Tinnitus behavior and hearing function correlate with the reciprocal expression patterns of BDNF and Arg3.1/arc in auditory neurons following acoustic trauma

Behavioral models of tinnitus and hyperacusis in animals

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

Insult-induced adaptive plasticity of the auditory system

The brain displays a remarkable capacity for both widespread and region-specific modifications in response to environmental challenges, with adaptive processes bringing about the reweighing of connections in neural networks putatively required for optimizing performance and behavior. As an avenue for investigation, studies centered around changes in the mammalian auditory system, extending from the brainstem to the cortex, have revealed a plethora of mechanisms that operate in the context of sensory disruption after insult, be it lesion-, noise trauma, drug-, or age-related. Of particular interest in recent work are those aspects of auditory processing which, after sensory disruption, change at multiple—if not all—levels of the auditory hierarchy. These include changes in excitatory, inhibitory and neuromodulatory networks, consistent with theories of homeostatic plasticity; functional alterations in gene expression and in protein levels; as well as broader network processing effects with cognitive and behavioral implications. Nevertheless, there abounds substantial debate regarding which of these processes may only be sequelae of the original insult, and which may, in fact, be maladaptively compelling further degradation of the organism's competence to cope with its disrupted sensory context. In this review, we aim to examine how the mammalian auditory system responds in the wake of particular insults, and to disambiguate how the changes that develop might underlie a correlated class of phantom disorders, including tinnitus and hyperacusis, which putatively are brought about through maladaptive neuroplastic disruptions to auditory networks governing the spatial and temporal processing of acoustic sensory information.

Animal models of subjective tinnitus

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

Acoustical stimulus changes the expression of stromal cell-derived factor-1 in the spiral ganglion neurons of the rat cochlea

Neural stem cell (NSC) transplantation into the cochlea has been tested as a treatment for spiral ganglion neuron (SGN) degenerative disease and injury in various animal models. A recent study has shown evidence of functional recovery after transplantation of the stem cells into a degenerated-SGN model. Chemokine stromal cell-derived factor-1 (SDF-1, or known as CXC chemokine ligand-12, CXCL-12) signaling through CXCR4 has previously been identified as a key step in the homing of the stem cells within the injury areas; meanwhile, studies have revealed that the SDF-1/CXCR4 axis is also involved in axon guidance and pathfinding. A study found that transplanted neural precursor cells can migrate to the root of the auditory nerve when animals are subjected to an augmented acoustic environment (AAE). In accordance with these studies, we hypothesize that AAE will up-regulate the expression of SDF-1 in acoustic nerves. We tested our hypothesis by examining the expression of SDF-1 in different acoustic environments, and the results were confirmed by the auditory brainstem response (ABR), immunohistochemical and RT-PCR analyses. The results showed that SDF-1 was expressed at a relatively low level in the SGNs under normal animal unit acoustic conditions (40-50 dB). Moreover, it was significantly up-regulated in the SGNs under the 75 dB (augmented physiological process without hearing loss) and 90 dB AAE (pathological process with light hearing loss) conditions; however, under the 115 dB AAE (pathological process with severe hearing loss) condition, the expression of SDF-1 was not up-regulated. The results confirmed that appropriately augmented acoustical stimuli lead to the up-regulation of SDF-1, which may assist in the migration of the transplanted cells and the subsequent establishment of essential synaptic contacts between the exogenous cells and the host auditory pathway.

Effects of early and late treatment with L-baclofen on the development and maintenance of tinnitus caused by acoustic trauma in rats

Subjective tinnitus is a chronic neurological disorder in which phantom sounds are perceived. Recent evidence supports the hypothesis that tinnitus is related to neuronal hyperactivity in auditory brain regions, and consequently drugs that increase GABAergic neurotransmission in the CNS, such as the GABA(B) receptor agonist L-baclofen, may be effective as a treatment. The aim of this study was to investigate the effects of early (5 mg/kg s.c., 30 min and then every 24 h for 5 days following noise exposure) and late treatment (3 mg/kg/day s.c. for 4.5 weeks starting at 17.5 weeks following noise exposure) with l-baclofen on the psychophysical attributes of tinnitus in a conditioned lick suppression model following acoustic trauma in rats. Acoustic trauma (a 16-kHz, 115-dB pure tone presented unilaterally for 1h) resulted in a significant decrease in the suppression ratio (SR) compared to sham controls in response to 20-kHz tones at 2, 10 and 17.5 weeks post-exposure (P ≤ 0.009, P ≤ 0.02 and P ≤ 0.03, respectively). However, l-baclofen failed to prevent the development of tinnitus when administered during the first 5 days following the acoustic trauma and also failed to reverse it when treatment was carried out every day for 4.5 weeks. We also found that treatment with L-baclofen did not alter the expression of the GABA(B)-R2 subunit in the cochlear nucleus of noise-exposed animals.

