Effects of sodium salicylate on spontaneous and evoked spike rate in the dorsal cochlear nucleus

Neural Adaptation at Stimulus Onset and Speed of Neural Processing as Critical Contributors to Speech Comprehension Independent of Hearing Threshold or Age

Background: It is assumed that speech comprehension deficits in background noise are caused by age-related or acquired hearing loss. Methods: We examined young, middle-aged, and older individuals with and without hearing threshold loss using pure-tone (PT) audiometry, short-pulsed distortion-product otoacoustic emissions (pDPOAEs), auditory brainstem responses (ABRs), auditory steady-state responses (ASSRs), speech comprehension (OLSA), and syllable discrimination in quiet and noise. Results: A noticeable decline of hearing sensitivity in extended high-frequency regions and its influence on low-frequency-induced ABRs was striking. When testing for differences in OLSA thresholds normalized for PT thresholds (PTTs), marked differences in speech comprehension ability exist not only in noise, but also in quiet, and they exist throughout the whole age range investigated. Listeners with poor speech comprehension in quiet exhibited a relatively lower pDPOAE and, thus, cochlear amplifier performance independent of PTT, smaller and delayed ABRs, and lower performance in vowel-phoneme discrimination below phase-locking limits (/o/-/u/). When OLSA was tested in noise, listeners with poor speech comprehension independent of PTT had larger pDPOAEs and, thus, cochlear amplifier performance, larger ASSR amplitudes, and higher uncomfortable loudness levels, all linked with lower performance of vowel-phoneme discrimination above the phase-locking limit (/i/-/y/). Conslusions: This study indicates that listening in noise in humans has a sizable disadvantage in envelope coding when basilar-membrane compression is compromised. Clearly, and in contrast to previous assumptions, both good and poor speech comprehension can exist independently of differences in PTTs and age, a phenomenon that urgently requires improved techniques to diagnose sound processing at stimulus onset in the clinical routine.

Salicylate- and Noise-induced Tinnitus. Different Mechanisms Producing the same Result? An Experimental Model

PURPOSE

Tinnitus, the generation of phantom sounds, can be the result of noise exposure, however, understanding of its underlying mechanisms is limited. Purpose of the study was is to determine whether different concentrations of salicylate can cause tinnitus of different intensity.

METHODS

For the purposes of this study 50 male Wistar rats were used. The animals were divided into 5 groups (10 rats in each group). The animals that did not receive any substance were allocated to the control group (Group A). The second group (Group B) of rats received salicylate (Sigma Aldrich) intraperitoneally for 7 days (300 mg/Kg/day). The 3rd group (Group C) received salicylate intraperitoneally for 7 days, but at twice the concentration of the animals in the second group (600 mg/kg/d). The 4th group (Group D) simultaneously received salicylate (300 mg/Kg/day) and pure Memantine (Sigma Aldrich, 10 mg/kg/d) intraperitoneally for 7 days. The 5th group (Group E) did not receive any substance but was exposed for 168 consecutive hours (7 days) to sound to induce tinnitus. Cochlear activity was evaluated with the use of Distortion Product Otoacoustic Emissions (DPOAEs). At the end of the experimental period, the animals were sacrificed, and the right cochlea was removed and prepared for further histological and immunohistochemical studies.

RESULTS

The DPOAEs of animals treated either with salicylate as monotherapy or salicylate combined with memantine were indistinguishable from the noise floor, did not differ significantly compared to the animals of the control group or those expose to constant noise. The cochlear structures of Group E remained anatomically and functionally unaffected from the exposure to constant noise. Memantine does not seem to offer substantial protection to the cochlear structures, according to histological examination and hearing tests, however, the rats receiving it exhibited better results in behavioral tests.

CONCLUSIONS

The administration of memantine does not contribute significantly to the reduction of tinnitus.

