The conserved neuropeptide Y (NPY) signaling pathway has been strongly implicated in the stimulation of food uptake in vertebrates as well as in the regulation of food conditioned foraging behaviors of Caenorhabditis elegans. Using in situ RNA hybridization and immunocytochemistry, the study reports the neuronal network of Drosophila neuropeptide F (dNPF), a human NPY homologue, in the larval central nervous system and its food-dependent modifications. Indications are provided that gustatory stimulation by sugar, but not its ingestion or metabolism, is sufficient to trigger long-term, dose-dependent alterations of the dNPF neuronal circuit through both dnpf activation and increased synaptic transmission. These results strongly suggest that the dNPF neuronal circuit is an integral part of the sensory system that mediates food signaling, providing the neural basis for understanding how invertebrate NPY regulates food response (Shen, 2001).

The Npf neural system is comprised of four to six Npf neurons located in the brain and subesophageal ganglia. In response to chemosensory stimulation by sugar, the Npf neuronal circuit undergoes long-term, dose-dependent modifications through npf activation and an increase in the number of Npf-positive varicosities. These properties of the Npf neurons support its potential role in the regulation of food-related behaviors (Shen, 2001; Brown 1999).

The possible role of Npf in regulating feeding activity was investigated in the third instar larva. Young third instar larvae feed voraciously, but their feeding activity subsides as they mature and become increasingly mobile. Under controlled growth conditions, the larval transition from feeding to nonfeeding is largely completed by the first 24 hr, when larvae are moving away from yeast paste on apple juice-agar. To determine the relationship between the feeding activity and npf neural expression, the npf RNA level in the CNS tissues of feeding and nonfeeding larvae was examined by whole-mount in situ RNA hybridization using a digoxigenin-labeled antisense npf RNA probe. Two-hour-old third instars were either harvested immediately or withheld from food for an additional 24 hr before tissue dissection and fixation. In both cases, strong fluorescence staining was detected in the four neurons in the brain lobes, indicating that the npf RNA level remains high in larvae that are attracted to food, regardless of their age and feeding state. Quantification of fluorescence staining in the four neurons showed that the npf RNA levels were comparable in both types of tissues. The npf expression was examined in synchronized 24-hr-old third instars that were fed continuously. Before tissue collection, the natural cessation of food intake was confirmed by the absence of dyed yeast paste in the gut. The fluorescence staining in the brain was undetectable in most of the nonfeeding larvae, while the rest showed greatly diminished staining, indicating that the downregulation of npf expression in the brain coincides with the cessation of larval feeding activity (Wu, 2003).

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date revised: 20 January 2010

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