Mechanisms of movement in outer hair cells and a possible structural basis

Changes in size and shape of auditory hair cells in vivo during noise-induced temporary threshold shift

In this study we describe changes in the size and shape of auditory hair cells of the alligator lizard in vivo during noise-induced temporary threshold shift. These changes consist of a decrease in cell volume, a decrease in cell length and an increase in cell width. We speculate that these changes are due to relaxation of cytoskeletal contractile elements and osmotic loss of intracellular water. We also describe a decrease in the surface area of the hair cell plasmalemma, and speculate that it is related to the endocytosis and intracellular accumulation of cell membrane during synaptic vesicle recycling. Finally we describe an increase in the endolymphatic surface area of the hair cell, and speculate that this could alter the micromechanics of the stereociliary tuft to attenuate the effective stimulus.

Effects of free radicals on the intracellular calcium concentration in the isolated outer hair cell of the guinea pig cochlea

The cytosolic free calcium concentration ([Ca2+]i) isolated from the cochlear outer hair cell (OHC) of the guinea pig was measured using microfluorimetric imaging technique and the effects of free radicals were investigated. Hypoxanthine (HX) plus xanthine oxidase (XO) induced a rise in [Ca2+]i in the presence of external Ca2+. Elimination of external Ca2+ (pCa = 7) did not show an increase in [Ca2+i, indicating that the increased [Ca2+]i is dependent on external Ca2+. The elevation of [Ca2+]i induced by HX-XO was reduced by addition of superoxide dismutase or nifedipine but not by addition of catalase. A single admission of HX or XO failed to affect [Ca2+]i. These findings suggest that superoxide anion generated in the OHC increases the Ca2+ influx across the membrane, presumably leading to some pathological changes in the acoustic transduction by modulating the OHC motility.

Fodrin is a constituent of the cortical lattice in outer hair cells of the guinea pig cochlea: immunocytochemical evidence

Localization of fodrin, a membrane skeletal protein, in the outer hair cell of the guinea pig cochlea was examined by immunocytochemical techniques. By immunofluorescence microscopy, fodrin was observed in the cuticular plate, in the infracuticular network and along the lateral wall. By immunoelectron microscopy of ultrathin cryosections, labeling for fodrin along the lateral wall was localized between the cell membrane and the outermost layer of the subsurface cisternae. Furthermore, pre-embedding immunoelectron microscopy of permeabilized specimens showed that most immunogolds for fodrin were on the thin cross-linking component of the cortical lattice. The results indicate that fodrin is a constituent of the cortical lattice which is thought to play an important role in outer hair cell motility.

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)

The effect of endocochlear potential suppression upon susceptibility to acoustic trauma

In the present study we investigated the influence of decreased endocochlear potential (EP) on acoustic trauma. Guinea pigs with decreased EP following i.v. administration of furosemide (FUR) and normal EP given physiological saline solution (PSS) i.v. instead of FUR were exposed to 2 kHz pure tone ranging from 110 to 130 dB SPL for 5 min. The input-output curves of the compound action potential (CAP) before acoustic overstimulation were compared with those at 2 h after the exposure. In 125 and 130 dB SPL exposure there was no difference in threshold shift between the FUR and PSS groups. In 115 and 120 dB SPL exposure, however, the threshold shift of the FUR group was significantly smaller than that of the PSS group. There was no significant difference between the two groups in 110 dB SPL exposure. These results indicate that the threshold shift of the CAP is mainly due to excessive vibration of the basilar membrane in the greater stimulus groups (125-130 dB SPL) but that the energy exhaustion of the hair cells to a certain extent participates in the threshold shift when the stimulus is milder (115-120 dB SPL). It is concluded that susceptibility to acoustic trauma of a milder intensity (115-120 dB SPL) is reduced when the energy consumption rate of the organ of Corti is decreased by EP suppression.

Structural variability of the sub-surface cisternae in intact, isolated outer hair cells shown by fluorescent labelling of intracellular membranes and freeze-fracture

The intracellular membrane systems in intact, isolated outer hair cells were visualised using the fluorescent membrane probe 3,3'-dihexyloxacarbocyanine iodide (DiOC6) and by freeze-fracture, and f-actin distribution was examined with rhodamine-phalloidin. DiOC6 stained the sub-surface cisternal membranes in the lateral wall and revealed a membrane system running in the centre of the cell from the nucleus to the sub-cuticular region. In optical sections of the lateral wall of fluorescently labelled cells, obtained by scanning laser confocal microscopy, the sub-surface membrane appeared as a fenestrated sheet or a fine network of tubules. Freeze-fracture replicas of rapidly-frozen, unfixed outer hair cells also showed the sub-surface membrane as a fenestrated sheet in some cells or as a network of tubules in others. These combined studies indicate that the interruptions within the cisternal membranes as seen in normal thin sections of outer hair cells are not fixation artefacts but may reflect the dynamic and plastic properties of this membrane system. Double staining of cells with rhodamine-phalloidin and DiOC6 showed substantial co-localisation of intracellular membranes and f-actin. The results suggest there may be a continuous, dynamic endoplasmic reticulum system, forming a core in the centre of the cell, broadening in the subcuticular region and extending down the lateral wall, that may have a role in the turnover and distribution of cytoskeletal assemblies within the outer hair cell.

