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Entrainment of the Melatonin Rhythms in Early Postnatal Lambs and Their Mothers

Reproductive Sciences Program, Departments of Physiology, Obstetrics and Gynecology, and Biology, The University of Michigan, Ann Arbor, Michigan 48109-0404

Lee E. Claypool

Wisconsin Regional Primate Research Center, 1223 Capitol Court, Madison, Wis consin 53715.

Francis J. P. Ebling

Department of Anatomy, University of Cambridge, Cambridge, UK.

Douglas L. Foster

Room 1101SW, 300 North Ingalls Building, The University of Michigan, Ann Arbor, Michigan 48109-0404.

Although the developing sheep can produce an appropriately timed melatonin rhythm as early as 1 week after birth, it is not known whether the lamb is able to adjust its melatonin rhythm to a change in daylength. The ability of the young lamb to entrain its pattern of melatonin secretion to a new photoperiod was determined in the present study. Eight female lambs and their mothers were raised in long days (LD 16:8) beginning 2 weeks post partum. At 7 weeks of age, the time of lights-off was advanced 8 hr, the short-day photoperiod then being LD 8:16; the time of lights-on remained unchanged. Concentrations of melatonin were measured in blood samples collected hourly on days - 1, 0, 2, 4, 6, and 13 relative to the light change. On day 0, all mothers and daughters had advanced the onset of melatonin secretion by at least 1 hr, and by day 13, 12 of 16 had completely entrained to the new photoperiod. The rate of entrainment among individuals varied; the mean rate for lambs and mothers did not differ. This study provides evidence that the melatonin-rhythm-generating system matures shortly after birth.

Journal of Biological Rhythms, Vol. 4, No. 4, 457-465 (1989)
DOI: 10.1177/074873048900400405


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