Distortion product otoacoustic emissions in mouse models of early-onset and late-onset presbycusis
Presbycusis, age-related hearing loss, is currently a major source of limitation in the functional status of the elderly. One of the earliest signs of presbycusis is postulated to be an impairment of function of the outer hair cells (OHCs) in the cochlea. Otoacoustic emissions (OEs), which are believed to be associated with OHC function, have strong potential to serve as a useful tool for the understanding of effects of age on the cochlear function and assessment of age-related hearing loss. In the present study, $/rm 2f/sb1$-$f/sb2$ distortion product (DP)OE was measured at $/rm f/sb2 = 11.3$ kHz by systematically varying $/rm f/sb2/f/sb1$, L$/sb1$ and L$/sb2$ in three age groups of C57BL/6J and four groups of CBA/J mice, mouse models of early-onset and late-onset presbycusis, respectively. This study examined effects of the stimulus parameters on DPOE level and interactions among the parameters in the young and aged mice. The recommended stimulus parameters to yield largest reduction in DPOE level being sensitive to age-related impairment (i.e., optimally age-sensitive stimulus parameters) are: $/rm f/sb2/f/sb1 = 1.35$ for both strains; $/rm L/sb1/L/sb2 = 55/35$ dB SPL ($/rm L/sb1$- $/rm L /sb2 = 20/ dB$) for C57; and $/rm L/sb1/L/sb2 = 45/20$ dB SPL ($/rm L/sb1$-$/rm L/sb2 = 25/ dB$) for CBA. DPOE behavior in various ages of mice was examined using the above parameters. The study found that DPOE revealed progressively age-related deterioration of cochlear function in the mice. In addition, efferent-mediated DPOE adaptation was observed in the two strains of mice. The adaptation had a magnitude of 0.5 to 1.6 dB and a time constant of 0.5 to 2.3 sec in the young mice, and was observed to be weakened in the aged mice. A major contribution of the present study is determination of the optimally age-sensitive stimulus parameters for DPOE measurement. The present study supports the conclusion that C57 and CBA mice, where DPOEs serve as an index of cochlear function, can be employed as useful models of presbycusis.