Pituitary and gonadal responses to the long-term pulsatile administration of gonadotrophin-releasing hormone in fetal sheep

Abstract

The fetal hypothalamo-pituitary-gonadal axis reaches a peak in activity at mid-gestation and this is followed by a period of suppression which persists until the onset of puberty. The decline in gonadotrophic activity during late gestation is thought to reflect the maturation of central and peripheral feedback signals. In order to establish if sustained pituitary responsiveness is rate limiting to the reinstatement of reproductive function, we have examined the endocrine consequences of repeated pulsatile GnRH administration to male and fetal sheep during late gestation. Beginning on day 121 of gestation (term = 145 days) chronically catheterized fetal sheep were given i.v. pulses of either 500 ng GnRH or saline every 2 h for 14 days. Pituitary and gonadal responses were assessed by measuring changes in plasma concentrations of LH, FSH, inhibin and testosterone (in male fetuses) in response to the first pulse of GnRH on day 1 and to the corresponding pulse on days 4, 7, 10 and 14. In response to the first pulse of GnRH there was an immediate release of LH, with the peak response being significantly (P < 0.01) greater than on subsequent days. In male fetuses each pulse of LH was followed by a rise in plasma testosterone concentrations within 40-60 min. The amplitude of these testosterone responses increased significantly (P < 0.01) after 9 days of treatment despite a decline in the plasma LH response. Basal FSH concentrations increased progressively (P < 0.05) during pituitary stimulation with GnRH in both male and female fetuses. Immunoreactive inhibin concentrations were significantly (P < 0.05) higher in males than in females, and there was a gradual increase throughout the experimental period irrespective of treatment. We observed no inverse correlation between inhibin and FSH concentrations. These data show that pulsatile administration of GnRH to fetal sheep during late gestation results in sustained re-activation of pituitary-gonadal function. The decline in fetal gonadotrophins, which is a characteristic feature of late gestation, is therefore likely to result from inadequate GnRH secretion from the fetal hypothalamus rather than an inhibition of pituitary function by peripheral feedback signals.