Effects of prenatal alcohol exposure and aging on auditory function in the rat: preliminary results

This study investigated select aspects of peripheral and central auditory dysfunction, as well as the pathological effects of aging, In an animal model of fetal alcohol syndrome (FAS). Pregnant rats consumed liquid alcohol diets containing 0, 17.5, or 35% ethanol-derived calories, from gestation day 7 to parturition. A fourth group was untreated. Offspring of these mothers were tested for auditory and neurological function, using the auditory brainstem response at 6, 12, and 18 months of age. Some animals in the alcohol-exposed groups showed a peripheral auditory disorder in the form of congenital sensorineural hearing loss. This was correlated with punctate lesions and malformed stereocilia on the auditory sensory receptor cells of the inner ear. Alcohol-exposed animals also showed a central auditory processing disorder characterized by prolonged transmission of neural potentials along the brainstem portion of the auditory pathway. Animals in the highest dose group also showed an augmentation in the age-related deterioration of auditory acuity. Thus, increased peripheral and central auditory dysfunctions and pathological deterioration of auditory function in old age may be sequelae of FAS. Such morbidities have important implications for the long-term clinical assessment and management of FAS patients.

Quantitative evaluation of myelinated nerve fibres and hair cells in cochleae of humans with age-related high-tone hearing loss

In this study 9 human temporal bones from 8 individuals were fixed with Karnovsky solution by perilymphatic perfusion within 1-3 h after death and examined using the "block-surface method' (Spoendlin and Brun, 1974; Spoendlin and Schrott, 1987) and the "micro-dissection method' (Johnsson and Hawkins, 1967). The audiogram of 7 individuals showed high-tone hearing loss, typical for sensory-neural presbycusis. The inner (IHC) and outer hair cells (OHC) and the myelinated nerve fibers in the osseous spiral lamina were counted to correlate audiometric curves with hair-cell and nerve-fibre densities. The "block-surface' method allows accurate hair-cell and myelinated nerve-fibre enumeration with maximal preservation of cochlear structures. The most significant change in the cochlea was not the expected loss of hair cells but an evident loss of nerve fibres in the spiral lamina along the entire length of the cochlea. This loss of nerve fibres was found to be age-related. Reductions up to 30-40% in comparison to normal-hearing middle-aged persons were found in cochleae from persons older than 60 years. In 2 cases only 13% of the fibres remained in some regions of the cochlea. The hair-cell counts showed a reduction of approximately 80% of the OHCs, mainly in the apical parts of the cochlea, and only little differences in the number of IHCs as compared with a group of normal-hearing middle-aged persons. We conclude that neither loss of hair cells nor primary degeneration of nerve fibres alone can fully explain the high-tone loss. Probably injuries of hair cells or neuronal elements at the cellular level can cause threshold elevation.

Patterns of degeneration in the human cochlear nerve

The patterns of neural degeneration of the spiral ganglion were studied in 12 human pathologic specimens and 2 normal neonatal specimens. Morphometric analysis of spiral ganglion cells included the maximum cross-sectional areas of both large (type 1) and small (type II) spiral ganglion cells. The organ of Corti in segments corresponding to the spiral ganglion, was evaluated for the presence or absence of inner (IHC) and outer (OHC) hair cells and supporting cells. The relationship between degeneration of spiral ganglion cells and degeneration in the organ of Corti, the age, sex, duration of deafness, cochlear location and delay between death and fixation was evaluated statistically. Both primary and secondary degeneration of the spiral ganglion were more severe in the basal than apical half of the cochlea. Degeneration of the spiral ganglion was most severe when both IHCs and OHCs were absent in the organ of Corti. No survival advantage was identified for type II ganglion cells as has been previously reported. That is, there was no correlation between the degree of degeneration of the spiral ganglion and the prevalence of type II ganglion cells. In fact, there was more severe degeneration of type II cells when the corresponding organ of Corti was severely degenerated. These findings in the human were compared with animal models of degeneration of the spiral ganglion, and the implications for cochlear implantation were discussed.

Deafness induced cell size changes in rostral AVCN of the guinea pig

The right cochleae of 250-350 g guinea pigs were lesioned by topical administration of neomycin in the middle ear cavity. Eight weeks after the lesion, the cochleae and cochlear nuclei were analyzed. Cochlear hair cell loss was assessed, and cell areas of spherical bushy cells in the rostral anteroventral cochlear nucleus (AVCN) were compared between the lesioned and normal hearing sides for each animal. In five animals with both inner and outer hair cell loss in the lesioned cochlea, the average area of neuronal somata in the rostral AVCN in the lesioned side was 22% smaller than the average area of these cells in the normal hearing side. In two animals with outer hair cell loss but inner hair cells remaining, there was no difference in cell size between the lesioned and non-lesioned AVCN. These results provide evidence that there is significant shrinkage in AVCN cell size in the mature mammal after hearing loss associated with inner hair cell loss.

An assessment of cochlear hair-cell loss in insulin-dependent diabetes mellitus diabetic and noise-exposed rats

OBJECTIVE

The purpose of this study was to investigate if insulin-dependent diabetes mellitus causes degenerative changes in the inner ear and whether these changes are exacerbated by noise exposure.

METHODS

Insulin-dependent diabetes mellitus was induced in male rats using streptozotocin (65 mg/kg of body weight, intravenously). Half the animals were exposed to 95 dB of random noise for 12 hours per day over a period of 6 months. The cochleae were removed, fixed, decalcified, dissected, and the hair cells counted.

