Protein Kinase-C Inhibition Within The Periaqueductal Gray Counteracts The Development Of Morphine Antinociceptive Tolerance

The ventrolateral periaqueductal gray (vlPAG) is an important midbrain region that regulates the descending pain modulatory system and is implicated in the development of antinociceptive tolerance to opioid analgesics. Protein Kinase-C (PKC) is a Ser/Thr kinase that phosphorylates the μ-opioid receptor (MOPr) following morphine activation. This phosphorylation is responsible for desensitization of the receptor, resulting in the termination of G-Protein signaling. It is speculated that PKC-mediated desensitization of the MOPr does not readily lead to the internalization of the receptor, thus contributing to tolerance. Previous studies have shown that intracerebroventricular administration of PKC inhibitors prevented the development of morphine tolerance in mice. Further, a non-specific PKC/GRK inhibitor was able to block the development of morphine tolerance in the vlPAG. However, the contribution of PKC to morphine tolerance in the vlPAG has not been directly explored. Using immunofluorescence staining, we observed increased phosphorylated PKC that is co-localized with the MOPr at the cell membrane of vlPAG neurons in morphine-tolerant animals. Further, we assessed the development of morphine antinociceptive tolerance following pre-treatment with the PKC inhibitor, Gö-7874, which targets conventional PKC isoforms. Repeated microinjections of the compound into the vlPAG of wild-type mice in combination with morphine led to a reduction in the development of morphine tolerance. Interestingly, repeated microinjections of Gö-7874 alone led to an enhancement of subsequent morphine antinociception. Lastly, morphine-induced changes in intracellular cAMP levels were assessed in-vitro, following pre-treatment with Gö-7874. This study contributes to the understanding of PKC-mediated MOPr desensitization processes leading to morphine tolerance.