and 6, which might bind ligands or proteins (Ganguly, 2021).

Together, these data point to a complex role of the OtopLA channel in the gustatory system of Drosophila, where it is expressed in multiple types of sensory cells and can mediate both attractive and aversive responses. Interestingly, humans also find acids appetitive at low concentrations and aversive at higher concentrations. The elucidation of the cellular and molecular mechanism of acid-sensing that we describe here can serve as the basis for further understanding as to how animals assign valence to stimuli that vary only in intensity (Ganguly, 2021).

Deciphering the genes for taste receptors for fructose in Drosophila

Taste sensitivity to sugars plays an essential role in the initiation of feeding behavior. In Drosophila melanogaster, recent studies have identified several gustatory receptor (Gr) genes required for sensing sweet compounds. However, it is as yet undetermined how these GRs function as taste receptors tuned to a wide range of sugars. Among sugars, fructose has been suggested to be detected by a distinct receptor from other sugars. While GR43A has been reported to sense fructose in the brain, it is not expressed in labellar gustatory receptor neurons that show taste response to fructose. In contrast, the Gr64a-Gr64f gene cluster was recently shown to be associated with fructose sensitivity. This study sought to decipher the genes required for fructose response among Gr64a-Gr64f genes. Unexpectedly, the qPCR analyses for these genes show that labellar expression levels of Gr64d and Gr64e are higher in fructose low-sensitivity flies than in high-sensitivity flies. Moreover, gustatory nerve responses to fructose in labellar sensilla are higher in Gr64d and Gr64f mutant lines than in mutant flies of the other Gr64a-Gr64f genes. These data suggest the possibility that deletion of GR64D or GR64F may indirectly induce enhanced fructose sensitivity in the labellum. Finally, it is concluded that response to fructose cannot be explained by a single one of the Gr64a-Gr64f genes (Uchizano, 2017).

Enhancing perception of contaminated food through acid-mediated modulation of taste neuron responses

Natural foods contain not only nutrients, but also nonnutritious and potentially harmful chemicals. Thus, animals need to evaluate food content in order to make adequate feeding decisions. This study investigated the effects of acids on the taste neuron responses and on taste behavior of desirable, nutritious sugars and sugar/bitter compound mixtures in Drosophila melanogaster. Using Ca2+ imaging, acids were shown to activate neither sweet nor bitter taste neurons in tarsal taste sensilla. However, they suppress responses to bitter compounds in bitter-sensing neurons. Moreover, acids reverse suppression of bitter compounds exerted on sweet-sensing neurons. Consistent with these observations, behavioral analyses show that bitter-compound-mediated inhibition on feeding behavior is alleviated by acids. To investigate the cellular mechanism by which acids modulate these effects, bitter-sensing gustatory neurons were silenced. Surprisingly, this intervention had little effect on acid-mediated derepression of sweet neuron or feeding responses to either sugar/bitter compound mixtures or sugar/bitter compound/acid mixtures, suggesting that there are two independent pathways by which bitter compounds are sensed. These investigations reveal that acids, when presented in dietary relevant concentrations, enhance the perception of sugar/bitter compound mixtures. Drosophila's natural food sources - fruits and cohabitating yeast - are rich in sugars and acids but are rapidly colonized by microorganisms, such as fungi, protozoan parasites, and bacteria, many of which produce bitter compounds. It is proposed that the acids present in most fruits counteract the inhibitory effects of these bitter compounds during feeding (Chen, 2014).

Presynaptic gain control drives sweet and bitter taste integration in Drosophila

The sense of taste is critical in determining the nutritional suitability of foods. Sweet and bitter are primary taste modalities in mammals, and their behavioral relevance is similar in flies. Sweet taste drives the appetitive response to energy sources, whereas bitter taste drives avoidance of potential toxins and also suppresses the sweet response. Despite their importance to survival, little is known about the neural circuit mechanisms underlying integration of sweet and bitter taste. This study describes a presynaptic gain control mechanism in Drosophila that differentially affects sweet and bitter taste channels and mediates integration of these opposing stimuli. Gain control is known to play an important role in fly olfaction, where GABAB receptor (GABABR) mediates intra- and interglomerular presynaptic inhibition of sensory neuron output. In the taste system, gustatory receptor neurons (GRNs) responding to sweet compounds were found to express GABABR, whereas those that respond to bitter do not. GABABR mediates presynaptic inhibition of calcium responses in sweet GRNs, and both sweet and bitter stimuli evoke GABAergic neuron activity in the vicinity of GRN axon terminals. Pharmacological blockade and genetic reduction of GABABR both lead to increased sugar responses and decreased suppression of the sweet response by bitter compounds. A model is proposed in which GABA acts via GABABR to expand the dynamic range of sweet GRNs through presynaptic gain control and suppress the output of sweet GRNs in the presence of opposing bitter stimuli (Chu, 2014).

Temperature and Sweet Taste Integration in Drosophila

Sugar-containing foods offered at cooler temperatures tend to be less appealing to many animals. However, the mechanism through which the gustatory system senses thermal input and integrates temperature and chemical signals to produce a given behavioral output is poorly understood. To study this fundamental problem, the fly, Drosophila melanogaster, was used. It was found that the palatability of sucrose is strongly reduced by modest cooling. Using Ca(2+) imaging and electrophysiological recordings, it was demonstrated that bitter gustatory receptor neurons (GRNs) and mechanosensory neurons (MSNs) are activated by slight cooling, although sugar neurons are insensitive to the same mild stimulus. A rhodopsin, Rh6, is expressed and required in bitter GRNs for cool-induced suppression of sugar appeal. These findings reveal that the palatability of sugary food is reduced by slightly cool temperatures through different sets of thermally activated neurons, one of which depends on a rhodopsin (Rh6) for cool sensation (Li, 2020).

Starvation-induced depotentiation of bitter taste in Drosophila

Nutrient deprivation can lead to dramatic changes in feeding behavior, including acceptance of foods that are normally rejected. In flies, this behavioral shift depends in part on reciprocal sensitization and desensitization of sweet and bitter taste, respectively. However, the mechanisms for bitter taste modulation remain unclear. This study identified a set of brain ventrolateral octopaminergic/tyraminergic neurons, named OA-VLs, that directly modulate bitter sensory neuron output in response to starvation. OA-VLs are in close proximity to bitter sensory neuron axon terminals and show reduced tonic firing following starvation. It was found that octopamine and tyramine potentiate bitter sensory neuron responses, suggesting that starvation-induced reduction in OA-VL activity depotentiates bitter taste. Consistent with this model, artificial silencing of OA-VL activity induces a starvation-like reduction in bitter sensory neuron output. These results demonstrate that OA-VLs mediate a critical step in starvation-dependent bitter taste modulation, allowing flies to dynamically balance the risks associated with bitter food consumption against the threat of severe starvation (LeDue, 2016).

Taste representations in the Drosophila brain

Fruit flies taste compounds with gustatory neurons on many parts of the body, suggesting that a fly detects both the location and quality of a food source. For example, activation of taste neurons on the legs causes proboscis extension or retraction, whereas activation of proboscis taste neurons causes food ingestion or rejection. Whether the features of taste location and taste quality are mapped in the fly brain was studied using molecular, genetic, and behavioral approaches. Projections were found to be segregated by the category of tastes that they recognize: neurons that recognize sugars project to a region different from those recognizing noxious substances. Transgenic axon labeling experiments also demonstrate that gustatory projections are segregated based on their location in the periphery. These studies reveal the gustatory map in the first relay of the fly brain and demonstrate that taste quality and position are represented in anatomical projection patterns (Wang, 2004).

Sixty-eight gustatory receptor (GR) genes have been identified in the sequenced Drosophila genome. These receptors are likely to recognize subsets of taste cues and therefore serve as molecular markers to distinguish neurons recognizing different taste stimuli. To determine whether there is a map of taste quality in the fly brain, the distribution of GRs in sensory neurons was examined. The potential number of tastes that a neuron may recognize was investigated and then the projections of different taste neurons in the brain were examined (Wang, 2004).

One of the difficulties in determining receptor expression patterns in the Drosophila taste system is that GR genes are expressed at very low levels. Most GR genes are not detectable by in situ hybridization experiments, and it has been necessary to generate transgenic flies in which GR promoters drive expression of reporters using the Gal4/UAS system to determine receptor expression. Two transgenic reporter systems were used to simultaneously detect the expression of different receptors. The Gal4/UAS system was used to label one set of neurons, using Gr-Gal4 to drive expression of UAS-CD2. Nine different GR promoters were used that have been reported to drive robust reporter expression in subsets of taste neurons (Dunipace, 2001; Scott, 2001; Chyb, 2003). To label the second set of GR-bearing neurons, transgenic flies were generated in which a GR promoter drives expression of multiple copies of GFP (e.g., Gr66a-GFP-IRES-GFP-IRES-GFP; for simplification, subsequently referred to as Gr-GFP) (Halfon, 2002). Although it is not known how much amplification multiple copies provide, this approach successfully allowed visualization of taste projections whereas direct promoter fusions to a single GFP did not. Transgenic flies for three different GR promoters (Gr32a, Gr47a, Gr66a) were generated and crossed to seven different Gr-Gal4, UAS-CD2 lines to generate a matrix of 21 double receptor combinations (Wang, 2004).

GRs are expressed in subsets of taste neurons, suggesting that one or a few receptors are expressed per cell. This hypothesis was tested by direct comparison of reporter expression for the matrix of three Gr-GFP by seven Gr-Gal4, UAS-CD2 receptor combinations. Focus was placed on the proboscis to compare reporter expression driven by different GR promoters. These studies revealed several surprising findings. (1) Many GR promoters drive reporter expression in partially overlapping cell populations. Gr66a-Gal4 drives expression in approximately 25 neurons per labial palp, in a single neuron in most or all sensilla. Of the six other GRs tested, five show expression in subsets of Gr66a-positive neurons. Of these five, four show largely overlapping expression with each other and one shows mostly non-overlapping expression. Therefore, Gr66a defines a population of gustatory neurons that express overlapping patterns of multiple receptors. (2) Some receptors are segregated into different cells. Gr5a-Gal4 drives reporter expression in approximately 30 neurons per labial palp, in one neuron in most sensilla. Gr5a is not expressed in Gr66a-positive cells. Thus, two non-overlapping neural populations can be identified by Gr66a and Gr5a. Together, these cells account for two of the four gustatory neurons in each taste sensillum (Wang, 2004).

Extracellular recordings of taste responses from proboscis chemosensory bristles have suggested that all taste sensilla are equivalent and that each of the four taste neurons within a sensillum recognizes a different taste modality, with one neuron responding to sugars, two to salts, and one to water. However, more recent experiments suggest a greater diversity of responsiveness. Because Gr5a and Gr66a are expressed in different cells in a sensillum, it was wondered whether they might mark neurons recognizing different classes of tastes. Interestingly, Drosophila defective in Gr5a show reduced responses to the sugar trehalose both in behavioral and electrophysiological studies, and heterologous expression of Gr5a in tissue culture cells confers trehalose responsiveness, strongly arguing that the ligand for Gr5a is trehalose. Given that Gr5a marks a cell that responds to a sugar, it was hypothesized that Gr66a might mark a cell responding to a different taste category. This type of segregation has been demonstrated in the mammalian taste system, where taste cells that respond to sweet are different from those responding to bitter or umami tastants. The taste ligands that Gr5a and Gr66a cells recognize were examined using genetic cell ablation and behavioral studies. It was discovered that Gr66a cells participate in the recognition of bitter compounds (Wang, 2004).

How is taste quality represented in the brain? Because Drosophila taste receptor neurons need not only recognize different tastes but most likely also the gustatory source (e.g., proboscis, internal mouthparts, legs, and wings), gustatory projections were examined to determine whether taste quality or location is represented in sensory projection patterns (Wang, 2004).

The adult Drosophila brain contains approximately 100,000 neurons, with cell bodies in an outer shell surrounding the dense fibrous core. The primary gustatory relay is the subesophageal ganglion/tritocerebrum (SOG) located in the ventral region of the fly brain. It receives input from three peripheral nerves. Neurons from the proboscis labellum project through the labial nerve; mouthpart neurons project through the pharyngeal/accessory pharyngeal nerve, and neurons from thoracic ganglia project via the cervical connective. Early studies employing cobalt backfills provided evidence that mouthpart neurons project more anteriorly in the SOG than proboscis neurons, suggesting that there might be a map of different taste organs in the fly brain (Wang, 2004).

To examine whether taste neurons in different locations project to different brain regions, GR promoters that drive reporter expression in different peripheral tissues were exploited to follow gustatory projections from the proboscis, mouthparts, or leg. Brains of Gr-Gal4, UAS-GFP were stained by anti-GFP immunohistochemistry, and a series of 1 μm optical sections through the SOG was collapsed to produce a two-dimensional representation of projections. These studies reveal differences in projections for neurons in different peripheral tissues. For instance, Gr2a is expressed only in the mouthparts and these neurons exit the pharyngeal nerve and arborize anteriorly. Gr59b, however, is expressed only in proboscis neurons that arborize in a ringed web. Notably, some receptors are expressed both in the proboscis and mouthparts. Interestingly, their neural projections seem to be the composite of Gr2a and Gr59b projections (Wang, 2004).

Two color labeling approaches were used to examine whether projections are segregated by peripheral tissue. For example, differential labeling of Gr2a neural projections, expressed in the mouthparts, and Gr66a neurons expressed in the proboscis, mouthparts, and legs illustrate overlap of the mouthpart projections but not of proboscis projections. Similarly, when the projections of Gr59b, expressed only in the proboscis, and Gr66a are differentially labeled, there is overlap of projections in the ventral proboscis region but not the dorsal mouthparts region (Wang, 2004).

The different axonal patterns from mouth, proboscis, and leg are also seen in different optical sections through the SOG of Gr32a-Gal4, UAS-GFP flies, arguing that projections are segregated by peripheral tissue even if they contain the same receptor. To better resolve the projections of individual neurons with the same receptor, taste neurons were labeled using a genetic mosaic strategy that relies on postmitotic recombination to induce expression of reporters in single cells. The Gr32a receptor is expressed in proboscis, mouthpart, and leg neurons. Single Gr32a-positive neurons from each tissue were labeled and their arborizations were examined in the SOG. A single mouthpart neuron sends an axon that arborizes in a discrete arbor in the most anterior aspect of the SOG. However, a proboscis neuron with the same receptor sends an axon that shows diffuse branching in the medial SOG, a region different from mouthpart projections. Gr32a-positive leg neurons project through the thoracic ganglia and directly terminate in the most posterior part of the SOG (Wang, 2004).

Overall, these studies demonstrate that taste neurons in different tissues project to different locations in the SOG, with mouthpart projections more anterior than proboscis projections, which are more anterior than leg projections. The demonstration that neurons that express the same receptor in different parts of the body project to distinct locations argues that they elicit different spatial patterns of brain activity and provide a means for encoding different behaviors in response to the same tastant (Wang, 2004).

It was next asked whether neurons from the same peripheral tissue that recognize different tastes project to the same or a different brain region to evaluate if taste quality is encoded in sensory projection patterns. Two-color labeling strategy was used to differentially label projections from Gr5a neurons that recognize sugars and Gr66a neurons that recognize bitter compounds. Remarkably, the projections of proboscis neurons with these receptors are clearly segregated in the SOG: Gr5a projections are more lateral and anterior to Gr66a projections. The Gr5a projections are ipsilateral and resemble two hands holding onto the medial, ringed web of Gr66a projections. Interestingly, leg taste projections for Gr5a and Gr66a neurons are segregated: Gr66a neurons project to the SOG whereas Gr5a neurons project to thoracic ganglia (Wang, 2004).

By contrast, when receptors are contained in partially overlapping populations, there is no obvious segregation of projections. For example, Gr32a is contained in a small fraction of Gr66a-positive cells in the proboscis, yet Gr32a-positive fibers colocalize with Gr66a-positive fibers in all optical sections. Moreover, Gr32a and Gr47a are expressed in mostly non-overlapping subsets of Gr66a-positive proboscis neurons, and their projections overlap, showing that smaller populations of Gr66a-positive cells are not spatially segregated. The lack of segregation suggests that these cell types are not functionally distinct (Wang, 2004).

These studies demonstrate that receptors that are expressed in subsets of cells that recognize bitter substances do not show segregated projections. However, different projection patterns are clearly discernible for proboscis neurons that recognize bitter compounds versus those that recognize sugars. The segregated projections from Gr5a and Gr66a cells reveal that there is a spatial map of taste quality in the brain (Wang, 2004).

The patterns of Drosophila taste receptor expression resemble those of the mammalian taste system and the C. elegans chemosensory system, where multiple receptors are also expressed per cell. In the mammalian taste system, multiple bitter receptors are coexpressed in one population of cells on the tongue whereas receptors for sugars are expressed in a different population of taste cells, arguing that different sensory cells recognize different taste modalities. Remarkably, the concept of distinct sweet and bitter cells also applies to the fly (Wang, 2004).

This study identified two populations of proboscis neurons that show spatially segregated projection patterns in the SOG. These different patterns correspond to different taste categories: neurons that recognize bitter substances are mapped differently in the fly brain from those that recognize sugars, suggesting that there is a map of taste modalities or behaviors in the fly brain. In addition, several subpopulations of Gr66a-positive cells show convergent projections. Two different models could account for this convergence. (1) In the simplest model, convergence could imply similar function. For example, all neurons mediating avoidance behaviors might project to the same region and synapse on a second order neuron that conveys avoidance. (2) The apparent convergence could still yield segregated gustatory information if there is a molecular identity code such that second order neurons synapse exclusively with gustatory neurons containing the same receptors. This second model is akin to what is seen in the mammalian pheromone system and sensory-motor connectivity in the spinal cord. Future experiments examining synaptic connectivity will be essential to determine how gustatory information is transmitted to higher brain centers. Nevertheless, the observation that there is spatial segregation of Gr5a sugar cells and Gr66a bitter cells, but not of smaller populations of Gr66a-positive cells, suggests that the diversity of recognition afforded by 68 or so receptor genes may be simplified into only a few different taste categories in the fly brain (Wang, 2004).

Gustatory projections are also segregated according to the peripheral position of the neuron. Early studies employing cobalt backfills argue that mouthpart neurons project more anteriorly in the SOG than proboscis neurons. The results are consistent with, and extend, these observations. Using genetic mosaic approaches, single taste neurons were labeled, and it was found that projections from different organs are segregated even from neurons containing the same receptor. These studies argue that the same taste stimulus will produce different patterns of brain activity depending on the stimulus' location in the periphery and may mediate different behaviors, consistent with the observation that sugar on the leg causes proboscis extension whereas sugar on the ovipositor causes egg laying. An organotopic map of gustatory projections may provide a means for the fly to distinguish different taste locations (Wang, 2004).

Imaging taste responses in the fly brain reveals a functional map of taste category and behavior

The sense of taste allows animals to distinguish nutritious and toxic substances and elicits food acceptance or avoidance behaviors. In Drosophila, taste cells that contain the Gr5a receptor are necessary for acceptance behavior, and cells with the Gr66a receptor are necessary for avoidance. To determine the cellular substrates of taste behaviors, taste cell activity in vivo was monitored with the genetically encoded calcium indicator G-CaMP. These studies reveal that Gr5a cells selectively respond to sugars and Gr66a cells to bitter compounds. Flies are attracted to sugars and avoid bitter substances, suggesting that Gr5a cell activity is sufficient to mediate acceptance behavior and that Gr66a cell activation mediates avoidance. As a direct test of this hypothesis, different taste neurons were inducibly activated by expression of an exogenous ligand-gated ion channel, and it was found that cellular activity is sufficient to drive taste behaviors. These studies demonstrate that taste cells are tuned by taste category and are hardwired to taste behaviors (Marella, 2006).

In Drosophila, cells with the Gr5a taste receptor are necessary for sugar acceptance behaviors, and those with Gr66a are necessary for avoidance. These taste cells selectively recognize different taste modalities, such that there is functional segregation of taste qualities in the periphery and at the first relay in the brain. Moreover, activation of these different taste neurons is sufficient to elicit different taste behaviors. Thus, activity of the sensory neuron, rather than the receptor, is the arbiter of taste behavior. These studies argue that animals distinguish different tastes by activation of dedicated neural circuits that dictate behavioral outputs (Marella, 2006).

The patterns of sensory projections provide internal representations of the external world. For example, there is an odotopic map of olfactory projections in flies and mammals. Drosophila gustatory projections are segregated by taste organ such that there is an anterior-posterior map in the subesophageal ganglion of mouthpart, proboscis, and leg projections. Within the proboscis, two different populations of taste neurons can be defined by their expression of either the Gr5a receptor or the Gr66a receptor. Neurons with these different receptors show segregated projections in the brain, with Gr5a projections lateral and anterior to Gr66a projections. Genetic cell ablation experiments revealed that Gr5a cells are required for sugar acceptance behavior and Gr66a for avoidance of bitter compounds. These experiments suggest that in addition to the organotopic map of taste projections, there is also an anatomical map of different taste modalities (Marella, 2006).

This study directly demonstrates that there is functional segregation of different taste modalities in the fly brain. Taste responses were monitored in the living fly by expressing the calcium-sensitive indicator G-CaMP in different classes of taste neurons; Gr5a projections respond to a large number of sugars and Gr66a termini respond to several bitter compounds. Monitoring the responses of subsets of Gr66a cells to a panel of bitter compounds did not reveal striking differences in ligand recognition profiles. Although the possibility that different subsets of Gr5a or Gr66a cells show more selective responses cannot be rule out, clear spatial segregation of sugar and bitter responses was found in the SOG. This argues that there is a spatial activity map of different taste modalities in the fly brain that corresponds to the anatomical projections of Gr5a and Gr66a cells (Marella, 2006).

The character of the taste map in the fly brain is very different from the olfactory map. In the olfactory system, 70 receptors in flies and ~1000 in mammals are used to detect odors. Neurons generally express one receptor, and neurons with the same odorant receptor in the periphery form functional synapses at the same glomerulus in the first relay of the brain. Functional imaging experiments demonstrate that a given odor will activate multiple glomeruli, and one glomerulus will respond to multiple odors. This has led to a spatial model for odor coding in the brain in which the unique combination of activated glomeruli specifies a smell. An animal is thus able to distinguish thousands of different smells by the activation of thousands of different combinations of glomeruli. By contrast, in the fly taste system, sugars activate Gr5a taste projections and bitter compounds activate Gr66a projections. This suggests that there is not a combinatorial code for different tastes in the fly. Instead, the activation of segregated neural populations encodes different taste modalities. This simple map may allow the fly to distinguish sugars from bitter compounds, but may limit the ability to distinguish compounds within the same modality (Marella, 2006).

Sugars elicit food acceptance behavior, and bitter compounds elicit avoidance. The segregation of sugar and bitter responses in the fly brain suggests that activation of different classes of sensory neurons may be sufficient to generate different taste behaviors. This hypothesis was directly tested. Gr5a or Gr66a cells were inducibly activated by expression of a cationic ion channel, VR1E600K, in taste cells and application of its ligand, capsaicin, at the proboscis. G-CaMP imaging experiments demonstrated that taste cells show calcium increases in response to capsaicin. Behavioral studies showed altered taste preferences in flies containing the VR1E600K channel: flies with VR1E600K in Gr5a cells are attracted to capsaicin, and those with VR1E600K in Gr66a cells avoid it. This demonstrates that activation of different taste neurons is sufficient to generate different taste behaviors. Recent studies in C. elegans chemosensory neurons and mammalian gustatory cells demonstrate that exogenous activation of these cells is sufficient to generate acceptance and avoidance behaviors as well. The picture that is emerging from these studies is that the activity of selective sensory cells in the periphery generates behavioral programs through the activation of dedicated neural circuits (Marella, 2006).

G-CaMP imaging and behavioral studies have important implications for understanding how taste information is encoded in the periphery. Three different models have been suggested for how taste information is encoded in the brain: the labeled-line model, the population-coding model (or mixed-lines model), and the temporal-coding model. In the labeled-line model of taste coding, cells are dedicated to detecting different taste ligands, and this information remains segregated as it is relayed to the brain, such that different tastes are distinguished by the selective activation of nonoverlapping cells. In population-coding models, the comparative activity of many cell types rather than activation of one type conveys taste information. This model proposes that the ensemble activity encodes taste quality. In temporal-coding models, it is the precise pattern of action potentials that communicates taste quality (Marella, 2006).

The labeled-line model can be distinguished from the other models by the requirement for a neuron to have a unique identity in terms of recognition properties and behavior. The observation that neurons express subsets of receptors and selectively recognize different taste categories argues that taste neurons have different identities. Moreover, the finding that activation of an exogenous ion channel in discrete taste cell populations elicits specific behaviors argues that selective cell activation is sufficient to mediate behavior, under conditions that do not activate the entire taste cell population and are unlikely to mimic endogenous firing patterns. Taken together, these studies strongly favor the labeled-line model of taste coding in the periphery, although they cannot rule out a role for spike timing or ensemble encoding in fine-tuning the responses (Marella, 2006).

Seminal studies in the gustatory system of mammals strongly argue in favor of the labeled-line model of taste coding in the mammalian gustatory system in the periphery as well. Taste cells on the tongue selectively express either sugar, bitter, or amino acid receptors, such that different taste qualities are detected by different cells in the periphery. Activation of these different taste cells is sufficient to generate specific taste behaviors, with artificial activation of sugar cells eliciting acceptance behavior and artificial activation of bitter cells eliciting avoidance. The observation that cells are dedicated to detecting a specific taste modality and mediate a specific behavior suggests that there are labeled lines of taste information from peripheral detection to behavior. Thus, taste behaviors are hardwired to selective cell activation on the tongue in mammals and the proboscis in flies (Marella, 2006).

The advantage of having taste cell activation innately coupled to behavioral outputs via labeled lines is that the valence of a taste compound is dictated by the neural circuit and requires no previous association. The stereotypy of taste behaviors affords the opportunity to examine how neural connectivity elicits distinct behaviors. It is anticipated that live imaging of neural responses will be a powerful approach to dissect higher-order taste processing in the fly brain (Marella, 2006).

An inhibitory sex pheromone tastes bitter for Drosophila males

Sexual behavior requires animals to distinguish between the sexes and to respond appropriately to each of them. In Drosophila, as in many insects, cuticular hydrocarbons are thought to be involved in sex recognition and in mating behavior, but there is no direct neuronal evidence of their pheromonal effect. Using behavioral and electrophysiological measures of responses to natural and synthetic compounds, this study shows that Z-7-tricosene, a Drosophila male cuticular hydrocarbon, acts as a sex pheromone and inhibits male-male courtship. These data provide the first direct demonstration that an insect cuticular hydrocarbon is detected as a sex pheromone. Intriguingly, this study shows that a particular type of gustatory neurons of the labial palps respond both to Z-7-tricosene and to bitter stimuli. Cross-adaptation between Z-7-tricosene and bitter stimuli further indicates that these two very different substances are processed by the same neural pathways. Furthermore, the two substances induced similar behavioral responses both in courtship and feeding tests. It is concluded that the inhibitory pheromone tastes bitter to the fly (Lacaille, 2007).

Sexual behavior requires animals to distinguish between the sexes and respond appropriately to each of them. The genetic potential of Drosophila has made it a focus of study to investigate the role of genes that affect male courtship behavior and sexual orientation. Most studies have centred on the stereotyped male courtship, rather than on the range of sensory cues that are integrated to produce changes in the male's behavior. In Drosophila melanogaster, as in many insect species, chemical signals likely play a key role in sex recognition, and in the initiation and progress of courtship: males are thought to be excited by a range of chemical substances produced by the females, including long-chain hydrocarbons, and to be inhibited by hydrocarbons on the male cuticle (Ferveur, 1996), especially by Z-7-tricosene (7-T) (Lacaille, 2007).

These substances, which have low or very low volatility are thought to be detected by gustatory receptors (Gr) on the male's legs and proboscis, but there is no direct neuronal evidence of their pheromonal role. The strongest evidence thus far obtained comes from the manipulation of male-specific neurons expressing the GR68A gustatory receptor protein, found in gustatory sensilla on fore-tarsi, which apparently altered the duration of male courtship of females (Bray, 2003). In support of this finding, a putative pheromone-binding protein expressed specifically in these gustatory sensilla has been shown to be required for male discrimination of sex objects (Xu, 2002; Svetec, 2005; Park, 2006). Strikingly, the pheromone(s) causing these effects has not been identified, nor has the nature of the responses shown by these sensilla been determined (Lacaille, 2007).

Cuticular hydrocarbons are lipophilic and difficult to manipulate using the classical electrophysiological 'tip-recording' method, which uses a single electrode filled with hydrophilic solution eventually mixed with a detergent. A device consisting of two electrodes was designed, with which taste sensilla could be separately stimulated and their physiological activity in response to synthetic 7-T recorded. The pheromonal effect of pure 7-T on male courtship intensity was investigated. It was found that (1) the same taste neurons respond to both 7-T and to bitter substances and (2) these two types of molecules similarly inhibit male behavior in a dose-dependent manner (Lacaille, 2007).

To assess the effect of sex pheromones on male courtship behavior, the courtship index (CI) that wild-type Canton-S (WT) males directed towards immobilized target flies with various cuticular hydrocarbon profiles was measured. All tests were carried out under dim red light and with decapitated targets so as to enhance the behavioral effect of pheromones. Under these experimental conditions, WT males produced relatively high levels of homosexual courtship to sibling target WT males which produce high levels of 7-T. However, as expected, these tester males showed significantly higher CIs toward WT females, and also to Tai and desat1 (encoding a stearoyl-CoA 9-desaturase) males. These three types of fly all have low levels of 7-T. However, in other respects they have different cuticular hydrocarbon profiles: Tai males (from West Africa) are rich in Z-7-pentacosene (7-P, a stimulatory pheromone for D. melanogaster males), whereas males from the desat1 mutant line have very low levels of 7-P. WT females have also very low levels of 7-P but produce high levels of 7,11 heptacosadiene (7,11-HD) and 7,11 nonacosadiene (7,11-ND) which tend to enhance male courtship stimulation (Lacaille, 2007).

These data confirm previous suggestions that 7-T tends to inhibit homosexual courtship by WT males. To demonstrate this, 7-T was synthesized, and various doses of this substance were placed on the cuticle of a desat1 mutant male which show almost no 7-T. The results further supported the hypothesis: tester WT males showed a negative correlation between their courtship of 7-T covered desat1 males, and the dose of synthetic 7-T placed on the target male. 0.5 micrograms 7-T did not affect the CI whereas 1 microgram 7-T—which roughly corresponds to one WT male-equivalent—induced a CI that was similar to that induced by WT target males, and 2 micrograms 7-T induced a strong inhibition of WT male courtship. The solvent (pentane) used to dissolve 7-T had no significant effect on male courtship. These data demonstrate that 7-T induces a dose-dependent inhibition of the homosexual courtship shown by WT tester males (Lacaille, 2007).

To investigate the role of gustatory neurons in detecting male inhibitory pheromones, the electrophysiological activity of these neurons in response to pure synthetic 7-T was measured. The tungsten electrode method allowed separate stimulation and recording of the electrophysiological activity of a sensillum on the labial palp. The stimulating electrode was filled with a lipophilic buffer (paraffin oil) containing 7-T, and the tip of the taste sensillum was tapped, while the tungsten recording electrode was inserted at the base of the sensillum (Lacaille, 2007).

Recordings were taken from the three types of sensilla present on the labial palps: i-type (which have two gustatory receptor neurons), s- and l-type taste sensilla (which have four gustatory receptor neurons), and the sensilla were mapped according to their physiological responses. Six out of six s- and four out of eight i-type sensilla so far examined consistently responded to 7-T at the 10-8M concentration. This provides the first direct evidence of a neuronal response to a cuticular hydrocarbon sex pheromone in an insect. In i-type sensilla, only one of the two gustatory neurons responded to 7-T. The active cell was the L2 cell, which produced small spikes. In contrast, the second gustatory neuron (the L1-S or L1 cell), housed in the same i-type sensilla, did not respond to 7-T but responded normally to salt and sucrose. The firing frequency of the L2 cell, measured in a s-type sensilla, increased with 7-T concentration (between 10-12 and 10-8 M). Conversely, l-type sensilla on the labial palp responded to NaCl, KCl and sucrose, but not to 10-8M 7-T (Lacaille, 2007).

The L2 cell in i-type sensilla is known to respond to bitter molecules. To investigate whether the same neuron processes both 7-T and bitter substances, a series of experiments was performed. When stimulated with a mixture of 7-T and caffeine, the L2 cell elicited an increased number of spikes with the same amplitude than when stimulated with either substance alone. 7-T and bitter substances also show cross-adaptation: pre-stimulation with 7-T significantly reduced the response of L2 cells to caffeine but not to sucrose. Taken together, these additive and cross-adaptative effects strongly indicate that 7-T and bitter stimuli are processed by the same taste neuron, the L2 cell, in the responsive i-type sensilla of the labial palps (Lacaille, 2007). thumbnail

To confirm this apparent dual sensory processing at the neuronal level, the reciprocal cross-effects were observed of (1) bitter stimuli on sexual behavior and (2) 7-T on feeding-related behavior. Painting desat1 males with any of three bitter substances-caffeine, quinine or berberine-strongly inhibited male courtship of these painted flies by WT control males. Tester males were sensitive to these bitter stimuli just as they were to synthetic 7-T: the three bitter substances induced dose-dependent effects very similar to those induced by 7-T. Berberine induced a more potent inhibition than the two other bitter substances: 1 µg and 2 µg of the former molecule respectively inhibited 50% and 100% males; similar doses of the latter molecules respectively inhibited 35% and 80% males. These results clearly show that bitter substances induce dose-dependent inhibition of male homosexual courtship (Lacaille, 2007).

