Abstract
The development of estrogen feedback system ongonadotropin release during sexual maturation in female rats was studied. Animals (Wistar strain rats) were divided into 6groups according to their ages; 10, 15, 20, 25, 30, and 35 days. Both LH and FSH levels in serum increased significantly in response to ovariectomy in all age-groups studied when measured one week postoperatively, though in the rats aged 10-15 days the increase in FSH following castration was only slight. In rats older than 25 days, the postcastrationgonadotropin rise, calculated as a percent increase from the basal figure, decreasedgradually with increasing age. Ovariectomized rats injected with estradiol benzoate (EB, 5 μg/100g BW) showed significantly lower levels of both LH and FSH than those in castrated controls. However, the inhibitory action of EB on postcastrationgonadotropin output was found to be relatively less effective in rats older than 25 days.
Ovariectomized rats primed with EB were again injected with a 2nd dose of EB (5 μg/100g BW) at noon 3 days after priming. The 2nd EB injection induced a significant rise in LH 6 h later in 30-and 35-day-old, though not in younger, animals. On the other hand, the FSH response to EB was markedly enhanced during days 15-25 of age. These results indicate that the estrogen negative feedback action on gonadotropin release is already operating in female rats at a very early age, and that the brain sensitivity to estrogen decreases slightly during the late prepubertal phase. In addition, the stimulatory effect of estrogen on LH release fully develops shortly before the onset of puberty, whereas the stimulatory estrogen control of FSH is initiated around the 2nd week of postnatal life.
