In vivo action of a single injection of thyroxine on the cerebellar protein synthesis and cellular multiplications, in normal and hypothyroid young rats. Study of the respective effects of the dose and time action of the hormone.

Abstract

The effect of a single dose of thyroxine on the cerebellar protein synthesis has been studied in 6, 10 and 14-day-old normal and hypothyroid rats. Doses of 10, 30 and 50 μg thyroxine were studied 24 hours after injection. Furthermore, a dose of 30 μg thyroxine was studied 3, 12, 24, 48 and 72 h post-injection. Hypothyroidism reduced the cerebellar weight compared to normal animals. A single dose of thyroxine did not change this parameter in the normal and hypothyroid animals. But the cerebellar protein content increased significantly 12 h after thyroxine injection in hypothyroid animals and 48 h later in normal animals. In hypothyroid rats, the effect of thyroxine on the relative specific radioactivity (RSA) of protein was optimal for a dose of 30 μg thyroxine. This RSA of proteins decreased at 6 days and increased at 10 and 14 days postinjection. In normal animals the effect of thyroxine on the RSA of proteins was maximal for a dose of 10 μg in 6 and 10–day-old animals only. In 6–day-old normal, as well as in hypothyroid animals, thyroxine 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. Thus, the effect of a single dose of thyroxine 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 thyroxine than those of normal animals. During the ontogenetic development, thyroid hormones rather seemed to stimulate cell multiplications and its effect on protein synthesis seemed subsequent to cell multiplications.