Noise-induced tinnitus using individualized gap detection analysis and its relationship with hyperacusis, anxiety, and spatial cognition

Abstract

Tinnitus has a complex etiology that involves auditory and non-auditory factors and may be accompanied by hyperacusis, anxiety and cognitive changes. Thus far, investigations of the interrelationship between tinnitus and auditory and non-auditory impairment have yielded conflicting results. To further address this issue, we noise exposed rats and assessed them for tinnitus using a gap detection behavioral paradigm combined with statistically-driven analysis to diagnose tinnitus in individual rats. We also tested rats for hearing detection, responsivity, and loss using prepulse inhibition and auditory brainstem response, and for spatial cognition and anxiety using Morris water maze and elevated plus maze. We found that our tinnitus diagnosis method reliably separated noise-exposed rats into tinnitus((+)) and tinnitus((-)) groups and detected no evidence of tinnitus in tinnitus((-)) and control rats. In addition, the tinnitus((+)) group demonstrated enhanced startle amplitude, indicating hyperacusis-like behavior. Despite these results, neither tinnitus, hyperacusis nor hearing loss yielded any significant effects on spatial learning and memory or anxiety, though a majority of rats with the highest anxiety levels had tinnitus. These findings showed that we were able to develop a clinically relevant tinnitus((+)) group and that our diagnosis method is sound. At the same time, like clinical studies, we found that tinnitus does not always result in cognitive-emotional dysfunction, although tinnitus may predispose subjects to certain impairment like anxiety. Other behavioral assessments may be needed to further define the relationship between tinnitus and anxiety, cognitive deficits, and other impairments.

Figure 1. GAP, PPI, and startle only ratios from a representative tinnitus⁽⁺⁾, tinnitus^{( −)} and control rat.

Gap-detection data showed tinnitus at 6–8 and 26–28 kHz in the tinnitus⁽⁺⁾ rat (A), which was unaccompanied by same-frequency impairment in PPI (B), although PPI showed auditory detection impairment at 14–16 kHz. Neither the tinnitus⁽⁻⁾ (C–D) nor the control rat (E–F) demonstrated tinnitus or auditory detection deficits. Error bars represent the standard error of the mean (SEM). * indicates p<0.05 between pre- and post-GAP, and p>0.05 between post-GAP and post-Stl-Only.

Figure 2. GAP and PPI ratios for the tinnitus⁽⁺⁾, tinnitus^{( −)} and control groups.

Gap-detection data showed tinnitus across all frequency bands and BBN at 1 to 2 and 5 to 6 weeks post-exposure in the tinnitus⁽⁺⁾ group (A). PPI data showed auditory detection impairment at 6–8 and 10–12 kHz at 1 to 2 weeks post-exposure, indicating that 1 to 2 week gap impairments at these frequencies may not be specifically due to tinnitus (B). PPI, however, recovered by 5 to 6 weeks post-exposure, except for BBN, which may indicate sensitivity at BBN PPI. The tinnitus⁽⁻⁾ group exhibited 10–12 kHz gap impairment at 1 to 2 weeks post-exposure and BBN impairment at 1 to 2 and 5 to 6 weeks (C). PPI, however, also showed 10–12 kHz impairment at 1 to 2 weeks, negating the alleged 10–12 kHz tinnitus (D). The BBN GAP impairment, on the other hand, may be due to hearing loss, since the individual frequency bands were not impaired for the tinnitus⁽⁻⁾ group yet their BBN impairment matched that of the tinnitus⁽⁺⁾ group, which has similarly elevated hearing thresholds (see Figure 4). No tinnitus or auditory detection impairments were seen in the control group (E–F), although a decrease in BBN PPI ratio was observed, which again may indicate sensitivity changes at this parameter. Error bars represent SEM. * indicates p<0.05.

Figure 3. Startle force for the tinnitus⁽⁺⁾, tinnitus^{( −)} and control groups.

The tinnitus⁽⁺⁾ group only demonstrated enhanced startle force during BBN background noise at 1 to 2 weeks post-exposure, but showed a dramatic increase during all carrier bands at 5 to 6 weeks (A). Enhanced startle force without background noise was also seen to a small extent at 1 to 2 weeks post-exposure, but to a much greater extent at 5 to 6 weeks (B). The tinnitus⁽⁻⁾ group demonstrated no startle force changes (C–D) except for a decrease at 1 to 2 weeks near 26–28 kHz PPI (C–D). The control group showed no startle force changes (E–F) except for an increase at 5–6 weeks during BBN noise (E–F). The tinnitus⁽⁺⁾ group by far showed the greatest change in startle force, suggesting hyperacusis-like behavior. All groups showed a sensitization to startle force during background noise, as evidenced by stronger startle force during gap-detection testing (background noise present) compared to PPI testing (background noise absent). For PPI tests, all startle only conditions were identical and were organized by the closest frequency of prepulse incidence to maintain similar comparison to gap-detection. Error bars represent SEM. * indicates p<0.05.

Figure 4. Auditory brainstem responses from the exposed left ear (A) and unexposed right ear (B).

In the exposed ear (A), both the tinnitus⁽⁺⁾ and tinnitus⁽⁻⁾ groups showed significant threshold shifts across tone-burst frequencies at 1 and 8 weeks post-exposure, with the strongest elevations occurring at 12 and 16 kHz. Overall, the tinnitus⁽⁺⁾ group had significantly higher hearing thresholds than the tinnitus⁽⁻⁾ group, although the thresholds were not significantly higher at any individual frequency or click. (B) No significant threshold shifts were seen in the unexposed ear for tinnitus⁽⁺⁾ and tinnitus⁽⁻⁾ groups. The control group showed no changes in either ear (A–B). Error bars represent SEM.

Figure 5. Correlation between ABR thresholds and GAP ratios for tinnitus⁽⁺⁾ (A), tinnitus⁽⁻⁾ (B), and control (C) groups.

No groups exhibited a significant correlation, suggesting that although tinnitus⁽⁺⁾ rats had more overall hearing loss (see Figure 4), it was not the only factor accounting for elevated GAP ratios and thus behavioral manifestation of tinnitus.

Figure 6. Morris water maze escape latency and probe trial data.

No significant differences were seen between the tinnitus⁽⁺⁾, tinnitus⁽⁻⁾ and control groups in escape latency (A), probe trial target zone entries (B), probe trial target zone time (C), and velocity (D). This indicated similar spatial learning and memory across groups. Error bars represent SEM.

Figure 7. Percent of open-arm entries and open-arm time in the elevated plus maze.

No significant differences were seen between the tinnitus⁽⁺⁾, tinnitus⁽⁻⁾, and control groups in percent of open-arm entries or percent of open-arm time, indicating similar anxiety level across groups. Error bars represent SEM.