Noise overexposure alters long-term somatosensory-auditory processing in the dorsal cochlear nucleus--possible basis for tinnitus-related hyperactivity?

Could DTI Unlock the Mystery of Subjective Tinnitus: It's Time for Parameters That Go A Little Out of the Routine

In this study, it was aimed to assess the microstructural changes in the main central auditory pathway in cases with subjective tinnitus. In total, 101 subjects (52 cases with bilateral subjective non-pulsatile tinnitus and 49 healthy cases as the control group) were included in the study. Participants underwent pure tone audiogram and Diffusion Tensor Imaging-Magnetic Resonance Imaging (DTI-MRI) examination with a 3 Tesla MRI device. The number of tracts, tract length, volume, and quantitative anisotropy (QA) and normalized quantitative anisotropy' (nQA) values were calculated by plotting cochleocortical pathways from the cochlear nerve to ipsilateral and contralateral Heschl's gyrus (HG). In pure tone audiometry, the control group had lower hearing thresholds than cases with tinnitus. Fibres and nQA values from the right cochlear nerve to the right HG were significantly lower in the tinnitus group than in the control group. Cochlear nuclei voxel counts were significantly decreased in the tinnitus group. Both cochlear nucleus volumes were higher in the tinnitus group than in the control group. nQA values in both cochlear nuclei were decreased in the tinnitus group. This study showed that the most commonly affected part in subjective non-pulsatile tinnitus cases is the cochlear nucleus. Therefore, the cochlear nucleus should be evaluated more carefully in cases presenting with subjective tinnitus.

Transcriptional profile changes caused by noise-induced tinnitus in the cochlear nucleus and inferior colliculus of the rat

INTRODUCTION

Tinnitus is a prevalent and disabling condition characterized by the perception of sound in the absence of external acoustic stimuli. The hyperactivity of the auditory pathway is a crucial factor in the development of tinnitus. This study aims to examine genetic expression variations in the dorsal cochlear nucleus (DCN) and inferior colliculus (IC) following the onset of tinnitus using transcriptomic analysis. The goal is to investigate the relationship between hyperactivity in the DCN and IC.

METHODS

To confirm the presence of tinnitus behavior, we utilized the gap pre-pulse inhibition of the acoustic startle (GPIAS) response paradigm. In addition, we conducted auditory brainstem response (ABR) tests to determine the baseline hearing thresholds, and repeated the test one week after subjecting the rats to noise exposure (8-16 kHz, 126 dBHL, 2 h). Samples of tissue were collected from the DCN and IC in both the tinnitus and non-tinnitus groups of rats. We employed RNA sequencing and quantitative PCR techniques to analyze the changes in gene expression between these two groups. This allowed us to identify any specific genes or gene pathways that may be associated with the development or maintenance of tinnitus in the DCN and IC.

RESULTS

Our results demonstrated tinnitus-like behavior in rats exposed to noise, as evidenced by GPIAS measurements. We identified 61 upregulated genes and 189 downregulated genes in the DCN, along with 396 upregulated genes and 195 downregulated genes in the IC. Enrichment analysis of the DCN revealed the involvement of ion transmembrane transport regulation, synaptic transmission, and negative regulation of neuron apoptotic processes in the development of tinnitus. In the IC, the enrichment analysis indicated that glutamatergic synapses and neuroactive ligand-receptor interaction pathways may significantly contribute to the process of tinnitus development. Additionally, protein-protein interaction (PPI) networks were constructed, and 9 hub genes were selected based on their betweenness centrality rank in the DCN and IC, respectively.

CONCLUSIONS

Our findings reveal enrichment of differential expressed genes (DEGs) associated with pathways linked to alterations in neuronal excitability within the DCN and IC when comparing the tinnitus group to the non-tinnitus group. This indicates an increased trend in neuronal excitability within both the DCN and IC in the tinnitus model rats. Additionally, the enriched signaling pathways within the DCN related to changes in synaptic plasticity suggest that the excitability changes may propagate to IC.

NEW AND NOTEWORTHY

Our findings reveal gene expression alterations in neuronal excitability within the DCN and IC when comparing the tinnitus group to the non-tinnitus group at the transcriptome level. Additionally, the enriched signaling pathways related to changes in synaptic plasticity in the differentially expressed genes within the DCN suggest that the excitability changes may propagate to IC.

Hearing loss-related altered neuronal activity in the inferior colliculus

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

Candidate Key Proteins in Tinnitus-A Bioinformatic Study of Synaptic Transmission in the Cochlear Nucleus

The aim of this study was to identify key proteins of synaptic transmission in the cochlear nucleus (CN) that are involved in normal hearing, acoustic stimulation, and tinnitus. A gene list was compiled from the GeneCards database using the keywords "synaptic transmission" AND "tinnitus" AND "cochlear nucleus" (Tin). For comparison, two gene lists with the keywords "auditory perception" (AP) AND "acoustic stimulation" (AcouStim) were built. The STRING protein-protein interaction (PPI) network and the Cytoscape data analyzer were used to identify the top two high-degree proteins (HDPs) and their high-score interaction proteins (HSIPs), together referred to as key proteins. The top1 key proteins of the Tin-process were BDNF, NTRK1, NTRK3, and NTF3; the top2 key proteins are FOS, JUN, CREB1, EGR1, MAPK1, and MAPK3. Highly significant GO terms in CN in tinnitus were "RNA polymerase II transcription factor complex", "late endosome", cellular response to cadmium ion", "cellular response to reactive oxygen species", and "nerve growth factor signaling pathway", indicating changes in vesicle and cell homeostasis. In contrast to the spiral ganglion, where important changes in tinnitus are characterized by processes at the level of cells, important biological changes in the CN take place at the level of synapses and transcription.

Neurodegeneration after repeated noise trauma in the mouse lower auditory pathway

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

Predictive coding and stochastic resonance as fundamental principles of auditory phantom perception

Mechanistic insight is achieved only when experiments are employed to test formal or computational models. Furthermore, in analogy to lesion studies, phantom perception may serve as a vehicle to understand the fundamental processing principles underlying healthy auditory perception. With a special focus on tinnitus-as the prime example of auditory phantom perception-we review recent work at the intersection of artificial intelligence, psychology and neuroscience. In particular, we discuss why everyone with tinnitus suffers from (at least hidden) hearing loss, but not everyone with hearing loss suffers from tinnitus. We argue that intrinsic neural noise is generated and amplified along the auditory pathway as a compensatory mechanism to restore normal hearing based on adaptive stochastic resonance. The neural noise increase can then be misinterpreted as auditory input and perceived as tinnitus. This mechanism can be formalized in the Bayesian brain framework, where the percept (posterior) assimilates a prior prediction (brain's expectations) and likelihood (bottom-up neural signal). A higher mean and lower variance (i.e. enhanced precision) of the likelihood shifts the posterior, evincing a misinterpretation of sensory evidence, which may be further confounded by plastic changes in the brain that underwrite prior predictions. Hence, two fundamental processing principles provide the most explanatory power for the emergence of auditory phantom perceptions: predictive coding as a top-down and adaptive stochastic resonance as a complementary bottom-up mechanism. We conclude that both principles also play a crucial role in healthy auditory perception. Finally, in the context of neuroscience-inspired artificial intelligence, both processing principles may serve to improve contemporary machine learning techniques.

