The effect of neonatal thyroidectomy on myelination in the rat brain

Abstract

The deposition of lipids in rat brain has been studied after neonatal thyroidectomy. The amounts of phospholipids and cholesterol per g wet wt. of brain were unaffected during the first fortnight of life; they were reduced at 23 days, but approached the normal values at 35 days or later. On the other hand, a lasting and significant reduction was observed in the concentration of cerebrosides. The content of gangliosides was similar to the controls in the cerebrum of 23-day-old hypothyroid rats. The amounts of phospholipids and cholesterol in the cerebrum were somewhat reduced in the 35- and 43-day-old rats after neonatal thyroidectomy but the reduction was similar in scale to that observed in brain weigth. The amount of cerebroside was, however, reduced to a greater extent (40%). Confirmation that the effect of thyroid deprivation on the deposition of lipids was related to a decrease in the extent of myelination has come from experiments in which the myelin containing fraction of brain homogenates was isolated. The dry weight of myelin obtained from the brain of 43-day-old rats thyroidectomized at birth was reduced by more than 30%, but the lipid composition of myelin was unaffected. The yield of myelin per g brain assessed by determining the cholesterol content of the myelin fraction was also reduced in the 25- and 35-day-old hypothyroid rats, but the timing of myelination was apparently unaffected. Replacement therapy instituted from the time of thyroidectomy reversed completely the reduction in myelin deposition as assessed by the amount of cerebrosides per unit weight of cerebrum.