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

Developmental hearing loss-induced perceptual deficits are rescued by genetic restoration of cortical inhibition

Even a transient period of hearing loss during the developmental critical period can induce long-lasting deficits in temporal and spectral perception. These perceptual deficits correlate with speech perception in humans. In gerbils, these hearing loss-induced perceptual deficits are correlated with a reduction of both ionotropic GABAA and metabotropic GABAB receptor-mediated synaptic inhibition in auditory cortex, but most research on critical period plasticity has focused on GABAA receptors. Therefore, we developed viral vectors to express proteins that would upregulate gerbil postsynaptic inhibitory receptor subunits (GABAA, Gabra1; GABAB, Gabbr1b) in pyramidal neurons, and an enzyme that mediates GABA synthesis (GAD65) presynaptically in parvalbumin-expressing interneurons. A transient period of developmental hearing loss during the auditory critical period significantly impaired perceptual performance on two auditory tasks: amplitude modulation depth detection and spectral modulation depth detection. We then tested the capacity of each vector to restore perceptual performance on these auditory tasks. While both GABA receptor vectors increased the amplitude of cortical inhibitory postsynaptic potentials, only viral expression of postsynaptic GABAB receptors improved perceptual thresholds to control levels. Similarly, presynaptic GAD65 expression improved perceptual performance on spectral modulation detection. These findings suggest that recovering performance on auditory perceptual tasks depends on GABAB receptor-dependent transmission at the auditory cortex parvalbumin to pyramidal synapse and point to potential therapeutic targets for developmental sensory disorders.

Tinnitus-related increases in single-unit activity in awake rat auditory cortex correlate with tinnitus behavior

Tinnitus is known to affect 10-15 % of the population, severely impacting 1-2 % of those afflicted. Canonically, tinnitus is generally a consequence of peripheral auditory damage resulting in maladaptive plastic changes in excitatory/inhibitory homeostasis at multiple levels of the central auditory pathway as well as changes in diverse nonauditory structures. Animal studies of primary auditory cortex (A1) generally find tinnitus-related changes in excitability across A1 layers and differences between inhibitory neuronal subtypes. Changes due to sound-exposure include changes in spontaneous activity, cross-columnar synchrony, bursting and tonotopic organization. Few studies in A1 directly correlate tinnitus-related changes in neural activity to an individual animal's behavioral evidence of tinnitus. The present study used an established condition-suppression sound-exposure model of chronic tinnitus and recorded spontaneous and driven single-unit responses from A1 layers 5 and 6 of awake Long-Evans rats. A1 units recorded from animals with behavioral evidence of tinnitus showed significant increases in spontaneous and sound-evoked activity which directly correlated to the animal's tinnitus score. Significant increases in the number of bursting units, the number of bursts/minute and burst duration were seen for A1 units recorded from animals with behavioral evidence of tinnitus. The present A1 findings support prior unit recording studies in auditory thalamus and recent in vitro findings in this same animal model. The present findings are consistent with sensory cortical studies showing tinnitus- and neuropathic pain-related down-regulation of inhibition and increased excitation based on plastic neurotransmitter and potassium channel changes. Reducing A1 deep-layer tinnitus-related hyperactivity is a potential target for tinnitus pharmacotherapy.

Neurodiagnostic and neurotherapeutic potential of graphene nanomaterials

Graphene has emerged as a highly promising nanomaterial for a variety of advanced technologies, including batteries, energy, electronics, and biotechnologies. Its recent contribution to neurotechnology is particularly noteworthy because its superior conductivity, chemical resilience, biocompatibility, thermal stability, and scalable nature make it well-suited for measuring brain activity and plasticity in health and disease. Graphene-mediated compounds are microfabricated in two central methods: chemical processes with natural graphite and chemical vapor deposition of graphene in a film form. They are widely used as biosensors and bioelectronics for neurodiagnostic and neurotherapeutic purposes in several brain disorders, such as Parkinson's disease, stroke, glioma, epilepsy, tinnitus, and Alzheimer's disease. This review provides an overview of studies that have demonstrated the technical advances of graphene nanomaterials in neuroscientific and clinical applications. We also discuss current limitations and future demands in relation to the clinical application of graphene, highlighting its potential technological and clinical significance for treating brain disorders. Our review underscores the potential of graphene nanomaterials as powerful tools for advancing the understanding of the brain and developing new therapeutic strategies.