Ontogenesis of F-actin in hair cells

This report describes the ontogenesis of cochlear stereocilia using scanning electron microscopy for analysis of cilia appearance, and fluorescence microscopy of phalloidin, a label for F-actin, to determine the maturation of the cilia framework. Surface and frozen-sectioned preparations of the otic capsule were obtained from several stages of rat pup development beginning at the 16th gestational day and at various stages until adulthood. In the earliest stage investigated, strong fluorescence labeling was visible on the apical part of Kölliker's organ, revealing a reticular outline of cell junctions. Hair cells started to differentiate at the 18th day of gestation from cells within the primordial receptor area. Phalloidin labeling revealed a sequential appearance of F-actin as the hair cells differentiated from the cells with the Kölliker's organ. The differentiation of receptor cells occurred first with the appearance of a junctional complex between the hair cell and the surrounding cells. Then a cuticular plate appeared followed by the progressive emergence of stereocilia. The F-actin labeling also revealed a progressive differentiation of receptor cells from the cochlear base to its apex. There was also an inner to outer hair cell developmental gradient of label. Inner hair cells developed stereocilia before outer hair cells. The third row of outer hair cells was the last to acquire stereocilia. The adult pattern of stereocilia was reached around the 6th postnatal day. We conclude that the appearance of actin filaments in developing receptor cells and the emergence of stereocilia can be regraded as markers for correlating function and other structural differentiation.

Mechanical demodulation of hydrodynamic stimuli performed by the lateral line organ

Tonic displacements of the fish lateral line cupula were observed during stimulation of the organ with amplitude-modulated water motion. The modulation frequency was fixed at 2.4 Hz and the carrier frequency was varied from 25 to 500 Hz. The time waveforms of the cupular displacement at carrier frequencies below 280 Hz and above 470 Hz were essentially amplitude-modulated waves. Between 350 Hz and 410 Hz the magnitude at the modulation frequency increased sharply and the predominant shape of the displacement waveform changed to that of the modulating frequency. The mechanism for extraction of the modulation component may play a key role in the decoding of sensory information.

Actin distribution along the lateral wall of gerbil outer hair cells

Outer hair cells can contract parallel to their long axis, and it has been hypothesized that actin may play a role in this contraction. In this study, actin distribution was examined in the gerbil organ of Corti using postembedment immunoelectron microscopy. In addition to regions typically labelled by actin antibodies and observed by epifluorescence--the cuticular plate, stereocilia, and supporting cell processes--these procedures preserved the ultrastructure of the cell and allowed us to demonstrate actin reactivity along the lateral wall of the outer hair cells between the subsurface cisterns and the plasma membrane. This region is the location of structures (pillars and cortical cytoskeleton) though to be associated with contraction of the outer hair cells.

Isolated brush cells of the rat stomach retain their structural polarity

The brush cells (BC) are highly polarized elements occurring in epithelia of endodermal origin. They have a preferential topographical distribution in the organs in which they reside. In the stomach of the rat, BC prevail near the transitional zone separating the forestomach from the glandular stomach. Thus, a method was developed to isolate and recover BC from this organ with the aim of investigating the changes they may undergo after dissociation. Strips of the rat stomach were severed from the very proximal border of the glandular region and incubated in Hanks' balanced salt solution containing pronase. After sedimentation of the dissociated cells (crude sediment containing all stomach epithelial cell types) two successive cell fractions were prepared on performed Percoll gradient in an attempt to enrich BC in a defined layer. BC were recovered in a fraction at a density close to 1.03 g/ml where they represented about 2% of all cells. The isolated BC changed their form from columnar to pear-shaped; however, they maintained their structural polarity over 2 h as demonstrated by light microscopy, transmission-and scanning-electron microscopy. The fine structure of BC was always satisfactorily preserved. Maintenance of the structural polarity of isolated BC is contrary to the general rule according to which all conventional epithelial cells examined to date lose their polarity after isolation. This result is discussed in relation to morphological findings in isolated sensory cells (hair cells, photoreceptor cells) leading to the suggestion that BC are more similar to these than to conventional epithelial cells.

Effect of reversible hypoxia on the compared time courses of endocochlear potential and 2f1-f2 distortion products

In order to study the effects of a controlled hypoxia on the cochlear active mechanisms, the 2f1-f2 distortion product (DP) and the endocochlear potential (EP) were recorded simultaneously in the same ear, in guinea pigs artificially respired with gas mixtures containing different percentages of oxygen. The data show an important difference in the behavior of the two parameters. While the EP undergoes a reduction of amplitude starting shortly after the establishment of the hypoxia, reaches a steady state, and recovers monotonically after a return to normoxic conditions, the time course of the DP is more complex. Its level also declines shortly after the beginning of the hypoxia though it slightly lags behind the EP decline. After switching back to normoxic conditions, the DP rises with an eventual delay with respect to the EP, overshoots, and then dramatically falls again. A slow recovery subsequently takes place and normal values are reached within 5 to 10 min. These results indicate a certain independence of the DP versus the EP. During the exposure to hypoxic conditions, differences in the time course and in the variation of amplitude of the two recorded parameters seem to indicate that the DPs could be more related to the OHC physiology than to the EP. The DP post-hypoxia effect observed after a return to normoxic conditions, indicates that a normal EP is not sufficient for the generation of normal DPs. Different hypotheses which could explain the DP post-hypoxia effect are discussed.

A membrane-based force generation mechanism in auditory sensory cells

Auditory outer hair cells can elongate and shorten at acoustic frequencies in response to changes of plasma membrane potential. We show that this fast bidirectional contractile activity consists of an electromechanical transduction process that occurs at the lateral plasma membrane and can be activated and analyzed independently in small membrane patches inside a patch electrode. Bidirectional forces are generated by increases and decreases in membrane area in response to hyperpolarization and depolarization, respectively. We suggest that the force generation mechanism is driven by voltage-dependent conformational changes within a dense array of large transmembrane proteins associated with the site of electromechanical transduction.

