Advancement of Puberty in the Female Rat by Estrogen

Abstract

Puberty, as defined by vaginal opening, ovulation and the initiation of estrous cycles, has been advanced more than a week in the immature female rat by treatment with low, approximately physiological dosages of estrogen (estradiol benzoate, 0.05 μg/100 g body wt/day). These results were achieved either by short-term treatment with estrogen starting at 26 days of age or by withdrawal of estrogen after vaginal opening at 30 days of age following long-term treatment from day 5. To study the dynamics of gonadotrophin secretion under natural and precocious pubertal conditions, both pituitary and plasma LH levels were measured by the ovarian ascorbic acid depletion method. In early puberty induced by estradiol benzoate, as in natural puberty 7-9 days later, there was a sharp drop in pituitary LH coincident with an equally marked rise in plasma LH, confirming the release of the pituitary trophin into the blood. In the rat ovariectomized at 26 days and treated with estrogen for 4 days the pituitary LH remained elevated and the plasma level remained undetectable, suggesting that in the intact rat the estrogen stimulus may involve synergism with some other ovarian hormone such as progestin. Testosterone propionate in low dosages (10 μg/100 g body wt/day X5 days) precipitated neither ovulation nor the release of pituitary LH; the latter was undetectable in plasma and high in pituitary extracts. Inasmuch as exogenous estrogen causes the precocious appearance of brain-pituitaryovarian changes characteristic of natural puberty, the results suggest that endogenous estrogen may evoke these changes during the normal maturation process, and that the secretion of estrogen may constitute a key limiting factor in the natural onset of puberty. (Endocrinology76: 1158, 1965)