Tinnitus pitch does not always fall within the frequency range of hearing loss - a cross-sectional study on the mechanism of tinnitus production

BACKGROUND

Clinically, we find that tinnitus patients often have hearing loss. According to the most accepted mechanism of tinnitus, that is, the spontaneous discharge and abnormal synchronization of neurons after afferent reduction, tinnitus frequency is closely related to the frequency of hearing loss.

OBJECTIVE

The purpose of this study was to investigate the correlation of tinnitus pitch with the frequency of hearing loss.

MATERIALS AND METHODS

A total of 500 patients with unilateral or bilateral chronic tinnitus were enrolled in this study. All patients underwent pure tone audiometry (PTA) and tinnitus acoustic examination. Hearing loss levels and frequencies were recorded. The relationship between tinnitus pitch and hearing loss level and frequency was statistically analyzed.

RESULTS

Our results showed that 96.6% of the 500 tinnitus patients had hearing loss. Statistical analysis showed that low frequency (LF) tinnitus was correlated with LF hearing loss, but moderate frequency & high frequency (MF&HF) tinnitus was not significantly associated with MF&HF hearing loss. The coincidence of tinnitus pitch with the highest hearing threshold correlated with the degree of hearing loss.

CONCLUSION AND SIGNIFICANCE

The vast majority of patients with chronic subjective tinnitus had hearing loss, and the frequency of tinnitus correlated with the degree and frequency of hearing loss but not exactly fall within the frequency range of hearing loss.

The human pupil and face encode sound affect and provide objective signatures of tinnitus and auditory hypersensitivity disorders

Sound is jointly processed along acoustic and emotional dimensions. These dimensions can become distorted and entangled in persons with sensory disorders, producing a spectrum of loudness hypersensitivity, phantom percepts, and - in some cases - debilitating sound aversion. Here, we looked for objective signatures of disordered hearing (DH) in the human face. Pupil dilations and micro facial movement amplitudes scaled with sound valence in neurotypical listeners but not DH participants with chronic tinnitus (phantom ringing) and sound sensitivity. In DH participants, emotionally evocative sounds elicited abnormally large pupil dilations but blunted and invariant facial reactions that jointly provided an accurate prediction of individual tinnitus and hyperacusis questionnaire handicap scores. By contrast, EEG measures of central auditory gain identified steeper neural response growth functions but no association with symptom severity. These findings highlight dysregulated affective sound processing in persons with bothersome tinnitus and sound sensitivity disorders and introduce approaches for their objective measurement.

Mice prone to tinnitus after acoustic trauma show increased pre-exposure sensitivity to background noise

Noise-induced tinnitus is generally associated with hearing impairment caused by traumatic acoustic overexposure. Previous studies in laboratory animals and human subjects, however, have observed differences in tinnitus susceptibility, even among individuals with similar hearing loss. The mechanisms underlying increased sensitivity or, conversely, resistance to tinnitus are still incompletely understood. Here, we used behavioral tests and ABR audiometry to compare the sound-evoked responses of mice that differed in the presence of noise-induced tinnitus. The aim was to find a specific pre-exposure neurophysiological marker that would predict the development of tinnitus after acoustic trauma. Noise-exposed mice were screened for tinnitus-like behavior with the GPIAS paradigm and subsequently divided into tinnitus (+T) and non-tinnitus (-T) groups. Both groups showed hearing loss after exposure, manifested by elevated audiometric thresholds along with reduced amplitudes and prolonged latencies of ABR waves. Prior to exposure, except for a slightly increased slope of growth function for ABR amplitudes in +T mice, the two groups did not show significant audiometric differences. Behavioral measures, such as the magnitude of the acoustic startle response and its inhibition by gap pre-pulse, were also similar before exposure in both groups. However, +T mice showed significantly increased suppression of the acoustic startle response in the presence of background noise of moderate intensity. Thus, increased modulation of startle by background sounds may represent a behavioral correlate of susceptibility to noise-induced tinnitus, and its measurement may form the basis of a simple non-invasive method for predicting tinnitus development in laboratory rodents.

