Active movements of the cuticular plate induce sensory hair motion in mammalian outer hair cells

Outer hair cells isolation from postnatal Sprague-Dawley rats

Outer hair cells (OHCs) damage is a general phenomenon in clinical disorders such as noise-induced hearing loss and drug-induced hearing loss. In order to elucidate the mechanism underlying these disorders, OHCs – its diseased region needs to be deeply investigated. However, OHCs array on the basilar membrane which contains massive cells with different types. Therefore, to isolate OHCs from this huge population is significant for revealing its pathological and molecular changes during disease processing. In the present study, we tried to isolate OHCs from the commonly used animal model –Sprague-Dawley (SD) rats. By separating outer hair cells from SD rats with different day ages, we found that 9 days after birth was a suitable time for the separation of the OHCs. At this time, the number of OHCs isolated from rats was large, and the cell morphology was typical of cylindrical shape. OHCs isolated using this method are histologically suitable and quantitatively adequate for molecular biological and electrophysiological analyses.

Cochlear outer hair cell motility

Normal hearing depends on sound amplification within the mammalian cochlea. The amplification, without which the auditory system is effectively deaf, can be traced to the correct functioning of a group of motile sensory hair cells, the outer hair cells of the cochlea. Acting like motor cells, outer hair cells produce forces that are driven by graded changes in membrane potential. The forces depend on the presence of a motor protein in the lateral membrane of the cells. This protein, known as prestin, is a member of a transporter superfamily SLC26. The functional and structural properties of prestin are described in this review. Whether outer hair cell motility might account for sound amplification at all frequencies is also a critical question and is reviewed here.

Contribution of outer hair cell bending to stereocilium deflection in the cochlea

The outer hair cell (OHC) in the cochlea is believed to actively enhance the cochlear sensitivity and frequency selectivity. Besides the well-known axial length change of the OHC, the bending mode of the OHC may also contribute to the stereocilium deflection. To investigate the contribution of the OHC bending to the stereocilium deflection, and the active process in the cochlea, we develop a simple kinematic model of the organ of Corti, consisting of the reticular lamina, the stereocilia and tectorial membrane. The electrically evoked axial length change and bending of the OHC are simulated, and their contributions to the stereocilium deflection are obtained. At the apical turn of the cochlea, the bending mode of the OHC results in stereocilium deflection comparable to that due to the axisymmetric length change of the OHC. At the basal turn, the contribution of the bending mode to the stereocilium deflection becomes insignificant compared to that of the axisymmetric mode.

Sensors, motors, and tuning in the cochlea: interacting cells could form a surface acoustic wave resonator

The outer hair cells of the cochlea occur in three distinct and geometrically precise rows and, unusually, display both sensing and motor properties. As well as sensing sound, outer hair cells (OHCs) undergo cycle-by-cycle length changes in response to stimulation. OHCs are central to the way in which the cochlea processes and amplifies sounds, but how they do so is presently unknown. In explanation, this paper proposes that outer hair cells act like a single-port surface acoustic wave (SAW) resonator in which the interdigital electrodes--the three distinctive rows--exhibit the required electromechanical and mechanoelectrical properties. Thus, frequency analysis in the cochlea might occur through sympathetic resonance of a bank of interacting cells whose microscopic separation largely determines the resonance frequency. In this way, the cochlea could be tuned from 20 Hz at the apex, where the spacing is largest, to 20 kHz at the base, where it is smallest. A suitable candidate for a wave that could mediate such a short-wavelength interaction--a 'squirting wave' known in ultrasonics--has recently been described. Such a SAW resonator could thereby underlie the 'cochlear amplifier'--the device whose action is evident to auditory science but whose identity has not yet been established.

Evidence for an active process and a cochlear amplifier in nonmammals

The last two decades have produced a great deal of evidence that in the mammalian organ of Corti outer hair cells undergo active shape changes that are part of a "cochlear amplifier" mechanism that increases sensitivity and frequency selectivity of the hearing epithelium. However, many signs of active processes have also been found in nonmammals, raising the question as to the ancestry and commonality of these mechanisms. Active movements would be advantageous in all kinds of sensory hair cells because they help signal detection at levels near those of thermal noise and also help to overcome fluid viscosity. Such active mechanisms therefore presumably arose in the earliest kinds of hair cells that were part of the lateral line system of fish. These cells were embedded in a firm epithelium and responded to relative motion between the hair bundle and the hair cell, making it highly likely that the first active motor mechanism was localized in the hair-cell bundle. In terrestrial nonmammals, there are many auditory phenomena that are best explained by the presence of a cochlear amplifier, indicating that in this respect the mammalian ear is not unique. The latest evidence supports siting the active process in nonmammals in the hair-cell bundle and in intimate association with the transduction process.

Selective aspects of human pathology in high-tone hearing loss of the aging inner ear

Accompanied with aging, the thresholds for high frequency sounds may elevate and result in a progressive hearing loss described as presbycusis. Based on correlations between audiometric measures of aged patients and histologic findings garnered from postmortem examinations, four types of presbycusis have been characterized: sensory-neural, neural, strial, and conductive [Schuknecht, H.F., Gacek, M.R., 1993. Ann. Otol. Rhinol. Laryngol. 102, 1--16]. Otopathologic changes to the inner ear as a direct function of age, however, remain controversial. The focus of this investigation involves the pathological impact on remaining sensory structures in patients having sensory--neural degeneration. The current study presents seven human temporal bones extracted from patients aged 53--67 years with high-tone hearing loss and with no known history of extraordinary environmental events involving head or noise trauma, acoustic overstimulation, or ototoxicity. In previously published findings of these specimens, all but one temporal bone failed to demonstrate a meaningful correlation between audiometric measurements and loss of functional hair cell populations with secondary retrograde degeneration of nerve fibers. Using the block surface method, electron microscopic micrographs demonstrate ultrastructural changes in the cuticular plate, stereocilia, pillar cells, stria vascularis, and the spiral ligament. In all pathological specimens, the greatest incidence of degeneration was seen at the cuticular plate. Conclusively, our findings present three implications in the aging human cochlea: firstly, audiometric measures that represent a high-tone hearing loss may take various forms with respect to ultrastructural patterns of degeneration and surviving structures; secondly, the incidence of lipofuscin and lysosome granules does not correlate with the degree of hearing loss and; thirdly, as shown only in guinea pigs [Anniko, M., 1988. Scanning Microsc. 2, 1035--1041], high-tone hearing loss can be associated with deformation of the cuticular plate.

