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Studies were undertaken to compare the physicochemical characteristics of solubilized nuclear T3 receptors obtained from rat liver and brain and to establish the regional distribution of receptor sites in various areas of the brain. Scatchard analysis revealed that nuclear receptors from brain and liver had similar equilibrium association constants (2.3 ± 0.51 and 1.8 ± 0.54 × 1010 M-1, respectively). Simultaneous chromatography of mixed extracts of the soluble receptor sites on DEAE-Sephadex A-50 resulted in an identical elution pattern with a single peak at 0.15 M NaCl in a continuous saline gradient. Indistinguishable sedimentation patterns were observed on a 5–20% sucrose gradient. The relative affinities of several analogs of T3 including L-T4, 3′-isopropyl-3,5-L-diiodothyronine, 3,3′-5-triiodothyroacetic acid, and L-rT3 were nearly identical. These findings suggest that the nuclear T3 re eptor in brain and liver are the same. The concentrations of T3 specifically bound to nuclear sites in various regions of the brain were studied by in vivo displacement techniques. The cortex contained the highest concentration (0.73 ng T3/mg DNA). The hypothalamus and telencephalon bound approximately 0.1–0.2 ng T3/mg DNA, whereas the cerebellum contained the lowest concentration of T3 (0.016 ng/mg DNA). A low concentration of receptor sites per mg DNA in cerebellum as compared to the cerebrum was confirmed with in vitro studies of nuclear extracts. The low concentration of receptors per mg DNA in the cerebellum is probably due to a significantly reduced concentration of receptors in the tightly packed granule cells, which constitute the bulk of cell types in this tissue and account for the high concentration of DNA per g tissue. Other cell types, such as the Purkinje cell, may therefore still contain a substantial number of receptor sites and constitute a target for thyroid hormone action.
Schwartz et al. (Sat,) studied this question.