OPIOID RECEPTORS FROM A LOWER VERTEBRATE (CATOSTOMUS-COMMERSONI) - SEQUENCE, PHARMACOLOGY, COUPLING TO A G-PROTEIN-GATED INWARD-RECTIFYING POTASSIUM CHANNEL (GIRK1), AND EVOLUTION

The molecular evolution of the opioid receptor family has been studied by isolating cDNAs that encode six distinct opioid receptor-like prot eins from a lower vertebrate, the teleost fish Catostomus commersoni. One of these, which has been obtained in full-length form, encodes a 3 83-amino acid protein that exhibits greatest sequence similarity to ma mmalian mu-opioid receptors; the corresponding gene is expressed predo minantly in brain and pituitary, Transfection of the teleost cDNA into HEK 293 cells resulted in the appearance of a receptor having high af finity for the mu-selective agonist [D-Ala(2), MePhe(4)-Gly-ol(5)] enk ephalin (DAMGO) (K-d = 0.63 +/- 0.15 nM) and for the nonselective anta gonist naloxone (K-d = 3.1 +/- 1.3 nM), The receptor had negligible af finity for U50488 and [D-Pen(2), D-Pen(5)]enkephalin (DP-DPE), which a re kappa- and delta-opioid receptor selective agonists, respectively. Stimulation of transfected cells with 1 mu M DAMGO lowered forskolin-i nduced cAMP levels, an effect that could be reversed by naloxone, Expe riments in Xenopus oocytes have demonstrated that the fish opioid rece ptor can, in an agonist-dependent fashion, activate a coexpressed mous e G-protein-gated inward-rectifying potassium channel (GIRK1), The ide ntification of six distinct fish opioid receptor-like proteins suggest s that additional mammalian opioid receptors remain to be identified a t the molecular level, Furthermore, our data indicate that the mu-opio id receptor arose very early in evolution, perhaps before the appearan ce of vertebrates; and that the pharmacological and functional propert ies of this receptor have been conserved over a period of approximate to 400 million years implying that it fulfills an important physiologi cal role.