InteractiveFly: GeneBrief

Octopamine receptor in mushroom bodies: Biological Overview | References


Gene name - Octopamine receptor in mushroom bodies

Synonyms -

Cytological map position - 92F6-92F8

Function - receptor

Keywords - brain, oogenesis, ovulation induced by mating, adult oviduct epithelium

Symbol - Oamb

FlyBase ID: FBgn0024944

Genetic map position - 3R:16,514,740..16,540,263 [-]

Classification - 7 transmembrane receptor (rhodopsin family)

Cellular location - transmembrane



NCBI links: Precomputed BLAST | EntrezGene
Recent literature
Deady, L. D. and Sun, J. (2015). A follicle rupture assay reveals an essential role for follicular adrenergic signaling in Drosophila ovulation. PLoS Genet 11: e1005604. PubMed ID: 26473732
Summary:
Ovulation is essential for the propagation of the species and involves a proteolytic degradation of the follicle wall for the release of the fertilizable oocyte. However, the precise mechanisms for regulating these proteolytic events are largely unknown. There are several parallels between Drosophila and mammalian ovulation at both the cellular and molecular levels. During ovulation in Drosophila, posterior follicle cells surrounding a mature oocyte are selectively degraded and the residual follicle cells remain in the ovary to form a corpus luteum after follicle rupture. Like in mammals, this rupturing process also depends on matrix metalloproteinase 2 (Mmp2) activity localized at the posterior end of mature follicles, where oocytes exit. This study shows that Mmp2 activity is regulated by the octopaminergic signaling in mature follicle cells. Exogenous octopamine (OA; equivalent to norepinephrine, NE) is sufficient to induce follicle rupture when isolated mature follicles are cultured ex vivo, in the absence of the oviduct or ovarian muscle sheath. Knocking down the alpha-like adrenergic receptor Oamb (Octoampine receptor in mushroom bodies) in mature follicle cells prevents OA-induced follicle rupture ex vivo and ovulation in vivo. Follicular OA-Oamb signaling induces Mmp2 enzymatic activation but not Mmp2 protein expression, likely via intracellular Ca2+ as the second messenger. This work develops a novel ex vivo follicle rupture assay and demonstrates the role for follicular adrenergic signaling in Mmp2 activation and ovulation in Drosophila, which is likely conserved in other species.
Dacanay, F. N. D., Ladra, M., Junio, H. A. and Nellas, R. B. (2017). Molecular affinity of Mabolo extracts to an Octopamine receptor of a fruit fly. Molecules 22(10). PubMed ID: 29064449
Summary:
Essential oils extracted from plants are composed of volatile organic compounds that can affect insect behavior. Identifying the active components of the essential oils to their biochemical target is necessary to design novel biopesticides. In this study, essential oils extracted from Diospyros discolor (Willd.) were analyzed using gas chromatography mass spectroscopy (GC-MS) to create an untargeted metabolite profile. Subsequently, a conformational ensemble of the Drosophila melanogaster Octopamine receptor in mushroom bodies (OAMB) was created from a molecular dynamics simulation to resemble a flexible receptor for docking studies. GC-MS analysis revealed the presence of several metabolites, i.e. mostly aromatic esters. Interestingly, these aromatic esters were found to exhibit relatively higher binding affinities to OAMB than the receptor's natural agonist, octopamine. The molecular origin of this observed enhanced affinity is the pi -stacking interaction between the aromatic moieties of the residues and ligands. This strategy, computational inspection in tandem with untargeted metabolomics, may provide insights in screening the essential oils as potential OAMB inhibitors.
Deady, L. D., Li, W. and Sun, J. (2017). The zinc-finger transcription factor Hindsight regulates ovulation competency of Drosophila follicles. Elife 6. PubMed ID: 29256860
Summary:
Follicle rupture, the final step in ovulation, utilizes conserved molecular mechanisms including matrix metalloproteinases (Mmps), steroid signaling, and adrenergic signaling. It is still unknown how follicles become competent for follicle rupture/ovulation. This study identified a zinc-finger transcription factor Hindsight (Hnt) as the first transcription factor regulating follicle's competency for ovulation in Drosophila. Hnt is not expressed in immature stage-13 follicle cells but is upregulated in mature stage-14 follicle cells, which is essential for follicle rupture/ovulation. Hnt upregulates Mmp2 expression in posterior follicle cells (essential for the breakdown of the follicle wall) and Oamb expression in all follicle cells (the receptor for receiving adrenergic signaling and inducing Mmp2 activation). Hnt's role in regulating Mmp2 and Oamb can be replaced by its human homolog Ras-responsive element-binding protein 1 (RREB-1). These data suggest that Hnt/RREB-1 plays conserved role in regulating follicle maturation and competency for ovulation.