L-type CaV1.2 deletion in the cochlea but not in the brainstem reduces noise vulnerability: implication for CaV1.2-mediated control of cochlear BDNF expression

Voltage-gated L-type Ca²⁺ channels (L-VGCCs) like Ca_V1.2 are assumed to play a crucial role for controlling release of trophic peptides including brain-derived neurotrophic factor (BDNF). In the inner ear of the adult mouse, besides the well-described L-VGCC Ca_V1.3, Ca_V1.2 is also expressed. Due to lethality of constitutive Ca_V1.2 knock-out mice, the function of this ion channel as well as its putative relationship to BDNF in the auditory system is entirely elusive. We recently described that BDNF plays a differential role for inner hair cell (IHC) vesicles release in normal and traumatized condition. To elucidate a presumptive role of Ca_V1.2 during this process, two tissue-specific conditional mouse lines were generated. To distinguish the impact of Ca_V1.2 on the cochlea from that on feedback loops from higher auditory centers Ca_V1.2 was deleted, in one mouse line, under the Pax2 promoter (Ca_V1.2^Pax2) leading to a deletion in the spiral ganglion neurons, dorsal cochlear nucleus, and inferior colliculus. In the second mouse line, the Egr2 promoter was used for deleting Ca_V1.2 (Ca_V1.2^Egr2) in auditory brainstem nuclei. In both mouse lines, normal hearing threshold and equal number of IHC release sites were observed. We found a slight reduction of auditory brainstem response wave I amplitudes in the Ca_V1.2^Pax2 mice, but not in the Ca_V1.2^Egr2 mice. After noise exposure, Ca_V1.2^Pax2 mice had less-pronounced hearing loss that correlated with maintenance of ribbons in IHCs and less reduced activity in auditory nerve fibers, as well as in higher brain centers at supra-threshold sound stimulation. As reduced cochlear BDNF mRNA levels were found in Ca_V1.2^Pax2 mice, we suggest that a Ca_V1.2-dependent step may participate in triggering part of the beneficial and deteriorating effects of cochlear BDNF in intact systems and during noise exposure through a pathway that is independent of Ca_V1.2 function in efferent circuits.

The function of BDNF in the adult auditory system

The inner ear of vertebrates is specialized to perceive sound, gravity and movements. Each of the specialized sensory organs within the cochlea (sound) and vestibular system (gravity, head movements) transmits information to specific areas of the brain. During development, brain-derived neurotrophic factor (BDNF) orchestrates the survival and outgrowth of afferent fibers connecting the vestibular organ and those regions in the cochlea that map information for low frequency sound to central auditory nuclei and higher-auditory centers. The role of BDNF in the mature inner ear is less understood. This is mainly due to the fact that constitutive BDNF mutant mice are postnatally lethal. Only in the last few years has the improved technology of performing conditional cell specific deletion of BDNF in vivo allowed the study of the function of BDNF in the mature developed organ. This review provides an overview of the current knowledge of the expression pattern and function of BDNF in the peripheral and central auditory system from just prior to the first auditory experience onwards. A special focus will be put on the differential mechanisms in which BDNF drives refinement of auditory circuitries during the onset of sensory experience and in the adult brain. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

The reduced cochlear output and the failure to adapt the central auditory response causes tinnitus in noise exposed rats

Tinnitus is proposed to be caused by decreased central input from the cochlea, followed by increased spontaneous and evoked subcortical activity that is interpreted as compensation for increased responsiveness of central auditory circuits. We compared equally noise exposed rats separated into groups with and without tinnitus for differences in brain responsiveness relative to the degree of deafferentation in the periphery. We analyzed (1) the number of CtBP2/RIBEYE-positive particles in ribbon synapses of the inner hair cell (IHC) as a measure for deafferentation; (2) the fine structure of the amplitudes of auditory brainstem responses (ABR) reflecting differences in sound responses following decreased auditory nerve activity and (3) the expression of the activity-regulated gene Arc in the auditory cortex (AC) to identify long-lasting central activity following sensory deprivation. Following moderate trauma, 30% of animals exhibited tinnitus, similar to the tinnitus prevalence among hearing impaired humans. Although both tinnitus and no-tinnitus animals exhibited a reduced ABR wave I amplitude (generated by primary auditory nerve fibers), IHCs ribbon loss and high-frequency hearing impairment was more severe in tinnitus animals, associated with significantly reduced amplitudes of the more centrally generated wave IV and V and less intense staining of Arc mRNA and protein in the AC. The observed severe IHCs ribbon loss, the minimal restoration of ABR wave size, and reduced cortical Arc expression suggest that tinnitus is linked to a failure to adapt central circuits to reduced cochlear input.

The Mozart effect in patients suffering from tinnitus

CONCLUSION

The study suggests that Mozart therapy could be a valid alternative to the common sound therapy methods in tinnitus patients.

OBJECTIVES

The aim of the study was to evaluate the presence of the Mozart effect as indexed by a variation in tinnitus intensity and tolerability.

