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. 2018 Apr;19(2):133-146.
doi: 10.1007/s10162-017-0651-7. Epub 2018 Jan 2.

Effects of Acoustic Environment on Tinnitus Behavior in Sound-Exposed Rats

Aikeen Jones  1 , Bradford J May  2
Affiliations

Effects of Acoustic Environment on Tinnitus Behavior in Sound-Exposed Rats

Aikeen Jones et al. J Assoc Res Otolaryngol. 2018 Apr.

Abstract

Laboratory studies often rely on a damaging sound exposure to induce tinnitus in animal models. Because the time course and ultimate success of the induction process is not known in advance, it is not unusual to maintain sound-exposed animals for months while they are periodically assessed for behavioral indications of the disorder. To demonstrate the importance of acoustic environment during this period of behavioral screening, sound-exposed rats were tested for tinnitus while housed under quiet or constant noise conditions. More than half of the quiet-housed rats developed behavioral indications of the disorder. None of the noise-housed rats exhibited tinnitus behavior during 2 months of behavioral screening. It is widely assumed that the "phantom sound" of tinnitus reflects abnormal levels of spontaneous activity in the central auditory pathways that are triggered by cochlear injury. Our results suggest that sustained patterns of noise-driven activity may prevent the injury-induced changes in central auditory processing that lead to this hyperactive state. From the perspective of laboratory studies of tinnitus, housing sound-exposed animals in uncontrolled noise levels may significantly reduce the success of induction procedures. From a broader clinical perspective, an early intervention with sound therapy may reduce the risk of tinnitus in individuals who have experienced an acute cochlear injury.

Keywords: hyperactivity; hyperacusis; sound therapy.

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Conflict of interest statement

Conflict of Interest

The authors declare that they have no conflict of interest.

Figures

Fig. 1
Fig. 1
Acquisition of conditioned suppression. The upper plot illustrates the temporal pattern of drinking for a representative rat. The conditioned response to silent safe trials (open circles) and broadband noise warning trials (BBN, filled circles) is measured during the 2.5-s interval immediately preceding the delivery of water or shocks (gray region). The lower plot shows the progressive separation of conditioned responses to sound and silence as the rat learned to avoid the spout during warning trials. Pure-tone warning trials were added when the rat reached stable performance for BBN warning trials
Fig. 2
Fig. 2
Ambient sound levels for the quiet environment of quiet-treated rats and the noisy environment of treated rats. Noise spectrum levels were measured at the location of the housing cage. Note that high-frequency components are truncated at the upper frequency limits of our sound level meter
Fig. 3
Fig. 3
Examples of tinnitus tests in the quiet-treatment and noise-treatment groups. Symbols indicate the conditioned response to BBN warning trials, silent safe trials, and unreinforced probe trials. Numerical labels indicate probe frequency (8, 12, 16, 22 kHz). Rats were assigned a positive tinnitus status when a probe elicited lick rates that approximated responses to silent safe trials (rat 127, 16 kHz)
Fig. 4
Fig. 4
Click-evoked auditory brainstem responses (ABRs) for representative rats in the quiet-treatment and noise-treatment groups. The rats have been matched in terms of thresholds in the protected ear (left panel) and unprotected ear (right panel). For illustrative purposes, signal levels have been selected to show the full dynamic range of responses
Fig. 5
Fig. 5
Auditory brainstem response (ABR) audiograms of unilaterally sound-exposed rats. Audiograms plot absolute thresholds at each frequency relative to the average threshold of 174 unexposed ears. Results from the quiet-treatment group (N = 9) and noise-treatment group (N = 9) are compared in the left and right columns. Exposed and protected ears are compared in the upper and lower panels
Fig. 6
Fig. 6
Box plots comparing the distribution of EDGE frequencies (upper panel) and maximum threshold shifts (lower panel) for the exposed ears of rats in the quiet-treatment and noise-treatment groups. The upper and lower limits of the boxes indicate the interquartile range of the distribution (1–3 quartile range). The line bisecting the box indicates the median. Error bars (“whiskers”) represent the highest and lowest values that fall within the ± 1.5 interquartile range. Outliers are plotted as individual symbols
Fig. 7
Fig. 7
Summary of tinnitus testing in the two treatment groups. Box plots (upper panel) compare the distributions of peak scores, regardless of probe frequency. Frequency profiles (lower panel) relate average tinnitus scores to probe frequency. Error bars indicate 95 % confidence intervals
Fig. 8
Fig. 8
Summary of tinnitus testing in individual rats. Frequency profiles were interpreted as a positive (upper panel), ambiguous (middle panel), or negative test (lower panel) based on the peak tinnitus score. Dashed lines indicate the criteria for classification. Legends report the number of rats in each treatment group that received the classification
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