THE LOCALIZATION OF I131-LABELED TRIIODOTHYRONINE IN THE PITUITARY AND BRAIN OF NORMAL AND THYROIDECTOMIZED MALE RATS

Abstract

The central nervous system and pituitary lobes of normal and hypothyroid male rats were both shown to be capable of concentrating triiodothyronine following intravenous injection of I131-labelled triiodothyronine. The pituitary site of maximal localization was similar to that observed in the guinea pig, being greatest in the anterior lobe. An autoradiographic study of the central nervous system and pituitary lobes showed a localization pattern similar to that seen in the guinea pig, being primarily associated with nuclear or cellular areas. However, the pronounced localization seen in the regions occupied by the neurosecretory structures in the guinea pig and rabbit was not observed in the rat. In addition, it was noted that the hippocampus, an area not actually studied in the other species, was practically autoradiographically negative in the rat. Thyroidectomy decreased the rate of uptake of labelled hormone in the central nervous system. However, the peak concentration was increased in the central nervous system as well as in the adenohypophysis. The delay in attaining a peak concentration and the increased amount taken up were attributed to a deficiency in deiodinating mechanisms. A possible role for the thyroid hormone in maintaining cellular enzyme systems is suggested. A possible relationship between the uptake curves of labelled triiodothyronine and the conjugated form of triiodothyronine is discussed. Injections with radio iodinated serum albumen showed that the vascular space of the rat brain is very small, similar to the rabbit, and somewhat enlarged by hypothyroidism.