Influence of hypothyroidism on the ontogenic development of tyrosine hydroxylase induction in the adrenal glands of the rat.

The ontogenic development of the transsynaptic induction of adrenal tyrosine hydroxylase (TH), evoked by reserpine and nicotine was studied in control and hypothyroid young rats, aged 3-52 days. The enzymatic induction was measured as an increase in the enzyme activity, since this increase was shown to be impaired either by an inhibitor of RNA synthesis or by a ganglionic blocker. In the control animals, TH induction elicited by reserpine increases between 3 and 32 days of age. In the hypothyroid rats, the enzymatic induction is impaired up to 32 days; at 52 days the induction is similar in both groups of animals. When nicotine is used as a stimulating agent, hypothyroidism still impairs the enzymatic induction at 5 and 21 days, indicating that at least one of the mechanisms inhibited by hypothyroidism is localized in the adrenal chromaffin cells. The present results, taken together with previous findings dealing with adrenal epinephrine secretion, show that the thyroid hormones play a crucial role in the responses of the adrenal medulla to a stimulation in the developing rat, while they have no effect in the adult.