Localization of NKCC1 in the cochlea and morphology of the cochlea in NKCC1-knockout mice

The distribution of the Na-K-2Cl co-transporter (NKCC1) in the cochlear K+ cycling pathway in cochlea and cochlear histological changes in the NKCC1 knockout mice were investigated. By using immunohistochemistry and toluidine blue staining, the localization of NKCC1 in cochlea of the C57BL/6J mice and the cochlear histological changes in the NKCC1 knockout mice were observed. It was found that the NKCC1 was expressed mainly in the stria marginal cells and the fibrocytes in the inferior portion of the spiral ligament in the adult C57BL/6J mice. Subpopulation of the fibrocytes in the suprastrial region and the limbus was also moderately immunoreactive. While in the cochlea of the NKCC1 knockout mice, Reissner's membrane was collapsed and scala media disappeared, accompanied with the loss of inner hair cells, outer hair cells and the support cells. The tunnel of Corti was often absent. All the findings suggested the localization of NKCC1 in the cochlea was closely correlated with cochlear K+ cycling. Loss of NKCC1 led to the destruction of the cochlear structures, and subsequently influenced the physiological function of cochlea.

Deletion of SLC19A2, the high affinity thiamine transporter, causes selective inner hair cell loss and an auditory neuropathy phenotype

Mutations in the gene coding for the high-affinity thiamine transporter Slc19a2 underlie the clinical syndrome known as thiamine-responsive megaloblastic anemia (TRMA) characterized by anemia, diabetes, and sensorineural hearing loss. To create a mouse model of this disease, a mutant line was created with targeted disruption of the gene. Cochlear function is normal in these mutants when maintained on a high-thiamine diet. When challenged with a low-thiamine diet, Slc19a2-null mice showed 40-60 dB threshold elevations by auditory brainstem response (ABR), but only 10-20 dB elevation by otoacoustic emission (OAE) measures. Wild-type mice retain normal hearing on either diet. Cochlear histological analysis showed a pattern uncommon for sensorineural hearing loss: selective loss of inner hair cells after 1-2 weeks on low thiamine and significantly greater inner than outer hair cell loss after longer low-thiamine challenges. Such a pattern is consistent with the observed discrepancy between ABR and OAE threshold shifts. The possible role of thiamine transport in other reported cases of selective inner hair cell loss is considered.

Cochlear compression in listeners with moderate sensorineural hearing loss

Psychophysical estimates of basilar membrane (BM) responses suggest that normal-hearing (NH) listeners exhibit constant compression for tones at the characteristic frequency (CF) across the CF range from 250 to 8000 Hz. The frequency region over which compression occurs is broadest for low CFs. This study investigates the extent that these results differ for three hearing-impaired (HI) listeners with sensorineural hearing loss. Temporal masking curves (TMCs) were measured over a wide range of probe (500-8000 Hz) and masker frequencies (0.5-1.2 times the probe frequency). From these, estimated BM response functions were derived and compared with corresponding functions for NH listeners. Compressive responses for tones both at and below CF occur for the three HI ears across the CF range tested. The maximum amount of compression was uncorrelated with absolute threshold. It was close to normal for two of the three HI ears, but was either slightly (at CFs < or =1000 Hz) or considerably (at CFs > or =4000 Hz) reduced for the third ear. Results are interpreted in terms of the relative damage to inner and outer hair cells affecting each of the HI ears. Alternative interpretations for the results are also discussed, some of which cast doubts on the assumptions of the TMC-based method and other behavioral methods for estimating human BM compression.

Efficacy of intratympanic methylprednisolone acetate in treatment of drill-induced sensorineural hearing loss in guinea pigs

Intratympanic steroids offer direct access to the inner ear with high concentration and without systemic effects. In this study, the efficacy of intratympanic methylprednisolone acetate (IT-MPA) was evaluated in a guinea-pig model of drill-induced inner ear trauma.

Twenty-five guinea pigs were divided into a control group to document the baseline distortion product otoacoustic emissions (DPOAEs) and the normal scanning electron microscopic (SEM) morphology of the inner ear. The animals in the study group were subdivided into a steroid-only group (S), a trauma-only group(T), a trauma-plus-time group (TT), and a trauma-plus-steroid (TS) group.

IT-MPA was found to have no damaging effect on the inner ear. Twelve days after trauma, there was spontaneous although incomplete recovery of the DPOAEs amplitudes and SEM morphology with scar tissue replacing lost outer hair cells. Statistically higher DPOAEs amplitudes (p < 0.05) were recorded in the TS group that had nearly normal SEM morphology compared to the TT group. The authors conclude that IT-MPA significantly improves drill-induced sensorineural hearing loss and inner ear morphological changes in guinea pigs.

Damage and threshold shift resulting from cochlear exposure to paraquat-generated superoxide

Superoxide has been implicated as a contributing factor to cochlear pathology from a number of sources, including noise and ototoxic drugs. The effects of NADPH oxidase-dependent superoxide on the cochlea were investigated in the current study using paraquat (PQ). PQ is a toxic herbicide that causes tissue damage by generating superoxide through reduction of molecular oxygen in a reaction catalyzed by NADPH oxidase. The current study examined the effects of round window PQ administration on inferior colliculus (IC) evoked potential thresholds (EVP) and hair cell damage. Using implanted IC electrodes, chinchillas were tested for IC EVP thresholds before and after PQ exposure. Ears were exposed to PQ at one of four concentrations: 10, 5, 3 mM, and vehicle control. Thresholds were increased in a dose-dependent manner, and peaked between one and seven days post-exposure. Thresholds then showed a small amount of recovery before reaching PTS by Day 22. Outer and inner hair cell losses were consistent with PTS. The similarities between PQ ototoxicity and noise-induced hearing loss suggest the possibility of similar biochemical pathways involving superoxide.

Leupeptin, a Calpain Inhibitor, Protects Inner Ear Hair Cells from Aminoglycoside Ototoxicity

Inner ear hair cells play a major role in the auditory pathway that converts sound stimulation into electrical signals, and then into a neural code. However this function is often lost by aminoglycoside ototoxicity. The injury of inner ear hair cells from aminoglycoside treatment is considered apoptosis, and caspase is an important participant in the apoptosis pathway in many organs. It has been reported that calpain, a calcium-dependent protease, is essential for mediation and promotion of cell death. The purpose of the present study was to investigate effects of caspase and calpain inhibitors on the inner ear hair cells after aminoglycoside treatment, and to explore the cell death pathway. Cochlea explant cultures were prepared from mice of postnatal 6 days, cultured with neomycin and/or protease inhibitors, and then stained with phalloidin-fluorescein isothiocyanate (phalloidin-FITC), which was used as a marker to identify surviving hair cells. We demonstrated that neomycin (0.1-1 mM) reduced the number of outer hair cells in a dose-dependent manner. Furthermore, we showed that leupeptin, a calpain inhibitor, significantly protects against the neomycin-induced loss of outer hair cells, whereas a caspase inhibitor was effective only against a lower concentration of neomycin (0.2 mM). Using the TdT-mediated dUTP-biotin nick and labeling method, we also found that a calpain inhibitor, but not a caspase inhibitor, prevents apoptotic DNA fragmentation after treatment with 1 mM neomycin. These results suggest that calpain, rather than caspase, may be responsible for apoptosis induced by aminoglycoside. Thus, leupeptin may prevent hearing loss from aminoglycoside ototoxity.

