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Suppression of neonatal rat pituitary-testis function by gonadotrophin-releasing hormone (GnRH) antagonists results in delayed sexual maturation and infertility. Since the mechanism is not understood, the acute effects of a GnRH antagonist on gonadotrophin secretion in neonatal male rats has been studied in more detail. Treatment with a GnRH antagonist analogue, N-Ac-D-Nal(2)¹,D-p-Cl-Phe²,D-Trp³,D-hArg(ET₂)⁶,D-Ala¹⁰-GnRH (2 mg/kg per day) on days 1–10 of life had prolonged effects on gonadotrophin secretion; serum LH and FSH recovered in 1 week, but the pituitary content took 2 weeks to recover. Likewise, LH and FSH responses to acute in-vivo stimulation with a GnRH agonist were still suppressed 1 week after the treatment. Interestingly, a rebound (86% increase) in basal serum FSH was found 16 days after treatment with the antagonist.

Whether testis factors influence gonadotrophin secretion during treatment with the GnRH antagonist and/or in the subsequent recovery period was also assessed. Neonatal rats were castrated on days 1, 5 or 10 of the 10-day period of antagonist treatment. Orchidectomy on days 1 and 5 only marginally affected gonadotrophin secretion. When orchidectomy was performed at the beginning of the recovery period, no effects on pituitary recovery were seen within 1 week of castration. After 16 days, serum LH and FSH in the antagonist-treated and control castrated rats were equally increased but the pituitary contents of the antagonist-treated rats were still suppressed.

Finally, the effect of testosterone treatment on the recovery of gonadotrophin secretion after antagonist suppression was studied in intact and orchidectomized animals. The rats were implanted with testosterone capsules for 7 days after treatment with the GnRH antagonist in the neonatal period. Testosterone suppressed pituitary LH contents similarly in all groups of animals, but had no effects on serum LH. Paradoxically, serum FSH was suppressed 50% by testosterone in intact and castrated antagonist-treated rats and in castrated controls but not in intact controls. These findings suggest that suppression of FSH by testosterone is only seen in neonatal animals with low endogenous levels of this androgen, whether due to GnRH antagonist treatment or castration.

It is concluded that neonatal treatment with a GnRH antagonist results in prolonged suppression of LH and FSH secretion, that testis factors play only a minor role in pituitary modulation during the antagonist suppression and that more disturbances are observed in the post-treatment recovery of FSH secretion than in that of LH.

Journal of Endocrinology (1989) 122, 519–526