Cortical surface plasticity promotes map remodeling and alleviates tinnitus in adult mice

Tinnitus induced by hearing loss is caused primarily by irreversible damage to the peripheral auditory system, which results in abnormal neural responses and frequency map disruption in the central auditory system. It remains unclear whether and how electrical rehabilitation of the auditory cortex can alleviate tinnitus. We hypothesize that stimulation of the cortical surface can alleviate tinnitus by enhancing neural responses and promoting frequency map reorganization. To test this hypothesis, we assessed and activated cortical maps using our newly designed graphene-based electrode array with a noise-induced tinnitus animal model. We found that cortical surface stimulation increased cortical activity, reshaped sensory maps, and alleviated hearing loss-induced tinnitus behavior in adult mice. These effects were likely due to retained long-term synaptic potentiation capabilities, as shown in cortical slices from the mice model. These findings suggest that cortical surface activation can be used to facilitate practical functional recovery from phantom percepts induced by sensory deprivation. They also provide a working principle for various treatment methods that involve electrical rehabilitation of the cortex.

An MR spectroscopy study of temporal areas excluding primary auditory cortex and frontal regions in subjective bilateral and unilateral tinnitus

Previous studies indicate changes in neurotransmission along the auditory pathway in subjective tinnitus. Most authors, however, investigated brain regions including the primary auditory cortex, whose physiology can be affected by concurrent hearing deficits. In the present MR spectroscopy study we assumed increased levels of glutamate and glutamine (Glx), and other Central Nervous System metabolites in the temporal lobe outside the primary auditory cortex, in a region involved in conscious auditory perception and memory. We studied 52 participants with unilateral (n = 24) and bilateral (n = 28) tinnitus, and a control group without tinnitus (n = 25), all with no severe hearing losses and a similar hearing profile. None of the metabolite levels in the temporal regions of interest were found related to tinnitus status or laterality. Unexpectedly, we found a tendency of increased concentration of Glx in the control left medial frontal region in bilateral vs unilateral tinnitus. Slightly elevated depressive and anxiety symptoms were also shown in participants with tinnitus, as compared to healthy individuals, with the bilateral tinnitus group marginally more affected. We discuss no apparent effect in the temporal lobes, as well as the role of frontal brain areas, with respect to hearing loss, attention and psychological well-being in chronic tinnitus. We furthermore elaborate on the design-related and technical obstacles of MR spectroscopy.

Increased pyramidal and VIP neuronal excitability in rat primary auditory cortex directly correlates with tinnitus behaviour

Tinnitus affects roughly 15%-20% of the population while severely impacting 10% of those afflicted. Tinnitus pathology is multifactorial, generally initiated by damage to the auditory periphery, resulting in a cascade of maladaptive plastic changes at multiple levels of the central auditory neuraxis as well as limbic and non-auditory cortical centres. Using a well-established condition-suppression model of tinnitus, we measured tinnitus-related changes in the microcircuits of excitatory/inhibitory neurons onto layer 5 pyramidal neurons (PNs), as well as changes in the excitability of vasoactive intestinal peptide (VIP) neurons in primary auditory cortex (A1). Patch-clamp recordings from PNs in A1 slices showed tinnitus-related increases in spontaneous excitatory postsynaptic currents (sEPSCs) and decreases in spontaneous inhibitory postsynaptic currents (sIPSCs). Both measures could be correlated to the rat's behavioural evidence of tinnitus. Tinnitus-related changes in PN excitability were independent of changes in A1 excitatory or inhibitory cell numbers. VIP neurons, part of an A1 local circuit that can control the excitation of layer 5 PNs via disinhibitory mechanisms, showed significant tinnitus-related increases in excitability that directly correlated with the rat's behavioural tinnitus score. That PN and VIP changes directly correlated to tinnitus behaviour suggests an important role in A1 tinnitus pathology. Tinnitus-related A1 changes were similar to findings in studies of neuropathic pain in somatosensory cortex suggesting a common pathology of these troublesome perceptual impairments. Improved understanding between excitatory, inhibitory and disinhibitory sensory cortical circuits can serve as a model for testing therapeutic approaches to the treatment of tinnitus and chronic pain. KEY POINTS: We identified tinnitus-related changes in synaptic function of specific neuronal subtypes in a reliable animal model of tinnitus. The findings show direct and indirect tinnitus-related losses of normal inhibitory function at A1 layer 5 pyramidal cells, and increased VIP excitability. The findings are similar to what has been shown for neuropathic pain suggesting that restoring normal inhibitory function at synaptic inputs onto A1 pyramidal neurons (PNs) could conceptually reduce tinnitus discomfort.

