Pudendal nerve somatosensory evoked potentials in probable multiple sclerosis

One of the most frequent causes of sensorineural hearing loss in childhood is damage to outer hair cells of the cochlea. The presence of otoacoustic emissions, generated by outer hair cells, provides evidence for normal hearing. This finding, however, may give rise to false reassurance, because even severe hearing loss, localized behind the cochlea, can be associated with normal otoacoustic emissions. The coexistence of otoacoustic emissions and hearing loss calls for the prompt exclusion of neurological disease.     

Ultrastructural, physiological, and molecular defects in the inner ear of a gene-knockout mouse model for autosomal Alport syndrome

It is well documented that damage to the chick cochlea caused by acoustic overstimulation or ototoxic drugs is reversible. Second-order auditory neurons in nucleus magnocellularis (NM) are sensitive to changes in input from the cochlea. However, few experiments studying changes in NM during cochlear hair cell loss and regeneration have been reported. Chicks were given a single systemic dose of gentamicin, which results in maximal hair cell loss in the base of the cochlea after 5 days. Many new hair cells are present by 9 days. These new hair cells are mature but not completely recovered in organization by 70 days. We counted neurons in Nissl-stained sections of the brainstem within specific tonotopic regions of NM, comparing absolute cell number between gentamicin- and saline-treated animals at both short and long survival times. Our data suggest that neuronal number in rostral NM parallels hair cell number in the base of the cochlea. That is, after a single dose of gentamicin, we see a loss of both cochlear hair cells and NM neurons early, followed by a recovery of both cochlear hair cells and NM neurons later. These results suggest that neurons, like cochlear hair cells, can recover following gentamicin-induced damage.     

Ionic currents and electromotility in inner ear hair cells from humans

The upright posture and rich vocalizations of primates place demands on their senses of balance and hearing that differ from those of other animals. There is a wealth of behavioral, psychophysical, and CNS measures characterizing these senses in primates, but no prior recordings from their inner ear sensory receptor cells. We harvested human hair cells from patients undergoing surgical removal of life-threatening brain stem tumors and measured their ionic currents and electromotile responses. The hair cells were either isolated or left in situ in their sensory epithelium and investigated using the tight-seal, whole cell technique. We recorded from both type I and type II vestibular hair cells under voltage clamp and found four voltage-dependent currents, each of which has been reported in hair cells of other animals. Cochlear outer hair cells demonstrated electromotility in response to voltage steps like that seen in rodent animal models. Our results reveal many qualitative similarities to hair cells obtained from other animals and justify continued investigations to explore quantitative differences that may be associated with normal or pathological human sensation.     

Distortion products in normal hearing patients with tinnitus

Otoacoustic emissions are the result of cochlear active non-linear micromechanical mechanisms which probably originate within the OHC. OAE findings in patients with tinnitus are not univoque and there is no clear correlation between OAE modifications and tinnitus. We investigated distortion products in 20 normal hearing patients with tinnitus; all patients were selected with restrictive criteria (audiogram within 20 dB for all the frequencies, ABR and other tests normal, no history of ototoxic, nootropic drug intake, normal psychological evaluation, etc.). 12 patients out of 20 (60%) showed DP alterations. This finding is interpreted as an abnormality or a dysregulation of the efferent system (olivo-cochlear pathways) or of the other structures of the control loop which could modify outer hair cell activity in an otherwise normal cochlea with the development of tinnitus.     

Pathologic mechanoelectric transduction of outer hair cells as the cause of recruitment

It is believed that the sound-induced travelling wave in the mammalian cochlea is enhanced and sharpened by a positive feedback mechanism. This causes the passive linear basilar membrane growth function to become non-linear. The present paper shows that nonlinear basilar membrane vibration is due to the nonlinear growth function of the receptor potential of outer hair cells, which can be described by a 2nd-order Boltzmann function. Since intensity coding in the inner ear depends on an interaction of nonlinear basilar membrane motion and nerve fibers with three different types of synaptic threshold and growth function, the process is directly dependent on an intact mechanoelectrical transduction of outer hair cells. According to the proposed model, a loss in efficiency of outer hair cell mechanoelectrical transduction must lead to both a reduction in gain (i.e., hearing loss) and a linearizing of the response. As a result, once above threshold, the changes of stereociliary displacement, basilar membrane displacement and neural firing rate per unit change of sound intensity must be larger than for the healthy cochlea with its compressive nonlinearity.     

