Ontogeny of hypothalamus-pituitary function in the fetal pig: gonadotropin release in response to electrical and electrochemical stimulation of the hypothalamus.

The ontogeny of hypothalamic control of anterior pituitary gonadotropin secretion was studied in anesthetized fetal pigs at different gestational ages (60, 80, and 105 days gestation; term, 114 days). Three or four fetuses from one mother simultaneously received either no treatment (control), sham operation, electrical stimulation (EL), or electrochemical stimulation (EC). Electrodes were implanted unilaterally into the hypothalamus. Fetuses remained in utero during surgery. A distinct difference in the development of basal LH and FSH secretion was observed. Basal plasma LH concentrations almost doubled (P less than or equal to 0.001) between days 60 and 80, with no further significant increase between 80 and 105. Plasma FSH concentrations did not change significantly between days 60 and 80, but increased more than 5-fold (P less than or equal to 0.001) between days 80 and 105. EL or EC did not affect LH secretion at 60 days. At 80 days, EL and EC significantly (P less than or equal to 0.05) elevated plasma LH concentrations 30 and 50 min after the onset of stimulation. At 105 days, EL and EC caused a rise in plasma LH levels within 10 min; the maximum level, reached 30 min after the onset of stimulation, was double that in 80-day-old fetuses. Plasma FSH values were not significantly affected by EL or EC in any age group. The results indicate that the fetal pig hypothalamus is able to influence pituitary LH secretion by day 80 (70% of gestation). Further, maturation of hypothalamic control of LH secretion becomes demonstrable between days 80 and 105. The development of hypothalamic control of FSH secretion is delayed relative to LH.