Central auditory plasticity after carboplatin-induced unilateral inner ear damage in the chinchilla: up-regulation of GAP-43 in the ventral cochlear nucleus

Ventral cochlear nucleus bushy cells encode hyperacusis in guinea pigs

Psychophysical studies characterize hyperacusis as increased loudness growth over a wide-frequency range, decreased tolerance to loud sounds and reduced behavioral reaction time latencies to high-intensity sounds. While commonly associated with hearing loss, hyperacusis can also occur without hearing loss, implicating the central nervous system in the generation of hyperacusis. Previous studies suggest that ventral cochlear nucleus bushy cells may be putative neural contributors to hyperacusis. Compared to other ventral cochlear nucleus output neurons, bushy cells show high firing rates as well as lower and less variable first-spike latencies at suprathreshold intensities. Following cochlear damage, bushy cells show increased spontaneous firing rates across a wide-frequency range, suggesting that they might also show increased sound-evoked responses and reduced latencies to higher-intensity sounds. However, no studies have examined bushy cells in relationship to hyperacusis. Herein, we test the hypothesis that bushy cells may contribute to the neural basis of hyperacusis by employing noise-overexposure and single-unit electrophysiology. We find that bushy cells exhibit hyperacusis-like neural firing patterns, which are comprised of enhanced sound-driven firing rates, reduced first-spike latencies and wideband increases in excitability.

Dynamic Changes in Synaptic Plasticity Genes in Ipsilateral and Contralateral Inferior Colliculus Following Unilateral Noise-induced Hearing Loss

Unilateral noise-induced hearing loss reduces the input to the central auditory pathway disrupting the excitatory and inhibitory inputs to the inferior colliculus (IC), an important binaural processing center. Little is known about the compensatory synaptic changes that occur in the IC as a consequence of unilateral noise-induced hearing loss. To address this issue, Sprague-Dawley rats underwent unilateral noise exposure resulting in severe unilateral hearing loss. IC tissues from the contralateral and ipsilateral IC were evaluated for acute (2-d) and chronic (28-d) changes in the expression of 84 synaptic plasticity genes on a PCR array. Arc and Egr1 genes were further visualized by in situ hybridization to validate the PCR results. None of the genes were upregulated, but many were downregulated post-exposure. At 2-d post-exposure, more than 75% of the genes were significantly downregulated in the contralateral IC, while only two were downregulated in the ipsilateral IC. Many of the downregulated genes were related to long-term depression, long-term potentiation, cell adhesion, immediate early genes, neural receptors and postsynaptic density. At 28-d post-exposure, the gene expression pattern was reversed with more than 85% of genes in the ipsilateral IC now downregulated. Most genes previously downregulated in the contralateral IC 2-d post-exposure had recovered; less than 15% remained downregulated. These time-dependent, asymmetric changes in synaptic plasticity gene expression could shed new light on the perceptual deficits associated with unilateral hearing loss and the dynamic structural and functional changes that occur in the IC days and months following unilateral noise-induced hearing loss.

Apoptosis in the cochlear nucleus and inferior colliculus upon repeated noise exposure

The time course of apoptosis and the corresponding neuronal loss was previously shown in central auditory pathway of mice after a single noise exposure. However, repeated acoustic exposure is a major risk factor for noise-induced hearing loss. The present study investigated apoptosis by terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick end labeling (TUNEL) assay after a second noise trauma in the ventral and dorsal cochlear nucleus and central nucleus of the inferior colliculus. Mice [Naval Medical Research Institute (NMRI) strain] were noise exposed [115 dB sound pressure level, 5-20 kHz, 3 h) at day 0. A double group received the identical noise exposure a second time at day 7 post-exposure and apoptosis was either analyzed immediately (7-day group-double) or 1 week later (14-day group-double). Corresponding single exposure groups were chosen as controls. No differences in TUNEL were seen between 7-day or 14-day single and double-trauma groups. Interestingly, independent of the second noise exposure, apoptosis increased significantly in the 14-day groups compared to the 7-day groups in all investigated areas. It seems that the first noise trauma has a long-lasting effect on apoptotic mechanisms in the central auditory pathway that were not largely influenced by a second trauma. Homeostatic mechanisms induced by the first trauma might protect the central auditory pathway from further damage during a specific time slot. These results might help to understand the underlying mechanisms of different psychoacoustic phenomena in noise-induced hearing loss.

