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Photoperiodic versus metabolic signals as determinants of seasonal anestrus in the mare.
Abstract: The objectives of this study were to compare the timing and mechanisms controlling the onset of anestrus in young and mature mares treated either continuously with melatonin and in those that remained untreated. Changes in body weight, subcutaneous body fat measured to provide an estimate of total body fat, and circulating concentrations of leptin were compared throughout the 1-yr experimental period. The results demonstrate that in young mares the timing of anestrus occurs significantly earlier in the year than in mature mares and that mature mares are more likely to exhibit continuous reproductive activity during the nonbreeding season. The propensity of mature mares to exhibit this phenomenon is not modified by continuous treatment with melatonin but is associated with higher mean circulating concentrations of leptin, body weight, and estimated percent of body fat. In both young and mature mares, body weight, percent of body fat, and circulating concentrations of leptin are higher during summer than winter months. We conclude that, in the mare, the reproductive response to a decrease in photoperiod or a presumptive inhibitory melatonin signal is modified by energy availability, which may be signaled to the hypothalamus-pituitary axis via a change in the circulating concentration of leptin. An additional observation confirmed that the prolactin axis is responsive to continuous treatment with melatonin but that a suppression of prolactin secretion is limited to the spring months.
Publication Date: 2000-06-22 PubMed ID: 10859276DOI: 10.1095/biolreprod63.1.335Google Scholar: Lookup The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
- Comparative Study
- Journal Article
- Research Support
- Non-U.S. Gov't
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 study investigates the factors influencing seasonal anestrus or non-breeding periods in mares, comparing the role of light exposure and metabolic influences such as body fat and hormone levels. The results show age and body composition affect the onset of anestrus, and continuous melatonin treatment doesn’t change this. However, a hormone named leptin potentially modulates the mare’s reproductive response to light.
Study Objectives and Methods
- The study aimed to discern the primary controlling mechanisms for the onset of anestrus in young and mature mares. The researchers compared mares subject to continuous melatonin treatment with untreated mares.
- The study examined changes in body weight, subcutaneous body fat (as a proxy for total body fat), and circulating levels of the hormone leptin over a one-year experimental period.
Key Findings
- Younger mares started their anestrus period significantly earlier than mature mares. Furthermore, mature mares were more prone to maintain reproductive activity during the non-breeding season.
- Continuous melatonin treatment did not change mature mare’s likelihood of continuing reproductive activity during anestrus. However, sustained reproduction in mature mares corresponded with higher body weight, body fat percentage, and levels of the hormone leptin.
- In both young and mature mares, body weight, body fat percentage, and leptin concentrations peaked during the summer, falling in winter.
Conclusions and Additional Observations
- The researchers concluded that in mares, the reproductive response to decreased daylight or a possible inhibiting melatonin signal could be tempered by energy availability, potentially signaled via changes in leptin concentration.
- The study also indicated that the prolactin hormone system in mares reacts to continuous melatonin treatment. Still, its secretion suppression only occurs in the spring.
Cite This Article
APA
Fitzgerald BP, McManus CJ.
(2000).
Photoperiodic versus metabolic signals as determinants of seasonal anestrus in the mare.
Biol Reprod, 63(1), 335-340.
https://doi.org/10.1095/biolreprod63.1.335 Publication
Researcher Affiliations
- Department of Veterinary Science, Maxwell Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky 40546-0099, USA. bfitz@pop.uky.edu
MeSH Terms
- Adipose Tissue / metabolism
- Age Factors
- Anestrus / drug effects
- Anestrus / metabolism
- Anestrus / physiology
- Animals
- Body Weight
- Female
- Horses / physiology
- Leptin / metabolism
- Melatonin / blood
- Melatonin / metabolism
- Melatonin / pharmacology
- Photoperiod
- Prolactin / metabolism
- Seasons
Citations
This article has been cited 15 times.- Hallman I, Karikoski N, Kareskoski M. The effects of obesity and insulin dysregulation on mare reproduction, pregnancy, and foal health: a review. Front Vet Sci 2023;10:1180622.
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- D'Fonseca NMM, Gibson CME, Hummel I, van Doorn DA, Roelfsema E, Stout TAE, van den Broek J, de Ruijter-Villani M. Overfeeding Extends the Period of Annual Cyclicity but Increases the Risk of Early Embryonic Death in Shetland Pony Mares. Animals (Basel) 2021 Feb 1;11(2).
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- Parmantier S, Kyriazopoulou P, McClendon M, Adams A, Murphy BA. Influence of Extended Photoperiod Using Blue Light Masks on Hypertrichosis, Coat Condition and General Health Parameters in Horses with Pituitary Pars Intermedia Dysfunction. Animals (Basel) 2025 Oct 5;15(19).
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