RESULTS

A significant loss of outer hair cells was exhibited in both noise-exposed groups; however, although there was no significant difference between these two groups, the noise-exposed diabetic animals had significant loss in more turns than did the noise-exposed control animals. The diabetic animals were not statistically different from the control animals.

CONCLUSION

These results suggest that insulin-dependent diabetes mellitus may increase the hair-cell loss caused by noise overstimulation.

Sound-induced displacement responses in the plane of the organ of Corti in the isolated guinea-pig cochlea

Sound-induced displacement responses in the plane of the organ of Corti were studied in the apical turn in the isolated temporal-bone preparation of the guinea-pig cochlea. Swept sinusoidal sound stimuli (100-500 Hz) were delivered closed-field to the external auditory meatus. The surface of the organ of Corti was continuously monitored using a CCD video camera. Displacement responses in the plane of the organ of Corti were determined by analyzing the change of the location of the cells (pixel-by-pixel) within the visual field of the microscope. Displacement responses followed the stimulus amplitude and were observable at Hensen's cells, three rows of outer hair cells and inner hair cells. The most prominent displacement responses were over the outer hair cells; the maximum amplitude was 0.6-1.7 microns at 100 dB SPL. Tuned displacement responses were found; the Q10 dB was 1.3 +/- 0.6. The best frequency was tonotopically organized, decreasing toward the apex with a space constant of 0.4-0.9 mm/oct. The motion was directed either strial-apically or strial-basally in a frequency dependent manner. With the aid of laser interferometric measurements of the transverse displacement, it was concluded that sound stimulation does not induce slow DC motion in the organ of Corti for the isolated temporal-bone preparation.

Extra inner hair cells: prevalence and noise susceptibility

A total of 39 cochleae, 10 non-exposed and 29 noise exposed cochleae, were analyzed in a scanning electronmicroscope. Inner hair cells (IHCs) localized on the modiolar side of the ordinary row of IHCs were described. The mean number of such extra IHCs was 11 per cochlea both for non-exposed and exposed ears, ranging from 0-37. They showed a bimodal distribution with a maximum number at apex and 6-9 mm from apex. The dimension of the cuticular plate was slightly but significantly larger than for the ordinary IHCs. The extra IHCs suffered significantly less damage to their stereocilia than the ordinary IHCs after short-term high-level noise exposure. There was a significant negative correlation between the number of extra IHCs and the size of the post exposure permanent threshold shift. There was no evidence of postexposure generation of extra IHCs in or near the region of hair cell damage. It was concluded that extra IHCs normally occur in rabbits and that they are less susceptible to noise trauma than ordinary IHCs. The negative correlation between extra IHC occurrence and noise susceptibility needs further analysis.

[Morphologic study of the guinea pig cochlea after insertion of cochlear prosthesis electrode materials]

Experience has shown that a special cochlear-implant procedure, the so-called "soft surgery technique", may help to retain the remaining function of the inner ear. The morphological changes in the organ of Corti after an atraumatic implant procedure were examined using an animal model. The insertion was facilitated by coating the electrode with Healon. Scanning electron microscopy of the organ of Corti revealed only minor changes in the inner and outer hair cells. In all cases connective tissue was found sealing the implant as a reaction of the cochlea to the intracochlear implant.

[Detection of GABA(A) receptor mRNA in cochlear tissue. An in situ hybridization study]

Gamma-aminobutyric acid (GABA) and the GABAergic system play an important role in the efferent modulation of cochlear function. We examined surface preparations of guinea pig and mouse cochleae by in situ hybridization using radioactive labelled oligonucleotides for several subunits of the GABAA receptor. Frozen sections of rat and guinea pig brain (cortex, hippocampus, cerebellum) served as controls. In the mouse cochlea the mRNA of the alpha-1 and alpha-5, beta-1 and gamma-1 subunit were detected, while in guinea pig cochlea mRNA of the alpha-1, alpha-4, alpha-5, and gamma-1 subunit of the GABAA receptor were found. Positive signals were located in the regions of the outer hair cells and had a weaker intensity in the inner hair cells. In the brain sections the several subunits were detected in a variable distribution in the cerebellum, hippocampus and cortical regions. Rat specimens exhibited stronger signals than guinea pig brain sections. These investigations have extended previous results of immunocytochemical experiments from our laboratory demonstrating mRNA sequences of GABAA receptor subunits in the mammalian inner ear. Detection of these nucleotide sequences using surface preparations of the cochlea on a molecular level by in situ hybridization supports the importance of GABA as a cochlear neurotransmitter. Furthermore, it can be concluded that the mammalian cochlea is able to express a GABA-dependent neurotransmission system.

The ototoxicity of 3,3'-iminodipropionitrile: functional and morphological evidence of cochlear damage

Previous reports have suggested that IDPN may be ototoxic (Wolff et al., 1977; Crofton and Knight, 1991). The purpose of this research was to investigate the ototoxicity of IDPN using behavioral, physiological and morphological approaches. Three groups of adult rats were exposed to IDPN (0-400 mg/kg/day) for three consecutive days. In the first group, at 9-10 weeks post-exposure, thresholds for hearing of 5.3- and 38-kHz filtered clicks were measured electrophysiologically and brainstem auditory evoked responses (BAERs) were also recorded to a suprathreshold broadband click stimulus. A second set of animals was tested at 9 weeks for behavioral hearing thresholds (0.5- to 40-kHz tones) and at 11-12 weeks post-exposure for BAER thresholds (5- to 80-kHz filtered clicks). A third group of animals was exposed (as above), and killed at 12-14 weeks post-exposure for histological assessment. Kanamycin sulfate was used as a positive control for high-frequency selective hearing loss. Surface preparations of the organ of Corti were prepared in order to assess hair cells, and mid-modiolar sections of the cochlea were used to examine Rosenthal's canal and the stria vascularis. Functional data demonstrate a broad-spectrum hearing loss ranging from 0.5 kHz (30 dB deficit) to 80 kHz (40 dB deficit), as compared to a hearing deficit in kanamycin-exposed animals that was only apparent at frequencies greater than 5 kHz. Surface preparations revealed IDPN-induced hair cell loss in all turns of the organ of Corti, with a basal-to-apical gradient (more damage in the basal turns) at the lower dosages. At higher dosages there was complete destruction of the organ of Corti. There was also a dosage-related loss of spiral ganglion cells in all turns of the cochlea, again with a basal-to-apical gradient at the lower dosages. These data demonstrate that IDPN exposure in the rat results in extensive hearing loss and loss of neural structures in the cochlea.

