Selective labeling of κ2 opioid receptors in rat brain by [ 125I]IOXY: Interaction of opioid peptides and other drugs with multiple κ2a binding sites

Abstract

Recent studies from our laboratory resolved two subtypes of the κ 2 binding site, termed κ 2a and κ 2b, using guinea pig, rat, and human brain membranes depleted of μ and δ receptors by pretreatment with the site-directed acylating agents BIT (μ-selective) and FIT (δ-selective). 6β-Iodo-3,14-dihydroxy-17-cyclopropylmethyl-4,5α-epoxymorphinan (IOXY), an opioid antagonist that has high affinity for κ 2 sites, was radioiodinated to maximum specific activity (2200 Ci/mmol) and purified by high pressure liquid chromotography and used to characterize multiple κ 2 binding sites. The results indicated that [ 125I]IOXY, like [ 3H]bremazocine, selectively labels κ 2 binding sites in rat brain membranes pretreated with BIT and FIT. Using 100 n M [ d-Ala 2-MePhe 4,Gly-ol 5]enkephalin to block [ 125I]IOXY binding to the κ 2b site, two subtypes of the κ 2a binding site were resolved, both in the absence and presence of 50 μ M 5′-guanylyimidodiphosphate. Viewed collectively, these results provide further evidence for heterogeneity of the κ opioid receptor, which may provide new targets for drug design, synthesis, and therapeutics.