Abstract
In the guinea pig colon, chronic sympathetic denervation entails supersensitivity to inhibitory μ-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 μ-opioid receptors on myenteric neurons facilitates PKC activity. Such coupling may counteract the inhibitory action of μ-opioid agents on acetylcholine overflow, since PKC, per se, increases this parameter. After chronic sympathetic denervation such restraint abates, representing a possible mechanism for development of supersensitivity to μ-opioid agents. In the present study, this hypothesis was further investigated. After chronic sympathetic denervation, Ca 2+-dependent PKC activity was reduced in colonic myenteric plexus synaptosomes. The μ-opioid agent, DAMGO, increased Ca 2+-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 2+-dependent PKC isoforms βI, βII and γ decreased, whereas α levels increased. In whole-mount preparations, the four Ca 2+-dependent PKC isoforms co-localized with μ-opioid receptors on subpopulations of colonic myenteric neurons. The percentage of neurons staining for PKCβII, as well as the number of μ-opioid receptor-positive neurons staining for PKCβII, decreased in denervated preparations. The same parameters related to PKCα, βI or γ remained unchanged. Overall, the present data strengthen the concept that μ-opioid receptors located on myenteric neurons are coupled to Ca 2+-dependent PKCs. After chronic sympathetic denervation, a reduced efficiency of this coupling may predominantly involve PKCβII, although also PKCβI and γ, but not PKCα, may be implicated.
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