LH Release Following Steroid Administration in Ovariectomized Rats12

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Abstract
The effect of time following ovariectomy on pituitary response to hypothalamic extractwas investigated in rats pre-treated with 50ug estradiol benzoate (EB) and 25 mg progesterone.The increment in circulating LH in response to administration of hypothalamic extract on thethird day after steroid treatment increased linearly up to 35 days following ovariectomy. The minimal dose of EB required to suppress plasma LH levels was examined in rats ovariectomized 12 to 20 weeks previously. There was no measurable change in plasma LH levels within 48 hrafter injection of 0.05 μ EB. Administration of0.5, 5, or 50 μg EB caused a decrease in circulating LH, the magnitude and duration of which increased with dose. Pituitary responsiveness to LRF following administration of EB (35 daysafter ovariectomy) was greater in rats treated with 50 μg EB than in those receiving 5 μg EB, a difference attributable to both lower pre-LRF plasma LH levels and higher post-LRF concentrationsof LH. In addition, injection of 50 μg EBalone, or in combination with progesterone, between 8 and 20 weeks after ovariectomy, caused a release of LH between 1400 and 1700 hr on the second day after injection. This estrogen-induced LH release precludes the use of this animal forassay of LRF activity during limited periods of time related to the critical period. (Endocrinology93: 781, 1973)

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