Auditory sensori-neural alterations induced by salicylate

Sodium salicylate improves detection of amplitude-modulated sound in mice

Salicylate is commonly used to induce tinnitus in animals, but its underlying mechanism of action is still debated. We therefore tested its effects on the firing properties of neurons in the mouse inferior colliculus (IC). Salicylate induced a large decrease in the spontaneous activity and an increase of ∼20 dB SPL in the minimum threshold of single units. In response to sinusoidally modulated noise (SAM noise) single units showed both an increase in phase locking and improved rate coding. Mice also became better at detecting amplitude modulations, and a simple threshold model based on the IC population response could reproduce this improvement. The responses to dynamic random chords (DRCs) suggested that the improved AM encoding was due to a linearization of the cochlear output, resulting in larger contrasts during SAM noise. These effects of salicylate are not consistent with the presence of tinnitus, but should be taken into account when studying hyperacusis.

A Central Auditory Test reveals differences between drug treatment regimens in adults living with HIV

OBJECTIVE

This exploratory study examined whether central auditory tests show differences between people living with HIV (PLWH) treated with two predominant antiretroviral drug therapy (ART) regimens.

DESIGN

Cross-sectional.

STUDY SAMPLE

253 PLWH (mean age 39.8 years) from the Shanghai Public Health Clinical Centre, China.

METHODS

The Hearing in Noise Test speech reception threshold (SRT) assessed central auditory function and the Montreal Cognitive Assessment (MoCA) assessed cognition. The relationship between ART regimen and SRT was evaluated with multivariable linear regression incorporating age, HIV duration, and peripheral hearing ability. Multivariable logistic regression was used to ascertain if SRT and ART regimen predicted MoCA impairment.

RESULTS

The two predominant ART regimens differed by one drug (zidovudine or tenofovir). Participants taking the zidovudine-containing regimen had poorer SRT performance (p=.012) independent of age and hearing thresholds. MoCA scores did not differ between drug regimens, but a negative relationship was found between SRT and MoCA impairment (p=.048).

CONCLUSIONS

ART regimens differed in their association with central auditory test performance likely reflecting neurocognitive changes in PLWH taking the zidovudine-containing regimen. Central auditory test performance also marginally predicted cognitive impairment, supporting further assessment of central auditory tests to detect neurocognitive deficits in PLWH.

What is the role of the hippocampus and parahippocampal gyrus in the persistence of tinnitus?

The hippocampus and parahippocampal gyrus have been implicated as part of a tinnitus network by a number of studies. These structures are usually considered in the context of a "limbic system," a concept typically invoked to explain the emotional response to tinnitus. Despite this common framing, it is not apparent from current literature that this is necessarily the main functional role of these structures in persistent tinnitus. Here, we highlight a different role that encompasses their most commonly implicated functional position within the brain-that is, as a memory system. We consider tinnitus as an auditory object that is held in memory, which may be made persistent by associated activity from the hippocampus and parahippocampal gyrus. Evidence from animal and human studies implicating these structures in tinnitus is reviewed and used as an anchor for this hypothesis. We highlight the potential for the hippocampus/parahippocampal gyrus to facilitate maintenance of the memory of the tinnitus percept via communication with auditory cortex, rather than (or in addition to) mediating emotional responses to this percept.

Functional Connectivity Alterations and Molecular Characterization of the Anterior Cingulate Cortex in Tinnitus Pathology without Hearing Loss

Compared with individuals with hearing loss, tinnitus patients without hearing loss have more psychological or emotional problems. Tinnitus is closely associated to abnormal metabolism and function of the limbic system, a key brain region for emotion experience, but the underlying molecular mechanism remains unknown. Using whole-brain microvasculature dynamics imaging, the anterior cingulate cortex (ACC) is identified as a key brain region of limbic system involve in the onset of salicylate-induced tinnitus in mice. In the tinnitus group, there is enhanced purine metabolism, oxidative phosphorylation, and a distinct pattern of phosphorylation in glutamatergic synaptic pathway according to the metabolome profiles, quantitative proteomic, and phosphoproteomic data of mice ACC tissue. Electroencephalogram in tinnitus patients with normal hearing thresholds show that the functional connectivity between pregenual anterior cingulate cortex and the primary auditory cortex is significantly increased for high-gamma frequency band, which is positively correlated with the serum glutamate level. These findings indicate that ACC plays an important role in the pathophysiology of tinnitus by interacting with the primary auditory cortex and provide potential molecular targets in the ACC for tinnitus treatment.

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.

Novel Nanoprobe with Combined Ultrasonography/Chemical Exchange Saturation Transfer Magnetic Resonance Imaging for Precise Diagnosis of Tumors

Given that cancer mortality is usually due to a late diagnosis, early detection is crucial to improve the patient's results and prevent cancer-related death. Imaging technology based on novel nanomaterials has attracted much attention for early-stage cancer diagnosis. In this study, a new block copolymer, poly(ethylene glycol)-poly(l-lactide) diblock copolymer (PEG-PLLA), was synthesized by the ring-opening polymerization method and thoroughly characterized using Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance spectroscopy (H-NMR), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The obtained PEG-PLLA was used to prepare nanoparticles encapsulated with perfluoropentane and salicylic acid by the emulsion-solvent evaporation method, resulting in a new dual-mode nano-image probe (PEG-PLLA@SA·PFP). The zeta potential and mean diameter of the obtained nanoparticles were measured using dynamic light scattering (DLS) with a Malvern Zetersizer Nano. The in vitro biocompatibility of the PEG-PLLA nanoparticles was evaluated with cell migration, hemolysis, and cytotoxicity assays. Ultrasonic imaging was performed using an ultrasonic imaging apparatus, and chemical exchange saturation transfer (CEST) MRI was conducted on a 7.0 T animal scanner. The results of IR and NMR confirmed that the PEG-PLLA was successfully synthesized. The particle size and negative charge of the nanoparticles were 223.8 ± 2.5 nm and -39.6 ± 1.9 mV, respectively. The polydispersity of the diameter was 0.153 ± 0.020. These nanoparticles possessed good stability at 4 °C for about one month. The results of cytotoxicity, cell migration, and hemolysis assays showed that the carrier material was biocompatible. Finally, PEG-PLLA nanoparticles were able to significantly enhance the imaging effect of tumors by the irradiation of ultrasound and saturation by a radiofrequency pulse, respectively. In conclusion, these nanoparticles exhibit promising dual-mode capabilities for US/CEST MR imaging.

