Opioid peptide receptor studies. 3. Interaction of opioid peptides and other drugs with four subtypes of the kappa 2 receptor in guinea pig brain.
Using guinea pig, rat, and human brain membranes depleted of mu and delta receptors by pretreatment with the site-directed acylating agents BIT (mu selective) and FIT (delta selective), previous studies from our laboratory resolved two subtypes of the kappa 2 binding site, termed kappa 2a and kappa 2b. In more recent studies, we used 6 beta-[125Iodo]-3,14-dihydroxy-17-cyclopropylmethyl-4,5 alpha-epoxymorphinan ([125I]IOXY) to characterize multiple kappa 2 binding sites in rat brain. The results indicated that [125I]IOXY, like [3H]bremazocine, selectively labels kappa 2 binding sites in rat brain membranes pretreated with BIT and FIT. In the rat brain, using 100 nM [D-Ala2-MePhe4,Gly-ol5]enkephalin to block [125I]IOXY binding to the kappa 2b site, we resolved two subtypes of the kappa 2a binding site. In the present study we examined the binding of [125I]IOXY to the kappa 2 receptors of guinea pig brain. As observed in rat brain, [125I]IOXY, under appropriate assay conditions, selectively labels kappa 2 binding sites. Quantitative binding studies readily demonstrated the presence of kappa 2a and kappa 2b binding sites. The kappa 2a binding sites were selectively assayed using 5 microM [Leu5]enkephalin to block [125I]IOXY binding to the kappa 2b sites, and kappa 2b sites were selectively assayed using 5 microM (-)-(1S,2S)-U50,488 to block [125I]IOXY binding to the kappa 2a sites. Under these conditions, two subtypes of the kappa 2a site were resolved with high (kappa 2a-1) and low (kappa 2a-2) affinity for nor-BNI (Ki values = 0.88 and 476 nM) and CI977 (Ki values = 17.5 and 95,098 nM). Similarly, two subtypes of the kappa 2b site were observed with high (kappa 2b-1) and low (kappa 2b-2) affinity for [D-Ala2-MePhe4,Gly-ol5]enkephalin (DAMGO) (Ki values = 97 and 12,321 nM) and alpha-neoendorphin (Ki values = 33 and 5308 nM). Two-site models were also resolved in the presence of 100 microM 5'-guanylyimidodiphosphate (GppNHp). We carried out detailed ligand selectivity analysis of the multiple kappa 2 binding sites. Most test agents were either nonselective or selective for the kappa 2a-1 site. Nalbuphine was moderately selective for the kappa 2a-2 site. Similarly, although most test agents were either nonselective or selective for the kappa 2b-1 site, butorphanol, and the delta antagonists naltrindole, naltriben, and 7-benzylidene-7-dehydronaltrexone were moderately selective for the kappa 2b-2 site.(ABSTRACT TRUNCATED AT 400 WORDS)
Clinical Psychopharmacology Section, NIDA, NIH, Baltimore, MD 21224, USA.
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D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000378:metabolism*; D004723:Endorphins / Q000737:chemistry / Q000378:metabolism; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D004745:Enkephalins / Q000737:chemistry / Q000378:metabolism; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006168:Guinea Pigs; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008024:Ligands; D008954:Models, Biological; D008969:Molecular Sequence Data; D009019:Morphinans / Q000378:metabolism; D018847:Opioid Peptides / Q000737:chemistry / Q000378:metabolism*; D011498:Protein Precursors / Q000737:chemistry / Q000378:metabolism; D051381:Rats; D017473:Receptors, Opioid, kappa / Q000145:classification / Q000378:metabolism*; D013552:Swine
D004723:Endorphins; D004745:Enkephalins; D008024:Ligands; D009019:Morphinans; D018847:Opioid Peptides; D011498:Protein Precursors; D017473:Receptors, Opioid, kappa; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; C075927:6-iodo-3,14-dihydroxy-17-(cyclopropylmethyl)-4,5-epoxymorphinan; D006165:Guanylyl Imidodiphosphate; C025681:alpha-neoendorphin