The influence of hypothermia on outer hair cells of the cochlea and its efferents

Transient evoked otoacoustic emissions (TEOAE) were recorded in 21 guinea-pigs undergoing hypothermia. The minimal average body temperature during cooling was 26 degrees C/24.9 degrees C measured orally or rectally, respectively. The animals were subsequently warmed to normal body temperature. A clear influence of body temperature on TEOAE could be documented. During cooling the amplitude and reproducibilities decreased, disappearing completely at a mean temperature below 28.5 degrees C (oral) and 27.3 degrees C (rectal). The emissions reappeared during rewarming at a mean temperature of 30.1 degrees C (oral) and 30.8 degrees C (rectal). Contralateral auditory stimulation (CAS) led to a decrease of the amplitudes of TEOAE during cooling down to a mean of 33 degrees C/32 degrees C (oral/rectal temperature). During rewarming, influences of the CAS could be recognized, again at an oral temperature above 35 degrees C. The changes to the TEOAE observed in these experiments suggest that hypothermia affects not only the outer hair cells (OHC) of the cochlea but also the efferent supply to the cochlea.

P2 purinoceptor of the globular substance in the otoconial membrane of the guinea pig inner ear

The biological characteristics of the globular substance, a precursor of otoconia, are unclear. In the present study, the ATP-induced internal free Ca2+ concentration ([Ca2+]i) changes of the globular substance and the ATP distribution in the vestibular organ were investigated using a Ca2+ indicator, fluo 3, and an adenine nucleotide-specific fluorochrome, quinacrine, by means of confocal laser scanning microscopy. [Ca2+]i showed a rapid and dose-dependent increase in response to ATP with a 50% effective concentration (EC50) of 16.7 microM. This reaction was independent of external Ca2+, indicating the presence of an internal Ca2+ reservoir. Neither adenosine, alpha, beta-methylene-ATP, 3'-O-(4-benzoylbenzoyl)-ATP, ADP, nor UTP evoked this reaction, whereas 2-methylthio-ATP induced an increase of [Ca2+]i with an EC50 of 14.4 microM. Moreover, P2 antagonists, reactive blue 2 and suramin, and a phospholipase C inhibitor, U-73122, inhibited the ATP-induced [Ca2+]i increase. These findings indicate the presence of a P2Y purinoceptor on the globular substance. In addition, granular fluorescence was observed in the quinacrine-stained macular sensory epithelium, indicating the presence of ATP-containing granules in this tissue. These results suggest that a paracrine mechanism involving ATP may exist in the macula and that this mechanism regulates the biological behavior of the globular substance.

The voltage dependence of the mechanoelectrical transducer modifies low frequency outer hair cell electromotility in vitro

The fast outer hair cell (OHC) electromotility is voltage dependent and is driven by changes in the OHC transmembrane potential. Those changes include the receptor potential generated by the variable conductance of the mechanoelectrical transducer (Evans and Dallos, 1993). In the experiments described here, we show that the voltage dependence of the mechanoelectrical transducer influences the low frequency motile responses of OHCs to an external electrical field. OHCs were fully inserted into a glass suction pipette, the microchamber, so that only the cuticular plate and hair bundle were exposed to the bath solution. With this technique, a rectification of the mechanical response, equivalent to an excitatory displacement of the hair bundle, was observed when the command voltage inside the microchamber depolarized the apical membrane. The shape of the response persisted when the OHC voltage-gated conductances were blocked. Following treatment of the hair bundle with BAPTA or dihydrostreptomycin, which are known to impair transduction function (Assad et al., 1991; Kroese et al., 1989), rectification of the motile response disappeared.

A theoretical basis for the high-frequency performance of the outer hair cell's receptor potential

The frequency response of the outer hair cell (OHC) was studied theoretically. An electrical model of the OHC was analyzed mathematically, taking into account the effect of its inherent voltage-dependent capacitance. It was found that the variations of the capacitance dependent on the membrane potential could enhance the high-frequency response of the OHC, so that its cutoff frequency could be extended into the audio range. It was found further that the enhancement of the frequency response of the OHC was strongly dependent on its resting potential and on the ratio of the maximum voltage-dependent capacitance to the membrane linear capacitance.

Preservation of the non-rectangular cuticular plate/cell axis angle of outer hair cells

Motile properties of outer hair cells (OHCs) may contribute to sharp tuning and amplification in the mammalian cochlea. Shape changes of isolated OHCs in response to various physical and chemical influences have been investigated intensively. However, determinations of shape may have been influenced by unanticipated effects of preparation and preservation of the OHCs investigated. Thus, in a first step, lengths of freshly isolated OHCs from the guinea pig cochlea were determined using a video-enhancing magnification system. The cuticular plate/cell axis angle (CP/CA angle) was then measured in native cells and under the influence of potassium chloride and potassium gluconate incubation. To show the influence of glutaraldehyde (GA) fixation on the isolated OHCs, fixative-dependent changes on cell length and CP/CA angle were recorded in native and preincubated OHCs. In these experiments, the cell length of vital isolated OHCs was between 41.5 micrometers, in the basal turn, and 103.7 micrometers, in the apical turn. The average CP/CA angle was 106 degrees +/- 4.2 degrees (n = 324 cells, turns 1-4) with no statistically significant differences for the four turns. Under the influence of potassium chloride, cell length was reduced by 8.1%. Potassium gluconate incubation led to a shortening of cell length, followed by a 5.3% increase after 5 min. The CP/CA angle under potassium chloride was decreased (97.0 degrees) and was then increased under the influence of potassium gluconate (110.7 degrees) as a result of cuticular plate tilting. Cell shrinkage after fixation depended on the fixative's osmolarity and on the GA concentration. Increased GA levels amplified cell shrinkage from 34% for hypo-osmolar solutions to 15% in iso-osmolar and 29% in hyperosmolar solutions. The CP/CA angle of native and incubated OHCs was not different from those fixed with GA. The present data provide a rational basis for isolated OHC shape parameters. Moreover, functionally induced changes can be better interpreted when OHCs are influenced by fixatives, as shown in the GA experiments.

