Modulation of inhibitory and excitatory synaptic transmission in rat inferior colliculus after unilateral cochleectomy: an in situ and immunofluorescence study

Noise-induced hearing loss alters potassium-chloride cotransporter KCC2 and GABA inhibition in the auditory centers

Homeostatic plasticity, the ability of neurons to maintain their averaged activity constant around a set point value, is thought to account for the central hyperactivity after hearing loss. Here, we investigated the putative role of GABAergic neurotransmission in this mechanism after a noise-induced hearing loss larger than 50 dB in high frequencies in guinea pigs. The effect of GABAergic inhibition is linked to the normal functioning of K + -Cl- co-transporter isoform 2 (KCC2) which maintains a low intracellular concentration of chloride. The expression of membrane KCC2 were investigated before and after noise trauma in the ventral and dorsal cochlear nucleus (VCN and DCN, respectively) and in the inferior colliculus (IC). Moreover, the effect of gabazine (GBZ), a GABA antagonist, was also studied on the neural activity in IC. We show that KCC2 is downregulated in VCN, DCN and IC 3 days after noise trauma, and in DCN and IC 30 days after the trauma. As expected, GBZ application in the IC of control animals resulted in an increase of spontaneous and stimulus-evoked activity. In the noise exposed animals, on the other hand, GBZ application decreased the stimulus-evoked activity in IC neurons. The functional implications of these central changes are discussed.

Noise-Induced Hearing Loss Alters Potassium-Chloride CoTransporter KCC2 and GABA Inhibition in the auditory centers

Homeostatic plasticity, the ability of neurons to maintain their averaged activity constant around a set point value, is thought to account for the central hyperactivity after hearing loss. Here, we investigated the putative role of GABAergic neurotransmission in this mechanism after a noise-induced hearing loss larger than 50 dB in high frequencies in guinea pigs. The effect of GABAergic inhibition is linked to the normal functioning of K+-Cl- co-transporter isoform 2 (KCC2) which maintains a low intracellular concentration of chloride. The expression of membrane KCC2 were investigated before after noise trauma in the ventral and dorsal cochlear nucleus (VCN and DCN, respectively) and in the inferior colliculus (IC). Moreover, the effect of gabazine (GBZ), a GABA antagonist, was also studied on the neural activity in IC. We show that KCC2 is downregulated in VCN, DCN and IC 3 days after noise trauma, and in DCN and IC 30 days after the trauma. As expected, GBZ application in the IC of control animals resulted in an increase of spontaneous and stimulus-evoked activity. In the noise exposed animals, on the other hand, GBZ application decreased the stimulus-evoked activity in IC neurons. The functional implications of these central changes are discussed.

Downregulated miRNAs associated with auditory deafferentation and compensatory neural plastic changes following single-sided deafness in the inferior colliculi of rats

BACKGROUND AND AIMS

Deafferentation and compensatory neural plastic changes in the inferior colliculus (IC) have been suggested following single-sided deafness (SSD). We explored related miRNA changes in the IC of SSD rats using miRNA microarray analyses.

METHODS

Eight-week-old rats were divided into control and SSD rats (n = 8 for each group). SSD rats underwent right-side cochlear ablation surgery, with the IC harvested two weeks post-surgery. miRNA microarray analysis was performed using GeneChip miRNA 4.0, microarray (Affymetrix Inc.). miRNAs whose expression levels differed between SSD and control rats with a fold-change ≥1.5 and P < 0.05 were examined using quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Target genes of differentially expressed miRNAs were predicted using TargetScan software. The pathways related to predicted target genes were analyzed. mRNA levels of predicted target genes were estimated using qRT-PCR.

