Identifying tinnitus-related genes based on a side-effect network analysis

Tinnitus: Clinical Insights in Its Pathophysiology-A Perspective

Tinnitus, the perception of sound without a corresponding external sound source, and tinnitus disorder, which is tinnitus with associated suffering, present a multifaceted clinical challenge due to its heterogeneity and its incompletely understood pathophysiology and especially due to the limited therapeutic options. In this narrative review, we give an overview on various clinical aspects of tinnitus including its heterogeneity, contributing factors, comorbidities and therapeutic pathways with a specific emphasis on the implications for its pathophysiology and future research directions. Tinnitus exhibits high perceptual variability between affected individuals (heterogeneity) and within affected individuals (temporal variability). Hearing loss emerges as predominant risk factor and the perceived pitch corresponds to areas of hearing loss, supporting the compensatory response theory. Whereas most people who have tinnitus can live a normal life, in 10-20% tinnitus interferes severely with quality of life. These patients suffer frequently from comorbidities such as anxiety, depression or insomnia, acting as both risk factors and consequences. Accordingly, neuroimaging studies demonstrate shared brain networks between tinnitus and stress-related disorders shedding light on the intricate interplay of mental health and tinnitus. The challenge lies in deciphering causative relationships and shared pathophysiological mechanisms. Stress, external sounds, time of day, head movements, distraction, and sleep quality can impact tinnitus perception. Understanding these factors provides insights into the interplay with autonomic, sensory, motor, and cognitive processes. Counselling and cognitive-behavioural therapy demonstrate efficacy in reducing suffering, supporting the involvement of stress and anxiety-related networks. Hearing improvement, especially through cochlear implants, reduces tinnitus and thus indirectly validates the compensatory nature of tinnitus. Brain stimulation techniques can modulate the suffering of tinnitus, presumably by alteration of stress-related brain networks. Continued research is crucial for unravelling the complexities of tinnitus. Progress in management hinges on decoding diverse manifestations, identifying treatment-responsive subtypes, and advancing targeted therapeutic approaches.

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

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

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

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

Abnormal Functional Connectivity Within Default Mode Network and Salience Network Related to Tinnitus Severity

BACKGROUND

Previous studies have demonstrated that tinnitus is associated with neural changes in the cerebral cortex. This study is aimed at investigating the central nervous characteristics of tinnitus patients with different severity by using a rs-EEG.

PARTICIPANTS AND METHODS

rs-EEG was recorded in fifty-seven patients with chronic tinnitus and twenty-seven healthy controls. Tinnitus patients were divided into moderate-to-severe tinnitus group and slight-to-mild tinnitus group based on their Tinnitus Handicap Inventory (THI) scores. Source localization and functional connectivity analyses were used to measure the changes in central levels and examine the altered network patterns. The correlation between functional connectivity and tinnitus severity was analyzed.

RESULT

Compared to the healthy controls, all tinnitus patients showed significant activation in the auditory cortex (middle temporal lobe, BA 21), while moderate-to-severe tinnitus group showed enhanced connectivity between the parahippocampus and posterior cingulate gyrus. Moreover, the moderate-to-severe tinnitus group had enhanced functional connectivity between auditory cortex and insula compared to the slight-to-mild tinnitus group. The connections between the insula and the parahippocampal and posterior cingulate gyrus were positively correlated with THI scores.

CONCLUSION

The current study reveals that patients with moderate-to-severe tinnitus demonstrate greater changes in the central brain areas, including the auditory cortex, insula, parahippocampus and posterior cingulate gyrus. In addition, enhanced connections were found between the insula and the auditory cortex, as well as the posterior cingulate gyrus and the parahippocampus, which suggests abnormality in the auditory network, salience network, and default mode network. Specifically, the insula is the core region of the neural pathway that is composed of the auditory cortex, insula, and parahippocampus/posterior cingulate gyrus. This suggests that the severity of tinnitus is affected by multiple brain regions.

The α9α10 nicotinic acetylcholine receptor: a compelling drug target for hearing loss?

INTRODUCTION

Hearing loss is a major health problem, impacting education, communication, interpersonal relationships, and mental health. Drugs that prevent or restore hearing are lacking and hence novel drug targets are sought. There is the possibility of targeting the α9α10 nicotinic acetylcholine receptor (nAChR) in the prevention of noise-induced, hidden hearing loss and presbycusis. This receptor mediates synaptic transmission between medial olivocochlear efferent fibers and cochlear outer hair cells. This target is key since enhanced olivocochlear activity prevents noise-induced hearing loss and delays presbycusis.

AREAS COVERED

The work examines the α9α10 nicotinic acetylcholine receptor (nAChR), its role in noise-induced, hidden hearing loss and presbycusis and the possibility of targeting. Data has been searched in Pubmed, the World Report on Hearing from the World Health Organization and the Global Burden of Disease Study 2019.

