Salicylate induced tinnitus: Behavioral measures and neural activity in auditory cortex of awake rats

Map plasticity following noise exposure in auditory cortex of rats: implications for disentangling neural correlates of tinnitus and hyperacusis

Introduction

Both tinnitus and hyperacusis, likely triggered by hearing loss, can be attributed to maladaptive plasticity in auditory perception. However, owing to their co-occurrence, disentangling their neural mechanisms proves difficult. We hypothesized that the neural correlates of tinnitus are associated with neural activities triggered by low-intensity tones, while hyperacusis is linked to responses to moderate- and high-intensity tones.

Methods

To test these hypotheses, we conducted behavioral and electrophysiological experiments in rats 2 to 8 days after traumatic tone exposure.

Results

In the behavioral experiments, prepulse and gap inhibition tended to exhibit different frequency characteristics (although not reaching sufficient statistical levels), suggesting that exposure to traumatic tones led to acute symptoms of hyperacusis and tinnitus at different frequency ranges. When examining the auditory cortex at the thalamocortical recipient layer, we observed that tinnitus symptoms correlated with a disorganized tonotopic map, typically characterized by responses to low-intensity tones. Neural correlates of hyperacusis were found in the cortical recruitment function at the multi-unit activity (MUA) level, but not at the local field potential (LFP) level, in response to moderate- and high-intensity tones. This shift from LFP to MUA was associated with a loss of monotonicity, suggesting a crucial role for inhibitory synapses.

Discussion

Thus, in acute symptoms of traumatic tone exposure, our experiments successfully disentangled the neural correlates of tinnitus and hyperacusis at the thalamocortical recipient layer of the auditory cortex. They also suggested that tinnitus is linked to central noise, whereas hyperacusis is associated with aberrant gain control. Further interactions between animal experiments and clinical studies will offer insights into neural mechanisms, diagnosis and treatments of tinnitus and hyperacusis, specifically in terms of long-term plasticity of chronic symptoms.

Sodium salicylate improves detection of amplitude-modulated sound in mice

Salicylate is commonly used to induce tinnitus in animals, but its underlying mechanism of action is still debated. We therefore tested its effects on the firing properties of neurons in the mouse inferior colliculus (IC). Salicylate induced a large decrease in the spontaneous activity and an increase of ∼20 dB SPL in the minimum threshold of single units. In response to sinusoidally modulated noise (SAM noise) single units showed both an increase in phase locking and improved rate coding. Mice also became better at detecting amplitude modulations, and a simple threshold model based on the IC population response could reproduce this improvement. The responses to dynamic random chords (DRCs) suggested that the improved AM encoding was due to a linearization of the cochlear output, resulting in larger contrasts during SAM noise. These effects of salicylate are not consistent with the presence of tinnitus, but should be taken into account when studying hyperacusis.

Characterization of the neural circuitry of the auditory thalamic reticular nucleus and its potential role in salicylate-induced tinnitus

Introduction

Subjective tinnitus, the perception of sound without an external acoustic source, is often subsequent to noise-induced hearing loss or ototoxic medications. The condition is believed to result from neuroplastic alterations in the auditory centers, characterized by heightened spontaneous neural activities and increased synchrony due to an imbalance between excitation and inhibition. However, the role of the thalamic reticular nucleus (TRN), a structure composed exclusively of GABAergic neurons involved in thalamocortical oscillations, in the pathogenesis of tinnitus remains largely unexplored.

Methods

We induced tinnitus in mice using sodium salicylate and assessed tinnitus-like behaviors using the Gap Pre-Pulse Inhibition of the Acoustic Startle (GPIAS) paradigm. We utilized combined viral tracing techniques to identify the neural circuitry involved and employed immunofluorescence and confocal imaging to determine cell types and activated neurons.

Results

Salicylate-treated mice exhibited tinnitus-like behaviors. Our tracing clearly delineated the inputs and outputs of the auditory-specific TRN. We discovered that chemogenetic activation of the auditory TRN significantly reduced the salicylate-evoked rise in c-Fos expression in the auditory cortex.

Discussion

This finding posits the TRN as a potential modulatory target for tinnitus treatment. Furthermore, the mapped sensory inputs to the auditory TRN suggest possibilities for employing optogenetic or sensory stimulations to manipulate thalamocortical activities. The precise mapping of the auditory TRN-mediated neural pathways offers a promising avenue for designing targeted interventions to alleviate tinnitus symptoms.

Transcriptional-profile changes in the medial geniculate body after noise-induced tinnitus

Tinnitus is a disturbing condition defined as the occurrence of acoustic hallucinations with no actual sound. Although the mechanisms underlying tinnitus have been explored extensively, the pathophysiology of the disease is not completely understood. Moreover, genes and potential treatment targets related to auditory hallucinations remain unknown. In this study, we examined transcriptional-profile changes in the medial geniculate body after noise-induced tinnitus in rats by performing RNA sequencing and validated differentially expressed genes via quantitative polymerase chain reaction analysis. The rat model of tinnitus was established by analyzing startle behavior based on gap-pre-pulse inhibition of acoustic startles. We identified 87 differently expressed genes, of which 40 were upregulated and 47 were downregulated. Pathway-enrichment analysis revealed that the differentially enriched genes in the tinnitus group were associated with pathway terms, such as coronavirus disease COVID-19, neuroactive ligand-receptor interaction. Protein-protein-interaction networks were established, and two hub genes (Rpl7a and AC136661.1) were identified among the selected genes. Further studies focusing on targeting and modulating these genes are required for developing potential treatments for noise-induced tinnitus in patients.

Electrophysiological and molecular changes following neuroprotective placental protein administration on tinnitus-induced rats

Objective

Despite 6%-20% of the adult population suffering from tinnitus, there is no standard treatment for it. Placenta extract has been used for various therapeutic purposes, including hearing loss. Here, we evaluate the effect of a novel neuroprotective protein composition (NPPC) extract on electrophysiological and molecular changes in the medial geniculate body (MGB) of tinnitus-induced rats.

Methods

To evaluate the protein analysis by western blot, the rats were divided into three groups: (1) saline group (intraperitoneal injection of 200 mg/kg saline twice a day for 28 consecutive days, (2) chronic Na-Sal group received sodium salicylate as in the first group, and (3) chronic treatment group (received salicylate 200 mg/kg twice daily for 2 weeks, followed by 0.4 mg NPPC daily from day 14 to day 28). Single-unit recordings were performed on a separate group that was treated as in group 4. Gap-prepulse inhibition of the acoustic startle (GPIAS) and pre-pulse inhibition (PPI) was performed to confirm tinnitus in all groups at the baseline, 14th and 28th days.

Results

Western blot analysis showed that the expression of γ-Aminobutyric acid Aα1 subunit (GABA Aα1), N-methyl-d-aspartate receptor subtype 2B (NR2B or NMDAR2B), α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors subunit GluR1 (GluR1), and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors subunit GluR2 (GluR2) decreased after Na-Sal injection, while NPPC upregulated their expression. MGB units in rats with tinnitus showed decreased spontaneous firing rate, burst per minute, and a spike in a burst. After NPPC administration, neural activity patterns showed a significant positive effect of NPPC on tinnitus.

Conclusion

NPPC can play an effective role in the treatment of tinnitus in salicylate-induced rats, and MGB is one of the brain areas involved in these processes.

Level of Evidence

NA.

