Ultrastructural correlates of selective outer hair cell destruction following kanamycin intoxication in the chinchilla

Kanamycin ototoxicity, combined with behavioral audiometry to evaluate threshold shifts, was used to destroy outer hair cells (OHCs) in the basal cochlea of the chincilla while leaving the inner hair cell (IHC) population largely intact. After survival times of four weeks to one year, transmission electron microscopy was employed to determine the condition of surviving hair cells and neural elements. Throughout the region of OHC loss, IHCs and their innervation were normal in appearance if their adjacent supporting cells were undamaged. When IHC supporting cells, specifically the inner pillar cells, were damaged or absent, damage to IHCs was commonly obsereved. Such supporting cell-related damage included extrusion of the cuticular plate from the surface of the reticular lamina, encapsulation and/or fusion of stereocilla, and gross distortion of hair cell shape. When the outer supporting cells of the organ of Corti were undamaged following OHC loss, outer spiral fibers were found to have survived in near-normal numbers in the region from 0.5–1.0 mm basalmost surviving OHC, but suffered progressive attrition toward the basal end of the cochlea. It is concluded that kanamycin-induced OHC loss can occur without concommitant IHC damage or outer spinal fiber loss.     

Ultrastructural damage in cochleas used for studies of basilar membrane mechanics

Cat cochleas used for interferometric studies of basilar membrane mechanics were examined with the electron microscope. The structures most severely damaged in the experimental cochleas are the outer hair cells and the radial afferent fibers to the inner hair cells. Since the basilar membrane and other supporting structures appear to be normal, mechanical changes observed in the experimental cochleas are most probably due to outer hair cell damage. Individual animals with varying degrees of damage showed large differences in the frequency of basilar membrane resonance at the same place in the cochlea. Shifts in tuning of this magnitude could occur as a consequence of hair cell damage only if the stiffness of the stereocilia and associated structures was greater initially than the stiffness of the basilar membrane and gradually decreased with damage. The present series of observations, therefore, suggest that the stiffness of the outer hair cell stereocilia determines basilar membrane tuning.     

卡铂导致毛细胞及其传出神经损害的耳蜗分析图

目的：介绍一种同时评估耳蜗传出神经和毛细胞的简便的组织化学技术。方法：首先应用脱氢酶染色选择性标记毛细胞,再用乙酰胆碱醌酶染色标记传出神经纤维,双重染色的耳蜗铺片样品在光学显微镜下沿着耳蜗基底膜的全长分别对毛细胞和穿越Ｃｏｒｔｉ隧道的传出神经纤维计数,根据卡铂耳中毒灰鼠的毛细胞及其传出神经纤维损伤的百分比制备耳蜗图。结果：耳蜗分析图充分显示;当绝大多数内毛细胞坏死以及部分外毛细胞坏死时,越隧道的传     

Correlation of changes in compound action potential (CAP) tuning curves and cochlear lesion in guinea pigs after explosion

The purpose of the present study was to investigate the sensitivity of compound action potential (CAP) tuning curves to changes of the cochlear status in guinea pigs after explosion and their ability to reflect specific histological variations. The results were as follows: 1. The CAP tuning curves were abnormally broad and the Q 10 dB values were reduced by a factor of 1 after explosion, indicating wider tuning. 2. The degree of broadening of the CAP tuning curves seemed to increase as the hair cell loss increased. 3. After explosion, the tip of the tuning curve shifted to frequencies significantly higher or lower than that of the signal, it might be related to the location of hair cell loss in the cochlea. 4. In animals for which damage was restricted to only three rows of outer hair cells, changes of the CAP tuning curves were observed. It provides further evidence that the tuning properties of cochlear nerve fibers are dependent upon the integrity of the outer hair cells even though the great majority of fibers innervate inner hair cells only.     