The use of non-invasive brain stimulation in auditory perceptual learning: A review

Auditory perceptual learning is an experience-dependent form of auditory learning that can improve substantially throughout adulthood with practice. A key mechanism associated with perceptual learning is synaptic plasticity. In the last decades, an increasingly better understanding has formed about the neural mechanisms related to auditory perceptual learning. Research in animal models found an association between the functional organization of the primary auditory cortex and frequency discrimination ability. Several studies observed an increase in the area of representation to be associated with improved frequency discrimination. Non-invasive brain stimulation techniques have been related to the promotion of plasticity. Despite its popularity in other fields, non-invasive brain stimulation has not been used much in auditory perceptual learning. The present review has discussed the application of non-invasive brain stimulation methods in auditory perceptual learning by discussing the mechanisms, current evidence and challenges, and future directions.

Sensory gating functions of the auditory thalamus: adaptation and modulations through noise-exposure and high-frequency stimulation in rats

The medial geniculate body (MGB) of the thalamus is an obligatory relay for auditory processing. A breakdown of adaptive filtering and sensory gating at this level may lead to multiple auditory dysfunctions, while high-frequency stimulation (HFS) of the MGB might mitigate aberrant sensory gating. To further investigate the sensory gating functions of the MGB, this study (i) recorded electrophysiological evoked potentials in response to continuous auditory stimulation, and (ii) assessed the effect of MGB HFS on these responses in noise-exposed and control animals. Pure-tone sequences were presented to assess differential sensory gating functions associated with stimulus pitch, grouping (pairing), and temporal regularity. Evoked potentials were recorded from the MGB and acquired before and after HFS (100Hz). All animals (unexposed and noise-exposed, pre- and post-HFS) showed gating for pitch and grouping. Unexposed animals also showed gating for temporal regularity not found in noise-exposed animals. Moreover, only noise-exposed animals showed restoration comparable to the typical EP amplitude suppression pattern following MGB HFS. The current findings confirm adaptive thalamic sensory gating based on different sound characteristics and provide evidence that temporal regularity affects MGB auditory signaling.

Intrinsic Noise Improves Speech Recognition in a Computational Model of the Auditory Pathway

Noise is generally considered to harm information processing performance. However, in the context of stochastic resonance, noise has been shown to improve signal detection of weak sub- threshold signals, and it has been proposed that the brain might actively exploit this phenomenon. Especially within the auditory system, recent studies suggest that intrinsic noise plays a key role in signal processing and might even correspond to increased spontaneous neuronal firing rates observed in early processing stages of the auditory brain stem and cortex after hearing loss. Here we present a computational model of the auditory pathway based on a deep neural network, trained on speech recognition. We simulate different levels of hearing loss and investigate the effect of intrinsic noise. Remarkably, speech recognition after hearing loss actually improves with additional intrinsic noise. This surprising result indicates that intrinsic noise might not only play a crucial role in human auditory processing, but might even be beneficial for contemporary machine learning approaches.

Disturbed Balance of Inhibitory Signaling Links Hearing Loss and Cognition

Neuronal hyperexcitability in the central auditory pathway linked to reduced inhibitory activity is associated with numerous forms of hearing loss, including noise damage, age-dependent hearing loss, and deafness, as well as tinnitus or auditory processing deficits in autism spectrum disorder (ASD). In most cases, the reduced central inhibitory activity and the accompanying hyperexcitability are interpreted as an active compensatory response to the absence of synaptic activity, linked to increased central neural gain control (increased output activity relative to reduced input). We here suggest that hyperexcitability also could be related to an immaturity or impairment of tonic inhibitory strength that typically develops in an activity-dependent process in the ascending auditory pathway with auditory experience. In these cases, high-SR auditory nerve fibers, which are critical for the shortest latencies and lowest sound thresholds, may have either not matured (possibly in congenital deafness or autism) or are dysfunctional (possibly after sudden, stressful auditory trauma or age-dependent hearing loss linked with cognitive decline). Fast auditory processing deficits can occur despite maintained basal hearing. In that case, tonic inhibitory strength is reduced in ascending auditory nuclei, and fast inhibitory parvalbumin positive interneuron (PV-IN) dendrites are diminished in auditory and frontal brain regions. This leads to deficits in central neural gain control linked to hippocampal LTP/LTD deficiencies, cognitive deficits, and unbalanced extra-hypothalamic stress control. Under these conditions, a diminished inhibitory strength may weaken local neuronal coupling to homeostatic vascular responses required for the metabolic support of auditory adjustment processes. We emphasize the need to distinguish these two states of excitatory/inhibitory imbalance in hearing disorders: (i) Under conditions of preserved fast auditory processing and sustained tonic inhibitory strength, an excitatory/inhibitory imbalance following auditory deprivation can maintain precise hearing through a memory linked, transient disinhibition that leads to enhanced spiking fidelity (central neural gain⇑) (ii) Under conditions of critically diminished fast auditory processing and reduced tonic inhibitory strength, hyperexcitability can be part of an increased synchronization over a broader frequency range, linked to reduced spiking reliability (central neural gain⇓). This latter stage mutually reinforces diminished metabolic support for auditory adjustment processes, increasing the risks for canonical dementia syndromes.

Detecting Noise-Induced Cochlear Synaptopathy by Auditory Brainstem Response in Tinnitus Patients With Normal Hearing Thresholds: A Meta-Analysis

Noise-induced cochlear synaptopathy (CS) is defined as a permanent loss of synapses in the auditory nerve pathway following noise exposure. Several studies using auditory brainstem response (ABR) have indicated the presence of CS and increased central gain in tinnitus patients with normal hearing thresholds (TNHT), but the results were inconsistent. This meta-analysis aimed to review the evidence of CS and its pathological changes in the central auditory system in TNHT. Published studies using ABR to study TNHT were reviewed. PubMed, EMBASE, and Scopus databases were selected to search for relevant literature. Studies (489) were retrieved, and 11 were included for meta-analysis. The results supported significantly reduced wave I amplitude in TNHT, whereas the alternations in wave V amplitude were inconsistent among the studies. Consistently increased V/I ratio indicated noise-induced central gain enhancement. The results indicated the evidence of noise-induced cochlear synaptopathy in tinnitus patients with normal hearing. However, inconsistent changes in wave V amplitude may be explained by that the failure of central gain that triggers the pathological neural changes in the central auditory system and/or that increased central gain may be necessary to generate tinnitus but not to maintain tinnitus.

