Die isolierte, lebende Haarzelle - Ein neues Modell zur Untersuchung der Hörfunktion

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.

Isolation of outer hair cells from the cochlear sensory epithelium in whole-mount preparation using laser capture microdissection

Outer hair cells (OHCs) play an important role in frequency selectivity and signal amplification in the mammalian cochlea. Because OHCs are relatively few in number and a minority of the cells in the cochlea, separating and isolating them for applications such as cDNA library creation and proteomic studies is a challenging task. Laser capture microdissection (LCM) is designed to capture cells from very thin tissue sections, it can accurately isolate specific cells from large regions of tissue for RNA, DNA, and proteomic studies. Due to the constraints of cochlear anatomy, thin sections of the cochlea contain small numbers of OHCs. Therefore, we adapted the LCM technique to isolate OHCs from organ of Corti whole-mounts, each of which contain hundreds of OHCs that are simultaneously accessible and collectable. For comparison, we also used a more traditional mechanical dissection. The quality of cDNA derived from the OHCs collected with LCM and with the traditional mechanical method are compared and the merits and limitations of the techniques discussed. A similar approach can also be used to isolate large quantities of inner hair cells and selected supporting cells from the whole-mount cochlear preparation.

Nitric oxide distribution and production in the guinea pig cochlea

Production sites and distribution of nitric oxide (NO) were detected in cochlear lateral wall tissue, the organ of Corti and in isolated outer hair cells (OHCs) from the guinea pig using the fluorescent dye, 4,5-diaminofluorescein diacetate. Fluorescent signal, indicating the presence of NO, was found in the afferent nerves and their putative endings near inner hair cells (IHCs) and putative efferent nerve endings near OHCs, the IHCs and OHCs, the endothelial cells of blood vessels of the spiral ligament, the stria vascularis, and the spiral blood vessels of the basilar membrane. An increased NO signal was observed following exposure to the substrate for NO, L-arginine, while exposure to NO synthase inhibitors resulted in a decrease in NO signal. Observation of OHCs at the subcellular level revealed differentially strong fluorescent signals at the locations of cuticular plate, the subcuticular plate region, the infranuclear region, and the region adjacent to the lateral wall. The findings indicate the presence of NO in the cochlea and suggest that NO may play an important role in both regulating vascular tone and mediating neurotransmission in guinea pig cochlea.

Isolation of cochlear inner hair cells

In order to identify hair cell specific genes, it is essential to obtain isolated hair cells in quantity. While whole-cell recordings have been made from isolated inner hair cells (IHCs) from guinea pigs, detailed methods for obtaining a fairly large amount of isolated inner hair cells have not been published. Here we describe a protocol that can yield a fairly large amount of isolated gerbil IHCs. This technique can provide sufficient numbers of solitary IHCs for either electrophysiological studies of the cell's membrane properties or identifying genes related to IHC functions using techniques of molecular biology.

In vitro effects of hydrogen peroxide on the cochlear neurosensory epithelium of the guinea pig

Reactive oxygen species (ROS) have been postulated to be involved in drug ototoxicity and noise-induced hearing loss. Hydrogen peroxide (H(2)O(2))-induced cell damage in the inner ear was investigated using the neurosensory epithelium of a guinea pig cochlea. Hair cells and supporting cells of the epithelium incubated in Hanks' balanced salt solution were viable up to 6 h. After 2 h of treatment with 0.2 mM H(2)O(2) about 85% of the outer hair cells lost their viability. In contrast inner hair cells slowly began to die after 2 h of H(2)O(2) treatment. The Deiters cells and Hensen cells did not show any signs of damage in the presence of H(2)O(2). Nifedipine, a calcium channel blocker, Quin-2 AM, an intracellular calcium chelator, and 2,2'-dipyridyl, a membrane-permeable iron chelator, all provided partial protection against H(2)O(2)-induced outer hair cell death. The combination of both chelators showed an additional protective effect. The antioxidants N-acetylcysteine and glutathione-monoethyl ester completely protected against H(2)O(2) damage. These results suggest that calcium, iron, and thiol homeostasis play a crucial role in hair cell death caused by H(2)O(2).

