Regulation of circulating thyroid hormone levels by hypothalamic tanycytes and hypophysial pars tuberalis‐specific cells and their morphological and gene‐ and protein‐expression changes under different photoperiods

Abstract Thyroid hormone in the hypothalamus acts as a key determinant of seasonal transitions. Thyroid hormone‐levels in the brain are mainly regulated by the hypothalamic tanycytes and pituitary pars tuberalis (PT)‐specific cells. TSHβ produced by the PT‐specific cells stimulates Dio2 expression and decreases Dio3 expression of the tanycytes. Both tanycytes and PT‐specific cells in photosensitive animals exhibit remarkable changes of morphological appearance and expressions of genes and proteins under different photoperiods. Long photoperiods induce increased gene‐ and protein‐expressions and active features. Short photoperiods cause the decreased gene‐ and protein‐expressions and inactive features. In the PT, expressions of TSHβ, common α‐subunit of glycoprotein hormones (α‐GSU), and MT1 receptor of melatonin receptors and eyes absent 3 change under different photoperiods. Diurnal rhythms of α‐GSU mRNA expression are observed in the PT of Djungarian hamsters. Hes1, Nkx2.1, and LIM homeodomain gene 2 (Lhx2) are involved in the differentiation of PT. In the hypothalamic tanycytes, expressions of Dio2, Dio3, vimentin, serine/threonine kinase 33, GPR50, Nestin, Retinoid signaling genes (retinaldehyde dehydrogenase 1, cellular retinol binding protein 1, and Stra6), monocarboxylate transporter 8, and neural cell adhesion molecule change under different photoperiods. Rax, Lhx2, Nfia/b/x, and fibroblast growth factor 10 are involved in the differentiation of tanycytes.