Tinnitus-like "hallucinations" elicited by sensory deprivation in an entropy maximization recurrent neural network

Sensory deprivation has long been known to cause hallucinations or "phantom" sensations, the most common of which is tinnitus induced by hearing loss, affecting 10-20% of the population. An observable hearing loss, causing auditory sensory deprivation over a band of frequencies, is present in over 90% of people with tinnitus. Existing plasticity-based computational models for tinnitus are usually driven by homeostatic mechanisms, modeled to fit phenomenological findings. Here, we use an objective-driven learning algorithm to model an early auditory processing neuronal network, e.g., in the dorsal cochlear nucleus. The learning algorithm maximizes the network's output entropy by learning the feed-forward and recurrent interactions in the model. We show that the connectivity patterns and responses learned by the model display several hallmarks of early auditory neuronal networks. We further demonstrate that attenuation of peripheral inputs drives the recurrent network towards its critical point and transition into a tinnitus-like state. In this state, the network activity resembles responses to genuine inputs even in the absence of external stimulation, namely, it "hallucinates" auditory responses. These findings demonstrate how objective-driven plasticity mechanisms that normally act to optimize the network's input representation can also elicit pathologies such as tinnitus as a result of sensory deprivation.

Using Extracochlear Multichannel Electrical Stimulation to Relieve Tinnitus and Reverse Tinnitus-Related Auditory-Somatosensory Plasticity in the Cochlear Nucleus

OBJECTIVES

Tinnitus has no reliable cure but may be significantly relieved by the usage of cochlear implants. However, not all tinnitus patients necessitate cochlear implantation that can impair hearing. This study was to investigate whether a novel extracochlear electrical stimulation (EES) strategy could relieve tinnitus of guinea pigs without hearing impairment, and the roles of auditory-somatosensory plasticity in the cochlear nucleus in the tinnitus relief.

MATERIALS AND METHODS

We used a novel four-electrode extracochlear implant to electrically stimulate the cochlea of tinnitus guinea pigs. Tinnitus was assessed by the gap-prepulse inhibition of the acoustic startle reflex (GPIAS) ratios and the tinnitus index. The plasticity of auditory and somatosensory innervation in the different subdivisions of cochlear nucleus was evaluated by immunostaining of vesicular glutamate transporter 1 (VGLUT1) and VGLUT2, respectively.

RESULTS

The EES induced significant decreases of GPIAS ratios and the tinnitus index of tinnitus guinea pigs, indicating reductions of tinnitus behavioral manifestations. Meanwhile, the EES reversed the abnormal auditory-somatosensory innervation in the cochlear nucleus of tinnitus animals but did not change the hearing and the numbers of inner hair cell synapses.

CONCLUSIONS

This study demonstrated that the novel EES strategy could effectively relieve tinnitus without impairment to hearing and cochlear structure of tinnitus animals. The reversal of tinnitus-related auditory-somatosensory plasticity in the cochlear nucleus was correlated with the tinnitus relief induced by the EES.

Simulated transient hearing loss improves auditory sensitivity

Recently, it was proposed that a processing principle called adaptive stochastic resonance plays a major role in the auditory system, and serves to maintain optimal sensitivity even to highly variable sound pressure levels. As a side effect, in case of reduced auditory input, such as permanent hearing loss or frequency specific deprivation, this mechanism may eventually lead to the perception of phantom sounds like tinnitus or the Zwicker tone illusion. Using computational modeling, the biological plausibility of this processing principle was already demonstrated. Here, we provide experimental results that further support the stochastic resonance model of auditory perception. In particular, Mongolian gerbils were exposed to moderate intensity, non-damaging long-term notched noise, which mimics hearing loss for frequencies within the notch. Remarkably, the animals developed significantly increased sensitivity, i.e. improved hearing thresholds, for the frequency centered within the notch, but not for frequencies outside the notch. In addition, most animals treated with the new paradigm showed identical behavioral signs of phantom sound perception (tinnitus) as animals with acoustic trauma induced tinnitus. In contrast, animals treated with broadband noise as a control condition did not show any significant threshold change, nor behavioral signs of phantom sound perception.

The Pathological Mechanisms and Treatments of Tinnitus

Tinnitus is defined as the ringing, hissing, clicking or roaring sounds an individual consciously perceives in the absence of an external auditory stimulus. Currently, the literature on the mechanism of tinnitus pathology is multifaceted, ranging from tinnitus generation at the cellular level to its perception at the system level. Cellular level mechanisms include increased neuronal synchrony, neurotransmission changes and maladaptive plasticity. At the system level, the role of auditory structures, non-auditory structures, changes in the functional connectivities in higher regions and tinnitus networks have been investigated. The exploration of all these mechanisms creates a holistic view on understanding the changes the pathophysiology of tinnitus undertakes. Although tinnitus percept may start at the level of cochlear nerve deafferentation, the neuronal changes in the central auditory system to the neuronal and connectivity changes in non-auditory regions, such as the limbic system, become cardinal in chronic tinnitus generation. At the present moment, some tinnitus generation mechanisms are well established (e.g., increased neuronal synchrony) whereas other mechanisms have gained more traction recently (e.g., tinnitus networks, tinnitus-distress networks) and therefore, require additional investigation to solidify their role in tinnitus pathology. The treatments and therapeutics designed for tinnitus are numerous, with varied levels of success. They are generally two-fold: some treatments focus on tinnitus cessation (including cochlear implants, deep brain stimulation, transcranial direct current stimulation and transcranial magnetic stimulation) whereas the other set focuses on tinnitus reduction or masking (including hearing aids, sound therapy, cognitive behavioral therapy, tinnitus retraining therapy, and tailor made notched musical training).  Tinnitus management has focused on implementing tinnitus masking/reducing therapies more than tinnitus cessation, since cessation treatments are still lacking in streamlined treatment protocols and long-term sustainability and efficacy of the treatment. This review will focus on concisely exploring the current and most relevant tinnitus pathophysiology mechanisms, treatments and therapeutics.

Effect of Unilateral Acoustic Trauma on Neuronal Firing Activity in the Inferior Colliculus of Mice

Neural hyperactivity induced by sound exposure often correlates with the development of hyperacusis and/or tinnitus. In laboratory animals, hyperactivity is typically induced by unilateral sound exposure to preserve one ear for further testing of hearing performance. Most ascending fibers in the auditory system cross into the superior olivary complex and then ascend contralaterally. Therefore, unilateral exposure should be expected to mostly affect the contralateral side above the auditory brain stem. On the other hand, it is well known that a significant number of neurons have crossing fibers at every level of the auditory pathway, which may spread the effect of unilateral exposure onto the ipsilateral side. Here we demonstrate that unilateral sound exposure causes development of hyperactivity in both the contra and ipsilateral inferior colliculus in mice. We found that both the spontaneous firing rate and bursting activity were increased significantly compared to unexposed mice. The neurons with characteristic frequencies at or above the center frequency of exposure showed the greatest increase. Surprisingly, this increase was more pronounced in the ipsilateral inferior colliculus. This study highlights the importance of considering both ipsi- and contralateral effects in future studies utilizing unilateral sound exposure.

