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AbstractIn the guinea pig colon, chronic sympathetic denervation entails supersensitivity to inhibitory -opioid agents modulating cholinergic neurons. The mechanism underlying such adaptive change has not yet been unravelled, although protein kinase C (PKC) may be involved. A previous study indirectly demonstrated that activation of -opioid receptors on myenteric neurons facilitates PKC activity. Such coupling may counteract the inhibitory action of -opioid agents on acetylcholine overflow, since PKC, per sè, increases this parameter. After chronic sympathetic denervation such restraint abates, representing a possible mechanism for development of supersensitivity to -opioid agents. In the present study, this hypothesis was further investigated. After chronic sympathetic denervation, Ca ++ -dependent PKC activity was reduced in colonic myenteric plexus synaptosomes. The -opioid agent, DAMGO, increased Ca ++ -dependent PKC activity in synaptosomes obtained from normal, but not from denervated animals. In myenteric synaptosomes obtained from this experimental group, protein levels of Ca ++ -dependent PKC isoforms I, II and decreased, whereas levels increased. In wholemount preparations, the four Ca ++ -dependent PKC isoforms co-localized with -opioid receptors on subpopulations of colonic myenteric neurons. The percentage of neurons staining for PKCII, as well as the number of -opioid receptor-positive neurons staining for PKCII, decreased in denervated preparations. The same parameters related to PKC, I or remained unchanged. Overall, the present data strengthen the concept that -opioid receptors located on myenteric neurons are coupled to Ca ++ -dependent PKCs. After chronic sympathetic denervation, a reduced efficiency of this coupling may predominantly involve PKCII, although also PKCI and , but not PKC, may be implicated.