Acoustic trauma causes reversible stiffness changes in auditory sensory cells

A common cause of hearing impairment is exposure to loud noise. Recent research has demonstrated that the auditory mechanosensory cells are essential for normal hearing sensitivity and frequency selectivity. However, little is known about the effect of noise exposure on the mechanical properties of the auditory sensory cells. Here we report a significant reduction in the stiffness and cell length of the outer hair cells after impulse noise exposure, suggesting that mechanical changes at the cellular level are involved in noise-induced hearing loss. There is a recovery of the cellular stiffness and cell length over a two-week period, indicating an activation of cellular repair mechanisms for restoring the auditory function following noise trauma. The reduced stiffness observed at the cellular level is likely to be the cause for the downward shift of the characteristic frequency seen following acoustic trauma. The deterioration and the recovery of the mechanical properties of outer hair cells may form important underlying factors in all kinds of noise-induced hearing loss.     

Does the organ of Corti attempt to differentiate new hair cells after antibiotic intoxication in rat pups?

In the adult mammalian cochlea, post-injury hair cell losses are considered to be irreversible. Recent studies in cochlear explants of embryonic rodents show that the organ of Corti can replace lost hair cells after injury. We have investigated this topic in vivo during the period of cochlear development. Rat pups were treated with a daily subcutaneous injection of 500 mg/kg amikacin for eight consecutive days between postnatal day 9 (PND 9) and PND 16. During this period the organ of Corti is not fully mature, but hair cells are hyper-sensitive to aminoglycoside antibiotics. Scanning and transmission electron microscopy was used to evaluate morphological changes in the organs of Corti during the treatment and at different post-treatment periods, up until PND 90. A massive loss in outer and inner hair cells was observed at least as early as PND 14. A prominent feature in the apical part of cochleas at PND 21 and 35 was the transient presence of small atypical cells in the region of pre-existing outer hair cells. These atypical cells had tufts of microvilli reminiscent of nascent stereociliary bundles. A second striking observation was the replacement of degenerating inner hair cells by pear-shaped supporting cells throughout the cochlea. These cells were covered with long microvilli, and their basal pole was contacted by both afferent and efferent fibers, as in the early stages of inner hair cell maturation. At PND 55 and 90, these features were not clearly observed due to further cytological changes in the organ of Corti. It is possible that an attempt at hair cell neodifferentiation could occur in vivo after an amikacin treatment in the rat during the period of cochlear hyper-sensitivity to antibiotic.     

Hair cell loss as a function of age in the normal cochlea of the guinea pig

The surface specimen technique was used to study both spiral organs of 28 normal guinea pigs of four age groups: less than 24 hours, 6 weeks, 3 months and 1 year. Damaged hair cells were recorded for the whole of each spiral organ on cochleograms. The mean percentage number of outer hair cells damaged per age group was found to increase as a power function of age. In the animals aged less than 24 hours the mean percentage of damaged outer hair cells was 0.45%; in the 6-week animals, 1.85%; in the 3-month animals, 3.19%; and in the 1-year animals, 6.82%. At all ages outer hair cell loss was maximal in the third row, and towards the apex of the cochlea. Inner hair cell loss was very slight, with a maximum of 9 damaged inner hair cells per cochlea.     

