Plasma melatonin in the horse: measurements in natural photoperiod and in acutely extended darkness throughout the year.
Abstract: Plasma melatonin was measured at the winter and summer solstices and the autumn and spring equinoxes in four mares held under natural conditions at 35 degrees S. At all seasons the onset of the nightly elevated melatonin was coincident with or after the time of sunset and the melatonin offset after the time of sunrise. The duration of elevated melatonin was not different from the duration of natural scotophase for each season, with the duration of elevated melatonin longer in winter than the other seasons. Immediately following each 24 hr sampling two mares were resampled in acutely extended darkness to determine the melatonin profile of the endogenous rhythm of the circadian pacemaker, originating from the suprachiasmatic nuclei (SCN). At each season melatonin secretion commenced earlier and decreased later than that measured under the natural photoperiodic condition, suggesting that the expression of the melatonin rhythm is normally gated by natural environmental light both at dusk and dawn. The interval from the onset of melatonin measured under acutely extended darkness to the time of sunset was greater in the spring/summer than the autumn/winter suggesting a possible alternating signal throughout the year. Thus the mare appears to exhibit a similar interaction between endogenous circadian rhythmic activity and the natural photoperiod as the ewe which may underlie the mechanism for timing reproductive activity through the year.
Publication Date: 1995-08-01 PubMed ID: 8609596DOI: 10.1111/j.1600-079x.1995.tb00165.xGoogle Scholar: Lookup
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- Journal Article
Summary
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The study tracks the relationship between plasma melatonin levels in horses and their exposure to light and darkness over different seasons. It found that the onset and duration of melatonin production was influenced by natural light and dark cycles, and was also associated with seasonal changes, which may affect reproductive activity in horses.
Sampling and Test Environment
- The study was conducted using four mares, and the plasma melatonin levels were measured during the winter and summer solstices and the autumn and spring equinoxes.
- All the measurements were taken under natural conditions at a location situated at 35 degrees south of the equator.
Observations and Findings
- At all seasons, the researchers observed that the start of elevated melatonin levels coincided with or occurred after sunset, while the drop in melatonin levels happened after sunrise.
- The duration of elevated melatonin levels was found to be consistent with the duration of darkness (scotophase) for each season, and this duration was observed to be longer during winter compared to other seasons.
Extended Darkness Experiment
- Following each 24-hour sampling period, two mares were then subjected to extended darkness to observe the changes in the melatonin profile of the endogenous rhythm of the circadian pacemaker, which originates from the suprachiasmatic nuclei (SCN).
- Under these conditions, melatonin secretion started earlier and reduced later than the measurements observed under normal photoperiodic conditions, suggesting that natural light levels both at dusk and dawn influence the expression of the melatonin rhythm.
- The interval from the initiation of melatonin production under extended darkness to sunset was found to be longer in spring/summer than in autumn/winter, indicating an alternating signal throughout the year.
Conclusion
- The results of this study suggest that horses, like sheep (ewes), exhibit a similar interaction between endogenous circadian rhythms and the natural photoperiod.
- This may form the underlying mechanism for timing reproductive activity in horses throughout the year.
Cite This Article
APA
Guerin MV, Deed JR, Kennaway DJ, Matthews CD.
(1995).
Plasma melatonin in the horse: measurements in natural photoperiod and in acutely extended darkness throughout the year.
J Pineal Res, 19(1), 7-15.
https://doi.org/10.1111/j.1600-079x.1995.tb00165.x Publication
Researcher Affiliations
- Department of Obstetrics and Gynaecology, University of Adelaide, Australia.
MeSH Terms
- Animals
- Circadian Rhythm / physiology
- Darkness
- Female
- Horses / blood
- Melatonin / blood
- Photoperiod
- Pineal Gland / physiology
- Reproduction / physiology
- Seasons
- Suprachiasmatic Nucleus / physiology
Citations
This article has been cited 5 times.- Salazar-Ortiz J, Camous S, Briant C, Lardic L, Chesneau D, Guillaume D. Effects of nutritional cues on the duration of the winter anovulatory phase and on associated hormone levels in adult female Welsh pony horses (Equus caballus).. Reprod Biol Endocrinol 2011 Sep 29;9:130.
- Murphy BA, Martin AM, Furney P, Elliott JA. Absence of a serum melatonin rhythm under acutely extended darkness in the horse.. J Circadian Rhythms 2011 May 10;9:3.
- Haritou SJ, Zylstra R, Ralli C, Turner S, Tortonese DJ. Seasonal changes in circadian peripheral plasma concentrations of melatonin, serotonin, dopamine and cortisol in aged horses with Cushing's disease under natural photoperiod.. J Neuroendocrinol 2008 Aug;20(8):988-96.
- Murphy BA, Elliott JA, Sessions DR, Vick MM, Kennedy EL, Fitzgerald BP. Rapid phase adjustment of melatonin and core body temperature rhythms following a 6-h advance of the light/dark cycle in the horse.. J Circadian Rhythms 2007 Aug 24;5:5.
- Price MR, Kruse JA, Galvez ME, Lorincz AM, Avigdor M, Heideman PD. Failure to respond to endogenous or exogenous melatonin may cause nonphotoresponsiveness in Harlan Sprague Dawley rats.. J Circadian Rhythms 2005 Sep 14;3:12.
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