The functional role of thyroid hormone (TH) in the cortex and hippocampus of mouse during neuronal development was investigated in this study. TH insufficiency showed a decrease in the expression of parvalbumin (PV) in the cortex and hippocampus of pups at postnatal day (PD) 14, while treatment with thyroxine from PD 0 to PD 14 ameliorated the PV loss. On the other hand, treatment with antithyroid agents in adulthood did not result in a decrease in the expression of PV in these areas. These results indicate the existence of a critical period of TH action during the early postnatal period. A decrease in MeCP2-positive neuronal nuclei was also observed in the cortical layers II–IV of the cerebral cortex. The brains were then stained with CUX1, a marker for cortical layers II–IV. In comparison with normal mice, CUX1 signals were decreased in the somatosensory cortex of the hypothyroid mice, and the total thickness of cortical layers II–IV of the mice was lower than that of normal mice. These results suggest that TH insufficiency during the perinatal period strongly and broadly affects neuronal development.
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