Sodium salicylate reduces gamma aminobutyric acid-induced current in rat spinal dorsal horn neurons

Map plasticity following noise exposure in auditory cortex of rats: implications for disentangling neural correlates of tinnitus and hyperacusis

Introduction

Both tinnitus and hyperacusis, likely triggered by hearing loss, can be attributed to maladaptive plasticity in auditory perception. However, owing to their co-occurrence, disentangling their neural mechanisms proves difficult. We hypothesized that the neural correlates of tinnitus are associated with neural activities triggered by low-intensity tones, while hyperacusis is linked to responses to moderate- and high-intensity tones.

Methods

To test these hypotheses, we conducted behavioral and electrophysiological experiments in rats 2 to 8 days after traumatic tone exposure.

Results

In the behavioral experiments, prepulse and gap inhibition tended to exhibit different frequency characteristics (although not reaching sufficient statistical levels), suggesting that exposure to traumatic tones led to acute symptoms of hyperacusis and tinnitus at different frequency ranges. When examining the auditory cortex at the thalamocortical recipient layer, we observed that tinnitus symptoms correlated with a disorganized tonotopic map, typically characterized by responses to low-intensity tones. Neural correlates of hyperacusis were found in the cortical recruitment function at the multi-unit activity (MUA) level, but not at the local field potential (LFP) level, in response to moderate- and high-intensity tones. This shift from LFP to MUA was associated with a loss of monotonicity, suggesting a crucial role for inhibitory synapses.

Discussion

Thus, in acute symptoms of traumatic tone exposure, our experiments successfully disentangled the neural correlates of tinnitus and hyperacusis at the thalamocortical recipient layer of the auditory cortex. They also suggested that tinnitus is linked to central noise, whereas hyperacusis is associated with aberrant gain control. Further interactions between animal experiments and clinical studies will offer insights into neural mechanisms, diagnosis and treatments of tinnitus and hyperacusis, specifically in terms of long-term plasticity of chronic symptoms.

Salicylate Induced GABAAR Internalization by Dopamine D1-Like Receptors Involving Protein Kinase C (PKC) in Spiral Ganglion Neurons

BACKGROUND Sodium salicylate (SS) induces excitotoxicity of spiral ganglion neurons (SGNs) by inhibiting the response of γ-aminobutyric acid type A receptors (GABAARs). Our previous studies have shown that SS can increase the internalization of GABAARs on SGNs, which involves dopamine D1-like receptors (D1Rs) and related signaling pathways. In this study, we aimed to explore the role of D1Rs and their downstream molecule protein kinase C (PKC) in the process of SS inhibiting GABAARs. MATERIAL AND METHODS The expression of D1Rs and GABARγ2 on rat cochlear SGNs cultured in vitro was tested by immunofluorescence. Then, the SGNs were exposed to SS, D1R agonist (SKF38393), D1R antagonist (SCH23390), clathrin/dynamin-mediated endocytosis inhibitor (dynasore), and PKC inhibitor (Bisindolylmaleimide I). Western blotting and whole-cell patch clamp technique were used to assess the changes of surface and total protein of GABARγ2 and GABA-activated currents. RESULTS Immunofluorescence showed that D1 receptors (DRD1) were expressed on SGNs. Data from western blotting showed that SS promoted the internalization of cell surface GABAARs, and activating D1Rs had the same result. Inhibiting D1Rs and PKC decreased the internalization of GABAARs. Meanwhile, the phosphorylation level of GABAARγ2 S327 affected by PKC was positively correlated with the degree of internalization of GABAARs. Moreover, whole-cell patch clamp recording showed that inhibition of D1Rs or co-inhibition of D1Rs and PKC attenuated the inhibitory effect of SS on GABA-activated currents. CONCLUSIONS D1Rs mediate the GABAAR internalization induced by SS via a PKC-dependent manner and participate in the excitotoxic process of SGNs.

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.

