To investigate the cellular and molecular basis for using methadone in substitution therapy for morphine addiction, the difference between methadone and morphine in causing desensitization of delta-opioid receptors was examined, and the effects of methadone pretreatment on opiate-induced inhibition of forskolin-stimulated cAMP accumulation was studied. Methadone substantially attenuated the ability of [D-Ala2,D-Leu5]enkephalin (DADLE), morphine and methadone to inhibit forskolin-stimulated cAMP accumulation. Methadone was able to block the morphine-induced compensatory increase in intracellular cAMP levels and naloxone-precipitated cAMP overshoot after chronic exposure to morphine. The protein kinase inhibitor (1-5-isoquinolinesulfony)-2-methylpiperazine) (H7) could significantly block the chronic methadone treatment-induced loss of the ability of DADLE to inhibit adenylate cyclase. The protein kinase inhibitor chelerythrine was able to block the acute methadone treatment-induced loss of the ability of DADLE to inhibit adenylate cyclase. In contrast, morphine did not cause a substantial desensitization of the delta-opioid receptor. These results indicate that methadone is different from morphine in its regulation of the delta-opioid receptor. In addition, these results also indicate that the mechanisms of delta-opioid receptor desensitization induced by acute and chronic methadone treatment are different.