Abstract

The goal of the present study was to investigate a possible role for regulators of G protein-signaling (RGS) proteins in opioid receptor (OR) desensitization using cultured Xenopus laevis dermal melanophores. Morphine-induced pigment aggregation in a melanophore cell line stably expressing the murine μ OR (μOR) was quantified over time. Responses of the μOR (a G_i-linked receptor) exhibited a time-dependent desensitization, which varied with the concentration of morphine used. In contrast, much less desensitization was observed in response to melatonin, effects mediated through the cells’ endogenous melatonin receptor (which is also G_i-linked). To further study OR desensitization, melanophores lacking a μOR were transiently transfected with plasmids encoding the μOR alone or in combination with plasmids encoding one of several RGS subtypes (RGS1, RGS2, RGS3, or RGS4). Overexpression of RGS2, but not the other RGS subtypes, produced a rightward shift in the morphine concentration-response curve. RGS protein overexpression also decreased the magnitude of morphine-induced responses. Finally, the effect of a mutant form of Gα_i1, which is insensitive to RGS action, was investigated with respect to its ability to alter the response of the μOR to morphine. Expression of the mutant Gα_i1prolonged morphine-induced pigment aggregation and produced leftward shifts in concentration-response curves, compared with expression of wild-type Gα_i1. These results demonstrate that specific RGS proteins can dampen signals initiated by agonist activation of the μOR, and support a possible role for RGS proteins in OR desensitization.