Cochlear Hair Cell Damage Research Articles

Preparing Future Professionals—What’s the Real Story?

Preparing Future Professionals—What’s the Real Story?

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

The ototoxic action of aminoglycoside antibiotics leading to the loss of hair cells of the inner ear is well documented. However, the molecular mechanisms are poorly defined. We have previously shown that in neomycin-exposed organotypic cultures of the cochlea, the c-Jun N-terminal kinase (JNK) pathway – associated with stress, injury and apoptosis – is activated in hair cells and leads to their death. We have also shown that hair cell death can be attenuated by CEP-1347, an inhibitor of JNK signalling [Pirvola et al., J. Neurosci. 20 (2000) 43–50]. In the present study, we demonstrate that gentamicin-induced ototoxicity leads to JNK activation and apoptosis in the inner ear hair cells in vivo. We also show that systemic administration of CEP-1347 attenuates gentamicin-induced decrease of auditory sensitivity and cochlear hair cell damage. In addition, CEP-1347 treatment reduces the extent of hair cell loss in the ampullary cristae after gentamicin intoxication. Particularly, the inner hair cells of the cochlea and type I hair cells of the vestibular organs are protected. We have previously shown that also acoustic overstimulation leads to apoptosis of cochlear hair cells and that CEP-1347 can attenuate noise-induced sensory cell loss. These results suggest that activation of the JNK cascade may be a common molecular outcome of cellular stress in the inner ear sensory epithelia, and that attenuation of the lesion can be provided by inhibiting JNK activation.

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

The ototoxic action of aminoglycoside antibiotics leading to the loss of inner ear hair cells is well documented. However, the molecular mechanisms are poorly defined. We have previously shown that in neomycin-exposed cochlear organotypic cultures, the c-Jun N-terminal kinase (JNK) pathway – associated with stress, injury and apoptosis – is activated in hair cells. We have shown that hair cell death can be attenuated by CEP-1347, an inhibitor of JNK signaling ( Pirvola et al., 2000). In the present study, we demonstrate that gentamicin-induced ototoxicity leads to JNK activation and apoptosis in the inner ear hair cells in vivo. We show that systemic administration of CEP-1347 attenuates gentamicin-induced decrease of auditory sensitivity and cochlear hair cell damage. In addition, CEP-1347 treatment reduces the extent of hair cell loss in the ampullary cristae after gentamicin intoxication. Particularly, the inner hair cells of the cochlea and type I hair cells of the vestibular organs are protected. Our previous data have shown that also acoustic overstimulation can cause apoptotic death of cochlear hair cells and that CEP-1347 can attenuate noise-induced hair cell loss. Thus, our results imply that activation of JNK cascade may be a common molecular outcome of cellular stress in the inner ear sensory epithelia and that attenuation of the lesion can be provided by inhibiting JNK activation.

Psychophysical evaluation of cochlear hair cell damage due to the A3243G mitochondrial DNA mutation.

Mitochondrial dysfunction is an important cause of human deafness, implicated in genetic deafness, toxin and noise damage. We assessed the mechanism of cochlear dysfunction in a population of 11 subjects with a specific mitochondrial disorder caused by the A3243G mitochondrial DNA mutation. Psychophysical tests were carried out to assess the inner and outer hair cell functions in vivo. Inner hair cell function was assessed using a measure of hearing threshold in the presence of "threshold-equalizing noise" which can indicate "dead regions" where the transduction mechanism fails. Outer hair cell function was assessed by using the notched-noise method to measure auditory filter width, dependent on active mechanisms in the outer hair cell. The results support the conclusion that this mitochondrial disorder causes both inner and outer hair cell dysfunctions. Evidence of inner hair cell dysfunction was found mainly in basal (high frequency) regions of the cochlea and occurred even in some subjects with only mild hearing loss. Evidence of outer hair cell dysfunction was found in some instances where pure tone threshold was at or close to normal. The common occurrence of dead regions in the basal cochlea has treatment implication for this form of deafness; such people may not be helped by amplification of high frequencies.

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.

Comparison of distortion-product and transient evoked otoacoustic emissions with ABR threshold shift in chinchillas with ototoxic damage

In this study we compare distortion product otoacoustic emissions (DPOAEs), transient evoked otoacoustic emissions (TEOAEs) and ABR threshold shifts in an animal model (chinchilla) of cochlear hearing loss. Subjects were treated with an aminoglycoside (amikacin) to produce basal cochlear lesions of various degree. DPOAE and TEOAE were measured throughout the treatment period and until hearing thresholds stabilized. ABR thresholds to tone pip stimuli were determined. Cytocochleograms of cochleas were prepared using scanning microscopy. DPOAEs (2f1–f2) were compared to fast Fourier transform (FFT)-analyzed TEOAEs components in the 1-, 2-, and 4-kHz frequency regions. Both types of emission were compared with corresponding ABR thresholds. There was no significant linear correlation between these different measures of cochlear function. Moreover, the amplitudes of DPOAEs reflected smaller regions of cochlear outer hair cell (OHC) damage better than TEOAEs. These results suggest that DPOAEs can be used to more accurately monitor hair cell function at specific hearing locations than TEOAEs.

