Agonist-induced desensitization and down-regulation of the human kappa opioid receptor expressed in Chinese hamster ovary cells.
J Pharmacol Exp Ther 1998;
285:28-36. [PMID:
9535991]
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Abstract
In this study, we examined whether the human kappa opioid receptor stably expressed in Chinese hamster ovary cells underwent desensitization and down-regulation after prolonged exposure to the agonist (-)U50,488H. Pretreatment with (-)U50,488H led to a reduction in the magnitude of increase in [35S]GTPgammaS binding by the subsequent application of (-)U50,488H. The extent of desensitization was related to duration of exposure and (-)U50,488H concentration. Pretreatment with (-)U50,488H also reduced the potency of (-)U50,488H in inhibiting forskolin-stimulated adenylate cyclase. In membranes of (-)U50,488H-pretreated cells, the affinity of (-)U50,488H was lower than that in the untreated control, and GTPgammaS had no effect on (-)U50,488H affinity, consistent with the notion of uncoupling of the receptor-G protein complex by (-)U50, 488H treatment. Down-regulation of the kappa opioid receptor also occurred on exposure to (-)U50,488H. Higher (-)U50,488H concentrations and/or longer incubation periods were required for down-regulation than for desensitization. The degree of down-regulation depended on the agonist concentration and incubation time. (-)U50,488H-induced desensitization and down-regulation were blocked by naloxone. (+)U50,488H, an inactive stereoisomer, did not cause desensitization or down-regulation. These results indicate that both processes were receptor-mediated. After incubation with (-)U50,488H and removal of (-)U50,488H, both (-)U50,488H-induced [35S]GTPgammaS binding and receptor number returned to the control level, which indicates that both processes were reversible. Thus, desensitization and down-regulation of the kappa opioid receptor occur after agonist exposure and represent two different adaptation mechanisms.
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