The action of FMRFamide (Phe-Met-Arg-Phe-NH2) and related peptides on mammals.

First purified 11 years ago from clam ganglia, FMRFamide (Phe-Met-Arg-Phe-NH2) was quickly demonstrated to be cardioactive in several molluscan species. Subsequent discovery that FMRFamide, or FMRFamide-related peptides (FaRPs), were present in mammalian central nervous system and gastrointestinal tract prompted investigations into the effect of FMRFamide on mammals. FMRFamide has now been shown to be cardioexcitatory in mammals, to inhibit morphine-induced antinociception, and to block morphine-, defeat-, and deprivation-induced feeding. It also inhibits colonic propulsive motility, induces behavioral effects when administered intrathecally, and has been reported to have amnesic effects in rodents. A proposal has arisen that a FMRFamide-like substance is an endogenous opioid antagonist and has stimulated a search for such a substance. However, FMRFamide has only weak affinity for opioid receptors and not all the actions of FMRFamide appear to be explained by actions at opioid receptors. Alternative mechanisms have been proposed which suggest that FMRFamide acts as a neuromodulator.