METHOD

Sixty-two individuals aged between 22 and 78 years, reporting tinnitus for at least 1 year, were enrolled for the study. All patients attended a 1 h cognitive behavioral counseling session and listened to Mozart's sonata k448 for 1 h per day for a month. Afterwards patients listened to Beethoven's Für Elise sonata for 1 h per day for a month. To evaluate the general stress level, the impact of tinnitus on patients' quality of life, and the intensity of tinnitus, patients were invited to participate in three tests: the Measure du Stress Psychologique (MSP) questionnaire, the Tinnitus Handicap Inventory (THI), and a 0 to 10 visual analog scale (VAS).

RESULTS

For all the parameters investigated, MSP, THI, and intensity, there was a general significant improvement between the pre- and post-listening evaluation. A significant improvement, as regards THI and intensity, could already be appreciated after a single exposure to Mozart's sonata.

Lack of brain-derived neurotrophic factor hampers inner hair cell synapse physiology, but protects against noise-induced hearing loss

The precision of sound information transmitted to the brain depends on the transfer characteristics of the inner hair cell (IHC) ribbon synapse and its multiple contacting auditory fibers. We found that brain derived neurotrophic factor (BDNF) differentially influences IHC characteristics in the intact and injured cochlea. Using conditional knock-out mice (BDNF(Pax2) KO) we found that resting membrane potentials, membrane capacitance and resting linear leak conductance of adult BDNF(Pax2) KO IHCs showed a normal maturation. Likewise, in BDNF(Pax2) KO membrane capacitance (ΔC(m)) as a function of inward calcium current (I(Ca)) follows the linear relationship typical for normal adult IHCs. In contrast the maximal ΔC(m), but not the maximal size of the calcium current, was significantly reduced by 45% in basal but not in apical cochlear turns in BDNF(Pax2) KO IHCs. Maximal ΔC(m) correlated with a loss of IHC ribbons in these cochlear turns and a reduced activity of the auditory nerve (auditory brainstem response wave I). Remarkably, a noise-induced loss of IHC ribbons, followed by reduced activity of the auditory nerve and reduced centrally generated wave II and III observed in control mice, was prevented in equally noise-exposed BDNF(Pax2) KO mice. Data suggest that BDNF expressed in the cochlea is essential for maintenance of adult IHC transmitter release sites and that BDNF upholds opposing afferents in high-frequency turns and scales them down following noise exposure.

Evidence that Memantine Reduces Chronic Tinnitus Caused by Acoustic Trauma in Rats

Subjective tinnitus is a chronic neurological disorder in which phantom sounds are perceived. Increasing evidence suggests that tinnitus is caused by neuronal hyperactivity in auditory brain regions, either due to a decrease in synaptic inhibition or an increase in synaptic excitation. One drug investigated for the treatment of tinnitus has been the uncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, memantine, although the evidence relating to it has been unconvincing to date. We re-investigated the effects of memantine on the behavioral manifestations of tinnitus induced by acoustic trauma (a 16-kHz, 110-dB pure tone presented unilaterally for 1 h) in rats. We used a conditioned lick suppression model in which lick suppression was associated with the perception of high frequency sound resembling tinnitus and a suppression ratio (SR) was calculated by comparing the number of licks in the 15-s period preceding the stimulus presentation (A) and the 15-s period during the stimulus presentation (B), i.e., SR = B/(A + B). Acoustic trauma resulted in a significant increase in the auditory brainstem-evoked response (ABR) threshold in the affected ear (P ≤ 0.0001) and a decrease in the SR compared to sham controls in response to 32 kHz tones in five out of eight acoustic trauma-exposed animals. A 5-mg/kg dose of memantine significantly reduced the proportion of these animals which exhibited tinnitus-like behavior (2/5 compared to 5/5; P ≤ 0.006), suggesting that the drug reduced tinnitus. These results suggest that memantine may reduce tinnitus caused by acoustic trauma.

Long-term, but not transient, threshold shifts alter the morphology and increase the excitability of cortical pyramidal neurons

Partial hearing loss often results in enlarged representations of the remaining hearing frequency range in primary auditory cortex (AI). Recent studies have implicated certain types of synaptic plasticity in AI map reorganization in response to transient and long-term hearing loss. How changes in neuronal excitability and morphology contribute to cortical map reorganization is less clear. In the present study, we exposed adult rats to a 4-kHz tone at 123 dB, which resulted in increased thresholds over their entire hearing range. The threshold shift gradually recovered in the lower-frequency, but not the higher-frequency, range. As reported previously, two distinct zones were observed 10 days after the noise exposure, an enlarged lower-characteristic frequency (CF) zone displaying normal threshold and enhanced cortical responses and a higher-CF zone showing higher threshold and a disorganized tonotopic map. Membrane excitability of layer II/III pyramidal neurons increased only in the higher-CF, but not the lower-CF, zone. In addition, dendritic morphology and spine density of the pyramidal neurons were altered in the higher-CF zone only. These results indicate that membrane excitability and neuronal morphology are altered by long-term, but not transient, threshold shift. They also suggest that these changes may contribute to tinnitus but are unlikely to be involved in map expansion in the lower-CF zone.