Evidence for opening of hair-cell transducer channels after tip-link loss

The mechanosensitive transducer channels of hair cells have long been proposed to be gated directly by tension in the tip links. These are thin, elastic extracellular elements connecting the tips of adjacent stereocilia located on the apical surface of the cell. If this hypothesis is true, the channels should close after destruction of tip links. The hypothesis was tested pharmacologically using receptor currents obtained in response to mechanical stimulation of the stereociliary bundle of outer hair cells isolated from the adult guinea pig cochlea. Application of elastase (20 U/ml) or 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetra-acetic acid (BAPTA; 5 mM), both of which are known to disrupt tip links in other hair-cell preparations, led to the expected irreversible loss of receptor currents. However, the cells then displayed a maintained inward current, implying that channels were left permanently open. This current was similar in magnitude to the receptor current before treatment and was reduced reversibly by known blockers of mechanosensitive channels, namely, dihydrostreptomycin (100 microM), amiloride (300 microM), and gadolinium ions (1 mM). These observations suggest that the maintained current flows through the mechanosensitive channels. Electron microscopical analysis of isolated hair cells, exposed to the same concentrations of elastase or BAPTA as in the electrophysiological experiments, demonstrated an almost total loss of tip links in hair bundles that showed no evidence of other mechanical damage. It is concluded that although the tip links are required for mechanoelectrical transduction, the channels are not gated directly by the tip links.     

Correlation of clinical characteristics and small bowel histopathology in celiac disease

The reversible hearing loss in the nonoperated ear noted by patients after ear surgery remains unexplained. This study proposes that this hearing loss is caused by drill noise conducted to the nonoperated ear by vibrations of the intact skull. This noise exposure results in dysfunction of the outer hair cells, which may produce a temporary hearing loss. Estimations of outer hair cell function in the nonoperated ear were made by recording the change in amplitude of the distortion-product otoacoustic emissions before and during ear surgery. Reversible drill-related outer hair cell dysfunction was seen in 2 of 12 cases. The changes in outer hair cell function and their clinical implications are discussed.     

Upper airway motor outputs during vomiting versus swallowing in the decerebrate cat

Hair cells in the basilar papilla of birds have the capacity to regenerate after injury. Methods commonly used to induce cochlear damage are systemic application of ototoxic substances such as aminoglycoside antibiotics or loud sound. Both methods have disadvantages. The systemic application of antibiotics results in damage restricted to the basal 50% of the papilla and has severe side effects on the kidneys. Loud sound damages only small parts of the papilla and is restricted to the short hair cells. The present study was undertaken to determine the effect of local aminoglycoside application on the physiology and morphology of the avian basilar papilla. Collagen sponges loaded with gentamicin were placed at the round window of the cochlea in adult pigeons. The time course of hearing thresholds was determined from auditory brain stem responses elicited with pure tone bursts within a frequency range of 0.35-5.565 kHz. The condition of the basilar papilla was determined from scanning electron micrographs. Five days after application of the collagen sponges loaded with gentamicin severe hearing loss, except for the lowest frequency tested, was observed. Only at the apical 20% of the basilar papilla hair cells were left intact, all other hair cells were missing or damaged. At all frequencies there was little functional recovery until day 13 after implantation. At frequencies above 1 kHz functional recovery occurred at a rate of up to 4 dB/day until day 21, beyond that day recovery continued at a rate below 1 dB/day until day 48 at the 5.6 kHz. Below 1 kHz recovery occurred up to day 22, the recovery rate was below 2 dB/day. A residual hearing loss of about 15-25 dB remained at all frequencies, except for the lowest frequency tested. At day 20 new hair cells were seen on the basilar papilla. At day 48 the hair cells appeared to have recovered fully, except for the orientation of the hair cell bundles. The advantage of the local application of the aminoglycoside drug over systemic application is that it damages almost all hair cells in the basilar papilla and it has no toxic side effects. The damage is more extensive than with systemic application.     