Acoustic and optoacoustic stimulations in auditory brainstem response test in salicylate induced tinnitus

As a common debilitating disorder worldwide, tinnitus requires objective assessment. In the auditory brainstem response (ABR) test, auditory potentials can be evoked by acoustic or optoacoustic (induced by laser light) stimulations. In order to use the ABR test in the objective assessment of tinnitus, in this study, acoustic ABR (aABR) and optoacoustic ABR (oABR) were compared in the control and tinnitus groups to determine the changes caused by sodium salicylate (SS)-induced tinnitus in rat. In both aABR and oABR, wave II was the most prominent waveform, and the amplitude of wave II evoked by oABR was significantly higher than that of aABR. Brainstem transmission time (BTT), which represents the time required for a neural stimulation to progress from the auditory nerve ending to the inferior colliculus, was significantly shorter in oABR. In the tinnitus group, there was a significant increase in the threshold of both ABRs and a significant decrease in the amplitude of wave II only in the oABR. Based on our findings, the ABR test has the potential to be used in the assessment of SS-induced tinnitus, but oABR has the advantages of producing more prominent waveforms and significantly reducing the amplitude of wave II in tinnitus.

The effect of systemic administration of salicylate on the auditory cortex of guinea pigs

Objective

To investigate the effect of systemic administration of salicylate as a tinnitus inducing drug in the auditory cortex of guinea pigs.

Methods

Extracellular recording of spikes of the primary auditory cortex and dorsocaudal areas in healthy male albino Hartley guinea pigs was continuously performed (pre- and post-salicylate).

Results

We recorded 160 single units in the primary auditory cortex from five guinea pigs and 156 single units in the dorsocaudal area from another five guinea pigs. The threshold was significantly elevated after the administration of salicylate in both the primary auditory cortex and dorsocaudal areas. The Q_10dB value was significantly increased in the primary auditory cortex, whereas it has significantly decreased in the dorsocaudal area. Spontaneous firing activity was significantly decreased in the primary auditory cortex, whereas it has significantly increased in the dorsocaudal area.

Conclusion

Salicylate induces significant changes in single units of both stimulated and spontaneous activity in the auditory cortex of guinea pigs. The spontaneous activity changed differently depending on its cortical areas, which may be due to the neural elements that generate tinnitus.

Increased risk of tinnitus following a trigeminal neuralgia diagnosis: a one-year follow-up study

Background

Tinnitus due to hyperactivity across neuronal ensembles along the auditory pathway is reported. We hypothesized that trigeminal neuralgia patients may subsequently suffer from tinnitus. Using nationwide, population-based data and a retrospective cohort study design, we investigated the risk of tinnitus within 1 year following trigeminal neuralgia.

Methods

We used the Taiwan National Health Insurance Research Dataset, a claims database, to identify all patients diagnosed with trigeminal neuralgia from January 2001 to December 2014, 12,587 patients. From the remaining patients, we identified 12,587 comparison patients without trigeminal neuralgia by propensity score matching, using sex, age, monthly income, geographic region, residential urbanization level, and tinnitus-relevant comorbidities (hyperlipidemia, diabetes, coronary heart disease, hypertension, cervical spondylosis, temporomandibular joint disorders and injury to head and neck and index year). All study patients (n = 25,174) were tracked for a one-year period to identify those with a subsequent diagnosis of tinnitus over 1-year follow-up.

Results

Among total 25,174 sample patients, the incidence of tinnitus was 18.21 per 100 person-years (95% CI = 17.66 ~ 18.77), the rate being 23.57 (95% CI = 22.68 ~ 24.49) among patients with trigeminal neuralgia and 13.17 (95% CI = 12.53 ~ 13.84) among comparison patients. Furthermore, the adjusted Cox proportional hazard ratio for tinnitus in the trigeminal neuralgia group was 1.68 (95% CI = 1.58 ~ 1.80) relative to the comparison cohort.

Conclusions

We found a significantly increased risk of tinnitus within 1 year of trigeminal neuralgia diagnosis compared to those without the diagnosis. Further studies in other countries and ethnicities are needed to explore the relationship between trigeminal neuralgia and subsequent tinnitus.

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.

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.

