PHARMACOLOGICAL CHARACTERIZATION OF THE CLONED KAPPA-OPIOID RECEPTOR AS A KAPPA(1B) SUBTYPE

SUBSTANTIAL pharmacological evidence in vitro and in vivo has suggested the existence of subtypes of the kappa opioid receptor. Quantitative radioligand binding techniques resolved the presence of two high affinity binding sites for the kappa(1) ligand [H-3]U69,593 in mouse brain membranes, termed kappa(1a) and kappa(1b), respectively. Whereas the kappa(1a) site has high affinity for fedotozine and oxymorphindole and low affinity for bremazocine and alpha-neoendorphin, site kappa(1b) has high affinity for bremazocine and alpha-neoendorphin and low affinity for fedotozine and oxymorphindole. CI-977 and U69,593 bind equally well at both sites. To determine the relationship between these kappa(1) receptor subtypes and the recently cloned mouse kappa(1) receptor (KOR), we examined [H-3]U69,593 binding to the KOR in stably transfected cells (KORCHN-8). Competition of [H-3]U69,593 binding to the KOR by bremazocine, alpha-neoendorphin, fedotozine and oxymorphindole resolved a single class of binding sites at which these agents had binding affinities similar to that of the kappa(1b) site present in mouse brain. These results suggest that the cloned KOR corresponds to the kappa(1) site in mouse brain defined as kappa(1b).