We plan to study the effects of chronic morphine exposure on the opioid receptors in the hippocampal region of the rat central nervous system (CNS). Our hypothesis is that cellular tolerance to chronic morphine exposure is the result of a reduction in opioid receptor coupling to its signal transduction system rather than a reduction in receptor affinity or receptor number. This proposal describes a series of experiments designed to test that hypothesis. Using extracellular electrophysiological recording methods, the opioid receptor types mediating the response of hippocampal pyramidal cells to morphine-like (opioid) compounds will be defined. Quantitative dose-effect relationships will be determined for selected opioid agonists to measure their potency changes in the morphine-tolerant hippocampal preparation. Using electrophysiological measures, the molecular basis for the reduction in agonist potency will be determined by comparing the opioid receptor affinities and agonist efficacies of selected opioids in hippocampal slices prepared from morphine-tolerant and drug-naive rats. This will require the adaptation of partial receptor inactivation methods originally developed for use in opioid sensitive smooth muscle bioassays. Using in vitro ligand binding assays selective for mu and delta sites, we will also compare the opioid receptor affinities and binding site densities in morphine-tolerant and naive rat hippocampus. If tolerance is a consequence of a change in the post receptor binding events as we predict, a likely site of change would be the coupling between the receptor and the signal transduction system. The proposed link between opioid receptors and GTP-coupling protein (Ni) will be determined by measuring the sensitivity of opioid effect to pertussis toxin treatment of the rat hippocampal slice. We will determine whether morphine-tolerance increases the pertussis toxin sensitivity as might be predicted by the expected change in agonist efficacy. The primary goal of these studies is an understanding of the cellular and molecular mechanisms used by the brain to adapt to chronic opiate exposure. Opiate addiction is known to have behavioral, cellular, and molecular components. Information will be obtained in this study to describe the latter mechanisms. By defining the specific molecular events underlying tolerance development in a model CNS system, we expect ultimately to have a clearer concept of the relevant clinical processes.
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