Selective Impairment in Frequency Discrimination in a Mouse Model of Tinnitus

<div><p>Tinnitus is an auditory disorder, which affects millions of Americans, including active duty service members and veterans. It is manifested by a phantom sound that is commonly restricted to a specific frequency range. Because tinnitus is associated with hearing deficits, understanding how tinnitus affects hearing perception is important for guiding therapies to improve the quality of life in this vast group of patients. In a rodent model of tinnitus, prolonged exposure to a tone leads to a selective decrease in gap detection in specific frequency bands. However, whether and how hearing acuity is affected for sounds within and outside those frequency bands is not well understood. We induced tinnitus in mice by prolonged exposure to a loud mid-range tone, and behaviorally assayed whether mice exhibited a change in frequency discrimination acuity for tones embedded within the mid-frequency range and high-frequency range at 1, 4, and 8 weeks post-exposure. A subset of tone-exposed mice exhibited tinnitus-like symptoms, as demonstrated by selective deficits in gap detection, which were restricted to the high frequency range. These mice exhibited impaired frequency discrimination both for tones in the mid-frequency range and high-frequency range. The remaining tone exposed mice, which did not demonstrate behavioral evidence of tinnitus, showed temporary deficits in frequency discrimination for tones in the mid-frequency range, while control mice remained unimpaired. Our findings reveal that the high frequency-specific deficits in gap detection, indicative of tinnitus, are associated with impairments in frequency discrimination at the frequency of the presumed tinnitus.</p></div

Gap detection of the pre-pulse induced inhibition of the acoustic startle response (ASR) for Control (N = 6), Tinnitus(+) (N = 14), and Tinnitus(-) (N = 8) mice at baseline, and 1, 4, and 8 weeks post-exposure.

<p>(A) Control mice exhibited no significant changes within each test frequency when comparing baseline and post-exposure ASR-inhibition. (B) Tinnitus(+) mice exhibited evidence of decreased performance on gap detection in high frequency bands after tone exposure. ASR-inhibition was significantly attenuated at 4 weeks post-exposure for the 2–32 kHz (BBN), 18–20 kHz, and 26–28 kHz bands, and at 8 weeks post-exposure for the 2–32 kHz and 26–28 kHz bands. (C) Tone exposure did not attenuate gap detection at high frequencies in Tinnitus(-) mice. Instead, Tinnitus(-) mice demonstrated gap detection deficits for BBN at 4 and 8 weeks post-exposure, and improved detection in the 6–8 kHz band at 8 weeks post-exposure. Each data point represents population mean ± SEM. *: p<0.05; **: p<.01.</p

Timeline of the experimental protocol and testing.

<p>(A) Habituation to the test environment and apparatus, baseline recording of auditory brainstem responses (ABRs), and behavioral testing for gap detection and frequency discrimination; (B) 60 min tone exposure to a 10kHz tone; (C) Post-exposure ABR recording, gap detection, and frequency discrimination testing at 1 week, 4 weeks, and 8 weeks.</p

Tone evoked ABR thresholds were measured in a subset of exposed mice at baseline, and at 1, 4, and 8 weeks post-exposure.

<p>(A) Tone exposure in Tinnitus(+) mice induced significant hearing loss at 28 kHz at all post-exposure timepoints, and at 16 kHz 1 week post-exposure. No hearing impairments developed for frequencies between 8 kHz and 22 kHz at 4 and 8 weeks post-exposure. (B) Tone exposure also induced hearing impairments in Tinnitus(-) mice for 28 kHz tone 1 and 4 weeks post-exposure, but not at 8 weeks post-exposure. (C-D) Frequency discrimination thresholds are shown for the subset of mice used for ABR recordings. Thresholds were defined as the frequency shift that caused 50% inhibition of the maximum ASR. (C) Tone exposure in Tinnitus(+) mice led to a decrease in frequency discrimination for the 12 kHz tone at 8 weeks post-exposure, and at 4 weeks post-exposure for the 22 kHz tone. (D) Frequency discrimination thresholds did not significantly change after tone exposure in Tinnitus(-) mice. Each data point represents population mean ± SEM. Open circles represent a significant difference from baseline and closed circles a non-significant difference from baseline (significance at p<0.05); <i>n</i> refers to the number of mice; *: p<0.05.</p

Frequency discrimination threshold, in % frequency change (<i>Th</i>_<i>40</i>).

<p>Data from panels A, B, C: Frequency discrimination in mid-frequency range. D, E, F: Frequency discrimination in high-frequency range. A, D: Control group. B, E: Tinnitus(+) group. C, F: Tinnitus(-) group. Error bars: standard deviation taken from 1000 repeats generated using parametric bootstrap method.</p

Average pre-pulse inhibition (PPI) of the acoustic-startle response to pre-pulse frequency shifts.

<p>(A-C) PPI due to increasing frequency shifts from a 12 kHz background tone for Control (N = 6), Tinnitus(+) (N = 14), and Tinnitus(-) (N = 8) groups at baseline and at 1, 4, and 8 weeks post-exposure. (A) Frequency shift detection remained unchanged in Control mice. (B) Tone exposure in Tinnitus(+) mice led to impaired frequency shift detection at 4 weeks post-exposure, but not at 8 weeks post-exposure. (C) In Tinntius(-) mice, tone exposure also led to a significant decrease in frequency shift detection at 4 weeks post-exposure, but not at 8 weeks post-exposure. (D-F) PPI due to decreasing frequency shifts from a 22 kHz background tone at baseline and at 1, 4, and 8 weeks post-exposure. (D) Frequency shift detection did not change over time in Control mice. (E) Tinnitus(+) demonstrated sustained impairments in frequency shift detection at 4 weeks and 8 weeks post-exposure. (F) There was no significant difference in post-exposure frequency shift detection relative to baseline in Tinnitus(-) mice. Each data point represents population mean ± SEM. *: denotes frequency discrimination thresholds</p