Involvement of Ca2+-dependent PKCs in the adaptive changes of mu-opioid pathways to sympathetic denervation in the guinea pig colon.

In the guinea pig colon, chronic sympathetic denervation entails supersensitivity to inhibitory m-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 m-opioid receptors on myenteric neurons facilitates PKC activity. Such coupling may counteract the inhibitory action of m-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 m-opioid agents. In the present study, this hypothesis was further investigated. After chronic sympathetic denervation, Ca2+- dependent PKC activity was reduced in colonic myenteric plexus synaptosomes. The m-opioid agent, DAMGO, increased Ca2+-dependent PKC activity in synaptosomes obtained from normal, but not from denervated animals. In myenteric synaptosomes obtained from this experimental group, protein levels of Ca2+-dependent PKC isoforms bI, bII and g decreased, whereas a levels increased. In whole-mount preparations, the four Ca2+-dependent PKC isoforms co-localized with m-opioid receptors on subpopulations of colonic myenteric neurons. The percentage of neurons staining for PKCbII, as well as the number of m-opioid receptor-positive neurons staining for PKCbII, decreased in denervated preparations. The same parameters related to PKCa, bI or g remained unchanged. Overall, the present data strengthen the concept that m-opioid receptors located on myenteric neurons are coupled to Ca2+- dependent PKCs. After chronic sympathetic denervation, a reduced efficiency of this coupling may predominantly involve PKCbII, although also PKCbI and g, but not PKCa, may be implicated.