Early hair-cell degeneration in the extreme apex of the guinea pig cochlea

Guinea pigs, aged from 3 weeks before term to 31 weeks after birth, were prepared for scanning electron microscopy. Examination of the extreme apex of the cochlea showed apparently pathological hair cells, even 3 weeks before term. The pathologies included loss and fusion of stereocilia, and the formation of giant stereocilia. The pathologies were most prevalent on row 3 of outer hair cells, declining to outer hair cell rows 2 and 1, with the inner hair cells being least affected. The abnormalities increased with time, increasing rapidly over the first few weeks of life, and more slowly thereafter. It is suggested that early degeneration in the extreme cochlear apex forms a novel model for spontaneous hair cell degeneration, with applicability to other types of spontaneous hair cell degeneration.

Cochlear synaptic development and morphology in a genetically induced type of progressive hair cell degeneration

In mice with genetically induced inner ear abnormalities it is conceivable that in the morphogenetic types and in mutants with the spotting kind of pigmentary anomaly, the genes act through the developing nervous system. It has been suggested that in degenerative (neuroepithelial) mutants the influence of the gene is also reflected in the inner ear through the agency of the nervous system. The jerker mouse belongs to the neuroepithelial type of mutants which in homozygotes results in early postnatal degeneration of the sensory epithelium of the inner ear, initially confined to the cuticular plate and the stereocilia. In spite of well-advanced hair cell degeneration, these mutants developed morphologically normal afferent and efferent nerve terminals at cochlear hair cells.

Selective inner hair cell ototoxicity induced by carboplatin

Carboplatin is a second-generation platinum antineoplastic agent. It has biological activity similar to cisplatin and is currently recommended for the treatment of ovarian cancer. In clinical use, carboplatin appears less ototoxic than cisplatin. This paper reports the ototoxic effects of carboplatin, in doses equivalent to the clinical use of the drug, in the chinchilla. Intravenous carboplatin 200 to 400 mg/m2 by bolus injection caused significant ototoxicity in this model as revealed by brainstem evoked responses (ABR audiometry). The cochlear pathology as seen by scanning electron microscopy revealed predominantly inner hair cell (IHC) stereocilia damage. Furthermore, the extent (grade) of the morphological lesions appears to be well correlated with the auditory brainstem response pattern of threshold elevation.

Damage to cochlear efferents following AF64A intoxication

Damage to cochlear efferents in chinchillas was assessed using transmission electron microscopy following unilateral treatment with the cholinotoxin ethylcholine mustard aziridinium ion (AF64A). AF64A was diluted in artificial perilymph to concentrations ranging from 0.5 to 100 microM. Survival times ranged from 1 to 12 weeks. At concentrations above 10 microM, widespread damage was noted to efferent fibers within the inner spiral bundle (ISB), tunnel spiral bundle (TSB), tunnel radial fibers (TRF) and efferent terminals at the base of OHCs. This damage included degeneration of fibers and terminals, delamination of mitochondria, vacuolization, and loss of cell membrane. However, at high concentrations, non-specific damage was also noted as thinnings or discontinuities of the membrane of OHCs and afferent fibers. At concentrations between 3 and 10 microM, selective damage was observed to efferent fibers within the ISB, TSB, TRF, and to terminals at the base of the OHCs, with all other structures appearing normal. At concentrations of 0.5 and 1 microM, damage was limited to efferent fibers within the TSB and ISB below the inner hair cells. In general, insult was greatest to middle- and basal-turn efferents, and longer survival times did not produce greater damage to, or loss of, efferents. These data suggest that at low concentrations, AF64A produces a partial yet selective degeneration of cochlear efferents within both the medial and lateral tracts, and that at the lowest concentrations used in these studies, AF64A produces a preferential insult on lateral olivocochlear efferents.

Height changes in the organ of Corti after noise exposure

To determine whether or not exposure to noise causes an alteration in the height of the organ of Corti (OC), 16 cochleas which had been exposed for one or two hours to an octave band of noise with a center frequency of 4 kHz and a sound pressure level of 108 dB were examined microscopically as whole mounts. These specimens were divided into four groups: early ears (N = 3) recovered less than 0.6 hours following the exposure; intermediate ears (N = 5) recovered 0.6-4.0 hours; 1-day ears (N = 3) recovered 24 hours; and late ears (N = 5) recovered 2-21 days. Height was measured at three positions across the OC and at multiple percentage locations from apex to base. The OC-height data from the noise-exposed cochleas were compared statistically to those from ten control cochleas. A significant reduction (P < or = 0.01) in OC height at the third outer hair cell (OHC) was first evident in the early ears in the region 65-95% distance from the apex. The height was reduced even further in the intermediate ears and included a region from 15-25% distance from the apex as well as the 65-95% region. In the late ears, heights had returned to control values, except within focal OC lesions. Height at the first row of OHCs was less affected than at the third row, and height at the inner hair cell (IHC) was least affected. These height changes were accompanied by distortion of the shape and position of OHCs, the shape of Deiters' cells and buckling of inner and outer pillar bodies. Sometimes IHCs had distorted shapes and were displaced from their usual positions. Although no functional measures were obtained from these ears, data from the literature indicate that the exposure described above would have produced a sizable threshold shift. Transient reduction in OC height likely accounts for some portion of noise-induced threshold shifts.