[Possible molecular mechanisms of spontaneous remission in sudden idiopathic hearing loss]

According to current knowledge, it must be assumed that temporary idiopathic hearing loss and its spontaneous remission are based on mechanical and/or pathological alterations in the inner ear. The causal mechanisms might be based on inter-individual variations. Induced by dose-dependent activators, temporary as well as permanent damage might occur. Sudden hearing loss may be initiated by an increase in the local nitric oxide (NO) concentration. Spontaneous remission, i.e. functional restoration, can be explained by a local decrease in the NO concentration. In this context, regulatory systems such as the gap-junction system, blood vessels or synapses might be affected. In addition, alterations in the hormone level of estrogen and mineralocorticoids, as well as cellular glutathione and vitamin levels, might lead to temporary alterations in the inner ear. Recent experimental findings indicate a role for the shuttle protein Survivin in the spontaneous remission of sudden hearing loss.

Tracking the expression of excitatory and inhibitory neurotransmission-related proteins and neuroplasticity markers after noise induced hearing loss

Excessive exposure to loud noise can damage the cochlea and create a hearing loss. These pathologies coincide with a range of CNS changes including reorganisation of frequency representation, alterations in the pattern of spontaneous activity and changed expression of excitatory and inhibitory neurotransmitters. Moreover, damage to the cochlea is often accompanied by acoustic disorders such as hyperacusis and tinnitus, suggesting that one or more of these neuronal changes may be involved in these disorders, although the mechanisms remain unknown. We tested the hypothesis that excessive noise exposure increases expression of markers of excitation and plasticity, and decreases expression of inhibitory markers over a 32-day recovery period. Adult rats (n = 25) were monaurally exposed to a loud noise (16 kHz, 1/10(th) octave band pass (115 dB SPL)) for 1-hour, or left as non-exposed controls (n = 5). Animals were euthanased at either 0, 4, 8, 16 or 32 days following acoustic trauma. We used Western Blots to quantify protein levels of GABA(A) receptor subunit α1 (GABA(A)α1), Glutamic-Acid Decarboxylase-67 (GAD-67), N-Methyl-D-Aspartate receptor subunit 2A (NR2A), Calbindin (Calb1) and Growth Associated Protein 43 (GAP-43) in the Auditory Cortex (AC), Inferior Colliculus (IC) and Dorsal Cochlear Nucleus (DCN). Compared to sham-exposed controls, noise-exposed animals had significantly (p<0.05): lower levels of GABA(A)α1 in the contralateral AC at day-16 and day-32, lower levels of GAD-67 in the ipsilateral DCN at day-4, lower levels of Calb1 in the ipsilateral DCN at day-0, lower levels of GABA(A)α1 in the ipsilateral AC at day-4 and day-32. GAP-43 was reduced in the ipsilateral AC for the duration of the experiment. These complex fluctuations in protein expression suggests that for at least a month following acoustic trauma the auditory system is adapting to a new pattern of sensory input.

Various levels of plasma brain-derived neurotrophic factor in patients with tinnitus

OBJECTIVE

Thus far, no objective measure has been developed to evaluate tinnitus severity. There is a close relationship between tinnitus and depression, in which brain-derived neurotrophic factor (BDNF) has a pathophysiological role. To determine whether BDNF levels could be used to evaluate tinnitus severity, we evaluated plasma BDNF levels in patients with tinnitus.

METHODS

Plasma BDNF levels were measured in 43 tinnitus patients and 30 healthy control patients. The severities of tinnitus, depression, and anxiety were measured using the tinnitus handicap inventory (THI) and the hospital anxiety and depression scale (HADS), respectively. Patients with tinnitus were divided into 2 groups depending on their THI scores: mildly handicapped (<36) and severely handicapped (>38). We also divided our subjects into 2 groups depending on the HADS score, which represents patient mood, including depression and anxiety.

RESULTS

Plasma BDNF levels were significantly higher in the mildly handicapped group than in the severely handicapped and control groups (P<0.01). Patients with HADS scores of ≤14 had significantly lower THI scores (P<0.05) and higher BDNF levels (P<0.01).

CONCLUSIONS

Our findings show for the first time that plasma BDNF levels vary with the severity of tinnitus, suggesting that plasma BDNF level is a useful tool for objective evaluation of tinnitus.

