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Biology of Reproduction 63, 769-774 (2000)
© 2000 Society for the Study of Reproduction, Inc.

Regular Article

Importance of Photoperiodic Signal Quality to Entrainment of the Circannual Reproductive Rhythm of the Ewe1

Graham K. Barrell3,a, Lori A. Thruna, Martha E. Browna, Catherine Viguiéa, and Fred J. Karsch2,a

a Reproductive Sciences Program, Department of Physiology, University of Michigan, Ann Arbor, Michigan 48109-0404

ABSTRACT

An endogenous circannual rhythm drives the seasonal reproductive cycle of a broad spectrum of species. This rhythm is synchronized to the seasons (i.e., entrained) by photoperiod, which acts by regulating the circadian pattern of melatonin secretion from the pineal gland. Prior work has revealed that melatonin patterns secreted in spring/summer entrain the circannual rhythm of reproductive neuroendocrine activity in sheep, whereas secretions in winter do not. The goal of this study was to determine if inability of the winter-melatonin pattern to entrain the rhythm is due to the specific melatonin pattern secreted in winter or to the stage of the circannual rhythm at that time of year. Either a summer- or a winter-melatonin pattern was infused for 70 days into pinealectomized ewes, centered around the summer solstice, when an effective stimulus readily entrains the rhythm. The ewes were ovariectomized and treated with constant-release estradiol implants, and circannual cycles of reproductive neuroendocrine activity were monitored by serum LH concentrations. Only the summer-melatonin pattern entrained the circannual reproductive rhythm. The inability of the winter pattern to do so indicates that the mere presence of a circadian melatonin pattern, in itself, is insufficient for entrainment. Rather, the characteristics of the melatonin pattern, in particular a pattern that mimics the photoperiodic signals of summer, determines entrainment of the circannual rhythm of reproductive neuroendocrine activity in the ewe.

FOOTNOTES

First decision: 30 March 2000.

1 Supported by NSF-IBN grant 92-06510, the Sheep Research and Assays and Reagents Core Facilities of the P30 Center for the Study of Reproduction, HD-18258, and the Office of the Vice President for Research at the University of Michigan.

2 Correspondence: Fred J. Karsch, Reproductive Sciences Program, 300 N. Ingalls Building, University of Michigan, Ann Arbor, MI 48109-0404. FAX: 734 936 8620; fjkarsch{at}umich.edu

3 Current address: Animal & Food Sciences Division, P.O. Box 84, Lincoln University, Canterbury, New Zealand.




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