Role of microRNA‑375‑3p‑mediated regulation in tinnitus development

Changes in the dorsal cochlear nucleus (DCN) following exposure to noise play an important role in the development of tinnitus. As the development of several diseases is known to be associated with microRNAs (miRNAs/miRs), the aim of the present study was to identify the miRNAs that may be implicated in pathogenic changes in the DCN, resulting in tinnitus. A previously developed tinnitus animal model was used for this study. The study consisted of four stages, including identification of candidate miRNAs involved in tinnitus development using miRNA microarray analysis, validation of miRNA expression using reverse transcription-quantitative PCR (RT-qPCR), evaluation of the effects of candidate miRNA overexpression on tinnitus development through injection of a candidate miRNA mimic or mimic negative control, and target prediction of candidate miRNAs using mRNA microarray analysis and western blotting. The miRNA microarray and RT-qPCR analyses revealed that miR-375-3p expression was significantly reduced in the tinnitus group compared with that in the non-tinnitus group. Additionally, miR-375-3p overexpression via injection of miR-375-3p mimic reduced the proportion of animals with persistent tinnitus. Based on mRNA microarray and western blot analyses, connective tissue growth factor (CTG.) was identified as a potential target for miR-375-3p. Thus, it was inferred that CTGF downregulation by miR-375-3p may weaken with the decrease in miRNA expression, and the increased pro-apoptotic activity of CTGF may result in more severe neuronal damage, contributing to tinnitus development. These findings are expected to contribute significantly to the development of a novel therapeutic approach to tinnitus, thereby bringing about a significant breakthrough in the treatment of this potentially debilitating condition.

Modulation of Neuronal Excitability by Low- Intensity Ultrasound in Two Principal Neurons of Rat Anteroventral Cochlear Nucleus

Ultrasonic neuromodulation has proved to be a promising new approach for direct neuromodulation or potential noninvasive deep brain stimulation technology for treating various neurological disorders. Previous studies have demonstrated that ultrasonic waves can noninvasively diffuse through the intact skull and thus precisely target specific brain regions with high spatial resolution. However, its neuromodulatory effects over different cell types of target nuclei have not been fully elucidated. In the present study, we investigated the neuronal excitability resulted from ultrasound stimulation on the two major neurons of anteroventral cochlear nucleus (AVCN) in vitro. Our results demonstrated that bushy cells (BCs) were well maintaining one action potential (AP) in response to the pairing of a sequence of depolarizing current pulses and 60-s continuous low-intensity ultrasound (LIUS), and meanwhile, stellate cells (SCs) significantly increased the firing rate. The ultrasonic waves with an acoustic pressure of 0.13 MPa were elicited by an on-chip ultrasonic stimulation system compatible with patch-clamp recording. Furthermore, LIUS significantly improved the neuronal excitability in both BCs and SCs based on their intrinsic excitability. Modulation of membrane properties among cell types was due to the LIUS-induced increase in the total inward sodium currents ( I_Na ) and outward potassium currents ( I_Kv ). LIUS significantly, at a similar rate, increased the amplitude of total inward sodium currents in both cell types. Meanwhile, LIUS induces a higher rate of the outward potassium currents in the BCs compared with SCs. Therefore, this study could provide new evidence for safe use of ultrasonic neuromodulation and its potential therapy for many auditory diseases, such as the central auditory processing disorder.

The stochastic resonance model of auditory perception: A unified explanation of tinnitus development, Zwicker tone illusion, and residual inhibition

Stochastic resonance (SR) has been proposed to play a major role in auditory perception, and to maintain optimal information transmission from the cochlea to the auditory system. By this, the auditory system could adapt to changes of the auditory input at second or even sub-second timescales. In case of reduced auditory input, somatosensory projections to the dorsal cochlear nucleus would be disinhibited in order to improve hearing thresholds by means of SR. As a side effect, the increased somatosensory input corresponding to the observed tinnitus-associated neuronal hyperactivity is then perceived as tinnitus. In addition, the model can also explain transient phantom tone perceptions occurring after ear plugging, or the Zwicker tone illusion. Vice versa, the model predicts that via stimulation with acoustic noise, SR would not be needed to optimize information transmission, and hence somatosensory noise would be tuned down, resulting in a transient vanishing of tinnitus, an effect referred to as residual inhibition.

Using Cortical Neuron Markers to Target Cells in the Dorsal Cochlear Nucleus

The dorsal cochlear nucleus (DCN) is a region of particular interest for auditory and tinnitus research. However, lack of useful genetic markers for in vivo manipulations hinders elucidation of the DCN contribution to tinnitus pathophysiology. This work assesses whether adeno-associated viral vectors (AAV) containing the calcium/calmodulin-dependent protein kinase 2α (CaMKIIα) promoter and a mouse line of nicotinic acetylcholine receptor α2 subunit (Chrna2)-Cre can target specific DCN populations. We found that CaMKIIα cannot be used to target excitatory fusiform DCN neurons as labeled cells showed diverse morphology indicating they belong to different classes of DCN neurons. Light stimulation after driving Channelrhodopsin2 (ChR2) by the CaMKIIα promoter generated spikes in some units but firing rate decreased when light stimulation coincided with sound. Expression and activation of CaMKIIα-eArchaerhodopsin3.0 in the DCN produced inhibition in some units but sound-driven spikes were delayed by concomitant light stimulation. We explored the existence of Cre+ cells in the DCN of Chrna2-Cre mice by hydrogel embedding technique (CLARITY). There were almost no Cre+ cell bodies in the DCN; however, we identified profuse projections arising from the ventral cochlear nucleus (VCN). Anterograde labeling in the VCN revealed projections to the ipsilateral superior olive and contralateral medial nucleus of the trapezoid body (MNTB; bushy cells), and a second bundle terminating in the DCN, suggesting the latter to be excitatory Chrna2+ T-stellate cells. Exciting Chrna2+ cells increased DCN firing. This work shows that cortical molecular tools may be useful for manipulating the DCN especially for tinnitus studies.

Auditory Noise Leads to Increased Visual Brain-Computer Interface Performance: A Cross-Modal Study

Noise has been proven to have a beneficial role in non-linear systems, including the human brain, based on the stochastic resonance (SR) theory. Several studies have been implemented on single-modal SR. Cross-modal SR phenomenon has been confirmed in different human sensory systems. In our study, a cross-modal SR enhanced brain–computer interface (BCI) was proposed by applying auditory noise to visual stimuli. Fast Fourier transform and canonical correlation analysis methods were used to evaluate the influence of noise, results of which indicated that a moderate amount of auditory noise could enhance periodic components in visual responses. Directed transfer function was applied to investigate the functional connectivity patterns, and the flow gain value was used to measure the degree of activation of specific brain regions in the information transmission process. The results of flow gain maps showed that moderate intensity of auditory noise activated the brain area to a greater extent. Further analysis by weighted phase-lag index (wPLI) revealed that the phase synchronization between visual and auditory regions under auditory noise was significantly enhanced. Our study confirms the existence of cross-modal SR between visual and auditory regions and achieves a higher accuracy for recognition, along with shorter time window length. Such findings can be used to improve the performance of visual BCIs to a certain extent.