Lowering extracellular chloride concentration alters outer hair cell shape

In general, increasing external K+ concentration, as well as exposure to hypotonic medium, induces a shortening of outer hair cells (OHCs) accompanied by an increase in width and volume. One possible mechanism suggested for these changes is a movement of Cl- and/or water across the cell membrane. We therefore examined the role of Cl- in OHC volume maintenance by testing the effect of decreasing extracellular Cl- concentration on OHC length and shape. In addition, the effect of hypotonic medium was examined. OHCs were isolated from guinea pig cochleae, mechanically dissociated and dispersed, and placed in a modified Hanks balanced salt solution (HBS). Exposing the cells to a Cl(-)-free HBS produced an initial shortening, which was rapidly followed by an increase in length. After about 9 min of exposure to Cl(-)-free HBS, the cells appeared to lose all water and collapsed. Upon return to normal HBS, the OHCs returned to their normal shape. We speculate that the collapse of the OHCs may be due to the loss of intracellular Cl-, which, in turn, resulted in the loss of intracellular K+ and water. The results indicate that Cl- contributes greatly to the maintenance of OHC volume. In addition, we confirmed that isolated OHCs swell in hypotonic medium and maintain their swollen state until returned to normal medium. The mechanism for maintenance of the swollen state is unknown.

The tuned displacement response of the hearing organ is generated by the outer hair cells

The motile responses of the guinea-pig hearing organ in response to a tone applied to the ear were measured by laser interferometry. Two types of responses can be recorded: (i) a vibration at the frequency of the applied tone; and (ii) a displacement response consisting of a shift in the position of the organ surface. The purpose of this study is to characterize the displacement response. The results are as follows. There is a relationship between the frequency of highest sensitivity (best-frequency) of the displacement response and the site from which it is recorded. High best-frequencies are noticed at more basal locations, low best-frequencies towards the apex. The displacement response is more frequency-selective than the vibration response. The displacement response is observed within physiological sound pressure levels. Its sharpness is dependent on the stimulus intensity, it shows biological variability and can be manipulated by drugs that are known to modify the receptor potential of the sensory cells, or to interfere with outer hair cell motility. These results suggest that the displacement response is an important step in the transduction process in the mammalian hearing organ and that it is generated by the motile action of the outer hair cells.

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.

Characterization of inner ear sensory hair cells after rapid-freezing and freeze-substitution

In order to determine the ultrastructural organization of normal cells and to understand better the anatomical substrates for outer hair cell motility, cryofixation was performed on the sensory epithelium of the inner ear of guinea pigs prior to substitution of frozen water with organic solvents containing chemical fixatives. In this way cells would not be altered by the direct application of the chemicals commonly used for preservation, which are also known to cause fixation-induced shape changes in outer hair cells. Following rapid freezing and freeze-substitution, preservation of cells within the isolated sensory epithelium containing the organ of Corti was similar to that seen in conventionally fixed cells. However, in rapidly frozen and freeze-substituted outer hair cells the cytoplasm and the cellular membranes differed from those seen in conventionally fixed preparations. The cytoplasmic matrix was densely packed with filaments and stained homogeneously, suggesting better preservation of the cytoskeleton and less extraction of the soluble ground substance. Cell membranes were smooth, indicating that fixation-induced shape changes and shrinkage had been avoided. The subsurface cisternal system of intracellular membranes lining the lateral wall of the outer hair cells was composed of continuous, tightly packed, parallel rows of membranous lamellae. Thus rapid-freezing and freeze-substitution are important techniques by which structural alterations correlated with outer hair cell motility can be separated from fixation-induced cell shape changes.

Cholinergic regulation of the phosphoinositide second messenger system in the guinea pig organ of Corti

The effect of cholinergic agents on the phosphoinositide second messenger system was investigated in the cochlea of the adult guinea pig in vivo and in vitro. In vivo, phospholipids were labeled with [32P]-orthophosphate by perilymphatic perfusion and their hydrolysis assayed in 'chase' experiments with non-radioactive orthophosphate. Carbachol (1 mM) reduced the content of 32P-labeled phosphatidylinositol 4,5-bisphosphate in the organ of Corti from 31% to 21% of total 32P-lipids, indicating stimulated hydrolysis. The pharmacology of this effect was studied in detail in vitro via the release of inositol phosphates from phosphoinositides pre-labeled with 3H-inositol. Release was increased 2-fold by 1 mM carbachol, 1.6-fold by 1 mM muscarine, but was unaffected by dimethylphenylpiperazinium; the stimulation was blocked by 1 microM atropine but not mecamylamine. These responses indicate the coupling of phosphoinositides to a muscarinic receptor. Furthermore, stimulated inositol phosphate release was higher in the base of the organ of Corti than in the apex which correlates with the increased cholinergic efferent innervation of outer hair cells in the basal region. These results suggest that muscarinic-stimulated inositol phosphate release occurs at the level of the outer hair cell and thus may have an important modulatory role in auditory transduction.

Ultrastructure of the horseshoe bat's organ of Corti. II. Transmission electron microscopy

The fine structure of the organ of Corti was investigated in the echolocating horseshoe bat (Rhinolophus rouxi) by transmission electron microscopy. Particular emphasis was placed on the receptor cells and their supporting cells. The receptor cells, inner hair cells (IHC) and outer hair cells (OHC), possess the typical mammalian shape, but OHCs are extremely short (length: 12-15 microns in the basal turn and up to 28-30 microns in the apical turn). The afferent innervation of both types of receptor cells and the efferent innervation of the IHC system conform to the general mammalian scheme; however, confirming earlier reports, an efferent innervation to the OHCs is absent. Throughout the cochlea, IHCs and OHCs possess a single layer of subsurface cisternae. Above the level of the nucleus of the OHCs, the arrangements of the subsurface cisternae and their connection to the lateral cell membrane via pillars are highly regular, whereas in IHCs, the cisternae are of irregular shape and the pillar system is much less distinct. In the basal turn of the cochlea, the attachment sites of the OHCs to the supporting cells possess specialized features: (a) in the reticular lamina, the contact sites of the cuticular plates of OHCs with the outer pillar cells and the Deiters cell phalanges are of exaggerated length, and (b) the cup formation of the Deiters cell body, which houses the bottom of the OHC, has a specialized shape and is packed with electron-dense material and microtubules. The results are discussed in relation to cochlear ultrastructure in other mammals and in the context of active processes in cochlear mechanics.

