[35S]GTPγS binding stimulated by endomorphin-2 and morphiceptin analogs

The ability of several μ-selective opioid peptides to activate G-proteins was measured in rat thalamus membrane preparations. The μ-selective ligands used in this study were three structurally related peptides, endomorphin-1, endomorphin-2 and morphiceptin, and their analogs modified in position 3 or 4 by introducing 3-(1-naphthyl)-d-alanine (d-1-Nal) or 3-(2-naphthyl)-d-alanine (d-2-Nal). The results obtained for these peptides in [35S]GTPγS binding assay were compared with those obtained for a standard μ-opioid agonist DAMGO. [d-1-Nal3]Morphiceptin was more potent in G-protein activation (EC50 value of 82.5 ± 4.5 nM) than DAMGO (EC50 = 105 ± 9 nM). [d-2-Nal3]Morphiceptin, as well as endomorphin-2 analogs substituted in position 4 by either d-1-Nal or d-2-Nal failed to stimulate [35S]GTPγS binding and were shown to be potent antagonists against DAMGO. It seems that the topographical location of the aromatic ring of position 3 and 4 amino acid residues can result in a completely different mode of action, producing either agonists or antagonists.