On the Feedback Actions of Estrogen on Gonadotropin and Prolactin Release in Infantile Female Rats1

Abstract

A fetoneonatal estradiol-binding plasma protein, often referred to as alpha fetoprotein (AFP), is present in high levels in serum of neonatal rats. The existence of elevated serum FSH levels, in spite of high estrogen titers and the presence of adult type estrogen receptors in the hypothalamus and pituitary, suggests that the lack of a fully expressed estrogen negative feedback in these animals is not due to a decreased function of the receptors, but rather to binding of estrogen to AFP. To evaluate this hypothesis, rats were ovariectomized at day 10 and injected sc daily for 3 days with different doses of estradiol benzoate (Eb), for which AFP has a very high binding affinity, diethylstilbestrol (DES), which binds very weakly to AFP, or ll-β-methoxyethynylestradiol (RU2858), which does not bind to AFP. Only the largest doses of EB (0.05 and 0.10 μg/100 g BW) were effective in preventing the postcastration rise in FSH. The smallest dose (0.0025 μg/100 g BW) of DES significantly reduced the postcastration ...