Progress on mechanisms of age-related hearing loss

Age-related hearing loss, or presbycusis, is a common cause of hearing loss in elderly people worldwide. It typically presents as progressive, irreversible, and usually affects the high frequencies of hearing, with a tremendous impact on the quality of life. Presbycusis is a complex multidimensional disorder, in addition to aging, multiple factors including exposure to noise, or ototoxic agents, genetic susceptibility, metabolic diseases and lifestyle can influence the onset and severity of presbycusis. With the aging of the body, its ability to clean up deleterious substances produced in the metabolic process is weakened, and the self-protection and repair function of the body is reduced, which in turn leads to irreversible damage to the cochlear tissue, resulting in the occurrence of presbycusis. Presently, oxidative stress (OS), mitochondrial DNA damage, low-grade inflammation, decreased immune function and stem cell depletion have been demonstrated to play a critical role in developing presbycusis. The purpose of this review is to illuminate the various mechanisms underlying this age-related hearing loss, with the goal of advancing our understanding, prevention, and treatment of presbycusis.

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.

Curcumin attenuates gentamicin and sodium salicylate ototoxic effects by modulating the nuclear factor-kappaB and apoptotic pathways in rats

This study aimed to investigate the effectiveness of curcumin (CCM) against gentamicin (GEN) and sodium salicylates (NaS)-induced ototoxic effects in rats. For 15 consecutive days, seven rat groups were given 1 mL/rat physiological saline orally, 1 mL/rat olive oil orally, 50 mg/kg bwt CCM orally, 120 mg/kg bwt GEN intraperitoneally, 300 mg/kg bwt NaS intraperitoneally, CCM+GEN, or CCM+NaS. The distortion product otoacoustic emission measurements were conducted. The rats' hearing function and balance have been behaviorally assessed using auditory startle response, Preyer reflex, and beam balance scale tests. The serum lipid peroxidation and oxidative stress biomarkers have been measured. Immunohistochemical investigations of the apoptotic marker caspase-3 and the inflammatory indicator nuclear factor kappa (NF-κB) in cochlear tissues were conducted. GEN and NaS exposure resulted in deficit hearing and impaired ability to retain balance. GEN and NaS exposure significantly decreased the reduced glutathione level and catalase activity but increased malondialdehyde content. GEN and NaS exposure evoked pathological alterations in cochlear and vestibular tissues and increased caspase-3 and NF-κB immunoexpression. CCM significantly counteracted the GEN and NaS injurious effects. These outcomes concluded that CCM could be a naturally efficient therapeutic agent against GEN and NaS-associated ototoxic side effects.

Longitudinal Study of Analgesic Use and Risk of Incident Persistent Tinnitus

BACKGROUND

Persistent tinnitus is common, disabling, and difficult to treat. High-dose aspirin may precipitate tinnitus, but longitudinal data on typical dose aspirin and other analgesics are scarce.

OBJECTIVE

To investigate independent associations of aspirin, NSAIDs, and acetaminophen and risk of incident persistent tinnitus.

DESIGN

Longitudinal cohort study.

SETTING

Nurses' Health Study II (1995-2017).

PARTICIPANTS

A total of 69,455 women, age 31-48 years, without tinnitus at baseline.

MAIN MEASURES

Information on analgesic use and tinnitus obtained by biennial questionnaires.

KEY RESULTS

After 1,120,936 person-years of follow-up, 10,452 cases of incident persistent tinnitus were reported. For low-dose aspirin, the risk of developing persistent tinnitus was not elevated among frequent low-dose aspirin users. For moderate dose aspirin, frequent use was associated with higher risk of tinnitus among women aged < 60 years, but not among older women (p-interactionage = 0.003). Compared with women aged < 60 using moderate-dose aspirin < 1 day/week, the multivariable-adjusted hazard ratio (MVHR, 95% CI) among women using moderate-dose aspirin 6-7 days per week was 1.16 (1.03, 1.32). Among all women, frequent non-aspirin non-steroidal anti-inflammatory drug (NSAID) or acetaminophen use was associated with higher risk. Compared with women using NSAIDs <1 day/week, the MVHR for use 4-5days/week was 1.17 (1.08, 1.28) and for 6-7days/week was 1.07 (1.00, 1.16) (p-trend=0.001). For acetaminophen, compared with use <1 day/week, the MVHR for use 6-7days/week was 1.18 (1.07, 1.29) (p-trend=0.002).

LIMITATIONS

Information on tinnitus and analgesic use was self-reported. Information on indications for analgesic use was not available. Studies in non-White women and men are needed.

CONCLUSION

The risk of developing persistent tinnitus was not elevated among frequent low-dose aspirin users. Among younger women, frequent moderate-dose aspirin use was associated with higher risk. Frequent NSAID use and frequent acetaminophen use were associated with higher risk of incident persistent tinnitus among all women, and the magnitude of the risks tended to be greater with increasing frequency of use. Our results suggest analgesic users are at higher risk for developing tinnitus and may provide insight into the precipitants of this challenging disorder, but additional investigation to determine whether there is a causal association is needed.

[The professional ear user-implications for the prevention, diagnosis, and treatment of ear diseases]

BACKGROUND

Perfect hearing is crucial to the practice of various professions, such as instrument makers, musicians, sound engineers, and other professions not related to music, such as sonar technicians. For people of these occupational groups, we propose the term "professional ear user" (PEU) in analogy to "professional voice user". PEUs have special requirements for their hearing health, as they have well-known above-average auditory perceptual abilities on which they are professionally dependent.

OBJECTIVE

The purpose of this narrative review is to summarize selected aspects of the prevention, diagnosis, and treatment of ear disorders in PEUs.

