The development of auditory callosal connections in normal and hypothyroid rats

Previous studies have shown that hypothyroidism modifies the development ofcallosal connections. In particular, adult hypothyroid rats have fewercallosally projecting neurons in layers II-III of the auditory cortex andmore in layer V. This might be due to disturbance in thestabilization/elimination of juvenile callosal axons, or to abnormalneuronal migration during cortical histogenesis. To distinguish betweenthese possibilities we have studied the distribution of callosallyprojecting auditory neurons at different postnatal ages using retrogradelytransported tracers, and the cortical neurogenetic gradients using DNAlabelling with 5-bromo-2'- deoxiuridine. In hypothyroid rats, injected atpostnatal day 5 (P5) and killed at P18-20, most of the neurons retrogradelylabelled from the contralateral hemisphere are distributed between layersIV and VI, as in older rats. In hypothyroid rats, many neurons are atlocations inappropriate for their birthdate, including the subcorticalwhite matter, resulting in more diffuse radial neurogenetic gradients.These results indicate that early induced hypothyroidism alters neuronalmigration and prevents the establishment of callosal connections fromcortical layers II-III.