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Ontogenetic development of GRF-containing neurons in the rat hypothalamus was studied employing antisera which were generated against hpGRF (1-44)NH2 and rhGRF(1-43)OH: anti-hpGRF-C and -rhGRF sera recognize the species-specific C-terminal portions of the peptides, and anti-hpGRF-MC and -N sera recognize hpGRF(27-44)NH2 and the N-terminal portion of hpGRF(1-44)NH2, respectively. The anti-hpGRF-C and -rhGRF sera stained different neuronal cell bodies, which were localized in distinct hypothalamic areas. The former serum did not stain the axonal terminals in the median eminence, but the latter stained them strongly. The anti-hpGRF-MC and -N sera stained neuronal cell bodies, some of which corresponded to those immunolabelled with anti-hpGRF-C or -rhGRF serum. The anti-rhGRF serum first demonstrated immunoreactive perikarya in the ventral-lateral border of the arcuate nucleus of 19.5-day-old fetuses that had received an intraventricular colchicine administration 24 h previously. The immunoreactive fibers were recognized first in the external layer of the median eminence of untreated fetuses on day 19.5 of gestation, and then they increased in amount with development. No immunoreactive fibers, however, were found in the median eminence of colchicine-treated animals during the fetal period. It is concluded that in rats GRF may be synthesized in the perikarya on day 18.5 of gestation and conveyed to the median eminence without delay via axonal flow.

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