Effects of Steroids on the Release of Luteinizing Hormone in the Rat

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A single subcutaneous injection of progesterone (3mg/rat) into 4-day cyclic rats at 11a.m. on the 2nd day of diestrus caused an approximately 48-hr. delay in ovulation. Estrogen administered 24 to 30hrs. after the progesterone treatment restored the delayed ovulation in approximately 24hrs. The minimal effective doses of various estrogens to cause a 24-hr. recovery of the progesterone-delayed ovulation were 0.2mg for estradiol-17β and estrone and 0.05mg for estriol, suggesting that LH releasing activities of estrogens are not related to their estrogenicity but to their metabolism in the liver. Estrogens administered 6hrs. after the progesterone treatment restored the ovulation completely, which was otherwise expected to delay by 48hrs. Hexestrol and mestranol (17α-ethinyl-3-methoxyestra-1, 3, 5 (10)-trien-17β-ol) also showed a similar effect, but they were less active than estradiol. The ovulation was advanced by 24hrs. when 10 or 20mg of progesterone was injected again 48 to 54hrs. after progesterone pretreatment, but it was not advanced by the administration 24hrs. after the pretreatment. The blood LH level always showed the peak at 7p.m. on the day before ovulation regardless of administration of ovarian steroid. It was concluded, therefore, that both estrogen and progesterone were able to provide a 24-hr. acceleration for the mechanism of LH release, but unable to produce an hourly change in the time of LH release. It is suggested, therefore, that in normal estrous cycle the estrogen started to secrete on the afternoon of the day before proestrus triggers somehow the initial discharge of LH at the critical time which has been fixed already by the neural mechanism of CNS under certain environmental factors, mainly lighting schedule, and that progesterone discharged under the influence of initial release of LH, in turn, facilitates the LH releasing mechanism in CNS to induce further release of LH, the amount of which is sufficient to cause ovulation.

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