Agonist Induced Homologous Desensitization of μ-Opioid Receptors Mediated by G Protein-Coupled Receptor Kinases Is Dependent on Agonist Efficacy

Abstract

Using Xenopus laevis oocytes coexpressing mammalian μ-opioid receptors (MORs), β-adrenergic receptor kinase 2 (β-ARK2) [also called G protein-coupled receptor kinase (GRK3)], and β-arrestin 2 (β-arr 2), we compared the rates of β-ARK2 (GRK3)- and β-arr 2-mediated homologous receptor desensitization produced by treatment with opioid agonists of different efficacies. The response to MOR activation was measured using two-electrode voltage clamp as an increase in the conductance of the coexpressed G protein-coupled inwardly rectifying potassium (heteromultimer of K_IR3.1 and K_IR3.4) channels. Treatment with opioids of high efficacy, either [d-Ala²,N-MePhe⁴,Gly-ol⁵]-enkephalin, fentanyl, or sufentanyl, produced a GRK3- and β-arr 2-dependent reduction in response in <20 min, whereas treatment with the partial agonist morphine produced receptor desensitization at a significantly slower rate. Because GRK3 requires activation and membrane targeting by free G protein βγ subunits released after agonist-mediated activation of G proteins, a low efficacy agonist such as morphine may produce weak receptor desensitization as a consequence of poor GRK3 activation. To address this hypothesis, we substituted GRK5, a GRK that does not require activation by G protein βγ. In oocytes expressing GRK5 instead of GRK3, both [d-Ala²,N-MePhe⁴,Gly-ol⁵]enkephalin and fentanyl, but not morphine, produced desensitization of MOR-activated potassium conductance. Thus, μ-opioid agonists produced significant receptor desensitization, mediated by either GRK3 or GRK5, at a rate dependent on agonist efficacy.