To assess whether the sensory processing of 7-T affected feeding behavior the Proboscis Extension Reflex (PER) was performed. When sensilla on the tarsi were unilaterally stimulated with 0.1M sucrose, a positive PER was shown by a majority of male and female flies. When flies were bilaterally stimulated, on one side with 0.1M sucrose and on the contralateral side with 10-8M 7-T, PER was highly reduced in both sexes. PER was also significantly reduced in both sexes by a contralateral application of 0.1M caffeine. These data are supported by a previous PER experiment carried out with berberine. Together, they indicate that both 7-T and bitter compounds similarly affect the appetitive behavior of male flies (Lacaille, 2007).

Finally, courtship behavior was measured in males with genetically altered taste neurons. To target neurons potentially involved in the perception of bitter substances and of 7-T, the Gr66a-Gal4 transgene, which contains the promoter of the gustatory receptor Gr66a gene fused with the yeast Gal4 sequence, was used. This choice was based on two observations: firstly, both the GR66A-receptor protein and Gr66a-Gal4-expressing neurons are involved in the detection of caffeine. Second, using UAS-GFP, the expression of the Gr66a-Gal4 transgene was visualized and found in approximately 22 i- and s-type taste sensilla symmetrically arranged on each labial plate (these were the same sensilla in which a dual response to 7-T and caffeine was observed) and in 7 to 8 taste sensilla on each front leg. In the labial palps, a single neuron in each sensillum expressed GFP under the control of Gr66a-Gal4 (Lacaille, 2007).

To assess the effect of Gr66a-Gal4 expressing neurons on male courtship and taste perception, the courtship index (CI) that males with targeted Gr66a-Gal4 expressing neurons directed towards immobilized target WT males was measured. When Gr66a-Gal4 expressing cells were deleted using the pro-apoptotic transgene reaper (UAS-rpr), Gr66a-Gal4; UAS-rpr males showed a significantly enhanced homosexual CI to WT target males. The amplitude of this effect was similar to that shown by Gr66a-Gal4/WT males carrying the Gr66a-Gal4 transgene alone, indicating that the GAL4 protein directly or indirectly affects the targeted taste cells and changes pheromonal perception. Strikingly, the effect of the transgene alone was specific to WT target males: the CIs of Gr66a-Gal4/WT tester males toward WT females and toward Tai and desat1 males were not significantly different to those shown by WT tester males, reinforcing the suggestion that Gr66a-Gal4-expressing neurons are specifically involved in detecting inhibitory chemical stimuli (Lacaille, 2007).

The inhibitory effect induced by pure 7-T was measured on Gr66a-Gal4/WT males using desat1 target males painted with various amounts of 7-T. The results further supported the hypothesis: contrary to the strong dose-dependent effect induced in WT males, 7-T had a much weaker (if any) inhibitory effect on Gr66a-Gal4/WT flies. As with the effects of 7-T, the three bitter molecules had less (if any) effect on Gr66a-Gal4/WT males compared to WT males. Taken together, these data suggest that Gr66a-Gal4 expressing neurons are required to detect the aversive effect induced by 7-T and by bitter stimuli on male courtship behavior (Lacaille, 2007).

This study has provided the first direct neuronal evidence of cuticular hydrocarbon sex pheromone processing in an insect. This study has also shown that an inhibitory sex pheromone and repulsive gustatory stimuli are processed by the same neurons (The L2 cells corresponding to some Gr66a-Gal4 expressing neurons) and induce similar behavioral responses. For the male fly, there is apparently no difference between the sensation induced by bitter stimuli and that induced by the inhibitory pheromone 7-T (Lacaille, 2007).

Given that the two types of substances have little structural similarity (7-T is a straight chain hydrocarbon whereas berberine, caffeine, and quinine are oxygenated, oligocyclic alkaloids) it is unlikely that they are detected by the same receptor molecule. Since multiple types of GR molecules are probably co-expressed in Gr66a-Gal4 expressing labellar taste neurons, and the GR66A receptor molecule is involved in the detection of caffeine, this suggests that 7-T and perhaps other bitter substances are detected by other, unknown receptor molecule(s). In this case, cross-adaptation would presumably take place through common activation of second-messenger systems or of calcium trafficking (Lacaille, 2007).

These findings provide experimental support for Darwin's hypothesis that sexual selection operates on pre-existing structures and behaviors, co-opting them into new functions. In the present case, it is proposed that pre-existing neuronal networks responsible for detecting and responding to bitter substances became able to detect stimuli produced by other males, with the result that these stimuli inhibited male courtship by activating aversive behaviors that were previously solely induced by bitter stimuli. A similar—likely convergent—process may have taken place in vertebrates, when some semiochemicals produced by male mice are detected by specialized receptor molecules in the main olfactory bulb, not in the accessory olfactory system, which had previously been considered to be the sole site of pheromone processing. This study has shown that identical peripheral neurons are involved in detecting inhibitory pheromones and aversive gustatory stimuli. The next challenge will be to understand how these signals are represented in the fly brain (Lacaille, 2007).

Molecular and cellular organization of the taste system in the Drosophila larva

This study examined the molecular and cellular basis of taste perception in the Drosophila larva through a comprehensive analysis of the expression patterns of all 68 Gustatory receptors (Grs). Gr-GAL4 lines representing each Gr are examined, and 39 show expression in taste organs of the larval head, including the terminal organ (TO), the dorsal organ (DO), and the pharyngeal organs. A receptor-to-neuron map is constructed. The map defines 10 neurons of the TO and DO, and it identifies 28 receptors that map to them. Each of these neurons expresses a unique subset of Gr-GAL4 drivers, except for two neurons that express the same complement. All of these neurons express at least two drivers, and one neuron expresses 17. Many of the receptors map to only one of these cells, but some map to as many as six. Conspicuously absent from the roster of Gr-GAL4 drivers expressed in larvae are those of the sugar receptor subfamily. Coexpression analysis suggests that most larval Grs act in bitter response and that there are distinct bitter-sensing neurons. A comprehensive analysis of central projections confirms that sensory information collected from different regions (e.g., the tip of the head vs the pharynx) is processed in different regions of the s ganglion, the primary taste center of the CNS. Together, the results provide an extensive view of the molecular and cellular organization of the larval taste system (Kwon, 2011).

Of the 67 Gr-GAL4 transgenes, 43 showed expression in the larva, of which 39 were expressed in the major taste organs of the head. The 39 Gr-GAL4 drivers are expressed in combinatorial fashion. Individual Gr-GAL4 drivers are expressed in up to 12 cells, in the case of Gr33a- and Gr66a-GAL4; approximately one-half, however, are expressed in only one cell (Kwon, 2011).

For some Gr-GAL4 drivers the observed pattern of expression may not be identical with that of the endogenous Gr gene. It was precisely with this concern in mind that a mean of 7.6 independent lines were analyzed for each of the 67 Gr drivers, and a rigorous, quantitative protocol was establised for identifying a representative line for each gene. In the absence of an effective in situ hybridization protocol, the approach used here seemed likely to be the most informative in providing a comprehensive systems-level view of larval taste reception (Kwon, 2011).

The Gr receptor-to-neuron map of the dorsal and terminal organs identified 10 neurons. Two neurons have cell bodies in the DOG and innervate the DO, two have cell bodies in the DOG and innervate the TO, and six have cell bodies in the TOG and innervate the TO (Kwon, 2011).

28 receptors were mapped to these 10 neurons. All of these neurons express at least two Gr-GAL4 drivers. Two receptors, Gr21a and Gr63a, are coreceptors for CO2; neither is sufficient to confer chemosensory function alone. It is conceivable that many other Grs may also require a coreceptor, which may explain the lack of neurons expressing a single Gr-GAL4. The number of receptors per neuron ranges up to 17, in the case of C1. This number is comparable with the maximum number of Gr-GAL4s observed in a labellar neuron (29), and much greater than the number of Ors observed in individual neurons of either the larval or adult olfactory system (Kwon, 2011).

Among the 10 identified cells, individual Gr-GAL4 drivers are expressed in as few as one cell and as many as six cells. Most of the drivers are expressed in only one of these 10 cells. The drivers expressed in six cells, Gr33a-GAL4 and Gr66a-GAL4, are expressed in all bitter neurons of the adult labellum. It is noted that Gr33a-GAL4 and Gr66a-GAL4 are the only drivers expressed in B1, arguing against the possibility that both of these receptors function exclusively as chaperones or as coreceptors that require another Gr for ligand specificity (Kwon, 2011).

There is little cellular redundancy. Only two neurons, A1 and A2, express the same complement of receptors. All other neurons contain a unique subset of the Gr repertoire. In this respect, the larval taste system differs from the adult taste system but is similar to the larval olfactory system, which also contains little if any cellular redundancy (Kwon, 2011).

Analysis of the central projections of all 39 Gr-GAL4 drivers provided evidence for a systematic difference among projection patterns between TO/DO neurons and pharyngeal neurons. These results support the conclusion that sensory information collected from the tip of the head is processed in different regions of the SOG than information collected in the pharynx, i.e., that evaluation of a potential food source before ingestion and the testing of food quality during ingestion are functionally partitioned in the brain. Similar inferences were drawn in an elegant study of a limited number of Gr-GAL4 transgenes (Colomb, 2007; Kwon, 2011 and references therein).

Conspicuously absent from the list of Gr-GAL4 drivers expressed in the larval taste system are those representing the eight members of the sugar receptor subfamily (Gr5a, Gr61a, Gr64a-f). The founding member of this family, Gr5a, mediates response to the sugar trehalose, and two other members of the subfamily have been shown to encode sugar receptors as well. No GFP expression for these genes was observed in cells of the taste organs or in neural fibers in the brain or ventral ganglion. Most of these Gr-GAL4 transgenes drive expression in the adult, but it is acknowledged that these transgenes may not faithfully reflect expression in the larva (Kwon, 2011).

Given that Drosophila larvae respond to sugars, as do larvae of other insect species, how do they detect them without members of the sugar receptor subfamily? Other Grs, including the recently identified fructose receptor Gr43a, may underlie sugar detection in the larva. It is noted that Gr59e-GAL4 and Gr59f-GAL4 are coexpressed in a cell that does not express the bitter cell markers Gr33a-GAL4 or Gr66a-GAL4. Sugar reception may also be mediated by other kinds of receptors, such as those of the TRPA family (Kwon, 2011).

In adult Drosophila, Gr33a-GAL4 and Gr66a-GAL4 are coexpressed with other Gr-GAL4s in bitter neurons; the simplest interpretation of expression and functional analysis is that multiple bitter receptors are coexpressed (Kwon, 2011).

In the larva, it ws found that most larval Gr-GAL4s are coexpressed with Gr33a- and Gr66a-GAL4, suggesting the possibility that most larval Grs act in bitter response. It is noted that, of the 17 Gr-GAL4s coexpressed in the C1 neuron, 15 are coexpressed in a bitter neuron of the labellum. It was also establish that there are distinct molecular classes of Gr33a-GAL4, Gr66a-GAL4-expressing neurons. The simplest interpretation of these results is that there are distinct bitter-sensing neurons in larvae (Kwon, 2011).

Larvae must determine whether to accept or reject a food source, and in principle this determination could be made by a simple binary decision-making circuit. However, the existence of six Gr33a-GAL4, Gr66a-GAL4-expressing neurons expressing distinct subsets of Gr-GAL4s suggests a greater level of complexity in the processing of gustatory information. One possibility is that C1, which expresses the largest subset of drivers among the TO/DO neurons, may activate an aversive behavior in response to many of the bitter compounds that the larva encounters, while C2, C3, C4, or B2 either potentiates the response or activates a different motor program in response to chemical cues of particular biological significance or exceptional toxicity. The existence of heterogenous bitter-sensing cells, some more specialized than others, is a common theme in insect larvae. In particular, many species contain a taste cell that responds physiologically to many aversive compounds and whose activity deters feeding. C1 could be such a cell, and its coexpression of many receptors may provide the molecular basis of a broad response spectrum (Kwon, 2011).

It is striking that the number of TO/DO neurons that express Gr-GAL4s is small compared with the total number of TO/DO neurons. Gr-GAL4 expression was mapped to only 10 cells in the TO/DO (although Gr2a-GAL4 and Gr28a-GAL4 were each expressed in two TO neurons that were not mapped). The DOG and TOG contain 36-37 and 32 sensory neurons, respectively, among which 21 in the DOG are olfactory. Thus, of the nonolfactory cells, on the order of 20%-30% express Gr-GAL4 drivers. It will be interesting to determine how many of the other DOG/TOG cells express other chemoreceptor genes, such as Ppk, Trp, or IR genes, and how many of the other neurons have mechanosensory, thermoreceptive, hygrosensory, or other sensory functions (Kwon, 2011).

The role of Gr genes in the larval pharyngeal organs is unknown. In adult pharyngeal sensilla, the TRPA1 channel, which detects irritating compounds, regulates proboscis extension. It is possible that Grs expressed in larval pharyngeal organs may also play a role in modulating feeding behavior. Of the 24 Gr-GAL4 drivers expressed in the larval pharyngeal organs, 9 are coexpressed with Gr33a-GAL4 and Gr66a-GAL4 in the TO/DO; it seems plausible that they may monitor ingested food for the presence of aversive compounds (Kwon, 2011).

In summary, this study has analyzed essential features of the molecular and cellular organization of a numerically simple taste system in a genetic model organism. Ten gustatory receptor neurons were described and evidence was provided that they express Grs in combinatorial fashion, with most of these neurons and receptors acting in the perception of bitter compounds. The results lay a foundation for a molecular and genetic analysis of how these receptors and neurons, and the downstream circuitry, underlie a critical decision: whether to accept or reject a food source (Kwon, 2011).

Temporal response dynamics of Drosophila olfactory sensory neurons depends on receptor type and response polarity

Insect olfactory sensory neurons (OSN) express a diverse array of receptors from different protein families, i.e. ionotropic receptors (IR), gustatory receptors (GR) and odorant receptors (OR). It is well known that insects are exposed to a plethora of odor molecules that vary widely in both space and time under turbulent natural conditions. In addition to divergent ligand specificities, these different receptors might also provide an increased range of temporal dynamics and sensitivities for the olfactory system. To test this, different Drosophila OSNs were challenged with both varying stimulus durations (10-2000 ms), and repeated stimulus pulses of key ligands at various frequencies (1-10 Hz). The results show that OR-expressing OSNs responded faster and with higher sensitivity to short stimulations as compared to IR- and Gr21a-expressing OSNs. In addition, OR-expressing OSNs could respond to repeated stimulations of excitatory ligands up to 5 Hz, while IR-expressing OSNs required ~5x longer stimulations and/or higher concentrations to respond to similar stimulus durations and frequencies. Nevertheless, IR-expressing OSNs did not exhibit adaptation to longer stimulations, unlike OR- and Gr21a-OSNs. Both OR- and IR-expressing OSNs were also unable to resolve repeated pulses of inhibitory ligands as fast as excitatory ligands. These differences were independent of the peri-receptor environment in which the receptors were expressed and suggest that the receptor expressed by a given OSN affects both its sensitivity and its response to transient, intermittent chemical stimuli. OR-expressing OSNs are better at resolving low dose, intermittent stimuli, while IR-expressing OSNs respond more accurately to long-lasting odor pulses. This diversity increases the capacity of the insect olfactory system to respond to the diverse spatiotemporal signals in the natural environment (Getahun, 2012).

The Drosophila IR20a clade of ionotropic receptors are candidate taste and pheromone receptors

Insects use taste to evaluate food, hosts, and mates. Drosophila has many "orphan" taste neurons that express no known taste receptors. The Ionotropic Receptor (IR) superfamily is best known for its role in olfaction, but virtually nothing is known about a clade of approximately 35 members, the IR20a clade. Here, a comprehensive analysis of this clade reveals expression in all taste organs of the fly. Some members are expressed in orphan taste neurons, whereas others are coexpressed with bitter- or sugar-sensing Gustatory receptor (Gr) genes. Analysis of the closely related IR52c and IR52d genes reveals signatures of adaptive evolution, roles in male mating behavior, and sexually dimorphic expression in neurons of the male foreleg, which contacts females during courtship. These neurons are activated by conspecific females and contact a neural circuit for sexual behavior. Together, these results greatly expand the repertoire of candidate taste and pheromone receptors in the fly (Koh, 2014).

Ir56d-dependent fatty acid responses in Drosophila uncovers taste discrimination between different classes of fatty acids

Chemosensory systems are critical for evaluating the caloric value and potential toxicity of food prior to ingestion. While animals can discriminate between 1000's of odors, much less is known about the discriminative capabilities of taste systems. Fats and sugars represent calorically potent and innately attractive food sources that contribute to hedonic feeding. Despite the differences in nutritional value between fats and sugars, the ability of the taste system to discriminate between different rewarding tastants is thought to be limited. In Drosophila, sweet taste neurons expressing the Ionotropic Receptor 56d (IR56d) are required for reflexive behavioral responses to the medium-chain fatty acid, hexanoic acid. Further, it was found that flies can discriminate between a fatty acid and a sugar in aversive memory assays, establishing a foundation to investigate the capacity of the Drosophila gustatory system to differentiate between various appetitive tastants. This study tested whether flies can discriminate between different classes of fatty acids using an aversive memory assay. The results indicate that flies are able to discriminate medium-chain fatty acids from both short- and long-chain fatty acids, but not from other medium-chain fatty acids. While IR56d neurons are broadly responsive to short-, medium-, and long-chain fatty acids, genetic deletion of IR56d selectively disrupts response to medium-chain fatty acids. Further, IR56d+GR64f+ neurons are necessary for proboscis extension response (PER) to medium-chain fatty acids, but both IR56d and GR64f neurons are dispensable for PER to short- and long-chain fatty acids, indicating the involvement of one or more other classes of neurons. Together, these findings reveal that IR56d is selectively required for medium-chain fatty acid taste, and discrimination of fatty acids occurs through differential receptor activation in shared populations of neurons. Our study uncovers a capacity for the taste system to encode tastant identity within a taste category (Brown, 2021).

The molecular and cellular basis of bitter taste in Drosophila

The extent of diversity among bitter-sensing neurons is a fundamental issue in the field of taste. Data are limited and conflicting as to whether bitter neurons are broadly tuned and uniform, resulting in indiscriminate avoidance of bitter stimuli, or diverse, allowing a more discerning evaluation of food sources. This study provides a systematic analysis of how bitter taste is encoded by the major taste organ of the Drosophila head, the labellum. Each of 16 bitter compounds is tested physiologically against all 31 taste hairs, revealing responses that are diverse in magnitude and dynamics (see The Drosophila labellum and its physiological responses). Four functional classes of bitter neurons are defined. Four corresponding classes are defined through expression analysis of all 68 gustatory taste receptors. A receptor-to-neuron-to-tastant map is constructed. Misexpression of one receptor confers bitter responses as predicted by the map. These results reveal a degree of complexity that greatly expands the capacity of the system to encode bitter taste (Weiss, 2011).

This study has defined five distinct classes of sensilla in the Drosophila labellum on the basis of their responses to bitter compounds (see Labellar sensilla fall into five expression classes that are similar to the functional classes). Four of these sensillar classes contain bitter-sensing neurons; other sensilla did not respond physiologically to any of our bitter tastants. This analysis, then, has defined four classes of bitter-sensing neurons that are diverse in their response profiles. Some are broadly tuned with respect to a panel of bitter compounds and some are more narrowly tuned. The neurons also vary in the temporal dynamics of their responses. Different neurons respond to the same tastant with different onset kinetics, and an individual neuron responds to distinct tastants with diverse dynamics. The functional diversity of bitter-sensing neurons expands the coding capacity of the system: different tastants elicit responses from different subsets of neurons, and distinct tastants elicit diverse temporal patterns of activity from these neurons (Weiss, 2011).

This systematic analysis does not support previous models that suggest functional uniformity among bitter neurons. A previous physiological study of the labellum did not reveal functionally distinct neuronal classes, but was limited in the number of sensilla and tastants that were examined. There are major technical challenges in recording from I and S sensilla; the S sensilla in particular are small, curved, and difficult to access because of their position on the labellar surface. The finding of functional heterogeneity in labellar sensilla is consistent with the finding that two taste sensilla on the prothoracic leg responded to berberine but not quinine, whereas another sensillum responded to quinine but not berberine. A recent study found that DEET elicited different responses from several labellar sensilla tested. Functionally distinct bitter neurons have also been described in taste organs of caterpillars, and in the case of the Manduca larva, aristolochic acid and salicin activate spike trains that differ in dynamics (Weiss, 2011).

The functional differences among neurons in the Drosophila labellum suggested underlying molecular differences. In particular, it was asked whether the four classes of bitter taste neurons defined by physiological analysis could be distinguished by molecular analysis. A receptor-to-neuron map of the entire Gr repertoire was constructed, and it was found that four classes of bitter taste neurons emerged on the basis of receptor expression, classes that coincided closely with the four functional classes. Moreover, the neuronal classes that were more broadly tuned expressed more receptors (Weiss, 2011).

While the physiological and molecular analyses support each other well, there are limitations to each analysis that raise interesting considerations. The functional analysis is based on a limited number of taste stimuli. Bitter tastants were selected that were structurally diverse, but bitter compounds vary enormously in structure and only a small fraction of them can be sampled. It is possible that by testing more tastants, by testing them over a greater concentration range, or by analyzing temporal dynamics in greater detail, that even more diversity would become apparent among the bitter-sensing neurons (Weiss, 2011).

There are also limitations to the receptor-to-neuron map. First, the map considers exclusively the 68 Grs. There are at least two additional receptors that can mediate bitter taste. DmXR, a G-protein coupled receptor, is expressed in bitter neurons of the labellum and is required for behavioral avoidance of L-canavanine, a naturally occurring insecticide (Mitri, 2009); the TRPA1 cation channel, also expressed in a subset of bitter neurons in the labellum, is required for behavioral and electrophysiological responses to aristolochic acid (Kim, 2010). Second, Gr-GAL4 drivers may not provide a fully accurate representation of Gr gene expression in every case. Genetic analysis has shown that Gr64a is required for the physiological responses of labellar sensilla to some sugars and is therefore expected to be expressed in labellar sugar neurons. The Gr64a-GAL4 driver, however, is not expressed in these neurons, suggesting the lack of a regulatory element. In light of the limitations to the use of the GAL4 system to assess receptor expression, it was encouraging that drivers representing all 68 Grs were expressed in chemosensory neurons, with very few exceptions, and that the expression patterns in the labellum agreed well with the patterns of physiological responses. In addition, it was possible to integrate the functional and expression data and predict a function for one Gr (Weiss, 2011).

While the data support the hypothesis that Gr59c encodes a bitter receptor for berberine, denatonium and lobeline, Gr59c is not sufficient for responses to these compounds in sugar neurons. It is also apparently not necessary, in the sense that physiological responses to these tastants were observed in S-a sensilla that do not express the Gr59c driver. These observations suggest that there is another receptor for berberine, denatonium and lobeline that may recognize a different moiety of these tastants, providing multiple means of detecting some of the most behaviorally aversive bitter tastants in the panel (Weiss, 2011).

It is noted that 38 of the Gr-GAL4 drivers, slightly more than half, showed expression in the labellum. The other Grs are likely expressed in other chemosensory neurons of the adult and larva. Of the 38 labellar Gr-GAL4 drivers, 33 are expressed in bitter neurons, and only a few in sugar neurons. It seems likely that a high fraction of Grs are devoted to bitter perception because of the number and structural complexity of bitter compounds. Sugars are simpler and more similar in structure. In order to detect the wide diversity of noxious bitter substances that an animal may encounter, a larger and more versatile repertoire of receptors is likely needed. It is noted that in mice and rats, 36 bitter receptors have been identified (Weiss, 2011).

Among the Grs mapped to bitter neurons, five map to all bitter neurons: Gr32a, Gr33a, Gr39a.a, Gr66a, and Gr89a. Some or all of these 'core bitter Grs' may function as coreceptors, perhaps forming multimers with other Grs. These core Grs might play a role analogous to Or83b, an Or that is broadly expressed in olfactory receptor neurons and that functions in the transport of other Ors and as a channel, rather than conferring odor-specificity per se. If so, the core Grs may be useful in deorphanizing other Grs in heterologous expression systems. It is noted that in mammals, T1R3 functions as a common coreceptor with either T1R1 or T1R2 to mediate gustatory responses to amino acids or sugars, respectively (Weiss, 2011).

Finally it is noted that the receptor-to-neuron map defines intriguing developmental problems. How do the five classes of sensilla acquire their diverse functional identities? How does an individual taste neuron select, from among a large Gr repertoire, which receptor genes to express? In the olfactory system of the fly, the expression of each receptor gene is dictated by a combinatorial code of cis-regulatory elements and by a combinatorial code of transcription factors. Mechanisms of receptor gene choice were elucidated in part by identifying upstream regulatory elements that were common to coexpressed Or genes. The receptor-to-neuron map that this study has established for the taste system lays a foundation for identifying regulatory elements shared by coexpressed Gr genes, which in turn may elucidate mechanisms of receptor gene choice in the taste system. It will be interesting to determine whether the mechanisms used in the olfactory and taste systems are similar (Weiss, 2011).

In principle the design of the Drosophila taste system could have been extremely simple. Every sensillum could be identical, and all sensilla could report uniformly the valence of each tastant, e.g. positive for most sugars and negative for bitter compounds. Such a design would be economical to encode in the genome and to execute during development (Weiss, 2011).

The design of the Drosophila olfactory system is not so simple. Physiological analysis of the fly has identified ~17 functionally distinct types of olfactory sensilla. This design allows for the combinatorial coding of odors. A recent study of the Drosophila larva defined an odor space in which each dimension represents the response of each component of olfactory input . The distance between two odors in this space was proportional to the perceptual relationship between them. In principle, a coding space of high dimension may enhance sensory discrimination and allow for a more adaptive behavioral response to a sensory stimulus (Weiss, 2011).

This study has found that the fly's taste system is similar to its olfactory system in that its sensilla fall into at least five functionally distinct types, four of which respond to bitter stimuli. This heterogeneity provides the basis for a combinatorial code for tastes and for a multidimensional taste space. A recent report has suggested that flies can not discriminate between pairs of bitter stimuli when applied to leg sensilla (Masek, 2010); it will be interesting to extend such analysis to the labellum, and especially to examine pairs of stimuli that have been shown to activate distinct populations of neurons. Physiological analysis thus invites an extensive behavioral analysis, beyond the scope of the current study, which explores the extent to which such a taste space supports taste discrimination in the fly (Weiss, 2011).

Why might there be selective pressure to enhance the coding of bitter taste? Why not simply coexpress all bitter receptors in one type of neuron that activates a single circuit, thereby triggering equivalent avoidance of all bitter compounds? Not all bitter compounds are equally toxic, and it is not clear that there is a direct correlation between bitterness and toxicity. It is even possible that in certain contexts, such as the selection of egg-laying sites or self-medication, some bitter tastants may have a positive valence. It is noted that in the behavioral analysis carried out in this study, flies tended to be more sensitive to bitter compounds that activate I-a than I-b neurons, suggesting that I-a ligands are perceived to be more bitter than those of I-b ligand, as if I-a ligands were more toxic. A more nuanced behavioral decision based on the intensities of bitter compounds may exist within the complex milieu of rotting fruit (Weiss, 2011).

The olfactory and taste systems of the fly differ in the anatomy of their projections to the brain. Olfactory receptor neurons (ORNs) project to the antennal lobe, which consists of spherical modules called glomeruli. ORNs of a particular functional specificity converge upon a common glomerulus, and there is a distinct glomerulus for each type of ORN. Taste neurons project from the labellum to a region of the ventral brain called the subesophageal ganglion (SOG) that does not have such an obviously modular structure. A study using Gr66a-GAL4, which marks all or almost all bitter cells in the labellum, and Gr5a-GAL4, which marks all or almost all sugar cells, revealed that the two classes of cells project to spatially segregated regions of the SOG (Thorne, 2004; Wang, 2004). However, subsets of bitter cells labeled by Gr-GAL4 drivers did not show obvious spatial segregation within the region of the SOG labeled by Gr66a-GAL4. Markers of different subsets of sugar cells also showed overlapping projections in the SOG. These studies did not, then, reveal at a gross level the kind of spatially discrete projections that are characteristic of the olfactory system (Weiss, 2011).

However, analysis of the SOG at higher resolution has recently revealed more detailed substructure (Miyazaki, 2010). Different sets of Gr66a-expressing neurons, such as those expressing Gr47a, an I-b-specific receptor, showed distinguishable projection patterns, leading to the suggestion that different subregions process different subsets of bitter compounds. Moreover, similarity in projection patterns does not imply identity of function. For example, in the antennal lobe, ORNs that express the odor receptor Or67d converge on the DA1 glomerulus in both males and females, but the projections from DA1 to the protocerebrum are sexually dimorphic. Activation of these ORNs elicits different behaviors in males and females. Taste neurons that project to similar locations in the SOG could also activate different circuits, with distinguishable behavioral consequences. Like the fly taste system, the C. elegans olfactory system does not contain glomeruli and its sensory neurons coexpress many receptors, yet the worm is able to discriminate odors. Finally, it is noted that different sensory neurons that project to similar positions may carry distinguishable information by virtue of differences in the temporal dynamics of their firing. Differences have been identified in the temporal dynamics elicited by different tastants. In summary, it is difficult to draw definitive conclusions about the functional roles of taste neurons from the currently available anatomical analysis (Weiss, 2011).

A final consideration raised by this analysis is how the responses of the different functional classes of taste sensilla are temporally integrated to control feeding behavior. The different functional classes of sensilla differ in length and are located in different regions of the labellar surface. Moreover, during the course of feeding the labellum expands, changing the positions of the various sensilla with respect to the food source. It seems likely that there is a temporal order in which labellar taste sensilla send information to the CNS (Weiss, 2011).

In summary, this study has provided a systematic behavioral, physiological, and molecular analysis of the primary representation of bitter compounds in a major taste organ. This study has defined the molecular and cellular organization of the bitter-sensitive neurons, and extensive functional diversity was found in their responses. The results provide a foundation for investigating how this primary tastant representation is transformed into successive representations in the CNS and ultimately into behavior (Weiss, 2011).

Functional dissociation in sweet taste receptor neurons between and within taste organs of Drosophila

Finding food sources is essential for survival. Insects detect nutrients with external taste receptor neurons. Drosophila possesses multiple taste organs that are distributed throughout its body. However, the role of different taste organs in feeding remains poorly understood. By blocking subsets of sweet taste receptor neurons, receptor neurons in the legs were shown to be required for immediate sugar choice. Furthermore, two anatomically distinct classes of sweet taste receptor neurons were identified in the leg. The axonal projections of one class terminate in the thoracic ganglia, whereas the other projects directly to the brain. These two classes are functionally distinct: the brain-projecting neurons are involved in feeding initiation, whereas the thoracic ganglia-projecting neurons play a role in sugar-dependent suppression of locomotion. Distinct receptor neurons for the same taste quality may coordinate early appetitive responses, taking advantage of the legs as the first appendages to contact food (Thoma, 2016).

A Drosophila gustatory receptor required for strychnine sensation
Strychnine is a potent, naturally occurring neurotoxin that effectively protects plants from animal pests by deterring feeding behavior. In insects, such as the fruit fly, Drosophila melanogaster, bitter-tasting aversive compounds are detected primarily through a family of
gustatory receptors (GRs), which are expressed in gustatory receptor neurons. Multiple GRs that eliminate the behavioral avoidance to several bitter compounds, with the exception of strychnine, have been previously described. This study reports the identity of a strychnine receptor, referred to as GR47a. A mutation in Gr47a, eliminates strychnine repulsion and strychnine-induced action potentials. GR47a is narrowly tuned, as the responses to other avoidance compounds are unaffected in the mutant animals. This analysis supports an emerging model that Drosophila GRs fall broadly into two specificity classes-one class is comprised of core receptors that are broadly required, whereas the other class, which includes GR47a, consists of narrowly tuned receptors that define chemical specificity (Lee, 2015).

Visualizing neuromodulation in vivo: TANGO-mapping of dopamine signaling reveals appetite control of sugar sensing

Behavior cannot be predicted from a 'connectome' because the brain contains a chemical 'map' of neuromodulation superimposed upon its synaptic connectivity map. Neuromodulation changes how neural circuits process information in different states, such as hunger or arousal. This study describes a genetically based method to map, in an unbiased and brain-wide manner, sites of neuromodulation under different conditions in the Drosophila brain. This method, and genetic perturbations, reveal that the well-known effect of hunger to enhance behavioral sensitivity to sugar is mediated, at least in part, by the release of dopamine onto primary gustatory sensory neurons, which enhances sugar-evoked calcium influx. These data reinforce the concept that sensory neurons constitute an important locus for state-dependent gain control of behavior and introduce a methodology that can be extended to other neuromodulators and model organisms (Inagaki, 2012).

The Tango system transforms a transient ligand/receptor interaction into a stable, anatomical readout of reporter gene expression. The reporter gene is activated by a 'private,' synthetic signal transduction pathway, using a bacterial transcription factor (lexA) that is covalently coupled (via a specific tobacco etch virus [TEV] protease-sensitive cleavage site) to the exogenous DA receptor expressed in the cells of interest. The transcription factor is cleaved from the DA receptor following ligand binding, by recruitment of an arrestin-TEV protease fusion protein, and translocates to the nucleus where it activates a lexAop-driven reporter. This system was originally developed to detect receptor activation in cultured mammalian cell lines, but it used to detect receptor activation in this system (Inagaki, 2012).

The data presented here provide proof-of-principle for the utility of a new method, called TANGO-map, to identify, in a brain-wide and relatively unbiased manner, circuit-level substrates of neuromodulation relevant to a particular state-dependent influence on behavior (Inagaki, 2012).