Salicylate-Induced Suppression of Electrically Driven Activity in Brain Slices from the Auditory Cortex of Aging Mice

The prevalence of tinnitus is known to increase with age. The age-dependent mechanisms of tinnitus may have important implications for the development of new therapeutic treatments. High doses of salicylate can be used experimentally to induce transient tinnitus and hearing loss. Although accumulating evidence indicates that salicylate induces tinnitus by directly targeting neurons in the peripheral and central auditory systems, the precise effect of salicylate on neural networks in the auditory cortex (AC) is unknown. Here, we examined salicylate-induced changes in stimulus-driven laminar responses of AC slices with salicylate superfusion in young and aged senescence-accelerated-prone (SAMP) and -resistant (SAMR) mice. Of the two strains, SAMP1 is known to be a more suitable model of presbycusis. We recorded stimulus-driven laminar local field potential (LFP) responses at multi sites in AC slice preparations. We found that for all AC slices in the two strains, salicylate always reduced stimulus-driven LFP responses in all layers. However, for the amplitudes of the LFP responses, the two senescence-accelerated mice (SAM) strains showed different laminar properties between the pre- and post-salicylate conditions, reflecting strain-related differences in local circuits. As for the relationships between auditory brainstem response (ABR) thresholds and the LFP amplitude ratios in the pre- vs. post-salicylate condition, we found negative correlations in layers 2/3 and 4 for both older strains, and in layer 5 (L5) in older SAMR1. In contrast, the GABAergic agonist muscimol (MSC) led to positive correlations between ABR thresholds and LFP amplitude ratios in the pre- vs. post-MSC condition in younger SAM mice from both strains. Further, in younger mice, salicylate decreased the firing rate in AC L4 pyramidal neurons. Thus, salicylate can directly reduce neural excitability of L4 pyramidal neurons and thereby influence AC neural circuit activity. That we observed age-dependent effects of salicylate and varied GABAergic sensitivity in the AC among mouse strains with hearing loss implies that potential therapeutic mechanisms for tinnitus may operate differently in young vs. aged subjects. Therefore, scientists developing new therapeutic modalities for tinnitus treatment should consider using both aged and young animals.

Can Animal Models Contribute to Understanding Tinnitus Heterogeneity in Humans?

The brain activity of humans with tinnitus of various etiologies is typically studied with electro- and magneto-encephalography and functional magnetic resonance imaging-based imaging techniques. Consequently, they measure population responses and mostly from the neocortex. The latter also underlies changes in neural networks that may be attributed to tinnitus. However, factors not strictly related to tinnitus such as hearing loss and hyperacusis, as well as other co-occurring disorders play a prominent role in these changes. Different types of tinnitus can often not be resolved with these brain-imaging techniques. In animal models of putative behavioral signs of tinnitus, neural activity ranging from auditory nerve to auditory cortex, is studied largely by single unit recordings, augmented by local field potentials (LFPs), and the neural correlates of tinnitus are mainly based on spontaneous neural activity, such as spontaneous firing rates and pair-wise spontaneous spike-firing correlations. Neural correlates of hyperacusis rely on measurement of stimulus-evoked activity and are measured as increased driven firing rates and LFP amplitudes. Connectivity studies would rely on correlated neural activity between pairs of neurons or LFP amplitudes, but are only recently explored. In animal models of tinnitus, only two etiologies are extensively studied; tinnitus evoked by salicylate application and by noise exposure. It appears that they have quite different neural biomarkers. The unanswered question then is: does this different etiology also result in different tinnitus?

Specific immediate early gene expression induced by high doses of salicylate in the cochlear nucleus and inferior colliculus of the rat

INTRODUCTION

Salicylate at high doses induces tinnitus in humans and experimental animals. However, the mechanisms and loci of action of salicylate in inducing tinnitus are still not well known. The expression of Immediate Early Genes (IEG) is traditionally associated with long-term neuronal modifications but it is still not clear how and where IEGs are activated in animal models of tinnitus.

OBJECTIVES

Here we investigated the expression of c-fos and Egr-1, two IEGs, in the Dorsal Cochlear Nucleus (DCN), the Inferior Colliculus (IC), and the Posterior Ventral Cochlear Nucleus (pVCN) of rats.

METHODS

Rats were treated with doses known to induce tinnitus in rats (300mg/kg i.p. daily, for 3 days), and c-fos and Egr-1 protein expressions were analyzed using western blot and immunocytochemistry.

RESULTS

After administration of salicylate, c-fos protein expression increased significantly in the DCN, pVCN and IC when assayed by western blot. Immunohistochemistry staining showed a more intense labeling of c-fos in the DCN, pVCN and IC and a significant increase in c-fos positive nuclei in the pVCN and IC. We did not detect increased Egr-1 expression in any of these areas.

