Project Summary All animals share motivated behaviors to fulfill their basic needs for survival, including food, water, sleep, and social interactions, etc. The homeostatic regulatory system energizes behaviors to defend a target level for these needs (the homeostatic setpoint). For example, human adults, on average, aim to sleep 7-8 hours daily. What defines the homeostatic setpoint and how is it modulated remain unanswered questions for all motivated behaviors. Do dedicated neural circuits exist that determine the setpoint? How are changes in physiological needs detected and translated into modified setpoints? These questions have been difficult to address, largely due to a lack of simple manipulations that rapidly and profoundly change the homeostatic setpoints. Protein is a crucially important macronutrient, and behavioral studies indicate that a wide range of species, including humans, seek to consume a fixed amount of protein: the protein intake target. In Drosophila fruit flies, mating induces a robust increase for the consumption of protein-enriched food in pregnant female flies, due to the greater need for protein in egg production. That change in needs rapidly reset the protein intake target and thus provides an elegant model to study the homeostatic setpoint for motivated behaviors. Using Drosophila, we recently identified and characterized the first neural circuit encoding protein-specific hunger. In this proposed study, we seek to delineate the circuit and molecular mechanisms underlying the determination and modulation of the protein intake setpoint. Specifically, we plan to 1) test the hypothesis that the protein intake setpoint is encoded in the hunger neurons by the membrane excitability; 2) identify the neural circuit feeds into the hunger neurons and resets the protein intake target; and 3) elucidate the molecular sensors detecting the changes of protein needs. To achieve these goals, we will employ a multidisciplinary approach, including large-scale genetic analyses, quantitative behavioral measurements, immunohistochemistry, patch-clamp electrophysiology, and functional imaging. These investigations will shed light on the fundamental principles underlying the organization and modulation of homeostatic setpoint for motivated behaviors.