RESULTS

The expression of miR-15b-5p, miR-202-5p, and miR-212-3p was lower in the contralateral (left) IC of SSD rats than that of control rats. In SSD rats, miRNA expression levels in the contralateral IC were 0.45-, 0.25-, and 0.50-fold lower for miR-15b-5p, miR-202-5p, and miR-212-3p, respectively (P < 0.05). The expression of predicted target genes (Spred1, Rasa1, Lsm11, and Srsf1) was higher in the contralateral IC of SSD rats than in control rats. The targets were predicted to be related with cleavage of growing transcripts in the termination region, mitogen-activated protein kinase family signaling cascades, RAF/AMP kinase cascade, regulation of RAS by GTPase activating proteins (GAPs), and RNA polymerase II transcription termination. For ipsilateral ICs, miR-425-3p, miR-199a-5p, and miR-134-3p showed lower expressions in SSD rats than in control rats, which were 0.55-, 0.61-, and 0.69-fold lower, respectively (P < 0.05). The expression of predicted target genes (Atp2b2, Grin2b, Foxp1, Ztbt20, Zfp91, and Strn) was higher in the ipsilateral IC of SSD rats; the regulation of synaptic plasticity, cAMP signaling pathway, metal ion binding, and calcium ion transport can be associated with these target genes.

CONCLUSION

Adult rats with unilateral auditory deprivation showed miRNA changes in the IC. The contralateral IC showed decreased miRNA expression predicted to be related to MAPK and RAS signaling, whereas the ipsilateral IC revealed decreased miRNA expression predicted to be associated with synaptic plasticity and calcium ion transport.

Establishing an Animal Model of Single-Sided Deafness in Chinchilla lanigera

OBJECTIVES

(1) To characterize changes in brainstem neural activity following unilateral deafening in an animal model. (2) To compare brainstem neural activity from unilaterally deafened animals with that of normal-hearing controls.

STUDY DESIGN

Prospective controlled animal study.

SETTING

Vivarium and animal research facilities.

SUBJECTS AND METHODS

The effect of single-sided deafness on brainstem activity was studied in Chinchilla lanigera. Animals were unilaterally deafened via gentamycin injection into the middle ear, which was verified by loss of auditory brainstem responses (ABRs). Animals underwent measurement of ABR and local field potential in the inferior colliculus.

RESULTS

Four animals underwent chemical deafening, with 2 normal-hearing animals as controls. ABRs confirmed unilateral loss of auditory function. Deafened animals demonstrated symmetric local field potential responses that were distinctly different than the contralaterally dominated responses of the inferior colliculus seen in normal-hearing animals.

CONCLUSION

We successfully developed a model for unilateral deafness to investigate effects of single-sided deafness on brainstem plasticity. This preliminary investigation serves as a foundation for more comprehensive studies that will include cochlear implantation and manipulation of binaural cues, as well as functional behavioral tests.

Mitotic activity, modulation of DNA processing, and purinergic signalling in the adult rat auditory brainstem following sensory deafferentation

A complex scenario of cellular network reorganization is caused by unilateral sensory deafferentation (USD) in the adult rat central auditory system. We asked whether this plasticity response involves mitosis. Immunohistochemistry was applied to brainstem sections for the detection and localization of mitotic markers Ki67 and PCNA, the growth-associated protein Gap43 and purine receptor P2X4. Fluorescent double staining was done for Ki67:PCNA and for both of them with HuC/HuD (neurons), S100 (astrocytes), Iba1 (microglia) and P2X4. Inquiring 1-7 days after USD, we found Ki67 expression to be changed in cellular profiles of cochlear nucleus (CN) with a significant increase in number by 1-3 days, followed by reset to control level within 1 week. USD-induced mitosis exclusively occurred in microglia and was absent elsewhere in the auditory brainstem. PCNA staining of small cellular profiles increased similarly but remained elevated. PCNA staining intensity also changed in CN, superior olive and inferior colliculus in neuronal nuclei, suggesting shifts in DNA processing. No apoptotic cell death was detected in any region of the adult auditory brainstem after USD. A comparison of anterograde and retrograde effects of nerve damage revealed proliferating microglia expressing P2X4 receptors in CN upon USD, but not in the facial nucleus after facial nerve transection. In conclusion, the deafferentation model studied here permits insight into the capacity of the adult mammalian brain to invoke mitosis among glia cells, adjustment of gene processing in neurons and purinergic signalling between them, jointly accounting for a multilayered neuro- and glioplastic response.