Plasticity of serotonergic innervation of the inferior colliculus in mice following acoustic trauma

Acoustic trauma often results in permanent damage to the cochlea, triggering changes in processing within central auditory structures such as the inferior colliculus (IC). The serotonergic neuromodulatory system, present in the IC, is responsive to chronic changes in the activity of sensory systems. The current study investigated whether the density of serotonergic innervation in the IC is changed following acoustic trauma. The trauma stimulus consisted of an 8 kHz pure tone presented at a level of 113 dB SPL for six consecutive hours to anesthetized CBA/J mice. Following a minimum recovery period of three weeks, serotonergic fibers were visualized via histochemical techniques targeting the serotonin reuptake transporter (SERT) and quantified using stereologic probes. SERT-positive fiber densities were then compared between the traumatized and protected hemispheres of unilaterally traumatized subjects and those of controls. A significant effect of acoustic trauma was found between the hemispheres of unilaterally traumatized subjects such that the IC contralateral to the ear of exposure contained a lower density of SERT-positive fibers than the IC ipsilateral to acoustic trauma. No significant difference in density was found between the hemispheres of control subjects. Additional dimensions of variability in serotonergic fibers were seen among subdivisions of the IC and with age. The central IC had a slightly but significantly lowered density of serotonergic fibers than other subdivisions of the IC, and serotonergic fibers also declined with age. Overall, the results indicate that acoustic trauma is capable of producing modest but significant decreases in the density of serotonergic fibers innervating the IC.

A dose-response analysis of the effects of L-baclofen on chronic tinnitus caused by acoustic trauma in rats

Subjective tinnitus is a chronic neurological disorder in which phantom sounds are perceived. Drugs that increase GABAergic neurotransmission in the CNS are sometimes used as a treatment. One such drug is the GABA(B) receptor agonist L-baclofen. The aim of this study was to investigate the effects of L-baclofen on the psychophysical attributes of tinnitus in rats.The effects of 1, 3 or 5 mg/kg L-baclofen (s.c.) on the psychophysical attributes of tinnitus were investigated using a conditioned lick suppression model, following acoustic trauma (a 16 kHz, 110 dB pure tone presented unilaterally for 1 h) in rats. In pre-drug testing, acoustic trauma resulted in a significant increase in the auditory brainstem-evoked response (ABR) threshold in the affected ear (P < 0.008) and a significant decrease in the suppression ratio (SR) compared to sham controls in response to the 20 kHz tones, but not the broadband noise or the 10 kHz tones (P < 0.002). The 3 and 5 mg/kg doses of L-baclofen significantly reversed the frequency-specific decrease in the SR in the acoustic trauma group, indicating that the drug reduced tinnitus. Following washout from the 3 mg/kg dose, but not the 5 mg/kg dose, the significant decrease in the SR for the acoustic trauma group returned, suggesting a return of the tinnitus. These results suggest that L-baclofen should be reconsidered as a drug treatment for tinnitus. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

The effects of acoustic trauma that can cause tinnitus on spatial performance in rats

Previous studies have shown that acoustic trauma can disrupt the firing of place cells in the hippocampus and also inhibit hippocampal neurogenesis, suggesting that such trauma might impair spatial memory. In this study, we investigated performance in the alternating T maze and Morris water maze of rats exposed to acoustic trauma (16 kHz, 110 dB SPL pure tone for 1 h), who had elevated auditory brainstem response thresholds and the psychophysical attributes of tinnitus (using a conditioned lick suppression task). To our surprise, we found that rats with tinnitus did not perform significantly differently from sham control animals in either the alternating T maze task or any aspect of the reference or working memory versions of the Morris water maze task except for a faster acquisition in T maze alternation. These results suggest that acoustic trauma and tinnitus may not impair spatial memory in rats.

Acoustic trauma that can cause tinnitus impairs impulsive control but not performance accuracy in the 5-choice serial reaction time task in rats

Although tinnitus is an auditory disorder, it is often associated with attentional and emotional problems. Functional neuroimaging studies in humans have revealed that the hippocampus, amygdala and anterior cingulate, areas of the brain involved in emotion, attention and spatial processing, are also involved in auditory memory and tinnitus perception. However, few studies of tinnitus-evoked emotional and cognitive changes have been reported using animal models of tinnitus. In the present study, we investigated whether acoustic trauma that could cause tinnitus would affect attention and impulsivity in rats. Eight male Wistar rats were exposed to unilateral acoustic trauma (110 dB, 16 kHz for 1 h under anaesthesia) and eight rats underwent the same anaesthesia without acoustic trauma. Tinnitus was tested in noise-exposed rats using a frequency-specific shift in a discrimination function with a conditioned lick suppression paradigm. At 4 months after the noise exposure, the rats were tested in a 5-choice serial reaction time task. The behavioural procedure involved training the rats to discriminate a brief visual stimulus presented randomly in one of the five spatial locations and responding by poking its nose through the illuminated hole and collecting a food pellet from the magazine. While all of the animals performed equally well in making correct responses, the animals exposed to acoustic trauma made significantly more premature responses. The results suggest that rats exposed to acoustic trauma and some of which have chronic tinnitus are impaired in impulsive control, but not performance accuracy.

