The present study was designed to explore further the functional antagonism between gonadotrophin-releasing hormone (GnRH) and the ovarian factor, gonadotrophin surge-inhibiting factor (GnSIF). In all experiments, pituitary tissue was exposed to various amounts of GnSIF, after which the self-priming action of GnRH was studied. GnSIF was increased in vivo by FSH treatment and increased in vitro by adding various amounts of follicular fluid (FF) to cultured pituitary cells.
Treatment with 3 or 10 IU FSH suppressed the initial LH response and delayed the maximally primed LH response to GnRH. Treatment with FSH was only effective in intact rats on days 1 and 2 of dioestrus. There was no difference in the rate of maximal LH release irrespective of treatment with either FSH or saline. Since FSH treatment was ineffective in long-term ovariectomized rats, it was concluded that the initial suppressive effect of FSH on LH release was mediated by GnSIF.
Cycloheximide prevented the self-priming action of GnRH by inhibiting GnRH-induced protein synthesis. The initial protein synthesis-independent GnRH-stimulated LH release, which was already suppressed by FSH treatment, remained suppressed in the presence of cycloheximide. Pretreatment with GnRH in vivo increased the protein synthesis-independent GnRH-induced LH release during subsequent incubation of the glands. This increase did not occur after FSH treatment.
Pituitary cells, cultured for 20 h in medium only, failed to elicit the self-priming effect of GnRH. Preincubation with FF maintained the self-priming effect. This was independent of the concomitant presence of various amounts of oestradiol. Preincubation with bovine FF suppressed the initial GnRH-stimulated LH release dose-dependently. Porcine FF, human FF and testicular extract suppressed the release of LH in a similar way.
It was concluded that GnSIF suppresses the initial LH response to continuous GnRH stimulation. Increased levels of GnSIF caused by FSH treatment also delayed the primed LH release. The mechanism of functional antagonism between GnSIF and GnRH could give rise to the occurrence of the phenomenon of GnRH self-priming.