Acoustic trauma evokes hyperactivity and changes in gene expression in guinea-pig auditory brainstem

From sound waves to molecular and cellular mechanisms: Understanding noise‑induced hearing loss and pioneering preventive approaches (Review)

Noise-induced hearing loss (NIHL) is a significant and urgent global public health concern, arising from prolonged exposure to elevated levels of noise. This auditory impairment harms delicate inner ear structures, particularly the essential hair cells transmitting auditory signals to the brain. Recognized by the World Health Organization as a major contributor to worldwide hearing loss, NIHL requires a comprehensive examination of its molecular and cellular mechanisms. Animal models emerge as indispensable tools for unraveling these intricacies, allowing researchers to simulate and study the impact of noise exposure on auditory structures, shedding light on the interplay of oxidative stress, inflammation and immune responses-crucial factors in NIHL progression. The present review focuses on elucidating the molecular mechanisms of NIHL, with a specific emphasis on findings derived from animal models, alongside the exploration of thorough preventive strategies, including protective measures and probing potential interventions. Understanding the molecular underpinnings not only provides insight into targeted treatment approaches, but also unlocks pathways for exploring and implementing preventive actions. This approach not only deepens the current comprehension of NIHL, but also has the potential to influence the shaping of public health policies, offering a nuanced perspective on this prevalent auditory disorder.

Hearing loss-related altered neuronal activity in the inferior colliculus

Hearing loss is well known to cause plastic changes in the central auditory system and pathological changes such as tinnitus and hyperacusis. Impairment of inner ear functions is the main cause of hearing loss. In aged individuals, not only inner ear dysfunction but also senescence of the central nervous system is the cause of malfunction of the auditory system. In most cases of hearing loss, the activity of the auditory nerve is reduced, but that of the successive auditory centers is increased in a compensatory way. It has been reported that activity changes occur in the inferior colliculus (IC), a critical nexus of the auditory pathway. The IC integrates the inputs from the brainstem and drives the higher auditory centers. Since abnormal activity in the IC is likely to affect auditory perception, it is crucial to elucidate the neuronal mechanism to induce the activity changes of IC neurons with hearing loss. This review outlines recent findings on hearing-loss-induced plastic changes in the IC and brainstem auditory neuronal circuits and discusses what neuronal mechanisms underlie hearing-loss-induced changes in the activity of IC neurons. Considering the different causes of hearing loss, we discuss age-related hearing loss separately from other forms of hearing loss (non-age-related hearing loss). In general, the main plastic change of IC neurons caused by both age-related and non-age-related hearing loss is increased central gain. However, plastic changes in the IC caused by age-related hearing loss seem to be more complex than those caused by non-age-related hearing loss.

Noise-induced hearing loss alters potassium-chloride cotransporter KCC2 and GABA inhibition in the auditory centers

Homeostatic plasticity, the ability of neurons to maintain their averaged activity constant around a set point value, is thought to account for the central hyperactivity after hearing loss. Here, we investigated the putative role of GABAergic neurotransmission in this mechanism after a noise-induced hearing loss larger than 50 dB in high frequencies in guinea pigs. The effect of GABAergic inhibition is linked to the normal functioning of K + -Cl- co-transporter isoform 2 (KCC2) which maintains a low intracellular concentration of chloride. The expression of membrane KCC2 were investigated before and after noise trauma in the ventral and dorsal cochlear nucleus (VCN and DCN, respectively) and in the inferior colliculus (IC). Moreover, the effect of gabazine (GBZ), a GABA antagonist, was also studied on the neural activity in IC. We show that KCC2 is downregulated in VCN, DCN and IC 3 days after noise trauma, and in DCN and IC 30 days after the trauma. As expected, GBZ application in the IC of control animals resulted in an increase of spontaneous and stimulus-evoked activity. In the noise exposed animals, on the other hand, GBZ application decreased the stimulus-evoked activity in IC neurons. The functional implications of these central changes are discussed.

Time-dependent effects of acoustic trauma and tinnitus on extracellular levels of amino acids in the inferior colliculus of rats

Chronic tinnitus is a debilitating condition with very few management options. Acoustic trauma that causes tinnitus has been shown to induce neuronal hyperactivity in multiple brain areas in the auditory pathway, including the inferior colliculus. This neuronal hyperactivity could be attributed to an imbalance between excitatory and inhibitory neurotransmission. However, it is not clear how the levels of neurotransmitters, especially neurotransmitters in the extracellular space, change over time following acoustic trauma and the development of tinnitus. In the present study, a range of amino acids were measured in the inferior colliculus of rats during acoustic trauma as well as at 1 week and 5 months post-trauma using in vivo microdialysis and high-performance liquid chromatography. Amino acid levels in response to sound stimulation were also measured at 1 week and 5 months post-trauma. It was found that unilateral exposure to a 16 kHz pure tone at 115 dB SPL for 1 h caused immediate hearing loss in all the animals and chronic tinnitus in 58 % of the animals. Comparing to the sham condition, extracellular levels of GABA were significantly increased at both the acute and 1 week time points after acoustic trauma. However, there was no significant difference in any of the amino acid levels measured between sham, tinnitus positive and tinnitus negative animals at 5 months post-trauma. There was also no clear pattern in the relationship between neurochemical changes and sound frequency/acoustic trauma/tinnitus status, which might be due to the relatively poorer temporal resolution of the microdialysis compared to electrophysiological responses.

Neurodegeneration after repeated noise trauma in the mouse lower auditory pathway

High intensity noise exposure leads to a permanent shift in auditory thresholds (PTS), affecting both peripheral (cochlear) tissue and the central auditory system. Studies have shown that a noise-induced hearing loss results in significant cell loss in several auditory structures. Degeneration can be demonstrated within hours after noise exposure, particularly in the lower auditory pathway, and continues to progress over days and weeks following the trauma. However, there is limited knowledge about the effects of recurring acoustic trauma. Repeated noise exposure has been demonstrated to increase neuroplasticity and neural activity. Thus, the present study aimed to investigate the influence of a second noise exposure on the cytoarchitecture of key structures of the auditory pathway, including spiral ganglion neurons (SGN), the ventral and dorsal cochlear nucleus (VCN and DCN, respectively), and the inferior colliculus (IC). In the experiments, young adult normal hearing mice were exposed to noise once or twice (with the second trauma applied one week after the initial exposure) for 3 h, using broadband white noise (5 - 20 kHz) at 115 dB SPL. The cell densities in the investigated auditory structures significantly decreased in response to the initial noise exposure compared to unexposed control animals. These findings are consistent with earlier research, which demonstrated degeneration in the auditory pathway within the first week after acoustic trauma. Additionally, cell densities were significantly decreased after the second trauma, but this effect was only observed in the VCN, with no similar effects seen in the SGN, DCN, or IC. These results illustrate how repeated noise exposure influences the cytoarchitecture of the auditory system. It appears that an initial noise exposure primarily damages the lower auditory pathway, but surviving cellular structures may develop resistance to additional noise-induced injury.

Comprehensive behavioral and physiologic assessment of peripheral and central auditory function in individuals with mild traumatic brain injury

Auditory complaints are frequently reported by individuals with mild traumatic brain injury (mTBI) yet remain difficult to detect in the absence of clinically significant hearing loss. This highlights a growing need to identify sensitive indices of auditory-related mTBI pathophysiology beyond pure-tone thresholds for improved hearing healthcare diagnosis and treatment. Given the heterogeneity of mTBI etiology and the diverse peripheral and central processes required for normal auditory function, the present study sought to determine the audiologic assessments sensitive to mTBI pathophysiology at the group level using a well-rounded test battery of both peripheral and central auditory system function. This test battery included pure-tone detection thresholds, word understanding in quiet, sentence understanding in noise, distortion product otoacoustic emissions (DPOAEs), middle-ear muscle reflexes (MEMRs), and auditory evoked potentials (AEPs), including auditory brainstem responses (ABRs), middle latency responses (MLRs), and late latency responses (LLRs). Each participant also received magnetic resonance imaging (MRI). Compared to the control group, we found that individuals with mTBI had reduced DPOAE amplitudes that revealed a compound effect of age, elevated MEMR thresholds for an ipsilateral broadband noise elicitor, longer ABR Wave I latencies for click and 4 kHz tone burst elicitors, longer ABR Wave III latencies for 4 kHz tone bursts, larger MLR Na and Nb amplitudes, smaller MLR Pb amplitudes, longer MLR Pa latencies, and smaller LLR N1 amplitudes for older individuals with mTBI. Further, mTBI individuals with combined hearing difficulty and noise sensitivity had a greater number of deficits on thalamic and cortical AEP measures compared to those with only one/no self-reported auditory symptoms. This finding was corroborated with MRI, which revealed significant structural differences in the auditory cortical areas of mTBI participants who reported combined hearing difficulty and noise sensitivity, including an enlargement of left transverse temporal gyrus (TTG) and bilateral planum polare (PP). These findings highlight the need for continued investigations toward identifying individualized audiologic assessments and treatments that are sensitive to mTBI pathophysiology.

