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Does Season Alter Responsiveness of the Reproductive Neuroendocrine Axis to the Suppressive Actions of Cortisol in Ovariectomized Ewes?¹

Reproductive Sciences Program and Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, Michigan 48109

ABSTRACT

Season can profoundly influence activity of the hypothalamic-pituitary-adrenal axis and alter reproductive neuroendocrine responsiveness to stress and gonadal steroids. Here we tested the hypothesis that the inhibitory effect of a stress-like increment in plasma concentration of the adrenal steroid cortisol on pulsatile LH secretion varies with season. LH pulse patterns were monitored prior to and during the administration of cortisol in the same seven ovariectomized ewes during three stages of the yearly breeding cycle: breeding season, transition to anestrus, and midanestrus. The elevation in cortisol mimicked the rise in plasma level of cortisol in response to an immune/inflammatory stress. During all three seasons, cortisol acutely suppressed the pulsatile release of LH. This inhibition reflected a marked reduction of LH pulse amplitude and a minimal suppression of LH pulse frequency. Of interest, the suppressive effect of this physiologic increment in cortisol did not vary across seasons. This provides initial evidence that, in ovariectomized ewes, cortisol-induced suppression of pulsatile LH secretion differs from that of gonadal steroids in that it is not profoundly influenced by season.

cortisol, gonadotropin-releasing hormone, luteinizing hormone, seasonal reproduction, stress

¹ Supported by NIH-HD-30773, T32-HD07048, and the Office of the Vice President for Research at the University of Michigan. A preliminary report was presented at the 37th annual meeting of the Society for the Study of Reproduction, Vancouver, BC, Canada, 1–4 August 2004.

² Correspondence: Kellie M. Breen, Reproductive Sciences Program, University of Michigan, 300 N. Ingalls Building, Room 1115 SW, Ann Arbor, MI 48109-0404. FAX: 734 936 8620; breenk{at}umich.edu

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