A novel behavioral assay for the assessment of acute tinnitus in rats optimized for simultaneous recording of oscillatory neural activity

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

Introduction

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

Methods

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

Results

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

Discussion

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

Targeting the Limbic System: Insights into Its Involvement in Tinnitus

Tinnitus is originally derived from the Latin verb tinnire, which means "to ring". Tinnitus, a complex disorder, is a result of sentient cognizance of a sound in the absence of an external auditory stimulus. It is reported in children, adults, and older populations. Patients suffering from tinnitus often present with hearing loss, anxiety, depression, and sleep disruption in addition to a hissing and ringing in the ear. Surgical interventions and many other forms of treatment have been only partially effective due to heterogeneity in tinnitus patients and a lack of understanding of the mechanisms of tinnitus. Although researchers across the globe have made significant progress in understanding the underlying mechanisms of tinnitus over the past few decades, tinnitus is still deemed to be a scientific enigma. This review summarises the role of the limbic system in tinnitus development and provides insight into the development of potential target-specific tinnitus therapies.

Chronic stress induced loudness hyperacusis, sound avoidance and auditory cortex hyperactivity

Hyperacusis, a debilitating loudness intolerance disorder, has been linked to chronic stress and adrenal insufficiency. To investigate the role of chronic stress, rats were chronically treated with corticosterone (CORT) stress hormone. Chronic CORT produced behavioral evidence of loudness hyperacusis, sound avoidance hyperacusis, and abnormal temporal integration of loudness. CORT treatment did not disrupt cochlear or brainstem function as reflected by normal distortion product otoacoustic emissions, compound action potentials, acoustic startle reflexex, and auditory brainstem responses. In contrast, the evoked response from the auditory cortex was enhanced up to three fold after CORT treatment. This hyperactivity was associated with a significant increase in glucocorticoid receptors in auditory cortex layers II/III and VI. Basal serum CORT levels remained normal after chronic CORT stress whereas reactive serum CORT levels evoked by acute restraint stress were blunted (reduced) after chronic CORT stress; similar changes were observed after chronic, intense noise stress. Taken together, our results show for the first time that chronic stress can induce hyperacusis and sound avoidance. A model is proposed in which chronic stress creates a subclinical state of adrenal insufficiency that establishes the necessary conditions for inducing hyperacusis.

Using an appetitive operant conditioning paradigm to screen rats for tinnitus induced by intense sound exposure: Experimental considerations and interpretation

In an effort to help elucidate the neural mechanisms underlying tinnitus in humans, researchers have often relied on animal models; a preclinical approach which ultimately required that behavioral paradigms be designed to reliably screen animals for tinnitus. Previously, we developed a two-alternative forced-choice (2AFC) paradigm for rats that allowed for the simultaneous recording of neural activity at the very moments when they were reporting the presence/absence of tinnitus. Because we first validated our paradigm in rats experiencing transient tinnitus following a high-dose of sodium salicylate, the present study now sought to evaluate its utility to screen for tinnitus caused by intense sound exposure; a common tinnitus-inducer in humans. More specifically, through a series of experimental protocols, we aimed to (1) conduct sham experiments to ensure that the paradigm was able to correctly classify control rats as not having tinnitus, (2) confirm the time course over which the behavioral testing could reliably be performed post-exposure to assess chronic tinnitus, and (3) determine if the paradigm was sensitive to the variable outcomes often observed after intense sound exposure (e.g., hearing loss with our without tinnitus). Ultimately, in accordance with our predictions, the 2AFC paradigm was indeed resistant to false-positive screening of rats for intense sound-induced tinnitus, and it was able to reveal variable tinnitus and hearing loss profiles in individual rats following intense sound exposure. Taken together, the present study documents the utility of our appetitive operant conditioning paradigm to assess acute and chronic sound-induced tinnitus in rats. Finally, based on our findings, we discuss important experimental considerations that will help ensure that our paradigm is able to provide a suitable platform for future investigations into the neural basis of tinnitus.

Recent-onset and persistent tinnitus: Uncovering the differences in brain activities using resting-state functional magnetic resonance imaging technologies

Objective

This study aimed to investigate the differences in intra-regional brain activity and inter-regional functional connectivity between patients with recent-onset tinnitus (ROT) and persistent tinnitus (PT) using resting-state functional magnetic resonance imaging (rs-fMRI), including the amplitude of low-frequency fluctuations (ALFF), regional homogeneity (ReHo), and voxel-wise functional connectivity (FC).

Method

We acquired rs-fMRI scans from 82 patients (25 without recent-onset tinnitus, 28 with persistent tinnitus, and 29 healthy controls). Age, sex, and years of education were matched across the three groups. We performed ALFF, ReHo, and voxel-wise FC analyses for all patients.

