THE HUMAN MU-OPIOID RECEPTOR - MODULATION OF FUNCTIONAL DESENSITIZATION BY CALCIUM CALMODULIN-DEPENDENT PROTEIN-KINASE AND PROTEIN-KINASE-C

Opioids are some of the most efficacious analgesics used in humans. Pr olonged administration of opioids, however, often causes the developme nt of drug tolerance, thus limiting their effectiveness. To explore th e molecular basis of those mechanisms that may contribute to opioid to lerance, we have isolated a cDNA for the human mu opioid receptor, the target of such opioid narcotics as morphine, codeine, methadone, and fentanyl. The receptor encoded by this cDNA is 400 amino acids long wi th 94% sequence similarity to the rat mu opioid receptor. Transient ex pression of this cDNA in COS-7 cells produced high-affinity binding si tes to mu-selective agonists and antagonists. This receptor displays f unctional coupling to a recently cloned G-protein-activated K+ channel . When both proteins were expressed in Xenopus oocytes, functional des ensitization developed upon repeated stimulation of the mu opioid rece ptor, as observed by a reduction in K+ current induced by the second m u receptor activation relative to that induced by the first. The exten t of desensitization was potentiated by both the multifunctional calci um/calmodulin-dependent protein kinase and protein kinase C. These res ults demonstrate that kinase modulation is a molecular mechanism by wh ich the desensitization of mu receptor signaling may be regulated at t he cellular level, suggesting that this cellular mechanism may contrib ute to opioid tolerance in humans.