mu 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 that homologous opioid receptor desensitization results from receptor phosphorylation by G protein-coupled receptor kinases. Activation of delta (DOR), mu (MOR) opioid, or beta(2)-adrenergic receptors increased K+ conductance in oocytes coexpressing the G protein-gated inwardly rectifying K+ channel subunits GIRK1 and GIRK4, and the intrinsic rate of desensitization was small. Coexpression of beta-adrenergic receptor kinase 2 (beta-ARK2) and beta-arrestin 2 (beta-arr2) synergistically produced a rapid desensitization of both DOR and beta 2-adrenergic receptor signaling with a t(1/2) < 4 min. beta-ARK2 and beta-arr2 more slowly desensitized MOR responses; a similar synergistic effect on MOR required 2-3 h of agonist treatment. DOR mutants lacking serine and threonine residues at the end of the cytoplasmic tail coupled effectively to GIRK channels but were insensitive to beta-ARK2 and beta-arr2. However, a DOR mutant having serine residues mutated to alanine in the third cytoplasmic loop was indistinguishable in coupling and desensitization from the wild type DOR. These studies establish that opioid receptors can be regulated by beta-ARK2 and beta-arr2 and that a portion of the COOH terminus of DOR enhances sensitivity to this modulation.