OCCIPITAL CORTICO-PYRAMIDAL PROJECTION IN HYPOTHYROID RATS

It is well established that the progressive disappearance of a transie nt occipitospinal projection in neonatal rats involves the selective e limination of axonal collaterals. We studied whether the development o f the occipitospinal pathway was affected by hypothyroidism induced by treatment with the goitrogen 6n-propyl-2-thiouracil (PTU) beginning p renatally. Using both anterograde (biocytin and Dil) and retrograde (h orseradish peroxidase and Fast Blue) tracing techniques in adult hypot hyroid rats, we found that many cells with projections into the pyrami dal tract are present in regions of visual cortex that are devoid of s uch cells in normal adult rats. Our results suggest that hypothyroidis m induced by PTU treatment leads to the maintenance of occipitospinal projections that are normally transient.