Relation between changes in compound action potential tuning curves and the pathology of cochlear hair cells stereocilia]

Relations of the changes of compound action potential tuning curve (AP-TC) and AP(N1) threshold to the pathology of the cochlear hair cells stereocilia were examined in acoustic-traumatized guinea pigs. The results showed that all animals with hair cell stereocilia damaged showed deteriorations of AP-TC. It supports the suggestion that mechanical properties of hair cell stereocilia contribute to the frequency selectivity of cochlea. After exposure, the characteristic frequency (CF) shift of AP-TC was observed. The mechanism of CF shift may be related to the lesion of hair cell stereocilia which may change normal tono-topic on cochlear level. Correlation between the damages of inner/outer hair cell stereocilia and changes of AP-TC and AP threshold suggested that the threshold and tuning properties of the cochlea are dependent upon the integrity of the inner and outer hair cells and their inter-relationship.     

灰鼠畸变产物耳声发射改变和外毛细胞缺失程度的相关分析

目的 探讨灰鼠畸变产物耳声发射(DPOAE)幅值的降低与外毛细胞缺失率之间的关系.方法 联合应用顺铂和利尿酸钠,建立灰鼠耳蜗毛细胞损伤模型.12只灰鼠在静脉注射利尿酸钠(40 mg/kg)同时,腹腔注射顺铂(0.2 mg/kg),分别于用药前和用药后的1周、2周及3周在清醒状态下检测动物DPOAE,并在施行最后1次DPOAE检测后处死动物.常规制备灰鼠耳蜗基底膜铺片并进行全耳蜗毛细胞计数.计算用药前和处死前DPOAE幅值的改变程度,通过Pearson相关分析对DPOAE幅值的降低与外毛细胞缺失程度之间的相关性进行分析.结果 顺铂和利尿酸钠引起灰鼠耳蜗外毛细胞缺失的特点是病变始发于基底膜的底回并逐渐向顶回发展,外毛细胞的缺损程度沿着基底膜从底回向顶回逐渐减轻,而不同频率DPOAE幅值降低的程度亦遵循从高频向低频发展的规律,各频率DPOAE在注射顺铂和利尿酸钠后的幅值改变与基底膜上相应部位的外毛细胞缺损程度存在一定的对应关系(r=0.796,P<0.05).外毛细胞每损失1%,可引起约0.24 dB的DPOAE幅值降低.结论 在灰鼠耳蜗损伤模型中,从DPOAE幅值的变化可以大致评估耳蜗外毛细胞的缺失程度.     

A model of the generation of the cochlear microphonic with nonlinear hair cell transduction and nonlinear basilar membrane mechanics

A phenomenological model of the generation of the cochlear microphonic of the guinea pig is presented which incorporates sharp tuning and nonlinear growth of the vibration of the cochlear partition, and nonlinear transduction of this response by a single linear array of hair cells (assumed to be the outer hair cells). The contribution to the microphonic potential from each hair cell generator is also assumed to decay exponentially with distance from the hair cell. Using experimentally obtained estimates for the parameters a good agreement between the theoretical microphonic tuning curves and growth functions was obtained without recourse to complex micromechanical motion of the cochlear partition, electrical tuning of the hair cells or a contribution from the inner hair cells of the cochlea.     

Dimensions of the cochlear stereocilia in man and the guinea pig

The tuning properties of the basilar membrane and the presence of acoustic emissions from the cochlea suggest that an energy consuming, mechanically active cochlear amplifier exists. Some models of this amplifier demand a mechanical resonator within the cochlea. The lengths of the stereocilia of the inner and outer hair cells in man and the guinea pig have been measured from scanning electron micrographs using a stereometric technique. In both species there is a linear increase in the length of the longest inner hair cell stereocilia with distance along the cochlea. There are, however, marked differences between the dimensions of the outer hair cell stereocilia in the two species. In man, there is an increase in length which is is a hyperbolic function of distance along the cochlear duct. The picture is more complicated in the guinea pig. This could account for some of the differences in auditory physiology between the two groups. The mechanical resonance properties of the human OHC stereocilia have been assessed, and, with certain assumptions, these properties are such that resonance of the stereocilia of the OHCs could form part of the cochlea amplifier, at least in man.     