Chemical synaptic transmission in the cochlea

The last two decades have witnessed major progress in the understanding of cochlear mechanical functioning, and in the emergence of cochlear neurochemistry and neuropharmacology. Recent models describe active processes within the cochlea that amplify and sharpen the mechanical response to sound. Although it is widely accepted that outer hair cells (OHCs) contribute to these processes, the nature of the medial efferent influence on cochlear mechanics needs further clarification. Acetylcholine (ACh) is the major transmitter released onto OHCs during the stimulation of these efferents. The inhibitory influence of this system is mediated by post- and presynaptic nicontinic and muscarinic receptors and the role of other neuroactive substances [gamma-aminobutyric acid (GABA), calcitonin gene-related peptide (CGRP), adenosine 5'-triphosphate (ATP) or nitric oxide (NO)] remains to be determined. The inner hair cells (IHCs) that transduce the mechanical displacements into neural activity, release glutamate on receptor-activated channels of AMPA, kainate, and NMDA types. This synapse is in turn controlled and/or regulated by the lateral efferents containing a cocktail of neuroactive substances (ACh, GABA, dopamine, enkephalins, dynorphin, CGRP). This glutamatergic nature of the IHCs is responsible for the acute destruction of the nerve endings and subsequently for neuronal death, damage usually described in various cochlear diseases (noise-induced hearing losses, neural presbycusis and certain forms of sudden deafness or peripheral tinnitus). These pathologies also include a regrowth of new dendritic processes by surviving neurons up to IHCs. Understanding the subtle molecular mechanisms which underly the control of neuronal excitability, synaptic plasticity and neuronal death in cochlear function and disease is a very important issue for the development of future therapies.

The arrangements of F-actin, tubulin and fodrin in the organ of Corti of the horseshoe bat (Rhinolophus rouxi) and the gerbil (Meriones unguiculatus)

The composition of cytoskeletal elements in hair cells and non-sensory cells was studied in paraformaldehyde fixed cochleae of the horseshoe bat and the gerbil using phallotoxins and antibodies directed against actin, alpha-tubulin and fodrin. In both species, cryostat sections of the organ of Corti were studied using confocal fluorescence microscopy; in the bat, ultrathin sections were investigated using actin-immunoelectron and classical electron microscopy. F-actin was found in stereocilia and cuticular plates of inner and outer hair cells (IHCs and OHCs) of both species. In fixed material from both species, no F-actin staining was detected in the cytoplasm or along the lateral cell membrane of OHCs, whereas in freshly isolated OHCs of the gerbil, a faint F-actin staining was detected along the lateral wall. In the bat, the patterns of F-actin staining were confirmed with actin-immunoelectron microscopy. The alpha-tubulin antibody strongly labeled IHCs of both species. They contained a complex network of microtubules especially in the neck portion. In the bat, OHCs showed no distinct alpha-tubulin reactivity, as would be expected given the scarcity of microtubules observed at the ultrastructural level. In the gerbil, alpha-tubulin reactivity was found throughout the OHC body with highest intensity in the cell apex. In Deiters cells, pillar cells and Boettcher cells of both species, F-actin and microtubules were colocalized at contact zones with the basilar membrane. In Deiters cups, F-actin staining was most pronounced in the basal turn of the bat cochlea. In the gerbil, a distinct baso-apical gradient was found in immunostaining properties and morphology of the Deiters cells. Intense fodrin reactivity was found in the cuticular plates and along the lateral cell membrane of both types of hair cells of the bat. Cytoplasmic fodrin staining was localized within the IHCs of the bat. In the gerbil, intense fodrin staining was only found in cuticular plates of hair cells and staining of the lateral cell membrane of hair cells was faint. A faint fodrin staining was also seen in Deiters cells of both species. The basic arrangement of the cytoskeletal elements in the batś organ of Corti is similar to that of other mammals, however, certain features suggest the presence of subtle differences in micromechanical properties: there is an increased concentration of microtubules in the neck portion of IHCs, an increase in the amount of F-actin within the Deiters cups and a reduced amount of microtubules in the OHCs.

Electromotile responses and frequency tuning of isolated outer hair cells of the guinea pig cochlea

Isolated outer hair cells (OHC) of the guinea pig cochlea were exposed to external alternating electric fields parallel to the longitudinal axis of the cells. This resulted in oscillations of the cells' length that were measured photoelectrically using a ratiometric light amplifier. At 5 Hz and elongations up to 300 nm, amplitude of the cell length during oscillation was a linear function of the amplitude of the sinusoidal electric field. When increasing the stimulus frequency up to 32 kHz, OHC length changes followed the stimulus cycle-by-cycle. Oscillations at frequencies above 32 kHz escaped the experimental approach by their small amplitudes and could not be excluded. The frequency dependence of the motile response measured at 5-12,000 Hz had low-pass filter characteristics in cells of the second, third and fourth turns of the cochlea. However, frequency tuning of the motile response was absent in each OHC and systematic differences between different turns were not observed.

Localization of F-actin and fodrin along the organ of Corti in the chinchilla

The distribution of the two cytoskeletal proteins, filamentous actin (F-actin) and fodrin, was investigated along the organ of Corti of the chinchilla using laser scanning confocal fluorescence microscopy. High intensity labeling of F-actin was seen in outer and inner hair cells, including the stereocilia. High intensity staining was also seen for fodrin in outer and inner hair cells, but not in their stereocilia. Staining intensity of both proteins along the lateral cell wall of the outer hair cells appeared to be greater in the middle and basal cochlear turns than in the apical turn. Pillars and Deiters cells also exhibited high intensity labeling of F-actin. The lack of significant differences in the distribution of fodrin between outer and inner hair cells makes the role of this protein in the active processes still unclear. Comparison of the distribution of F-actin and fodrin in the chinchilla with those reported in the guinea pigs suggest possible species differences.

