Changes in Proinflammatory Cytokines in the Cochlea in Relation to Hearing Thresholds in Noise-Exposed Rats.

Abstract

Our aim was to investigate time effects in proinflammatory cytokines and the auditory brainstem response (ABR) thresholds of rat cochlea exposed to noise. Twenty-one rats were divided into two groups: the control group and the noise group. As high as 115 dB sound pressure of white noise was administered to the noise group of 16 rats for 3 h a day for 10 days. This group was further split into four subgroups based on the timing of sacrifice: 3rd hour group, 12th hour group, 24th hour group, and 28th day group. ABR thresholds were measured in all the rats, after the noise exposure and right before being sacrificed. Proinflammatory cytokine levels (IL-6, IL-1β, and TNF-α) at the cochlea were measured. We found a significant difference between the first ABR thresholds (5 dB nHL) and the post-exposure ABR thresholds in each group (25 dBnHL, 35 dBnHL, 15 dBnHL, and 17.50 dBnHL for the 3rd hour group, 12th hour group, 24th hour group, and 28th day group, respectively). The IL-1β levels in the 3rd hour group and 12th hour group were significantly higher than those in the control group and other noise subgroups. The TNF-α level in the 3rd hour group was significantly higher than that in the control group and other noise subgroups. It seems reasonable to point out a direct correlation between the cytokine levels and hearing threshold levels after the noise exposure. This correlation was the highest for IL-1β. This result suggested a significant role of proinflammatory cytokines in hearing deterioration after noise exposure.