RESULTS AND CONCLUSION

Prevention of hearing disorders and other ear diseases includes protection from excessive sound levels, avoidance of ototoxins and nicotine, and a safe manner of cleaning the outer auditory canal. Diagnosing hearing disorders in PEUs can be challenging, since subclinical but relevant changes in hearing cannot be reliably objectified by conventional audiometric methods. Moreover, the fact that a PEU is affected by an ear disease may influence treatment decisions. Further, physicians must be vigilant for non-organic ear diseases in PEUs. Lastly, measures to promote comprehensive ear health in PEUs as part of an educational program and to maintain ear health by means of a specialized otolaryngology service are discussed. In contrast to existing concepts, we lay the attention on the entirety of occupational groups that are specifically dependent on their ear health in a professional setting. In this context, we suggest avoiding a sole focus on hearing disorders and their prevention, but rather encourage the maintenance of a comprehensive ear health.

Cryo-EM structures of thermostabilized prestin provide mechanistic insights underlying outer hair cell electromotility

Outer hair cell elecromotility, driven by prestin, is essential for mammalian cochlear amplification. Here, we report the cryo-EM structures of thermostabilized prestin (PresTS), complexed with chloride, sulfate, or salicylate at 3.52-3.63 Å resolutions. The central positively-charged cavity allows flexible binding of various anion species, which likely accounts for the known distinct modulations of nonlinear capacitance (NLC) by different anions. Comparisons of these PresTS structures with recent prestin structures suggest rigid-body movement between the core and gate domains, and provide mechanistic insights into prestin inhibition by salicylate. Mutations at the dimeric interface severely diminished NLC, suggesting that stabilization of the gate domain facilitates core domain movement, thereby contributing to the expression of NLC. These findings advance our understanding of the molecular mechanism underlying mammalian cochlear amplification.

Identifying targets to prevent aminoglycoside ototoxicity

Aminoglycosides are potent antibiotics that are commonly prescribed worldwide. Their use carries significant risks of ototoxicity by directly causing inner ear hair cell degeneration. Despite their ototoxic side effects, there are currently no approved antidotes. Here we review recent advances in our understanding of aminoglycoside ototoxicity, mechanisms of drug transport, and promising sites for intervention to prevent ototoxicity.

The conformational cycle of prestin underlies outer-hair cell electromotility

The voltage-dependent motor protein prestin (also known as SLC26A5) is responsible for the electromotive behaviour of outer-hair cells and underlies the cochlear amplifier1. Knockout or impairment of prestin causes severe hearing loss2-5. Despite the key role of prestin in hearing, the mechanism by which mammalian prestin senses voltage and transduces it into cellular-scale movements (electromotility) is poorly understood. Here we determined the structure of dolphin prestin in six distinct states using single-particle cryo-electron microscopy. Our structural and functional data suggest that prestin adopts a unique and complex set of states, tunable by the identity of bound anions (Cl- or SO42-). Salicylate, a drug that can cause reversible hearing loss, competes for the anion-binding site of prestin, and inhibits its function by immobilizing prestin in a new conformation. Our data suggest that the bound anion together with its coordinating charged residues and helical dipole act as a dynamic voltage sensor. An analysis of all of the anion-dependent conformations reveals how structural rearrangements in the voltage sensor are coupled to conformational transitions at the protein-membrane interface, suggesting a previously undescribed mechanism of area expansion. Visualization of the electromotility cycle of prestin distinguishes the protein from the closely related SLC26 anion transporters, highlighting the basis for evolutionary specialization of the mammalian cochlear amplifier at a high resolution.

Diabetes mellitus and hearing loss: A review

Diabetes (type 2) and sensorineural hearing loss are common health problems manifested with ageing. While both type 1 and type 2 diabetes have been associated with hearing loss, a causal link has been difficult to establish. Individuals with diabetes have twice the incidence of hearing loss compared to those without diabetes and those with prediabetes have a 30% higher rate of hearing loss. Whether hearing loss is associated with diabetes independent of glycemic control remains to be determined. Hearing loss has its own set of risk factors and shares others with diabetes. This review will summarize the complex relationship between diabetes and sensorineural hearing loss.

Review: Neural Mechanisms of Tinnitus and Hyperacusis in Acute Drug-Induced Ototoxicity

Purpose Tinnitus and hyperacusis are debilitating conditions often associated with age-, noise-, and drug-induced hearing loss. Because of their subjective nature, the neural mechanisms that give rise to tinnitus and hyperacusis are poorly understood. Over the past few decades, considerable progress has been made in deciphering the biological bases for these disorders using animal models. Method Important advances in understanding the biological bases of tinnitus and hyperacusis have come from studies in which tinnitus and hyperacusis are consistently induced with a high dose of salicylate, the active ingredient in aspirin. Results Salicylate induced a transient hearing loss characterized by a reduction in otoacoustic emissions, a moderate cochlear threshold shift, and a large reduction in the neural output of the cochlea. As the weak cochlear neural signals were relayed up the auditory pathway, they were progressively amplified so that the suprathreshold neural responses in the auditory cortex were much larger than normal. Excessive central gain (neural amplification), presumably resulting from diminished inhibition, is believed to contribute to hyperacusis and tinnitus. Salicylate also increased corticosterone stress hormone levels. Functional imaging studies indicated that salicylate increased spontaneous activity and enhanced functional connectivity between structures in the central auditory pathway and regions of the brain associated with arousal (reticular formation), emotion (amygdala), memory/spatial navigation (hippocampus), motor planning (cerebellum), and motor control (caudate/putamen). Conclusion These results suggest that tinnitus and hyperacusis arise from aberrant neural signaling in a complex neural network that includes both auditory and nonauditory structures.

