Melatonin receptors and thyroid stimulating hormone in the equine pars tuberalis: Potential modulators of seasonal reproduction in the mare

The mechanism by which photoperiod influences the hypothalamic-pituitary-gonadal (HPG) axis and regulates seasonal reproduction in horses has yet to be fully elucidated. The hypophyseal pars tuberalis (PT) has been indicated as a critical site for the transduction of melatonin signals through melatonin-responsive, PT-specific cells that produce thyroid-stimulating hormone (TSH) in many mammalian species. However, this has yet to be investigated in horses. The objective of this study was to explore the interaction of melatonin and thyroid-stimulating hormone in the equine HPG axis. Pituitaries from mares of light-horse breeds (Quarter Horse, Thoroughbred, etc.) categorized as either breeding season or non-breeding season based on season, gross examination of ovarian structures, and plasma progesterone concentrations were collected post-mortem. In situ hybridization revealed melatonin receptor (MT1r) mRNA abundantly expressed in glandular cells of the PT in both breeding and non-breeding season mares. Immunofluorescent analysis revealed a higher abundance of TSH-ir cells (p = .0043) in PT obtained during the breeding season compared to the non-breeding season. In cycling mares, MT1r mRNA co-localized with TSH-producing cells in the PT, suggesting a role for TSH as a modulator of seasonal reproduction in the mare. These findings support a role for melatonin and TSH in modulating seasonal reproduction in the mare, further evidenced by the increased TSH immunosignal observed during the breeding season. Altogether, this study endorses the PT as the key site for integrating multiple cues to regulate seasonal reproduction in the horse, as this study marks the first investigation of the relationship between melatonin and PT-specific TSH cells and release in an equine model.