Subcellular Levels of L-T3 and L-T4 in Adult Rat Brain Cerebral Cortex

Background. Thyroid hormones exert major well-known functions in the growth and differentiation of most types of mammalian tissues. The hormones exert these effects via interaction with nuclear thyroid-hormone receptors, which regulate target gene expression. Thyroid dysfunctions in adult individuals have been linked to several neuropsychological disorders that can be corrected by appropriate adjustments of circulating thyroid hormones. However, the actions of thyroid hormones in the mature brain are unclear. As the mammalian brain approaches adulthood, thyroid hormone concentrations within nuclei decrease and in nerve terminals increase. These ontogenic differences and the changes in the subcellular localization and concentration of thyroid hormones are important in the exploration of thyroid hormone function in the adult mammalian brain. Aim. The aim of the present study was to compare the concentrations of L-triiodothyronine 3 4 (L-T ) and L-tetraiodothyronine (L-T ) in two preparations from adult rat brain. Procedure. 3 4 L-T and L-T concentrations were determined by radioimmunoassay. Comparisons were made of the concentrations of the hormones in two different preparations derived from young adult rat brain cerebral cortex: (1) purified synaptosomes (an artificial preparation of a subcellular fraction considered to have most biological properties of nerve terminals) and (2) non-synaptic mitochondria. Results. 3 The concentration of L-T was found to be 3 ~3-fold higher in non-synaptic mitochondria compared to synaptosomal L-T concentrations. The assay used 4 did not detect L-T in either fraction. Conclusion. 3 4 The higher concentrations of L-T compared to L-T in 3 the two different prepared fractions indicates a pivotal biological role of L-T in adult mammalian brain.