Modulation of G protein coupled receptor function

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Opioid agonists like morphine, codeine and meperidine remain the most commonly used and effective treatment for chronic pain conditions. Despite the fact that chronic opioid treatment can produce high levels of tolerance and physical dependence. Mechanisms of tolerance and dependence for in brain are not well understood, but it is clear that chronic opioid treatment produces significant receptor desensitization in specific brain regions. Moreover, pain itself, as well as concurrent treatment with drugs like adenosine and alpha2- adrenergic agonists, alter the sensitivity of patients to opioid treatment. Opioid receptors (including mu, delta and kappa types), as well as adenosine A1 and alpha2-adrenergic receptors, are G-protein-coupled receptors, and their ability to activate signal transduction systems can be determined by the ability of agonists to stimulate [35S]GTPgammaS binding in both membranes and section autoradiography. This project will examine regulation of several receptor/G-protein interactions in rat spinal cord, using models of drug treatment, self-administration and chronic pain developed by other Center components. First, after determining the acute efficacies of opioid and adenosine A1 agonists in activating G-proteins in spinal cord, the ability of chronic drug exposure to produce receptor desensitization will be examined in both spinal cord membranes and by autoradiography. These treatments will include chronic intrathecal opioid administration to desensitize opioid receptor-activated G-proteins, and chronic intrathecal administration of adenosine and clonidine to desensitize agonist-stimulated incorporation of [32P]AAGTP into specific G-protein subunits. Second, various receptor-activated G-protein activities will be determined in both brains ans spinal cords from spinal nerve ligated rats to determine whether chronic pain and hypersensitivity affect receptor/G-protein coupling. These studies will also determine how chronic pain states modulate basal levels and activities of G-proteins in spinal cord. Third, the ability of NMDA antagonists and protein kinase C inhibitors to modulate receptor desensitization will be tested after chronic intrathecal administration of drugs. Information from this project will help design studies in the Clinical Core to test the use of opioid agonists of differing efficacies in treating chronic pain. Moreover, these studies will provide information to minimize tolerance in long-term drug treatment of chronic pain.