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Alterations During the Estrous Cycle in the Responsiveness of the Pituitary to Subcutaneous Administration of Synthetic LH-Releasing Hormone (LHRH)¹

Department of Physiology, The University of Texas Health Science Center at Dallas, Southwestern Medical School Dallas, Texas 75235

Abstract

To further evaluate the alterations in responsiveness of the pituitary to synthetic LHRH during the proestrous discharge of gonadotropins, LHRH was given SC in the hope of producing a release of FSH as well as LH, and blood samples were removed prior to and at various intervals after the injections of the neurohormone while the rats were anesthetized with tribromoethanol. The procedure of anesthesia and blood sampling produced small declines in both FSH and LH, but these only achieved significance in the case of FSH on the morning of estrus and diestrus day 2. An increase in plasma LH occurred in response to LHRH at all stages of the estrous cycle but was minimal during diestrus. The increment in plasma LH was already increased by 9 AM on proestrus and the titer remained elevated for 2 h. A further increase in response to LHRH occurred at 2 PM associated with an increase in initial plasma LH. The maximum response occurred at 5 PM together with maximal initial plasma LH, and the characteristics of the response changed such that there was a much larger increase in plasma LH at 20 min and a rapid decline thereafter, so that the response became pulselike. Initial plasma LH had already declined significantly by 9 PM, and this was associated with a diminished pulselike release of LH in response to LHRH. By the morning of estrus, an even smaller pulse-like release occurred. Significant FSH release in response to LHRH occurred at all stages of the cycle as indicated by the increments in plasma FSH. The relative increase in plasma titers was much less than that for LH and responsiveness to LHRH did not increase until 5 PM on proestrus. Responsiveness had already begun to decline by 9 PM on proestrus and by the morning of estrus was similar to that obtained on diestrus. Initial plasma FSH titers were first elevated by 2 PM on proestrus and they remained elevated on the morning of estrus. The results indicate that maximal responsiveness to LHRH in terms of LH release is associated with the proestrous discharge of LH, but that the maximal responsiveness in terms of FSH release terminates long before the endogenous release of hormone ceases sometime on estrus. The increased responsiveness in terms of LH release may be related to prior estrogen secretion and to the priming action of endogenous LHRH. The continued release of FSH in the face of a return of responsiveness to LHRH to low levels indicates that release of FSH on late proestrus and early estrus is not caused by endogenous release to LHRH. (Endocrinology 96: 1377, 1975)

Footnotes

¹ A preliminary report of this work has appeared, Fed Proc 33: 50, 1974 (Abstract).

² Postdoctoral Fellow of the Ford Foundation.

³ Supported by grants from NIH (AM 10073 and HD 05151), the Ford Foundation and the Texas Population Research Institute.

Received November 15, 1974.

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