Mechanical and electromechanical properties of the stereovillar bundles of isolated and cultured hair cells of the chicken

Isolated single chicken hair cells and pieces of epithelium without the tectorial membrane, either freshly isolated or in tissue culture, were studied using water-jet stimulation of their stereovillar bundles and current injection. Responses were measured under enhanced video-microscopic observation or while using a differential photodiode technique sensitive down into the nanometer range. When stimulated with a water jet at low displacement amplitudes up to about 200 nm, the stereovillar bundle displacement was asymmetrical, indicating a lower stiffness in the excitatory direction, but the reverse was true at higher displacement amplitudes. Undamaged bundles showed no mechanical resonances below 1 kHz. In damaged bundles, however, such resonances were prominent and accompanied by splaying of the stereovilli. Hair cells in the epithelium showed small bundle movements (0.6 nm/mV) whose polarity depended on the polarity of the injected current. These movements probably resulted from activation of the bundle's adaptation motors.

Effects of adenosine 5'-triphosphate and related agonists on cochlear function

Several lines of evidence implicate a neurotransmitter/modulator role for ATP in the cochlea. Most of the work supporting such a notion has been accomplished using in vitro preparations of sensory hair cells or other cochlear tissues. Little is known regarding the functional consequences of ATP receptor activation in vivo. In the present experiments, we tested ATP and related agonist analogs for their effects on sound-evoked responses of the cochlea (cochlear microphonic, CM; summating potential, SP; distortion product otoacoustic emissions, DPOAE) and auditory nerve (compound action potential, CAP) in vivo and on outer hair cell (OHC) currents and cell length in vitro. In vivo, local application of these compounds was associated with concentration- and intensity-dependent response alterations. The slowly-hydrolyzable P2y agonist, ATP-gamma-S, was clearly of greatest in vivo potency: At low to moderate stimulus intensities, micromolar concentrations of this drug reduced all responses, in particular CAP and DPOAEs, which fell to the level of the noise floor. At high intensities, response suppression was smaller and SP was increased. In vivo effects of ATP, ATP-alpha-S and 2-Me-S-ATP were qualitatively similar to, but smaller in magnitude and requiring higher concentrations than those observed for ATP-gamma-S. Adenosine was without significant effect on responses of the cochlea and auditory nerve. In vitro, effects of ATP-gamma-S and ATP were similar: both induced inward currents in OHCs held at -60 mV without producing observable (> 0.3 micron) changes in OHC length. Results suggest that endogenous ATP influences cochlear function through receptors at several sites in the cochlea. Results suggest further that these response alterations are mediated, at least in part, by receptors of the P2y subtype.

The cuticular plate of the hair cell in relation to morphological gradients of the chicken basilar papilla

The aim of the present study was to provide details on the diversity and morphological gradients in the anatomy of the cuticular plate of hair cells in the chicken basilar papilla. The structure of the cuticular plate, which is mainly made up of a network of actin filaments, may be related to differences in the mechanical demands on the anchorage of the stereovillar bundle. We describe the morphological gradients in the cuticular plates as seen in transverse section for four positions along the basilar papilla. Three different shapes of cuticular plate could be distinguished. In general, cuticular plates in neurally-lying hair cells have their main mass on the neural side of the cells; for abneural cells, the converse is true. The shape of the plates changes gradually across the papilla; symmetrical forms exist. The hair-cell bundle orientation (and thus the preferred direction of stimulation of the bundle), as measured using scanning EM preparations, does not correlate with the shape of the plate in transverse section. The present data confirm the notion developed from other studies that (1) there are no distinct populations of hair cells, (2) there are no linear or monotonic morphological gradients, and (3) the gradients on the papilla are species- and position-specific.

Differences of inner and outer hair cells in the organ of Corti of the guinea pig in respect to the cellular content of precipitable calcium

Differences between inner and outer hair cells in the cellular content of precipitable calcium were detected using a potassium pyroantimonate precipitation method and the electron spectroscopic imaging (ESI-) technique. The cytoplasm of the inner hair cells was scattered with a high number of calcium precipitates in all analysed animals, but only a few reaction products could be identified in the cytoplasm of the outer hair cells in all analyzed specimens. Even the well developed system of the subsurface fenestrated cisternae in the outer hair cells was nearly empty of calcium precipitates. A relatively high amount of reaction products could be identified in the nuclei of both types of nerve endings of the receptor cells. Significant differences regarding the content of precipitable calcium were found in the different types of nerve endings, which come into contact with the basal parts of both receptor cells. The observed differences in the content of precipitable calcium between the two types of hair cells are discussed with respect to their probable different roles in signal transduction processes.

Confocal laser microscopical images of calcium distribution and intracellular organelles in the outer hair cell isolated from the guinea pig cochlea

We report the use of a confocal laser fluorescence microscope to observe the distribution of cytosolic Ca2+ and the localization of intracellular organelles and cytoskeleton in the isolated outer hair cell (OHC). Membrane-bound Ca2+ stained by chlortetracycline was mainly seen in the subcuticular region, the infranuclear region, and the region adjacent to the lateral wall. In contrast, the central portion of the cytoplasm and nucleus were devoid of detectable fluorescence of membrane-associated Ca2+, but were relatively rich in free Ca2+. The cuticular plate showed a lack of both membrane-bound and free Ca2+. Fluorescent clusters of mitochondria and endoplasmic reticulum were predominantly seen in the infracuticular and infranuclear regions, and some were associated with the lateral wall. These two types of cytosolic organelles which fluoresced upon chlortetracycline treatment are therefore presumed to sequester calcium. The characteristic distribution of the endoplasmic reticulum was observed in coincidence with the infracuticular network of F-actin. The subsurface cistern, which was shown to be analogous to the endoplasmic reticulum in terms of its biological function, is likely to be the source of Ca2+ for the actin-mediated process.

Possible roles of outer hair cell d.c. movements in the cochlea

Outer hair cells are expected to change the position of the cochlear partition in response to sound stimulation. This may contribute to the extended dynamic range of the cochlea. This paper explores the possible underlying mechanisms and the consequences for models of cochlear function of such position changes.