Temporal Changes in Functional and Structural Neuronal Activities in Auditory System in Non-Severe Blast-Induced Tinnitus

Background and Objectives: Epidemiological data indicate that blast exposure is the most common morbidity responsible for mild TBI among Service Members (SMs) during recent military operations. Blast-induced tinnitus is a comorbidity frequently reported by veterans, and despite its wide prevalence, it is also one of the least understood. Tinnitus arising from blast exposure is usually associated with direct structural damage that results in a conductive and sensorineural impairment in the auditory system. Tinnitus is also believed to be initiated by abnormal neuronal activities and temporal changes in neuroplasticity. Clinically, it is observed that tinnitus is frequently accompanied by sleep disruption as well as increased anxiety. In this study, we elucidated some of the mechanistic aspects of sensorineural injury caused by exposure to both shock waves and impulsive noise. The isolated conductive auditory damage hypothesis was minimized by employing an animal model wherein both ears were protected. Materials and Methods: After the exposure, the animals' hearing circuitry status was evaluated via acoustic startle response (ASR) to distinguish between hearing loss and tinnitus. We also compared the blast-induced tinnitus against the well-established sodium salicylate-induced tinnitus model as the positive control. The state of the sensorineural auditory system was evaluated by auditory brainstem response (ABR), and this test helped examine the neuronal circuits between the cochlea and inferior colliculus. We then further evaluated the role of the excitatory and inhibitory neurotransmitter receptors and neuronal synapses in the auditory cortex (AC) injury after blast exposure. Results: We observed sustained elevated ABR thresholds in animals exposed to blast shock waves, while only transient ABR threshold shifts were observed in the impulsive noise group solely at the acute time point. These changes were in concert with the increased expression of ribbon synapses, which is suggestive of neuroinflammation and cellular energy metabolic disorder. It was also found that the onset of tinnitus was accompanied by anxiety, depression-like symptoms, and altered sleep patterns. By comparing the effects of shock wave exposure and impulsive noise exposure, we unveiled that the shock wave exerted more significant effects on tinnitus induction and sensorineural impairments when compared to impulsive noise. Conclusions: In this study, we systematically studied the auditory system structural and functional changes after blast injury, providing more significant insights into the pathophysiology of blast-induced tinnitus.

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.

The Auditory Brainstem Response (ABR) Test, Supplementary to Behavioral Tests for Evaluation of the Salicylate-Induced Tinnitus

Tinnitus is a symptom of various disorders that affects the quality of life of millions people. Given the significance of the access to an objective and non-invasive method for tinnitus detection, in this study the auditory brainstem response (ABR) electrophysiological test was used to diagnose salicylate-induced tinnitus, in parallel with common behavioral tests. Wistar rats were divided into saline (n = 7), and salicylate (n = 7) groups for behavioral tests, and salicylate group (n = 5) for the ABR test. The rats were evaluated by pre-pulse inhibition (PPI), gap pre-pulse inhibition of the acoustic startle (GPIAS), and ABR tests, at baseline, 14 and 62 h after salicylate (350 mg/kg) or vehicle injection. The mean percentage of GPIAS test was significantly reduced following salicylate administration, which confirms the induction of tinnitus. The ABR test results showed an increase in the hearing threshold at click and 8, 12, and 16 kHz tones. Moreover, a decline was observed in the latency ratio of II-I waves in all tone burst frequencies with the highest variation in 12 and 16 kHz as well as a decrement in the latency ratio of III-I and IV-I only in 12 and 16 kHz. ABR test is able to evaluate the salicylate induced tinnitus pitch and confirm the results of behavioral tinnitus tests. GPIAS reflexive response is dependent on brainstem circuits and the auditory cortex while, ABR test can demonstrate the function of the auditory brainstem in more details, and therefore, a combination of these two tests can offer a more accurate tinnitus evaluation.

Graphical Abstract

Online ascorbate sensing reveals oxidative injury occurrence in inferior colliculus in salicylate-induced tinnitus animal model

Tinnitus is a widespread and serious clinical and social problem. Although oxidative injury has been suggested to be one of pathological mechanisms in auditory cortex, whether this mechanism could be applied to inferior colliculus remains unclear. In this study, we used an online electrochemical system (OECS) integrating in vivo microdialysis with selective electrochemical detector to continuously monitor the dynamics of ascorbate efflux, an index of oxidative injury, in inferior colliculus of living rats during sodium salicylate-induced tinnitus. We found that OECS with a carbon nanotubes (CNTs)-modified electrode as the detector selectively responses to ascorbate, which is free from the interference from sodium salicylate and MK-801 that were used to induce tinnitus animal model and investigate the N-methyl-d-aspartate (NMDA) receptor mediated excitotoxicity, respectively. With the OECS, we found that the extracellular ascorbate level in inferior colliculus significantly increases after salicylate administration and such increase was suppressed by immediate injection of NMDA receptor antagonist MK-801. In addition, we found that salicylate administration significantly increases the spontaneous and sound stimuli evoked neural activity in inferior colliculus and that the increases were inhibited by the injection of MK-801. These results suggest that oxidative injury may occur in inferior colliculus following salicylate-induced tinnitus, which is closely relevant to the NMDA-mediated neuronal excitotoxicity. This information is useful for understanding the neurochemical processes in inferior colliculus involved in tinnitus and its related brain diseases.

Small molecule modulation of the large-conductance calcium-activated potassium channel suppresses salicylate-induced tinnitus in mice

Tinnitus is the phantom perception of sound that has no external source. A neurological signature of tinnitus, and the frequently associated hyperacusis, is an imbalance between excitatory and inhibitory activity in the central auditory system (CAS), leading to dysregulated network excitability. The large conductance, calcium-activated potassium (BK) channel is a key player in pre- and post-synaptic excitability through its mediation of K⁺ currents. Changes in BK channel activity are associated with aberrant network activity in sensory regions of the CNS, raising the possibility that BK channel modulation could regulate activity associated with tinnitus and hyperacusis. To test whether BK channel openers are able to suppress biomarkers of drug-induced tinnitus and hyperacusis, the 1,3,4 oxadiazole BMS-191011 was given to young adult CBA mice that had been administered 250 mg/kg sodium salicylate (SS). Systemic treatment with BMS-191011 reduced behavioral manifestations of SS-induced tinnitus, but not hyperacusis, probed via the gap-in-noise startle response method. Systemic BMS-191011 treatment did not influence SS-induced increases in auditory brainstem response functions, but local application at the inferior colliculus did reverse SS-suppressed spontaneous activity, particularly in the frequency region of the tinnitus percept. Thus, action of BMS-191011 in the inferior colliculus may contribute to the reduction in behaviorally measured tinnitus. Together, these findings support the utility of BK channel openers in reducing central auditory processing changes associated with the formation of the tinnitus percept.

Decreasing dorsal cochlear nucleus activity ameliorates noise-induced tinnitus perception in mice

Background

The dorsal cochlear nucleus (DCN) is a region known to integrate somatosensory and auditory inputs and is identified as a potential key structure in the generation of phantom sound perception, especially noise-induced tinnitus. Yet, how altered homeostatic plasticity of the DCN induces and maintains the sensation of tinnitus is not clear. Here, we chemogenetically decrease activity of a subgroup of DCN neurons, Ca²⁺/Calmodulin kinase 2 α (CaMKII α)-positive DCN neurons, using Gi-coupled human M4 Designer Receptors Exclusively Activated by Designer Drugs (hM4Di DREADDs), to investigate their role in noise-induced tinnitus.

