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Does Seasonal Reproductive State Affect the Neuroendocrine Response of the Ewe to a Long-Day Pattern of Melatonin?

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

Nancy L. Wayne

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

Fred J. Karsch

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

This study examined whether or not the reproductive response of female sheep to photoperiod varies with seasonal reproductive state. The specific objective was to test the hypothesis that the reproductive response to a long-day pattern of melatonin varies with the reproductive state of the ewe. The response examined was the synchronization of reproductive neuroendocrine induction (rise in serum luteinizing hormone, or LH) following nocturnal infusion of melatonin into pinealectomized ewes for 35 consecutive nights. This infusion restored a pattern of circulating melatonin similar to that in pineal-intact ewes maintained in a long photoperiod (LD 16:8). The ewes had been pinealectomized and without melatonin replacement for 16-25 months prior to the study. They were in differing reproductive states at the start of the infusion, as their endogenous reproductive rhythm had become desynchronized among individuals and with respect to time of year. Noninfused pinealectomized ewes served as controls. Regardless of the reproductive state at the start of the 35-day infusion of the long-day pattern of melatonin, all treated ewes exhibited the same reproductive neuroendocrine response after the infusion was ended. This consisted of a synchronized rise in LH some 6-8 weeks after the infusion was terminated, the maintenance of a high level of serum LH for some 15 weeks, and a subsequent precipitous fall in LH to a very low level. These results provide evidence that a long-day pattern of melatonin can synchronize reproductive neuroendocrine induction in the ewe, regardless of reproductive condition, and thus do not support the hypothesis that this response differs with seasonal reproductive state.

Key Words: circannual rhythm • photoperiod • photoperiodic history • photoperiodism • pineal gland • seasonal reproduction • sheep • luteinizing hormone

Journal of Biological Rhythms, Vol. 7, No. 1, 1-11 (1992)
DOI: 10.1177/074873049200700101


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