Deafness Due to Degeneration of Cochlear Neurons in Caspase-3-Deficient Mice

Mice that lack caspase-3, which functions in apoptosis, were generated by gene targeting and shown to undergo hearing loss. The ABR threshold of the caspase-3(-/-) mice was significantly elevated compared to that of caspase-3(+/+) mice at 15 days of age and was progressively elevated further by 30 days. Distortion product otoacoustic emissions were not detectable in caspase-3(-/-) mice at 15 days of age. Caspase-3(-/-) mice exhibited marked degeneration of spiral ganglion neurons and a loss of inner and outer hair cells in the cochlea at 30 days of age, although no such changes were apparent at 15 days. The degenerating neurons manifested features, including cytoplasmic vacuolization, distinct from those characteristic of apoptosis. Spiral ganglion neurons and cochlear hair cells thus appear to require caspase-3 for survival but not for initial development. The mapping of both the human caspase-3 gene and the locus responsible for an autosomal dominant, nonsyndromic form of hearing loss (DFNA24) to chromosome 4q35 suggests that the caspase-3(-/-) mice may represent a model of this human condition.

Selective inner hair cell loss in premature infants and cochlea pathological patterns from neonatal intensive care unit autopsies

BACKGROUND

Deafness and handicapping sensorineural hearing impairment occur frequently in neonatal intensive care unit survivors for unknown reasons.

PATIENTS AND METHODS

Hearing was tested early and repeatedly in neonatal intensive care unit patients with an auditory brainstem response (ABR) screener. The temporal bones of 15 nonsurvivors (30 ears) were fixed promptly (average, 5 hours) after death for histological evaluation.

RESULTS

Among these patients, 12 failed the ABR screen bilaterally, 1 passed unilaterally, and 2 passed bilaterally. Cochlear histopathologic conditions that could contribute to hearing loss included bilateral selective outer hair cell loss in 2 patients, bilateral selective inner hair cell loss in 3 (all premature), and a combination of both outer and inner hair cell loss in 2. Other hair cell abnormalities were noted; the 2 infants who had passed the ABR screen demonstrated normal histological features. Neuronal counts were normal.

CONCLUSIONS

Auditory brainstem response failure among these neonatal intensive care unit infants who died was extremely common in part owing to an unexpected histological alteration, selective inner hair cell loss among premature newborns, that should be detectable uniquely by the ABR testing method. Additional histological patterns suggest more than one cause for neonatal intensive care unit hearing loss. Hair cell loss patterns seem frequently compatible with in utero damage.

Hypothermia prevents hearing loss and progressive hair cell loss after transient cochlear ischemia in gerbils

The effects of hypothermia on ischemia-reperfusion injury of the cochlea were studied in gerbils. Hearing was assessed by sequentially recording compound action potentials 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 the dye Hoechst 33342. Ischemic insult was applied to the animals by occluding the bilateral vertebral arteries for 15 min under normothermic or hypothermic (rectal temperature 32 degrees C) conditions. Interruption of the blood supply to the cochlea caused a tremendous increase in the compound action potential threshold, which usually recovered to some extent with reperfusion. In the ischemia/normothermic group, the threshold did not return to the pre-ischemic level. The average increase in the threshold seven days after ischemia was 20.0 dB. Histologically, the hair cell loss increased gradually until four days after the ischemic insult. On the seventh day, the mean loss of inner and outer hair cells at the basal turn was 31.1 % and 2.4 %, respectively. In the ischemia/hypothermic group, the threshold returned to the pre-ischemic level within 30 min after reperfusion and remained stable thereafter. The mean loss of inner and outer hair cells on the seventh day was 0.1 % and 0.2 %, respectively. These results indicate that hypothermia can prevent inner ear damage, which otherwise occurs after transient ischemia of the cochlea.

Caspase-3-deficiency induces hyperplasia of supporting cells and degeneration of sensory cells resulting in the hearing loss

Caspase-3 is one of the cystein proteases that play essential roles in programmed cell death. As such, brain development is profoundly affected by caspase-3-deficiency, resulting in hyperplasia and abnormal cell organization (Kuida et al., Nature 1996;384:368-372). In the present study, we used caspase-3 (-/-) mice to show that caspase-3 deficiency results in severe hearing loss, hyperplasia of supporting cells and degeneration of sensory hair cells. The greater epithelial ridge, a remnant of the primordial organ of Corti, persists throughout all of the turns of cochlea in 2-week-old caspase-3 (-/-) mice, which indicates that the morphology of the cochlea is immature. The number of border cells, that develop from the greater epithelial ridge and are one of the supporting cells of the inner hair cell, increase significantly in both 2- and 5-week-old caspase-3 (-/-) mice. On the other hand, abnormal fused stereocilia can be seen in both 2- and 5-week-old caspase-3 (-/-) mice, and disarrangement and loss of sensory hair cells are observed in 5-week-old caspase-3 (-/-) mice. Taken together, both hyperplasia and degeneration occur simultaneously in the inner ear of the caspase-3 (-/-) mice, suggesting that caspase-3-dependent apoptosis is necessary for the development and formation of a properly functioning auditory system in mammals.

NeuroD-null mice are deaf due to a severe loss of the inner ear sensory neurons during development

A key factor in the genetically programmed development of the nervous system is the death of massive numbers of neurons. Therefore, genetic mechanisms governing cell survival are of fundamental importance to developmental neuroscience. We report that inner ear sensory neurons are dependent on a basic helix-loop-helix transcription factor called NeuroD for survival during differentiation. Mice lacking NeuroD protein exhibit no auditory evoked potentials, reflecting a profound deafness. DiI fiber staining, immunostaining and cell death assays reveal that the deafness is due to the failure of inner ear sensory neuron survival during development. The affected inner ear sensory neurons fail to express neurotrophin receptors, TrkB and TrkC, suggesting that the ability of NeuroD to support neuronal survival may be directly mediated through regulation of responsiveness to the neurotrophins.

Succinic dehydrogenase histochemistry as an early marker for hair cell pathology

Density measurements of succinic dehydrogenase (SDH) activity were obtained from the inner and outer hair cells on surface preparations obtained from the guinea pig cochlea. Guinea pigs were exposed to noise (3.85 kHz, 120 dB SPL, 22.5 min) and sacrificed 0, 4 or 24 h after the exposure. By 4 h after exposure, the first- and second-row outer hair cells already demonstrated an altered SDH activity. By 24 h after exposure, a significant decrease in SDH staining in both the inner and outer hair cells at a distance of 10-12 mm from the cochlear apex was demonstrated. After a 1-month recovery period, scanning electron microscopy confirmed the main lesion site to be at a distance of 10-12 mm. In addition, Hensen's cells (supporting cells) at a distance of 10-12 mm from the apex were intensely stained by SDH after noise exposure, indicating an increase in oxidative metabolism. SDH staining in the Hensen's cells from the unexposed cochleae was not found. In conclusion, our findings suggest that the early use of SDH histochemistry can predict later permanent damage to the organ of Corti.

Mutations in Cdh23, encoding a new type of cadherin, cause stereocilia disorganization in waltzer, the mouse model for Usher syndrome type 1D

Mouse chromosome 10 harbors several loci associated with hearing loss, including waltzer (v), modifier-of deaf waddler (mdfw) and Age-related hearing loss (Ahl). The human region that is orthologous to the mouse 'waltzer' region is located at 10q21-q22 and contains the human deafness loci DFNB12 and USH1D). Numerous mutations at the waltzer locus have been documented causing erratic circling and hearing loss. Here we report the identification of a new gene mutated in v. The 10.5-kb Cdh23 cDNA encodes a very large, single-pass transmembrane protein, that we have called otocadherin. It has an extracellular domain that contains 27 repeats; these show significant homology to the cadherin ectodomain. In v(6J), a GT transversion creates a premature stop codon. In v(Alb), a CT exchange generates an ectopic donor splice site, effecting deletion of 119 nucleotides of exonic sequence. In v(2J), a GA transition abolishes the donor splice site, leading to aberrant splice forms. All three alleles are predicted to cause loss of function. We demonstrate Cdh23 expression in the neurosensory epithelium and show that during early hair-cell differentiation, stereocilia organization is disrupted in v(2J) homozygotes. Our data indicate that otocadherin is a critical component of hair bundle formation. Mutations in human CDH23 cause Usher syndrome type 1D and thus, establish waltzer as the mouse model for USH1D.