Salicylate-Induced Changes in Hearing Thresholds in Mongolian Gerbils Are Correlated With Tinnitus Frequency but Not With Tinnitus Strength

Tinnitus is an auditory phantom percept without external sound sources. Despite the high prevalence and tinnitus-associated distress of affected patients, the pathophysiology of tinnitus remains largely unknown, making prevention and treatments difficult to develop. In order to elucidate the pathophysiology of tinnitus, animal models are used where tinnitus is induced either permanently by noise trauma or transiently by the application of salicylate. In a model of trauma-induced tinnitus, we have suggested a central origin of tinnitus-related development of neuronal hyperactivity based on stochastic resonance (SR). SR refers to the physiological phenomenon that weak subthreshold signals for given sensors (or synapses) can still be detected and transmitted if appropriate noise is added to the input of the sensor. The main objective of this study was to characterize the neurophysiological and behavioral effects during salicylate-induced tinnitus and compare these to the conditions within the trauma model. Our data show, in line with the pharmacokinetics, that hearing thresholds generally increase 2 h after salicylate injections. This increase was significantly stronger within the region of best hearing compared to other frequencies. Furthermore, animals showed behavioral signs of tinnitus during that time window and frequency range as assessed by gap prepulse inhibition of the acoustic startle reflex (GPIAS). In contrast to animals with noise trauma-induced tinnitus, salicylate-induced tinnitus animals showed no correlation between hearing thresholds and behavioral signs of tinnitus, indicating that the development of tinnitus after salicylate injection is not based on SR as proposed for the trauma model. In other words, salicylate-induced tinnitus and noise trauma-induced tinnitus are not based on the same neurophysiological mechanism.

Effect of Sodium Salicylate on Calcium Currents and Exocytosis in Cochlear Inner Hair Cells: Implications for Tinnitus Generation

Sodium salicylate is an anti-inflammatory medication with a side-effect of tinnitus. Here, we used mouse cochlear cultures to explore the effects of salicylate treatment on cochlear inner hair cells (IHCs). We found that IHCs showed significant damage after exposure to a high concentration of salicylate. Whole-cell patch clamp recordings showed that 1-5 mmol/L salicylate did not affect the exocytosis of IHCs, indicating that IHCs are not involved in tinnitus generation by enhancing their neuronal input. Instead, salicylate induced a larger peak amplitude, a more negative half-activation voltage, and a steeper slope factor of Ca²⁺ current. Using noise analysis of Ca²⁺ tail currents and qRT-PCR, we further found that salicylate increased the number of Ca²⁺ channels along with Ca_V1.3 expression. All these changes could act synergistically to enhance the Ca²⁺ influx into IHCs. Inhibition of intracellular Ca²⁺ overload significantly attenuated IHC death after 10 mmol/L salicylate treatment. These results implicate a cellular mechanism for tinnitus generation in the peripheral auditory system.

The increase in the degree of neural forward masking of cochlea following salicylate application

BACKGROUND

High doses of salicylate are known to reduce cochlear response amplitude and raise threshold. However, its effect on the cochlear forward masking, reflecting temporal resolution, is still unclear.

METHODS

The neural forward masking of cochlea was evaluated using double-tone stimulation. The first tone burst (5ms) was named the "masker" and the second tone burst (5 ms) was named the "probe". The frequency and intensity of the masker and probe were equal, and the masker-probe interval varied from 2 to 32 ms. The reduction (%) of the probe-evoked cochlear compound action potential caused by the addition of the masker tone was used to represent cochlear forward masking. The data obtained before and 2 h following the injection of sodium salicylate (250 mg/kg) were compared.

RESULTS

The neural forward masking of cochlea in the normal rats increased as the masker-probe interval decreased. At 16 kHz, for example, it increased from ~5% to 32ms masker-probe interval to ~85% at 2ms masker-probe interval. Two hours post salicylate injection, the neural forward masking was significantly enhanced except at 32 ms masker-probe interval. Interestingly, this enhancement was only observed in the limited frequency range of 16-30 kHz.

DISCUSSION

The enhancement of forward masking of cochlea following salicylate administration may reflect defective neurotransmitter release. This frequency-dependent injury in the cochlea may lead to the development of central plasticity observed after salicylate administration, likely through the increase in central gain, leading to perceptual consequences.

A contribution to the debate on tinnitus definition

Tinnitus is generally defined as an auditory perception in the absence of environmental sound stimulation. However, this definition is quite incomplete as it omits an essential aspect, the patient's point of view. This point of view constitutes, first and foremost, a global and unified lived experience, which is not only sensory (localization, loudness, pitch and tone), but also cognitive (thoughts, attentiveness, behaviors) and emotional (discomfort, suffering). This experience can be lived in a very unpleasant way and consequently have a very negative impact on quality of life. This article proposes and justifies a new definition for tinnitus elaborated by a group of French clinicians and researchers, which is more in line with its phenomenology. It also provides a minimum knowledge base, including possibilities for clinical care, hoping to eradicate all misinformation, misconceptions and inappropriate attitudes or practices toward this condition. Here is the short version of our definition: Tinnitus is an auditory sensation without an external sound stimulation or meaning, which can be lived as an unpleasant experience, possibly impacting quality of life.

What's the buzz? The neuroscience and the treatment of tinnitus

Tinnitus is a pervasive public health issue that affects ∼15% of the United States population. Similar estimates have also been shown on a global scale, with similar prevalence found in Europe, Asia, and Africa. The severity of tinnitus is heterogeneous, ranging from mildly bothersome to extremely disruptive. In the United States, ∼10-20% of individuals who experience tinnitus report symptoms that severely reduce their quality of life. Due to the huge personal and societal burden, in the last 20 yr a concerted effort on basic and clinical research has significantly advanced our understanding and treatment of this disorder. Yet, neither full understanding, nor cure exists. We know that tinnitus is the persistent involuntary phantom percept of internally generated nonverbal indistinct noises and tones, which in most cases is initiated by acquired hearing loss and maintained only when this loss is coupled with distinct neuronal changes in auditory and extra-auditory brain networks. Yet, the exact mechanisms and patterns of neural activity that are necessary and sufficient for the perceptual generation and maintenance of tinnitus remain incompletely understood. Combinations of animal model and human research will be essential in filling these gaps. Nevertheless, the existing progress in investigating the neurophysiological mechanisms has improved current treatment and highlighted novel targets for drug development and clinical trials. The aim of this review is to thoroughly discuss the current state of human and animal tinnitus research, outline current challenges, and highlight new and exciting research opportunities.

