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.

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. TSHß 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 TSHß, 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.

Expression of neuropeptide Y and agouti-related peptide in the hypothalamic arcuate nucleus of newborn neurogenin3 null mutant mice.

Mice deficient in neurogenin 3 (Ngn3) fail to generate pancreatic endocrine cells and intestinal endocrine cells. Hypothalamic neuropeptides implicated in the control of energy homeostasis might also be affected in Ngn3 homozygous null mutant mice. We investigated the expression of two prominent orexigenic neuropeptides, neuropeptide Y (NPY) and agouti-related protein (AgRP), in the hypothalamic arcuate nucleus of newborn wild-type and Ngn3 null mutant mice. Immunohistochemical analysis demonstrated that, in Ngn3 null mutants, the number of NPY-immunoreactive neurons and nerve fibers was markedly increased in the arcuate nucleus, and the nerve fibers were widely distributed in the hypothalamic area, including the paraventricular and dorsomedial nuclei. Little increase of AgRP immunoreactivity was detected in the arcuate nucleus of mutant mice. In situ hybridization analysis confirmed the increased population of the NPY neurons in the arcuate nucleus of the mutants. The NPY mRNA level, as estimated by laser capture microdissection and real-time quantitative polymerase chain reaction, was 371% higher in Ngn3 null mutants than in wild-type mice. AgRP mRNA levels did not differ significantly between the null mutants and wild-type mice. Thus, up-regulation of the hypothalamic NPY system is probably a feature characteristic of Ngn3 null mice.

Ultrastructural localization of vimentin immunoreactivity and gene expression in tanycytes and their alterations in hamsters kept under different photoperiods.

Tanycytes are located in the basolateral walls of the third ventricle. By light- and electron-microscopic immunohistochemistry, we demonstrated that the tanycytes of Djungarian hamsters were intensely immunostained with the vimentin monoclonal antibody V9. The cells always extended long radial processes in which aggregations of vimentin-immunoreactive intermediate filaments were prominent. The tanycytes showed photoperiod-dependent changes. The population of vimentin-immunoreactive tanycytes was increased in hamsters exposed to continuous lighting for 1 month or to a long photoperiod (16 h light:8 h dark) for 2 months. On the other hand, the immunoreactive cells were significantly decreased in hamsters exposed to complete darkness for 1 month or to a short photoperiod (8 h light:16 h dark) for 2 months. The pericapillary spaces of the primary plexus of the portal circulation system were lined by the end-feet of tanycytic processes. Electron microscopy confirmed that the tanycytic processes were markedly decreased in number and size after exposure to complete darkness. Expression of vimentin mRNA in the hamster mediobasal hypothalamus was detected by the reverse transcription-polymerase chain reaction (RT-PCR). The alterations of vimentin mRNA expression under different photoperiods were analyzed using laser capture microdissection and real-time quantitative PCR. The level of vimentin mRNA in the mediobasal hypothalamus was enhanced after exposure to continuous lighting for 1 month or to a long photoperiod for 2 months, whereas it was significantly diminished after exposure to constant darkness or short photoperiod. These changes in the tanycytes under different photoperiods may influence the portal circulation system and also the cell activity of the pars tuberalis, and may thus participate in photoperiodic regulation of the endocrine system.