A defect in harmonin, a PDZ domain-containing protein expressed in the inner ear sensory hair cells, underlies Usher syndrome type 1C

Usher syndrome type 1 (USH1) is an autosomal recessive sensory defect involving congenital profound sensorineural deafness, vestibular dysfunction and blindness (due to progressive retinitis pigmentosa)1. Six different USH1 loci have been reported. So far, only MYO7A (USH1B), encoding myosin VIIA, has been identified as a gene whose mutation causes the disease. Here, we report a gene underlying USH1C (MIM 276904), a USH1 subtype described in a population of Acadian descendants from Louisiana and in a Lebanese family. We identified this gene (USH1C), encoding a PDZ-domain-containing protein, harmonin, in a subtracted mouse cDNA library derived from inner ear sensory areas. In patients we found a splice-site mutation, a frameshift mutation and the expansion of an intronic variable number of tandem repeat (VNTR). We showed that, in the mouse inner ear, only the sensory hair cells express harmonin. The inner ear Ush1c transcripts predicted several harmonin isoforms, some containing an additional coiled-coil domain and a proline- and serine-rich region. As several of these transcripts were absent from the eye, we propose that USH1C also underlies the DFNB18 form of isolated deafness.

A test for the diagnosis of dead regions in the cochlea

Hearing impairment may sometimes be associated with complete loss of inner hair cells (IHCs) over a certain region of the basilar membrane. We call this a 'dead region'. Amplification (using a hearing aid) over a frequency range corresponding to a dead region may not be beneficial and may even impair speech intelligibility. However, diagnosis of dead regions is not easily done from the audiogram. This paper reports the design and evaluation of a method for detecting and delimiting dead regions. A noise, called 'threshold equalizing noise' (TEN), was spectrally shaped so that, for normally hearing subjects, it would give equal masked thresholds for pure tone signals at all frequencies within the range 250-10,000 Hz. Its level is specified as the level in a one-ERB (132 Hz) wide band centred at 1000 Hz. Measurements obtained from 22 normal-hearing subjects and TEN levels of 30, 50 and 70 dB/ERB confirmed that the signal level at masked threshold was approximately equal to the noise level/ERB and was almost independent of signal frequency. Masked thresholds were measured for 20 ears of 14 subjects with sensorineural hearing loss, using TEN levels of 30, 50 and 70 dB/ERB. Psychophysical tuning curves (PTCs) were measured for the same subjects. When there are surviving IHCs corresponding to a frequency region with elevated absolute thresholds, a signal in that frequency region is detected via IHCs with characteristic frequencies (CFs) close to that region. In such a case, threshold in the TEN is close to that for normal-hearing listeners, provided that the noise intensity is sufficient to produce significant masking. Also, the tip of the PTC lies close to the signal frequency. When a dead region is present, the signal is detected via IHCs with CFs different from that of the signal frequency. In such a case, threshold in the TEN is markedly higher than normal, and the tip of the PTC is shifted away from the signal frequency. Generally, there was a very good correspondence between the results obtained using the TEN and the PTCs. We conclude that the measurement of masked thresholds in TEN provides a quick and simple method for the diagnosis of dead regions.

Auditory deprivation of the central auditory system resulting from selective inner hair cell loss: animal model of auditory neuropathy

Auditory neuropathy is often characterized by normal thresholds, present otoacoustic emissions, poor speech discrimination, absent acoustic reflexes, absent or abnormal auditory brainstem response waveform, but normal late cortical potential. This paper describes an animal model that has many characteristics of auditory neuropathy. Chinchillas can be deprived of a significant portion of the neural inputs to the central auditory system by administering carboplatin, an antineoplastic agent that selectively destroys inner hair cells (IHCs) and type I auditory nerve fibers. Selective IHC loss has no effect on distortion product otoacoustic emissions or the cochlear microphonic potential, implying normal outer hair cell function. However, selective IHC loss causes the amplitude of the compound action potential to decrease in proportion to the degree of IHC loss. However, the threshold of the CAP shows little increase with mild to moderate IHC loss. Acoustically responsive auditory nerve fibers in ears with mild to moderate IHC loss have normal thresholds and tuning curves with narrowly tuned tips. Although the central auditory pathway is deprived of much of its sensory inputs, the amplitude of the local field potential in the auditory cortex was normal or enhanced, while those from the inferior colliculus were slightly reduced. The results are related to those of a patient with auditory neuropathy.

Characterization of the ototoxicity of difluoromethylornithine and its enantiomers

Difluoromethylornithine (DFMO) is an irreversible inhibitor of ornithine decarboxylase (ODC), the essential enzyme in mammalian polyamine biosynthesis (Pasic et al., 1997, Arch. Otolaryngol. Head Neck Surg. 123[12], 1281-1286). This cancer chemotherapeutic agent has significant ototoxic potential. Because the DFMO enantiomers differ in their ability to block ODC, the present study was designed to compare the ototoxic potential of each enantiomer with the racemic form of this drug for the rat and guinea pig. Determining differential ototoxicity of the enantiomers is one preliminary step in determining the optimal form of DFMO to use in human cancer chemotherapy. Daily intubation with D,L-DFMO does not produce any auditory dysfunction in rats with doses between 200 mg/kg/day and 1. 2 g/kg/day for up to 8 weeks, despite the fact that doses of 800 and 1200 mg/kg/day depressed body weight gain. In contrast to the data observed in rats, substantial ototoxicity was observed when guinea pigs were injected ip with doses of D,L-DFMO between 500 mg/kg/day and 1 g/kg/day. D,L-DFMO produced loss of compound action potential sensitivity, but not of cochlear microphonic amplitude. This finding correlated with histological data revealing loss of both outer and inner hair cells in the cochlea with inner more affected than outer hair cells, particularly in the basal turn. Higher exposure doses (2-3 g/kg/day) resulted in significant general toxicity including impaired growth and some mortality. When the enantiomers were evaluated in the guinea pig, it was found that 1 g/kg/day D-DFMO did not produce any significant hearing impairment, whereas 1 g/kg/day of the L-enantiomer of DFMO generated a threshold shift that surpassed that of 1 g/kg/day of the D,L-DFMO treatment.

Structural/audiometric correlations in a human inner ear with noise-induced hearing loss

A morphological analysis was performed on a human cochlea removed during skull base surgery. The patient experienced a noise-induced hearing loss following 30 years of mechanical exposure. The tissue was processed according to the block surface technique and the organ of Corti, osseous spiral lamina and spiral ganglion were analyzed at different levels. There was a circumscribed lesion approx. 10 mm from the round window extending to about 13 mm. At this site, the dominant pathological feature was the loss of outer hair cells that was comprehensive in the centermost area and partial in the peripheral region of the damage. The degradation of inner hair cells was less severe with signs of cell atrophy yet with limited loss. Outer pillar cells were often collapsed leading to deformation of the acoustic ridge. The Deiters cells were often present and physically interactive with remaining nerve fibers. In the reticular lamina, surgical manipulation and dissection resulted in tears which may be attributed to a reduction of intercellular strength between cells. In the damaged area, there was a 45% loss of myelinated nerve fibers measured at the osseous spiral lamina. Pathological changes could not be observed in the spiral ganglion with certainty although the type II cells innervating the outer hair cells were often difficult to discern.

Retinal degeneration but not obesity is observed in null mutants of the tubby-like protein 1 gene

The tub gene is a member of a small, well conserved neuronal gene family of unknown function. Mutations within this gene lead to early-onset blindness and deafness, as well as late-onset obesity and insulin resistance. To test the hypothesis that mutations within other members of this gene family would lead to similar phenotypes as observed in tubby mice, and hence have similar functional properties, we have generated null mutants of the tubby-like protein ( Tulp ) 1 gene by homologous recombination. Similarly to tubby mice, Tulp1 (-/-)mice exhibit an early-onset retinal degeneration with a progressive, rapid loss of photoreceptors, further supporting the notion that previously identified mutations within the human TULP1 gene are indeed causative of retinitis pigmentosa. However, in contrast to tubby mice, Tulp1 (-/-)mice exhibited normal hearing ability and, surprisingly, normal body weight despite the fact that both TUB and TULP1 are expressed in the same neurons within the hypothalamus in areas known to be involved in feeding behavior and energy homeo stasis. However, TUB and TULP1 show a distinctly different staining pattern in the nucleus of these neurons, perhaps explaining the difference in body weight between the Tulp1 (-/-)and tubby mutant mice.

