Inner Hair Cell Loss Research Articles

Neuronal and transneuronal degeneration of auditory axons in the brainstem after cochlear lesions in the chinchilla: cochleotopic and non-cochleotopic patterns

Terminal axonal degeneration in the brain following cochlear lesions was studied with the Nauta-Rasmussen method. Losses of hair cells and myelinated cochlear fibers were assessed. The cochleotopic map projected, from apex to base, on the ventral-to-dorsal axes of the cochlear nuclei. The cochleotopic correspondence was better for loss of cochlear nerve fibers and inner hair cells, than for outer hair cells. Cochlear fibers were traced to all parts of the cochlear nucleus, including the small-cell shell, also to cell-group Y and the flocculus. Terminal axonal degeneration in nuclei of the superior olivary complex, lateral lemniscus, and inferior colliculus was interpreted as transynaptic, since degenerated axons could not be traced to these locations from the cochlear nerve or trapezoid body. Moreover, biotinylated dextran amine injection in the basal turn of scala media of a normal cochlea labeled cochlear nerve fibers projecting to the high-frequency regions of the cochlear nuclei and to the flocculus, but not to more central auditory nuclei. This is the first detailed account of transynaptic degeneration in the ascending auditory pathway resulting from cochlear damage in an adult mammal. These findings are consistent with a dystrophic process depending on hair-cell loss and/or direct damage to cochlear nerve fibers.

Selective inner hair cell loss does not alter distortion product otoacoustic emissions

Outer hair cells (OHC) are believed to be the dominant source of distortion product otoacoustic emissions (DPOAE) in mammals; however, some studies in genetic mutants suggest that inner hair cell (IHC) loss may lead to a significant reduction of DPOAE amplitude. In the present study, we determined the extent to which IHC loss altered DPOAE amplitude by using carboplatin to destroy selectively the IHCs in the chinchilla while sparing virtually all of the OHCs. IHC losses of 80–100% with normal retention of OHCs did not reduce the amplitude of the DPOAEs or the cochlear microphonic potential (CM); however, it completely abolished the compound action potential (CAP). The only time that the amplitude of the DPOAEs and CM were reduced was in cases where both the IHCs and OHCs were destroyed in the base of the cochlea. We conclude that the total loss of IHCs does not lead to a significant change in DPOAE amplitude. DPOAE amplitude was only reduced when there was a significant loss of OHCs.

Evoked-potential thresholds and cubic distortion product otoacoustic emissions in the chinchilla following carboplatin treatment and noise exposure

Twenty-two chinchillas were given either a single intraperitoneal (i.p.) or intravenous (i.v.) injection (50 or 75 mg/kg) of Paraplatin TM, an asymptotic threshold shift-producing noise or a combination of the drug and noise in series. Auditory evoked potential (pure-tone) audiograms and cubic distortion product otoacoustic emissions were obtained on each animal before and after treatment, and the sensory epithelium of the cochlea was evaluated using the surface preparation method. Anatomical analysis indicated that the carboplatin alone caused relatively severe but scattered losses of inner hair cells throughout most of the cochlea which were dependent on dose and administration route. The outer sensory cell population remained essentially intact. In animals that had up to 40% scattered losses of only inner hair cells, evoked potential thresholds were near normal and the emission functions either were normal or showed an enhanced output. The severe losses of inner hair cells produced by the drug had no effect on the threshold shift dynamics produced by a five-day uninterrupted noise exposure. In general, there was not a consistent relation between the emission data and both the permanent threshold shift and outer hair cell losses.

Recording from the inferior colliculus following cochlear inner hair cell damage.

The anti-cancer drug carboplatin has been used to generate inner hair cell (IHC) lesions in the cochleae of chinchilla. This model has provided a valuable physiological tool for the study of the auditory system, particularly concerning the relative roles of IHCs and outer hair cells (OHCs). We recorded responses to contralateral sound stimuli of single units (SU) in the central nucleus (CN) of the inferior colliculus (IC) from normal and carboplatin treated animals. Normal single unit thresholds and frequency tuning curves (FTCs) were found, despite gross IHC damage within the cochleae of carboplatin treated animals. No evoked afferent responses could be detected in CN regions which represented cochlear loci where total IHC loss had occurred. Normal frequency selectivity in the auditory system is possible with small numbers of surviving IHCs provided OHCs remain normal.