Neurotoxicity of sodium salicylate to the spiral ganglion neurons: GABAA receptor regulates NMDA receptor by Fyn-dependent phosphorylation

The purpose of this study was to observe the regulatory effects of GABA_A (γ-aminobutyric acid A) receptor on the N-methyl-D-aspartate (NMDA) receptor during excitotoxicity in spiral ganglion neurons in the rat cochlea induced by sodium salicylate (SS). Western blot illustrated SS decreased the expression of NMDA receptor 2B subunit (NR2B) surface protein through affecting GABA_A receptor, but the total protein content did not significantly change. Y1472 and S1480 are important phosphorylation sites in NR2B, SS downregulated the Fyn-dependent phosphorylation of Y1472 in a manner not related to the CK2 (Casein Kinase 2) dependent phosphorylation of S1480, thus regulating the surface distribution and internalization of NMDA receptor through GABA_A receptor. These results suggest that the modified pattern of dynamic balance between excitation and inhibition by coactivation of the GABA_A receptor can attenuate the excitatory NMDA receptor under the action of SS, via inhibiting the Fyn-dependent phosphorylation of Y1472.

Salicylate-Induced Ototoxicity of Spiral Ganglion Neurons: Ca2+/CaMKII-Mediated Interaction Between NMDA Receptor and GABAA Receptor

Sodium salicylate (SS) is one of the nonsteroidal anti-inflammatory drugs and widely used in clinical practice. Therefore, we aimed to investigate the potential ototoxicity mechanism of sodium salicylate: the influence of Ca²⁺/calmodulin-dependent protein kinase II (Ca²⁺/CaMKII) in interaction between NMDA receptors (NMDAR) and GABA_A receptors (GABA_AR) in rat cochlear spiral ganglion neurons (SGNs). After treatment with SS, NMDA, and an NMDAR inhibitor (APV), the changes of GABA_AR β3 (GABR β3) mRNA, surface and total protein, and GABA_AR currents in SGNs were assessed by quantitative PCR, Western blot, and whole-cell patch clamp. Mechanistically, SS and/or NMDA increased the GABR β3 mRNA expression, while decreased GABR β3 surface protein levels and GABA_AR-mediated currents. Moreover, application of SS and/or NMDA showed promotion in phosphorylation levels at S383 of GABR β3. Collectively, Ca²⁺ chelator (BAPTA) or Ca²⁺/CaMKII inhibitor (KN-93) reversed the effects of SS and/or NMDA on GABA_AR. Therefore, we hypothesize that the interaction between NMDAR and GABA_AR is involved in the SGNs damage induced by SS. In addition, the underlying molecular mechanism is related to Ca²⁺/CaMKII-mediated signaling pathway, which suggests that the interaction between calcium signal-regulated receptors mediates SS ototoxicity.

Plastic changes along auditory pathway during salicylate-induced ototoxicity: Hyperactivity and CF shifts

High dose of salicylate, the active ingredient in aspirin, has long been known to induce transient hearing loss, tinnitus and hyperacusis making it a powerful experimental tool. These salicylate-induced perceptual disturbances are associated with a massive reduction in the neural output of the cochlea. Paradoxically, the diminished neural output of the cochlea is accompanied by a dramatic increase in sound-evoked activity in the auditory cortex (AC) and several other parts of the central nervous system. Exactly where the increase in neural activity begins and builds up along the central auditory pathway are not fully understood. To address this issue, we measured sound-evoked neural activity in the cochlea, cochlear nucleus (CN), inferior colliculus (IC), and AC before and after administering a high dose of sodium salicylate (SS, 300 mg/kg). The SS-treatment abolished low-level sound-evoked responses along the auditory pathway resulting in a 20-30 dB threshold shift. While the neural output of the cochlea was substantially reduced at high intensities, the neural responses in the CN were only slightly reduced; those in the IC were nearly normal or slightly enhanced while those in the AC considerably enhanced, indicative of a progress increase in central gain. The SS-induced increase in central response in the IC and AC was frequency-dependent with the greatest increase occurring in the mid-frequency range the putative pitch of SS-induced tinnitus. This frequency-dependent hyperactivity appeared to result from shifts in the frequency receptive fields (FRF) such that the response areas of many FRF shifted/expanded toward the mid-frequencies. Our results suggest that the SS-induced threshold shift originates in the cochlea. In contrast, enhanced central gain is not localized to one region, but progressively builds up at successively higher stage of the auditory pathway either through a loss of inhibition and/or increased excitation.