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.

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.

Ototoxicity of topical otomicrobial agents.

Topical antimicrobial solutions are used commonly for prevention or treatment of purulent otorrhea even though most solutions contain ototoxic agents. This study compares the cochlear and middle ear toxicity of cortisporin otic solution (COS), 0.3% gentamicin ophthalmic solution (GOS), benzalkonium chloride (0.026% and 0.05%), and 1.0% Ofloxacin, a new quinolone antibiotic. Saline 0.9% was used as a control. The agent was instilled daily for 7 days into the bulla of juvenile guinea pigs. The animals were sacrificed on the 14th day. The organ of Corti was examined using surface preparation light microscopy and scanning electron microscopy. The tympanic membrane (TM) and adjacent middle ear mucosa were examined with light microscopy. The average cochlear hair cell damage was 66% for COS, 6.5% for GOS, and 1% for Ofloxacin, benzalkonium and saline. COS and benzalkonium 0.05% produced moderate mucosal thickening and inflammation. However, this was not statistically different from the mild mucosal thickening produced by saline, GOS, Ofloxacin and 0.026% benzalkonium. There was statistically significant mild to moderate thickening of the TM for all agents compared to the saline control.

Histological evaluation of cochlear hair cell damage from noise-induced hearing loss in chinchillas

In studies which attempt to define the influence of various factors on recovery of hair cell integrity after acoustic trauma, an experimental and a control ear which initially have equal degrees of damage are required. With in a group of animals receiving an identical level of acoustic trauma there is more symmetry between the ears of each individual, in respect to function, than between animals. Figure 1 illustrates this, left and right cochlear evoked potential (CAP) audiograms are shown for two chinchillas receiving identical trauma. For this reason the contralateral ear is used as control.To compliment such functional evaluations we have devised a scoring system, based on the condition of hair cell stereocilia as revealed by scanning electron microscopy, which permits total stereociliar damage to be expressed numerically. This quantification permits correlation of the degree of structural pathology with functional changes. In this paper wereport experiments to verify the symmetry of stereociliar integrity between two ears, both for normal (non-exposed) animals and chinchillas in which each ear has received identical noise trauma.

Cochlear hair cell damage from intermittent noise exposure in young and adult guinea pigs

A comparative study of age-dependent damage to cochlea from intermittent noise exposure was carried out on five-week-old and one-year-old pigmented guinea pigs. Hair cell loss in the organ of Corti was studied after five weeks' exposure to a pure tone stimulus (95 dB SPL at 2 kHz, one hour per day for five weeks). The noise-induced damage was sharply limited to the 7- and 11-mm marks from the apex. Damage was more marked in younger guinea pigs and was distinct from natural age-induced cell loss. When the 7- to 11-mm zone was further analyzed, outer hair cell damage appeared highly significant in both age groups but more severe in younger animals. Inner hair cell damage in this area was severe in both groups but statistically insignificant.

Antibiotics and Sensorineural InteractionsIn vitro Studies

Organ culture of the embryonic inner ear of the mouse results, under normal conditions, in a good morphological morphogenesis and cytodifferentiation of individual cells and tissues. Organ culture is performed during a period corresponding to the embryonic development in vivo. Continuing the culture period causes a deterioration of the organ culture explant. Only a minor portion of the total innervation of vestibular and cochlear parts of the labyrinth occurs prior to birth. Low dose exposure (1-10 micrograms/ml) of ototoxic antibiotics in the culture medium causes a primary morphologic damage of vestibular and cochlear hair cells and a markedly reduced number of nerve fibers and statoacoustic ganglion cells. Likewise the ingrowing nerve fibers are even more vulnerable to aminoglycoside antibiotics than the hair cells. Such a drug exposure in the in vitro system confirms (by a chemical deprivation of nerve fiber ingrowth to developing hair cells) that hair cell differentiation occurs at least initially independent of morphologic contact with ingrowing nerve fibers as was earlier shown using a mechanical excision of the statoacoustic ganglion.

低音障害型感音難聴のか電図

The cases with low tone sensorineural deafness were studied using the transtympanic-electrode technique of electrocochleography.The results obtained were as follows:1) Low tone sensorineural deafness of sudden onset without vertigo:All the cases showed high AP and -SP amplitude and satisfactory CM response. These findings resembled the electrocochleographic findings of type I of Meniere's disease suggesting of the relationship with endolymphatic hydrops. The hearing returned to normal range in a half of the cases but remained hardly changed in the other half. The difference between these two groups could not be clarified electrocochleographically.2) Familial progressive deafness showed low tone sensorineural deafness:This case showed a high threshold and a low amplitude of CM at low frequency, and showed cochlear hair cell damage at low frequency range.3) Low tone sensorineural deafness due to retrolabyrinthine lesion:This case showed a normal CM but a low AP amplitude, and lacked a rapid increase of AP amplitude with increasing click intensity in the intensity region of AP input-output functions.