[Central and peripheral aspects of noise-induced hearing loss]

Noise is an important socioeconomic problem in industrialized countries. Development of efficient treatment options for the audiological phenomena resulting from noise-induced hearing loss requires in-depth understanding of the underlying damage mechanisms causing peripheral and central nervous changes. Mechanical damage, ischemia and excitotoxicity are mainly responsible for noise-induced cell death and biophysical changes in the cochlea. Auditory synaptopathy is an additional consequence. Besides these cochlear pathologies, noise exposure leads to extensive changes within the central auditory pathway. Overstimulation causes early cell loss in the ventral cochlear nucleus just after noise exposure, which is in accordance with enhancement of apoptotic mechanisms within the corresponding timeframe. In contrast to the cell loss in lower auditory structures due to overstimulation, the later significant reduction of cell density in higher auditory structures is due to sensory deprivation. Changes in network homeostasis seem to partially compensate structural losses by modulation of spontaneous activity. However, central nervous processing of auditory information is permanently impaired by the neuroplastic changes. Unfortunately, the various noise-induced peripheral and central pathologies are difficult to treat. New therapeutic approaches are required, particularly for treatment of central nervous processing disorders and auditory synaptopathy, which contribute to audiological phenomena such as tinnitus, hyperacusis and poor speech perception in noise.

Synaptic Reorganization Response in the Cochlear Nucleus Following Intense Noise Exposure

The cochlear nucleus, located in the brainstem, receives its afferent auditory input exclusively from the auditory nerve fibers of the ipsilateral cochlea. Noise-induced neurodegenerative changes occurring in the auditory nerve stimulate a cascade of neuroplastic changes in the cochlear nucleus resulting in major changes in synaptic structure and function. To identify some of the key molecular mechanisms mediating this synaptic reorganization, we unilaterally exposed rats to a high-intensity noise that caused significant hearing loss and then measured the resulting changes in a synaptic plasticity gene array targeting neurogenesis and synaptic reorganization. We compared the gene expression patterns in the dorsal cochlear nucleus (DCN) and ventral cochlear nucleus (VCN) on the noise-exposed side versus the unexposed side using a PCR gene array at 2 d (early) and 28 d (late) post-exposure. We discovered a number of differentially expressed genes, particularly those related to synaptogenesis and regeneration. Significant gene expression changes occurred more frequently in the VCN than the DCN and more changes were seen at 28  d versus 2 d post-exposure. We confirmed the PCR findings by in situ hybridization for Brain-derived neurotrophic factor (Bdnf), Homer-1, as well as the glutamate NMDA receptor Grin1, all involved in neurogenesis and plasticity. These results suggest that Bdnf, Homer-1 and Grin1 play important roles in synaptic remodeling and homeostasis in the cochlear nucleus following severe noise-induced afferent degeneration.

Acute Noise Exposure Is Associated With Intrinsic Apoptosis in Murine Central Auditory Pathway

Noise that is capable of inducing the hearing loss (NIHL) has a strong impact on the inner ear structures and causes early and most obvious pathophysiological changes in the auditory periphery. Several studies indicated that intrinsic apoptotic cell death mechanisms are the key factors inducing cellular degeneration immediately after noise exposure and are maintained for days or even weeks. In addition, studies demonstrated several changes in the central auditory system following noise exposure, consistent with early apoptosis-related pathologies. To clarify the underlying mechanisms, the present study focused on the noise-induced gene and protein expression of the pro-apoptotic protease activating factor-1 (APAF1) and the anti-apoptotic B-cell lymphoma 2 related protein a1a (BCL2A1A) in the cochlear nucleus (CN), inferior colliculus (IC) and auditory cortex (AC) of the murine central auditory pathway. The expression of Bcl2a1a mRNA was upregulated immediately after trauma in all tissues investigated, whereas the protein levels were significantly reduced at least in the auditory brainstem. Conversely, acute noise has decreased the expression of Apaf1 gene along the auditory pathway. The changes in APAF1 protein level were not statistically significant. It is tempting to speculate that the acoustic overstimulation leads to mitochondrial dysfunction and induction of apoptosis by regulation of proapoptotic and antiapoptotic proteins. The inverse expression pattern on the mRNA level of both genes might reflect a protective response to decrease cellular damage. Our results indicate the immediate presence of intrinsic apoptosis following noise trauma. This, in turn, may significantly contribute to the development of central structural deficits. Auditory pathway-specific inhibition of intrinsic apoptosis could be a therapeutic approach for the treatment of acute (noise-induced) hearing loss to prevent irreversible neuronal injury in auditory brain structures and to avoid profound deficits in complex auditory processing.