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.

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.

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

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

Sodium salicylate suppresses GABAergic inhibitory activity in neurons of rodent dorsal raphe nucleus

Sodium salicylate (NaSal), a tinnitus inducing agent, can activate serotonergic (5-HTergic) neurons in the dorsal raphe nucleus (DRN) and can increase serotonin (5-HT) level in the inferior colliculus and the auditory cortex in rodents. To explore the underlying neural mechanisms, we first examined effects of NaSal on neuronal intrinsic properties and the inhibitory synaptic transmissions in DRN slices of rats by using whole-cell patch-clamp technique. We found that NaSal hyperpolarized the resting membrane potential, decreased the input resistance, and suppressed spontaneous and current-evoked firing in GABAergic neurons, but not in 5-HTergic neurons. In addition, NaSal reduced GABAergic spontaneous and miniature inhibitory postsynaptic currents in 5-HTergic neurons. We next examined whether the observed depression of GABAergic activity would cause an increase in the excitability of 5-HTergic neurons using optogenetic technique in DRN slices of the transgenic mouse with channelrhodopsin-2 expressed in GABAergic neurons. When the GABAergic inhibition was enhanced by optical stimulation to GABAergic neurons in mouse DRN, NaSal significantly depolarized the resting membrane potential, increased the input resistance and increased current-evoked firing of 5-HTergic neurons. However, NaSal would fail to increase the excitability of 5-HTergic neurons when the GABAergic synaptic transmission was blocked by picrotoxin, a GABA receptor antagonist. Our results indicate that NaSal suppresses the GABAergic activities to raise the excitability of local 5-HTergic neural circuits in the DRN, which may contribute to the elevated 5-HT level by NaSal in the brain.

Local Application of Sodium Salicylate Enhances Auditory Responses in the Rat's Dorsal Cortex of the Inferior Colliculus

Sodium salicylate (SS) is a widely used medication with side effects on hearing. In order to understand these side effects, we recorded sound-driven local-field potentials in a neural structure, the dorsal cortex of the inferior colliculus (ICd). Using a microiontophoretic technique, we applied SS at sites of recording and studied how auditory responses were affected by the drug. Furthermore, we studied how the responses were affected by combined local application of SS and an agonists/antagonist of the type-A or type-B γ-aminobutyric acid receptor (GABA_A or GABA_B receptor). Results revealed that SS applied alone enhanced auditory responses in the ICd, indicating that the drug had local targets in the structure. Simultaneous application of the drug and a GABAergic receptor antagonist synergistically enhanced amplitudes of responses. The synergistic interaction between SS and a GABA_A receptor antagonist had a relatively early start in reference to the onset of acoustic stimulation and the duration of this interaction was independent of sound intensity. The interaction between SS and a GABA_B receptor antagonist had a relatively late start, and the duration of this interaction was dependent on sound intensity. Simultaneous application of the drug and a GABAergic receptor agonist produced an effect different from the sum of effects produced by the two drugs released individually. These differences between simultaneous and individual drug applications suggest that SS modified GABAergic inhibition in the ICd. Our results indicate that SS can affect sound-driven activity in the ICd by modulating local GABAergic inhibition.

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.

Salicylate-induced cochlear impairments, cortical hyperactivity and re-tuning, and tinnitus

High doses of sodium salicylate (SS) have long been known to induce temporary hearing loss and tinnitus, effects attributed to cochlear dysfunction. However, our recent publications reviewed here show that SS can induce profound, permanent, and unexpected changes in the cochlea and central nervous system. Prolonged treatment with SS permanently decreased the cochlear compound action potential (CAP) amplitude in vivo. In vitro, high dose SS resulted in a permanent loss of spiral ganglion neurons and nerve fibers, but did not damage hair cells. Acute treatment with high-dose SS produced a frequency-dependent decrease in the amplitude of distortion product otoacoustic emissions and CAP. Losses were greatest at low and high frequencies, but least at the mid-frequencies (10-20 kHz), the mid-frequency band that corresponds to the tinnitus pitch measured behaviorally. In the auditory cortex, medial geniculate body and amygdala, high-dose SS enhanced sound-evoked neural responses at high stimulus levels, but it suppressed activity at low intensities and elevated response threshold. When SS was applied directly to the auditory cortex or amygdala, it only enhanced sound evoked activity, but did not elevate response threshold. Current source density analysis revealed enhanced current flow into the supragranular layer of auditory cortex following systemic SS treatment. Systemic SS treatment also altered tuning in auditory cortex and amygdala; low frequency and high frequency multiunit clusters up-shifted or down-shifted their characteristic frequency into the 10-20 kHz range thereby altering auditory cortex tonotopy and enhancing neural activity at mid-frequencies corresponding to the tinnitus pitch. These results suggest that SS-induced hyperactivity in auditory cortex originates in the central nervous system, that the amygdala potentiates these effects and that the SS-induced tonotopic shifts in auditory cortex, the putative neural correlate of tinnitus, arises from the interaction between the frequency-dependent losses in the cochlea and hyperactivity in the central nervous system.

