Influence of thyroid hormones on rat cerebellar cell aggregation and survival in culture

In order to study the effect of thyroid hormones on rat cerebellar granule cells, cerebellar perikarya isolated from 5-7-day-old rats were cultured in the presence of normal fetal calf serum, fetal calf serum treated with Dowex-2 ion exchange resin to remove thyroid hormones, or ion exchange-treated serum resupplemented with physiological concentrations of T4 and T3. Granule cells grown under hypothyroid conditions showed abnormal cellular aggregation, neurite fasciculation and cell survival. Cell aggregation, which may be an in vitro correlate of cerebellar granule cell migration to the internal granule layer, was reduced by over 50% under hypothyroid conditions. The rate of protein synthesis in hypothyroid cultures was stimulated by as much as 3-fold compared to thyroid hormone-containing cultures consistent with a proposal that the normal developmental pattern of diminishing protein synthesis specific activity is delayed or prevented under hypothyroid conditions. In addition ultrastructural studies revealed abnormalities in the density of cold-stable microtubules in thyroid hormone-deficient cultures. These results demonstrate that thyroid hormones can influence granule neuron behavior in the absence of Purkinje cells. Furthermore, the observed defects bear numerous similarities to documented abnormalities within the hypothyroid cerebellum, suggesting that this in vitro culture system may serve as a useful model for studying the mechanism of action of thyroid hormones on the cells of the cerebellum.