Regeneration of cochlear efferent nerve terminals after gentamycin damage

Chickens recover auditory function after hair cell loss caused by ototoxic drug damage or acoustic overstimulation, indicating that mechanisms exist to reestablish appropriate neuronal connections to regenerated hair cells. However, despite similar hair cell regeneration times, hearing recovery takes substantially longer after aminoglycoside than after sound damage. We have therefore begun examining damage and regeneration of efferent nerve terminals by immunolabeling whole-mount cochleae for differentially localized synaptic proteins and by visualizing the distribution of label with confocal microscopy. In undamaged cochleae, the synaptic proteins synapsin and syntaxin show similar distribution patterns corresponding to the large cup-like terminals on short hair cells. After gentamycin administration, these terminals are disrupted as hair cells are lost, leaving smaller, more numerous synapsin-reactive structures in the sensory epithelium. Syntaxin reactivity remains associated with the extruded hair cells, indicating that the presynaptic membrane is still attached to the postsynaptic site. In contrast, after sound damage, both synapsin and syntaxin reactivity are lost from the epithelium with extruded hair cells. As regenerated hair cells differentiate after gentamycin treatment, the synapsin labeling associated with cup-like efferent endings reappears but is not completely restored even after 60 d of recovery. Thus, efferent terminals are reestablished much more slowly than after sound damage (), consistent with the prolonged loss of hearing function. This in vivo model system allows comparison of axonal reconnection after either complete loss (sound damage) or partial disruption (gentamycin treatment) of axon terminals. Elucidating the differences in recovery between these injuries can provide insights into reinnervation mechanisms.     

Anticholinergic effects of strychnine in the cochlea do not involve muscarinic receptors

Central control of cochlear function is mediated by the cholinergic (medial) efferent system and both muscarinic and nicotinic acetylcholine receptors are thought to be present on outer hair cells. All the physiological effects of acetylcholine in the cochlea are blocked by strychnine and we therefore investigated whether strychnine interacts with muscarinic receptors in the cochlea. The effects of strychnine on both (3H)-quinuclidinyl benzylate binding and atropine sensitive carbachol-induced (3H)-inositol phosphate formation were examined. Strychnine (1 to 50 microM) has no effect on either quinuclidinyl benzylate binding or carbachol (1 mM)-induced inositol phosphate synthesis. Moreover, strychnine does not change basal inositol phosphate metabolism. These data indicate that muscarinic receptors are not sensitive to strychnine at concentrations which are known to block the effects of acetylcholine on outer hair cells.     

Ototoxicity of sodium nitroprusside

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

Histopathological findings in clinical gentamicin ototoxicity

The temporal bone histopathological findings in a case of gentamicin sulfate-induced hearing loss and vertigo in an anephric patient undergoing hemodialysis are presented. A study of the sensory neuroepithelium of the cristae and maculae disclosed the presence of vacuoles with clubbing of the sensory cells. In the cochlea, loss of the innermost row of outer hair cells in the basal turn was the most prominent feature. These findings are discussed in light of reports of similar morphological changes in laboratory studies of gentamicin ototoxicity.     

Measurements and a model of the outer hair cell hydraulic conductivity

The dimensions of the apical surfaces of hair cells were measured in guinea pigs, aged from 3 weeks before term to 25 weeks after birth. In the basal two-thirds of the cochlea, the apical surfaces of the outer hair cells and their supporting cells changed with age, shrinking in a direction radial across the cochlear duct. There was an associated widening of the angle of the 'V' of the rows of stereocilia. Further apically, between 12 and 16 mm from the base of the cochlea, the outer hair cells and their supporting cells underwent the opposite change, becoming wider in a radial direction with age. The changes were seen before birth and continued for more than 3 weeks after birth. The results suggest that the guinea pig cochlea continues certain developmental processes for a considerable time after birth.     

Distortion-product otoacoustic emissions in kanamycin-treated guinea pig cochlea

Measurement of distortion-product otoacoustic emissions (DPOAE) is widely accepted as one of the most valuable tools for evaluating the frequency of specific cochlear pathology. Previous studies have revealed that distortion-product levels in the ear canal are definitely correlated with degree of damage in the cochlea. However, there seem to be no clear data of help in predicting the distribution and grade of damage in the cochlea quantitatively on the basis of the results of this non-invasive test. The present study is designed to assess correlations between degree of outer hair cell (OHC) damage by a potent ototoxic antibiotic, kanamycin, and DPOAE levels at the characteristic frequency at the site. Guinea pigs were used after daily intramuscular administration of kanamycin for 7 or 10 days. DPOAE levels were measured using a system (CUBDIS: Etymotic Research) with 78 frequency combinations of iso-intensity primaries from 0.5kHz to 16kHz of f2. The frequency ratio (f2/f1) was set at 1.2. Distortion-product level plots versus f2 (DP-grams) were constructed. The integrity of the OHC system was evaluated histologically by the succinic dehydrogenase (SDH) method under a light microscope. Cochleograms were constructed by calculating percentages of intact OHCs along the basilar membrane in 1-mm blocks. The DP-grams and the histopathological cochleograms showed essentially identical patterns in the kanamycin-damaged guinea pig cochlea. The results suggest that: 1) The generation of DPOAE requires functioning OHCs. 2) DPOAE measurement provides information allowing prediction of OHC damage distribution in the cochlea without histological investigations. 3) Careful setting of primary levels and other parameters is necessary to reliably predict the pathology. 4) Attempts to detect of minimal OHC damage could fail. 5) DPOAE seem very useful for monitoring cochlear function in clinically.     