BIOLOGICAL OVERVIEW

Ovulation is an essential physiological process in sexual reproduction; however, the underlying cellular mechanisms are poorly understood. OAMB, a Drosophila G-protein-coupled receptor for octopamine (the insect counterpart of mammalian norepinephrine), is required for ovulation induced upon mating. OAMB is expressed in the nervous and reproductive systems and has two isoforms (OAMB-AS and OAMB-K3) with distinct capacities to increase intracellular Ca2+ or intracellular Ca2+ and cAMP in vitro. This study investigated tissue specificity and intracellular signals required for OAMB's function in ovulation. Restricted OAMB expression in the adult oviduct epithelium, but not the nervous system, reinstated ovulation in oamb mutant females, in which either OAMB isoform was sufficient for the rescue. Consistently, strong immunoreactivities for both isoforms were observed in the wild-type oviduct epithelium. To delineate the cellular mechanism by which OAMB regulates ovulation, protein kinases functionally interacting with OAMB were explored by employing a new GAL4 driver with restricted expression in the oviduct epithelium. Conditional inhibition of Ca2+/Calmodulin-dependent protein kinase II (CaMKII), but not protein kinase A or C, in the oviduct epithelium inhibited ovulation. Moreover, constitutively active CaMKII, but not protein kinase A, expressed only in the adult oviduct epithelium fully rescued the oamb female's phenotype, demonstrating CaMKII as a major downstream molecule conveying the OAMB's ovulation signal. This is consistent with the ability of both OAMB isoforms, whose common intracellular signal in vitro is Ca2+, to reinstate ovulation in oamb females. These observations reveal the critical roles of the oviduct epithelium and its cellular components OAMB and CaMKII in ovulation. It is conceivable that the OAMB-mediated cellular activities stimulated upon mating are crucial for secretory activities suitable for egg transfer from the ovary to the uterus (Lee, 2009).

Mating activates highly coordinated physiological processes in the Drosophila female. During copulation, the female receives somatosensory stimulation, sperm and seminal proteins from the male. These mating signals act at multiple sites in the mated female to activate post-mating responses required for successful reproduction. For example, the seminal protein Ovulin stimulates egg-laying for 1 day after mating (Herndon, 1995). Ovulin is not only present in the base of the ovary right after copulation but it also enters the circulatory system, possibly acting at additional sites. Moreover, the seminal sex peptides Acp70A and DUP99B reduce sexual receptivity and stimulate egg-laying. While sex peptides have widespread binding sites in the central nervous system, endocrine glands, and reproductive tissues in the female, it is the sex peptide receptor SPR in the neurons expressing the sex determination factor Fruitless that is indispensable for reduced receptivity as well as increased egg-laying (Lee, 2009).

The downstream targets and mechanisms that Ovulin and SPR activate in the mated female are unknown. Several studies, nonetheless, indicate octopamine as a key neuromessenger for ovulation (Lee, 2003; Monastirioti, 2003; Cole, 2005; Rodriguez-Valentin, 2006; Middleton, 2006), suggesting it being a downstream signal of Ovulin or SPR for egg-laying. Octopamine is a major monoamine in insects and has similar functions to mammalian norepinephrine. Octopamine is synthesized from tyrosine by sequential actions of tyrosine decarboxylase (dTdc) and tyramine beta-hydroxylase (Tβh). The females defective in dTdc2 encoding neuronal dTdc or tβh are sterile due to defective egg-laying (Monastirioti, 2003; Cole, 2005). Octopaminergic neurons innervate numerous brain and thoracico-abdominal ganglion (TAG) areas (Monastirioti, 1995; Sinakevitch, 2006). In addition, octopaminergic neurons in the TAG project to reproductive tissues such as the ovaries, oviducts, sperm storage organs and uterus (Rodriguez-Valentin, 2006; Middleton, 2006). Indeed, the sterility of tβh females is rescued by restored Tβh expression in a subset of neurons including the TAG neurons that innervate the reproductive system (Monastirioti, 1995). Consistently, octopamine, when applied to the dissected reproductive system, modulates muscle activities in a tissue-specific manner: it enhances muscle contraction in the ovary but inhibits it in the oviduct (Rodriguez-Valentin, 2006; Middleton, 2006). This suggests that distinct octopamine receptors present in the ovary and oviduct mediate the opposite actions of octopamine on muscle activity (Lee, 2009).

Drosophila has four known octopamine receptors: OAMB, Octβ1R, Octβ2R and Octβ3R (Han, 1998; Maqueira, 2005; Balfanz, 2005). The oamb gene encodes two isoforms OAMB-K3 (K3) and OAMB-AS (AS), which are produced by alternative splicing of the last exon, and differ in the third cytoplasmic loop and downstream sequence. Both K3 and AS transcripts are found in the brain, TAG and reproductive system (Lee, 2003; Han, 1998). When assayed in the heterologous cell lines, both isoforms activate an increase in intracellular Ca2+ (Han, 1998; Balfanz, 2005) while K3 also stimulates a cAMP increase (Han 1998). This implies that the two isoforms may activate distinct combinations of signal transduction pathways in vivo. To investigate OAMB's in vivo functions, several oamb mutants defective in both K3 and AS have been generated, and their prominent phenotype is female sterility (Lee, 2003). While oamb mutant females show normal mating, they are impaired in ovulation, causing abnormal retention of mature eggs in the ovary (Lee, 2003). This raises several important questions regarding mechanism of OAMB activity: where (brain, TAG or reproductive system) does OAMB regulate ovulation? Which isoform is critical for this process and what are the downstream signals? This study shows that the critical site for the OAMB's function in ovulation is the oviduct epithelium, in which transgenic expression of either K3 or AS isoform is sufficient to rescue the oamb female's ovulation defect. Moreover, OAMB recruits CaMKII as a key downstream effector for this function (Lee, 2009).