[Molecular biological aspects of neuroplasticity: approaches for treating tinnitus and hearing disorders]

Peripheral and central structures are involved in the onset of tinnitus. Neuronal plasticity is of special importance for the occurrence of central tinnitus and its persistent form. Neuronal plasticity is the ability of the brain to adapt its own structure (synapses, nerve cells, or even whole areas of the brain) and its organization to modified biological requirements. Neuroplasticity is an ongoing dynamic process. Generally speaking, there are two types of plasticity: synaptic and cortical. Cortical plasticity involves activity-dependent changes in size, connectivity, or in the activation pattern of cortical networks. Synaptic plasticity refers to the activity-dependent change in the strength of synaptic transmission and can affect both the morphology and physiology of the synapse. The stimulation of afferent fibers leads to long-lasting changes in synaptic transmission. This phenomenon is called long-term potentiation (LTP) or long-term depression (LTD). From the perspective of molecular biology, synaptic plasticity is of particular importance for the development of tinnitus and its persistence. Ultimately, the damage to the hair cells, auditory nerve, and excitotoxicity results in an imbalance between LTP and LTD and thus in changes of synaptic plasticity. After excessive acoustic stimulation, LTP can be induced by the increase of afferent inputs, whereas decreased afferent inputs generate LTD. The imbalance between LTP and LTD leads to changes in gene expression and involves changes in neurotransmission, in the expression of the receptors, ion channels, regulatory enzymes, and in direct changes on the synapses. This causes an increase of activity on the cellular level. As a result, the imbalance can lead to hyperactivity in the dorsal cochlear nucleus, inferior colliculus, and in the auditory cortex and, later on, to changes in cortical plasticity leading to tinnitus.

Arc/Arg3.1 mRNA expression reveals a subcellular trace of prior sound exposure in adult primary auditory cortex

Acquiring the behavioral significance of sound has repeatedly been shown to correlate with long term changes in response properties of neurons in the adult primary auditory cortex. However, the molecular and cellular basis for such changes is still poorly understood. To address this, we have begun examining the auditory cortical expression of an activity-dependent effector immediate early gene (IEG) with documented roles in synaptic plasticity and memory consolidation in the hippocampus: Arc/Arg3.1. For initial characterization, we applied a repeated 10 min (24 h separation) sound exposure paradigm to determine the strength and consistency of sound-evoked Arc/Arg3.1 mRNA expression in the absence of explicit behavioral contingencies for the sound. We used 3D surface reconstruction methods in conjunction with fluorescent in situ hybridization (FISH) to assess the layer-specific subcellular compartmental expression of Arc/Arg3.1 mRNA. We unexpectedly found that both the intranuclear and cytoplasmic patterns of expression depended on the prior history of sound stimulation. Specifically, the percentage of neurons with expression only in the cytoplasm increased for repeated versus singular sound exposure, while intranuclear expression decreased. In contrast, the total cellular expression did not differ, consistent with prior IEG studies of primary auditory cortex. Our results were specific for cortical layers 3-6, as there was virtually no sound driven Arc/Arg3.1 mRNA in layers 1-2 immediately after stimulation. Our results are consistent with the kinetics and/or detectability of cortical subcellular Arc/Arg3.1 mRNA expression being altered by the initial exposure to the sound, suggesting exposure-induced modifications in the cytoplasmic Arc/Arg3.1 mRNA pool.

Impact of sound exposure and aging on brain-derived neurotrophic factor and tyrosine kinase B receptors levels in dorsal cochlear nucleus 80 days following sound exposure

Recent studies suggested that acute sound exposure resulting in a temporary threshold shift in young adult animals within a series of maladaptive plasticity changes in central auditory structures. Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, is involved in post-trauma peripheral hair cell and spiral ganglion cell survival and has been shown to modulate synaptic strength in cochlear nucleus following sound exposure. The present study evaluated levels of BDNF and its receptor (tyrosine kinase B, [TrkB]) in the dorsal cochlear nucleus (DCN) following a unilateral moderate sound exposure in young (7-8 months) and aged (28-29 months) Fischer Brown Norway (FBN) rats. Eighty days post-exposure, auditory brainstem response (ABR) thresholds for young exposed rats approached control values while aged exposed rats showed residual permanent threshold shifts (PTS) relative to aged controls. BDNF protein levels were significantly up-regulated by 9% in young exposed fusiform cells ipsilateral to the exposure. BDNF levels in aged sound-exposed fusiform cells increased 31% ipsilateral to the exposure. Protein levels of the BDNF receptor, TrkB, were also significantly increased in aged but not in young sound-exposed DCN fusiform cells. The present findings suggest a relationship between the up-regulation of BDNF/TrkB and the increase in spontaneous and driven activity previously observed for aged and sound-exposed fusiform cells. This might be due to a selective maladaptive compensatory down-regulation of glycinergic inhibition in DCN fusiform cells.