Amino acid concentrations in chinchilla cochlear nucleus at different times after carboplatin treatment

Amino acid concentrations were measured in the cochlear nucleus for a group of 20 chinchillas: four each of control and 4, 8, 29, and 85 days after treatment with the ototoxic anti-tumor drug carboplatin (100 mg/kg, i.p.). The treated chinchillas showed various extents of inner hair cell loss, generally more complete at longer survival times, but little loss of outer hair cells. Aspartate concentration in rostral anteroventral cochlear nucleus (AVCN) showed a decline to 28% less than the control value at 29 and 85 days after treatment, whereas glutamate concentration showed little change through 29 days, then dropped by 22% at 85 days after treatment. In caudal posteroventral cochlear nucleus (PVCN), the aspartate concentration decreased by 32% at 29 days, in animals with significant inner hair cell loss, and 48% at 85 days after treatment, while the glutamate concentration showed no decrease through 29 days and 40% decrease at 85 days. The concentration of gamma-aminobutyrate (GABA) was about 18% lower than control in caudal PVCN at all survival times. Significant correlations were found between the proportion of inner hair cells remaining and glutamate and aspartate concentrations in PVCN and AVCN, but not GABA or other amino acids.

Usher I syndrome: unravelling the mechanisms that underlie the cohesion of the growing hair bundle in inner ear sensory cells

Defects in myosin VIIa, the PDZ-domain-containing protein harmonin, cadherin 23 and protocadherin 15 (two cadherins with large extracellular regions), and the putative scaffolding protein Sans underlie five genetic forms of Usher syndrome type I (USH1), the most frequent cause of hereditary deafness-blindness in humans. All USH1 proteins are localised within growing stereocilia and/or the kinocilium that make up the developing auditory hair bundle, the mechanosensitive structure receptive to sound stimulation. Cadherin 23 has been shown to be a component of fibrous links interconnecting the growing stereocilia as well as the kinocilium and the nearest tall stereocilia. A similar function is anticipated for protocadherin 15. Multiple direct interactions between USH1 proteins have been demonstrated. In particular, harmonin b can bind to the cytoplasmic regions of cadherin 23 and protocadherin 15, and to F-actin, and thus probably anchors these cadherins to the actin filaments filling the stereocilia. Myosin VIIa and Sans are both involved in the sorting and/or targeting of harmonin b to the stereocilia. Together, this suggests that the disorganisation of the hair bundles observed in mice mutants lacking orthologues of USH1 proteins may result from a defect of hair-bundle-link-mediated adhesion forces. Moreover, several recent evidences suggest that some genes defective in Usher type II syndrome also encode interstereocilia links, thus bridging the pathogenic pathways of USH1 and USH2 hearing impairment. Additional functions of USH1 proteins in the inner ear and the retina are evident from other phenotypic abnormalities observed in these mice. In particular, myosin VIIa could act at the interface between microtubule- and actin-based transport.

The aminoglycoside antibiotic dihydrostreptomycin rapidly enters mouse outer hair cells through the mechano-electrical transducer channels

The most serious side-effect of the widely used aminoglycoside antibiotics is irreversible intracellular damage to the auditory and vestibular hair cells of the inner ear. The mechanism of entry into the hair cells has not been unequivocally resolved. Here we report that extracellular dihydrostreptomycin not only blocks the mechano-electrical transducer channels of mouse outer hair cells at negative membrane potentials, as previously shown, but also enters the cells through these channels, which are located in the cells' mechanosensory hair bundles. The voltage-dependent blocking kinetics indicate an open-channel block mechanism, which can be well described by a two barrier-one binding site model, quantifying the antibiotic's block of the channel as well as its permeation in terms of the associated rate constants. The results identify the open transducer channels as the main route for aminoglycoside entry. Intracellularly applied dihydrostreptomycin also blocks the transducer channels, but at positive membrane potentials. However, the potency of the block was two orders of magnitude lower than that due to extracellular dihydrostreptomycin. Extracellular Ca2+ increases the free energy of the barrier nearest the extracellular side and of the binding site for dihydrostreptomycin. This reduces both the entry of dihydrostreptomycin into the channel and the channel's affinity for the drug. In vivo, where the extracellular Ca2+ concentration in the endolymph surrounding the hair bundles is < 100 microM, we predict that some 9000 dihydrostreptomycin molecules per second enter each hair cell at therapeutic drug concentrations.

Syndrome de Usher de type 1 et développement de la touffe ciliaire des cellules sensorielles de l’oreille interne

Defects in myosin VIIa, the PDZ-domain-containing protein harmonin, cadherin 23, protocadherin 15, and the putative scaffolding protein sans, underlie five genetic forms of Usher syndrome type I (USH1), the most frequent cause of hereditary deafness-blindness in humans. Mice mutants defective for any of these proteins have a severe hearing impairment and display similar inner ear phenotypes characterized by the abnormal spreading of the sensory cells' stereocilia. These are highly specialized mechanoreceptive organelles derived from microvilli, that normally form a well-structured hair bundle at the apex of inner ear sensory cells. All the USH1 proteins, except sans, have been detected in the growing stereocilia. Moreover, biochemical studies have started to unravel the multiple direct molecular interactions between USH1 proteins. In particular, harmonin can bind to the other four USH1 proteins and to F-actin. Finally, cell biology studies have provided the first insights into the functions of these proteins, and revealed that cadherin 23, and probably protocadherin 15 also, are associated with transient lateral links that interconnect growing stereocilia. These connectors play a critical role in the differentiating hair bundle.

Progressive degeneration of stereocilia in cochlear hair cells in hearing-impaired kuru2 mice

BACKGROUND

We previously isolated a mouse strain, kuru2, which exhibits abnormal behavior and hearing impairment. To investigate the etiology of this impairment, the ultrastructure of the inner ear was examined.

MATERIALS AND METHODS

The morphologies of the cochlea and the vestibule of control (Jcl:ICR) and mutant mice were analyzed by electron microscopy. In some experiments, the mice were cross-mated and their offspring examined.

RESULTS

The mutant mice displayed progressive degeneration of the stereocilia in the cochlea. The stereocilia started to degenerate on post-natal day 10 and, subsequently, the hair bundles continued to degenerate. On day 18, degeneration of the stereocilia was complete. In contrast, the vestibule was intact.

DISCUSSION

Many mutant mice display hearing impairment These mice demonstrate a characteristic morphology of the inner ear and, since correlations may be made with corresponding human diseases, the current results could contribute to the further understanding of hearing impairment mechanisms.

