PROJECT SUMMARY/ABSTRACT This proposal aims to determine how interoceptive nutrient detection modulates chemosensory responses and innate behavior. Protein-rich foods are widely considered beneficial to postpartum mothers by supplying energy and cellular building materials, or improving moods by acting as precursors for neurotransmitters. However, it remains unclear whether the nutrient’s restorative effect can also arise from other means: specifically, whether and how dietary amino acids can trigger signaling events to modulate neuronal function and behavior. To address these complex questions, it is necessary to dissociate the multifaceted functions of dietary proteins in neuromodulation or nutrition. Gut enteroendocrine cells, found in both flies and mammals, present excellent opportunities to address these questions, because these cells release peptide hormones in a nutrient-specific manner and, with modern genetic tools, can be specifically manipulated to release their hormones without ingestion of the nutrients. Using fruit flies as a model, this project leverages the powerful genetic toolkit and tractable nervous & gut enteroendocrine systems of D. melanogaster. Preliminary data showed that high-protein diet can accelerate receptivity recovery in mated females. At the molecular level, a subset of enteroendocrine cells exhibits heightened sensitivity to amino acids, thereby increasing the likelihood of releasing a specific peptide hormone from those cells upon protein ingestion. At the neuronal level, high-protein diet heightens the sensitivity of two types of olfactory receptor neurons (ORNs) to external social cues. Importantly, the specific gut-released peptide hormone is necessary for the ORN sensitization as well as the restorative effect of high protein on postmating receptivity. This proposal will test the hypothesis that mated females employ a “gut-to-nose” signaling pathway whereby dietary amino acids trigger the release of a peptide hormone from the gut to regulate sensitivity to social cues and restore critical innate behavior. The proposed experiments will characterize the regulation and function of the gut-released peptide hormone (Aim 1), identify the molecular mechanism by which the specific enteroendocrine cell subset responds to amino acids (Aim 2), and determine the role of peripheral chemosensory neuromodulation on receptivity recovery (Aim 3). The mechanistic insights expected from this research will shed light on how macronutrients impinge on particular internal physiological states to influence neuronal function and behavior—in a manner beyond nutrient assimilation or energy supply. Importantly, the proposed research reveals that nutrient responses of gut enteroendocrine cells can be modulated by mating status, and that the outcome of this modulated interoception (elevated release of a peptide hormone by dietary proteins) can in turn impact exteroception of social cues (ORNs’ sensitization to pheromones).