Octopamine, as a major neurotransmitter, neuromodulator and neurohormone, regulates diverse physiological processes in invertebrates that include sensory information processing, egg-laying, fight or flight responses, and complex neural functions such as learning and memory (Roeder, 2005). These astonishingly diverse effects of octopamine are initiated by the binding of octopamine to G-protein-coupled receptors expressed in distinct tissue or cell types; however, very little is known about relevant octopamine receptors and underlying cellular mechanisms that mediate octopamine's physiological functions. This work has shown that OAMB regulates ovulation in the oviduct epithelium and recruits CaMKII for this function. This role of OAMB is physiological, as opposed to developmental, since restored OAMB expression in the oviduct epithelium at the adult stage is sufficient for reinstating ovulation in oamb females. This is consistent with the findings observed in the octopamine-less dTdc2 and tβh females, in which feeding octopamine only at the adult stage rescues the sterility phenotype of both mutants (Cole, 2005; Lee, 2009 and references therein).

Sex peptides transferred to the female during copulation enhance egg-laying upon binding to the receptor SPR expressed in the Fruitless neurons. The mechanism by which the Fruitless neurons stimulate egg-laying is unknown; however, octopaminergic neurons in the TAG likely represent a downstream target since the egg-laying phenotype of tβh females is rescued by restored TβH expression in these neurons (Monastirioti, 2003). The TAG octopaminergic neurons project axons to various areas in the reproductive track including the ovary, lateral and common oviducts, sperm storage organs and the uterus (Rodriguez-Valentin, 2006; Middleton, 2003). Mating induces distinctive changes in vesicle release at the nerve terminals in different areas of the reproductive track (Heifetz, 2004), some of which may represent the TAG octopamine neuronal activities. In the dissected reproductive system, octopamine application augments the amplitude of myogenic contractions of the peritoneal sheath in the ovary while it inhibits stimulated muscle contractions of the oviduct (Rodriguez-Valentin, 2006; Middleton, 2003). These opposite effects of octopamine may be crucial for coordinated constriction and relaxation of the ovary and oviduct, respectively, in transferring a mature egg to the uterus. OAMB may serve as a receptor processing the octopamine's input in the oviduct while another octopamine receptor may mediate the constriction signal in the ovarian peritoneal sheath, which lacks OAMB expression (Lee, 2009).

Remarkably, OAMB's activity is required in the epithelium rather than the muscle for normal ovulation. Consistent with this, the histochemical analysis reported here reveals extensive innervation of the TAG octopamine neuronal processes into the oviduct epithelial layer where both OAMB isoforms are enriched, in addition to the muscle. This raises an important question regarding the nature of an OAMB's role in ovulation. While no information is available on the oviduct epithelium in Drosophila or other insects, studies of the mammalian oviduct indicate active roles of the epithelium in fluid secretion and ciliary activity for gamete and embryo transport. Similarly, it is possible that OAMB in the Drosophila oviduct epithelium is involved in regulating fluid secretion to establish proper luminal environment and possibly ciliary action for egg transport. The capacity of either OAMB-K3 or OAMB-AS to reinstate ovulation in oamb females strongly implicates intracellular Ca2+ rather than cAMP as a downstream effector. This is corroborated by findings demonstrating CaMKII as a key epithelial component downstream of OAMB. It is uncertain whether individual isoforms or two isoforms together have comparable efficacies in activating CaMKII and ovulation. Future studies employing quantitative manipulation of transgenic OAMB expression may clarify this issue. Taken together, the epithelial OAMB stimulated upon mating likely activates CaMKII via increased intracellular Ca2+, which may in turn trigger biochemical changes necessary for fluid secretion. Potential molecules involved in this process may include transporters, ion channels, Na+-K+-ATPase and the molecules involved in cilia movements. In the absence of OAMB, epithelial cell activities and fluid may be inadequate for egg movement, leading to ovulation failure. Since octopamine induces relaxation in the dissected oviduct, relaxation may involve another octopamine receptor in the muscle, and concerted activities of OAMB and a muscle receptor may be crucial for successful egg transport. This working model is currently under test (Lee, 2009).

Octopamine regulates oviduct activities in other insects as well. In the locust oviduct, octopamine inhibits the basal tonus and neurally evoked muscle contractions, which are mediated by cAMP-dependent mechanisms (Nykamp, 2000; Orchard, 1986). These effects of octopamine may be mediated by an OAMB-like receptor with the different intracellular effector cAMP. Alternatively, they may involve another octopamine receptor(s) present in the muscle. Drosophila has three octopamine receptors (OctβR1, 2 and 3) that can also stimulate cAMP increases (Evans, 2005). Spatial expression patterns of three OctβRs are as yet unknown. It is conceivable that an OctβR or OctβR-like receptor, possibly present in the Drosophila or locust oviduct muscle, respectively, is additionally involved in ovulation by inducing muscle relaxation through a cAMP signaling pathway. At present, molecular components and cellular pathways controlling ovulation are largely unknown and likewise very little is known about the oviduct functions and mechanisms. The current findings uncover the critical roles of the oviduct epithelium and its cellular components OAMB and CaMKII in ovulation. Future studies to identify additional downstream effectors of OAMB and their functions should help further understanding of the important reproductive process ovulation and provide novel insights into the development of effective insecticides. Typically, intracellular signals activated by G-protein-coupled receptors are characterized in in vitro cell lines. This study has identified the intracellular signal activated by the G-protein-coupled receptor OAMB in vivo that has functional significance. Similar approaches could be applied to other receptors to investigate rather poorly defined cellular mechanisms that G-protein-coupled receptors activate for their in vivo functions (Lee, 2009).

