Absence of a serum melatonin rhythm under acutely extended darkness in the horse.
Abstract: 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. The present study investigates whether equine melatonin is endogenously rhythmic in extended darkness (DD). Methods: Six 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. Results: Two-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). Conclusions: The 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.
Publication Date: 2011-05-10 PubMed ID: 21569251PubMed Central: PMC3103467DOI: 10.1186/1740-3391-9-3Google Scholar: Lookup
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Summary
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The study aims to investigate the endogenous rhythmicity of melatonin in horses under extended darkness. It found that unlike other species, melatonin in horses does not follow a 24-hour oscillation and is not endogenously rhythmic. This suggests that light, rather than an internal clock, strongly influences melatonin secretion in these animals.
Study Design and Methods
- The research involved six healthy young mares that were kept in a lightproof barn following the natural ambient light/dark cycle outside.
- Over a period of 48 hours, blood samples were taken every 2 hours. The first 24 hours followed a light/dark (LD) cycle and the second 24 hours were under extended darkness (DD).
- The blood samples were then stored at -20 degrees Celsius until tested for melatonin and cortisol using commercial Radioimmunoassay (RIA) kits.
Results
- A significant interaction between circadian time and lighting conditions was found for melatonin, where the levels did not follow a rhythmic pattern and remained consistently high during extended darkness.
- Cortisol, on the other hand, showed significant variation throughout the light/dark and extended darkness cycle, but there was no interaction with light treatment.
- Further analysis confirmed that melatonin in horses displayed a significant 24-hour temporal variation under light/dark conditions, but not in extended darkness.
- Interestingly, cortisol showed a semblance of circadian rhythm that was nearly significant under light/dark conditions and highly significant under extended darkness.
Conclusions
- The absence of a 24-hour oscillation in melatonin under extended darkness indicates that melatonin secretion in horses is not governed by a self-sustained circadian rhythm. Therefore, it’s not a suitable marker of circadian phase in this species.
- This finding, along with similar observations in reindeer, suggests that in some ungulates (hoofed mammals), melatonin biosynthesis in the pineal glands is primarily influenced by the environmental light cycle rather than any endogenous circadian oscillator residing in the Suprachiasmatic Nucleus (SCN) of the mammalian hypothalamus.
Cite This Article
APA
Murphy BA, Martin AM, Furney P, Elliott JA.
(2011).
Absence of a serum melatonin rhythm under acutely extended darkness in the horse.
J Circadian Rhythms, 9, 3.
https://doi.org/10.1186/1740-3391-9-3 Publication
Researcher Affiliations
- School of Agriculture, Food Science and Veterinary medicine, University College Dublin, Belfield, Dublin 4, Ireland. barbara.murphy@ucd.ie.
References
This article includes 46 references
Citations
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- Harmat L, Nagy J, Somoskői B, Alpár A, Fekete SG, Gáspárdy A. Determination of Rhythmicity and Gestational Stage-Related Distribution of Blood Plasma Melatonin Concentrations in Donkey Mares. Vet Sci 2024 Jul 11;11(7).
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