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.

Bimodal Auditory Electrical Stimulation for the Treatment of Tinnitus: Preclinical and Clinical Studies

Tinnitus, or the phantom perception of sound, arises from pathological neural activity. Neurophysiological research has shown increased spontaneous firing rates and synchronization along the auditory pathway correlate strongly with behavioral measures of tinnitus. Auditory neurons are plastic, enabling external stimuli to be utilized to elicit long-term changes to spontaneous firing and synchrony. Pathological plasticity can thus be reversed using bimodal auditory plus nonauditory stimulation to reduce tinnitus. This chapter discusses preclinical and clinical evidence for efficacy of bimodal stimulation treatments of tinnitus, with highlights on sham-controlled, double-blinded clinical trials. The results from these studies have shown some efficacy in reducing the severity of tinnitus, based on subjective and objective outcome measures including tinnitus questionnaires and psychophysical tinnitus measurements. While results of some studies have been positive, the degree of benefit and the populations that respond to treatment vary across the studies. Directions and implications of future studies are discussed.

Tinnitus and Brain Stimulation

The pathophysiological mechanisms that underlie the generation and maintenance of tinnitus are being unraveled progressively. Based on this knowledge, a large variety of different neuromodulatory interventions have been developed and are still being designed, adapting to the progressive mechanistic insights in the pathophysiology of tinnitus. rTMS targeting the temporal, temporoparietal, and the frontal cortex has been the mainstay of non-invasive neuromodulation. Yet, the evidence is still unclear, and therefore systematic meta-analyses are needed for drawing conclusions on the effectiveness of rTMS in chronic tinnitus. Different forms of transcranial electrical stimulation (tDCS, tACS, tRNS), applied over the frontal and temporal cortex, have been investigated in tinnitus patients, also without robust evidence for universal efficacy. Cortex and deep brain stimulation with implanted electrodes have shown benefit, yet there is insufficient data to support their routine clinical use. Recently, bimodal stimulation approaches have revealed promising results and it appears that targeting different sensory modalities in temporally combined manners may be more promising than single target approaches.While most neuromodulatory approaches seem promising, further research is required to help translating the scientific outcomes into routine clinical practice.

Ventral cochlear nucleus bushy cells encode hyperacusis in guinea pigs

Psychophysical studies characterize hyperacusis as increased loudness growth over a wide-frequency range, decreased tolerance to loud sounds and reduced behavioral reaction time latencies to high-intensity sounds. While commonly associated with hearing loss, hyperacusis can also occur without hearing loss, implicating the central nervous system in the generation of hyperacusis. Previous studies suggest that ventral cochlear nucleus bushy cells may be putative neural contributors to hyperacusis. Compared to other ventral cochlear nucleus output neurons, bushy cells show high firing rates as well as lower and less variable first-spike latencies at suprathreshold intensities. Following cochlear damage, bushy cells show increased spontaneous firing rates across a wide-frequency range, suggesting that they might also show increased sound-evoked responses and reduced latencies to higher-intensity sounds. However, no studies have examined bushy cells in relationship to hyperacusis. Herein, we test the hypothesis that bushy cells may contribute to the neural basis of hyperacusis by employing noise-overexposure and single-unit electrophysiology. We find that bushy cells exhibit hyperacusis-like neural firing patterns, which are comprised of enhanced sound-driven firing rates, reduced first-spike latencies and wideband increases in excitability.

Age-Dependent Auditory Processing Deficits after Cochlear Synaptopathy Depend on Auditory Nerve Latency and the Ability of the Brain to Recruit LTP/BDNF

Age-related decoupling of auditory nerve fibers from hair cells (cochlear synaptopathy) has been linked to temporal processing deficits and impaired speech recognition performance. The link between both is elusive. We have previously demonstrated that cochlear synaptopathy, if centrally compensated through enhanced input/output function (neural gain), can prevent age-dependent temporal discrimination loss. It was also found that central neural gain after acoustic trauma was linked to hippocampal long-term potentiation (LTP) and upregulation of brain-derived neurotrophic factor (BDNF). Using middle-aged and old BDNF-live-exon-visualization (BLEV) reporter mice we analyzed the specific recruitment of LTP and the activity-dependent usage of Bdnf exon-IV and -VI promoters relative to cochlear synaptopathy and central (temporal) processing. For both groups, specimens with higher or lower ability to centrally compensate diminished auditory nerve activity were found. Strikingly, low compensating mouse groups differed from high compensators by prolonged auditory nerve latency. Moreover, low compensators exhibited attenuated responses to amplitude-modulated tones, and a reduction of hippocampal LTP and Bdnf transcript levels in comparison to high compensators. These results suggest that latency of auditory nerve processing, recruitment of hippocampal LTP, and Bdnf transcription, are key factors for age-dependent auditory processing deficits, rather than cochlear synaptopathy or aging per se.

Lidocaine injections to the otic ganglion for the treatment of tinnitus-A pilot study

Tinnitus is defined as the perception of noise without an external acoustic stimulus. Due to the heterogeneity of tinnitus, no treatment has proven equally beneficial to every single of these patients. Previous studies have shown that trigeminal input can interfere with the perception of tinnitus. Therefore, we aimed to explore the therapeutic potential of lidocaine injections in trigeminal structures. We conducted a pilot study with 19 participants to explore feasibility and tolerability of this approach. The intervention consisted of three injections of lidocaine in the anatomical area of the trigeminal ganglion and the ganglion oticum via an oral approach corresponding to the affected side of tinnitus. We performed an assessment that included the Mini-TQ, CGII, and numeric rating scales of tinnitus loudness and severity at different time points over a follow-up period of 12 weeks. In addition to changes of subjective tinnitus complaints, potential adverse events were documented. Patients were treated at the Centre of Otorhinolaryngology in Traunstein, Germany, and data were analyzed at the University of Regensburg, Germany. We did not observe any relevant side effects. There was a significant reduction of tinnitus distress (Mini-TQ, Tinnitus severity) and loudness (measured subjectively) over time. Our pilot data suggests that lidocaine injections targeting trigeminal structures may be able to reduce tinnitus complaints. Future studies should investigate the effects of lidocaine injections in placebo-controlled trials in an extended sample size to further explore the potential benefits of this therapeutic approach on tinnitus.

