Targeted Deletion of the Cytosolic Cu/Zn-Superoxide Dismutase Gene (<i>Sod1</i>) Increases Susceptibility to Noise-Induced Hearing Loss

Reactive oxygen species (ROS) such as superoxide, peroxide and hydroxyl radicals are generated during normal cellular metabolism and are increased in acute injury and in many chronic disease states. When their production is inadequately regulated, ROS accumulate and irreversibly damage cell components, causing impaired cellular function and death. Antioxidant enzymes such as superoxide dismutase (SOD) play a vital role in minimizing ROS levels and ROS-mediated damage. The cytosolic form of Cu/Zn-SOD appears specialized to remove superoxide produced as a result of injury. 'Knockout' mice with targeted deletion of Sod1, the gene that codes for Cu/Zn-SOD, develop normally but show enhanced susceptibility to central nervous system injury. Since loud noise is injurious to the cochlea and is associated with elevated cochlear ROS, we hypothesized that Sod1 knockout mice would be more susceptible to noise-induced permanent threshold shifts (PTS) than wild-type and heterozygous control mice. Fifty-nine mice (15 knockout, 29 heterozygous and 15 wild type for Sod1) were exposed to broad-band noise (4.0-45.0 kHz) at 110 dB SPL for 1 h. Hearing sensitivity was evaluated at 5, 10, 20 and 40 kHz using auditory brainstem responses before exposure and 1, 14 and 28 days afterward. Cu/Zn-SOD deficiency led to minor (0-7 dB) threshold elevations prior to noise exposure, and about 10 dB of additional noise-induced PTS at all test frequencies, compared to controls. The distribution of thresholds at 10 and 20 kHz at 28 days following exposure contained three modes, each showing an effect of Cu/Zn-SOD deficiency. Thus another factor, possibly an additional unlinked gene, may account for the majority of the observed PTS. Our results indicate that genes involved in ROS regulation can impact the vulnerability of the cochlea to noise-induced hearing loss.