Norepinephrine, a mammalian counterpart of octopamine, also plays profound roles in female reproduction by acting on the reproductive and nervous systems. Sympathetic nerve terminals containing norepinephrine innervate the ovaries, oviducts, and uterus. Moreover, norepinephrine levels in the human fallopian tube vary in a region- and estrous cycle-dependent manner being the highest in the isthmus and the fimbriated end at the time of ovulation. When assayed in vitro, adrenergic receptor agonists not only modulate oviduct muscle activities but they also stimulate fluid secretion possibly via Ca2+-dependent mechanisms. Oviduct fluid in mammals is critical for egg transport, maturation and fertilization; however, the cellular process regulating its secretion is largely unknown. Damage in the oviduct epithelium is associated with pelvic inflammatory disorder, leading to hydrosalpinx formation and reduced fertility. Thus, enhanced understanding of physiological and cellular factors and processes controlling oviduct fluid will provide significant insights into healthy reproduction as well as impaired fertility associated with pelvic inflammatory disorder and other related disorders (Lee, 2009).

Sweet taste and nutrient value subdivide rewarding dopaminergic neurons in Drosophila

Dopaminergic neurons provide reward learning signals in mammals and insects. Recent work in Drosophila has demonstrated that water-reinforcing dopaminergic neurons are different to those for nutritious sugars. This study tested whether the sweet taste and nutrient properties of sugar reinforcement further subdivide the fly reward system. They found that dopaminergic neurons expressing the OAMB octopamine receptor specifically conveyed the short-term reinforcing effects of sweet taste. These dopaminergic neurons projected to the β'2 and γ4 regions of the mushroom body lobes. In contrast, nutrient-dependent long-term memory required different dopaminergic neurons that project to the γ5b regions, and it could be artificially reinforced by those projecting to the β lobe and adjacent α1 region. Surprisingly, whereas artificial implantation and expression of short-term memory occurred in satiated flies, formation and expression of artificial long-term memory required flies to be hungry. These studies suggest that short-term and long-term sugar memories have different physiological constraints. They also demonstrate further functional heterogeneity within the rewarding dopaminergic neuron population (Huetteroth, 2015).

These results demonstrate that the sweet taste and nutrient properties of sugars are independently processed and reinforce memories of different duration. Sweet taste is transduced through octopaminergic neurons whose released octopamine, via the OAMB receptor, activates dopaminergic neurons that project to the β'2am and γ4 regions of the mushroom body. Octopaminergic reinforcement also modulates the state dependence of STM via the OCTβ2R receptor that is required in the dopaminergic MB-MP1 neurons (Huetteroth, 2015).

Nutrient-dependent LTM does not involve octopamine or sweet-taste-reinforcing dopaminergic neurons. Nutrient reinforcement instead requires dopaminergic neurons innervating γ5b of the mushroom body, whereas those going to β1, β2, and the adjacent α1 region are sufficient. More work will be required to understand this distributed process, which apparently has an immediate and delayed dynamic (Huetteroth, 2015).

Whereas formation and expression of sweet-taste-reinforced STM is insensitive to satiety state, artificial formation and expression of nutrient-relevant memory require flies to be hungry. Even direct stimulation of the relevant rewarding dopaminergic neurons cannot implant appetitive LTM in food-satiated flies. These experiments suggest that hunger establishes an internal state that permits the nutrient-reinforcing signals to be effective. It will be interesting to understand what the permissive state involves and where it is required. Others have previously described a role for CREB-regulated transcription co-activator 1 (CRTC) in enabling hunger-dependent LTM in the fly and promoting persistent memory in the mouse. It therefore seems plausible that such a mechanism might be required in the mushroom body neurons to permit nutrient-dependent reinforcement (Huetteroth, 2015).

Genetic and neurobiological analyses of the noradrenergic-like system in vulnerability to sugar overconsumption using a Drosophila model

Regular overconsumption of sugar is associated with obesity and type-2 diabetes, but how genetic factors contribute to variable sugar preferences and intake levels remains mostly unclear. This study provides evidence for the usefulness of a Drosophila larva model to investigate genetic influence on vulnerability to sugar overconsumption. Using genetic and RNA interference approaches, this study shows that the activity of the Oamb gene, which encodes a receptor for octopamine (OA, the invertebrate homologue of norepinephrine), plays a major role in controlled sugar consumption. Furthermore, Oamb appears to suppress sugar food intake in fed larvae in an acute manner, and neurons expressing this Oamb receptor do not overlap with neurons expressing Octbeta3R, another OA receptor previously implicated in hunger-driven exuberant sugar intake. Together, these results suggest that two separate sub-circuits, defined by Oamb and Octbeta3R respectively, co-regulate sugar consumption according to changes in energy needs. It is proposed that the noradrenergic-like system defines an ancient regulatory mechanism for prevention of sugar overload (Branch, 2017).