Interaction of auditory and pain pathways: Effects of stimulus intensity, hearing loss and opioid signaling

Moderate intensity sounds can reduce pain sensitivity (i.e., audio-analgesia) whereas intense sounds can induce aural pain, evidence of multisensory interaction between auditory and pain pathways. To explore auditory-pain pathway interactions, we used the tail-flick (TF) test to assess thermal tail-pain sensitivity by measuring the latency of a rat to remove its tail from 52 °C water. In Experiment 1, TF latencies were measured in ambient noise and broadband noise (BBN) presented from 80 to 120 dB SPL. TF latencies gradually increased from ambient to 90 dB SPL (audio-analgesia), but then declined. At 120 dB, TF latencies were significantly shorter than normal, evidence for audio-hyperalgesia near the aural threshold for pain. In Experiment II, the opioid pain pathway was modified by treating rats with a high dose of fentanyl known to induce post-treatment hyperalgesia. TF latencies in ambient noise were normal 10-days post-fentanyl. However, TF latencies became shorter than normal from 90 to 110 dB indicating that fentanyl pre-treatment had converted audio-analgesia to audio-hyperalgesia. In Experiment III, we tested the hypothesis that hearing loss could alter pain sensitivity by unilaterally exposing rats to an intense noise that induced a significant hearing loss. TF latencies in ambient noise gradually declined from 1- to 4-weeks post-exposure indicating that noise-induced hearing loss had increased pain sensitivity. Our results suggest that auditory and pain pathways interact in ways that depend on intensity, hearing loss and opioid pain signaling, results potentially relevant to pain hyperacusis.

Transient Conductive Hearing Loss Regulates Cross-Modal VGLUT Expression in the Cochlear Nucleus of C57BL/6 Mice

Auditory nerve fibers synapse onto the cochlear nucleus (CN) and are labeled using the vesicular glutamate transporter-1 (VGLUT-1), whereas non-auditory inputs are labeled using the VGLUT-2. However, the underlying regulatory mechanism of VGLUT expression in the CN remains unknown. We examined whether a sound level decrease, without primary neural damage, induces cellular and VGLUT expression change in the CN, and examined the potential for neural plasticity of the CN using unilateral conductive hearing loss models. We inserted earplugs in 8-week-old mice unilaterally for 4 weeks and subsequently removed them for another 4 weeks. Although the threshold of an auditory brainstem response significantly increased across all tested frequencies following earplug insertion, it completely recovered after earplug removal. Auditory deprivation had no significant impact on spiral ganglion and ventral CN (VCN) neurons’ survival. Conversely, although the cell size and VGLUT-1 expression in the VCN significantly decreased after earplug insertion, VGLUT-2 expression in the granule cell lamina significantly increased. These cell sizes decreased and the alterations in VGLUT-1 and -2 expression almost completely recovered at 1 month after earplug removal. Our results suggested that the cell size and VGLUT expression in the CN have a neuroplasticity capacity, which is regulated by increases and decreases in sound levels. Restoration of the sound levels might partly prevent cell size decrease and maintain VGLUT expression in the CN.

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.

A proof-of-concept study comparing tinnitus and neural connectivity changes following multisensory perceptual training with and without a low-dose of fluoxetine

Background. This proof-of-concept study investigated a method of multisensory perceptual training for tinnitus, and whether a short, low-dose administration of fluoxetine enhanced training effects and changed neural connectivity.Methods. A double-blind, randomized placebo controlled design with 20 participants (17 male, 3 female, mean age = 57.1 years) involved 30 min daily computer-based, multisensory training (matching visual, auditory and tactile stimuli to perception of tinnitus) for 20 days, and random allocation to take 20 mg fluoxetine or placebo daily. Behavioral measures of tinnitus and correlations between pairs of a priori regions of interest (ROIs), obtained using resting-state functional magnetic resonance imaging (rs-fMRI), were performed before and after the training.Results. Significant changes in ratings of tinnitus loudness, annoyance, and problem were observed with training. No statistically significant changes in Tinnitus Functional Index, Tinnitus Handicap Inventory or Depression Anxiety Stress Scales were found with training. Fluoxetine did not alter any of the behavioural outcomes of training compared to placebo. Significant changes in connectivity between ROIs were identified with training; sensory and attention neural network ROI changes correlated with significant tinnitus rating changes. Rs-fMRI results suggested that the direction of functional connectivity changes between auditory and non-auditory networks, with training and fluoxetine, were opposite to the direction of those changes with multisensory training and placebo.Conclusions. Improvements in tinnitus measures were correlated with changes in sensory and attention networks. The results provide preliminary evidence for changes in rs-fMRI accompanying a multisensory training method in persons with tinnitus.

Single-Session of Combined tDCS-TMS May Increase Therapeutic Effects in Subjects With Tinnitus

To treat motor and psychiatric disorders, transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS) are used in clinics worldwide. We combined these two types of neuromodulation technique to increase the effective response of a single session of neuromodulation in subjective tinnitus. Eighty tinnitus subjects were split into four different treatment groups: tDCS, tDCS with sham TMS, tDCS-TMS, and TMS group. Subjects were given 1.5 mA tDCS on the bi-frontal area and TMS stimulated the contralateral single side of the temporo-parietal cortex with 200 pulses at 1 Hz stimulation. Comparing pre-treatment questionnaire scores to post-treatment questionnaire scores, all four groups showed statistically significant improvements. Although there was no significant difference among group comparison, the largest mean difference was shown in the combined group, especially for tinnitus intensity and tinnitus-related distress. Responders in the combined group were the highest for VAS intensity, with a maximum of 80% of twenty subjects. To summarize, dual-neuromodulation responders could consist of responders of frontal tDCS and temporal TMS. In addition, abnormal activity in the frontal or temporal area of the responders is presumed to be modulated by treatment and will be suggested as the target areas in future studies.

Separate auditory pathways for the induction and maintenance of tinnitus and hyperacusis?

Tinnitus and hyperacusis often occur together, however tinnitus may occur without hyperacusis or hyperacusis without tinnitus. Based on animal research one could argue that hyperacusis results from noise exposures that increase central gain in the lemniscal, tonotopically organized, pathways, whereas tinnitus requires increased burst firing and neural synchrony in the extra-lemniscal pathway. However, these substrates are not sufficient and require involvement of the central nervous system. The dominant factors in changing cortical networks in tinnitus patients are foremost the degree and type of hearing loss, and comorbidities such as distress and mood. So far, no definite changes have been established for tinnitus proper, albeit that changes in connectivity between the dorsal attention network and the parahippocampal area, as well as the default-mode network-precuneus decoupling, appear to be strong candidates. I conclude that there is still a strong need for further integrating animal and human research into tinnitus and hyperacusis.

