Vagal CCK-A receptors exist in high and low affinity states. In the last funding period we demonstrated that CCK at physiological levels acts on vagal high affinity CCK-A receptors to mediate pancreatic secretion. Vagal CCK receptors also appear to play an important role in short term control of food intake, likely mediated by low affinity CCK-A receptors. The doses of CCK required to induce satiety however is much higher than postprandial levels. Studies indicate a synergistic interaction between vagal CCK and leptin receptors to regulate short term food intake, which may lower the dose of CCK required to induce satiety. We hypothesize CCK mediates pancreatic secretion and satiety using two different vagal afferent signaling pathways. One group of nodose neurons contain high affinity CCK-A receptors, which mediate pancreatic secretion via the vago-vagal cholinergic reflex. A second group of nodose ganglia neurons contain both low affinity CCK and leptin receptors; CCK enhances the leptin signal transduction pathway by amplifying the signaling of STAT 3 through Src kinase. These neurons project to the NTS and then the hypothalamus to control short term eating behavior. This proposal characterizes the neurons containing both CCK and leptin receptors and investigates the intracellular mechanisms by which CCK amplifies the leptin signal transduction pathway. We will demonstrate that leptin receptors co-localize with low but not high affinity CCK-A receptors, then show that pretreatment enhances leptin responsiveness of a specific group of gastric vagal afferent fibers utilizing an in vitro isolated stomach-vagus nerve preparation. Electrophysiological and biological characteristics of these fibers will be characterized and contrasted with fibers responding only to the high affinity CCK-A receptor agonist JMV 180. We will then characterize the chemical codings utilized by the two groups of nodose ganglia neurons by intracellular recording and labeling techniques. The mechanism by which CCK enhances the leptin signal transduction pathway will be examined by patch clamp studies using isolated neurons from nodose ganglia various Src and PI3 antagonists. Neuroblastoma SY5Y cell lines transfected with CCK-A receptor and Src dominant negative gene will be used to dissect the interaction between Src and STAT 3 for the amplification of electrical firings. Lastly, chemical studies to examine the mechanism by which CCK enhances STAT 3 phosphorylation evoked by leptin will be done. These studies will provide a detailed characterization of how vagal low affinity CCK receptors mediate short- term satiety and delineate the intracellular mechanism by which CCK interacts with leptin to enhance the STAT 3 signaling cascade. Failure of this neural pathway may result in hyperphagia resulting in obesity.