Analogues of F8Famide resistant to degradation, with high affinity and in vivo effects.

Four analogues of Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2, a mammalian FMRFamide-like peptide with antiopiate properties, were synthesized with N-terminus modifications and were shown to have high affinity for F8Famide binding sites. The degradation rate of these analogues in mouse brain slices was 3 times lower than that of the natural peptide. One analogue, (2DME)Y8Fa (D.Tyr-D.Leu-[N-Me]Phe-Gln-Pro-Gln-Arg-Phe-NH2), produced a clear hyperalgic effect and inhibited morphine analgesia in the mouse tail-flick test at lower doses than did the parent compound. (3D)Y8Fa (D.Tyr-D.Leu-D.Phe-Gln-Pro-Gln-Arg-Phe-NH2) and (2D)Y8Fa (D.Tyr-D.Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2) in contrast did not decrease morphine analgesia but were analgesic alone. The analgesic effects of 22 nmol (2D)Y8Fa and (3D)Y8Fa were decreased by (1DME)Y8Fa (D.Tyr-Leu-[N-Me]Phe-Gln-Pro-Gln-Arg-Phe-NH2) or (2DME)Y8Fa and were reversed by naloxone. These results indicate opioid modulating properties of F8Famide. These analogues may prove to be useful tools for studying the modulation of pain by F8Famide.