Serum levels of FSH, LH and estradiol-17 beta in female rats around the time of puberty onset.

Groups of female rats were autopsied at daily intervals from 27 days of age through the first vaginal cycle and the serum FSH and LH and plasma estradiol-17 beta (E2) concentrations were estimated. Fluctuating hormone concentrations were recorded between days 27 and 31. Both gonadotropins and E2 dropped to a very low level on days 32 and 33 and showed then a progressive increase that commenced on day 34 and terminated for LH and E2 on day 39, i.e. in proestrus, whereas FSH declined from day 37 to day 39. The circulating gonadotropin level was lower during the first postpubertal diestrus than during most of the prepubertal phase. The findings suggest that a high sensitivity to the gonadotropin -inhibiting effect of estrogen prevented completion of an ovarian cycle up to day 33. After that, desensitization to the negative estrogen feedback as indicated by the simultaneous rise of E2 and gonadotropins makes final maturation of ovarian follicles and the first ovulation possible. In part of the rats aged 27-38 days cell nuclear volumes of medial preoptic neurons were evaluated, because recent results suggest that estrogen induces the desensitization process by an inhibitory action on the medial preoptic area. In accordance with this assumption, an inverse relationship between nuclear size and the E2 level in the blood could be revealed.