Effects of persistent perilymph fistula on the inner ear

To study the effects of a persistent perilymph fistula on the cochlea, a small cannula was inserted into the scala tympani of the basal turn of cochlea in guinea pigs. A month later, cochlear morphology and blood flow were studied using either histological evaluation or the microsphere surface preparation technique. Some animals showed no cochlear morphologic changes or no cochlear blood-flow reduction, even if tubal patency was maintained and perilymph leakage lasted for 1 month. This suggests that a prolonged perilymph fistula, per se, causes no permanent cochlear damage. However, in some animals, hair cell damage and cochlear blood-flow disorders were observed. These observations and the causes of hearing loss in clinical cases of perilymph fistula were studied.

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.

Progressive hereditary deafness with predominant inner hair cell loss

This paper presents the first case of human genetic hearing loss due to predominant inner hair cell damage investigated by psychoacoustic audiometry, auditory brainstem responses (ABR), evoked acoustic emissions (EAEs), and distortion product otoacoustic emissions (DPOEs). The genetic transmission was an autosomal dominant inheritance. The audiogram pattern was a dominant high-frequency hearing loss. EAE and DPOE properties were normal. Bronx waltzer mutant mouse with genetically determined predominant inner hair cell defects provide an interesting model to study this congenital deafness. This rare congenital deafness with predominant damage to inner hair cells cannot be detected by neonatal screening techniques using EAEs or DPOEs.

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 correlates of aging in the chinchilla cochlea

The inner ears from 80 chinchillas ranging in age from premature to 19.2 years were examined as plastic-embedded flat preparations to determine the morphological changes associated with aging. Three of the four forms of human presbycusis defined by Schuknecht were found in the chinchillas. All animals had losses of sensory cells or sensory presbycusis. Inner (IHCs) and outer hair cells (OHCs) degenerated at a rate of about 0.29% and 1.0% per year, respectively. Age-related degeneration of inner (IPs) and outer pillars (OPs) occurred at a much slower rate. In four animals (5%) the dendritic processes of some of the spiral ganglion cells had degenerated in areas where the loss of sensory cells was minimal. This pathological change is likely equivalent to neural presbycusis. Six animals (7.5%) had regions of degeneration of the stria vascularis or strial presbycusis. The other common finding in the aging cochleas was the presence of lipofuscin or age pigment. Lipofuscin deposits were found to accumulate in the subcuticular region of OHCs, IPs and OPs, near the endolymphatic surfaces of many of the supporting cells and in the epithelial cells of Reissner's membrane. The IHCs accumulated much less lipofuscin. The morphological changes seen in the ears of aging chinchillas were qualitatively similar to those seen in the temporal bones of aging humans although the magnitude of the changes was considerably less. These results suggest that some of the damage found in aging human cochleas may be due to aging plus exposure to one or more ototraumatic agents.

Auditory brainstem response thresholds in a mouse mutant with selective outer hair cell loss

Mutant animals with a particular type of cochlear pathology are an excellent model for studying the functional role of various cells of the cochlea. In homozygous WV/WV mutant mice we found a selective loss of outer hair cells as a constant defect with no progressive degeneration of the organ of Corti. The mice were followed throughout their lives and exhibited auditory brainstem responses that were elevated to about 50 dB SPL as compared to normal control animals. Sequential temporal bone studies showed that there was a selective loss of outer hair cells throughout the entire cochlea as seen in surface preparations. The inner hair cells were present in normal numbers and appeared to be essentially normal.

The quantitative relation between sensory cell loss and hearing thresholds

On the basis of experimental data obtained from 420 noise-exposed animals (chinchilla), the amount of sensory cell loss has been quantitatively related to the amount of permanent threshold shift at eight audiometric test frequencies between 0.125 and 16 kHz. The noise exposures, which varied extensively in spectrum, intensity and duration, produced permanent threshold shifts that ranged from 0 to 70 dB across a broad range of test frequencies. These data show: (1) consistent outer hair cell losses with less than 5 dB permanent threshold shifts (PTS) across all the test frequencies; (2) the first approximately 30 dB of PTS is established by losses of primarily outer hair cells; (3) in regions of the cochlea that transduce frequencies higher than or equal to 2 kHz, the three rows of outer hair cells show the same degree of loss for a given PTS, while in the 0.5 to 1.0 kHz region of the cochlea, the third row of outer hair cells (OHC) consistently shows less loss than do rows one and two; (4) appreciable inner hair cell (IHC) loss does not begin to appear until PTS exceeds approximately 30 dB; (5) in the virtual absence of OHC, hearing thresholds are least sensitive to IHC loss in the octave band centered at 4 kHz, i.e., the 4 kHz region can be as functional as other areas of the cochlea in spite of a greater amount of damage. The quantitative relation between cell loss and PTS varies as a function of test frequency in an orderly fashion.

