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

Quantitative binding studies resolved two high-affinity [³H][D-Ala²,D- Leu⁵]enkephalin binding sites in rat brain membranes depleted of μ binding sites by pretreatment with the irreversible agent BIT. - The two binding sites had lower (δ(nc-x), Ki = 96.6 nM) and higher ({dncx-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, [³H][D-Ala²,D-Leu⁵]enkephalin binding to the cells expressing the cloned μ receptor is very low and pretreatment of cell membranes with BIT almost completely inhibits [³H]DAMGO and [³H][D-Ala²,D-Leu⁵]enkephalin binding. Intracerebroventricular administration of antisense DNA to the cloned delta receptor selectively decreased [³H][D-Ala²,D-Leu⁵]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.