PRELIMINARY LIGAND-BINDING DATA FOR SUBTYPES OF THE DELTA OPIOID RECEPTOR IN RAT-BRAIN MEMBRANES

Delta opioid binding sites were assayed using [H-3][D-ala2,D-leu5]enkephalin and rat brain membranes depleted of mu-binding sites with the site-directed acylating agent, 2-(pethoxybenzyl)-1-diethylaminoethyl-5-isothiocyanatobenzimidazole-HCI. [D-Pen2,D-Pen5]enkephalin (DPDPE), [D-Pen2,L-Pen5]enkephalin, [D-Ala2]deltorphin-I and [D-Ala2]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-mu-M 5'-guanylyimidodiphosphate in the assay Quantitative analysis of [D-ala2,D-leu5]enkephalin, DPDPE and [D-Ala2]deltorphin-I binding surfaces resolved two binding sites. Whereas [D-ala2,D-leu5]enkephalin had equal affinity for both sites, DPDPE and [D-Ala2]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 delta-receptor subtypes which mediate antinociception.