Effect of ascorbic acid on the numerical hair cell loss in noise exposed guinea pigs

Two groups of guinea pigs were exposed to 1/3 octave band noise centered at 4 kHz, 113-118 dB SPL, for 2 h. The animals of the first group were treated with ascorbic acid (AA), 0.5 mg per 1 g of body mass injected intraperitoneally before noise exposure. The second group (control) was exposed without being treated. By means of the surface specimen method and consequent assessment of numerical atrophy of cochlear hair cells it was found that application of ascorbic acid before the noise exposure resulted in a lower or no loss of hair cells especially within the respective frequency segment of the basilar membrane. Possible protective effect of AA and/or the negative effect of hypovitaminosis "C" are discussed.

Impulse noise and continuous noise of equivalent frequency spectrum and total sound energy

It has been proposed that impulse noise and continuous noise affect the inner ear differently and investigations have found impulse noise to be harmful to both the inner hair cells and the outer hair cells. Scanning electron microscopy and non-standard methods for statistical analysis have facilitated the evaluation of different types of morphological changes after exposure to various kinds of noise. Morphological differences were compared in groups of guinea pigs exposed to either impulse noise or continuous noise of equivalent duration, spectral content and energy. Functionally, the groups also showed similar threshold elevations. In order to separate the two groups, subtle hair cell changes were recorded and evaluated either alone, in combination with each other or with hair cell loss. It was found that both the inner hair cells and the outer hair cells were affected differently by impulse noise than by continuous noise even though the auditory thresholds were similar.

Effects on the inner and outer hair cells

Scanning electron microscopy has improved the quality of the information obtained on morphological changes in the organ of Corti caused by noise exposure and the development of non-standard statistical methods has made accurate quantitative evaluation of these morphological changes possible. Many different types of hair cell damage were observed in noise-exposed cochleas, and the changes found in the inner and outer hair cells were found to differ considerably. Both analysis of impulse noise and continuous noise gave different results when applied to the inner hair cells as compared to when applied to the outer hair cells. However, of the outer hair cell rows the outermost was most frequently damaged by both impulse noise and continuous noise. The results of this investigation support previous suggestions that the inner and outer hair cell rows should be evaluated separately in experimental research.

Loss of hair cells and threshold sensitivity during prolonged noise exposure in normotensive albino rats

The relation between hearing loss and loss of hair cells after prolonged exposure to a simulated industrial noise environment was determined in normotensive albino rats. Hearing loss was assessed behaviorally by a conditioned suppression technique and electrophysiologically by auditory brainstem response to pulses of 1/3 octave filtered full cycle sine waves. The ears were analyzed in surface preparations and the hair cells counted. Sixty-two ears from 46 animals were analyzed after 1, 3, 7, and 15 months of exposure. A hearing loss of up to approximately 30 dB was consistently found with minimal loss of hair cells. Between 30 and 60 dB hearing loss there was a fair correspondence between loss of function and loss of hair cells. After 15 months exposure a relatively larger loss of hair cells was observed than expected from decrease of threshold sensitivity. The results indicate the existence of a systematic although not directly proportional relation between hair cell loss and loss of sensitivity during prolonged noise exposure. It is suggested that the correlation between hearing loss and hair cell loss is influenced, e.g., by duration and level of exposure, degree of hearing loss, and location along the basilar membrane.

Acute ultrastructural changes in acoustic trauma: serial-section reconstruction of stereocilia and cuticular plates

Single-unit recordings were made from populations of auditory-nerve fibers in 12 cats before and after acoustic overstimulation. Cats were killed 4 to 16 h after exposure, and the cochleas were analyzed at the light- and electron-microscopic levels. The exposures were designed to create 40 to 60 dB of acute threshold shift. Physiological changes were similar to those seen in cases of permanent threshold shift: tuning curves with elevation of 'tips' and 'tails' were associated with significant decreases in the mean spontaneous discharge rates; tuning curves with elevated tips but hypersensitive tails were associated with clear elevation of the mean spontaneous rates. At the light-microscopic level, none of the ears showed any significant stereociliary pathology. Some of the ears showed no light-microscopic pathology whatsoever, while others showed signs of swelling and vacuolization in both inner and outer hair cell areas in cochlear regions appropriate to the CF regions showing threshold shifts. The presence or absence of these light-microscopic changes was, to some extent, dependent on the nature of the exposure stimulus. At the electron-microscopic level, in addition to apparent swelling of radial afferent terminals, the inner hair cells themselves were swollen. In two cochlear regions (from two ears) which showed acute threshold shifts of 20 to 40 dB, but no light-microscopic changes, serial-section ultrastructural analysis of stereocilia and cuticular plates was performed. In contrast to the situation in ears with permanent threshold shifts [(1986) Hear Res. 26, 65-88], there was no pathology in the intracuticular portion of the stereocilia rootlets. There were, however, significant changes in the lengths of the supracuticular portion of the rootlets. It is suggested that this attenuation of the supracuticular rootlet could decrease the stiffness of the stereocilia tufts and thereby change the tuning properties and sensitivity of the cochlear partition.

Inner hair cell loss and intracochlear clot in the preterm infant

Little is known about the pathological changes in the cochlea of preterm infants which might be responsible for the hearing loss which the infants are at risk of developing. This study looks at the cochleas of 5 preterm infants who died at ages up to 2 weeks using light and electronmicroscopic techniques, and shows changes in the inner hair cell population in association with intracochlear clot which may be responsible for subsequent hearing loss.

Inner ear damage induced by Mycobacterium fortuitum

We used electron microscopy to investigate Mycobacterium fortuitum-induced changes in the inner ears of mice. We found that the inner and outer hair cells had degenerated and disappeared in the organ of Corti. Changes in the lower turn of the cochlea were more severe than those of the upper turn while the changes of the outer hair cells were more severe than those of the inner hair cells. Disappearance, fusion and ballooning of the sensory hairs were observed in the vestibular organs. The bacterial extract also induced inner ear damage which was similar to that caused by live M. fortuitum.