Na(+)-Ca2+ exchange in the isolated cochlear outer hair cells of the guinea-pig studied by fluorescence image microscopy

The outer hair cell isolated from the guinea-pig was superfused in vitro and the cytosolic calcium concentration ([Ca2+]i) and sodium concentration ([Na+]i) were measured using fluorescence indicators. Under the resting condition, [Ca2+]i and [Na+]i were 91 +/- 9 nM (n = 51) and 110 +/- 5 mM (n = 12), respectively. Removal of external Na+ by replacing with N-methyl-D-glucamine (NMDG+) increased [Ca2+]i by 270 +/- 79% (n = 27) and decreased [Na+]i by 23 +/- 4 mM (n = 6). Both changes in [Ca2+]i and [Na+]i were totally reversible on returning external Na+ to the initial value and were inhibited by addition of 0.1 mM La3+ or 100 microM amiloride 5-(N,N-dimethyl) hydrochloride. Elevation of external Ca2+ ions to 20 mM reversibly decreased [Na+]i by 8 +/- 6 mM (n = 5). Moreover, the chelation of the intracellular Ca2+ with 1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA) exerted an inhibitory action on the NMDG(+)-induced reduction in [Na+]i. Exposure to 5 mM NaCN for 2 min significantly and reversibly increased [Ca2+]i by 290 +/- 37% (n = 5), but did not affect the [Ca2+]i elevation induced by the NMDG+ solution. The rise in [Ca2+]i induced by the NMDG+ solution was not enhanced by ouabain pretreatment. Addition of ouabain did not alter the [Na+]i. The present results are best explained by the presence of an Na(+)-Ca2+ exchanger in cell membrane and indicate that the activity of Na+/K+ pump is poor in outer hair cells.

Functional and morphological comparisons between cochlear outer hair cells and muscle tissues in the guinea-pig

The effect of polylysine on the motility of outer hair cells and various muscle types was compared. Poly-L-lysine and its stereoisomer, poly-D-lysine, inhibited acoustically induced length changes of isolated outer hair cells from the guinea-pig hearing organ. The frequency specific displacements of the hearing organ in response to a tone stimulus are also inhibited to polylysine (Brundin et al. 1991). Poly-L-lysine, and its stereoisomer, irreversibly attenuated motile responses to transmural stimulation of guinea-pig ileum, vas deferens and taenia coli in a dose dependent manner, but were without significant effect on motile responses in skeletal and heart muscle. L-lysine, D-lysine, and the negatively charged polyaminoacid poly-L-aspartate, were without significant effect on outer hair cell and smooth muscle motility. The inhibitory effect of polylysine in smooth muscle is a direct effect on the muscle cell since polylysine attenuated acetylcholine- and adenosine triphosphate-induced contractions in the ileum, and ATP- or noradrenaline-induced contractions in the vas deferens. Pillar structures, believed to be of importance to excitation contraction coupling, were compared. In heart and skeletal muscle the pillars span the gap between sarcoplasmic reticulum and T-tubuli, deeply recessed into the muscle cell. In smooth muscle and outer hair cell the pillars are in closer relation to the cell exterior. The length of the pillars of the outer hair cells exceeds by two times that of smooth and skeletal muscle. The susceptibility of outer hair cells and smooth muscle tissue to the positively charged polylysine may indicate similarities in membrane or channel composition.

Intracellular pH regulation in isolated cochlear outer hair cells of the guinea-pig

1. The intracellular pH (pHi) regulation mechanisms of the outer hair cell (OHC) isolated from the guinea-pig were studied using fluorescence ratio imaging microscopy. 2. The OHC pHi in the resting condition was 7.26 +/- 0.08 (mean +/- S.D., n = 49) when the standard solution buffered with HEPES-Tris was superfused. 3. Exposure to 25 mM-NH4+ in the absence of HCO3- caused biphasic changes in pHi; a transient increase (7.89 +/- 0.14, n = 22) followed by a slow decrease (7.57 +/- 0.12; mean +/- S.D.). Removal of external NH4+ by introducing the N-methyl-D-glucamine (NMDG+) solution in the absence of HCO3- markedly acidified the pHi to 6.38 +/- 0.12 with little pHi recovery. Subsequent application of the standard Na+ solution restored the pHi to the initial value. The recovery was inhibited by 0.5 mM-amiloride but not by 0.3 mM-DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid). 4. In the presence of HCO3-, removal of both external NH4+ and Na+ promptly caused an intracellular acidification followed by a pHi recovery. The pHi recovery from an acid load was inhibited by 0.3 mM-DIDS or 10 microM-NPPB (5-nitro-2-(3-phenylpropyl-amino)-benzoate). However, the pHi in the steady state in the presence or absence of HCO3- was not altered by addition of 0.5 mM-amiloride or NMDG+ solution. 5. The intracellular buffering power obtained from the NH4+ exposure and withdrawal was -15.1 +/- 8.7 mM (pH unit)-1 (n = 6) and -14.3 +/- 5.8 mM (pH unit)-1, respectively. 6. Replacement of external Cl- with gluconate in the HCO3- solution increased the pHi from 7.22 +/- 0.12 to 7.51 +/- 0.20 (n = 6), which was inhibited by 0.3 mM-DIDS. Moreover, addition of DIDS to the HCO3- solution increased the pHi by 0.13 +/- 0.08 (n = 8). 7. When the external standard solution buffered with HEPES-Tris was replaced with the HCO3- solution, the basal pHi (7.27 +/- 0.10) was promptly acidified to 6.87 +/- 0.10 then relaxed slowly to 7.00 +/- 0.15 (n = 16). 8. The pHi showed an initial alkalinization and a subsequent slow acidification after the HCO3(-)-free standard solution replaced the HCO3- solution. The slow acidification was inhibited by low external Cl- concentration or by addition of 0.3 mM-DIDS.(ABSTRACT TRUNCATED AT 400 WORDS)

A simple in vitro method for raising monoclonal antibodies to cochlear proteins

Monoclonal antibodies have been produced to mammalian hair cell antigens using a simple in vitro kit. Antigen was crudely prepared from dissected cochlear tissue by detergent extraction. There was no need to purify hair cells. Hybridoma supernatents were screened most efficiently on dissociated cells fixed with acetone. The immunisation method is sensitive to nanograms of antigen and can generate responses to conserved or weak antigens. The kit requires very little previous experience with cell culture and generates monoclonal antibodies within 3-4 weeks. It has overcome a number of problems with production of antibodies to hair cells and it should prove to be a very valuable tool in many laboratories.

