Abstract
An appropriate conditioning noise exposure may reduce a subsequent noise-induced threshold shift. Although this “toughening” effect helps to protect the auditory system from a subsequent traumatic noise exposure, the mechanisms that regulate this protective process are not fully understood yet. Accordingly, the goal of the present study was to characterize physiological processes associated with “toughening” and to determine their relationship to metabolic changes in the cochlea and cochlear nucleus (CN). Auditory brainstem responses (ABR) were evaluated in Wistar rats before and after exposures to a sound conditioning protocol consisting of a broad-band white noise of 118 dB SPL for 1 h every 72 h, four times. After the last ABR evaluation, animals were perfused and their cochleae and brains removed and processed for the activity markers calretinin (CR) and neuronal nitric oxide synthase (nNOS). Toughening was demonstrated by a progressively faster recovery of the threshold shift, as well as wave amplitudes and latencies over time. Immunostaining revealed an increase in CR and nNOS levels in the spiral ganglion, spiral ligament, and CN in noise-conditioned rats. Overall, these results suggest that the protective mechanisms of the auditory toughening effect initiate in the cochlea and extend to the central auditory system. Such phenomenon might be in part related to an interplay between CR and nitric oxide signaling pathways, and involve an increased cytosolic calcium buffering capacity induced by the noise conditioning protocol.
Highlights
It is well-known that noise overexposure induces hearing dysfunction and pathologies of the inner ear (Clark and Bohne, 1999; Daniel, 2007; Sliwinska-Kowalska and Davis, 2012; Basner et al, 2014)
The highest auditory thresholds were found at the earliest noise exposure at the earliest time (1S1H24H; Figures 1B,C), reflecting the progressive resistance or toughening effect that occurred under the sound conditioning protocol
The findings in the present study demonstrate that following exposure to a non-damaging repetitive sound conditioning protocol in Wistar rats there is a toughening effect expressed as a smaller threshold shift, faster recovery of the threshold shift, and a reduction of noise exposure effect on Auditory brainstem responses (ABR) wave amplitudes and latencies
Summary
It is well-known that noise overexposure induces hearing dysfunction and pathologies of the inner ear (Clark and Bohne, 1999; Daniel, 2007; Sliwinska-Kowalska and Davis, 2012; Basner et al, 2014). When experimental subjects are exposed to a conditioning/toughening noise stimulation protocol, the auditory threshold shift induced by the noise exposure is progressively reduced (Canlon et al, 1988; Subramaniam et al, 1991; Pukkila et al, 1997; Brown et al, 1998; Attanasio et al, 1999; Hamernik and Ahroon, 1999a; Kujawa and Liberman, 1999; Niu and Canlon, 2002a,b; Gourévitch et al, 2009)
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