Synergistic Transcriptional Changes in AMPA and GABAA Receptor Genes Support Compensatory Plasticity Following Unilateral Hearing Loss

Debilitating perceptual disorders including tinnitus, hyperacusis, phantom limb pain and visual release hallucinations may reflect aberrant patterns of neural activity in central sensory pathways following a loss of peripheral sensory input. Here, we explore short- and long-term changes in gene expression that may contribute to hyperexcitability following a sudden, profound loss of auditory input from one ear. We used fluorescence in situ hybridization to quantify mRNA levels for genes encoding AMPA and GABA_A receptor subunits (Gria2 and Gabra1, respectively) in single neurons from the inferior colliculus (IC) and auditory cortex (ACtx). Thirty days after unilateral hearing loss, Gria2 levels were significantly increased while Gabra1 levels were significantly decreased. Transcriptional rebalancing was more pronounced in ACtx than IC and bore no obvious relationship to the degree of hearing loss. By contrast to the opposing, synergistic shifts in Gria2 and Gabra1 observed 30 days after hearing loss, we found that transcription levels for both genes were equivalently reduced after 5 days of hearing loss, producing no net change in the excitatory/inhibitory transcriptional balance. Opposing transcriptional shifts in AMPA and GABA receptor genes that emerge several weeks after a peripheral insult could promote both sensitization and disinhibition to support a homeostatic recovery of neural activity following auditory deprivation. Imprecise transcriptional changes could also drive the system toward perceptual hypersensitivity, degraded temporal processing and the irrepressible perception of non-existent environmental stimuli, a trio of perceptual impairments that often accompany chronic sensory deprivation.

Astrocyte Hypertrophy and Microglia Activation in the Rat Auditory Midbrain Is Induced by Electrical Intracochlear Stimulation

Neuron–glia interactions contribute to tissue homeostasis and functional plasticity in the mammalian brain, but it remains unclear how this is achieved. The potential of central auditory brain tissue for stimulation-dependent cellular remodeling was studied in hearing-experienced and neonatally deafened rats. At adulthood, both groups received an intracochlear electrode into the left cochlea and were continuously stimulated for 1 or 7 days after waking up from anesthesia. Normal hearing and deafness were assessed by auditory brainstem responses (ABRs). The effectiveness of stimulation was verified by electrically evoked ABRs as well as immunocytochemistry and in situ hybridization for the immediate early gene product Fos on sections through the auditory midbrain containing the inferior colliculus (IC). Whereas hearing-experienced animals showed a tonotopically restricted Fos response in the IC contralateral to electrical intracochlear stimulation, Fos-positive neurons were found almost throughout the contralateral IC in deaf animals. In deaf rats, the Fos response was accompanied by a massive increase of GFAP indicating astrocytic hypertrophy, and a local activation of microglial cells identified by IBA1. These glia responses led to a noticeable increase of neuron–glia approximations. Moreover, staining for the GABA synthetizing enzymes GAD65 and GAD67 rose significantly in neuronal cell bodies and presynaptic boutons in the contralateral IC of deaf rats. Activation of neurons and glial cells and tissue re-composition were in no case accompanied by cell death as would have been apparent by a Tunel reaction. These findings suggest that growth and activity of glial cells is crucial for the local adjustment of neuronal inhibition to neuronal excitation.

Auditory thalamic circuits and GABAA receptor function: Putative mechanisms in tinnitus pathology