Synaptic loss in the central nucleus of the inferior colliculus correlates with sensorineural hearing loss in the C57BL/6 mouse model of presbycusis

Between 3 and 25 months of age, light and electron microscopic features of principal neurons in the central nucleus of the inferior colliculus of the C57BL/6 mouse were quantitated. This mouse strain has a genetic defect producing progressive sensorineural hearing loss which starts during young adulthood (2 months of age) with high-frequency sounds. During the second year of life, hearing is severely impaired, progressively involving all frequencies. The hearing loss was documented in the present study by auditory brainstem recordings of the mice at various ages. The cochleas from many of the same animals showed massive loss of both inner and outer hair cells beginning at the base (high-frequency region) and progressing with age along the entire length to the apex (low-frequency region). In the inferior colliculi, there was a significant decrease in the size of principal neurons in the central nucleus. There was a dramatic decrease in the number of synapses of all morphologic types on principal neuronal somas. The percentage of somatic membrane covered by synapses decreased by 67%. A ventral (high frequency) to dorsal (low frequency) gradient of synaptic loss could not be identified within the central nucleus. These synaptic changes may be related to the equally dramatic physiologic changes which have been noted in the central nucleus of the inferior colliculus, in which response properties of neurons normally sensitive to high-frequency sounds become more sensitive to low-frequency sounds. The synaptic loss noted in this study may be due to more than the loss of primary afferent pathways. It may represent alterations of the complex synaptic circuitry related to the central deficits of presbycusis

Pure-tone thresholds and cubic distortion product otoacoustic emissions in the chinchilla following carboplatin treatment

In the chinchilla, carboplatin has an unusual ototoxic effect on the sensory epithelium of the cochlea [Takono et al., Hear. Res. 75, 93–102 (1994)]. Twelve chinchillas were treated with a single IP or IV injection (50 or 75 mg/kg) of carboplatin. Baseline auditory evoked potential audiograms and cubic distortion product otoacoustic emissions (3DPE) were obtained on each animal. Threshold and 3DPE functions were also acquired at regular intervals between one hour and 30 days post-injection. The sensory epithelium of the cochlea was evaluated using the surface preparation method. Anatomical analysis indicated that the carboplatin caused relatively severe but scattered losses of inner hair cells (IHC) throughout most of the cochlea. The outer sensory cell population was intact and the cells appeared normal at the level of the light microscope. Despite the IHC pathology, which also included vacuolization in the area of the IHCs, evoked potential thresholds, measured at the level of the inferior colliculus were very near normal and 3DPE functions showed very little change. [Work supported by Bristol-Myers Oncology Division and SUNY-Plattsburgh.]

Age-related cochlear degeneration in senescence-accelerated mouse

Age-related hair cell loss and strial atrophy were investigated in an accelerated senescence-prone strain, SAMP1 mice, and an accelerated senescence-resistant strain, SAMR1 mice. The loss of inner and outer hair cells in SAMP1 progressed more rapidly than that in SAMR1 with age. In both strains, areas of the loss of inner and outer hair cells were located mainly in the apex and base. Atrophy of the stria vascularis was observed in both strains, but in SAMP1 it appeared to increase earlier than in SAMR1. These results reveal that age-related hair cell loss and atrophy of the stria vascularis comparable to that in the human cochlea occur earlier and progress more rapidly in SAMP1 than in SAMR1. Hearing impairment in SAM may be due to a combination of sensory and strial presbycusis as well as to neural presbycusis, as reported previously. The morphological changes in the cochlea observed in SAMP1 and SAMR1 make these strains suitable for the study of the mechanisms of presbycusis.

Degenerative changes in the organ of Corti and lateral cochlear wall in experimental endolymphatic hydrops and human Menière's disease.

The pathogenesis of sensorineural hearing loss in Menière's disease and experimental endolymphatic hydrops is not fully understood. At the light microscopic level, there is poor correlation between the histopathology and loss of sensitivity and speech discrimination. The results of electron microscopic investigation of histopathology and alterations in immunoreactivity in the organ of Corti and lateral cochlear wall in the hydropic guinea pig are presented. Loss of outer and inner hair cells and spiral ganglion cells, particularly in the apical turn was evident by light microscopy. By electron microscopy, further evidence of degeneration was detected in the cuticular plate of outer hair cells, neural endings of both inner and outer hair cells, myelinated dendritic fibers, spiral ganglion cells, and types I and II fibrocytes of the lateral cochlear wall. There was a marked decrease in immunoreactivity for a variety of enzymes, calcium binding proteins, structural proteins, and integral membrane proteins of gap junctions, particularly among type I and type II fibrocytes of the lateral cochlear wall. The evidence suggests that dysfunction and degeneration of hair cells, afferent neurons and fibrocytes of the lateral cochlear wall are involved in the pathogenesis of hearing loss in endolymphatic hydrops.

