Pre- and postnatal developmental changes in hypothalamic content of rat growth hormone-releasing factor

The ontogenesis of hypothalamic GH-releasing factor (GRF) in pre- and postnatal rats was examined by means of a specific rat GRF RIA. Whereas GRF content was undetectable (less than 10 pg/hypothalamus) on day 17 of gestation, it increased to 30-65 pg/hypothalamus during days 18-20. During postnatal life, hypothalamic GRF content increased more rapidly during days 20-50 than during days 0-20 or 50-90. GRF content was 900-1300 pg/hypothalamus in 50- to 90-day-old rats, and there was no consistent sex difference during postnatal life. Hypothalamic somatostatin levels, as measured by RIA, showed a developmental pattern similar to that of rat GRF. GRF immunoreactivity in hypothalamic extracts from fetal as well as adult rats exhibited HPLC retention times identical to that of synthetic rat GRF. Administration of antirat GRF serum produced a significant decrease in plasma GH levels in fetal rats on day 21 of gestation and in newborn pups 4 h after birth. Passive immunization against GRF caused a more marked suppression of plasma GH (75-85%) 6-9 h after birth and on postnatal day 3. The results demonstrate that immunoreactive GRF is present in measurable levels in the hypothalami of fetal and newborn rats, is chemically indistinguishable from synthetic rat GRF, and exhibits biological effects as early as day 21 of fetal life.