Abstract

The human μ-opioid receptor (HμOR) is a G-protein coupled receptor that mediates analgesia, euphoria and other important central and peripheral neurological functions. In this study, we found in a yeast two-hybrid screen that a protein kinase C-interacting protein (PKCI) specifically interacts with the C terminus of HμOR. The interaction of PKCI with HμOR was recapitulated in Chinese hamster ovary cells that express the full-length HμOR and PKCI proteins. The affinity of HμOR for an opioid ligand and its ability to mediate the activation of a G-protein were not altered by their interaction. However, the association of PKCI with HμOR reduced agonist-induced inhibition of adenylyl cyclase and suppressed HμOR desensitization partially at the G protein level and completely at the adenylyl cyclase level. Furthermore, PMA-induced, but not DAMGO-induced, HμOR phosphorylation was partially inhibited by the coexpression of PKCI, suggesting that PKCI exerts a selective regulatory effect on HμOR signaling. This effect was specific to the μ-opioid receptor because δ-opioid receptor desensitization was unaffected by PKCI. In addition, behavioral studies revealed that both basal and morphine-induced analgesia were significantly enhanced in the mutant mice that lacked expression of PKCI gene, and these mice developed a greater extent of tolerance to morphine analgesia. Taken together, these results suggest that PKCI functions as a negative regulator in HμOR desensitization, phosphorylation, and in mediating morphine analgesia.