Results

Compared with the control group, participants with ROT and PT manifested significantly reduced ALFF and ReHo activity within the left and right dorsolateral superior frontal gyrus (SFG) and gyrus rectus (GR). Additional voxel-wise FC revealed decreased connectivity between the dorsolateral SFG (left and right) and the right superior parietal gyrus (SPG), right middle frontal gyrus (MFG), and left medial superior frontal gyrus (mSFG) within these two groups. Significant differences were observed between the ROT and PT groups, with the ROT group demonstrating reduced FC.

Conclusion

Our data suggest that patients with PT have more difficulty monitoring external stimuli and reorienting attention than patients with ROT. In addition, patients who perceive higher levels of disruption from tinnitus are more likely to develop persistent and debilitating tinnitus once the tinnitus lasts longer than six months. Therefore, we strongly recommend that clinicians implement effective tinnitus management strategies in patients with ROT as soon as possible.

Hearing loss and brain plasticity: the hyperactivity phenomenon

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

Alterations of brain activity and functional connectivity in transition from acute to chronic tinnitus

The objective of this study was to investigate alterations to brain activity and functional connectivity in patients with tinnitus, exploring neural features in the transition from acute to chronic phantom perception. Twenty‐four patients with acute tinnitus, 23 patients with chronic tinnitus, and 32 healthy controls were recruited. High‐density electroencephalography (EEG) was used to explore changes in brain areas and functional connectivity in different groups. When compared with healthy subjects, acute tinnitus patients had a significant reduction in superior frontal cortex activity across all frequency bands, whereas chronic tinnitus patients had a significant reduction in the superior frontal cortex at beta 3 and gamma frequency bands as well as a significant increase in the inferior frontal cortex at delta‐band and superior temporal cortex at alpha 1 frequency band. When compared to the chronic tinnitus group, the acute tinnitus group activity was significantly increased in the middle frontal and parietal gyrus at the gamma‐band. Functional connectivity analysis showed that the chronic tinnitus group had increased connections between the parahippocampus gyrus, posterior cingulate cortex, and precuneus when compared with the healthy group. Alterations of local brain activity and connections between the parahippocampus gyrus and other nonauditory areas appeared in the transition from acute to chronic tinnitus. This indicates that the appearance and development of tinnitus is a dynamic process involving aberrant local neural activity and abnormal connectivity in multifunctional brain networks.

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

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

Functional Neuroanatomy of Salicylate- and Noise-Induced Tinnitus and Hyperacusis

Tinnitus and hyperacusis are debilitating conditions often associated with aging or exposure to intense noise or ototoxic drugs. One of the most reliable methods of inducing tinnitus is with high doses of sodium salicylate, the active ingredient in aspirin. High doses of salicylate have been widely used to investigate the functional neuroanatomy of tinnitus and hyperacusis. High doses of salicylate have been used to develop novel behavioral methods to detect the presence of tinnitus and hyperacusis in animal models. Salicylate typically induces a hearing loss of approximately 20 dB which greatly reduces the neural output of the cochlea. As this weak neural signal emerging from the cochlea is sequentially relayed to the cochlear nucleus, inferior colliculus, medial geniculate, and auditory cortex, the neural response to suprathreshold sounds is progressively amplified by a factor of 2-3 by the time the signal reaches the auditory cortex, a phenomenon referred to as enhanced central gain. Sound-evoked hyperactivity also occurred in the amygdala, a region that assigns emotional significance to sensory stimuli. Resting state functional magnetic imaging of the BOLD signal revealed salicylate-induced increases in spontaneous neural activity in the inferior colliculus, medial geniculate body, and auditory cortex as well as in non-auditory areas such as the amygdala, reticular formation, cerebellum, and other sensory areas. Functional connectivity of the BOLD signal revealed increased neural coupling between several auditory areas and non-auditory areas such as the amygdala, cerebellum, reticular formation, hippocampus, and caudate/putamen; these strengthened connections likely contribute to the multifaceted dimensions of tinnitus. Taken together, these results suggest that salicylate-induced tinnitus disrupts a complex neural network involving many auditory centers as well as brain regions involved with emotion, arousal, memory, and motor planning. These extra-auditory centers embellish the basic auditory percepts that results in tinnitus and which may also contribute to hyperacusis.

Pharmacological Evaluation of Drugs in Animal Models of Tinnitus

Despite the pressing need for effective drug treatments for tinnitus, currently, there is no single drug that is approved by the FDA for this purpose. Instead, a wide range of unproven over-the-counter tinnitus remedies are available on the market with little or no benefit for tinnitus but with potential harm and adverse effects. Animal models of tinnitus have played a critical role in exploring the pathophysiology of tinnitus, identifying therapeutic targets and evaluating novel and existing drugs for tinnitus treatment. This review summarises and compares the studies on pharmacological evaluation of tinnitus treatment in different animal models based on the pharmacological properties of the drug and provides insights into future directions for tinnitus drug discovery.