Kanamycin ototoxicity in glutamate transporter knockout mice

Glutamate-aspartate transporter (GLAST), a powerful glutamate uptake system, removes released glutamate from the synaptic cleft and facilitates the re-use of glutamate as a neurotransmitter recycling system. Aminoglycoside-induced hearing loss is mediated via a glutamate excitotoxic process. We investigated the effect of aminoglycoside ototoxicity in GLAST knockout mice using the recorded auditory brainstem response (ABR) and number of hair cells in the cochlea. Kanamycin (100 mg/mL) was injected directly into the posterior semicircular canal of mice. Before the kanamycin treatment, there was no difference in the ABR threshold average between the wild-type and knockout mice. Kanamycin injection aggravated the ABR threshold in the GLAST knockout mice compared with the wild-type mice, and the IHC degeneration was more severe in the GLAST knockout mice. These findings suggest that GLAST plays an important role in preventing the degeneration of inner hair cells in aminoglycoside ototoxicity.

Sox2 is required for sensory organ development in the mammalian inner ear

Sensory hair cells and their associated non-sensory supporting cells in the inner ear are fundamental for hearing and balance. They arise from a common progenitor, but little is known about the molecular events specifying this cell lineage. We recently identified two allelic mouse mutants, light coat and circling (Lcc) and yellow submarine (Ysb), that show hearing and balance impairment. Lcc/Lcc mice are completely deaf, whereas Ysb/Ysb mice are severely hearing impaired. We report here that inner ears of Lcc/Lcc mice fail to establish a prosensory domain and neither hair cells nor supporting cells differentiate, resulting in a severe inner ear malformation, whereas the sensory epithelium of Ysb/Ysb mice shows abnormal development with disorganized and fewer hair cells. These phenotypes are due to the absence (in Lcc mutants) or reduced expression (in Ysb mutants) of the transcription factor SOX2, specifically within the developing inner ear. SOX2 continues to be expressed in the inner ears of mice lacking Math1 (also known as Atoh1 and HATH1), a gene essential for hair cell differentiation, whereas Math1 expression is absent in Lcc mutants, suggesting that Sox2 acts upstream of Math1.

[Carboplatin-induced injury patterns of inner hair cells missing in chinchilla cochlea]

OBJECTIVE

To explore carboplatin-induced injury patterns of inner hair cells (IHCs) in cochlea of chinchilla, namely, when injury of IHCs occurs, what extension injury of IHCs exists at different time after drug administration, where maximal injury of IHCs locates on basal membrane and how long most IHCs lose after drug administration.

METHOD

Carboplatin of moderate concentration (100 mg/kg) was used to inject peritoneally one time and animals were decapitated on schedule, surface preparations were made and cochleograms were obtained to observe injury of IHCs in the cochleae of chinchillas.

RESULT

Loss of outer hair cells was not observed on basal membrane from 24 hours to 3 months after drug administration except one ear. The characteristic of IHCs injury patterns was found to be: (1) IHCs were intact at 24 hours after drug administration. IHCs injury was firstly observed within 3 days. (2) The maximal rate of IHCs injury located around boundary of turn one and turn two on basal membrane from 24 hours to 3 months. (3) IHCs injury began from boundary of turn one and turn two and extended to basal turn and apical turn gradually. Compared to apical turn, IHCs injury on basal turn occurred earlier and more severely. (4) Most IHCs lost in 3 months.

CONCLUSION

Carboplatin of 100 mg/kg unequally destroyed IHCs from different turns. Temporal variance of injury of IHCs from given turn is most likely due to their discrepant sensitivity to carboplatin.

Characterization of a new allele of Ames waltzer generated by ENU mutagenesis

Mutation in the protocadherin 15 (Pcdh15) gene causes hair cell dysfunction and is associated with abnormal stereocilia development. We have characterized the first allele (Pcdh15(av-nmf19)) of Ames waltzer (av) obtained by N-ethyl-N-nitrosourea (ENU) mutagenesis. Pcdh15(av-nmf19) was generated in the Neuroscience Mutagenesis Facility (NMF) at The Jackson Lab (Bar Habor, USA). Pcdh15(av-nmf19) mutants display circling and abnormal swimming behavior along with lack of auditory-evoked brainstem response at the highest intensities tested. Mutation analysis shows base substitution (A--> G) in the consensus splice donor sequence linked to exon 14 resulting in the skipping of exon 14 and the splicing of exon 13-15. This results in the introduction of a stop codon in the coding sequence of exon 15 due to shift in the reading frame. The effect of nmf19 mutation is expected to be severe since the expressed Pcdh15 protein is predicted to truncate in the 5th cadherin domain. Abnormalities of cochlear hair cell stereocilia are apparent in Pcdh15(av-nmf19) mutants near the time of birth and by about P15 (15 days after birth) there is evidence of sensory cell degeneration. Disorganization of outer hair cell stereocilia is observed as early as P2. Inner hair cell stereocilia are also affected, but less severely than those of the outer hair cells. These results are consistent with characteristics of the mutation in the Pcdh15(av-nmf19) allele and they support our previous finding that Protocadherin 15 plays an important role in hair-bundle morphogenesis.

Na+ influx triggers bleb formation on inner hair cells

Large blebs form rapidly on apical membranes of sensory inner hair cells (IHCs) when the organ of Corti is freshly isolated from adult guinea pigs. Bleb formation had two distinguishable phases. Initially, we identified small particles labeled with fluorescent annexin V; these rapidly coalesced into larger aggregates. After particle aggregation, a single membrane bleb emerged from cuticular plate at the vestigial kinocilium location, eventually reaching approximately 10 microm maximum spherical diameter; blebs this size often detached from IHCs. Development of blebs was associated with elevated concentration of intracellular Na(+); blocking Na(+) influx through mechanotransduction and ATP channels in the apical pole of IHCs or by replacement of Na(+) with N-methyl-D-glucamine prevented Na(+) loading and bleb formation. Depletion of intracellular ATP, blocking cAMP synthesis, inhibition of vesicular transport with brefeldin A, or inhibition of phosphatidylinositol 3-kinase with 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one (LY-294002) significantly reduced bleb formation in the presence of a Na(+) load. Neither the mechanism of blebbing nor the size growth of the IHC blebs was associated with cellular apoptosis or necrosis. Bleb formation was not significantly reduced by disassembling microtubules or decreasing intracellular hydrostatic pressure. Moreover, no polymerized actin was observed in the lumen of blebs. We conclude that IHC bleb formation differs from classic blebbing mechanisms and that IHC blebs arise from imbalance of endocytosis and exocytosis in the apical plasma membrane, linked to Na(+) loading that occurs in vitro.

Targeted point mutagenesis of mouse Kcnq1: phenotypic analysis of mice with point mutations that cause Romano-Ward syndrome in humans

Inherited long QT syndrome is most frequently associated with mutations in KCNQ1, which encodes the primary subunit of a potassium channel. Patients with mutations in KCNQ1 may show only the cardiac defect (Romano-Ward syndrome or RWS) or may also have severe deafness (Jervell and Lange-Nielsen syndrome or JLNS). Targeted disruption of mouse Kcnq1 models JLNS in that mice are deaf and show abnormal ECGs. However, the phenotype is broader than that seen in patients. Most dramatically, the inner ear defects result in a severe hyperactivity/circling behavior, which may influence cardiac function. To understand the etiology of the cardiac phenotype in these mice and to generate a potentially more useful model system, we generated new mouse lines by introducing point mutations associated with RWS. The A340E line phenocopies RWS: the repolarization phenotype is inherited in a dominant manner and is observed independent of any inner ear defect. The T311I line phenocopies JLNS, with deafness associated with inner hair cell malfunction.

