Project Summary/Absract Obesity is a leading cause of preventable death in this country. However, despite widespread knowledge of the prevalence and health disparities linked to chronic obesity, effective treatments remain elusive – most people who lose weight will re-gain it. Though prior research has often explored metabolic or energy adaptations to explain weight re-gain, this does not address the proven contribution of hedonic feeding mechanisms to weight re-gain after obesity. Human and rodent studies show that weight loss after obesity causes increased motivation to consume palatable foods, and that activity in brain areas involved in food motivation – namely the medial prefrontal cortex (mPFC) and the nucleus accumbens (NAc) – is altered with obesity. Yet both enhancements and reductions in synaptic connectivity between mPFC and NAc during obesity have been reported, rendering conclusions about how hedonic feeding circuits change with obesity difficult to draw, and therefore challenging to target with potential therapies. The long-term objective of the experiments proposed here is to define the neural mechanisms governing chronic, obesity-linked changes in food motivation in the mPFC-NAc circuit. I propose to do this by investigating how activity in NAc neural ensembles tuned to food seeking is affected by obesity and subsequent weight loss. I hypothesize that obesity increases mPFC-NAc connectivity onto discrete NAc neural ensembles modulated during food seeking, and that this persists after weight loss. Here I propose to test this hypothesis in three aims: Aim 1) I will identify how weight gain and weight loss alter NAc ensemble activity during food seeking, Aim 2) I will identify how activity of a genetically identified subset of NAc neurons associated with reward changes during food seeking as a result of weight gain and subsequent weight loss, and Aim 3) I will investigate how weight loss after obesity disrupts synaptic plasticity mechanisms. These aims will be investigated using novel behavioral techniques, in vivo recording of single neuron activity, and optogenetic stimulation to induce and study corticostriatal synaptic connectivity mechanisms. The data from these experiments will elucidate a persistent neural mechanism through which obesity increases food motivation and ultimately allow for the generation of targeted therapies to combat obesity.