Sodium salicylate enhances neural excitation via reducing GABAergic transmission in the dentate gyrus area of rat hippocampus in vivo

Sodium salicylate, one of the non-steroidal anti-inflammatory drugs, is widely prescribed in the clinic, but a high dose of usage can cause hyperactivity in the central nervous system, including the hippocampus. At present, the neural mechanism underlying the induced hyperactivity is not fully understood, in particular, in the hippocampus under an in vivo condition. In this study, we found that systemic administration of sodium salicylate increased the field excitatory postsynaptic potential slope and the population spike amplitude in a dose-dependent manner in the hippocampal dentate gyrus area of rats with in vivo field potential extracellular recordings, which indicates that sodium salicylate enhances basal synaptic transmission and neural excitation. In the presence of picrotoxin, a GABA-A receptor antagonist, sodium salicylate failed to increase the initial slope of the field excitatory postsynaptic potential and the amplitude of the population spike in vivo. To further explore how sodium salicylate enhances the neural excitation, we made whole-cell patch-clamp recordings from hippocampal slices. We found that perfusion of the slice with sodium salicylate decreased electrically evoked GABA receptor-mediated currents, increased paired-pulse ratio, and lowered frequency and amplitude of miniature inhibitory postsynaptic currents. Together, these results demonstrate that sodium salicylate enhances the neural excitation through suppressing GABAergic synaptic transmission in presynaptic and postsynaptic mechanisms in the hippocampal dentate gyrus area. Our findings may help understand the side effects caused by sodium salicylate in the central nervous system.

Salicylate decreases the spontaneous firing rate of guinea pig auditory nerve fibres

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Spontaneous firing rates were recorded from single auditory fibres in vivo.

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Salicylate was injected at 350 mg/kg by the subcutaneous route.

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Median firing rate decreased by 32 spikes/s in nerve fibres after salicylate injection.

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The high spontaneous rate fibres (type 1A) showed the main reduction.

Tinnitus has similarities to chronic neuropathic pain where there are changes in the firing rate of different types of afferent neurons. We postulated that one possible cause of tinnitus is a change in the distribution of spontaneous firing rates in at least one type of afferent auditory nerve fibre in anaesthetised guinea pigs. In control animals there was a bimodal distribution of spontaneous rates, but the position of the second mode was different depending upon whether the fibres responded best to high (> 4 kHz) or low (≤4 kHz) frequency tonal stimulation. The simplest and most reliable way of inducing tinnitus in experimental animals is to administer a high dose of sodium salicylate. The distribution of the spontaneous firing rates was different when salicylate (350 mg/kg) was administered, even when the sample was matched for the distribution of characteristic frequencies in the control population. The proportion of medium spontaneous rate fibres (MSR, 1≤ spikes/s ≤20) increased while the proportion of the highest, high spontaneous firing rate fibres (HSR, > 80 spikes/s) decreased following salicylate. The median rate fell from 64.7 spikes/s (control) to 35.4 spikes/s (salicylate); a highly significant change (Kruskal-Wallis test p < 0.001). When the changes were compared with various models of statistical probability, the most accurate model was one where most HSR fibres decreased their firing rate by 32 spikes/s. Thus, we have shown a reduction in the firing rate of HSR fibres that may be related to tinnitus.

Curcumin mitigates neurotoxic and neurobehavioral changes of gentamicin and sodium salicylate in rats by adjusting oxidative stress and apoptosis

Currently, antibiotics and salicylates are the most highly consumed medications worldwide. The side effects of these pharmaceuticals on the nervous system have been little investigated. Thus, this study aimed to examine the influence of the gentamicin (GM) and sodium salicylates (SS) on neurobehavioral functions, including locomotors function, memory, and sensorimotor functions together with gamma-aminobutyric acid (GABA) neurotransmitter levels. Also, oxidative stress, lipid peroxidation, and apoptotic indicators of brain tissue were assessed. Additionally, the histopathological architecture of brain tissues was investigated. This study also evaluated the curcumin (CUR) efficacy to counteract the GM or SS induced neurotoxic impacts in rats. For this purpose, seven groups were administered physiological saline (1 ml/rat; orally), olive oil (1 ml/rat; orally), CUR (50 mg/kg bwt; orally), GM (120 mg/kg bwt; intraperitoneally), SS (300 mg /kg bwt; intraperitoneally), CUR + GM, or CUR + SS for consecutive 15 days. The results revealed that GM and SS exposure evoked impaired memory, sensorimotor deficit functions, and depressive-like behavior together with the depletion of GABA. GM and SS exposure elevated malondialdehyde and Caspase-3 levels, but total antioxidant capacity and Bcl-2 levels were reduced. Besides, GM and SS exposure induced distinct pathological perturbations in cerebral cortices and hippocampus tissues. CUR significantly reversed the GM and SS harmful impacts. In conclusion, these findings verified that CUR could be a biologically efficient protective intervention against GM and SS induced neurotoxic impacts and neurobehavioral aberrations.

Aberrant expression of Nav1.6 in the cochlear nucleus correlates with salicylate-induced tinnitus in rats

Hyperactivity in cochlear nucleus (CN) is one of the major neural correlates for tinnitus induction, yet the molecular factors that participate in the neuronal hyperexcitability remain unclear. The present study showed that acute and chronic administrations of salicylate were both capable of inducing reversible tinnitus in rats. The number of GAD 65/67-immunoreactive neurons in the AVCN and DCN was decreased, while the number of VGLUT 1/2-immunoreactive neurons in the AVCN and DCN was increased when rats were experiencing tinnitus, providing evidence for excitatory-inhibitory imbalance in CN is correlated with tinnitus. Interestingly, the expression level of Nav1.6, an important subtype of voltage-gated sodium channels was significantly increased in the DCN and AVCN of rats experiencing tinnitus, the up-regulation of Nav1.6 was returned to normal level following the disappearance of tinnitus. Double-labeling experiments revealed that Nav1.6 expression was down-regulated in the GAD 65/67-positive neurons in the DCN and AVCN of rats experiencing tinnitus. Notably, the percentage of co-localization of Nav1.6 and NeuN-labeling fusiform neurons was markedly increased in the DCN during tinnitus. These findings uncover the tinnitus-associated alteration in Nav1.6, a potential key contributor that can lead to hyperexcitability in CN and contribute to salicylate-induced tinnitus.