Results

Mice were exposed to loud noise (9–11kHz, 90dBSPL, 1h, followed by 2h of silence), and auditory brainstem responses (ABRs) and gap prepulse inhibition of acoustic startle (GPIAS) were recorded 2 days before and 2 weeks after noise exposure to identify animals with a significantly decreased inhibition of startle, indicating tinnitus but without permanent hearing loss. Neuronal activity of CaMKII α+ neurons expressing hM4Di in the DCN was lowered by administration of clozapine-N-oxide (CNO). We found that acutely decreasing firing rate of CaMKII α+ DCN units decrease tinnitus-like responses (p = 3e −3, n = 11 mice), compared to the control group that showed no improvement in GPIAS (control virus; CaMKII α-YFP + CNO, p = 0.696, n = 7 mice). Extracellular recordings confirmed CNO to decrease unit firing frequency of CaMKII α-hM4Di+ mice and alter best frequency and tuning width of response to sound. However, these effects were not seen if CNO had been previously administered during the noise exposure (n = 6 experimental and 6 control mice).

Conclusion

We found that lowering DCN activity in mice displaying tinnitus-related behavior reduces tinnitus, but lowering DCN activity during noise exposure does not prevent noise-induced tinnitus. Our results suggest that CaMKII α-positive cells in the DCN are not crucial for tinnitus induction but play a significant role in maintaining tinnitus perception in mice.

Supplementary Information

The online version contains supplementary material available at (10.1186/s12915-022-01288-1).

Behavioral and Immunohistochemical Evidence for Suppressive Effects of Goshajinkigan on Salicylate-Induced Tinnitus in Rats

Many people are affected by tinnitus, a sensation of ringing in the ear despite the absence of external sound. Goshajinkigan (GJG) is one of the formulations of Japanese traditional herbal medicine and is prescribed for the palliative treatment of patients with tinnitus. Although GJG is clinically effective in these patients, its behavioral effects and the underlying neuroanatomical substrate have not been modeled in animals. We modeled tinnitus using salicylate-treated rats, demonstrated the effectiveness of GJG on tinnitus, and examined the underlying neuronal substrate with c-Fos expression. Intraperitoneal injection of sodium salicylate (400 mg/kg) into rats for three consecutive days significantly increased false positive scores, which were used to assess tinnitus behavior. When GJG was orally administered one hour after each salicylate injection, the increase in tinnitus behavior was suppressed. The analysis of c-Fos expression in auditory-related brain areas revealed that GJG significantly reduced the salicylate-induced increase in the number of c-Fos-expressing cells in the auditory cortices, inferior colliculus, and dorsal cochlear nucleus. These results suggest a suppressive effect of GJG on salicylate-induced tinnitus in animal models.

Changes in the spatiotemporal pattern of spontaneous activity across a cortical column after noise trauma

In the auditory modality, noise trauma has often been used to investigate cortical plasticity as it causes cochlear hearing loss. One limitation of these past studies, however, is that the effects of noise trauma have been mostly documented at the granular layer, which is the main cortical recipient of thalamic inputs. Importantly, the cortex is composed of six different layers each having its own pattern of connectivity and specific role in sensory processing. The present study aims at investigating the effects of acute and chronic noise trauma on the laminar pattern of spontaneous activity in primary auditory cortex of the anesthetized guinea pig. We show that spontaneous activity is dramatically altered across cortical layers after acute and chronic noise-induced hearing loss. First, spontaneous activity was globally enhanced across cortical layers, both in terms of firing rate and amplitude of spike-triggered average of local field potentials. Second, current source density on (spontaneous) spike-triggered average of local field potentials indicates that current sinks develop in the supra- and infragranular layers. These latter results suggest that supragranular layers become a major input recipient and that the propagation of spontaneous activity over a cortical column is greatly enhanced after acute and chronic noise-induced hearing loss. We discuss the possible mechanisms and functional implications of these changes.

Current topics in hearing research: Deafferentation and threshold independent hearing loss

Hearing research findings in recent years have begun to change how we think about hearing loss and how we consider the risk of auditory damage from noise exposure. These findings include evidence of noise-induced cochlear damage in the absence of corresponding permanent threshold elevation or evidence of hair cell loss. Animal studies in several species have shown that noise exposures that produce robust but only temporary threshold shifts can permanently damage inner hair cell synaptic ribbons. This type of synaptic degeneration has also been shown to occur as a result of aging in animals and humans. The emergence of these data has motivated a number of clinical studies aimed at identifying the perceptual correlates associated with synaptopathy. The deficits believed to arise from synaptopathy include poorer hearing in background noise, tinnitus and hyperacusis (loudness intolerance). However, the findings from human studies have been mixed. Key questions remain as to whether synaptopathy reliably produces suprathreshold perceptual deficits or whether it serves as an early indicator of auditory damage with suprathreshold deficits emerging later as a function of further cochlear damage. Here, we provide an overview of both human and animal studies that explore the relationship among inner hair cell damage, including loss of afferent synapses, auditory thresholds, and suprathreshold measures of hearing.

Chemogenetic Activation of Cortical Parvalbumin-Positive Interneurons Reverses Noise-Induced Impairments in Gap Detection

Exposure to loud noises not only leads to trauma and loss of output from the ear but also alters downstream central auditory circuits. A perceptual consequence of noise-induced central auditory disruption is impairment in gap-induced prepulse inhibition, also known as gap detection. Recent studies have implicated cortical parvalbumin (PV)-positive inhibitory interneurons in gap detection and prepulse inhibition. Here, we show that exposure to loud noises specifically reduces the density of cortical PV but not somatostatin (SOM)-positive interneurons in the primary auditory cortex in mice (C57BL/6) of both sexes. Optogenetic activation of PV neurons produced less cortical inhibition in noise-exposed than sham-exposed animals, indicative of reduced PV neuron function. Activation of SOM neurons resulted in similar levels of cortical inhibition in noise- and sham-exposed groups. Furthermore, chemogenetic activation of PV neurons with the hM3-based designer receptor exclusively activated by designer drugs completely reversed the impairments in gap detection for noise-exposed animals. These results support the notions that cortical PV neurons encode gap in sound and that PV neuron dysfunction contributes to noise-induced impairment in gap detection.SIGNIFICANCE STATEMENT Noise-induced hearing loss contributes to a range of central auditory processing deficits (CAPDs). The mechanisms underlying noise-induced CAPDs are still poorly understood. Here we show that exposure to loud noises results in dysfunction of PV-positive but not somatostatin-positive inhibitory interneurons in the primary auditory cortex. In addition, cortical PV inhibitory neurons in noise-exposed animals had reduced expression of glutamic acid decarboxylases and weakened inhibition on cortical activity. Noise exposure resulted in impaired gap detection, indicative of disrupted temporal sound processing and possibly tinnitus. We found that chemogenetic activation of cortical PV inhibitory interneurons alleviated the deficits in gap detection. These results implicate PV neuron dysfunction as a mechanism for noise-induced CAPDs.