The ototoxic interaction of viomycin, capreomycin and polymyxin B with ethacrynic acid.

The ototoxic interaction between the aminoglycoside antibiotics (streptomycin, kanamycin, etc.) and the loop-inhibiting diuretics (ethacrynic acid, furosemide and bumetanide) has been well documented. This interaction causes extensive destruction of the hair cells of the cochlea. Brummett et al. (1974) demonstrated that this interaction did not occur with the non-loop-inhibiting diuretics and kanamycin. The present study was undertaken to determine if antibiotics other than the aminoglycosides could produce the ototoxic interaction when combined with a loop-inhibiting diuretic. Three antibiotics-viomycin, capreomycin, and polymyxin B- when given with ethacrynic acid were found to produce cochlear hair cell damage that was similar to that produced by aminoglycoside antibiotics administered with ethacrynic acid. Therefore, the interaction appears to be specific to the loop-inhibiting diuretics but not specific for the aminoglycoside antibiotics.

III. Comparative Quantitative Analysis

Hypotheses to explain the selective ototoxicity of aminoglycoside antibiotics may be grouped into two categories: 1) a partition effect and 2) innate hair cell selectivity. The results of this study give evidence that, in fact, both mechanisms may interplay. This conclusion is induced from the observations that following the transtympanic administration of streptomycin, dihydrostreptomycin and neomycin: 1) All three drugs elicited nearly the same topographic pattern and degree of damage in the vestibular compartment. 2) All three drugs elicited nearly the same topographic pattern but widely different degrees of damage in the cochlear compartment (neomycin > > streptomycin > dihydrostreptomycin). 3) Neomycin caused the same devastating cochlear hair cell damage as reported for its systemic administration. 4) Dihydrostreptomycin was significantly more vestibulotoxic than cochleotoxic, and streptomycin was both significantly more cochleotoxic than vestibulotoxic and significantly more cochleotoxic than was dihydrostreptomycin. The latter observations are discussed with regard to contrasts with the literature reporting the drug effects following systemic administration.

Some vascular effects of noise exposure in the chinchilla cochlea.

Chinchillas were exposed to pink noise at levels ranging from 110 dB (for 8 h) to 125 dB (for 1/4 h). After a 3-week survival period, the animals were terminated without anesthesia and tissues were prepared using a soft-surface preparation technique [Axelsson et al., Acta Otolaryngol. 80, 362 (1975)]. Cochlear hair cell damage as well as vascular pathology was slight. On occasion, a vessel or vessel segment was obviously atypical, thus allowing noise-exposed animals to be differentiated from their non-noise-exposed controls. More often, however, differences between animals were more discrete and only evident through a computer analysis of the data. The specific effects of these particular noise-exposure parameters on the cochlear blood supply of the chinchilla will be discussed.

Some vascular effects of noise exposure in the chinchilla cochlea.

Chinchillas were exposed to pink noise at levels ranging from 110 dB (for 8 hours) to 125 dB (for 15 min). After a 3-week survival period, the animals were killed and cochlear tissues were prepared using a soft-surface preparation technique. To evaluate discrete changes of the cochlear vasculature and minimize bias with respect to what was judged pathological, a method was used whereby experimental and control animals were mixed and randomly assessed without prior knowledge of the group to which the specimen belonged. The results were analysed by computer. Cochlear hair cell damage was slight. Statistically significant differences in the vasculature between noise-exposed and control animals were few. For example, in some cochlear vessels in the experimental animals, red blood cells were found to be less frequent; other vessels showed an increase or decrease in endothelial or periendothelial cells. A more prominent difference between control and noise-exposed animals was an increased variability in several...

Some vascular effects of noise exposure in the chinchilla

Chinchillas were exposed to pink noise at levels ranging from 110 dB (for 8 h) to 125 dB (for 14 h). After a three‐week survival period, the animals were terminated without anesthesia and tissues were prepared using a soft‐surface preparation technique [Axelsson et al., Ann. Otol. 83, 537 (1974)]. Cochlear hair cell damage as well as vascular pathology was slight. On occasion, a vessel or vessel segment was obviously atypical, thus allowing noise‐exposed animals to be differentiated from their non‐noise‐exposed controls. More often, however, differences between animals were more discrete and only evident through a computer analysis of the data. The specific effects of these particular noise‐exposure parameters on the cochlear blood supply of the chinchilla will be discussed.

Cochlear hair-cell damage in guinea pigs after exposure to impulse noise.

Fourteen young guinea pigs were individually exposed to 500 rounds of paper “caps” tired in a toy gun at a distance of 30 cm. The gun was fired at intervals of 1 5 sec. After seven weeks survival, the cochleas were removed and each complete organ of Corti examined, using Engström's surface-preparation technique. Cochleograms (tabular forms showing position and condition of each cell) were prepared for each row of hair cells. Curves were plotted showing number of damaged cells as a function of distance from the base. In approximately 80% of the cases, total destruction of hair cells occurred in a narrow band midway along the organ of Corti. The severity and distribution of damage for the group are compared with similar data for another group, which was exposed to high-intensity pure tones.