Drug-induced hearing loss: a case/non-case study in the French pharmacovigilance database

Hearing loss is defined as a decrease in the ability to perceive sounds which can occur suddenly or gradually and affects one ear or both. It is related to various etiologies, in particular drugs. The identification of all drugs that could be associated with hearing loss is essential for the patients' life quality. The objective of our study was to identify signals of hearing loss involving drugs approved in the last 20 years. The occurrence in association with drugs known for their ototoxicity was also analyzed. We used a case/non-case method in the French Pharmacovigilance Database (FPVD). The cases were reports of hearing loss in the FPVD between January 2007 and August 2017. Non-cases were all reports over the same period. We calculated the reporting odds ratio (ROR) with 95% confidence intervals. Among the 555 reports of hearing loss, significant RORs were found for 68 drugs. The main therapeutic classes implicated were antineoplastic agents (n = 240), systemic anti-infective agents (n = 182), immunosuppressants (n = 42) loop diuretics (n = 26), and salicylate analgesics (n = 26). We found signals of hearing loss with azacitidine, vaccines and nevirapine, immunosuppressants such as leflunomide, and biotherapies such as panitumumab and vandetanib. Prescribers should be informed about the potential associations with all these drugs. The role of the pathology itself and the known ototoxic drugs that can be associated do not allow to conclude definitively. Audiograms for the early detection of hearing loss induced by drugs known to be ototoxic are rarely carried out. Preventive treatments exist and must be considered.

Plasticity in Limbic Regions at Early Time Points in Experimental Models of Tinnitus

Tinnitus is one of the most prevalent auditory disorders worldwide, manifesting in both chronic and acute forms. The pathology of tinnitus has been mechanistically linked to induction of harmful neural plasticity stemming from traumatic noise exposure, exposure to ototoxic medications, input deprivation from age-related hearing loss, and in response to injuries or disorders damaging the conductive apparatus of the ears, the cochlear hair cells, the ganglionic cells of the VIIIth cranial nerve, or neurons of the classical auditory pathway which link the cochlear nuclei through the inferior colliculi and medial geniculate nuclei to auditory cortices. Research attempting to more specifically characterize the neural plasticity occurring in tinnitus have used a wide range of techniques, experimental paradigms, and sampled at different windows of time to reach different conclusions about why and which specific brain regions are crucial in the induction or ongoing maintenance of tinnitus-related plasticity. Despite differences in experimental methodologies, evidence reveals similar findings that strongly suggest that immediate and prolonged activation of non-classical auditory structures (i.e., amygdala, hippocampus, and cingulate cortex) may contribute to the initiation and development of tinnitus in addition to the ongoing maintenance of this devastating condition. The overarching focus of this review, therefore, is to highlight findings from the field supporting the hypothesis that abnormal early activation of non-classical sensory limbic regions are involved in tinnitus induction, with activation of these regions continuing to occur at different temporal stages. Since initial/early stages of tinnitus are difficult to control and to quantify in human clinical populations, a number of different animal paradigms have been developed and assessed in experimental investigations. Reviews of traumatic noise exposure and ototoxic doses of sodium salicylate, the most prevalently used animal models to induce experimental tinnitus, indicate early limbic system plasticity (within hours, minutes, or days after initial insult), supports subsequent plasticity in other auditory regions, and contributes to the pathophysiology of tinnitus. Understanding this early plasticity presents additional opportunities for intervention to reduce or eliminate tinnitus from the human condition.

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.

Protection of Hair Cells from Ototoxic Drug-Induced Hearing Loss

Hair cells are specialized sensory epithelia cells that receive mechanical sound waves and convert them into neural signals for hearing, and these cells can be killed or damaged by ototoxic drugs, including many aminoglycoside antibiotics, platinum-based anticancer agents, and loop diuretics, leading to drug-induced hearing loss. Studies of therapeutic approaches to drug-induced hearing loss have been hampered by the limited understanding of the biological mechanisms that protect and regenerate hair cells. This review briefly discusses some of the most common ototoxic drugs and describes recent research concerning the mechanisms of ototoxic drug-induced hearing loss. It also highlights current developments in potential therapies and explores current clinical treatments for patients with hearing impairments.

Electrophysiological changes in auditory evoked potentials in rats with salicylate-induced tinnitus

Early-response auditory evoked potentials (AEPs) in humans are significantly altered in tinnitus. These changes are closely related to that seen in animals, leading to new approaches to study tinnitus based on objective parameters. The purpose of this study was to characterize the AEPs in animals with tinnitus, by assessing early to late latency responses. For behavioral evaluation, rats were trained using positive reinforcement to press a lever in the presence of an auditory stimulus and to not press during silence. The auditory brainstem response (ABR), middle latency response (MLR) and auditory late latency response (LLR) were correlated to the false-positive responses (pressing the lever during silence), after oral administrations of Sodium Salicylate (SS, 350 mg/kg). In the present study, SS significantly increased the hearing thresholds and reduced ABR peak I amplitudes across the frequency range (4-32 kHz). In contrast, increased amplitudes were observed for several peaks in ABR, MLR, and LLR. Moreover, reduced ABR latencies in response to 8, 16 and 24 kHz tone bursts were observed after SS administration. Similarly, the central evaluation also revealed significantly reduced latencies in MLR and LLR during SS administration. In contrast, increased latencies were observed for ABR latencies in response to 32 kHz tone bursts, and at the P1-N1 component of LLR. Correlational analysis revealed that latencies and amplitudes of peaks II and IV (8 and 16 kHz) of ABR, and N2 latency and P2-N2 amplitude of LLR were associated with behavioral tinnitus. We suggest that AEPs can be used in the rat to evaluate the reduced sensory input and the increased central gain in SS-induced tinnitus, as well as reduced latencies (8-16 kHz) to distinguish between hearing loss and tinnitus.