Blast-induced tinnitus: Animal models

Blast-induced tinnitus is a prevalent problem among military personnel and veterans, as blast-related trauma damages the vulnerable microstructures within the cochlea, impacts auditory and non-auditory brain structures, and causes tinnitus and other disorders. Thus far, there is no effective treatment of blast-induced tinnitus due to an incomplete understanding of its underlying mechanisms, necessitating development of reliable animal models. This article focuses on recent animal studies using behavioral, electrophysiological, imaging, and pharmacological tools. The mechanisms underlying blast-induced tinnitus are largely similar to those underlying noise-induced tinnitus: increased spontaneous firing rates, bursting, and neurosynchrony, Mn++ accumulation, and elevated excitatory synaptic transmission. The differences mainly lie in the data variability and time course. Noise trauma-induced tinnitus mainly originates from direct peripheral deafferentation at the cochlea, and its etiology subsequently develops along the ascending auditory pathways. Blast trauma-induced tinnitus, on the other hand, results from simultaneous impact on both the peripheral and central auditory systems, and the resultant maladaptive neuroplasticity may also be related to the additional traumatic brain injury. Consequently, the neural correlates of blast-induced tinnitus have different time courses and less uniform manifestations of its neural correlates.

The rat as a model for studying noise injury and otoprotection

A major challenge for those studying noise-induced injury pre-clinically is the selection of an animal model. Noise injury models are particularly relevant in an age when people are constantly bombarded by loud noise due to occupation and/or recreation. The rat has been widely used for noise-related morphological, physiological, biochemical, and molecular assessment. Noise exposure resulting in a temporary (TTS) or permanent threshold shift (PTS) yields trauma in peripheral and central auditory related pathways. While the precise nature of noise-related injuries continues to be delineated, both PTS and TTS (with or without hidden hearing loss) result in homeostatic changes implicated in conditions such as tinnitus and hyperacusis. Compared to mice, rats generally tolerate exposure to loud sounds reasonably well, often without exhibiting other physical non-inner ear related symptoms such as death, loss of consciousness, or seizures [Skradski, Clark, Jiang, White, Fu, and Ptacek (2001). Neuron 31, 537-544; Faingold (2002). Hear. Res. 168, 223-237; Firstova, Abaimov, Surina, Poletaeva, Fedotova, and Kovalev (2012). Bull Exp. Biol. Med. 154, 196-198; De Sarro, Russo, Citraro, and Meldrum (2017). Epilepsy Behav. 71, 165-173]. This ability of the rat to thrive following noise exposure permits study of long-term effects. Like the mouse, the rat also offers a well-characterized genome allowing genetic manipulations (i.e., knock-out, viral-based gene expression modulation, and optogenetics). Rat models of noise-related injury also provide valuable information for understanding mechanistic changes to identify therapeutic targets for treatment. This article provides a framework for selection of the rat as a model for noise injury studies.

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

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

Aberrant thalamocortical coherence in an animal model of tinnitus

Electrophysiological and imaging studies from humans suggest that the phantom sound of tinnitus is associated with abnormal thalamocortical neural oscillations (dysrhythmia) and enhanced gamma band activity in the auditory cortex. However, these models have seldom been tested in animal models where it is possible to simultaneously assess the neural oscillatory activity within and between the thalamus and auditory cortex. To explore this issue, we used multichannel electrodes to examine the oscillatory behavior of local field potentials recorded in the rat medial geniculate body (MBG) and primary auditory cortex (A1) before and after administering a dose of sodium salicylate (SS) that reliably induces tinnitus. In the MGB, SS reduced theta, alpha, and beta oscillations and decreased coherence (synchrony) between electrode pairs in theta, alpha, and beta bands but increased coherence in the gamma band. Within A1, SS significantly increased gamma oscillations, decreased theta power, and decreased coherence between electrode pairs in theta and alpha bands but increased coherence in the gamma band. When coherence was measured between one electrode in the MGB and another in A1, SS decreased coherence in beta, alpha, and theta bands but increased coherence in the gamma band. SS also increased cross-frequency coupling between the phase of theta oscillations in the MGB and amplitude of gamma oscillations in A1. Altogether, our results suggest that SS treatment fundamentally alters the manner in which thalamocortical circuits communicate, leading to excessive cortical gamma power and synchronization, neurophysiological changes implicated in tinnitus. Our data provide support for elements of both the thalamocortical dysrhythmia (TD) and synchronization by loss of inhibition (SLIM) models of tinnitus, demonstrating that increased cortical gamma band activity is associated with both enhanced theta-gamma coupling as well as decreases alpha power/coherence between the MGB and A1. NEW & NOTEWORTHY There are no effective drugs to alleviate the phantom sound of tinnitus because the physiological mechanisms leading to its generation are poorly understood. Neural models of tinnitus suggest that it arises from abnormal thalamocortical oscillations, but these models have not been extensively tested. This article identifies abnormal thalamocortical oscillations in a drug-induced tinnitus model. Our findings open up new avenues of research to investigate whether cellular mechanisms underlying thalamocortical oscillations are causally linked to tinnitus.

