title: Methadone-induced desensitization of the delta-opioid receptor is mediated by uncoupling of receptor from G protein.
abstract: Chronic exposure of neuroblastoma x glioma (NG108-15) hybrid cells and rat mu-receptor-transfected Chinese hamster ovary (CHO) cells to 10 microM morphine resulted in a compensatory and antagonist-precipitated increase in cAMP accumulation. However, incubation of these cells with 10 microM methadone during chronic exposure to morphine substantially prevented the actions of morphine. Chronic methadone treatment caused a pronounced reduction in agonist-stimulated binding of [35S]GTPgammaS to G proteins, but it did not produce significant down-regulation of delta-opioid receptors, whereas chronic morphine treatment failed to induce either uncoupling of delta-opioid receptors from G proteins or down-regulation of delta-opioid receptors. In contrast to chronic treatment with morphine alone, treatment of cells with morphine and methadone simultaneously resulted in a significant decrease in agonist-stimulated binding of [35S]GTPgammaS to G proteins. The action of methadone-mediated uncoupling of the receptor from the G protein was blocked by the nonselective protein kinase inhibitor [1-(5-isoqinolinesulfony)-2-methylpiprazine](H7), but not by the specific protein kinase C inhibitor, chelerythrine. The data demonstrate that methadone desensitizes the delta-opioid receptor by uncoupling the receptor from the G protein. In this way, methadone antagonizes the morphine-mediated adaptive sensitization and overshoot of adenylate cyclase. The functional desensitization of opioid receptors by methadone may explain why methadone is effective in the treatment of morphine dependence.
authors.affiliation: Department of Neuropharmacology, Beijing Institute of Pharmacology and Toxicology, China. jgliu@mailexcite.com
authors.forename: J G
authors.initials: JG
authors.lastname: Liu
authors.forename: X P
authors.initials: XP
authors.lastname: Liao
authors.forename: Z H
authors.initials: ZH
authors.lastname: Gong
authors.forename: B Y
authors.initials: BY
authors.lastname: Qin
mesh_terms: D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D000242:Cyclic AMP / Q000378:metabolism; D015536:Down-Regulation / Q000187:drug effects; D004347:Drug Interactions; D019204:GTP-Binding Proteins / Q000378:metabolism*; D005910:Glioma / Q000473:pathology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism*; D008691:Methadone / Q000494:pharmacology*; D009020:Morphine / Q000494:pharmacology; D009447:Neuroblastoma / Q000473:pathology*; D011485:Protein Binding; D051381:Rats; D017465:Receptors, Opioid, delta / Q000187:drug effects*; D013997:Time Factors; D014407:Tumor Cells, Cultured
chemical_list: D017465:Receptors, Opioid, delta; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D009020:Morphine; D000242:Cyclic AMP; D019204:GTP-Binding Proteins; D008691:Methadone