Hormone Ontogeny in the Ovine Fetus. VIII. The Effect of Thyrotropin-Releasing Factor on Prolactin and Growth Hormone Release in the Fetus and Neonate*

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Abstract
As part of studies on the ontogeny of PRL secretion and regulation, the effect of TRF on PRL release was studied in the chronically catheterized ovine fetus and neonate. After a 2-h control period, 250 μg TRF were given by rapid iv injection to seven fetuses between 88—133 days gestation (term, 147 days) and five lambs between 4—17 days old. All animals exhibited a prompt increase in the concentration of plasma PRL. In three fetuses with basal PRL concentrations less than 10 ng/ml (88, 89, and 111 days old), PRL rose from a mean value of 3.1 ± 2.0 to 11.5 ± 5.6 (SE) ng/ml 15 min after TRF and was 10.7 ± 5.0 ng/ml at 90 min. In four fetuses with basal PRL concentrations greater than 10 ng/ml (112, 121, 124, and 133 days old), significantly (P ≪ 0.05) more PRL was secreted; the mean concentration of PRL increased from 36.9 ± 12.1 to 88.2 ± 21.7 ng/ml at 15 min and was 77.8 ± 22.8 ng/ml at 90 min. In five neonates (5, 6, 10, 10, and 17 days old), PRL rose from 13.1 ± 2.3 to 81.6 ± 20.1 ng/ml at 15 min and fell to 38.2 ± 4.5 ng/ml at 90 min. The plasma GH concentration did not increase after TRF administration. TRF evokes PRL secretion as early as 88 days gestation and the response increases with advancing gestational age; late gestation fetuses have peak PRL levels similar to those in neonates. However, neonates have lower basal levels and a less sustained (P ≪ 0.05) PRL response to TRF than late gestation fetuses. These differences reflect changes in PRL reserve and may be modulated by changing estrogen levels which rise in late pregnancy and fall after birth. The differences in the pattern of the PRL response to TRF in neonates may also reflect increased dopaminergic activity in the neonate compared to the late gestation fetus. (Endocrinology106: 1074, 1980)