Does thyroid hormone influence the maturation of cerebellar granule neurones?

The following hypotheses were tested: is the degeneration of differentiating granule cells in the internal granular layer of the thyroid deficient cerebellum due to a direct requirement of these cells for thyroid hormone, or is it mediated through the failure of some of these cells to make synaptic contact with the hypoplastic Purkinje cells? The effect of thyroid hormone (T3) was studied in rat cerebellar cultures which contain predominantly granule cells. The cultures were grown in a chemically defined medium (S-) in the presence or absence of T3, and were also compared with serum (and thus thyroid hormone) containing cultures (S+). It would appear that T3 is not essential for the relatively long-term survival of the granule cells. Furthermore, cell growth in terms of protein accretion, and the morphological appearance of the cultures were also similar in S- in the presence and absence of T3. Maturation of granule cells was followed by estimating indices, which in the cerebellum in vivo are influenced by the hormone. However, developmental changes affecting the D2 protein, which is implicated in adhesion among nerve cells, and muscarinic receptor binding were not influenced by T3 in vitro. The voltage (veratridine)-sensitive uptake of 22Na was also unaffected, although T3 increased the rate of the relatively small veratridine insensitive component of the 22Na-influx. However, in comparison with cells grown in S+, the rate of both the veratridine sensitive and insensitive component of 22Na-influx was similar under serum-free conditions, whereas the maturation of the D2 protein and muscarinic receptor binding was retarded. The failure of thyroid hormone to influence the differentiation of granule cells, is not due to the in vitro conditions, since T3 is known to have a marked effect on the maturation of certain other classes of neural cells in culture. The results are consistent with the view that the effect of thyroid hormone on neural maturation is cell-type specific, and that granule cells are not targets of thyroid hormone action. They support our second hypothesis, that is, that the degeneration of granule cells in thyroid deficiency is a consequence of the reduction in available postsynaptic sites for the granule cell axons due to the retarded differentiation of Purkinje cells.