A 7 Tesla fMRI investigation of human tinnitus percept in cortical and subcortical auditory areas

Tinnitus is a clinical condition defined by hearing a sound in the absence of an objective source. Early experiments in animal models have suggested that tinnitus stems from an alteration of processing in the auditory system. However, translating these results to humans has proven challenging. One limiting factor has been the insufficient spatial resolution of non-invasive measurement techniques to investigate responses in subcortical auditory nuclei, like the inferior colliculus and the medial geniculate body (MGB). Here we employed ultra-high field functional magnetic resonance imaging (UHF-fMRI) at 7 Tesla to investigate the frequency-specific processing in sub-cortical and cortical regions in a cohort of six tinnitus patients and six hearing loss matched controls. We used task-based fMRI to perform tonotopic mapping and compared the magnitude and tuning of frequency-specific responses between the two groups. Additionally, we used resting-state fMRI to investigate the functional connectivity. Our results indicate frequency-unspecific reductions in the selectivity of frequency tuning that start at the level of the MGB and continue in the auditory cortex, as well as reduced thalamocortical and cortico-cortical connectivity with tinnitus. These findings suggest that tinnitus may be associated with reduced inhibition in the auditory pathway, potentially leading to increased neural noise and reduced functional connectivity. Moreover, these results indicate the relevance of high spatial resolution UHF-fMRI for the investigation of the role of sub-cortical auditory regions in tinnitus.

Mechanisms of Noise-Induced Tinnitus: Insights from Cellular Studies

Tinnitus, sound perception in the absence of physical stimuli, occurs in 15% of the population and is the top-reported disability for soldiers after combat. Noise overexposure is a major factor associated with tinnitus but does not always lead to tinnitus. Furthermore, people with normal audiograms can get tinnitus. In animal models, equivalent cochlear damage occurs in animals with and without behavioral evidence of tinnitus. But cochlear-nerve-recipient neurons in the brainstem demonstrate distinct, synchronized spontaneous firing patterns only in animals that develop tinnitus, driving activity in central brain regions and ultimately giving rise to phantom perception. Examining tinnitus-specific changes in single-cell populations enables us to begin to distinguish neural changes due to tinnitus from those that are due to hearing loss.

Axonal sprouting in the dorsal cochlear nucleus affects gap‑prepulse inhibition following noise exposure

One of the primary theories of the pathogenesis of tinnitus involves maladaptive auditory-somatosensory plasticity in the dorsal cochlear nucleus (DCN), which is assumed to be due to axonal sprouting. Although a disrupted balance between auditory and somatosensory inputs may occur following hearing damage and may induce tinnitus, examination of this phenomenon employed a model of hearing damage that does not account for the causal relationship between these changes and tinnitus. The present study aimed to investigate changes in auditory-somatosensory innervation and the role that axonal sprouting serves in this process by comparing results between animals with and without tinnitus. Rats were exposed to a noise-inducing temporary threshold shift and were subsequently divided into tinnitus and non-tinnitus groups based on the results of gap prepulse inhibition of the acoustic startle reflex. DCNs were collected from rats divided into three sub-groups according to the number of weeks (1, 2 or 3) following noise exposure, and the protein levels of vesicular glutamate transporter 1 (VGLUT1), which is associated with auditory input to the DCN, and VGLUT2, which is in turn primarily associated with somatosensory inputs, were assessed. In addition, factors related to axonal sprouting, including growth-associated protein 43 (GAP43), postsynaptic density protein 95, synaptophysin, α-thalassemia/mental retardation syndrome X-linked homolog (ATRX), growth differentiation factor 10 (GDF10), and leucine-rich repeat and immunoglobulin domain-containing 1, were measured by western blot analyses. Compared to the non-tinnitus group, the tinnitus group exhibited a significant decrease in VGLUT1 at 1 week and a significant increase in VGLUT2 at 3 weeks post-exposure. In addition, rats in the tinnitus group exhibited significant increases in GAP43 and GDF10 protein expression levels in their DCN at 3 weeks following noise exposure. Results from the present study provided further evidence that changes in the neural input distribution to the DCN may cause tinnitus and that axonal sprouting underlies these alterations.

Neural plasticity and its initiating conditions in tinnitus

BACKGROUND AND OBJECTIVE

Deafferentation caused by cochlear pathology (which can be hidden from the audiogram) activates forms of neural plasticity in auditory pathways, generating tinnitus and its associated conditions including hyperacusis. This article discusses tinnitus mechanisms and suggests how these mechanisms may relate to those involved in normal auditory information processing.

MATERIALS AND METHODS

Research findings from animal models of tinnitus and from electromagnetic imaging of tinnitus patients are reviewed which pertain to the role of deafferentation and neural plasticity in tinnitus and hyperacusis.

RESULTS

Auditory neurons compensate for deafferentation by increasing their input/output functions (gain) at multiple levels of the auditory system. Forms of homeostatic plasticity are believed to be responsible for this neural change, which increases the spontaneous and driven activity of neurons in central auditory structures in animals expressing behavioral evidence of tinnitus. Another tinnitus correlate, increased neural synchrony among the affected neurons, is forged by spike-timing-dependent neural plasticity in auditory pathways. Slow oscillations generated by bursting thalamic neurons verified in tinnitus animals appear to modulate neural plasticity in the cortex, integrating tinnitus neural activity with information in brain regions supporting memory, emotion, and consciousness which exhibit increased metabolic activity in tinnitus patients.

DISCUSSION AND CONCLUSION

The latter process may be induced by transient auditory events in normal processing but it persists in tinnitus, driven by phantom signals from the auditory pathway. Several tinnitus therapies attempt to suppress tinnitus through plasticity, but repeated sessions will likely be needed to prevent tinnitus activity from returning owing to deafferentation as its initiating condition.

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

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

Pathophysiology of Subjective Tinnitus: Triggers and Maintenance

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

Glutamatergic Projections to the Cochlear Nucleus are Redistributed in Tinnitus

Tinnitus alters auditory-somatosensory plasticity in the cochlear nucleus (CN). Correspondingly, bimodal auditory-somatosensory stimulation treatment attenuates tinnitus, both in animals and humans (Marks et al., 2018). Therefore, we hypothesized that tinnitus is associated with altered somatosensory innervation of the CN. Here, we studied the expression of vesicular glutamate transporters 1 and 2 (VGLUT1 and VGLUT2) in the CN, which reveals glutamatergic projections from the cochlea as well as somatosensory systems to this brainstem auditory center. Guinea pigs were unilaterally exposed to narrowband noise and behaviorally tested for tinnitus using gap-prepulse inhibition of the acoustic startle. Following physiological and behavioral measures, brain sections were immunohistochemically stained for VGLUT1 or VGLUT2. Puncta density was determined for each region of the ipsilateral and contralateral CN. Tinnitus was associated with an ipsilateral upregulation of VGLUT2 puncta density in the granule cell domain (GCD) and anteroventral CN (AVCN). Furthermore, there was a tinnitus-associated interaural asymmetry for VGLUT1 expression in the AVCN and deep layer of the dorsal CN (DCN3), due to contralateral downregulation of VGLUT1 expression. These tinnitus-related glutamatergic imbalances were reversed upon bimodal stimulation treatment. Tinnitus-associated ipsilateral upregulation of VGLUT2-positive projections likely derives from somatosensory projections to the GCD and AVCN. This upregulation may underlie the neurophysiological hallmarks of tinnitus in the CN. Reversing the increased ipsilateral glutamatergic innervation in the CN is likely a key mechanism in treating tinnitus.