Tinnitus is defined as a phantom sound (ringing in the ears), and can significantly reduce the quality of life for those who suffer its effects. Ten to fifteen percent of the general adult population report symptoms of tinnitus with 1-2% reporting that tinnitus negatively impacts their quality of life. Noise exposure is the most common cause of tinnitus and the military environment presents many challenging high-noise situations. Military noise levels can be so intense that standard hearing protection is not adequate. Recent studies suggest a role for inhibitory neurotransmitter dysfunction in response to noise-induced peripheral deafferentation as a key element in the pathology of tinnitus. The auditory thalamus, or medial geniculate body (MGB), is an obligate auditory brain center in a unique position to gate the percept of sound as it projects to auditory cortex and to limbic structures. Both areas are thought to be involved in those individuals most impacted by tinnitus. For MGB, opposing hypotheses have posited either a tinnitus-related pathologic decrease or pathologic increase in GABAergic inhibition. In sensory thalamus, GABA mediates fast synaptic inhibition via synaptic GABAA receptors (GABAARs) as well as a persistent tonic inhibition via high-affinity extrasynaptic GABAARs and slow synaptic inhibition via GABABRs. Down-regulation of inhibitory neurotransmission, related to partial peripheral deafferentation, is consistently presented as partially underpinning neuronal hyperactivity seen in animal models of tinnitus. This maladaptive plasticity/Gain Control Theory of tinnitus pathology (see Auerbach et al., 2014; Richardson et al., 2012) is characterized by reduced inhibition associated with increased spontaneous and abnormal neuronal activity, including bursting and increased synchrony throughout much of the central auditory pathway. A competing hypothesis suggests that maladaptive oscillations between the MGB and auditory cortex, thalamocortical dysrhythmia, predict tinnitus pathology (De Ridder et al., 2015). These unusual oscillations/rhythms reflect net increased tonic inhibition in a subset of thalamocortical projection neurons resulting in abnormal bursting. Hyperpolarizing de-inactivation of T-type Ca2+ channels switches thalamocortical projection neurons into burst mode. Thalamocortical dysrhythmia originating in sensory thalamus has been postulated to underpin neuropathies including tinnitus and chronic pain. Here we review the relationship between noise-induced tinnitus and altered inhibition in the MGB.

Reactive Neurogenesis and Down-Regulation of the Potassium-Chloride Cotransporter KCC2 in the Cochlear Nuclei after Cochlear Deafferentation

While many studies have been devoted to investigating the homeostatic plasticity triggered by cochlear hearing loss, the cellular and molecular mechanisms involved in these central changes remain elusive. In the present study, we investigated the possibility of reactive neurogenesis after unilateral cochlear nerve section in the cochlear nucleus (CN) of cats. We found a strong cell proliferation in all the CN sub-divisions ipsilateral to the lesion. Most of the newly generated cells survive up to 1 month after cochlear deafferentation in all cochlear nuclei (except the dorsal CN) and give rise to a variety of cell types, i.e., microglial cells, astrocytes, and neurons. Interestingly, many of the newborn neurons had an inhibitory (GABAergic) phenotype. This result is intriguing since sensory deafferentation is usually accompanied by enhanced excitation, consistent with a reduction in central inhibition. The membrane potential effect of GABA depends, however, on the intra-cellular chloride concentration, which is maintained at low levels in adults by the potassium chloride co-transporter KCC2. The KCC2 density on the plasma membrane of neurons was then assessed after cochlear deafferentation in the cochlear nuclei ipsilateral and contralateral to the lesion. Cochlear deafferentation is accompanied by a strong down-regulation of KCC2 ipsilateral to the lesion at 3 and 30 days post-lesion. This study suggests that reactive neurogenesis and down-regulation of KCC2 is part of the vast repertoire involved in homeostatic plasticity triggered by hearing loss. These central changes may also play a role in the generation of tinnitus and hyperacusis.

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

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

Tinnitus-related changes in the inferior colliculus

Tinnitus is highly complex, diverse, and difficult to treat, in part due to the fact that the underlying causes and mechanisms remain elusive. Tinnitus is generated within the auditory brain; however, consolidating our understanding of tinnitus pathophysiology is difficult due to the diversity of reported effects and the variety of implicated brain nuclei. Here, we focus on the inferior colliculus (IC), a midbrain structure that integrates the vast majority of ascending auditory information and projects via the thalamus to the auditory cortex. The IC is also a point of convergence for corticofugal input and input originating outside the auditory pathway. We review the evidence, from both studies with human subjects and from animal models, for the contribution the IC makes to tinnitus. Changes in the IC, caused by either noise exposure or drug administration, involve fundamental, heterogeneous alterations in the balance of excitation and inhibition. However, differences between hearing loss-induced pathology and tinnitus-related pathology are not well understood. Moreover, variability in tinnitus induction methodology has a significant impact on subsequent neural and behavioral changes, which could explain some of the seemingly contradictory data. Nonetheless, the IC is likely involved in the generation and persistence of tinnitus perception.