Species Differences in the Organization of the Ventral Cochlear Nucleus

The mammalian cochlear nuclei (CN) consist of two major subdivisions, the dorsal (DCN) and ventral (VCN) nuclei. We previously reported differences in the structural and neurochemical organization of the human DCN from that in several other species. Here we extend this analysis to the VCN, considering both the organization of subdivisions and the types and distributions of neurons. Classically, the VCN in mammals is composed of two subdivisions, the anteroventral (VCA) and posteroventral cochlear nuclei (VCP). Anatomical and electrophysiological data in several species have defined distinct neuronal types with different distributions in the VCA and VCP. We asked if VCN subdivisions and anatomically defined neuronal types might be distinguished by patterns of protein expression in humans. We also asked if the neurochemical characteristics of the VCN are the same in humans as in other mammalian species, analyzing data from chimpanzees, macaque monkeys, cats, rats and chinchillas. We examined Nissl- and immunostained sections, using antibodies that had labeled neurons in other brainstem nuclei in humans. Nissl-stained sections supported the presence of both VCP and VCA in humans and chimpanzees. However, patterns of protein expression did not differentiate classes of neurons in humans; neurons of different soma shapes and dendritic configurations all expressed the same proteins. The patterns of immunostaining in macaque monkey, cat, rat, and chinchilla were different from those in humans and chimpanzees and from each other. The results may correlate with species differences in auditory function and plasticity. Anat Rec, 301:862-886, 2018. © 2017 Wiley Periodicals, Inc.

Time course of cell death due to acoustic overstimulation in the mouse medial geniculate body and primary auditory cortex

It has previously been shown that acoustic overstimulation induces cell death and extensive cell loss in key structures of the central auditory pathway. A correlation between noise-induced apoptosis and cell loss was hypothesized for the cochlear nucleus and colliculus inferior. To determine the role of cell death in noise-induced cell loss in thalamic and cortical structures, the present mouse study (NMRI strain) describes the time course following noise exposure of cell death mechanisms for the ventral medial geniculate body (vMGB), medial MGB (mMGB), and dorsal MGB (dMGB) and the six histological layers of the primary auditory cortex (AI 1-6). Therefore, a terminal deoxynucleotidyl transferase dioxyuridine triphosphate nick-end labeling assay (TUNEL) was performed in these structures 24 h, 7 days, and 14 days after noise exposure (3 h, 115 dB sound pressure level, 5-20 kHz), as well as in unexposed controls. In the dMGB, TUNEL was statistically significant elevated 24 h postexposure. AI-1 showed a decrease in TUNEL after 14 days. There was no statistically significant difference between groups for the other brain areas investigated. dMGB's widespread connection within the central auditory pathway and its nontonotopical organization might explain its prominent increase in TUNEL compared to the other MGB subdivisions and the AI. It is assumed that the onset and peak of noise-induced cell death is delayed in higher areas of the central auditory pathway and takes place between 24 h and 7 days postexposure in thalamic and cortical structures.

Salicylate-Induced Hearing Loss Trigger Structural Synaptic Modifications in the Ventral Cochlear Nucleus of Rats via Medial Olivocochlear (MOC) Feedback Circuit

