Recent studies have showed that inflammatory responses occur in inner ear under various damaging conditions including noise-overstimulation. We evaluated the time-dependent expression of proinflammatory cytokines in noise-exposed rat cochlea. Among several detected cytokines, real-time RT-PCR showed that interleukin-1beta (IL-1beta) and interleukin-6 (IL-6) were significantly induced 3 hr after noise exposure, and quickly downregulated to the basal level. Tumor necrosis factor-alpha (TNF-alpha) was also slightly upregulated immediately after noise exposure. Immunohistochemical analysis showed that IL-6 expression was distinctively induced within the lateral side of the spiral ligament. Sequential expression analysis showed that IL-6 immunoreactivity was initially found in the cytoplasm of lateral wall cells, including Type IV and III fibrocytes, and expanded broader throughout the lateral wall, finally to the stria vascularis. Because of the negative Iba-1 staining, IL-6 expression in the early-phase was not due to macrophage or microglia activation. IL-6 was also detected in spiral ganglion neurons at 12 and 24 hr after noise exposure. Our data demonstrates the production of proinflammatory cytokines, including TNF-alpha, IL-1beta, and IL-6, in early phase of noise overstimulated cochlea. IL-6 expression was observed in the spiral ligament, stria vascularis, and spiral ganglion neurons. These cytokines, produced by the cochlear structure itself in response to noise exposure, may initiate an inflammatory response and have some role in the mechanism of noise-induced cochlear damage.

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As with other cranial nerves and many CNS neurons, primary auditory neurons degenerate as a consequence of loss of input from their target cells, the inner hair cells (IHCs). Electrical stimulation (ES) of spiral ganglion cells (SGCs) has been shown to enhance their survival. Glial cell line-derived neurotrophic factor (GDNF) has also been shown to increase survival of SGCs following IHC loss. In this study, the combined effects of the GDNF transgene delivered by adenoviral vectors (Ad-GDNF) and ES were tested on SGCs after first eliminating the IHCs. Animal groups received Ad-GDNF or ES or both. Ad-GDNF was inoculated into the cochlea of guinea pigs after deafening, to overexpress human GDNF. ES-treated animals were implanted with a cochlear implant electrode and chronically stimulated. A third group of animals received both Ad-GDNF and ES (GDNF/ES). Electrically evoked auditory brainstem responses were recorded from ES-treated animals at the start and end of the stimulation period. Animals were sacrificed 43 days after deafening and their ears prepared for evaluation of IHC survival and SGC counts. Treated ears exhibited significantly greater SGC survival than nontreated ears. The GDNF/ES combination provided significantly better preservation of SGC density than either treatment alone. Insofar as ES parameters were optimized for maximal protection (saturated effect), the further augmentation of the protection by GDNF suggests that the mechanisms of GDNF- and ES-mediated SGC protection are, at least in part, independent. We suggest that GDNF/ES combined treatment in cochlear implant recipients will improve auditory perception. These findings may have implications for the prevention and treatment of other neurodegenerative processes. .

Neutrophil counts above the reference range of a facility will be a useful indicator of poor prognosis of ISHL. Synchronism of different types of NF-κB activation pathways could be required to cause severe ISHL. An NKCA decrease, an acute neutrophil count increase, and an IL-6 increase can induce NF-κB activation in the cochlea and cause severe ISHL. Further epidemiologic surveys should be conducted to evaluate whether stressful life events increase the risk of severe ISHL onset.