Neuropeptide Y Signaling Regulates Recurrent Excitation in the Auditory Midbrain

Neuropeptides play key roles in shaping the organization and function of neuronal circuits. In the inferior colliculus (IC), which is in the auditory midbrain, Neuropeptide Y (NPY) is expressed by a class of GABAergic neurons that project locally and outside the IC. Most neurons in the IC have local axon collaterals; however, the organization and function of local circuits in the IC remain unknown. We previously found that excitatory neurons in the IC can express the NPY Y1 receptor (Y1R+) and application of the Y1R agonist, [Leu31, Pro34]-NPY (LP-NPY), decreases the excitability of Y1R+ neurons. As NPY signaling regulates recurrent excitation in other brain regions, we hypothesized that Y1R+ neurons form interconnected local circuits in the IC and that NPY decreases the strength of recurrent excitation in these circuits. To test this hypothesis, we used optogenetics to activate Y1R+ neurons in mice of both sexes while recording from other neurons in the ipsilateral IC. We found that nearly 80% of glutamatergic IC neurons express the Y1 receptor, providing extensive opportunities for NPY signaling to regulate local circuits. Additionally, Y1R+ neuron synapses exhibited modest short-term synaptic plasticity, suggesting that local excitatory circuits maintain their influence over computations during sustained stimuli. We further found that application of LP-NPY decreased recurrent excitation in the IC, suggesting that NPY signaling strongly regulates local circuit function in the auditory midbrain. Our findings show that Y1R+ excitatory neurons form interconnected local circuits in the IC, and their influence over local circuits is regulated by NPY signaling.SIGNIFICANCE STATEMENT Local networks play fundamental roles in shaping neuronal computations in the brain. The IC, localized in the auditory midbrain, plays an essential role in sound processing, but the organization of local circuits in the IC is largely unknown. Here, we show that IC neurons that express the Neuropeptide Y1 receptor (Y1R+ neurons) make up most of the excitatory neurons in the IC and form interconnected local circuits. Additionally, we found that NPY, which is a powerful neuromodulator known to shape neuronal activity in other brain regions, decreases the extensive recurrent excitation mediated by Y1R+ neurons in local IC circuits. Thus, our results suggest that local NPY signaling is a key regulator of auditory computations in the IC.

Noise-Induced Hearing Loss Alters Potassium-Chloride CoTransporter KCC2 and GABA Inhibition in the auditory centers

Homeostatic plasticity, the ability of neurons to maintain their averaged activity constant around a set point value, is thought to account for the central hyperactivity after hearing loss. Here, we investigated the putative role of GABAergic neurotransmission in this mechanism after a noise-induced hearing loss larger than 50 dB in high frequencies in guinea pigs. The effect of GABAergic inhibition is linked to the normal functioning of K+-Cl- co-transporter isoform 2 (KCC2) which maintains a low intracellular concentration of chloride. The expression of membrane KCC2 were investigated before after noise trauma in the ventral and dorsal cochlear nucleus (VCN and DCN, respectively) and in the inferior colliculus (IC). Moreover, the effect of gabazine (GBZ), a GABA antagonist, was also studied on the neural activity in IC. We show that KCC2 is downregulated in VCN, DCN and IC 3 days after noise trauma, and in DCN and IC 30 days after the trauma. As expected, GBZ application in the IC of control animals resulted in an increase of spontaneous and stimulus-evoked activity. In the noise exposed animals, on the other hand, GBZ application decreased the stimulus-evoked activity in IC neurons. The functional implications of these central changes are discussed.

The role of GABAB receptors in the subcortical pathways of the mammalian auditory system

GABAB receptors are G-protein coupled receptors for the inhibitory neurotransmitter GABA. Functional GABAB receptors are formed as heteromers of GABAB1 and GABAB2 subunits, which further associate with various regulatory and signaling proteins to provide receptor complexes with distinct pharmacological and physiological properties. GABAB receptors are widely distributed in nervous tissue, where they are involved in a number of processes and in turn are subject to a number of regulatory mechanisms. In this review, we summarize current knowledge of the cellular distribution and function of the receptors in the inner ear and auditory pathway of the mammalian brainstem and midbrain. The findings suggest that in these regions, GABAB receptors are involved in processes essential for proper auditory function, such as cochlear amplifier modulation, regulation of spontaneous activity, binaural and temporal information processing, and predictive coding. Since impaired GABAergic inhibition has been found to be associated with various forms of hearing loss, GABAB dysfunction could also play a role in some pathologies of the auditory system.

Developmental Exposure to Polychlorinated Biphenyls Prevents Recovery from Noise-Induced Hearing Loss and Disrupts the Functional Organization of the Inferior Colliculus

Exposure to combinations of environmental toxins is growing in prevalence; and therefore, understanding their interactions is of increasing societal importance. Here, we examined the mechanisms by which two environmental toxins, polychlorinated biphenyls (PCBs) and high-amplitude acoustic noise, interact to produce dysfunction in central auditory processing. PCBs are well established to impose negative developmental impacts on hearing. However, it is not known whether developmental exposure to this ototoxin alters the sensitivity to other ototoxic exposures later in life. Here, male mice were exposed to PCBs in utero, and later as adults were exposed to 45 min of high-intensity noise. We then examined the impacts of the two exposures on hearing and the organization of the auditory midbrain using two-photon imaging and analysis of the expression of mediators of oxidative stress. We observed that developmental exposure to PCBs blocked hearing recovery from acoustic trauma. In vivo two-photon imaging of the inferior colliculus (IC) revealed that this lack of recovery was associated with disruption of the tonotopic organization and reduction of inhibition in the auditory midbrain. In addition, expression analysis in the inferior colliculus revealed that reduced GABAergic inhibition was more prominent in animals with a lower capacity to mitigate oxidative stress. These data suggest that combined PCBs and noise exposure act nonlinearly to damage hearing and that this damage is associated with synaptic reorganization, and reduced capacity to limit oxidative stress. In addition, this work provides a new paradigm by which to understand nonlinear interactions between combinations of environmental toxins.SIGNIFICANCE STATEMENT Exposure to common environmental toxins is a large and growing problem in the population. This work provides a new mechanistic understanding of how the prenatal and postnatal developmental changes induced by polychlorinated biphenyls (PCBs) could negatively impact the resilience of the brain to noise-induced hearing loss (NIHL) later in adulthood. The use of state-of-the-art tools, including in vivo multiphoton microscopy of the midbrain helped in identifying the long-term central changes in the auditory system after the peripheral hearing damage induced by such environmental toxins. In addition, the novel combination of methods employed in this study will lead to additional advances in our understanding of mechanisms of central hearing loss in other contexts.

Increased central auditory gain in 5xFAD Alzheimer's disease mice as an early biomarker candidate for Alzheimer's disease diagnosis

Alzheimer's Disease (AD) is a neurodegenerative illness without a cure. All current therapies require an accurate diagnosis and staging of AD to ensure appropriate care. Central auditory processing disorders (CAPDs) and hearing loss have been associated with AD, and may precede the onset of Alzheimer's dementia. Therefore, CAPD is a possible biomarker candidate for AD diagnosis. However, little is known about how CAPD and AD pathological changes are correlated. In the present study, we investigated auditory changes in AD using transgenic amyloidosis mouse models. AD mouse models were bred to a mouse strain commonly used for auditory experiments, to compensate for the recessive accelerated hearing loss on the parent background. Auditory brainstem response (ABR) recordings revealed significant hearing loss, a reduced ABR wave I amplitude, and increased central gain in 5xFAD mice. In comparison, these effects were milder or reversed in APP/PS1 mice. Longitudinal analyses revealed that in 5xFAD mice, central gain increase preceded ABR wave I amplitude reduction and hearing loss, suggesting that it may originate from lesions in the central nervous system rather than the peripheral loss. Pharmacologically facilitating cholinergic signaling with donepezil reversed the central gain in 5xFAD mice. After the central gain increased, aging 5xFAD mice developed deficits for hearing sound pips in the presence of noise, consistent with CAPD-like symptoms of AD patients. Histological analysis revealed that amyloid plaques were deposited in the auditory cortex of both mouse strains. However, in 5xFAD but not APP/PS1 mice, plaque was observed in the upper auditory brainstem, specifically the inferior colliculus (IC) and the medial geniculate body (MGB). This plaque distribution parallels histological findings from human subjects with AD and correlates in age with central gain increase. Overall, we conclude that auditory alterations in amyloidosis mouse models correlate with amyloid deposits in the auditory brainstem and may be reversed initially through enhanced cholinergic signaling. The alteration of ABR recording related to the increase in central gain prior to AD-related hearing disorders suggests that it could potentially be used as an early biomarker of AD diagnosis.

