Salicylate-induced Tinnitus Research Articles

Carbamazepine reduces the behavioural manifestations of tinnitus following salicylate treatment in rats

Conclusion. The results are consistent with the hypothesis that carbarmazepine (CBZ) has efficacy against tinnitus in humans. Objective. CBZ is an anti-epileptic drug that is widely used for the treatment of tinnitus. Despite this, there are relatively few clinical trials or preclinical studies supporting its efficacy. In an effort to increase the amount of information available on CBZ, the aim of this study was to investigate the efficacy of CBZ in salicylate-induced tinnitus in rats Materials and methods. We investigated the effects of CBZ in an animal model of tinnitus induced by the injection of salicylate using a conditioned lick suppression paradigm. Results. We found that CBZ, at a dose of 15mg/kg i.p., but not at 5mg/kg or 30mg/kg, significantly suppressed the behavioural manifestations of tinnitus.

Salicylate induced tinnitus: Behavioral measures and neural activity in auditory cortex of awake rats

Neurophysiological studies of salicylate-induced tinnitus have generally been carried out under anesthesia, a condition that abolishes the perception of tinnitus and depresses neural activity. To overcome these limitations, measurement of salicylate induced tinnitus were obtained from rats using schedule induced polydipsia avoidance conditioning (SIPAC) and gap pre-pulse inhibition of acoustic startle (GPIAS). Both behavioral measures indicated that tinnitus was present after treatment with 150 and 250 mg/kg of salicylate; measurements with GPIAS indicated that the pitch of the tinnitus was near 16 kHz. Chronically implanted microwire electrode arrays were used to monitor the local field potentials and spontaneous discharge rate from multiunit clusters in the auditory cortex of awake rats before and after treatment with 150 mg/kg of salicylate. The amplitude of the local field potential elicited with 60 dB SPL tone bursts increased significantly 2 h after salicylate treatment particularly at 16–20 kHz; frequencies associated with the tinnitus pitch. Field potential amplitudes had largely recovered 1–2 days post-salicylate when behavioral results showed that tinnitus was absent. The mean spontaneous spike recorded from the same multiunit cluster pre- and post-salicylate decreased from 22 spikes/s before treatment to 14 spikes/s 2 h post-salicylate and recovered 1 day post-treatment. These preliminary physiology data suggest that salicylate induced tinnitus is associated with sound evoked hyperactivity in auditory cortex and spontaneous hypoactivity.

Salicylate- and quinine-induced tinnitus and effects of memantine

Conclusion: Memantine, an antiglutamatergic drug, has been proposed as a treatment for tinnitus. Objectives: The purpose of this study was to determine if memantine would prevent salicylate-induced tinnitus. Local field potentials were also recorded from auditory cortex to determine what effect salicylate, memantine, and the combination of both drugs would have on evoked potential amplitudes. Materials and methods: Schedule induced polydipsia-avoidance conditioning was used to identify the doses of salicylate or quinine that reliably induced tinnitus in rats. Rats were trained to lick for water during quiet intervals and avoid licking during sound intervals. Results: Rats injected with saline or a low dose of sodium salicylate or quinine failed to develop tinnitus-like behaviors. However, high doses of salicylate (150–300 mg/kg/day) or quinine (100–150 mg/kg/day) greatly reduced licks-in-quiet, behavior consistent with the presence of tinnitus. Licks-in-quiet increased slightly when memantine (1.5 or 3 mg/kg/day) was co-administered with salicylate; however, the effect was not statistically significant or dose-dependent. These results indicate that memantine does not completely suppress salicylate-induced tinnitus. Cortical auditory evoked potential amplitude increased after salicylate treatment; co-administration of memantine failed to block this salicylate-induced increase.

M‐Chlorophenylpiperazine exacerbates perception of salicylate‐induced tinnitus in rats