Histopathological differences between temporary and permanent threshold shift

The structural changes associated with noise-induced temporary threshold shift (TTS) were compared to the damage associated with permanent threshold shift (PTS). A within-animal paradigm involving survival-fixation was used to minimize problems with data interpretation from interanimal variability in response to noise. Auditory brainstem response thresholds for clicks and tone pips were determined pre- and 1-2 h post-exposure in 11 chinchillas. The animals were exposed for 24 h to an octave band of noise with a center frequency of 4 kHz and a sound pressure level of 86 dB. Three animals (0/0-day) had both cochleas terminal-fixed 2-3 h post-exposure. Two animals (27/27-day) had threshold shifts determined every other day for 1 week, every week thereafter, and underwent terminal-fixation of both cochleas 27 days after exposure. Six animals (0/n-day) had threshold shifts determined in both ears upon removal from the noise; their left cochlea was then survival-fixed 2-3 h post-exposure. Threshold shifts were determined in their right ear every 2-3 days until their hearing either returned to pre-exposure values or stabilized at a reduced level at which time their right cochlea was terminal-fixed (4-13 days post-exposure). All cochleas were prepared as plastic-embedded flat preparations. Missing hair cells were counted and supporting cells and nerve fibers were evaluated throughout the organ of Corti using phase-contrast microscopy. Post-exposure, all animals had moderate TTSs in their left and right ears which averaged 43 dB for 4-12 kHz. In the 0/0-day animals, the only abnormality which correlated with TTS was a buckling of the pillar bodies. In the 0/n-day animals, their left cochlea (survival-fixed 2-3 h post-exposure) had outer hair cell (OHC) stereocilia which were not embedded in the tectorial membrane in the region of the TTS whereas OHC stereocilia were embedded in the tectorial membrane throughout the cochleas of non-noise-exposed, survival-fixed controls. Three of six right cochleas (terminal-fixed 4-13 days post-exposure) from the 0/n-day animals developed a PTS and two of these cochleas had focal losses of inner and outer hair cells and afferent nerve fibers at the corresponding frequency location. The other cochlea with PTS had buckled pillars in the corresponding frequency region. These results suggest that with moderate levels of noise exposure, buckling of the supporting cells results in an uncoupling of the OHC stereocilia from the tectorial membrane which results in a TTS. The mechanisms resulting in TTS appear to be distinct from those that produce permanent hair cell damage and a PTS.

Medial olivocochlear efferent terminals are protected by sound conditioning

Synaptophysin immunoreactivity was used as a marker for the olivocochlear efferent system that innervates the outer hair cells of the cochlea. An intense noise exposure at either 6.3 kHz or 1.0 kHz caused a significant reduction in anti-synaptophysin immunoreactivity within the 8-6 mm or 14-11 mm distance from the round window, respectively. In the region of the main lesion, the reduction in synaptophysin immunoreactivity for both the 6.3 and 1.0 kHz exposures correlated well with outer hair cell loss. In regions peripheral to the main lesion, some remnants of efferent nerve endings could remain even when their associated outer hair cells were missing. Pre-treatment with a low level sound conditioner (either at 6.3 tone or 1.0 kHz) effectively reduced the efferent and outer hair cell pathology induced by the 6.3 and 1.0 kHz intense noise exposures, respectively. The results demonstrate the feasibility of using anti-synaptophysin immunoreactivity as an effective means of quantifying pathological alterations to the medial cochlear efferent terminals throughout the cochlea. Furthermore, the results show that sound conditioning significantly reduces damage to the efferent terminals.

Cytoskeletal integration in a highly ordered sensory epithelium in the organ of Corti: reponse to loss of cell partners in the Bronx waltzer mouse

This report is concerned with control of cell shaping, positioning, and cytoskeletal integration in a highly ordered cochlear neuroepithelium. It is largely based on investigations of events that occur during abnormal morphogenesis of the organ of Corti in the Bronx waltzer (bv/bv) mutant mouse. The organ's sensory hair cells and adjacent supporting cells ordinarily construct a spatially elaborate and supracellularly integrated cytoskeletal framework. Large microtubule bundles are connected to cytoskeletal components in neighbouring cells by actin-containing meshworks that link them to substantial arrays of adherens junctions. In bv/bv mice, degeneration and loss of most inner hair cells and outer pillar cells occurs during organ development. These cells flank each side of a row of inner pillar cells that respond by upregulating assembly of their actin-containing meshworks. This only occurs in surface regions where they no longer contact cell types involved in construction of the cytoskeletal framework. The meshworks are larger and exhibit a more extensive sub-surface deployment than is normally the case. Hence, assembly of intercellular cytoskeletal connecting components can proceed without contact with appropriate cell neighbours but termination of assembly is apparently subject to a negative feedback control triggered by successful completion of intercellular connection with the correct cell neighbours. In addition, inner pillar cells compensate for loss of cell neighbours by interdigitating and overlapping each other more extensively than is usually the case to increase opportunities for generating adherens junctions. Certain adherens junctions in the organs of +/+ and bv/bv mice exhibit features that distinguish them from all previously described cell junctions. The dense plaques on their cytoplasmic faces are composed of aligned ridges. We suggest that they are called ribbed adherens junctions. Perturbations of cell shaping and positioning indicate that loss of inner hair cells is the primary consequence of the bv mutation. Most of the other abnormalities can be understood in terms of a secondary sequence of morphogenetic aberrations (precipitated by loss of inner hair cells). These aberrations provide new information about the ways in which supporting cells help to control hair cell positioning.

Cytotoxic changes in hair cells secondary to pneumococcal middle-ear infection

To determine whether the bacterial toxins associated with otitis media could induce morphologic changes in the organ of Corti, we inoculated the middle-ear cavities of healthy guinea pigs with either Streptococcus pneumoniae or sterile saline and then examined the organ of Corti histologically at 1, 2, and 3 weeks postinoculation. We found that the outer hair cells (OHCs) in the infected ears underwent several changes that were dependent on both the length of time following inoculation and also the position of the OHCs in the cochlea. At 2 weeks postinoculation, 7.0 to 20% of the OHC nuclei from the infected animals became very swollen, with the most significant swelling occurring in the basal turn. At 3 weeks postinoculation, 2.5 to 3.5% of the OHCs were missing in the infected animals, with the most significant loss occurring in the basal and middle turns. These results suggest that bacterial otitis media can produce cytotoxic changes in the cochlea. These changes may be a clinically significant factor in the temporary and permanent sensorineural hearing loss that has been associated with bacterial otitis media.

Cu/Zn SOD deficiency potentiates hearing loss and cochlear pathology in aged 129,CD-1 mice

Copper/zinc superoxide dismutase (Cu/Zn SOD) is a first-line defense against free radical damage in the cochlea and other tissues. To determine whether deficiencies in Cu/Zn SOD increase age-related hearing loss and cochlear pathology, we collected auditory brainstem responses (ABRs) and determined cochlear hair cell loss in 13-month-old 129/CD-1 mice with (a) no measurable Cu/Zn SOD activity (homozygous knockout mice), (b) 50% reduction of Cu/Zn SOD (heterozygous knockout mice), and (c) normal levels of Cu/Zn SOD (wild-type mice). ABRs were obtained by using 4-, 8-, 16-, and 32-kHz tone bursts. Cochleas were harvested immediately after testing, and separate counts were made of inner and outer hair cells. Compared with wild-type mice, homozygous and heterozygous knockout mice exhibited significant threshold elevations and greater hair cell loss. Phenotypic variability was higher among heterozygous knockout mice than among wild-type or homozygous knockout mice. Separate groups of wild-type and homozygous knockout mice were examined for loss of spiral ganglion cells and eighth nerve fibers. At 13 months of age, both wild-type and knockout mice had significantly fewer nerve fibers than did 2-month-old wild-type mice, with significantly greater loss in aged knockout mice than in aged wild-type mice. Thirteen-month-old knockout mice also had a significant loss of spiral ganglion cells compared with 2-month-old wild-type mice. The results indicate that Cu/Zn SOD deficiencies increase the vulnerability of the cochlea to damage associated with normal aging, presumably through metabolic pathways involving the superoxide radical.

Apoptosis of inner hair cells caused by laser ablation of their spiral ganglion neurons in cultures of the mouse organ of Corti

Laser beam ablation of spiral ganglion neurons was performed in seven organotypic cultures of the newborn mouse cochlea between 5 and 8 days in vitro, with a recovery period of from 18 hours to 3 days. Direct somatic injury (laser or mechanical) inflicted on hair cells does not necessarily cause their death; many of them survive, repair damage and re-establish their neurosensory connections. By contrast, laser irradiation and ablation of their afferent spiral ganglion neurons causes a most spectacular degeneration of sensory cells within 18-48 hours after the insult. Ultrastructurally, the degenerated hair cells-characteristically the inner hair cells-display "dark-cell vacuolar degeneration" that combines the signs of apoptotic death (the peripheral condensation of nuclear chromatin and nuclear pyknosis) with signs of cell edema, vacuolization and necrosis. The ultimate condensation of the cytoplasm gives the dead cells a jet black appearance. The irradiated spiral ganglion neurons die displaying similar pathological characteristics. The extent and locus of inner hair cell degeneration correspond to that of ablated spiral ganglion neurons: ultimately the ablation of one neuron causes degeneration of a single inner hair cell within the closest radial segment of the afferent innervation. The elimination of spiral ganglion neurons by mechanical means does not affect hair cell survival. It is inferred that the laser pulse acts as a stimulus depolarizing the neuronal membrane of the spiral ganglion neurons and their radial fibers and causing the excitotoxic death of their synaptic sensory cells through excessive stimulation of the glutamatergic receptors. Reciprocal pre-and postsynaptic synapses between the afferent dendrites and inner hair cells in culture could possibly serve as entryways of the stimulus. The pathogenesis of this apparent transsynaptically-induced apoptotic death of inner hair cells will be further examined in culture.

