Temporary and permanent noise-induced changes in distortion product otoacoustic emissions in CBA/CaJ mice

A number of studies have shown that the ear can be protected from sound over-exposure, either by activating the cochlear efferent system, or by sound 'conditioning' in which the role of the efferent system is less certain. To study more definitively the molecular basis of deliberately induced cochlear protection from excessive sounds, it is advantageous to determine, for an inbred mouse strain, a range of noise exposure parameters that effectively alter cochlear function. As an initial step towards this goal, young CBA/CaJ mice were exposed to a 105-dB SPL octave-band noise (OBN), centered at 10 kHz, for various lengths of time consisting of 10 min, or 0.5, 1, 3, or 6 h. Distortion product otoacoustic emissions (DPOAEs) at the 2f1-f2 frequency, in response to equilevel primary tones of low to moderate levels, were used to quantify the damaging effects of these sound over-exposures on cochlear function. In addition, staining for acetylcholinesterase (AChE) activity to assess for noise-induced changes in the pattern of efferent-nerve innervation to the cochlea was also performed in a subset of mice that were exposed to the longest-lasting 6-h OBN. The 10-min OBN resulted in only temporary reductions in DPOAE levels, which recovered to pre-exposure values within 5 days. Increasing the exposure to 0.5 h resulted in permanent DPOAE losses that, for low primary-tone levels, were still present at 31 days post-exposure. Additionally, the 1-h and longer exposures caused permanent reductions in DPOAEs for all test levels, which were measurable at 31 days following exposure. Light-microscopic observations restricted to the 11-18-kHz frequency region of the organ of Corti, for a subset of mice exposed to the 6-h OBN, uncovered a significant loss of outer hair cells (OHCs). However, despite the OHC loss in this region, the AChE activity associated with the related pattern of efferent innervation remained largely intact.