Stimulatory Effects of Estradiol on Lh Secretion During the Estrous Cycle of the Ewe (Progesterone, Lh Surge).

Abstract

Estradiol is known to have both stimulatory and inhibitory effects on LH secretion by the ewe. The importance of the former in orchestrating the hormonal events of the follicular phase of the ovine estrous cycle was investigated. In the first study, estradiol was shown to be capable of stimulating LH-pulse frequency in long-term castrated ewes. An experimental model was then developed in which endogenous GnRH ecretion in the ewe was abolished while pituitary responsiveness to oxogenous GnRH was not significantly altered (Progesterone Suppressed Model; PSM). This model, consisting of ovariectomy and treatment with progesterone during anestrus, was applied to the study of the inverse relationship between frequency and amplitude of LH-pulses. The results showed that GnRH-pulse frequency can affect pituitary responsiveness to GnRH and thus account for the inverse relationship. LH-surge responses were induced in the PSM by simulation of the pituitary with estradiol and GnRH treatments thus demonstrating that the blockade of the LH surge by progesterone is affected at a neural rather than an hypophyseal level. Further investigation showed that gradual increases in GnRH-pulse frequency were ineffective in producing gonadotropin surges whereas abrupt increases to the same end-point were effective. This suggested that an abrupt increase in GnRH stimulation of the pituitary is needed to induce the surge; a further study showed that attainment of peak follicular phase estradiol concentrations is also required. These observations are consistent with the hypothesis that, prior to the surge, a positive feedback relationship between estradiol and LH allows development and maturation of the ovarian follicles. High levels of estradiol present late in the follicular phase then affect neural and hypophyseal function to produce the LH surge.