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.

Central Compensation in Auditory Brainstem after Damaging Noise Exposure

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

Small Arms Fire-like noise: Effects on Hearing Loss, Gap Detection and the Influence of Preventive Treatment

A noise-induced loss of inner hair cell (IHC) - auditory nerve synaptic connections has been suggested as a factor that can trigger the progression of maladaptive plastic changes leading to noise-induced tinnitus. The present study used a military relevant small arms fire (SAF)-like noise (50 biphasic impulses over 2.5 min at 152 dB SPL given unilaterally to the right ear) to induce loss (∼1/3) of IHC synaptic ribbons (associated with synapse loss) in rat cochleae with only minor (less than 10%) loss of outer hair cells. Approximately half of the noise-exposed rats showed poorer Gap Detection post-noise, a behavioral indication suggesting the presence of tinnitus. There was significantly greater loss of IHC ribbons in noise-exposed rats with reduced Gap Detection compared to noise-exposed rats retaining normal Gap Detection. We have previously shown systemic administration of piribedil, memantine, and/or ACEMg significantly reduced loss of IHC ribbons induced by a 3 h 4 kHz octave band 117 dB (SPL) noise. The present study examined if this treatment would also reduce ribbon loss from the SAF-like noise exposure and if this would prevent the reduced Gap Detection. As in the previous study, piribedil, memantine, and ACEMg treatment significantly reduced the noise-induced loss of ribbons, such that it was no longer significantly different from normal. However, it did not prevent development of the reduced Gap Detection indication of tinnitus in all treated noise-exposed rats, reducing the incidence but not reaching significance.

Losartan Prevents Maladaptive Auditory-Somatosensory Plasticity After Hearing Loss via Transforming Growth Factor-β Signaling Suppression

OBJECTIVES

Hearing loss disrupts the balance of auditory-somatosensory inputs in the cochlear nucleus (CN) of the brainstem, which has been suggested to be a mechanism of tinnitus. This disruption results from maladaptive auditory-somatosensory plasticity, which is a form of axonal sprouting. Axonal sprouting is promoted by transforming growth factor (TGF)-β signaling, which can be inhibited by losartan. We investigated whether losartan prevents maladaptive auditory-somatosensory plasticity after hearing loss.

METHODS

The study consisted of two stages: determining the time course of auditory-somatosensory plasticity following hearing loss and preventing auditory-somatosensory plasticity using losartan. In the first stage, rats were randomly divided into two groups: a control group that underwent a sham operation and a deaf group that underwent cochlea ablation on the left side. CNs were harvested 1 and 2 weeks after surgery. In the second stage, rats were randomly divided into either a saline group that underwent cochlear ablation on the left side and received normal saline or a losartan group that underwent cochlear ablation on the left side and received losartan. CNs were harvested 2 weeks after surgery. Hearing was estimated with auditory brainstem responses (ABRs). Western blotting was performed for vesicular glutamate transporter 1 (VGLUT1), reflecting auditory input; vesicular glutamate transporter 2 (VGLUT2), reflecting somatosensory input; growth-associated protein 43 (GAP-43), reflecting axonal sprouting; and p-Smad2/3.

RESULTS

Baseline ABR thresholds before surgery ranged from 20 to 35 dB sound pressure level. After cochlear ablation, ABR thresholds were higher than 80 dB. In the first experiment, VGLUT2/VGLUT1 ratios did not differ significantly between the control and deaf groups 1 week after surgery. At 2 weeks after surgery, the deaf group had a significantly higher VGLUT2/VGLUT1 ratio compared to the control group. In the second experiment, the losartan group had a significantly lower VGLUT2/VGLUT1 ratio along with significantly lower p-Smad3 and GAP-43 levels compared to the saline group.

CONCLUSION

Losartan might prevent axonal sprouting after hearing loss by blocking TGF-β signaling thereby preventing maladaptive auditory-somatosensory plasticity.

Serotonergic Modulation of Sensory Representation in a Central Multisensory Circuit Is Pathway Specific

Many studies have explored how neuromodulators affect synaptic function, yet little is known about how they modify computations at the microcircuit level. In the dorsal cochlear nucleus (DCN), a region that integrates auditory and multisensory inputs from two distinct pathways, serotonin (5-HT) enhances excitability of principal cells, predicting a generalized reduction in sensory thresholds. Surprisingly, we found that when looked at from the circuit level, 5-HT enhances signaling only from the multisensory input, while decreasing input from auditory fibers. This effect is only partially explained by an action on auditory nerve terminals. Rather, 5-HT biases processing for one input pathway by simultaneously enhancing excitability in the principal cell and in a pathway-specific feed-forward inhibitory interneuron. Thus, by acting on multiple targets, 5-HT orchestrates a fundamental shift in representation of convergent auditory and multisensory pathways, enhancing the potency of non-auditory signals in a classical auditory pathway.

Spatial hearing ability of the pigmented Guinea pig (Cavia porcellus): Minimum audible angle and spatial release from masking in azimuth

Despite the common use of guinea pigs in investigations of the neural mechanisms of binaural and spatial hearing, their behavioral capabilities in spatial hearing tasks have surprisingly not been thoroughly investigated. To begin to fill this void, we tested the spatial hearing of adult male guinea pigs in several experiments using a paradigm based on the prepulse inhibition (PPI) of the acoustic startle response. In the first experiment, we presented continuous broadband noise from one speaker location and switched to a second speaker location (the "prepulse") along the azimuth prior to presenting a brief, ∼110 dB SPL startle-eliciting stimulus. We found that the startle response amplitude was systematically reduced for larger changes in speaker swap angle (i.e., greater PPI), indicating that using the speaker "swap" paradigm is sufficient to assess stimulus detection of spatially separated sounds. In a second set of experiments, we swapped low- and high-pass noise across the midline to estimate their ability to utilize interaural time- and level-difference cues, respectively. The results reveal that guinea pigs can utilize both binaural cues to discriminate azimuthal sound sources. A third set of experiments examined spatial release from masking using a continuous broadband noise masker and a broadband chirp signal, both presented concurrently at various speaker locations. In general, animals displayed an increase in startle amplitude (i.e., lower PPI) when the masker was presented at speaker locations near that of the chirp signal, and reduced startle amplitudes (increased PPI) indicating lower detection thresholds when the noise was presented from more distant speaker locations. In summary, these results indicate that guinea pigs can: 1) discriminate changes in source location within a hemifield as well as across the midline, 2) discriminate sources of low- and high-pass sounds, demonstrating that they can effectively utilize both low-frequency interaural time and high-frequency level difference sound localization cues, and 3) utilize spatial release from masking to discriminate sound sources. This report confirms the guinea pig as a suitable spatial hearing model and reinforces prior estimates of guinea pig hearing ability from acoustical and physiological measurements.