This study has shown that two of the four OA receptors encoded by the Drosophila genome mediate the dual role of the OA system in modulation of feeding of readily available sugar food under different motivational states. An α-adrenergic-like receptor Oamb is acutely required for prevention of sugar overconsumption in fed larvae, while a β-adrenergic-like receptor Octβ3R is required for hunger-driven responses to the sugar food. These findings suggest that the adrenergic-like system of invertebrate animals is a crucial regulator that links the motivational state to the adaptive consumption of sugar, a vital energy source (Branch, 2017).

Sugar food preference is known to vary among individuals, and understanding of how genetic factors contribute to such variations remain limited. This study has shown that functional deficiency of the Oamb gene caused significant increases in the sugar food consumption in fed larvae. These results raise the possibility that mutations in an array of genes involved in the OA/Oamb pathway may also have similar effects on sugar food consumption. Therefore, these findings suggest that the fly larva may be a useful platform for investigating the contributions of genetic factors to variations in sugar consumption among individual animals. It would also be interesting to test whether genetic variations that affect the function of norepinephrine system may underlie the genetic predisposition to crave for sugar-rich food in mammals (Branch, 2017).

A previous study provided evidence for a potential interaction between the OA/Oamb- and OA/Octβ3R-mediated sub-circuits in modulation of sugar consumption by fly larvae (Zhang, 2013). It has shown that two separate subsets of OA neurons (named VUM1 and VUM2, respectively) in the hindbrain-like region are required for the control of sugar food ingestion. Targeted lesioning of VUM1 resulted in sugar overconsumption in fed larvae, while targeted lesioning of VUM2 attenuated Octβ3R-dependent feeding of sugar food in hungry larvae. Further, targeted lesioning of VUM2 also attenuated Octβ3R-dependent feeding response to sugar food. However, how VUM1 and VUM2 neurons functionally interact with each other remains unclear. This work supports the notion that VUM1 neurons are acutely active in fed larvae but silenced under prolonged food deprivation. In fed larvae, VUM1 may indirectly suppress a VUM2-dependent sub-circuit through its signaling to Oamb neurons. It is possible that the VUM1/Oamb neuronal pathway may exert the inhibitory effect on the VUM2/Octβ3R neuronal pathway at the level of the Octβ3R neurons or their downstream targets. Further experiments will be needed to determine how the OA/Oamb and OA/Octβ3R sub-circuits interact to co-regulate sugar consumption under different motivational states (Branch, 2017).

Carbohydrates are vital energy sources to animals across evolution. Despite considerable evolutionary divergence, the control mechanisms for carbohydrate intake in insects and mammals may share similar molecular and neural mechanisms. For example, OA neurons from the hindbrain-like SOG region are known to be associated with sugar sensation in insects. Treatment of OA promotes honey bee's feeding response toward sucrose, and is able to increase the reward value of food resources. It has also been reported that OA is necessary and can even replace sugar stimuli in forming appetitive olfactory memories in Drosophila1. Similarly, a group of norepinephrine (the vertebrate counterpart of OA) neurons in the brainstem of rats are responsive to glucose level required for regulating carbohydrates-specific food ingestion (Branch, 2017).

It is proposed that precise control of feeding is achieved through different affinities between agonists and different receptors, and the relative activity level of α1 and α2 receptor neurons determines the feeding consequences. In rats, antagonistic effects of altering food intake are mediated through different downstream receptor neurons located in the paraventricular nucleus of hypothalamus. NE signaling promotes feeding through α1 receptors, while its activation of α2 receptors inhibits food intake. In Drosophila larvae, this study has also identified two separate OA circuits exerting opposite effects in regulating feeding. Similar to mammalian models, two different downstream receptors are found exhibiting antagonistic effects. Both 1.6-Oamb-GAL4 and 1.8-Octβ3R-GAL4 neurons are present in a larval brain region anterior to the OA neurons. It would be interesting to determine whether this region represents a functional equivalence of the mammalian hypothalamus. Furthermore, satiation status in rats affects an animal's feeding decisions by altering both NE release adrenoceptor levels. It is postulated that the OA system is also subject to modulation by endocrine hormones and nutrients levels, and it may define a key control site in the central nervous system where multi-sensory integration and feeding regulation takes place (Branch, 2017).

Layered reward signalling through octopamine and dopamine in Drosophila

Dopamine is synonymous with reward and motivation in mammals. However, only recently has dopamine been linked to motivated behaviour and rewarding reinforcement in fruitflies. Instead, octopamine has historically been considered to be the signal for reward in insects. This study shows, using temporal control of neural function in Drosophila, that only short-term appetitive memory is reinforced by octopamine. Moreover, octopamine-dependent memory formation requires signalling through dopamine neurons. Part of the octopamine signal requires the alpha-adrenergic-like OAMB receptor in an identified subset of mushroom-body-targeted dopamine neurons. Octopamine triggers an increase in intracellular calcium in these dopamine neurons, and their direct activation can substitute for sugar to form appetitive memory, even in flies lacking octopamine. Analysis of the beta-adrenergic-like OCTbeta2R receptor reveals that octopamine-dependent reinforcement also requires an interaction with dopamine neurons that control appetitive motivation. These data indicate that sweet taste engages a distributed octopamine signal that reinforces memory through discrete subsets of mushroom-body-targeted dopamine neurons. In addition, they reconcile previous findings with octopamine and dopamine and suggest that reinforcement systems in flies are more similar to mammals than previously thought (Burke, 2012).