Neuropeptide Y signaling regulates recurrent excitation in the auditory midbrain

Neuropeptides play key roles in shaping the organization and function of neuronal circuits. In the inferior colliculus (IC), which is located in the auditory midbrain, Neuropeptide Y (NPY) is expressed by a large class of GABAergic neurons that project locally as well as outside the IC. The IC integrates information from numerous auditory nuclei making the IC an important hub for sound processing. Most neurons in the IC have local axon collaterals, however the organization and function of local circuits in the IC remains largely unknown. We previously found that neurons in the IC can express the NPY Y1 receptor (Y 1 R + ) and application of the Y 1 R agonist, [Leu 31 , Pro 34 ]-NPY (LP-NPY), decreases the excitability of Y 1 R + neurons. To investigate how Y 1 R + neurons and NPY signaling contribute to local IC networks, we used optogenetics to activate Y 1 R + neurons while recording from other neurons in the ipsilateral IC. Here, we show that 78.4% of glutamatergic neurons in the IC express the Y1 receptor, providing extensive opportunities for NPY signaling to regulate excitation in local IC circuits. Additionally, Y 1 R + neuron synapses exhibit modest short-term synaptic plasticity, suggesting that local excitatory circuits maintain their influence over computations during sustained stimuli. We further found that application of LP-NPY decreases recurrent excitation in the IC, suggesting that NPY signaling strongly regulates local circuit function in the auditory midbrain. Together, our data show that excitatory neurons are highly interconnected in the local IC and their influence over local circuits is tightly regulated by NPY signaling.

Developmental exposure to polychlorinated biphenyls prevents recovery from noise-induced hearing loss and disrupts the functional organization of the inferior colliculus

Exposure to combinations of environmental toxins is growing in prevalence, and therefore understanding their interactions is of increasing societal importance. Here, we examined the mechanisms by which two environmental toxins - polychlorinated biphenyls (PCBs) and high-amplitude acoustic noise - interact to produce dysfunction in central auditory processing. PCBs are well-established to impose negative developmental impacts on hearing. However, it is not known if developmental exposure to this ototoxin alters the sensitivity to other ototoxic exposures later in life. Here, male mice were exposed to PCBs in utero, and later as adults were exposed to 45 minutes of high-intensity noise. We then examined the impacts of the two exposures on hearing and the organization of the auditory midbrain using two-photon imaging and analysis of the expression of mediators of oxidative stress. We observed that developmental exposure to PCBs blocked hearing recovery from acoustic trauma. In vivo two-photon imaging of the inferior colliculus revealed that this lack of recovery was associated with disruption of the tonotopic organization and reduction of inhibition in the auditory midbrain. In addition, expression analysis in the inferior colliculus revealed that reduced GABAergic inhibition was more prominent in animals with a lower capacity to mitigate oxidative stress. These data suggest that combined PCBs and noise exposure act nonlinearly to damage hearing and that this damage is associated with synaptic reorganization, and reduced capacity to limit oxidative stress. In addition, this work provides a new paradigm by which to understand nonlinear interactions between combinations of environmental toxins.

Significance statement

Exposure to common environmental toxins is a large and growing problem in the population. This work provides a new mechanistic understanding of how the pre-and postnatal developmental changes induced by polychlorinated biphenyls could negatively impact the resilience of the brain to noise-induced hearing loss later in adulthood. The use of state-of-the-art tools, including in vivo multiphoton microscopy of the midbrain helped in identifying the long-term central changes in the auditory system after the peripheral hearing damage induced by such environmental toxins. In addition, the novel combination of methods employed in this study will lead to additional advances in our understanding of mechanisms of central hearing loss in other contexts.

Change in gene expression levels of GABA, glutamate and neurosteroid pathways due to acoustic trauma in the cochlea

The characteristic feature of noise-induced hearing loss (NIHL) is the loss or malfunction of the outer hair cells (OHC) and the inner hair cells (IHC) of the cochlea. 90-95% of the spiral ganglion neurons, forming the cell bodies of cochlear nerve, synapse with the IHCs. Glutamate is the most potent excitatory neurotransmitter for IHC-auditory nerve synapses. Excessive release of glutamate in response to acoustic trauma (AT), may cause excitotoxicity by causing damage to the spiral ganglion neurons (SGN) or loss of the spiral ganglion dendrites, post-synaptic to the IHCs. Another neurotransmitter, GABA, plays an important role in the processing of acoustic stimuli and central regulation after peripheral injury, so it is potentially related to the regulation of hearing function and sensitivity after noise. The aim of this study is to evaluate the effect of AT on the expressions of glutamate excitotoxicity, GABA inhibition and neurosteroid synthesis genes.We exposed 24 BALB/c mice to AT. Controls were sacrificed without exposure to noise, Post-AT(1) and Post-AT(15) were sacrificed on the 1st and 15th day, respectively, after noise exposure. The expressions of various genes playing roles in glutamate, GABA and neurosteroid pathways were compared between groups by real-time PCR.Expressions of Cyp11a1, Gls, Gabra1, Grin2b, Sult1a1, Gad1, and Slc1a2 genes in Post-AT(15) mice were significantly decreased in comparison to control and Post-AT(1) mice. No significant differences in the expression of Slc6a1 and Slc17a8 genes was detected.These findings support the possible role of balance between glutamate excitotoxicity and GABA inhibition is disturbed during the post AT days and also the synthesis of some neurosteroids such as pregnenolone sulfate may be important in this balance.

Electrophysiological assessment and pharmacological treatment of blast-induced tinnitus

Tinnitus, the phantom perception of sound, often occurs as a clinical sequela of auditory traumas. In an effort to develop an objective test and therapeutic approach for tinnitus, the present study was performed in blast-exposed rats and focused on measurements of auditory brainstem responses (ABRs), prepulse inhibition of the acoustic startle response, and presynaptic ribbon densities on cochlear inner hair cells (IHCs). Although the exact mechanism is unknown, the “central gain theory” posits that tinnitus is a perceptual indicator of abnormal increases in the gain (or neural amplification) of the central auditory system to compensate for peripheral loss of sensory input from the cochlea. Our data from vehicle-treated rats supports this rationale; namely, blast-induced cochlear synaptopathy correlated with imbalanced elevations in the ratio of centrally-derived ABR wave V amplitudes to peripherally-derived wave I amplitudes, resulting in behavioral evidence of tinnitus. Logistic regression modeling demonstrated that the ABR wave V/I amplitude ratio served as a reliable metric for objectively identifying tinnitus. Furthermore, histopathological examinations in blast-exposed rats revealed tinnitus-related changes in the expression patterns of key plasticity factors in the central auditory pathway, including chronic loss of Arc/Arg3.1 mobilization. Using a formulation of N-acetylcysteine (NAC) and disodium 2,4-disulfophenyl-N-tert-butylnitrone (HPN-07) as a therapeutic for addressing blast-induced neurodegeneration, we measured a significant treatment effect on preservation or restoration of IHC ribbon synapses, normalization of ABR wave V/I amplitude ratios, and reduced behavioral evidence of tinnitus in blast-exposed rats, all of which accorded with mitigated histopathological evidence of tinnitus-related neuropathy and maladaptive neuroplasticity.

Does cannabis alleviate tinnitus? A review of the current literature

OBJECTIVES

Endocannabinoid pathways have been proposed to affect the underlying pathophysiology of tinnitus. The aim of this study is to evaluate the scope and findings of existing literature on the relationship between cannabis and cannabinoid pathways and tinnitus.

METHODS

We conducted a review of animal, clinical and survey studies investigating the relationship between the use of cannabis-derived agents and tinnitus. Using pertinent keywords and MeSH terms on PubMed, relevant studies were identified, yielding four animal studies, two large cross-sectional survey studies, one clinical cross-over study, and one case report.

RESULTS

Animal studies revealed that cannabinoid receptor expression in the cochlear nucleus varied with tinnitus symptomatology and the use of cannabinoid agents either increased or had no effect on tinnitus-related behavior. Survey studies yielded conflicting results between cannabis use and tinnitus in the general population. Clinical data is largely lacking, although a small cohort study showed a dose-dependent relationship between tetrahydrocannabinol consumption and frequency of tinnitus episodes in patients receiving treatment for cancer.

CONCLUSION

While animal studies have revealed that cannabinoid receptors likely have a role in modulating auditory signaling, there is no compelling data either from animal or human studies for the use of cannabinoids to alleviate tinnitus. Further research is necessary to elucidate their precise role to guide development of therapeutic interventions.

LEVEL OF EVIDENCE

NA.

Objective evidence of temporal processing deficits in older adults

The older listener's ability to understand speech in challenging environments may be affected by impaired temporal processing. This review summarizes objective evidence of degraded temporal processing from studies that have used the auditory brainstem response, auditory steady-state response, the envelope- or frequency-following response, cortical auditory-evoked potentials, and neural tracking of continuous speech. Studies have revealed delayed latencies and reduced amplitudes/phase locking in subcortical responses in older vs. younger listeners, in contrast to enhanced amplitudes of cortical responses in older listeners. Reconstruction accuracy of responses to continuous speech (e.g., cortical envelope tracking) shows over-representation in older listeners. Hearing loss is a factor in many of these studies, even though the listeners would be considered to have clinically normal hearing thresholds. Overall, the ability to draw definitive conclusions regarding these studies is limited by the use of multiple stimulus conditions, small sample sizes, and lack of replication. Nevertheless, these objective measures suggest a need to incorporate new clinical measures to provide a more comprehensive assessment of the listener's speech understanding ability, but more work is needed to determine the most efficacious measure for clinical use.

