CLONING AND PHARMACOLOGICAL CHARACTERIZATION OF A RAT KAPPA-OPIOID RECEPTOR

A full-length cDNA was isolated from a rat striatal library by using l ow-stringency screening with two PCR fragments, one spanning transmemb rane domains 3-6 of the mouse delta opioid receptor and the other unid entified but homologous to the mouse delta receptor from rat brain. Th e novel cDNA had a long open reading frame encoding a protein of 380 r esidues with 59% identity to the mouse delta receptor and topography c onsistent with a seven-helix guanine nucleotide-binding protein-couple d receptor. COS-1 cells transfected with the coding region of this clo ne showed high-affinity binding to kappa opioid receptor-selective lig ands such as dynorphin A and U-50,488 and also nonselective opioid lig ands such as bremazocine, ethylketocyclazocine, and naloxone. Not boun d at all (or bound with low affinity) were dynorphin A-(2-13), enantio mers of naloxone and levophanol [i.e., (+)-naloxone and dextrorphan], and selective mu and delta opioid receptor ligands. Activation of the expressed receptor by kappa receptor agonists led to inhibition of cAM P. Finally, in situ hybridization revealed a mRNA distribution in rat brain that corresponded well to the distribution of binding sites labe led with kappa-selective ligands. These observations indicate that we have cloned a cDNA encoding a rat kappa receptor of the kappa1 subtype .