Immunohistochemical localization of intracellular Ca-ATPase in outer hair cells, neurons and fibrocytes in the adult and developing inner ear

Intracellular isoforms of the enzyme Ca-ATPase were identified in the inner ear by immunostaining paraffin sections with a polyclonal antiserum against rabbit cardiac muscle Ca-ATPase. In the adult cochlea, intense staining was present at the lateral border of outer hair cells in regions corresponding with the distribution of the subsurface cisternal system. Other cell types containing high levels of Ca-ATPase were skeletal muscle fibers in the tensor tympani, vascular smooth muscle, spiral ganglion neurons and subpopulations of fibrocytes in the limbus, spiral ligament and underlying vestibular neurosensory epithelium. In neonatal gerbils, staining of tensor tympani muscle fibers was observed at 4 days after birth and approached adult levels by 8 days after birth. Ca-ATPase was first detected in other cell types between postnatal days 12 and 14 but immunostaining still remained well below the intensity seen in adults at 20 days after birth. The demonstration of abundant calcium pumps in the subsurface cisternae confirms the role of this organelle as an intracellular reservoir for Ca2+ in outer hair cells. The presence of high levels of Ca-ATPase in spiral ganglion neurons and in fibrocytes specialized for ion transport points to a role for the enzyme in regulating the activity of other cell types of importance to normal hearing.

Sound preprocessing by ac and dc movements of cochlear outer hair cells

In inner and outer hair cells, a sound event results mechano-electrically in a receptor potential from the hair cells by the functioning of apical and lateral K(+)-channels. However, after this point, the signal transfer is divided. Inner hair cells (IHC) release an unknown afferent transmitter. By contrast, outer hair cells (OHC) are proposed to produce mechanical ac and dc responses. In our model, the ac components of the sound signal, the carrier frequencies, determine the response of the OHC. Usually, they respond by ac and dc movements. The rapid ac movements of OHC, for which the underlying mechanism is unknown, may respond cycle-by-cycle to and interfere with the carrier frequency of the traveling wave. Near hearing threshold, they could drastically amplify the traveling wave thus contributing to the postulated cochlear amplifier. Active dc movements of the cytoskeleton of the cell body, as well as of the cuticular plate with the sensory hairs, are proposed to respond to millisecond changes of the sound stimulus over time. Such changes could be a modulation of the amplitude (AM), i.e., an increase or decrease of the sound pressure level (SPL), which is reflected in the envelope of the traveling wave. The active mechanical dc response of OHC to the amplitude (AM) and frequency modulation (FM) pattern is then expected to result in dc position changes of the reticular lamina (RL). These should control the operation point of the stereocilia, thus influencing their transfer function and sensitivity. In addition, experimental data suggest that there are modulations of the compliance of the organ of Corti (OC) and changes of its geometry. This dc modulation of micromechanical properties and geometry of the OC by active force generation of OHCs might contribute to automatic gain control, adaptation, TTS, as well as to the homeostasis of the basilar membrane location. In particular, the motile mechanism may protect the vulnerable cochlear partition against high sound pressure levels. Moreover, according to this model, changes of the sound signal with time are expected to be encoded in the actively produced dc movements of the RL. As the signal changes may carry important information (e.g., complex sound signal modulations such as formant transitions in speech), this is extracted and mechanically encoded by the proposed active dc mechanism. It cannot be excluded that the information-carrying dc signal is transferred to inner hair cells contributing to their adequate stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)

High frequency radial movements of the reticular lamina induced by outer hair cell motility

Recently, it was shown in cochlear explants from the guinea pig cochlea that electrokinetic motile responses of outer hair cells can induce radial and transverse motion of the reticular lamina. Here we demonstrate, that the radial component of these motions can be measured up to high frequencies (15 kHz). Cochlear explants were taken from guinea pig inner ears and exposed to a sinusoidal electric field. A double photodiode was used as a linear position detector with high spatial and temporal resolution to detect radial movements in the plane of the reticular lamina. The organ of Corti of the second, third and fourth cochlear turns was stimulated with frequencies of the electrical field between 0.5 Hz and 20 kHz. Sinusoidal movements of up to 15 kHz were recorded. At higher frequencies the signal-to-noise ratio became too small. The largest responses were measured at the three rows of outer hair cells. If the strength of the electrical field was 2 kV/m, into which the cochlear explants were placed, the amplitudes of outer hair cell movements were around 1 micron at 1 Hz and 10 nm at 10 kHz. Uncoupling of the outer hair cells from the tunnel of Corti and from the inner hair cells decreased the oscillations of inner hair cells but did not affect outer hair cells. The movements showed frequency dependent amplitudes like a complex low-pass filter but no best frequency was observed.

Distribution of F-actin and fodrin in the hair cells of the guinea pig cochlea as revealed by confocal fluorescence microscopy

We double-stained paraformaldehyde fixed guinea pig cochleas with rhodaminated phalloidin to detect F-actin and with a monoclonal antibody against non-erythroid spectrin (fodrin). The hair cells were studied in surface specimens of the organ of Corti with confocal fluorescence microscopy. In serial optical sections, phalloidin stained the stereocilia, cuticular plate, and a circumferential ring beneath it in the inner and outer hair cells (IHCs and OHCs). The cytoplasm of the IHCs and the OHCs was unlabelled, but the infracuticular network of the OHCs in the upper turns showed a strong reaction. The lateral plasma membrane was unreactive with phalloidin in the IHCs and OHCs, except in the basal turn, where a moderate reaction, probably representing actin of Deiter's cups, was seen along the lateral walls of the basal pole of the OHCs. Fodrin was similarly seen in the cuticular plate, in a circumferential ring beneath it, and in the infracuticular network of the apical OHCs. The most interesting finding was the fodrin-specific distinct labelling of the lateral cell surface in the OHCs of the basal cochlear turn. This staining diminished towards the apex and was practically absent in the OHCs located above the level of 15 mm from the round window. The lateral cell surface of IHCs showed moderate fodrin labelling in all cochlear turns. This staining was much weaker than that seen in the basal OHCs. Fodrin labelling revealed deformation from the regular cylindrical shape in midportion of the OHC bodies in the basal turn of the cochlea.

