Stimulation of Endogenous Follicle-Stimulating Hormone Release during Estrus by Exogenous Follicle-Stimulating Hormone or Luteinizing Hormone at Proestrus in the Phenobarbital-Blocked Rat*

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Abstract
We previously reported that administration of LHRH on proestrus restores the two phases of increased plasma FSH during the periovulatory FSH surge in plasma in phenobarbital- blocked rats. In this study, we determined if the LHRHinduced restoration of the second phase of the FSH surge, which occurs during late proestrus and the morning of estrus, might be mediated by pituitary LH and/or FSH release during proestrus. In phenobarbital-blocked animals, rat FSH or LH was injected between 1500–1800 h on proestrus. Blood was collected by decapitation or through indwelling atrial cannulae during proestrus and estrus for RIA of FSH and LH. After sc or iv injection of FSH, plasma FSH but not LH was elevated during the late afternoon and evening of proestrus and during the morning of estrus. The FSH elevations during estrus reflect endogenous FSH release as hypophysectomy before the injection of FSH blocked the response. Subcutaneous or iv injection of rat LH did not elevate plasma FSH acutely. After sc inection of a dose of rat LH which caused plasma LH to rise about 5-fold, plasmaFSH remained low during proestrus and estrus. After two iv injections of rat LH which caused plasma LH to rise more than 75-fold, plasma FSH rose during late proestrus and remained elevated during the morning of estrus. The stimulatory effects of plasma gonadotropins on FSH secretion were ovulation independent. The results indicate that in the phenobarbitalblocked rat, small selective elevations in plasma rat FSH or large selective elevations in plasma rat LH during proestrus, which re similar to FSH and LH levels observed in nonblocked rats, can stimulate pituitary FSH release during the time interval in which the second phase of FSH release would normally occur. The results also suggest that 1) the spontaneous LH surge and the associated first phase of increased plasma FSH are both involved in mediating the second phase of increased plasma FSH, and 2) the magnitude and/or duration of a rise in plasma gonadotropin during proestrus s important in effecting a threshold response which results in stimulating FSH release during estrus.

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