Actin-binding and microtubule-associated proteins in the organ of Corti

Actin-binding and microtubule-associated proteins regulate microfilament and microtubule number, length, organization and location in cells. In freeze-dried preparations of the guinea pig cochlea, both actin and tubulin are found in the sensory and supporting cells of the organ of Corti. Fodrin (brain spectrin) co-localized with actin in the cuticular plates of both inner and outer hair cells and along the lateral wall of the outer hair cells. Alpha-actinin co-localized with actin in the cuticular plates of the hair cells and in the head and foot plates of the supporting cells. It was also found in the junctional regions between hair cells and supporting cells. Profilin co-localized with actin in the cuticular plates of the sensory hair cells. Myosin was detected only in the cuticular plates of the outer hair cells and in the supporting cells in the region facing endolymph. Gelsolin was found in the region of the nerve fibers. Tubulin is found in microtubules in all cells of the organ of Corti. In supporting cells, microtubules are bundled together with actin microfilaments and tropomyosin, as well as being present as individual microtubules arranged in networks. An intensely stained network of microtubules is found in both outer and inner sensory hair cells. The microtubules in the outer hair cells appear to course throughout the entire length of the cells, and based on their staining with antibodies to the tyrosinated form of tubulin they appear to be more dynamic structures than the microtubules in the supporting cells. The microtubule-associated protein MAP-2 is present only in outer hair cells within the organ of Corti and co-localizes with tubulin in these cells. No other MAPs (1,3,4,5) are present. Tau is found in the nerve fibers below both inner and outer hair cells and in the osseous spiral lamina. It is clear that the actin-binding and microtubule-associated proteins present in the cochlea co-localize with actin and tubulin and that they modulate microfilament and microtubule structure and function in a manner similar to that seen in other cell types. The location of some of these proteins in outer hair cells suggests a role for microfilaments and microtubules in outer hair cell motility.

The sensory epithelium and its innervation in the mole rat cochlea

The mole rat (Spalax ehrenbergi) burrows throughout its life in subterranean tunnels. Several structural and functional features which adapt the mole rat to its habitat have been discerned. The goal of this study was to elucidate the structural basis for adaptation of the auditory end-organ to an environment where low-frequency acoustical signals prevail. For this purpose, cochleae of adult mole rats were studied with light and electron microscopy. Inner hair cells throughout the cochlea, and outer hair cells in the basal (high-frequency) portion of the cochlea, were similar in structure to those seen in other mammals. In contrast, outer hair cells in the apical (low-frequency) portion displayed unique structural features. These features resembled the structure of inner hair cells or immature outer hair cells. The innervation of outer hair cells was most uncommon, in that classical medial efferent innervation was not found throughout the cochlear spiral.

First appearance and development of motile properties in outer hair cells isolated from guinea-pig cochlea

Cochleae from fetal guinea-pigs (37 to 64 gestation days, gd) were used to correlate the appearance of motile properties of isolated outer hair cells (OHCs) with the development of specific morphological features. Both the 'fast' electrically-driven and the 'slow' calcium-induced motilities appeared first in OHCs from basal turn of 52 gd fetuses. At 56 gd, most of basal and some apical OHCs responded positively to both types of stimulation. All tested cells were positive at 64 gd. It is noteworthy that this period closely corresponds to the onset and maturation of the gross cochlear potentials. Some structural changes in the organ of Corti may be correlated with the development of OHC motile properties: the acquisition of an adult-like cylindrical shape by the OHC, its lateral detachment from neighboring Deiters cells, and its surrounding by fluid spaces. At the ultrastructural level, the formation of a first layer of laminated cisternae regularly aligned along the OHC plasma membrane from the cuticular plate down to the nuclear level, temporally coincided with the onset of in vitro motility (52 gd). The following days, pillars and a sub-membrane lattice were clearly noticed between the outermost cisternal membrane and the plasma membrane. The results support the ideas that: motile properties observed in vitro reflect the in vivo active mechanisms, and that one single layer of laminated cisternae and its associated sub-plasma membrane material may be needed for OHC motility.

Muscarinic receptor hyperpolarizes cochlear hair cells of chick by activating Ca(2+)-activated K+ channels

