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Domestic animal endocrinology1994; 11(2); 227-238; doi: 10.1016/0739-7240(94)90030-2

Factors affecting the circadian rhythm in plasma cortisol concentrations in the horse.

Abstract: In horses, a circadian rhythm in plasma cortisol concentrations has been reported in some but not all studies. When a rhythm occurred, horses were accustomed to a management routine, comprising stabling, feeding and sometimes exercise, which may entrain a circadian pattern. In this work, we monitored plasma cortisol by collecting jugular blood through indwelling cannulae from four groups: 1): 10 untrained, unperturbed mares grazing excess pasture, bled hourly for 26 hr; 2) 4 mares housed in a barn for 48 hr before sampling every 15 min for 20-24 hr; 3) 5 mares placed in an outdoor yard for sampling every 30 min from 0930-2100 hr; and 4) 4 stabled racehorses in training, bled every 30 min from 0730-2000 hr and once the following morning at 0830 hr. Plasma cortisol showed a similar-timed circadian rhythm (P or = 20 hr); however nadir values were higher (P < 0.05) in Group 2. In Group 4, cortisol declined (P = 0.004) during the sampling period but had returned to initial concentrations the next morning. Values did not differ from those for Group 1, except between 1000 and 1300 hr when cortisol in Group 4 was lower (P < 0.05). We conclude that a circadian cortisol rhythm exists in horses in the absence of any known cues imposed by humans. However, this rhythm can be obliterated by the minor perturbation of removing the horse from its accustomed environment. By contrast, the rhythm occurs in trained racehorses, suggesting either that they have adapted to their environment thereby allowing an endogenous rhythm to emerge, or that the rhythm is entrained by their daily routine. These observations highlight the difficulties in determining the cortisol status of a horse, since measurements will be affected by time of day, the occurrence of short-term fluctuations, and how accustomed the horse is to its environment.
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Publication Date: 1994-04-01 PubMed ID: 8045104DOI: 10.1016/0739-7240(94)90030-2Google Scholar: Lookup
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  • Journal Article
  • Research Support
  • Non-U.S. Gov't
  • Research Support
  • U.S. Gov't
  • P.H.S.

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The research article examines the circadian rhythm (biological clock that follows a 24-hour cycle) in the plasma cortisol concentrations of horses and how this rhythm may be influenced by their environment and routines. The study finds that the rhythm could be disrupted when the horses are removed from their familiar environment; but it seems to be present in trained racehorses, which may indicate adaptation to their environment or entrainment by their daily routine.

Research Design and Methodology

  • The study monitored the plasma cortisol levels of horses in four different groups with different living conditions and routines:
  • Group 1 consisted of 10 untrained, unperturbed mares grazing on an excess pasture who were bled hourly for 26 hours.
  • Group 2 consisted of 4 mares who were housed in a barn for 48 hours before being bled every 15 minutes for 20 to 24 hours.
  • Group 3 was of 5 mares placed in an outdoor yard, sampled every 30 minutes from 0930 to 2100 hours.
  • Group 4 comprised 4 stabled racehorses in training who were bled every 30 minutes from 0730 to 2000 hours and once the next morning at 0830 hours.

Key Findings

  • All horses in Group 1 showed a consistent circadian rhythm, with cortisol levels peaking between 0600-0900 hours and reaching the lowest point (nadir) between 1800-2100 hours.
  • Unlike Group 1, the cortisol concentrations in Groups 2 and 3 did not display any significant variations with time, indicating no clear circadian rhythm.
  • In Group 4, cortisol levels decreased during the sampling period but returned to the initial levels the next morning. The cortisol levels in this group were found to be lower between 1000-1300 hours compared to Group 1.
  • This suggests that a circadian cortisol rhythm can be observed in horses that are not exposed to any known cues imposed by humans. However, this rhythm is disrupted when the horses are moved from their familiar environment. In trained racehorses, the rhythm persists, possibly because these horses have adapted or their rhythm is influenced by their daily routine.

Implications of the Study

  • The results highlight the difficulties in determining the cortisol status of horses as multiple factors influence these measurements – such as the time of day, short-term fluctuations, and the familiarity of the horse with its environment.
  • Understanding these rhythms and their influential factors can help in better understanding the metabolic and physical health issues in horses and provide possible ways to address them.

Cite This Article

APA
Irvine CH, Alexander SL. (1994). Factors affecting the circadian rhythm in plasma cortisol concentrations in the horse. Domest Anim Endocrinol, 11(2), 227-238. https://doi.org/10.1016/0739-7240(94)90030-2

Publication

Domestic animal endocrinology
ISSN: 0739-7240
NlmUniqueID: 8505191
Country: United States
Language: English
Volume: 11
Issue: 2
Pages: 227-238

Researcher Affiliations

Irvine, C H
  • Animal & Veterinary Sciences Group, Lincoln University, New Zealand.
Alexander, S L

    MeSH Terms

    • Adaptation, Physiological
    • Animal Feed
    • Animals
    • Circadian Rhythm
    • Environment
    • Female
    • Horses / blood
    • Hydrocortisone / blood
    • Physical Exertion

    Grant Funding

    • DK-38322 / NIDDK NIH HHS

    Citations

    This article has been cited 38 times.
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