Diet composition plays a significant role in the development of human obesity. A genetic propensity towards dietary obesity has been identified in rodents, and obese individuals demonstrate differences in food preferences from lean individuals. In rodent models, dietary-induced obesity is associated with decreased sensitivity to exogenous administration of the candidate adiposity signal insulin. This suggests that some difference-metabolic, neurochemical, or both-as a consequence of eating a highly palatable, high energy diet is present, resulting in a change in the physiological regulation of body adiposity. Activation of the CNS mesolimbic dopamine (DA) neurons is implicated in the reinforcing or rewarding aspects of several classes of stimuli, including food. We have obtained preliminary evidence that insulin can downregulate the activity of these DA neurons, both at the level of the synapse and at the level of behavior. In this proposal, we pursue the hypothesis that activity of these neurons is altered in association with dietary obesity, and that there is a loss of insulin-induced downregulation of DA neuronal activity. To test this hypothesis, we will evaluate performance in two behavioral tasks in which DA has been implicated: lick rates of sweet and fat solutions, and the conditioned place preference (CPP) paradigm; release of DA from the mesolimbic DA neurons by in vivo microdialysis; and the cellular mechanisms underlying altered DA release, using our established methodologies. All studies will utilize normal weight and dietary obese rats (both outbred rats, and the inbred dietary-induced obese (DIO)/dietary obese-resistant (DR) rats, which are genotypically distinct in their propensity to develop dietary obesity), infused with intraventricular (IVT) vehicle or insulin. Together, these studies will evaluate the ability of a candidate adiposity signal to interact with brain pathways associated with reward; and the influence of palatable high energy diets on the function and regulation of the mesolimbic DA neurons.