Induction of Delayed or Advanced Ovulation by Estrogen in 4-Day Cyclic Rat

概要

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When estradiol-17β dissolved in sesame oil was administered subcutaneously on the day of estrus in 4-day cyclic rats, spontaneous ovulation expected to occur on the next estrus was postponed for 3 days in maximum according to the dosage of estrogen injected. On the contrary, when estradiol was injected on the morning of diestrus I, ovulation occurred on the day of coming proestrus. This 1-day advancement of ovulation was demonstrated to be specific for estrogen, and it occurred only when a large amount of the steroid, 50μg per rat or more, was administered within a limited period of time on the morning of diestrus I, between 10:00 and 12:00 noon. The results indicate that estrogen as well as progesterone has a biphasic effect, either inhibitory or facilitatory, on the ovulating hormone release, although the effective period of time for estrogen during the estrous cycle is entirely different from that for progesterone, and it is emphasized that the advancement of ovulation can be induced by estrogen even in 4-day cyclic rats as well as in 5-day cyclers.