Thyroid Hormones and Neurotubule Assembly in vitro during Brain Development

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Abstract
A new model was used to evaluate the effects of thyroid hormones on brain development. This model is based on the assumption that the major effect of thyroid hormones is in regulating the rate of neurite growth of the rat brain at early stages of postnatal development. Microtubules were chosen as markers of neurite growth. Whether the rate of microtubule assembly in vitro is under thyroid hormone control was tested. The rate of tubulin assembly into microtubules in vitro seems to be thyroid hormone dependent. In 15-day-old hypothyroid rats the rates of tubulin assembly in vitro were low, comparable to those levels found in normal rats on day 3. Normal rates of assembly in vitro were restored upon addition of very small amounts of microtubule fragments which act as nucleating centers in the process of microtubule formation. Addition of microtubule-associated proteins to a hypothyroid preparation restored maximal assembly rates. Similar results were obtained on adding one of the microtubule-associated proteins (purified .tau. protein). Physiological amounts of thyroid hormones completely restored normal assembly rates provided that they were administered very early after birth. The ability of tubulin to assemble maximally did not seem to be permanently impaired, since normal assembly rates were spontaneously restored when hypothyroidism was maintained until an adult stage. Normal microtubule assembly was observed when hypothyroidism was produced at an adult stage. The model which may be constructed from these results implies that thyroid hormones are required briefly after birth to accelerate the rate of microtubule assembly, thus allowing intensive neurite growth during the critical period of brain development.