Ovarian stimulation with FSH in the rat reduces proestrous GnRH-dependent LH secretion through a dual mechanism: inhibition of LH synthesis and release.

Administration of human follicle-stimulating hormone (hFSH) to female rats during diestrus phase attenuates the spontaneous luteinizing hormone (LH) surge in proestrous afternoon. The inhibition of LH secretion is associated with a decreased pituitary LH content in intact, but not in ovariectomized rats injected with 25mug estradiol benzoate (EB). This suggests that the mechanism of action of the putative non-steroidal ovarian bioactive FSH-dependent gonadodotropin surge attenuating factor (GnSAF) might, in addition, involve a reduction in LH synthesis. The present experiments studied, in proestrous pituitaries, the effects of different doses of hFSH, with or without EB on: (i) basal and GnRH-stimulated LH release and GnRH self-priming, (ii) LHbeta mRNA values, and (iii) LH content. Results showed that bioactive GnSAF reduced mainly GnRH self-priming, but also GnRH-stimulated LH secretion and pituitary LH content in a dose-dependent manner. GnSAF had no effect on basal LH secretion or pituitary LHbeta mRNA values. EB increased pituitary sensitivity to GnRH in controls, and overcame the inhibitory effects of GnSAF after low doses of hFSH but not after 10IU of hFSH. In contrast with the sensitizing action of EB on LH secretion, EB had no effect on pituitary LHbeta mRNA content or LH protein. It is concluded that the putative GnSAF blunted the LH surge by reducing LH synthesis at post-transcriptional level and antagonizing the GnRH-dependent LH secretion and the sensitizing effect of estradiol to GnRH.