Hair cell regeneration in the inner ear.

Hearing and balance disorders caused by the loss of inner ear hair cells is a common problem encountered in otolaryngology-head and neck surgery. The postembryonic production of hair cells in cold-blooded vertebrates has been known for several decades, and recent studies in the avian inner ear after ototoxic drug and noise damage have demonstrated a remarkable capacity for both anatomic and functional recovery. The regeneration of sensory hair cells has been shown to be integral to this repair process. Current work is focusing on the cellular progenitor source of new hair cells and the trigger mechanism responsible for inducing hair cell regeneration. Preliminary studies suggest that reparative proliferation may also occur in the mammalian inner ear. Work in this field is moving at a rapid pace. The results thus far have yielded optimism that direct stimulation of hair cell production or transplantation of living hair cells may eventually become treatment modalities for the damaged human inner ear. These proposals would have been considered unrealistic less than 10 years ago, but they now have caught the full attention of both clinician and researcher.

Progressive hair cell loss induced by toluene exposure

Rats were exposed to toluene by inhalation (1400 ppm, 16 h/d, 8 days) and sacrificed for morphological investigations at 3 and 5 days after the start of the exposure, and 4 days and 6 weeks after the end of the exposure. The cochleae were removed and prepared for light microscopy and scanning electron microscopy. After 3 days of tolucne exposure no loss of hair cells was found. A slight loss in the third row outer hair cells was observed after 5 days of exposure. Four days after the 8-day long exposure a loss of hair cells was found in all 3 rows of outer hair cells, mainly in the middle and upper turns of the cochlea. Six weeks post-exposure the damage on the hair cells had progressed towards the basal part of the cochlea, and a 50–100% loss of outer hair cells together with some loss of inner hair cells were seen. A fairly good correlation was found between the frequency regions showing loss of hair cells and the threshold shifts previously measured by auditory brainstem responses and distortion product otoacoustic emissions in the same rats at corresponding times (Johnson and Canlon, 1994). These results indicate that the outer hair cells in the middle frequency region of the cochlea, were primarily affected by toluene exposure. However, after a long post-exposure period the damage extended basally and apically and some damage to the inner hair cells was seen.

Cochlear function after selective inner hair cell degeneration induced by carboplatin

The ototoxicity of carboplatin, a second generation anti-cancer agent, was examined using the chinchilla as an animal model. In animals treated with a clinical therapeutic dose (400 mg/m 2), the dominant degenerative change is to inner hair cells (IHCs). This is in sharp contrast to most other ototoxic agents, which damage primarily the outer hair cells (OHCs). Functional changes to the cochlea have been evaluated in carboplatin treated subjects by recording cochlear action potentials (CAP) and cochlear microphonics (CM); cochlear lesions were evaluated using scanning electron microscopy. In carboplatin treated animals, CAP thresholds to tone-pip stimuli were elevated in proportion to IHC damage in corresponding cochlear regions. In contrast, CM amplitudes and ‘thresholds’ remained close to normal in most cases, reflecting the preservation of OHCs in the basal turn. These results indicate a high degree of independence between the inner and outer hair cell systems in the cochlear transduction mechanism. We suggest that this species-specific preparation with selective IHC loss will provide a valuable tool for studying, separately, the role of OHCs in both afferent and efferent cochlear function.

Effects of Noise and Salicylate on Hair Cell Loss in the Chinchilla Cochlea

Chinchillas were exposed to octave band noise, sodium salicylate (300 mg/kg per day intraperitoneally), or the combination of both agents for 15 days. The octave band noise exposure was centered at 500 Hz at an intensity of either 80 or 105 dB sound pressure level. The effects of the experimental treatments were evaluated by determining the number of missing hair cells after recovery as a function of location within the cochlea using a surface preparation technique. Average cochleograms were calculated for each of five experimental groups. Animals given salicylate alone showed little or no hair cell loss. Noise exposure at 80 dB resulted in a mild (less than 30%) outer hair cell loss in the apical turn of the cochlea, whereas exposure at 105 dB resulted in moderate (50%) outer hair cell loss (outer hair cell first row particularly) in the apical half of the cochlea, mild outer hair cell loss in the basal region of the cochlea, and a mild loss of inner hair cells. The amount of hair cell loss in the groups exposed to the combination of salicylates and noise was not significantly different from the corresponding groups exposed to noise alone. Statistical analysis of the data suggest that the combination of salicylate plus noise does not produce any greater hair cell loss than noise alone.

