Changes in Hypothalamic Luteinizing Hormone Releasing Factor (LHRF) in the Female Rat During Puberty

Abstract

In earlier work it was found that the natural or estrogen-advanced onset of puberty in the female rat was accompanied by a sharp drop in pituitary LH [luteinizing hormone] and the appearance of LH in peripheral blood. The present investigation is a parallel study of puberal changes in hypothalamic LHRF. Acid extracts of stalk-median eminence of test animals were boiled to destroy LH and assayed for LHRF in immature rats primed with gonadotropin after the manner of McCann and Ramirez. Shortly prior to the natural opening of the vagina, which occurs at about 38 days of age in Holtzman rats under laboratory conditions, there was an abrupt rise in hypothalamic LHRF followed by an equally sharp drop to low LHRF values early on the first day of vaginal opening. The whole pattern could be advanced more than a week by daily treatment with low dosages of estrogen (estradiol benzoate 0.05 [ug]/day) starting at day 26. Under these conditions LHRF dropped to undetectable levels by the time the vagina opened (day 29-30) but recovered later that day and rose to higher levels the next day. These alterations in LHRF level at puberty are similar to those observed at estrus in the estrous cycle and are consistent with the hypothesis that LHRF is controlled by feedback mechanisms involving both luteinizing hormone and estrogen.