Preliminary ligand binding data for subtypes of the delta opioid receptor in rat brain membranes

Abstract

Delta opioid binding sites were assayed using [ 3H][D-ala 2,D-leu 5]enkephalin and rat brain membranes depleted of μ binding sites with the site-directed acylating agent, 2-(p-ethoxybenzyl)-1-diethylaminoethyl-5 -isothiocyanatobenzimidazole-HCI. [D-Pen 2, D-Pen 5]enkephalin (DPDPE), [D-Pen 2,L-Pen 5]enkephalin, [D-Ala 2]deltorphin-I and [D-Ala 2]deltorphin-II inhibition curves were characterized by slope factors (Hill coefficients) less than 1. The low slope factor of DPDPE persisted in the presence of 50 μM 5'-guanylyimidodiphosphate in the assay. Quantitative analysis of [D-ala 2,D-leu 5]enkephalin, DPDPE and [D-Ala 2]deltorphin-I binding surfaces resolved two binding sites. Whereas [D-ala 2,D-leu 5]enkephalin had equal affinity for both sites, DPDPE and [D-Ala 2]deltorphin-I had high affinity for the high capacity binding site, and low affinity for the low capacity binding site. These data support pharmacological studies demonstrating δ receptor subtyes which mediate antinociception.