Our laboratory studies the organization of spinal cord circuits that contribute to the generation of persistent pain and to the mechanisms through which opioids regulate the transmission of nociceptive messages. During the previous years of this proposal, we used Fos expression as a marker of neuronal activity and demonstrated that analgesia produced by acute administration of mu, delta or kappa selective opioid agonists is associated with inhibition of Fos expression in different populations of spinal cord neurons. By contrast, chronic morphine resulted in enhanced noxious stimulus-evoked Fos expression. Based on the pattern of withdrawal induced Fos expression, we hypothesized that morphine tolerance is associated with the development of compensatory responses that occur in superficial dorsal horn neurons that express the opioid receptor or in related neuronal circuits and that the compensatory response underlies a latent sensitization that is comparable to the central sensitization of dorsal horn neurons produced by injury stimuli. To test the hypothesis that dorsal horn neurons are sensitized in the setting of tolerance we will perform electrophysiological studies of the magnitude of injury-induced sensitization in the spinal cord of morphine-tolerant animals. We will also study the patterns of withdrawal-induced spinal cord Fos expression rendered tolerant to receptor selective opioid agonists. Based on our studies in mice that lack the gene that encodes the gamma isoform of protein kinase C (PKCgamma) we hypothesize that PKCgamma -containing interneurons of the superficial dorsal horn are critical to the development of injury-induced persistent pain states, central sensitization and to the magnitude of opioid tolerance and dependence.We will perform a series of pharmacological and electrophysiological studies to test this hypothesis and will begin to address the neurochemistry and the afferent connections of the PKCgamma-containing interneurons. Taken together these studies will provide important information on the spinal cord mechanisms through which injury and opioid tolerance-associated changes in nociceptive processing are produced.
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