Horseradish peroxidase injection of physiologically characterized afferent and efferent neurones in the guinea pig spiral ganglion

Single afferent and efferent neurones in the guinea pig spiral ganglion were injected with horseradish peroxidase. They could be recovered in subsequent histological processing and traced from the injection site in the ganglion to their final termination in the organ of Corti. All responsive primary afferents innervated the inner hair cells (58 neurones). One outer spiral fibre innervating the outer hair cells was recovered. This cell was non-spiking and unresponsive to acoustic stimulation. Neurones having properties previously attributed to cochlear efferents, terminated on the outer hair cells in regions of the cochlea consistent with their characteristic frequencies.     

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.     

Absence of Voltage-Dependent Compliance in High-Frequency Cochlear Outer Hair Cells

Cochlear outer hair cells are the key element in a mechanical amplification process that enhances auditory sensitivity and tuning in the mammalian inner ear. The electromotility of outer hair cells, that is, their ability to extend or contract at acoustic frequencies, is proposed to be the source of the mechanical amplification. For amplification to take place, some stiffness is required for outer hair cells to communicate force to the organ of Corti, the sensory epithelium of the inner ear. Modulation of this stiffness would be expected to have a significant effect on inner ear function. Outer hair cell compressive stiffness has recently been shown to be dependent on membrane potential, but this has only been demonstrated for cells originating in the apical, low-frequency segment of the cochlea, whereas cochlear amplification is arguably more important in the more basal high-frequency segment. The voltage-dependent compliance (the reciprocal of stiffness) of high-frequency outer hair cells was investigated by two methods in cells isolated from the basal turns of the guinea pig cochlea. In contrast to previous findings, no evidence was found for voltage-dependent changes in compliance. The results call into question the importance of outer hair cell voltage-dependent compliance as a component of cochlear amplification.     

Noise-induced inner ear damage in newborn and adult guinea pigs

Two-day (Group A), eight-day (Group B), and eight-month (Group C) old guinea pigs were exposed to 30 continuous hours of white noise at 119–120 db SPL. One month later pathology of the organ of Corti was evaluated and quantitated by use of the surface preparation technique. Percent cell damage was determined for outer hair, inner hair, outer pillar, and inner pillar cells at each of the four turns of the cochlea and for the cochlea as a whole. Comparisons of pathology of each cell type were made between groups. Mean percent outer hair cell damage per cochlea (± 1 S.E.) was 23.72 ± 3.69 for Group A, 36.98 ± 5.76 for Group B, and 7.24 ± 1.75 for Group C. There was no significant difference in outer hair cell damage between Groups A and B. Outer hair cells of Group A were significantly more damaged than those of Group C when damage in the cochlea as a whole was considered due to significantly greater damage in Group A at three and one half turns; likewise, outer hair cells of Group B were significantly more damaged than those of Group C when damage in the cochlea as a whole was considered due to significantly greater damage in Group B at two and one half and at three and one half turns. A similar effect was observed in terms of pathology of inner hair cells and pillar cells: there was a trend toward increased damage in animals of Groups A and B compared with C. Group C showed no outer or inner pillar cell damage, and only one of six animals had alterations in inner hair cells. In contrast, outer and inner pillar cells were damaged in Groups A and B, and four of six animals of Group A and six of eight of Group B showed inner hair cell damage. Recent electrophysiological and audiometric studies are discussed which, with the results of the present study, indicate greater susceptibility of young cochleas when compared with older cochleas, to noise-induced physiological and pathological alterations. It would seem medically prudent to take special precautions to avoid exposing newborns to excessive noise.     

Effects of trigeminal ganglion stimulation on the central auditory system

It is very important to determine if recombinant adenoviral vector (Ad) can damage the auditory hair cells in guinea pig cochlea after transgene expression. In this study, the scanning electron microscope was used to determine if there was loss of the auditory hair cells after Ad.LacZ (Ad5 containing Escherichia coli galactosidase) was inoculated into the cochlea through the round window membrane. Seven days later all inner and outer hair cells were found to express the LacZ gene. Except for the sparse loss of outer hair cells in the basal turn and the second turn, there was insignificant loss in the other turns. All inner hair cells were present. The damage to auditory hair cells resulting from intracochlear inoculation of Ad is limited, and this vector can be used as one of the ideal delivery tools in gene therapy of the cochlea.     