1. Electrical responses to extracellularly applied acetylcholine (ACh) and to intracellularly introduced substances were studied in isolated short and tall hair cells from the chick cochlear organ by a whole-cell voltage clamp technique using a patch electrode. These cells were isolated without using proteolytic enzymes. 2. Short hair cells generated a transient outward current at -50 mV in normal saline in response to puff-applied 100 microM-ACh, when the patch electrode was filled with a 160 mM-K+ and 100 microM-EGTA-based intracellular medium. The amplitude was 317.1 +/- 97.1 pA (n = 32). When ACh was applied ionophoretically, the outward current was generated with a delay of about 10 ms. 3. The amplitude of ACh-induced current was dose dependent with a KD of 19 microM and a Hill coefficient of 1.6 when measured at -50 mV. 4. The ACh (100 microM)-induced current was suppressed by 1 microM-atropine. ACh-induced current was generated in a Ca(2+)-free extracellular medium; however, the second ACh puff in the Ca(2+)-free medium generated a much reduced response. ACh-induced current was suppressed reversibly by 100 microM-quinine. 5. Intracellular injections of guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S), inositol 1,4,5-trisphosphate (IP3) or Ca2+ (1 microM) via the patch pipette activated outward currents at -50 mV. 6. When the internal medium with strong Ca(2+)-buffering capacity (5 mM-EGTA) was used, the ACh-induced current was reduced to 39.3 +/- 6.8 pA (n = 4) at -50 mV (12.3% of the response in the low-EGTA medium). 7. The reversal potential of the ACh-induced current was -85.7 +/- 4.2 mV (n = 3) in normal saline containing 5 mM-K+. The reversal potential was dependent on the extracellular K+ concentration ([K+]o) and was shifted 57 mV by a 10-fold increase in [K+]o at room temperature (20-25 degrees C). 8. These results (points 4-7) indicate that ACh induces a K+ conductance by releasing Ca2+ intracellularly, probably by activating the pathway of muscarine receptor, G-protein and IP3. 9. Channel activities were recorded using cell-attached patch electrodes. Channel activities were rarely observed when ACh was applied to the extra-patch membrane, while robust channel activities were observed when ACh was included in the patch pipette medium. It is therefore suggested that Ca(2+)-activated K+ channels exist in the membrane in close vicinity to muscarinic receptor molecules and intracellular Ca2+ release sites.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Light- and electron microscopy of isolated vestibular hair cells from the guinea pig

Cells isolated from the guinea-pig vestibular sensory epithelia were studied using light- and electron-microscopic techniques. The cells maintained their characteristic shapes when they had been separated. Mammalian vestibular cells are traditionally divided into two classes, type-I and type-II hair cells. It was, however, found that the population of isolated cells consisted of hair cells with a striking variability in shape and size. This was most conspicuous for the type-I hair cells. Isolated hair cells processed for electron microscopy showed that the isolation process caused minor ultrastructural damage but that the separation often was incomplete in that the large calyx-like nerve endings were still attached to type-I cells. The results suggest that the distinction of only two classes might be insufficient to describe mammalian vestibular hair cells.

Concomitant salicylate-induced alterations of outer hair cell subsurface cisternae and electromotility

Isolated cochlear outer hair cells undergo rapid, force-generating length changes in response to electrical stimulation. The cellular mechanism responsible for electromotility and its structural substrate is not yet known. Salicylates reduce and block electromotility in vitro. Therefore, we exposed isolated outer hair cells from the guinea pig cochlea to various doses of sodium salicylate and evaluated both ultrastructural changes and responses to electrical stimulation. Following salicylate superfusion, the subsurface cisternae showed dilatation, vesiculation and a deviation from their normal, unfenestrated, axial orientation below the plasma membrane. These changes were time and dose dependent and reversible over a time course of about 30 min. Electromotility was blocked and showed recovery following the same time course as the salicylate-induced reversible structural changes. These results indicate that intact, unfenestrated subsurface cisternae are required for the optimal generation of electrically-induced motility in mammalian outer hair cells.

Effects of pH on intracellular calcium levels in isolated cochlear outer hair cells of guinea pigs

Effects of intracellular pH (pHi) on intracellular Ca2+ (Cai2+) in the outer hair cell (OHC) were investigated using fluorescence ratio imaging microscopy. Cai2+ and pHi were determined with fluorescence indicators fura-2 and 2,7-bis(2-carboxyethyl)- 5,6-carboxyfluorescein, respectively. Intracellular alkalinization from the basal pHi of 7.22 +/- 0.03 to 7.82 +/- 0.09 (n = 7) induced by 25 mM NH4+ caused a rise in Cai2+ from 102 +/- 15 to 195 +/- 28 nM (n = 8). The elevation of Cai2+ was inhibited by removing external Ca2+ or by 50 microM nifedipine. On the other hand, the cytosolic acidification produced by the removal of NH4+ (delta pHi = 0.53 +/- 0.06, n = 7) and the admission of 5% CO2 (delta pHi = 0.72 +/- 0.05, n = 4) elicited a slight reduction in Cai2+. The depolarization of the membrane potential by exposure to 100 mM K+ induced an increase in Cai2+ that was susceptible to nifedipine, indicating the presence of voltage-sensitive Ca2+ channels. The elevation in Cai2+ induced by 100 mM K(+)-containing solution was increased by 138 +/- 6% (n = 4) by external alkalinization to pH 8.4 and was decreased by 87 +/- 3% (n = 4) by external acidification to pH 6.4. These results demonstrate that both internal and external alkalinization of OHCs facilitate the influx of Ca2+ through the Ca2+ channel, leading to a rise of Cai2+. Thus pH, extra- and intracellular, may modulate the OHC motility by regulating the Cai2+.

Frequency-specific position shift in the guinea pig organ of Corti

The organ of hearing is tuned as expressed both in the vibratory response of the cochlear partition and in the resulting receptor potentials of the sensory cells. We now demonstrate a sharply tuned response, consisting of a position shift of the surface of the organ of Corti, occurring during the presentation of a tone. The magnitude of the position shift exceeds that of the vibratory response to the stimulus. The shift is most pronounced in the region of the outer hair cells, and its affected by an inhibitor of outer hair cell motility. We conclude that the response is induced by the action of the outer hair cells.

