DIFFERENTIAL OPIOID AGONIST REGULATION OF THE MOUSE MU-OPIOID RECEPTOR

mu opioid receptors mediate the analgesia induced by morphine. Prolong ed use of morphine causes tolerance development and dependence. To inv estigate the molecular basis of tolerance and dependence, the cloned m ouse mu opioid receptor with an amino-terminal epitope tag was stably expressed in human embryonic kidney (HEK) 293 cells, and the effects o f prolonged opioid agonist treatment on receptor regulation were exami ned, In HEK 293 cells the expressed mu receptor showed high affinity, specific, saturable binding of radioligands and a pertussis toxin-sens itive inhibition of adenylyl cyclase. Pretreatment (1 h, 3 h, or overn ight) of cells with 1 mu M morphine or [D-Ala(2)MePhe(4),Gly(ol)(5)] e nkephalin (DAMGO) resulted in no apparent receptor desensitization, as assessed by opioid inhibition of forskolin-stimulated cAMP levels. In contrast, the morphine and DAMGO pretreatments (3 h) resulted in a 3- 4-fold compensatory increase in forskolin-stimulated cAMP accumulation . The opioid agonists methadone and buprenorphine are used in the trea tment of addiction because of a markedly lower abuse potential. Pretre atment of mu receptor-expressing HEK 293 cells with methadone or bupre norphine abolished the ability of opioids to inhibit adenylyl cyclase. No compensatory increase in forskolin-stimulated cAMP accumulation wa s found with methadone or buprenorphine; these opioids blocked the com pensatory effects observed with morphine and DAMGO. Taken together, th ese results indicate that methadone and buprenorphine interact differe ntly with the mouse mu receptor than either morphine or DAMGO. The abi lity of methadone and buprenorphine to desensitize the mu receptor and block the compensatory rise in forskolin stimulated cAMP accumulation may be an underlying mechanism by which these agents are effective in the treatment of morphine addiction.