Comparison of Acute Effects of Neurotoxic Compounds on Network Activity in Human and Rodent Neural Cultures

Assessment of neuroactive effects of chemicals in cell-based assays remains challenging as complex functional tissue is required for biologically relevant readouts. Recent in vitro models using rodent primary neural cultures grown on multielectrode arrays allow quantitative measurements of neural network activity suitable for neurotoxicity screening. However, robust systems for testing effects on network function in human neural models are still lacking. The increasing number of differentiation protocols for generating neurons from human-induced pluripotent stem cells (hiPSCs) holds great potential to overcome the unavailability of human primary tissue and expedite cell-based assays. Yet, the variability in neuronal activity, prolonged ontogeny and rather immature stage of most neuronal cells derived by standard differentiation techniques greatly limit their utility for screening neurotoxic effects on human neural networks. Here, we used excitatory and inhibitory neurons, separately generated by direct reprogramming from hiPSCs, together with primary human astrocytes to establish highly functional cultures with defined cell ratios. Such neuron/glia cocultures exhibited pronounced neuronal activity and robust formation of synchronized network activity on multielectrode arrays, albeit with noticeable delay compared with primary rat cortical cultures. We further investigated acute changes of network activity in human neuron/glia cocultures and rat primary cortical cultures in response to compounds with known adverse neuroactive effects, including gamma amino butyric acid receptor antagonists and multiple pesticides. Importantly, we observed largely corresponding concentration-dependent effects on multiple neural network activity metrics using both neural culture types. These results demonstrate the utility of directly converted neuronal cells from hiPSCs for functional neurotoxicity screening of environmental chemicals.

Auditory thalamus dysfunction and pathophysiology in tinnitus: a predictive network hypothesis

Tinnitus is the perception of a 'ringing' sound without an acoustic source. It is generally accepted that tinnitus develops after peripheral hearing loss and is associated with altered auditory processing. The thalamus is a crucial relay in the underlying pathways that actively shapes processing of auditory signals before the respective information reaches the cerebral cortex. Here, we review animal and human evidence to define thalamic function in tinnitus. Overall increased spontaneous firing patterns and altered coherence between the thalamic medial geniculate body (MGB) and auditory cortices is observed in animal models of tinnitus. It is likely that the functional connectivity between the MGB and primary and secondary auditory cortices is reduced in humans. Conversely, there are indications for increased connectivity between the MGB and several areas in the cingulate cortex and posterior cerebellar regions, as well as variability in connectivity between the MGB and frontal areas regarding laterality and orientation in the inferior, medial and superior frontal gyrus. We suggest that these changes affect adaptive sensory gating of temporal and spectral sound features along the auditory pathway, reflecting dysfunction in an extensive thalamo-cortical network implicated in predictive temporal adaptation to the auditory environment. Modulation of temporal characteristics of input signals might hence factor into a thalamo-cortical dysrhythmia profile of tinnitus, but could ultimately also establish new directions for treatment options for persons with tinnitus.

Hearing loss and brain plasticity: the hyperactivity phenomenon

Many aging adults experience some form of hearing problems that may arise from auditory peripheral damage. However, it has been increasingly acknowledged that hearing loss is not only a dysfunction of the auditory periphery but also results from changes within the entire auditory system, from periphery to cortex. Damage to the auditory periphery is associated with an increase in neural activity at various stages throughout the auditory pathway. Here, we review neurophysiological evidence of hyperactivity, auditory perceptual difficulties that may result from hyperactivity, and outline open conceptual and methodological questions related to the study of hyperactivity. We suggest that hyperactivity alters all aspects of hearing-including spectral, temporal, spatial hearing-and, in turn, impairs speech comprehension when background sound is present. By focusing on the perceptual consequences of hyperactivity and the potential challenges of investigating hyperactivity in humans, we hope to bring animal and human electrophysiologists closer together to better understand hearing problems in older adulthood.

The increase in the degree of neural forward masking of cochlea following salicylate application

BACKGROUND

High doses of salicylate are known to reduce cochlear response amplitude and raise threshold. However, its effect on the cochlear forward masking, reflecting temporal resolution, is still unclear.

METHODS

The neural forward masking of cochlea was evaluated using double-tone stimulation. The first tone burst (5ms) was named the "masker" and the second tone burst (5 ms) was named the "probe". The frequency and intensity of the masker and probe were equal, and the masker-probe interval varied from 2 to 32 ms. The reduction (%) of the probe-evoked cochlear compound action potential caused by the addition of the masker tone was used to represent cochlear forward masking. The data obtained before and 2 h following the injection of sodium salicylate (250 mg/kg) were compared.

RESULTS

The neural forward masking of cochlea in the normal rats increased as the masker-probe interval decreased. At 16 kHz, for example, it increased from ~5% to 32ms masker-probe interval to ~85% at 2ms masker-probe interval. Two hours post salicylate injection, the neural forward masking was significantly enhanced except at 32 ms masker-probe interval. Interestingly, this enhancement was only observed in the limited frequency range of 16-30 kHz.

DISCUSSION

The enhancement of forward masking of cochlea following salicylate administration may reflect defective neurotransmitter release. This frequency-dependent injury in the cochlea may lead to the development of central plasticity observed after salicylate administration, likely through the increase in central gain, leading to perceptual consequences.

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.

Role of the caudate-putamen nucleus in sensory gating in induced tinnitus in rats

Tinnitus can be described as the conscious perception of sound without external stimulation, and it is often accompanied by anxiety, depression, and insomnia. Current clinical treatments for tinnitus are ineffective. Although recent studies have indicated that the caudate-putamen nucleus may be a sensory gating area involved in noise elimination in tinnitus, the underlying mechanisms of this disorder are yet to be determined. To investigate the potential role of the caudate-putamen nucleus in experimentally induced tinnitus, we created a rat model of tinnitus induced by intraperitoneal administration of 350 mg/kg sodium salicylate. Our results revealed that the mean spontaneous firing rate of the caudate-putamen nucleus was increased by sodium salicylate treatment, while dopamine levels were decreased. In addition, electrical stimulation of the caudate-putamen nucleus markedly reduced the spontaneous firing rate of neurons in the primary auditory cortex. These findings suggest that the caudate-putamen nucleus plays a sensory gating role in sodium salicylate-induced tinnitus. This study was approved by the Institutional Animal Care and Use Committee of Peking University Health Science Center (approval No. A2010031) on December 6, 2017.

The gap prepulse inhibition of the acoustic startle (GPIAS) paradigm to assess auditory temporal processing: Monaural versus binaural presentation

The Gap Prepulse Inhibition of the Acoustic Startle Reflex (GPIAS) is a paradigm used to assess auditory temporal processing in both animals and humans. It consists of the presentation of a silent gap embedded in noise and presented a few milliseconds before a startle sound. The silent gap produces the inhibition of the startle reflex, a phenomenon called gap-prepulse inhibition (GPI). This paradigm is also used to detect tinnitus in animal models. The lack of inhibition by the silent gaps is suggested to be indicative of the presence of tinnitus "filling-in" the gaps. The current research aims at improving the GPIAS technique by comparing the GPI produced by monaural versus binaural silent gaps in 29 normal-hearing subjects. Two gap durations (5 or 50 ms), each embedded in two different frequency backgrounds (centered around 500 or 4 kHz). Both low- and high- frequency narrowband noises had a bandwidth of half an octave. Overall, the startle magnitude was greater for the binaural versus the monaural presentation, which might reflect binaural loudness summation. In addition, the GPI was similar between the monaural and the binaural presentations for the high-frequency background noise. However, the GPI was greater for the low-frequency background noise for the binaural, compared to the monaural, presentation. These findings suggest that monaural GPIAS might be more suited to detect tinnitus compared to the binaural presentation.