EXPERT OPINION

The design of positive allosteric modulators of α9α10 nAChRs is proposed because of the advantage of reinforcing the medial olivocochlear (MOC)-hair cell endogenous neurotransmission without directly stimulating the target receptors, therefore avoiding receptor desensitization and reduced efficacy. The time is right for the discovery and development of α9α10 nAChRs targeting agents and high throughput screening assays will support this.

Multidisciplinary Tinnitus Research: Challenges and Future Directions From the Perspective of Early Stage Researchers

Tinnitus can be a burdensome condition on both individual and societal levels. Many aspects of this condition remain elusive, including its underlying mechanisms, ultimately hindering the development of a cure. Interdisciplinary approaches are required to overcome long-established research challenges. This review summarizes current knowledge in various tinnitus-relevant research fields including tinnitus generating mechanisms, heterogeneity, epidemiology, assessment, and treatment development, in an effort to highlight the main challenges and provide suggestions for future research to overcome them. Four common themes across different areas were identified as future research direction: (1) Further establishment of multicenter and multidisciplinary collaborations; (2) Systematic reviews and syntheses of existing knowledge; (3) Standardization of research methods including tinnitus assessment, data acquisition, and data analysis protocols; (4) The design of studies with large sample sizes and the creation of large tinnitus-specific databases that would allow in-depth exploration of tinnitus heterogeneity.

Emerging Topics in the Behavioral Neuroscience of Tinnitus

This volume has highlighted the many recent advances in tinnitus theory, models, diagnostics, therapies, and therapeutics. But tinnitus knowledge is far from complete. In this chapter, contributors to the Behavioral Neuroscience of Tinnitus consider emerging topics and areas of research needed in light of recent findings. New research avenues and methods to explore are discussed. Issues pertaining to current assessment, treatment, and research methods are outlined, along with recommendations on new avenues to explore with research.

Genetic Inheritance and Its Contribution to Tinnitus

Tinnitus is the abnormal perception of sound that affects more than 15% of adult population around the globe. Severe tinnitus is considered a complex disorder that arises as result of the interaction of genetic and environmental factors, and it is associated with several comorbidities such as hearing loss, anxiety, and insomnia. We begin this review with an introduction to human molecular genetics and the role of genetic variation on the inheritance. There are some genetic reports on tinnitus heritability including concordance studies in twins and adoptees or aggregation in families providing some evidence for familial aggregation in patients with severe tinnitus and high concordance in monozygotic twins with bilateral tinnitus. So, sex differences in familial aggregation and heritability of bilateral tinnitus suggest a potential sexual dimorphism in tinnitus inheritance.Molecular genetic studies have been demonstrated to be a useful tool to understand the role of genetic variation in rare diseases and complex disorders. The reported associations in common variants in neurotrophic factors such as GDNF, BDNF, or potassium channels genes were underpowered, and the lack of replication questions these findings. Although candidate gene approaches have failed in replicating these genetic associations, the development of high throughput sequencing technology and the selection of extreme phenotypes are strategies that will allow the clinicians and researchers to combine genetic information with clinical data to implement a personalized diagnosis and therapy in patients with tinnitus.

Pharmacotherapy of Tinnitus

Tinnitus is a common symptom for which there is in most cases no causal therapy. The search for an improvement of tinnitus through pharmacological interventions has a long tradition. The observation that tinnitus can be transiently suppressed by the use of lidocaine has shown that the symptom is susceptible to pharmacotherapy. So far, however, no medication has been found for either acute or chronic subjective tinnitus that reliably leads to a long-term reduction or even complete disappearance of the symptom for the majority of tinnitus sufferers. Nevertheless, in everyday clinical life, drugs are frequently used, usually off-label, to relieve tinnitus or tinnitus-associated symptoms (e.g. sleep disturbance, depression, anxiety disorder or hearing loss). This chapter shows the different approaches to acute and chronic subjective tinnitus by means of pharmacotherapeutic interventions. Furthermore, this review reports on the scientific studies carried out in this area in recent years and explains the difficulties in finding a suitable medication for most forms of tinnitus. In addition, it reports on the pharmacotherapeutic options for objective tinnitus and describes the development of tinnitus as a side effect of certain drugs. Finally, possible target structures are mentioned, which should possibly be addressed in pharmacological studies in the near future.