Transcriptional Profile Changes after Noise-Induced Tinnitus in Rats

Tinnitus is an unpleasant symptom characterized by detective hearing without the actual sound input. Despite numerous studies elucidating a variety of pathomechanisms inducing tinnitus, the pathophysiology of tinnitus is not fully understood. The genes that are closely associated with this subtype of the auditory hallucination that could be utilized as potential treatment targets are still unknown. In this study, we explored the transcriptional profile changes of the auditory cortex after noise-induced tinnitus in rats using high throughput sequencing and verification of the detected genes using quantitative PCR (qPCR). Tinnitus models were established by analyzing startle behaviors through gap pre-pulse inhibition (PPI) of the acoustic startle. Two hundred and fifty-nine differential genes were identified, of which 162 genes were up-regulated and 97 genes were down-regulated. Analysis of the pathway enrichment indicated that the tinnitus group exhibited increased gene expression related to neurodegenerative disorders such as Huntington’s disease and Amyotrophic lateral sclerosis. Based on the identified genes, networks of protein–protein interaction were established and five hub genes were identified through degree rank, including Fos, Nr4a1, Nr4a3, Egr2, and Egr3. Therein, the Fos gene ranked first with the highest degree after noise exposure, and may be a potential target for the modulation of noise-induced tinnitus.

Post-Mortem Analysis of Neuropathological Changes in Human Tinnitus

Tinnitus is the phantom perception of a sound, often accompanied by increased anxiety and depressive symptoms. Degenerative or inflammatory processes, as well as changes in monoaminergic systems, have been suggested as potential underlying mechanisms. Herein, we conducted the first post-mortem histopathological assessment to reveal detailed structural changes in tinnitus patients’ auditory and non-auditory brain regions. Tissue blocks containing the medial geniculate body (MGB), thalamic reticular nucleus (TRN), central part of the inferior colliculus (CIC), and dorsal and obscurus raphe nuclei (DRN and ROb) were obtained from tinnitus patients and matched controls. Cell density and size were assessed in Nissl-stained sections. Astrocytes and microglia were assessed using immunohistochemistry. The DRN was stained using antibodies raised against phenylalanine hydroxylase-8 (PH8) and tyrosine-hydroxylase (TH) to visualize serotonergic and dopaminergic cells, respectively. Cell density in the MGB and CIC of tinnitus patients was reduced, accompanied by a reduction in the number of astrocytes in the CIC only. Quantification of cell surface size did not reveal any significant difference in any of the investigated brain regions between groups. The number of PH8-positive cells was reduced in the DRN and ROb of tinnitus patients compared to controls, while the number of TH-positive cells remained unchanged in the DRN. These findings suggest that both neurodegenerative and inflammatory processes in the MGB and CIC underlie the neuropathology of tinnitus. Moreover, the reduced number of serotonergic cell bodies in tinnitus cases points toward a potential role of the raphe serotonergic system in tinnitus.

