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Department of Zoology, 3029 Cordley Hall, Oregon State University, Corvallis, Oregon 97331, USA

(Requests for offprints should be addressed to S Bradford; Email: bradforc{at}science.oregonstate.edu)

A full-length cDNA that encodes a kappa () opioid receptor has been isolated from the brain of a urodele amphibian, the rough-skinned newt Taricha granulosa. The deduced protein contains 385 amino acids and possesses features commonly attributed to G protein-coupled receptors, such as seven putative transmembrane domains. The newt receptor has 75% sequence identity to opioid receptors cloned from mammals, and 66% sequence identity to the opioid receptor reported for the zebrafish, with the greatest divergence in the extracellular N-terminus, the second and third extracellular loops and the intracellular C-terminus. Membranes isolated from COS-7 cells expressing the newt receptor possessed a single, high-affinity (K_d =1.5 nM) binding site for the -selective agonist U69593. In competition binding assays, the expressed newt receptor displayed high affinity for the -selective agonists GR89696, dynorphin A(1–13), U69593, U50488 and BRL52537 as well as the -selective antagonist nor-binaltorphimine and the non-selective antagonist naloxone. Rank order potencies and affinity constants were similar in competition binding studies that used either whole brain homogenates or membranes isolated from COS-7 cells expressing the newt receptor. The expressed receptor displayed essentially no affinity for the -selective agonist DPDPE ([d-penicillamine, d-penicillamine]enkephalin), but showed moderate affinity for the µ-selective agonist DAMGO ([d-Ala-MePhe, Gly-ol]enkephalin) and moderately high affinity for nociceptin (orphanin FQ), the endogenous ligand for the opioid receptor-like (ORL)1 receptor. These findings support the conclusions that a gene for the opioid receptor is expressed in amphibians and that the pharmacology of the newt receptor closely matches mammalian opioid receptors. However, the functional dichotomy between the classic opioid receptors (, , µ) and ORL1 appears less strict in amphibians than in mammals.

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				S. Dreborg, G. Sundstrom, T. A. Larsson, and D. Larhammar Evolution of vertebrate opioid receptors PNAS, October 7, 2008; 105(40): 15487 - 15492. [Abstract] [Full Text] [PDF]