Therapeutic Approaches to the Treatment of Tinnitus

Tinnitus is a highly prevalent condition that is associated with hearing loss in most cases. In the absence of external stimuli, phantom perceptions of sounds emerge from alterations in neuronal activity within central auditory and nonauditory structures. Pioneering studies using lidocaine revealed that tinnitus is susceptible to pharmacological interventions. However, lidocaine is not effective in all patients, and no other drug has been identified with clear efficacy for the long-term treatment of tinnitus. In this review, we present recent advances in tinnitus research, including more detailed knowledge of its pathophysiology and involved neurotransmitter systems. Moreover, we summarize results from animal and clinical treatment studies as well as from studies that identified tinnitus as a side effect of pharmacological treatments. Finally, we focus on challenges in the development of pharmacological compounds for the treatment of tinnitus, namely the limitations of available animal models and of standardized clinical research methodologies.

Tinnitus Treatment with Oxytocin: A Pilot Study

Introduction

Tinnitus is the perception of sound in the absence of an external stimulus. It is a frequent condition for which there is as yet no pharmacological treatment approved. Auditory and non-auditory pathways are involved in tinnitus’ pathophysiology. Oxytocin is a neurohormone and eventual neurotransmitter that plays a complex role in social cognition and behavior.

Objective

To evaluate the potential of oxytocin as a tinnitus treatment.

Study design

Two studies were performed. Study 1 was a long-term open pilot study, while study 2 investigated short-term effects with a double-blinded placebo-controlled cross-over study.

Setting

Ambulatory ENT care.

Subjects and method

In study 1, 15 patients were investigated over a 10-week period in an open pilot study. In study 2, 16 patients were included in a placebo-controlled crossover trial to investigate short-term effects following a single dose.

Results

For the long-term study (study 1), analysis of variance revealed a significant decrease in tinnitus sensation, both for the Tinnitus Handicap Inventory and Clinical Global Impression (CGI). Also, the short-term effects in study 2 revealed a significant reduction of tinnitus because of the oxytocin nasal spray as measured with the Visual Analog Scale and the CGI Scale.

Conclusion

These preliminary studies demonstrated that oxytocin may represent a helpful tool for treating tinnitus and further larger controlled studies are warranted.

The Role of MRI in Diagnosing Neurovascular Compression of the Cochlear Nerve Resulting in Typewriter Tinnitus

BACKGROUND AND PURPOSE

Typewriter tinnitus, a symptom characterized by paroxysmal attacks of staccato sounds, has been thought to be caused by neurovascular compression of the cochlear nerve, but the correlation between radiologic evidence of neurovascular compression of the cochlear nerve and symptom presentation has not been thoroughly investigated. The purpose of this study was to examine whether radiologic evidence of neurovascular compression of the cochlear nerve is pathognomonic in typewriter tinnitus.

MATERIALS AND METHODS

Fifteen carbamazepine-responding patients with typewriter tinnitus and 8 control subjects were evaluated with a 3D T2-weighted volume isotropic turbo spin-echo acquisition sequence. Groups 1 (16 symptomatic sides), 2 (14 asymptomatic sides), and 3 (16 control sides) were compared with regard to the anatomic relation between the vascular loop and the internal auditory canal and the presence of neurovascular compression of the cochlear nerve with/without angulation/indentation.

RESULTS

The anatomic location of the vascular loop was not significantly different among the 3 groups (all, P > .05). Meanwhile, neurovascular compression of the cochlear nerve on MR imaging was significantly higher in group 1 than in group 3 (P = .032). However, considerable false-positive (no symptoms with neurovascular compression of the cochlear nerve on MR imaging) and false-negative (typewriter tinnitus without demonstrable neurovascular compression of the cochlear nerve) findings were also observed.

CONCLUSIONS

Neurovascular compression of the cochlear nerve was more frequently detected on the symptomatic side of patients with typewriter tinnitus compared with the asymptomatic side of these patients or on both sides of control subjects on MR imaging. However, considering false-positive and false-negative findings, meticulous history-taking and the response to the initial carbamazepine trial should be regarded as more reliable diagnostic clues than radiologic evidence of neurovascular compression of the cochlear nerve.