Low endolymph calcium concentrations in deafwaddler2J mice suggest that PMCA2 contributes to endolymph calcium maintenance

In vertebrates, transduction of sound into an electrochemical signal is carried out by hair cells that rely on calcium to perform specialized functions. The apical surfaces of hair cells are surrounded by endolymphatic fluid containing calcium at concentrations that must be maintained by active transport. The mechanism of this transport is unknown, but an ATP-dependent pump is believed to participate. Mutation of the Atp2b2 gene that encodes plasma membrane calcium ATPase type 2 (PMCA2) produces the deaf, ataxic mouse: deafwaddler2J (dfw2J). We hypothesized that PMCA2 might transport calcium into the endolymph and that dfw2J mice would have low endolymph calcium concentrations, possibly contributing to their deafness and ataxia. First, using immunocytochemistry, we demonstrated that PMCA2 is present in control mice inner and outer hair cell stereocilia where it could pump calcium into the endolymph and that PMCA2 is absent in dfw2J stereocilia. Second, using an aspirating microelectrode and calcium-sensitive fluorescent dye, we found that dfw2J mice endolymph calcium concentrations are significantly lower than those of control mice. These findings suggest that PMCA2, located in hair cell stereocilia, contributes significantly to endolymph calcium maintenance.

Overexpression of Bcl-2 prevents neomycin-induced hair cell death and caspase-9 activation in the adult mouse utricle in vitro

Mechanosensory hair cells of the inner ear are especially sensitive to death induced by exposure to aminoglycoside antibiotics. This aminoglycoside-induced hair cell death involves activation of an intrinsic program of cellular suicide. Aminoglycoside-induced hair cell death can be prevented by broad-spectrum inhibition of caspases, a family of proteases that mediate apoptotic and programmed cell death in a wide variety of systems. More specifically, aminoglycoside-induced hair cell death requires activation of caspase-9. Caspase-9 activation requires release of mitochondrial cytochrome c into the cytoplasm, indicating that aminoglycoside-induced hair cell death is mediated by the mitochondrial (or "intrinsic") cell death pathway. The Bcl-2 family of pro-apoptotic and anti-apoptotic proteins are important upstream regulators of the mitochondrial apoptotic pathway. Bcl-2 is an anti-apoptotic protein that localizes to the mitochondria and promotes cell survival by preventing cytochrome c release. Here we have utilized transgenic mice that overexpress Bcl-2 to examine the role of Bcl-2 in neomycin-induced hair cell death. Overexpression of Bcl-2 significantly increased hair cell survival following neomycin exposure in organotypic cultures of the adult mouse utricle. Furthermore, Bcl-2 overexpression prevented neomycin-induced activation of caspase-9 in hair cells. These results suggest that the expression level of Bcl-2 has important effects on the pathway(s) important for the regulation of aminoglycoside-induced hair cell death.

The role of Pax2 in mouse inner ear development

The paired box transcription factor, Pax2, is important for cochlear development in the mouse inner ear. Two mutant alleles of Pax2, a knockout and a frameshift mutation (Pax21Neu), show either agenesis or severe malformation of the cochlea, respectively. In humans, mutations in the PAX2 gene cause renal coloboma syndrome that is characterized by kidney abnormalities, optic nerve colobomas and mild sensorineural deafness. To better understand the role of Pax2 in inner ear development, we examined the inner ear phenotype in the Pax2 knockout mice using paint-fill and gene expression analyses. We show that Pax2-/- ears often lack a distinct saccule, and the endolymphatic duct and common crus are invariably fused. However, a rudimentary cochlea is always present in all Pax2 knockout inner ears. Cochlear outgrowth in the mutants is arrested at an early stage due to apoptosis of cells that normally express Pax2 in the cochlear anlage. Lack of Pax2 affects tissue specification within the cochlear duct, particularly regions between the sensory tissue and the stria vascularis. Because the cochlear phenotypes observed in Pax2 mutants are more severe than those observed in mice lacking Otx1 and Otx2, we postulate that Pax2 plays a key role in regulating the differential growth within the cochlear duct and thus, its proper outgrowth and coiling.

Dose and time-dependent protection of the antioxidant N-l-acetylcysteine against impulse noise trauma

Noise-induced hearing loss is one of the most common causes of hearing disability, and at present there is no effective biological protection or cure. Firearms and some industrial equipment can generate very high levels of impulse noise, which is known to cause sensorineural hearing loss. It has been shown that antioxidants such as N-L-acetylcysteine (NAC) can protect the inner ear from oxidative damage. The present study investigates whether NAC (i.p.) can protect the cochlea from impulse noise trauma. Rats were exposed to 50 noise pulses at 160 dB SPL peak value. Electrophysiological hearing thresholds were assessed with auditory brainstem response (ABR) up to 4 weeks after noise exposure. Animals exposed to impulse noise, without treatment of NAC, had larger threshold shifts in the frequency range 4-40 kHz than animals injected with NAC. Hair cell loss was significantly reduced using a schedule of three NAC injections in the rats. These results suggest that NAC can partially protect the cochlea against impulse noise trauma.

Changes of Hair Cell Stereocilia and Threshold Shift after Acoustic Trauma in Guinea Pigs: Comparison between Inner and Outer Hair Cells

The vulnerability of inner hair cells (IHCs) and outer hair cells (OHCs) to acoustic overstimulation is still controversially discussed. The present study was undertaken to investigate the vulnerability of IHCs and OHCs and the relation between chronological changes of auditory threshold shifts and stereocilia damages on IHCs and OHCs in guinea pigs after moderate acoustic trauma, caused by a single continuous exposure to pink noise (20 Hz to 20 kHz) of around 106 +/- 2 dB SPL for 44 h. Stereocilia changes and threshold shifts of auditory brainstem responses (ABR) were assessed at regular intervals after noise exposure for 9 weeks. Scanning electron microscopy demonstrated the morphological changes of stereocilia as early as 1 day after noise exposure. The morphological changes included fused, bent, collapsed, and even missing stereocilia. These damages were more prominent on IHCs than on OHCs. The shift of ABR threshold was not parallel to the chronological change of the stereocilia on IHCs as well as OHCs. The elevation of the ABR threshold (40-60 dB SPL) was greatest on the 1st day after noise exposure, whereas the stereocilia showed the most damage 7 days after noise exposure. Combined with the results from previous studies, we conclude that moderate-level (around 105-110 dB) noise tends to induce more damage to the stereocilia of IHCs than of OHCs. Other damage (e.g., metabolic disturbance) than morphological damage of hair cell stereocilia may contribute partially to the hearing threshold shift induced by moderate acoustic overstimulation.