Lower glutamate and GABA levels in auditory cortex of tinnitus patients: a 2D-JPRESS MR spectroscopy study

We performed magnetic resonance spectroscopy (MRS) on healthy individuals with tinnitus and no hearing loss (n = 16) vs. a matched control group (n = 17) to further elucidate the role of excitatory and inhibitory neurotransmitters in tinnitus. Two-dimensional J-resolved spectroscopy (2D-JPRESS) was applied to disentangle Glutamate (Glu) from Glutamine and to estimate GABA levels in two bilateral voxels in the primary auditory cortex. Results indicated a lower Glu concentration (large effect) in right auditory cortex and lower GABA concentration (medium effect) in the left auditory cortex of the tinnitus group. Within the tinnitus group, Glu levels positively correlated with tinnitus loudness measures. While the GABA difference between groups is in line with former findings and theories about a dysfunctional auditory inhibition system in tinnitus, the novel finding of reduced Glu levels came as a surprise and is discussed in the context of a putative framework of inhibitory mechanisms related to Glu throughout the auditory pathway. Longitudinal or interventional studies could shed more light on interactions and causality of Glu and GABA in tinnitus neurochemistry.

Correlation of Electrophysiological and Gene Transcriptional Dysfunctions in Single Cortical Parvalbumin Neurons After Noise Trauma

Parvalbumin-expressing (PV+) interneurons in the sensory cortex form powerful inhibitory synapses on the perisomatic compartments and axon initial segments of excitatory principal neurons (PNs), and perform diverse computational functions. Impaired PV+ interneuron functions have been reported in neural developmental and degenerative disorders. Expression of the unique marker parvalbumin (PV) is often used as a proxy of PV+ interneuron functions. However, it is not entirely clear how PV expression is correlated with PV+ interneuron properties such as spike firing and synaptic transmission. To address this question, we characterized electrophysiological properties of PV+ interneurons in the primary auditory cortex (AI) using whole-cell patch clamp recording, and analyzed the expression of several genes in samples collected from single neurons using the patch pipettes. We found that, after noise induced hearing loss (NIHL), the spike frequency adaptation increased, and the expression of PV, glutamate decarboxylase 67 (GAD67) and Shaw-like potassium channel (KV3.1) decreased in PV+ neurons. In samples prepared from the auditory cortical tissue, the mRNA levels of the target genes were all pairwise correlated. At the single neuron level, however, the expression of PV was significantly correlated with the expression of GAD67, but not KV3.1, maximal spike frequency, or spike frequency adaptation. The expression of KV3.1 was correlated with spike frequency adaptation, but not with the expression of GAD67. These results suggest separate transcriptional regulations of PV/GAD67 vs. KV3.1, both of which are modulated by NIHL.

Subjective tinnitus: lesion-induced pathological central homeostasis remodeling

Subjective tinnitus is the most common type of tinnitus, which is the manifestation of pathological activities in the brain. It happens in a substantial portion of the general population and brings significant burden to the society. Severe subjective tinnitus can lead to depression and insomnia and severely affects patients' quality of life. However, due to poor understanding of its etiology and pathogenesis, treatment of subjective tinnitus remains challenging. In recent decades, a growing number of studies have shown that subjective tinnitus is related to lesion-induced neural plasticity of auditory and non-auditory central systems. This article reviews cellular mechanisms of neural plasticity in subjective tinnitus to provide further understanding of its pathogenesis.

Effect of Sodium Salicylate on Calcium Currents and Exocytosis in Cochlear Inner Hair Cells: Implications for Tinnitus Generation

Sodium salicylate is an anti-inflammatory medication with a side-effect of tinnitus. Here, we used mouse cochlear cultures to explore the effects of salicylate treatment on cochlear inner hair cells (IHCs). We found that IHCs showed significant damage after exposure to a high concentration of salicylate. Whole-cell patch clamp recordings showed that 1-5 mmol/L salicylate did not affect the exocytosis of IHCs, indicating that IHCs are not involved in tinnitus generation by enhancing their neuronal input. Instead, salicylate induced a larger peak amplitude, a more negative half-activation voltage, and a steeper slope factor of Ca²⁺ current. Using noise analysis of Ca²⁺ tail currents and qRT-PCR, we further found that salicylate increased the number of Ca²⁺ channels along with Ca_V1.3 expression. All these changes could act synergistically to enhance the Ca²⁺ influx into IHCs. Inhibition of intracellular Ca²⁺ overload significantly attenuated IHC death after 10 mmol/L salicylate treatment. These results implicate a cellular mechanism for tinnitus generation in the peripheral auditory system.

