Effects of neonatal deafening and chronic intracochlear electrical stimulation on the cochlear nucleus in cats

Four newborn kittens were deafened by daily intramuscular injections of neomycin sulfate, beginning the day after birth and continuing for 14-16 days. At 10-16 weeks of age the deaf kittens were implanted unilaterally with a four wire intracochlear electrode array. The animals were stimulated daily (starting at 13-18 weeks of age), for a period of one hour, at 6 dB above the electrically evoked auditory brainstem response threshold. After 3 months of chronic intracochlear electrical stimulation, animals were studied in acute electrophysiological experiments and euthanized for histological studies. This study compares the stimulated and control cochlear nuclei (CN) of these deafened animals to the CN of four normal adult cats. Statistical comparisons of spherical cell densities in the anteroventral cochlear nucleus (AVCN), cross-sectional spherical cell areas, and volumes of the cochlear nucleus subdivisions were included in the analysis. The results indicate that, by all of these measures, the cochlear nuclei in neonatally deafened animals were significantly different from the cochlear nuclei of control animals. As a result of deafening, the density of spherical cells was decreased by 30%, the cross-sectional areas of spherical cells were reduced by 20%, and the volume of the cochlear nucleus was reduced by 25%. These changes were observed in both cochlear nuclei (ipsilateral to both stimulated and unstimulated ears) of the deafened animals. With the measures employed, no significant difference was demonstrated in comparisons between the deafened/unstimulated and the deafened/stimulated cochlear nuclei. That is, no reversal of the profound effects of deafening was observed in the cochlear nuclei as a consequence of chronic intracochlear electrical stimulation which was begun 11 to 16 weeks after deafening.