Hormonal Events Surrounding the Natural Onset of Puberty in Female Rats1

In an effort to more completely define the hormonal events associated with the onset of puberty in the female rat, serum levels of a number of steroids, cytosol estradiol receptors of the anterior pituitary-hypothalamic complex, and pituitary responsiveness to exogenous LH-RH were quantified. We found that an increase (P<0.01) in blood concentrations of estradiol, progesterone and testosterone preceded vaginal opening and the initial ovulation in Holtzman rats. Serum levels of 17-hydroxyprogesterone and Δ4-androstenedione were not significantly increased until the day of ovulation. Coincident with the increase in serum estradiol was a significant decline in cytosol estradiol receptor levels of the anterior pituitary and hypothalamus. Basal serum LH levels and pituitary responsiveness to exogenous LH-RH (10 ng/100 g BW) were both increased after the initial elevation in serum estradiol. The changes in blood steroids and cytosol estradiol receptors documented in this study at the time of the initial ovulation are similar to those noted in the PMSG-primed immature female rat and the cycling adult rat, suggesting that the preovulatory gonadotropin surge is induced via common mechanisms.