Nitric oxide increases gain in the ventral cochlear nucleus of guinea pigs with tinnitus

Previous work has led to the hypothesis that, during the production of noise-induced tinnitus, higher levels of nitric oxide (NO), in the ventral cochlear nucleus (VCN), increase the gain applied to a reduced input from the cochlea. To test this hypothesis, we noise-exposed 26 guinea pigs, identified evidence of tinnitus in 12 of them and then compared the effects of an iontophoretically applied NO donor or production inhibitor on VCN single unit activity. We confirmed that the mean driven firing rate for the tinnitus and control groups was the same while it had fallen in the non-tinnitus group. By contrast, the mean spontaneous rate had increased for the tinnitus group relative to the control group, while it remained the same for the non-tinnitus group. A greater proportion of units responded to exogenously applied NO in the tinnitus (56%) and non-tinnitus groups (71%) than a control population (24%). In the tinnitus group, endogenous NO facilitated the driven firing rate in 37% (7/19) of neurons and appeared to bring the mean driven rate back up to control levels by a mechanism involving N-methyl-D-aspartic acid (NMDA) receptors. By contrast, in the non-tinnitus group, endogenous NO only facilitated the driven firing rate in 5% (1/22) of neurons and there was no facilitation of driven rate in the control group. The effects of endogenous NO on spontaneous activity were unclear. These results suggest that NO is involved in increasing the gain applied to driven activity, but other factors are also involved in the increase in spontaneous activity.

Dynamic Changes in Synaptic Plasticity Genes in Ipsilateral and Contralateral Inferior Colliculus Following Unilateral Noise-induced Hearing Loss

Unilateral noise-induced hearing loss reduces the input to the central auditory pathway disrupting the excitatory and inhibitory inputs to the inferior colliculus (IC), an important binaural processing center. Little is known about the compensatory synaptic changes that occur in the IC as a consequence of unilateral noise-induced hearing loss. To address this issue, Sprague-Dawley rats underwent unilateral noise exposure resulting in severe unilateral hearing loss. IC tissues from the contralateral and ipsilateral IC were evaluated for acute (2-d) and chronic (28-d) changes in the expression of 84 synaptic plasticity genes on a PCR array. Arc and Egr1 genes were further visualized by in situ hybridization to validate the PCR results. None of the genes were upregulated, but many were downregulated post-exposure. At 2-d post-exposure, more than 75% of the genes were significantly downregulated in the contralateral IC, while only two were downregulated in the ipsilateral IC. Many of the downregulated genes were related to long-term depression, long-term potentiation, cell adhesion, immediate early genes, neural receptors and postsynaptic density. At 28-d post-exposure, the gene expression pattern was reversed with more than 85% of genes in the ipsilateral IC now downregulated. Most genes previously downregulated in the contralateral IC 2-d post-exposure had recovered; less than 15% remained downregulated. These time-dependent, asymmetric changes in synaptic plasticity gene expression could shed new light on the perceptual deficits associated with unilateral hearing loss and the dynamic structural and functional changes that occur in the IC days and months following unilateral noise-induced hearing loss.

Pharmacological Evaluation of Drugs in Animal Models of Tinnitus

Despite the pressing need for effective drug treatments for tinnitus, currently, there is no single drug that is approved by the FDA for this purpose. Instead, a wide range of unproven over-the-counter tinnitus remedies are available on the market with little or no benefit for tinnitus but with potential harm and adverse effects. Animal models of tinnitus have played a critical role in exploring the pathophysiology of tinnitus, identifying therapeutic targets and evaluating novel and existing drugs for tinnitus treatment. This review summarises and compares the studies on pharmacological evaluation of tinnitus treatment in different animal models based on the pharmacological properties of the drug and provides insights into future directions for tinnitus drug discovery.

Early Onset Region and Cell Specific Alterations of Doublecortin Expression in the CNS of Animals with Sound Damage Induced Hearing Loss

Sound damage induced hearing loss has been shown to elicit changes in auditory and non-auditory brain regions. A protein critical for neuronal migration and brain development, doublecortin (DCX), has been used as a marker of central nervous system (CNS) neuroplasticity. DCX is expressed in unipolar brush cells (UBCs) of the dorsal cochlear nucleus (DCN), cerebellar parafloccular lobe (PFL) and neuronal precursor cells in the sub-granular zone of the hippocampal dentate gyrus (DG). Sound damage induced hearing loss has been shown to differentially impact DCX expression months later. To identify earlier alterations in DCX expression, we utilized immunohistochemistry to detect DCX protein in three brain regions (DCN, PFL, DG) approximately one month following unilateral sound damage. Auditory brainstem response was used to measure hearing loss. Unilateral hearing loss was evident in all sound damaged animals. Hearing loss related decreases in DCX expression were evident bilaterally in the DG while hearing loss related increases in DCX expression were evident bilaterally in the PFL. No changes to DCX expression were evident in the auditory DCN. Gap detection was used to assess whether this sound damage paradigm induced tinnitus-like behavior. However, results obtained from this behavioral test as used here were inconclusive and are presented here only as a guide to others wishing to design similar studies.

The rat as a model for studying noise injury and otoprotection

A major challenge for those studying noise-induced injury pre-clinically is the selection of an animal model. Noise injury models are particularly relevant in an age when people are constantly bombarded by loud noise due to occupation and/or recreation. The rat has been widely used for noise-related morphological, physiological, biochemical, and molecular assessment. Noise exposure resulting in a temporary (TTS) or permanent threshold shift (PTS) yields trauma in peripheral and central auditory related pathways. While the precise nature of noise-related injuries continues to be delineated, both PTS and TTS (with or without hidden hearing loss) result in homeostatic changes implicated in conditions such as tinnitus and hyperacusis. Compared to mice, rats generally tolerate exposure to loud sounds reasonably well, often without exhibiting other physical non-inner ear related symptoms such as death, loss of consciousness, or seizures [Skradski, Clark, Jiang, White, Fu, and Ptacek (2001). Neuron 31, 537-544; Faingold (2002). Hear. Res. 168, 223-237; Firstova, Abaimov, Surina, Poletaeva, Fedotova, and Kovalev (2012). Bull Exp. Biol. Med. 154, 196-198; De Sarro, Russo, Citraro, and Meldrum (2017). Epilepsy Behav. 71, 165-173]. This ability of the rat to thrive following noise exposure permits study of long-term effects. Like the mouse, the rat also offers a well-characterized genome allowing genetic manipulations (i.e., knock-out, viral-based gene expression modulation, and optogenetics). Rat models of noise-related injury also provide valuable information for understanding mechanistic changes to identify therapeutic targets for treatment. This article provides a framework for selection of the rat as a model for noise injury studies.

The Ras Superfamily of Small GTPases in Non-neoplastic Cerebral Diseases

The small GTPases from the Ras superfamily play crucial roles in basic cellular processes during practically the entire process of neurodevelopment, including neurogenesis, differentiation, gene expression, membrane and protein traffic, vesicular trafficking, and synaptic plasticity. Small GTPases are key signal transducing enzymes that link extracellular cues to the neuronal responses required for the construction of neuronal networks, as well as for synaptic function and plasticity. Different subfamilies of small GTPases have been linked to a number of non-neoplastic cerebral diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), intellectual disability, epilepsy, drug addiction, Huntington’s disease (HD), amyotrophic lateral sclerosis (ALS) and a large number of idiopathic cerebral diseases. Here, we attempted to make a clearer illustration of the relationship between Ras superfamily GTPases and non-neoplastic cerebral diseases, as well as their roles in the neural system. In future studies, potential treatments for non-neoplastic cerebral diseases which are based on small GTPase related signaling pathways should be explored further. In this paper, we review all the available literature in support of this possibility.

Speech-in-noise representation in the aging midbrain and cortex: Effects of hearing loss

Age-related deficits in speech-in-noise understanding pose a significant problem for older adults. Despite the vast number of studies conducted to investigate the neural mechanisms responsible for these communication difficulties, the role of central auditory deficits, beyond peripheral hearing loss, remains unclear. The current study builds upon our previous work that investigated the effect of aging on normal-hearing individuals and aims to estimate the effect of peripheral hearing loss on the representation of speech in noise in two critical regions of the aging auditory pathway: the midbrain and cortex. Data from 14 hearing-impaired older adults were added to a previously published dataset of 17 normal-hearing younger adults and 15 normal-hearing older adults. The midbrain response, measured by the frequency-following response (FFR), and the cortical response, measured with the magnetoencephalography (MEG) response, were recorded from subjects listening to speech in quiet and noise conditions at four signal-to-noise ratios (SNRs): +3, 0, -3, and -6 dB sound pressure level (SPL). Both groups of older listeners showed weaker midbrain response amplitudes and overrepresentation of cortical responses compared to younger listeners. No significant differences were found between the two older groups when the midbrain and cortical measurements were analyzed independently. However, significant differences between the older groups were found when investigating the midbrain-cortex relationships; that is, only hearing-impaired older adults showed significant correlations between midbrain and cortical measurements, suggesting that hearing loss may alter reciprocal connections between lower and higher levels of the auditory pathway. The overall paucity of differences in midbrain or cortical responses between the two older groups suggests that age-related temporal processing deficits may contribute to older adults' communication difficulties beyond what might be predicted from peripheral hearing loss alone; however, hearing loss does seem to alter the connectivity between midbrain and cortex. These results may have important ramifications for the field of audiology, as it indicates that algorithms in clinical devices, such as hearing aids, should consider age-related temporal processing deficits to maximize user benefit.