Uncovering the contribution of enhanced central gain and altered cortical oscillations to tinnitus generation

Various theories and their associated mechanisms have been proposed as the neural basis of phantom sound perception (tinnitus), including central gain enhancement and altered cortical oscillations. However, it remains unknown whether these cortical changes directly cause tinnitus, or simply coexist with the phantom percept. Using chronically-implanted electrodes and drug delivery cannulae in rats, we examined whether enhanced central gain and cortical oscillations are consistent across different tinnitus induction methods (noise exposure; salicylate), and if directly-inducing enhanced central gain or altered cortical oscillations via pharmacologic manipulation of inhibition along the auditory pathway would cause behavioral evidence of tinnitus. We show that, while there appeared to be no clear link between tinnitus and the presence of enhanced sound-evoked cortical activity or altered spontaneous cortical oscillations, pharmacologic impairment of GABAergic neurotransmission in the auditory cortex was sufficient to cause tinnitus; collective findings which further advance our understanding of the neural basis of tinnitus.

The effect of fibromyalgia treatment on tinnitus

OBJECTIVES

We investigated the frequency of tinnitus in fibromyalgia patients and the effect of drugs used for routine fibromyalgia on tinnitus.

METHODS

We included 101 diagnosed fibromyalgia patients. After detailed ear nose throat examination, audiometric tests and tinnitus handicap index (THI) were performed. After the tests, routine treatment for fibromyalgia was started by the physical therapy and rehabilitation department. Two months after the beginning of the treatment, THI were repeated again and the results were statistically evaluated.

RESULTS

All patients included in the study were women. 74.3% of the patients had tinnitus. Pregabalin and selective serotonin and norepinephrine reuptake inhibitor (SNRI) antidepressant-treated patients were evaluated; In both groups, there was a statistically significant difference between pre- and post-treatment tinnitus levels (p < .001). However, there was no statistically significant difference between pregabalin group and diloxetine group according to treatment results.

CONCLUSIONS

The incidence of tinnitus is high in fibromyalgia patients. That pregabalin and duloxetine agents routinely used in fibromyalgia require further experimental and human studies in order to be able to use in tinnitus.

Does Tinnitus Fill in the Gap Using Electrophysiology? A Scoping Review

The results showed a trend of increased post-gap amplitudes and reduced gap salience; however, the small number of articles yield and limited consensus prohibit any conclusions for clinical use. Nevertheless, gap-induced EPs may be further explored as a potential tool for tinnitus detection.

Animal Models of Tinnitus: A Review

Animal models have significantly contributed to understanding the pathophysiology of chronic subjective tinnitus. They are useful because they control etiology, which in humans is heterogeneous; employ random group assignment; and often use methods not permissible in human studies. Animal models can be broadly categorized as either operant or reflexive, based on methodology. Operant methods use variants of established psychophysical procedures to reveal what an animal hears. Reflexive methods do the same using elicited behavior, for example, the acoustic startle reflex. All methods contrast the absence of sound and presence of sound, because tinnitus cannot by definition be perceived as silence.

Aberrant expression of Nav1.6 in the cochlear nucleus correlates with salicylate-induced tinnitus in rats

Hyperactivity in cochlear nucleus (CN) is one of the major neural correlates for tinnitus induction, yet the molecular factors that participate in the neuronal hyperexcitability remain unclear. The present study showed that acute and chronic administrations of salicylate were both capable of inducing reversible tinnitus in rats. The number of GAD 65/67-immunoreactive neurons in the AVCN and DCN was decreased, while the number of VGLUT 1/2-immunoreactive neurons in the AVCN and DCN was increased when rats were experiencing tinnitus, providing evidence for excitatory-inhibitory imbalance in CN is correlated with tinnitus. Interestingly, the expression level of Nav1.6, an important subtype of voltage-gated sodium channels was significantly increased in the DCN and AVCN of rats experiencing tinnitus, the up-regulation of Nav1.6 was returned to normal level following the disappearance of tinnitus. Double-labeling experiments revealed that Nav1.6 expression was down-regulated in the GAD 65/67-positive neurons in the DCN and AVCN of rats experiencing tinnitus. Notably, the percentage of co-localization of Nav1.6 and NeuN-labeling fusiform neurons was markedly increased in the DCN during tinnitus. These findings uncover the tinnitus-associated alteration in Nav1.6, a potential key contributor that can lead to hyperexcitability in CN and contribute to salicylate-induced tinnitus.

Anxiety-like behavior induced by salicylate depends on age and can be prevented by a single dose of 5-MeO-DMT

Salicylate intoxication is a cause of tinnitus and comorbidly associated with anxiety in humans. In a previous work, we showed that salicylate induces anxiety-like behavior and hippocampal type 2 theta oscillations (theta2) in mice. Here we investigate if the anxiogenic effect of salicylate is dependent on age and previous tinnitus experience. We also tested whether a single dose of DMT can prevent this effect. Using microwire electrode arrays, we recorded local field potential in young (4-5- month-old) and old (11-13-month-old) mice to study the electrophysiological effect of tinnitus in the ventral hippocampus (vHipp) and medial prefrontal cortex (mPFC) in an open field arena and elevated plus maze 1h after salicylate (300mg/kg) injection. We found that anxiety-like behavior and increase in theta2 oscillations (4-6 Hz), following salicylate pre-treatment, only occurs in young (normal hearing) mice. We also show that theta2 and slow gamma oscillations increase in the vHipp and mPFC in a complementary manner during anxiety tests in the presence of salicylate. Finally, we show that pre-treating mice with a single dose of the hallucinogenic 5-MeO-DMT prevents anxiety-like behavior and the increase in theta2 and slow gamma oscillations after salicylate injection in normal hearing young mice. This work further support the hypothesis that anxiety-like behavior after salicylate injection is triggered by tinnitus and require normal hearing. Moreover, our results show that hallucinogenic compounds can be effective in treating tinnitus-related anxiety.

Noise-Induced loudness recruitment and hyperacusis: Insufficient central gain in auditory cortex and amygdala

Noise-induced hearing loss generally induces loudness recruitment, but sometimes gives rise to hyperacusis, a debilitating condition in which moderate intensity sounds are perceived abnormally loud. In an attempt to develop an animal model of loudness hyperacusis, we exposed rats to a 16-20 kHz noise at 104 dB SPL for 12 weeks. Behavioral reaction time-intensity functions were used to assess loudness growth functions before, during and 2-months post-exposure. During the exposure, loudness recruitment (R) was present in the region of hearing loss, but subtle evidence of hyperacusis (H) started to emerge at the border of the hearing loss. Unexpectedly, robust evidence of hyperacusis appeared below and near the edge of the hearing loss 2-months post-exposure. To identify the neural correlates of hyperacusis and test the central gain model of hyperacusis, we recorded population neural responses from the cochlea, auditory cortex and lateral amygdala 2-months post-exposure. Compared to controls, the neural output of the cochlea was greatly reduced in the noise group. Consistent with central gain models, the gross neural responses from the auditory cortex and amygdala were proportionately much larger than those from the cochlea. However, despite central amplification, the population responses in the auditory cortex and amygdala were still below the level needed to fully account for hyperacusis and/or recruitment. Having developed procedures that can consistently induce hyperacusis in rats, our results set the stage for future studies that seek to identify the neurobiological events that give rise to hyperacusis and to develop new therapies to treat this debilitating condition.

