Well confirmed evidence has demonstrated that the cerebellum is an important target of thyroid hormone action during development. Moreover, the presence of nuclear receptors and strong 5′-deiodinase activity in cerebella of adult rats have suggested that this region may continue to respond to thyroid hormones during maturity. Recent autoradiographic observations have focused attention on the cerebellar granular layer, in that [¹²⁵I]T₃ administered iv to adult rats was found to be selectively and saturably concentrated there.

To determine the specificity of iodothyronine localization in the granular layer, we have now compared film autoradiographic observations made after iv [¹²⁵I]T₄ and iv [¹²⁵I]rT₃ with those found after iv [¹²⁵I]T₃. The results demonstrated that, as in the case of the latter hormone, labeling within the cerebellar cortex after iv [mI]T₄ was both selective and saturable. Moreover, except for a lag in time to resolution and a longer retention time, the distribution of cerebellar radioactivity after iv labeled T₄ was qualitatively similar to that seen after iv [¹²⁵I]T₃. However, the ability of T₄ to become differentially concentrated in the granular layer of cerebellum was absolutely dependent on its ability to be converted intracerebrally to T₃. Thus, pretreatment with ipodate, which blocks brain 5′-deiodinase activity and, therefore, the intracerebral formation of T₃ from T₄, completely prevented cerebellar granular layer labeling after iv [¹²⁵I]T₄ even though it did not interfere with differential labeling of this region by iv delivered [¹²⁵I]T₃. In the same experiments, propylthiouracil, a potent peripheral, but not central, 5′-deiodinase inhibitor, had no qualitative effect on the distribution of either T₄ or T₃ in cerebellum.

By contrast with the results obtained after administering labeled T₃ or T₄, brain labeling after iv delivered [¹²⁵I]rT₃ was found to be no different from that produced by markers of cerebral blood flow, which rapidly enter and leave the brain without becoming incorporated into brain cells. This was so even during treatment with propylthiouracil and ipodate, both of which markedly prolonged the normally brief residence time of this iodothyronine in serum and brain.

Overall, the autoradiographic results served to highlight the importance of the morphological approach for investigating thyroid hormone action and metabolism in brain. They demonstrated that only T₃, whether entering as such from the circulation or formed in situ from T₄ (but neither T₄ itself nor iv administered rT₃) was strongly, selectively, and saturably concentrated in the cerebellar granular layer of adult rats. The high degree of iodothyronine specificity as well as morphological specificity revealed by these studies suggest that constituents of the granular layer may be important histological sites of putative thyroid hormone actions in both developing and adult cerebellum.