The serotonin receptor 5-HT2c agonist m-chlorophenylpiperazine (mCPP) has been widely used to induce anxiety-like states in animals and anxiety in humans. We here addressed the effect of an intraperitoneal injection of mCPP on the perception of tinnitus in a behavioural protocol based on an active avoidance paradigm. In control saline-treated animals, mCPP did not change compound action potential audiograms or measurable tinnitus. In contrast, mCPP to animals demonstrating salicylate-induced tinnitus exacerbated tinnitus perception by nearly twofold. We went on to test whether manipulation of the peripheral generator of tinnitus (i.e. the cochlea) could extinguish this exacerbated perception by applying the N-methyl-d-aspartate (NMDA) antagonist 7-chlorokynurenate (7-CK) into cochlear fluids using a 7-CK-soaked Gelfoam ball placed on the round window of each of the animals' two ears. In addition to blocking the tinnitus induced by salicylate alone, 50 microm of 7-CK clearly abolished tinnitus in animals receiving salicylate and an injection of mCPP. The demonstration that cochlear NMDA receptor blockade abolishes the exacerbated perception of tinnitus is highly relevant in terms of treatment. In addition to psychotherapeutic treatment that may help to attenuate an individual's perception of tinnitus, targeting cochlear NMDA receptors represents a promising therapeutic strategy, even in depressed or chronically anxious patients.

Salicylate blocks L-type calcium channels in rat inferior colliculus neurons

To investigate the effects of the tinnitus inducer sodium salicylate on L-type voltage-gated calcium channels, we studied freshly dissociated inferior colliculus neurons of rats by the whole-cell voltage clamp method. Salicylate’s blocking of L-type calcium channels was concentration dependent, and the IC 50 value of salicylate was estimated to be 1.99 mM. An amount of 1 mM salicylate significantly shifted the steady-state inactivation curve of L-type calcium channels about 9 mV in the hyperpolarizing direction and significantly delayed calcium channel recovery. Our results suggest that salicylate’s blocking of L-type calcium channels may contribute to salicylate-induced tinnitus by decreasing GABA release in the inferior colliculus.

Paradoxical Enhancement of Active Cochlear Mechanics in Long-Term Administration of Salicylate

Aspirin (salicylate) is a common drug and frequently used long term in the clinic. It has been well documented that salicylate can cause reversible hearing loss and tinnitus and diminish outer hair cell (OHC) electromotility, which is capable of actively boosting the basilar membrane vibration and producing acoustic emission. However, aspirin's ototoxic mechanisms still remain largely unclear. In this experiment, the effects of long-term salicylate administration on cochlear hearing functions were investigated by measuring distortion product otoacoustic emissions (DPOAEs) in awake guinea pigs. A single injection of sodium salicylate (200 mg/kg) could reduce the amplitude of the cubic distortion product of 2f1-f2 within 2 h. The reduction was significant at 20-50 dB SPL stimulus levels and recovered after 8 h. However, following daily injections of sodium salicylate (200 mg/kg, b.i.d.), the distortion product of 2f1-f2 progressively increased. After injection for 14 days, the distortion product increased about 2-3.5 dB SPL. The increase rate was about 0.2 dB SPL/day. The DP-I/O function remained nonlinear. The increase was greater at 40-70 dB SPL primary sound intensities and reversible. After cessation of salicylate treatment for 4 wk, the increased distortion product returned to the initial normal levels. The rate of recovery was 0.1 dB SPL/day. In the control animals with saline injection, there was no change in DPOAEs. The data revealed that long-term administration of salicylate could paradoxically enhance active cochlear mechanics. The data also suggested that salicylate-induced tinnitus might be generated at the OHC level.

Effects of salicylate on transient outward and delayed rectifier potassium channels in rat inferior colliculus neurons

The effects of salicylate (a tinnitus inducer) were studied on the transient outward potassium current ( I K(A)) and the delayed rectifier potassium current ( I K(DR)) in acutely dissociated rat inferior colliculus neurons by the whole-cell voltage-clamp method. Salicylate's inhibition of the amplitude of I K(A) and I K(DR) was concentration-dependent. The IC 50 values for the blocking action of salicylate on I K(A) and I K(DR) were 2.27 and 0.80 mM, respectively. At a concentration of 1 mM, salicylate significantly shifted the activation and inactivation curves of I K(DR) negatively by approximately 11 and 24 mV, respectively, but did not shift the I K(A) curves. In conclusion, salicylate inhibits both I K(A) and I K(DR) in rat inferior colliculus neurons but only significantly affects the activation and inactivation kinetics of I K(DR). Depression of I K(DR) by salicylate may play an important role in salicylate-induced tinnitus.