Distal effects in a model of proximal axonopathy: 3,3'-iminodipropionitrile causes specific loss of neurofilaments in rat vestibular afferent endings

3,3'-Iminodipropionitrile (IDPN) is a neurotoxic compound that causes both a proximal neurofilamentous axonopathy and loss of the vestibular sensory hair cells. We used immunocytochemistry to examine changes in the expression of heavy, medium and light neurofilament (NF-H, NF-M, NF-L) proteins in the afferent terminals of vestibular sensory epithelia after IDPN exposure in rats. Acute, repeated and subchronic IDPN exposure induced a marked loss of NFs in the nerve terminals. The effect of subchronic IDPN was specific, as demonstrated by comparison with the synaptic membrane protein SNAP-25. In addition, Western blot analysis indicated specific loss of NFs in the vestibular receptors. Ultrastructural analysis revealed that afferent endings in the vestibular receptors were significantly preserved in animals exposed to subchronic IDPN, but that these endings showed NF segregation from microtubules followed by NF loss. These effects were closely paralleled by ultrastructural changes in the nerve terminals, particularly in the afferent contacts with the hair cells, and preceded hair cell loss. Thus, distal NF loss and nerve terminal pathology occur in the IDPN model of proximal neurofilamentous axonopathy. Similar distal pathology could also occur in human diseases characterized by proximal axonal swellings, particularly in amyotrophic lateral sclerosis.

Fate of neural stem cells grafted into injured inner ears of mice

Loss of sensory hair cells in the inner ear is a major cause of permanent hearing loss, since regeneration of hair cells rarely occurs in mammals. The aim of this study was to examine the potential of neural stem cell transplantation to restore inner ear hair cells in mice. Fetal neural stem cells were transplanted into the mouse inner ear after drug-induced injury. Histological analysis demonstrates that the majority of grafted cells differentiated into glial or neural cells in the inner ear. Strikingly, however, we show that grafted cells integrate in vestibular sensory epithelia and express specific markers for hair cells. This finding suggests that transplanted neural stem cells have the potential to differentiate and restore inner ear hair cells.

AMPA/kainate-type glutamate receptor antagonist reduces progressive inner hair cell loss after transient cochlear ischemia

We investigated the effect of glutamate receptor antagonists on progressive inner hair cell (IHC) loss following transient cochlear ischemia in gerbils. Transient cochlear ischemia was induced by 15-min bilateral vertebral artery occlusion. An alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate-type glutamate receptor antagonist, 6-7-dinitroquinoxaline-2,3-dione (DNQX), or an N-methyl-D-aspartate (NMDA)-type receptor antagonist, MK-801, was administered 10 min before the ischemic insult. Hearing was assessed by sequentially recording compound action potentials (CAPs) before, during, and after the ischemia. The degree of hair cell loss in the organ of Corti was evaluated in specimens stained with rhodamine-phalloidin and Hoechst 33342. On the seventh day after ischemia, the increases in the CAP threshold and the progressive IHC loss were significantly reduced in cochleae treated with DNQX, while MK-801 was ineffective. These results suggest that the AMPA receptor plays a critical role in the development of the progressive IHC loss induced by ischemia/reperfusion injury in the cochlea.

Carboplatin-induced early cochlear lesion in chinchillas

Carboplatin preferentially damages inner hair cells (IHC) and type I spiral ganglion neurons (SGNs) in the chinchilla; however, the temporal sequence of events leading to the destruction of these structures is poorly understood. To better understand the mechanisms leading up to the destruction of IHCs and type I SGNs, we measured the activity in single auditory nerve fibers for the first 8 h following carboplatin treatment and also monitored the development of histopathologies in SGNs and IHCs using a dose of carboplatin that killed approximately 50% of the IHCs. The spontaneous discharge rate (SDR) showed a slight increase around 3 h post carboplatin followed by a significant decline at 4-5 h. The saturation driven discharge rate (DDR) showed a significant increase 1-5 h post carboplatin. These physiological changes were associated with the formation of small vacuoles in type I afferent terminals and proximal nerve fibers 1-6 h post carboplatin; signs of IHC damage were first observed around 24-48 h. Thus, the neurotoxic effects of carboplatin occur approximately a day before the IHCs are damaged. The large fluctuations in SDR and DDR that occur several hours after carboplatin treatment are most likely due to the neurotoxic effects of carboplatin.

The effects of the amplitude distribution of equal energy exposures on noise-induced hearing loss: the kurtosis metric

Seventeen groups of chinchillas with 11 to 16 animals/group (sigmaN = 207) were exposed for 5 days to either a Gaussian (G) noise or 1 of 16 different non-Gaussian (non-G) noises at 100 dB(A) SPL. All exposures had the same total energy and approximately the same flat spectrum but their statistical properties were varied to yield a series of exposure conditions that varied across a continuum from G through various non-G conditions to pure impact noise exposures. The non-G character of the noise was produced by inserting high level transients (impacts or noise bursts) into the otherwise G noise. The peak SPL of the transients, their bandwidth, and the intertransient intervals were varied, as was the rms level of the G noise. The statistical metric, kurtosis (beta), computed on the unfiltered noise beta(t), was varied 3 < or = beta(t) < or = 105. Brainstem auditory evoked responses were used to estimate hearing thresholds and surface preparation histology was used to determine sensory cell loss. Trauma, as measured by asymptotic and permanent threshold shifts (ATS, PTS) and by sensory cell loss, was greater for all of the non-G exposure conditions. Permanent effects of the exposures increased as beta(t) increased and reached an asymptote at beta(t) approximately 40. For beta(t) > 40 varying the interval or peak histograms did not alter the level of trauma, suggesting that, in the chinchilla model, for beta(t) > 40 an energy metric may be effective in evaluating the potential of non-G noise environments to produce hearing loss. Reducing the probability of a transient occurring could reduce the permanent effects of the non-G exposures. These results lend support to those standards documents that use an energy metric for gauging the hazard of exposure but only after applying a "correction factor" when high level transients are present. Computing beta on the filtered noise signal [beta(f)] provides a frequency specific metric for the non-G noises that is correlated with the additional frequency specific outer hair cell loss produced by the non-G noise. The data from the abundant and varied exposure conditions show that the kurtosis of the amplitude distribution of a noise environment is an important variable in determining the hazards to hearing posed by non-Gaussian noise environments.

Protective effect of magnesium and MK 801 on hypoxia-induced hair cell loss in new-born rat cochlea

Hypoxia is a pathogenetic factor in various inner ear diseases, and increasing importance is attached to the protection of the cochlea from traumatic influences. It was recently demonstrated in guinea pigs that magnesium can significantly reduce ischemia- and impulse noise-induced hearing loss. The aim of this study was to evaluate if magnesium has a protective effect on hypoxia-induced hair cell loss using an in vitro model of the new-born rat cochlea In view of the NMDA receptor-antagonistic action of magnesium, we tested MK 801, a highly potent and selective non-competitive NMDA receptor antagonist. Organotypic cochlea cultures were exposed to hypoxia (pO2 = 10-20 mm Hg at 37 degrees C) in DMEM medium containing magnesium (0.75 or 3.0 mmol/l) or MK801 (1 or 10 micromol/l) for 24 or 36 h. The cultures were phalloidin-labeled for counting the number of outer and inner hair cells (OHC/IHC). The mean damage in normoxic controls was 1-4%. IHC revealed a significantly higher susceptibility to hypoxia than OHC. In the normal magnesium group (0.75 mmol/l), 36-hour exposure to hypoxia caused a mean loss of about 25% OHC and 60% IHC. In the groups treated with either 3.0 mmol magnesium or 10 microm MK 801, the damage was significantly reduced to about 10% in OHC and 35% in IHC. This study supports previous in vivo observations in the guinea pig demonstrating the protective effects of magnesium on noise-induced impairment of inner ear oxygenation.