Molecular genetic analysis of sexual rejection: roles of octopamine and its receptor OAMB in Drosophila courtship conditioning

After Drosophila males are rejected by mated females, their subsequent courtship is inhibited even when encountering virgin females. Molecular mechanisms underlying courtship conditioning in the CNS are unclear. This study found that tyramine beta hydroxylase (TbetaH) mutant males unable to synthesize octopamine (OA) show impaired courtship conditioning, which can be rescued by transgenic TbetaH expression in the CNS. Inactivation of octopaminergic neurons mimics the TbetaH mutant phenotype. Transient activation of octopaminergic neurons in males not only decreases their courtship of virgin females, but also produces courtship conditioning. Single cell analysis revealed projection of octopaminergic neurons to the mushroom bodies. Deletion of the OAMB gene encoding an OA receptor expressed in the mushroom bodies disrupts courtship conditioning. Inactivation of neurons expressing OAMB also eliminates courtship conditioning. OAMB neurons respond robustly to male-specific pheromone cis-vaccenyl acetate in a dose-dependent manner. These results indicate that OA plays an important role in courtship conditioning through its OAMB receptor expressed in a specific neuronal subset of the mushroom bodies (Zhou, 2012).

In Drosophila, OA has been proposed to mediate the unconditioned reward stimuli in appetitive olfactory learning where sugar was used as the unconditioned stimulus while olfactory cues were used as conditioned stimuli. In honeybees, electric stimulation of VUMmx1 neurons could substitute food reward in proboscis extension reflex to modify olfactory responses. It is unclear whether OA mediates general reward signals in insects as dopamine does in mammals, or it simply associates a specific type of unconditioned stimuli (sugar in case of Drosophila) with odorant cues. An inactive mutant with reduced activity of tyrosine decarboxylase, which catalyzes the conversion of tyrosine to tyramine, displays abnormal courtship learnin. TβH mutants are unable to repress the initiation of courtship behavior in the presence of mated females. Either TβH mutation or activation of octopaminergic neurons enhanced male-male courtship when a male tester was presented with male and female targets in a competitive courtship assay, suggesting that a balanced OA signaling is important for sexual discrimination in flies (Zhou, 2012).

The results support a role for OA in courtship conditioning. In courtship conditioning, anti-aphrodisiac pheromones such as cVA are associated with appetitive pheromones to modify male response to mated females. This study provides evidence supporting the notion that OA mediates the sensing of aversive stimuli in courtship conditioning: (1) activation of octopaminergic neurons mimics anti-aphrodisiac pheromones in repressing male-female courtship and (2) activation of octopaminergic neurons in the presence of virgins could induce courtship memory, which subsequently reduces male courtship of virgin females (Zhou, 2012).

Mushroom bodies are known to be important for learning and memory. It is hypothesized that the OAMB receptor may act downstream of OA signaling to regulate courtship conditioning. This has been supported by the data that either loss-of-function mutation in the oamb gene or inactivation of OAMB neurons leads to defective courtship conditioning. The OAMB-Gal4 labels a cluster of α/β lobe neurons. It has been reported that DA1 glomeruli in antenna lobe respond to cVA and only send axon branches to the calyx region of early α/β lobe neurons. The fact that OAMB neurons in α/β lobes of the mushroom bodies respond to cVA suggests OAMB neurons as a potential sensory integration site of courtship conditioning. Interestingly, OAMB neurons show enhanced response to cVA in trained males compared with naive males, suggesting that neuronal plasticity in OAMB circuits might mediate courtship conditioning. However, Activation of OAMB neurons does not reduce male courtship behavior, suggesting that the inhibitory effects of octopaminergic neurons may be mediated by other downstream neuronal circuits (Zhou, 2012).

It is possible that different subsets of octopaminergic neurons act upon different downstream receptor neurons to regulate different behaviors. In Drosophila, anterior superior medial octopaminergic cells in the medial protocerebrum promote wakefulness, whereas a few ventral unpaired median (VUM) neurons with neurites ramifying the subesophageal ganglion were suggested to regulate aggression. This study has characterized two classes of octopaminergic neurons innervating mushroom bodies, both of which are morphologically distinct from the VUM neurons involved in aggression. These suggest that innate social behaviors and experience-dependent behaviors are mediated by separate populations of octopaminergic cells. However, It remains to be established which classes of octopaminergic cells mediate reward olfactory learning or social learning in Drosophila (Zhou, 2012).

Identification of a neural circuit that underlies the effects of octopamine on sleep:wake behavior

An understanding of sleep requires the identification of distinct cellular circuits that mediate the action of specific sleep:wake-regulating molecules, but such analysis has been very limited. This study identifies a circuit that underlies the wake-promoting effects of octopamine in Drosophila. Using MARCM, the ASM cells in the medial protocerebrum were identified as the wake-promoting octopaminergic cells. Octopamine signaling was then blocked in random areas of the fly brain, and the postsynaptic effect was mapped to insulin-secreting neurons of the pars intercerebralis (PI). These PI neurons show altered potassium channel function as well as an increase in cAMP in response to octopamine, and genetic manipulation of their electrical excitability alters sleep:wake behavior. Effects of octopamine on sleep:wake are mediated by the cAMP-dependent isoform of the OAMB receptor. These studies define the cellular and molecular basis of octopamine action and suggest that the PI is a sleep:wake-regulating neuroendocrine structure like the mammalian hypothalamus (Crocker, 2010).

