In vivo action of a single injection of thyroxine on the cerebellar protein synthesis and cellular multiplications, in normal and hypothyroid young rats

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Abstract
The effect of a single dose of thyroxine [T4] on cerebellar protein synthesis was studied in 6, 10 and 14 day old normal and hypothyroid rats. Doses of 10, 30 and 50 .mu.g T4 were studied 24 h after injection. A dose of 30 .mu.g T4 was studied 3, 12, 24, 48 and 72 h post-injection. Hypothyroidism reduced the cerebellar weight compared to normal animals. A single T4 dose did not change this parameter in the normal and hypothyroid animals. But the cerebellar protein content increased significantly 12 h after T4 injection in hypothyroid animals and 48 h later in normal animals. In hypothyroid rats, the effect of T4 on the relative specific radioactivity (RSA) of protein was optimal for a dose of 30 .mu.g T4. This RSA of proteins decreased at 6 days and increased at 10 and 14 days post-injection. In normal animals the effect of T4 on the RSA of proteins was maximal for a 10 .mu.g dose in 6 and 10 day old animals only. In 6 day old normal, as well as in hypothyroid animals, T4 treatment increased RSA of DNA throughout the experimental period, whereas the RSA of DNA increased during the first 72 h of the hormonal treatment in the hypothyroid animals, but only after 48 h in the normal ones. The effect of a single T4 dose on both cerebellar protein synthesis and cellular divisions was more rapid and marked in hypothyroid animals than in normal ones. Consequently, the nervous structures of hypothyroid animals were more sensitive to T4 than those of normal animals. During the ontogenetic development, thyroid hormones rather seemed to stimulate cell multiplications and their effect on protein synthesis seemed subsequent to cell multiplications.

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