MU-OPIOID AND DELTA-OPIOID RECEPTORS ARE DIFFERENTIALLY DESENSITIZED BY THE COEXPRESSION OF BETA-ADRENERGIC-RECEPTOR KINASE-2 AND BETA-ARRESTIN-2 IN XENOPUS OOCYTES

The Xenopus oocyte expression system was used to test the hypothesis t hat homologous opioid receptor desensitization results from receptor p hosphorylation by G protein-coupled receptor kinases. Activation of de lta (DOR), mu (MOR) opioid, or beta(2)-adrenergic receptors increased K+ conductance in oocytes coexpressing the G protein-gated inwardly re ctifying K+ channel subunits GIRK1 and GIRK4, and the intrinsic rate o f desensitization was small. Coexpression of beta-adrenergic receptor kinase 2 (beta-ARK2) and beta-arrestin 2 (beta-arr2) synergistically p roduced a rapid desensitization of both DOR and beta 2-adrenergic rece ptor signaling with a t(1/2) < 4 min. beta-ARK2 and beta-arr2 more slo wly desensitized MOR responses; a similar synergistic effect on MOR re quired 2-3 h of agonist treatment. DOR mutants lacking serine and thre onine residues at the end of the cytoplasmic tail coupled effectively to GIRK channels but were insensitive to beta-ARK2 and beta-arr2. Howe ver, a DOR mutant having serine residues mutated to alanine in the thi rd cytoplasmic loop was indistinguishable in coupling and desensitizat ion from the wild type DOR. These studies establish that opioid recept ors can be regulated by beta-ARK2 and beta-arr2 and that a portion of the COOH terminus of DOR enhances sensitivity to this modulation.