Ligand binding profiles of δ-opioid receptor in human cerebral cortex membranes

In this study, receptor binding profiles of opioid ligands for subtypes of opioid δ-receptors were examined employing [ 3 H]D-Pen 2 ,D-Pen 5 -enkephalin ([ 3 H]DPDPE) and [ 3 H]Ile 5,6 -deltorphin II ([ 3 H]Ile-Delt II) in human cerebral cortex membranes. [ 3 H]DPDPE, a representative ligand for δ 1 sites, labeled a single population of binding sites with apparent affinity constant (Kd) of 2.72 ± 0.21 nM and maximal binding capacity (Bmax) value of 20.78 ± 3.13 fmol/mg protein. Homologous competition curve of [ 3 H]Ile-Delt II, a representative ligand for δ 2 sites, was best fit by the one-site model (Kd = 0.82 ± 0.07 nM). Bmax value (43.65 ± 2.41 fmol/mg) for [ 3 H]Ile-Delt II was significantly greater than that for [ 3 H]DPDPE. DPDPE, [D-Ala 2 ,D-Leu 5 ]enkephalin (DADLE) and 7-benzylidenaltrexone (BNTX) were more potent in competing for the binding sites of [ 3 H]DPDPE than for those of [ 3 H]Ile-Delt II. On the other hand, deltorphin II (Delt II), [D-Ser 2 ,Leu 5 ,Thr 6 ]enkephalin (DSLET), naltriben (NTB) and naltrindole (NTI) were found to be equipotent in competing for [ 3 H]DPDPE and [ 3 H]Ile-Delt II binding sites. These results indicate that both subtypes of opioid δ-receptors, δ 1 and δ 2 , exist in human cerebral cortex with different ligand binding profiles.