Mixed type inhibition of [D-Ala2, D-Leu5]enkephalin binding to mu-opiate binding sites by mu-, but not by kappa-opiate ligands.

The interaction of the delta-opiate agonist [D-Ala2,D]Leu5]enkephalin (DADL) with the mu-opiate ligands dihydromorphine (DHM) and naloxone and the kappa-opiate ligand ethylketocyclazocine (EK) was studied in rat diencephalic membranes which contain mainly mu-opiate binding sites. Saturation binding experiments performed with tritiated DADL in the presence of DHM and naloxone were indicative of mixed type inhibition whereas EK behaved as a competitive inhibitor. This result indicates differences in the interaction of mu-, delta- and kappa-ligands with mu-opiate binding sites.