Brain Metabolic Changes in Rats following Acoustic Trauma

Acoustic trauma is the most common cause of hearing loss and tinnitus in humans. However, the impact of acoustic trauma on system biology is not fully understood. It has been increasingly recognized that tinnitus caused by acoustic trauma is unlikely to be generated by a single pathological source, but rather a complex network of changes involving not only the auditory system but also systems related to memory, emotion and stress. One obvious and significant gap in tinnitus research is a lack of biomarkers that reflect the consequences of this interactive "tinnitus-causing" network. In this study, we made the first attempt to analyse brain metabolic changes in rats following acoustic trauma using metabolomics, as a pilot study prior to directly linking metabolic changes to tinnitus. Metabolites in 12 different brain regions collected from either sham or acoustic trauma animals were profiled using a gas chromatography mass spectrometry (GC/MS)-based metabolomics platform. After deconvolution of mass spectra and identification of the molecules, the metabolomic data were processed using multivariate statistical analysis. Principal component analysis showed that metabolic patterns varied among different brain regions; however, brain regions with similar functions had a similar metabolite composition. Acoustic trauma did not change the metabolite clusters in these regions. When analyzed within each brain region using the orthogonal projection to latent structures discriminant analysis sub-model, 17 molecules showed distinct separation between control and acoustic trauma groups in the auditory cortex, inferior colliculus, superior colliculus, vestibular nucleus complex (VNC), and cerebellum. Further metabolic pathway impact analysis and the enrichment overview with network analysis suggested the primary involvement of amino acid metabolism, including the alanine, aspartate and glutamate metabolic pathways, the arginine and proline metabolic pathways and the purine metabolic pathway. Our results provide the first metabolomics evidence that acoustic trauma can induce changes in multiple metabolic pathways. This pilot study also suggests that the metabolomic approach has the potential to identify acoustic trauma-specific metabolic shifts in future studies where metabolic changes are correlated with the animal's tinnitus status.

A Pilot Genome-Wide Association Study Identifies Potential Metabolic Pathways Involved in Tinnitus

Tinnitus, the perception of an auditory phantom sound in the form of ringing, buzzing, roaring, or hissing in the absence of an external sound source, is perceived by ~15% of the population and 2.5% experiences a severely bothersome tinnitus. The contribution of genes on the development of tinnitus is still under debate. The current manuscript reports a pilot Genome Wide Association Study (GWAS) into tinnitus, in a small cohort of 167 independent tinnitus subjects, and 749 non-tinnitus controls, who were collected as part of a cross-sectional study. After genotyping, imputation, and quality checking, the association between the tinnitus phenotype and 4,000,000 single-nucleotide polymorphisms (SNPs) was tested followed by gene set enrichment analysis. None of the SNPs reached the threshold for genome-wide significance (p < 5.0e–8), with the most significant SNPs, situated outside coding genes, reaching a p-value of 3.4e–7. By using the Genetic Analysis of Complex Traits (GACT) software, the percentage of the variance explained by all SNPs in the GWAS was estimated to be 3.2%, indicating that additive genetic effects explain only a small fraction of the tinnitus phenotype. Despite the lack of genome-wide significant SNPs, which is, at least in part, due to the limited sample size of the current study, evidence was found for a genetic involvement in tinnitus. Gene set enrichment analysis showed several metabolic pathways to be significantly enriched with SNPs having a low p-value in the GWAS. These pathways are involved in oxidative stress, endoplasmatic reticulum (ER) stress, and serotonin reception mediated signaling. These results are a promising basis for further research into the genetic basis of tinnitus, including GWAS with larger sample sizes and considering tinnitus subtypes for which a greater genetic contribution is more likely.

The Management and Outcomes of Pharmacological Treatments for Tinnitus

Tinnitus, a phantom sensation experienced by people around the world, currently is endured without a known cure. Some find the condition tolerable, while others are tortured on a daily basis from the incessant phantom noises. For those who seek treatment, oftentimes, they have a comorbid condition (e.g., depression, anxiety, insomnia), which is treated pharmaceutically. These products aim to reduce the comorbities associated with tinnitus thereby minimizing the overall burden present. Because of the phantom nature of tinnitus, it is often compared to neurologic pain. Since pain can be managed with pharmaceutical options, it is reasonable to assume that similar agents might work to alleviate tinnitus. The effects of antidepressants, benzodiazepines, anticonvulsants, and glutamate antagonists are reviewed in this paper. Table 1 summarizes the pharmaceutical products discussed. Due to the variety of comorbid factors and potential causes of tinnitus, there may not be one pharmaceutical treatment that will combat every type of tinnitus. Nevertheless, a product that finally addresses the true cause of tinnitus, and not just its comorbidities, will benefit millions of people worldwide.

Treatment of tinnitus

PURPOSE OF REVIEW

Tinnitus, the perception of sound in the absence of a corresponding external acoustic stimulus, is a highly prevalent and frequently severely impairing disorder with worldwide impact. In this article after a short overview about epidemiology and pathophysiology the currently available treatment options will be discussed with specific consideration of the available evidence, their mechanisms of action and their limitations.

RECENT FINDINGS

During the last decades, advances in neuroimaging methods and the development of animal models have contributed to an increasing understanding of the neuronal correlates of tinnitus and have motivated the development of innovative brain-based treatment approaches for directly targeting the neuronal correlates of tinnitus. A further important development has been the insight that there exist different forms of tinnitus that differ in their pathophysiology and their response to specific treatments.

SUMMARY

Treatment of tinnitus should be based on a comprehensive diagnosis of etiologic and concomitant aspects of an individual's tinnitus. Already today a large variety of therapeutic interventions are available, which can efficiently reduce tinnitus severity. Several innovative treatment approaches are currently under development.