Age-related declines in distortion product otoacoustic emissions utilizing pure tone contralateral stimulation in CBA/CaJ mice.

One role of the medial olivocochlear (MOC) auditory efferent system is to suppress cochlear outer hair cell (OHC) responses when presented with a contralateral sound. Using distortion product otoacoustic emissions (DPOAEs), the effects of active changes in OHC responses due to the MOC as a function of age can be observed when contralateral stimulation with a pure tone is applied. Previous studies have shown that there are age-related declines of the MOC when broad band noise is presented to the contralateral ear. In this study, we measured age-related changes in CBA/CaJ mice by comparing DPOAE generation with and without a contralateral pure tone at three different frequencies (12, 22, and 37 kHz). Young (n = 16), middle (n = 10) and old-aged (n = 10) CBA mice were tested. DPOAE-grams were obtained using L1 = 65 and L2 = 50 dB SPL, F1/F2 = 1.25, using eight points per octave covering a frequency range from 5.6-44.8 kHz. The pure tone was presented contralaterally at 55 dB SPL. DPOAE-grams and ABR levels indicated age-related hearing loss in the old mice. In addition, there was an overall change in DPOAEs in the middle-aged and old groups relative to the young. Pure tone stimulation was not as effective as a suppressor compared to broadband noise. An increase in pure tone frequency from 12 to 22 kHz induced greater suppression of DPOAEs, but the 37 kHz was least effective. These results indicate that as the mouse ages, there are significant changes in the efficiency of the suppression mechanism as elicited by contralateral narrowband stimuli. These findings reinforce the idea that age-related changes in the MOC or the operating points of OHCs play a role in the progression of presbycusis - age-related hearing loss in mammals.