Effects of BDNF and NT-3 on hair cell survival in guinea pig cochlea damaged by kanamycin treatment

The aim of this study was to determine whether neurotrophic factors such as brain derived neurotrophic factor (BDNF) and neurotrophin 3 (NT-3) would protect auditory hair cells from ototoxicity by aminoglycoside antibiotic. Twenty-seven Wistar guinea pigs were divided into three groups of nine animals each. BDNF and NT-3 (100 microg/ml) were delivered into the right scala tympani of guinea pig cochlea through a cannula-osmotic pump device. Artificial perilymph (AP) was used as control. Immediately after implantation of the device, each animal was given five successive doses of kanamycin (400 mg/kg). At 15, 30 and 60 days after infusion, surviving inner and outer hair cells were counted at each turn of every cochlea with a Philips 515 scanning electron microscope. Multiple comparison tests were carried out among the groups, using ANOVA and Dunnett T3/Tukey HSD. Protective effects of NT-3 on hair cells were observed at 30 and 60 days after kanamycin injection. BDNF had no protective effect on hair cells at 15 and 60 days, but some at 30 days. This study suggests that NT-3 and BDNF may protect against cochlear hair cell damage caused by kanamycin treatment. Possible mechanisms for the otoprotective effects were discussed. No single mechanism postulated can explain fully the results seen in this study. It is possible that the mechanisms act in concert to produce the observed effects, or there are as yet undiscovered mechanisms or secondary messengers responsible for the otoprotective effects.

Does electrical stimulation of deaf cochleae prevent spiral ganglion degeneration?

Thirty-six drug deafened guinea pigs were studied to determine how electrical stimulation of the cochlea affects spiral ganglion cell (SGC) survival. Animals were divided into two groups, extracochlear and intracochlear stimulation, and each group was further divided into four stimulus subgroups: no stimulation (implanted controls), the inferior colliculus electrically evoked potential (ICEEP) threshold-2 dB, ICEEP threshold+2 dB, and ICEEP threshold+6 dB. Stimuli consisted of 200 micros/phase charge balanced biphasic current pulses presented at 100 pulses per second using monopolar stimulation. Animals were stimulated 5 h/day, 5 days per week, for 8 weeks. The animals were then perfused and the cochleae serially sectioned at 4 microm saving every 8th section. We counted the number of intact SGCs, those containing a nucleus with chromatin, in each 20% segment of the cochlea and also measured SGC densities (number of neurons per mm2 of Rosenthal's canal). The number of surviving spiral ganglion neurons was not significantly different (P > 0.05) between the implanted and the unimplanted ears in any of the experimental groups. However, the spiral ganglion neuron densities were significantly elevated in the electrically stimulated ears (P < 0.001) but not in the implanted but not chronically stimulated ears (P > 0.05). We measured the volume of Rosenthal's canal in one subgroup (ICEEP threshold+2 dB) and found a decrease in this volume in the stimulated ear compared to the unstimulated ear (P < 0.01). These findings support the hypothesis that chronic monopolar electrical intracochlear or extracochlear stimulation is not a neurotrophic factor, increasing spiral ganglion neuron survival, but instead causes a narrowing of Rosenthal's canal that accounts for the increased spiral ganglion neuronal densities seen in the stimulated cochleae.

The effects of kanamycin injection into the fetal lamb cochlea

Mammalian auditory hair cells have minimal capacity for repair or regeneration after a variety of insults, including acoustic trauma and aminoglycoside exposure. Although fetal tissues have a greater potential for repair and regeneration than adult tissues, there have been no reported studies on fetal hair cell response to injury in mammals. The purpose of this research was to investigate the effects of local application of kanamycin on fetal lamb cochlear hair cells. Eleven fetal lambs in the early third trimester underwent kanamycin injection through the left round-window membrane. The right ear served as a control. Click-evoked compound action potentials (CAPs) were serially recorded in 8 fetuses. CAPs were observed in all control ears. None of the 8 kanamycin-injected ears had measurable CAPs on postoperative day 1. One kanamycin-injected ear demonstrated definite CAPs, beginning on postoperative day 6. Hair cells were found to be intact in 6 of 9 kanamycin-treated ears. Hair cells were missing only in animals that went into premature labor. The presence of intact hair cells despite the loss of measurable CAPs in kanamycin-perfused lamb cochleae was striking. This finding may indicate that the fetal auditory epithelium is relatively resistant to aminoglycoside injury or may be capable of prompt repair or regeneration. Further studies on the effects of aminoglycoside injury in the fetal cochleae seem to be warranted.

Intracochlear infusion of buthionine sulfoximine potentiates carboplatin ototoxicity in the chinchilla

The aim of this experiment was to determine if buthionine sulfoximine (BSO), an inhibitor of glutathione (GSH) synthesis, enhances the ototoxicity of carboplatin. Osmotic pumps were used to infuse BSO into the right cochleas of 12 adult chinchillas for 14 days. The left cochleas served as controls. Animals were assigned to three groups: a drug control group that did not receive carboplatin, a group that received a single dose of carboplatin (25 mg/kg i.p.), and a group that received a double dose of carboplatin (25 mg/kg i.p. x 2), with 4 days between injections. Carboplatin was administered after three days of BSO pre-treatment. Ototoxicity was assessed with evoked potentials recorded from electrodes implanted in the inferior colliculi (ICPs), distortion product otoacoustic emissions (DPOAEs), and cochleograms. BSO infusion itself caused no long-term functional or morphological changes. One of four animals treated with it single dose of carboplatin showed a significant loss of inner hair cells (IHCs), with greater loss in the BSO-treated ear. All animals in the double-dose carboplatin group showed marked differences between BSO-treated and control ears. Average IHC losses were 59% in BSO-treated ears vs. 18% in control ears. Moreover, BSO-treated ears sustained significantly greater outer hair cell (OHC) losses than control ears (37% vs. 2%, respectively). ICP and DPOAE response amplitudes were reduced slightly in BSO-treated ears relative to control ears, consistent with their greater hair cell loss. The results clearly show that BSO can enhance carboplatin ototoxicity in the chinchilla, supporting a role of GSH and reactive oxygen species in platinum ototoxicity.

Cisplatin ototoxicity and the possibly protective effect of alpha-melanocyte stimulating hormone

It is known that adrenocorticotrophic hormone (ACTH)-derived peptides, the so-called melanocortins, can reduce cisplatin-induced neurotoxicity. Recently, our group has found that cisplatin-induced ototoxicity can also be reduced or prevented by treatment with the synthetic melanocortin-like peptide, ORG 2766 (Hamers et al., 1994; De Groot et al., 1997). The present study was designed to investigate the possibly ameliorating effects of the physiologically more relevant naturally occurring neuropeptide alpha-melanocyte stimulating hormone (alpha-MSH) upon cisplatin ototoxicity and to compare its protective effects to those of ORG 2766. For eight consecutive days guinea pigs were treated with cisplatin at a concentration of either 1.5 mg/kg/day or 2 mg/kg/day. Animals were co-treated with either alpha-MSH (75 microg/kg/day), ORG 2766 (75 microg/kg/day), or a sham injection containing physiological saline. Electrocochleography and hair cell counts were performed. Treatment with 1.5 mg/kg/day cisplatin resulted in a large variability of the morphological and electrophysiological data, a variability that might have masked possible effects of ORG 2766 and alpha-MSH. Treatment with 2 mg/kg/day cisplatin caused less variable, severe reductions in the compound action potentials and cochlear microphonics combined with basal and middle-turn outer hair cell loss in five out of six animals. However, in the alpha-MSH co-treated groups, two out of six animals could be classified as normal, two animals as moderately affected and two animals as severely affected. In the ORG 2766 co-treated group we found three animals that were not affected and three animals that were severely affected. We conclude that the protective effects of alpha-MSH and ORG 2766 co-treatment are comparable and that alpha-MSH might be clinically useful in protecting against cisplatin-induced ototoxicity.

