Opioid peptide receptor studies. 9. Identification of a novel non-μ- non-δ-like opioid peptide binding site in rat brain

Abstract The two binding sites had lower (δ ncx-2 , Ki = 96.6 nM) and higher (δ ncx-1 , Ki = 1.55 nM) affinity for DPDPE. The ligand-selectivity profile of the δ ncx-1 site was that of a classic δ binding site. The ligand-selectivity profile of the δ ncx-2 site was neither μ- or δ-like. The Ki values of selected agents for the δ ncx-2 site were: [pCl]DPDPE (3.9 nM), DPLPE (140 nM), and DAMGO (2.6 nM). Under these assay conditions, [ 3 H][ d -Ala 2 , d -Leu 5 ]enkephalin binding to the cells expressing the cloned μ receptor is very low and pretreatment of cell membranes with BIT almost completely inhibits [ 3 H]DAMGO and [ 3 H][ d -Ala 2 , d -Leu 5 ]enkephalin binding. Intracerebroventricular administration of antisense DNA to the cloned delta receptor selectively decreased [ 3 H][ d -Ala 2 , d -Leu 5 ]enkephalin binding to the δ ncx-1 site. Administration of buprenorphine to rats 24 h prior to preparation of membranes differentially affected μ, δ ncx-1 , and δ ncx-2 binding sites. Viewed collectively, these studies have identified a novel non-μ- non-δ-like binding site in rat brain.