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C-terminal splice variants of the mouse mu-opioid receptor differ in morphine-induced internalization and receptor
1Department of Pharmacology and Toxicology, Otto-von-Guericke University, 39120 Magdeburg, Germany. Thomas.Koch@Medizin.Uni-Magdeburg.de
Abstract:
The main analgesic effects of the opioid alkaloid morphine are mediated by the mu-opioid receptor. In contrast to endogenous opioid peptides, morphine activates the mu-opioid receptor without causing its rapid endocytosis. Recently, three novel C-terminal splice variants (MOR1C, MOR1D, and MOR1E) of the mouse mu-opioid receptor (MOR1) have been identified. In the present study, we show that these receptors differ substantially in their agonist-selective membrane trafficking. MOR1 and MOR1C stably expressed in human embryonic kidney 293 cells exhibited phosphorylation, internalization, and down-regulation in the presence of the opioid peptide [d-Ala(2),Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO) but not in response to morphine. In contrast, MOR1D and MOR1E exhibited robust phosphorylation, internalization, and down-regulation in response to both DAMGO and morphine. DAMGO elicited a similar desensitization (during an 8-h exposure) and resensitization (during a 50-min drug-free interval) of all four mu-receptor splice variants. After morphine treatment, however, MOR1 and MOR1C showed a faster desensitization and no resensitization as compared with MOR1D and MOR1E. These results strongly reinforce the hypothesis that receptor phosphorylation and internalization are required for opioid receptor reactivation thus counteracting agonist-induced desensitization. Our findings also suggest a mechanism by which cell- and tissue-specific C-terminal splicing of the mu-opioid receptor may significantly modulate the development of tolerance to the various effects of morphine.
Insights
Novel mu-opioid receptor (MOR1) splice variants MOR1D and MOR1E traffic differently than MOR1 and MOR1C. This differential trafficking influences morphine
Area of Science:
- Pharmacology
- Neuroscience
- Molecular Biology
Background:
- The analgesic effects of morphine are primarily mediated by the mu-opioid receptor (MOR1).
- Morphine's interaction with MOR1 differs from endogenous opioid peptides, notably in receptor endocytosis.
- Three new C-terminal splice variants of MOR1 (MOR1C, MOR1D, MOR1E) have been identified.
Purpose of the Study:
- To investigate the differential membrane trafficking of MOR1 splice variants in response to agonists.
- To determine how C-terminal splicing influences receptor phosphorylation, internalization, and desensitization.
- To explore the implications of these trafficking differences for morphine tolerance.
Main Methods:
- Stable expression of MOR1 and its splice variants (MOR1C, MOR1D, MOR1E) in human embryonic kidney 293 cells.
- Treatment with the opioid peptide [d-Ala(2),Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO) and morphine.
- Assessment of receptor phosphorylation, internalization, and down-regulation.
- Analysis of receptor desensitization and resensitization kinetics.
Main Results:
- MOR1 and MOR1C showed phosphorylation, internalization, and down-regulation with DAMGO but not morphine.
- MOR1D and MOR1E exhibited robust responses to both DAMGO and morphine.
- All variants desensitized with DAMGO, but MOR1 and MOR1C desensitized faster and did not resensitize with morphine compared to MOR1D and MOR1E.
Conclusions:
- Receptor phosphorylation and internalization are crucial for opioid receptor reactivation and counteracting desensitization.
- Cell- and tissue-specific C-terminal splicing of MOR1 significantly impacts the development of morphine tolerance.
- Differential trafficking of MOR1 splice variants provides a mechanism for varied responses to morphine.