Review ArticleCircadian and Circannual Regulation in the Horse: Internal Timing in an Elite Athlete
Section snippets
Introduction to Chronobiology
Every life form that evolved on our rotating planet was subject to the evolutionary pressures provided by rhythmically fluctuating light–dark (LD) cycles. The daily and seasonal changes in photoperiod represented the dominant environmental cues that gave rise to the development of cellular clock mechanisms, allowing the simplest to the most advanced organisms to tell physiological time. The evolution of biological timekeeping mechanisms meant survival to those organisms most capable of
The Circadian System
The circadian system consists of a network of clocks that drive self-sustained circadian rhythms in mammalian rest-activity cycles [20], hormone secretion [21], cardiovascular activity [22], metabolism [23], immune function [24], alertness [25], and musculoskeletal performance [26]. Suffice to say that few physiological processes are outside the control of the circadian clock. By definition, a circadian rhythm is endogenously generated, has a period length close to 24 hours that persists under
The Circannual Clock
Endogenous circannual rhythms allow anticipation of seasons and drive long-term physiological cycles in long-lived mammals including reproduction, pelage growth, metabolism, and migration [107], [108]. Photoreception by the retina records the annual cycle in day length and acts as the primary synchronizer of these rhythms. Unlike temperature, availability of food, rainfall, or other environmental cues, photoperiod provides seasonal information that remains constant from year to year. The
Lighting for Circadian and Circannual Entrainment
Within the equine industry, there is a general understanding of the application of extended daily lighting for manipulation of seasonal reproduction and coat growth. However, what is lacking is an appreciation for the impact of daily lighting regimes on maintenance of a horse's circadian rhythms. All light is not equal, especially when it comes to the capacity to stimulate and entrain the circadian system. The natural daylight spectral composition encompasses the visible, ultraviolet, and
Conclusions
Circadian rhythmicity is a feature of nearly every physiological, metabolic, and behavioral system. As its primary synchronizer, light reception by the retina directly and indirectly regulates a wide array of biological processes. The concept of “circadian lighting”—lighting that promotes alertness by day and rest and recovery by night—is gaining increasing popularity within human society. The health benefits of regularly timed changes in spectral intensity and composition that mimic the
Acknowledgment
The author would like to acknowledge the late Barry P. Fitzgerald, whose work and interest in equine seasonality and melatonin regulation, sparked my interest for the field of biological rhythms and ultimately led to much of the described research on equine circadian regulation.
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2023, Applied Animal Behaviour ScienceThe effect of month and breed on plasma adrenocorticotropic hormone concentrations in equids
2022, Veterinary JournalCitation Excerpt :Rather than simply considering there to be an autumnal peak in ACTH, there would appear to be gradually increasing secretory activity from a nadir in April and then a decline from the peak in September (Figs. 1 and 3). Studies of circannual rhythmicity of reproductive activity in mares indicates a critical long day length of 14.5–16 h for triggering onset of early seasonal ovarian activity, this being a consequence of an apparently crucial light sensitive phase approximately 9.5 h after dusk (Murphy, 2019). In this respect it is interesting that the time at which > 14.5 h of daylight first occurs in the UK lies between 16th and 28th April (depending on latitude1), corresponding to the nadir in annual ACTH concentrations.
Physiological role of circadian clock gene on the energetic metabolism in horses
2022, Journal of Veterinary BehaviorCitation Excerpt :Many studies have been conducted in horses to investigate the circadian rhythm of its physiology (Piccione and Giannetto, 2011), starting from its activity behavior (Piccione et al., 2008; Giannetto et al., 2016), the rectal temperature circadian rhythm (Piccione et al., 2011) and the melatonin daily rhythm (Piccione et al., 2013a). An overview of this field, highlighting the recent discoveries related to biological timing in horses, and discusses the implications that these findings may have for the production and management of the elite equine athlete has been performed by Murphy (2019). Considering the evidence that disruption of the circadian system increases the risk of metabolic diseases (Li et al., 2012) and the increasing interest in understanding of the design principles of the mammalian circadian clock by the identification of key clock genes; the aim of the present study was to improve the knowledge on the circadian system control of glucose, protein and lipid metabolisms in healthy horses.
Genetics of Equine Behavioral Traits
2020, Veterinary Clinics of North America - Equine PracticeCitation Excerpt :Genomic tools are not limited to the study of heritable variations. Circadian and circannual behaviors influence several important economic traits, such as reproductive seasonality, feeding, and athletic performance.77,78 Gene expression provides a valuable tool for tracking changes in physiologic processes, and the signaling pathways responsible for entraining them.77
No-Contact Microchip Monitoring of Body Temperature in Yearling Horses
2020, Journal of Equine Veterinary ScienceCitation Excerpt :Like other mammals, horses are homeothermic with core body temperature remaining in a constant range, whatever external environment variations. However, the maintenance of a constant body temperature may represent a high energy cost [7]. Regulation mechanisms depend on both ambient temperature and humidity index as a high humidity can impair thermal regulation [8–10].
Animal welfare/ethical statement: Where relevant, all studies were conducted in accordance with the “Code of Good Practice in Research” (University College Dublin, Ireland) and “Directive 2010/63/EU of the European Parliament and the Council on the Protection of Animals used for Scientific Purposes” and had approval of the institutional Animal Research Ethics Committee. Additional studies referred to in the review article have ethical statements that appear in the respective publications.
Conflict of interest statement: B.A.M. is a member of the Board of Directors of Equilume Ltd. Equilume Ltd. has been granted a license for commercial use of information that is contained in the following granted patents: Practices US Pat. 9,839,791; UK Pat. GB2504244B, and GB2549682B. B.A.M. is named on these patents.