Effects of salicylate on voltage-gated sodium channels in rat inferior colliculus neurons

To investigate the effects of the tinnitus inducer, sodium salicylate, on voltage-gated sodium channels, we studied freshly dissociated inferior colliculus neurons of rats by the whole-cell voltage clamp method. Salicylate blocked sodium channels in concentration-dependent manner (0.1–10 mM), and the IC 50 value of salicylate was estimated to be 1.43 mM after application. The sodium conductance–voltage curve did not shift along the voltage axis with salicylate application. In contrast, the steady-state sodium channel inactivation curve was shifted by about 9 mV in the hyperpolarizing direction. In addition, salicylate delayed the sodium channel recovery from inactivation by increasing the slow time constant. It was concluded that salicylate bound to the resting and inactivated sodium channels to cause blocking, with a higher affinity for the latter state. Our results suggest that salicylate causes a concentration-dependent blockade of voltage-gated sodium channels and shifts the inactivation curve to more hyperpolarized potentials, which could be related to the mechanism of salicylate-induced tinnitus.

Salicylate Induces Tinnitus through Activation of Cochlear NMDA Receptors

Salicylate, the active component of aspirin, is known to induce tinnitus. However, the site and the mechanism of generation of tinnitus induced by salicylate remains unclear. Here, we developed a behavioral procedure to measure tinnitus in rats. The behavioral model was based on an active avoidance paradigm in which rats had to display a motor task (i.e., to jump on a climbing pole when hearing a sound). Giving salicylate led to a decrease in the percentage of correct responses (score) and a drastic increase in the number of false positive responses (i.e., animals execute the motor task during a silent period). Presentation of the sound at a constant perceptive level prevents decrease of the score, leading to the proposal that score is related to hearing performance. In contrast, the increase of false positive responses remained unchanged. In fact, animals behaved as if they hear a sound, indicating that they are experiencing tinnitus. Mefenamate in place of salicylate also increased the number of false positive responses, suggesting that salicylate-induced tinnitus is related to an inhibition of cyclooxygenase. One physiological basis of salicylate ototoxicity is likely to originate from altered arachidonic acid metabolism. Because arachidonic acid potentiates NMDA receptor currents, we tested the involvement of cochlear NMDA receptors in the occurrence of tinnitus. Application of NMDA antagonists into the perilymphatic fluids of the cochlea blocked the increase in pole-jumping behavior induced by salicylate, suggesting that salicylate induces tinnitus through activation of cochlear NMDA receptors.

Differential changes in Fos-immunoreactivity at the auditory brainstem after chronic injections of salicylate in rats

In human, salicylate-induced tinnitus sometimes occurs a few days after its administration, but the chronic effects of salicylate in animal models are not fully known. In this study, we revealed the distribution of active cells in the rat auditory brainstem by staining an activity marker Fos-protein after multiple daily injections of salicylate. Experimental animals were first given five daily doses of sodium salicylate (250 mg/kg, i.p.). On day 6 they were placed inside a sound room for 8 h before sacrifice. Immunohistochemistry showed a significant increase in the number of Fos-positive cells at the inferior colliculus (IC), particularly its central division. At the cochlear nucleus (CN), only a few Fos-stains were found at the dorsal nucleus while no Fos-stain appeared at the ventral nucleus. The scarcity of Fos-stains at the CN reflected more a lack of external sound inputs than an adaptation in Fos-expression. Since Fos-stains in CN could still be induced on day 6 following brief tonal stimulation. Results are consistent with the hypothesis that salicylate-induced tinnitus is a phantom sound perception related to overactivity of cells at the IC.

Effects of salicylate on serotoninergic activities in rat inferior colliculus and auditory cortex

In vivo microdialysis offers a unique approach to monitor biochemical events related to brain function and metabolism, and has been used extensively in many systems to measure the release of endogenous transmitters and other neuroactive substances during normal and pathological conditions. The characterization of neurotransmitters’ changes induced by salicylate in the inferior colliculus (IC) and the auditory cortex (AC) may provide insight into the action of salicylate on the auditory system and, through this, provide a better understanding of neurological mechanism of salicylate-induced tinnitus. In the present study, the effect of salicylate on 5-HT system in IC and AC has been monitored by microdialysis in salicylate-induced tinnitus animal models. Glucose and lactate levels in IC and AC were significantly increased after application of salicylate (350 mg/kg, i.p.), indicating a salicylate-related increase in regional neuronal activity. The 5-HT level increased to a maximum of 268±27% basal level in IC 2 h after application and of 277±24% basal level in AC around 3 h after application. These data suggest that the increases of 5-HT levels in IC and AC may be involved in the tinnitus generation.