Bioluminescence imaging of Arc expression enables detection of activity-dependent and plastic changes in the visual cortex of adult mice

Induction of the activity-regulated cytoskeleton-associated protein gene (Arc), one of the immediate early genes, in the brain correlates with various sensory processes, natural behaviors, and pathological conditions. Arc is also involved in synaptic plasticity during development. Thus, in vivo monitoring of Arc expression is useful for the analysis of physiological and pathological conditions in the brain. Recently, in vivo imaging of Arc expression using various green fluorescent protein-based probes has been reported; however, these probes can only be applied for the detection of fluorescence signals from superficial layers of the cortex with some autofluorescence noise. Here, we generated a novel transgenic mouse strain to monitor the neuronal-activity-dependent Arc expression using bioluminescence signals in vivo. Because of the very high sensitivity with a high signal-to-noise ratio, we detected neuronal-activity-dependent plastic changes in the bioluminescence signal intensity in the mouse visual cortex after visual deprivation, suggesting structural plasticity after peripheral lesions in adults. We also detected drastic changes in bioluminescence signals after seizure induction with kainic acid. Our novel mouse strain will be valuable for the continuous monitoring of neuronal-activity-dependent Arc expression in the brain under physiological and pathological conditions.

Thyroxine treatments do not correct inner ear defects in tmprss1 mutant mice

Complete deficiency of a member of the type II transmembrane serine protease family, tmprss1 (also known as hepsin), is associated with severe to profound hearing loss in mice and a gross enlargement of the tectorial membrane in the cochlea. Levels of thyroxine in these mice have been shown to be significantly lower when compared with wild-type controls. As thyroxine is critical for inner ear development, we delivered thyroxine to these mice during the prenatal or postnatal stage of development. Both the treatments could not ameliorate hearing loss or correct deformities in the tectorial membrane of these mutant mice, suggesting that a deficiency in tmprss1 affects thyroxine responsiveness in the inner ear in vivo.

Tinnitus psychopharmacology: A comprehensive review of its pathomechanisms and management

BACKGROUND

Subjective tinnitus is a frequent, impairing condition, which may also cause neurotransmitter imbalance at the cochlea. Psychopharmacologic agents, although not being the first-line treatment for tinnitus, may modulate cochlear neurotransmission, thereby influencing the subjective tinnitus experience.

METHOD

A comprehensive review of MEDLINE literature (from January 1990-January 2010) was performed searching for: "tinnitus", major classes of psychopharmacological agents, and psychiatric disorders. The most relevant clinical evidence is reported briefly along with a concise description of the main neurotransmitters purported to be involved in tinnitus, in order to provide the reader with a rational evaluation of tinnitus therapy with psychopharmacological agents.

RESULTS

Although strong methodological issues limit the reliability of the current results, a broad number of psychopharmacological agents have already been considered for tinnitus, both as candidate triggers or potential therapies.

CONCLUSIONS

Selected psychopharmacological drugs may play a role in the clinical management of this disorder. While the rational use of these agents for the treatment of tinnitus should not be overlooked, research should be undertaken on their neuromodulating actions at the cochlea.

Tonotopic changes in GABA receptor expression in guinea pig inferior colliculus after partial unilateral hearing loss

Immunohistochemistry was used to investigate the topographic distribution of the alpha1 subunit of the GABA receptor (GABRA1) in guinea pig inferior colliculus after treatments that caused a unilateral loss of peripheral neural sensitivity in the high-frequency regions of the cochlea. Both forms of treatment (direct mechanical lesion of the cochlea and acoustic overstimulation) resulted in a significant decrease in GABRA1 labeling in regions of the contralateral inferior colliculus in which high-frequency sound stimuli are represented. This localized region of reduced inhibitory receptor expression corresponds to the region in which hyperactivity of inferior colliculus neurons has been shown to develop after such treatments. The results strengthen the notion of a causal link between reduced GABRA1 expression and neural hyperactivity in central auditory nuclei and provide a possible mechanism for the development of phantom auditory sensations, or tinnitus.

Sustained delivery of lidocaine into the cochlea using poly lactic/glycolic acid microparticles

OBJECTIVES/HYPOTHESIS

Lidocaine is a local anesthetic that is known to suppress tinnitus via systemic or local application; however, this effect has only limited duration. The current study aimed to establish a method for the sustained delivery of lidocaine into the cochlea using poly lactic/glycolic acid (PLGA) microparticles.

STUDY DESIGN

Experimental study.

METHODS

Lidocaine-loaded PLGA microparticles were produced and their in vitro-release profiles were examined. The lidocaine concentrations in the perilymph were measured at different time points following the application of the lidocaine-loaded PLGA microparticles to the round-window membranes of guinea pigs. The possible adverse effects of the local application of lidocaine-loaded PLGA microparticles were also examined.

RESULTS

The in vitro analyses revealed that the microparticles were capable of the sustained delivery of lidocaine. The in vivo experiments demonstrated the sustained delivery of lidocaine into the cochlear fluid, and the maintenance of high lidocaine concentrations in the perilymph for up to 3 days after application. Nystagmus and inflammation in the middle ear mucosa were not detected after the local application of lidocaine-loaded PLGA microparticles, although temporary hearing loss was observed.

CONCLUSIONS

Lidocaine-loaded PLGA microparticles were shown to be capable of the sustained delivery of lidocaine into the cochlea, suggesting that they could be used for the attenuation of peripheral tinnitus.

