Effects of noise traumata of different strengths on hearing functions of C57BL/6N mice

Abstract

Noise-induced hearing loss is a common pathology in humans. Mice serve as model organism for unraveling the underlying mechanisms. Moderate noise (100 dB SPL, 2 h, 8 -16 kHz) can irreversibly damage the cochlea by destroying part of the presynapses and/or postsynapses of the inner hair cells in the high frequency region without permanently increasing hearing thresholds. This phenomenon termed hidden hearing loss was first demonstrated by Kujawa and Liberman (2009) in CBA/CaJ mice. The C57BL/6N mouse line has been widely used for genetic manipulation, e.g. for generating knock-out and knock-in mice, which can help to gain mechanistic insights into noise-induced hearing loss. The aim of this thesis was to explore hearing function after application of two different noise traumata - 106 and 112 dB SPL - in C57BL/6N mice. Hearing function was assessed two days before, directly after trauma and up to four weeks after trauma by auditory brainstem response (ABR) audiometry with click and pure tone stimulation. A previously applied noise trauma of 100 dB SPL (2 h, 8-16 kHz octave band, (Nasri, 2023)) caused a permanent threshold shift at 45 kHz in C57BL/6N mice. The more intense traumata applied in this study (106 and 112 dB SPL) caused a larger damage. Both the amount of temporary (TTS) as well as permanent threshold shifts (PTS) and the range of affected frequencies scaled with trauma strength. In the case of TTS, recovery was almost complete on day 14. Four weeks after the 106 dB SPL trauma, the amplitude growth functions of ABR waves I were reduced for the frequencies of 11, 16, and 22 kHz, which indicates loss of functional synaptic connections between inner hair cells and spiral ganglion neurons. In a fellow dissertation, Philipp Schätzle showed trauma-induced loss of ~50 % of synaptic ribbons in inner hair cells in the mid-basal region (Blum et al., 2024), which was largely responsi-ble for the reduction of the amplitude of ABR wave I by 72 % at 22 kHz four weeks after the 106 dB SPL trauma. Similarly, growth functions of ABR wave I amplitudes evoked by click stimulation were reduced four weeks after both the 106 and the 112 dB SPL trauma. Mean amplitudes of click-ABR 40 dB over threshold were reduced for the waves I to V four weeks after the 106 dB SPL trauma and for waves I, III, IV, and V four weeks after the 112 dB SPL trauma. By contrast, peak latencies of waves I to V at 40 dB over threshold were not altered by either trauma suggesting that latencies of click responses relative to threshold are not a sensitive measure for loss of functional connections between inner hair cells and spiral ganglion neurons. Although both the 106 and the 112 dB SPL trauma did not cause cellular outer hair cell loss (Blum et al., 2024) a functional impairment of outer hair cells is likely, which will lead to increased hearing thresholds independent of the loss of threshold-determining auditory nerve Ia fibers. Future studies on hidden and overt hearing loss in genetic mouse models therefore should not only analyze ABR properties and the fate of pre- and postsynapses but also include measurements of the distortion product otoacoustic emissions as an indicator of outer hair cell function.