ORGANIZATION OF AUDITORY CALLOSAL CONNECTIONS IN HYPOTHYROID ADULT-RATS

Callosal connections were studied with tracers (horseradish peroxidase (HRP) and wheat germ agglutinin - horseradish peroxidase (WGA - HRP)) in normal rats and rats deprived of thyroid hormones with methimazole (Sigma) since embryonic day 14 and thyroidectomized at postnatal day 6. In hypothyroid rats, the auditory areas, in particular the primary auditory area, showed cytoarchitectonic changes including blurred lami nation and decrease in the size of layer V pyramidal neurons. In contr ol rats, callosally-projecting neurons were found between layers II an d VI with a peak in layer III and upper layer IV. In hypothyroid rats, labelled neurons were found between layers IV and VI with two peaks c orresponding to layer IV and upper layer V, and in upper layer VI. Qua ntitative analysis of radial distribution of callosally-projecting neu rons confirmed their shift to infragranular layers in hypothyroid rats . Three-dimensional reconstructions showed a more continuous tangentia l distribution of callosally-projecting neurons in hypothyroid rats wh ich may be due to the maintenance of a juvenile 'exuberant' pattern of projections. These changes in cortical connectivity may be relevant f or understanding epilepsy and mental retardation associated with early hypothyroidism in humans and to clarify basic mechanisms of cortical development.