The Octopamine receptor Octβ2R regulates ovulation in Drosophila melanogaster

Oviposition is induced upon mating in most insects. Ovulation is a primary step in oviposition, representing an important target to control insect pests and vectors, but limited information is available on the underlying mechanism. This study reports that the beta adrenergic-like octopamine receptor Octβ2R serves as a key signaling molecule for ovulation and recruits Protein kinase A and Ca2+/calmodulin-sensitive kinase II as downstream effectors for this activity. The octβ2r homozygous mutant females are sterile. They displayed normal courtship, copulation, sperm storage and post-mating rejection behavior but are unable to lay eggs. It has been shown previously that octopamine neurons in the abdominal ganglion innervate the oviduct epithelium. Consistently, restored expression of Octβ2R in oviduct epithelial cells is sufficient to reinstate ovulation and full fecundity in the octβ2r mutant females, demonstrating that the oviduct epithelium is a major site of Octβ2R's function in oviposition. It was also found that overexpression of the protein kinase A catalytic subunit or Ca2+/calmodulin-sensitive protein kinase II leads to partial rescue of octβ2r's sterility. This suggests that Octβ2R activates cAMP as well as additional effectors including Ca2+/calmodulin-sensitive protein kinase II for oviposition. All three known β adrenergic-like octopamine receptors stimulate cAMP production in vitro. Octβ1R, when ectopically expressed in the octβ2r's oviduct epithelium, fully reinstated ovulation and fecundity. Ectopically expressed Octβ3R, on the other hand, partly restores ovulation and fecundity while OAMB-K3 and OAMB-AS that increase Ca2+ levels yielded partial rescue of ovulation but not fecundity deficit. These observations suggest that Octβ2R have distinct signaling capacities in vivo and activate multiple signaling pathways to induce egg laying. The findings reported in this study narrow the knowledge gap and offer insight into novel strategies for insect control (Lim, 2014; PubMed).

Octopamine receptor OAMB is required for ovulation in Drosophila melanogaster

Octopamine is a major monoamine in invertebrates and affects many physiological processes ranging from energy metabolism to complex behaviors. Octopamine binds to receptors located on various cell types and activates distinct signal transduction pathways to produce these diverse effects. One of the Drosophila octopamine receptors named OAMB produces increases in cAMP and intracellular Ca2+ upon ligand binding. It is expressed at high levels in the brain. To explore OAMB's physiological roles, deletions were generated in the OAMB locus. The resultant oamb mutants were viable without gross anatomical defects. The oamb females displayed normal courtship and copulation; however, they were impaired in ovulation with many mature eggs retained in their ovaries. RT-PCR, in situ hybridization, and expression of a reporter gene revealed that OAMB was also expressed in the thoracicoabdominal ganglion, the female reproductive system, and mature eggs in the ovary. Moreover, analysis of various alleles pinpointed the requirement for OAMB in the body, but not in the brain, for female fecundity. The novel expression pattern of OAMB and its genetic resource described in this study will help advance understanding on how the neuromodulatory or endocrine system controls reproductive physiology and behavior (Lee, 2003; Full text of article).

A novel octopamine receptor with preferential expression in Drosophila mushroom bodies

Octopamine is a neuromodulator that mediates diverse physiological processes in invertebrates. In some insects, such as honeybees and fruit flies, octopamine has been shown to be a major stimulator of adenylyl cyclase and to function in associative learning. To identify an octopamine receptor mediating this function in Drosophila, putative biogenic amine receptors were cloned by a novel procedure using PCR and single-strand conformation polymorphism. One new receptor, octopamine receptor in mushroom bodies (OAMB), was identified as an octopamine receptor because human and Drosophila cell lines expressing OAMB showed increased cAMP and intracellular Ca2+ levels after octopamine application. Immunohistochemical analysis using an antibody made to the receptor revealed highly enriched expression in the mushroom body neuropil and the ellipsoid body of central complex, brain areas known to be crucial for olfactory learning and motor control, respectively. The preferential expression of OAMB in mushroom bodies and its capacity to produce cAMP accumulation suggest an important role in synaptic modulation underlying behavioral plasticity (Han, 1998. PubMed ID: Full text of article).


