Influence of neonatal cochlear removal on the development of mouse cochlear nucleus. III. Its efferent projections to inferior colliculus.

In order to determine the transneuronal developmental influences of auditory deafferentation, the right cochleas, with the first order spiral ganglion neurons, were removed in 6-day-old mice to eliminate all peripheral input to the right cochlear nucleus (CN). At 45 days, some of the efferent projections of right CN in these unilaterally lesioned mice and their unoperated controls were identified by retrograde transport of horseradish peroxidase from the contralateral (left) inferior colliculus (IC). In both groups of animals, reaction product was observed in neurons of the right CN, contralateral to the injection, and no labeling was seen in the ipsilateral left CN in either group. Contralaterally labeled were the fusiform cells of dorsal CN, the globular cells of ventral CN, and neurons within the nucleus of the intermediate acoustic stria. Quantification revealed significantly fewer fusiform and globular cells labeled in the deafferented CN, whereas the number of labeled acoustic stria neurons was the same in both groups. Although the deafferented CN had 65.4% fewer labeled neurons, the proportions projecting to IC were similar in the two groups, 7.8%. Because of this significant reduction in the number of deafferented CN neurons projecting to the contralateral IC, it was concluded that the transneuronal effects of deafferentation would be to deprive or deafferent developing neurons within the higher auditory brainstem nuclei.