Mu opioid receptor mutant, T394A, abolishes opioid-mediated adenylyl cyclase superactivation.

This study was to characterize the effects of a point-mutant at C-terminal of mu opioid receptor (MOR), namely MOR T394A, in chronic opioid-induced cellular responses. After 18 h of exposure to [D-Ala, N-Me-Phe, Gly-ol] enkephalin (DAMGO), adenylyl cyclase (AC) superactivation, a hallmark for the cellular adaptive response after chronic opioid stimulation, was observed in the cells expressing wild-type receptor, but was totally abolished in the cells expressing MOR T394A. Receptor phosphorylation was also attenuated in cells with MOR T394A after prolonged preexposure to agonist. Furthermore, MAP kinase kinase-1 (MKK1) overexpression was able to rescue AC superactivation in cells with MOR T394A, but showed no effect in the wild-type MOR-expressing cells. These results indicated that the amino acid T394 at C-terminus of MOR played a critical role in chronic agonist-induced AC superactivation and receptor phosphorylation.