Central Compensation in Auditory Brainstem after Damaging Noise Exposure

Abstract

Noise exposure is one of the most common causes of hearing loss and peripheral damage to the auditory system. A growing literature suggests that the auditory system can compensate for peripheral loss through increased central neural activity. The current study sought to investigate the link between noise exposure, increases in central gain, synaptic reorganization, and auditory function. All axons of the auditory nerve project to the cochlear nucleus, making it a requisite nucleus for sound detection. As the first synapse in the central auditory system, the cochlear nucleus is well positioned to respond plastically to loss of peripheral input. To investigate noise-induced compensation in the central auditory system, we measured auditory brainstem responses (ABRs) and auditory perception and collected tissue from mice exposed to broadband noise. Noise-exposed mice showed elevated ABR thresholds, reduced ABR wave 1 amplitudes, and spiral ganglion neuron loss. Despite peripheral damage, noise-exposed mice were hyperreactive to loud sounds and showed nearly normal behavioral sound detection thresholds. Ratios of late ABR peaks (2-4) relative to the first ABR peak indicated that brainstem pathways were hyperactive in noise-exposed mice, while anatomical analysis indicated there was an imbalance between expression of excitatory and inhibitory proteins in the ventral cochlear nucleus. The results of the current study suggest that a reorganization of excitation and inhibition in the ventral cochlear nucleus may drive hyperactivity in the central auditory system. This increase in central gain can compensate for peripheral loss to restore some aspects of auditory function.

Keywords: Auditory; brainstem; compensation; hyperactivity.

Figure 1.

ABR thresholds. Average thresholds for responses to clicks and tones of frequencies ranging from 6 to 32 kHz. Threshold was met when the peak-to-peak amplitude of the response was equal to two standards deviations above the average baseline noise amplitude. Error bars represent SEM. Asterisks indicate significant differences between noise-exposed and sham-exposed subjects.

Figure 2.

Measurements of ABR peak 1. A, Amplitudes. B, Latencies of peak 1 in response to clicks and tones of 8, 16, and 32 kHz. Legend in A also applies to B. Axis in B also applies to A. Solid lines indicate predictions of means made by linear mixed models fit to the data. C, Waveforms averaged across all subjects for each exposure condition. Solid lines represent means, and lighter shading indicates the SEM.

Figure 3.

Amplitudes and latencies of ABR peaks 2–4. A, Amplitudes of p2 (top), p3 (middle), and p4 (bottom) in response to clicks and tones of 8, 16, and 32 kHz. B, Latencies for the same corresponding peaks in A. Legend in A corresponds to all panels. Axis in B (bottom) corresponds to all panels. Solid lines indicate predictions made by linear mixed models.

Figure 4.

Hyperactivity in bushy cell-driven pathways. Plotted are average ratios of amplitudes of ABR peak 2 relative to peak 1 (A), peak 3 relative to peak 1 (B), and peak 4 relative to peak 1 (C). Ratios are plotted for responses to clicks and tones of frequencies from 6 to 32 kHz at stimulus levels of 60 dB SPL (left), 70 dB SPL (middle), and 80 dB SPL (right). Error bars represent SEM. Legend applies to all panels. Asterisks indicate significant differences between noise-exposed and sham-exposed mice. Ratios larger than those of sham-exposed animals indicate hyperactivity, while smaller ratios indicate hypoactivity.

Figure 5.

Acoustic startle reflex. Average relative startle responses to broadband noise bursts ranging from 70 to 105 dB SPL. Error bars represent SEM. Asterisks indicate significant differences between groups.

Figure 6.

Behavioral detection thresholds. A, Lick suppression rates for noise-exposed and sham-exposed mice for detection of 16 kHz tone. B, Response times for the data in A. C, CLS thresholds for noise-exposed and sham-exposed mice. Error bars indicate SEM. Asterisks indicate significant differences between groups. D, Difference between thresholds of noise-exposed and sham-exposed mice as determined from CLS or ABR. Error bars indicate SEM. Asterisks indicate significant differences between the threshold differences for CLS and ABR.

Figure 7.

Inner ear immunolabeling. A, Example maximum intensity images of cochleae from a sham-exposed (top) and noise-exposed (bottom) mouse at 32 kHz. Cochleae were immunolabeled for CTBP2 (green), myosin 6 (red), and neurofilament (blue). Scale bar equals 10 µm. B, Average counts of ribbon synapses per inner hair cell as a function of frequency. Error bars indicate SEM. Asterisks indicate significant differences between groups. C, Average percentages of hair cells remaining in noise-exposed cochleae relative to the averages for sham-exposed cochleae. Error bars indicate SEM.

Figure 8.

Overall label density in the VCN. A, Quantification of VGLUT1 labeling, calculated as the total number of thresholded pixels divided by the total number of pixels within the entire VCN. Example sections in right panel from a sham-exposed and a noise-exposed individual. Scale bars equal 10 µm. The overall amount of VGLUT1 expression decreased by ∼25% in noise-exposed mice relative to sham-exposed mice, but this reduction was not statistically significant. B, Quantification and examples of GAD65 labeling in sham-exposed and noise-exposed individuals. Noise-exposed animals exhibited a significant reduction of labeling (indicated by asterisk) of ∼50% relative to sham-exposed mice.

Figure 9.

Frequency mapping of immunolabeling. A, An example coronal section of the VCN stained for GAD65. B, The same section in A following auto-thresholding. Positively-labeled particles are black. The border of the VCN ROI for this section is also shown in black. C, The same section in A after its corresponding 3D-reconstructed CN was mapped to the frequency model (Muniak et al., 2013b). Colors indicate assigned frequency values. Contour lines indicate edges of 1/4-octave bins used in subsequent analyses. D, Merging of auto-thresholding data (B) with frequency mapping (C) produces frequency specific GAD65 labeling in the CN. Orientation axes apply to A–D. E, 3D reconstruction of frequency-specific GAD65 labeling for a single case. All coronal slices are shown at their corresponding locations within the reconstructed CN. Black arrowhead indicates position of section shown in panels A–D. The color gradient is a reflection of the tonotopic organization of the VCN. F, Same as in E, but shown from a lateral viewpoint.

Figure 10.

Frequency-specific analysis of immunolabeling. A, Mean density of VGLUT1 expression in the VCN of sham and noise-exposed mice in 1/4-octave bins. B, Mean density of GAD65 expression in the VCN of sham and noise-exposed mice in 1/4-octave bins. Asterisks indicate significant differences following post hoc testing.