Molecular cloning and expression of a rat κ opioid receptor

Abstract

At least three types of opioid receptors have been identified in the nervous system. In this paper we report molecular cloning and expression of a rat K opioid receptor. PCR was performed on double-stranded cDNA derived from poly(A)+ RNA of the rat striatum with primers similar to those of Libert and co-workers. One of the PCR products, which had 65 % sequence similarity to the mouse δ opioid receptor, was used to screen a rat striatum cDNA library. Two positive clones were isolated and found to be identical. The clone had a 2.1-kb insert, which was termed RKOR-1. RKOR-1 has an open reading frame of 1140 bp and encodes a 380-amino-acid protein. Hydropathy analysis indicates that RKOR-1 has seven putative transmembrane domains with short intra- and extra-cellular loops. Membranes of Cos-7 cells transfected with RKOR-1 exhibited high specific binding for [3H]diprenorphine ([3H]DIP), a non-selective opioid ligand. Naloxone inhibited [3H]DIP binding with stereospecificity. [3H]DIP binding was potently inhibited by selective K opioid ligands, with K1 values in the nanomolar or subnanomolar range but much less potently inhibited by drugs selective for μ or δ receptors. Thus, RKOR-1 represents an opioid receptor with κ characteristics.