Tinnitus Correlates with Downregulation of Cortical Glutamate Decarboxylase 65 Expression But Not Auditory Cortical Map Reorganization

Hearing loss is the biggest risk factor for tinnitus, and hearing-loss-related pathological changes in the auditory pathway have been hypothesized as the mechanism underlying tinnitus. However, due to the comorbidity of tinnitus and hearing loss, it has been difficult to differentiate between neural correlates of tinnitus and consequences of hearing loss. In this study, we dissociated tinnitus and hearing loss in FVB mice, which exhibit robust resistance to tinnitus following monaural noise-induced hearing loss. Furthermore, knock-down of glutamate decarboxylase 65 (GAD65) expression in auditory cortex (AI) by RNA interference gave rise to tinnitus in normal-hearing FVB mice. We found that tinnitus was significantly correlated with downregulation of GAD65 in the AI. By contrast, cortical map distortions, which have been hypothesized as a mechanism underlying tinnitus, were correlated with hearing loss but not tinnitus. Our findings suggest new strategies for the rehabilitation of tinnitus and other phantom sensation, such as phantom pain.SIGNIFICANCE STATEMENT Hearing loss is the biggest risk factor for tinnitus in humans. Most animal models of tinnitus also exhibit comorbid hearing loss, making it difficult to dissociate the mechanisms underlying tinnitus from mere consequences of hearing loss. Here we show that, although both C57BL/6 and FVB mice exhibited similar noise-induced hearing threshold increase, only C57BL/6, but not FVB, mice developed tinnitus following noise exposure. Although both strains showed frequency map reorganization following noise-induced hearing loss, only C57BL/6 mice had reduced glutamate decarboxylase 65 (GAD65) expression in the auditory cortex (AI). Knocking down GAD65 expression in the AI resulted in tinnitus in normal-hearing FVB mice. Our results suggest that reduced inhibitory neuronal function, but not sensory map reorganization, underlies noise-induced tinnitus.

Tinnitus Impacts on Speech and Non-speech Stimuli

OBJECTIVE

To investigate how tinnitus affects the processing of speech and non-speech stimuli at the subcortical level.

STUDY DESIGN

Cross-sectional analytical study.

SETTING

Academic, tertiary referral center.

PATIENTS

Eighteen individuals with tinnitus and 20 controls without tinnitus matched based on their age and sex. All subjects had normal hearing sensitivity.

INTERVENTION

Diagnostic.

MAIN OUTCOME MEASURES

The effect of tinnitus on the parameters of auditory brainstem responses (ABR) to non-speech (click-ABR), and speech (sABR) stimuli was investigated.

RESULTS

Latencies of click ABR in waves III, V, and Vn, as well as inter-peak latency (IPL) of I to V were significantly longer in individuals with tinnitus compared with the controls. Individuals with tinnitus demonstrated significantly longer latencies of all sABR waves than the control group. The tinnitus patients also exhibited a significant decrease in the slope of the V-A complex and reduced encoding of the first and higher formants. A significant difference was observed between the two groups in the spectral magnitudes, the first formant frequency range (F1) and a higher frequency region (HF).

CONCLUSIONS

Our findings suggest that maladaptive neural plasticity resulting from tinnitus can be subcortically measured and affects timing processing of both speech and non-speech stimuli. The findings have been discussed based on models of maladaptive plasticity and the interference of tinnitus as an internal noise in synthesizing speech auditory stimuli.

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.

[Differentiating cochlear synaptopathies into different hearing disorders]

Due to demographic change and altered recreational behavior, a rapid increase in hearing deficits is expected in the next 20-30 years. Consequently, the risk of age-related loss of speech discrimination, tinnitus, hyperacusis, or-as recently shown-dementia, will also increase. There are increasing indications that the loss of specific hearing fibers in humans and animals is involved in various hearing disorders. This fiber loss can be caused by cochlear synaptopathy or deafferentation and does not necessarily lead to clinically measurable threshold changes. Animal experiments have shown that reduced auditory nerve activity due to acoustic trauma or aging can be centrally compensated by disproportionately elevated and faster auditory brainstem responses (ABR). The analysis of the suprathreshold amplitudes of auditory evoked brain stem potentials and their latency in combination with non-invasive imaging techniques such as magnetic resonance imaging can help to identify the central compensatory ability of subjects and to assign defined hearing deficits.

Testing the Central Gain Model: Loudness Growth Correlates with Central Auditory Gain Enhancement in a Rodent Model of Hyperacusis

The central gain model of hyperacusis proposes that loss of auditory input can result in maladaptive neuronal gain increases in the central auditory system, leading to the over-amplification of sound-evoked activity and excessive loudness perception. Despite the attractiveness of this model, and supporting evidence for it, a critical test of the central gain theory requires that changes in sound-evoked activity be explicitly linked to perceptual alterations of loudness. Here we combined an operant conditioning task that uses a subject's reaction time to auditory stimuli to produce reliable measures of loudness growth with chronic electrophysiological recordings from the auditory cortex and inferior colliculus of awake, behaviorally-phenotyped animals. In this manner, we could directly correlate daily assessments of loudness perception with neurophysiological measures of sound encoding within the same animal. We validated this novel psychophysical-electrophysiological paradigm with a salicylate-induced model of hearing loss and hyperacusis, as high doses of sodium salicylate reliably induce temporary hearing loss, neural hyperactivity, and auditory perceptual disruptions like tinnitus and hyperacusis. Salicylate induced parallel changes to loudness growth and evoked response-intensity functions consistent with temporary hearing loss and hyperacusis. Most importantly, we found that salicylate-mediated changes in loudness growth and sound-evoked activity were correlated within individual animals. These results provide strong support for the central gain model of hyperacusis and demonstrate the utility of using an experimental design that allows for within-subject comparison of behavioral and electrophysiological measures, thereby making inter-subject variability a strength rather than a limitation.

Electrophysiological changes in auditory evoked potentials in rats with salicylate-induced tinnitus

Early-response auditory evoked potentials (AEPs) in humans are significantly altered in tinnitus. These changes are closely related to that seen in animals, leading to new approaches to study tinnitus based on objective parameters. The purpose of this study was to characterize the AEPs in animals with tinnitus, by assessing early to late latency responses. For behavioral evaluation, rats were trained using positive reinforcement to press a lever in the presence of an auditory stimulus and to not press during silence. The auditory brainstem response (ABR), middle latency response (MLR) and auditory late latency response (LLR) were correlated to the false-positive responses (pressing the lever during silence), after oral administrations of Sodium Salicylate (SS, 350 mg/kg). In the present study, SS significantly increased the hearing thresholds and reduced ABR peak I amplitudes across the frequency range (4-32 kHz). In contrast, increased amplitudes were observed for several peaks in ABR, MLR, and LLR. Moreover, reduced ABR latencies in response to 8, 16 and 24 kHz tone bursts were observed after SS administration. Similarly, the central evaluation also revealed significantly reduced latencies in MLR and LLR during SS administration. In contrast, increased latencies were observed for ABR latencies in response to 32 kHz tone bursts, and at the P1-N1 component of LLR. Correlational analysis revealed that latencies and amplitudes of peaks II and IV (8 and 16 kHz) of ABR, and N2 latency and P2-N2 amplitude of LLR were associated with behavioral tinnitus. We suggest that AEPs can be used in the rat to evaluate the reduced sensory input and the increased central gain in SS-induced tinnitus, as well as reduced latencies (8-16 kHz) to distinguish between hearing loss and tinnitus.

