Characterization and co-localization of thyroid stimulating hormone and melatonin receptors in the pars tuberalis of mares during the breeding and non-breeding seasons

Control of seasonal reproduction in horses has yet to be fully elucidated; however, duration of melatonin secretion is a seasonal regulator of the hypothalamic-pituitary-gonadal (HPG) axis. The hypophyseal pars tuberalis (PT) appears to play a key role in the transduction of melatonin signal through melatonin responsive, PT-specific cells that produce thyroid stimulating hormone (TSH). The objectives of these experiments were to characterize and colocalize TSH-producing cells and melatonin receptors (MT1r) in the PT during breeding and non-breeding seasons. Pituitaries from nine light-horse type mares, aged 6 to 26 years, were obtained following euthanasia during the breeding and non-breeding seasons. Mares were determined to be either cycling (n = 5) or seasonally anovulatory (n = 4) based on season, gross inspection of ovarian structures, and plasma progesterone concentrations. Immediately following euthanasia, the carotid artery was perfused with sodium nitrite and 4% paraformaldehyde before decapitation and dissection of PT from the hypophysis. Immunofluorescent detection of TSH was carried out on fixed-frozen PT sections and number of immunopositive cells per 1 mm2 were counted and averaged for three replications per horse to compare TSH distribution between breeding and non-breeding season using Mann Whitney U test. TSH-immunoreactive cells were present in PT and were greater (P < 0.05) in PT during the breeding season comparedto non-breeding season; few to no immunoreactive cells were seen in PT from non-breeding season. In situ hybridization (ISH) was carried out using RNAscope HD Red Detection Kit and a custom MT1r probe (Ec-MTNR1A) to confirm MT1r localization and compare MT1r distribution between breeding and non-breeding seasons. Mean percentage of MT1r immunopositive cells, determined as a percentage of total number of cells, and expression of MT1r mRNA per cell, determined as mean number of dots (representing mRNA) per cell, were analyzed using Mann Whitney U test. Melatonin receptor mRNA was densely expressed in glandular cells of the PT and no differences were found in either percent immunopositive cells or mean number of mRNA dots per cell between breeding and non-breeding seasons. To determine if MT1r colocalize with TSH-producing cells, ISH was carried out alongside immunofluorescent detection of TSH as previously described. In cycling mares, MT1r mRNA colocalized with TSH-producing cells in the PT; however, not all TSH-producing cells colocalized with MT1r and vice versa. Lack of TSH-producing cells accounted for absence of colocalization in seasonally anovulatory mares. The PT is considered a major target for melatonin, and colocalization with TSH supports a role for TSH as a modulator of seasonal reproduction in the mare. This is further evidenced by increased TSH immunosignal during the breeding season.