The effect of estrogen on the release of luteinizing hormone (LH) and folliclestimulating hormone (FSH) was studied in spayed rats. The concentration of the hormones in serum was determined by a radioimmunoassay procedure. After a single injection of 20 μg estradiol benzoate (EB), the elevated values of LH in ovariectomized rats were reduced the following day and remained low for at least 8 days. A second injection of EB given at noon 3 or 4 days after the first injection significantly increased serum LH a few hours later and a second peak of LH also occurred the next day. When EB was injected on alternate days, beginning 3 days after the priming dose, high serum LH levels were found every day up to the eighth day. The positive feedback effect of EB on LH release was observed only in the afternoon and not in the morning. No significant changes in serum FSH were found. It is concluded that estrogen injection is effective in triggering the release of LH in ovariectomized rats primed with estrogen. (Endocrinology88: 810, 1971)
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Caligaris et al. (1971) studied this question.