Sweet taste sensitivity in the labellum is enhanced with increasing duration of food deprivation in Drosophila. This observation confirms and extends previous reports in Drosophila and is consistent with observations in many other animal species. This phenomenon was used as a prototypic case of a state-dependent change in behavior to investigate the ability of TANGO-map to identify underlying neuromodulatory mechanisms (Inagaki, 2012).

The results indicate that starvation enhances endogenous DA release onto primary gustatory receptor neurons (GRNs) in the sub-esophageal ganglion, as detected by increased expression of the DopR-Tango reporter in vivo. In contrast, starvation did not increase the DopR-Tango reporter in the MB or AL, although L-dopa feeding did so. These data indicate that DopR-Tango is capable of revealing selective sites of endogenous DA release in a brain-wide manner, under specific behavioral conditions (Inagaki, 2012).

The results indicate that a mutation in the DA receptor DopEcR, as well as specific knockdown of this receptor in sugar-sensing GRNs, eliminates the effect of starvation to enhance the sucrose sensitivity of the proboscis extension reflex (PER). However, this phenotype was only observed at 6 hr of starvation; after 24 hr of food deprivation, these genetic manipulations no longer had an effect. This is not because these manipulations themselves became ineffective at later times, as the same manipulations did attenuate the increased PER sensitivity caused by L-dopa feeding for 24 hr. This suggests that at an early stage of starvation, DA is necessary to enhance the sugar sensitivity of the PER, whereas at later stages additional factors come into play. The slow kinetics of Tango reporter accumulation preclude the detection of statistically significant increases in signal as early as 6 hr following an experimental manipulation. However, the level of reporter expression detected in animals examined after 48 hr of treatment likely reflects the integration of increases in dopaminergic signaling occurring throughout the first 12-24 hr of the treatment period. Thus, although an increase was detected in DopR-Tango signal at a starvation time point when genetic reduction of DopEcR levels no longer impaired the behavioral effect of starvation and observed a behavioral phenotype at a time point too early to be evaluated directly by the TANGO-map method, this should not be taken to imply that no DA release occurred after 6 hr of starvation. Importantly, given the kinetics of the system, the DopR-Tango signals detected in vivo are likely to reflect primarily changes in tonic levels of DA signaling, rather than brief episodes of phasic DA release. Further improvements of the TANGO-map method are required to increase its temporal resolution. Nevertheless, the present methodology provides a powerful method to identify sites where dopaminergic modulation of a given behavior may occur, even if it cannot reveal precisely how quickly such regulation is exerted (Inagaki, 2012).

Several lines of evidence suggest that the dopaminergic modulation of sugar-sensing GRNs revealed in this study may involve an enhancement of Ca2+ influx at the nerve terminal. Both starvation and L-dopa feeding increased sucrose-evoked Ca2+ influx, without changing the frequency of action potentials measured extracellularly at GRN somata. Furthermore, it was found that direct exposure of the brain to DA increased Ca2+ influx at the presynaptic terminals of sugar-sensing GRNs in a DopEcR-dependent manner. A model consistent with these data is that starvation leads to increased DA release, which increases calcium influx into sugar-sensing GRNs via DopEcR, leading to increased neurotransmitter release. The fact that DopEcR signals via the cAMP/PKA pathway, and that this pathway has been reported to increase Ca2+ channel currents in Drosophila, is also consistent with this scenario. Nevertheless, the genetic data suggest that there are additional pathways through which starvation modulates feeding behavior in this system. The finding that DA modulates primary GRNs to control starvation-dependent changes in behavioral sensitivity to sugar echoes the observation of a similar influence of food deprivation on odorant sensitivity in Drosophila. Such neuromodulatory gain control at the level of primary sensory neurons has also been reported in a variety of other invertebrate as well as vertebrate species. Although the possibility that hunger also influences PER behavior at higher-order synapses in the circuit cannot be excluded, the data add to a growing body of information indicating that modulation of primary sensory neurons is a general mechanism for implementing state-dependent changes in behavioral responses to the stimuli detected by these neurons (Inagaki, 2012).

TANGO-map affords a number of unique advantages to study neuronal modulation in the brain. First, and most importantly, it permits the detection of increases in endogenous neuromodulator release in vivo, in an organism in which the application of conventional methods is not feasible. Second, it provides an anatomical readout of neuromodulation at the neural circuit level. The use of a pan-neuronal GAL4 driver to express the sensor permits, in principle, an unbiased survey of potential sites of neuromodulatory activity throughout the brain. Third, the sensor has ligand specificity. The modular design of the Tango system affords the ability to develop in vivo Tango reporters for other biogenic amines and neuropeptides that work via G protein-coupled receptors (GPCRs). Importantly, because the method employs a synthetic, 'private' signal transduction pathway, the readout of the reporter should be relatively insensitive to interference from conventional signal transduction pathways activated by other endogenous receptors. Systematic and comprehensive application of this approach could, in principle, provide an overview of anatomic patterns of neuromodulation in the brain in a given behavioral setting. Finally, because the Tango system is transcriptionally based, in principle it permits the expression not only of neutral reporters but also of effectors such as RNAi’s or ion channels in the neurons receiving neuromodulatory input. Although the TANGO-map system can certainly benefit from improvements in its kinetics and SNR, it affords a means of identifying points-of-entry for studying circuit-level mechanisms of behaviorally relevant neuromodulation that are currently difficult to access in any otherway. The extension of this methodology to other neuromodulators and model organisms should further understanding of state-dependent control of neural activity and behavior (Inagaki, 2012).

Independent, reciprocal neuromodulatory control of sweet and bitter taste sensitivity during starvation in Drosophila

An organism's behavioral decisions often depend upon the relative strength of appetitive and aversive sensory stimuli, the relative sensitivity to which can be modified by internal states like hunger. However, whether sensitivity to such opposing influences is modulated in a unidirectional or bidirectional manner is not clear. Starved flies exhibit increased sugar and decreased bitter sensitivity. It is widely believed that only sugar sensitivity changes, and that this masks bitter sensitivity. This study used gene- and circuit-level manipulations to show that sweet and bitter sensitivity are independently and reciprocally regulated by starvation in Drosophila. Orthogonal neuromodulatory cascades were detected that oppositely control peripheral taste sensitivity for each modality. Moreover, these pathways are recruited at increasing hunger levels, such that low-risk changes (higher sugar sensitivity) precede high-risk changes (lower sensitivity to potentially toxic resources). In this way, state-intensity-dependent, reciprocal regulation of appetitive and aversive peripheral gustatory sensitivity permits flexible, adaptive feeding decisions (Inagaki, 2014).

A fructose receptor functions as a nutrient sensor in the Drosophila brain

Internal nutrient sensors play important roles in feeding behavior, yet their molecular structure and mechanism of action are poorly understood. Using Ca2+ imaging and behavioral assays, this study shows that the Gustatory receptor 43a (Gr43a) functions as a narrowly tuned fructose receptor in taste neurons. Remarkably, Gr43a also functions as a fructose receptor in the brain. Interestingly, hemolymph fructose levels are tightly linked to feeding status: after nutritious carbohydrate consumption, fructose levels rise several fold and reach a concentration sufficient to activate Gr43a in the brain. By using different feeding paradigms and artificial activation of Gr43a-expressing brain neurons, this study shows that Gr43a is both necessary and sufficient to sense hemolymph fructose and promote feeding in hungry flies but suppress feeding in satiated flies. Thus, these studies indicate that the Gr43a-expressing brain neurons function as a nutrient sensor for hemolymph fructose and assign opposing valence to feeding experiences in a satiation-dependent manner (Miyamoto, 2012).

The taste sensory system plays a central role in identifying and evaluating potential foods by discriminating between nutritious chemicals that promote feeding, and structurally diverse, harmful or even toxic compounds, that inhibit feeding. Despite the distinct evolutionary origin of taste receptors of mammals and invertebrates, the cellular organization underlying taste discrimination is largely conserved. In Drosophila, gustatory receptor neurons (GRNs) express distinct sets of gustatory receptors (GRs), providing the basis for discrimination between sweet and bitter taste, respectively. Specifically, the sweet taste of sugars is thought to be exclusively mediated by members of a small conserved subfamily of eight putative sugar Gr (psGr) genes (Gr5a, Gr61a and Gr64a-f), which are partially co-expressed in a single GRN of each taste sensillum, the 'sweet' neuron. Conversely, the bitter taste of alkaloids, terpenoids and phenols is mediated by receptors encoded by most other Gr genes, including Gr66a, Gr33a, and Gr93a, which are partially co-expressed in a second GRN of each sensillum, the 'bitter/high salt' neuron. Most sensillae have two additional GRNs not associated with any characterized Gr gene and are thought to detect low salt solutions and water, respectively (Miyamoto, 2012).

In addition to evaluating external chemicals by the taste sensory system, cells located in internal organs, including the gut, liver/fat body and the brain, express receptors that detect nutrients or their metabolically processed derivates to regulate energy homeostasis and feeding behaviors. Interestingly, internal nutrient sensing in the gut of rodents is in part mediated by taste receptors. Moreover, some bitter taste receptors were shown to be expressed in the mammalian brain and glucose sensing neurons were identified in the hypothalamus. In Drosophila, evidence of an internal nutrient sensor was recently suggested by work of several laboratories. Two groups showed that flies are able to evaluate tasteless carbohydrates based solely on their nutritional content. Suh and co-workers reported that hungry flies with severely impaired sugar-sensing ability can still discriminate sweet tasting sugars based on their nutritious content. However, the molecular identity of the proposed nutrient sensor, the anatomical structure in which it resides, and its ligand, are not known (Miyamoto, 2012).

This paper reports that Gr43a, one of the most conserved insect gustatory receptor genes, is expressed in the brain. Using a Ca2+ imaging assay, GR43a was found to be a narrowly tuned fructose receptor. Although circulating fructose is approximately 100 times less abundant than the main hemolymph sugars glucose/trehalose, it rises to levels high enough after a sugar meal to activate the Gr43a-expressing brain neurons. Feeding experiments reveal that these neurons promote feeding in hungry flies, but suppress feeding in satiated flies. It was shown that artificial activation of Gr43a expressing brain neurons assign positive valence in hungry flies, but negative valence in satiated flies. Thus, this work establishes a precedent of a new nutrient sensing system that regulates food consumption in a satiation-dependent manner (Miyamoto, 2012).

Fructose is the most abundant dietary carbohydrate in many fruits, including apples, grapes and blueberries, major food sources of many Drosophila species. This study has shown that flies have two receptors for this sugar, GR43a, and one encoded by the putative sugar Gr genes (Gr5a, Gr61a and Gr64a-f). While Ca2+ imaging experiments indicate that GR43a is more potent in sensing fructose, it has a minor role as an external sugar receptor in adult flies. However, Gr43a plays a prominent role in sugar sensing during the larval stage. Recent patch-clamp recordings in heterologous expression systems showed that the bombyx mori gustatory receptor BmGr-9 and its Drosophila ortholog GR43a function as a ligand-gated ion channel that is selectively activated by fructose. Curiously, BmGr-9 was not activated by sucrose (GR43a was not tested), and it will be interesting to see whether this difference is due to sequence variation between the two orthologs, or a reflection of suboptimal conditions in a heterologous expression system (Miyamoto, 2012).

Like blood glucose in mammals, levels of glucose and trehalose, the main hemolymph sugars in Drosophila, are kept relatively stable, regardless of feeding state. Analogous to the insulin and glucagon pathways in mammals, flies counteract rising or decreasing glucose/trehalose in the hemolymph by secreting insulin-like peptides or adipokinetic hormone to maintain energy homeostasis. Interestingly, glucose responsive neurons have been identified in the hypothalamus, and it has been proposed that an internal sugar sensor in mice modulates feeding behavior via the dopamine reward system). However, the neural circuits and the molecular nature of internal nutrient sensors are unknown, both in mammals and insects (Miyamoto, 2012).

The internal fructose sensing system described in this study establishes a precedent. About six Gr43aGAL4 neurons in the posterior superior lateral protocerebrum are specifically activated by fructose. Although GR43a functions independently of any of the known psGRs, ectopic expression of Gr43a in other brain neurons does not render them fructose sensitive, implying that this receptor acts in concert with another GR protein, or requires a cell-specific transducer. Regardless, these observations indicate that Gr43aGAL4 brain neurons detect fructose in the hemolymph that derive directly from dietary fructose, or indirectly from other nutritious carbohydrates (Miyamoto, 2012).

Why do flies use fructose, rather glucose (or its disaccharide, trehalose), which is 100 fold more abundant than fructose in the hemolymph, as a signal for an internal nutrient sensor? The data indicate that a glucose sensor would be difficult to activate in a robust fashion because a single sugar meal, even if completely absorbed and converted into glucose would increase overall amount of hemolymph glucose by less than 1/3. Specifically, the amount of hemolymph glucose in a fly (1 mg weight) is ~9.4 μg, compared to ~3 μg of glucose or ~2.4 μg of fructose present in a single meal. In contrast, the amount of hemolymph fructose is very low and therefore increases several fold after a single sugar meal. For example, the ~2.4 μg of fructose in a meal is more than 30 times the amount of fructose present in the hemolymph (0.07 μg). The actual increase observed in the hemolymph sugar assay is ~10 fold, which nevertheless is sufficient for strong activation of the fructose sensor GR43a. Therefore, the internal fructose sensor described here provides a robust system that takes advantage of a steep increase in hemolymph fructose after a sugar meal to evaluate the nutritious content of food (Miyamoto, 2012).

A remarkable property of the Gr43aGAL4 brain neurons is their ability to both promote feeding in hungry flies and suppress feeding in satiated flies. This was directly demonstrated in the odor-conditioning paradigm, which revealed that these neurons assign diametrically opposite valence that is dependent on the satiation status. These observations lead to proposal of the following model: Ingestion and conversion of nutritious carbohydrates leads to an increase of hemolymph fructose, resulting in activation of Gr43aGAL4 brain neurons. In hungry flies, this activation is perceived positively, thereby reinforcing feeding behavior. In contrast, the activation in satiated flies is perceived negatively and leads to feeding termination. How can the satiation status so drastically change the behavioral output through a single group of neurons? At least two distinct mechanisms could account for these opposite effects. The first mechanism invokes direct modulation of some, but not all of the Gr43aGAL4 brain neurons. For example a subset of these neurons may respond to a factor present in hemolymph of satiated, but not hungry flies. Such a factor may act on a second receptor co-expressed in these neurons and modulate GR43a-mediated activity. Alternatively, Gr43aGAL4 brain neurons may target distinct regions of the brain. Consistent with this idea is the observation that axons of Gr43aGAL4 brain neurons project along two separate paths, connecting to distinct, spatially segregated neural ensembles. Distinct output of Gr43aGAL4 brain neurons could be achieved if one of these neural ensembles is regulated by a satiation-dependent signal such as dopamine or octopamine, which are required for aversive and appetitive learning, respectively. Even though Gr43aGAL4 neurons are neither dopaminergic nor octopaminergic, it is possible that their downstream targets use these neurotransmitters to convert output from Gr43aGAL4 neurons into positive or negative valence (Miyamoto, 2012).

In both scenarios, the satiety signal may derive from the fat cells or neurosecretory cells, such as the Drosophila insulin like peptide (DILP) expressing cells in the brain. Interestingly, TOR signaling of larvae fed with a protein rich diet was shown to induce secretion of a humoral signal from fat cells that acts on the DILP expressing cells to control growth and glucose homeostasis. The same signaling pathway might be used to regulate the intake of carbohydrates in adult flies by imposing positive and negative valence via the Gr43aGAL4 brain neurons. Alternatively, satiation-dependent factors, such as neuropeptide F (NPF), may modulate the Gr43aGAL4 brain circuitry. NPF and the mammalian ortholog, neuropeptide Y (NPY) are known to increase food consumption. Moreover, the NPF/NPF receptor system also provides the neural framework that integrates the state of satiety and appetitive memory in adult flies (Miyamoto, 2012).

Gr43a is likely to have additional roles, due to its highly specific expression in proventriculus- and uterus-associated neurons. For example, detection of dietary fructose in the foregut may induce digestive processes, such as peristaltic movements of the gut musculature, or activation of metabolic enzymes in secretory cells. Gr43a expression in the uterus suggests a role for this receptor in female physiology and/or behavior that are linked to mating or reproduction. Fructose, like sex-peptide and other male-specific proteins, might be present in seminal fluid and serve as a ligand to modulate female behaviors associated with reproduction. Whatever the roles of Gr43a may be, the biological functions of this receptor are predicted to be conserved across insect species (Miyamoto, 2012).

Drosophila learn opposing components of a compound food stimulus

Dopaminergic neurons provide value signals in mammals and insects. During Drosophila olfactory learning, distinct subsets of dopaminergic neurons appear to assign either positive or negative value to odor representations in mushroom body neurons. However, it is not known how flies evaluate substances that have mixed valence. This study shows that flies form short-lived aversive olfactory memories when trained with odors and sugars that are contaminated with the common insect repellent DEET. This DEET-aversive learning required the MB-MP1 dopaminergic neurons that are also required for shock learning. Moreover, differential conditioning with DEET versus shock suggests that formation of these distinct aversive olfactory memories relies on a common negatively reinforcing dopaminergic mechanism. Surprisingly, as time passed after training, the behavior of DEET-sugar-trained flies reversed from conditioned odor avoidance into odor approach. In addition, flies that were compromised for reward learning exhibited a more robust and longer-lived aversive-DEET memory. These data demonstrate that flies independently process the DEET and sugar components to form parallel aversive and appetitive olfactory memories, with distinct kinetics, that compete to guide learned behavior (Das, 2014).

DEET has been reported to drive aversive behavior in flies through olfactory and gustatory pathways. Therefore a low concentration presented in solid medium (1% agar) was used to decrease the effects of volatile DEET and increase the chance that flies would taste and perhaps ingest it. To further encourage flies to sample DEET, its palatability was increased by adding it to a mixture of sweet sugars-3 M xylose and 100 mM sucrose (from here on referred to as 'carrier'). Xylose is detected by sweet-sensitive gustatory neurons and is palatable to flies, but it contributes no measurable nutrient value. The low concentration of sweet and nutritious sucrose was added to further increase palatability. The optimum DEET concentration was determined by adding increasing amounts to sugar carrier and conditioning hungry flies by pairing the exposure of the second of two odors with DEET presentation (Das, 2014).

Flies trained with only the sugar carrier showed a significant appetitive memory. In contrast, those trained with increasing amounts of DEET formed aversive memory, with the score rising in line with the increase in DEET concentration, up to 0.4%. Surprisingly, flies trained with 0.8% DEET did not exhibit significantly negative aversive memory scores, suggesting a change in the flies' perception of DEET at this concentration. Therefore the effect of 0.4% and 0.8% DEET on fly feeding was tested by measuring ingestion marked with blue food dye. Whereas flies ate significant amounts of food containing sugar carrier, both 0.4% and 0.8% DEET strongly suppressed feeding behavior. However, whereas flies ate a measurable amount of dye with 0.4% DEET, ingestion was abolished with 0.8% DEET. These data suggest that the failure to train flies with 0.8% DEET reflects an inhibition of sampling by the proboscis and perhaps ingestion of DEET and sugar. To further test a requirement for feeding in learning, attempts were made to train flies that were not hungry or with 0.4% DEET without sugar carrier. Both of these conditions significantly impaired aversive learning when compared to hungry flies trained with 0.4% DEET in sugar carrier. A similar concentration-dependent aversive memory formation was also observed when flies were trained with bitter-tasting quinine that was mixed with sugar carrier. Furthermore, flies that were defective in the IR40a olfactory route of DEET detection displayed normal DEET learning. It was therefore concluded that robust learning with 0.4% DEET-laced sugar requires the flies to attempt to eat DEET and that low DEET concentrations convert the conditioned approach that is formed when flies are trained with the sugar carrier into a conditioned aversion (Das, 2014).

The persistence of DEET memory was tested by conditioning flies and testing their odor preference at extended times after training. Whereas aversive memory performance was robust immediately after training, no statistically significant performance was evident 15 min later. Aversive memory formed with 0.4% DEET is therefore surprisingly labile. DEET and quinine can be sensed by bitter-taste neurons, and ablation of bitter-sensing neurons with Gr66a-GAL4-directed expression of cell-death genes partially impaired DEET, but not sugar, learning. Therefore whether flies could be aversively conditioned by pairing odor presentation with artificial bitter-taste neuron activation, achieved by expression of UAS-dTrpA1, was tested. The dTrpA1 gene encodes a transient receptor potential (TRP) channel that conducts Ca2+ and depolarizes neurons when flies are exposed to temperature >25°C. Gr66a-GAL4, UAS-dTrpA1, and Gr66a-GAL4; UAS-dTrpA1 flies were conditioned by presentation of the first odor with activating 32°C and were immediately tested for memory. Gr66a-GAL4; UAS-dTrpA1 flies exhibited aversive memory that was statistically different from that of all other groups. However, unlike flies conditioned with DEET, significant memory remained 3 hr after training. The differing persistence could result from artificial stimulation of bitter neurons being stronger than DEET activation, in addition to lacking plausible competition from a copresented sugar stimulus (Das, 2014).

Octopamine is required to convey the reinforcing effects of sweet taste. Therefore DEET learning was tested in TbhM18 mutant flies that cannot synthesize octopamine. Whereas appetitive conditioning with 1 M sucrose was significantly impaired in TbhM18 flies, aversive learning with 0.4% DEET was indistinguishable from that of wild-type flies. Therefore, octopamine is not required for DEET learning (Das, 2014).

Electric-shock-reinforced aversive memory formation also requires specific dopaminergic neurons and the DopR1 dopamine receptor. It was therefore first determined whether DEET learning required the DopR1 receptor. Mutant dumb1 flies that are defective for the DopR1 dopamine receptor did not display aversive learning with DEET. Similarly, aversive learning with artificial activation of bitter-taste neurons was abolished in dumb1 flies (Das, 2014).

The MB-MP1, MB-MV1, and MB-M3 classes of dopamine neuron have been previously implicated in shock learning. To test whether either of these neurons were required for DEET learning, the dominant temperature-sensitive UAS-shibirets1 transgene was tested in MP1, MV1, and M3 neurons using the c061; MBGAL80, R73F07, and NP5272 and NP1528 GAL4 drivers, respectively. The shibirets1 transgene permitted blockade of the respective neurons by performing DEET conditioning experiments at the restrictive temperature of 31°C. This analysis revealed significantly impaired DEET learning performance when MP1 neurons were blocked but nonsignificant effects when either MV1 or M3 neurons were compromised. Blockade of MP1 neurons, however, did not significantly affect DEET avoidance in naive flies. To further support a role for the dopaminergic MP1 neurons in c061; MBGAL80, they were removed from the expression pattern by including a TH-GAL80 transgene. When the remaining cells were blocked during conditioning, flies exhibited levels of DEET learning that were indistinguishable from those of wild-type flies. It is therefore concluded that MP1 neurons are critical for DEET learning, whereas MV1 and M3 neurons contribute a lesser role. It was noted that prior work implicated the MV1 and M3 neurons in the formation of more persistent forms of shock-reinforced aversive memory (Das, 2014).

Next, live imaging was used to determine whether DEET ingestion activated the MP1 dopamine neurons. UAS-GCaMP3 was expressed in dopaminergic neurons with TH-GAL4 and imaged DEET-evoked changes in fluorescence in the dopaminergic neuron processes on the mushroom body. These analyses revealed strong activation of the MP1 innervated heel and MV1 innervated junction regions of the mushroom body while presenting flies with both 0.4% DEET in sugar carrier, sugar carrier alone, and DEET alone. In comparison, water presentation did not activate the MP1 and MV1 neurons. Therefore, functional imaging does not reveal obvious valence specificity of MP1 and MV1 signals, being activated by both sugar and DEET. It should be noted that the MP1 neurons have been previously implicated in shock- and sugar-reinforced learning and memory expression. Since a strong requirement was observed for MP1 neurons in behavioral DEET learning, it is concluded that MP1 activity is likely to represent aversive reinforcement signals to mushroom body neurons. As expected, transmission from mushroom body neurons is required for the expression of DEET memory (Das, 2014).

Finding a role in DEET learning for dopamine neurons that are also required for shock learning suggests a common reinforcement process, despite the different nature of the external unconditioned stimulus. Therefore a differential conditioning paradigm was designed to further test this model. Flies were trained by pairing of one odor with DEET and the other odor with a varying intensity of electric shock. These experiments revealed an avoidance of the previously DEET-associated odor when countered with 30 or 60 V but an avoidance of the shock-paired odor when countered with 80 or 90 V. Extrapolation of a curve fit between the tested points predicted 70 V as being equivalent to 0.4% DEET-which was subsequently confirmed in direct experiments. Having established the point of reinforcer equivalence, it was reasoned that if the shock and DEET reinforcement processes were common, blocking some of the responsible dopamine neurons would equally impair shock and DEET learning and therefore not alter equivalence. If, on the other hand, MP1 neurons contribute differently to DEET and shock reinforcement, blocking them would unevenly affect learned behavior and would skew performance toward one or the other, reflecting the imbalance. Strikingly, differential learning remained balanced in c061; MBGAL80; UAS-shits1 flies in which MP1 neurons were blocked. Importantly, this balanced valuation does not reflect a 'zero versus zero' learning because the same c061; MBGAL80; UAS-shits1 flies only display a partial defect if they were trained with 70 V shock alone. Therefore, these experiments support a model in which the reinforcing systems for 0.4% DEET and 70 V shock are similar, with MP1 being part of the system for both. In addition, it is notable that despite the relative magnitude of immediate memory scores (∼0.6 for 70 V shock and <0.3 for DEET) and the difference in respective memory persistence (hours for shock and minutes for DEET), the immediate learned value of these two aversive stimuli is comparable (Das, 2014).

Next whether the apparent fragility of aversive DEET memory could be explained by the coformation of a more persistent sugar memory was tested. Reasoning that these analyses would benefit from the induction of a more robust sugar memory, optimal conditions for aversive memory formation was first established with DEET-laced 1 M sucrose. Flies trained with DEET in 1 M sucrose showed a similar dose-dependent aversive learning to those trained in prior experiments with DEET in xylose and sucrose carrier, although the optimal DEET concentration for learning shifted from 0.4% to 0.6%. The DEET memory performance was next thested of TbhM18 mutant flies that are impaired in appetitive learning. Strikingly, whereas the behavior of wild-type flies became conditioned approach within 30 min, TbhM18 flies showed a more persistent aversive memory performance, with scores remaining significantly negative 30 and 60 min after training. However, the performance still converted from odor avoidance to approach by 24 hr. Since octopamine only provides short-term sweet-taste reinforcement, it was hypothesized that persistent nutrient-dependent memory must be independently formed in TbhM18 flies. Indeed, TbhM18 flies trained with 1 M sucrose did not display immediate memory, but significant performance emerged 1 hr after training and remained for at least 24 hr. These data support the prior model of octopamine specifically conveying short-term appetitive reinforcement and not the nutrient-dependent long-term signal. In addition, they suggest that the DEET learning protocols form parallel aversive and appetitive memories. To further test a parallel memory trace model, flies were trained with 0.3% DEET and 1 M sucrose, a combination with which no immediate odor avoidance or approach performance is evident, and either the rewarding or aversive dopaminergic neurons were blocked during training. Strikingly, blockade of the rewarding dopaminergic neurons with 0104; UAS-shits1 revealed significant conditioned avoidance. In contrast, blockade of the negatively reinforcing MB-MP1 dopaminergic neurons with c061; MBGAL80; UAS-shits1 uncovered significant conditioned odor approach performance. It was therefore concluded that training with the compound DEET and sugar stimulus leads to the independent formation of aversive and appetitive memories. The differing stability of these competing memories subsequently determines which one of them guides learned behavior after training (Das, 2014).

The extent to which rewarding and aversive stimuli are coded in mammalian dopaminergic neurons is hotly debated. Recordings in the monkey have shown that some dopaminergic neurons respond to either bitter taste or an aversive air puff, suggesting that the quality of an aversive reinforcer may be represented. Work in flies has functionally split dopaminergic neurons into groups that are critical for reward learning and others for aversive learning. However, recent studies suggested a requirement for modulation of the aversive system in appetitive learning and demonstrated a role for rewarding dopaminergic neurons in relative aversive learning. In addition, imaging activity in negatively reinforcing MB-MP1 neurons revealed responses to both sweet sugar and bitter DEET. Nevertheless, the DEET reinforcement data presented in this study, when taken with published knowledge of shock reinforcement, imply that flies utilize the same, or at least an overlapping, evaluation system to convey the reinforcing effects of discrete aversive stimuli. It will be interesting to determine the respective input pathways to the negatively reinforcing dopaminergic neurons. These experiments also highlight the importance of being able to both record from and control recognizable subpopulations of dopaminergic neurons. Without intervention, it is difficult to understand whether a given dopaminergic neuron provides a reinforcement or motivational salience signal (Das, 2014).

Perhaps most surprisingly, the data demonstrate that during learning flies independently assign the value of individual components of a compound food stimulus to an odor. Rather than forming a single memory of the relative quality of the tainted sugar, they learn the bitter and sugar components in parallel. This multiplexing is further illustrated by sugars in which octopamine distinguishes between memories of sweet taste and nutrient components. These results suggest that despite the integration of tastant information that occurs within the first layers of the gustatory system and provides control over food ingestion, each component also gains unprocessed access to the negative and positive arms of the reinforcement system. The fly therefore appears to retain as much information of foraging history as possible, while allowing the relative persistence of the resultant constituent memories to inform later behavior. Such a mechanism might help the fly to direct short-term foraging away from food sources that happen to be unpalatable but remember that they are usually nutritious (Das, 2014).

Octopamine-mediated circuit mechanism underlying controlled appetite for palatable food in Drosophila

The easy accessibility of energy-rich palatable food makes it difficult to resist food temptation. Drosophila larvae are surrounded by sugar-rich food most of their lives, raising the question of how these animals modulate food-seeking behaviors in tune with physiological needs. This study describes a circuit mechanism defined by neurons expressing tdc2-Gal4 (a tyrosine decarboxylase 2 promoter-directed driver) that selectively drives a distinct foraging strategy in food-deprived larvae. Stimulation of this otherwise functionally latent circuit in tdc2-Gal4 neurons was sufficient to induce exuberant feeding of liquid food in fed animals, whereas targeted lesions in a small subset of tdc2-Gal4 neurons in the subesophageal ganglion blocked hunger-driven increases in the feeding response. Furthermore, regulation of feeding rate enhancement by tdc2-Gal4 neurons requires a novel signaling mechanism involving the VEGF2-like receptor, octopamine, and its receptor. These findings provide fresh insight for the neurobiology and evolution of appetitive motivation (T. Zhang, 2013).

Modulation of feeding responses to food sources is heavily influenced by nutritional quality, taste, and the energy costs of foraging. The current findings suggest that Drosophila larvae have evolved a complex neural network to regulate appetitive motivations. In hungry fly larvae, OA neurons seem to mediate a specialized circuit that selectively promotes persistent feeding of readily ingestible sugar food. This OA circuit functions in parallel to the previously characterized mechanism coregulated by the fly insulin and NPY-like systems that drives feeding response to non-preferred solid food. Because food deprivation triggers simultaneous activation of both circuits, hungry larvae become capable of adaptively responding to diverse energy sources of high or low quality. It remains to be determined how OA signaling promotes persistent feeding response to liquid sugar food in hungry larvae. One possible scenario is that OA neurons in the SOG may be conditionally activated by gustatory cues associated with rich palatable food to promote appetitive motivation (T. Zhang, 2013).

This study has has provided evidence, at both molecular and neuronal levels, that the OA-mediated feeding circuit has two opposing effects on food motivation. When surrounded by liquid sugar media, the OA circuit is essential to prevent fed animals from excessive feeding. Because targeted lesions in VUM1 neurons caused excessive feeding response, these neurons may define an inhibitory subprogram within the OA feeding circuit. However, targeted lesions in VUM2 neurons attenuated hunger-induced increases of feeding response, suggesting that VUM2 neurons, along with the OA receptor Octβ3R, may define a subprogram that enhances feeding in fasted larvae. Several lines of evidence suggest that the VUM2 neuron-mediated subprogram may be suppressed by the VUM1 neuron-mediated subprogram. First, fed larvae with double lesions in both VUM1 and VUM2 neurons failed to display excessive feeding, suggesting that increased feeding response of fed larvae deficient for VUM1 neuronal signaling requires VUM2 neurons. Second, targeted lesions in VUM2 neurons of fed tdc2-Gal4/UAS- dTrpA1 larvae completely blocked the increased feeding response induced by genetic activation of tdc2-Gal4 neurons. Finally, the anatomical data also show that VUM1 and VUM2 neurons project to many common regions of the larval brain implicated in the control of feeding. Future work will be needed to determine whether VUM1 neurons inhibit directly or indirectly the activity of VUM2 neurons (T. Zhang, 2013).

Genetic and pharmacological evidence has been obtained for the critical role of OA in the regulation of acquiring readily accessible sugar media. OA has been reported to mediate diverse neurobiological functions including appetitive memory formation and modulation of the dance of honey bee foragers to communicate floral or sucrose rewards. It is postulated that the different OA receptors may mediate diverse OA-dependent behavioral responses to high-quality foods (T. Zhang, 2013).

Norepinephrine (NE), the vertebrate counterpart of OA, has been shown to promote ingestion of carbohydrate-rich food at the beginning of a natural feeding cycle. This feeding activity of NE resides in the paraventricular nucleus (PVN) of the feeding control center. In the PVN, α1 and α2 adrenergic receptors are organized in an antagonistic pattern. Activation of α1 receptor inhibits food intake, whereas activation of the α2 receptor stimulates food intake. The current results suggest that the insect OA system, like the NE system in mammals, exerts both positive and negative effects on the intake of preferred food. The activity of NE in PVN has been shown to antagonize that of 5-HT, which suppresses intake of carbohydrate- rich food. In Drosophila, 5-HT is also known to suppress feeding response. These findings suggest that the homeostatic control of intake of preferred food is likely mediated by a conserved neural network in flies and mammals (T. Zhang, 2013).