A review of hyperacusis and future directions: part II. Measurement, mechanisms, and treatment

PURPOSE

Hyperacusis can be extremely debilitating, and at present, there is no cure. In this detailed review of the field, we consolidate present knowledge in the hope of facilitating future research.

METHOD

We review and reference the literature on hyperacusis and related areas. This is the 2nd of a 2-part review.

RESULTS

Hyperacusis encompasses a wide range of reactions to sounds, which can be grouped into the categories of excessive loudness, annoyance, fear, and pain. Reasonable approaches to assessing the different forms of hyperacusis are emerging, including brain-imaging studies. Researchers are only beginning to understand the many mechanisms at play, and valid animal models are still evolving. There are many counseling and sound-therapy approaches that some patients find helpful, but well-controlled studies are needed to measure their long-term efficacy and to test new approaches.

CONCLUSIONS

Hyperacusis can make life difficult in this increasingly noisy world, forcing sufferers to dramatically alter their work and social habits. We believe this is an opportune time to explore approaches to better understand and treat hyperacusis.

Cochlear damage affects neurotransmitter chemistry in the central auditory system

Tinnitus, the perception of a monotonous sound not actually present in the environment, affects nearly 20% of the population of the United States. Although there has been great progress in tinnitus research over the past 25 years, the neurochemical basis of tinnitus is still poorly understood. We review current research about the effects of various types of cochlear damage on the neurotransmitter chemistry in the central auditory system and document evidence that different changes in this chemistry can underlie similar behaviorally measured tinnitus symptoms. Most available data have been obtained from rodents following cochlear damage produced by cochlear ablation, intense sound, or ototoxic drugs. Effects on neurotransmitter systems have been measured as changes in neurotransmitter level, synthesis, release, uptake, and receptors. In this review, magnitudes of changes are presented for neurotransmitter-related amino acids, acetylcholine, and serotonin. A variety of effects have been found in these studies that may be related to animal model, survival time, type and/or magnitude of cochlear damage, or methodology. The overall impression from the evidence presented is that any imbalance of neurotransmitter-related chemistry could disrupt auditory processing in such a way as to produce tinnitus.

Acoustic trauma triggers upregulation of serotonin receptor genes

Hearing loss induces plasticity in excitatory and inhibitory neurotransmitter systems in auditory brain regions. Excitatory-inhibitory balance is also influenced by a range of neuromodulatory regulatory systems, but less is known about the effects of auditory damage on these networks. In this work, we studied the effects of acoustic trauma on neuromodulatory plasticity in the auditory midbrain of CBA/J mice. Quantitative PCR was used to measure the expression of serotonergic and GABAergic receptor genes in the inferior colliculus (IC) of mice that were unmanipulated, sham controls with no hearing loss, and experimental individuals with hearing loss induced by exposure to a 116 dB, 10 kHz pure tone for 3 h. Acoustic trauma induced substantial hearing loss that was accompanied by selective upregulation of two serotonin receptor genes in the IC. The Htr1B receptor gene was upregulated tenfold following trauma relative to shams, while the Htr1A gene was upregulated threefold. In contrast, no plasticity in serotonin receptor gene expression was found in the hippocampus, a region also innervated by serotonergic projections. Analyses in the IC demonstrated that acoustic trauma also changed the coexpression of genes in relation to each other, leading to an overexpression of Htr1B compared to other genes. These data suggest that acoustic trauma induces serotonergic plasticity in the auditory system, and that this plasticity may involve comodulation of functionally-linked receptor genes.

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.