Lesion-induced cochlear damage can result in synaptic outgrowth in the ventral cochlear nucleus (VCN). Tinnitus may be associated with the synaptic outgrowth and hyperactivity in the VCN. However, it remains unclear how hearing loss triggers structural synaptic modifications in the VCN of rats subjected to salicylate-induced tinnitus. To address this issue, we evaluated tinnitus-like behavior in rats after salicylate treatment and compared the amplitude of the distortion product evoked otoacoustic emission (DPOAE) and auditory brainstem response (ABR) between control and treated rats. Moreover, we observed the changes in the synaptic ultrastructure and in the expression levels of growth-associated protein (GAP-43), brain-derived neurotrophic factor (BDNF), the microglial marker Iba-1 and glial fibrillary acidic protein (GFAP) in the VCN. After salicylate treatment (300 mg/kg/day for 4 and 8 days), analysis of the gap prepulse inhibition of the acoustic startle showed that the rats were experiencing tinnitus. The changes in the DPOAE and ABR amplitude indicated an improvement in cochlear sensitivity and a reduction in auditory input following salicylate treatment. The treated rats displayed more synaptic vesicles and longer postsynaptic density in the VCN than the control rats. We observed that the GAP-43 expression, predominantly from medial olivocochlear (MOC) neurons, was significantly up-regulated, and that BDNF- and Iba-1-immunoreactive cells were persistently decreased after salicylate administration. Furthermore, GFAP-immunoreactive astrocytes, which is associated with synaptic regrowth, was significantly increased in the treated groups. Our study revealed that reduced auditory nerve activity triggers synaptic outgrowth and hyperactivity in the VCN via a MOC neural feedback circuit. Structural synaptic modifications may be a reflexive process that compensates for the reduced auditory input after salicylate administration. However, massive increases in excitatory synapses in the VCN may represent a detrimental process that causes central hyperactivity, leading to tinnitus.

Central plasticity and dysfunction elicited by aural deprivation in the critical period

The acoustic signal is crucial for animals to obtain information from the surrounding environment. Like other sensory modalities, the central auditory system undergoes adaptive changes (i.e., plasticity) during the developmental stage as well as other stages of life. Owing to its plasticity, auditory centers may be susceptible to various factors, such as medical intervention, variation in ambient acoustic signals and lesion of the peripheral hearing organ. There are critical periods during which auditory centers are vulnerable to abnormal experiences. Particularly in the early postnatal development period, aural inputs are essential for functional maturity of auditory centers. An aural deprivation model, which can be achieved by attenuating or blocking the peripheral acoustic afferent input to the auditory center, is ideal for investigating plastic changes of auditory centers. Generally, auditory plasticity includes structural and functional changes, some of which can be irreversible. Aural deprivation can distort tonotopic maps, disrupt the binaural integration, reorganize the neural network and change the synaptic transmission in the primary auditory cortex or at lower levels of the auditory system. The regulation of specific gene expression and the modified signal pathway may be the deep molecular mechanism of these plastic changes. By studying this model, researchers may explore the pathogenesis of hearing loss and reveal plastic changes of the auditory cortex, facilitating the therapeutic advancement in patients with severe hearing loss. After summarizing developmental features of auditory centers in auditory deprived animals and discussing changes of central auditory remodeling in hearing loss patients, we aim at stressing the significant of an early and well-designed auditory training program for the hearing rehabilitation.

Cochlear injury and adaptive plasticity of the auditory cortex

Growing evidence suggests that cochlear stressors as noise exposure and aging can induce homeostatic/maladaptive changes in the central auditory system from the brainstem to the cortex. Studies centered on such changes have revealed several mechanisms that operate in the context of sensory disruption after insult (noise trauma, drug-, or age-related injury). The oxidative stress is central to current theories of induced sensory-neural hearing loss and aging, and interventions to attenuate the hearing loss are based on antioxidant agent. The present review addresses the recent literature on the alterations in hair cells and spiral ganglion neurons due to noise-induced oxidative stress in the cochlea, as well on the impact of cochlear damage on the auditory cortex neurons. The emerging image emphasizes that noise-induced deafferentation and upward spread of cochlear damage is associated with the altered dendritic architecture of auditory pyramidal neurons. The cortical modifications may be reversed by treatment with antioxidants counteracting the cochlear redox imbalance. These findings open new therapeutic approaches to treat the functional consequences of the cortical reorganization following cochlear damage.

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.

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.

