DELTA-OPIOID RECEPTOR DOWN-REGULATION IS INDEPENDENT OF FUNCTIONAL G-PROTEIN YET IS DEPENDENT ON AGONIST EFFICACY

Chronic treatment of C6 glioma cells stably expressing the rat delta o pioid receptor (C6 delta) with full agonists resulted in receptor down -regulation. Chronic [D-Ser(2),L-Leu(5)]enkephalyl-Thr treatment cause d a decrease in cell surface as well as a decrease in agonist-stimulat ed [S-35]guanosine-5'-O-(3-thio)-triphosphate binding. Treatment with full agonists for 12 hr resulted in a 90% decrease in receptor number that was paralleled by a decrease in the ability of agonist to stimula te [S-35]guanosine-5-O-(3-thio)triphosphate binding and inhibit forsko lin-stimulated adenylyl cyclase. Of the remaining receptors, a smaller fraction of receptors (41 +/- 4 vs. 56 +/- 4% in control) exhibited h igh affinity for agonist as compared to receptors in control membranes . Elimination of functional guanosine triphosphate binding protein (G protein) by Pertussis toxin pretreatment did not alter the ability of agonist to down regulate receptor. We hypothesized that agonist affini ty (not efficacy) would be a predictor of an agonist's ability to down regulate receptor. However, we found that only full agonists were able to down-regulate receptor number, G protein activation and adenylyl c yclase inhibition. Chronic exposure to partial agonist 7-spiroindinoox ymorphone, which has a very high affinity for the receptor, as well as morphine, did not cause receptor down-regulation. Taken together, the se results suggest that full agonists alter receptor conformation such that the altered conformation is recognized by G protein as well as p roteins involved in receptor down-regulation. in addition, down-regula tion is independent of agonist-mediated G protein activation and subse quent down-stream signaling.