INTERACTION OF ENDOGENOUS OPIOID-PEPTIDES AND OTHER DRUGS WITH 4 KAPPA-OPIOID BINDING-SITES IN GUINEA-PIG BRAIN

Guinea pig brain membranes depleted of μ and δ receptors by pretreatment with the site-directed acylating agents, 2-(4-ethoxybenzyl)-1-diethylaminoethyl-5-isothiocyanatobenzimidazole·HCl (BIT) and N-phenyl-N-[1-(2-(4-isothiocyanato)phenethyl)-4-piperidinyl]-propanamide·HCl (FIT), were used in this study to test the hypothesis that guinea pig brain possesses subtypes of κ receptors. Pretreatment of membranes with either (-)-(1S,2S)-U50,488 or the κ selective acylating agent, (1S,2S)-trans-2-isothiocyanato-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide, caused a wash-resistant inhibition of κ1 binding sites labeled by [3H]U69,593 binding, but not κ2 binding sites labeled by [3H]bremazocine. Binding surface analysis of [3H]bremazocine binding resolved two binding sites, termed κ2a and κ2b, present at densities of 212 and 225 fmol/mg protein, which had low affinity for (-)-(1S,2S)-U50,488 and U69,593. The κ2b site had high affinity for β-endorphin(1-31) (Kd=5.5 nM) and [D-Ala2,D-Leu5]enkephalin (Kd=14 nM), and lower affinity for [D-Ala2-MePhe4,Gly-ol5]enkephalin (Kd=147 nM) and [Leu5]enkephalin (Kd=46.0 nM). Binding surface analysis of [3H]U69,593 binding also resolved two binding sites, termed κ1a and κ1b, present at densities of 6.0 and 40.0 fmol/mg protein. The κ1a binding site was characterized by very high affinity for α-neoendorphin. Quantitative autoradiographic studies demonstrated that κ2a and κ2b binding sites are heterogeneously distributed in guinea pig brain, and that the anatomical distribution of κ1 binding sites reported in the literature is different from that observed in this study for the κ2 binding sites. Viewed collectively, these data provide evidence for four κ receptor subtypes in guinea pig brain. © 1990.