This study has identified a unique role of Pvr in physiological regulation of hunger-motivated feeding of preferred food (liquid sugar media). The feeding-related activity of the Pvr pathway involves two regulatory proteins, Drk and Ras, and oral introduction of OA restores the hunger-driven feeding response in tdc2-Gal4/ UAS-drkdsRNA larvae. Together, these results suggest that the Pvr pathway positively regulates OA release by tdc2-Gal4 neurons. Among the three identified ligands of Pvr, Pvf2 is enriched in the larval CNS. The current finding suggests that Pvf2 regulates the feeding-related activity of the Pvr pathway. It is possible that Pvf2 may transduce a metabolic stimulus to Pvr/tdc2-Gal4 neurons that signals the energy state of larvae. In the honey bee brain, OA neurons from the SOG have been reported to respond to sugar stimulation. Therefore, it would be interesting to test whether the Pvf2/ Pvr pathway is responsive to sugar stimuli (T. Zhang, 2013).

Previous studies have shown that the fly insulin and NPY-like systems coregulate hunger-elicited motivation to acquire solid sugar media. This study has now provided evidence that the fly VEGFR2- and NE-like systems control larval motivation to acquire liquid sugar media. These findings strongly suggest that the neural activities of different RTK systems play critical roles in different aspects of adaptive feeding decisions under various food and metabolic conditions. Therefore, further investigation of the mechanistic details of the food-related functions of RTK systems in the Drosophila model may provide novel insights into the neurobiology and evolution of appetitive control as well as pathophysiology of eating-related disorders (T. Zhang, 2013).

Cellular Basis of Bitter-Driven Aversive Behaviors in Drosophila Larva

Feeding, a critical behavior for survival, consists of a complex series of behavioral steps. In Drosophila larvae, the initial steps of feeding are food choice, during which the quality of a potential food source is judged, and ingestion, during which the selected food source is ingested into the digestive tract. It remains unclear whether these steps employ different mechanisms of neural perception. This study provides insight into the two initial steps of feeding in Drosophila larva. Substrate choice and ingestion were found to be determined by independent circuits at the cellular level. First, 22 candidate bitter compounds were taken, and their influence on choice preference and ingestion behavior was examined. Interestingly, certain bitter tastants caused different responses in choice and ingestion, suggesting distinct mechanisms of perception. Evidence is provided that certain gustatory receptor neurons (GRNs) in the external terminal organ (TO) are involved in determining choice preference, and a pair of larval pharyngeal GRNs is involved in mediating both avoidance and suppression of ingestion. These results show that feeding behavior is coordinated by a multistep regulatory process employing relatively independent neural elements. These findings are consistent with a model in which distinct sensory pathways act as modulatory circuits controlling distinct subprograms during feeding (Choi, 2020).

A general assumption would be that a tastant would cause a similar response in ingestion and choice preference behavior, in either a positive or negative manner. However, the current findings corroborate that certain tastants elicit divergent ingestion and choice preference behavior. Combining molecular genetic tools, behavioral assays, and genetically encoded calcium sensors to assess neuronal activity, the results provide evidence that relatively independent neural systems appear to regulate the two initial processes of feeding in Drosophila larva: searching for palatable food, i.e., choice preference, and eating the selected food, i.e., ingestion. A subset of gustatory neurons housed in the TO, the external gustatory organ of Drosophila larva, detect denatonium and induce avoidance behavior, and DP1, a specific pair of GRNs in the dorsal pharyngeal organ, plays a major role in regulating both ingestion and avoidance in response to CAF (Choi, 2020).

The TO of Drosophila larva is located at the tip of the cephalic lobes, and is thus anatomically likely to be the first organ to contact external stimuli and subsequently cause a change in movement to regulate the initial step of feeding. Similarly, pharyngeal sensilla are located between the external sense organs and digestive organs, and are thus anatomically likely to act in maintaining the ingestion of appetitive foods while stopping ingestion and causing avoidance of aversive cues such as bitter toxins. It could be advantageous for ingestion to be predominantly controlled by pharyngeal sense organs, rather than by external organs, since animals can try out a potential food source before making their decision, rather than blindly avoiding it. This could be a particularly advantageous strategy for insect larvae whose main purpose is to feed. Also, the difference in behavioral responses elicited by the C1 and C7 neurons in the TO and DP1 in the pharyngeal sense organs is likely linked to the difference in brain projection patterns of GRNs from the TO and pharyngeal GRNs from the larval SEZ, with the distinct projection areas of the brain taste center likely being linked to different circuits, resulting in distinct behavioral outputs (Choi, 2020).

In Drosophila larvae, choice and ingestion have generally been grouped together and studied as a group of reflexive behaviors. Sugar processing provides another intriguing example of divergence between choice and ingestion. Larvae generally show increased preference and feeding when exposed to increasing concentrations of fructose or sucrose. At extremely high concentrations such as 2 M or 4 M, larvae still exhibit preference in terms of choice, but show suppression of feeding (or ingestion, as is denoted in this study). Since this suppression of ingestion could be due to high viscosity and/or osmolarity, a direct comparison to the processing of aversive tastants such as bitter chemicals is difficult. However, this example nonetheless provides evidence that relatively independent circuits exist to determine choice and ingestion. Using bitter tastants, this study found that choice and ingestion can manifest in clearly divergent behaviors to the same compounds and elucidate the cellular basis of these observations. Similarities to the observation that external sense organs and pharyngeal organs appear to be involved in somewhat independent behavioral output can be seen in sugar consumption in the adult fly. The activation of sweet GRNs in the legs and labellum initiates feeding behaviors including the proboscis extension reflex, and pharyngeal sweet GRNs play an important role in directing the sustained consumption of sweet compounds (Choi, 2020).

Most of the 22 putative bitter tastants tested in this study, including CAF, cause negative effects in choice preference and ingestion. Nicotine caused a positive P.I. in the choice preference assay. It cannot be completely rule out that nicotine could act as an attractive chemosensory cue at low concentrations, this study found that nicotine inhibits the movement of larvae in the experimental setup. Larvae strongly avoid denatonium, but once they sample denatonium-containing food, they ingest it. This ingestion likely occurs because denatonium is added to the agarose of the entire plate, whereas larvae probably would not ingest as much if they had the choice. Nevertheless, the results suggest that this larval response to denatonium is due to the existence of a functional receptor complex for denatonium in the TO, which does not exist in the pharyngeal sense organs, or at the very least the DP1 neuron. Consistently, ectopic expression of GR59c in DP1 caused a novel calcium response to denatonium and suppression of ingestion in response to denatonium. Some remaining questions regarding sensing of denatonium merit further study. Avoidance to denatonium is defective when either C1 or C7 is inactivated, indicating that C1 and C7 are not redundant in terms of behavior. It is possible that a certain threshold of neuronal activity is required to elicit behavior, or inactivation of one neuron may cause a change in the functions of other GRNs. Although a numerically simple system, larval GRNs also have a multimodal character, and as such a more complicated mechanism might be involved. Also, in the bitter sensing neurons of the adult labellum, two complexes, GR32a/GR66a/GR59c and GR32a/GR66a/GR22e, are each sufficient to confer a response to denatonium. Based on Gr-GAL4 expression, the larval DP1 neuron expresses Gr22e, but not Gr59c, but is not capable of detecting denatonium. This suggests that the GRNs of the larva and adult fly possess different cellular contexts, which could be interesting to unravel. An interesting remaining question is if Gr59c is solely responsible for denatonium sensing in the larval C1 neuron or if the existing Gr22e can rescue denatonium sensing in Gr59c mutants. This would indicate that Gr22e needs a specific co-receptor repertoire for denatonium detection and could help elucidate coding differences in the larva versus the adult fly (Choi, 2020).

The levels at which distinct bitter compounds are detected might reflect the ecological niche of the animal and the toxicity level of a given tastant. The results suggest that information from the DP1 neuron is processed in a circuit that results in negative and aversive behavior in ingestion and choice preference to CAF. The C1 and C7 neuron in the TO elicit avoidance to denatonium in choice preference behavior. Thus, these results suggest that distinct sensory neurons appear to have distinct sensory roles, likely through the expression of specific receptors or specific groups of receptors. Sensory information detected by these sensory neurons appears to be processed through distinct circuits in the central nervous system to mediate changes in ingestion or choice behavior. It is yet unclear whether the different circuits interact to result in a final behavioral output. Further examination of the potential connections between the external and pharyngeal gustatory neurons and interneurons or motor neurons in the brain may provide insight into the overall neural circuit that regulates feeding and locomotion (Choi, 2020).

Dopaminergic modulation of sucrose acceptance behavior in Drosophila

For an animal to survive in a constantly changing environment, its behavior must be shaped by the complex milieu of sensory stimuli it detects, its previous experience, and its internal state. Although taste behaviors in the fly are relatively simple, with sugars eliciting acceptance behavior and bitter compounds avoidance, these behaviors are also plastic and are modified by intrinsic and extrinsic cues, such as hunger and sensory stimuli. This study shows that dopamine modulates a simple taste behavior, proboscis extension to sucrose. Conditional silencing of dopaminergic neurons reduces proboscis extension probability, and increased activation of dopaminergic neurons increases extension to sucrose, but not to bitter compounds or water. One dopaminergic neuron with extensive branching in the primary taste relay, the subesophageal ganglion, triggers proboscis extension, and its activity is altered by satiety state. These studies demonstrate the marked specificity of dopamine signaling and provide a foundation to examine neural mechanisms of feeding modulation in the fly (Marella, 2012).

Invertebrate models with less complex nervous systems and robust sensory-motor behaviors may illuminate simple neural modules that regulate behavior. This study examined flexibility in a gustatory-driven behavior and found that a dopaminergic neuron is a critical modulator. Loss-of-function studies involving dopamine receptor mutants and gain-of-function studies argue that increased dopaminergic activity promotes proboscis extension to sucrose, and decreased dopaminergic activity inhibits it. These studies show that a single dopaminergic neuron in the SOG, TH-VUM, can drive proboscis extension. TH-VUM does not respond to sugars, arguing that it is not directly in the pathway from taste detection to behavior, but instead acts over a longer timescale or in response to other cues to modulate proboscis extension to sucrose. Consistent with this idea, satiety state influences TH-VUM activity, promoting activity when the animal is food deprived and the probability of proboscis extension is increased. These studies suggest that dopaminergic activity regulates the probability of extension according to an animal's nutritional needs (Marella, 2012).

The finding that dopamine neural activity affects proboscis extension to sucrose, but not water, argues that dopamine regulation occurs upstream of shared motor neurons involved in proboscis extension. The pathway selectivity also argues that different molecular mechanisms modulate food and water intake independently in the fly, with parallels to hunger and thirst drives in mammals. Where dopamine acts in the sugar pathway is not known. Experiments to test for proximity between sugar sensory neurons and TH-VUM using the GRASP approach suggested that a few fibers are in close proximity, but the significance is unclear. The broad arborizations of TH-VUM suggest it may have many targets (Marella, 2012).

Dopamine is a potent modulator of a variety of behaviors in mammals and flies. In mammals, functions of dopamine include motor control, reward, arousal, motivation, and saliency. Dopamine also critically regulates feeding behavior. Mice mutant for tyrosine hydroxylase fail to initiate feeding, although they distinguish sucrose concentrations and have the motor ability to consume (Szczypka, 1999). Dopamine pathways that regulate feeding are complex, with the tuberoinfundibular, nigrostriatal, and mesolimbic and mesocortical pathways implicated in different aspects of feeding regulation (Vucetic and Reyes, 2010). Although several studies show that dopamine promotes positive aspects of feeding, there is debate over whether dopamine is involved in pleasure ('liking'), motivation or salience ('wanting'), associative learning, or sensory-motor activation (Berridge, 2007). With 20,000–30,000 TH-positive neurons in mice and 400,000-600,000 in humans (Björklund and Dunnett, 2007), the complexity of dopaminergic regulation makes it difficult to parse the function of different neurons (Marella, 2012).

In Drosophila, as in mammals, dopamine participates in conditioning and arousal, and this study highlights a shared role in feeding regulation. There are only a few hundred TH-positive neurons in Drosophila, and recent studies have begun to elucidate the function of different dopaminergic neural subsets. This work demonstrates that a single dopaminergic neuron in the SOG potently modulates proboscis extension behavior. Other dopaminergic neurons have cell bodies near TH-VUM and extensive projections in the SOG, yet activation of these neurons is not associated with proboscis extension. It is possible that additional dopaminergic neurons regulate other aspects of taste behavior, but they are insufficient to drive proboscis extension (Marella, 2012).

In mammals, dopamine levels in the nucleus accumbens, the target of the mesolimbic pathway, increase upon sugar detection in the absence of consumption or upon nutrient consumption in the absence of detection, suggesting that dopamine encodes multiple rewarding aspects of sugar: intensity on the tongue and nutritional value. Recent studies in Drosophila also show that they sense nutritional content independent of taste detection, and this influences ingestion. It will be interesting to determine whether dopamine plays a role in sensing internal nutritional state and regulates other aspects of ingestion in addition to its role in proboscis extension (Marella, 2012).

The anatomical location of the dopaminergic interneuron highlights the central role of the SOG in taste processing and suggests that local SOG circuits may control proboscis extension behavior. Future studies identifying the downstream targets of TH-VUM will ultimately enable a deeper understanding of how dopamine achieves spatial and temporal modulation of extension probability. Our current study identifies an essential role for dopamine in gain control of proboscis extension to sucrose and underscores the exquisite specificity of single neurons as thin threads to behavior (Marella, 2012).

Food experience-induced taste desensitization modulated by the Drosophila TRPL channel

Animals tend to reject bitter foods. However, long-term exposure to some unpalatable tastants increases acceptance of these foods. This study shows that dietary exposure to an unappealing but safe additive, camphor, caused the fruit fly to decrease camphor rejection. The transient receptor potential-like (TRPL) cation channel is a direct target for camphor in gustatory receptor neurons, and long-term feeding on a camphor diet leads to reversible downregulation of TRPL protein concentrations. The turnover of TRPL is controlled by an E3 ubiquitin ligase, Ube3a. The decline in TRPL levels and increased acceptance of camphor reversed after returning the flies to a camphor-free diet long term. It is proposed that dynamic regulation of taste receptors by ubiquitin-mediated protein degradation comprises an important molecular mechanism that allows an animal to alter its taste behavior in response to a changing food environment (Y. V. Zhang, 2013).

Depending on the properties of a food, an animal decides to accept or reject it. In most terrestrial animals, sweet substances are assumed to provide nutrients, whereas many bitter compounds are correlated with poisons. However, this latter assumption is flawed, as many bitter foods are safe and nutritious7. Consequently, many animals learn to accept formerly unpalatable, bitter tasting foods, but only if they are safe and if more appealing options are unavailable. Although changes in the mammalian gustatory cortex have been found to be associated with diet-induced changes in taste preference, the nature of the molecular modifications in taste-receptor cells that contribute to environmentally induced modifications of food selection was previously unclear (Y. V. Zhang, 2013).

This study found that fruit flies, similarly to many other animals, including insects such as locusts and moths, decrease food avoidance to certain bitter foods after prolonged exposure. The flies decreased their aversion to the unappealing tastant, camphor, in response to dietary experience. However, the flies did not form adaptation to all bitter tastants, including quinine, strychnine and lobeline (Y. V. Zhang, 2013).

The decline in rejection of camphor was controlled in the peripheral sensory neurons through a reversible decline in the concentration of the camphor-activated TRPL channel in dendrites. Because TRPL was activated by camphor but not other unpalatable tastants such as quinine, downregulation of this channel selectively affected aversion to camphor. Two observations support the conclusion that the downregulation of TRPL contributes to taste adaptation. First, removal of camphor from the diet resulted in a return to the original TRPL levels and a restoration of the formerly held aversion to camphor. Second, during camphor exposure, an E3 ubiquitin ligase, Ube3a, targeted TRPL for degradation, thereby decreasing TRPL expression levels in the GRNs. Loss of Ube3a eliminated the camphor diet–induced downregulation of TRPL and prevented taste desensitization. This finding also highlights that the diet-induced reduction in TRPL expression is mediated by protein turnover. Consistent with this mechanism underlying the decline in TRPL levels rather than a reduction in trpl transcription, the activity of the trpl reporter was indistinguishable between flies maintained on normal as compared to camphor diets. Thus, this study identified a molecular mechanism in peripheral sensory neurons that underlies plastic, diet-induced alterations in food preference. A question that remains open concerns the link between TRPL activation and downregulation of the channel. Given that TRPL is a Ca2+-permeable channel, Ube3a activity might be directly or indirectly activated by a rise in Ca2+ levels (Y. V. Zhang, 2013).

During the formation of taste adaptation at the behavioral level, there was a second change that occurred. After downregulation of TRPL, the number of boutons at the GRN axonal terminals in the SOG declined. Thus, synapse loss appeared to be a secondary consequence of the decline in TRPL levels. In further support of this conclusion, the number of synapses was unchanged in the ube3a mutant, which did not show a reduction in TRPL protein levels. Because the morphological change was reversible by withdrawal of a sustained camphor diet, it is concluded that GRNs in adult Drosophila undergo cellular modification. An important question still to be answered concerns the identity of the molecular pathway bridging the long-distance communication between the decline of TRPL levels in the dendrites and bouton pruning in the axons. It is suggested that synapse elimination may be insufficient to cause camphor desensitization, but it might synergize with TRPL downregulation to decrease distaste for camphor. In summary, this study revealed that food experience can modify behavior by altering signaling at both the dendrites and axons of the GRNs (Y. V. Zhang, 2013).

Mechanisms similar to those described here in this study represent an evolutionarily conserved strategy that contributes to chemosensory desensitization. It is noteworthy that a worm TRP vanilloid (TRPV) channel, OSM-9, functions in both olfactory adaptation and adaptation to sodium chloride. Thus, a similar mechanism of ubiquitination-mediated downregulation of OSM-9 might contribute to chemosensory desensitization in Caenorhabditis elegans. In addition, diet-induced morphological changes in mammalian taste buds have been reported. The current work raises the possibility that changes in taste preference in other animals, including vertebrates, may be mediated by alterations in the concentration of receptors and channels in taste-receptor cells and subsequent modifications in synaptic connections (Y. V. Zhang, 2013).

A gustatory receptor paralogue controls rapid warmth avoidance in Drosophila

Behavioural responses to temperature are critical for survival, and animals from insects to humans show strong preferences for specific temperatures. Preferred temperature selection promotes avoidance of adverse thermal environments in the short term and maintenance of optimal body temperatures over the long term, but its molecular and cellular basis is largely unknown. Recent studies have generated conflicting views of thermal preference in Drosophila, attributing importance to either internal or peripheral warmth sensors. This study reconciles these views by showing that thermal preference is not a singular response, but involves multiple systems relevant in different contexts. Previously it was found that the transient receptor potential channel TRPA1 acts internally to control the slowly developing preference response of flies exposed to a shallow thermal gradient. This study now finds that the rapid response of flies exposed to a steep warmth gradient does not require TRPA1; rather, the gustatory receptor GR28B(D), one of five splice variants of Gustatory receptor 28b), drives this behaviour through peripheral thermosensors. Gustatory receptors are a large gene family, widely studied in insect gustation and olfaction, and are implicated in host-seeking by insect disease vectors, but have not previously been implicated in thermosensation. At the molecular level, GR28B(D) misexpression confers thermosensitivity upon diverse cell types, suggesting that it is a warmth sensor. These data reveal a new type of thermosensory molecule and uncover a functional distinction between peripheral and internal warmth sensors in this tiny ectotherm reminiscent of thermoregulatory systems in larger, endothermic animals. The use of multiple, distinct molecules to respond to a given temperature, as observed in this study, may facilitate independent tuning of an animal's distinct thermosensory responses (Ni, 2013).

In Drosophila two sets of warmth-sensing neurons (activated above ~25°C) have been proposed to control thermal preference: the anterior cell (AC) neurons, located inside the head, and the hot cell (HC) neuron, located peripherally in the arista. However, different studies suggest conflicting cellular and molecular mechanisms for thermal preference control. At the cellular level, primary importance has been attributed to either internal warmth sensors. At the molecular level, the internal AC neurons sense warmth via TrpA1, which encodes a warmth-activated transient receptor potential (TRP) channel, whereas the peripheral HC neurons seem to be TrpA1-independent. To clarify the mechanisms of thermal preference, this study sought to discover the molecular basis of HC neuron function (Ni, 2013).

The arista contains six neurons: three warmth-responsive HC neurons (which can be labelled using cell-specific Gal4 expression in the HC-GAL4 strain) and three cool-responsive (cold cell; CC) neurons (labelled in the CC-GAL4 strain). Three unidentified cells in the arista have been reported to express Gr28b.d-GAL4, a transgene in which promoter sequences upstream of the gustatory receptor GR28B(D) control Gal4 expression. This study found that these Gr28b.d-GAL4-expressing cells resembled thermoreceptors, with cell bodies near the arista base and thin processes in the shaft. To determine the thermoreceptor subset labelled, Gr28b.d-GAL4 was combined with each thermoreceptor-specific Gal4. Gr28b.d-GAL4 plus HC-GAL4 labelled three neurons, whereas Gr28b.d-GAL4 plus CC-GAL4 labelled six neurons, indicating that Gr28b.d-GAL4 is expressed in the HC neurons. Although in situ hybridization was unsuccessful (common for gustatory receptors, GR28B(D) transcripts were robustly detected in dissected antennae/aristae from wild-type, but not Gr28b mutant, animals by reverse transcriptase PCR (RT–PCR), demonstrating expression in this tissue (Ni, 2013).

Gustatory receptors are a large family of seven transmembrane proteins present in invertebrates, with 68 members in Drosophila melanogaster. Insects also contain multiple gustatory receptor-related odorant receptors (62 in D. melanogaster. Gustatory receptors and odorant receptors form a gene family distinct from, and apparently unrelated to, the G-protein-coupled receptor superfamily. Gustatory receptors and odorant receptors have been studied extensively as chemoreceptors for sweet and bitter tastants, food odours, carbon dioxide and other chemicals, but have not previously been implicated in thermosensation. This study examined gustatory receptor involvement in thermosensation using a two-temperature choice assay, exposing flies for 1 min to a steep thermal gradient (initially >5°C per cm) created using tubes of ~25.5 and ~31.0°C air (a preferred and an increased-but-innocuous temperature, respectively) separated by 1 cm. Flies normally prefer the cooler tube, a behaviour termed 'rapid negative thermotaxis'. This and a previous study showed that inhibiting HC neurons by cell-specific expression of tetanus toxin light chain (TNT), a vesicle release inhibitor, using HC-GAL4 strongly reduced such behaviour). In agreement with the importance of HC neurons, and in addition to previous studies, third antennal segment/arista removal strongly reduced this behaviour, whereas ablating other tissues expressing HC-GAL4 and Gr28b.d-GAL4 did not. By contrast, inhibiting AC neurons by TNT expression using TrpA1GAL4, a Gal4 knock-in at the TrpA1 locus, had no effect. (This manipulation disrupted a previously reported AC-dependent thermosensory behaviour) These data indicate that rapid negative thermotaxis depends on the peripheral HC warmth sensors (Ni, 2013).

To probe the molecular basis of rapid negative thermotaxis, its dependence on TrpA1, which is required for AC neuron warmth-sensing, was examined. Consistent with the TrpA1-independence of HC neuron thermosensitivity, a strong loss-of-function TrpA1 mutation did not affect this behaviour. By contrast, strong loss-of-function mutations in the gene encoding GR28B(D) eliminated the response; Gr28b mutants distributed nearly equally between ~25.5°C and ~31.0°C. The defect was specific: excising the transposon in the Gr28bMi allele restored thermotaxis, and both a Gr28b-containing genomic transgene and Gr28b(D) complementary DNA expression rescued the mutant. Attempts were made rescue by expressing cDNAs for the other Drosophila GR28 family members (four other Gr28b isoforms and Gr28a) under Gr28b.d-GAL4 control. Although a negative result could reflect a failure to be properly expressed, only Gr28b(E) yielded significant rescue. However, endogenous Gr28b(E) transcripts were not detected in the antenna/arista, consistent with a previous analysis indicating that GR28B(E) is not expressed there. Together, these data demonstrate that rapid negative thermotaxis depends not on TrpA1, but on Gr28b, consonant with the specific dependence of this behaviour on HC neuron function. Notably, cell-specific GR28B(D) expression using HC-GAL4 strongly rescued the Gr28b mutant, indicating that GR28B(D) function in the HC thermosensors is sufficient to restore rapid negative thermotaxis (Ni, 2013).

To test whether GR28B(D) might act as a thermosensor, whether it conferred warmth-sensitivity when ectopically expressed was examined. Unlike controls, flies broadly expressing GR28B(D) under Actin5C-GAL4 control were incapacitated when heated to 37°C for 3 min, recovering when returned to 23°C. This dramatic effect suggested that GR28B(D) might promote warmth-responsive neuronal activation. It has been shown previously that ectopic expression of the warmth-activated cation channel TRPA1(B), a product of Drosophila TrpA1, renders fly chemosensors warmth-responsive. Like TRPA1(B), chemosensor expression of GR28B(D) (using Gr5a-GAL4) conferred robust warmth-responsiveness. The behavioural consequences of such GR28B(D) expression were examined. When chemically activated, sweet-responsive chemosensors promote proboscis extension. When GR28B(D) was expressed in these cells, strong proboscis extension was elicited by warming to ~32°C. This ability to confer warmth-responsiveness is consistent with GR28B(D) acting as a warmth sensor (Ni, 2013).

Whether GR28B(D) requires sensory neuron-specific cofactors was examined in the neuromuscular system. Unlike controls, motor neurons expressing GR28B(D) (using OK371-GAL4) triggered warmth-responsive excitatory junction potentials at the neuromuscular junction. Thus, GR28B(D)-mediated warmth-responsiveness does not require sensory neuron-specific cofactors. The threshold for GR28B(D)-dependent muscle stimulation was 26.0± 0.3°C, just above TRPA1(B)’s ~25°C threshold in this system, indicating that both molecules mediate responses to innocuous warming (Ni, 2013).

To quantify the thermosensitivity of GR28B(D)-dependent responses, currents were monitored using whole-cell patch-clamp electrophysiology. Unlike controls, voltage-clamped motor neurons expressing GR28B(D) exhibited warmth-responsive inward currents. The response's temperature coefficient (Q10, fold change in current per 10°C change) was calculated by Arrhenius analysis. GR28B(D)-dependent currents were highly thermosensitive (Q10 of 25 ± 5, similar to mammalian neurons expressing thermosensitive TRP channels. Substituting N-methyl-d-glucamine (NMDG)+ for Na+ in the extracellular solution eliminated heat-responsiveness, consistent with cation channel activation (Ni, 2013).

The potential dependence of GR28B(D) on neuron-specific cofactors was tested in muscle. Although control muscles voltage-clamped at −60 mV exhibited modest warmth-responsive outward currents, muscles expressing GR28B(D) (using Mhc-GAL4) exhibited robust warmth-responsive inward currents. The ability of GR28B(D) to confer warmth sensitivity across diverse cell types supports the hypothesis that GR28B(D) acts as a molecular thermoreceptor. It further suggests GR28B(D) as a new class of tool for thermogenetic neuronal activation, adding to the TRP-based toolbox currently used in Drosophila (Ni, 2013).

Although GR28B(D) resembles TRPA1(B) in conferring warmth-sensitivity, these two proteins have distinct functions in the fly, with only Gr28b controlling rapid negative thermotaxis. TrpA1 was found previously to control the slowly developing thermal preference response of flies on a shallow, broad thermal gradient (~0.5°C per cm, 18-32°C). The contribution of Gr28b to this long-term body temperature selection behaviour was tested. As reported previously, TrpA1 mutants selected unusually warm temperatures after 30 min on the gradient, with many accumulating at ≥28°C. By contrast, strong loss-of-function Gr28b mutants behaved indistinguishably from wild type. This neatly distinguishes Gr28b and TrpA1, with the former controlling rapid negative thermotaxis and the latter long-term body temperature selection (Ni, 2013).

These findings reconcile previously disparate views of Drosophila thermosensation by demonstrating that thermal preference is not a singular behaviour, but involves multiple systems relevant in different contexts. It suggests a model in which Gr28b, acting peripherally, controls rapid responses to ambient temperature jumps, whereas TrpA1, acting internally, controls responses to sustained temperature increases reaching the core. In the arista, Gr28b could experience ambient temperature fluctuations in advance of core changes, eliciting rapid avoidance. Such behaviour could be critical for a tiny animal in which ambient and core temperatures equalize rapidly. The dispensability of Gr28b for responses on the shallow gradient could relate to observations in other insects where peripheral thermoreceptors respond more to temperature fluctuations than absolute values. The fly's reliance on distinct sensors for distinct aspects of thermal preference is reminiscent of complex thermosensory systems of larger, endothermic animals. In the fly, these warmth-responsive pathways potentially converge in the brain, where both sets of sensors innervate overlapping regions (Ni, 2013).

Finally, whether Gr28b and TrpA1 were uniquely suited to their roles in the fly was tested. Although TrpA1 was normally not required for rapid negative thermotaxis, when expressed in the arista using Gr28b.d-GAL4, TRPA1(B) significantly rescued the Gr28b mutant defect. (As expected, a less thermosensitive TrpA1 isoform, TRPA1(A), did not rescue the defect). Conversely, although Gr28b was not normally required for slowly developing thermal preference on the shallow gradient, GR28B(D) expression under TrpA1GAL4 control significantly rescued the TrpA1 mutant defect. Thus, when their expression is manipulated appropriately, GR28B(D) and TRPA1(B) can act in the same cells and support the same behaviours, indicating fundamental functional similarities (Ni, 2013).

Although studied extensively, the mechanisms of gustatory receptor action are not fully resolved. Gustatory receptors have been reported to act as cation channels and via G-proteins. Whether GR28B(D) acts by either mechanism remains unknown. Although attempts to study GR28B(D) in heterologous cells (including Xenopus laevis oocytes and HEK cells) were unsuccessful, the ability of GR28B(D) to confer warmth-responsiveness upon diverse cell types argues against a requirement for cell-type-specific cofactors in the fly. Gr28b has been implicated in responses to strong illumination (Xiang, 2010). This seems to be unrelated to GR28B(D)-dependent thermosensation, as Gr28b-dependent photosensors are unresponsive to innocuous warming (Xiang, 2010) and appear to express other Gr28b isoforms. GR28B(D)-expressing muscles were not light-responsive (Ni, 2013).

Previous studies have demonstrated the importance of TRP channels in Drosophila thermosensation, stimulating interest in their potential involvement in warmth-dependent host-seeking by insect disease vectors. This work raises the possibility that gustatory receptors, including GR28 receptors in disease vectors such as tsetse flies and mosquitoes, regulate thermosensation more broadly. GR28B(D) adds to a growing list of highly thermosensitive membrane proteins including not only TRPs, but the mammalian ANO1 chloride channel. The presence of exceptional thermosensitivity in diverse proteins may facilitate temperature-responsive modulation of diverse physiological responses. Furthermore, using multiple molecules to mediate behavioural responses to similar temperatures may facilitate independent tuning of distinct thermosensory responses (Ni, 2013).

Two distinct types of neuronal asymmetries are controlled by the Caenorhabditis elegans zinc finger transcription factor die-1

Left/right asymmetric features of animals are either randomly distributed on either the left or right side within a population ('antisymmetries') or found stereotypically on one particular side of an animal ('directional asymmetries'). Both types of asymmetries can be found in nervous systems, but whether the regulatory programs that establish these asymmetries share any mechanistic features is not known. This study describes an unprecedented molecular link between these two types of asymmetries in Caenorhabditis elegans. The zinc finger transcription factor die-1 is expressed in a directionally asymmetric manner in the gustatory neuron pair ASE left (ASEL) and ASE right (ASER), while it is expressed in an antisymmetric manner in the olfactory neuron pair AWC left (AWCL) and AWC right (AWCR). Asymmetric die-1 expression is controlled in a fundamentally distinct manner in these two neuron pairs. Importantly, asymmetric die-1 expression controls the directionally asymmetric expression of gustatory receptor proteins in the ASE neurons and the antisymmetric expression of olfactory receptor proteins in the AWC neurons. These asymmetries serve to increase the ability of the animal to discriminate distinct chemosensory inputs (Cochella, 2014).

Histone methyltransferase G9a is a key regulator of the starvation-induced behaviors in Drosophila melanogaster

Organisms have developed behavioral strategies to defend themselves from starvation stress. Despite of their importance in nature, the underlying mechanisms have been poorly understood. This study shows that Drosophila G9a (dG9a), one of the histone H3 Lys 9-specific histone methyltransferases, functions as a key regulator for the starvation-induced behaviors. RNA-sequencing analyses utilizing dG9a null mutant flies revealed that the expression of some genes relating to gustatory perception are regulated by dG9a under starvation conditions. Reverse transcription quantitative-PCR analyses showed that the expression of gustatory receptor genes for sensing sugar are up-regulated in starved dG9a null mutant. Consistent with this, proboscis extension reflex tests indicated that dG9a depletion increased the sensitivity to sucrose under starvation conditions. Furthermore, the locomotion activity was promoted in starved dG9a null mutant. It was also found that dG9a depletion downregulates the expression of insulin-like peptide genes that are required for the suppression of starvation-induced hyperactivity. Furthermore, refeeding of wild type flies after starvation conditions restores the hyperactivity and increased sensitivity to sucrose as well as dG9a expression level. These data suggest that dG9a functions as a key regulator for the decision of behavioral strategies under starvation conditions (Shimaji, 2017).

