Desensitization of endogenously expressed δ-opioid receptors: no evidence for involvement of G protein-coupled receptor kinase 2

Abstract

The involvement of G protein-coupled receptor kinase 2 (GRK2) in the agonist-induced desensitization of δ-opioid receptor-mediated inhibition of cAMP formation in NG108-15 mouse neuroblastoma×rat glioma hybrid cells was investigated. Pretreatment of wild-type cells with the δ-opioid receptor agonist [ d-Pen 2,5]-enkephalin (DPDPE; 100 nM) for as little as 5 min produced marked desensitization of subsequent DPDPE-mediated inhibition of iloprost (300 nM)-stimulated cAMP formation. In NG108-15 cells stably overexpressing wild-type GRK2 or dominant negative mutant GRK2 (DNM GRK2), the DPDPE-induced desensitization of cAMP inhibition was the same as in plasmid-transfected control cells. Pretreatment of wild-type cells with the inhibitors of receptor internalization, concanavalin A (0.25 mg ml −1) or hypertonic sucrose (0.4 M), also failed to inhibit DPDPE-mediated desensitization. Finally, in NG108-15 cells stably overexpressing G protein-coupled receptor kinase 6 (GRK6), DPDPE-induced desensitization was significantly increased as compared to plasmid-transfected control cells. These results indicate that GRK2 is unlikely to mediate the desensitization of endogenous δ-opioid receptors in NG108-15 cells, but that other GRKs, such as GRK6, may be more important.