Reversible morphological alterations of cortical neurons in juvenile and adult hypothyroidism in the rat.

We have studied: (i) whether or not hypothyroidism induced experimentally in the adult rat affects the number and distribution of spines along the apical shaft of pyramidal cells from the visual cortex; and (ii) whether treatment with thyroid hormone would reverse such changes. Two experiments were performed. Experiment A involved the study of rats which were thyroidectomized (T) at 40 or at 120 days of age, killed at 120 and 220 days of age, respectively, data being compared to those of adequate sex and age-paired controls. Experiment B involved rats which were T at 40 days of age; they were subdivided into two groups, one of which was left untreated and the other received 0.2 microgram of thyroxine/100 g body weight/day for 25 days. Both groups were killed at 90 days of age. We found that: (i) T at 40 and at 120 days of age resulted in a decrease of the number of spines and a derangement of their distribution along the shaft, the observed changes being qualitatively comparable to those previously described for rats T at 10 days of age; and (ii) preliminary results from experiment B indicate that the observed changes in neuronal morphology might be reversible. It is suggested that age-independent and reversible alterations of learning capacity and electrocortical activity of hypothyroid rats might have age-independent and reversible morphological correlates at the level of the cortex.