Regional distribution of nuclear T3 receptors in rat brain and evidence for preferential localization in neurons1

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Abstract
We examined the distribution of nuclear T3 in mature rat brain with the aim of determining specific targets of thyroid hormones within this tissue. Saturation experiments, performed in 9 different structures of the brain and in 4 parts of the cortex, revealed the presence of a single class of binding sites with a mean Ka of 0.53 × 1010 M1. The highest concentrations of receptors were found in the amygdala (0.523 +0.025 ng T3/mg DNA, Mean ± SE) and the hippocampus (0.438 ± 0.071 ng T3/mg DNA) while the lowest were in the brain stem (0.058 ± 0.003 ngT3/mg DNA) and the cerebellum (0.079 ± 0.026 ngT3/ml DNA). The receptor was not uniformal-ly distributed within the cerebrel cortex, its concentration being relatively high in the central sections and intermediate in the remaining portions. The cell type distribution of the T3 receptor was studied by separating glial and neuronal nuclei on a discontinuous sucrose gradient. There was no detectable specific T3binding in the fraction of oligodendrocyte nuclei (~ 95% pure). Conversely, the neuron-enriched fraction (~ 60%) showed a significant increase in receptor concentration compared to total nuclei (35-40% neurons): 0.857 + 0.196 vs 0.511 + 0.095 ng T3/mg DNA (p < 0.01) in the cortex and 0.425 ± 0.018 vs 0.234 ± 0.24 ngT3/mg DNA (p < 0.01) in the forebrain. The absence of nuclear T3 receptors in oligodendrocytes may have important implications on the mechanism of action of thyroid hormone in myelination.