Canavan's leukodystrophy is associated with defects in cochlear neurodevelopment and deafness

The authors present the temporal bone histopathology of two siblings (4 months old and 6 months old at autopsy) with Canavan's disease, an autosomal recessive leukodystrophy that is variably associated with sensorineural hearing loss. The histopathology demonstrated bilateral absence of the organ of Corti throughout the apical and basal cochlea and mild secondary atrophy of the spiral ganglia neurons. The vestibular end organs and ganglia were normal. These findings implicate a role of aminoacylase II in the neurodevelopment of the organ of Corti.

Auditory peripheral influences on calcium binding protein immunoreactivity in the cochlear nucleus during aging in the C57BL/6J mouse

The C57BL/6J (C57) mouse was selected as a suitable model for early presbyacusis to determine if there were correlations between peripheral pathology (spiral ganglion loss, inner and outer hair cell loss) and calcium binding immunoreactivity in the cochlear nucleus during aging. The quantitative stereological method, the optical fractionator, was used for determining the total number of neurons and calcium binding immunopositive neurons (calbindin, parvalbumin and calretinin) during aging in the posteroventral- and dorsal cochlear nucleus (PVCN and DCN) in C57 mice. Comparing 30-month-old to 1-month-old C57 mice, a percent increase in parvalbumin and calbindin immunoreactivity was evident in both the PVCN and DCN. Correlations were made between peripheral pathology (spiral ganglion and inner and outer hair cell loss) and calcium binding protein expression. Significant correlations between cochlear pathology and the percentage of parvalbumin and calretinin immunoreactive neurons were demonstrated in the DCN. Moreover, significant correlations were found between cochlear pathology and parvalbumin and calbindin in the PVCN. In summary, the findings imply that degenerative changes in the auditory periphery can modulate neuronal homeostasis by increasing calcium binding proteins in the PVCN and DCN during aging. Taken together, these findings suggest a role for calcium binding proteins in protecting against age-induced calcium toxicity.

Noise-induced hearing loss in chinchillas pre-treated with glutathione monoethylester and R-PIA

The protective effects of glutathione monoethylester (GEE) and GEE in combination with R-N6-phenylisopropyladenosine (R-PIA) were evaluated in the chinchilla when exposed to impulse (145 dB pSPL) or continuous (105 dB SPL, 4 kHz OB) noise. Six groups of 10 chinchillas were used as subjects. Before exposure to noise, the subjects were anesthetized, a 30 microl drop of drug was placed on the round window (GEE [50, 100, 150 mM], GEE 50 mM and R-PIA). Forty minutes later the subject was exposed to either impulse or continuous noise. The 50 mM treatment provided significant protection from impulse noise, but not from continuous noise exposure. The combination provided significant protection from both the continuous and impulse noise. In a separate set of experiments, glutathione (GSH) levels were measured in the perilymph. All the drug treatments elevated GSH levels. The results are discussed in terms of antioxidant treatments as a prophylactic measure against noise-induced hearing loss.

Cochlear damage caused by continuous and intermittent noise exposure

We compared the extent of permanent threshold shifts (PTS) and cochlear hair cell damage caused by continuous noise exposure with those caused by intermittent noise exposure. Twenty male pigmented guinea pigs that had been exposed to a one-octave band of noise at 4 kHz for 5 h were placed in four groups: exposure to 115 dB SPL continuous noise (group 1, n=5), 115 dB SPL intermittent noise (group 2, n=5), 125 dB SPL continuous noise (group 3, n=5), and 125 dB SPL intermittent noise (group 4, n=5). PTS at 2, 4, 8, and 16 kHz were assessed by means of auditory brainstem responses measured before noise exposure and 10 days after. The guinea pigs were killed 15 days after noise exposure, and the number of hair cells missing counted in surface preparations of the organs of Corti stained with rhodamine phalloidin. Groups 1 and 3 had significantly greater PTS (P<0.05) at all frequencies than intermittent groups 2 and 4. Group 3 had the greatest PTS at all the frequencies. Intermittent 125 dB noise total energy was greater than that of continuous 115 dB noise, but the latter elicited more PTS than the former. The extent of hair cell damage was comparable to the physiological findings. This indicates that continuous noise causes greater damage to the cochlea than intermittent noise of the same intensity and that, at the intensities tested, damage to the cochlea is not proportional to the total noise energy.

Adenovirus-mediated overexpression of a gene prevents hearing loss and progressive inner hair cell loss after transient cochlear ischemia in gerbils

The use of adenoviral vectors has recently provided a novel strategy for direct gene transfer into the cochlea. In this study, we assessed the utility of an adenoviral vector expressing glial-cell-derived neurotrophic factor (GDNF) in ischemia-reperfusion injury of the gerbil cochlea. The vector was injected through the round window 4 days before ischemic insult. The distribution of a reporter transgene was confirmed throughout the cochlea from the basal to the apical turn and Western blot analysis indicated significant upregulation of GDNF protein 11 days following virus inoculation. Hearing ability was assessed by sequentially recording compound action potentials (CAP), and the degree of hair cell loss in the organ of Corti was evaluated in specimens stained with rhodamine-phalloidin and Hoechst 33342. On the seventh day of ischemia, the CAP threshold shift and inner hair cell loss were remarkably suppressed in the Ad-GDNF group compared with the control group. These results suggest that adenovirus-mediated overexpression of GDNF is useful for protection against hair cell damage, which otherwise eventually occurs after transient ischemia of the cochlea.

Neuronal responses in the inferior colliculus of mutant mice (Bronx waltzer) with hereditary inner hair cell loss

Bronx waltzer mice lose a great proportion of their cochlear inner hair cells during early development. Hair cell counts revealed that these mice lacked on average 86% of their inner hair cells. Outer hair cells were present in a normal number, but appeared disarranged. The effect of this inner hair cell loss on the properties of central auditory neurons was investigated by recording neuronal responses in the inferior colliculus. Neuronal thresholds were on average elevated by 40 dB compared to CBA controls. The frequency tuning curves of the mutants were broad, and in part (18.5%) multi-peaked. The tonotopy found in the inferior colliculus of the Bronx waltzer mice appeared diffuse. Both the driven and spontaneous discharge rates were not statistically significantly different from the controls. However, the average first spike latency was significantly longer in the Bronx waltzer mice.

