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The human mu opioid receptor: modulation of functional desensitization by calcium/calmodulin-dependent protein kinase
A Mestek1, J H Hurley, L S Bye
1Department of Medical and Molecular Genetics, Indiana University School of Medicine, Indianapolis 46202.
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.
Insights
Researchers identified the human mu opioid receptor and found that kinase modulation of its signaling desensitization may contribute to opioid tolerance. This cellular mechanism is crucial for understanding long-term opioid effectiveness.
Area of Science:
- Neuroscience
- Molecular Biology
- Pharmacology
Background:
- Opioids are effective analgesics but prolonged use leads to tolerance, reducing efficacy.
- Understanding the molecular basis of opioid tolerance is critical for developing effective pain management strategies.
Purpose of the Study:
- To isolate and characterize the human mu opioid receptor (hMOR).
- To investigate the cellular mechanisms underlying mu opioid receptor desensitization and its potential role in opioid tolerance.
Main Methods:
- Isolated cDNA for the human mu opioid receptor.
- Expressed hMOR in COS-7 cells and Xenopus oocytes.
- Assessed functional coupling with a G-protein-activated K+ channel.
- Investigated the role of protein kinases (CaMKII, PKC) in receptor desensitization.
Main Results:
- Successfully expressed functional hMOR with high-affinity binding sites.
- Demonstrated functional desensitization of hMOR signaling upon repeated stimulation.
- Showed that CaMKII and PKC potentiate hMOR desensitization.
Conclusions:
- Kinase modulation of mu opioid receptor desensitization is a key cellular mechanism.
- This mechanism likely contributes to the development of opioid tolerance in humans.
- Findings provide insights into the molecular underpinnings of opioid efficacy and tolerance.