Alternating current induced otoacoustic emissions in the guinea pig

Injection of alternating current (AC) into the scala media of the guinea pig cochlea induced otoacoustic emissions (OAEs) at the frequency of the AC fundamental, together with harmonic and intermodulation distortion products. Although the waveform of the injected ACs was distorted, probably due to nonlinear polarization of the metal electrodes, and was composed of the fundamental plus distortion products of every order, only a few of the lowest order distortion products were selectively emitted with the fundamental. AC injection at a basal site extended the high frequency limit of OAEs. Electrical stimulation of the crossed olivocochlear bundle inhibited the sideband emissions with little change in the fundamental. OAE was reduced reversibly by temporary impairment of the cochlea due to exposure to fatiguing sound, by intravenous application of furosemide and by temporary anoxia. Irreversible reduction resulted from intracochlear perfusion with excess K+ solution, acoustic trauma and cardiac arrest. These facts imply that AC-induced OAE is not an artifact generated electrically; rather, such emissions originate in the cochlea and normal metabolic activity in the cochlea is essential. A proposed mechanism of generation includes two components: 1) electromechanical transduction from AC to mechanical vibration in the cochlea and 2) a distortion-producing process; the contribution of each component to the receptor mechanism is discussed.

Anatomical mapping of choline acetyltransferase (ChAT)-like and glutamate decarboxylase (GAD)-like immunoreactivity in outer hair cell efferents in adult rats

The distribution of choline acetyltransferase (ChAT)-like and glutamate decarboxylase (GAD)-like immunoreactivity in the cochleae of 15 adult Wistar white rats was investigated using the peroxidase-antiperoxidase (PAP) technique. A monoclonal antibody to ChAT and a polyclonal antiserum to GAD were used. Immunoreaction was investigated quantitatively, in the electron microscope, on tangential sections of the tunnel of Corti and the rows of outer hair cells. ChAT-like and GAD-like immunoreactivity was found in all efferent nerve fibres in the tunnel of Corti and in all efferent synapses on the outer hair cells. A coexistence of ChAT and GAD in the efferent system to the outer hair cells of the rat is therefore assumed.

The organ of Corti in the bat Hipposideros bicolor

The bat Hipposideros bicolor (Hipposideridae, Microchiroptera) is the mammalian species with the highest upper limit of hearing in which the structure of the organ of Corti has been studied. H. bicolor emits pure tone echo-locating signals of 153 kHz, compensates for Doppler shifts in the echo and hears ultrasonic frequencies up to 200 kHz (Neuweiler et al., 1984). The organ of Corti was investigated qualitatively and quantitatively using the technique of semi-thin sectioning. Some complementary ultra-thin sections were also examined. Length, width and cross-sectional area of the basilar membrane, the tectorial membrane, the hair cells with their stereocilia and the organ of Corti were measured at equi-distant positions on the basilar membrane. The organ of Corti of H. bicolor is composed of elements similar to those found in the cochleae of other eutherian mammals studied. However, in H. bicolor some of these elements show species-specific differences when compared to auditorily unspecialized mammals. The most basal region of the cochlea is characterized by miniaturization and re-inforcement of macro- and micro-mechanically important elements. This is interpreted as an adaptation for hearing extremely high frequencies. Specialized structures as well as local maxima of 'normal' elements in the basal and middle cochlear region are associated with evaluation of the echos of emitted pure tones. Besides the basal specializations. Hipposideros also shows specializations in the apical, low frequency, region which can be correlated with passive acoustic orientation.

Lowering extracellular calcium decreases the length of isolated outer hair cells

Transduction by the inner hair cells is hypothesized to be modulated through a change in the length of the outer hair cells (OHC). It has been suggested that the slow change occurring in OHC length is mediated by an actin-myosin system requiring Ca2+ and ATP. This study was designed to systematically examine the effects of lowering extracellular Ca2+ on OHC length. OHCs were isolated from guinea pig cochleae, mechanically dissociated and dispersed, and placed in a Hank's balanced salt solution (HBS). Exposing the cells to a Ca(2+)-free HBS supplemented with 200 microns EDTA produced a shortening in OHC length with a concomitant increase in cell width. The shortening was reversed successfully by bathing the cells in 8 mM Ca2+. We speculate that the decrease in length due to lowering extracellular Ca2+ may be caused by a relaxation of a circumferential contractile mechanism which is thought to cause elongation of intact OHCs (Slepecky, 1989; Dulon et al., 1990).

Isolation of and calcium kinetics in cochlear inner hair cells of the guinea pig

Single inner hair cells of the guinea pig cochlea were isolated using enzymatic and mechanical techniques. The intracellular free calcium ion concentrations [( Ca2+]i) of the isolated inner hair cells were determined using the Ca2+ sensitive dye fura-2 and digital imaging microscopy. In the presence of 1 micron ionomycin, a Ca2+ ionophore, there was an irreversible increase in [Ca2+]i. The 150 mM KCl stimulation, which induces a depolarization, resulted in a temporary increase in [Ca2+]i. This increase in [Ca2+]i was not observed under conditions of depolarization, in Ca(2+)-free medium. These observations are interpreted to mean that the [Ca2+]i during membrane depolarization mainly originates from an influx of extracellular Ca2+ into the cytoplasm.

Influence of collagenase on tip links in hair cells of the chick basilar papilla

Chick basilar papillae were incubated in collagenase, using concentrations previously employed for the isolation of viable hair cells. When assessed by scanning electron microscopy, the hair bundles had a normal conformation, with no loss of tip links compared with control incubations. The results suggest that collagenase dose not destroy the integrity of structures on the apical surface of hair cells, and that tip links are not composed of collagen. The results are in agreement with the hypothesis that tip links are involved in mechanotransduction.