Cochlear origin of 2f1-f2 distortion products assessed by using 2 types of mutant mice

Mutant mice with a particular type of cochlear pathology are excellent models to study the functional role of various structures in the cochlea. In order to assess the contribution of inner and outer hair cells to the generation of distortion product emissions (DPEs) we have recorded the 2f1-f2 DPE in a control group of CBA mice, which have normal numbers of inner and outer hair cells and two different types of mutant mice: the Bronx-waltzer mice and the Wv/Wv mice. In the Bronx waltzer mutant mice, 70% of inner hair cells are missing whereas the outer hair cells are present in normal number. The distortion product emissions 2f1-f2 is clearly recordable with a 10-20 dB lower magnitude as compared to normal CBA control mice. The homozygous Wv/Wv mutant mice on the other hand present a selective outer hair cell loss as a constant defect with no progressive degeneration of the organ of Corti and an essentially normal inner hair cell population. The cubic distortion products 2f1-f2 could not be detected in all but one animal. Therefore, the present study strongly suggests, that the outer hair cells are critically involved in the production of DPEs.     

Abrupt (sharp cut) type sensorineural hearing loss--a human temporal bone study--

Histopathology of a case of bilateral sensorineural hearing loss of abrupt (sharp cut) type is reported. While there was a 45 dB gap in threshold between 1,000 Hz and 2,000 Hz bilaterally, the patient had a good hearing at 1,000 Hz and lower frequencies. The patient was suffering from Takayasu's arteritis. Major histopathological findings were as follows: Almost complete loss of the outer hair cells from the basal end to 12 mm area in the left cochlea (length: 30.5 mm) and 13 mm in the right (length: 31.5 mm). The inner hair cell of the same region was also missing in the left cochlea, and to a lesser degree in the right. There was a clear separation between the normal and the pathological organ of Corti. Marked loss of the cochlear neuron was noted in the same region. Blood vessels within the cochlea and the internal auditory meatus were normal. Bilateral abrupt (sharp cut) type sensorineural hearing loss with unknown etiology is a group of inner ear disease due to abiotrophy of the organ of Corti and cochlear neuron. Disposition or hereditary factor possibly plays an important role in the development of hearing loss.     

Chinchilla models of selective cochlear hair cell loss

Although it is well known that ethacrynic acid (EA) can enhance gentamicin (GM) ototoxicity, there has been no systematic study of the relationship between dosing parameters and inner ear pathology. We examined the effects of two parameters, GM dose and time delay between GM and EA administration, on cochlear and vestibular hair cell loss in chinchillas. 'No delay' groups received one injection of GM (125, 40, 20, or 10 mg/kg i.m.) followed immediately by EA (40 mg/kg i.v.); 'delay' groups received GM (10 mg/kg i.m.) followed by EA 1 or 1.5 h later. Animals were sacrificed 7 days later for evaluation of hair cell loss in the cochlea and vestibular end organs (cristae, saccule and utricle). Vestibular function was assessed prior to sacrifice by measuring the duration of nystagmus induced by cold caloric stimulation. No delay groups had approximately 100% loss of outer hair cells and dose-dependent losses of inner hair cells, ranging from approximately 100% to 58%. In 1 and 1.5 h delay groups, inner hair cell losses were approximately 19% and 0%, outer hair cell losses were approximately 74% and 47%, and outer hair cell loss followed a typical base to apex gradient. Two results were remarkable. First, the three groups with partial inner hair cell loss showed an atypical lesion pattern in which losses were substantially greater in the apical half than in the basal half of the cochlea. Second, there was no vestibular pathology in any group. The results establish dosing parameters that can be used to produce animal models with defined patterns and magnitudes of cochlear hair cell damage, but normal vestibular function and morphology.     

A micromechanical model of the cochlea with radial movement of the tectorial membrane.

The function of the tectorial membrane in the cochlear micromechanics is uncertain. In modeling approaches some models have assumed it to be a resonator that participates in the sharp tuning mechanisms of the cochlea with its mass coupled to the ciliary stiffness of outer hair cells, being driven by the shear force between the reticular lamina and itself. This paper presents a different type of micromechanical model which assumes that the tectorial membrane is driven by a lymphatic fluid flow that can be shown to have a substantial radial component. It also assumes that the reticular lamina is relatively stiff and thereby restrains the top end of outer hair cells that exert a force to the basilar membrane via Deiters cells. When combined with a three-dimensional block model, it can simulate the sharp tuning mechanisms of the cochlea well.     