Behavioral Plasticity of Audiovisual Perception: Rapid Recalibration of Temporal Sensitivity but Not Perceptual Binding Following Adult-Onset Hearing Loss

The ability to accurately integrate or bind stimuli from more than one sensory modality is highly dependent on the features of the stimuli, such as their intensity and relative timing. Previous studies have demonstrated that the ability to perceptually bind stimuli is impaired in various clinical conditions such as autism, dyslexia, schizophrenia, as well as aging. However, it remains unknown if adult-onset hearing loss, separate from aging, influences audiovisual temporal acuity. In the present study, rats were trained using appetitive operant conditioning to perform an audiovisual temporal order judgment (TOJ) task or synchrony judgment (SJ) task in order to investigate the nature and extent that audiovisual temporal acuity is affected by adult-onset hearing loss, with a specific focus on the time-course of perceptual changes following loud noise exposure. In our first series of experiments, we found that audiovisual temporal acuity in normal-hearing rats was influenced by sound intensity, such that when a quieter sound was presented, the rats were biased to perceive the audiovisual stimuli as asynchronous (SJ task), or as though the visual stimulus was presented first (TOJ task). Psychophysical testing demonstrated that noise-induced hearing loss did not alter the rats' temporal sensitivity 2-3 weeks post-noise exposure, despite rats showing an initial difficulty in differentiating the temporal order of audiovisual stimuli. Furthermore, consistent with normal-hearing rats, the timing at which the stimuli were perceived as simultaneous (i.e., the point of subjective simultaneity, PSS) remained sensitive to sound intensity following hearing loss. Contrary to the TOJ task, hearing loss resulted in persistent impairments in asynchrony detection during the SJ task, such that a greater proportion of trials were now perceived as synchronous. Moreover, psychophysical testing found that noise-exposed rats had altered audiovisual synchrony perception, consistent with impaired audiovisual perceptual binding (e.g., an increase in the temporal window of integration on the right side of simultaneity; right temporal binding window (TBW)). Ultimately, our collective results show for the first time that adult-onset hearing loss leads to behavioral plasticity of audiovisual perception, characterized by a rapid recalibration of temporal sensitivity but a persistent impairment in the perceptual binding of audiovisual stimuli.

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.

Gap-induced reductions of evoked potentials in the auditory cortex: A possible objective marker for the presence of tinnitus in animals

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Gap-suppression of startle responses is regularly used as a measure for tinnitus.

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We studied this phenomenon in auditory cortical evoked potentials in awake animals.

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Gap-suppression of evoked potentials was also examined following noise exposure.

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120 dB SPL noise exposure, but not 105 dB, resulted in deficits in gap-suppression.

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Results are discussed in the context of a potential correlate of tinnitus.

Animal models of tinnitus are essential for determining the underlying mechanisms and testing pharmacotherapies. However, there is doubt over the validity of current behavioural methods for detecting tinnitus. Here, we applied a stimulus paradigm widely used in a behavioural test (gap-induced inhibition of the acoustic startle reflex GPIAS) whilst recording from the auditory cortex, and showed neural response changes that mirror those found in the behavioural tests. We implanted guinea pigs (GPs) with electrocorticographic (ECoG) arrays and recorded baseline auditory cortical responses to a startling stimulus. When a gap was inserted in otherwise continuous background noise prior to the startling stimulus, there was a clear reduction in the subsequent evoked response (termed gap-induced reductions in evoked potentials; GIREP), suggestive of a neural analogue of the GPIAS test. We then unilaterally exposed guinea pigs to narrowband noise (left ear; 8–10 kHz; 1 h) at one of two different sound levels – either 105 dB SPL or 120 dB SPL – and recorded the same responses seven-to-ten weeks following the noise exposure. Significant deficits in GIREP were observed for all areas of the auditory cortex (AC) in the 120 dB-exposed GPs, but not in the 105 dB-exposed GPs. These deficits could not simply be accounted for by changes in response amplitudes. Furthermore, in the contralateral (right) caudal AC we observed a significant increase in evoked potential amplitudes across narrowband background frequencies in both 105 dB and 120 dB-exposed GPs. Taken in the context of the large body of literature that has used the behavioural test as a demonstration of the presence of tinnitus, these results are suggestive of objective neural correlates of the presence of noise-induced tinnitus and hyperacusis.