Maturation profile of inferior olivary neurons expressing ionotropic glutamate receptors in rats: role in coding linear accelerations

Using sinusoidal oscillations of linear acceleration along both the horizontal and vertical planes to stimulate otolith organs in the inner ear, we charted the postnatal time at which responsive neurons in the rat inferior olive (IO) first showed Fos expression, an indicator of neuronal recruitment into the otolith circuit. Neurons in subnucleus dorsomedial cell column (DMCC) were activated by vertical stimulation as early as P9 and by horizontal (interaural) stimulation as early as P11. By P13, neurons in the β subnucleus of IO (IOβ) became responsive to horizontal stimulation along the interaural and antero-posterior directions. By P21, neurons in the rostral IOβ became also responsive to vertical stimulation, but those in the caudal IOβ remained responsive only to horizontal stimulation. Nearly all functionally activated neurons in DMCC and IOβ were immunopositive for the NR1 subunit of the NMDA receptor and the GluR2/3 subunit of the AMPA receptor. In situ hybridization studies further indicated abundant mRNA signals of the glutamate receptor subunits by the end of the second postnatal week. This is reinforced by whole-cell patch-clamp data in which glutamate receptor-mediated miniature excitatory postsynaptic currents of rostral IOβ neurons showed postnatal increase in amplitude, reaching the adult level by P14. Further, these neurons exhibited subthreshold oscillations in membrane potential as from P14. Taken together, our results support that ionotropic glutamate receptors in the IO enable postnatal coding of gravity-related information and that the rostral IOβ is the only IO subnucleus that encodes spatial orientations in 3-D.

Two-channel recording of auditory-evoked potentials to detect age-related deficits in temporal processing

Auditory brainstem responses (ABRs), and envelope and frequency following responses (EFRs and FFRs) are widely used to study aberrant auditory processing in conditions such as aging. We have previously reported age-related deficits in auditory processing for rapid amplitude modulation (AM) frequencies using EFRs recorded from a single channel. However, sensitive testing of EFRs along a wide range of modulation frequencies is required to gain a more complete understanding of the auditory processing deficits. In this study, ABRs and EFRs were recorded simultaneously from two electrode configurations in young and old Fischer-344 rats, a common auditory aging model. Analysis shows that the two channels respond most sensitively to complementary AM frequencies. Channel 1, recorded from Fz to mastoid, responds better to faster AM frequencies in the 100-700 Hz range of frequencies, while Channel 2, recorded from the inter-aural line to the mastoid, responds better to slower AM frequencies in the 16-100 Hz range. Simultaneous recording of Channels 1 and 2 using AM stimuli with varying sound levels and modulation depths show that age-related deficits in temporal processing are not present at slower AM frequencies but only at more rapid ones, which would not have been apparent recording from either channel alone. Comparison of EFRs between un-anesthetized and isoflurane-anesthetized recordings in young animals, as well as comparison with previously published ABR waveforms, suggests that the generators of Channel 1 may emphasize more caudal brainstem structures while those of Channel 2 may emphasize more rostral auditory nuclei including the inferior colliculus and the forebrain, with the boundary of separation potentially along the cochlear nucleus/superior olivary complex. Simultaneous two-channel recording of EFRs help to give a more complete understanding of the properties of auditory temporal processing over a wide range of modulation frequencies which is useful in understanding neural representations of sound stimuli in normal, developmental or pathological conditions.

Reconsidering the role of neuronal intrinsic properties and neuromodulation in vestibular homeostasis

The sensorimotor transformations performed by central vestibular neurons constantly adapt as the animal faces conflicting sensory information or sustains injuries. To ensure the homeostasis of vestibular-related functions, neural changes could in part rely on the regulation of 2° VN intrinsic properties. Here we review evidence that demonstrates modulation and plasticity of central vestibular neurons’ intrinsic properties. We first present the partition of Rodents’ vestibular neurons into distinct subtypes, namely type A and type B. Then, we focus on the respective properties of each type, their putative roles in vestibular functions, fast control by neuromodulators and persistent modifications following a lesion. The intrinsic properties of central vestibular neurons can be swiftly modulated by a wealth of neuromodulators to adapt rapidly to temporary changes of ecophysiological surroundings. To illustrate how intrinsic excitability can be rapidly modified in physiological conditions and therefore be therapeutic targets, we present the modulation of vestibular reflexes in relation to the variations of the neuromodulatory inputs during the sleep/wake cycle. On the other hand, intrinsic properties can also be slowly, yet permanently, modified in response to major perturbations, e.g., after unilateral labyrinthectomy (UL). We revisit the experimental evidence, which demonstrates that drastic alterations of the central vestibular neurons’ intrinsic properties occur following UL, with a slow time course, more on par with the compensation of dynamic deficits than static ones. Data are interpreted in the framework of distributed processes that progress from global, large-scale coping mechanisms (e.g., changes in behavioral strategies) to local, small-scale ones (e.g., changes in intrinsic properties). Within this framework, the compensation of dynamic deficits improves over time as deeper modifications are engraved within the finer parts of the vestibular-related networks. Finally, we offer perspectives and working hypotheses to pave the way for future research aimed at understanding the modulation and plasticity of central vestibular neurons’ intrinsic properties.

