Ontogeny of the FMRFamide-immunoreactivity in the rat forebrain and diencephalon

Ontogeny of the FMRFamide (molluscan cardioexcitatory neuropeptide)-containing structures in the forebrain and diencephalon of the rat was investigated by employing immunohistochemical methods. FMRFamide-like immunoreacted (FMRF-IR) fibers first appeared in the borders of the periventricular zone and the preoptic area at embryonic day 18 (E18). Toward birth, the FMRF-IR fibers gradually increased both in immunoreactivity and in number in these areas. A pronounced increase in FMRF-IR was also found in the septum, the arcuate nucleus, the median eminence, the paraventricular nucleus and the amygdaloid complex. A few FMRF-IR fibers appeared at the prenatal stage in the caudate nucleus, the bed nucleus of the stria terminalis, the dorsomedial nucleus and the cortex. The first FMRFamide-immunoreactive neurons were seen in the caudate-putamen and the amygdaloid complex at E21. These FMRF-IR cells increased in immunoreactivity and a significant number of cells was noted in these nuclei in the adult rat. The highest density of FMRF-IR neurons, especially in the amygdala and tuberal hypothalamic area, was detected at postnatal two weeks (P15). FMRFamide-like immunoreactivity in the forebrain and diencephalon appeared in the cell fibers prior to that observed in the cell bodies. This may suggest that some of the immunoreacted fibers may have originated from the lower areas of the rat brain. High densities of FMRF-IR cells present in the embryonic and early postnatal stages may indicate that FMRFamide is an important factor involved in developmental organization of the central nervous system. These results also indicate a differential genesis of FMRF-IR neuronal groups in different regions.