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.

Amygdala hyperactivity and tonotopic shift after salicylate exposure

The amygdala, important in forming and storing memories of aversive events, is believed to play a major role in debilitating tinnitus and hyperacusis. To explore this hypothesis, we recorded from the lateral amygdala (LA) and auditory cortex (AC) before and after treating rats with a dose of salicylate that induces tinnitus and hyperacusis-like behavior. Salicylate unexpectedly increased the amplitude of the local field potential (LFP) in the LA making it hyperactive to sounds≥60 dB SPL. Frequency receptive fields (FRFs) of multiunit (MU) clusters in the LA were also dramatically altered by salicylate. Neuronal activity at frequencies below 10 kHz and above 20 kHz was depressed at low intensities, but was greatly enhanced for stimuli between 10 and 20 kHz (frequencies near the pitch of the salicylate-induced tinnitus in the rat). These frequency-dependent changes caused the FRF of many LA neurons to migrate towards 10-20 kHz thereby amplifying activity from this region. To determine if salicylate-induced changes restricted to the LA would remotely affect neural activity in the AC, we used a micropipette to infuse salicylate (20 μl, 2.8 mM) into the amygdala. Local delivery of salicylate to the amygdala significantly increased the amplitude of the LFP recorded in the AC and selectively enhanced the neuronal activity of AC neurons at the mid-frequencies (10-20 kHz), frequencies associated with the tinnitus pitch. Taken together, these results indicate that systemic salicylate treatment can induce hyperactivity and tonotopic shift in the amygdala and infusion of salicylate into the amygdala can profoundly enhance sound-evoked activity in AC, changes likely to increase the perception and emotional salience of tinnitus and loud sounds. This article is part of a Special Issue entitled: Tinnitus Neuroscience.

High doses of salicylate and aspirin are inhibitory on acid-sensing ion channels and protective against acidosis-induced neuronal injury in the rat cortical neuron

Aspirin and its main metabolite salicylate are widely used to relieve pain, treat inflammatory diseases, and prevent ischemic stroke. Multiple pathways are responsible for the therapeutic actions exerted by these drugs. One of the pathways is targeting neuronal receptors/ion channels in the central nervous system. Correspondingly, increasing evidence has implicated acid-sensing ion channels (ASICs) in the processes of the diseases that are medicated by aspirin and salicylate. We therefore employed whole-cell patch-clamp recordings to examine the effects of salicylate as well as aspirin on ASICs in cultured cortical neurons of the rat. We recorded rapid and reversible inhibition of ASIC current by millimolar concentrations of aspirin and salicylate and found that salicylate reduced acidosis-induced membrane depolarization. These data suggest that ASICs in the cortex are molecular targets of high doses of aspirin and salicylate. In addition, the results from lactate dehydrogenase release measurement showed that high doses of aspirin and salicylate protected the cortical neuron from acidosis-induced neuronal injury. These findings may contribute to a better understanding of the therapeutic mechanisms of aspirin and salicylate actions in the brain and provide new evidence on aspirin and salicylate used as neuroprotective agents in the treatment of ischemic stroke.