Effects of the cannabinoid CB1 agonist ACEA on salicylate ototoxicity, hyperacusis and tinnitus in guinea pigs

Cannabinoids have been suggested as a therapeutic target for a variety of brain disorders. Despite the presence of their receptors throughout the auditory system, little is known about how cannabinoids affect auditory function. We sought to determine whether administration of arachidonyl-2′-chloroethylamide (ACEA), a highly-selective CB₁ agonist, could attenuate a variety of auditory effects caused by prior administration of salicylate, and potentially treat tinnitus. We recorded cortical resting-state activity, auditory-evoked cortical activity and auditory brainstem responses (ABRs), from chronically-implanted awake guinea pigs, before and after salicylate + ACEA. Salicylate-induced reductions in click-evoked ABR amplitudes were smaller in the presence of ACEA, suggesting that the ototoxic effects of salicylate were less severe. ACEA also abolished salicylate-induced changes in cortical alpha band (6–10 Hz) oscillatory activity. However, salicylate-induced increases in cortical evoked activity (suggestive of the presence of hyperacusis) were still present with salicylate + ACEA. ACEA administered alone did not induce significant changes in either ABR amplitudes or oscillatory activity, but did increase cortical evoked potentials. Furthermore, in two separate groups of non-implanted animals, we found no evidence that ACEA could reverse behavioural identification of salicylate- or noise-induced tinnitus. Together, these data suggest that while ACEA may be potentially otoprotective, selective CB₁ agonists are not effective in diminishing the presence of tinnitus or hyperacusis.

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CB₁ agonist (ACEA) effects were assessed in awake guinea pigs following salicylate.

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Salicylate-induced decreases in brainstem response amplitudes were tempered by ACEA.

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Decreases in alpha band oscillations were not evident following salicylate + ACEA.

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ACEA did not eliminate salicylate-induced increases in cortical evoked potentials.

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ACEA failed to prevent or reverse salicylate- or noise-induced tinnitus behaviour.

Salicylate-induced hyperacusis in rats: Dose- and frequency-dependent effects

The use of auditory reaction time is a reliable measure of loudness perception in both animals and humans with reaction times (RT) decreasing with increasing stimulus intensity. Since abnormal loudness perception is a common feature of hyperacusis, a potentially debilitating auditory disorder in which moderate-intensity sounds are perceived as uncomfortable or painfully loud, we used RT measures to assess rats for salicylate-induced hyperacusis. A previous study using an operant conditioning RT procedure found that high-dose sodium salicylate (SS) induced hyperacusis-like behavior, i.e., faster than normal RTs to moderate and high level sounds, when rats were tested with broadband noise stimuli. However, it was not clear from that study if salicylate induces hyperacusis-like behavior in a dose- or frequency-dependent manner. Therefore, the goals of the current study were to determine how RT-intensity functions were altered by different doses of salicylate, and, using tone bursts, to determine if salicylate induces hyperacusis-like behavior across the entire frequency spectrum or only at certain frequencies. Similar to previous physiological studies, we began to see faster than normal RTs for sounds 60 dB SPL and greater with salicylate doses of 150 mg/kg and higher; indicating the rats were experiencing hyperacusis at high salicylate doses. In addition, high-dose salicylate significantly reduced RTs across all stimulus frequencies tested which suggests that a central neural excitability mechanism may be a potential driver of salicylate-induced changes in loudness perception and hyperacusis.

Audiovisual Temporal Processing and Synchrony Perception in the Rat

Extensive research on humans has improved our understanding of how the brain integrates information from our different senses, and has begun to uncover the brain regions and large-scale neural activity that contributes to an observer’s ability to perceive the relative timing of auditory and visual stimuli. In the present study, we developed the first behavioral tasks to assess the perception of audiovisual temporal synchrony in rats. Modeled after the parameters used in human studies, separate groups of rats were trained to perform: (1) a simultaneity judgment task in which they reported whether audiovisual stimuli at various stimulus onset asynchronies (SOAs) were presented simultaneously or not; and (2) a temporal order judgment task in which they reported whether they perceived the auditory or visual stimulus to have been presented first. Furthermore, using in vivo electrophysiological recordings in the lateral extrastriate visual (V2L) cortex of anesthetized rats, we performed the first investigation of how neurons in the rat multisensory cortex integrate audiovisual stimuli presented at different SOAs. As predicted, rats (n = 7) trained to perform the simultaneity judgment task could accurately (~80%) identify synchronous vs. asynchronous (200 ms SOA) trials. Moreover, the rats judged trials at 10 ms SOA to be synchronous, whereas the majority (~70%) of trials at 100 ms SOA were perceived to be asynchronous. During the temporal order judgment task, rats (n = 7) perceived the synchronous audiovisual stimuli to be “visual first” for ~52% of the trials, and calculation of the smallest timing interval between the auditory and visual stimuli that could be detected in each rat (i.e., the just noticeable difference (JND)) ranged from 77 ms to 122 ms. Neurons in the rat V2L cortex were sensitive to the timing of audiovisual stimuli, such that spiking activity was greatest during trials when the visual stimulus preceded the auditory by 20–40 ms. Ultimately, given that our behavioral and electrophysiological results were consistent with studies conducted on human participants and previous recordings made in multisensory brain regions of different species, we suggest that the rat represents an effective model for studying audiovisual temporal synchrony at both the neuronal and perceptual level.