Behavioral epigenetics has attracted attentions for the last decade, because epigenetic regulation can induce rapid and long-lasting effects on gene expression in response to environmental changes. Although a numerous studies have revealed the biological roles of epigenetic factors over the last 40 years, how epigenetic regulation affects behaviors of organisms and how behaviors affect epigenetic regulation have just started to be investigated. Previous studies reported that several epigenetic factors are relevant to behaviors included in learning/memory, neurodevelopmental disorders, drug addiction, parenting and stress responses in mammals. Some of these relationships have also been found in Drosophila melanogaster, the model organism extensively used for genetic studies because of its short life span and the high homology to human genes. For example, some Drosophila epigenetic factors like Ash1 and Suppressor of variegation 4–20-homolog 1 are associated with autism spectrum disorders, one of the neurodevelopmental disorders characterized by the impaired communication, restricted interests and hyperactivity. Especially it is revealed that the dG9a, a Drosophila homolog of mammalian G9a functions as the key regulator of learning and memory through alteration of histone modification (Shimaji, 2017 and references therein).

G9a has been identified in mammals as one of the histone H3 Lysine 9 (H3K9) specific methyltransferases (HMTases) that catalyzes both H3K9 mono-methylation and H3K9 di-methylation. G9a has various biological roles including DNA replication, developmental reprogramming and substance addiction. Especially, G9a plays a critical role in embryogenesis, since G9a knockout mice show embryonic lethality in early stages due to severe growth defects. Generally, G9a functions to suppress expression of its target genes through H3K9 methylation, although from some reports it may act as a co-activator to positively regulate some genes such as targets of the hormone-activated glucocorticoid receptor. However, studies on in vivo functions of G9a utilizing the mouse model have not advanced efficiently because of the embryonic lethality of the G9a knockout mice. On the other hand, dG9a depletion exerted no effect on fly viability at all. Therefore, Drosophila melanogaster is suitable for the functional analyses of G9a in adults. dG9a can catalyze the methylation of H3K9 in euchromatin regions and the methylated H3K9 contributes to heterochromatin formation and transcriptional repression of specific genes in vivo. Although the dG9a depleted flies show no viability defect previous reports utilizing dG9a knockdown flies or knockout flies revealed that dG9a has a regulatory role in the developmental process of germ cell line like spermatogenesis and oocyte specification as well as in learning and memory. In contrast to the mammalian G9a, dG9a was not essential for Drosophila viability. Because of this unexpected finding, one can consider that the epigenetic regulation through H3K9 methylation is not developed to be critical for Drosophila viability and therefore dG9a plays no important role for Drosophila viability under laboratory conditions. However, it was important to note that while there are various environmental stresses in the wild, Drosophila was always maintained under optimal conditions in the laboratory. Therefore, this study focused on the analyses under stressed conditions and recently revealed that dG9a has a critical role in acquisition of tolerance under starvation stress in adult stage through regulating the activity of autophagy (Shimaji, 2017).

In terms of behavioral changes under starvation stress, Drosophila developed two behavioral strategies to increase the possibility that they can find new food sources. Firstly, they increase responsiveness for a sugar taste by means of up-regulating the expression of Gustatory receptor (Gr) 64a, a well-known gustatory receptor for sensing sugar. Secondary, starvation stress induces hyperactivity through activating octopaminergic neurons whose functions can be regulated by glucagon and insulin signals. However, it remains totally unknown whether there is a key regulator for the decision of these starvation-induced behaviors in spite of its importance in nature (Shimaji, 2017).

In this study, RNA-sequencing (RNA-seq) analyses followed by gene ontology (GO) analyses revealed that the expression of genes encoding gustatory receptors and odorant binding proteins are altered in dG9a mutants under starvation conditions. Further genetic analyses revealed that dG9a depletion increases the expression levels of gustatory receptor genes for sensing sucrose. Behavioral analyses revealed that dG9a depletion up-regulates the sucrose sensitivity in response to starvation stress. These data suggest that dG9a regulates the starvation-induced shift of locomotion activity through controlling the expression of insulin-like peptide (Ilp) genes that are required for the suppression of starvation-induced hyperactivity. Refeeding of wild type flies after starvation conditions restored the hyperactivity and increased sensitivity to sucrose as well as dG9a expression level. These data suggest that dG9a functions as a key regulator for the decision of behavioral strategies under starvation conditions (Shimaji, 2017).

Prior to this study, dG9a null mutant flies were found to be sensitive to starvation and the underlying mechanism were explored. dG9a is functional for saving energy through recycling cellular components by regulating the expression of genes required for autophagy (An, 2017). In addition to this process, it was further found that dG9a functions as a suppressor of starvation-induced hyperactivity. This is also preferable for saving energy under starvation conditions. In nature, animals are exposed to starvation frequently, however, foraging require the costs of food-seeking energy as well as the threats from predation and environmental changes along with their migration. Therefore, this foraging strategy requires assumption that the nutrient-poor conditions do not last long. Moreover, the strategy appears to be not effective under the conditions that there is no food available near them. Another way to survive under starvation is saving energy without moving like the hibernation associated with seasonal fluctuations of food availability, which can be observed in a wide range of animals including Drosophila. The current data indicated that dG9a suppresses the starvation-induced hyperactivity and that wild type flies exhibit the hyperactivity along with reduction of dG9a expression. These data suggest that the wild type flies save energy without moving at the early phase of starvation and they become active to seek foods with risks along with the reduction of dG9a expression at the late phase of starvation. Therefore, these data suggest that dG9a functions as a key regulator for flies to decide these strategies depending on the time course of starvation and has an adaptive advantage to survive starvation conditions (Shimaji, 2017).

RNA-seq analyses was performed to identify which genes are regulated by dG9a under starvation stress. This analyses indicated that the most enriched term was 'innate immune response' and the second was 'response to bacterium', which suggests that there is strong relationship between dG9a and innate immune responses. In Drosophila, starved conditions and the following disruption of insulin signaling induce the expression of four antimicrobial peptides (AMPs), Metchnikowin (Mtk), Drosocin (Dro), Drosomycin (Drs) and Attacin-A (AttA). RNA-seq analyses detected the significant increase in the expression of all of these four antimicrobial genes under 12-h starved dG9a null mutant. The expression of these representative antibacterial genes by was examined by RT-qPCR using Canton S and dG9aRG5 kept under same conditions. The expression of three representative genes, Mtk, Dro and Drs, showed similar expression pattern to that observed in the results of RNA-seq analyses. These results indicate that the expression pattern of antibacterial genes was not due to accidental infection of one set of the flies, but truly due to dG9a mutation. These observations suggest a link between activation of innate immune system and starvation. Previous reports suggested that the induction of AMPs may help maintaining and enhancing the defense activity in particular when organisms are exposed to poor conditions like starvation. The data indicates the possibility that dG9a null mutant acquires the excess defense activity to bacterial infection under starvation conditions (Shimaji, 2017).

This study revealed that the dG9a depletion increases taste sensitivity to sucrose under 24-h starved conditions. Our RT-qPCR results indicated that dG9a depletion significantly increased the expression levels of genes encoding gustatory receptors for sensing sugar taste under 24-h starved conditions. These results suggest that the sucrose sensitivity under 24-h starved conditions is dependent on the expression levels of gustatory receptor genes regulated by dG9a. However, the dG9a depletion does not increase sucrose sensitivity under earlier (8-h) starved conditions, although the significant increases were detected in gustatory receptor genes under 0-h and 6-h starved dG9a null mutant. Moreover, none of these genes in wild type flies are up-regulated under 12-h and 24-h starvation conditions in compared to non-starved conditions. These results suggest that the starvation-induced increase of the sucrose sensitivity in wild type flies as well as increase of the sucrose sensitivity under non-starved conditions are dependent on underlying mechanisms other than expression changes of gustatory receptor genes, including the alterations of translation levels of Gr mRNAs, localization of Gr mRNA/proteins and the responses of higher order gustatory circuits in the brain like the feeding behavior control by neuropeptides like dRYamides. Further analyses are required to clarify these mechanisms (Shimaji, 2017).

Drosophila Life Span and Physiology Are Modulated by Sexual Perception and Reward

Sensory perception modulates aging and physiology across taxa. This study found that perception of female sexual pheromones through a specific gustatory receptor expressed in a subset of foreleg neurons in male fruit flies rapidly and reversibly decreases fat stores, reduces resistance to starvation, and limits life span together with neurons that express the reward-mediating neuropeptide F. High-throughput RNA-seq experiments revealed a set of molecular processes that were impacted by the activity of the longevity circuit, thereby identifying new candidate cell non-autonomous aging mechanisms. Mating reversed the effects of pheromone perception, suggesting a model where life span is modulated through integration of sensory and reward circuits and where healthy aging may be compromised when the expectations defined by sensory perception are discordant with ensuing experience (Gendron, 2013).

Sensory perception can modulate aging and physiology in multiple species. In Drosophila, exposure to food-based odorants partially reverses the anti-aging effect of dietary restriction, whereas broad reduction in olfactory function promotes longevity and alters fat metabolism. Even the well-known relation between body temperature and life span may have a sensory component (Gendron, 2013).

To identify sensory cues and neuronal circuitry that underlie the effects of sensory perception on aging, this study focused on the perception of potential mates. Social interactions are prevalent throughout nature, and the influence of social context on health and longevity is well-known in several species, including humans. Such influences include behavioral interactions with mates and broader physiological 'costs of reproduction,' which often form the basis for evolutionary models of aging (Gendron, 2013).

In Drosophila, the presence of potential mates is perceived largely through non-volatile cuticular hydrocarbons, which are produced by cells called oenocytes and are secreted to the cuticular surface where they function as pheromones. To test whether differential pheromone exposure influenced life span or physiology, 'experimental' flies of the same genotype were housed with 'donor' animals of the same sex that either expressed normal pheromone profiles or were genetically engineered to express pheromone profiles characteristic of the opposite sex. Donor males with feminized pheromone profiles were generated by targeting expression of the sex determination gene, tra, to the oenocytes (via OK72-GAL4 or Prom-E800-Gal4), whereas masculinization of female flies was accomplished by expressing tra-RNAi in a similar way. This design allowed manipulation of the experimental animals' perceived sexual environment without introducing complications associated with mating itself (Gendron, 2013).

In Drosophila, sensory manipulations can affect life span, fat storage (as determined by baseline measures of triacylglyceride-TAG), and certain aspects of stress resistance. This study has found that flies exposed to pheromones of the opposite sex showed differences in these phenotypes. Experimental male flies exposed to male donor pheromone had higher amounts of TAG, were substantially more resistant to starvation, and exhibited a significantly longer life span than genetically identical male siblings exposed to female donor pheromone. Females exhibited similar phenotypes in response to male donor pheromone, but the magnitude of the effects was smaller. Subsequent experiments were therefore focused on males (Gendron, 2013).

The characteristics of pheromone exposure were indicative of a mechanism involving sensory perception. Effects were similar in several genetic backgrounds, including a strain recently collected in the wild, and were largely unaffected by cohort composition. Pheromone-induced phenotypes were detected after as little as two days exposure to donor animals, persisted with longer manipulations, and were progressively reversed when female donor pheromone was removed. Pheromone effects appeared not to be mediated by aberrant or aggressive interactions with donor flies because no significant differences were observed in such behaviors and because continuous, vigorous agitation of the vials throughout the exposure period, which effectively disrupted observed behaviors, had no effect on the impact of donor pheromone. Furthermore, exposure of experimental males to the purified female pheromone 7-11-heptacosadiene (7-11 HD) produced physiological changes in the absence of donor animals (Gendron, 2013).

To explore the sensory modality through which donor pheromone exerts its effects, this study tested whether a broadly-expressed olfactory co-receptor, Or83b, whose loss of function renders flies largely unable to smell, was required for pheromone effects. Or83b mutant flies exhibited similar changes in starvation resistance in response to donor pheromone as did control animals, indicating that olfaction was not required. To test whether taste perception was involved, flies were tested that were mutant for the gene Pox neuro (Poxn), a null mutation that putatively transforms all chemosensory neurons into mechanosensory neurons. Drosophila taste neurons are present in the mouthparts and distributed on different body parts including the wings, legs, and genitals, which allow sensation by contact. When the Poxn null mutation is coupled with a partially rescuing transgene, Poxn ΔM22-B5-ΔXB, flies are generally healthy, but gustatory perception is eliminated in the labelum, the legs, and the wing margins. Poxn ΔM22-B5XB flies showed no pheromone-induced changes in starvation resistance, TAG amounts, or life span. However, Poxn mutant flies that carried a transgene that restores taste function to the legs and wing margins (but not labelum; PoxnΔM22-B5-Full1 responses were similar to those of control flies. Thus, the effects of pheromone exposure appear to be mediated by taste perception through gustatory neurons outside of the mouthparts (Gendron, 2013).

To identify specific gustatory receptors and neurons that might mediate the pheromone effects, candidate pheromone receptors were tested. Of the mutants that were examined, only flies that carried a loss of function mutation in the gene pickpocket 23 (ppk23) were resistant to the effects of pheromone exposure. Further analysis verified that ppk23 was required for the effects of pheromone exposure on starvation resistance, TAG amounts, and life span. Silencing ppk23-expressing neurons only during exposure to donor males by expressing a temperature-sensitive dominant negative allele of the dynamin gene shibire (via ppk23-GAL4; UAS-shits) also eliminated the differential response to pheromones. In male Drosophila, the transcription factor fruitless (fru) is expressed with ppk23 in pheromone-sensing neurons located in the animals' forelegs, and silencing fru-expressing neurons during exposure (via fru-GAL4;UAS-shits) abrogated pheromone effects. Consistent with a requirement for these neurons, it was found that surgical amputation of the forelegs, but not injury alone, was sufficient to reproducibly eliminate the effects of pheromone exposure. Moreover, acute, targeted activation of ppk23-expressing neurons using a temperature-sensitive TRPA1 channel (ppk23-GAL4;UAS-TRPA1) was sufficient to mimic the effects of female pheromone without exposure. Together, these data indicate that pheromone-sensing neurons in the foreleg of the male fly that express the gustatory receptor, ppk23, and the transcription factor, fruitless, influence stress resistance, physiology, and life span in response to perception of female pheromones (Gendron, 2013).

To examine brain circuits that may function in transducing pheromone perception, UAS-shits was selectively expressed to block synaptic transmission in various neuro-anatomical regions with the goal of disrupting the physiological effects of donor pheromone exposure. The effects were abrogated when UAS-shits was driven in neurons characterized by expression of neuropeptide F (NPF, as represented by npf-GAL4). Further analysis verified that pheromone-induced changes in starvation resistance and TAG abundance were lost following silencing of npf-expressing neurons. Consistent with a possible role in transducing pheromone information, npf expression was significantly increased by 30% in experimental males after exposure to feminized donor males, and activation of npf-expressing neurons was sufficient to decrease life span in the absence of pheromone exposure (Gendron, 2013).

NPF may function as a mediator of sexual reward in Drosophila, and its mammalian counterpart, neuropeptide Y (NPY), has been associated with sexual motivation and psychological reward. Tests were performed to see whether the effects of pheromone perception might be rescued by allowing males to successfully mate with females. Neither a small number of conjugal visits with virgin females nor housing with wild-type females in a 1:1 ratio was sufficient to ameliorate the effects of pheromone exposure. In this context, decreased longevity may be a consequence of pheromone perception and not of mating itself. Male Drosophila are willing and able to copulate up to five times in rapid succession before requiring a refractory period. It was found that supplementing donor cohorts with an excess of mating females (in a 5:1 ratio) was sufficient to significantly reduce the effects on mortality and TAG caused by female donor pheromone early in life. The benefits of mating on age-specific mortality decreased with age, suggesting that aging may reduce mating efficiency or may diminish effective mating reward (Gendron, 2013).

To identify how sexual perception and reward may alter physiological responses in peripheral tissues, changes in gene expression were examined using whole-genome RNA-seq technology. 195 genes were found with significantly different expression (using an experiment-wise error rate of 0.05) in control male flies that were exposed to feminized or control donor males for 48 hours. Nearly all (188/195 = 96%) of the changes appeared to be due to pheromone perception because they were not observed in identical experiments using ppk23 mutant flies. Males exposed to female pheromones decreased transcription of genes encoding odorant-binding proteins and increased transcription of several genes with lipase activity. A significant enrichment was observed in secreted molecules, which includes genes encoding proteins mediating immune- and stress-responses. Many of these genes and pathways were highlighted in a recent meta-analysis of gene expression changes in response to stress and aging (Gendron, 2013).

Activities of insulin and target of rapamycin (TOR) signaling, which modulate aging across taxa, increase sexual attractiveness in flies. The current demonstration that perception of sexual characteristics is sufficient to modulate life span and physiology suggests aging pathways in one individual may modulate health and life span in another. These types of indirect genetic effects have the potential to be influential agents of natural selection, suggesting that expectation/reward imbalance may have broad effects on health and physiology in humans and may present a potent evolutionary force in nature (Gendron, 2013).

Gr33a modulates Drosophila male courtship preference

In any gamogenetic species, attraction between individuals of the opposite sex promotes reproductive success that guarantees their thriving. Consequently, mate determination between two sexes is effortless for an animal. However, choosing a spouse from numerous attractive partners of the opposite sex needs deliberation. In Drosophila melanogaster, both younger virgin females and older ones are equally liked options to males; nevertheless, when given options, males prefer younger females to older ones. Non-volatile cuticular hydrocarbons, considered as major pheromones in Drosophila, constitute females' sexual attraction that act through males' gustatory receptors (Grs) to elicit male courtship. To date, only a few putative Grs are known to play roles in male courtship. This study reports that loss of Gr33a function or abrogating the activity of Gr33a neurons does not disrupt male-female courtship, but eliminates males' preference for younger mates. Furthermore, ectopic expression of human amyloid precursor protein (APP; see Drosophila Appl) in Gr33a neurons abolishes males' preference behavior. Such function of APP is mediated by the transcription factor Forkhead box O (dFoxO). These results not only provide mechanistic insights into Drosophila male courtship preference, but also establish a novel Drosophila model for Alzheimer's disease (AD) (Xue, 2015).

To avoid futile reproductive efforts, an animal must distinguish conspecifics from other species and differentiate the sex of conspecific partners. It must also determine the most suitable mates from large amounts of available partners in order to maximize reproductive efficiency. Evolution endows Drosophila melanogaster males with the instinct to discriminate conspecifics from other Drosophila species1 and to discern females from males. It also bestows on them the ability to select the most favorable mates among masses of desirable virgin females (Xue, 2015).

In Drosophila, non-volatile cuticular hydrocarbons (CHCs) have been recognized as a type of major sex pheromone, which convey information of an individual such as species and sex. Female-specific CHCs are detected by male gustatory receptors (Grs), a chemosensory receptor family mainly responsible for detecting non-volatile chemicals, during tapping and licking steps in stereotypical male courtship behavior. So far, only a few Grs, including Gr32a, Gr33a and Gr39a are reported to be engaged in Drosophila male courtship behavior. While Gr32a acts to assist males to discriminate conspecifics from other species, Gr39a is required for males to distinguish females from males. On the other hand, Gr33a functions to inhibit male-male courtship. Despite these findings, roles of the Grs in males' choices for the most favorable mates have remained largely unknown (Xue, 2015).

A previous study (Hu, 2014) set a paradigm of choice model in which both options (younger virgin females and older ones) are proved to be attractive to Drosophila males, but males still intensely prefer younger mates to older ones. Using this model, this study explored the mechanisms by which males bias their potential mates. Gene loss-of-function, gain-of-function, and cell-inactivation experiments demonstrated that Gr33a and Gr33a neurons are essential for males' preference for younger mates. Since the previous data indicated that pan-neuronal expression of human amyloid precursor protein (APP) ablates males' preference for younger mates, this study sought to investigate whether APP expression in Gr33a neurons would affect this behavior. Indeed, it was found that Gr33a neurons-specific expression of APP abolished males' preference for younger mates, and this function of APP is mediated by the transcription factor forkhead box O (dFoxO) (Xue, 2015).

Drosophila male courtship choice has been frequently applied for studying decision making in animals, yet most of the past studies have focused on male courtship choices between likes and dislikes, such as court towards females vs. males, or virgin vs. non-virgin females. Previous study has characterized a choice behavior between two equally-liked options: mature virgin females, whether younger or older, were similarly attractive to naive males; nevertheless, when given the option, males turn out to be picky and prefer younger virgin females to older ones. This study found that a gustatory receptor, Gr33a, is necessary for males' preference for younger mates. Gr33a is thought to be necessary to inhibit homosexual behavior; its role in heterosexual behavior, however, is rarely pondered. This study has revealed the critical role of Gr33a in males' preference for younger mates. Furthermore, ectopic expression of APP in Gr33a neurons eliminates males' preference behavior, and such function is mediated by dFoxO, a recently reported downstream factor of APP27. Therefore, this work demonstrates the genetic interaction of APP and dFoxO in Gr33a neurons, which modulates males' preference for younger mates. APP is identified as a potential causative protein of AD, a common progressive neurodegenerative disorder, in which cognitive decline is the prime symptom. Although Drosophila has long been utilized for building AD models to investigate the pathogenesis and possible cure for AD, accepted Drosophila AD models are limited to locomotion model and life span model, which have little correlation with cognitive ability. The current findings, however, have offered the possibility for establishing a novel Drosophila AD model that is related to cognitive ability (Xue, 2015).

In all CHCs produced by files, 7, 11-HD and 7, 11-ND have been identified as female specific aphrodisiac pheromones to Drosophila melanogaster males. GC and MS studies suggest that both 7, 11-HD and 7, 11-ND are expressed at lower concentration in younger virgin females than the older ones. Hence, it appears unlikely that 7, 11-HD or 7, 11-ND is the cause that leads males to court younger virgin females more vigorously than the older ones. On the contrary, since Gr33a has been reported as a receptor of aversive odors, it is more likely that older females produce certain aversive odors that can be recognized by males and repel them. Consistent with this explanation, this study found that the concentrations of most detected CHCs are significantly higher on the older virgin females than the younger ones. Nevertheless, at this stage it was not possible to identify the CHC(s) that serves as the aversive pheromone to males. Besides, it cannot be excluded that younger virgin females produce unknown attractive pheromones other than 7, 11-HD or 7, 11-ND. However, all CHCs detected on younger virgin females also present on older ones at a similar or higher lever. Thus, the tentative conclusion is drawn that younger virgin females do not produce more attractive pheromones than the older ones. The results, taken together, unravel the role of bitter sensory Gr33a neurons in males' preference for younger mates and infer that older females might produce certain aversive odors that cause males to turn to younger mates (Xue, 2015).

High-NaCl perception in Drosophila melanogaster

Salt is a fundamental nutrient that is required for many physiological processes, including electrolyte homeostasis and neuronal activity. In mammals and Drosophila, the detection of NaCl induces two different behaviors: low-salt concentrations provide an attractive stimulus, whereas high-salt concentrations are avoided. A gene called serrano (sano) was identified as being expressed in the sensory organs of Drosophila larvae. A transgenic reporter line showed that sano was coexpressed with Gr66a in a subset of gustatory neurons in the terminal organ of third-instar larvae. The disruption of sano gene expression in gustatory neurons led to the specific loss of high-salt concentration avoidance in larvae, whereas the detection of other attractive or aversive substances was unaffected. Moreover, using a cellular marker sensitive to calcium levels, Sano function was shown to be required for neuronal activity in response to high-salt concentrations. In these neurons, the loss of the DEG/ENaC channel PPK19 function also eliminated the cellular response to high-salt concentrations. This study revealed that PPK19 and Sano are required in the neurons of the larval gustatory organs for the detection of high-salt concentrations (Alves, 2014).

Representations of taste modality in the Drosophila brain

Gustatory receptors and peripheral taste cells have been identified in flies and mammals, revealing that sensory cells are tuned to taste modality across species. How taste modalities are processed in higher brain centers to guide feeding decisions is unresolved. This study developed a large-scale calcium-imaging approach coupled with cell labeling to examine how different taste modalities are processed in the fly brain. These studies reveal that sweet, bitter, and water sensory cells activate different cell populations throughout the subesophageal zone, with most cells responding to a single taste modality. Pathways for sweet and bitter tastes are segregated from sensory input to motor output, and this segregation is maintained in higher brain areas, including regions implicated in learning and neuromodulation. This work reveals independent processing of appetitive and aversive tastes, suggesting that flies and mammals use a similar coding strategy to ensure innate responses to salient compounds (Harris, 2015).

A central question in taste processing is how different taste modalities are encoded in the brain. In the mammalian gustatory system, labeled lines, mixed lines, and temporal dynamics have all been proposed as fundamental stategies used by the nervous system to process tastes, with recent evidence favoring labeled line encoding. In Drosophila, the limited understanding of neural circuits beyond sensory neurons and MNs has precluded examination of modality processing. This study takes advantage of recent improvements in genetically encoded calcium indicators and high-speed, multi-plane imaging to examine taste-induced activity throughout the SEZ and anterior brain, similar to whole-brain calcium-imaging approaches taken previously in zebrafish and C. elegans. This approach enabled examination of the neurons activated by different taste modalities and probe models of taste coding (Harris, 2015).

Monitoring brain activity in response to stimulation of different gustatory classes revealed that the majority of central neurons responded selectively to bitter, sweet, or water sensory cell activation. In addition, some neurons responded to both water and sucrose and may represent positive valence or taste acceptance behavior, while a few responded to other taste pairs. This demonstrates that most central taste-processing neurons in the fly do not respond to multiple taste modalities and argues against models of taste coding based on broadly tuned cells. Instead, it was found that taste representations are largely modality specific, indicating separate processing streams for different taste qualities. Moreover, bitter and sweet stimulation activate different sets of proboscis MNs and activate different neurons in the higher brain. This argues that sweet and bitter tastes are processed by segregated pathways, suggesting a strategy that ensures innate responses to essential compounds (Harris, 2015).

Examining responses to taste mixtures in the SEZ revealed that no additional cells are activated by mixtures that are not activated by single components, again arguing against non-selective cells. Instead, mixtures activated only a fraction of the cells that responded to the individual components. Mixture suppression occurred for sugar/bitter mixes and sugar/water mixes, demonstrating inhibition between appetitive and aversive stimuli as well between two appetitive stimuli. Recent studies have identified two mechanisms by which bitter compounds inhibit sweet sensory activation: (1) bitter compounds bind a chemosensory binding protein that acts on sweet sensory cells to inhibit activity; and (2) bitter compounds activate GABAergic neurons, causing GABAB-receptor-mediated pre-synaptic inhibition of sweet sensory axons. The current study shows that taste mixtures reduce sensory activation and reduce the number of taste-responsive cells in the SEZ, consistent with the notion that decreased sensory activity contributes to mixture suppression. Cross-inhibition of different gustatory pathways is an effective strategy to weight the activation of acceptance versus avoidance pathways based on the ratio of sugars versus bitter compounds present in food (Harris, 2015).

This study represents the first large-scale analysis of pan-neural activity in the fly brain. The advantages of this approach are that it is possible to monitor the activity of large populations at single-cell resolution, which is not feasible by other approaches; it enables unbiased sampling that does not require specific Gal4 lines expressed in already known cells of interest; and it provides rapid evaluation of brain-wide activity. Whereas previous studies had identified four classes of taste-responsive cells, this study uncovered more than 100 taste-responsive cells, the vast majority of which are modality specific. As this approach relies on calcium imaging in the soma, the sensitivity of GCaMP6, and the expression levels driven byn Synaptobrevin-Gal4, detection limitations may exist. Nevertheless, this study provides a population overview of gustatory processing in the fly brain and a framework for future studies to determine the anatomy and connectivity of taste-responsive neurons (Harris, 2015).

This study shows that different taste modalities in the periphery activate different pathways in the brain, consistent with labeled line taste processing. Information is processed in separate streams for appetitive and aversive tastes, which are maintained in the higher brain and are mutually inhibitory. Recent studies in the mammalian gustatory system argue for modality-specific representations in the gustatory cortex and are consistent with the labeled line model, suggesting that dedicated pathways may be a general strategy to process tastes used throughout evolution (Harris, 2015).

Immediate perception of a reward is distinct from the reward's long-term salience

Reward perception guides all aspects of animal behavior. However, the relationship between the perceived value of a reward, the latent value of a reward, and the behavioral response remains unclear. This study report that, given a choice between two sweet and chemically similar sugars-L- and D-arabinose-Drosophila melanogaster prefers D- over L-arabinose, but forms long-term memories of L-arabinose (the isomer present in ripening fruits) more reliably. Behavioral assays indicate that L-arabinose-generated memories require sugar receptor Gr43a, and calcium imaging and electrophysiological recording indicate that L- and D-arabinose differentially activate Gr43a-expressing neurons. It is posited that the immediate valence of a reward is not always predictive of the long-term reinforcement value of that reward, and that a subset of sugar-sensing neurons may generate distinct representations of similar sugars, allowing for rapid assessment of the salient features of various sugar rewards and generation of reward-specific behaviors. However, how sensory neurons communicate information about L-arabinose quality and concentration-features relevant for long-term memory-remains unknown (McGinnis, 2016).

The observation that two similar sugars generate strikingly different behavioral responses can perhaps be best understood using the framework of 'incentive salience' in rewards, formulated by Berridge and Robinson (2003), who divided reward percepts into 'liking' (conscious pleasure, hedonic) and 'wanting' (incentive salience). According to Berridge and Robinson, 'wanting' (incentive salience) is a component of rewards that transforms mere sensory information about rewards and their cues into 'attractive, desired, riveting incentives' and 'emerged early in evolution as an elementary form of stimulus-guided goal direction, to mediate pursuit of a few innate food or sex unconditioned stimuli' (Berridge and Robinson, 2003). In most cases, rewards that are 'liked' are usually also 'wanted', and in conventional formulations, they are considered effectively identical. But work on addiction and monetary reward on human suggest that 'wanting' and 'liking' are in fact dissociable, and while, in many cases, a behavioral response to an experience can predict the likelihood of memory formation, people can be motivated by cues remaining outside conscious awareness. This study reports that a similar distinction in reward perception may also exist in Drosophila: D-arabinose appears to preferentially involve the 'liking' component of the reward percept and L-arabinose the 'wanting'. For Drosophila, the incentive to remember L-arabinose is perhaps owing to the fact that it can inform a specific attribute of food, such as the ripening status of a fruit. Moreover, work in humans suggests that although 'liking' and 'wanting' both represent a positive reward, they utilize distinct neural processing. The observations with D- and L-arabinose now provide an opportunity to explore the neural basis of 'liking' and 'wanting', and how these reward percepts strengthen memory in the accessible nervous system of Drosophila (McGinnis, 2016).

The caloric value of a sugar has been found to be an important determinant of long-term appetitive memory, implying that flies quickly metabolize the sugar and that caloric evaluation somehow provides cues necessary to elicit long-term memory. This study found that sugar with no caloric value can also produce long-term appetitive memories. One obvious possibility is that memories of sweet nutritious sugars are distinct from memories of sweet non-nutritious sugars. However, this seems so far not to be the case: a subset of higher order dopaminergic neurons (R58E02GAL4) necessary for long-term memory of nutritious sucrose is also required for non-nutritious L-arabinose. Similarly, addition of sorbitol, a tasteless but nutritious sugar, enhances the memory of non-nutritious sugars like xylose and D-arabinose, but does not enhance the memory of nutritious sugars. Adding sorbitol to L-arabinose had no additive effect on long-term memory. It therefore appears that L-arabinose memory uses at least some of the same downstream neural circuitry as memory of nutritious sugars (McGinnis, 2016).

Whether memory of L-arabinose, a non-nutritious sugar, is an exception or represents a more general phenomenon is unclear since this study tested only a limited number of sugars in a particular behavioral paradigm. However, in addition to L-arabinose, L-fucose can also produce memory; both are components of the pectin in many fruits' cell walls. It is therefore possible that these sugars may signal some specific attributes of ripening fruit ripening is accompanied by breakdown of the fruit's cell walls although neither of these sugars are present in fruits near the concentrations (1 M) used in memory assays. Nonetheless, these observations suggest that flies can quickly assess salient features of sugars a sort of leading indicator of nutritional value without the sugar's metabolic breakdown. This approach to memory formation may allow flies to quickly recognize and remember potential foods using specific cues, a time advantage that could be vital in natural contexts (McGinnis, 2016).

Do insects distinguish structurally similar sugars? The taste modality of insects, particularly Drosophila, is reported to have limited discriminatory power and be primarily based on the intensity of the stimuli as opposed to the chemical nature of the sugar. Indeed it was found that, apart from flies' differential preference for various sugars at equal concentrations, for immediate and short-term behavior this is largely true. However, no obvious correlation was observed between immediate behavior and long-term memories: flies immediate preference is L-fucose > D-arabinose > L-arabinose > L-sorbose; for short-term memory, L-sorbose = D-arabinose ≥ L-arabinose = L-fucose; but in order of long-term memory score, L-arabinose ≥ L-fucose ≥ D-arabinose = L-sorbose. These results indicate that while short-term responses are guided by palatability, long-term behavioral responses are guided by additional attributes of the sugars. It is not yet clear why D-arabinose is a less effective stimulus. Since D- and L-arabinose are both sweet, they may generate positive sensations in a different manner, or perhaps D-arabinose carries a negative value that over time reduces the positive association formed initially (or dampens the behavioral output) (McGinnis, 2016).

