The human mu opioid receptor: modulation of functional desensitization by calcium/calmodulin-dependent protein kinase and protein kinase C

TLDR

Opioids are powerful analgesics, yet chronic use frequently induces tolerance that limits their clinical effectiveness. The study aimed to uncover the molecular mechanisms underlying opioid tolerance by cloning the human mu opioid receptor cDNA. The cloned receptor was expressed in COS‑7 cells and Xenopus oocytes, where it bound mu‑selective ligands with high affinity and coupled to a G‑protein‑activated K⁺ channel, enabling assessment of receptor desensitization. The 400‑residue receptor, 94 % identical to the rat counterpart, exhibited functional desensitization that was enhanced by calcium/calmodulin‑dependent protein kinase and protein kinase C, indicating kinase‑mediated modulation of mu‑receptor signaling may contribute to tolerance.

Abstract

Opioids are some of the most efficacious analgesics used in humans. Prolonged administration of opioids, however, often causes the development of drug tolerance, thus limiting their effectiveness. To explore the molecular basis of those mechanisms that may contribute to opioid tolerance, 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 with 94% sequence similarity to the rat mu opioid receptor. Transient expression of this cDNA in COS-7 cells produced high-affinity binding sites to mu-selective agonists and antagonists. This receptor displays functional coupling to a recently cloned G-protein-activated K+ channel. When both proteins were expressed in Xenopus oocytes, functional desensitization developed upon repeated stimulation of the mu opioid receptor, as observed by a reduction in K+ current induced by the second mu receptor activation relative to that induced by the first. The extent of desensitization was potentiated by both the multifunctional calcium/calmodulin-dependent protein kinase and protein kinase C. These results demonstrate that kinase modulation is a molecular mechanism by which the desensitization of mu receptor signaling may be regulated at the cellular level, suggesting that this cellular mechanism may contribute to opioid tolerance in humans.