Reductions in cortical alpha activity, enhancements in neural responses and impaired gap detection caused by sodium salicylate in awake guinea pigs

Tinnitus chronically affects between 10-15% of the population but, despite its prevalence, the underlying mechanisms are still not properly understood. One experimental model involves administration of high doses of sodium salicylate, as this is known to reliably induce tinnitus in both humans and animals. Guinea pigs were implanted with chronic electrocorticography (ECoG) electrode arrays, with silver-ball electrodes placed on the dura over left and right auditory cortex. Two more electrodes were positioned over the cerebellum to monitor auditory brainstem responses (ABRs). We recorded resting-state and auditory evoked neural activity from awake animals before and 2 h following salicylate administration (350 mg/kg; i.p.). Large increases in click-evoked responses (> 100%) were evident across the whole auditory cortex, despite significant reductions in wave I ABR amplitudes (in response to 20 kHz tones), which are indicative of auditory nerve activity. In the same animals, significant decreases in 6-10 Hz spontaneous oscillations (alpha waves) were evident over dorsocaudal auditory cortex. We were also able to demonstrate for the first time that cortical evoked potentials can be inhibited by a preceding gap in background noise [gap-induced pre-pulse inhibition (PPI)], in a similar fashion to the gap-induced inhibition of the acoustic startle reflex that is used as a behavioural test for tinnitus. Furthermore, 2 h following salicylate administration, we observed significant deficits in PPI of cortical responses that were closely aligned with significant deficits in behavioural responses to the same stimuli. Together, these data are suggestive of neural correlates of tinnitus and oversensitivity to sound (hyperacusis).

A Conditioned Behavioral Paradigm for Assessing Onset and Lasting Tinnitus in Rats

Numerous behavioral paradigms have been developed to assess tinnitus-like behavior in animals. Nevertheless, they are often limited by prolonged training requirements, as well as an inability to simultaneously assess onset and lasting tinnitus behavior, tinnitus pitch or duration, or tinnitus presence without grouping data from multiple animals or testing sessions. To enhance behavioral testing of tinnitus, we developed a conditioned licking suppression paradigm to determine the pitch(s) of both onset and lasting tinnitus-like behavior within individual animals. Rats learned to lick water during broadband or narrowband noises, and to suppress licking to avoid footshocks during silence. After noise exposure, rats significantly increased licking during silent trials, suggesting onset tinnitus-like behavior. Lasting tinnitus-behavior, however, was exhibited in about half of noise-exposed rats through 7 weeks post-exposure tested. Licking activity during narrowband sound trials remained unchanged following noise exposure, while ABR hearing thresholds fully recovered and were comparable between tinnitus⁽⁺⁾ and tinnitus^(-) rats. To assess another tinnitus inducer, rats were injected with sodium salicylate. They demonstrated high pitch tinnitus-like behavior, but later recovered by 5 days post-injection. Further control studies showed that 1): sham noise-exposed rats tested with footshock did not exhibit tinnitus-like behavior, and 2): noise-exposed or sham rats tested without footshocks showed no fundamental changes in behavior compared to those tested with shocks. Together, these results demonstrate that this paradigm can efficiently test the development of noise- and salicylate-induced tinnitus behavior. The ability to assess tinnitus individually, over time, and without averaging data enables us to realistically address tinnitus in a clinically relevant way. Thus, we believe that this optimized behavioral paradigm will facilitate investigations into the mechanisms of tinnitus and development of effective treatments.

Psychophysical and neural correlates of noised-induced tinnitus in animals: Intra- and inter-auditory and non-auditory brain structure studies

