Koch T, Schulz S, Pfeiffer M, Klutzny M, Schröder H, Kahl E, Höllt V
Department of Pharmacology and Toxicology, Otto-von-Guericke University, 39120 Magdeburg, Germany.
J Biol Chem. 2001 Aug 17;276(33):31408-14. doi: 10.1074/jbc.M100305200. Epub 2001 May 18.
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.
阿片类生物碱吗啡的主要镇痛作用是由μ-阿片受体介导的。与内源性阿片肽不同，吗啡激活μ-阿片受体而不会导致其快速内吞。最近，已鉴定出小鼠μ-阿片受体（MOR1）的三种新型C末端剪接变体（MOR1C、MOR1D和MOR1E）。在本研究中，我们表明这些受体在激动剂选择性膜转运方面存在显著差异。在人胚肾293细胞中稳定表达的MOR1和MOR1C在阿片肽[d-Ala(2),Me-Phe(4),Gly(5)-ol]脑啡肽（DAMGO）存在下表现出磷酸化、内化和下调，但对吗啡无反应。相比之下，MOR1D和MOR1E对DAMGO和吗啡均表现出强烈的磷酸化、内化和下调。DAMGO在8小时暴露期间对所有四种μ-受体剪接变体引发了类似的脱敏作用（在50分钟无药间隔期间）和再敏化作用。然而，与MOR1D和MOR1E相比，吗啡处理后，MOR1和MOR1C表现出更快的脱敏作用且无再敏化作用。这些结果有力地支持了以下假设：受体磷酸化和内化是阿片受体重新激活所必需的，从而抵消激动剂诱导的脱敏作用。我们的研究结果还提出了一种机制，通过该机制μ-阿片受体的细胞和组织特异性C末端剪接可能会显著调节对吗啡各种作用的耐受性发展。
