Ontogenic changes in the expression of cytochrome c oxidase subunit I gene in the cerebellar cortex of the perinatal hypothyroid rat.

The thyroid hormone plays a critical role in normal development of the mammalian central nervous system. This study was designed to examine the effect of perinatal hypothyroidism on ontogenic change in cytochrome c oxidase subunit I (COX I) gene expression in the rat cerebellum by using quantitative in situ hybridization histochemistry (ISH). Newborn rats were rendered hypothyroid by continuous administration of methimazole in the mothers' drinking water. The pups were then killed by decapitation on 1, 5, 10, 15, 20, and 30 days after birth (P1, P5, P10, P15, P20, and P30). Their cerebella were removed, and frozen sections were cut and processed for ISH with 35S-labeled RNA probe for COX I messenger RNA. After hybridization, emulsion autoradiography was performed. The numbers of grains within the external granule cell layer, molecular layer, and internal granule cell layer were then counted. A significant decrease in grain density was detected in the hypothyroid animal in all these areas on P5, P10, and P15. On P15, in the molecular layer, a greater hybridization signal was detected in the inner portion than in the outer portion in the euthyroid animal. No such difference was seen in the hypothyroid animal. Daily T4 treatment for 15 days restored the effect of methimazole treatment. The significant effect of perinatal hypothyroidism on COX I gene expression was not detected after P20. These results indicate that altered thyroid states affect the COX I gene expression in the cerebellar cortex during development, suggesting that the COX I gene is one of the key genes regulated by the thyroid hormone and plays an important role in the morphogenetic changes observed in the perinatal hypothyroid cerebellum.