Noise-induced threshold shift dynamics measured with distortion-product otoacoustic emissions and auditory evoked potentials in chinchillas with inner hair cell deficient cochleas

Abstract

Chinchillas ( n=6) were treated with carboplatin and, following a 30-day recovery period, were exposed to a 115 dB peak SPL impact noise presented at a rate of 1/s for 6 h/day for 10 days. A second group ( n=6) received only the noise treatment. Cubic distortion product otoacoustic emissions (2f 1−f 2) and auditory evoked potential (AEP) detection thresholds in response to tone bursts were measured before and 30 days after drug treatment and following the first and 10th day of the noise exposure. Thirty days after the final exposure day, permanent changes in AEP detection thresholds and emissions were measured and cochleograms constructed. The drug treatment eliminated over 80% of the inner hair cells (IHC) in the cochlea, leaving the outer hair cell (OHC) population essentially intact prior to the interrupted noise exposure. The drug treatment alone had very little or no effect on AEP detection thresholds and emission metrics. Following the noise exposure, the IHC-deficient animals showed clear `toughening' effects in the AEP and emission measures which were the same as measured in the group receiving only the noise. After a 30-day post-exposure recovery period, AEP thresholds were elevated about 10 dB at the low frequencies in the drug-noise group whereas emissions returned to near normal despite the massive IHC losses. These results are consistent with the idea that an intact OHC population is required for toughening. However, sound-evoked efferent pathways activated by the few remaining IHCs (∼20%) which, in this preparation, are distributed throughout the cochlea, may still contribute significantly to the toughening phenomena.