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.