Abstract
BackgroundIn contrast to studies showing gradual adaptation of melatonin (MT) rhythms to an advanced photoperiod in humans and rodents, we previously demonstrated that equine MT rhythms complete a 6-h light/dark (LD) phase advance on the first post-shift day. This suggested the possibility that melatonin secretion in the horse may be more strongly light-driven as opposed to endogenously rhythmic and light entrained. The present study investigates whether equine melatonin is endogenously rhythmic in extended darkness (DD).MethodsSix healthy, young mares were maintained in a lightproof barn under an LD cycle that mimicked the ambient natural photoperiod outside. Blood samples were collected at 2-h intervals for 48 consecutive h: 24-h in LD, followed by 24-h in extended dark (DD). Serum was harvested and stored at -20°C until melatonin and cortisol were measured by commercial RIA kits.ResultsTwo-way repeated measures ANOVA (n = 6/time point) revealed a significant circadian time (CT) x lighting condition interaction (p < .0001) for melatonin with levels non-rhythmic and consistently high during DD (CT 0-24). In contrast, cortisol displayed significant clock-time variation throughout LD and DD (p = .0009) with no CT x light treatment interaction (p = .4018). Cosinor analysis confirmed a significant 24-h temporal variation for melatonin in LD (p = .0002) that was absent in DD (p = .51), while there was an apparent circadian component in cortisol, which approached significance in LD (p = .076), and was highly significant in DD (p = .0059).ConclusionsThe present finding of no 24 h oscillation in melatonin in DD is the first evidence indicating that melatonin is not gated by a self-sustained circadian process in the horse. Melatonin is therefore not a suitable marker of circadian phase in this species. In conjunction with recent similar findings in reindeer, it appears that biosynthesis of melatonin in the pineal glands of some ungulates is strongly driven by the environmental light cycle with little input from the circadian oscillator known to reside in the SCN of the mammalian hypothalamus.
Highlights
In contrast to studies showing gradual adaptation of melatonin (MT) rhythms to an advanced photoperiod in humans and rodents, we previously demonstrated that equine MT rhythms complete a 6-h light/ dark (LD) phase advance on the first post-shift day
Two-way repeated measures analysis of variance (ANOVA) (n = 6/time point) of hormone levels revealed a significant circadian time (ZT/CT) x light treatment interaction (p < .0001) for melatonin with mean levels remaining consistently high during DD, and elevated relative to LD throughout the subjective day
Observing substantial individual differences in the amplitude and pattern of temporal variation of melatonin in horse serum (Figure 2A), we normalized the individual data by expressing the value at each time point as a percentage of the ZT16-ZT22 mean, an elevation representing the nocturnal LD peak
Summary
In contrast to studies showing gradual adaptation of melatonin (MT) rhythms to an advanced photoperiod in humans and rodents, we previously demonstrated that equine MT rhythms complete a 6-h light/ dark (LD) phase advance on the first post-shift day This suggested the possibility that melatonin secretion in the horse may be more strongly light-driven as opposed to endogenously rhythmic and light entrained. Melatonin is synthesized and secreted primarily during the dark period of the light/dark (LD) cycle, thereby encoding the duration of darkness and reflecting seasonal change in the length of day and night In doing so it provides a neuroendocrine signal (from clock to body) that conveys seasonal timing and regulates reproduction in seasonal breeding animals [8,9]. The 24-h pattern of plasma cortisol concentration peaks in the early morning, declines in the afternoon and remains low most of the night displaying a diurnal rhythm in humans [10] and horses [13,14,15,16] that provides temporal regulation of mammalian immune parameters through powerful immuno-suppressant activity [17]
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