A study of postmortem autolysis in the human organ of Corti

Forty-six human temporal bones from 24 individuals were removed at autopsy and prepared for electron microscopy. The adequacy of histologic preservation was evaluated by light and electron microscopy. Characteristic autolytic changes included vacuolization of afferent neurons and neural poles of inner and outer hair cells, lysis of limiting membranes of hair and supporting cells, swelling of endoplasmic reticulum, and dissolution of mitochondrial cristae. The rate of autolysis varied significantly within cellular components of the inner ear. The neural poles of hair cells demonstrated more rapid autolysis than apical poles and nerve terminals showed more autolysis than myelinated nerve fibers. Postmortem time and the cause of death affected the adequacy of histologic preservation. Fixation in patients dying of pneumonia, hypoxia, head injury, or malignancy tended to be poor, whereas the fixation achieved in patients dying of cardiac disease with postmortem time of under 140 minutes was generally good.

Differences in the cochlear degeneration pattern in the guinea pig as a result of gentamicin and cis-platinum intoxication

This study gives the results of a series of studies on inner ears carried out with the help of microdissections and surface preparations. The total degeneration patterns of the cochlea of the guinea pig due to two different ototoxic drugs were investigated with this technique. Clear differences between the results of the ototoxic drugs (gentamicin and cis-platinum) in the inner ear were observed. Gentamicin has a 'typical starting point' of degeneration on the basilar membrane. From this 'degeneration point' the loss of hair cells progressed towards the round window (fast) and the apex (slowly). The stria vascularis showed no signs of degeneration due to gentamicin. Reissner's membrane, on the other hand, showed cellular vacuolization of the endolymphatic cells. Cis-platinum (DDP) showed no 'degeneration point'; the loss of hair cells was found over the complete length of the basilar membrane, with a preference for the basilar turn. The stria vascularis, on the other hand, showed severe degenerative changes due to DDP, whereas Reissner's membrane showed no change. In using the microdissection technique we were in a position to discover these differences between the two ototoxic drugs.

Distribution of cochlear damage caused by the removal of the round window membrane

The distribution of damage that occurs in the cochlea after removal of the round window membrane was examined in the apical, middle and basal regions with light and electron microscopy. The damage resembles that seen after acoustic trauma in many respects. The outer hair cells are often disrupted in damaged zones, and the radial afferent fibers to the inner hair cells swell enormously to form large vacuoles. 16 h after opening of round window, there is conspicuous swelling of myelinated axons in the osseous spiral lamina of the apical region. This swelling is associated with large vacuoles underneath the inner hair cells. 10 h after opening the round window, much smaller vacuoles are seen in the apical region. The distribution of the damage is not uniform throughout the cochlea. Damage is usually less severe and is not uniform in the middle region but is pronounced in the base. The nature of the damage is also variable in different animals. For example, sharply delimited, discontinuous damage to the inner hair cells was occasionally observed in the apical region. The most likely cause for the damage to the cochlea is a pressure differential across the organ of Corti that appears after removing the round window membrane. The damage apparently causes low frequency random movements of the basilar membrane that are observed in the experimental cochleas using a reflected laser beam.

Single-neuron labeling and chronic cochlear pathology. II. Stereocilia damage and alterations of spontaneous discharge rates

The spontaneous discharge rates (SRs) sampled from auditory-nerve fibers in cases of chronic cochlear pathology are often abnormally low [17]. The application of intracellular labeling techniques to noise-exposed ears makes it possible to accurately correlate fiber populations showing SR abnormalities with the cochlear locations from which these responses originate. The correlations reveal that a decrease in the mean rates of spontaneous discharge is typically associated with selective loss of the tallest row of stereocilia from the inner hair cells. In cochlear regions where virtually all of the tall stereocilia are missing from the inner hair cells, the maximum rates of spontaneous discharge are less than 1/3 normal values. We suggest that the loss of tall stereocilia causes the decrease in SR because much of the resting current in the inner hair cell normally flows through the stereocilia membrane. Thus, the loss of that membrane leads to a hyperpolarization of the inner hair cell which, in turn, decreases the spontaneous release of vesicles at the synapse. An interpretation is also suggested for the "compression" of the SR distribution commonly seen among high-frequency neurons in normal animals [9].

Histological evaluation of damage in cat cochleas used for measurement of basilar membrane mechanics

Cochleas utilized in basilar membrane vibration measurements were examined histologically using an epon-embedded surface preparation technique. The amount of damage observed at both the apical and basal ends of the cochleas was variable. The apical damage was probably caused by large, low-frequency movements of the basilar membrane. The basal damage was due to trauma produced directly by the surgical and experimental procedure. The sharpness of turning observed in the basilar membrane frequency response was found to be inversely related to the extent of histological damage.

Fusion of stereocilia on inner hair cells in man and in the rabbit, rat and guinea pig

The complexity of the problem of sensorineural hearing loss is illustrated by the well-known poor correlation between pure tone thresholds and loss of sensory cells in the organ of Corti and between pure tone thresholds and other auditory functions such as discrimination of speech and thresholds of the intra-aural muscle reflex (MER). The present work demonstrates that two types of inner ear pathology are present in several mammalian species, including man. Both types can be induced by noise. In addition to the earlier well-known loss of outer hair cells, it is shown that abnormal sensory hairs on inner hair cells are an important and frequent finding in acoustically damaged ears. Fusion and inclination of inner hair cell cilia were predominant in rabbits with moderate hearing loss and a rather common finding in rats and humans. In guinea pigs, damage to inner hair cell stereocilia was not common and was seen only in conjunction with considerable loss of outer hair cells. In summary, fusion of the stereocilia of inner hair cells was a common finding in several mammalian species.