The gustatory receptors Gr5a, Gr43a, Gr61a, and Gr64a-f have been implicated in sugar detection. Although exactly which Gr receptors are responsible for detecting which sugar remains somewhat controversial, two features of sweet-sensing gustatory receptors are generally agreed upon: first, different gustatory neurons express a number of Gr receptors in unique combinations; second, more than one receptor is typically involved in detecting a sugar. However, the physiological consequences of this combinatorial expression of semi-redundant gustatory receptors remain uncertain. This study raises the possibility that gustatory neurons in different locations, expressing unique combinations of receptors, are responsible for discriminating chemically similar sugars and eliciting different behavioral responses. Consistent with this idea, previous studies suggested that Gr43a neurons in the central brain monitor hemolymph fructose levels and modulate feeding behavior, while this study found that these neurons are dispensable for L-arabinose memory, and that peripheral Gr43a-neurons are likely sufficient to signal the presence of a rewarding sugar and generate associative memories. These differences likely arise from the locations of these neurons, differentially expressed receptors, the presence or absence of various co-receptors, and the second-order neurons to which these neurons project. Exactly which or how many Gr43a-, Gr61a-, and Gr5a-expressing neurons in the periphery are sufficient for L-arabinose memory is currently unclear (McGinnis, 2016).

This study also found that activation of Gr43a-expressing neurons by ReaChR but not dTrpA1 is able to generate appetitive memory, while artificially activating a subset of dopaminergic neurons (R58E02GAL4) by heat (dTrpA1) or light (ReaChR) both led to long-term memory. How a difference in activity at the sensory level is conveyed to higher-order neurons, and how that difference is interpreted by the higher-order neurons, remains unclear. More concretely, why is dTrpA1 activation of a subset of dopamine neurons sufficient to generate memory, but dTrpA1 activation of Gr43a-expressing neurons is not? One possibility is that the activity requirements of neuromodulatory systems are less stringent than those for sensory coding, and that temporal selectivity occurs before the signal reaches these dopamine neurons. Alternatively, recent studies have indicated that dopaminergic neurons are functionally diverse, and that distinct population of dopaminergic neurons are involved in appetitive associative memory. These reports raise the possibility that differing sensory inputs could activate different subsets of dopaminergic neurons (McGinnis, 2016).

How can structurally similar sugars generate differential activation? It is likely that although these sugars bind to some of the same receptors, the relative affinity of the receptors vary. In this regard, the fly sweet taste system may be similar to that of the mammalian system, where a single heteromeric receptor (T1R2 and T1R3) is responsible for detecting a large number of sweet substances, with multiple discrete ligand-binding sites in each receptor responsible for generating diverse responses. It is suspected that the differential engagement of multiple gustatory receptors leads similar chemicals to generate differential activation of the same neurons, and that differential activation and different ensembles of activated neurons allows higher-order neurons to decode the relevant features of sugars. It is speculated that, at least in Drosophila, evaluation of a sugar's long-term salience may be encoded in the activation pattern of subsets of gustatory neurons, which allows rapid evaluation and remembering of nutritious food in complex environments (McGinnis, 2016).

Bitter taste receptors confer diverse functions to neurons

Bitter compounds elicit an aversive response. In Drosophila, bitter-sensitive taste neurons coexpress many members of the Gr family of taste receptors. However, the molecular logic of bitter signaling is unknown. This study used an in vivo expression approach to analyze the logic of bitter taste signaling. Ectopic or overexpression of bitter Grs increased endogenous responses or conferred novel responses. Surprisingly, expression of Grs also suppressed many endogenous bitter responses. Conversely, deletion of an endogenous Gr led to novel responses. Expression of individual Grs conferred strikingly different effects in different neurons. The results support a model in which bitter Grs interact, exhibiting competition, inhibition, or activation. The results have broad implications for the problem of how taste systems evolve to detect new environmental dangers (Delventhal, 2016).

Gustatory receptor 22e is essential for sensing chloroquine and strychnine in Drosophila melanogaster

Chloroquine, an amino quinolone derivative commonly used as an anti-malarial drug, is known to impart an unpleasant taste. Little research has been done to study chloroquine taste in insects; therefore, this study examined both the deterrant properties and mechanisms underlying chloroquine perception in fruit flies. The antifeedant effect of chloroquine was identified by screening 21 gustatory receptor (Grs) mutants through behavioral feeding assays and electrophysiology experiments. Two molecular sensors, GR22e and GR33a, were found to act as chloroquine receptors, and chloroquine-mediated activation of GRNs was found to occur through S-type sensilla. At the same time, the chloroquine receptor was successfully recapitulated by expressing GR22e in ectopic gustatory receptor neurons. GR22e was found to form a part of the strychnine receptor. It is suggested that the Drosophila strychnine receptor might have a very complex structure since five different GRs are required for strychnine-induced action potentials (Poudel, 2017).

Neofunctionalization of "Juvenile Hormone Esterase Duplication" in Drosophila as an odorant-degrading enzyme towards food odorants

Odorant degrading enzymes (ODEs) are thought to be responsible, at least in part, for olfactory signal termination in the chemosensory system by rapid degradation of odorants in the vicinity of the receptors. A carboxylesterase, specifically expressed in Drosophila antennae, called "juvenile hormone esterase duplication (JHEdup)" has been previously reported to hydrolyse different fruit esters in vitro. This study functionally characterized JHEdup in vivo. The jhedup gene is highly expressed in large basiconic sensilla, housed in the the maxillary palps, that have been reported to detect several food esters. An electrophysiological analysis demonstrates that ab1A olfactory neurons of jhedup mutant flies exhibit an increased response to certain food acetates. Furthermore, mutant flies show a higher sensitivity towards the same odorants in behavioural assays. A phylogenetic analysis reveals that jhedup arose as a duplication of the juvenile hormone esterase gene during the evolution of Diptera, most likely in the ancestor of Schizophora, and has been conserved in all the 12 sequenced Drosophila species. Jhedup exhibits also an olfactory-predominant expression pattern in other Drosophila species. These results support the implication of JHEdup in the degradation of food odorants in D. melanogaster and propose a neofunctionalization of this enzyme as a bona fide ODE in Drosophilids (Steiner, 2017).

Physiological responses of the Drosophila labellum to amino acids

This study has systematically studied the physiological responses elicited by amino acids from the principal taste organ of the Drosophila head. Although the detection and coding of sugars and bitter compounds have been examined extensively in this organism, little attention has been paid to the physiology of amino acid taste. One class of sensilla, the labellar basiconic S sensilla, were found to yield the strongest responses to amino acids, although these responses were much weaker than the most robust responses to sugar or bitter compounds. S sensilla are heterogeneous in their amino acid responses and amino acids differ in the responses they elicit from individual sensilla. Tryptophan elicited relatively strong responses from S sensilla and these responses were eliminated when bitter-sensing neurons were ablated. Although tryptophan yielded little if any response in a behavioral paradigm, phenylalanine elicited a relatively strong response in the same paradigm and had a different physiological profile, supporting the notion that different amino acids are differentially encoded by the repertoire of taste neurons (Park, 2017).

Molecular and cellular organization of taste neurons in adult Drosophila pharynx

The Drosophila pharyngeal taste organs are poorly characterized despite their location at important sites for monitoring food quality. Functional analysis of pharyngeal neurons has been hindered by the paucity of molecular tools to manipulate them, as well as their relative inaccessibility for neurophysiological investigations. This study generated receptor-to-neuron maps of all three pharyngeal taste organs by performing a comprehensive chemoreceptor-GAL4/LexA expression analysis. The organization of pharyngeal neurons reveals similarities and distinctions in receptor repertoires and neuronal groupings compared to external taste neurons. The mapping results were validated by pinpointing a single pharyngeal neuron required for feeding avoidance of L-canavanine. Inducible activation of pharyngeal taste neurons reveals functional differences between external and internal taste neurons and functional subdivision within pharyngeal sweet neurons. These results provide roadmaps of pharyngeal taste organs in an insect model system for probing the role of these understudied neurons in controlling feeding behaviors (Chen, 2017).

In Drosophila, taste neurons located in sensilla in several body regions sense and distinguish nutritive substances such as sugars, amino acids, and low salt, and potentially harmful ones such as high salt, acids, and a diverse variety of bitter compounds. Hair-like sensilla on the labellum, distal segments of the legs (tarsi), anterior wing margins, and ovipositor have access to chemicals in external substrates. Pit-like sensilla (taste pegs) on the oral surface have access only once the fly extends its proboscis and opens the labellar palps; similar sensilla in the pharynx have access only when food intake is initiated. Based on its anatomical position, the pharynx is considered to act as a gatekeeper to control ingestion, promoting the intake of appetitive foods and blocking that of toxins (Chen, 2017).

Three distinct internal taste organs are present in the adult fly pharynx: the labral sense organ (LSO), the ventral cibarial sense organ (VCSO), and dorsal cibarial sense organ (DCSO). The VCSO and DCSO are paired on opposite sides of the rostrum, whereas the LSO is located in the haustellum. The organization and neuronal composition of all three organs, based on both light and electron microscopy data, have been described in detail. Nine separate sensilla are present in the LSO, of which 1-6 are innervated by a single mechanosensory neuron each. The remaining three, named 7-9, are uniporous sensilla, a feature that ascribes chemosensory function to them. Sensillum 7 is the largest one, with eight chemosensory neurons. Sensilla 8 and 9 have two neurons each (one mechanosensory and one chemosensory). Although one study reported two sensilla in the VCSO, this and other studies have observed three sensilla in the VCSO, innervated by a total of eight chemosensory neurons. The DCSO has two sensilla, each containing three chemosensory neurons. Notwithstanding the availability of detailed anatomical descriptions of pharyngeal taste organs, little is known about their function. The internal location of these organs poses challenges for electrophysiological analysis of taste neurons located within them. Additionally, few molecular tools are currently described to manipulate the function of selected pharyngeal taste neurons (Chen, 2017).

The expression and function of members of several chemosensory receptor gene families such as gustatory receptors (Grs), ionotropic receptors (Irs), Pickpocket (Ppk) channels, and transient receptor potential channels (Trps) have been found in external gustatory receptor neurons (GRNs) of the labellum and the tarsal segments. A number of Gr- and Ir-GAL4 drivers are also shown to label pharyngeal organs, but only a few, including Gr43a and members of sweet Gr clade, Gr2a, Ir60b, and TrpA1, have been mapped to specific taste neurons (Chen, 2017).

This study generated receptor-to-neuron maps for three pharyngeal taste organs by a systematic expression analysis of chemoreceptor reporter lines that represent Gr, Ir, and Ppk receptor families. The maps reveal a large and diverse chemoreceptor repertoire in the pharynx. Some receptors are expressed in combinations that are predictive of neuronal sweet or bitter taste function based on analysis of external GRNs. By contrast, some pharyngeal taste neurons express receptor combinations that are distinct from any that have been reported in other organs, leaving open questions about their functional roles. This study validated he receptor-to-neuron maps derived from reporter gene expression by assessing roles of pharyngeal GRNs predicted to detect L-canavanine, a bitter tastant for which a complete receptor repertoire has been reported. Interestingly, a systematic activation analysis of different classes of pharyngeal taste neurons reveals functional differences between external and internal taste neurons for bitter avoidance and functional subdivision within pharyngeal sweet neurons for sweet acceptance. Together, this study provides a molecular map of pharyngeal taste organs, which will serve as a resource for future studies of the roles of pharyngeal taste neurons in food evaluation (Chen, 2017).

Internal pharyngeal taste organs are the least explored taste organs, despite their obvious importance in insect feeding behaviors, which are crucial drivers for damaging crops and vectoring disease. The receptor-to-neuron maps of pharyngeal taste organs suggest a high degree of molecular complexity, with co-expression of different chemoreceptor family members in many pharyngeal GRNs. In particular, none of the pharyngeal GRNs were found to express Gr genes alone; rather, one or more Ir genes were always expressed in the same neurons. Gr and Ir genes are also co-expressed in some external sweet and bitter-sensing GRNs. Thus, both classes of receptors are likely to contribute to responses of Gr/Ir-expressing neurons in the LSO and VCSO, but whether they interact functionally or act independently remains to be determined. In the LSO, expression of sweet Grs and Ir76b overlaps in pharyngeal sweet GRNs, as observed in tarsi as well. In the pharynx, this study also found co-expression of ppk28 with Ir genes, which has not been described for external GRNs. These observations invite explorations of possible crosstalk, and its functional significance, between the two classes of receptors (Chen, 2017).

Pharyngeal GRNs also exhibit distinctive functional groupings. All external bitter GRNs have always been found grouped with sweet GRNs in taste hairs. By contrast, canonical sweet and bitter GRNs appear to segregate in different sensilla in the LSO, which is most well characterized for this perspective. L8 and L9 may be functionally identical and house only one Gr66a-expressing bitter GRN each, whereas L7 contains two sweet GRNs (L7-1 and L7-2). Moreover, external hairs typically have two to four GRNs, each of which has a distinct functional profile. In the LSO duplications are found (L7-1 and L7-2 are identical, as are L7-4 and L7-5), although differences between these pairs of GRNs may emerge as additional chemoreceptors are mapped in the pharynx. Finally, it is difficult to ascribe putative functions to most pharyngeal GRNs based on existing knowledge of receptor function in external counterparts. The L7-3 Gr-expressing neuron, for example, does not express members of the sweet clade, but neither does it express any of the common bitter Grs (Gr32a, Gr66a, and Gr89a) that would corroborate its role as a bitter GRN. Similarly, with the exception of salt neurons that may express Ir76b alone, there are few known functions for GRNs that solely express Ir genes. One possibility is that some of these GRNs possess novel chemoreceptor family ligand interactions. For example, L7-7 is involved in sensing sucrose but limiting sugar ingestion, representing an Ir neuron that operates in a negative circuit module for sugar intake. In addition, another recent study suggests that TRPA1 expression in L8 and L9 of the LSO is involved in feeding avoidance to bacterial endotoxins lipopolysaccharides (LPS). Alternatively, some pharyngeal GRNs may evaluate characteristics other than palatability, such as temperature or viscosity. Ir25a, which is broadly expressed in all 24 pharyngeal GRNs, is required for cool sensing and thermosensing. It will be worth investigating whether one or more pharyngeal GRNs act to integrate information about temperature and chemical quality of food substrates (Chen, 2017).

Expression analyses also hint at some functional subdivisions between pharyngeal taste organs. The LSO contains a smaller proportion of Gr-expressing neurons than the VCSO, which also expresses a larger number of Gr genes that are co-expressed with Gr66a. Thus, broader bitter taste function might be expected in the VCSO. By contrast, sweet taste function appears to be more dominant in the LSO; its sweet GRNs express more sweet Gr-GAL4 drivers than the ones in the VCSO, and their activation is sufficient to drive feeding preference. VCSO sweet GRNs fail to promote ingestion by themselves but may contribute to an increase in feeding preference when activated simultaneously with those in the LSO. Thus, there may be synergistic or hierarchical interactions between LSO and VCSO sweet taste circuits, with the latter coming into play only once the former is activated. The finding that Gr and Ir genes are expressed in the LSO and VCSO but only Ir genes in the DCSO is also striking and raises the possibility that the DCSO, which is present at the most internal location relative to the others, may serve a unique role in controlling ingestion (Chen, 2017).

Based on its molecular signature, the V5 neuron was identified as an L-canavanine-sensing neuron in the pharynx. As predicted, feeding avoidance of L-canavanine is dependent on V5. It was thus unexpected that capsaicin-mediated activation of bitter pharyngeal GRNs, which include V5, did not induce strong feeding avoidance either in the absence or presence of sugar. Because the strength and pattern of pharyngeal neuronal activation by bitter tastants or capsaicin is unknown, it is possible that capsaicin response may be weaker than that of canonical bitter tastants. Alternatively, sweet and bitter inputs from internal and external neurons may be summed differently. It is known that activation of one or few external sweet neurons can lead to proboscis extension, for example, but a larger number of bitter neurons may need to be activated for avoidance (Chen, 2017).

The afferents of pharyngeal GRNs target regions of the SEZ that are distinct from areas in which afferents from labellar and tarsal GRNs terminate. Interestingly, pharyngeal GRN projections between molecularly different classes of neurons, as well as between GRNs of the LSO and VCSO, are also distinct. Projections of sugar-sensing GRNs were found in separate ipsilateral regions, whereas those of neurons predicted to detect aversive tastants were found at the midline, suggesting the presence of contralateral termini. These observations may inform future functional studies of pharyngeal GRNs. L7-6 neurons, for example, would be predicted to sense aversive compounds based on the presence of their termini at the midline. Analysis of pharyngeal GRN projections also suggests distinct connectivity to higher order neuronal circuits. With the molecular tools described here, future investigations of pharyngeal GRNs and pharyngeal taste circuits will provide insight into how internal taste is integrated with external taste to control various aspects of feeding behavior (Chen, 2017).

Internal amino acid state modulates yeast taste neurons to support protein homeostasis in Drosophila

To optimize fitness, animals must dynamically match food choices to their current needs. For drosophilids, yeast fulfils most dietary protein and micronutrient requirements. While several yeast metabolites activate known gustatory receptor neurons (GRNs) in Drosophila melanogaster, the chemosensory channels mediating yeast feeding remain unknown. This study identified a class of proboscis GRNs required for yeast intake. Within this class, taste peg GRNs are specifically required to sustain yeast feeding. Sensillar GRNs, however, mediate feeding initiation. Furthermore, the response of yeast GRNs, but not sweet GRNs, is enhanced following deprivation from amino acids, providing a potential basis for protein-specific appetite. Although nutritional and reproductive states synergistically increase yeast appetite, reproductive state acts independently of nutritional state, modulating processing downstream of GRNs. Together, these results suggest that different internal states act at distinct levels of a dedicated gustatory circuit to elicit nutrient-specific appetites towards a complex, ecologically relevant protein source (Steck, 2018).

The Drosophila Gr28bD product is a non-specific cation channel that can be used as a novel thermogenetic tool

A good candidate for developing new thermogenetic tools is the Drosophila gustatory receptor family, which has been implicated in high-temperature avoidance behavior. Five members of the alternatively spliced Gr28b gene cluster were examined for temperature-dependent properties via three approaches: biophysical characterization in Xenopus oocytes, functional calcium imaging in Drosophila motor neurons, and behavioral assays in adult Drosophila. Gr28bD expression in Xenopus oocytes produced a non-specific cationic current that is activated by elevated temperatures. This current is non-inactivating and non-voltage dependent. When expressed in Drosophila motor neurons, Gr28bD can be used to change the firing pattern of individual cells in a temperature-dependent fashion. Pan-neuronal or motor neuron expression of Gr28bD can be used to alter fruit fly behavior with elevated temperatures. Together, these results validate the potential of the Gr28bD gene as a founding member of a new class of thermogenetic tools (Mishra, 2018).

The taste of ribonucleosides: Novel macronutrients essential for larval growth are sensed by Drosophila gustatory receptor proteins

Animals employ various types of taste receptors to identify and discriminate between different nutritious food chemicals. These macronutrients are thought to fall into 3 major groups: carbohydrates/sugars, proteins/amino acids, and fats. This study reports that Drosophila larvae exhibit a novel appetitive feeding behavior towards ribose, ribonucleosides, and RNA. Members of the gustatory receptor (Gr) subfamily 28 (Gr28), expressed in both external and internal chemosensory neurons were identified as molecular receptors necessary for cellular and appetitive behavioral responses to ribonucleosides and RNA. Specifically, behavioral preference assays show that larvae are strongly attracted to ribose- or RNA-containing agarose in a Gr28-dependent manner. Moreover, Ca2+ imaging experiments reveal that Gr28a-expressing taste neurons are activated by ribose, RNA and some ribonucleosides and that these responses can be conveyed to Gr43aGAL4 fructose-sensing neurons by expressing single members of the Gr28 gene family. Lastly, a critical role in behavioral fitness for the Gr28 genes was established by showing that Gr28 mutant larvae exhibit low survival rates when challenged to find ribonucleosides in food. Together, this work identifies a novel taste modality dedicated to the detection of RNA and ribonucleosides, nutrients that are essential for survival during the accelerated growth phase of Drosophila larvae (Mishra, 2018).

A complex peripheral code for salt taste in Drosophila

Each taste modality is generally encoded by a single, molecularly defined, population of sensory cells. However, salt stimulates multiple taste pathways in mammals and insects, suggesting a more complex code for salt taste. This study examined salt coding in Drosophila. After creating a comprehensive molecular map comprised of five discrete sensory neuron classes across the fly labellum, four were found to be activated by salt: two exhibiting characteristics of 'low salt' cells, and two 'high salt' classes. Behaviorally, low salt attraction depends primarily on 'sweet' neurons, with additional input from neurons expressing the ionotropic receptor IR94e. High salt avoidance is mediated by 'bitter' neurons and a population of glutamatergic neurons expressing Ppk23. Interestingly, the impact of these glutamatergic neurons depends on prior salt consumption. These results support a complex model for salt coding in flies that combinatorially integrates inputs from across cell types to afford robust and flexible salt behaviors (Jaeger, 2018).

Sodium is essential to survival, but its intake must be carefully regulated to maintain ionic homeostasis. It is therefore unsurprising that taste systems have evolved robust mechanisms for detecting salt, and that salt palatability depends on its concentration. In general, sodium concentrations below 100 mM tend to be attractive, while any salt present at higher concentrations becomes increasingly aversive (Jaeger, 2018).

Although there is considerable debate about modes of central taste coding, there is strong evidence that most taste modalities activate a single, molecularly defined, population of peripheral taste receptor cells. However, research in both mammals and insects has favoured a dual-pathway model for salt taste: a low-threshold sodium-specific population of 'low salt' cells mediates attraction, which is overridden at higher concentrations by ion non-specific 'high salt' cells that drive avoidance. Moreover, two distinct aversive taste receptor cell (TRC) types (bitter and sour) contribute to high salt taste in mammals. Thus, peripheral coding of salt taste appears more complex than other primary taste modalities (Jaeger, 2018).

The Drosophila labellum contains three types of gustatory sensilla, each of which harbors 2-4 gustatory receptor neurons (GRNs). Short (S-type) and long (L-type) sensilla have four molecularly and physiologically distinct GRNs, while intermediate (I-type) sensilla have only two. Extracellular 'tip-recordings' of different sensilla have identified four GRN types: a water (W) cell that responds to low osmolarity; a sugar (S) cell that responds to sweet compounds; a low salt (L1) cell that is sodium-specific; and a high salt (L2) cell that responds to high ionic concentrations (>250 mM). S- and L-type sensilla are thought to have one of each GRN type, with S-type L2 cells responding to bitter compounds in addition to high salt. I-type sensilla were shown to have an S/L1 hybrid cell that responds to sugars and low salt, and an L2 cell that responds to bitters and high salt (Jaeger, 2018).

The early physiological recordings have been mostly borne out by molecular characterization of GRN types. S- and L-type sensilla each have a single GRN that expresses the low osmolarity sensor Pickpocket28 (Ppk28) and corresponds to the W cell. The S cell is labelled by the sugar receptor Gr64f, along with other members of the gustatory receptor (GR) family. Similarly, Gr66a is co-expressed with other Grs in a single bitter responsive neuron per S-type and I-type sensillum, corresponding to the L2 cell. The degenerin/epithelial sodium channel (Deg/ENaC) family member Ppk23, which is required for pheromone detection in leg gustatory sensilla, is known to be expressed in a labellar neuron population that partially overlaps with Gr66a/bitter GRNs. Ppk23 neurons are necessary for calcium avoidance, but details of the labellar Ppk23 expression map, as well as the physiology and function of these neurons are largely unknown (Jaeger, 2018).

In contrast to water, sweet, and bitter tastes, the principles of peripheral salt coding in flies remain unclear. Early calcium imaging experiments revealed low salt responses in Gr5a-Gal4 GRNs, suggesting that sweet neurons may mediate low salt attraction. However, Gr5a-Gal4 was later shown to label additional GRNs outside the sweet class, and the Ionotropic receptor (IR) family member IR76b was proposed to specifically mediate low salt taste via a dedicated low salt cell distinct from sweet GRNs. This view was challenged by the recent demonstration that IR76b is also required for high salt taste, raising questions about its utility as a marker for one defined GRN population. Moreover, although Gr66a GRNs showed calcium responses to high salt concentrations and electrophysiology suggested that bitter and high salt are encoded by the same sensory neurons, genetically eliminating these cells left behavioral aversion to high salt largely intact (Jaeger, 2018).

This study probes the logic of salt coding across the labellum by systematically characterizing the physiological and behavioral roles of molecularly-defined GRN types covering the entire labellar GRN map. All GRN types showed dose-dependent excitation or inhibition by salt, indicating a complex model for salt coding. Of particular interest is that, like mammals, flies have two distinct high salt cells. In addition to activating canonical bitter neurons, high salt concentrations excite a glutamatergic GRN population expressing Ppk23. Salt responses of these Ppk23glut GRNs require IR76b, whereas those of bitter GRNs do not. Both bitter and Ppk23glut GRNs are necessary for behavioral avoidance of high salt when flies have been reared on a salt-containing diet. However, salt deprivation reduces high salt avoidance by specifically suppressing the impact of Ppk23glut neurons, suggesting that these GRNs mediate internal state-dependent modulation of salt consumption. Consistent with this idea, closed-loop optogenetic activation of Ppk23glut neurons reduces feeding by salt-fed flies, but not those that have been salt deprived. The results support a model where the combinatorial excitation and inhibition of various taste pathways mediates the behavioral valence of salt, with one pathway conferring the ability to specifically modulate salt consumption based on internal state (Jaeger, 2018).

The results suggest a complex model for how the fly peripheral gustatory system encodes salt taste. In contrast to other known taste modalities, which typically activate a single, molecularly defined population of sensory neurons, salt taste is encoded by the combined activity of most to all GRN classes (see Model for salt encoding across different GRN classes in the labellum). By identifying markers that cumulatively cover virtually every labellar GRN, this study found that two cell types -- Gr64f and IR94e -- display low salt tuning properties and mediate salt attraction. Although Gr64f neurons are also activated by higher concentrations of salt, their relatively low threshold for activation and specificity for sodium are consistent with a low salt GRN identity. Two other cell types -- Gr66a and Ppk23glut -- act as high salt GRNs, responding ion non-selectively to high concentrations of salt and driving avoidance. Moreover, the impact of Ppk23glut activation is suppressed upon salt deprivation, providing a means to reduce salt avoidance when need is elevated (Jaeger, 2018).

Prior salt coding models have primarily relied on correlations between neural activity and behavioural responses to different salt stimuli, as well as changes to those properties in mutants that may have wide ranging effects. These studies led to the important idea that there are distinct low salt and high salt cells present, and that at increasing salt concentrations, the aversive high salt cell progressively dominates over the attractive low salt cell. However, without the molecular tools to identify and manipulate individual GRN classes, the identity and number of salt-responsive cell types were unclear. This examination of salt coding across a comprehensive set of molecularly defined GRN classes provides new insight into the complexity of salt coding in flies, but is not without limitations. Most notably, possible heterogeneity within defined GRN classes was not detected. For example, a Gr64f (sweet) neuron is present in each bristle of all sensillum types. Because the population was examined as a whole, it cannot be confidently concluded that every Gr64f cell acts as a low salt cell; it is known only that there are sodium-specific salt responses from the Gr64f population and that this population drives low salt attraction (Jaeger, 2018).

Electrophysiological recordings of individual labellar taste sensilla identified high salt responses in the bitter-sensing neurons of S- and I-type sensilla, and previous GRN calcium imaging confirmed that Gr66a neurons respond to 1 M NaCl and KCl. However, two key results suggested that bitter GRNs did not account for all high salt taste. First, high salt neurons have been identified in L-type sensilla (which don't have Gr66a neurons) via tip recordings, although this has subsequently been debated (Hiroi, 2002; Ishimoto and Tanimura, 2004; Zhang, 2013). Second, genetically ablating Gr66a GRNs did not block the inhibition of PER by high salt (Wang et al., 2004). The existence of Ppk23glut high salt cells likely explains both of these observations and provides a mechanism by which flies can specifically modulate their salt behavior in response to need (Jaeger, 2018).

In addition to the modulation of their behavioral impact, Ppk23glut neurons display some notable characteristics, the most conspicuous being they are the only GRN class to express a marker for glutamatergic, rather than cholinergic, neurons. This adds a potential new dimension to the gustotopic GRN map formed in the fly brain and may be a key mechanism by which the output of Ppk23glut neurons remains functionally distinct from other GRN classes targeting postsynaptic neurons in the same area. Indeed, the aversive nature of Ppk23glut output stands in contrast to what one would predict from their projection morphology, which looks qualitatively similar to known appetitive (Gr64f, Ppk28), rather than aversive (Gr66a) GRNs. It is possible that Ppk23glut aversiveness is mediated through inhibition of appetitive taste pathways, as glutamate can have excitatory or inhibitory postsynaptic effects, depending on the receptor present (Jaeger, 2018).

Although Ppk23glut defines a novel high salt cell, it is important to note that the Ppk23 channel is not required for its salt responsiveness. This raises questions about what Ppk23-dependent responses these cells may exhibit. Since Ppk23 is required for leg GRN pheromone-evoked activity that regulates courtship, a related function for Ppk23 labellar GRNs cannot be excluded. Indeed, weak, but significant Ppk23-dependent pheromone responses have been observed in labellar GRNs, although it's unclear whether these were from Ppk23glut or Ppk23chat cells. Moreover, the interaction between salt taste and mating suggests that perhaps there is a need to co-modulate salt and social cues based on salt diet (Jaeger, 2018).

In contrast to the strong salt responses in Ppk23glut cells, the other uncharacterized GRN class that was identified, IR94e, displayed only weak salt-evoked activity. It is therefore expected that this class primarily responds to other, yet unidentified, taste ligands. Given the lack of strong effects that were observe upon activation of IR94e GRNs in the STROBE, it is also suspected that the behavioral impact of IR94e activation is, like Ppk23glut, state- or context-dependent (Jaeger, 2018).

To date, IR76b has been shown to be necessary for gustatory responses to low salt, high salt, calcium, acids, amino acids, fatty acids, and polyamines. Consistent with these widespread roles in the taste system, this study found expression of IR76b-Gal4 in every GRN type tested. Nonetheless, it was important to clarify the role of IR76b in salt taste, given the apparent complexities in salt responses across labellar GRN types, and the previous demonstration that IR76b can function as a sodium leak channel (Jaeger, 2018).

Gr64f salt responses were found to be completely dependent on IR76b, consistent with its proposed role in low salt taste. Ppk23glut salt responses also require IR76b, but those in Gr66a GRNs do not, indicating two different salt transduction mechanisms in these two high salt cells. This may explain why prior reports differed on whether high salt responses remain intact in IR76b mutants. Interestingly, the IR76b-dependent salt responses in Ppk23glut GRNs are not sodium specific, as loss of high sodium and potassium salt-evoked activity is seen. This suggests that, although IR76b is primarily permeable to sodium when expressed in heterologous cells, it may function in complexes with other subunits that confer different ion selectivity in different GRN classes (Jaeger, 2018).

Recently, Ppk23 GRNs were identified as underlying IR76b-dependent calcium taste avoidance. Although it isn't clear whether Ppk23glut or Ppk23chat (or both) subpopulations are responsible, the results indicate that this effect is not specific to calcium, but rather a general salt avoidance mechanism. Indeed, Ppk23 GRNs respond to high concentrations of all salts tested. Moreover, IR25a, which was implicated in Ppk23-mediated calcium taste was found to be necessary for salt responses in Gr64f and Ppk23glut GRNs, similar to the requirements for IR76b. This stands in contrast to results reported in another study, which suggested that IR25a did not play a role in sodium taste. The similar requirements for IR76b and IR25a also suggest that these two receptors may act in a complex to mediate salt taste, consistent with previous evidence that IR25a is a broadly expressed coreceptor (Jaeger, 2018).

Changes in gustatory sensitivity based on internal state are a widespread feature of the fly taste system: starvation potentiates sweet GRN sensitivity and suppresses bitter GRN responses; mating increases taste peg GRN sensitivity to polyamines and behavioral sensitivity to low salt in females; and protein deprivation sensitizes taste peg GRNs to yeast and increases behavioral sensitivity to amino acids. Although modulation of salt taste has not been previously examined in flies, salt depletion in humans increases salt palatability. In line with all these results, significant modulation was seen of fly salt taste behavior by salt deprivation (Jaeger, 2018).

In contrast to most taste modalities, which activate a single GRN population, modulation of salt taste presents a complicated problem, because tuning the gain of Gr64 or Gr66a GRN output would have side effects on sweet and bitter taste sensitivity that may be situationally inappropriate. This study has presented evidence that the fly gustatory system solves this problem by specifically modulating the effects downstream of Ppk23glut activation. Salt deprivation suppresses the aversiveness of these neurons, allowing the fly to be less repulsed (or more attracted) to salty foods (Jaeger, 2018).

Thus, the fly taste system appears to encode salt as a complex mixture of attractive and repulsive sensory responses. Two GRN classes -- Gr64f and Gr66a -- provide a baseline level of attraction or avoidance, and this response is then adjusted to need via modulation of a third class of salt-responsive GRNs, Ppk23glut. The apparent specificity of labellar Ppk23glut GRNs to salt may also provide an important neural substrate for discrimination between salt and other taste modalities. Continued exploration of how salt, and other, taste signals are integrated higher in the brain will provide insight into how an apparently low-dimensional sensory system can successfully encode a variety of diverse chemical cues (Jaeger, 2018).

Ir56b is an atypical ionotropic receptor that underlies appetitive salt response in Drosophila

Salt taste is one of the most ancient of all sensory modalities. However, the molecular basis of salt taste remains unclear in invertebrates. This study shows that the response to low, appetitive salt concentrations in Drosophila depends on Ir56b, an atypical member of the ionotropic receptor (Ir) family. Ir56b acts in concert with two coreceptors, Ir25a and Ir76b. Mutation of Ir56b virtually eliminates an appetitive behavioral response to salt. Ir56b is expressed in neurons that also sense sugars via members of the Gr (gustatory receptor) family. Misexpression of Ir56b in bitter-sensing neurons confers physiological responses to appetitive doses of salt. Ir56b is unique among tuning Irs in containing virtually no N-terminal region, a feature that is evolutionarily conserved. Moreover, Ir56b is a "pseudo-pseudogene": its coding sequence contains a premature stop codon that can be replaced with a sense codon without loss of function. This stop codon is conserved among many Drosophila species but is absent in a number of species associated with cactus in arid regions. Thus, Ir56b serves the evolutionarily ancient function of salt detection in neurons that underlie both salt and sweet taste modalities (Dweck, 2022).