Abortive synaptogenesis as a factor in the inner hair cell degeneration in the Bronx Waltzer (bv) mutant mouse

The Bronx Waltzer (vb) mutation in the mouse results in the degeneration of most but not all of the primary auditory receptors, the inner hair cells, and their afferent neurons. We analyzed the ultrastructure of 94 inner hair cells in the intact postnatal mutant mouse and in neonatal cochleas in culture to understand the pathogenesis of hair cell death and to detect factors that may prevent it. The vb spiral neurons of the Bronx Waltzer display two distinctive features: some of them continue to divide mitotically for at least seven postnatal days, and the type I radial fibers that innervate inner hair cells display a deficiency in immunoexpression of GAD. The growing endings of spiral neurons converge around the inner hair cells or, in their absence, invade the outer hair cell region. Their profuse sprouting among inner spiral sulcus cells contributes to the characteristic ultrastructural picture of the bv cochlea. During the first three days after birth, 40% of the inner hair cells appear normal and innervated, 40% are mostly denervated and degenerating, and 20% are immature, with minimal or no neuronal appositions. However, in mutants 6 days and older only a few inner hair cells survive, and these show either normal or superfluous afferent innervation and axosomatic GABAergic efferent innervation. Degeneration of inner hair cells begins with a distention of the nuclear envelope and the ribosomal endoplasmic reticulum. The outer nuclear membrane eventually breaks, and exudate fills the cell interior. The cellular edema leads to cell death. We propose that success or failure in synaptic acquisition is a decisive factor in the survival or decline of the mutant inner hair cells. We also suggest that the developmental delay in maturation of the spiral ganglion neurons (type I) and the failure in their synaptogenesis may be caused by an impairment in neurotrophin (NT3/BDNF) synthesis by their mutant receptor cells.

Increased otoacoustic-emission amplitude secondary to cochlear lesions

OBJECTIVE

The measurements of transient evoked otoacoustic emissions and distortion-product otoacoustic emissions are being used increasingly, both as an objective hearing test clinically, and as a research tool to investigate the micromechanical aspects of cochlear function. We hypothesized that localized damage in the apical or middle cochlear turns may have an influence on the micromechanics and the function of adjacent, apparently normal cochlea. For that purpose, we used an animal model of localized apical and middle-turn cochlear lesions.

METHOD

Extent of damage was assessed by scanning electron microscopy and the function of the damaged cochlea by change in the otoacoustic emission (OAE) levels.

RESULTS

We found that localized damage to the apical or middle turn may be accompanied by an increase in OAE measured from adjacent apparently normal cochlea.

CONCLUSION

Explanations to this phenomenon are suggested, and possible clinical associations such as to Meniere's disease and to sudden hearing loss are reviewed.

An animal model of auditory neuropathy

OBJECTIVE

We describe an animal model of auditory neuropathy in which subjects have extensive, scattered inner haircell loss but with a relatively intact outer haircell population.

DESIGN

Such a pattern of cochlear haircell damage can be produced in the chinchilla by treatment with the anticancer agent carboplatin.

RESULTS

In these subjects, otoacoustic emissions (OAEs) and cochlear microphonics remain normal while auditory brain stem evoked potential (ABR) thresholds are significantly elevated. However, in the same subjects, central auditory neurons (in the inferior colliculus) have response thresholds that are considerably lower (by up to 50 dB) than ABR thresholds. These findings parallel the characteristics of auditory neuropathy in humans, in which absent or abnormal ABRs are recorded in patients with only mild to moderate audiometric thresholds and preserved OAEs.

CONCLUSIONS

We suggest that scattered inner haircell lesions also can result from long-term cochlear hypoxia, and we propose that this is a likely candidate for the etiology of many types of auditory neuropathy in human subjects.

Degeneration of spiral ganglion cells in the chinchilla after inner hair cell loss induced by carboplatin

The anticancer drug carboplatin has been used to generate inner hair cell (IHC) lesions in the cochlea of chinchillas. This has provided a valuable model for the study of the relative roles of IHCs and outer hair cells (OHCs). In the present study, we examined the pathological and temporal relationships between the degeneration of the cochlear IHCs and type I spiral ganglion cells (SGCs). A single intravenous dose of 200 mg/m2 carboplatin produced extensive IHC loss with no apparent effect on the OHCs. The auditory brainstem response threshold was significantly elevated by 2 weeks following treatment and remained stable through 12 weeks. Elevated thresholds were well correlated with morphological lesions. On the other hand, the SGC population progressively decreased from 2 to 12 weeks after treatment, to about half of the control density values. A positive correlation existed between the density of SGC and the number of surviving IHCs. These results indicate that selective damage to IHCs causes a distinct loss of SGCs.

Effects of inner hair cell loss on inferior colliculus evoked potential thresholds, amplitudes and forward masking functions in chinchillas

The effects of outer hair cell (OHC) loss on evoked potential (EVP) thresholds, amplitudes and forward masking (FWM) functions have been fairly well characterized. In contrast, the effects of inner hair cell (IHC) losses are largely unknown, primarily due to the difficulty of producing selective IHC lesions. Recent studies have shown that IHCs of the chinchilla are preferentially damaged by the anticancer drug, carboplatin. In this study, we administered a single 100 mg/kg dose of carboplatin to four chinchillas, to examine the effects of IHC lesions on EVPs measured from the inferior colliculus (IC-EVPs). Thresholds and amplitude functions were measured for 0.25-16 kHz tone bursts, and FWM functions were measured at 1, 2 and 4 kHz, using masker probe intervals of 2, 5, 10, 20, 40 and 80 ms, before and 1-2 months after carboplatin treatment. Histology revealed IHC lesions ranging from approximately 15 to 90%, with virtually no loss of OHCs. Surprisingly, even massive IHC lesions were not associated with elevations of IC-EVP thresholds. IC-EVP amplitudes at suprathreshold levels were sometimes depressed, sometimes enhanced, and in some cases unchanged. IHC lesions increased susceptibility to FWM, particularly at intermediate (10-20 ms) masker-probe intervals, without significantly changing the overall time course of FWM. The results provide new perspectives on the contribution of IHCs to FWM, and on the ability of the central auditory system to adapt to a significant reduction of neural input from the cochlea.

Tone burst-evoked otoacoustic emissions in cats with acoustic overstimulation and anoxia

Transient evoked otoacoustic emissions (TEOAE) produced by a 2 kHz tone burst could be detected in 30 out of 37 ears (81% detectability) in 21 cats. The amplitude of tone burst-evoked TEOAE was saturated at a stimulus level between 45 and 50 dB SPL and the latency time of peak amplitude was 6.23 ms on average (5.53-7.28 ms). The effects of pure tone overstimulation and short-term anoxia on the tone burst-evoked TEOAE in cats were evaluated. A permanent detection threshold shift of the TEOAE was confirmed at 24 h and 1 week after the overstimulation at 125 dB SPL. In these cases, damaged first row outer hair cells and inner hair cells were observed over an average length of 3.3 mm (16% of the entire cochlear length) by scanning electron microscopy. These findings suggested that the TEOAE can detect localized cochlear hair cell damage. A temporary detection threshold shift of the TEOAE was observed after the overstimulation at 105 dB SPL, and the threshold shift recovered in 107.5 min on average. In the short-term anoxia trial, the TEOAE amplitude started to decrease 45-90 s after the anoxia and recovered completely when the duration of anoxia was under 1 min. However, the TEOAE amplitude did not recover pre-anoxia values (it remained below 80% of its initial value) after 5 min when the anoxia was over 2 min. These findings demonstrated that the detection threshold and amplitude of the TEOAE were also affected by metabolic changes of the cochlear hair cells. Tone burst-evoked TEOAE are useful for the evaluation of localized histological and functional damage of the cochlear hair cells.

Effects of noise on inferior colliculus evoked potentials and cochlear anatomy in young and aged chinchillas

Like many aging humans, the aging chinchilla tends to lose high-frequency sensitivity at a faster rate than low-frequency sensitivity. This feature, combined with its excellent low-frequency hearing, makes the chinchilla attractive as an animal model for studying the relationship between noise-induced hearing loss (NIHL) and age-related hearing loss (AHL). In the present study, we examined susceptibility to noise in 15 aged (10-15 years old) and 15 young chinchillas. Two levels of noise were used, with the aim of determining whether age-related differences exist in the magnitude and rate of recovery from temporary threshold shifts produced by a moderate-level (95 dB) noise exposure, or in susceptibility to permanent threshold shifts and cochlear damage caused by a high-level (106 dB) noise exposure. Thresholds and response amplitudes at 0.5, 1, 2, 4, 8 and 16 kHz were determined from evoked potentials recorded from the inferior colliculus. Cochlear histology was performed on animals exposed to high-level noise. The results suggest that older animals are equally vulnerable to moderate-level noise, but may be slightly more vulnerable to high-level noise. For moderate-level exposures, there appears to be a simple additive relationship (in dB) between AHL and NIHL. For high-level exposures, the relationship may be more complex.

[Quantitation of nerve fibers in habenula perforata in chinchilla]

OBJECTIVE

To examine the number of nerve fibers in the habenula perforata of the cochlea in normal and carboplatin-treated chinchillas.