Pathophysiological mechanisms of hearing loss

An understanding of auditory transduction in the ear can contribute to a better comprehension of the pathophysiological mechanisms which give rise to hearing loss. The incoming sound sets up a mechanical traveling wave which begins at the base and progresses along the basilar membrane, reaching a point of maximal displacement. The region of maximal displacement is a function of stimulus frequency. The mechanical displacement, by directly opening ion channels in the stereocilia of the hair cells, induces changes in the electrical potential of the hair cells. This initial stage is called mechano-electrical transduction, and in the normal ear, is followed by a stage of electro-mechanical transduction based on the ability of the outer hair cells to respond to the electrical changes induced in them with a change in their length. This "electromotility" presumably provides mechanical feedback to the basilar membrane, augmenting its mechanical displacement. This is called the cochlear amplifier, providing the ear with improved sensitivity and frequency discrimination. Most forms of sensori-neural hearing losses (affecting the inner ear) are due to a lesion to some part of this cochlear amplifier (e.g. noise induced hearing loss, ototoxic drugs) and are therefore characterized by auditory threshold elevations and poorer frequency discrimination.     

Basilar papilla explants: a model to study hair cell regeneration-repair and protection

Explants of basilar papillae from 6-7 days posthatch chicks were cultured in growth medium for a period of 1-8 days. Hair cells were counted following staining of stereocilia bundles with FITC-phalloidin, and the percentage of hair cell survival was determined by comparison to control (i.e. uncultured) specimens. Hair cell integrity was evaluated by scanning electron microscopy. Although previous studies have utilized organotypic culture of the basilar papilla to assess cell proliferation and ototoxicity, viability and integrity of hair cells was documented for periods of up to only 2 3 days. Our results demonstrate substantive auditory hair cell viability for a period of 7 days in vitro. We describe a pattern of natural hair cell loss in organotypic culture that progresses along a proximal-distal, abneural-neural gradient, mimicking the pattern of hair cell loss that occurs following ototoxic insult to the chick basilar papilla in vivo and the pattern we observed during a 48-h period of exposure of basilar papilla explants to an ototoxic dose of neomycin. Our results provide an important quantitative step for the use of organotypic culture of the chick basilar papilla as a purposeful model to investigate the process of hair cell regeneration-repair in the avian auditory system.     

Evidence for differential regulation of calcium by outer versus inner hair cells: plasma membrane Ca-ATPase gene expression

The expression of mRNA encoding plasma membrane calcium ATPase (PMCA) subunit isoforms (1-4) and splice variants was examined in the adult and developing rat cochlea by PCR and in situ hybridization. High levels of PMCA mRNA expression were observed in the neurons of the spiral ganglion, and in hair cells. Spiral ganglion neurons expressed PMCA 1-3 beginning in embryonic development, reaching high levels shortly after birth, and continuing into adulthood. Inner hair cells expressed PMCA 1 at moderate levels from birth to the time of onset of cochlear function on postnatal day 12, and strongly from then until adulthood. Outer hair cells expressed PMCA 2 at high levels from shortly after birth through adulthood. The data suggest that the calcium clearance requirements of inner and outer hair cells are distinct. PMCA 2 is the isoform with the highest affinity for calmodulin, and has also been associated with high levels of inositol triphosphate. Its presence in outer hair cells suggests that regulation of the enzyme by calmodulin may be particularly important for this hair cell type. It further suggests that inositol phosphate may play a unique role in the outer hair cell.