Adaptations to chronic agonist exposure of mu-opioid receptor-expressing Chinese hamster ovary cells.

To investigate cellular adaptation responses induced by chronic agonist treatment of the mu-opioid receptor, Chinese hamster ovary (CHO) cells were stably transfected with the rat mu-opioid receptor cDNA. Chronic treatment with agonists selective for the mu-opioid receptor, [D-Ala2, N-MePhe4, Gy-ol5]enkephalin (DAMGO), morphine and fentanyl, time- and dose-dependently induced down-regulation of the mu-opioid receptor. The down-regulation was not significantly affected by pretreatment with pertussis toxin, but was completely blocked by treatment with hypertonic sucrose, suggesting that receptor internalization mediated by clathrin-coated vesicles is an essential step in the mu-opioid receptor down-regulation. On the other hand, forskolin-stimulated cyclic AMP formation was increased by chronic DAMGO treatment, which was inhibited by pertussis toxin pretreatment. These results indicate that two adaptation responses induced by chronic agonist treatment of the mu-opioid receptor-expressing CHO cells, down-regulation of the mu-opioid receptor and supersensitization of adenylate cyclase, are mediated by distinct mechanisms.