Synopsis

Tinnitus involves the perception of sound in the absence of external acoustic stimuli. The underlying mechanisms of tinnitus have been investigated using noninvasive, whole brain functional imaging methods. However, the changes in the subcortical auditory system are less well understood. To address this knowledge gap, we conduct an auditory fMRI on a well-established sodium salicylate-induced rodent tinnitus and hyperacusis model. The results reveal frequency-dependent changes in auditory subcortical structures, inferior colliculus and lateral lemniscus by salicylate induced tinnitus. This study has advanced our understanding of subcortical auditory processing during tinnitus/hyperacusis and advances the rat model for further fMRI investigations.

Introduction

Methods

Animal preparation (N=9): Sodium salicylate (SS) dissolved in saline solution (dosage: 250mg/kg; concentration: 50mg/ml) was intraperitoneally injected into adult SD rats. All subjects underwent auditory fMRI under light isoflurane (~1.0%), 2 hrs after saline and SS administration.

Auditory stimulation: Sound stimulation was delivered via a custom nylon tube inserted in the left ear and through a block-design paradigm (20s sound-on, 40s sound-off). Three frequencies (8, 16, 32khz) were presented in pseudorandom order for equal durations (Figure 1). Sound pressure level (SPL) 72 dB was used before salicylate administration and increased to 87, 87 and 92 dB (8, 16 and 32 kHz, respectively) after administration to compensate for the hearing threshold shift (10).

fMRI acquisition and analysis: fMRI data was acquired at 7T using GE-EPI (FOV=32×32mm², matrix=64×64, α=56°, TE/TR=20/1000ms, 6 slices with 1.2mm thickness and 0.2mm gap). Standard fMRI preprocessing was performed before GLM analysis was applied to identify significant BOLD responses (p<0.001).

Results

Figure 2 shows the sound-evoked BOLD maps before and after sodium salicylate (SS) injection. Positive BOLD responses are observed in IC and LL at all frequencies. In general, SS injection slightly reduces fMRI responses during 8kHz, slightly enhances the responses at 32kHz and leads to a great enhancement at 16kHz in IC and LL.

Figure 3 shows the averaged t-value difference maps before and after SS injection during 8, 16, and 32kHz. The differences were computed by subtracting the saline map from the salicylate map. In general, SS results in increased BOLD responses in the IC and LL during 16 and 32kHz and reduced responses during 8kHz.

Figure 4 shows the BOLD signal amplitudes in the IC and LL before and after SS. The amplitudes are significantly increased at 16 (IC: p<0.001; LL: p<0.01) and 32kHz (IC: p<0.05; LL: p<0.01) and decreased at 8kHz (IC: p<0.05; LL: p<0.01) after SS. The largest difference observed in the IC is during 16kHz.

Discussion

This study demonstrates the roles of two major subcortical auditory structures, the inferior colliculus (IC) and lateral lemniscus (LL) (11-14), in tinnitus/hyperacusis induced by acute sodium salicylate (SS) administration. BOLD signal amplitudes increased significantly during 16 and 32kHz stimulation after SS and reduced slightly during 8kHz. The results suggest that SS induced tinnitus/hyperacusis leads to larger enhancements of subcortical responses at 16kHz, smaller enhancements at 32kHz, and reduced responses at 8kHz.

In our results, the BOLD signal amplitude difference is frequency dependent. The greatest increase in the IC after SS was during 16kHz, which had previously been behaviorally observed as approximately the salicylate-induced tinnitus pitch (15,16). Tonotopic reorganization in IC after salicylate administration may play a role for inducing such frequency dependent fMRI signal changes. Previous electrophysiological recordings in the rat auditory cortex observed tonotopic reorganization following SS. Neurons with characteristic frequency (CF) below 10kHz and above 20kHz shifted their CFs into the 10 to 20kHz range while the neurons with initial CFs between 10 and 20kHz showed minimal shift (17). Similar cortical tonotopic reorganization of tinnitus frequency had been observed in tinnitus patients (18). Furthermore, frequency receptive fields in the amygdala were found to be altered by local salicylate exposure. Neural activity was depressed at frequencies below 10kHz and above 20kHz, but greatly enhanced at frequency from 10 to 20kHz (19).

Conclusion

Acknowledgements

References

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