Opioid peptide receptor studies. 8. One of the mouse brain δ(NCX) binding sites is similar to the cloned mouse opioid δ receptor: Further evidence for heterogeneity of δ opioid receptors

Quantitative ligand binding studies resolved two subtypes of the δ opioid receptor, termed δ(ncx1), and δ(ncx2), in mouse brain membranes depleted of μ receptors by pretreatment with the irreversible ligand, BIT. The purpose of the present study was to compare the binding parameters, ligand-selectivity profile and pharmacological properties of the cloned mouse δ receptor (MDOR) stably expressed in a cell line to the δ(ncx) binding sites of mouse brain. [³H][D-Ala²,D-Leu⁵]enkephalin labeled a single binding site in membranes prepared from MDOR cells under several different assay conditions including BIT-pretreatment. The MDOR had high affinity for δ agonists and antagonists. [³H][D-Ala²,D-Leu⁵]enkephalin labeled two binding sites in mouse brain membranes depleted of p. receptors by pretreatment with BIT: the δ(ncx1) site (high affinity for DPDPE and deltorphin) and the δ(ncx2) site (low affinity for DPDPE and deltorphin). Some agents were moderately selective for the delta(ncx2) site: [pCl]DPDPE (10.9-fold), JP41 (5.9-fold) and JP45 (3.8-fold). The K(i) values of 12 opioids at the mouse MDOR were determined. These values were highly correlated with their values at the delta(ncx1) site but not the delta(ncx2) site. These data suggest that the delta(ncx2) site may be distinct from the cloned delta opioid receptor.