Sodium salicylate alters temporal integration measured through increasing stimulus presentation rates

OBJECTIVE

High doses of sodium salicylate (SS) are known to induce tinnitus, general hyperexcitability in the central auditory system, and to cause mild hearing loss. We used the auditory brainstem response (ABR) to assess the effects of SS on auditory sensitivity and temporal processing in the auditory nerve and brainstem. ABRs were evoked using tone burst stimuli varying in frequency and intensity with presentation rates from 11/s to 81/s.

DESIGN

ABRs were recorded and analysed prior to and after SS treatment in each animal, and peak 1 and peak 4 amplitudes and latencies were determined along with minimal response threshold.

STUDY SAMPLE

Nine young adult CBA/CaJ mice were used in a longitudinal within-subject design.

RESULTS

No measurable effects of presentation rate were found on ABR threshold prior to SS; however, following SS administration increasing stimulus rates lowered ABR thresholds by as much as 10 dB and compressed the peak amplitude by intensity level functions.

CONCLUSIONS

These results suggest that SS alters temporal integration and compressive nonlinearity, and that varying the stimulus rate of the ABR may prove to be a useful diagnostic tool in the study of hearing disorders that involve hyperexcitability.

Prolonged Exposure of CBA/Ca Mice to Moderately Loud Noise Can Cause Cochlear Synaptopathy but Not Tinnitus or Hyperacusis as Assessed With the Acoustic Startle Reflex

Hearing loss changes the auditory brain, sometimes maladaptively. When deprived of cochlear input, central auditory neurons become more active spontaneously and begin to respond more strongly and synchronously to better preserved sound frequencies. This spontaneous and sound-evoked central hyperactivity has been postulated to trigger tinnitus and hyperacusis, respectively. Localized hyperactivity has also been observed after long-term exposure to noise levels that do not damage the cochlea. Adult animals exposed to bands of nondamaging noise exhibited suppressed spontaneous and sound-evoked activity in the area of primary auditory cortex (A1) stimulated by the exposure band but had increased spontaneous and evoked activity in neighboring A1 areas. We hypothesized that the cortically suppressed frequencies should for some time after exposure be perceived as less loud than before (hypoacusis), whereas the hyperactivity outside of the exposure band might lead to frequency-specific hyperacusis or tinnitus. To investigate this, adult CBA/Ca mice were exposed for >2 months to 8 to 16 kHz noise at 70 or 75 dB sound pressure level and tested for hypo-/hyperacusis and tinnitus using tone and gap prepulse inhibition of the acoustic startle reflex. Auditory brainstem responses and distortion product otoacoustic emissions showed evidence of cochlear synaptopathy after exposure at 75 but not 70 dB, putting a lower bound on damaging noise levels for CBA/Ca mice. Contrary to hypothesis, neither exposure significantly shifted startle results from baseline. These negative findings nevertheless have implications for startle test methodology and for the putative role of central hyperactivity in hyperacusis and tinnitus.

Neuroprotective effects of MK-801 on auditory cortex in salicylate-induced tinnitus: Involvement of neural activity, glutamate and ascorbate

Tinnitus may cause anxiety, depression, insomnia, which impair the quality of life of millions worldwide. However, the mechanism of tinnitus remains to be understood, it has been previously hypothesized that the activation of N-methyl-D-aspartate (NMDA) receptor is involved in the tinnitus processes and blockade of the NMDA receptor is regarded as a therapeutic strategy for tinnitus treatment even if the rescue treatment is still proved invalid in some cases. To demonstrate the therapeutic effect of the NMDA receptor blocker on tinnitus, we examined here the spontaneous firing rate (SFR) and the neurochemical dynamics in the auditory cortex (AC) of rats after sodium salicylate (SS) injection, which is a widely used model for tinnitus research. We also recorded their responses to MK-801 treatment. Electrophysiological studies showed that MK-801 significantly suppresses SFR in AC of rats with SS-induced tinnitus. In addition, by using a technique that combining in vivo microdialysis with an online electrochemical system (OECS) and a high-performance liquid chromatography (HPLC), we found that the levels of both glutamate and ascorbate in AC dramatically increased after SS injection and that MK-801 administration attenuated those response. Further studies found that MK-801 given at a time point of 30 min pre- or post-injection of SS were more effective than that given at a time point of 60 min post-SS injection, indicating that the time point of MK-801 intervention has a critical impact on the therapeutic effect. These findings suggest that MK-801 plays a neuroprotective role against hyperactivity during tinnitus induced by SS and that the therapeutic effect depends on the time point of MK-801 intervention, which would advance the studies on understanding of the therapeutic potential of NMDA receptor antagonist in tinnitus therapy.

Atypical Auditory Brainstem Response and Protein Expression Aberrations Related to ASD and Hearing Loss in the Adnp Haploinsufficient Mouse Brain

Autism is a wide spread neurodevelopmental disorder with growing morbidity rates, affecting more boys than girls worldwide. Activity-dependent neuroprotective protein (ADNP) was recently recognized as a leading gene accounted for 0.17% of autism spectrum disorder (ASD) cases globally. Respectively, mutations in the human ADNP gene (ADNP syndrome), cause multi-system body dysfunctions with apparent ASD-related traits, commencing as early as childhood. The Adnp haploinsufficient (Adnp^+/-) mouse model was researched before in relations to Alzheimer's disease and autism. Adnp^+/- mice suffer from deficient social memory, vocal and motor impediments, irregular tooth eruption and short stature, all of which corresponds with reported phenotypes in patients with the ADNP syndrome. Recently, a more elaborated description of the ADNP syndrome was published, presenting impediments such as hearing disabilities in > 10% of the studied children. Irregular auditory brainstem response (ABR) has been connected to ASD-related cases and has been suggested as a potential hallmark for autism, allowing diagnosis of ASD risk and early intervention. Herein, we present detriment hearing in the Adnp^+/- mice with atypical ABR and significant protein expression irregularities that coincides with ASD and hearing loss studies in the brain.

Noise-Induced Hypersensitization of the Acoustic Startle Response in Larval Zebrafish

Overexposure to loud noise is known to lead to deficits in auditory sensitivity and perception. We studied the effects of noise exposure on sensorimotor behaviors of larval (5-7 days post-fertilization) zebrafish (Danio rerio), particularly the auditory-evoked startle response and hearing sensitivity to acoustic startle stimuli. We observed a temporary 10-15 dB decrease in startle response threshold after 18 h of flat-spectrum noise exposure at 20 dB re·1 ms-2. Larval zebrafish also exhibited decreased habituation to startle-inducing stimuli following noise exposure. The noise-induced sensitization was not due to changes in absolute hearing thresholds, but was specific to the auditory-evoked escape responses. The observed noise-induced sensitization was disrupted by AMPA receptor blockade using DNQX, but not NMDA receptor blockade. Together, these experiments suggest a complex effect of noise exposure on the neural circuits mediating auditory-evoked behaviors in larval zebrafish.

