Comparison of functional and morphologic characteristics of mice models of noise-induced hearing loss

Objectives: This study was conducted to compare morphologic and audiologic changes after noise exposure in two different strains of mice (CBA and C57) and to create morphologically proven models of noise-induced hearing loss. Methods: Mice were exposed to white noise at 110-dB sound-pressure level for 60. minutes at the age of 1. month. Hearing thresholds and outer hair cell functions were evaluated by auditory brainstem response recordings and distortion product otoacoustic emission immediately and 22. days after noise exposure. Cochlear pathology was observed and compared by light and electron microscopic studies. Results: Both mice strains showed hearing threshold shifts with decreased outer hair cell function immediately and 22. days after noise exposure. More severe auditory brainstem response threshold shifts were observed in C57 mice compared with CBA mice at click, 8-, 16-, and 32-kHz tone-burst stimuli. A cochlear morphologic study demonstrated predominant outer hair cell degeneration at all turns of the cochlea; degeneration was most severe at the basal turn in both mice strains. A scanning electron microscopic study revealed more severe ultrastructural damage of outer hair cells at each turn of the cochlea in C57 mice. The lateral wall of the cochlea was more severely degenerated in CBA mice. Conclusion: Both mice strains showed consistent, permanent noise-induced hearing loss with different susceptibilities and site vulnerabilities. Further studies to investigate the mechanism of the different degree and cochlear site vulnerability to noise exposure between two mice strains are necessary.