Control of intracellular calcium by ATP in isolated outer hair cells of the guinea-pig cochlea

1. Intracellular calcium levels were monitored in isolated outer hair cells of the guinea-pig cochlea using the calcium-sensitive dye Fura-2. 2. The calcium in the cells was studied during application of ATP externally applied from a pipette. ATP induced a rise of intracellular calcium which could be separated into two components: a rapid rise, peaking in 20 s, localized around the apical end of the cell, and a slower rise, peaking in 50-150 s but spread throughout the cell. The effects were observed with 5, 25 and 100 microM-ATP concentrations. 3. In the absence of external Ca2+, ATP was still able to trigger a rise in Ca2+, but with a longer delay. Under these conditions, the cells did not show the initial rapid Ca2+ rise. The result suggests that ATP can mobilize intracellular stores. 4. A rise in intracellular Ca2+ was also observed when 5 mM-caffeine was applied to the bath. 5. Simultaneous measurements were made of whole-cell currents and intracellular calcium. ATP activated an inward current at resting potentials of -60 mV. Internal Ca2+ levels increased during the inward current. In current-clamped cells Ca2+ levels also increased during the associated depolarization produced by ATP. 6. Adenosine (150 microM) did not produce any measurable inward current. Acetylcholine (ACh, 100 microM-1 mM) produced only a small rise in Ca2+. However, applied simultaneously with ATP, ACh suppressed the rise in intracellular Ca2+ produced by ATP, with the kinetics of a competitive antagonist. 7. Intracellular Ca2+ increased with step depolarizations of the cell above -20 mV during whole-cell clamp. Large rises in Ca2+ were also observed on depolarizing the cell with isotonic KCl. 8. Calcium levels in supporting cells of the organ of Corti were sensitive to ATP. In these cells, rises in intracellular Ca2+ did not require the presence of extracellular Ca2+. 9. It is concluded that the organ of Corti contains receptors for ATP on a variety of the cells. ATP controls a direct entry of Ca2+ through the membrane and also may mobilize intracellular stores.

Acetylcholine, carbachol, and GABA induce no detectable change in the length of isolated outer hair cells

The mechanical and electrical properties of cochlear outer hair cells (OHCs) are suggested to modulate transduction by inner hair cells. These properties of OHCs are presumably regulated by efferent neurons which use several transmitters including acetylcholine (Ach) and gamma aminobutyric acid (GABA). Since it had been suggested that Ach causes isolated OHCs to shorten visibly, this study was designed to investigate whether GABA also alters the length of OHCs. OHCs were isolated from the guinea pig cochlea by mechanical dispersion after collagenase treatment. Cells were initially selected by strict morphological criteria. In addition they were only included in further studies if they attained a constant length during 10 min of superfusion with buffer solution. Neither GABA (20 microM: 100 microM), Ach (5 mM; 10 microM with 10 microM eserine) or carbachol (10 microM; 100 microM) altered OHC length when applied in iso-osmotic Hank's balanced salt solution (total number of cells tested, 72). If a change in length occurred it must have been smaller than 0.3 microns, our detection ability. In contrast, high potassium and variations in osmolarity changed hair cell length by 3-10% in agreement with other reports.

Phantom auditory perception (tinnitus): mechanisms of generation and perception

Phantom auditory perception--tinnitus--is a symptom of many pathologies. Although there are a number of theories postulating certain mechanisms of its generation, none have been proven yet. This paper analyses the phenomenon of tinnitus from the point of view of general neurophysiology. Existing theories and their extrapolation are presented, together with some new potential mechanisms of tinnitus generation, encompassing the involvement of calcium and calcium channels in cochlear function, with implications for malfunction and aging of the auditory and vestibular systems. It is hypothesized that most tinnitus results from the perception of abnormal activity, defined as activity which cannot be induced by any combination of external sounds. Moreover, it is hypothesized that signal recognition and classification circuits, working on holographic or neuronal network-like representation, are involved in the perception of tinnitus and are subject to plastic modification. Furthermore, it is proposed that all levels of the nervous system, to varying degrees, are involved in tinnitus manifestation. These concepts are used to unravel the inexplicable, unique features of tinnitus and its masking. Some clinical implications of these theories are suggested.

The cell potential of isolated inner hair cells in vitro

Inner hair cells (IHCs) were isolated from the 3rd and 4th turn of the guinea-pig cochlea using a new microsurgical technique. Microelectrode impalements in vitro with conventional microelectrodes yielded intracellular potentials of -14 +/- 7 mV (mean +/- SD, N = 43) at 24 degrees C. This suggested depletion of intracellular K+ in the isolated IHCs. Whole-cell recordings with patch-clamp suction electrodes of 11 +/- 3 M omega when filled with KCl Ringer demonstrated reloading of the cytoplasmic compartment with K+ (time constant t = 26.3 +/- 3.5 s), resulting in stable intracellular potentials after equilibration of -58.7 +/- 7.3 mV (mean +/- SD, N = 6) which is close to the K+ Nernst potential. The results suggest that IHC membranes are predominantly potassium-permeable. Assuming that in vivo the apical membranes of IHCs are exposed to endolymph-like fluid, the lower resting potentials of IHCs can be explained with the potassium permeability ratio of their apical and basolateral membranes. In whole-cell recordings, injection of hyperpolarizing current induced larger changes of the intracellular potential than depolarizing current. Oscillatory potential fluctuations were not observed, neither spontaneous nor in response to rectangular current pulses.

Rotated stereociliary bundles and their relationship with the tectorial membrane in the guinea pig cochlea

Hair cells with rotated stereociliary bundles have been observed in the cochleae of control and kanamycin-treated guinea pigs. The affected outer hair cell bundles have a variable degree of rotation, with some being completely reversed. The inner hair cells are more rarely affected, and only small areas of an individual inner hair cell bundle are abnormal. In counts from ten cochleae, the number of outer hair cells with rotated bundles was most commonly between 10% and 20%, with almost 27% of all outer hair cells affected in the most extreme case. The rotated outer hair cell bundles often have distorted outlines but in other respects closely resemble normal bundles. In particular, they have the usual gradation in stereociliary height, intracellular cross-links and intercellular links to adjacent normally-orientated bundles. There are also corresponding imprints in the tectorial membrane which match the pattern of the stereocilia. In kanamycin-treated guinea pigs, imprints of both normal and rotated hair bundles are present, even when the corresponding bundle is absent, and there are frequently remnants of stereocilia inserted in the imprints. These observations suggest that, apart from their abnormal orientation, the rotated bundles are similar to normal bundles in both their organization and association with the tectorial membrane. The implications of this with respect to transduction and cochlear mechanics are discussed.