Dorsal Cochlear Nucleus Fusiform-cell Plasticity is Altered in Salicylate-induced Tinnitus

Following noise overexposure and tinnitus-induction, fusiform cells of the dorsal cochlear nucleus (DCN) show increased spontaneous firing rates (SFR), increased spontaneous synchrony and altered stimulus-timing-dependent plasticity (StDP), which correlate with behavioral measures of tinnitus. Sodium salicylate, the active ingredient in aspirin, which is commonly used to induce tinnitus, increases SFR and activates NMDA receptors in the ascending auditory pathway. NMDA receptor activation is required for StDP in many brain regions, including the DCN. Blocking NMDA receptors can alter StDP timing rules and decrease synchrony in DCN fusiform cells. Thus, systemic activation of NMDA receptors with sodium salicylate should elicit pathological changes to StDP, thereby increasing SFR and synchrony and induce tinnitus. Herein, we examined the action of salicylate in tinnitus generation in guinea pigs in vivo by measuring tinnitus using two behavioral measures and recording single-unit responses from DCN fusiform cells pre- and post-salicylate administration in the same animals. First, we show that animals administered salicylate show evidence of tinnitus using both behavioral paradigms, cross-validating the tests. Second, fusiform cells in animals with tinnitus showed increased SFR, synchrony and altered StDP timing rules, like animals with noise-induced tinnitus. These findings suggest that alterations to fusiform-cell plasticity are an essential component of tinnitus, regardless of induction technique.

Changes in spontaneous movement in response to silent gaps are not robust enough to indicate the perception of tinnitus in mice

Approaches to identify the perception of tinnitus in various animal models have been difficult to apply to mouse. As a result, mice have been underutilized to investigate the cellular, molecular, and genetic mechanisms underlying tinnitus. A recent study in guinea pigs identified a novel spontaneous behavior (unconditioned response), changes in movement during silent gaps, that identified a subgroup of animals presumably with tinnitus. Guinea pigs identified with tinnitus failed to “freeze” in response to silent gaps in sound. In the hope of developing a rapid and reliable assay for mice, we used a similar approach. C57BL/6J mice underwent three trials in which spontaneous movement was video recorded in the presence of white noise interrupted with six silent gaps. Movement metrics included velocity and body movement. Before the third trial, mice underwent either sham or noise exposure to induce hearing loss and tinnitus. Auditory brainstem responses before and after noise trauma confirmed normal hearing in sham-treated animals and hearing loss in the noise-exposed cohort. No differences in the various movement metrics were detected during the silent gaps either before or after sham/noise exposure. Variability in spontaneous movement both before and after sham/noise exposure was substantially greater in mice compared to guinea pigs. Thus, this assay is not sufficiently statistically powerful to identify changes in movement that might indicate tinnitus perception in mice. Previous observations also reported increased movement overall in guinea pigs identified as suffering tinnitus. In contrast, mice showed no statistically significant differences in movement between the three trials. Despite our results, other unconditioned (as well as conditioned) behaviors should be examined in mice to test their utility to detect changes that indicate the perception of tinnitus. Such assays are essential to accelerate the use of mouse models in tinnitus research.

Sensitivity to sounds in sport-related concussed athletes: a new clinical presentation of hyperacusis

Sensitivity to sounds is one frequent symptom of a sport-related concussion, but its assessment rarely goes beyond a single question. Here we examined sensitivity to sounds using psychoacoustic and psychometric outcomes in athletes beyond the acute phase of injury. Fifty-eight college athletes with normal hearing who either had incurred one or more sport-related concussions (N = 28) or who had never suffered head injury (N = 30) participated. Results indicated that the Concussed group scored higher on the Hyperacusis questionnaire and displayed greater sensitivity to sounds in psychoacoustic tasks compared to the Control group. However, further analyses that separated the Concussed group in subgroups with Sound sensitivity symptom (N = 14) and Without sound sensitivity symptom (N = 14) revealed that athletes with the sound complaint were the ones responsible for the effect: Concussed athletes with self-reported sound sensitivity had lower Loudness Discomfort Thresholds (LDLs), higher Depression and Hyperacusis scores, and shifted loudness growth functions compared to the other subgroup. A simple mediation model disclosed that LDLs exert their influence both directly on Hyperacusis scores as well as indirectly via depressive symptoms. We thus report a new clinical presentation of hyperacusis and discuss possible mechanisms by which it could arise from concussion.

Tinnitus and temporary hearing loss result in differential noise-induced spatial reorganization of brain activity

Loud noise frequently results in hyperacusis or hearing loss (i.e., increased or decreased sensitivity to sound). These conditions are often accompanied by tinnitus (ringing in the ears) and changes in spontaneous neuronal activity (SNA). The ability to differentiate the contributions of hyperacusis and hearing loss to neural correlates of tinnitus has yet to be achieved. Towards this purpose, we used a combination of behavior, electrophysiology, and imaging tools to investigate two models of noise-induced tinnitus (either with temporary hearing loss or with permanent hearing loss). Manganese (Mn2+) uptake was used as a measure of calcium channel function and as an index of SNA. Manganese uptake was examined in vivo with manganese-enhanced magnetic resonance imaging (MEMRI) in key auditory brain regions implicated in tinnitus. Following acoustic trauma, MEMRI, the SNA index, showed evidence of spatially dependent rearrangement of Mn2+ uptake within specific brain nuclei (i.e., reorganization). Reorganization of Mn2+ uptake in the superior olivary complex and cochlear nucleus was dependent upon tinnitus status. However, reorganization of Mn2+ uptake in the inferior colliculus was dependent upon hearing sensitivity. Furthermore, following permanent hearing loss, reduced Mn2+ uptake was observed. Overall, by combining testing for hearing sensitivity, tinnitus, and SNA, our data move forward the possibility of discriminating the contributions of hyperacusis and hearing loss to tinnitus.

Salicylate-Induced Suppression of Electrically Driven Activity in Brain Slices from the Auditory Cortex of Aging Mice

The prevalence of tinnitus is known to increase with age. The age-dependent mechanisms of tinnitus may have important implications for the development of new therapeutic treatments. High doses of salicylate can be used experimentally to induce transient tinnitus and hearing loss. Although accumulating evidence indicates that salicylate induces tinnitus by directly targeting neurons in the peripheral and central auditory systems, the precise effect of salicylate on neural networks in the auditory cortex (AC) is unknown. Here, we examined salicylate-induced changes in stimulus-driven laminar responses of AC slices with salicylate superfusion in young and aged senescence-accelerated-prone (SAMP) and -resistant (SAMR) mice. Of the two strains, SAMP1 is known to be a more suitable model of presbycusis. We recorded stimulus-driven laminar local field potential (LFP) responses at multi sites in AC slice preparations. We found that for all AC slices in the two strains, salicylate always reduced stimulus-driven LFP responses in all layers. However, for the amplitudes of the LFP responses, the two senescence-accelerated mice (SAM) strains showed different laminar properties between the pre- and post-salicylate conditions, reflecting strain-related differences in local circuits. As for the relationships between auditory brainstem response (ABR) thresholds and the LFP amplitude ratios in the pre- vs. post-salicylate condition, we found negative correlations in layers 2/3 and 4 for both older strains, and in layer 5 (L5) in older SAMR1. In contrast, the GABAergic agonist muscimol (MSC) led to positive correlations between ABR thresholds and LFP amplitude ratios in the pre- vs. post-MSC condition in younger SAM mice from both strains. Further, in younger mice, salicylate decreased the firing rate in AC L4 pyramidal neurons. Thus, salicylate can directly reduce neural excitability of L4 pyramidal neurons and thereby influence AC neural circuit activity. That we observed age-dependent effects of salicylate and varied GABAergic sensitivity in the AC among mouse strains with hearing loss implies that potential therapeutic mechanisms for tinnitus may operate differently in young vs. aged subjects. Therefore, scientists developing new therapeutic modalities for tinnitus treatment should consider using both aged and young animals.