REFERENCES

Search PubMed for articles about Drosophila Oamb

Balfanz, S., Strunker, T., Frings, S. and Baumann, A. (2005). A family of octopamine receptors that specifically induce cyclic AMP production or Ca2+ release in Drosophila melanogaster. J. Neurochem. 93: 440-451. PubMed ID: 15816867

Branch, A., Zhang, Y. and Shen, P. (2017). Genetic and neurobiological analyses of the noradrenergic-like system in vulnerability to sugar overconsumption using a Drosophila model. Sci Rep 7(1): 17642. PubMed ID: 29247240

Burke, C. J., Huetteroth, W., Owald, D., Perisse, E., Krashes, M. J., Das, G., Gohl, D., Silies, M., Certel, S. and Waddell, S. (2012). Layered reward signalling through octopamine and dopamine in Drosophila. Nature 492: 433-437. PubMed ID: 23103875

Cole, S. H., et al. (2005). Two functional but noncomplementing Drosophila tyrosine decarboxylase genes: distinct roles for neural tyramine and octopamine in female fertility. J. Biol. Chem. 280: 14948-14955. PubMed ID: 15691831

Crocker, A., Shahidullah, M., Levitan, I. B. and Sehgal, A. (2010). Identification of a neural circuit that underlies the effects of octopamine on sleep:wake behavior. Neuron 65: 670-681. PubMed ID: 20223202

Evans, P. D. and Maqueira, B. (2005). Insect octopamine receptors: a new classification scheme based on studies of cloned Drosophila G-protein coupled receptors. Invert. Neurosci. 5: 111-118. PubMed ID: 16211376

Han, K. A., Millar, N. S. and Davis, R. L. (1998). A novel octopamine receptor with preferential expression in Drosophila mushroom bodies. J. Neurosci. 18: 3650-3658. PubMed ID: 9570796

Heifetz, Y. and Wolfner, M. F. (2004). Mating, seminal fluid components, and sperm cause changes in vesicle release in the Drosophila female reproductive tract. Proc. Natl. Acad. Sci. 101: 6261-6266. PubMed ID: 15071179

Herndon, L. A. and Wolfner, M. F. (1995). A Drosophila seminal fluid protein, Acp26Aa, stimulates egg laying in females for 1 day after mating. Proc. Natl. Acad. Sci. 92: 10114-10118. PubMed ID: 7479736

Huetteroth, W., Perisse, E., Lin, S., Klappenbach, M., Burke, C. and Waddell, S. (2015). Sweet taste and nutrient value subdivide rewarding dopaminergic neurons in Drosophila. Curr Biol 25(6):751-8. PubMed ID: 25728694

Lee, H. G., Seong, C. S., Kim, Y. C., Davis, R. L. and Han, K. A. (2003). Octopamine receptor OAMB is required for ovulation in Drosophila melanogaster. Dev. Biol. 264: 179-190. PubMed ID: 14623240

Lee, H. G., Rohila, S. and Han, K. A. (2009). The octopamine receptor OAMB mediates ovulation via Ca2+/calmodulin-dependent protein kinase II in the Drosophila oviduct epithelium. PLoS One 4(3): e4716. PubMed ID: 19262750

Lim, J., Sabandal, P. R., Fernandez, A., Sabandal, J. M., Lee, H. G., Evans, P. and Han, K. A. (2014). The Octopamine receptor Octβ2R regulates ovulation in Drosophila melanogaster. PLoS One 9: e104441. PubMed ID: 25099506

Maqueira, B., Chatwin, H. and Evans, P. D. (2005). Identification and characterization of a novel family of Drosophila beta-adrenergic-like octopamine G-protein coupled receptors. J. Neurochem. 94: 547-560. PubMed ID: 15998303

Middleton, C. A., et al. (2006). Neuromuscular organization and aminergic modulation of contractions in the Drosophila ovary. BMC Biol. 4: 17. PubMed ID: 16768790

Monastirioti, M., et al. (1995). Octopamine immunoreactivity in the fruit fly Drosophila melanogaster. J. Comp. Neurol. 356: 275-287. PubMed ID: 7629319

Monastirioti, M. (2003). Distinct octopamine cell population residing in the CNS abdominal ganglion controls ovulation in Drosophila melanogaster. Dev. Biol. 264: 38-49. PubMed ID: 14623230

Nykamp, D. A. and Lange, A. B. (2000). Interaction between octopamine and protolin on the oviducts of Locusta migratoria. J. Insect Physiol. 46: 809-816. PubMed ID: 10742530

Orchard, I. and Lange, A. B. (1986). Neuromuscular transmission in an insect visceral muscle. J. Neurobiol. 17: 359-372. PubMed ID: 2877049

Rodriguez-Valentin, R., et al. (2006). Oviduct contraction in Drosophila is modulated by a neural network that is both, octopaminergic and glutamatergic. J. Cell/ Physiol. 209: 183-198. PubMed ID: 16826564

Roeder, T. (2005). Tyramine and octopamine: ruling behavior and metabolism. Annu. Rev. Entomol. 50: 447-477. PubMed ID: 15355245

Sinakevitch I, Strausfeld NJ. Comparison of octopamine-like immunoreactivity in the brains of the fruit fly and blow fly. J. Comp. Neurol. 494: 460-475. PubMed ID: 16320256

Zhang, T., Branch, A. & Shen, P (2013). Octopamine-mediated circuit mechanism underlying controlled appetite for palatable food in Drosophila. PNAS 110: 15431–15436. PubMed ID: 24003139

Zhou, C., Huang, H., Kim, S. M., Lin, H., Meng, X., Han, K. A., Chiang, A. S., Wang, J. W., Jiao, R. and Rao, Y. (2012). Molecular genetic analysis of sexual rejection: roles of octopamine and its receptor OAMB in Drosophila courtship conditioning. J Neurosci 32: 14281-14287. PubMed ID: 23055498


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date revised: 25 April 2018

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