The First Proestrus in the Female Rat: Circulating Steroid Levels Preceding and Accompanying the Preovulatory LH Surge

On the day of the first proestrus in the female rat, serum LH levels were very low throughout the morning and increased markedly between 1200 and 1600 hr to reach maximum values at this time. By 1800 hr the LH titers, though still elevated, were already declining. Serum estradiol (E₂) levels were maximally increased at 0800 hr (76 pg/ml) and decreased somewhat between this time and 1600 hr to finally return to prepubertal values by 1800 hr. Although morning progesterone (P) levels were higher than those of prepubertal rats, the greatest increase in serum P occurred in the afternoon of proestrus with a time course that slightly lagged behind that of LH, so that maximal P values (56 ng/ml) were observed by 1800 hr. At 0800 hr on proestrus serum testosterone (T) was higher than in prepubertal rats, declined thereafter to a nadir at 1200 hr and increased again briefly at the time of the LH surge. Serum dihydrotestosterone (DHT) did not vary throughout the proestrus day and did not differ from levels in prepubertal animals. Ovarian weight increased dramatically between 0800 and 1800 hr, with some increment already being observed at a time in which serum LH was still at basal levels. The occurrence of elevated E₂ levels, in the absence of any apparent elevation in mean basal radioimmunoassayable LH levels prior to the afternoon LH surge, supports the view that the onset of puberty in the female rat is a consequence of the triggering of E₂ positive feedback on LH release.