Transient cochlear ischemia causes delayed cell death in the organ of Corti: an experimental study in gerbils

To elucidate whether ischemia-reperfusion can cause delayed cell death in the cochlea, the effects of transient cochlear ischemia on hearing and on neuronal structures in the cochlea were studied in Mongolian gerbils. Ischemia was induced by bilaterally occluding the vertebral arteries for 5 minutes in gerbils, which lack posterior cerebral communicating arteries. In gerbils, the labyrinthine arteries are fed solely by the vertebral arteries. Occlusion of the vertebral arteries caused a remarkable increase in the threshold of compound action potentials (CAPs), which recovered over the following day. However, 7 days after the onset of reperfusion, the threshold began to increase again. Morphologic changes in the hair cell stereocilia were revealed by electron microscopy. The number of nuclear collapses was counted in cells stained for DNA and F-actin to evaluate the degree of cell death in the organ of Corti. Changes in spiral ganglion cell (SGC) neuron number were detected, whether or not progressive neuronal death occurred in the SGC. These studies showed that sporadic fusion of hair cells and the disappearance of hair cell stereocilia did not begin until 4 days after ischemia. On subsequent days, the loss of hair cells, especially inner hair cells (IHCs), and the degeneration of SGC neurons became apparent. Ten days after ischemia, the mean percentage cell loss of IHCs was 6.4% in the basal turn, 6.4% in the second turn, and 0.8% in the apical turn, respectively, and the number of SGC neurons had decreased to 89% of preischemic status. These results indicate that transient ischemia causes delayed hearing loss and cell death in the cochlea by day 7 after ischemia.

Tinnitus suppression with threshold and subthreshold sound stimuli

In this preliminary report, we present the results from our investigation of 34 tinnitus patients for tinnitus suppression with frequency-specific sound stimuli within the auditory spectrum. Of this number, 22 (64.7%) experienced suppression, 5 (14.7%) had partial suppression, and 7 (20.6%) were nonresponders. Suppression of peripheral tinnitus may result when mechanosensitive outer hair cells are recruited by sound stimuli that can remain at subthreshold level. The suppression mechanism is possibly explained by the electromodel of the auditory system. This physiological model could be the basis of tinnitus suppression therapy in which a low-intensity, frequency-specific and tinnitus-suppressing sound stimulus is introduced instead of a wide-band masking noise.

Morphology, GluR1 and GRIP-C localization differ in octopus cells of C57BL6 and B6Cast mice

The C57BL6 mouse (B6) is homozygous for the gene for age-related hearing loss (ahl/ahl) and shows normal adult-like hearing before subtle changes in hearing begin at about 30 days of age. The B6Cast mouse is congenic to B6, having the wild type allele for normal hearing from Castaneous Ei on a B6 background. It has normal hearing throughout most of its lifespan. This study characterized the morphology of octopus cell (OC) somata in the posterior-ventral cochlear nucleus and of synaptic terminals on the OC somata in 8-week-old B6 and B6Cast mice, and the immunolocalization of antibodies to GluR1 (glutamate receptor subunit 1) and GRIP-C (glutamate receptor interacting protein-C terminus). By 8 weeks of age there are significant changes in the morphology of OCs and synaptic terminals around their somata in B6 mice compared to B6Cast mice. The distribution of immunoreactivity for the proteins GluR1 and GRIP is also significantly different in B6 mice from that in B6Cast mice. The modest degenerative changes reported in some B6 outer hair cells of the basal turn at this age do not seem adequate to explain the major changes observed in most OCs at a time when physiological studies show that many measures of the animals' hearing are still near normal. The findings suggest that changes in the alpha-amino-3-hydroxy-5-methyl-4-isoxazole glutamate receptor subunits and/or their binding proteins are part of the phenotype of ahl, and may reflect a role of the glutamate receptor pathway in the mechanism of ahl.

Hearing loss caused by progressive degeneration of cochlear hair cells in mice deficient for the Barhl1 homeobox gene

The cochlea of the mammalian inner ear contains three rows of outer hair cells and a single row of inner hair cells. These hair cell receptors reside in the organ of Corti and function to transduce mechanical stimuli into electrical signals that mediate hearing. To date, the molecular mechanisms underlying the maintenance of these delicate sensory hair cells are unknown. We report that targeted disruption of Barhl1, a mouse homolog of the Drosophila BarH homeobox genes, results in severe to profound hearing loss, providing a unique model for the study of age-related human deafness disorders. Barhl1 is expressed in all sensory hair cells during inner ear development, 2 days after the onset of hair cell generation. Loss of Barhl1 function in mice results in age-related progressive degeneration of both outer and inner hair cells in the organ of Corti, following two reciprocal longitudinal gradients. Our data together indicate an essential role for Barhl1 in the long-term maintenance of cochlear hair cells, but not in the determination or differentiation of these cells.

Fibroblast growth factors (FGFs) in the cochlear nucleus of the adult mouse following acoustic overstimulation

To see if fibroblast growth factors (FGFs) might function in the central changes following auditory overstimulation we tracked immunostaining in the cochlear nucleus of adult mice with monoclonal antibodies to FGFs (FGF-1, FGF-2) and FGF receptor. After exposure nearly all outer hair cells died, while inner hair cell and fiber loss were restricted to a region midway along the cochlear spiral. FGFs staining in the cochlear nucleus appeared in hypertrophied astrocytes in the regions of nerve fiber degeneration only. For normal-sized astrocytes there was an increase in the number stained and the intensity of staining across all frequency domains, but not in neurons. The increases were modest at 3-7 days, pronounced at 14 days, modest again by 30 days, and back to control levels by 60 days. FGF receptor staining of neurons occurred equally in all mice, exposed or not. The findings suggest that astrocytes play a role in the central responses to acoustic overstimulation and cochlear damage, involving FGFs, possibly regulating the activity of intrinsic neurons or signaling axonal growth. Not limited to regions of cochlear nerve fiber and inner hair cell loss, the changes in FGFs may represent a reaction to outer hair cell damage which spreads broadly across the central pathways.

Involvement of the mitochondrial permeability transition in gentamicin ototoxicity

Aminoglycosides may induce irreversible hearing loss in both animals and humans. In order to study the nature and mechanisms underlying gentamicin-induced cell death in the inner ear, the cochlear neurosensory epithelia were dissected from guinea pigs and incubated with 0.5-10 mM gentamicin. Concentration-dependent loss of cell viability was detected by the inability of damaged cells to exclude propidium iodide. Outer hair cells were most sensitive towards gentamicin toxicity, followed by inner hair cells whereas Deiters and Hensen cells were not affected by the gentamicin concentrations used. The iron chelators 2,2'-dipyridyl and deferoxamine provided partial protection against gentamicin-induced hair cell death while the calcium chelator Quin-2 AM had no effect. Gentamicin (0.5-1 mM) induced condensation of chromatin typical for apoptosis. Using the fluorescent dye tetramethyl-rhodamine methyl ester and laser scanning microscopy we could visualize a loss of the mitochondrial membrane potential in damaged outer hair cells about 1 h before cell death occurred. Cyclosporin A, an inhibitor of the mitochondrial permeability pore, provided partial protection against gentamicin toxicity. This strongly suggests an involvement of the mitochondrial permeability transition in gentamicin-induced apoptosis.

