The proposed research will explore the nature of the dynorphins, a class of endogenous opioid peptides, interaction with opioid receptors in a particular brain region of the rat and guinea pig. The goal of these studies is to provide a detailed understanding of the actions of an endogenous neuropeptide at a site of its synthesis and release, and thus ultimately to provide an important model of endogenous opioid action. Our specific hypothesis is that the dynorphin peptides are synthesized by dentate granule cells and released at their mossy fiber terminals to control the excitability of hippocampal pyramidal neurons. Preliminary evidence obtained has supported this idea: We have demonstrated the presence of the dynorphin opioid peptides in the rat hippocampus, the in vitro release of these peptides from hippocampal slices, and the effects of exogenously applied opioids on hippocampal cell excitability. These data strongly suggest that the dynorphin opioids are likely to be neurotransmitters in the rat hippocampus. We plan to test our hypothesis by further delineating the actions of opioids and the properties of the opioid receptors present in this brain region. Electrophysiological recording methods will be used to measure responses to the dynorphins in hippocampal slices; the pharmacological effects of opioids will be quantified; responses to endogenously released opioids will be identified, and the opioid receptor types mediating the effects of both exogenously applied and endogenously released opioids will be defined. To accomplish these goals we will record the effects of the dynorphins applied to likely hippocampal cell targets in the pyramidal and dentate granule cell layers. We propose to 1) define the effects of dynorphin by intracellularly recording cell excitability while applying peptide by micropipette; 2) characterize the specific opioid receptors that mediate the pharmacological effects of the dynorphins at opioid sensitive sites in the hippocampus; 3) identify the effects of endogenously released dynorphins by stimulating the peptide-containing fiber tracts and measuring the opioid antagonist sensitive effects.