Noise Exposure Alters Glutamatergic and GABAergic Synaptic Connectivity in the Hippocampus and Its Relevance to Tinnitus

Figure 2

Repeated noise exposure induces tinnitus in a subset of animals. (a) Timeline of the experimental procedures of the chronically exposed group. Nineteen animals were grouped into sham controls () and noise-exposed animals (). GPIAS was used for tinnitus assessment, and the baseline of GPIAS was acquired for four weeks before noise exposure. Animals were noise-exposed to the same noise band/sham for two hours twice in sessions conducted four weeks apart, and then assessed for tinnitus with GPIAS eight weeks following the first noise exposure. ABR measurements were performed before and after each noise exposure and GPIAS. Noise-exposed animals were divided into two further groups according to GPIAS assessment results: noise exposed animals that exhibit no behavioral evidence signs of tinnitus (ENT, ) and exposed animals that exhibit behavioral evidence of tinnitus (ET, ). (b) Mean (±SEM) ABR thresholds of animals with tinnitus (ET) and without tinnitus (ENT). ABR thresholds on the ipsilateral side were elevated immediately following noise exposure in both groups, but recovered to baseline levels at 8, 12, 16, and 20 kHz 12 weeks after the first noise exposure. (c) Mean (±SEM) ABR wave I amplitude-intensity functions for ENT and ET animals pre- (baseline) and postnoise exposure (12w) were not significantly different, suggesting no underlying cochlear synaptopathy in both ENT and ET animals after the noise exposure. (d) Mean (±SEM) normalized startle inhibition ratio (NSIR) was the ratio of the startle amplitudes for the gap (or prepulse inhibition, PPI) trials and those for the no-gap trials. NSIR for gap trials was significantly higher postexposure (post) relative to baseline levels (pre) for ET animals, but not for ENT or control animals. All animals exhibited stable responses to PPI trials both pre- and postnoise exposure. . This figure is reproduced with permission of authors in Zhang et al., 2019.

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