THE DEVELOPMENT OF AUDITORY CALLOSAL CONNECTIONS IN NORMAL AND HYPOTHYROID RATS

Previous studies have shown that hypothyroidism modifies the developme nt of callosal connections. In particular, adult hypothyroid rats have fewer callosally projecting neurons in layers II-III of the auditory cortex and more in layer V, This might be due to disturbance in the st abilization/elimination of juvenile callosal axons, or to abnormal neu ronal migration during cortical histogenesis. To distinguish between t hese possibilities we have studied the distribution of callosally proj ecting auditory neurons at different postnatal ages using retrogradely transported tracers, and the cortical neurogenetic gradients using DN A labelling with 5-bromo-2'-deoxiuridine. In hypothyroid rats, injecte d at postnatal day 5 (P5) and killed at P18-20, most of the neurons re trogradely labelled from the contralateral hemisphere are distributed between layers IV and VI, as in older rats. In hypothyroid rats, many neurons are at locations inappropriate for their birthdate, including the subcortical white matter, resulting in more diffuse radial neuroge netic gradients. These results indicate that early induced hypothyroid ism alters neuronal migration and prevents the establishment of callos al connections from cortical layers II-III.