Serum Melatonin Concentrations During Different Stages of the Annual Reproductive Cycle in Ewes

Abstract

To determine the effect of reproductive status on photo-induced alterations in serum concentrations of melatonin, samples of blood were collected at 2 h intervals for 96 h from ewes during estrous cycles, anestrus, the transition from estrus to anestrus and the transition from anestrus to estrus. Two groups of ewes were sampled during the normal breeding season. One group was sampled during the luteal phase of the estrous cycle (Days 8-11 of a 16 day cycle) and the other group was sampled during the follicular and estrual phases of the cycle (Days 15, 16, 1, 2). At each of the 4 times of the year sampled, there was a distinct circadian rhythm in peripheral concentrations of melatonin, Average night-time concentrations (297 .+-. 46.5, S.E.M. [standard error of the mean], pg/ml) were 2-3 times greater than average day-time concentrations (140 .+-. 16.8, S.E.M., pg/ml). Concentrations of melatonin remained constant throughout the dark phase. Due to alterations in duration of the photoperiod, the duration of elevated melatonin concentrations was longest when ewes were exhibiting estrous cycles and shortest when ewes were exhibiting anestrus. Mean concentrations of melatonin during the day or during the night did not differ significantly during different reproductive states. To determine precisely how serum melatonin concentrations reflect light-dark transitions, melatonin concentrations were quantified in samples of blood collected at 1 min intervals from 3 ewes during the mid-luteal phase of the estrous cycle. Lights were left on 2 h into the normal light, turned off 1 h and then turned on. Sampling was initiated 30 min before the initial light-dark transition and was terminated 30 min after the return to light. When ewes were exposed to light, plasma concentrations of melatonin remained at baseline levels of 10-100 pg/ml. Upon initiation of darkness, there was a short lag (5-10 min) after which melatonin concentrations elevated to levels of 150-300 pg/ml. This elevation occurred over a 2 min interval in 2 ewes and over a 10 min interval in the 3rd. Upon return to light, melatonin concentrations declined to baseline levels within 5-10 min.