POSITIVE FEEDBACK ACTION OF OESTRADIOL ON GONADOTROPHIN RELEASE IN 15 DAY OLD FEMALE RATS

Abstract

Female rats were ovariectomized (ovx), adrenalectomized (adx) or both (adx-ovx) on day 8 after birth. The serum gonadotropin concentrations on day 15 were higher in ovx and adx-ovx rats than in sham-operated or untreated controls of the same age. Intact animals on day 15 had higher LH [luteinizing hormone] and FSH [follicle stimulating hormone] levels compared with adult, diestrous levels, and a number of LH peaks were observed. After partial separation of estradiol (LH 20 column chromatography) from other lipid substances which interfere with the radioimmunoassay for estradiol, levels of estradiol were undetectable in ovx and in adx-ovx animals on day 15 but concentrations were relatively high in intact or adx rats. To test whether the high gonadotropin concentrations in 15 day old intact rats were due to a positive feedback action of estradiol, silastic tubes containing different amounts of estradiol were implanted on day 8 at the time of adx and ovx. The mean serum LH and FSH concentrations were increased on day 15 in those animals in which silastic tube implantation resulted in physiological estradiol levels. These elevated gonadotropin values were due to a number of peak levels. Injection of 600 .mu.g progesterone on day 15, 8 h before decapitation resulted in high FSH levels in all the implanted animals, whereas LH levels were still variable from one animal to another. This situation is very similar to that in intact control rats and it is concluded that the hypothalamo-pituitary axis in 15 day old female rats reacts to an estrogenic stimulus followed by a progestational reaction as does the adult gonadostat. This would account for the premature, pre-ovulatory type of LH peaks.