Thyroid hormones effects on RNA synthesis in brain and liver of neonatal hypothyroid rats

The effect of replacement therapy with thyroid hormones upon RNA transcription was studied in isolated nuclei of cerebrum and liver from hypothyroid rats. The chronic administration of T4 to hypothyroid rats, from the 5th day of life, during ten days, corrected the RNA synthesis in isolated nuclei from cerebrum and liver. The injection of a single dose of T3 to neonatally thyroidectomized rats, ten days after birth, enhanced the endogeneous activity of RNA polymerase I and II of cerebrum and liver. In cerebrum the transcription of ribosomal RNA increased 40 minutes after T3 injection, this activity being constant during the whole period studied. On the other hand, the synthesis of heterogeneous RNA showed an increase only 5 hrs. after the beginning of the treatment. In liver, the lag period is higher for both enzymes, being the maximal response achieved 5 hs. after the hormonal administration.