AGONIST-INDUCED HOMOLOGOUS DESENSITIZATION OF MU-OPIOID RECEPTORS MEDIATED BY G-PROTEIN-COUPLED RECEPTOR KINASES IS DEPENDENT ON AGONIST EFFICACY

Using Xenopus laevis oocytes coexpressing mammalian mu-opioid receptor s (MORs), beta-adrenergic receptor kinase 2 (beta-ARK2) [also called G protein-coupled receptor kinase (GRK3)], and beta-arrestin 2 (beta-ar r 2), we compared the rates of beta-ARK2 (GRK3)- and beta-arr 2-mediat ed homologous receptor desensitization produced by treatment with opio id agonists of different efficacies. The response to MOR activation wa s measured using two-electrode voltage clamp as an increase in the con ductance of the coexpressed G protein-coupled inwardly rectifying pota ssium (heteromultimer of K(IR)3.1 and K(IR)3.4) channels. Treatment wi th opioids of high efficacy, either [D-Ala(2),N-MePhe(4),Gly-ol(5)]-en kephalin, fentanyl, or sufentanyl, produced a GRK3- and beta-arr 2-dep endent reduction in response in <20 min, whereas treatment with the pa rtial agonist morphine produced receptor desensitization at a signific antly slower rate. Because GRK3 requires activation and membrane targe ting by free G protein py subunits released after agonist-mediated act ivation of G proteins, a low efficacy agonist such as morphine may pro duce weak receptor desensitization as a consequence of poor GRK3 activ ation. To address this hypothesis, we substituted GRK5, a GRK that doe s not require activation by G protein beta gamma. In oocytes expressin g GRK5 instead of GRK3, both [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin and fentanyl, but not morphine, produced desensitization of MOR-activ ated potassium conductance. Thus, mu-opioid agonists produced signific ant receptor desensitization, mediated by either GRK3 or GRK5, at a ra te dependent on agonist efficacy.