Because of some indication that FSH secretion is less dependent than LH secretion on GnRH in vivo, we performed experiments to examine the effects of a GnRH antagonist (antag) on LH and FSH secretion. We first showed that pituitary cells superfused with GnRH showed a similar pattern of suppressed secretion of both LH and FSH in response to addition of antag. In contrast, antag administration to ovariectomized rats had differing effects on LH and FSH secretion. Serum LH was suppressed in a dose-dependent fashion by 2 h (20–50% of control values). Recovery from the lower doses of antag was seen by 12 h, but the two highest doses maintained serum LH levels at 10% of control values for 72 h. In contrast, the effect on serum FSH was not manifested until 12 h. FSH was maximally decreased only to 40–60% of control values. The two highest doses maintained this effect for 72 h. These results reinforce previous suggestions that FSH secretion in vivo may occur independently of acute changes in GnRH secretion, and may have an GnRH-independ-ent component.

The differential role of porcine follicular fluid (pFF) in regulating follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release in vivo in situations of different gonadotropin releasing hormone (GnRH) backgrounds was studied. In experiment 1, 2-week ovariectomized rats injected intravenously with 4, 16 or 64 mg of protein from pFF, showed a dose-dependent suppression of FSH over time, with a maximal suppression to 40% of control values by 10 h. LH levels were slightly, but significantly, elevated by the two lower doses, but not by the highest dose of pFF. In experiment 2,64 mg pFF was superimposed (i.v. injection) in ovariectomized rats injected subcutaneously with a high dose of GnRH antagonist (500 µg) 24 h earlier. The pFF suppressed FSH 35% below the level achieved in the absence of GnRH stimulation, with no effect on LH. In experiment 3, the rise in FSH secretion in acutely ovariectomized rats was shown to be inhibited by 8 or 32 mg pFF administered intravenously 3.5 h after surgery. Injection of GnRH (250 or 1,000 ng) 4.5 h after pFF could not overcome the inhibitory action of pFF on FSH, although non-pFF-treated controls responded in a dose-dependent fashion to GnRH stimulation. The expected LH response to GnRH was not affected by pFF, except in the group receiving 1,000 ng GnRH and 8 mg pFF. In these rats, LH was enhanced in one trial, but suppressed in a replicate trial, illustrating the inconsistent effects of pFF on LH under conditions of high GnRH stimulation. These results demonstrate the existence of a component of FSH secretion which is independent of GnRH, but sensitive to the inhibitory effect of pFF, suggesting a site of action of the putative peptide hormone, folliculostatin, distal to the GnRH receptor. Also, pFF always suppresses FSH (due to the action of folliculostatin); its effect on LH depends on the dose and the state of GnRH stimulation, and is probably not a function of folliculostatin per se.

This study was performed to determine if two sex differences in gonadotropin responses to negative feedback, the acute postcastration rise and the effect of follicular fluid (FF) in the acute castrate, could be reversed by neonatal treatment with sex-opposite steroids. Female rats that received testosterone propionate (TP-females) and male rats that received estradiol benzoate (EB-males) neonatally were studied as adults. EB-males showed an LH response to gonadecto-my which was much less than control males, and did not differ from control females, which could suggest the hypothesis that neonatal estrogen ‘feminizes’ the male response to gonadectomy. However, as the postgonadectomy response in both LH and FSH was depressed in both TP-females and EB-males in comparison to their respective matched sex controls, neonatal steroid treatment appears simply to impair hypothalamic-pituitary function. This is not a result of decreased pituitary responsiveness to GnRH in TP-females and EB-males. On the other hand, neonatal steroid treatment does not change the sex-specific response to imposition of peptide negative feedback (i.e., FF administration). In control and TP-females, FF significantly suppressed serum FSH levels, both in intact animals and 9 h after gonadectomy. In both control and EB-males, FF suppressed FSH in intact animals, but failed to do so in acute castrates. Thus, neonatal steroid treatment does not reverse the sex differences in gonadal-gonadotropin interrelations, but rather causes an impairment in the acute recognition of loss of negative feedback at the hypothalamic level.

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