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

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.

[Noise-induced neurodegeneration in the central auditory pathway : An overview of experimental studies in a mouse model]

BACKGROUND

A noise trauma induces central nervous system pathologies, which generate deficits in hearing and perception of sound.

OBJECTIVE

Are degenerative mechanisms in the central auditory system a direct impact of overstimulation or an effect of acoustic deprivation?

MATERIALS AND METHODS

Detection of cell death in a mouse model of noise-induced hearing loss at different times after single or repeated noise exposure.

RESULTS

A single noise exposure (3 h, 115 dB SPL, 5-20 kHz) induces acute (≤1 day) and long-term (observation period 14 days) degeneration, particularly in subcortical structures. Repeated noise trauma is followed by pathologies in the auditory thalamus and cortex.

CONCLUSION

Noise has a direct impact on basal structures of the central auditory system; a protection of cortical areas is possibly due to inhibitory neuronal projections. Degenerative mechanisms in higher structures of the pre-damaged system point to an increased impairment of complex processing of acoustic information.

Up-regulation of Ca2+/CaMKII/CREB signaling in salicylate-induced tinnitus in rats

The purpose of the study was to investigate the changes of Ca²⁺/calmodulin-dependent protein kinases II (CaMKII)/cAMP response element-binding protein (CREB) signaling pathway in a rat tinnitus model. Eighteen Wistar rats were randomly divided into three groups: normal control (NC), normal saline (NS), and tinnitus model (TM) groups. Tinnitus model was induced by intraperitoneal injection of salicylate. The concentration of intracellular calcium level in auditory cortex cells was determined using Fura-2 acetoxymethyl ester (Fura-2 AM) method with fluorospectrophotometer. Expressions of calmodulin (CaM), N-methyl-D-aspartate receptor 2B subunit (NR2B), calcium-calmodulin kinase II (CaMKII), and cAMP response element-binding protein (CREB) were detected with Western blot. Tinnitus model was successfully established by the intraperitoneal administration of salicylate in rats. Compared with rats in NC and NS groups, salicylate administration significantly elevated CaM, NR2B, phospho-CaMKII and phospho-CREB expression in auditory cortex from tinnitus model group (p < 0.05), and the free intracellular Ca²⁺ concentrations (p < 0.05). Our data reveal that salicylate administration causes tinnitus symptoms and elevates Ca²⁺/CaMKII/CREB signaling pathway in auditory cortex cells. Our study likely provides a new understanding of the development of tinnitus.

Robust Subthreshold Cross-modal Modulation of Auditory Response by Cutaneous Electrical Stimulation in First- and Higher-order Auditory Thalamic Nuclei

Conventional extracellular recording has revealed cross-modal alterations of auditory cell activities by cutaneous electrical stimulation of the hindpaw in first- and higher-order auditory thalamic nuclei (Donishi et al., 2011). Juxta-cellular recording and labeling techniques were used in the present study to examine the cross-modal alterations in detail, focusing on possible nucleus and/or cell type-related distinctions in modulation. Recordings were obtained from 80 cells of anesthetized rats. Cutaneous electrical stimulation, which did not elicit unit discharges, i.e., subthreshold effects, modulated early (onset) and/or late auditory responses of first- (64%) and higher-order nucleus cells (77%) with regard to response magnitude, latency and/or burst spiking. Attenuation predominated in the modulation of response magnitude and burst spiking, and delay predominated in the modulation of response time. Striking alterations of burst spiking took place in higher-order nucleus cells, which had the potential to exhibit higher propensities for burst spiking as compared to first-order nucleus cells. A subpopulation of first-order nucleus cells showing modulation in early response magnitude in the caudal domain of the nucleus had larger cell bodies and higher propensities for burst spiking as compared to cells showing no modulation. These findings suggest that somatosensory influence is incorporated into parallel channels in auditory thalamic nuclei to impose distinct impacts on cortical and subcortical sensory processing. Further, cutaneous electrical stimulation given after early auditory responses modulated late responses. Somatosensory influence is likely to affect ongoing auditory processing at any time without being coincident with sound onset in a narrow temporal window.

Effects of Acoustic Environment on Tinnitus Behavior in Sound-Exposed Rats

Laboratory studies often rely on a damaging sound exposure to induce tinnitus in animal models. Because the time course and ultimate success of the induction process is not known in advance, it is not unusual to maintain sound-exposed animals for months while they are periodically assessed for behavioral indications of the disorder. To demonstrate the importance of acoustic environment during this period of behavioral screening, sound-exposed rats were tested for tinnitus while housed under quiet or constant noise conditions. More than half of the quiet-housed rats developed behavioral indications of the disorder. None of the noise-housed rats exhibited tinnitus behavior during 2 months of behavioral screening. It is widely assumed that the "phantom sound" of tinnitus reflects abnormal levels of spontaneous activity in the central auditory pathways that are triggered by cochlear injury. Our results suggest that sustained patterns of noise-driven activity may prevent the injury-induced changes in central auditory processing that lead to this hyperactive state. From the perspective of laboratory studies of tinnitus, housing sound-exposed animals in uncontrolled noise levels may significantly reduce the success of induction procedures. From a broader clinical perspective, an early intervention with sound therapy may reduce the risk of tinnitus in individuals who have experienced an acute cochlear injury.

Pharmacological modulation of Kv3.1 mitigates auditory midbrain temporal processing deficits following auditory nerve damage

Higher stages of central auditory processing compensate for a loss of cochlear nerve synapses by increasing the gain on remaining afferent inputs, thereby restoring firing rate codes for rudimentary sound features. The benefits of this compensatory plasticity are limited, as the recovery of precise temporal coding is comparatively modest. We reasoned that persistent temporal coding deficits could be ameliorated through modulation of voltage-gated potassium (Kv) channels that regulate temporal firing patterns. Here, we characterize AUT00063, a pharmacological compound that modulates Kv3.1, a high-threshold channel expressed in fast-spiking neurons throughout the central auditory pathway. Patch clamp recordings from auditory brainstem neurons and in silico modeling revealed that application of AUT00063 reduced action potential timing variability and improved temporal coding precision. Systemic injections of AUT00063 in vivo improved auditory synchronization and supported more accurate decoding of temporal sound features in the inferior colliculus and auditory cortex in adult mice with a near-complete loss of auditory nerve afferent synapses in the contralateral ear. These findings suggest modulating Kv3.1 in central neurons could be a promising therapeutic approach to mitigate temporal processing deficits that commonly accompany aging, tinnitus, ototoxic drug exposure or noise damage.

Tinnitus: Prospects for Pharmacological Interventions With a Seesaw Model

Chronic tinnitus, the perception of lifelong constant ringing in ear, is one capital cause of disability in modern society. It is often present with various comorbid factors that severely affect quality of life, including insomnia, deficits in attention, anxiety, and depression. Currently, there are limited therapeutic treatments for alleviation of tinnitus. Tinnitus can involve a shift in neuronal excitation/inhibition (E/I) balance, which is largely modulated by ion channels and receptors. Thus, ongoing research is geared toward pharmaceutical approaches that modulate the function of ion channels and receptors. Here, we propose a seesaw model that delineates how tinnitus-related ion channels and receptors are involved in homeostatic E/I balance of neurons. This review provides a thorough account of our current mechanistic understanding of tinnitus and insight into future direction of drug development.