Differential motile response of isolated inner and outer hair cells to stimulation by potassium and calcium ions

Inner and outer hair cells were mechanically isolated from the guinea pig cochlea and subjected to stimuli known to induce shape changes in outer hair cells. Depolarization by 70 mM KCl which causes osmotic swelling of outer hair cells also swelled inner hair cells by approximately 8% of their volume. The application of the calcium ionophore ionomycin which induces cortical contractions and elongation of outer hair cells, did not affect the shape of inner hair cells. Since ionomycin increased free intracellular calcium levels in both inner and outer hair cells, the results demonstrate that inner hair cells do not possess the mechanisms necessary for a contractile response to calcium. Thus, calcium is a specific regulator of outer hair cell motility making this mechanism a likely physiological modulator of a transduction feedback process.     

Sex differences in auditory sensitivity of chinchillas before and after exposure to impulse noise

OBJECTIVE: To determine if chinchillas exhibit sex differences in 1) basic auditory sensitivity and 2) susceptibility to cochlear damage and hearing loss from high-level impulse noise. DESIGN: The auditory sensitivity of 73 chinchillas was assessed by measuring evoked potentials from electrodes implanted in the inferior colliculus (IC-EVPs) and cubic (2f1-f2) distortion product otoacoustic emissions (CDPs). A subgroup of 16 chinchillas were retested after exposure to simulated M16 rifle fire (150 dB pSPL impulse noise). Thresholds and postexposure temporary and permanent threshold shifts were compared as a function of sex and frequency using analysis of variance procedures. Cochleograms, showing the percent of hair cells missing as a function of location on the basilar membrane, were constructed to show inner hair cell (IHC) and outer hair cell (OHC) losses for each group. RESULTS: Female chinchillas had slightly lower high-frequency thresholds, and slightly higher low-frequency thresholds than male chinchillas, but similar IC-EVP and CDP amplitude functions. Significant sex differences were observed after exposure to high-level impulse noise. Overall, female chinchillas developed approximately 10 dB more high-frequency hearing loss, but approximately 5 dB less low-frequency hearing loss than males. Hair cell losses, particularly IHC losses, were substantially less for females as compared with males. CONCLUSIONS: The results point to close similarities between chinchillas and humans with regard to sex/gender differences in basic auditory sensitivity before noise exposure, suggesting that the chinchilla may be a good model for exploring the anatomical and physiological bases of these differences. In addition, the results show significant sex differences in the physiological and anatomical response of the chinchilla cochlea to high-level noise. Similar differences in humans could have important implications with regard to military assignments and hearing conservation programs.     

Cisplatin-induced increases in spontaneous neural activity in the dorsal cochlear nucleus and associated outer hair cell loss

Tinnitus is one of the consequences of cisplatin chemotherapy, but its underlying mechanisms are not well understood. Since it has been shown that cisplatin causes outer hair cell loss, it is possible that loss of these cells might induce tinnitus by increasing spontaneous activity in the central auditory system. To test this possibility, the present study examined the effects of cisplatin treatment on cochlear hair cells and on spontaneous neural activity in the dorsal cochlear nucleus of hamsters. Recordings, carried out approximately 1 month after cisplatin treatment, demonstrated significant increases in spontaneous activity across broad regions of the dorsal cochlear nucleus relative to levels in saline-treated controls. Histological results showed that cisplatin-treated animals also displayed dramatic loss of outer hair cells over most of the basal turn of the cochlea. Inner hair cells remained intact, although some evidence of damage to their stereocilia was evident. These findings indicate that cisplatin treatment causes abnormalities in spontaneous activity in the dorsal cochlear nucleus that are associated with widespread damage to outer hair cells. However, since some damage to inner hair cells was also observed, the role of inner hair cell injury in contributing to higher spontaneous activity cannot be ruled out.