There is compelling evidence indicating that protein is the macronutrient that has the biggest impact on how much energy we eat. Across diets with various protein contents, total protein intake is tightly regulated, and this is achieved via wide variations in total energy intake. However, how the brain detects dietary protein to regulate appetite and metabolism remains poorly understood. Some specific amino acids, including the branched chain amino acid leucine, can reproduce some of the effects of protein on satiety. We hypothesize that postprandial leucine levels signal protein abundance to specialized brain sensors present in regions relevant to the control of energy balance. Using a multidisciplinary approach , our aim is to establish the neurochemical and molecular identity of these sensors, and characterize the downstream neurocircuits mediating central protein representation and controlling energy balance.