Effects of triiodothyronine (T3) on the development of rat cerebellar cells in culture.

To determine whether or not the effects of thyroxine on the cells of the external granular layer of rat cerebellum are direct or indirect, Purkinje cell-free dissociated cell cultures from 5-day-old rat cerebellum in serum-free medium were treated with triiodothyronine (T3) at concentrations of 20-3000 ng/ml. Cultures were assayed for uptake and synthesis of γ-aminobutyric acid (GABA) after 3 weeks, and for thymidine incorporation at 1, 2, 3 and 4 days. Specific (per mg protein) activity of glutamic acid decarboxylase and of GABA uptake into neurons was reduced by the more physiologic (50 or 500 ng/ml) concentrations of T3, probably due to a non-neuronal increase in overall protein synthesis, since the total per culture values seem unaffected by hormone. Thymidine uptake was significantly increased only at the highest (unphysiologic) concentration of T3 on the second day after treatment. None of the label appeared to be in the nuclei of the neuronal cells at any of the times and concentrations tested. These results are consistent with the hypothesis that T3 does not have a direct effect on the proliferation of neurons derived from the external granular layer, although it does seem to affect some non-neuronal cells. T3 also does not seem to enhance differentiated GABA functions (uptake or synthesis) beyond the levels achieved using insulin, progesterone, putrescine, selenium and transferrin.