Assessment of ultrastructure in isolated cochlear hair cells using a procedure for rapid freezing before freeze-fracture and deep-etching

Separated cochlear outer hair cells and isolated strips of organ of Corti containing hair cells and supporting cells have been rapidly frozen before freeze-fracture and deep-etching by immersion of samples sandwiched between two copper plates into liquid nitrogen-cooled propane: isopentane. Assessment of this procedure has shown that no significant freezing damage occurs. The ultrastructure of the hair cells revealed by freeze-fracture of these non-chemically fixed preparations was generally very similar to that seen in fixed material. This indicates that the processing of cochlear tissue normally used for electron microscopy produces few obvious structural artefacts. It also demonstrated that procedures for isolating cochlear hair cells generally do not affect cell structure significantly. However, some isolated hair cells did show abnormalities within the membranes of the lateral cisternae. Such membrane alterations, which would not be identified by light microscopy, occurred to a variable extent but were more commonly present after prolonged periods in maintenance medium. Deep-etching of the preparations to examine extracellular features around stereocilia revealed clearly lateral cross-links between stereocilia. However, tip-links could not be positively identified in either unfixed or prefixed preparations.

Evoked otoacoustic emissions in infant hearing screening

The main application of the ILO88 otodynamic otoacoustic analyser is clearly in detecting whether the peripheral auditory system is normal or pathological. The evoked otoacoustic emission technique in normal and hearing-impaired children for which audiometric data were known (n = 422 ears) was first performed. The correlation was excellent. The actual experimental group (n = 258 ears) consisted of children who are difficult to test with the conventional screening methods in school health services. The use of evoked otoacoustic emissions as screening instrument is new, and therefore several minimal practical criteria, advantages and limitations were defined. The evoked otoacoustic emission test tool is especially applicable in difficult to test children using conventional screening methods.

Spatiotemporal development of cochlear innervation and hair cell differentiation in the rat

The apical cytoskeleton of cochlear hair cells is largely comprised of actin microfilaments and actin-associated proteins, of which fodrin is one of the most prominent. We studied the development of this mechanosensory apical portion of cochlear hair cells of the rat by fluorescence microscopy using rhodamine conjugated phalloidin to detect F-actin and an antibody against alpha-fodrin. An antibody against the 160 kDa neurofilament polypeptide was used for tracing nerve fibers. The first sign of differentiation of the mechanosensory region, actin-containing stereocilia, was observed on the 19th gestational day in the inner hair cells of the basal coil. The appearance of expression of cytoskeletal actin in the cochlear hair cells proceeded gradientally from basal to apical coil and from inner to outer hair cells. Corresponding maturation sequences were observed in the development of fodrin immunoreactivity in the cuticular plates, but the first evidence of this reactivity was found one day later than the appearance of stereocilia in the hair cells at the same location. Also the penetration of neurofilament-positive neurites into the sensory epithelium followed the same kind of longitudinal and radial maturation gradients throughout the cochlea. Fibers were revealed beneath the sensory cells shortly before the first appearance of differentiation of their mechanosensory region. The results suggest that ingrowing nerve fibers may influence the timing of the apical cytoskeleton differentiation in cochlear hair cells or that both these processes could be controlled by the same external signals that are gradientally expressed throughout the cochlea.

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).

Nature of the motor element in electrokinetic shape changes of cochlear outer hair cells

It is the prevailing notion that cochlear outer hair cells function as mechanical effectors as well as sensory receptors. Electrically induced changes in the shape of mammalian outer hair cells, studied in vitro, are commonly assumed to represent an aspect of their effector process that may occur in vivo. The nature of the motile process is obscure, even though none of the established cellular motors can be involved. Although it is known that the motile response is under voltage control, it is uncertain whether the stimulus is a drop in the voltage along the long axis of the cell or variation in the transmembrane potential. We have now performed experiments with cells partitioned in differing degrees between two chambers. Applied voltage stimulates the cell membrane segments in opposite polarity to an amount dependent on the partitioning. The findings show, in accordance with previous suggestions, that the driving stimulus is a local transmembrane voltage drop and that the cellular motor consists of many independent elements, distributed along the cell membrane and its associated cortical structures. We further show that the primary action of the motor elements is along the longitudinal dimension of the cell without necessarily involving changes in intracellular hydrostatic pressure. This establishes the outer hair cell motor as unique among mechanisms that control cell shape.

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

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

High frequency force generation in outer hair cells from the mammalian ear

Mammalian outer hair cells generate mechanical forces at acoustic frequencies and can thus amplify the sound stimulus within the inner ear. The mechanism of force generation depends upon the plasma membrane potential but not upon either calcium or ATP. Forces are generated in the lateral cortex along the full length of the cell. The cortex includes a two-dimensional cytoskeletal lattice composed of circumferential filaments 6-7 nm thick that are cross-linked by filaments 3-4 nm thick and 40-60 nm long. The two filament types may, respectively, be actin and some form of spectrin. The lattice reinforces the cylindrical shape of the cell and permits limited changes in length. Beneath it lie the lateral cisternae, a regular system of multi-layered membranes. Force-generation may depend upon voltage-dependent shape changes in proteins that lie either in the plasma membrane or in the cytoskeletal lattice.

In vivo noise exposure alters the in vitro motility and viability of outer hair cells

The in vitro motility and viability of outer hair cells isolated from cochleae of normal control guinea pigs have been compared to that of guinea pigs exposed, just before sacrifice, to low-frequency high-intensity noise inducing acute 30 dB thresholds shifts at all frequencies below 10 kHz. The results indicate that the cells' viability is shortened, their contractile response to Ca2+/ATP reduced, while their electrically-induced motility is not modified. These experiments demonstrate that in vivo cochlear dysfunction can correlate with changes in in vitro outer hair cell's properties. Thus the morphological and "functional" investigation of hair cells in vitro can be a valuable approach to the study of cochlear physiopathology. Here the acoustic overstimulation seems to have modified the outer hair cells' Ca2+/ATP dependent slow contractile apparatus in a way which could modify in turn their mechanical excitation by the noise.

