Role of G Protein–Coupled Receptor Kinases 2 and 3 in m-Opioid Receptor Desensitization and Internalization

There is ongoing debate about the role of G protein–coupled receptor kinases (GRKs) in agonist-induced desensitization of the m-opioid receptor (MOPr) in brain neurons. In the present paper, we have used a novel membrane-permeable, small-molecule inhibitor of GRK2 and GRK3, Takeda compound 101 (Cmpd101; 3-[[[4-methyl-5-(4-pyridyl)-4H-1,2,4-triazole-3-yl] methyl] amino]N-[2-(trifuoromethyl) benzyl] benzamidehydrochloride), to study the involvement of GRK2/3 in acute agonist-induced MOPr desensitization. We observed that Cmpd101 inhibits the desensitization of the G protein–activated inwardly-rectifying potassium current evoked by receptor-saturating concentrations of methionineenkephalin (Met-Enk), [D-Ala, N-MePhe, Gly-ol]-enkephalin (DAMGO), endomorphin-2, andmorphine in rat andmouse locus coeruleus (LC) neurons. In LC neurons from GRK3 knockout mice, Met-Enk–induced desensitization was unaffected, implying a role for GRK2 in MOPr desensitization. Quantitative analysis of the loss of functional MOPrs following acute agonist exposure revealed that Cmpd101 only partially reversed MOPr desensitization. Inhibition of extracellular signal–regulated kinase 1/2, protein kinase C, c-Jun N-terminal kinase, or GRK5 did not inhibit the Cmpd101-insensitive component of desensitization. In HEK 293 cells, Cmpd101 produced almost complete inhibition of DAMGO-induced MOPr phosphorylation at Ser, arrestin translocation, and MOPr internalization. Our data demonstrate a role for GRK2 (and potentially also GRK3) in agonist-induced MOPr desensitization in the LC, but leave open the possibility that another, as yet unidentified, mechanism of desensitization also exists.