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 receptors (MORs), beta-adrenergic receptor kinase 2 (beta-ARK2) [also called G protein-coupled receptor kinase (GRK3)], and beta-arrestin 2 (beta-arr 2), we compared the rates of beta-ARK2 (GRK3)- and beta-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 KIR3.1 and KIR3.4) channels. Treatment with opioids of high efficacy, either [D-Ala2,N-MePhe4,Gly-ol5]-enkephalin, fentanyl, or sufentanyl, produced a GRK3- and beta-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 betagamma 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 betagamma. 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, mu-opioid agonists produced significant receptor desensitization, mediated by either GRK3 or GRK5, at a rate dependent on agonist efficacy.
Kovoor
A
A
Department of Pharmacology, University of Washington, Seattle, Washington 98195-7280, USA.
Celver
J P
JP
Wu
A
A
Chavkin
C
C
D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D004745:Enkephalins / Q000494:pharmacology; D005283:Fentanyl / Q000494:pharmacology; D054770:G-Protein-Coupled Receptor Kinase 3; D054774:G-Protein-Coupled Receptor Kinase 5; D006801:Humans; D007700:Kinetics; D009020:Morphine / Q000494:pharmacology; D015221:Potassium Channels / Q000096:biosynthesis / Q000187:drug effects / Q000502:physiology; D017346:Protein Serine-Threonine Kinases; D051381:Rats; D020794:Receptor Protein-Tyrosine Kinases / Q000187:drug effects / Q000502:physiology*; D017450:Receptors, Opioid, mu / Q000819:agonists* / Q000096:biosynthesis / Q000502:physiology; D012680:Sensitivity and Specificity; D014982:Xenopus laevis
D000701:Analgesics, Opioid; D004745:Enkephalins; D015221:Potassium Channels; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D009020:Morphine; D020794:Receptor Protein-Tyrosine Kinases; D017346:Protein Serine-Threonine Kinases; D054770:G-Protein-Coupled Receptor Kinase 3; C517848:GRK3 protein, human; C517851:Grk3 protein, rat; D054774:G-Protein-Coupled Receptor Kinase 5; C517898:GRK5 protein, human; C517897:Grk5 protein, rat; D005283:Fentanyl