The effect of quinine on psychoacoustic tuning curves, stapedius reflexes and evoked otoacoustic emissions in healthy volunteers

Quinine causes reversible hearing loss, closely related to the quinine plasma concentration. The effects of quinine on psychoacoustic tuning curves, stapedius reflex thresholds and evoked otoacoustic emissions were studied in healthy volunteers. The tuning curves became shallower, whereas reflex thresholds were unaffected. The shift in the emission thresholds paralleled that of the pure-tone thresholds. There were also qualitative changes in the emissions: 1) the exponent of the stimulus-response function changed from 0.34 to 0.56; 2) decay time shortened; 3) the power spectrum shifted towards lower frequencies. The results are discussed in relation to various aspects of cochlear performance and are suggested to depend on an outer hair cell dysfunction.

Quick-freeze, deep-etch visualization of the 'cytoskeletal spring' of cochlear outer hair cells

The lateral membrane system of the cochlear outer hair cell, consisting of the lateral plasma membrane, pillars, filamentous lattice and subsurface cisternae, is considered to be involved in the contractile movement of the isolated cochlear outer hair cell. The filamentous lattice, called the cytoskeletal spring, has been identified in the demembranated cochlear outer hair cell treated with the detergent Triton X-100. In this study, the quick-freeze, deep-etch method was applied to demonstrate the three-dimensional organization of both the filamentous and membranous structures of the lateral membrane system of cochlear outer hair cells. Treatment with saponin revealed that the inner leaflet of the lateral plasma membrane of the cochlear outer hair cell possesses more membrane particles than the outer leaflets, and that the pillars are closely associated with membrane particles in the inner leaflet of the lateral membrane. The presence of filamentous bridges between the filamentous lattice and the subsurface cisternae was also detected. We propose that the lateral membrane system in the cochlear outer hair cell may play an important role in the tuning mechanisms within the cochlea in normal hearing.

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.

Age-related changes in evoked otoacoustic emissions

Outer hair cells of the organ of Corti play an important part in the genesis of evoked otoacoustic emissions (EOAEs), which are related to cochlear biomechanics. The aim of this study was to investigate the age factor in relation to EOAEs in 166 ears of subjects between 6 weeks and 83 years of age. The results show that when age increases, the presence of EOAEs by age group and the frequency peak in spectral analysis decrease, and EOAE threshold increases. Thus, there is an effect of age upon EOAEs, and it seems linked with alteration of cochlear biomechanics and/or hair cell loss. Such an effect has to be taken into consideration when EOAEs are used in clinical applications, and limits the use of EOAEs in older subjects.

Hydrodynamic analysis of a two-dimensional model for micromechanical resonance of free-standing hair bundles

To investigate the role of inner ear fluids and structures on mechanical stimulation of the hair bundles of hair cells, we analyzed a two-dimensional structure that consists of: a rectangular flap (which represents a hair bundle) attached to a flat basal plate (which represents the surface of the epithelium that contains the hair cells) with a spring-loaded hinge (that represents the compliant attachment of a hair bundle to the hair cell body) and surrounded by a viscous fluid (that represents endolymph). We computed the fluid velocity as well as the forces on and motion of the flap in response to sinusoidal vibration of the plate by numerical integration of the hydrodynamic equations, and--at asymptotically low and high frequencies--by analytic methods. The results suggest that: (1) the surface of the sensory epithelium, from which hair bundles project into fluid, plays an important part in the production of fluid forces on hair bundles; (2) both fluid inertia and viscosity play a key role in hair bundle mechanics; (3) passive mechanical resonances are likely to contribute to both frequency selectivity and frequency-to-place coding in the inner ear.

Superposition of hydrodynamic forces on a hair bundle

Vertebrates sense sound, orientation, and motion by means of bundles of microscopic sensory hairs that protrude from the surfaces of receptor (hair) cells. To determine the effects of the sensory epithelium, tectorial structures, and fluids on the motions of hair bundles, we examine a class of mathematical models in which hair-cell organs are represented as a system of rigid mechanical structures surrounded by fluid. The epithelium and tectorial structures are represented by rigid basal and tectorial plates, respectively; the hair bundle by a rigid body hinged to the basal plate. When the displacements of these structures are small, the equations of motion for the fluid are predominately linear. Therefore, both the fluid velocity and the force of fluid origin on the body can be expressed as a sum of components; each component results from motion of a single structure while all others are stationary. This analysis leads to a network description of the motion of the rigid body in which hydrodynamic forces are segregated from mechanical forces. The separation of hydrodynamics and mechanics not only clarifies the effects of fluids on motion but also minimizes the number of hydrodynamic computations needed to analyze models of hair-bundle motion.

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.

Quinine causes isolated outer hair cells to change length

The outer hair cells have been shown to exhibit motile properties which are likely to participate in the cochlear performance. Quinine is known to induce hearing loss as well as contraction of skeletal muscle. Isolated outer hair cells were exposed to quinine and tetracaine. This resulted in a biphasic elongation-shortening response, quantitatively as well as qualitatively altered by tetracaine. These findings are in good agreement with similar studies on muscle.

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.

Intracellular distribution of actin in cells of the organ of Corti: a structural basis for cell shape and motility

Immunofluorescence staining and phalloidin labeling have provided localization of actin in the sensory and supporting cells of the inner ear at the light microscopic level. However, with electron microscopy, neither actin nor actin filaments have been found in the outer hair cell body. This paper describes various techniques utilized to preserve and identify cytoplasmic actin at the ultrastructural level. Post-embedding staining of Lowicryl K4M sections, pre-embedding staining of permeabilized cells of the organ of Corti, pre-embedding staining of vibratome sections, and pre-embedding staining of permeabilized dissociated cells documented the presence of actin, but each of these techniques was best suited to localize actin in specific parts of the cell. Cytoplasmic actin was labeled when isolated cells were lightly fixed and membranes were permeabilized with detergent--conditions under which the cell ultrastructure was compromised. Under conditions of optimal fixation, cytoplasmic filaments embedded in the dense granular matrix of the hair cell cytoplasm were observed.