Tinnitus, a ringing in the ear or head without an external sound source, is a prevalent health problem. It is often associated with a number of limbic-associated disorders such as anxiety, sleep disturbance, and emotional distress. Thus, to investigate tinnitus, it is important to consider both auditory and non-auditory brain structures. This paper summarizes the psychophysical, immunocytochemical and electrophysiological evidence found in rats or hamsters with behavioral evidence of tinnitus. Behaviorally, we tested for tinnitus using a conditioned suppression/avoidance paradigm, gap detection acoustic reflex behavioral paradigm, and our newly developed conditioned licking suppression paradigm. Our new tinnitus behavioral paradigm requires relatively short baseline training, examines frequency specification of tinnitus perception, and achieves sensitive tinnitus testing at an individual level. To test for tinnitus-related anxiety and cognitive impairment, we used the elevated plus maze and Morris water maze. Our results showed that not all animals with tinnitus demonstrate anxiety and cognitive impairment. Immunocytochemically, we found that animals with tinnitus manifested increased Fos-like immunoreactivity (FLI) in both auditory and non-auditory structures. The manner in which FLI appeared suggests that lower brainstem structures may be involved in acute tinnitus whereas the midbrain and cortex are involved in more chronic tinnitus. Meanwhile, animals with tinnitus also manifested increased FLI in non-auditory brain structures that are involved in autonomic reactions, stress, arousal and attention. Electrophysiologically, we found that rats with tinnitus developed increased spontaneous firing in the auditory cortex (AC) and amygdala (AMG), as well as intra- and inter-AC and AMG neurosynchrony, which demonstrate that tinnitus may be actively produced and maintained by the interactions between the AC and AMG.

Tinnitus and hyperacusis involve hyperactivity and enhanced connectivity in auditory-limbic-arousal-cerebellar network

Hearing loss often triggers an inescapable buzz (tinnitus) and causes everyday sounds to become intolerably loud (hyperacusis), but exactly where and how this occurs in the brain is unknown. To identify the neural substrate for these debilitating disorders, we induced both tinnitus and hyperacusis with an ototoxic drug (salicylate) and used behavioral, electrophysiological, and functional magnetic resonance imaging (fMRI) techniques to identify the tinnitus-hyperacusis network. Salicylate depressed the neural output of the cochlea, but vigorously amplified sound-evoked neural responses in the amygdala, medial geniculate, and auditory cortex. Resting-state fMRI revealed hyperactivity in an auditory network composed of inferior colliculus, medial geniculate, and auditory cortex with side branches to cerebellum, amygdala, and reticular formation. Functional connectivity revealed enhanced coupling within the auditory network and segments of the auditory network and cerebellum, reticular formation, amygdala, and hippocampus. A testable model accounting for distress, arousal, and gating of tinnitus and hyperacusis is proposed.

Gap-Prepulse Inhibition of the Acoustic Startle Reflex (GPIAS) for Tinnitus Assessment: Current Status and Future Directions

The progress in the field of tinnitus largely depends on the development of a reliable tinnitus animal model. Recently, a new method based on the acoustic startle reflex modification was introduced for tinnitus screening in laboratory animals. This method was enthusiastically adopted and now widely used by many scientists in the field due to its seeming simplicity and a number of advantages over the other methods of tinnitus assessment. Furthermore, this method opened an opportunity for tinnitus assessment in humans as well. Unfortunately, multiple modifications of data collection and interpretation implemented in different labs make comparisons across studies very difficult. In addition, recent animal and human studies have challenged the original “filling-in” interpretation of the paradigm. Here, we review the current literature to emphasize on the commonalities and differences in data collection and interpretation across laboratories that are using this method for tinnitus assessment. We also propose future research directions that could be taken in order to establish whether or not this method is warranted as an indicator of the presence of tinnitus.

Salicylate-induced hearing loss and gap detection deficits in rats

To test the "tinnitus gap-filling" hypothesis in an animal psychoacoustic paradigm, rats were tested using a go/no-go operant gap detection task in which silent intervals of various durations were embedded within a continuous noise. Gap detection thresholds were measured before and after treatment with a dose of sodium salicylate (200 mg/kg) that reliably induces tinnitus in rats. Noise-burst detection thresholds were also measured to document the amount of hearing loss and aid in interpreting the gap detection results. As in the previous human psychophysical experiments, salicylate had little or no effect on gap thresholds measured in broadband noise presented at high-stimulus levels (30-60 dB SPL); gap detection thresholds were always 10 ms or less. Salicylate also did not affect gap thresholds presented in narrowband noise at 60 dB SPL. Therefore, rats treated with a dose of salicylate that reliably induces tinnitus have no difficulty detecting silent gaps as long as the noise in which they are embedded is clearly audible.

Central gain control in tinnitus and hyperacusis

Sensorineural hearing loss induced by noise or ototoxic drug exposure reduces the neural activity transmitted from the cochlea to the central auditory system. Despite a reduced cochlear output, neural activity from more central auditory structures is paradoxically enhanced at suprathreshold intensities. This compensatory increase in the central auditory activity in response to the loss of sensory input is referred to as central gain enhancement. Enhanced central gain is hypothesized to be a potential mechanism that gives rise to hyperacusis and tinnitus, two debilitating auditory perceptual disorders that afflict millions of individuals. This review will examine the evidence for gain enhancement in the central auditory system in response to cochlear damage. Further, it will address the potential cellular and molecular mechanisms underlying this enhancement and discuss the contribution of central gain enhancement to tinnitus and hyperacusis. Current evidence suggests that multiple mechanisms with distinct temporal and spectral profiles are likely to contribute to central gain enhancement. Dissecting the contributions of these different mechanisms at different levels of the central auditory system is essential for elucidating the role of central gain enhancement in tinnitus and hyperacusis and, most importantly, the development of novel treatments for these disorders.

