Octopamine signaling for feeding regulation in Drosophila larvae
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As a basic instinct in living organisms, feeding behavior is essential across evolution and species. The ability to be attracted to palatable food enables animals to gain energy efficiently. Using Drosophila melanogaster (fruit fly) as the model organism, and feeding assays to measure larval feeding rate on palatable food, I have demonstrated that octopamine (OA), the insect equivalent of norepinephrine, is acutely required for the fly larva's hunger-elicited feeding response to palatable food. The inhibition of octopaminergic neurons blocked the hunger-elicited feeding rate increase in fly larvae. Oral treatment with OA also promotes the feeding response to palatable food of fly early third instar larvae. Using laser ablation, I was also able to map the octopaminergic neurons involved in feeding regulating functions to the subesophageal ganglion (SOG), which is known as the feeding control center of insects. Two clusters of octopaminergic neurons in the SOG, which are ventral unpaired medium (VUM) neurons VUM1 and VUM2, respectively, regulate feeding response to palatable food antagonistically. The VUM1 octopaminergic neurons inhibit feeding when the animals are fed, while the VUM2 octopaminergic neurons promote hunger-elicited feeding activity. OAMB and OCTß3R are the antagonistic receptors required for the two functions, respectively. In addition, I have demonstrated that the Pvr/drk/Ras pathway regulates OA function in the feeding response. Drk activity in the tdc2-Gal4 neurons positively regulated larval feeding rate in the palatable food. Expression of a dominant negative form of Ras in tdc2-Gal4 neurons blocked drkcDNA function in the food response. PDGF- and VEGF-related Receptor (Pvr) is the receptor receiving extracellular signals to activate this pathway, and PDGF- and VEGF-related factor 2 (Pvf2) may be the extracellular signal. My work is the first study to unveil and characterize the function of OA in Drosophila melanogaster feeding behavior, and provided molecular and genetic evidence for the neural circuits underlying the complex preferred-food response of fly larvae, which may suggest a conserved pathway in the mammalian system.