Projections from the cochlear nucleus to the inferior colliculus in normal and neonatally cochlea-ablated gerbils

The distribution of the projection from one cochlear nucleus (CN) within each inferior colliculus (IC) was studied in adult, normal gerbils and adult gerbils subjected to unilateral ablation of the contralateral cochlea at 2 days of age. The projection was studied by using the Fink-Heimer technique for impregnating degenerating axons and their terminal processes with silver. Following an extensive, unilateral lesion of the CN, degeneration was seen in both ICs of all animals. In normal animals, degeneration was both more widespread and heavier in the contralateral than in the ipsilateral central nucleus of IC (ICC). Degeneration was most widespread in the rostral and lateral parts of both ICCs and in the ventral part of the contralateral ICC. Degeneration was observed in 26% of the area examined in ipsilateral ICC and in 73% of the area examined in contralateral ICC. In cochlea-ablated animals there was a much greater similarity in the area of degeneration in the ICC ipsilateral (57%) and contralateral (67%) to the CN lesion. The same regional distributions of degeneration were observed as in the normal animals except that the distribution of degeneration in the ipsilateral ICC more closely resembled the normal contralateral than the normal ipsilateral profile. We conclude that the normal distribution of projections from the CN within the ipsilateral ICC is substantially modified by neonatal ablation of the contralateral cochlea.