Cochlear pathology induced by aminoglycoside ototoxicity during postnatal maturation in cats

Cochlear pathology resulting from neonatal administration of the aminoglycoside antibiotic, neomycin sulfate, was studied in young kittens at 15-24 days postnatal. Hearing thresholds showed severe to profound hearing loss in all but one animal. Scanning electron microscopy demonstrated that initial hair cell degeneration occurred in the extreme base (hook region) of the cochlea and sequentially progressed to the basal, middle, then the apical coil of the cochlea. The first row of outer hair cells degenerated first, followed by row 2, then row 3; the last cells to degenerate in a given region were the inner hair cells. This pattern of hair cell degeneration is similar to that seen in adults with neomycin ototoxicity. In contrast, the spiral ganglion exhibited a different pattern of degeneration with initial cell loss occurring in the middle of the cochlea, about 40-60% from the base (approximately 2.8-8 kHz). Thus, neuronal degeneration apparently is not secondary to sensory cell loss, but rather comprises an independent process in these neonatal animals. Taken together, the findings suggest that the spiral ganglion cell loss in the middle cochlear turn results from increased aminoglycoside sensitivity associated with an earlier initial onset of function in these neurons as compared to other cochlear regions.