Up-regulation of GAP-43 in the chinchilla ventral cochlear nucleus after carboplatin-induced hearing loss: correlations with inner hair cell loss and outer hair cell loss

Inner ear damage leads to nerve fiber growth and synaptogenesis in the ventral cochlear nucleus (VCN). In this study, we documented the relationship between hair cell loss patterns and synaptic plasticity in the chinchilla VCN using immunolabeling of the growth associated protein-43 (GAP-43), a protein associated with axon outgrowth and modification of presynaptic endings. Unilateral round window application of carboplatin caused hair cell degeneration in which inner hair cells (IHC) were more vulnerable than outer hair cells (OHC). One month after carboplatin treatment (0.5-5 mg/ml), we observed varying patterns of cochlear hair cell loss and GAP-43 expression in VCN. Both IHC loss and OHC loss were strongly correlated with increased GAP-43 immunolabeling in the ipsilateral VCN. We speculate that two factors might promote the expression of GAP-43 in the VCN; one is the loss of afferent input through IHC or the associated type I auditory nerve fibers. The other occurs when the medial olivocochlear efferent neurons lose their cochlear targets, the OHC, and may as compensation increase their synapse numbers in the VCN.

Differential molecular profiles of astrocytes in degeneration and re-innervation after sensory deafferentation of the adult rat cochlear nucleus

Ablating the cochlea causes total sensory deafferentation of the cochlear nucleus. Over the first postoperative week, degeneration of the auditory nerve and its synaptic terminals in the cochlear nucleus temporally overlaps with its re-innervation by axon collaterals of medial olivocochlear neurons. At the same time, astrocytes increase in size and density. We investigated the time courses of the expression of ezrin, polysialic acid, matrix metalloprotease-9 and matrix metalloprotease-2 within these astrocytes during the first week following cochlear ablation. All four proteins are known to participate in degeneration, regeneration, or both, following injury of the central nervous system. In a next step, stereotaxic injections of kainic acid were made into the ventral nucleus of the trapezoid body prior to cochlear ablation to destroy the neurons that re-innervate the deafferented cochlear nucleus by axon collaterals developing growth-associated protein 43 immunoreactivity. This experimental design allowed us to distinguish between molecular processes associated with degeneration and those associated with re-innervation. Under these conditions, astrocytic growth and proliferation showed an unchanged deafferentation-induced pattern. Similarly, the distribution and amount of ezrin and matrix metalloprotease-9 in astrocytes after cochlear ablation developed in the same way as under cochlear ablation alone. In sharp contrast, the astrocytic expression of polysialic acid and matrix metalloprotease-2 normally invoked by cochlear ablation collapsed when re-innervation of the cochlear nucleus was inhibited by lesioning medial olivocochlear neurons with kainic acid. In conclusion, re-innervation, including axonal growth and synaptogenesis, seems to prompt astrocytes to recompose their molecular profile, paving the way for tissue reorganisation after nerve degeneration and loss of synaptic contacts.

Long-term monaural auditory deprivation and bilateral cochlear implants

Long-term binaural auditory deprivation is associated with poorer speech recognition outcomes after cochlear implantation, even for postlingual hearing loss. It is, however, unknown to what extent the outcomes of implantation are related to the peripheral changes occurring monaurally or to changes at a higher level in the auditory system related to binaural deafness. This retrospective study aimed to unravel peripheral and central contributions to cochlear implantation outcomes by comparing outcomes obtained in individual ears for adults with long-term monaural auditory deprivation (i.e. unilateral use of hearing aid) who received bilateral cochlear implants. Results showed that similar outcomes can be obtained with the implant placed in the auditory-deprived or in the aided ear. This suggests that the peripheral changes related to monaural auditory deprivation have little effect on outcomes of cochlear implantation.

Ouabain-induced cochlear degeneration in rat

Ouabain, a potent inhibitor of the Na+/K+-ATPase pump, selectively destroys spiral ganglion neurons (SGNs) in gerbils and mice, whereas in guinea pigs it preferentially damages cochlear hair cells. To elucidate the effects of ouabain on the rat inner ear, a species widely used in research, 5 μl of 1 or 10 mM ouabain was applied to the round window membrane. Distortion product otoacoustic emissions (DPOAE) and auditory brainstem responses (ABR) were used to identify functional deficits in hair cells and neurons, respectively, and histological techniques were used to characterize cochlear pathologies. High-frequency ABR thresholds were elevated after treatment with 1 mM ouabain, whereas DPOAEs remained normal. In contrast, 10 mM ouabain increased ABR thresholds and reduced DPOAE amplitudes. Consistent with the physiological changes, 1 mM ouabain only damaged the SGNs and auditory nerve fibers in the basal turn of the cochlea whereas 10 mM ouabain destroyed both SGNs and cochlear hair cells; damage was greatest near the base and decreased toward the apex. The nuclei of degenerating SGNs and hair cells were condensed and fragmented and many cells were TUNEL-positive, morphological features of apoptotic cell death. Thus, ouabain-induced cochlear degeneration in rats is apoptotic and concentration dependent; low concentrations preferentially damage SGNs in the base of the cochlea, producing an animal model of partial auditory neuropathy, whereas high concentrations damage both hair cells and SGNs with damage decreasing from the base toward the apex.

