Effects of unilateral cochlea removal on anteroventral cochlear nucleus neurons in developing gerbils.

Afferent regulation of neurons in the cochlear nucleus as a function of age was investigated at the light microscope level. Unilateral cochlea removal was performed on Mongolian gerbils of three age groups: 1, 8, and 20 weeks postnatal. Animals survived for either 2 days or 2 weeks. An additional group of neonatally operated animals had a prolonged survival of 9 weeks. The number of neurons in anteroventral cochlear nucleus (AVCN) was counted, and cross-sectional area measurements of large spherical cells in AVCN were made. In animals 1 week old at the time of surgery, there was a 35% reduction in neuron number in AVCN after 2 days, a 58% reduction after 2 weeks, and a 59% reduction 9 weeks after inner ear destruction. However, in animals 20 weeks old at the time of surgery, there was no cell loss in AVCN either 2 days or 2 weeks after surgery. Animals in each age group showed a reduction in cross-sectional area of large spherical cells in AVCN after cochlea ablation. The gerbils that underwent cochlea removal at 8 and 20 weeks showed an average decrease of 14-18%. This effect was seen as early as 2 days after cochlea removal. Animals that underwent cochlea removal at 1 week exhibited the greatest change; a 25% decrease at 2 days progressed to 38% at 2 weeks following cochlea removal. No appreciable further changes were seen at 9 weeks after neonatal cochlea removal. The results indicate greater susceptibility of 1-week-old gerbil cochlear nucleus neurons to peripheral loss than found in older animals.(ABSTRACT TRUNCATED AT 250 WORDS)