Blockade of c-Jun N-terminal kinase pathway attenuates gentamicin-induced cochlear and vestibular hair cell death

The ototoxic action of aminoglycoside antibiotics leading to the loss of inner ear hair cells is well documented. However, the molecular mechanisms are poorly defined. We have previously shown that in neomycin-exposed cochlear organotypic cultures, the c-Jun N-terminal kinase (JNK) pathway - associated with stress, injury and apoptosis - is activated in hair cells. We have shown that hair cell death can be attenuated by CEP-1347, an inhibitor of JNK signaling (). In the present study, we demonstrate that gentamicin-induced ototoxicity leads to JNK activation and apoptosis in the inner ear hair cells in vivo. We show that systemic administration of CEP-1347 attenuates gentamicin-induced decrease of auditory sensitivity and cochlear hair cell damage. In addition, CEP-1347 treatment reduces the extent of hair cell loss in the ampullary cristae after gentamicin intoxication. Particularly, the inner hair cells of the cochlea and type I hair cells of the vestibular organs are protected. Our previous data have shown that also acoustic overstimulation can cause apoptotic death of cochlear hair cells and that CEP-1347 can attenuate noise-induced hair cell loss. Thus, our results imply that activation of JNK cascade may be a common molecular outcome of cellular stress in the inner ear sensory epithelia and that attenuation of the lesion can be provided by inhibiting JNK activation.

Early development and degeneration of vestibular hair cells in bronx waltzer mutant mice

In bronx waltzer mouse mutants, inner hair cells die at an early stage in their development, from around 17.5 days of gestation onwards. In contrast, outer hair cells appear to develop normally. Vestibular hair cells also degenerate, but the earliest signs of vestibular abnormalities have not yet been described. We looked at prenatal and early postnatal stages of vestibular development by scanning electron microscopy in the mutants, and established that vestibular hair cells (types I and II) never reach beyond the middle stages of differentiation (at least up to P2) and instead show signs of degeneration. Thus, it appears that the bronx waltzer gene product is required for the continued survival and differentiation of inner and vestibular hair cells past a set point in their development.

Mechanisms of tinnitus

The generation of tinnitus is a topic of much scientific enquiry. This chapter reviews possible mechanisms of tinnitus, whilst noting that the heterogeneity observed within the human population with distressing tinnitus means that there may be many different mechanisms by which tinnitus can occur. Indeed, multiple mechanisms may be at work within one individual. The role of the cochlea in tinnitus is considered, and in particular the concept of discordant damage between inner and outer hair cells is described. Biochemical models of tinnitus pertaining to the cochlea and the central auditory pathway are considered. Potential mechanisms for tinnitus within the auditory brain are reviewed, including important work on synchronised spontaneous activity in the cochlear nerve. Whilst the number of possible mechanisms of tinnitus within the auditory system is considerable, the identification of the physiological substrates underlying tinnitus is a crucial element in the design of novel and effective therapies.

Anatomical, metabolic and genetic aspects of age-related hearing loss in mice

Because of their short lifespan and genetic homogeneity, mice can provide valuable insights into the biological basis of age-related hearing loss. In C57BL/6 mice, hair cell loss begins around 1-2 months of age and progresses rapidly along a base-to-apex gradient, whereas CBA mice show relatively little hair cell loss until late in life. This anatomical difference is reflected in dehydrogenase histochemistry, an indirect measure of aerobic energy metabolism. A small, but significant, decrease in hair cell dehydrogenase staining occurred in CBA mice between 1.5 and 18 months of age. Significantly, dehydrogenase levels in 1.5-month C57 mice were substantially lower than in CBA mice of any age. Thus, deficits in aerobic energy metabolism presage degeneration of the hair cells. The superoxide radical, O2*-, a normal byproduct of cellular metabolism, is potentially toxic and can cause cellular damage if it is not inactivated by superoxide dismutases. Cytosolic copper/zinc superoxide dismutase (SOD1) is highly expressed in the cochlea. Knockout mutant mice with a single (HET) or double deletion (KO) of the Sod1 gene coding for SOD1 showed greater age-related losses than wild-type (WT) mice. KO mice had the worst hearing, WT the best, and HETs were intermediate. KOs exhibited considerably greater hair cell loss than WT mice; however, losses in KOs were only slightly greater than in HETs. KO mice showed significantly greater loss of spiral ganglion cells and nerve fibers than WT mice. These results indicate that SOD1 and O2*- play important roles in age-related hearing loss. Intervention strategies targeting O2*- may reduce age-related hearing loss.

Relationship between insult intensity and mode of hair cell loss in the vestibular system of rats exposed to 3,3'-iminodipropionitrile

A variety of stimuli cause sensory hair cell loss in the mammalian inner ear. This loss occurs by several differing processes, the significance of which remains undetermined. This study examines the relationship between the intensity of the damaging stimulus and the mode of hair cell loss found in the vestibular sensory epithelia of the rat. The ototoxin 3,3'-iminodipropionitrile (IDPN) was administered to rats at three different intoxication rates: acute exposure to high doses, repeated exposure to intermediate doses, and subchronic exposure to low doses. The morphology of the vestibular epithelia was examined by light microscopy and by scanning and transmission electron microscopy (SEM and TEM). In addition, DNA fragmentation in the epithelia was assessed by terminal deoxynucleotidyl transferase (tdt)-dUTP-nick-end-label (TUNEL). One day after acute IDPN, necrosis of hair cells was observed. However, at day 4 with this dose, and 1 and 4 days after repeated exposure, apoptotic figures and positive TUNEL labeling predominated. Subchronic IDPN resulted in a slowly evolving extrusion of basically intact hair cells in the crista and utricle. The data demonstrate that extrusion is a major mechanism of hair cell demise in mammals, that necrosis, apoptosis, and extrusion form a continuum of modes of hair cell loss, and that the intensity of the damaging stimulus determines the prevalence of each mode: Necrosis was most evident when the intensity was at its highest, whereas extrusion predominated when the intensity was at the lowest end of the scale.

Galectine-1 expression in cochleae of C57BL/6 mice during aging

Presbycusis is a progressive hearing loss related to the aging process associated with auditory receptor degeneration. Adult animals exhibited galectine-1 (GAL-1) expression in epithelial cells of the inner and outer spiral sulci and Hensen cells, which was absent from the sensory and supporting cells. The progressive age-related degeneration of auditory receptor was analyzed using an anti-GAL-1 monoclonal antibody on cochleae of C57BL/6 mice (1-24 months old). Mice < or = 9 months old showed adult and healthy auditory receptors, with a similar GAL-1 expression along the time. Conversely, animals > 1 year old exhibited a sensory degeneration at the cochleae basal coil that progressively reached the middle coil. Older animals (18-24 months old) showed a single layer epithelium that replaced the auditory receptor. The age-related degenerative process of the auditory receptor involves sensory but also supporting cells, which are both substituted by epithelial non-specialized cells.

Hypothermia reduces glutamate efflux in perilymph following transient cochlear ischemia

The effect of hypothermia on ischemic injury of the cochlea in gerbils was studied with particular regard to glutamate efflux in the perilymph. Under normothermic conditions interruption of the blood supply to the cochlea for 15 min caused a remarkable elevation of the compound action potential (CAP) threshold, and an increase in perilymphatic glutamate. The CAP threshold recovered to some extent with reperfusion, but not to preischemic levels. CAP thresholds, under hypothermic conditions and with reperfusion, recovered promptly to near pre-ischemic levels, while glutamate concentration did not change. These results, together with electron microscopy studies, suggest that hypothermia prevents hearing loss primarily through reduction of glutamate efflux at the synopses between inner hair cells and primary afferent auditory neurons.