METHODS

Three months following administration of carboplatin (100 mg/kg x 2), six carboplatin-treated animals, together with 6 controls, were sacrificed, prepared for semi-thin sectioning of the cochlea. The sections were stained with toluidine blue and basic fuchsin, examined under a light microscope and photographed.

RESULTS

In normal chinchillas, the numbers of the nerve fibers in each habenula opening varied across the cochlea, with 93.7 +/- 12.2(mean +/- s) fibers at the hook, 181.5 +/- 30.4 at the upper of basal turn, 129.7 +/- 27.8 at the second turn and 75.8 +/- 13.2 at the apical turn. In cochlear devoid of type I ganglion cells and inner hair cells as a result of carboplatin treatment, only one-seventh of the fiber population remained.

CONCLUSION

Approximately six-seventh of the nerve fibers in the habenula are type I afferent axons that innervate the inner hair cells. The remaining fibers are type II afferents and surviving efferents.

Quantitative measures reflect degeneration, but not regeneration, in the deafness mouse organ of Corti

The deafness mouse (dn/dn) is a well known model of hereditary deafness uncomplicated by behavioral and motor disturbances. The organ of Corti in this mouse develops a normal complement of sensory and supporting cell structures, yet animals homozygous for this gene never demonstrate any hearing capacity. They are profoundly deaf from birth. Soon after development, the organ of Corti rapidly degenerates, most sensory cells having vanished by 50 days of age. Published observations have suggested that apical regions of the organ of Corti may regenerate some supporting cell structures by 90 days of age. We have quantified changes in organ of Corti structure from 15 to 130 days of age using several different measures. Measures of peak height and total cross-sectional area. as well as a subjective rating scale, all demonstrate consistent degenerative changes during this time period. No evidence for regeneration of supporting or sensory cell structures is noted, although a surprising degree of variability is present in all regions of the organ of Corti which may account for previous claims.

Ototoxicity of sodium nitroprusside

Nitric oxide (NO) not only has normal physiological roles like vasodilation and neurotransmission in the living organism, it could also have possible neurodestructive effects under certain pathological conditions. The present study aimed to determine whether direct exposure of guinea pig cochlea to a NO donor like sodium nitroprusside (SNP), or a nitric oxide synthase (NOS) inhibitor like N(G)-nitro-L-arginine methyl ester (L-NAME), would cause damage to the auditory hair cells. A piece of gelfoam was placed on the round window of the right ear of adult albino guinea pigs. It was then soaked with 0.1 ml of SNP (3.4 microM), 0.1 ml of L-NAME (9.3 microM or 18.5 microM) or 0.1 ml of injection water, the vehicle used to dissolve the above chemicals. Twelve animals receiving SNP were perfused 1 day, 2, 3 and 7 days later, with three animals being used for each survival period. Six animals receiving L-NAME were allowed to survive up to 7 days before perfusion. Eight animals receiving injection water or 0.45% saline were used as controls. With the scanning electron microscope, the inner and outer hair cells were counted over a 1 mm length of the basilar membrane in each turn of every cochlea. The results showed that, in animals treated with L-NAME at both concentrations stated, no significant loss of either inner or outer hair cells was noted in any part of the cochlea studied. However, as early as 1 day after SNP treatment, a striking loss of inner and outer hair cells was observed in the three lower turns of the cochlea. Damage to the outer hair cells was extended to the apical turn with increasing survival period, but no significant loss of inner hair cells was evident in the apical turn at any of the survival periods studied. To rule out the possibility that the effects were due to the presence of cyanide, a metabolite of SNP, hydroxycobalamin was introduced into the scala tympani of three animals through a cannula-osmotic pump device during SNP treatment. There was no significant difference in the results between the groups with and without hydroxycobalamin infusion 7 days after SNP treatment. The present study suggests that an excessive production of NO in the inner ear could lead to extensive loss of hair cells.

R-phenylisopropyladenosine attenuates noise-induced hearing loss in the chinchilla

Reactive oxygen species, which are cytotoxic to living tissues, are thought to be partly responsible for noise-induced hearing loss. In this study R-phenylisopropyladenosine (R-PIA), a stable non-hydrolyzable adenosine analogue which has been found effective in upregulating antioxidant enzyme activity levels, was topologically applied to the round window of the right ears of chinchillas. Physiological saline was applied to the round window of the left ears (control). The animals were then exposed to a 4 kHz octave band noise at 105 dB SPL for 4 h. Inferior colliculus evoked potential thresholds and distortion product otoacoustic emissions (DPOAE) were measured and hair cell damage was documented. The mean threshold shifts immediately after the noise exposure were 70-90 dB at frequencies between 2 and 16 kHz. There were no significant differences in threshold shifts at this point between the R-PIA-treated and control ears. By 4 days after noise exposure, however, the R-PIA-treated ears showed 20-30 dB more recovery than saline-treated ears at frequencies between 4 and 16 kHz. More importantly, threshold measurements made 20 days after noise exposure showed 10-15 dB less permanent threshold shifts in R-PIA-treated ears. The amplitudes of DPOAE also recovered to a greater extent and outer hair cell losses were less severe in the R-PIA-treated ears. The results suggest that administration of R-PIA facilitates the recovery process of the outer hair cell after noise exposure.

Quantitative relationship of carboplatin dose to magnitude of inner and outer hair cell loss and the reduction in distortion product otoacoustic emission amplitude in chinchillas

The outer hair cells (OHCs) are thought to be the dominant source of distortion product otoacoustic emissions (DPOAEs) in the mammalian cochlea; however, little is known about the quantitative relationship between reduction in DPOAE amplitude and the degree of inner hair cell (IHC) and OHC loss. To examine this relationship, we measured the DPOAE input/output functions in the chinchilla before and after destroying the IHCs and/or OHCs with carboplatin. Low-to-moderate doses (38-150 mg/kg, i.p.) of carboplatin selectively destroyed some or all of the IHCs along the entire length of the cochlea while sparing the OHCs. Selective loss of all the IHCs had little effect on DPOAE amplitude as long as the OHCs were present. With high doses of carboplatin (200 mg/kg, i.p.), there was complete destruction of IHCs plus massive OHC loss that decreased from the base towards the apex of the cochlea. OHC loss resulted in a large decrease in DPOAE amplitude. DPOAE amplitude at 9.6 kHz decreased at the rate of 4.1 dB for every 10% loss of OHCs. At 7.2 and 4.8 kHz, DPOAE amplitude decreased 3.1 dB and 2.4 dB per 10% OHC loss, respectively. These results indicate that OHCs are the dominant source of DPOAEs.

Patterns of neural degeneration in the human cochlea and auditory nerve: implications for cochlear implantation

Although the identity of all the variables that may influence speech recognition after cochlear implantation is unknown, the degree of preservation of spiral ganglion cells is generally considered to be of primary importance. A series of experiments in our laboratories, directed at quantification of surviving spiral ganglion cells in the profoundly deaf, evaluation of the predictive value of a variety of clinical parameters, and the evaluation of the consequences of implantation in the inner ear, is summarized. Histologic study of the inner ears of patients who were deafened during life demonstrated that the cause of deafness accounted for 57% of the variability of spiral ganglion cell counts. Spiral ganglion cell counts were highest in individuals deafened by aminoglycoside toxicity or sudden idiopathic deafness and lowest in those deafened by postnatal viral labyrinthitis, congenital or genetic deafness, or bacterial meningitis. Study of the determinants of degeneration of the spiral ganglion revealed that degeneration is most severe in the basal compared with the apical turn and more severe when both inner and outer hair cells are absent. Unlike the findings in some experimental animal studies, no survival advantage of type II ganglion cells could be identified. There was a strong negative correlation between the degree of bony occlusion of the cochlea and the normality of the spiral ganglion cell count. However, even in specimens in which there was severe bony occlusion, significant numbers of spiral ganglion cells survived. A strong positive correlation between the diameter of the cochlear, vestibular, and eighth cranial nerves with the total spiral ganglion cell count (p < 0.001) was found. This would suggest that modern imaging techniques may be used to predict residual spiral ganglion cell population in cochlear implant candidates. Trauma from implantation of the electrode array was studied in both cadaveric human temporal bone models and temporal bones from individuals who received implants during life. A characteristic pattern of damage to the lateral cochlear wall and basilar membrane was identified in the upper basal turn. New bone formation and perielectrode fibrosis was common after cochlear implantation. Despite this significant trauma and reaction, there is no firm evidence that further degeneration of the spiral ganglion can be predicted as a consequence.

Total ablation of cochlear haircells by perilymphatic perfusion with water

Inner and outer haircells are destroyed within one day following perfusion of the perilymphatic spaces with water, otherwise the structural integrity of the cochlea and the organ of Corti are preserved. This technique is a reliable method to create an acute unilateral deafness model for investigations of auditory function.

