The Journal of comparative neurology
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Copper/zinc superoxide dismutase (Cu/Zn SOD) is a first-line defense against free radical damage in the cochlea and other tissues. To determine whether deficiencies in Cu/Zn SOD increase age-related hearing loss and cochlear pathology, we collected auditory brainstem responses (ABRs) and determined cochlear hair cell loss in 13-month-old 129/CD-1 mice with (a) no measurable Cu/Zn SOD activity (homozygous knockout mice), (b) 50% reduction of Cu/Zn SOD (heterozygous knockout mice), and (c) normal levels of Cu/Zn SOD (wild-type mice). ABRs were obtained by using 4-, 8-, 16-, and 32-kHz tone bursts. ⋯ At 13 months of age, both wild-type and knockout mice had significantly fewer nerve fibers than did 2-month-old wild-type mice, with significantly greater loss in aged knockout mice than in aged wild-type mice. Thirteen-month-old knockout mice also had a significant loss of spiral ganglion cells compared with 2-month-old wild-type mice. The results indicate that Cu/Zn SOD deficiencies increase the vulnerability of the cochlea to damage associated with normal aging, presumably through metabolic pathways involving the superoxide radical.
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This investigation examined the consequences of neonatal deafness and chronic intracochlear electrical stimulation delivered by a cochlear implant during maturation. Kittens were bilaterally deafened by an ototoxic drug administered daily for 2 weeks immediately after birth. Unilateral electrical stimulation was initiated at 7-10 weeks of age and continued over periods of 22-47 weeks (4 hours/day; 5 days/week). ⋯ Measurements of the SG cell somata revealed a pronounced decrease in cell diameter in neonatally deafened cats studied about 1 year after deafening, and an additional decrease after long-term deafness (2.5-6.5 years). Furthermore, in the cochlear regions with the greatest stimulation-induced differences in SG cell density, direct measurements of cross-sectional soma area of the largest cells revealed that cells were significantly larger in the stimulated ears. Thus, in addition to the marked increase in the number of surviving SG cells, larger soma area contributed modestly to the pronounced increase in neural density following chronic electrical stimulation.