Effects of salicylate on organ-of-Corti vibrations in the second turn of the gerbil cochlea.

Sound-evoked responses of the organ of Corti (OoC) in the living cochlea are enhanced by active processes driven by outer hair cells (OHCs). To assess how suppression of OHC function affects intracochlear mechanics, we applied salicylate-a blocker of OHC electromotility-to the round window of anesthetized Mongolian gerbils and measured both distortion product otoacoustic emissions (DPOAEs) and sound-evoked vibrations of the OoC in the second cochlear turn using optical coherence tomography (OCT). Salicylate reduced DPOAE amplitudes in a time- and frequency-dependent manner and decreased intracochlear vibration amplitudes throughout the OoC. The largest effects occurred for low-level stimuli near the best frequency, resulting in reduced cochlear gain consistent with suppression of OHC-mediated amplification. In contrast, phase relationships were largely preserved, suggesting attenuation of active feedback without major changes in intracochlear mechanical coupling. Salicylate affected responses over a broad frequency range extending more than two octaves below the best frequency, a pattern that differs from previous observations in the cochlear base. Similar reductions in basilar membrane (BM) and OHC-region vibrations indicate that OHC-generated forces contribute to motion throughout the local cochlear partition. DPOAE reductions closely matched intracochlear gain changes near the best frequency, supporting their use as a frequency-specific indicator of local cochlear amplification.