Histamine avoidance through three gustatory receptors in Drosophila melanogaster

Histamine is a fermented food product that exerts adverse health effects on animals when consumed in high amounts. This biogenic amine is fermented by microorganisms from histidine through the activity of histidine decarboxylase. Drosophila melanogaster can discriminate histidine and histamine using GR22e and IR76b in bitter-sensing gustatory receptor neurons (GRNs). In this study, RNA interference screens were conducted to examine 28 uncharacterized gustatory receptor genes using electrophysiology and behavioral experiments, including the binary food choice and proboscis extension response assays. GR9a and GR98a were first identified as specific histamine receptors by evaluating newly generated null mutants and recovery experiments by expressing their wild-type cDNA in the bitter-sensing GRNs. It was further determined that histamine sensation was mainly mediated by the labellum but not by the legs, as demonstrated by the proboscis extension response assay. These findings indicated that toxic histamine directly activates bitter-sensing GRNs in S-type sensilla, and this response is mediated by the GR9a, GR22e, and GR98a gustatory receptors (Aryal, 2022).

An inhibitory mechanism for suppressing high salt intake in Drosophila
High concentrations of dietary salt are harmful to health. Like most animals, Drosophila melanogaster are attracted to foods that have low concentrations of salt, but show strong taste avoidance of high salt foods. Salt in known on multiple classes of taste neurons, activating Gr64f sweet-sensing neurons that drive food acceptance and 2 others (Gr66a bitter and Ppk23 high salt) that drive food rejection. This study finds that NaCl elicits a bimodal dose-dependent response in Gr64f taste neurons, which show high activity with low salt and depressed activity with high salt. High salt also inhibits the sugar response of Gr64f neurons, and this action is independent of the neuron's taste response to salt. Consistent with the electrophysiological analysis, feeding suppression in the presence of salt correlates with inhibition of Gr64f neuron activity, and remains if high salt taste neurons are genetically silenced. Other salts such as Na2SO4, KCl, MgSO4, CaCl2, and FeCl3 act on sugar response and feeding behavior in the same way. A comparison of the effects of various salts suggests that inhibition is dictated by the cationic moiety rather than the anionic component of the salt. Notably, high salt-dependent inhibition is not observed in Gr66a neurons-response to a canonical bitter tastant, denatonium, is not altered by high salt. Overall, this study characterizes a mechanism in appetitive Gr64f neurons that can deter ingestion of potentially harmful salts.

LPS perception through taste-induced reflex in Drosophila melanogaster

In flies, grooming serves several purposes, including protection against pathogens and parasites. Previous work has shown that Escherichia coli or lipopolysaccharides (LPS) can induce grooming behavior via activation of contact chemoreceptors on Drosophila wing. This suggested that specific taste receptors may contribute to this detection. This study examined the perception of commercially available LPS on Drosophila wing chemoreceptors in grooming reflex. Behavioral tests conducted with bitter, sweet and salty gustation such as caffeine, sucrose and salt, using flies carrying a defect in one of their taste receptors related to the detection of bitter molecules (Gr66a, Gr33a), sugars (Gr5a, Gr64f), or salt (IR76b). LPS and tastants of each category were applied to wing sensilla of these taste defective flies and to wild-type Canton Special (CS) flies. The results indicate that the grooming reflex induced by LPS requires a wide range of gustatory genes, and the inactivation of any of tested genes expressing cells causes a significant reduction of the behavior. This suggests that, while the grooming reflex is strongly regulated by cues perceived as aversive, other sapid cues traditionally related to sweet and salty tastes are also contributing to this behavior (Yanagawa, 2018).

Mechanism of acetic acid gustatory repulsion in Drosophila

The decision to consume or reject a food based on the degree of acidity is critical for animal survival. However, the gustatory receptors that detect sour compounds and influence feeding behavior have been elusive. Using the fly, Drosophila melanogaster, it was revealed that a member of the ionotropic receptor family, IR7a, is essential for rejecting foods laced with high levels of acetic acid. IR7a is dispensable for repulsion of other acidic compounds, indicating that the gustatory sensation of acids occurs through a repertoire rather than a single receptor. The fly's main taste organ, the labellum, is decorated with bristles that house dendrites of gustatory receptor neurons (GRNs). IR7a is expressed in a subset of bitter GRNs rather than GRNs dedicated to sour taste. These findings indicate that flies taste acids through a repertoire of receptors, enabling them to discriminate foods on the basis of acid composition rather than just pH (Rimal, 2019).

Molecular and cellular basis of acid taste sensation in Drosophila

Acid taste, evoked mainly by protons (H(+)), is a core taste modality for many organisms. The hedonic valence of acid taste is bidirectional: animals prefer slightly but avoid highly acidic foods. However, how animals discriminate low from high acidity remains poorly understood. To explore the taste perception of acid, the fruit fly was used as a model organism. Flies were found to employ two competing taste sensory pathways to detect low and high acidity, and the relative degree of activation of each determines either attractive or aversive responses. Moreover, one member of the fly Otopetrin family, Otopetrin-like a (OtopLa), was established as a proton channel dedicated to the gustatory detection of acid. OtopLa defines a unique subset of gustatory receptor neurons and is selectively required for attractive rather than aversive taste responses. Loss of otopla causes flies to reject normally attractive low-acid foods. Therefore, the identification of OtopLa as a low-acid sensor firmly supports a competition model of acid taste sensation. Altogether, this study has discovered a binary acid-sensing mechanism that may be evolutionarily conserved between insects and mammals (Mi, 2021).

Chemosensory sensilla of the Drosophila wing express a candidate ionotropic pheromone receptor

The Drosophila wing has been proposed to be a taste organ. A differential RNA-seq analysis was carried out of a row of sensilla on the anterior wing margin; expression was found of many genes associated with pheromone and chemical perception. To ask whether these sensilla might receive pheromonal input, a dye-transfer paradigm was carried out; large, hydrophobic molecules comparable to pheromones were found to be transferred from one fly to the wing margin of another. One gene, Ionotropic receptor (IR)52a, is coexpressed in neurons of these sensilla with fruitless, a marker of sexual circuitry; IR52a is also expressed in legs. Mutation of IR52a and optogenetic silencing of IR52a+ neurons decrease levels of male sexual behavior. Optogenetic activation of IR52a+ neurons induces males to show courtship toward other males and, remarkably, toward females of another species. Surprisingly, IR52a is also required in females for normal sexual behavior. Optogenetic activation of IR52a+ neurons in mated females induces copulation, which normally occurs at very low levels. IR52a acts in both males and females, and can promote male-male as well as male-female interactions. Moreover, IR52a+ neurons can override the circuitry that normally suppresses sexual behavior toward unproductive targets. Circuit mapping and Ca2+ imaging using the trans-Tango system reveals second-order projections of IR52a+ neurons in the subesophageal zone (SEZ), some of which are sexually dimorphic. Optogenetic activation of IR52a+ neurons in the wing activates second-order projections in the SEZ. Taken together, this study provides a molecular description of the chemosensory sensilla of a greatly understudied taste organ and defines a gene that regulates the sexual circuitry of the fly (He, 2019).

Molecular sensor of nicotine in taste of Drosophila melanogaster

Nicotine is an alkaloid and potent parasympathomimetic stimulant found in the leaves of many plants including Nicotiana tabacum, which functions as an anti-herbivore chemical and an insecticide. Chemoreceptors embedded in the gustatory receptor neurons (GRNs) enable animals to judge the quality of bitter compounds and respond to them. Various taste receptors such as gustatory receptors (GRs), ionotropic receptors (IRs), transient receptor potential channels (TRPs), and pickpocket channels (PPKs) have been shown to have important roles in taste sensation. However, the mechanism underlying nicotine taste sensation has not been resolved in the insect model. This study identified molecular receptors to detect the taste of nicotine and provide electrophysiological and behavioral evidence that gustatory receptors are required for avoiding nicotine-laced foods. GR10a, GR32a and GR33a are necessary for nicotine sensing while TRP family member receptors are not required. The results demonstrate that gustatory receptors are reasonable targets to develop new pesticides that maximize the insecticidal effects of nicotine (Rimal, 2019).

Acetic acid activates distinct taste pathways in Drosophila to elicit opposing, state-dependent feeding responses

Taste circuits are genetically determined to elicit an innate appetitive or aversive response, ensuring that animals consume nutritious foods and avoid the ingestion of toxins. This study has examined the response of Drosophila melanogaster to acetic acid, a tastant that can be a metabolic resource but can also be toxic to the fly. The data reveal that flies accommodate these conflicting attributes of acetic acid by virtue of a hunger-dependent switch in their behavioral response to this stimulus. Fed flies show taste aversion to acetic acid, whereas starved flies show a robust appetitive response. These opposing responses are mediated by two different classes of taste neurons, the sugar- and bitter-sensing neurons. Hunger shifts the behavioral response from aversion to attraction by enhancing the appetitive sugar pathway as well as suppressing the aversive bitter pathway. Thus a single tastant can drive opposing behaviors by activating distinct taste pathways modulated by internal state (Devineni, 2019).

Searching for relief: Drosophila melanogaster navigation in a virtual bitter maze

Animals use relief-based place learning to pinpoint a specific location where noxious stimuli are diminished or abolished. This study shows how the optogenetically-induced activation of bitter-sensing neurons in Drosophila melanogaster elicits pain-like behavioural responses and stimulates the search for a place where this activation is relieved. Under this 'virtual' stimulation paradigm it would be feasible to test relief learning several times throughout an animal's lifespan, without the potentially damaging effects which may result from the repeated administration of 'real' heat or electrical shock. Furthermore, virtual bitter taste could be used in place of virtual pain stimulation to guide conditioned place preference and study learning processes. It is also proposed that spatially-specific reduction of locomotor velocity may provide immediate evidence of relief-based place learning and spatial memory (Meda, 2020).

Vitamin preference in Drosophila

Many animals rely on taste to identify nutritious foods and to avoid the consumption of harmful substances. The tastes of macronutrients, as well as of non-caloric micronutrients such as sodium and calcium, contribute to the regulation of ingestive behavior. Whether vitamins also affect feeding behavior through taste is less clear. This study shows that the fly Drosophila melanogaster has a strong preference for consuming a vitamin-containing diet: both sexes show a preference for folic acid, whereas only females show a preference for riboflavin. Females show a preference with vitamin concentrations as low as ∼10 nM - at least 50,000-fold lower than the concentration needed for sucrose preference. This female vitamin preference requires inputs from external and internal taste organs, suggesting that post-ingestive signals, in the absence of gustatory input, are insufficient to actuate preferential consumption of vitamin-containing diets. These studies demonstrate that vitamin perception is an important determinant of feeding behavior (Wu, 2021).

Serotonergic neurons translate taste detection into internal nutrient regulation

The nervous and endocrine systems coordinately monitor and regulate nutrient availability to maintain energy homeostasis. Sensory detection of food regulates internal nutrient availability in a manner that anticipates food intake, but sensory pathways that promote anticipatory physiological changes remain unclear. This study identified serotonergic (5-HT) neurons as critical mediators that transform gustatory detection by sensory neurons into the activation of insulin-producing cells and enteric neurons in Drosophila. One class of 5-HT neurons responds to gustatory detection of sugars, excites insulin-producing cells, and limits consumption, suggesting that they anticipate increased nutrient levels and prevent overconsumption. A second class of 5-HT neurons responds to gustatory detection of bitter compounds and activates enteric neurons to promote gastric motility, likely to stimulate digestion and increase circulating nutrients upon food rejection. These studies demonstrate that 5-HT neurons relay acute gustatory detection to divergent pathways for longer-term stabilization of circulating nutrients (Yao, 2022).

This study identified two classes of 5-HT neurons that play distinct roles in gustatory processing and function independently to promote nutrient homeostasis. Sugar-SELs, located in the lateral subesophageal ganglion (SEL) respond to sugar gustatory detection, promote insulin-producing cell (IPC) activity, and reduce feeding drive, suggesting that they prevent overconsumption in nutrient rich environments. Bitter-SELs respond to bitter taste detection and promote crop contractions, likely to utilize stored food upon food rejection. Thus, 5-HT neurons coordinate endocrine and digestive function in anticipation of altered food intake (Yao, 2022).

5-HT profoundly modulates appetite and feeding across animal species. In humans and rodents, the global effect of brain 5-HT signaling is suppression of food intake; however, the involvement of multiple brain regions (including the hypothalamus, solitary tract nucleus, and parabrachial nuclei) and multiple 5-HT receptors (e.g., 5-HT1B, 5-HT2C, 5-HT6) underscores the complex nature of 5-HT modulation of appetite and feeding. Studies in invertebrates have likewise demonstrated multiple, sometimes opposite, roles for 5-HT neurons in regulating feeding. For instance, in Drosophila adults, activating all 5-HT neurons suppresses food intake whereas activating a smaller yet diverse subset promotes food intake, suggesting heterogeneity in 5-HT feeding regulation. With the exception of a few cases where specific 5-HT neurons that influence feeding have been identified—for example, 5-HT neurons that promote pharyngeal pumping to enhance ingestion in C. elegans and Drosophila larvae—the diversity of 5-HT neurons that contributes to feeding regulation and nutrient homeostasis remains largely uncharacterized (Yao, 2022).

This study identified multiple classes of 5-HT neurons that are activated by gustatory detection and signal to the endocrine and digestive systems to influence nutrient availability. These studies reveal 5-HT neurons that have different projection patterns, relay sugar and bitter gustatory information to different downstream targets, and regulate internal nutrient availability by distinct mechanisms. These studies provide insight into the multifaceted roles of 5-HT neurons in gustatory processing, feeding regulation, and nutrient homeostasis, highlighting the importance of understanding the myriad functions of 5-HT at the neural circuit level (Yao, 2022).

Food-derived sensory cues are used as anticipatory signals to regulate endocrine function and feeding drive. Work in mammals has demonstrated that there are two phases of insulin release in response to food consumption, a pre-absorptive phase in response to sensory detection of food and a post-absorptive phase in response to elevated blood glucose levels. The pre-absorptive phase (cephalic phase) is triggered by sensory detection prior to food consumption and nutrient absorption. The neural circuit that underlies pre-absorptive insulin release, however, is not fully understood (Yao, 2022).

The current findings suggest that insulin release in anticipation of food consumption is a process shared in flies and mammals, perhaps indicating an effective strategy to promote rapid nutrient storage during feeding. Using in vivo calcium imaging, this study found that fly IPCs are rapidly excited by sugar gustatory detection independent of consumption. The findings that sugar-SELs respond to sugar taste detection and activate IPCs suggest a neural circuit mechanism for pre-absorptive insulin release independent of ingestion. Moreover, as gustatory sensory neurons detect both nutritive and non-nutritive sugars and are inputs to sugar-SELs and IPCs, it is anticipated that sugar-SELs, IPCs, and pre-absorptive insulin release are also activated by non-nutritive sugars, but this requires further investigation. The finding that fly IPCs are activated by sugar taste detection contrasts with previous whole-brain imaging studies, likely indicating signal detection limits. In addition to identifying sugar taste responses in IPCs, this study also identified the sugar-SELs as a specific neural pathway mediating the preparatory insulin response (Yao, 2022).

Knocking down 5-HT2A in IPCs reduces but does not abolish gustatory-induced IPC activity and does not impact consumption. One caveat of this approach is that RNAi reduces gene expression and may not produce complete loss-of-function phenotypes. The IPCs are activated by many nutritional state signals, including multiple pathways that report circulating sugars and signals from the intestine and fat body. The current work shows that external nutrients in the form of sugar taste detection also activate this important hub and identifies sugar-SELs as a defined pathway conveying the sugar taste signal. As IPCs release multiple peptides, the activation of IPCs by sugar-SELs may coordinate widespread changes in metabolism and behavior. Thus, this study has identified a specific class of 5-HT neurons that participates in the preparatory insulin response and the reduction of feeding drive in response to sugar gustatory detection, shedding light on the neural circuit mechanisms that anticipate sugar consumption (Yao, 2022).

A surprising finding from this study is that bitter-SELs, which respond to bitter gustatory detection, promote contractions of the crop food storage organ. While there is evidence that intestinal bitter detection modulates gastrointestinal physiology, the regulation of gastrointestinal function by bitter gustatory detection is less examined. Activation of bitter gustatory neurons, bitter-SELs, and 5-HT7 neurons all promote crop contractions, although rates differ, possibly based on optogenetic activation strength or propagation of activity to the crop (Yao, 2022).

Why do bitter compounds promote crop contractions? It was reasoned that because bitter compounds are feeding deterrents, frequent encounters with bitter compounds may prevent food intake, leading to depletion of internal nutrients. Under such conditions, bitter-SELs may promote crop motility to utilize food reserves in anticipation of limited food intake. It is therefore proposed that bitter gustatory compounds may have an unappreciated role in predicting food scarcity and stimulating digestion as a preparatory response (Yao, 2022).

Previous studies in Drosophila have demonstrated that bitter taste detection elicits inhibition of proboscis extension and suppression of consumption. In addition, detection of bitter compounds drives avoidance behavior and increased locomotion, likely to promote departure to new areas. The current findings suggest an additional role for bitter taste detection in promoting mobilization of food stores to increase circulating nutrients. These actions are aligned in mitigating the impact of a potentially toxic food source by limiting consumption, promoting relocation, and maintaining internal nutrient levels. Whereas bitter taste detection rapidly leads to consumption inhibition and increased locomotion, whether bitter taste detection activates crop contractions acutely or only under specific conditions of food deprivation is not resolved. In addition, although increased crop contractions were observed upon activation of bitter-SELs, it remains to be examined whether this impacts internal circulating nutrients. Bitter-SELs may also influence additional aspects of feeding behavior not tested in this study, including modulating consumption under different physiological states or when different feeding assays or different bitter compounds are used (Yao, 2022).

In addition to the hypocerebral ganglion (HCG), bitter-SELs project to diverse targets, suggesting that they may carry out other functions besides enteric modulation. For example, bitter-SELs broadly arborize on the dorsal surface of the VNC. Thus, bitter-SELs may also set the 5-HT tone in the VNC or secrete 5-HT into the hemolymph to modulate target tissues in a paracrine or endocrine fashion (Yao, 2022).

This study found that 5-HT neurons are critical nodes in the circuits that transform gustatory detection into changes in endocrine and digestive function. Although the timescale of activation of sugar-SELs and bitter-SELs and the dynamics of 5-HT release requires further investigation, 5-HT receptors are metabotropic receptors ideally suited for transforming transient neural signals into more sustained cellular responses. In this regard, neuromodulatory circuits are prime candidates for eliciting preparatory responses that require the transformation of neural signals across time scales. This work thus sheds light on neural circuit mechanisms that translate external sensory cues into preparatory physiological responses and suggests that neuromodulators such as 5-HT may contribute to anticipatory mechanisms in other animals (Yao, 2022).

Complex representation of taste quality by second-order gustatory neurons in Drosophila

Sweet and bitter compounds excite different sensory cells and drive opposing behaviors. However, it remains unclear how sweet and bitter tastes are represented by the neural circuits linking sensation to behavior. To investigate this question in Drosophila, this study devised trans-Tango(activity), a strategy for calcium imaging of second-order gustatory projection neurons based on trans-Tango, a genetic transsynaptic tracing technique. Spatial overlap was found between the projection neuron populations activated by sweet and bitter tastants. The spatial representation of bitter tastants in the projection neurons was consistent, while that of sweet tastants was heterogeneous. Furthermore, it wads discovered that bitter tastants evoke responses in the gustatory receptor neurons and projection neurons upon both stimulus onset and offset and that bitter offset and sweet onset excite overlapping second-order projections. These findings demonstrate an unexpected complexity in the representation of sweet and bitter tastants by second-order neurons of the gustatory circuit (Snell, 2022).

A central question in taste coding is how sweet and bitter tastes are represented beyond the first-order sensory neurons of the circuit. This study is the first to broadly characterize the taste responses of second-order projection neurons in Drosophila, a strategy enabled by trans-Tango(activity), a novel configuration of trans-Tango. Evidence was found for a variety of GPN response profiles, including some tuned specifically to either sweet or bitter tastants, and some that respond to both. It was also discovered that bitter tastants can evoke activity in GRNs and GPNs upon both stimulus onset and offset. The spatial distribution of the GPN offset response extends beyond the onset-responsive region and overlaps with regions responding to sweet tastants. These results thus demonstrate considerable complexity in the taste responses of GPNs (Snell, 2022).

Beyond these findings on taste representation, this study demonstrates more generally that trans-Tango can fill a niche in analyzing neural circuit function. Although a plethora of genetic tools exist for probing circuit function in Drosophila, they are limited by genetic access (e.g., the expression patterns of Gal4 driver lines). There is an obvious use in having a line that labels a broad population of neurons at a defined stage of the circuit; for example, the GH146-Gal4 driver has been extensively used because it labels a broad set of second-order olfactory projection neurons with little background expression (Snell, 2022).

In many other circuits, though, no comparable driver exists. Using trans-Tango initiated from a sparse set of starter neurons, one can selectively image the calcium responses of their downstream projection neurons, as has been done in this study in the gustatory system. The increasing availability of highly specific split-Gal4 drivers will enable this strategy in a wide variety of systems (Snell, 2022).

Moreover, this study demonstrates that using a nuclear-localized reporter as an output of trans-Tango can facilitate the counting of postsynaptic neurons. This strategy can be used to assess the degree of overlap between the second-order neurons of two different circuits, as has been done in this study for the sweet and bitter circuits (Snell, 2022).

A caveat of this analysis method is that the regions of interest (ROIs) obtained through the segmentation algorithm do not necessarily have a one-to-one correspondence with underlying neurons. As a result, it was not possible to definitively conclude whether a given ROI response profile matches the tuning of an individual gustatory projection neurons (GPNs). It is thus possible that ROIs exhibiting responses to multiple tastants result from the close proximity of multiple narrowly tuned neurons. Although proximity is unlikely to fully explain these findings, determining the extent to which ROIs reflect multiple cell types will require identifying cell-type-specific markers for different subclasses of GPNs (Snell, 2022).

Furthermore, this caveat implies that the proportion of ROIs responding to a given stimulus does not necessarily correspond to the proportion of GPNs responding to that stimulus. Thus, it is not possible to draw a strong conclusion about the relative number of GPNs exhibiting each observed response profile. Although ROIs responsive to both bitter and sweet tastants are described, the underlying GPNs with this response profile may be few in number. However, even one such neuron type would be notable, as no GRNs have been reported to encode both sweet and bitter taste (Snell, 2022).

The range of tastant responses observed suggests that second-order gustatory neurons may exhibit more diverse tuning profiles than first-order neurons. For example, some GPNs appear to encode both bitter taste and either water or mechanosensory features of the stimulus. Although weak calcium responses to water have been observed in bitter GRNs, responses of similar strength to bitter and water have been observed in many GPN ROIs. This suggests that GPNs may either nonlinearly transform the bitter GRN signal or perhaps integrate sensory inputs across different taste qualities and even across different sensory modalities. Additionally, the relative magnitudes of the bitter onset and offset responses varied across regions, with some responding similarly to these features and others responding far more strongly to offset. A similar dichotomy has not been observed in GRNs. Thus, these two types of GPN bitter responses likely arise from different circuit-level mechanisms. Furthermore, contrary to observed responses in GRNs, more homogeneity has been observed in GPN responses to bitter tastants than to sweet ones. These results suggest that substantial signal transformation occurs in the transition from the first to the second order of the taste circuit. (Snell, 2022).

The discovery of a bitter offset response is supported by an additional report of this phenomenon, along with another report of an offset response of sweet GRNs to lactic acid. The fact that the fly actively encodes onset and offset of some tastants raises questions about the relevance of these responses for the animal. Distinct onset and offset responses have been observed in other sensory modalities, such as vision. In vision, for example, stimulus offset may serve as a cue for the motion of an object. Bitter offset might similarly signal a change in the fly's gustatory environment. In the wild, flies can encounter a mixture of noxious and nutritious substances at a single feeding site: Drosophila feed on microbes that grow on decaying plant matter, and some of these microbes metabolize plant-derived compounds that are normally toxic to flies, thereby making them safe for the fly's consumption (Snell, 2022).

As many plant toxins are bitter, a fly sampling such a food source would encounter a series of bitter and non-bitter patches; a bitter offset response could thus signal movement off of a toxic substrate and onto a nutritive microbial one (Snell, 2022).

The data suggests that different bitter-responsive GPNs exhibit a different balance of onset- and offset-driven activity. Separate neural channels differentially encoding bitter onset and offset may endow the fly with greater sensitivity to changes in bitter concentration. In primate vision, separate ON and OFF pathways have been proposed to enable metabolically efficient coding of intensity changes to enhance contrast sensitivity and increase the dynamic range of the system (Snell, 2022).

Likewise, the cockroach has separate olfactory sensory neurons tuned to odor onset and offset, which increase the animal's ability to detect small concentration changes. The bitter offset pathway in Drosophila might perform a similar function, enabling the fly to efficiently navigate concentration gradients of bitter compounds on heterogeneous food sources. Finally, as this parallel ON/OFF pathway circuit motif has been observed in several sensory systems, including vision, olfaction, touch, and thermosensation, it may have general utility in sensory coding (Snell, 2022).

An alternative explanation is that bitter onset and offset may have opposing hedonic valences to the fly. The cessation of punishment is known to act as a reward in conditioning experiments (Snell, 2022).

Because bitter taste is aversive, bitter offset may thus be interpreted as rewarding. Separating these components of the bitter taste response may allow for differential input to the dopaminergic neurons of the mushroom body that mediate aversive and appetitive conditioning. The intriguing observation that the bitter offset response overlaps with the sweet response in lateral regions is consistent with this idea. If there is a GPN that underlies both the bitter offset and sweet responses in this area, this neuron may functionally encode the valence of the stimulus. Yet, even if these responses to bitter offset and sweet are caused by separate GPNs, their overlapping projections in this region may indicate that these GPNs converge onto common third-order neurons in the circuit. The results suggest that some GPNs integrate sweet and bitter taste information. Such integration may indicate that a population coding model is operative in the gustatory system of the fly. However, determining the coding scheme implemented by the gustatory system will require further investigation, including a determination of how that code is read out by downstream regions. It remains to be seen how downstream circuits use the taste information encoded by GPNs to sculpt the behavior of the fly (Snell, 2022).

A leaky integrate-and-fire computational model based on the connectome of the entire adult Drosophila brain reveals insights into sensorimotor processing

This study describes a leaky integrate-and-fire computational model of the entire Drosophila brain, based on neural connectivity and neurotransmitter identity, to study circuit properties of feeding and grooming behaviors. Activation of sugar-sensing or water-sensing gustatory neurons in the computational model accurately predicts neurons that respond to tastes and are required for feeding initiation. Computational activation of neurons in the feeding region of the Drosophila brain predicts those that elicit motor neuron firing, a testable hypothesis that this study validated by optogenetic activation and behavioral studies. Moreover, computational activation of different classes of gustatory neurons makes accurate predictions of how multiple taste modalities interact, providing circuit-level insight into aversive and appetitive taste processing. This computational model predicts that the sugar and water pathways form a partially shared appetitive feeding initiation pathway, which calcium imaging and behavioral experiments confirmed. Additionally, this model was applied to mechanosensory circuits and found that computational activation of mechanosensory neurons predicts activation of a small set of neurons comprising the antennal grooming circuit that do not overlap with gustatory circuits, and accurately describes the circuit response upon activation of different mechanosensory subtypes. These results demonstrate that modeling brain circuits purely from connectivity and predicted neurotransmitter identity generates experimentally testable hypotheses and can accurately describe complete sensorimotor transformations (Shiu, 2023).

Taste cues elicit prolonged modulation of feeding behavior in Drosophila

Taste cues regulate immediate feeding behavior, but their ability to modulate future behavior has been less well studied. Pairing one taste with another can modulate subsequent feeding responses through associative learning, but this requires simultaneous exposure to both stimuli. This study investigated whether exposure to one taste modulates future responses to other tastes even when they do not overlap in time. Using Drosophila, it was found that brief exposure to sugar enhanced future feeding responses, whereas bitter exposure suppressed them. This modulation relies on neural pathways distinct from those that acutely regulate feeding or mediate learning-dependent changes. Sensory neuron activity was required not only during initial taste exposure but also afterward, suggesting that ongoing sensory activity may maintain experience-dependent changes in downstream circuits. Thus, the brain stores a memory of each taste stimulus after it disappears, enabling animals to integrate information as they sequentially sample different taste cues that signal local food quality (Deere, 2022).

Meeting a threat of the Anthropocene: Taste avoidance of metal ions by Drosophila

The Anthropocene Epoch poses a critical challenge for organisms: they must cope with new threats at a rapid rate. These threats include toxic chemical compounds released into the environment by human activities. This study examined elevated concentrations of heavy metal ions as an example of anthropogenic stressors. The fruit fly Drosophila avoids nine metal ions when present at elevated concentrations that the flies experienced rarely, if ever, until the Anthropocene. This study characterized the avoidance of feeding and egg laying on metal ions, and this study identified receptors, neurons, and taste organs that contribute to this avoidance. Different subsets of taste receptors, including members of both Ir (Ionotropic receptor) and Gr (Gustatory receptor) families contribute to the avoidance of different metal ions. Metal ions activate certain bitter-sensing neurons and inhibit sugar-sensing neurons. Some behavioral responses are mediated largely through neurons of the pharynx. Feeding avoidance remains stable over 10 generations of exposure to copper and zinc ions. Some responses to metal ions are conserved across diverse dipteran species, including the mosquito Aedes albopictus. These results suggest mechanisms that may be essential to insects as they face challenges from environmental changes in the Anthropocene (Xiao, 2022).

Alkaline taste sensation through the alkaliphile chloride channel in Drosophila

The sense of taste is an important sentinel governing what should or should not be ingested by an animal, with high pH sensation playing a critical role in food selection. This study explored the molecular identities of taste receptors detecting the basic pH of food using Drosophila melanogaster as a model. A chloride channel has been identified, named alkaliphile (Alka), that is both necessary and sufficient for aversive taste responses to basic food. Alka forms a high-pH-gated chloride channel and is specifically expressed in a subset of gustatory receptor neurons (GRNs). Optogenetic activation of alka-expressing GRNs is sufficient to suppress attractive feeding responses to sucrose. Conversely, inactivation of these GRNs causes severe impairments in the aversion to high pH. Altogether, this discovery of Alka as an alkaline taste receptor lays the groundwork for future research on alkaline taste sensation in other animals (Mi, 2023).

The sense of taste, which lies at the interface between the interior and the exterior of the body, ensures that food of nutritional value is consumed, whereas potentially noxious substances are rejected. Acids and bases are opposite chemical substances that are broadly present in food sources. While it is generally accepted that animals use sour taste to assess the acidity, or low pH, of food, whether animals have a taste modality to sense the basicity, or high pH, of food is a long-standing open question. Given that acid, or low pH, has a sour taste, it would be logical to hypothesize that base, or high pH, also produces a gustatory sensation (Mi, 2023).

Previous studies in humans and animal models provide initial clues to the existence of alkaline taste sensations. In the 1940s, psychophysical studies conducted on human participants reported that the tip portion of the tongue, which is enriched with taste buds, exhibits a higher sensitivity to sodium hydroxide (NaOH) than the mid-dorsal part of the tongue with few taste buds, implying that basic solutions may have taste qualities. Furthermore, electrophysiological recordings of the taste nerves in cats document that a subpopulation of the chorda tympani nerves, which relay taste input from the oral cavity to the brain, can be activated by high pH but not by other stimuli such as temperature, indicating that cats interpret the stimulation by high-pH solutions as a sense of taste rather than as an irritating noxious chemical. Moreover, insects such as beetles show robust avoidance of alkaline environments associated with unfavorable habitats and food sources. The beetle's high-pH sensitivity is mediated by pH receptor cells in the beetle's taste organ, which display increased firing activities proportional to basic pH10. Collectively, these earlier studies imply but do not resolve whether the high-pH sensation is a discrete taste modality. Since these early studies, little mechanistic research had been conducted to unravel the molecular and cellular underpinnings of alkaline taste sensation. In particular, the molecular identities of taste receptors and taste receptor cells orchestrating alkaline taste sensation had not yet been established in animals (Mi, 2023).

Like mammals, the fruit fly, Drosophila melanogaster, employs different classes of taste receptors to detect sugars, salts, acids , bitter substances and other chemicals. Given that flies have such a remarkable capability to detect a wide range of substances through taste, it was inferred that flies are also able to sense the alkalinity of food. Indeed, a vital fly gene dubbed alkaliphile (alka) was discovered that regulates gustatory responses to strong alkalinity. Molecular genetic studies revealed that alka is both necessary and sufficient to avoid highly basic food. Extensive electrophysiological assays were performed, and it was discovered that the Alka protein forms a chloride (Cl-) channel, which is specifically activated by hydroxide (OH-). Moreover, this study found that alka is expressed in a subset of gustatory receptor neurons (GRNs) in the peripheral taste organ. At the sensory cell level, alka-expressing GRNs are both necessary and sufficient for the rejection of strongly alkaline food. In summary, this work establishes Alka as the long-sought-after taste receptor responsible for sensing the basic pH of food (Mi, 2023).

As most organisms' optimal physiological activities and enzymatic reactions can occur only in a narrow pH range (around 7.4), excessively high pH can disrupt the acid-base balance and lead to alkalosis of the body, a life-threatening condition. There are many places where organisms can encounter high-pH conditions in their ecosystem such as in the food and water they may consume. Moreover, many naturally occurring toxins, including alkaloids and aqueous ammonia, are quite basic. Ethological research has documented well-defined behavioral responses to basic pH in a large variety of species, such as nematodes, insects, fish and mammals (Mi, 2023).