Behavioral models of tinnitus and hyperacusis in animals

The phantom perception of tinnitus and reduced sound-level tolerance associated with hyperacusis have a high comorbidity and can be debilitating conditions for which there are no widely accepted treatments. One factor limiting the development of treatments for tinnitus and hyperacusis is the lack of reliable animal behavioral models of these disorders. Therefore, the purpose of this review is to highlight the current animal models of tinnitus and hyperacusis, and to detail the advantages and disadvantages of each paradigm. To date, this is the first review to include models of both tinnitus and hyperacusis.

Prolonged noise exposure-induced auditory threshold shifts in rats

Noise-induced hearing loss (NIHL) initially increases with exposure duration, but eventually reaches an asymptotic threshold shift (ATS) once the exposure duration exceeds 18-24 h. Equations for predicting the ATS have been developed for several species, but not for rats, even though this species is extensively used in noise exposure research. To fill this void, we exposed rats to narrowband noise (NBN, 16-20 kHz) for 5 weeks starting at 80 dB SPL in the first week and then increasing the level by 6 dB per week to a final level of 104 dB SPL. Auditory brainstem responses (ABR) were recorded before, during, and following the exposure to determine the amount of hearing loss. The noise induced threshold shift to continuous long-term exposure, defined as compound threshold shift (CTS), within and above 16-20 kHz increased with noise level at the rate of 1.82 dB threshold shift per dB of noise level (NL) above a critical level (C) of 77.2 dB SPL i.e. CTS = 1.82(NL-77.2). The normalized amplitude of the largest ABR peak measured at 100 dB SPL decreased at the rate of 3.1% per dB of NL above the critical level of 76.9 dB SPL, i.e., %ABR Reduction = 3.1%(NL-76.9). ABR thresholds measured >30 days post-exposure only partially recovered resulting in a permanent threshold shift of 30-40 dB along with severe hair cell loss in the basal, high-frequency region of the cochlea. In the rat, CTS increases with noise level with a slope similar to humans and chinchillas. The critical level (C) in the rat is similar to that of humans, but higher than that of chinchillas.

Salicylate-induced auditory perceptual disorders and plastic changes in nonclassical auditory centers in rats

Previous studies have shown that sodium salicylate (SS) activates not only central auditory structures, but also nonauditory regions associated with emotion and memory. To identify electrophysiological changes in the nonauditory regions, we recorded sound-evoked local field potentials and multiunit discharges from the striatum, amygdala, hippocampus, and cingulate cortex after SS-treatment. The SS-treatment produced behavioral evidence of tinnitus and hyperacusis. Physiologically, the treatment significantly enhanced sound-evoked neural activity in the striatum, amygdala, and hippocampus, but not in the cingulate. The enhanced sound evoked response could be linked to the hyperacusis-like behavior. Further analysis showed that the enhancement of sound-evoked activity occurred predominantly at the midfrequencies, likely reflecting shifts of neurons towards the midfrequency range after SS-treatment as observed in our previous studies in the auditory cortex and amygdala. The increased number of midfrequency neurons would lead to a relative higher number of total spontaneous discharges in the midfrequency region, even though the mean discharge rate of each neuron may not increase. The tonotopical overactivity in the midfrequency region in quiet may potentially lead to tonal sensation of midfrequency (the tinnitus). The neural changes in the amygdala and hippocampus may also contribute to the negative effect that patients associate with their tinnitus.

Animal models of subjective tinnitus

Tinnitus is one of the major audiological diseases, affecting a significant portion of the ageing society. Despite its huge personal and presumed economic impact there are only limited therapeutic options available. The reason for this deficiency lies in the very nature of the disease as it is deeply connected to elementary plasticity of auditory processing in the central nervous system. Understanding these mechanisms is essential for developing a therapy that reverses the plastic changes underlying the pathogenesis of tinnitus. This requires experiments that address individual neurons and small networks, something usually not feasible in human patients. However, in animals such invasive experiments on the level of single neurons with high spatial and temporal resolution are possible. Therefore, animal models are a very critical element in the combined efforts for engineering new therapies. This review provides an overview over the most important features of animal models of tinnitus: which laboratory species are suitable, how to induce tinnitus, and how to characterize the perceived tinnitus by behavioral means. In particular, these aspects of tinnitus animal models are discussed in the light of transferability to the human patients.