Degree

Doctor of Philosophy (PhD)

Department

Animal Science

Document Type

Dissertation

Abstract

Reproductive seasonality in the mare involves various neuroendocrine modulations, and a series of experiments were conducted to investigate the anatomical and physiological roles of some of these seasonal modulators. The first experiment incorporated kisspeptin-10 (Kp10) into a treatment with estradiol benzoate (EB) and sulpiride, aiming to induce early cyclicity in seasonally anovulatory mares. Incorporation of Kp10 resulted in more mares ovulating earlier compared to mares receiving EB-sulpiride. Although plasma prolactin increased in both groups, concentrations were notably higher in Kp10-treated mares. Therefore, the subsequent experiment explored the anatomical interaction of kisspeptin and prolactin, aiming to determine whether kisspeptin may regulate dopamine and, thus, prolactin at the hypothalamic or pituitary level, or both. Kisspeptin receptors were identified and quantified in the equine hypothalamus for the first time and co-localized with dopaminergic neurons. While kisspeptin receptors were also identified on the equine anterior pituitary, significant co-localization with prolactin-releasing cells was not observed. The third experiment aimed to investigate the interaction of melatonin and thyroid stimulating hormone (TSH) in the equine hypothalamic-pituitary-ovarian axis. Melatonin receptors were identified in the equine pars tuberalis and observed to co-localize with TSH-producing cells exclusively in breeding season mares. Thyroid stimulating hormone immunopositive cells were greater in the breeding than non-breeding season, providing anatomical evidence that supports a role for TSH as a modulator of seasonal reproduction in the horse. The final experiment aimed to assess the impact of melatonin on systemic thyroid and reproductive endocrine responses in summer cycling mares. Daily melatonin administration, to mimic a short-day photoperiod, did not alter plasma TSH or triiodothyronine concentrations, suggesting that the TSH signal related to season does not circulate systemically. In summary, these experiments proposed an additional role for kisspeptin as a modulator of prolactin release from the pituitary, provided anatomical evidence of pars tuberalis TSH production and co- localization with melatonin receptors, and determined that exogenous melatonin, at the dose and frequency administered, does not reduce circulating TSH or T3. Collectively, these findings contribute to the existing knowledge of neuroendocrine mechanisms governing the HPG axis and seasonal reproduction in horse.

Date

7-1-2024

Committee Chair

Erin Oberhaus

DOI

https://doi.org/10.31390/gradschool_dissertations.6509

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Available for download on Wednesday, July 01, 2026

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