Salicylate, an aspirin metabolite, specifically inhibits the current mediated by glycine receptors containing alpha1-subunits

BACKGROUND AND PURPOSE

Aspirin or its metabolite sodium salicylate is widely prescribed and has many side effects. Previous studies suggest that targeting neuronal receptors/ion channels is one of the pathways by which salicylate causes side effects in the nervous system. The present study aimed to investigate the functional action of salicylate on glycine receptors at a molecular level.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp and site-directed mutagenesis were deployed to examine the effects of salicylate on the currents mediated by native glycine receptors in cultured neurones of rat inferior colliculus and by glycine receptors expressed in HEK293T cells.

KEY RESULTS

Salicylate effectively inhibited the maximal current mediated by native glycine receptors without altering the EC(50) and the Hill coefficient, demonstrating a non-competitive action of salicylate. Only when applied simultaneously with glycine and extracellularly, could salicylate produce this antagonism. In HEK293T cells transfected with either alpha1-, alpha2-, alpha3-, alpha1beta-, alpha2beta- or alpha3beta-glycine receptors, salicylate only inhibited the current mediated by those receptors that contained the alpha1-subunit. A single site mutation of I240V in the alpha1-subunit abolished inhibition by salicylate.

CONCLUSIONS AND IMPLICATIONS

Salicylate is a non-competitive antagonist specifically on glycine receptors containing alpha1-subunits. This action critically involves the isoleucine-240 in the first transmembrane segment of the alpha1-subunit. Our findings may increase our understanding of the receptors involved in the side effects of salicylate on the central nervous system, such as seizures and tinnitus.

Salicylate increases the gain of the central auditory system

High doses of salicylate, the anti-inflammatory component of aspirin, induce transient tinnitus and hearing loss. Systemic injection of 250 mg/kg of salicylate, a dose that reliably induces tinnitus in rats, significantly reduced the sound evoked output of the rat cochlea. Paradoxically, salicylate significantly increased the amplitude of the sound-evoked field potential from the auditory cortex (AC) of conscious rats, but not the inferior colliculus (IC). When rats were anesthetized with isoflurane, which increases GABA-mediated inhibition, the salicylate-induced AC amplitude enhancement was abolished, whereas ketamine, which blocks N-methyl-d-aspartate receptors, further increased the salicylate-induced AC amplitude enhancement. Direct application of salicylate to the cochlea, however, reduced the response amplitude of the cochlea, IC and AC, suggesting the AC amplitude enhancement induced by systemic injection of salicylate does not originate from the cochlea. To identify a behavioral correlate of the salicylate-induced AC enhancement, the acoustic startle response was measured before and after salicylate treatment. Salicylate significantly increased the amplitude of the startle response. Collectively, these results suggest that high doses of salicylate increase the gain of the central auditory system, presumably by down-regulating GABA-mediated inhibition, leading to an exaggerated acoustic startle response. The enhanced startle response may be the behavioral correlate of hyperacusis that often accompanies tinnitus and hearing loss.

The aspirin metabolite salicylate enhances neuronal excitation in rat hippocampal CA1 area through reducing GABAergic inhibition

Salicylate is the major metabolite and active component of aspirin (acetylsalicylic acid), which is widely used in clinical medicine for treating inflammation, pain syndromes and cardiovascular disorders. The well-known mechanism underlying salicylate's action mainly involves the inhibition of cyclooxygenase and subsequent decrease in prostaglandin production. Recent evidence suggests that salicylate also affects neuronal function through interaction with specific membrane channels/receptors. However, the effect of salicylate on synaptic and neural network function remains largely unknown. In this study, we investigated the effect of sodium salicylate on the synaptic transmission and neuronal excitation in the hippocampal CA1 area of rats, a key structure for many complex brain functions. With electrophysiological recordings in hippocampal slices, we found that sodium salicylate significantly enhanced neuronal excitation through reducing inhibitory GABAergic transmission without affecting the basal excitatory synaptic transmission. Salicylate significantly inhibited the amplitudes of both evoked and miniature inhibitory postsynaptic currents, and directly reduced gamma-aminobutyric acid type A (GABA(A)) receptor-mediated responses in cultured rat hippocampal neurons. Together, our results suggest that the widely used aspirin might impair hippocampal synaptic and neural network functions through its actions on GABAergic neurotransmission. Given the capability of aspirin to penetrate the blood-brain barrier, the present data imply that aspirin intake may cause network hyperactivity and be potentially harmful in susceptible subpopulations.