Effects of chronic salicylate on GABAergic activity in rat inferior colliculus

It is well accepted that salicylate ototoxicity results in reversible tinnitus in humans. Salicylate-induced tinnitus may be an example of plasticity of the central auditory system and could potentially serve as a model to further understand mechanisms of tinnitus generation. This study examined levels of glutamic acid decarboxylase (GAD) and the binding characteristics of the GABA A receptor in auditory brainstem structures of Long–Evans rats chronically treated with salicylate. Western blotting revealed a significant 63% ( P<0.008) elevation of GAD levels in the inferior colliculus (IC) of salicylate-treated subjects. This occurred in subjects demonstrating behavioral evidence of tinnitus. Muscimol saturation analysis was indicative of a salicylate-related increase in receptor affinity. Linear regression of [ 3H]muscimol saturation analysis data revealed a significant ( P<0.05) reduction in K d values in whole IC (−48%), as well as in the central nucleus of IC (CIC, −58%) and combined external and dorsal cortex of IC (E/DCIC, −46%). The number of GABA A binding sites ( B max) were also significantly ( P<0.05) decreased. These changes were observed only in central auditory structures. This suggests that GAD expression and GABA A receptor binding characteristics may be altered with chronic exposure to sodium salicylate and these changes may represent aberrant plasticity clinically experienced as tinnitus.

Behavioral model of chronic tinnitus in rats

An animal model of tinnitus was developed to study chronic salicylate-induced tinnitus in rats. Novel features of the model included oral dosing of salicylate, test stimuli that included a range of pure tones and silence, and assessment of tinnitus for several months. Experimental subjects were given sodium salicylate in their drinking water while control subjects received normal tap water. Subjects were conditioned to press a lever for food in the presence of continuous white noise. At random intervals, offset of the noise was paired with a noxious stimulus, resulting in cessation of lever pressing during the silent test periods. At other randomly scheduled intervals, a test tone was substituted for the white noise, unpaired with noxious stimuli. When the test stimuli were pure tones, the salicylate-treated subjects suppressed less than the control subjects. One explanation for this result is that the experimental subjects’ sensations of tones were noisier than those of the controls because experimental subjects were experiencing tinnitus. (Otolaryngol Head Neck Surg 1999;121:457-62.)

Attenuation of salicylate-induced tinnitus by Ginkgo biloba extract in rats.

The effects of an extract from Ginkgo biloba, EGb 761, on tinnitus were tested using an animal model of tinnitus. Daily oral administration of EGb 761 in doses from 10 to 100 mg/ kg/day began 2 weeks before behavioral procedures and continued until the end of the experiment. Tinnitus was induced by daily administration of 321 mg/kg sodium salicylate s.c. (corresponding to 275 mg/kg/day of salicylate acid) in fourteen groups of pigmented rats, 6 animals/group. The results from salicylate- and EGb-761-treated animals were compared to control groups receiving either salicylate, saline, or EGb 761 only in doses of 100 mg/kg. Administration of EGb 761 resulted in a statistically significant decrease of the behavioral manifestation of tinnitus for doses of 25, 50 and 100 mg/kg/ day.

Salicylate-induced abnormal activity in the inferior colliculus of rats

The evaluation of the spontaneous activity of 471 units from the external nucleus of the IC revealed that salicylate induces an increase of the spontaneous activity and the emergence of a bursting type of activity longer than 4 spikes. For sharply tuned units, the affected cells were from the frequency range of 10–16 kHz, which corresponds to the behaviorally measured pitch of salicylate-induced tinnitus in rats. An exogenous calcium supplement, provided under the conditions shown to attenuate the behavioral manifestation of salicylate-induced tinnitus, abolished the modification of the spontaneous activity induced by salicylate. Finally, profound changes of activity were observed for cells not responding to contralateral sound. We propose that the observed long bursts of discharges represent tinnitus-related neuronal activity. The results are consistent with the hypothesis that GABA-mediated disinhibition is involved in the processing of tinnitus-related neuronal activity.

Evaluating the loudness of phantom auditory perception (tinnitus) in rats.

Using our behavioral paradigm for evaluating tinnitus, the loudness of salicylate-induced tinnitus was evaluated in 144 rats by comparing their behavioral responses induced by different doses of salicylate to those induced by different intensities of a continuous reference tone mimicking tinnitus. Group differences in resistance to extinction were linearly related to salicylate dose and, at moderate intensities, to the reference tone as well. Comparison of regression equations for salicylate versus tone effects permitted estimation of the loudness of salicylate-induced tinnitus. These results extend the animal model of tinnitus and provide evidence that the loudness of phantom auditory perception is expressed through observable behavior, can be evaluated, and its changes detected.