SELECTIVE LABELING OF KAPPA(2) OPIOID RECEPTORS IN RAT-BRAIN BY [I-125] IOXY - INTERACTION OF OPIOID-PEPTIDES AND OTHER DRUGS WITH MULTIPLEKAPPA(2A) BINDING-SITES

Recent studies from our laboratory resolved two subtypes of the kappa2 binding site, termed kappa2a and kappa2b, using guinea pig, rat, and human brain membranes depleted of mu and delta receptors by pretreatme nt with the site-directed acylating agents BIT (mu-selective) and FIT (delta-selective). droxy-17-cyclopropylmethyl-4,5alpha-epoxymorphinan (IOXY), an opioid antagonist that has high affinity for kappa2 Sites, was radioiodinated to maximum specific activity (2200 Ci/mmol) and pur ified by high pressure liquid chromotography and used to characterize multiple kappa2 binding sites. The results indicated that [I-125]IOXY, like [H-3]bremazocine, selectively labels kappa2 binding sites in rat brain membranes pretreated with BIT and FIT. Using 100 nM [D-ala2-MeP he4,Gly-ol5]enkephalin to block [I-125]IOXY binding to the kappa2b sit e, two subtypes of the kappa2a binding site were resolved, both in the absence and presence of 50 muM 5'-guanylyimidodiphosphate. Viewed col lectively, these results provide further evidence for heterogeneity of the kappa opioid receptor, which may provide new targets for drug des ign, synthesis, and therapeutics.