FMRFamide-like immunoreactivity in rat brain: development of a radioimmunoassay and its application in studies of distribution and chromatographic properties.

A radioimmunoassay is described for the molluscan neuropeptide, Phe-Met-Arg-Phe-NH2 (FMRFamide). The antibody used is C-terminal-specific and shows slight but significant (1-2%) cross-reactivity with chicken pancreatic polypeptide (APP). The assay has been used to identify in rat brain extracts a pair of molecules that may represent mammalian counterparts of FMRFamide. Their concentrations were highest in spinal cord and hypothalamus (greater than 10 pmol . g-1) and lowest in cerebellum and striatum (less than 3.5 pmol . g-1). The two immunoreactive peptides were separated on CM ion-exchange chromatography where they appeared to be less basic than FMRFamide. On Sephadex G50 gel filtration one eluted in a similar position of FMRFamide and the other slightly earlier suggesting it may be of higher molecular weight. The rat immunoreactive components do not correspond to previously described neuropeptides or hormones, and may be members of a new group of mammalian neuropeptides with transmitter or modulatory functions.