The Relationship Between Regional Distribution Of Manganese Superoxide Dismutase (Mn SOD) In Spiral Ganglion Cells Of Rats And Noise-induced Hearing Loss

1ObjectiveThe vulnerability of high frequency hearing is closely related to the oxidative damage vulnerability of hair cells in the basal region of cochlea. Spiral ganglion cells, however, the same as the sensory nerve cells in the cochlea, has no obvious morphological damage in the early stage of Sensorineural deafness. But also have been impacted by the role of oxidative stress, resulting in the transformation of oxidative stress related molecules. The regional differences of antioxidant capacity along cochlear spiral has certain relationship with the vulnerability of high-frequency hearing.In the present study, we explored the gradient expression of Mn-SOD through spiral ganglion and its correlationship with the vulnerability of high frequency hearing.2MethodsRats were divided into two groups:the experimental animals were exposed to the white noise115dB2h3d. Auditory brainstem responses (ABRs) were determined to observe acoustic trauma of experimental animals. Immunohistochemical and western-blot methods were used to identify the distribution of Mn-SOD in spiral ganglion, before and after noise exposure. The activities of Mn-SOD in different regions of the spiral ganglion were detected by xanthine oxidase method. The distribution of Mn-SOD mRNA in different regions of the spiral ganglion were detected by RT-PCR method.3Results(1) ABRs:3days after exposure, ABR thresholds in4,8,16,20,32kHz experienced significant elevation, especially in high-frequency audition.(2) Immunohistochemical staining and western-blot results:In normal spiral ganglion, the expression of Mn-SOD appeared to be greater at the cochlear apex than at the base, with roughly intermediate findings in the middle turn. After exposure, the expression of Mn-SOD in the spiral ganglion were significantly higher than normal (control group), and the change was more significant at the apex.(3) Mn-SOD activity results:After noise exposure, Mn-SOD activity declined compared with the corresponding parts of the control group, and the change was more significant in the basal region.(4) RT-PCR results:whether in normal spiral ganglion or after exposure, the expression level and variation of Mn-SOD mRNA are consistent with the expression of Mn-SOD.4ConclusionPrevious studies have shown that noise-induced hearing loss, is mainly due to the damages of hair cell, but no obvious morphological changes of spiral ganglios. Our findings suggest that spiral ganglion cells response to ROS exposure may vary along cochlear spiral, with lower response capacity at cochlear base contributing to cumulative susceptibility to high frequency hearing loss.This phenomenon provides a new way to explor the mechanism of sensorineural deafness and the factors of the vulnerability of high-frequency hearing.