Inner ear pathology associated with Reye's syndrome

Severe pathological changes were observed in the inner ear tissues of a 2-month-old patient who died of Reye's syndrome after 5 days of hospitalization. In the organ of Corti, the inner hair cells appeared to be more severely damaged than the outer hair cells. Various degrees of degeneration were observed in all non-sensory epithelial cells lining the cochlear duct. In most turns of the cochlear duct, Reissner's membrane was ruptured and/or collapsed onto the organ of Corti. Likewise, both sensory and non-sensory cells of the vestibular end organs were markedly degenerated. These observations suggest that the inner ear tissues are acutely affected in patients with Reye's syndrome, and that the changes may cause impairment of hearing and/or equilibrium in patients who recover.

[Chlorphentermine-induced lipidosis of the cochlea and the cochlear nucleus]

The anoretic drug chlorphentermine induces, after long-term feeding, an accumulation of phospholipids mainly stored in lysosomes. Inner hair cells and nerve running to these cells are especially affected, whereas the outer hair cells and their nerves are less changed. Supporting cells and ganglion cells of the spiral ganglion always exhibit inclusions. Changes in the perikarya of the cochlear nucleus are not so outstanding and are in the scope of the alterations found in other parts of the nervous system. The morphological alterations allow some conclusions about the phospholipid metabolism and the lysosomal capacity of the affected cells. Furthermore, experimental lipidosis is considered as an useful model for inherited lipidosis bearing in mind the different pathogenesis underlying both conditions. Last but not least, the clinical consequence has to be pointed out when treating with drugs having different main effects but the same side effects.

Damage to sensory hairs of inner hair cells after exposure to noise in rabbits without outer hair cells

We investigated if the noise-induced damage to the stereocilia of inner hair cells (IHCs) was dependent on the integrity of the outer hair cells (OHCs) in rabbit. Prior to the noise exposure a total loss of OHCs in the basal 1.5 to 2 turns was induced by administration of kanamycin (400 mg/kg for 10 days). This left the IHCs apparently normal as observed in the scanning electron microscope. These animals exhibited a 20-60 dB hearing loss before noise exposure. In spite of this pronounced hearing loss, the fusion and inclination of the IHC stereocilia were extensive in these noise-exposed ears. The stereocilia damage occurred at the same noise exposure and was as prominent or even more pronounced than has been noted in ears exposed to noise only. Under the assumption that kanamycin causes selective destruction of OHCs, the results can be interpreted as evidence that the OHCs facilitate the IHCs at low sound levels without being involved in the process which damages IHC stereocilia at high levels of noise.

Effects on guinea pig cochlea from exposure to moderately intense broad-band noise

Guinea pigs, which were either anesthetized (A) or conscious (U), were exposed to four 2 h sessions of broad-band noise of 96 dB SPL. Cochlear microphonics and N1 thresholds were measured prior to killing from 1 to 13 days later. The cochleas were examined by SEM and by section. The U series suffered less N1 threshold loss and recovered within 10-13 days, while the large initial loss in the A series did not completely reverse within the period of study. Initially, the IHC stereocilia in the basal half of the cochlea showed marked bending, the affected area being somewhat more extensive in the A group. These hairs gradually recovered, although not progressively. On the other hand, the disturbance to OHC stereocilia, which appeared to be less pronounced early on and was more apically centered, developed over time into marked permanent damage. The outermost row consistently showed the greatest effect with hairs becoming elongated or fused, and occasionally lost altogether. Susceptibility to noise varied between individuals of both groups. The recovery of N1 threshold was concomitant with the recovery of the erectness of the IHC hairs.

Alteration of aminoglycoside antibiotic ototoxicity: effect of semistarvation

The effect of semistarvation on the toxicity and ototoxicity of tobramycin sulfate (TO) and gentamicin sulfate (GE) was investigated in guinea pigs by electrophysiological and histopathological methods. The presented data has shown that the toxicity and ototoxicity of aminoglycoside antibiotics is substantially increased when guinea pigs were semistarved. Our results should also warn researchers using semistarvation in their conditioning experiments which investigate the toxicity of different chemicals. Toxicity was greater in GE- than TO-treated animals, which caused the GE-treated animals to die during treatment or shortly after treatment. Thus, TO should be preferentially used because it has been shown to be less toxic and ototoxic in normal and altered nutritional conditions.

Functional significance of dendritic swelling after loud sounds in the guinea pig cochlea

Exposure of the guinea pig cochlea to loud pure tones caused a dramatic swelling of afferent dendrites beneath the inner hair cell (IHC). This swelling occurred in a restricted region of the cochlea basalward of the exposure frequency location. For a 110 dB tone swelling was just detectable in 1 micron sections for a 18 3/4 min exposure and was clearly visible after a 22 1/2 min exposure. Swelling was reversible. Exposures which caused swelling produced a loss in sensitivity of the flat low frequency 'tail' of the frequency-threshold curves of single auditory neurons whose most sensitive frequency was a 1/2 octave higher than the exposure frequency. The findings are consistent with the notion that dendritic swelling causes a non-selective decrease in sensitivity to all frequencies of sound.