Patterns of hair cell survival and innervation in the cochlea of the Bronx waltzer mouse

A massive loss of inner hair cells typifies the cochleae of Bronx waltzer mutant mice. We have characterized the surviving inner hair cells and their modified innervation by immunocytochemistry using antibodies against neuron-specific enolase, with additional stains for neural cell adhesion molecule and neurofilaments, and by electron microscopy. The surviving inner hair cells vary in size, neuron-specific enolase content and innervation. All neuron-specific enolase-positive cells are innervated by neuron-specific enolase-positive endings. There is apparent correspondence between the neuron-specific enolase immunoreactivity of sensory cells and their innervation. Well-stained cells are richly innervated (and large) while lightly stained cells received fewer nerve endings. Neuron-specific enolase-negative inner hair cells innervated either by neuron-specific enolase-positive or -negative nerve endings are very rare. Ultrastructurally, the surviving inner hair cells vary from those of a normal morphological appearance to underdeveloped or vacuolated. Most of the apparently normal inner hair cells are associated with few nerve endings; instead nerve growth cones are abundant in the adjacent inner spiral sulcus epithelium. Cells forming ribbon synapses with afferent endings are rare. The contingent of efferent endings in the inner spiral bundle depends on the presence of afferent endings. The absence of inner hair cells and the uneven distribution of nerve endings on the surviving cells results in the disruption of normal innervation patterns, especially in the thinning out or discontinuation of the inner spiral bundle and an uneven distribution of tunnel fibres. We infer that the sprouting of nerve endings and their convergence on a selected population of the surviving inner hair cells represents a compensatory regenerative phenomenon in response to the loss and the genetic defect of the remaining inner hair cells.

Scanning electron microscopy of hair cells, stereocilia and cross-linkage systems in experimentally induced endolymphatic hydrops

In this study scanning electron microscopy was used to examine the stereocilia and their cross-linkages in guinea pig cochleas 1, 2, 4 and 8 months after endolymphatic sac obliteration. Initial changes were restricted to the stereocilia of the outer hair cells and consisted of a disarrangement and bulging of the stereocilia with an interruption of their cross-linkage systems. Subsequently, the stereocilia became fused and atrophied. Cross-linkages of inner hair cells remained intact. Loss of both outer and inner hair cells started in the apex and progressed towards the base of the cochlea. These findings indicate that early changes in the micro-architecture of the stereocilia may have a mechanical origin, with pressure fluctuations in the scala media possibly playing an important role.

Trimethyltin disrupts auditory function and cochlear morphology in pigmented rats

Trimethyltin (TMT) produces auditory deficits, presumably of cochlear origin, in rats. The present study identified pathological changes in the cochlea following treatment with TMT and correlated them with auditory threshold changes. Thresholds were determined by reflex-modulation audiometry, before and after treatment with TMT or with saline vehicle. Animals were then perfused and their cochleas embedded for examination as block-surface preparations or radial sections. In the first week following treatment, all TMT-treated rats showed threshold shifts of 40 to 60 dB at 40 kHz, and smaller threshold shifts (10–25 dB) at 2.5 and 10 kHz. At 3 weeks they showed threshold shifts similar to those identified one week following treatment, but with some recovery at 10 kHz. At 10 weeks, one animal showed complete recovery and three showed recovery of function at 10 but not at 40 kHz. TMT-treated animals showed losses of outer hair cells (OHC) in the basal turn of the cochlea as early as 48 hours following exposure. Comparable OHC pathology was seen at 9 days, along with some losses of inner hair cells. More extensive pathology occurred at longer survival times including the loss of type 1 spiral ganglion cells. The loss of auditory sensitivity at high frequencies was closely related to the loss of outer hair cells in the basal turn of the cochlea.

Morphological Changes of Cochlea in a Strain of New-mutant Mice

The hearing ability and histological characteristics of the cochlea of a strain of new-mutant mice were analyzed. This new mutant arose as a spontaneous mutation in the C3H/He stock. The genetic mode is autosomal recessive and the animals show abnormal behavior such as circling, head-tossing and hyperactivity. The audiological findings exhibited no recordable auditory brain stem response (ABR) in any homozygotes at ages ranging from 11 days to 117 days. For morphological examination, we used 36 homozygote with ages ranging from 10 days to 18 months. The primary morphological abnormalities were observed in the organ of Corti. The stereocilia of the outer hair cells showed disarray throughout the whole cochlea, although outer hair cell cytoplasm became fully developed, including the nerve terminals. Age-dependent degeneration of the outer hair cells subsequently occurred from the basal to the apical part of the cochlea. The earliest change demonstrated in the outer hair cells was cuticular degeneration. Although the abnormalities of the inner hair cells occurred late, a complete loss of inner and outer hair cells was demonstrated. The stria vascularis was well preserved at a later age as were spiral ganglion cells. These histological findings confirm that this mouse is classified as a neuroepithelial-type mutant. As this animal was expected to have a single gene abnormality, molecular genetic studies on this animal can provide important information on the nature of histological changes of the hair cell from a mode of gene action.

