On the feedback actions of estrogen on gonadotropin and prolactin release in infantile female rats
Andrews, W.W.; Ojeda, S.R.
Endocrinology 101(5): 1517-1523
1977
ISSN/ISBN: 0013-7227 PMID: 913323 DOI: 10.1210/endo-101-5-1517Document Number: 434487
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 ) were effective in preventing the postcastration rise in FSH. The smallest dose (0.0025 .mu.g/100 g BW) of DES significantly reduced the postcastration rise in FSH while larger doses brought FSH levels below those of intact sham-operated controls. Even the smallest dose (0.0025 .mu.g/100 g BW) of RU2858 was effective in reducing plasma FSH to less than half of that of control rats. To study estrogen positive feedback, intact female rats received a s.c. injection of Eb, DES, or RU2858 in different doses on the morning of day 10, followed by a 2nd injection 2 days later. The animals were killed at various times after the 1st and/or 2nd injection. As a control of the effectiveness of this procedure, 26 day old animals received an identical Eb treatment. No estrogen positive feedback on gonadotropin release was observed in infantile rats at any time studied regardless of dose or type or estrogen given. Moreover, plasma FSH was reduced following estrogen treatment. Plasma prolactin was not altered by Eb but was slightly increased by DES and further increased by RU2858. A marked increase in plasma LH, prolactin and, to a lesser extent FSH, was elicited by Eb in 26 day old rats. The results are consistent with the concept that although the infantile hypothalamic-pituitary unit is capable of sensing changes in estrogen levels, the manifestation of estrogen negative feedback is prevented by the presence of AFP. By contrast, immaturity of a central mechanism appears to be responsible for the failure of estrogen positive feedback to operate in neonatal rats.