[Animal experiment studies on the neurohumoral control of female puberty]

To study the mode of action of oestrogen in female sexual maturation, prepubertal female rats were treated in different ways with oestradiol benzoate (OB). Using an improved implantation method, former findings on a hypophysial site of oestrogen action in the positive feedback (Hohlweg effect) were confirmed. A single s.c. injection or intrahypophysial implantation of OB at 25 or 26 days of age induced precocious vaginal opening (VO) and one ovulation, but the subsequent ovarian cycle was significantly prolonged, so that the first spontaneous ovulation occurred only at the normal time of the onset of puberty. Further studies demonstrated that the sensitivity to the ovulation-inducing effect of oestrogen increases as the rats approach the age of puberty, and that the first pubertal ovulation can be suppressed by intrahypophysial, but not by intrahypothalamic implantation of progestereone. The conclusion is drawn that the precocious induction of one ovulation via the Hohlweg effect is not tantamount to an advancement of puberty. The results furthermore suggest that the positive oestrogen feedback forms the basis for the first pubertal ovulation as it does with regard to later cyclic ovulations. True acceleration of sexual maturation was achieved by daily injections of 0,05 microgram OB/100 g b.w. from 5 days of age to VO. In contrast to findings obtained by other authors in Sprague-Dawley rats, a shortened OB treatment from 26 days of age to VO was not effective in advancing the onset of puberty unless the rats had additionally been injected with OB from 5 to 10 days of age. A final experiment demonstrated that the first ovarian cycle was not prolonged after neonatal ovariectomy and implantation of ovaries at 24, 28 or 32 days of age. Thus, maturation of the neurohormonal mechanisms that are responsible for the cyclic ovarian function continues during the prepubertal development in the absence of ovarian steroids.