Effects of Ovariectomy, Estrogen and LHRH on Periovulatory Increases in Plasma Gonadotropins in the Cyclic Rat,

Abstract

We investigated the roles that ovarian secretions during proestrus and estrus and decreasing E₂ titers during proestrus may play in modulating the entire periovulatory gonadotropin surges in plasma in the 4 day cyclic rat. A cannula was implanted into the heart on the early afternoon of proestrus. Experimental protocols included injection of phenobarbital (phen), ovariectomy, implantation of E₂ -Silastic capsules, infusion of LHRH (50 ng/h for 3 h) or a combination of more than one of these treatments during the afternoon of proestrus. Blood samples were withdrawn through the cannula during proestrus and estrus for measurement of plasma LH and FSH or plasma estrogen. Phenobarbital blocked the proestrous LH surge and associated first phase of FSH release as well as the second phase of FSH release which normally occurs during late proestrus and estrus. Ovariectomy had no effect on the periovulatory plasma gonadotropin levels. In ovariectomized (ovx) rats, phen blocked the LH surge and suppressed the rise in plasma FSH during proestrus but the plasma FSH was elevated during estrus to levels observed in controls at this time. Administration of E₂ did not block the elevations in plasma LH and FSH at 1800 h proestrus. Also, treatment with E₂ did not reverse the suppression of LH and FSH at proestrus by phenobarbital in phen treated control or phen treated ovx rats. It did, however, cause a slight suppression of the second phase of increased plasma FSH and it markedly suppressed the magnitude of the plasma LH and FSH at 1800 h proestrus in ovx rats. LHRH infusion during proestrus restored the entire periovulatory gonadotropin surges in phen treated intact and phen treated ovx rats. It did not increase the magnitude of the LH or FSH responses observed at the end of infusion in control and in ovx rats. Administration of E₂ had no effect on the LH response to LHRH as observed at the end of infusion in any of the groups, but had a slight suppressive effect on the magnitude of the increased plasma FSH during estrus in rats infused with LHRH at proestrus. The results support those of previous studies (Blake, 1976a,b) which suggested that ovarian secretions during the time of the periovulatory gonadotropin surges are not required to stimulate the pattern or magnitude of the preovulatory LH surge or the first phase of FSH release and to extend them to include the second phase of FSH release. The results of this study also suggest that: 1) decreasing E₂ titers are not necessary for a substantial elevation in plasma LH and FSH to occur during the afternoon of proestrus in the intact rat; 2) elevated plasma E₂ during proestrus suppresses markedly the magnitude of the LH surge and first phase of FSH release in the acute ovx rat; and 3) a decreasing E₂ titer during proestrus is not involved in initiating the second phase of FSH release but it is involved to a small extent in determining the magnitude of the plasma FSH level during estrus.