Inhibition of voltage-gated channel currents in rat auditory cortex neurons by salicylate

Salicylate is a medicine for anti-inflammation with a side effect of tinnitus. To understand the mechanisms of tinnitus induced by salicylate, we studied the effects of salicylate on voltage-gated ion channels and action potential firing rates in freshly dissociated rat pyramidal neurons in auditory cortex (AC) using the whole-cell patch technique. We found that salicylate reduced the voltage-gated sodium current (I(Na)), the delayed rectifier potassium current (I(K(DR))) and the L-type voltage-gated calcium current (I(Ca,L)) in concentration-dependent manner. An amount of 1mM salicylate shifted the steady-state inactivation curve of I(Na) negatively by about 5mV, shifted the steady-state activation and inactivation curve of I(K(DR)) negatively by approximately 14mV and 17mV, respectively, and shifted the steady-state activation curve of I(Ca,L) negatively by about 10mV. 1mM salicylate significantly increased the action potential firing rates, ultimately. From the results, we speculated that through affecting the voltage-gated ion channels in AC, an important position in auditory system, salicylate increased the firing rate of neurons and enhanced neuronal excitability on the one hand, increased the excitatory transmitters release and reduced the inhibitory transmitter release on the other hand, thus finally induced tinnitus.

Amiloride attenuates glycine-induced currents in cultured neurons of rat inferior colliculus

Amiloride, a potassium sparing diuretic, is well known to interact with many ion transport systems and modulate the activity of several membrane receptors. However, relatively little information is available as to how amiloride affects membrane receptors of neurons in the brain areas. In the present study, we investigated the effects of amiloride on glycine-induced currents (I(Gly)) in cultured neurons of rat inferior colliculus with whole-cell patch-clamp recordings. Amiloride itself did not activate any current across the neuronal membrane but it reversibly inhibited the amplitude of the I(Gly) in a reversible and concentration-dependent manner, with an IC(50) of 487.4+/-25.3microM (n=5). Amiloride shifted the concentration-response relationship to the right without changing Hill coefficient and without changing the maximum response of the I(Gly). The pre-perfusion of amiloride produced an inhibitory effect on the I(Gly). In addition, amiloride was shown with a voltage ramp protocol to significantly reduce the conductance induced by glycine but not to change the reversal potential of the I(Gly). These results demonstrate that amiloride competitively inhibits the I(Gly) in rat inferior colliculus neurons by decreasing the affinity of glycine to its receptor. Our finding suggests that attention should be paid to the possible side effects of amiloride used as a drug on brain functions in the case of a defective blood-brain barrier and in the case of direct application of this drug into the cerebrospinal fluid for treatment of brain tumors.

Sodium salicylate reduces inhibitory postsynaptic currents in neurons of rat auditory cortex

Sodium salicylate (SS) is a medicine for anti-inflammation and for chronic pain relief with a side effect of tinnitus. To understand the cellular mechanisms of tinnitus induced by SS in the central auditory system, we examined effects of SS on evoked and miniature inhibitory postsynaptic currents (eIPSCs and mIPSCs) recorded from layer II/III pyramidal neurons of rat auditory cortex in a brain slice preparation with whole-cell patch-clamp techniques. Both eIPSCs and mIPSCs recorded from the auditory cortex could be completely blocked by bicuculline, a selective GABA(A) receptor antagonist. SS did not change the input resistance of neurons but was found to reversibly depress eIPSCs in a concentration-dependent manner. SS reduced eIPSCs to 82.3% of the control level at 0.5 mM (n=7) and to 60.9% at 1.4 mM (n=12). In addition, SS at 1.4 mM significantly reduced the amplitude of mIPSCs from 24.12+/-1.44 pA to 19.92+/-1.31 pA and reduced the frequency of mIPSCs from 1.34+/-0.23 Hz to 0.89+/-0.13 Hz (n=6). Our results demonstrate that SS attenuates inhibitory postsynaptic currents in the auditory cortex, suggesting that the alteration of inhibitory neural circuits may be one of the cellular mechanisms for tinnitus induced by SS in the central auditory region.