The impact of alkaline pH on fly physiology has been well documented across a variety of studies. It has been reported that the fly's overall body pH exhibits a dynamic change over the course of development: the body pH starts as approximately neutral at the larval stage and gradually becomes more acidic as the animal advances to the pupal and adult stages. In addition, there is remarkable variation in the luminal pH at different regions of the fly midgut, with its posterior segment more alkaline. As a result, alkaline pH sensation is strongly implicated in fly health and longevity. Flies fed moderately alkaline diets display reduced lifespan and survivability. Furthermore, chronic exposure to a highly alkaline environment impairs development, shortens lifespan and causes lethality. Consequently, female flies robustly avoid alkaline substrates when selecting a location to deposit eggs. Taken together, alkaline pH sensation serves as an essential self-protection strategy that enables flies and other animal species to effectively avoid toxic environments during food foraging and habitat selection. It is proposed that alkaline taste dramatically increases the fly's evolutionary fitness by enhancing its survival, growth and reproduction (Mi, 2023).

Alkaloids taste very bitter and are poorly soluble, meaning their strong bitterness can confound the investigation of the taste component solely contributed by high pH. In addition, ammonia is highly volatile and can interfere with the contact-dependent taste sensation by strongly activating the olfactory system. To avoid these limitations, NaOH and Na2CO3 were chosen in feeding assays because these two basic substances are simple and ecologically relevant. Molecular genetic study demonstrates that flies have the capability to avoid highly basic food mainly through their gustatory system. Based on these findings, the fly shows specific and robust taste responses to basic food, suggesting that it is a well-suited model organism to explore alkaline taste sensation. Several lines of evidence are provided supporting that Alka is a taste sensor specifically tuned to high pH. First, alka is expressed in a subset of GRNs in the fly labellum, which functions similarly to the mammalian tongue. The alka-expressing GRNs are both necessary and sufficient to detect basic food. Second, alka1 mutant flies show remarkably decreased aversion to basic food. Third, using in vivo electrophysiological analyses, we found that the S-type sensilla of alka1 flies exhibit considerably reduced responses to high pH but maintain normal responses to bitter compounds such as caffeine. Last, misexpressing alka in sweet GRNs attracts the flies to the otherwise aversive basic food. In summary, this work establishes that Alka is a bona fide taste receptor dedicated to sensing food basicity in Drosophila (Mi, 2023).

Over the past 20 years, the fly model has made enormous contributions to the discovery of various classes of ionotropic taste receptors. These include GRs, IRs, otopetrin (Otop) channels, TRP channels and the degenerin/epithelial Na+ channels (DEG/ENaC) or pickpocket (Ppk) channels. Notably, Alka shows remarkable differences from previously identified taste receptors because it is an anion Cl- channel, whereas the IRs, Otops, TRPs and Ppks are cation channels. Therefore, it is believed that identification of the Cl- channel Alka as an alkaline taste receptor is substantial and innovative, adding another class of receptors to the diverse taste receptor repertoire in Drosophila (Mi, 2023).

The combination of optogenetic and intersectional genetic approaches enables selective manipulation of the activity of alka-expressing GRNs at will. Acutely activating alka-expressing GRNs alone is sufficient to trigger aversive taste responses, supporting the conclusion that alka-expressing GRNs are both necessary and sufficient for alkaline taste sensation. Like bitter taste, alkaline taste can suppress sweet taste responses. Although the neuronal mechanism underlying this cross-modal inhibition is currently not clear, our assay serves as a robust behavioral paradigm allowing us to screen for the higher-order neurons mediating the integration between alkaline taste and sweet taste (Mi, 2023).

The membrane potential of a living cell is established and maintained by the flows of different cation and anion species, such as Na+, K+ and Cl-, across the plasma membrane. While extensive studies have focused on the roles played by cations and cation channels in the regulation of membrane excitability, the functional importance of the anion Cl- and Cl- channels had been overlooked. In recent years, Cl- has emerged as an essential player in electrical signal transduction and great progress has been made toward the molecular identification of various types of Cl-channels such as the acid-sensitive Cl- channel PAC/TMEM206. The curren work demonstrates that Alka functions as a distinct Cl- channel because it is strongly activated by highly basic pH (11.9). In contrast to other LGCCs such as pHCl44, Alka is insensitive to slightly basic or acidic pH. Thus, Alka is well suited to act as an external sensor to detect high pH in the natural ecosystem, which can become noxiously basic due to excessive carbonation or nitration (Mi, 2023).

Protein sequence analyses suggest that Alka is distantly related to glycine-gated Cl- channels in vertebrates. Nevertheless, this study found that Alka functions as a taste receptor rather than acting as a glycine or GABA receptor, as Alka is not activated by physiological concentrations of glycine or GABA. In support of this notion, Alka is selectively expressed in the chemosensory organs. The functional divergence of Alka from canonical GlyRs is reminiscent of the evolution of the ionotropic (IR) family in Drosophila, which adopted chemosensory functions as olfactory or gustatory receptors in the periphery rather than as glutamate neurotransmitter receptors in the brain. In summary, by discovering that Alka is a high-pH-activated Cl- channel in taste receptor cells, this work provides important insights into the highly diversified nature of Cl- channels in terms of gating and function (Mi, 2023).

Upon further interrogation of the channel properties of Alka using patch-clamp experiments, it is concluded that Alka mainly conducts Cl-. This creates a logical quandary, as mature neurons in the brain usually experience an intracellular-facing Cl- gradient and the Cl- influx in mature neurons is hyperpolarizing instead of depolarizing; however, the answer lies in the unusual distribution of the Cl- gradient across the fly GRN. From the perspective of comparative physiology, the extracellular receptor lymph of the fly gustatory receptor neuron (GRN) is analogous to the saliva bathing taste receptor cells in humans, which contains lower levels of Cl- than that of blood plasma. Furthermore, like mammalian olfactory sensory neurons and airway epithelial cells, the Cl- concentration of the extracellular receptor lymph seems to be even lower than in the cytosol of the taste receptor neuron in flies. Thus, Alka perfectly fits the unusual ionic milieu of fly GRNs. Based on this model, in response to OH- stimuli, Alka is induced to adopt an open conformation, which leads to the flow of intracellular Cl- out of the GRN. The Cl- efflux depolarizes the GRN and results in the production of action potentials, thereby enabling the animals to sense alkaline food. In summary, this work highlights the important roles played by Cl- and Cl- channels in regulating taste transduction (Mi, 2023).

In conclusion, it is proposed that Alka, which forms a previously unknown hydroxide-gated Cl- channel, is a taste receptor responsible for detecting alkaline food. As far as is known, Alka represents a notable Cl- channel identified in the animal kingdom, which is dedicated to sensing highly basic pH. Furthermore, by showing that basic pH has a discrete taste in Drosophila, this work resolves a long-standing debate as to whether alkaline taste really exists. Moreover, this study has demonstrated that Alka functions as a Cl- channel that is potently activated by highly alkaline pH. Therefore, this molecular discovery of Alka as a taste receptor of alkaline pH advances understanding of the roles and modes of activation of Cl- channels. Finally, given that detecting basic pH is crucial for food selection across many different species, this work provides the conceptual basis for investigating the neuronal mechanisms underlying alkaline taste sensations in other animals (Mi, 2023).

Characterization and its implication of a novel taste receptor detecting nutrients in the honey bee, Apis mellifera

Umami taste perception indicates the presence of amino acids, which are essential nutrients. Although the physiology of umami perception has been described in mammals, how insects detect amino acids remains unknown except in Drosophila melanogaster. This study functionally characterized a gustatory receptor responding to L-amino acids in the western honey bee, Apis mellifera. Using a calcium-imaging assay and two-voltage clamp recording, it was found that one of the honey bee's gustatory receptors, AmGr10, functions as a broadly tuned amino acid receptor responding to glutamate, aspartate, asparagine, arginine, lysine, and glutamine, but not to other sweet or bitter compounds. Furthermore, the sensitivity of AmGr10 to these L-amino acids was dramatically enhanced by purine ribonucleotides, like inosine-5'-monophosphate (IMP). Contact sensory hairs in the mouthpart of the honey bee responded strongly to glutamate and aspartate, which house gustatory receptor neurons expressing AmGr10. Interestingly, AmGr10 protein is highly conserved among hymenopterans but not other insects, implying unique functions in eusocial insects (Lim, 2019).

Molecular Logic and Evolution of Bitter Taste in Drosophila

Taste systems detect a vast diversity of toxins, which are perceived as bitter. When a species adapts to a new environment, its taste system must adapt to detect new death threats. This study deleted each of six commonly expressed bitter gustatory receptors (Grs) from Drosophila melanogaster. Systematic analysis revealed that requirements for these Grs differed for the same tastant in different neurons and for different tastants in the same neuron. Responses to some tastants in some neurons required four Grs, including Gr39a. Deletions also produced increased or novel responses, supporting a model of Gr-Gr inhibitory interactions. Coexpression of four Grs conferred several bitter responses to a sugar neuron. Bitter coding was examined in three other Drosophila species. Major evolutionary shifts were found. One shift depended on the concerted activity of seven Grs. This work shows how the complex logic of bitter coding provides the capacity to detect innumerable hazards and the flexibility to adapt to new ones (Dweck, 2019).

Cucurbitacin B Activates Bitter-Sensing Gustatory Receptor Neurons via Gustatory Receptor 33a in Drosophila melanogaster

The Gustatory system enables animals to detect toxic bitter chemicals, which is critical for insects to survive food induced toxicity. Cucurbitacin is widely present in plants such as cucumber and gourds that acts as an anti-herbivore chemical and an insecticide. Cucurbitacin has a harmful effect on insect larvae as well. Although various beneficial effects of cucurbitacin such as alleviating hyperglycemia have also been documented, it is not clear what kinds of molecular sensors are required to detect cucurbitacin in nature. Cucurbitacin B, a major bitter component of bitter melon, was applied to induce action potentials from sensilla of a mouth part of the fly, labellum. This study identified that only Gr33a is required for activating bitter-sensing gustatory receptor neurons by cucurbitacin B among available 26 Grs, 23 Irs, 11 Trp mutants, and 26 Gr-RNAi lines. The difference between control and Gr33a mutant was investigated by analyzing binary food choice assay. Toxic effect of Cucurbitacin B was measured over 0.01 mM range. These findings uncover the molecular sensor of cucurbitacin B in Drosophila melanogaster. It is proposed that the discarded shell of Cucurbitaceae can be developed to make a new insecticide (Rimal, 2020).

Robustness and plasticity in Drosophila heat avoidance

Simple innate behavior is often described as hard-wired and largely inflexible. This study shows that the avoidance of hot temperature, a simple innate behavior, contains unexpected plasticity in Drosophila. First, it was demonstrate that hot receptor neurons of the antenna and their molecular heat sensor, Gr28B.d, are essential for flies to produce escape turns away from heat. High-resolution fly tracking combined with a 3D simulation of the thermal environment shows that, in steep thermal gradients, the direction of escape turns is determined by minute temperature differences between the antennae (0.1°-1 °C). In parallel, live calcium imaging confirms that such small stimuli reliably activate both peripheral thermosensory neurons and central circuits. Next, based on these measurements, a fly/vehicle model with two symmetrical sensors and motors (a "Braitenberg vehicle") was evolved which closely approximates basic fly thermotaxis. Critical differences between real flies and the hard-wired vehicle reveal that fly heat avoidance involves decision-making, relies on rapid learning, and is robust to new conditions, features generally associated with more complex behavior (Simoes, 2021).

Cellular and molecular basis of IR3535 perception in Drosophila

IR3535 is among the most widely used synthetic insect repellents, particularly for the mitigation of mosquito-borne diseases such as malaria, yellow fever, dengue and Zika, as well as to control flies, ticks, fleas, lice and mites. These insects are well-known vectors of deadly diseases that affect humans, livestock and crops. Moreover, global warming could increase the populations of these vectors. This study performed IR3535 dose-response analyses on Drosophila melanogaster, a well-known insect model organism, using electrophysiology and binary food choice assays. The findings indicated that bitter-sensing gustatory receptor neurons (GRNs) are indispensable to detect IR3535. Further, potential candidate gustatory receptors were screened, among which GR47a was identified as a key molecular sensor. IR3535 concentrations in the range 0.1-0.4% affected larval development and mortality. In addition, N,N-diethyl-m-toluamide (DEET, another commonly used insecticide) was found to exert synergistic effects when co-administered with IR3535. These findings confirmed that IR3535 directly activates bitter-sensing GRNs, which are mediated by GR47a. This relatively safe and highly potent insecticide can be largely used in combination with DEET to increase its efficiency to protect livestock and crops. Collectively, these findings suggest that the molecular sensors elucidated herein could be used as targets for the development of alternative insecticides (Shrestha, 2021).

A subset of brain neurons controls regurgitation in adult Drosophila melanogaster

Taste is essential for animals to evaluate food quality and make important decisions about food choice and intake. How complex brains process sensory information to produce behavior is an essential question in the field of sensory neurobiology. Currently, little is known about higher order taste circuits in the brain as compared to those of other sensory systems. This study used the common vinegar fly, Drosophila melanogaster, to screen for candidate neurons labeled by different transgenic GAL4 lines in controlling feeding behaviors. Activation of one line (VT041723-GAL4) produces "proboscis holding" behavior (extrusion of the mouthpart without withdrawal). Further analysis shows that the proboscis holding phenotype indicates an aversive response, since flies pre-fed with either sucrose or water prior to neuronal activation exhibit regurgitation. Anatomical characterization of VT041723-GAL4 labeled neurons suggests that they receive sensory input from peripheral taste neurons. Overall, this study identifies a subset of brain neurons labeled by VT041723-GAL4 that may be involved in a taste circuit that controls regurgitation (Chen, 2019).

Molecular basis of fatty acid taste in Drosophila

Behavioral studies have established that Drosophila appetitive taste responses towards fatty acids are mediated by sweet sensing Gustatory Receptor Neurons (GRNs). This study shows that sweet GRN activation requires the function of the Ionotropic Receptor genes IR25a, IR76b and IR56d. The former two IR genes are expressed in several neurons per sensillum, while IR56d expression is restricted to sweet GRNs. Importantly, loss of appetitive behavioral responses to fatty acids in IR25a and IR76b mutant flies can be completely rescued by expression of respective transgenes in sweet GRNs. Interestingly, appetitive behavioral responses of wild type flies to hexanoic acid reach a plateau at ~1%, but decrease with higher concentration, a property mediated through IR25a/IR76b independent activation of bitter GRNs. With previous report on sour taste, these studies suggest that IR-based receptors mediate different taste qualities through cell-type specific IR subunits (Ahn, 2017).

IR genes have emerged as a second large gene family encoding chemoreceptors in insects. In the Drosophila olfactory system, IRs function as multimeric receptors in coeloconic olfactory sensory neurons (OSN) and are thought to sense volatile carboxylic acids, amines and aldehydes. Expression analyses have shown that each coeloconic OSN expresses up to four IR genes, including high levels of either IR8a or IR25a. IR25a and IR8a are distinct from other IRs in that they are more conserved to each other and iGluRs, and they share a long amino terminal domain absent in all other IRs. These observations, along with functional analyses of basiconic olfactory neurons that express combinations of IR genes, led to a model in which IR based olfactory receptors are tetrameric complexes thought to consist of up to three different subunits that contain at least one core unit (IR8a or IR25a) and two additional IRs that determine ligand binding specificity. The findings presented in this paper expand this concept to taste receptors that sense fatty acids through the sweet GRNs found in tarsal taste sensilla (Ahn, 2017).

This analysis extends the multimodal role of IR25a and IR76b to the taste systems. Consistent with gene expression arrays, this paper shows that up to three GRNs, including many sweet and bitter GRNs, co-express IR25a and IR76b. Functional studies have established a novel role for these two IR proteins in fatty acid taste, which revealed that these two subunits are not only critically important to elicit Proboscis Extension Reflex (PER) responses in flies when challenged with fatty acids, but are also necessary for fatty acid induced Ca2+ increases in tarsal sweet GRNs. Based on these findings and with consideration of their established role in other sensory systems, it is proposed that IR25a and IR76b play central roles in sweet GRNs in a multimeric receptor complex for initiating appetitive taste behavior to these chemicals. Intriguingly, both genes are also co-expressed in two other GRNs of most tarsal taste sensilla, strongly arguing for additional taste functions. While the subset of tarsal bitter GRNs activated by hexanoic acid does not require either gene, the third GRN (the sour GRN) is narrowly tuned to acids in an IR25a/IR76b dependent manner. These observations suggest that modality specific IRs are likely expressed in a cell-type specific fashion whereby they complement IR25a/IR76b to function as either a fatty acid or a sour taste receptor. Indeed, the screen identified IR56d, a gene that is expressed in sweet GRNs of tarsal taste sensilla, as a likely candidate encoding an IR subunit specific for a fatty acid taste receptor. It remains to be seen whether IR25a, IR76b and IR56d comprise all subunits that constitute this receptor or whether yet additional IRs are necessary to mediate responses to these chemicals (Ahn, 2017).

The fact that different food chemicals can activate a single class of neurons raises the question how flies discriminate between sugars and fatty acids. First, the difference is noted in sensitivity of appetitive GRNs to sugars and fatty acids, respectively: The most responsive GRN for sugars is the one associated with the 5v sensilla, followed by that with the 5s and finally the 5b sensilla, while the responsiveness for fatty acids is the reverse (5b > 5s > 5v). Second, fatty acids induces weaker PER responses from stimulation of the labial palps as opposed to tarsi, while sugars induce equally strong PER responses from stimulation of either taste organ. Third, at least some fatty acids activate bitter GRNs, and hence, generate more complex activation patterns in the brain than sugars, which are not known to activate neurons other than sweet GRNs. These properties may provide a rationale for differential coding of these two classes of chemicals in the brain. Finally, sugars but not fatty acids are soluble in water, and hence, the specific solvents in which these chemicals are presented provides different textural quality, which was recently shown to play a role in taste perception (Ahn, 2017).

NorpA, which encodes a phospholipase C (PLC), plays a critical role in sweet GRNs for appetitive feeding responses to fatty acids, but it is dispensable for behavioral responses to sugars. This study found that its absence also selectively abolishes Ca2+ responses to fatty acids, but not sugars, in sweet cells. NorpA is known for its role as downstream effector of G-protein coupled receptors in the fly's visual system, but interestingly it is also required for olfactory responses in neurons of the maxillary palps, which express ORs that are thought to function as ligand-gated ion channels. It is noted that fatty acid taste in mammals is in part mediated by two G-protein coupled receptors, GPR40 and GPR120, and that one of these (GPR120) was found to signal through a phospholipase C. Thus, future studies will be necessary to gain insights for how PLC mediates chemosensory responses through ORs and the phylogenetically unrelated IRs (Ahn, 2017).

Multimeric IR based receptors were recently shown to be required in non-chemosensory processes. Specifically, Dorsal Organ Cool Cells (DOCCs) located in the larval brain, express and require the function of three IRs (IR21a, IR25a and IR93a), thereby allowing larvae to avoid temperatures below ~20°C. Similarly, two sets of cells in the antennal sacculus of adult flies, requiring the functions of IR25a and IR93a and either IR40a or IR68a, were shown to mediate a fly's preferred humidity environment, which is generally in the dry range, but is also dependent on the fly's hydration state. Intriguingly, these non-chemosensory IR complexes share a common theme with the fatty acid and carboxylic acid taste receptors in that they all require a core unit (IR25a) and two additional IRs that mediate specificity for a particular stimulus type (i.e., temperature, humidity, fat, acid) (Ahn, 2017).

An IR76b based sodium channel and an IR76b based amino acid receptor appear to lack an obligate core unit (IR25a or IR8a) found in olfactory receptors or fatty acid and carboxylic acid taste receptors. The IR76b sodium channel mediates salt responses in a heterologous systems independently of any other IRs, while a proposed multimeric IR76b containing receptor mediates amino acids taste in wild type and IR25a mutant flies (IR8a is not expressed in taste neurons). It will be interesting to elucidate the compositions of complete IR based amino acid and sour taste receptors, and (with regard of amino acid receptors) to identify the neurons that mediate this taste modality (Ahn, 2017).

Central relay of bitter taste to the protocerebrum by peptidergic interneurons in the Drosophila brain

Bitter is a taste modality associated with toxic substances evoking aversive behaviour in most animals, and the valence of different taste modalities is conserved between mammals and Drosophila. Despite knowledge gathered in the past on the peripheral perception of taste, little is known about the identity of taste interneurons in the brain. This study shows that hugin neuropeptide-containing neurons in the Drosophila larval subesophageal zone are necessary for avoidance behaviour to caffeine, and when activated, result in cessation of feeding and mediates a bitter taste signal within the brain. Hugin neuropeptide-containing neurons project to the neurosecretory region of the protocerebrum and functional imaging demonstrates that these neurons are activated by bitter stimuli and by activation of bitter sensory receptor neurons. The study proposes that hugin neurons projecting to the protocerebrum act as gustatory interneurons relaying bitter taste information to higher brain centres in Drosophila larvae (Hückesfeld, 2016).

The bitter taste rejection response is important for all animals that encounter toxic or harmful food in their environment. This study showed that the hugin neurons in the Drosophila larval brain function as a relay between bitter sensory neurons and higher brain centres. Strikingly, activation of the hugin neurons, located in the subesophageal zone, made the animals significantly more insensitive to substrates with negative valence like bitter (caffeine) and salt (high NaCl), as well as positive valence like sweet (fructose). In other words, when the hugin neurons are active these animals 'think' they are tasting bitter and therefore become insensitive to other gustatory cues. This is in line with observations made in mice, in which optogenetically activating bitter cortex neurons caused animals to avoid an empty chamber illuminated with blue light. In this situation, although mice do not actually taste something bitter, they avoid the empty chamber since the bitter perception has been optogenetically induced in the central nervous system (CNS) and the mice 'think' they are tasting a bitter substance (Hückesfeld, 2016).

In previous work, activation of all hugin neurons led to behavioural and physiological phenotypes such as decreased feeding, decrease in neural activity of the antennal nerve (AN), and induction of a wandering-like behaviour (Schoofs, 2014). The neurons responsible specifically to those that project have now been pinpointed to the protocerebrum. These neurons not only respond to bitter stimuli, but also show a concentration dependent increase in calcium activity in response to caffeine. Dose dependent coding of bitter taste stimuli was previously shown to occur in peripheral bitter sensory neurons, where bitter sensilla exhibit dose dependent responses to various bitter compounds. Larvae in which the huginPC neurons have been ablated still showed some avoidance to caffeine. Whether this implies the existence of other interneurons being involved in caffeine taste processing remains to be determined. Interestingly, the huginPC neurons are inhibited when larvae taste other modalities like salt (NaCl), sugar (fructose) or protein (yeast). This may indicate that taste pathways in the brain are segregated, but influence each other, as previously suggested (Hückesfeld, 2016).

Bitter compounds may be able to inhibit the sweet-sensing response to ensure that bitter taste cannot be masked by sweet tasting food. This provides an efficient strategy for the detection of potentially harmful or toxic substances in food. For appetitive tastes like fructose and yeast, bitter interneurons neurons like the huginPC neurons in the CNS may become inhibited to ensure appropriate behaviour to pleasant food. Salt is a bivalent taste modality, that is, low doses of salt drive appetitive behaviour, whereas high doses of salt are aversive to larval and adult Drosophila. Inhibition of huginPC neurons when larvae are tasting salt might be due to a different processing circuit for different concentrations of salt and the decision to either take up low doses or reject high doses (Hückesfeld, 2016).

Taken together, it is proposed that hugin neuropeptide neurons projecting to the protocerebrum represent a hub between bitter gustatory receptor neurons and higher brain centres that integrate bitter sensory information in the brain, and through its activity, influences the decision of the animal to avoid a bitter food source. The identification of second order gustatory neurons for bitter taste will not only provide valuable insights into bitter taste pathways in Drosophila, but may also help in assigning a potentially novel role of its mammalian homologue, Neuromedin U, in taste processing (Hückesfeld, 2016).

Mechanosensory neurons control sweet sensing in Drosophila

Animals discriminate nutritious food from toxic substances using their sense of taste. Since taste perception requires taste receptor cells to come into contact with water-soluble chemicals, it is a form of contact chemosensation. Concurrent with that contact, mechanosensitive cells detect the texture of food and also contribute to the regulation of feeding. Little is known, however, about the extent to which chemosensitive and mechanosensitive circuits interact. This study shows Drosophila prefers soft food at the expense of sweetness and that this preference requires labellar mechanosensory neurons (MNs) and the mechanosensory channel Nanchung. Activation of these labellar MNs causes GABAergic inhibition of sweet-sensing gustatory receptor neurons, reducing the perceived intensity of a sweet stimulus. These findings expand understanding of the ways different sensory modalities cooperate to shape animal behaviour (Jeong, 2016).

Animals must eat to survive, but not all food sources are equally desirable. Animals use their sense of taste to discriminate nutritious foods and toxic substances. Although a food's taste is a major determinant of its acceptability, animals must assess a food's visual appearance, smell, temperature and texture as well. What humans call 'flavour' is actually a complex multisensory picture of a food's general desirability. In fact, each person has direct experience with the interaction of multiple sensory modalities in the general perception of food quality. Who hasn't noticed a change in a food's flavour on catching a cold severe enough to block their sense of smell (Jeong, 2016)?

Despite its obvious importance, the mechanisms by which multimodal sensory information is incorporated into feeding decisions are not well understood. Psychologists and neuroscientists have begun to explore the ways the individual channels of sensory input affect the perception of flavour, but understanding of cross-modal interactions lags behind. This is partly due to difficulties with parsing the individual components that make up the gestalt of flavour perception, and partly due to technical difficulties associated with the controlled delivery of precisely defined multimodal stimuli. Because of these difficulties, it is suggested that the exploration of simpler model systems can help extend understanding of the multisensory perception of flavour that directs feeding decisions (Jeong, 2016).

In particular, this study concerns itself with the ways neural circuits integrate taste and texture information. Texture is a product of mechanosensation. Animals, of course, use mechanosensory information to help determine their food's precise location, but it is the food's physical properties (for example, its hardness or viscosity) that contribute to determining its palatability. Several studies have demonstrated flavour perception can be altered by a food's hardness or viscosity. In particular, A negative correlation between food viscosity and perceived sweetness has been found in humans; as a food's viscosity increases, it is perceived as being less sweet. Since these sorts of interactions exist, they presumably offer some utility, but the neural mechanisms by which they help coordinate appropriate feeding behaviours are not understood in any system (Jeong, 2016).

Drosophila presents an especially attractive system for exploring interactions between taste and mechanosensation with regard to feeding decisions. Although both taste and olfaction are forms of chemosensation, because odorants are airborne and tastants are water-soluble, only taste requires contact with the stimulus. Indeed, while Drosophila olfactory sensilla lack mechanosensory neurons (MNs), the gustatory receptor neurons (GRNs) of each taste sensillum are accompanied by a MN. Thus, as a fly feeds the sensory sensilla on its labellum (mouthparts) unavoidably receive concurrent taste and mechanical activation. In addition, the molecular genetic tools available in the fly allow examination of the role each type of sensory information plays in directing feeding behaviour via selective activation or inactivation of each class of sensory neuron (Jeong, 2016).

This paper presents an exploration of the circuit-level interactions between the perception of gustatory and mechanical stimuli that help direct feeding decisions in Drosophila. It was discovered that Drosophila prefer soft food at the expense of sweetness and that this preference depends on labellar MNs and their expression of the mechanosensory channel Nanchung. Activation of these labellar MNs attenuates the perceived intensity of a sweet stimulus by suppressing the calcium responses of sweet GRN termini via the inhibitory neurotransmitter GABA. These findings expand understanding of the mechanisms by which the neural circuits responsible for the various modes of sensory perception can cooperate to shape animal behaviour (Jeong, 2016).

This study has uncovered a mechanism by which tactile sensation regulates feeding by controlling the presynaptic gain of phagostimulatory GRNs. Activation of MNs inhibits calcium responses in sweet GRNs via the inhibitory neurotransmitter GABA. This effect likely contributes to Drosophila's preference for ripe or overripe rather than fresh fruits, as both sweetness and hardness change with decay (Jeong, 2016).

The association of MNs with GRNs in labellar taste bristles and taste pegs was first observed several decades ago, but the physiologic significance of this association was never investigated. This study has shown labellar MNs produce GABA in the SEZ to inhibit signalling through the sweet GRNs. The activation and inhibition of R55B01-GAL4-expressing cells show similar effects on presynaptic gain in sweet GRNs as activation and inhibition of R41E11-GAL4-expressing cells and VT2692-GAL4-expressing cells. This implicates the taste bristle MNs labelled by all three of these lines rather than the taste peg MNs in the interaction between sweet sensing and mechanosensation. The projection of taste peg MNs to an area of the SEZ distinct from that innervated by sweet and bitter GRNs project further supports this idea (Jeong, 2016).

In flies, the tarsal segments of the legs also have chemosensory and mechanosensory sensilla that can be activated during food foraging. Two other groups recently explored the role these tarsal MNs play in behavioural regulation. Ramdya (2015) reported that tarsal MNs provide sensory information that drives collective behaviour, and Mann (2013) showed that tarsal MNs inhibit feeding via a population of thoracic ganglion interneurons. The fact that the R41E11-GAL4 and VT2692-GAL4 drivers used in this study are expressed not in the MNs of the legs but in their supporting cells, suggests the tarsal MNs play no role in food hardness detection. In further support of this conclusion, this study found inactivation of the tarsal MNs using Gr68a-GAL4 does not impair hardness-mediated food preference. Thus, it is clear the tarsal and labellar MNs play different roles in controlling animal behaviour (Jeong, 2016).

Although soft food preference is strongly affected by both silencing of the labellar MNs and the loss of Nan, both of these conditions still show a slight residual preference for soft food. This suggests the presence of another mechanosensory system involved in food hardness detection, perhaps the pharyngeal MNs or labellar multidendritic neurons (Jeong, 2016).

Despite being unable to detect any role for NompC in food hardness detection using a preference assay, NompC's expression in the labellar taste bristle MNs makes it a plausible secondary candidate for the labellar MN mechanosensor. In other words, while Nan may act as the mechanosensor in labellar MNs with NompC modulating its function, the reverse may also be true, as is the case in the chordotonal neurons (Jeong, 2016).

In Drosophila, GABABR2 is required in sweet GRN axon termini for the suppression of sweet responses by bitter stimuli when sweet and bitter tastants are mixed together. Knockdown of GABABR2 in sweet GRNs increases the PER to sugar as well as to sugar/bitter mixtures. In this study, knockdown of GABABR2 in sweet GRNs impairs soft food preference at the expense of sweetness, but it does not affect preference for sweetness in the absence of differences in food hardness. These data suggest sweet GRNs receive multiple GABAergic inputs from different sensory circuits (Jeong, 2016).

This study has shown taste-related mechanosensory information can inhibit sweet perception in the primary taste relay centre, the SEZ, but it remains unclear whether mechanosensation modulates the perception of sweet tastants only at the level of the GRNs or whether the tactile information is relayed to higher brain centres for integration. It will be interesting to see which other parts of the brain these MNs innervate and what other behaviours, apart from food hardness perception, they regulate. It will also be interesting to see whether these or any other MNs interact with taste information in any higher brain centres. During feeding, multiple modes of sensory information must be perceived and integrated to produce the perception of 'flavour'. This phenomenon is well-described in humans using mainly a psychophysiological approach, but the molecular mechanisms and neural circuits that produce it remain unclear. Using the Drosophila model system, this study has explored potential circuit motifs underlying multimodal sensory processing and has demonstrated an intriguing interaction between sweet GRNs and MNs that modulates feeding decision-making (Jeong, 2016).

Long-range projection neurons in the taste circuit of Drosophila

Taste compounds elicit innate feeding behaviors and act as rewards or punishments to entrain other cues. The neural pathways by which taste compounds influence innate and learned behaviors have not been resolved. This study identified three classes of taste projection neurons (TPNs) in Drosophila melanogaster distinguished by their morphology and taste selectivity. TPNs receive input from gustatory receptor neurons and respond selectively to sweet or bitter stimuli, demonstrating segregated processing of different taste modalities. Activation of TPNs influences innate feeding behavior, whereas inhibition has little effect, suggesting parallel pathways. Moreover, two TPN classes are absolutely required for conditioned taste aversion, a learned behavior. The TPNs essential for conditioned aversion project to the superior lateral protocerebrum (SLP) and convey taste information to mushroom body learning centers. These data identify taste pathways from sensory detection to higher brain that influence innate behavior and are essential for learned responses to taste compounds (Kim, 2017).

The taste response to ammonia in Drosophila

Ammonia is both a building block and a breakdown product of amino acids and is found widely in the environment. The odor of ammonia is attractive to many insects, including insect vectors of disease. The olfactory response of Drosophila to ammonia has been studied in some detail, but the taste response has received remarkably little attention. This study shows that ammonia is a taste cue for Drosophila. Nearly all sensilla of the major taste organ of the Drosophila head house a neuron that responds to neutral solutions of ammonia. Ammonia is toxic at high levels to many organisms, and it was found to have a negative valence in two paradigms of taste behavior, one operating over hours and the other over seconds. Physiological and behavioral responses to ammonia depend at least in part on Gr66a+ bitter-sensing taste neurons, which activate a circuit that deters feeding. The Amt transporter, a critical component of olfactory responses to ammonia, is widely expressed in taste neurons but is not required for taste responses. This work establishes ammonia as an ecologically important taste cue in Drosophila, and shows that it can activate circuits that promote opposite behavioral outcomes via different sensory systems (Delventhal, 2017).


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date revised: 25 August 2022

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