Blast overpressure induced structural and functional changes in the auditory system

Blast overpressure of sufficient intensity can produce injury to various organ systems. Unprotected ears result in the auditory system being the most susceptible. The injuries to the auditory system include: rupture of the tympanic membrane, dislocation or fracture of the ossicular chain, and damage to the sensory structures on the basilar membrane. All these injuries can be characterized as a form of mechanical damage to the affected structure. Injury to the sensory structures on the basilar membrane leads to temporary and permanent loss of hearing sensitivity. The temporary component of the hearing loss shows a time course after removal from the noise which frequently will include an initial increase in hearing loss followed by a recovery period during which threshold may return to preexposure levels or stabilize at a higher level which represents a permanent loss of hearing sensitivity. This type of recovery function suggests that there are damage processes which continue after the traumatic event and that intervention might mitigate some of the damage and hearing loss.

Processing and analyzing the mouse temporal bone to identify gross, cellular and subcellular pathology

A technique has been developed for preparing the mouse temporal bone for histopathological examination: first, as a whole mount to detect any gross malformations of the bony or membranous labyrinths; second, in dissected segments to localize damage in the different sensory organs and to quantify sensory- and supporting-cell losses; and finally, in semi-thick and thin sections to identify and characterize subcellular pathology. Examples are given of the successful application of this technique to mice with very different inner-ear problems, including those with an abnormally short cochlear spiral, a defective lateral semicircular canal, abnormal otoliths over the saccular macula, an increased susceptibility to noise damage and those which lack fibroblast growth factor receptor 3.

Co-administration of the neurotrophic ACTH(4-9) analogue, ORG 2766, may reduce the cochleotoxic effects of cisplatin

In this study the effect of the neurotrophic ACTH(4-9) analogue, ORG 2766, on cisplatin cochleotoxicity was investigated with both light- and transmission electron microscopy. Guinea pigs were treated with either cisplatin+ORG 2766 (n = 11) or cisplatin + physiological saline (n = 9). All animals treated with cisplatin + physiological saline showed complete loss of outer hair cells (OHC) and degeneration of the organ of Corti in the basal cochlear turns, while partial OHC loss was found in the middle and apical turns. The inner hair cells (IHC) and other cochlear tissues were not affected. Eight animals from the group treated with cisplatin + ORG 2766 demonstrated similar pathological changes, but to a lesser degree, especially in the middle turns. The three remaining animals demonstrated no cochlear alterations at all, light-microscopically, and only minor subcellular changes in the OHCs at the ultrastructural level. Electrophysiologically, these three animals showed normals compound action potential (CAP) amplitudes at stimulus frequencies from 0.5 to 16 kHz and normal cochlear microphonics (CM) in the frequency range from 0.5 to 8 kHz. The other animals treated with cisplatin + ORG 2766 showed a severe loss in their CAPs and CM, except for one showing intermediate loss. All animals from the group treated with cisplatin alone showed a severe loss in their CAPs and CM. Endolymphatic hydrops was present in all animals from the cisplatin- and the cisplatin + ORG 2766-treated groups. These data indicate that daily, concomitant administration of ORG 2766 may reduce OHC loss and subsequent degeneration of the organ of Corti in cisplatin-treated guinea pig cochleas.

Effects of chronic high-rate electrical stimulation on the cochlea and eighth nerve in the deafened guinea pig

This study was undertaken to examine the effects of chronic high-rate stimulation on the eighth nerve and cochlea. Fifty-four male pigmented guinea pigs were deafened and implanted with single ball electrodes in scala tympani. Four groups of animals received chronic electrical stimulation at a level of 5 microCol/cm2/ph for 1000 h as follows: Group A: 1000 Hz, 100 microseconds/ph duration, 100 microA peak; Group B: 250 Hz, 100 microseconds/ph duration, 100 microA peak; Group C: 2750 Hz, 36 microseconds/ph duration, 250 microA peak; Group D: 250 Hz, 400 microseconds/ph duration, 25 microA peak. Also, two control groups received 20 min stimulation during weekly electrically evoked auditory brainstem response (eABR) measurement (Group E) and about 5 s stimulation (Group F) during a brief eABR 3 day postimplantation and at perfusion. On Day 50, animals were perfused, midmodiolar sections cut and a quantitative assessment of spiral ganglion cells (SGC) performed. All stimulated subjects showed a similar decrease in eABR thresholds and dynamic range over time. No stimulation conditions induced pathology. All stimulation conditions enhanced survival of SGCs compared to unimplanted ears and implanted non-stimulated ears (Group F). There were no statistically significant differences in SGC survival between any stimulated groups, including Group E stimulated once a week. In conclusion, high-rate stimulation, under the conditions of this study, provides no additional risks and the same benefits to SGC survival as low-rate stimulation.

Recovery of structure and function of inner ear afferent synapses following kainic acid excitotoxicity

The present study was conducted to examine the re-establishment of IHC/VIII nerve synapses following kainic acid (KA) excitotoxicity and to discern if the re-organized afferent could render not only a normal auditory threshold but also a normal supra-threshold function. KA (60 mM) applied to the intact round window membrane in chinchilla destroyed postsynaptic endings of the auditory nerve, depressed the input-output (I/O) functions of auditory evoked potentials (EVP) and produced an average loss of sensitivity of over 80 dB at 4, 8, and 16 kHz, with less substantial losses (40-60 dB) at lower frequencies. However, there was no significant difference in 2f1-f2 distortion-product otoacoustic emissions (DPOAE) before and after the application of KA. The nerve endings went through a sequence of swelling, degeneration and recovery over a 3-5 day period at higher frequency. Auditory sensitivity and supra-threshold response returned accordingly. In contrast, complete recovery at lower frequencies (1 and 2 kHz) required more than 5 days. The results provide strong evidence that (1) excitotoxically damaged cochlear afferent neurons can recover and render both a normal EVP threshold and EVP I/O function and (2) afferent innervation to IHCs is not necessary for DPOAE generation.

Chronic electrical stimulation of the auditory nerve at high stimulus rates: a physiological and histopathological study

A major factor associated with recent improvements in the clinical performance of cochlear implant patients has been the development of speech-processing strategies based on high stimulation rates. While these processing strategies show clear clinical advantage, we know little of their long-term safety implications. The present study was designed to evaluate the physiological and histopathological effects of long-term intracochlear electrical stimulation using these high rates. Thirteen normal-hearing adult cats were bilaterally implanted with scala tympani electrode arrays and unilaterally stimulated for periods of up to 2100 h using either two pairs of bipolar or three monopolar stimulating electrodes. Stimuli consisted of short duration (25-50 microseconds/phase) charge-balanced biphasic current pulses presented at 1000 pulses per second (pps) per channel for monopolar stimulation, and 2000 pps/channel for bipolar stimulation. The electrodes were shorted between current pulses to minimize any residual direct current, and the pulse trains were presented using a 50% duty cycle (500 ms on; 500 ms off) in order to simulate speech. Both acoustic (ABR) and electrical (EABR) auditory brainstem responses were recorded periodically during the chronic stimulation program. All cochleas showed an increase in the click-evoked ABR threshold following implant surgery; however, recovery to near-normal levels occurred in approximately half of the stimulated cochleas 1 month post-operatively. The use of frequency-specific stimuli indicated that the most extensive hearing loss generally occurred in the high-frequency basal region of the cochlea (12 and 24 kHz) adjacent to the stimulating electrode. However, thresholds at lower frequencies (2, 4 and 8 kHz), appeared at near-normal levels despite long-term electrode implantation and electrical stimulation. Our longitudinal EABR results showed a statistically significant increase in threshold in nearly 40% of the chronically stimulated electrodes evaluated; however, the gradient of the EABR input/output (I/O) function (evoked potential response amplitude versus stimulus current) generally remained quite stable throughout the chronic stimulation period. Histopathological examination of the cochleas showed no statistically significant difference in ganglion cell densities between cochleas using monopolar and bipolar electrode configurations (P = 0.67), and no evidence of cochlear damage caused by high-rate electrical stimulation when compared with control cochleas. Indeed, there was no statistically significant relationship between spiral ganglion cell density and electrical stimulation (P = 0.459), or between the extent of loss of inner (IHC, P = 0.86) or outer (OHC, P = 0.30) hair cells and electrical stimulation. Spiral ganglion cell loss was, however, influenced by the degree of inflammation (P = 0.016) and electrode insertion trauma. These histopathological findings were consistent with the physiological data. Finally, electrode impedance, measured at completion of the chronic stimulation program, showed close correlation with the degree of tissue response adjacent to the electrode array. These results indicated that chronic intracochlear electrical stimulation, using carefully controlled charge-balanced biphasic current pulses at stimulus rates of up to 2000 pps/channel, does not appear to adversely affect residual auditory nerve elements or the cochlea in general. This study provides an important basis for the safe application of improved speech-processing strategies based on high-rate electrical stimulation.