Cochlear Synaptopathy Changes Sound-Evoked Activity Without Changing Spontaneous Discharge in the Mouse Inferior Colliculus

Tinnitus and hyperacusis are life-disrupting perceptual abnormalities that are often preceded by acoustic overexposure. Animal models of overexposure have suggested a link between these phenomena and neural hyperactivity, i.e., elevated spontaneous rates (SRs) and sound-evoked responses. Prior work has focused on changes in central auditory responses, with less attention paid to the exact nature of the associated cochlear damage. The demonstration that acoustic overexposure can cause cochlear neuropathy without permanent threshold elevation suggests cochlear neuropathy per se may be a key elicitor of neural hyperactivity. We addressed this hypothesis by recording responses in the mouse inferior colliculus (IC) following a bilateral, neuropathic noise exposure. One to three weeks post-exposure, mean SRs were unchanged in mice recorded while awake, or under anesthesia. SRs were also unaffected by more intense, or unilateral exposures. These results suggest that neither neuropathy nor hair cell loss are sufficient to raise SRs in the IC, at least in 7-week-old mice, 1-3 weeks post exposure. However, it is not clear whether our mice had tinnitus. Tone-evoked rate-level functions at the CF were steeper following exposure, specifically in the region of maximal neuropathy. Furthermore, suppression driven by off-CF tones and by ipsilateral noise were reduced. Both changes were especially pronounced in neurons of awake mice. This neural hypersensitivity may manifest as behavioral hypersensitivity to sound - prior work reports that this same exposure causes elevated acoustic startle. Together, these results indicate that neuropathy may initiate a compensatory response in the central auditory system leading to the genesis of hyperacusis.

Pathophysiology of Subjective Tinnitus: Triggers and Maintenance

Tinnitus is the conscious perception of a sound without a corresponding external acoustic stimulus, usually described as a phantom perception. One of the major challenges for tinnitus research is to understand the pathophysiological mechanisms triggering and maintaining the symptoms, especially for subjective chronic tinnitus. Our objective was to synthesize the published literature in order to provide a comprehensive update on theoretical and experimental advances and to identify further research and clinical directions. We performed literature searches in three electronic databases, complemented by scanning reference lists from relevant reviews in our included records, citation searching of the included articles using Web of Science, and manual searching of the last 6 months of principal otology journals. One-hundred and thirty-two records were included in the review and the information related to peripheral and central mechanisms of tinnitus pathophysiology was collected in order to update on theories and models. A narrative synthesis examined the main themes arising from this information. Tinnitus pathophysiology is complex and multifactorial, involving the auditory and non-auditory systems. Recent theories assume the necessary involvement of extra-auditory brain regions for tinnitus to reach consciousness. Tinnitus engages multiple active dynamic and overlapping networks. We conclude that advancing knowledge concerning the origin and maintenance of specific tinnitus subtypes origin and maintenance mechanisms is of paramount importance for identifying adequate treatment.

Synergistic Transcriptional Changes in AMPA and GABAA Receptor Genes Support Compensatory Plasticity Following Unilateral Hearing Loss

Debilitating perceptual disorders including tinnitus, hyperacusis, phantom limb pain and visual release hallucinations may reflect aberrant patterns of neural activity in central sensory pathways following a loss of peripheral sensory input. Here, we explore short- and long-term changes in gene expression that may contribute to hyperexcitability following a sudden, profound loss of auditory input from one ear. We used fluorescence in situ hybridization to quantify mRNA levels for genes encoding AMPA and GABA_A receptor subunits (Gria2 and Gabra1, respectively) in single neurons from the inferior colliculus (IC) and auditory cortex (ACtx). Thirty days after unilateral hearing loss, Gria2 levels were significantly increased while Gabra1 levels were significantly decreased. Transcriptional rebalancing was more pronounced in ACtx than IC and bore no obvious relationship to the degree of hearing loss. By contrast to the opposing, synergistic shifts in Gria2 and Gabra1 observed 30 days after hearing loss, we found that transcription levels for both genes were equivalently reduced after 5 days of hearing loss, producing no net change in the excitatory/inhibitory transcriptional balance. Opposing transcriptional shifts in AMPA and GABA receptor genes that emerge several weeks after a peripheral insult could promote both sensitization and disinhibition to support a homeostatic recovery of neural activity following auditory deprivation. Imprecise transcriptional changes could also drive the system toward perceptual hypersensitivity, degraded temporal processing and the irrepressible perception of non-existent environmental stimuli, a trio of perceptual impairments that often accompany chronic sensory deprivation.

Central Compensation in Auditory Brainstem after Damaging Noise Exposure

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.

Reduction of sound-evoked midbrain responses observed by functional magnetic resonance imaging following acute acoustic noise exposure

Short duration and high intensity acoustic exposures can lead to temporary hearing loss and auditory nerve degeneration. This study investigates central auditory system function following such acute exposures after hearing loss recedes. Adult rats were exposed to 100 dB sound pressure level noise for 15 min. Auditory brainstem responses (ABRs) were recorded with click sounds to check hearing thresholds. Functional magnetic resonance imaging (fMRI) was performed with tonal stimulation at 12 and 20 kHz to investigate central auditory changes. Measurements were performed before exposure (0D), 7 days after (7D), and 14 days after (14D). ABRs show an ∼6 dB threshold shift shortly after exposure, but no significant threshold differences between 0D, 7D, and 14D. fMRI responses are observed in the lateral lemniscus (LL) and inferior colliculus (IC) of the midbrain. In the IC, responses to 12 kHz are 3.1 ± 0.3% (0D), 1.9 ± 0.3% (7D), and 2.9 ± 0.3% (14D) above the baseline magnetic resonance imaging signal. Responses to 20 kHz are 2.0 ± 0.2% (0D), 1.4 ± 0.2% (7D), and 2.1 ± 0.2% (14D). For both tones, responses at 7D are less than those at 0D (p < 0.01) and 14D (p < 0.05). In the LL, similar trends are observed. Acute exposure leads to functional changes in the auditory midbrain with timescale of weeks.

Corelease of Inhibitory Neurotransmitters in the Mouse Auditory Midbrain

The central nucleus of the inferior colliculus (ICC) of the auditory midbrain, which integrates most ascending auditory information from lower brainstem regions, receives prominent long-range inhibitory input from the ventral nucleus of the lateral lemniscus (VNLL), a region thought to be important for temporal pattern discrimination. Histological evidence suggests that neurons in the VNLL release both glycine and GABA in the ICC, but functional evidence for their corelease is lacking. We took advantage of the GlyT2-Cre mouse line (both male and female) to target expression of ChR2 to glycinergic afferents in the ICC and made whole-cell recordings in vitro while exciting glycinergic fibers with light. Using this approach, it was clear that a significant fraction of glycinergic boutons corelease GABA in the ICC. Viral injections were used to target ChR2 expression specifically to glycinergic fibers ascending from the VNLL, allowing for activation of fibers from a single source of ascending input in a way that has not been previously possible in the ICC. We then investigated aspects of the glycinergic versus GABAergic current components to probe functional consequences of corelease. Surprisingly, the time course and short-term plasticity of synaptic signaling were nearly identical for the two transmitters. We therefore conclude that the two neurotransmitters may be functionally interchangeable and that multiple receptor subtypes subserving inhibition may offer diverse mechanisms for maintaining inhibitory homeostasis.SIGNIFICANCE STATEMENT Corelease of neurotransmitters is a common feature of the brain. GABA and glycine corelease is particularly common in the spinal cord and brainstem, but its presence in the midbrain is unknown. We show corelease of GABA and glycine for the first time in the central nucleus of the inferior colliculus of the auditory midbrain. Glycine and GABA are both inhibitory neurotransmitters involved in fast synaptic transmission, so we explored differences between the currents to establish a physiological foundation for functional differences in vivo In contrast to the auditory brainstem, coreleased GABAergic and glycinergic currents in the midbrain are strikingly similar. This apparent redundancy may ensure homeostasis if one neurotransmitter system is compromised.

Blast-Induced Tinnitus and Elevated Central Auditory and Limbic Activity in Rats: A Manganese-Enhanced MRI and Behavioral Study

Blast-induced tinitus is the number one service-connected disability that currently affects military personnel and veterans. To elucidate its underlying mechanisms, we subjected 13 Sprague Dawley adult rats to unilateral 14 psi blast exposure to induce tinnitus and measured auditory and limbic brain activity using manganese-enhanced MRI (MEMRI). Tinnitus was evaluated with a gap detection acoustic startle reflex paradigm, while hearing status was assessed with prepulse inhibition (PPI) and auditory brainstem responses (ABRs). Both anxiety and cognitive functioning were assessed using elevated plus maze and Morris water maze, respectively. Five weeks after blast exposure, 8 of the 13 blasted rats exhibited chronic tinnitus. While acoustic PPI remained intact and ABR thresholds recovered, the ABR wave P1-N1 amplitude reduction persisted in all blast-exposed rats. No differences in spatial cognition were observed, but blasted rats as a whole exhibited increased anxiety. MEMRI data revealed a bilateral increase in activity along the auditory pathway and in certain limbic regions of rats with tinnitus compared to age-matched controls. Taken together, our data suggest that while blast-induced tinnitus may play a role in auditory and limbic hyperactivity, the non-auditory effects of blast and potential traumatic brain injury may also exert an effect.

