Project Summary Thirsty animals quickly learn to drink fluids that rehydrate them. This reinforcement learning process is necessary for the initial development of drinking behavior and the continual development of fluid preferences. How this learning process works in the brain is not well understood, but likely involves dopamine-releasing ventral tegmental (VTA DA) neurons. As I show in preliminary data, these VTA DA neurons strongly respond to changes in blood osmolarity. I propose to examine how VTA DA projections transform fluid state changes into behavioral reinforcement. This work will significantly further our understanding of the neural circuitry involved in maintaining fluid balance. Additionally, homeostatic challenges demonstrate the physiological role of the mesolimbic dopamine system, which is exploited by recreational drugs to promote addictive behavior. Finally, this project will address the simple yet fundamental question of how the brain can transform an internal sensation into behavioral reinforcement.