What's the buzz? The neuroscience and the treatment of tinnitus

Tinnitus is a pervasive public health issue that affects ∼15% of the United States population. Similar estimates have also been shown on a global scale, with similar prevalence found in Europe, Asia, and Africa. The severity of tinnitus is heterogeneous, ranging from mildly bothersome to extremely disruptive. In the United States, ∼10-20% of individuals who experience tinnitus report symptoms that severely reduce their quality of life. Due to the huge personal and societal burden, in the last 20 yr a concerted effort on basic and clinical research has significantly advanced our understanding and treatment of this disorder. Yet, neither full understanding, nor cure exists. We know that tinnitus is the persistent involuntary phantom percept of internally generated nonverbal indistinct noises and tones, which in most cases is initiated by acquired hearing loss and maintained only when this loss is coupled with distinct neuronal changes in auditory and extra-auditory brain networks. Yet, the exact mechanisms and patterns of neural activity that are necessary and sufficient for the perceptual generation and maintenance of tinnitus remain incompletely understood. Combinations of animal model and human research will be essential in filling these gaps. Nevertheless, the existing progress in investigating the neurophysiological mechanisms has improved current treatment and highlighted novel targets for drug development and clinical trials. The aim of this review is to thoroughly discuss the current state of human and animal tinnitus research, outline current challenges, and highlight new and exciting research opportunities.

Noise-induced hippocampal oxidative imbalance and aminoacidergic neurotransmitters alterations in developing male rats: Influence of enriched environment during adolescence

Living in big cities might involuntarily expose people to high levels of noise causing auditory and/or extra-auditory impairments, including adverse effects on central nervous system (CNS) areas such as the hippocampus. In particular, CNS development is a very complex process that can be altered by environmental stimuli. We have previously shown that noise exposure of developing rats can induce hippocampal-related behavioral alterations. However, noise-induced biochemical alterations had not been studied yet. Thus, the aim of this work was to assess whether early noise exposure can affect rat hippocampal oxidative state and aminoacidergic neurotransmission tone. Additionally, the effectiveness of an enriched environment (EE) as a neuroprotective strategy was evaluated. Male Wistar rats were exposed to different noise schemes at 7 or 15 days after birth. Upon weaning, some animals were transferred to an EE whereas others were kept in standard cages. Short- and long-term measurements were performed to evaluate reactive oxygen species, thioredoxins levels and catalase activity as indicators of hippocampal oxidative status as well as glutamic acid decarboxylase and a subtype of glutamate transporter to evaluate aminoacidergic neurotransmission tone. Results showed noise-induced changes in hippocampal oxidative state and aminoacidergic neurotransmission markers that lasted until adolescence and differed according to the scheme and the age of exposure. Finally, EE housing was effective in preventing some of these changes. These findings suggest that CNS development seems to be sensitive to the effects of stressors such as noise, as well as those of an environmental stimulation, favoring prompt and lasting molecular changes.

Acute and Long-Term Circuit-Level Effects in the Auditory Cortex After Sound Trauma

Harmful environmental sounds are a prevailing source of chronic hearing impairments, including noise induced hearing loss, hyperacusis, or tinnitus. How these symptoms are related to pathophysiological damage to the sensory receptor epithelia and its effects along the auditory pathway, have been documented in numerous studies. An open question concerns the temporal evolution of maladaptive changes after damage and their manifestation in the balance of thalamocortical and corticocortical input to the auditory cortex (ACx). To address these issues, we investigated the loci of plastic reorganizations across the tonotopic axis of the auditory cortex of male Mongolian gerbils (Meriones unguiculatus) acutely after a sound trauma and after several weeks. We used a residual current-source density analysis to dissociate adaptations of intracolumnar input and horizontally relayed corticocortical input to synaptic populations across cortical layers in ACx. A pure tone-based sound trauma caused acute changes of subcortical inputs and corticocortical inputs at all tonotopic regions, particularly showing a broad reduction of tone-evoked inputs at tonotopic regions around the trauma frequency. At other cortical sites, the overall columnar activity acutely decreased, while relative contributions of lateral corticocortical inputs increased. After 4-6 weeks, cortical activity in response to the altered sensory inputs showed a general increase of local thalamocortical input reaching levels higher than before the trauma. Hence, our results suggest a detailed mechanism for overcompensation of altered frequency input in the auditory cortex that relies on a changing balance of thalamocortical and intracortical input and along the frequency gradient of the cortical tonotopic map.