Low-frequency 'conditioning' provides long-term protection from noise-induced threshold shifts in chinchillas

Studies have shown that loss of auditory sensitivity caused by exposure to high-level acoustic stimuli can be significantly reduced by pre-exposing the subject to moderate-level acoustic stimuli. Although the protective effects of such 'conditioning' exposures have been well documented, very little is known about the persistence of conditioning-induced protection, or about the biological mechanisms underlying it. In the present study, the persistence of conditioning-induced protection was examined in chinchillas by imposing either a 30- or 60-day recovery period between conditioning (10 days of exposure to 0.5 kHz noise at 90 or 95 dB, 6 h/day) and high-level (0.5 kHz noise at 106 dB for 48 h) exposures. Comparisons of threshold shifts between conditioned animals and control animals exposed only to high-level noise indicated that conditioning provided significant protection from noise-induced threshold shifts for at least 2 months. Conditioned animals sustained outer hair cell losses similar to controls, ranging from 15 to 30% in the apical half of the cochlea. The results suggest that low-frequency conditioning can trigger long-lasting changes in cochlear homeostasis rather than temporary changes in physiology or reductions in susceptibility to hair cell loss in chinchillas.

Degeneration of axons in the brainstem of the chinchilla after auditory overstimulation

The patterns of axonal degeneration following acoustic overstimulation of the cochlea were traced in the brainstem of adult chinchillas. The Nauta-Rasmussen method for axonal degeneration was used following survivals of 1-32 days after a 105 min exposure to an octave-band noise with a center frequency of 4 kHz and a sound pressure level of 108 dB. Hair-cell and myelinated nerve-fiber loss were assessed in the cochlea. The cochleotopic pattern of terminal degeneration in the ventral cochlear nucleus correlated with the sites of myelinated fiber and inner-hair-cell loss: this correlation was less rigorous with outer-hair-cell loss, especially in the dorsal cochlear nucleus. These results are consistent with a dystrophic process with a slow time course depending on hair-cell loss and/or direct cochlear nerve-fiber damage. However, in a number of cases with no damage in the apical cochlea, fine fiber degeneration occurred with a faster course in low-frequency regions in the dorsal cochlear nucleus and, transynaptically, in a non-cochleotopic pattern in the superior olive and inferior colliculus. These findings suggest that neuronal hyperactivity plays a role in the central degeneration following acoustic overstimulation, possibly by an excitotoxic process.

4-Aminopyridine effects on summating potentials in the guinea pig

DC receptor potentials measured in hair cells, and the associated extracellular DC potential known as the summating potential (SP), originate with nonlinear elements in the mechanoelectric transduction chain. Nonlinear electric conductance has been demonstrated in the basolateral membrane of the hair cell, and is commonly attributed to the presence of voltage- and time-dependent K+ conductances in this part of the hair cell membrane. To study a possible contribution of these K+ channels to the SP we perfused the perilymphatic spaces of the guinea pig cochlea with the K+ channel blocker 4-aminopyridine (4-AP). Since 4-AP might also affect the afferent fibers and, thus, interfere with SP measurement, we added tetrodotoxin (TTX) to the perfusion solutions to block the neuronal discharges. Sound-evoked (2-12 kHz) intracochlear potentials were recorded from the basal turn of both scala vestibuli and scala tympani. The results showed a frequency- and level-dependent effect of 4-AP on the magnitude of the SP. At low and moderate levels of 8 and 12 kHz stimuli 4-AP mostly reduced the SP amplitude, while at high levels of these stimuli and at all levels of 2 and 4 kHz stimuli 4-AP enlarged the SP amplitude. These effects were reversible and occurred in both scala vestibuli and scala tympani. We attribute these bi-directional effects on the SP amplitude to a differential effect of 4-AP on inner hair cell (IHC) and outer hair cell (OHC) physiology. The decrease in SP was found for stimulus conditions where the SP presumably depends mainly on contributions from basal turn IHCs. Blocking the 4-AP-sensitive K+ channel in the IHC membrane should lead to a reduced contribution from the IHCs to the SP, because of an increase in basolateral membrane resistance. The increase in SP was found for stimulus conditions where the SP is assumed to depend mainly on contributions from basal turn OHCs. In this case the OHCs seemed to respond to blocking of the 4-AP-sensitive K+ channel in the basolateral membrane with an increased contribution to the nonlinearity of the transduction chain. Administration of 4-AP did not affect the endocochlear potential. Light microscopic examination revealed no apparent changes in morphology after 4-AP perfusion.

Cochlear integrity in rats with experimentally induced audiogenic seizure susceptibility

While chronic susceptibility of rodents to audiogenic seizures (AGSs) is often accompanied by cochlear lesions, it has not been demonstrated whether cochlear hair cell losses are essential to pathogenesis in this epileptic disorder. An alternative possibility is that the neonatal timing of hearing losses is what unites various models of chronic AGS susceptibility. In the latter case, either transient or permanent hearing losses might induce susceptibility as long as they concur with a critical period of development. To address this issue, it was examined whether lesions were universally present in cochleas of adult rats after having been made susceptible to sound-triggered seizures by different types and severities of neonatal auditory trauma. Neonatal treatments included: (1) an 8 min exposure of rat pups to intense noise (125 dB SPL) on postnatal day (PND) 14; (2) injections of low doses of kanamycin (KM: 100 mg/kg) on PNDs 9-12; or (3) injections of high doses of KM (500 mg/kg) on PNDs 9-12. As adults, rats in all experimental groups, but not in sham-treated groups, exhibited sound-triggerable seizure responses. Nonetheless, this outcome did not depend on integrity of cochleas. Hair cells were rarely missing in the cochleas of noise-exposed, low-dosage KM-treated, or sham-treated rats. By contrast, all inner and outer hair cells were missing from the basal 75% of cochleas of adult rats which had been treated with high-dose KM on PNDs 9-12. Results indicate that cochlear lesions are not requisite for the induction or expression of AGS susceptibility. At the same time, however, significant hair cell losses do not necessarily preclude susceptibility. It appears that the neonatal timing rather than the permanence of hearing losses may be what engenders chronic AGS susceptibility.

The effects of moderate and low levels of acoustic overstimulation on stereocilia and their tip links in the guinea pig

Guinea pigs were exposed to pure tones of 10 kHz at intensities between 98 and 115 dB SPL for 5-30 min, to produce varying degrees of acoustic trauma. Changes in auditory thresholds were measured electrophysiologically, and the animals were immediately fixed for scanning electron microscopy. Correlation between morphological changes to the hair bundle and losses in threshold, showed that with the smallest degrees of trauma (98 dB SPL for 15 min, mean maximum threshold loss of 22 dB), damage was confined to a small stretch of inner hair cells (IHC), with only subtle changes to the stereocilia of the outer hair cells (OHC). At exposure intensities greater than 102 dB SPL (duration: 15 min) the IHC stereocilia in the centre of the lesion were always substantially disarrayed. Substantial damage to the OHC bundles was seen only with exposures above 110 dB SPL (duration: > or = 5 min), producing threshold losses of 50 dB or more. Tip links were lost only where the stereocilia were disarrayed. It is concluded that the tip links are not the most vulnerable components of the cochlear hair cell, but that relatively low levels of acoustic stimulation can cause significant damage to the stereociliary bundle of the IHCs.

A rat homolog of the mouse deafness mutant jerker (je)

An autosomal recessive deafness mutant was discovered in our colony of Zucker (ZUC) rats. These mutants behave like shaker-waltzer deafness mutants, and their inner ear pathology classifies them among neuroepithelial degeneration type of deafness mutants. To determine whether this rat deafness mutation (-) defines a unique locus or one that has been previously described, we mapped its chromosomal location. F2 progeny of (Pbrc:ZUC x BN/Crl) A/a B/b H/h +/- F1 rats were scored for coat color and behavioral phenotypes. Segregation analysis indicated that the deafness locus might be loosely linked with B on rat Chromosome (Chr) 5 (RNO5). Therefore, 40 -/- rats were scored for BN and ZUC alleles at four additional loci, D5Mit11, D5Mit13, Oprd1, and Gnb1, known to map to RNO5 or its homolog, mouse Chr 4 (MMU4). Linkage analysis established the gene order (cM distance) as D5Mit11-(19.3)-B-(17.9)-D5Mit13-(19. 2)-Oprd1-(21.5) - (1.2) Gnb1, placing the deafness locus on distal RNO5. The position of the deafness locus on RNO5 is similar to that ofjerker (je) on MMU4; the phenotypes and patterns of inheritance of the deafness mutation and je are also similar. It seems likely that the mutation affects the rat homolog of je. The rat deafness locus should, therefore, be named jerker and assigned the gene symbol Je.