Opioid binding profile of morphiceptin, Tyr-MIF-1 and dynorphin-related peptides in rat brain membranes

Opioid properties of several morphiceptin- (Tyr-Pro-Phe-Pro-NH2), Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) and dynorphin-derivatives were characterized in rat brain in vitro receptor binding assay and in electrically stimulated longitudinal muscle strip preparation of guinea pig ileum. In the case of morphiceptin-related peptides, an excellent correlation was found between the [H-3]-naloxone binding displacement data and the agonist potencies determined in the bioassay. The 'turning point' was the C-terminal amidation in the tri- and tetrapeptide pairs in both series. Tyr-MIF-1 derivatives showed weak affinity in the opioid receptor binding assay and none of them had any remarkable effect in the bioassay either as agonist or antagonist. The dynorphin A((1-10))-peptides modified at positions 5 and 8 retained their affinity with Pro(5)-, Pro(8)-, and Ala(8)- substituents, whereas some loss of affinity was observed in the case of Gly(8)-Dyn A((1-10)).