Conclusion. Among the three main gentamicin (GM) compounds following intratympanic application, the cochleotoxicity of C2 was the most severe, whereas that of C1a was the weakest. Understanding of the different cochleotoxicity characteristics of each compound may be of use in future custom-made intratympanic therapy for Ménière's disease. Objective. To investigate differences in cochleotoxicity among three major GM compounds following intratympanic application. Materials and methods. Three GM compounds (C1, C2, and C1a) were isolated. Sprague-Dawley rats were treated every 2 days for 2 weeks with intratympanic application of saline, GM complex, C1, C2, and C1a. The cochleotoxicity of each compound was assessed by measuring auditory brainstem response (ABR) and through morphological analyses using scanning electron microscopy. Results. The ABR threshold of the C2 group was found to be more impaired than those of the other groups. The C1a group showed the mildest elevation of the ABR thresholds. Morphological analyses revealed that the proportion of remaining outer hair cells (OHCs) was the lowest in animals treated with C2. Morphologically, the C1 and C1a groups showed the least damage to OHCs.
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