Active radial and transverse motile responses of outer hair cells in the organ of Corti

In isolated outer hair cells (OHCs) electrically induced movements of high frequencies have been described. The experiments, however, gave no information whether fast OHC motility exists in situ. In the present report, we developed a technique to prepare viable half turn explants from the guinea-pig cochlea which could be kept as organ culture. Several video imaging methods and pixel-by-pixel, digital-image subtractions allowed simultaneous observations and quantitative measurements at video rates of 688 x 512 localizations of investigated segments of the organ of Corti (OC). When living cochlea explants were exposed to an electrical a.c. field, the OHCs in the OC followed this field by shortenings and elongations of their cell bodies and by radial movements of their cuticular plates (CP). This was accompanied by radial displacements of the hair bundles. In the apical turns video stroboscopy allowed recording of in situ movements of OHCs up to auditory frequencies. In all experiments motile responses were most prominent in the three rows of the OHCs. No or less pronounced passive motile responses could be observed at the tunnel of Corti (TC) and in the inner hair cells (IHCs). Mechanical decoupling of OHCs and IHCs at the TC resulted in a loss of IHC movements, whereas OHCs were uneffected. Motility was detectable in the presence of physiological salt solutions (300 mOsm/l) and in iso-osmolar mannitol or sorbitol solutions. The electrically induced motile responses were not suppressed in the presence of dinitrophenol or cytochalasin B. Thus, the present report shows active transverse and radial motile responses of OHCs in the OC, which are electro-mechanical in situ processes. The results indicate how outer hair cell electromotility may influence hearing when it occurs within the mechanical framework of the OC.

The responses of cochlear hair cells to tonic displacements of the sensory hair bundle

Hair bundle displacements and receptor potentials were recorded from outer hair cells (OHCs) in organotypic cultures of the mouse cochlea during force steps applied to the bundles with a silica probe of known stiffness. The receptor potentials of some OHCs adapt for excitatory displacements and the time constants of receptor potential adaptation and hair bundle force relaxation for excitatory displacements are very similar. Thus in these OHCs, the receptor potentials correspond to the applied force for excitatory displacements. For inhibitory displacements, the receptor potentials correspond to hair bundle displacement. Some OHC receptor potentials are nonadapting and follow displacement in both the excitatory and inhibitory directions. The hair bundles of nonadapting OHCs are less stiff than those of adapting OHCs and nonadapting OHCs are an order of magnitude less sensitive to hair bundle displacement than adapting OHCs. In response to a combination of excitatory, tonic, hair bundle displacement and current injection, the receptor potentials of nonadapting OHCs decline as the membrane potential is made more positive and reverse near 0 mV. When the receptor potentials of adapting OHCs measured during current injection are compensated for constant input resistance and driving voltage across the transducer conductance, the receptor potential amplitude at the offset of the step displacement is independent of the level and polarity of the injected current. Before adaptation, at the onset of the step displacement of the hair bundle, the amplitude of the receptor potential increases as the injected current becomes more positive. For adapting OHCs, the receptor potential amplitude is a linear function of excitatory bundle displacement for amplitudes less than 50 nm. With negative, but not positive, current injection the receptor potentials at the onset of the displacement tend to saturate and the slope of the function decreases. This voltage dependent control of OHC transducer operating range is proposed to have a role in regulating the sensitivity of the cochlea.

How the ear's works work

The senses of hearing and equilibrium depend on sensory receptors called hair cells which can detect motions of atomic dimensions and respond more than 100,000 times a second. Biophysical studies suggest that mechanical forces control the opening and closing of transduction channels by acting through elastic components in each hair cell's mechanoreceptive hair bundle. Other ion channels, as well as the mechanical and hydrodynamic properties of hair bundles, tune individual hair cells to particular frequencies of stimulation.

Tubulin and microtubules in cochlear hair cells: comparative immunocytochemistry and ultrastructure

The distribution of tubulin has been investigated in surface preparations of the guinea pig organ of Corti using indirect immunofluorescence microscopy. Two different monoclonal antibodies to tubulin produce similar distinct patterns of labelling in hair cells. Labelling is greater in inner hair cells than outer hair cells. It occurs in rings around the cell apex, and in a meshwork below and channels through, the cuticular plate. In outer hair cells from the apical region of the cochlea, labelling occurs around the location of a basalward protrusion of the cuticular plate. These patterns correlate with the location of microtubules observed using transmission electron microscopy. A large patch of labelling occurs on the strial side of the cell corresponding to the largest channel through the cuticular plate and the kinociliary basal body. Strands of labelling are seen running parallel to the long axis of the cell between the subcuticular and synaptic region. Many more of these strands are seen in the inner hair cell than the outer hair cell and may correspond to tracks of microtubules transporting neurotransmitter vesicles or other organelles. In outer hair cells, intense labelling and many microtubules are seen in the subnuclear region. The possible roles of the different microtubule arrangements are discussed.

Functional role of the olivo-cochlear bundle: a motor unit control system in the mammalian cochlea

A fiber optic lever is applied to the measurement of the motion of the basilar membrane motion in guinea pigs. In response to intense tones from either ear, the motion includes a substantial summating shift in the mean position in addition to a travelling wave originally described by von Békésy. His stroboscopic technique and most techniques used since have been concentrated upon measuring vibrations of the basilar membrane synchronous with the stimulus and have been insensitive to variations in the baseline position such as a summating component of motion analogous to the extracellular summating potential. In addition to the role of the outer hair cells in providing normal hearing sensitivity, they evidently play a role in regulating the mean position of the basilar membrane. For a fixed frequency, the polarity of the mean position varies systematically with sound level and place and summates with time since onset. Since these cells are the target cells for the olivocochlear bundle, homeostasis in the cochlea would appear to be linked efferent function and involve cochlear mechanics. The negative damping hypothesis asserts that hair cell activity is necessary for low thresholds. The results presented here demonstrate that OHC activity exists independent of neural thresholds. The discussion develops the concept that threshold losses are due to a mismatch of opposing tonic forces which normally maintain the mean position of the basilar membrane. Structure is examined in relation to function and the group of outer hair cells innervated by a single medial efferent neuron is identified as a motor unit. Implications of central control of individual motor units include peripheral involvement in selective attention tasks.