The expression of mitogen-activated protein kinases and brain-derived neurotrophic factor in inferior colliculi after acoustic trauma

Acoustic trauma is well known to cause peripheral damage with subsequent effects in the central auditory system. The inferior colliculus (IC) is a major auditory center for the integration of ascending and descending information and is involved in noise-induced tinnitus and central hyperactivity. Here we show that the early effects of acoustic trauma, that eventually result in permanent damage to auditory system, lead to a transient activation of BDNF and mitogen-activated protein kinases (MAPK) including extracellular signal-regulated kinase (ERK), c-jun N-terminal kinase (JNK), and p38 in the IC. In contrast, the early effects of acoustic trauma that result in a temporary damage produced a reversible activation only of p38. The transient activation of MAPK and BDNF in the IC after permanent acoustic trauma is attributed to the plastic changes triggered by a decreased signal input from the damaged periphery. The pattern of MAPK and BDNF activation in the IC is different from that previously described for the cochlea from this laboratory. The differences in the pattern of MAPK and BDNF expression in the IC highlight unique molecular mechanisms underlying temporary and permanent acoustic damage to the central auditory system.

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.

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.

Effects of hearing aid fitting on the perceptual characteristics of tinnitus

Restoration of auditory input through the use of hearing aids has been proposed as a potentially important means of altering tinnitus among those tinnitus sufferers who experience significant sensorineural hearing loss. In animal models of neural plasticity induced by noise trauma, high-frequency stimulation in deafferented regions of the auditory spectrum has been shown to modulate cortical reorganization after hearing loss, a result which suggests that the neural basis of tinnitus is subject to interference by acoustic stimulation. This study drew on deafferentation models to investigate the effect of hearing aids on the psychoacoustic properties of the tinnitus sensation, using both conventional amplification and high-bandwidth amplification regimes. The tinnitus percept was affected only weakly in the conventional amplification group, and was not at all affected in the high-bandwidth group. The changes observed under conventional, low-to-medium frequency amplification may indicate that the perceptual characteristics of tinnitus depend on the pattern of sensory inputs - notably a contrast in activity between adjacent central auditory regions of more and less afferent activity - while the absence of modifications in the high-bandwidth amplification group suggests limit on the tractability of the tinnitus percept. This limit to the malleability of the tinnitus percept may arise from either the extent of hearing deficits or the duration and robustness of the neuroplastic changes that originally give rise to tinnitus.

Developmental hearing loss disrupts synaptic inhibition: implications for auditory processing

Hearing loss during development leads to central deficits that persist even after the restoration of peripheral function. One key class of deficits is due to changes in central inhibitory synapses, which play a fundamental role in all aspects of auditory processing. This review focuses on the anatomical and physiological alterations of inhibitory connections at several regions within the central auditory pathway following hearing loss. Such aberrant inhibitory synaptic function may be linked to deficits in encoding binaural and spectral cues. Understanding the cellular changes that occur at inhibitory synapses following hearing loss may provide specific loci that can be targeted to improve function.

