Cloning and pharmacological characterization of a rat κ opioid receptor

Abstract

A full-length cDNA was isolated from a rat striatal library by using low-stringency screening with two PCR fragments, one spanning transmembrane domains 3-6 of the mouse δ opioid receptor and the other unidentified but homologous to the mouse δ receptor from rat brain. The novel cDNA had a long open reading frame encoding a protein of 380 residues with 59% identity to the mouse δ receptor and topography consistent with a seven-helix guanine nucleotide-binding protein-coupled receptor. COS-1 cells transfected with the coding region of this clone showed high-affinity binding to κ opioid receptor-selective ligands such as dynorphin A and U-50,488 and also nonselective opioid ligands such as bremazocine, ethylketocyclazocine, and naloxone. Not bound at all (or bound with low affinity) were dynorphin A-(2-13), enantiomers of naloxone and levophanol [i.e., (+)-naloxone and dextrorphan], and selective μ and δ opioid receptor ligands. Activation of the expressed receptor by κ receptor agonists led to inhibition of cAMP. Finally, in situ hybridization revealed a mRNA distribution in rat brain that corresponded well to the distribution of binding sites labeled with κ-selective ligands. These observations indicate that we have cloned a cDNA encoding a rat κ receptor of the κ1 subtype.