Neuroinflammation and Tinnitus

Neuroinflammation is the central nervous system's response to: injury, infection, and abnormal neural activity. Inflammatory processes are known to mediate many diseases, and recently evidence indicates that neuroinflammation underlies hearing disorders such as presbyacusis, middle-ear disease, ototoxicity, noise-induced hearing loss, and tinnitus. This chapter provides a review of the role of neuroinflammation in the etiology and treatment of tinnitus. Specifically, our research team has demonstrated that both tumor necrosis factor alpha (TNF-α) and calpain signaling pathways are involved in noise-induced tinnitus and that blocking them yielded therapeutic effects on tinnitus. Other efforts such as controlling acute inflammatory response via specialized pro-resolving mediators may help provide insight into preventing and treating tinnitus-related inflammatory processes.

Animal Models of Tinnitus Treatment: Cochlear and Brain Stimulation

Neuromodulation, via stimulation of a variety of peripheral and central structures, is used to suppress tinnitus. However, investigative limitations in humans due to ethical reasons have made it difficult to decipher the mechanisms underlying treatment-induced tinnitus relief, so a number of animal models have arisen to address these unknowns. This chapter reviews animal models of cochlear and brain stimulation and assesses their modulatory effects on behavioral evidence of tinnitus and its related neural correlates. When a structure is stimulated, localized modulation, often presenting as downregulation of spontaneous neuronal spike firing rate, bursting and neurosynchrony, occurs within the brain area. Through anatomical projections and transmitter pathways, the interventions activate both auditory- and non-auditory structures by taking bottom-up ascending and top-down descending modes to influence their target brain structures. Furthermore, it is the brain oscillations that cochlear or brain stimulation evoke and connect the prefrontal cortex, striatal systems, and other limbic structures to refresh neural networks and relieve auditory, attentive, conscious, as well as emotional reactive aspects of tinnitus. This oscillatory neural network connectivity is achieved via the thalamocorticothalamic circuitry including the lemniscal and non-lemniscal auditory brain structures. Beyond existing technologies, the review also reveals opportunities for developing advanced animal models using new modalities to achieve precision neuromodulation and tinnitus abatement, such as optogenetic cochlear and/or brain stimulation.

Resistance to noise-induced gap detection impairment in FVB mice is correlated with reduced neuroinflammatory response and parvalbumin-positive neuron loss

Exposure to loud noises results in neuroinflammatory responses in the central auditory pathway. Noise-induced neuroinflammation is implicated in auditory processing deficits such as impairment in gap detection. In this study, we examined whether strain differences between the FVB and C57BL/6 mice in noise-induced impairment in gap detection are correlated with strain differences in neuroinflammatory responses. We found that noise induced more robust TNF-α expression in C57BL/6 than in FVB mice. Noise-induced microglial deramification was observed in C57BL/6 mice, but not in FVB mice. Furthermore, noise exposure resulted in a reduction in parvalbumin-positive (PV+) neuron density in the C57BL/6 mice, but not in FVB mice. These results suggest that neuroinflammatory responses and loss of PV+ neurons may contribute to strain differences in noise-induced impairment in gap detection.

Neuroglial activation in the auditory cortex and medial geniculate body of salicylate-induced tinnitus rats