Understanding tinnitus: the dorsal cochlear nucleus, organization and plasticity

Tinnitus, the perception of a phantom sound, is a common consequence of damage to the auditory periphery. A major goal of tinnitus research is to find the loci of the neural changes that underlie the disorder. Crucial to this endeavor has been the development of an animal behavioral model of tinnitus, so that neural changes can be correlated with behavioral evidence of tinnitus. Three major lines of evidence implicate the dorsal cochlear nucleus (DCN) in tinnitus. First, elevated spontaneous activity in the DCN is correlated with peripheral damage and tinnitus. Second, there are somatosensory inputs to the DCN that can modulate spontaneous activity and might mediate the somatic-auditory interactions seen in tinnitus patients. Third, we have found a subpopulation of DCN neurons in the adult rat that express doublecortin, a plasticity-related protein. The expression of this protein may reflect a role of these neurons in the neural reorganization causing tinnitus. However, there is a problem in extending the findings in the rodent DCN to humans. Classic studies state that the structure of the primate DCN is quite different from that of rodents, with primates lacking granule cells, the recipients of somatosensory input. To address the possibility of major species differences in DCN organization, we compared Nissl-stained sections of the DCN in five different species. In contrast to earlier reports, our data suggest that the organization of the primate DCN is not dramatically different from that of the rodents, and validate the use of animal data in the study of tinnitus. This article is part of a Special Issue entitled: Tinnitus Neuroscience.

Ototoxic destruction by co-administration of kanamycin and ethacrynic acid in rats

It is well known that ethacrynic acid (EA) can potentiate the ototoxicity of aminoglycoside antibiotics (AmAn) such as kanamycin (KM), if they were applied at the same time. Currently, to create the model of EA-KM-induced cochlear lesion in rats, adult rats received a single injection of EA (75 mg/kg, intravenous injection), or followed immediately by KM (500 mg/kg, intramuscular injection). The hearing function was assessed by auditory brainstem response (ABR) measurement in response to click and/or tone bursts at 4, 8, 12, 16, 20, 24, and 32 kHz. The static microcirculation status in the stria vascularis after a single EA injection was evaluated with eosin staining. The pathological changes in cochlear and vestibular hair cells were also quantified after co-administration of EA and KM. After a single EA injection, blood flow in vessels supplying the stria vascularis rapidly diminished. However, the blood supply to the cochlear lateral wall partially recovered 5 h after EA treatment. Threshold changes in ABR were basically parallel to the microcirculation changes in stria vascularis after single EA treatment. Importantly, disposable co-administration of EA and KM resulted in a permanent hearing loss and severe damage to the cochlear hair cells, but spared the vestibular hair cells. Since the cochlear lateral wall is the important part of the blood-cochlea barrier, EA-induced anoxic damage to the epithelium of stria vascularis may enhance the entry of KM to the cochlea. Thus, experimental animal model of selective cochlear damage with normal vestibular systems can be reliably created through co-administration of EA and KM.

Relationship between noise-induced hearing-loss, persistent tinnitus and growth-associated protein-43 expression in the rat cochlear nucleus: does synaptic plasticity in ventral cochlear nucleus suppress tinnitus?

