Hormone Ontogeny in the Ovine Fetus: XII. The Dopaminergic Regulation of Growth Hormone and Chorionic Somatomammotropin Release*

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Abstract
As part of studies on the development of neural regulation of pituitary function, we examined the ontogeny of dopaminergic regulation of GH secretion in the ovine fetus and newborn lamb. The dopaminergic agonist, apomorphine (40–100 μg/kg, iv) was infused into 15 fetuses (83–136 days gestation; term, 147 days) and 14 infant lambs. In the fetuses, apomorphine caused a prompt fall in ovine GH from 103.4 ± 11.6 to 72.2 ± 9.6 ng/ml (P > 0.01), with a rebound rise to 172.3 ± 19.0 ng/ml (P > 0.01) 30 min after the infusion ceased. There was no correlation between the degree of suppression or rebound and gestational age. In the infant lambs, basal GH concentrations were low (6.5 ±1.3 ng/ml), decreased during apomorphine infusion to 2.6 ± 0.8 ng/ml, and increased after termination of the infusion to 18.1 ±6.1 ng/ml (P < 0.01). The effect of apomorphine on fetal GH was blocked by prior treatment with the dopaminergic antagonist haloperidol, suggesting that the action of apomorphine was mediated by dopamine receptors. These results demonstrate that a dopaminergic system capable of inhibiting GH secretion has developed in the fetal hypothalamic-pituitary unit by 83 days (55%) of gestation. The administration of haloperidol (1 mg, iv) to 11 fetuses (92–140 days gestation) and 5 lambs did not alter the concentration of plasma GH; apparently, dopamine does not tonically inhibit GH secretion in the ovine fetus. Plasma chorionic somatomammotropin (oCS) levels were not changed during these experiments in either the pregnant ewe or fetus. Previous reports that the dopaminergic agonist bromocryptine inhibits oCS release may reflect an indirect effect rather than a direct dopaminergic action on oCS secretion by the placenta.