Speech in Noise Perception as a Marker of Cognitive Impairment in HIV Infection

OBJECTIVES

Human immunodeficiency virus positive (HIV+) individuals report hearing difficulties, but standard audiological tests show no, or small, changes in peripheral hearing ability. The hearing complaints may reflect central nervous system (CNS) auditory processing deficits, rather than middle or inner ear problems, and may result from CNS damage due to HIV infection or treatment. If central auditory task performance and cognitive deficits in HIV+ individuals are shown to be related, then central auditory tests might serve as a "window" into CNS function in these patients.

DESIGN

We measured cognitive performance (Mandarin Montreal Cognitive Assessment [MoCA]) and speech in noise perception (Mandarin hearing-in-noise test [HINT]) in 166 normal-hearing HIV+ individuals (158 men, 8 women, average age 36 years) at the Shanghai Public Health Clinical Center in Shanghai, China. Data collection included audiometry, tympanometry, and the Amsterdam Inventory of Auditory Handicap (AIAH), which assesses the subjective ability to understand speech and localize sound.

RESULTS

Subjects had no middle ear disease and met criteria for normal-hearing sensitivity (all thresholds 20 dB HL or less). A significant negative relationship between speech reception thresholds (SRT) and MoCA scores (r = 0.15, F = 28.2, p < 0.001) existed. Stepwise linear regression showed that when the factors of age, MoCA scores, hearing thresholds, and education level were considered, only age and MoCA scores contributed independently to the SRT results (overall model r = 0.30, F = 38.8, p < 0.001). Subjective hearing complaints from the AIAH supported the HINT results. AIAH and MoCA scores were also related (r = 0.05, F = 8.5, p = 0.004), with those with worse MoCA scores having more problems on the AIAH. When the cohort was divided into those with normal and abnormal performance on the MoCA, those with abnormal performance on the MoCA had significantly higher average SRTs (p < 0.001).

CONCLUSIONS

Understanding speech in noise measured both objectively with the HINT and subjectively with the AIAH was inversely related to cognitive abilities despite a normal ability to hear soft sounds determined by audiometry. Although age was also an important independent factor affecting speech perception, the age relationship within the speech findings in this study may represent more than just age-related declines in speech in noise understanding. Although reliable data on disease duration are not available, the older members of this cohort likely had HIV longer and probably had more severe symptoms at presentation than the younger members because early detection and treatment of HIV in Shanghai has improved over time. Therefore, the age relationship may also include elements of disease duration and severity. Speech perception, especially in challenging listening conditions, involves cortical and subcortical centers and is a demanding neurological task. The problems interpreting speech in noise HIV+ individuals have may reflect HIV-related or HIV treatment-related, central nervous damage, suggesting that CNS complications in HIV+ individuals could potentially be diagnosed and monitored using central auditory tests.

Sodium salicylate alters temporal integration measured through increasing stimulus presentation rates

OBJECTIVE

High doses of sodium salicylate (SS) are known to induce tinnitus, general hyperexcitability in the central auditory system, and to cause mild hearing loss. We used the auditory brainstem response (ABR) to assess the effects of SS on auditory sensitivity and temporal processing in the auditory nerve and brainstem. ABRs were evoked using tone burst stimuli varying in frequency and intensity with presentation rates from 11/s to 81/s.

DESIGN

ABRs were recorded and analysed prior to and after SS treatment in each animal, and peak 1 and peak 4 amplitudes and latencies were determined along with minimal response threshold.

STUDY SAMPLE

Nine young adult CBA/CaJ mice were used in a longitudinal within-subject design.

RESULTS

No measurable effects of presentation rate were found on ABR threshold prior to SS; however, following SS administration increasing stimulus rates lowered ABR thresholds by as much as 10 dB and compressed the peak amplitude by intensity level functions.

CONCLUSIONS

These results suggest that SS alters temporal integration and compressive nonlinearity, and that varying the stimulus rate of the ABR may prove to be a useful diagnostic tool in the study of hearing disorders that involve hyperexcitability.

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.

Acupuncture methods put to the test for a tinnitus study: A Bayesian analysis

BACKGROUND

This study evaluated the effectiveness of different methods of acupuncture in the treatment of tinnitus due to neurological causes. In total, eight treatment methods were selected for this study: traditional acupuncture, electroacupuncture, moxibustion acupuncture, medicine only without acupuncture, traditional acupuncture with supplementary medicine, electroacupuncture with supplementary medicine, moxibustion acupuncture with supplementary medicine, and an electroacupuncture and moxibustion acupuncture combination. All sample data come from the results of clinical treatment studies.

METHODS

Both Chinese- and English-language online databases were searched. The Chinese language databases included the Wanfang database, the China National Knowledge Infrastructure (CNKI) database, and the VIP Chinese Science and Technique Journals database. The English language databases included PubMed, Web of Science, Embase and Cochrane Library. After the previously mentioned eight interventions for the treatment of neurological tinnitus were tested in a randomized controlled trial (RCT), the data were extracted, and the effectiveness of each intervention was evaluated. A meta-analysis was performed using Stata14.0 and GeMTC 0.14.3 statistical software.

RESULTS

A total of 40 studies were included, which contained a total of 3657 patients and 8 intervention methods. There was a trend of greater effectiveness of moxibustion acupuncture, followed by moxibustion acupuncture combined with electroacupuncture, moxibustion acupuncture combined with supplementary medicine, acupuncture combined with drugs, electroacupuncture with supplementary medicine, electroacupuncture, traditional acupuncture, and medicine only without acupuncture. There was no significant difference between the results of indirect comparisons and direct comparisons.

CONCLUSIONS

Eight interventions are all effective in the treatment of neurological tinnitus, but moxibustion acupuncture seems to be a better trend treatment for tinnitus.

