Methadone-induced desensitization of the delta-opioid receptor is mediatedby uncoupling of receptor from G protein

Chronic exposure of neuroblastoma x glioma (NG108-15) hybrid cells and rat mu-receptor-transfected Chinese hamster ovary (CHO) cells to 10 mu M morphi ne resulted in a compensatory and antagonist-precipitated increase in cAMP accumulation. However. incubation of these cells with 10 mu M methadone dur ing chronic exposure to morphine substantially prevented the actions of mor phine. Chronic methadone treatment caused a pronounced I eduction in agonis t-stimulated binding of [S-35]GTP gamma S to G proteins, but it did not pro duce significant down-regulation of delta-opioid receptors, whereas chronic morphine treatment failed to induce either uncoupling of delta-opioid rece ptors from G proteins or down-regulation of delta-opioid receptors. In cont rast to chronic treatment with morphine alone, treatment of cells with morp hine and methadone simultaneously resulted in a significant decrease in ago nist-stimulated binding of [S-35]JGTP gamma S to G proteins. The action of methadone-mediated uncoupling of the receptor from the G protein was blocke d by the nonselective protein kinase inhibitor [1-(5-isoqinolinesulfony)-2- methylpiprazine](H-7), but not by the specific protein kinase C inhibitor, chelerythrine. The data demonstrate that methadone desensitizes the delta-o pioid receptor by uncoupling the receptor from the G protein. In this way, methadone antagonizes the morphine-mediated adaptive sensitization and over shoot of adenylate cyclase. The functional desensitization of opioid recept ors by methadone may explain why methadone is effective in the treatment of morphine dependence. (C) 1999 Elsevier Science B.V. All rights reserved.