Stimulation by cochlear implant in unilaterally deaf rats reverses the decrease of inhibitory transmission in the inferior colliculus

In the last decade, numerous studies have investigated synaptic transmission changes in various auditory nuclei after unilateral cochlear injury. However, few data are available concerning the potential effect of electrical stimulation of the deafferented auditory nerve on the inhibitory neurotransmission in these nuclei. We report here for the first time the effect of chronic electrical stimulation of the deafferented auditory nerve on alpha1 subunit of the glycinergic receptor (GlyRalpha1) and glutamic acid decarboxylase (GAD)67 expression in the central nucleus of inferior colliculus (CIC). Adult rats were unilaterally cochleectomized by intracochlear neomycin sulphate injection. Fifteen days later, the ipsilateral auditory nerve was chronically stimulated either 4, 8 or 22 h daily, for 5 days using intracochlear bipolar electrodes. GlyRalpha1 and GAD67 mRNA and protein were quantified in the CIC using in situ hybridization and immunohistofluorescence methods. Our data showed that as after surgical ablation, GlyRalpha1 and GAD67 expression were strongly decreased in the contralateral CIC after unilateral chemical cochleectomy. Most importantly, these postlesional down-modulations were significantly reversed by chronic electrical stimulation of the deafferented auditory nerve. This recovery, however, did not persist for more than 5 days after the cessation of the deafferented auditory nerve electrical stimulation. Thus, downregulations of GlyRalpha1 and GAD67 may be involved both in the increased excitability observed in the CIC after unilateral deafness and consequently in the tinnitus frequently observed in unilateral adult deaf patients. Electrical stimulation of the deafferented auditory nerve in patients may be a potential new approach for treating tinnitus with unilateral hearing loss.

Dynamic changes in level influence spatial coding in the lateral superior olive

It is well established that the responses of binaural auditory neurons can adapt and change dramatically depending on the nature of a preceding sound. Examples of how the effects of ensuing stimuli play a functional role in auditory processing include motion sensitivity and precedence-like effects. To date, these types of effects have been documented at the level of the midbrain and above. Little is known about sensitivity to ensuing stimuli below in the superior olivary nuclei where binaural response properties are first established. Here we report on single cell responses in the gerbil lateral superior olive, the initial site where sensitivity to interaural level differences is established. In contrast to our expectations we found a robust sensitivity to ensuing stimuli. The majority of the cells we tested (86%), showed substantial suppression and/or enhancement to a designated target stimulus, depending on the nature of a preceding stimulus. Hence, sensitivity to ensuing stimuli is already established at the first synaptic station of binaural processing.

Central neural activity in rats with tinnitus evaluated with manganese-enhanced magnetic resonance imaging (MEMRI)

The pathophysiology of tinnitus, the perception of sound in the absence of acoustic stimulation, is largely unknown, although several lines of research implicate long-term neuroplastic loss of inhibition. The evidence to date suggests that the neuroplastic alterations are likely to be found in multiple brain structures. The present study used manganese-enhanced magnetic resonance imaging (MEMRI) to assess the pattern of neural activity in the central auditory pathway of rats with psychophysical evidence of chronic acoustic-exposure-induced tinnitus. Manganese, an activity-dependent paramagnetic contrast agent, accumulates in active neurons through voltage-gated calcium channels, primarily at synapses, and serves as both a structural and functional indicator. Comparison images were obtained from normal subjects exposed to external tinnitus-like sound, and from tinnitus subjects treated with vigabatrin, a GABA agonist shown to eliminate the psychophysical evidence of tinnitus in rats. MEMRI indicated: (1) In rats with evidence of tinnitus, activity was generally elevated in the auditory brainstem, with significant elevation in the cerebellar paraflocculus, the posterior ventral cochlear nucleus, and the inferior colliculus; in general forebrain structures showed decreased activity, although MEMRI may be a less sensitive indicator of forebrain activity than brainstem activity; (2) in normal rats exposed to a tinnitus-like sound, a similar pattern of elevated brainstem activity and decreased forebrain activity was evident, with the notable exception of the paraflocculus, where artificial tinnitus had no effect and (3) vigabatrin, decreased brainstem activity to control levels, in rats with prior evidence of tinnitus, and decreased forebrain activity to below control levels. It was concluded that chronic tinnitus in rats is associated with focal activity elevation in the auditory brainstem and increased activity in the paraflocculus that may be unique to tinnitus.