Cross-Linking of Human [‘25I]3-Endorphin to Opiold Receptors in Rat Striatal Membranes: Biochemical Evidence for the Existence of a mu/delta Opioid Receptor Complex1

In a modified Krebs buffer at 37#{176}C,the selective mu agonist [3H] D-Ala2,MePhe4,Gly-ol5]enkephalin ([3H]DAMGO) and the nonse-lective mu/delta agonist human [125l]3-endorphin ([125l](-endH) bound to rat striatal membranes with a Kd of about 7 and 5 nM and a Bmax of about 95 and 260 fmol/mg of protein, respectively, consistent with labeling of mu receptors by the former ligand and labeling of both mu and delta receptors by the latter. The binding of 2 nM [125l]f1-endH was displaced by unlabeled DAMGO (IC50 30 nM), [D-A1a2-D-Leu5lenkephalin (IC50 60 nM) as well as by the selective delta agonists [D-Ser(O-tert-butyl),Leu5]enkephalyl-Thr (DSTBULET, IC50 500 nM) and Tyr-Ala-Phe-Asp-Val-Val-Gly-NH2 (IC50 700 nM) in a monophasic manner within 2 to 3 log concentration units, suggesting an allosteric interaction between mu and delta sites labeled by [125I1f3-endH under these conditions. Accordingly, 500 nM DSTBULET caused almost 40 % inhibitio