| Thyroid hormone is essential for proper development of the mammalian central nervous system. Hypothyroidism during human perinatal development results in profound alterations of mental capacity, neurological function, and metabolic process. However, the exact mechanism by which hypothyroidism affects brain development remains to be elucidated. Thyroid hormone effects on cell proliferation, migration, differentiation and maturation of the brain have been investigated in detail. It is well established that apoptosis plays an important role in the maturation of the central nervous system. However, apoptosis has received less attention in the context of thyroid hormone and brain development. The hippocampus participates in cognization and behavior. The nature of increased cell death attributed to deficiency of thyroid hormone during hippocampus development is not yet understood in the context of apoptosis. We studied on apoptosis of hippocampus neurons and mechanism of it, and the correlation between apoptosis and behaviour in rats of perinatal hypothyroidism. Our aim was to explore whether brain damage as a result of insufficient thyroid hormone during brain development is caused by an increased level of apoptotic cell death in the hippocampus.Part one Effects of perinatal hypothyroidism on apoptosis of hippocampus neurons in ratsMethods Hypothyroidism was induced by administration of propylthiouracil (PTU, 50mg/d) solution to the dams by gavage from gestational day 15. Pups from both hypothyroid and control groups were harvested at postnatal day 1,5, 10 and 15, respectively. The pups with significant lower serum free thyroxine (FT4) level were employed in this study. Apoptosis of hippocampus neurons was examined by light and transmissional electron microscopy, measurement of DNA fragmentation and terminal deoxynuleotidy transferase-mediated dUTP nick end labelling ( TUNEL) in rats of hypothyroidism, compared with normal controls.Results Significantly higher serum TSH and lower circulating FT4 and FT3 levels confirmed the hypothyroid status of the experimental pups. Shrunken and contracted degenerations increased were seen in hippocampus neurons of hypothyroid pups under light microscopy. Increased numbers of apoptotic cells were found in the hippocampus of hypothyroid pups under transmissional electron microscopy, especially at 10 and 15 days. Extensive DNA fragmentation was seen in all developmental stages in the hippocampus of hypothyroid pups, but not in the euthyroid controls, except for basal levels at 10 days. Apart from 1 day, positive TUNEL stain in CA3 was higher in the study group than control at other stages of development (all P<0.05), especially at 15 days.Part two The mechanism of hypothyroidism-induced apoptosis of rat hippocampus neurons during the perinatal periodMethods Mitochondrias of hippocampus neurons in pups of hypothyroid were examed under transmissional electron microscopy at 1, 5, 10 and 15 days. Theexpression of Bcl-2 protein and the translocation of apoptogenic molecules protein (Bax, cytochrome c and AIF) and activation of caspase-3 in hippocampus neurons were analysed by Western blotting, compared with normal controls. Results Electron microscopy showed that altered morphology of mitochondria significantly increased under hypothyroid conditions (P<0.05). The expression of Bcl-2 in the cytosol of hippocampus neurons of hypothyroid pups was significantly lower than that of euthyroid controls at all stages of development (all P<0.05). The expression of Bax in the cytosol of hypothyroid pups was higher than that of control pups at all stages of development (all P<0.05), and significantly higher in mitochondria (all P<0.001). The ratio of Bcl-2/Bax in the cytosol of hippocampus neurons of hypothyroid pups was lower than that of age-matched controls (all P<0.01). The expression of cytochrome c in the cytosol of hypothyroid pups was significantly higher than that of control pups at all stages of development (all P<0.05), and lower in mitochondria (all P<0.05). The expression of AIF in the cyto... |
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