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Serum concentrations of prolactin, thyroxine and triiodothyronine relative to season and the estrous cycle in the mare.
Abstract: Studies were conducted to characterize circulating concentrations of prolactin, thyroxine (T4) and triiodothyronine (T3) relative to season and the estrous cycle, and to evaluate the ability of thyrotropin-releasing hormone (TRH) to induce prolactin secretion in the horse mare. The increase in serum prolactin following the iv injection of 0, 5, 50 and 500 micrograms TRH was dose-related, while all three doses of TRH induced a significant and comparable increase in serum T4. Seasonal variations in serum prolactin were directly correlated (P less than .001) with changes in photoperiod (r = .80) and temperature (r = .61). By comparison, concentrations of T3 were inversely related to temperature (r = -.49; P less than .001), while changes in T4 were less closely associated with photoperiod (r = -.29; P = .04). There were no significant fluctuations in prolactin, T3 or T4 at any stage of the estrous cycle. These results demonstrate that concentrations of prolactin in the mare are highest during the summer and lowest during the winter, but it is unknown whether this annual rhythm is controlled by photoperiod, temperature and(or) other environmental factors.
Publication Date: 1986-04-01 PubMed ID: 3086267DOI: 10.2527/jas1986.6241012xGoogle 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
- U.S. Gov't
- P.H.S.
Summary
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The study investigates the varying levels of certain hormones (prolactin, thyroxine, and triiodothyronine) in horse mares relative to different seasons and the estrous cycle and examines the effect of thyrotropin-releasing hormone on prolactin secretion.
Hormone Level Measurements and TRH Effect
- The researchers conducted tests to measure the circulating concentrations of three hormones – prolactin, thyroxine (T4), and triiodothyronine (T3) – in the horse mares.
- Additionally, they also looked into the ability of thyrotropin-releasing hormone (TRH) to induce the secretion of prolactin. The response of serum prolactin to varied doses (0, 5, 50, and 500 micrograms) of TRH was examined. Findings showed that the increase in serum prolactin after intravenous injection of TRH was dose-related. All three doses of TRH led to a significant and comparable increase in the levels of T4 in the serum.
Seasonal Variations and Correlation with Photoperiod and Temperature
- The study found significant seasonal variations in the serum prolactin levels. The findings indicated a direct correlation (r = .80, P less than .001) with changes in photoperiod and temperature variations (r = .61). This means that as the photoperiod and temperature increase, the levels of prolactin in the serum also increase.
- However, the concentration changes of T3 were inversely related to temperature (i.e., as the temperature rises, T3 levels decrease), with a correlation coefficient of -.49.
- Changes in T4 levels were found to be less closely associated with changes in photoperiod (r = -.29; P = .04), suggesting a weak negative relation.
Estrous Cycle and Hormone Fluctuations
- The study also investigated whether there were any significant changes in the levels of prolactin, T3, or T4 during different stages of the estrous cycle. The results indicated that there were no considerable fluctuations in these hormones at any stage.
Environment Factors Impacting Annual Prolactin Rhythm
- The researchers also concluded that the findings demonstrate the highest prolactin concentrations in horse mares during the summer and the lowest during the winter.
- However, it was unclear whether this yearly rhythm was influenced by photoperiod, temperature, or other environmental factors.
Cite This Article
APA
Johnson AL.
(1986).
Serum concentrations of prolactin, thyroxine and triiodothyronine relative to season and the estrous cycle in the mare.
J Anim Sci, 62(4), 1012-1020.
https://doi.org/10.2527/jas1986.6241012x Publication
Researcher Affiliations
MeSH Terms
- Animals
- Estrus
- Female
- Horses / physiology
- Luteinizing Hormone / blood
- Progesterone / blood
- Prolactin / blood
- Seasons
- Thyrotropin-Releasing Hormone / administration & dosage
- Thyroxine / blood
- Triiodothyronine / blood
Grant Funding
- RR 07058-17 / NCRR NIH HHS
Citations
This article has been cited 6 times.- O'Brien C, Darcy-Dunne MR, Murphy BA. The effects of extended photoperiod and warmth on hair growth in ponies and horses at different times of year. PLoS One 2020;15(1):e0227115.
- Tangyuenyong S, Sato F, Nambo Y, Murase H, Endo Y, Tanaka T, Nagaoka K, Watanabe G. Comparison of physical body growth and metabolic and reproductive endocrine functions between north and south climates of Japan in trained Thoroughbred yearling horses. J Equine Sci 2017;28(3):77-86.
- Taylor WA, Evans NP, Hertz C, Skinner DC. Intra-pituitary administration revisited: development of a novel in vivo approach to investigate the ovine hypophysis. J Neurosci Methods 2011 Aug 15;199(2):175-82.
- Roig-Pons M, Bachmann I, Freymond SB. Impact of feeding strategies on the welfare and behaviour of horses in groups: An experimental study. PLoS One 2025;20(6):e0325928.
- Ishimaru M, Kume K, Murase H, Sato F, Matsui A, Ohmura H, Taya K. Effect of birth month on endocrine function in Thoroughbred foals born in Hokkaido, the northern part of Japan. J Vet Med Sci 2025 Jul 7;87(7):804-815.
- Bukhari SSUH, Urooj S. Retinol and cholecalciferol affect buserelin-induced estrous in anestrous mares. Trop Anim Health Prod 2025 Mar 17;57(3):124.
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