This proposal for R03 funding is the result of 5 years of K08 funding and preliminary data, which raised several new questions. This project's goals are to achieve a greater understanding of the hypothalamic regulation of feeding. Obesity in both adults and children is an epidemic with 65% of U.S. adults and 25% of children being overweight or obese. This project began with the identification of a novel form of monogenic obesity in a child with a balanced translocation disrupting a gene called SIM1. The Sim1 heterozygous knock out mouse is also hyperphagic, obese and sensitive to high fat. SIM1 is expressed in the hypothalamus, mostly in the paraventricular nucleus. We found that Sim1 mice have a profound deficiency in Oxytocin (Oxt) in the paraventricular nucleius (PVN) and Supraoptic Nucleus (SON). When treated with Oxt by central administration, their hyperphagia was rescued at doses that did not affect wild-type mice. When treated with Oxt antagonists, these mice were found to be hypersensitive. These studies suggested a role for Oxt in the hypothalamic regulation of feeding. Examining the literature on Oxt and feeding we found conflicting results. Pharmacologic and anatomic studies supported a role for Oxt neurons and Oxt in feeding regulation. Parvocellular Oxt neurons project from the PVN to the Nucleus of the Solitary tract (NTS), which is important in regulating satiety. Central injection of Oxt leads to reduction of food intake in rats and Oxt antagonists increase food intake. On the other hand, several lines of Oxt-/- mice are not hyperphagic. This discrepancy remains unexplained. Possible explanations are that physiologically, Oxt is not relevant in feeding regulation by the hypothalamus, or that Oxt is important but that compensatory mechanisms such as neuronal plasticity contribute to normalizing food intake in Oxt-/- mice. Our hypothesis is that Oxt neurons and Oxt peptide are necessary for the hypothalamic regulation of feeding. We aim to generate two mouse models, which will be useful tools in answering these questions. Aim 1 will cross Oxt-cre and Oxt-ires-cre mice with iDTR mice, which will be used to specifically ablate Oxt neurons using diphtheria toxin. These mice will be validated for the effectiveness and specificity of Oxt neuronal ablation and decrease in Oxt expression. If we are able to demonstrate effective and specific ablation of Oxt neurons we will use these data to write an R01 grant aim to study the effect of specifically ablating Oxt neurons on feeding regulation and body weight. Aim 2 will generate and validate Floxed Oxt mice, which will be validated for effective deletion using ubiquitously expressed EIIa- cre mice. Once generated, they will be useful to write a future R01 grant aim to effect conditional deletion Oxt in adult mice thereby eliminating developmental compensatory mechanisms.