OPIOID-CHOLINERGIC INTERACTIONS IN THE HIPPOCAMPUS

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The proposed work seeks to define the interaction between opioid peptides and acetylcholine in the hippocampus. Although there is evidence that both systems play important roles in memory and seizure activity in the hippocampus, there is little information on the interactions between the two systems. The effect of the major hippocampal opioid peptides--enkephalins and dynorphin A-- on biochemical and behavioral responses associated with the muscarinic cholinergic system will be investigated. The experiments are designed to distinguish between direct opioid effects and those that are mediated through the cholinergic system. Prototypical mu (DAGO), delta ((D-Pen 2,5) enkephalin) and kappa (dynorphin A) agonists will be examined for their abilities to affect the muscarinic system at different levels. Biochemical measures include second messenger systems, density and subtype distribution of muscarinic receptors and acetylcholine release. The second messenger systems to be examined are phosphtidylinositol turnover and adenylate cyclase and GTPase activities, which will be measured utilizing in vitro slice and membrane preparations. The effects of acute and chronic intrahippocampal injections of the opioid peptides on muscarinic receptors will be examined by autoradiography using (3H)- quinuclidinyl benzilate as a non-selective ligand and its inhibition by pirenzepine and gallamine as selective M1 and M2 muscarinic antagonists, respectively. Acetylcholine release is assessed by potassium-stimulated of superfused slices, which are preloaded with (3H)-choline. Effects of acute and chronic opioid treatment on representational memory (i.e., memory dependent on cues not present at the time of choice) is measured by performance of rats on a paired alternation task in a T-maze. The effects of chronic opioid treatment on memory impairments induced by pirenzepine and seizures induced by gallamine will provide information on muscarinic receptor-mediated behaviors that are subtype specific. The hippocampus is a structure known to be involved in memory and seizures in both humans and experimental animals. Interactions between opioid and muscarinic systems in the hippocampus may be operative in humans and contribute to behavioral problems associated with drug abuse.