Noise Trauma-Induced Behavioral Gap Detection Deficits Correlate with Reorganization of Excitatory and Inhibitory Local Circuits in the Inferior Colliculus and Are Prevented by Acoustic Enrichment

Hearing loss leads to a host of cellular and synaptic changes in auditory brain areas that are thought to give rise to auditory perception deficits such as temporal processing impairments, hyperacusis, and tinnitus. However, little is known about possible changes in synaptic circuit connectivity that may underlie these hearing deficits. Here, we show that mild hearing loss as a result of brief noise exposure leads to a pronounced reorganization of local excitatory and inhibitory circuits in the mouse inferior colliculus. The exact nature of these reorganizations correlated with the presence or absence of the animals' impairments in detecting brief sound gaps, a commonly used behavioral sign for tinnitus in animal models. Mice with gap detection deficits (GDDs) showed a shift in the balance of synaptic excitation and inhibition that was present in both glutamatergic and GABAergic neurons, whereas mice without GDDs showed stable excitation-inhibition balances. Acoustic enrichment (AE) with moderate intensity, pulsed white noise immediately after noise trauma prevented both circuit reorganization and GDDs, raising the possibility of using AE immediately after cochlear damage to prevent or alleviate the emergence of central auditory processing deficits.SIGNIFICANCE STATEMENT Noise overexposure is a major cause of central auditory processing disorders, including tinnitus, yet the changes in synaptic connectivity underlying these disorders remain poorly understood. Here, we find that brief noise overexposure leads to distinct reorganizations of excitatory and inhibitory synaptic inputs onto glutamatergic and GABAergic neurons and that the nature of these reorganizations correlates with animals' impairments in detecting brief sound gaps, which is often considered a sign of tinnitus. Acoustic enrichment immediately after noise trauma prevents circuit reorganizations and gap detection deficits, highlighting the potential for using sound therapy soon after cochlear damage to prevent the development of central processing deficits.

Auditory thalamic circuits and GABAA receptor function: Putative mechanisms in tinnitus pathology

Tinnitus is defined as a phantom sound (ringing in the ears), and can significantly reduce the quality of life for those who suffer its effects. Ten to fifteen percent of the general adult population report symptoms of tinnitus with 1-2% reporting that tinnitus negatively impacts their quality of life. Noise exposure is the most common cause of tinnitus and the military environment presents many challenging high-noise situations. Military noise levels can be so intense that standard hearing protection is not adequate. Recent studies suggest a role for inhibitory neurotransmitter dysfunction in response to noise-induced peripheral deafferentation as a key element in the pathology of tinnitus. The auditory thalamus, or medial geniculate body (MGB), is an obligate auditory brain center in a unique position to gate the percept of sound as it projects to auditory cortex and to limbic structures. Both areas are thought to be involved in those individuals most impacted by tinnitus. For MGB, opposing hypotheses have posited either a tinnitus-related pathologic decrease or pathologic increase in GABAergic inhibition. In sensory thalamus, GABA mediates fast synaptic inhibition via synaptic GABAA receptors (GABAARs) as well as a persistent tonic inhibition via high-affinity extrasynaptic GABAARs and slow synaptic inhibition via GABABRs. Down-regulation of inhibitory neurotransmission, related to partial peripheral deafferentation, is consistently presented as partially underpinning neuronal hyperactivity seen in animal models of tinnitus. This maladaptive plasticity/Gain Control Theory of tinnitus pathology (see Auerbach et al., 2014; Richardson et al., 2012) is characterized by reduced inhibition associated with increased spontaneous and abnormal neuronal activity, including bursting and increased synchrony throughout much of the central auditory pathway. A competing hypothesis suggests that maladaptive oscillations between the MGB and auditory cortex, thalamocortical dysrhythmia, predict tinnitus pathology (De Ridder et al., 2015). These unusual oscillations/rhythms reflect net increased tonic inhibition in a subset of thalamocortical projection neurons resulting in abnormal bursting. Hyperpolarizing de-inactivation of T-type Ca2+ channels switches thalamocortical projection neurons into burst mode. Thalamocortical dysrhythmia originating in sensory thalamus has been postulated to underpin neuropathies including tinnitus and chronic pain. Here we review the relationship between noise-induced tinnitus and altered inhibition in the MGB.

Long-Lasting forward Suppression of Spontaneous Firing in Auditory Neurons: Implication to the Residual Inhibition of Tinnitus

Tinnitus is the perception of a sound that has no external source. Sound stimuli can suppress spontaneous firing in auditory neurons long after stimulus offset. It is unknown how changes in sound stimulus parameters affect this forward suppression. Using in vivo extracellular recording in awake mice, we found that about 40 % of spontaneously active inferior colliculus (IC) neurons exhibited forward suppression of spontaneous activity after sound offset. The duration of this suppression increased with sound duration and lasted about 40 s following a 30-s stimulus offset. Pure tones presented at the neuron's characteristic frequency (CF) were more effective in triggering suppression compared to non-CF or wideband noise stimuli. In contrast, non-CF stimuli often induced forward facilitation. About one third of IC neurons exhibited shorter suppression durations with each subsequent sound presentation. These characteristics of forward suppression are similar to the psychoacoustic properties of residual inhibition of tinnitus: a phenomenon of brief (about 30 s) suppression of tinnitus observed in tinnitus patients after sound presentations. Because elevated spontaneous firing in central auditory neurons has been linked to tinnitus, forward suppression of this firing with sound might be an underlying mechanism of residual inhibition.

The effects of repetitive transcranial magnetic stimulation in an animal model of tinnitus

Tinnitus (phantom auditory perception associated with hearing loss) can seriously affect wellbeing. Its neural substrate is unknown however it has been linked with abnormal activity in auditory pathways. Though no cure currently exists, repetitive transcranial magnetic stimulation (rTMS) has been shown to reduce tinnitus in some patients, possibly via induction of cortical plasticity involving brain derived neurotrophic factor (BDNF). We examined whether low intensity rTMS (LI-rTMS) alleviates signs of tinnitus in a guinea pig model and whether this involves changes in BDNF expression and hyperactivity in inferior colliculus. Acoustic trauma was used to evoke hearing loss, central hyperactivity and tinnitus. When animals developed tinnitus, treatment commenced (10 sessions of 10 minutes 1 Hz LI-rTMS or sham over auditory cortex over 14 days). After treatment ceased animals were tested for tinnitus, underwent single-neuron recordings in inferior colliculus to assess hyperactivity and samples from cortex and inferior colliculus were taken for BDNF ELISA. Analysis revealed a significant reduction of tinnitus after LI-rTMS compared to sham, without a statistical significant effect on BDNF levels or hyperactivity. This suggests that LI-rTMS alleviates behavioural signs of tinnitus by a mechanism independent of inferior colliculus hyperactivity and BDNF levels and opens novel therapeutic avenues for tinnitus treatment.

Non-Monotonic Relation between Noise Exposure Severity and Neuronal Hyperactivity in the Auditory Midbrain

The occurrence of tinnitus can be linked to hearing loss in the majority of cases, but there is nevertheless a large degree of unexplained heterogeneity in the relation between hearing loss and tinnitus. Part of the problem might be that hearing loss is usually quantified in terms of increased hearing thresholds, which only provides limited information about the underlying cochlear damage. Moreover, noise exposure that does not cause hearing threshold loss can still lead to “hidden hearing loss” (HHL), i.e., functional deafferentation of auditory nerve fibers (ANFs) through loss of synaptic ribbons in inner hair cells. While it is known that increased hearing thresholds can trigger increases in spontaneous neural activity in the central auditory system, i.e., a putative neural correlate of tinnitus, the central effects of HHL have not yet been investigated. Here, we exposed mice to octave-band noise at 100 and 105 dB SPL to generate HHL and permanent increases of hearing thresholds, respectively. Deafferentation of ANFs was confirmed through measurement of auditory brainstem responses and cochlear immunohistochemistry. Acute extracellular recordings from the auditory midbrain (inferior colliculus) demonstrated increases in spontaneous neuronal activity (a putative neural correlate of tinnitus) in both groups. Surprisingly, the increase in spontaneous activity was most pronounced in the mice with HHL, suggesting that the relation between hearing loss and neuronal hyperactivity might be more complex than currently understood. Our computational model indicated that these differences in neuronal hyperactivity could arise from different degrees of deafferentation of low-threshold ANFs in the two exposure groups. Our results demonstrate that HHL is sufficient to induce changes in central auditory processing, and they also indicate a non-monotonic relationship between cochlear damage and neuronal hyperactivity, suggesting an explanation for why tinnitus might occur without obvious hearing loss and conversely why hearing loss does not always lead to tinnitus.