Neuroglial activation has been recognized as a pathological hallmark of a variety of neurological diseases, yet the role of neuroglia in tinnitus hasn't been well established so far. To explore the potential roles of two types of glia cells (astrocyte and microglia) in the development of tinnitus, we examined markers associated with them in the primary auditory (A1) cortex and medial geniculate body (MGB) of rats with salicylate-induced tinnitus. The results demonstrated that acute and chronic administrations of salicylate could cause reversible tinnitus-like behavior in rats. The expression level of GFAP markedly increased in the A1 cortex of rats following acute and chronic treatments of salicylate, accompanied by increased endpoint and process length of astrocyte. The expression level of GFAP and the morphology of astrocyte in the rat MGB remained almost constant following salicylate treatment. On the other hand, the expression level of Iba1 markedly increased in the rat A1 cortex and MGB following acute and chronic treatments of salicylate, together with increased endpoint and process length of microglia in the MGB. Additionally, interleukin 1β (IL-1β), a pro-inflammatory cytokine released by activated glia was significantly up-regulated in the A1 cortex and MGB of rats after salicylate treatments. These findings highlight astrocyte activation and microglia proliferation in the central auditory system of rats experiencing tinnitus, which potently implicate an indispensable glial regulation in tinnitus development.

Diffusible Tumor Necrosis Factor-Alpha (TNF-α) Promotes Noise-Induced Parvalbumin-Positive (PV+) Neuron Loss and Auditory Processing Impairments

Neuroinflammation has been implicated in noise-induced auditory processing disorder and tinnitus. Certain non-auditory neurological disorders can also increase the levels of proinflammatory cytokines in the brain. To investigate the impact of increased brain proinflammatory cytokine levels on the central auditory pathway, we infused recombinant TNF-α into the right lateral cerebral ventricle, and examined auditory processing and cytoarchitecture of the auditory cortex. Microglial deramification was observed in the auditory cortex of mice that had received both TNF-α infusion and exposure to an 86-dB noise, but not in mice that had received either TNF-α infusion or noise exposure alone. In addition, we observed reduced cortical PV+ neuron density and impaired performances in gap detection and prepulse inhibition (PPI) only in mice that received both TNF-α infusion and the noise exposure. These results suggest that disease-related increase in brain proinflammatory cytokine release could be a risk factor for noise-induced auditory processing disorder and tinnitus.

Auditory Neural Plasticity in Tinnitus Mechanisms and Management

Tinnitus, which is the perception of sound in the absence of a corresponding external acoustic stimulus, including change of hearing and neural plasticity, has become an increasingly important ailment affecting the daily life of a considerable proportion of the population and causing significant burdens for both the affected individuals and society as a whole. Here, we briefly review the epidemiology and classification of tinnitus, and the currently available treatments are discussed in terms of the available evidence for their mechanisms and efficacy. The conclusion drawn from the available evidence is that there is no specific medication for tinnitus treatment at present, and tinnitus management might provide better solutions. Therapeutic interventions for tinnitus should be based on a comprehensive understanding of the etiology and features of individual cases of tinnitus, and more high quality and large-scale research studies are urgently needed to develop more efficacious medications.

Regulation of auditory plasticity during critical periods and following hearing loss

Sensory input has profound effects on neuronal organization and sensory maps in the brain. The mechanisms regulating plasticity of the auditory pathway have been revealed by examining the consequences of altered auditory input during both developmental critical periods—when plasticity facilitates the optimization of neural circuits in concert with the external environment—and in adulthood—when hearing loss is linked to the generation of tinnitus. In this review, we summarize research identifying the molecular, cellular, and circuit-level mechanisms regulating neuronal organization and tonotopic map plasticity during developmental critical periods and in adulthood. These mechanisms are shared in both the juvenile and adult brain and along the length of the auditory pathway, where they serve to regulate disinhibitory networks, synaptic structure and function, as well as structural barriers to plasticity. Regulation of plasticity also involves both neuromodulatory circuits, which link plasticity with learning and attention, as well as ascending and descending auditory circuits, which link the auditory cortex and lower structures. Further work identifying the interplay of molecular and cellular mechanisms associating hearing loss-induced plasticity with tinnitus will continue to advance our understanding of this disorder and lead to new approaches to its treatment.

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During CPs, brain plasticity is enhanced and sensitive to acoustic experience.

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Enhanced plasticity can be reinstated in the adult brain following hearing loss.

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Molecular, cellular, and circuit-level mechanisms regulate CP and adult plasticity.

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Plasticity resulting from hearing loss may contribute to the emergence of tinnitus.

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Modifying plasticity in the adult brain may offer new treatments for tinnitus.