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

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Abstract
On the day of the 1st proestrus in the female rat, serum LH [lutropin] levels were very low throughout the morning and increased markedly between 1200 and 1600 h to reach maximum values at this time. By 1800 h the LH titers, though still elevated, were already declining. Serum estradiol (E2) levels were maximally increased at 0800 h (76 pg/ml) and decreased somewhat between this time and 1600 h to finally return to prepubertal values by 1800 h. 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 h. At 0800 h on proestrus serum testosterone was higher than in prepubertal rats, declined thereafter to a nadir at 1200 h and increased again briefly at the time of the LH surge. Serum dihydrotestosterone did not vary throughout the proestrus day and did not differ from levels in prepubertal animals. Ovarian weight increased dramatically between 0800 and 1800 h, 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, apparently 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.