Cochlear damage and increased threshold in alpha-difluoromethylornithine (DFMO) treated guinea pigs

Alpha-difluoromethylornithine (DFMO) inhibits polyamine synthesis and is used as an antineoplastic and antiparasitic drug. In early human trials DFMO unexpectedly caused a sensorineural hearing loss. In the current study DFMO was administered to guinea pigs to investigate its effects on organ of Corti histology and on auditory thresholds. Histologic examination revealed that DFMO caused greatest damage in the hook and first turn. Damage in the second and third turns was minimal. Animals treated for 12 weeks with DFMO differed significantly ( P < 0.05) from controls in the hook and first turn in that: 1) DFMO caused a loss of hair cells in all rows. Loss of inner hair cells was greater than that of outer hair cells. 2) The remaining outer hair cells were shorter and contained a greater number of Hensen bodies. 3) The Deiters' cell bodies were longer and this increased length was associated with the decreased length of the corresponding outer hair cells. Brainstem audiometry showed that DFMO produced a hearing loss and the magnitude of this loss increased over twelve weeks of DFMO administration.

Influence of age on hair cell loss in the rabbit cochlea

The surface preparation technique was used for a study of cochlear hair cell loss as a function of age in rabbits 1 week, 6 months, 1 year, and 4 years old. Cytocochleograms from these animals were prepared which indicated damaged or missing hair cells over the entire spiral organ from the apical to the basal coil. The loss in the mean percentage of total outer hair cells (OHC) and inner hair cells (IHC) appeared to increase linearly with increasing age, with a statistically significant correlation. The cell loss was similar in all three rows of OHC in each of the age groups. In animals up to 1 year of age, areas of OHC loss were localized in the apical and basal coils. In 4-year-old rabbits, OHC degeneration had become progressively accentuated and widespread. The pattern of IHC loss was somewhat similar to that of OHC. Age-related hair cell loss in the rabbit is comparable to that in the rat, monkey, and human cochlea.

Apical hair cells and hearing

Hair cells located in the cochlear apex were destroyed by low-frequency noise exposure or by application of a − 140 °C cryoprobe to the bony wall of the apical turn. Complementary lesions that spared apical hair cells were produced by treatment with ototoxic antibiotics. Behavioral thresholds were measured in monkeys, chinchillas, and guinea pigs before and after these treatments. The results showed (1) with loss of up to 80% of apical outer hair cells, thresholds were unchanged; (2) with complete loss of both outer and inner apical hair cells, thresholds shifted approximately 20 dB at low frequencies; and (3) with complete loss of hair cells except in the apex, some hearing remained, with better hearing at low frequencies. Addition of a high-pass masker always elevated high-frequency thresholds, but only elevated low-frequency thresholds when all apical hair cells had been destroyed, indicating that basal hair cells can respond to low-frequency stimuli. Compared to data from more basal portions of the cochlea, these data suggest that functional differences exist between the base and apex, with comparable damage in the apex resulting in less hearing impairment. [Work supported by NINCDS grants NS05785 and NS25564.]

Retrograde cochlear neuronal degeneration in human subjects.

The purpose of this study was to identify the structural changes in the organ of Corti that correlate with retrograde cochlear neuronal degeneration. Thirty-eight temporal bones with excellent histological preparation from 23 subjects having hearing losses caused by cochlear disease were selected for study. Cytohistograms were prepared for inner and outer hair cells, inner and outer pillar cells, inner phalangeal cells, and cochlear neurons. The extent of neuronal degeneration was found to be directly related to the extent of injury to inner pillar cells and inner phalangeal cells, but not to loss of inner or outer hair cells. In most cochleas the loss of dendritic nerve fibers exceeded the loss of cell bodies. The findings support the concept that retrograde neuronal degeneration is initiated by injury to the dendritic nerve fibers, secondary to collapse and/or degeneration of the inner pillar cells and inner phalangeal cells.