Opioid peptides. Synthesis and binding properties of dermorphin related heptapeptides

C-Terminal amino acid residues of dermorphin (H-Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2) were replaced by N alpha-methyl- or D-amino acids in order to examine the effect on opioid activity. In binding studies based on displacement of mu, delta, and kappa opioid receptor selective radiolabels from guinea pig brain membranes, the 13 new analogues showed, like dermorphin, a negligible affinity for the kappa binding site. The introduction of N alpha-methyl- or D-amino acid residues at position 5, 6, or 7 of dermorphin, when matched with C-terminal amide function modifications, generally produced analogues with reversed mu/delta specificity.