[D-Met2]-FMRFamide (DMFa): production of naloxone-sensitive antinociception in mouse tail-flick test.

Considerable data support the hypothesis that mammalian FMRFamide (Phe-Met-Arg-Phe-NH2) or mammalian FMRFamide-related peptides (FaRPs) function as endogenous antiopiates (for review see Raffa, 1988). We report here that central administration (i.c.v.) of a FaRP with D-amino acid substitution in the second position, i.e. [D-Met2]-FMRFamide (DMFa), produces dose-related, naloxone-reversible antinociception in the mouse tail-flick test. Hence, this modification appears to confer agonist-like activity.