Short- and long-term effects of an LHRH antagonist given during the prepubertal period on follicle dynamics in the rat

The effects of the suppression of the high gonadotrophin concentrations normally present by the end of the second week of life on ovarian follicle dynamics were studied in immature rats. Gonadotrophins were suppressed by treatment with an LHRH antagonist (LHRH-A; Org. 30276) on days 6, 9, 12 and 15, and the total population of ovarian follicles was studied at 15 and 28 days, on the day of first oestrus and on the day of oestrus at or following 90 and 300 days of age. Primordial follicles were counted and growing follicles were counted and measured. In rats treated with LHRH-A, follicle recruitment into the growing pool was clearly diminished; the number of growing follicles was significantly (P < 0.01) lower up to the day of first oestrus and the pool of primordial follicles was significantly (P < 0.05) larger at 15 and 28 days. Ovarian weights were significantly lower in rats treated with LHRH-A until at least 90 days of age. However, on the day of oestrus at or after 90 and 300 days of age, there were no differences in either the pool of primordial follicles or the pool of growing follicles between rats treated with LHRH-A and control rats. There was also no difference between groups in the number of fresh corpora lutea at these ages. It was concluded that the early peak in gonadotrophin concentrations in immature rats causes substantial recruitment of follicles into the growing pool. Thus, the number of follicles entering the growing pool is not solely dependent upon the size of the pool of primordial follicles but is clearly influenced by the level of circulating gonadotrophins. In contrast, the large gonadotrophic stimulation that normally takes place during the second and third week of life is neither a prerequisite for functional sexual maturation nor for later cyclic function. Shortly before the time of first ovulation a tight control of follicle dynamics is established which is largely independent of previous gonadotrophin concentrations and follicle dynamics.