Action of exogenous progesterone and estrogen on behavioral estrus and luteinizing hormone levels in the ovariectomized ewe.

Abstract

A series of experiments were con- ducted which involved the exogenous administra- tion of progesterone and estrogens to 15 ovariec- tomized ewes. Plasma LH levels were measured by radioimmunoassay and the occurrence of behavioral estrus was determined by vasec- tomized rams. Injection of estradiol benzoate (EDB) either at 2 or at 16 days after the end of progesterone priming produced 2 effects. In- itially, a depression (0-12 hr post injection) of plasma LH, followed by an LH surge (12-24 hr post injection). EDB (50 tig) 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 pro- gesterone 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 de- pression 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. (Endocrinology 88: 1184, 1971)