Electrically-evoked responses in animals with progressive spiral ganglion degeneration

The deafness (dn/dn) mouse has an hereditary cochlear dysfunction throughout its development, and spiral ganglion cell density decreases progressively over the three age groups we examined. We have used this mutant to examine inferior colliculus evoked responses to modiolar electrical stimulation as a function of spiral ganglion degeneration. No differences were found between mutants and control mice or between ages in either threshold for detection of the response or latency of the response. However, peak-to-peak amplitudes of the response were larger in the mutants than in the controls in the young and intermediate age groups. There was a poor correlation between spiral ganglion degeneration and size of the evoked response: for example, mutants in the old age group had similar amplitudes of response as controls while spiral ganglion cell density was reduced to 21% of the value in young mice, and mutants in the intermediate age group with 50% spiral ganglion degeneration showed response amplitudes more than double that in controls. These data may be relevant to the significant numbers of people with hereditary deafness among the hearing-impaired human population.