The goal of this project is to understand the neurobiological basis of food intake and body weight regulation. Leptin, a product of the obese gene in fat cells, is one of the most important peripheral satiety factors that inhibits food intake and body weight by acting in the brain. Since obese humans, and mice made obese by dietary manipulation, have elevated levels of circulating leptin but maintain a normal food intake, it is thought that obese individuals are relatively insensitive to endogenous leptin. In rodents, chronic leptin infusion produces resistance to the satiety action of leptin. The hypothalamic mechanisms for the development of resistance to leptin's satiety action are unknown. This problem will be addressed here in a chronic leptin infused rat. A simple hypothesis that alterations in the secretion and actions of a model orexigenic, neuropeptide Y (NPY), and anorectic, neurotensin (NT), peptide underlies the resistance to leptin's satiety action will be tested. The following Specific Aims will be addressed. Specific Aim 1: To determine whether decreased NPY secretion (synthesis and release) and/or increased NT secretion in the arcuate nucleus (ARC)- paraventricular nucleus (PVN) pathway can account for the acute leptin induced decrease in food intake; and whether changes in responsiveness to the actions of NPY and NT are also involved; Specific Aim 2: To determine whether the changes in secretion and action of NPY and NT that are established to underlie the acute effects of leptin are reversed with chronic leptin exposure and lead to the development of resistance to leptin's satiety action; Specific Aim 3: To determine the changes in leptin receptor activity and gene expression during the acute hypophagia induced by leptin and during the development of resistance to leptin's satiety action; Specific Aim 4: To describe the reversals in activities of the NPY and NT systems and in leptin receptor function that may be anticipated to occur in association with the hyperphagia and subsequent return to the control condition that follows withdrawl from chronic leptin infusion. Peptide neurosecretion will be evaluated by assessing synthetic potential (mRNA), availability for release (peptide content), and output (in vivo and in vitro release). Peptides and hormones will be measured by RIA. Prepro-mRNAs for neuropeptides and their receptors, and for leptin receptors will be measured by in situ hybridization technique. NPY and NT receptor activities will be assessed by ligand-binding assay. These studies will further our understanding of leptin signaling in the hypothalamus in relation to feeding, and therefore will be relevant to the development of therapeutic approaches to eating disorders.