Acute changes in pulsatile LH and FSH secretion after ovariectomy in rats: treatment with oestradiol for 24 h suppresses LH, but not FSH, for at least 48 h.

Concentrations of LH in female rats do not increase to concentrations seen in the male at 24 h after castration until up to one week after ovariectomy, while FSH concentrations increase rapidly in both sexes. We hypothesized that the lag in LH rise is due to the imposition of a prolonged suppression by the high concentrations of oestradiol seen during each cycle at pro-oestrus, which prevents a rapid response to the removal of negative feedback. This hypothesis was tested by studying the effect of exposure to pro-oestrus oestradiol concentrations, administered in Silastic capsules, for 24 h on the pulsatile release of LH and FSH. In the first experiment, treatment with oestradiol for 24 h began on day 3 after ovariectomy and resulted in a significant suppression of LH pulse frequency, which appeared to persist for up to 4 days after removal of the implant. In the second experiment, exposure for 24 h to high, pro-oestrous oestradiol concentrations beginning on day 3 or 7 after ovariectomy significantly suppressed mean LH concentrations and LH pulse amplitude compared with vehicle-treated controls, while FSH secretion did not differ between the two treatments. These results suggest that the acute lag in the rate of LH rise after ovariectomy depends on a prolonged suppressive effect of oestradiol and that the rise in oestradiol during the cycle, which exerts a positive feedback to trigger the preovulatory gonadotrophin surges, also exerts a negative feedback for 3-4 days, and this contributes to the relative stability of cycle duration in female rats.