Evidence of activity-dependent plasticity in the dorsal cochlear nucleus, in vivo, induced by brief sound exposure

The purpose of the present study was to investigate the immediate effects of acute exposure to intense sound on spontaneous and stimulus-driven activity in the dorsal cochlear nucleus (DCN). We examined the levels of multi- and single-unit spontaneous activity before and immediately following brief exposure (2 min) to tones at levels of either 109 or 85 dB SPL. Exposure frequency was selected to either correspond to the units' best frequency (BF) or fall within the borders of its inhibitory side band. The results demonstrate that these exposure conditions caused significant alterations in spontaneous activity and responses to BF tones. The induced changes have a fast onset (minutes) and are persistent for durations of at least 20 min. The directions of the change were found to depend on the frequency of exposure relative to BF. Transient decreases followed by more sustained increases in spontaneous activity were induced when the exposure frequency was at or near the units' BF, while sustained decreases of activity resulted when the exposure frequency fell inside the inhibitory side band. Follow-up studies at the single unit level revealed that the observed activity changes were found on unit types having properties which have previously been found to represent fusiform cells. The changes in spontaneous activity occurred despite only minor changes in response thresholds. Noteworthy changes also occurred in the strength of responses to BF tones, although these changes tended to be in the direction opposite those of the spontaneous rate changes. We discuss the possible role of activity-dependent plasticity as a mechanism underlying the rapid emergence of increased spontaneous activity after tone exposure and suggest that these changes may represent a neural correlate of acute noise-induced tinnitus.

Immediate manifestation of acoustic trauma in the auditory cortex is layer specific and cell type dependent

Exposure to loud sounds damages the auditory periphery and induces maladaptive changes in central parts of the auditory system. Diminished peripheral afferentation and altered inhibition influence the processing of sounds in the auditory cortex. It is unclear, however, which types of inhibitory interneurons are affected by acoustic trauma. Here we used single-unit electrophysiological recording and two-photon calcium imaging in anesthetized mice to evaluate the effects of acute acoustic trauma (125 dB SPL, white noise, 5 min) on the response properties of neurons in the core auditory cortex. Electrophysiological measurements suggested the selective impact of acoustic trauma on inhibitory interneurons in the auditory cortex. To further investigate which interneuronal types were affected, we used two-photon calcium imaging to record the activity of neurons in cortical layers 2/3 and 4, specifically focusing on parvalbumin-positive (PV+) and somatostatin-positive (SST+) interneurons. Spontaneous and pure-tone-evoked firing rates of SST+ interneurons increased in layer 4 immediately after acoustic trauma and remained almost unchanged in layer 2/3. Furthermore, PV+ interneurons with high best frequencies increased their evoked-to-spontaneous firing rate ratios only in layer 2/3 and did not change in layer 4. Finally, acoustic trauma unmasked low-frequency excitatory inputs only in layer 2/3. Our results demonstrate layer-specific changes in the activity of auditory cortical inhibitory interneurons within minutes after acoustic trauma.

Neural Hyperactivity of the Central Auditory System in Response to Peripheral Damage

It is increasingly appreciated that cochlear pathology is accompanied by adaptive responses in the central auditory system. The cause of cochlear pathology varies widely, and it seems that few commonalities can be drawn. In fact, despite intricate internal neuroplasticity and diverse external symptoms, several classical injury models provide a feasible path to locate responses to different peripheral cochlear lesions. In these cases, hair cell damage may lead to considerable hyperactivity in the central auditory pathways, mediated by a reduction in inhibition, which may underlie some clinical symptoms associated with hearing loss, such as tinnitus. Homeostatic plasticity, the most discussed and acknowledged mechanism in recent years, is most likely responsible for excited central activity following cochlear damage.

Noise trauma induced plastic changes in brain regions outside the classical auditory pathway

The effects of intense noise exposure on the classical auditory pathway have been extensively investigated; however, little is known about the effects of noise-induced hearing loss on non-classical auditory areas in the brain such as the lateral amygdala (LA) and striatum (Str). To address this issue, we compared the noise-induced changes in spontaneous and tone-evoked responses from multiunit clusters (MUC) in the LA and Str with those seen in auditory cortex (AC) in rats. High-frequency octave band noise (10-20 kHz) and narrow band noise (16-20 kHz) induced permanent threshold shifts at high-frequencies within and above the noise band but not at low frequencies. While the noise trauma significantly elevated spontaneous discharge rate (SR) in the AC, SRs in the LA and Str were only slightly increased across all frequencies. The high-frequency noise trauma affected tone-evoked firing rates in frequency and time-dependent manner and the changes appeared to be related to the severity of noise trauma. In the LA, tone-evoked firing rates were reduced at the high-frequencies (trauma area) whereas firing rates were enhanced at the low-frequencies or at the edge-frequency dependent on severity of hearing loss at the high frequencies. The firing rate temporal profile changed from a broad plateau to one sharp, delayed peak. In the AC, tone-evoked firing rates were depressed at high frequencies and enhanced at the low frequencies while the firing rate temporal profiles became substantially broader. In contrast, firing rates in the Str were generally decreased and firing rate temporal profiles become more phasic and less prolonged. The altered firing rate and pattern at low frequencies induced by high-frequency hearing loss could have perceptual consequences. The tone-evoked hyperactivity in low-frequency MUC could manifest as hyperacusis whereas the discharge pattern changes could affect temporal resolution and integration.

Re-Emergent Inhibition of Cochlear Inner Hair Cells in a Mouse Model of Hearing Loss

Hearing loss among the elderly correlates with diminished social, mental, and physical health. Age-related cochlear cell death does occur, but growing anatomical evidence suggests that synaptic rearrangements on sensory hair cells also contribute to auditory functional decline. Here we present voltage-clamp recordings from inner hair cells of the C57BL/6J mouse model of age-related hearing loss, which reveal that cholinergic synaptic inputs re-emerge during aging. These efferents are functionally inhibitory, using the same ionic mechanisms as do efferent contacts present transiently before the developmental onset of hearing. The strength of efferent inhibition of inner hair cells increases with hearing threshold elevation. These data indicate that the aged cochlea regains features of the developing cochlea and that efferent inhibition of the primary receptors of the auditory system re-emerges with hearing impairment.

SIGNIFICANCE STATEMENT

Synaptic changes in the auditory periphery are increasingly recognized as important factors in hearing loss. To date, anatomical work has described the loss of afferent contacts from cochlear hair cells. However, relatively little is known about the efferent innervation of the cochlea during hearing loss. We performed intracellular recordings from mouse inner hair cells across the lifespan and show that efferent innervation of inner hair cells arises in parallel with the loss of afferent contacts and elevated hearing threshold during aging. These efferent neurons inhibit inner hair cells, raising the possibility that they play a role in the progression of age-related hearing loss.

Selective Impairment in Frequency Discrimination in a Mouse Model of Tinnitus

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.

Enhanced GABAA-Mediated Tonic Inhibition in Auditory Thalamus of Rats with Behavioral Evidence of Tinnitus

Accumulating evidence suggests a role for inhibitory neurotransmitter dysfunction in the pathology of tinnitus. Opposing hypotheses proposed either a pathologic decrease or increase of GABAergic inhibition in medial geniculate body (MGB). In thalamus, GABA mediates fast synaptic inhibition via synaptic GABAA receptors (GABAARs) and persistent tonic inhibition via high-affinity extrasynaptic GABAARs. Given that extrasynaptic GABAARs control the firing mode of thalamocortical neurons, we examined tonic GABAAR currents in MGB neurons in vitro, using the following three groups of adult rats: unexposed control (Ctrl); sound exposed with behavioral evidence of tinnitus (Tin); and sound exposed with no behavioral evidence of tinnitus (Non-T). Tonic GABAAR currents were evoked using the selective agonist gaboxadol. Months after a tinnitus-inducing sound exposure, gaboxadol-evoked tonic GABAAR currents showed significant tinnitus-related increases contralateral to the sound exposure. In situ hybridization studies found increased mRNA levels for GABAAR δ-subunits contralateral to the sound exposure. Tin rats showed significant increases in the number of spikes per burst evoked using suprathreshold-injected current steps. In summary, we found little evidence of tinnitus-related decreases in GABAergic neurotransmission. Tinnitus and chronic pain may reflect thalamocortical dysrhythmia, which results from abnormal theta-range resonant interactions between thalamus and cortex, due to neuronal hyperpolarization and the initiation of low-threshold calcium spike bursts (Walton and Llinás, 2010). In agreement with this hypothesis, we found tinnitus-related increases in tonic extrasynaptic GABAAR currents, in action potentials/evoked bursts, and in GABAAR δ-subunit gene expression. These tinnitus-related changes in GABAergic function may be markers for tinnitus pathology in the MGB.