CONCLUSION

Our data show that a high dose of salicylate activates neurons in the DCN, pVCN and IC. The expression of these genes by high doses of salicylate strongly suggests that plastic changes in these areas are involved in the genesis of tinnitus.

Neural Hyperactivity of the Central Auditory System in Response to Peripheral Damage

It is increasingly appreciated that cochlear pathology is accompanied by adaptive responses in the central auditory system. The cause of cochlear pathology varies widely, and it seems that few commonalities can be drawn. In fact, despite intricate internal neuroplasticity and diverse external symptoms, several classical injury models provide a feasible path to locate responses to different peripheral cochlear lesions. In these cases, hair cell damage may lead to considerable hyperactivity in the central auditory pathways, mediated by a reduction in inhibition, which may underlie some clinical symptoms associated with hearing loss, such as tinnitus. Homeostatic plasticity, the most discussed and acknowledged mechanism in recent years, is most likely responsible for excited central activity following cochlear damage.

Sodium salicylate potentiates the GABAB-GIRK pathway to suppress rebound depolarization in neurons of the rat's medial geniculate body

Rebound depolarization (RD) is a voltage response to the offset from pre-hyperpolarization of neuronal membrane potential, which manifests a particular form of the postsynaptic membrane potential response to inhibitory presynaptic inputs. We previously demonstrated that sodium salicylate (NaSal), a tinnitus inducer, can drastically suppress the RD in neurons of rat medial geniculate body (MGB) (Su et al, 2012; PLoS ONE 7, e46969). The purpose of the present study was to investigate the underlying cellular mechanism by using whole-cell patch-clamp recordings in rat MGB slices. NaSal (1.4 mM) had no effects on the current mediated by T-type Ca(2+) channels, indicating that it does not target these channels to suppress the RD. Instead, NaSal was shown to hyperpolarize the resting membrane potential to suppress the RD. NaSal had no effects on the current mediated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, indicating that it does not target these channels to hyperpolarize the resting membrane potential. NaSal induced an outward leak current that could be abolished by CGP55845, a GABAB receptor blocker, or respectively by Ba(2+) and Tertiapin-Q, blockers for G-protein-gated inwardly rectifying potassium (GIRK) channels, indicating that NaSal potentiates the GABAB-GIRK pathway to hyperpolarize the resting membrane potential. Our study demonstrates that NaSal targets GABAB receptors to alter functional behaviors of MGB neurons, which may be implicated in NaSal-induced tinnitus.

Characteristic anatomical structures of rat temporal bone

As most gene sequences and functional structures of internal organs in rats have been well studied, rat models are widely used in experimental medical studies. A large number of descriptions and atlas of the rat temporal bone have been published, but some detailed anatomy of its surface and inside structures remains to be studied. By focusing on some unique characteristics of the rat temporal bone, the current paper aims to provide more accurate and detailed information on rat temporal bone anatomy in an attempt to complete missing or unclear areas in the existed knowledge. We also hope this paper can lay a solid foundation for experimental rat temporal bone surgeries, and promote information exchange among colleagues, as well as providing useful guidance for novice researchers in the field of hearing research involving rats.

Tinnitus: animal models and findings in humans

Chronic tinnitus (ringing of the ears) is a medically untreatable condition that reduces quality of life for millions of individuals worldwide. Most cases are associated with hearing loss that may be detected by the audiogram or by more sensitive measures. Converging evidence from animal models and studies of human tinnitus sufferers indicates that, while cochlear damage is a trigger, most cases of tinnitus are not generated by irritative processes persisting in the cochlea but by changes that take place in central auditory pathways when auditory neurons lose their input from the ear. Forms of neural plasticity underlie these neural changes, which include increased spontaneous activity and neural gain in deafferented central auditory structures, increased synchronous activity in these structures, alterations in the tonotopic organization of auditory cortex, and changes in network behavior in nonauditory brain regions detected by functional imaging of individuals with tinnitus and corroborated by animal investigations. Research on the molecular mechanisms that underlie neural changes in tinnitus is in its infancy and represents a frontier for investigation.

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.