Fluorescent Arc/Arg3.1 indicator mice: a versatile tool to study brain activity changes in vitro and in vivo

The brain-specific immediate early gene Arc/Arg3.1 is induced in response to a variety of stimuli, including sensory and behavior-linked neural activity. Here we report the generation of transgenic mice, termed TgArc/Arg3.1-d4EGFP, expressing a 4-h half-life form of enhanced green fluorescent protein (d4EGFP) under the control of the Arc/Arg3.1 promoter. We show that d4EGFP-mediated fluorescence faithfully reports Arc/Arg3.1 induction in response to physiological, pathological and pharmacological stimuli, and that this fluorescence permits electrical recording from activated neurons in the live mouse. Moreover, the fluorescent Arc/Arg3.1 indicator revealed activity changes in circumscribed brain areas in distinct modes of stress and in a mouse model of Alzheimer's disease. These findings identify the TgArc/Arg3.1-d4EGFP mouse as a versatile tool to monitor Arc/Arg3.1 induction in neural circuits, both in vitro and in vivo.

Prenatal dexamethasone impairs behavior and the activation of the BDNF exon IV promoter in the paraventricular nucleus in adult offspring

Prenatal manipulations to the hypothalamic-pituitary-adrenal axis are shown to affect auditory responses to an acoustic challenge as well as behavior in adult life. To achieve these results, we examined the effect of prenatal dexamethasone (DEX) treatment in male and female adult rat offspring by assessing body and adrenal weight, anxiety using the elevated plus maze (EPM), and acoustic startle responses as well as the effects of acoustic challenge in the paraventricular nucleus (PVN). DEX male offspring had reduced adrenal gland weight in adult life and demonstrated anxiolytic-like behavior when tested on the EPM. The acoustic startle amplitude in naive DEX-treated male offspring was significantly higher compared with saline (SAL)-treated males and females and DEX-treated females. When challenged with either a glucocorticoid agonist or antagonist, the startle response of the SAL-treated males and females significantly increased or decreased in the presence of agonist and antagonist treatment, respectively, whereas DEX males and females were not affected. Acoustic challenge caused an increase in c-fos mRNA and glucocorticoid receptor nuclear translocation in the PVN of all groups. BDNF and TrkB mRNA increased in the PVN after acoustic challenge in the SAL-treated males and females but not in the DEX males or females. These findings exemplify the differential sensitivity of the developing nervous and endocrine systems to prenatal hormonal stress and demonstrate that prenatal DEX treatment elicits long-term behavioral alterations related to anxiety and auditory processing.

Cochlear implants stimulate activity-dependent CREB pathway in the deaf auditory cortex: implications for molecular plasticity induced by neural prosthetic devices

Neural activity modulates the maturation of synapses and their organization into functional circuits by regulating activity-dependent signaling pathways. Phosphorylation of cyclic AMP/Ca(2+)-responsive element-binding protein (CREB) is widely accepted as a stimulus-inducible event driven by calcium influx into depolarized neurons. In turn, phosphorylated CREB (pCREB) activates the transcription of brain-derived neurotrophic factor (BDNF), which is needed for synaptic transmission and long-term potentiation. We examined how these molecular events are influenced by sensorineural hearing loss and long-term reactivation via cochlear implants. Sensorineural hearing loss reduced the expression of pCREB and BDNF. In contrast, deafened animals subject to long-term, unilateral intracochlear electrical stimulation exhibited an increased expression of pCREB and BDNF in the contralateral auditory cortical neurons, relative to ipsilateral ones. These changes induced by cochlear implants are further accompanied by the activation of the mitogen-activated protein kinase (MAPK) signaling pathway, which has been implicated in long-lasting forms of synaptic plasticity. Because CREB and BDNF are critical modulators of synaptic plasticity, our data describe for the first time possible molecular candidate genes, which are altered in the auditory cortex, following cochlear implantation. These findings provide insights into adaptive, molecular mechanisms recruited by the brain upon functional electrical stimulation by neural prosthetic devices.

Noise exposure-induced enhancement of auditory cortex response and changes in gene expression

Noise exposure is one of the most common causes of hearing loss. There is growing evidence suggesting that noise-induced peripheral hearing loss can also induce functional changes in the central auditory system. However, the physiological and biological changes in the central auditory system induced by noise exposure are poorly understood. To address these issues, neurophysiological recordings were made from the auditory cortex (AC) of awake rats using chronically implanted electrodes before and after acoustic overstimulation. In addition, focused gene microarrays and quantitative real-time polymerase chain reaction were used to identify changes in gene expression in the AC. Monaural noise exposure (120 dB sound pressure level, 1 h) significantly elevated hearing threshold on the exposed ear and induced a transient enhancement on the AC response amplitude 4 h after the noise exposure recorded from the unexposed ear. This increase of the cortical neural response amplitude was associated with an upregulation of genes encoding heat shock protein (HSP) 27 kDa and 70 kDa after several hours of the noise exposure. These results suggest that noise exposure can induce a fast physiological change in the AC which may be related to the changes of HSP expressions.