Fluctuations in Pituitary FSH During the Ovulatory Cycle in the Rat and a Possible Role of FSH in the Induction of Ovulation1

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Abstract
Measurements of pituitary gonadotropins in adult, female rats showed that a sharp drop in hypophyseal FSH occurs at approximately the same time as the proastrous drop in LH [luteinizing hormone]. Between the morning and late afternoon of proestrus pituitary LH and FSH both decreased by approximately 50%. A similar decrease in hypophyseal LH and FSH was found to occur in pubertal rats on the day preceding their first ovulation. In the pubertal rats FSH and LH levels continued to fall, reaching their lowest levels at estrus and subsequently rising again. The data suggest that an acute release of pituitary FSH occurs approximately concurrently with the ovulatory surge of LH. In further experiments follicular development was initiated in immature rats by PMS [pregnant mare''s serum gonadotropin] treatment and ovulation was obtained in response to injections of FSH given 2 days after the PMS. It was demonstrated that the ovulation could not have been solely due to LH contamination in the injected FSH. Also, the amount of FSH required for ovulation in the PMS-primed rats was compared with the proestrous drop in hypophyseal FSH. The results suggest that an acute release of FSH at proestrus may play a significant role in the ovulatory process.

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