Action of Exogenous Progesterone and Estrogen on Behavioral Estrus and Luteinizing Hormone Levels in the Ovariectomized Ewe

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Abstract
A series of experiments were conducted which involved the exogenous administration of progesterone and estrogens to 15 ovariectomized ewes. Plasma LH levels were measured by radioimmunoassay and the occurrence of behavioral estrus was determined by vasectomized rams. Injection of estradiol benzoate (EDB) either at 2 or at 16 days after the end of progesterone priming produced 2 effects. Initially, a depression (0-12 hr post injection) of plasma LH, followed by an LH surge (12-24 hr post injection). EDB (50 μg) injected at 2, 8 and 14 days after the end of progesterone priming resulted in typical LH surges to all 3 injections. However, the behavioral responses decreased with repeated injections of EDB. When progesterone was injected simultaneously with EDB on day 2 after the end of progesterone priming, both estrous behavior and the LH surge were totally inhibited but the initial depression in plasma LH was still apparent. These data indicate that estrogens exert both negative and positive actions on LH release from the hypothalamo-hypophyseal system, and that progesterone exerts a block on only the positive action of estrogen. (Endocrinology88: 1184, 1971)
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