Long‑term treatment with salicylate enables NMDA receptors and impairs AMPA receptors in C57BL/6J mice inner hair cell ribbon synapse

Salicylate is widely used to produce animal models of tinnitus in mice and/or rats. The side effects on auditory function, including hearing loss and tinnitus, are considered the results of the auditory nerve dysfunction. A recent study indicated that chronic treatment with salicylate for several weeks reduces compressed action potential amplitude, which is contradictory to the studies reporting excessive activation of N-methyl-D-aspartate receptors (NMDAR) in tinnitus-induced animals. The specific aims of the experiment were to detect the effect of salicylate on the inner hair cells (IHCs), ribbon synapse, as well as the association between the hearing threshold and the number of mismatched ribbon synapses. In the present study, mice were injected intraperitoneally with a low dose of salicylate (200 mg/kg) for 14 days. The auditory brainstem response and otoacoustic emission were measured to assess auditory function of the mice. The postsynaptic regions of IHC were identified with two types of immunostaining targets: Postsynaptic density protein 95 and Glu2/3. The number of spheres was counted and the synapses were reconstructed in 3-dimensional images. Increases in distortion product otoacoustic emissions amplitudes of the salicylate group were detected, however, an elevation in the hearing threshold was also observed. A mismatch between pre-and post-ribbon synapses was observed. In addition, the cochlear components, including the numbers of outer hair cells and IHCs, were unlikely to be affected by salicylate. IHC ribbon synapses were more susceptible to salicylate stimuli. Furthermore, mismatch of pre- and post-ribbon synapses may indicate a competitive inhibition between NMDAR and α-amino-3-hydroxy-5-methyl-4-isoxa-zole-propionate receptors and dysfunction of ribbon synapses.

Prevalence of ototoxic medication use among older adults in Beaver Dam, Wisconsin

BACKGROUND AND PURPOSE

Drug-related ototoxicity may exacerbate presbycusis (age-related hearing loss); yet, few data are available on the prevalence of ototoxic medication use by older adults. The purposes of this study were to assess the impact of aging and ototoxicity on hearing loss, the prevalence of ototoxic medication use, and select characteristics associated with ototoxic medication use among older adults.

METHODS

Cross-sectional analyses were conducted using select variables extracted from the baseline and 10-year follow-up assessments of the two population-based epidemiological studies to compare two points in time.

RESULTS

Ninety-one percent of the sample was taking a medication reported to be ototoxic. Nonsteroidal anti-inflammatory drugs were the most commonly used (75.2%), followed by acetaminophen (39.9%) and diuretics (35.6%). Hypertension, diabetes, cardiovascular disease, and history of smoking were associated with ototoxic medication use. Participants with hearing loss were taking a significantly greater number of ototoxic medications than those without hearing loss.

CONCLUSION

Known ototoxic medications are widely used. Any subsequent ototoxicity may interact with age changes and a more severe hearing loss than that associated with only age.

IMPLICATIONS FOR PRACTICE

Nurse practitioners should inform older adults about the possibility of drug-related ototoxicity and monitor hearing acuity of all older adults taking known ototoxic medications.

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.

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.

Functional data analysis of single-trial auditory evoked potentials recorded in the awake rat

Evoked potentials (EPs) reflect neural processing and are widely used to study sensory perception. However, methods of analyzing EP have been limited mostly to the conventional ensemble averaging of EP response trials to a repeated stimulus, and less so to single-trials analysis. Here we applied a new approach - functional data analysis (FDA) - to study auditory EP in the rat model of tinnitus, in which overdoses of salicylate (SS) are known to alter sound perception characteristically, as the same way as in humans. Single-trial auditory EPs were analyzed, after being collected on a daily basis from an awake rat, which had been surgically implanted with intracranial electrodes over its auditory cortex. Single-trial EP integrals were generated with sound stimuli (tones and clicks) presented systematically over an intensity range. The results were approximated using the cubic spline to give sets of smoothed response-level functions in dependence on the sound intensity. These functional data were analyzed using the methods of FDA. Comparisons between daily intensity series for each sound type were done using cross-distance measures based on the response-level functions in both the original form and the first-derivative form. From the results of FDA, the first-derivative form was found to provide a clearer separation when EP data from control groups were compared to the data from SS groups. This is also true when the daily data were compared within the more variable SS-group itself. In addition, at the high-intensity region where SS-action is presumably strong, we also observed characteristic changes in two statistical parameters, mean and skewness, of the cross-distance representations. Results suggested that FDA is a sensitive approach for EP studies, and it can become a powerful tool for the research in neural science, particularly neuropharmacology.

Salicylates promote mitochondrial biogenesis by regulating the expression of PGC-1α in murine 3T3-L1 pre-adipocytes

Mitochondrial dysfunction has been associated with insulin resistance and diabetes. Decreased mitochondrial density and mitochondrial copy numbers have been found in insulin-resistant individuals. Restoration of the number of mitochondria and normal mitochondrial function has become an important therapeutic target of diabetes. Salicylate, the main active ingredient in aspirin, has been in medicinal use since ancient times. Little information regarding the effects of salicylate on mitochondrial function has been reported. In this study, we assessed the effects of salicylate on the peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) signaling pathway and mitochondrial biogenesis in pre-adipocytes. Our findings demonstrate that treatment with salicylate promoted the expression of PGC-1α and its downstream targets nuclear respiratory factor 1 (NRF1) and mitochondrial transcription factor A (TFAM). Importantly, salicylate treatment significantly increased the number of mDNA, citrate synthase activity, expression of respiratory chain complex I, and mitochondrial mass, which were suppressed by the specific AMPK inhibitor Compound C. Indeed, salicylate treatment induced the phosphorylation of AMPK, which was involved in the induction of PGC-1α, NRF1, and TFAM. Importantly, inhibition of PGC-1α expression using PGC-1α small RNA interference abolished the effects of salicylate on mitochondrial biogenesis. These results suggest that salicylate has a potential therapeutic capacity against mitochondrial dysfunction in diabetes.