Expression of immediate-early genes in the inferior colliculus and auditory cortex in salicylate-induced tinnitus in rat

Tinnitus could be associated with neuronal hyperactivity in the auditory center. As a neuronal activity marker, immediate-early gene (IEG) expression is considered part of a general neuronal response to natural stimuli. Some IEGs, especially the activity-dependent cytoskeletal protein (Arc) and the early growth response gene-1 (Egr-1), appear to be highly correlated with sensory-evoked neuronal activity. We hypothesize, therefore, an increase of Arc and Egr-1 will be observed in a tinnitus model. In our study, we used the gap prepulse inhibition of acoustic startle (GPIAS) paradigm to confirm that salicylate induces tinnitus-like behavior in rats. However, expression of the Arc gene and Egr-1 gene were decreased in the inferior colliculus (IC) and auditory cortex (AC), in contradiction of our hypothesis. Expression of N-methyl d-aspartate receptor subunit 2B (NR2B) was increased and all of these changes returned to normal 14 days after treatment with salicylate ceased. These data revealed long-time administration of salicylate induced tinnitus markedly but reversibly and caused neural plasticity changes in the IC and the AC. Decreased expression of Arc and Egr-1 might be involved with instability of synaptic plasticity in tinnitus.

A novel behavioural approach to detecting tinnitus in the guinea pig

Tinnitus, the perception of sound in the absence of an external stimulus, is a particularly challenging condition to demonstrate in animals. In any animal model, objective confirmation of tinnitus is essential before we can study the neural changes that produce it. A gap detection method, based on prepulse inhibition of the whole-body startle reflex, is often used as a behavioural test for tinnitus in rodents. However, in the guinea pig the whole-body startle reflex is subject to rapid habituation and hence is not an ideal behavioural measure. By contrast, in this species the Preyer or pinna reflex is a very reliable indicator of the startle response and is much less subject to habituation. We have developed a novel adaptation of the gap detection paradigm, which uses the Preyer reflex to measure the startle response, rather than whole-body movement. Using this method, we have demonstrated changes in gap detection, in guinea pigs where tinnitus had been induced by the administration of a high dose of salicylate. Our data indicate that the Preyer reflex gap detection method is a reliable test for tinnitus in guinea pigs.

Altered neuronal intrinsic properties and reduced synaptic transmission of the rat's medial geniculate body in salicylate-induced tinnitus

Sodium salicylate (NaSal), an aspirin metabolite, can cause tinnitus in animals and human subjects. To explore neural mechanisms underlying salicylate-induced tinnitus, we examined effects of NaSal on neural activities of the medial geniculate body (MGB), an auditory thalamic nucleus that provides the primary and immediate inputs to the auditory cortex, by using the whole-cell patch-clamp recording technique in MGB slices. Rats treated with NaSal (350 mg/kg) showed tinnitus-like behavior as revealed by the gap prepulse inhibition of acoustic startle (GPIAS) paradigm. NaSal (1.4 mM) decreased the membrane input resistance, hyperpolarized the resting membrane potential, suppressed current-evoked firing, changed the action potential, and depressed rebound depolarization in MGB neurons. NaSal also reduced the excitatory and inhibitory postsynaptic response in the MGB evoked by stimulating the brachium of the inferior colliculus. Our results demonstrate that NaSal alters neuronal intrinsic properties and reduces the synaptic transmission of the MGB, which may cause abnormal thalamic outputs to the auditory cortex and contribute to NaSal-induced tinnitus.

Salicylate toxicity model of tinnitus

Salicylate, the active component of the common drug aspirin, has mild analgesic, antipyretic, and anti-inflammatory effects at moderate doses. At higher doses, however, salicylate temporarily induces moderate hearing loss and the perception of a high-pitch ringing in humans and animals. This phantom perception of sound known as tinnitus is qualitatively similar to the persistent subjective tinnitus induced by high-level noise exposure, ototoxic drugs, or aging, which affects ∼14% of the general population. For over a quarter century, auditory scientists have used the salicylate toxicity model to investigate candidate biochemical and neurophysiological mechanisms underlying phantom sound perception. In this review, we summarize some of the intriguing biochemical and physiological effects associated with salicylate-induced tinnitus, some of which occur in the periphery and others in the central nervous system. The relevance and general utility of the salicylate toxicity model in understanding phantom sound perception in general are discussed.