Aberrant, lesion-induced neuroplastic changes in the auditory pathway are believed to give rise to the phantom sound of tinnitus. Noise-induced cochlear damage can induce extensive fiber growth and synaptogenesis in the cochlear nucleus, but it is currently unclear if these changes are linked to tinnitus. To address this issue, we unilaterally exposed nine rats to narrow-band noise centered at 12 kHz at 126 dB sound pressure level (SPL) for 2 h and sacrificed them 10 weeks later for evaluation of synaptic plasticity (growth-associated protein 43 [GAP-43] expression) in the cochlear nucleus. Noise-exposed rats along with three age-matched controls were screened for tinnitus-like behavior with gap prepulse inhibition of the acoustic startle (GPIAS) before, 1-10 days after, and 8-10 weeks after the noise exposure. All nine noise-exposed rats showed similar patterns of severe hair cell loss at high- and mid-frequency regions in the exposed ear. Eight of the nine showed strong up-regulation of GAP-43 in auditory nerve fibers and pronounced shrinkage of the ventral cochlear nucleus (VCN) on the noise-exposed side, and strong up-regulation of GAP-43 in the medial ventral VCN, but not in the lateral VCN or the dorsal cochlear nucleus. GAP-43 up-regulation in VCN was significantly greater in Noise-No-Tinnitus rats than in Noise-Tinnitus rats. One Noise-No-Tinnitus rat showed no up-regulation of GAP-43 in auditory nerve fibers and only little VCN shrinkage, suggesting that auditory nerve degeneration plays a role in tinnitus generation. Our results suggest that noise-induced tinnitus is suppressed by strong up-regulation of GAP-43 in the medial VCN. GAP-43 up-regulation most likely originates from medial olivocochlear neurons. Their increased excitatory input on inhibitory neurons in VCN may possibly reduce central hyperactivity and tinnitus.

Differential impact of temporary and permanent noise-induced hearing loss on neuronal cell density in the mouse central auditory pathway

Although acoustic overstimulation has a major pathophysiological influence on the inner ear, central components of the auditory pathway can also be affected by noise-induced hearing loss (NIHL). The present study investigates the influence of a noise-induced temporary threshold shift (TTS) and/or permanent threshold shift (PTS) on neuronal cell densities in key structures of the central auditory pathway. Mice were noise-exposed (3 h, 5-20 kHz) at 115 dB sound pressure level (SPL) under anesthesia, and were investigated immediately (TTS group, n = 5) after the exposure, or 1 week later (PTS group, n = 6). Unexposed animals were used as controls (n = 7). Frequency-specific auditory brainstem responses (ABR) were recorded to examine auditory thresholds. Cell density was determined within the dorsal (DCN) and ventral (VCN) cochlear nucleus; the central nucleus of the inferior colliculus (ICC); the dorsal, ventral, and medial subdivisions of the medial geniculate body (MGBd, MGBv, and MGBm); and layer I to VI of the primary auditory cortex (AI I-VI). ABR thresholds were significantly elevated in the TTS group (52-69 dB SPL) and in the PTS group (33-42 dB SPL) compared to controls. There was a significant decrease in cell density only in the VCN of the TTS group (-10%), most likely induced by the acute overstimulation of neurons. Cell density was significantly reduced in all investigated auditory structures at 1 week post-exposure (PTS group), except in layer II of the AI (VCN: -30% and DCN: -30% (high-frequency); -39% (low-frequency); ICC: -31%; MGBd: -31%; MGBm: -28%; MGBv: -31%; AI: -10 to 14%). Thus there were dramatic changes within the neuronal cytoarchitecture of the central auditory pathway following a single noise exposure. The present findings should help clinicians to better understand the complex psychoacoustic phenomena of NIHL.

Poly (γ, L-glutamic acid)-cisplatin bioconjugate exhibits potent antitumor activity with low toxicity: a comparative study with clinically used platinum derivatives

We have recently synthesized a new platinum derivative, poly (γ, L-glutamic acid)-cisplatin conjugate (γ-PGA-CDDP), and shown that it displayed remarkable antitumor activity against breast tumor in a mouse model. The purpose of this study is to systematically compare this new drug with three platinum derivatives currently used in the clinic: cisplatin, carboplatin and oxaliplatin. Here, we show that γ-PGA-CDDP displays impressive antitumor activity over the current clinically used platinum drugs. More interestingly and more importantly, γ-PGA-CDDP conjugate significantly reduces cytotoxicity, mitigates oxidative stress and improves antioxidative capability in vivo. Animals treated with γ-PGA-CDDP display the same profile of body weight as the control animals, while the tumors in γ-PGA-CDDP-treated animals are significantly suppressed compared with those treated with carboplatin and oxaliplatin. Our data suggest that γ-PGA could be used as an effective carrier for drug delivery and that γ-PGA-CDDP conjugate may have potential therapeutic applications in human cancers that are sensitive to treatment with CDDP-based chemotherapy such as ovarian cancer.