Experiments on cochlear cryosurgery in the guinea pig. Light and surface microscopy

The present investigation indicates that with cryosurgery it is possible to produce well defined cochlear lesions. The point of complete ablation can be chosen at will. A great advantage also is the fact that the bony capsule of the labyrinth is left intact. The remarkably slight scarring caused as well as the morphologically intact walls of the cochlear duct indicate an effective healing process, already completed after 6 days. These findings explain the unsuccessful clinical experiences when trying to ablate the labyrinth completely or to induce labyrinthine fistula with the help of cryosurgery (House, 1966). We suggest that for studies on cochlear function after exogenic trauma, cryosurgery gives easy reproductive morphological changes that may help the understanding and correlation of hair cell loss with cochlear physiology.

The influence of chronic vitamin A deficiency on human and animal ears

After feeding young rats a diet deficient in vitamin A, we examined the inner ear with the electron microscope. There were changes in the cuticle of the outer and inner hair cells. Furthermore, there were changes in the reticular system of the intermediate zone and massive degenerative changes in the ganglion cells of the VIII nerve. In a second experiment with older animals we found no significant changes in the sensory cells, though there was new bone formation in Rosenthal's canal and damage to the ganglion cells, of a lesser extent than was evident in the first experiment, however. In a further clinical study, we carefully chose human subjects suffering from alcoholic liver disease who also had a negative history of ear infection, noise exposure, head injury and use of streptomycin. Normal auditory function in the family was also a criterion. A decreased auditory function associated with low vitamin A levels was found in these patients. Those with liver disease showed not only a significant auditory dysfunction in the higher frequencies, but as well a poorer performance in the tone decay test. They were compared to a control group with normal hepatic, renal and thyroid status.

Acoustic reflex after experimental lesions to inner and outer hair cells

Chronic effects of noise or kanamycin on the acoustic intra-aural reflex in nonanesthetized rabbits were studied. The intra-aural reflex activity was simultaneously recorded in both ears upon alternate stimulation of the left and the right side. The inner ears were analyzed in scanning and transmission electronmicroscopy. Exposure to bandlimited high-level noise was found to induce extensive deformations in the sensory hairs of the inner hair cells without loss of outer hair cells. Kanamycin caused a degeneration of outer hair cells in the basal 1-2 turns without affecting the morphology of the inner hair cells. The morphological changes, as a function of the location on the basilar membrane, were compared to physiological changes, as a function of frequency. In the noise-exposed animals the threshold of the intra-aural reflex was found to be raised in a frequency range corresponding to the extent of abnormal sensory hairs of the inner hair cells. In the kanamycin-treated animals reflex changes correlated to the loss of outer hair cells. The observations were interpreted as indicating that both types of hair cells cooperate in the activation of the intra-aural reflex in rabbits.

Imprints of the inner sensory cell hairs on the human tectorial membrane

Imprints indicating possible direct inner sensory cell hair contact with the tectorial membrane were observed in the cochlea of a 77-year-old woman under a scanning electron microscope (SEM). The imprints were seen in the lower and upper basal cochlear turns but not in the apical and middle turns. The small dot of imprints numbered from a few up to l2 and were arranged in various forms rather than straight lines. Contact between the tectorial membrane and inner and outer sensory cell hairs of the humancochlea was discussed from the SEM findings found in this case.

Stapes anomaly and cochlear sensory cell changes. A scanning electron microscope study

This paper reports the pathological findings in the cochleae of a 66-year-old man examined under a scanning electron microscope (SEM). Columella-shaped stapes, which were the only anomalous changes in the middle ear cleft, were found in both ears. Beside this, giant hair formation and fusion of stereocilia in the inner sensory cells were also observed in the apical part of the lower basal turn. Since the patient had no history of ototoxic drug use or other ear disease episodes, it is suggested that these inner hair cell changes might have been caused by the conductive disorder accompanying the stapes anomaly. Giant hair formation was discussed from the inner ear findings of 14 other cases examined by a SEM.

Hearing thresholds with outer and inner hair cell loss

Hearing impairment and related cochlear histopathologic changes were evaluated in experimental animals after treatment with aminoglycoside antibiotics or exposure to intense sound. In the course of treatment with kanamycin, neomycin, or dihydrostreptomycin, permanent hearing loss in monkeys and guinea pigs occurred first at the high frequencies and progressed toward the lows. Exposure to different octave bands of noise at 120 dB SPL in monkeys and chinchillas produced permanent hearing loss at frequencies related to the spectral characteristics of the octave band. In most instances loss of outer hair cells was substantially greater than that of inner hair cells. In fact, the pattern and location of missing outer hair cells on the basilar membrane were most often correlated with threshold shifts of 50 dB or less. Generally inner hair cell loss was observed when the threshold shift was greater than 50 dB. Our data support the place principle and the inference that the outer hair cells are essential for hearing from threshold to about 50 dB SL. The inner hair cells, if functioning normally, apparently take over above that level. Although there is little doubt that such a generalization will, in the long term, be found to have been greatly oversimplified, there is every reason to believe that a combination of behavioral and morphologic procedures, as used in this study, will play an important part in elucidating the differences in functional significance of the two types of hair cells.

Critical bands following the selective destruction of cochlear inner and outer hair cells

Critical bandwidths and absolute intensity thresholds were measured in cats before and after kanamycin treatment which induced selective inner and outer hair cell losses. Hair cell losses were measured from cochleograms constructed from surface preparations of the organ of Corti. Results suggested that, for the test frequencies and stimulus intensities employed, critical bandwidths were not affected for frequencies tonotopically located in cochlear regions where only outer hair cells were lost. Critical bands were widened or not measurable only when inner hair cell losses exceeding 40% were also associated with complete loss of outer hair cells. The experiment suggests that cochlear frequency selectivity can be mediated by inner hair cells alone.