Dopamine in the auditory brainstem and midbrain: co-localization with amino acid neurotransmitters and gene expression following cochlear trauma

Dopamine (DA) modulates the effects of amino acid neurotransmitters (AANs), including GABA and glutamate, in motor, visual, olfactory, and reward systems (Hnasko et al., 2010; Stuber et al., 2010; Hnasko and Edwards, 2012). The results suggest that DA may play a similar modulatory role in the auditory pathways. Previous studies have shown that deafness results in decreased GABA release, changes in excitatory neurotransmitter levels, and increased spontaneous neuronal activity within brainstem regions related to auditory function. Modulation of the expression and localization of tyrosine hydroxylase (TH; the rate limiting enzyme in the production of DA) in the IC following cochlear trauma has been previously reported (Tong et al., 2005). In the current study the possibility of co-localization of TH with AANs was examined. Changes in the gene expression of TH were compared with changes in the gene expression of markers for AANs in the cochlear nucleus (CN) and inferior colliculus (IC) to determine whether those deafness related changes occur concurrently. The results indicate that bilateral cochlear ablation significantly reduced TH gene expression in the CN after 2 months while in the IC the reduction in TH was observed at both 3 days and 2 months following ablation. Furthermore, in the CN, glycine transporter 2 (GLYT2) and the GABA transporter (GABAtp) were also significantly reduced only after 2 months. However, in the IC, DA receptor 1 (DRDA1), vesicular glutamate transporters 2 and 3 (VGLUT2, VGLUT3), GABAtp and GAD67 were reduced in expression both at the 3 days and 2 months time points. A close relationship between the distribution of TH and several of the AANs was determined in both the CN and the IC. In addition, GLYT2 and VGLUT3 each co-localized with TH within IC somata and dendrites. Therefore, the results of the current study suggest that DA is spatially well positioned to influence the effects of AANs on auditory neurons.

Effects of acoustic trauma on the auditory system of the rat: The role of microglia

Exposure to loud, prolonged sounds (acoustic trauma, AT) leads to the death of both inner and outer hair cells (IHCs and OHCs), death of neurons of the spiral ganglion and degeneration of the auditory nerve. The auditory nerve (8cn) projects to the three subdivisions of the cochlear nuclei (CN), the dorsal cochlear nucleus (DC) and the anterior (VCA) and posterior (VCP) subdivisions of the ventral cochlear nucleus (VCN). There is both anatomical and physiological evidence for plastic reorganization in the denervated CN after AT. Anatomical findings show axonal sprouting and synaptogenesis; physiologically there is an increase in spontaneous activity suggesting reorganization of circuitry. The mechanisms underlying this plasticity are not understood. Recent data suggest that activated microglia may have a role in facilitating plastic reorganization in addition to removing trauma-induced debris. In order to investigate the roles of activated microglia in the CN subsequent to AT we exposed animals to bilateral noise sufficient to cause massive hair cell death. We studied four groups of animals at different survival times: 30 days, 60 days, 6 months and 9 months. We used silver staining to examine the time course and pattern of auditory nerve degeneration, and immunohistochemistry to label activated microglia in the denervated CN. We found both degenerating auditory nerve fibers and activated microglia in the CN at 30 and 60 days and 6 months after AT. There was close geographic overlap between the degenerating fibers and activated microglia, consistent with a scavenger role for activated microglia. At the longest survival time, there were still silver-stained fibers but very little staining of activated microglia in overlapping regions. There were, however, activated microglia in the surrounding brainstem and cerebellar white matter.

Tinnitus: Maladaptive auditory-somatosensory plasticity

Tinnitus, the phantom perception of sound, is physiologically characterized by an increase in spontaneous neural activity in the central auditory system. However, as tinnitus is often associated with hearing impairment, it is unclear how a decrease of afferent drive can result in central hyperactivity. In this review, we first assess methods for tinnitus induction and objective measures of the tinnitus percept in animal models. From animal studies, we discuss evidence that tinnitus originates in the cochlear nucleus (CN), and hypothesize mechanisms whereby hyperactivity may develop in the CN after peripheral auditory nerve damage. We elaborate how this process is likely mediated by plasticity of auditory-somatosensory integration in the CN: the circuitry in normal circumstances maintains a balance of auditory and somatosensory activities, and loss of auditory inputs alters the balance of auditory somatosensory integration in a stimulus timing dependent manner, which propels the circuit towards hyperactivity. Understanding the mechanisms underlying tinnitus generation is essential for its prevention and treatment. This article is part of a Special Issue entitled <Tinnitus>.

Tinnitus-related changes in the inferior colliculus

Tinnitus is highly complex, diverse, and difficult to treat, in part due to the fact that the underlying causes and mechanisms remain elusive. Tinnitus is generated within the auditory brain; however, consolidating our understanding of tinnitus pathophysiology is difficult due to the diversity of reported effects and the variety of implicated brain nuclei. Here, we focus on the inferior colliculus (IC), a midbrain structure that integrates the vast majority of ascending auditory information and projects via the thalamus to the auditory cortex. The IC is also a point of convergence for corticofugal input and input originating outside the auditory pathway. We review the evidence, from both studies with human subjects and from animal models, for the contribution the IC makes to tinnitus. Changes in the IC, caused by either noise exposure or drug administration, involve fundamental, heterogeneous alterations in the balance of excitation and inhibition. However, differences between hearing loss-induced pathology and tinnitus-related pathology are not well understood. Moreover, variability in tinnitus induction methodology has a significant impact on subsequent neural and behavioral changes, which could explain some of the seemingly contradictory data. Nonetheless, the IC is likely involved in the generation and persistence of tinnitus perception.

Effects of unilateral acoustic trauma on tinnitus-related spontaneous activity in the inferior colliculus

This study describes the long-term effects of sound-induced cochlear trauma on spontaneous discharge rates in the central nucleus of the inferior colliculus (ICC). As in previous studies, single-unit recordings in Sprague-Dawley rats revealed pervasive increases in spontaneous discharge rates. Based on differences in their sources of input, it was hypothesized that physiologically defined neural populations of the auditory midbrain would reveal the brainstem sources that dictate ICC hyperactivity. Abnormal spontaneous activity was restricted to target neurons of the ventral cochlear nucleus. Nearly identical patterns of hyperactivity were observed in the contralateral and ipsilateral ICC. The elevation in spontaneous activity extended to frequencies well below and above the region of maximum threshold shift. This lack of frequency organization suggests that ICC hyperactivity may be influenced by regions of the brainstem that are not tonotopically organized. Sound-induced hyperactivity is often observed in animals with behavioral signs of tinnitus. Prior to electrophysiological recording, rats were screened for tinnitus by measuring gap pre-pulse inhibition of the acoustic startle reflex (GPIASR). Rats with positive phenotypes did not exhibit unique patterns of ICC hyperactivity. This ambiguity raises concerns regarding animal behavioral models of tinnitus. If our screening procedures were valid, ICC hyperactivity is observed in animals without behavioral indications of the disorder. Alternatively, if the perception of tinnitus is strictly linked to ongoing ICC hyperactivity, our current behavioral approach failed to provide a reliable assessment of tinnitus state.

Cochlear damage affects neurotransmitter chemistry in the central auditory system

Tinnitus, the perception of a monotonous sound not actually present in the environment, affects nearly 20% of the population of the United States. Although there has been great progress in tinnitus research over the past 25 years, the neurochemical basis of tinnitus is still poorly understood. We review current research about the effects of various types of cochlear damage on the neurotransmitter chemistry in the central auditory system and document evidence that different changes in this chemistry can underlie similar behaviorally measured tinnitus symptoms. Most available data have been obtained from rodents following cochlear damage produced by cochlear ablation, intense sound, or ototoxic drugs. Effects on neurotransmitter systems have been measured as changes in neurotransmitter level, synthesis, release, uptake, and receptors. In this review, magnitudes of changes are presented for neurotransmitter-related amino acids, acetylcholine, and serotonin. A variety of effects have been found in these studies that may be related to animal model, survival time, type and/or magnitude of cochlear damage, or methodology. The overall impression from the evidence presented is that any imbalance of neurotransmitter-related chemistry could disrupt auditory processing in such a way as to produce tinnitus.

Induction of enhanced acoustic startle response by noise exposure: dependence on exposure conditions and testing parameters and possible relevance to hyperacusis

There has been a recent surge of interest in the development of animal models of hyperacusis, a condition in which tolerance to sounds of moderate and high intensities is diminished. The reasons for this decreased tolerance are likely multifactorial, but some major factors that contribute to hyperacusis are increased loudness perception and heightened sensitivity and/or responsiveness to sound. Increased sound sensitivity is a symptom that sometimes develops in human subjects after acoustic insult and has recently been demonstrated in animals as evidenced by enhancement of the acoustic startle reflex following acoustic over-exposure. However, different laboratories have obtained conflicting results in this regard, with some studies reporting enhanced startle, others reporting weakened startle, and still others reporting little, if any, change in the amplitude of the acoustic startle reflex following noise exposure. In an effort to gain insight into these discrepancies, we conducted measures of acoustic startle responses (ASR) in animals exposed to different levels of sound, and repeated such measures on consecutive days using a range of different startle stimuli. Since many studies combine measures of acoustic startle with measures of gap detection, we also tested ASR in two different acoustic contexts, one in which the startle amplitudes were tested in isolation, the other in which startle amplitudes were measured in the context of the gap detection test. The results reveal that the emergence of chronic hyperacusis-like enhancements of startle following noise exposure is highly reproducible but is dependent on the post-exposure thresholds, the time when the measures are performed and the context in which the ASR measures are obtained. These findings could explain many of the discrepancies that exist across studies and suggest guidelines for inducing in animals enhancements of the startle reflex that may be related to hyperacusis.