Understanding tinnitus: the dorsal cochlear nucleus, organization and plasticity

Tinnitus, the perception of a phantom sound, is a common consequence of damage to the auditory periphery. A major goal of tinnitus research is to find the loci of the neural changes that underlie the disorder. Crucial to this endeavor has been the development of an animal behavioral model of tinnitus, so that neural changes can be correlated with behavioral evidence of tinnitus. Three major lines of evidence implicate the dorsal cochlear nucleus (DCN) in tinnitus. First, elevated spontaneous activity in the DCN is correlated with peripheral damage and tinnitus. Second, there are somatosensory inputs to the DCN that can modulate spontaneous activity and might mediate the somatic-auditory interactions seen in tinnitus patients. Third, we have found a subpopulation of DCN neurons in the adult rat that express doublecortin, a plasticity-related protein. The expression of this protein may reflect a role of these neurons in the neural reorganization causing tinnitus. However, there is a problem in extending the findings in the rodent DCN to humans. Classic studies state that the structure of the primate DCN is quite different from that of rodents, with primates lacking granule cells, the recipients of somatosensory input. To address the possibility of major species differences in DCN organization, we compared Nissl-stained sections of the DCN in five different species. In contrast to earlier reports, our data suggest that the organization of the primate DCN is not dramatically different from that of the rodents, and validate the use of animal data in the study of tinnitus. This article is part of a Special Issue entitled: Tinnitus Neuroscience.

GABAergic neural activity involved in salicylate-induced auditory cortex gain enhancement

Although high doses of sodium salicylate impair cochlear function, it paradoxically enhances sound-evoked activity in the auditory cortex (AC) and augments acoustic startle reflex responses, neural and behavioral metrics associated with hyperexcitability and hyperacusis. To explore the neural mechanisms underlying salicylate (SS)-induced hyperexcitability and "increased central gain," we examined the effects of GABA receptor agonists and antagonists on SS-induced hyperexcitability in the AC and startle reflex responses. Consistent with our previous findings, local or systemic application of SS significantly increased the amplitude of sound-evoked AC neural activity, but generally reduced spontaneous activity in the AC. Systemic injection of SS also significantly increased the acoustic startle reflex. S-baclofen or R-baclofen, GABA-B agonists, which suppressed sound-evoked AC neural firing rate and local field potentials, also suppressed the SS-induced enhancement of the AC field potential and the acoustic startle reflex. Local application of vigabatrin, which enhances GABA concentration in the brain, suppressed the SS-induced enhancement of AC firing rate. Systemic injection of vigabatrin also reduced the SS-induced enhancement of acoustic startle reflex. Collectively, these results suggest that the sound-evoked behavioral and neural hyperactivity induced by SS may arise from a SS-induced suppression of GABAergic inhibition in the AC.

Molecular layer inhibitory interneurons provide feedforward and lateral inhibition in the dorsal cochlear nucleus

In the outer layers of the dorsal cochlear nucleus, a cerebellum-like structure in the auditory brain stem, multimodal sensory inputs drive parallel fibers to excite both principal (fusiform) cells and inhibitory cartwheel cells. Cartwheel cells, in turn, inhibit fusiform cells and other cartwheel cells. At the microcircuit level, it is unknown how these circuit components interact to modulate the activity of fusiform cells and thereby shape the processing of auditory information. Using a variety of approaches in mouse brain stem slices, we investigated the synaptic connectivity and synaptic strength among parallel fibers, cartwheel cells, and fusiform cells. In paired recordings of spontaneous and evoked activity, we found little overlap in parallel fiber input to neighboring neurons, and activation of multiple parallel fibers was required to evoke or alter action potential firing in cartwheel and fusiform cells. Thus neighboring neurons likely respond best to distinct subsets of sensory inputs. In contrast, there was significant overlap in inhibitory input to neighboring neurons. In recordings from synaptically coupled pairs, cartwheel cells had a high probability of synapsing onto nearby fusiform cells or other nearby cartwheel cells. Moreover, single cartwheel cells strongly inhibited spontaneous firing in single fusiform cells. These synaptic relationships suggest that the set of parallel fibers activated by a particular sensory stimulus determines whether cartwheel cells provide feedforward or lateral inhibition to their postsynaptic targets.