R446 – Rostral Cochlear Nucleus Changes After Cochlear Ablation

Abstract

Problem Identification of neurotransmitter concentration changes occurring in the rostral anterior ventral cochlear nucleus (AVCN) following transection of the auditory nerve within the cochlea. Methods Chinchillas with cochlear ablations, as well as sham-lesioned chinchillas, were euthanized at times ranging from 3 to 84 days post ablation. Both temporal bones and brains were saved. Temporal bones were fixed, embedded in paraffin and sectioned to document the completeness of the cochlear lesion. Brain portions containing the cochlear nuclei were frozen-sectioned, and sections were freeze dried. Freeze-dried sections were microdissected into samples of AVCN for high performance liquid chromatography (HPLC) assay of 12 amino acid concentrations. Results The average concentration of glutamate, the most likely neurotransmitter of auditory nerve fibers, declined in the lesioned-side rostral AVCN by about 25% at 15 days. This decrease was maintained through 31 days post ablation and became bilateral at 83 days. There was no decrease in the adjacent granular region. Larger lesioned-side decreases, approaching 50%, were found more caudally in the AVCN at 31 days post ablation. The average concentration of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) decreased bilaterally by 15–25% at 3 through 15 days post ablation. Conclusion The degeneration of the central portion of the auditory nerve following mechanical ablation of the cochlea is accompanied by decreases of glutamate concentration on the lesioned side but bilateral decreases of GABA in the rostral part of the AVCN. These decreases were smaller than those reported previously for the posteroventral cochlear nucleus (PVCN). However, changes more caudally in AVCN approach those found in PVCN. Significance Our results are consistent with other evidence that damage to the cochlea leads to neurotransmitter changes in the central auditory system. The smaller changes in AVCN than in PVCN may correlate with different types of auditory processing in these two regions. Support The American Tinnitus Association and the University of Toledo Foundation.