Abolition of the receptor potential response of isolated mammalian outer hair cells by hair-bundle treatment with elastase: a test of the tip-link hypothesis

To test the hypothesis that the tip-links of hair-cell stereocilia are essential for mechanoelectrical transduction, tip-links of isolated outer hair cells (OHCs) of the guinea-pig cochlea were eliminated with a proteolytic enzyme, elastase, and the influence on the receptor potential measured with the whole-cell patch-clamp technique. Within 45 s of immersion of the hair bundle in 20 IU/ml elastase, the receptor potential in response to direct deflection of the hair bundle was irreversibly abolished. The electrical input impedance of the cell remained unchanged, implying that the channels of the basolateral membrane were not affected by elastase. The effect of elastase on the receptor potential was comparable to changes seen after mechanically induced hair-bundle damage. As a further control, a putative transduction-channel blocker, dihydrostreptomycin (68 microM), which does not affect tip-links, was applied to the hair bundle. Although the receptor potential was also blocked by dihydrostreptomycin, the effect was reversible. The results suggest that tip-links are required for mechanoelectrical transduction of mammalian OHCs.

Ligand-binding properties of an unusual nicotinic acetylcholine receptor subtype on isolated outer hair cells from guinea pig cochlea

Acetylcholine receptors on isolated guinea pig cochlear outer hair cells (OHC) were characterized by radioligand binding. Equilibrium binding of [125I]alpha-bungarotoxin revealed a KD of 62 +/- 2 nM, Bmax = 7.2 +/- 1.8 x 10(7) binding sites/OHC, and a slowly reversible dissociation rate constant, kappa-1 = 2.2 +/- 0.01 x 10(-4) min-1. L-[3H]Nicotine bound reversibly (estimated KD approximately 230 nM and Bmax approximately 5 x 10(7)) with kinetic rate constants of association kappa-1 = 6.2 +/- 0.06 x 10(4) min-1 nM-1 and dissociation kappa-1 = 0.23 +/- 0.003 min-1. [3H]Strychnine bound to OHC with a KD of 35 +/- 6 nM and Bmax = 2.6 +/- 0.5 x 10(7), and binding increased 3-4 fold after membrane depolarization with 56.2 mM [K+], suggesting additional binding sites. Binding, seen only at > nM concentrations, of [3H]3-quinuclidinyl benzilate (KD = 11.5 +/- 5 nM; Bmax = 2.5 +/- 0.6 x 10(6)) was competitively inhibited by the muscarinic antagonists atropine and 4-DAMP (IC50 of 6.1 +/- 0.5 and 6.5 +/- 0.4 nM). The OHC receptor is thus an atypical nicotinic acetylcholine receptor subtype with unusual pharmacological properties.

Immunological identification of candidate proteins involved in regulating active shape changes of outer hair cells

By employing immunological methods, it has been demonstrated that myosin, myosin light chain (MLC) and myosin light chain kinase (MLCK) proteins in outer hair cells (OHC) are immunologically different from isoforms in platelets, smooth muscle and heart muscle, and are probably more related to isoforms found in red blood cells (RBC). Moreover, proteins related to band 3 protein (b3p) and protein 4.1 (p 4.1), ankyrin as well as fodrin and spectrin, but not glycophorin, have been identified in isolated OHCs. Both OHCs and RBC differ from other motile non-muscle cells in their lack of smooth muscle isoforms of actin, their common high levels of spectrin-, ankyrin- and band 3-like proteins, as well as the expression of the 80 kDa protein 4.1 isoform. The data support the notion that motility of OHC may be based upon regulation of the b3p/p 4.1/ankyrin complex, and thus may be reminiscent to the active shape changes in RBC.

ATP-induced cytoplasmic [Ca2+] increases in isolated cochlear outer hair cells. Involved receptor and channel mechanisms

Outer hair cells (OHC) of the mammalian cochlea are thought to preprocess the sound signal by active movements, which can be induced by electrical or chemical stimulation, e.g. depolarization evoked by high [K+] or increased cytoplasmic [Ca2+]. Extracellular ATP has been found to induce cytoplasmic [Ca2+] increases in OHC but involved mechanisms have not been elucidated. Cytoplasmic [Ca2+] was measured in non-enzymatically isolated single OHC using Fura-2 microspectrometry. Results, using ATP/derivatives and other P2-purinergic receptor (P2R) ligands, as well as Ca(2+)-channel blockers and pertussis toxin, revealed several signal transduction pathways that increase cytoplasmic [Ca2+] in OHC: a P2-purinergic receptor (P2R)--G-protein--effector (phospholipase C or an ion channel) system and a voltage-dependent Ca2+ channel. Agonist potency studies denote a pattern analogous to that found in skeletal muscle, i.e. ATP-alpha-S > ATP = 2-methyl-S-ATP >> ADP > alpha,beta-methylene-ATP, but no activation by ADP beta F or UTP, leaving a choice of P2y or P2zR subtypes. The latter possibility gained strength from calculations showing that up to 8% of ATP may have formed the P2zR agonist ATP4- in the experimental medium. Experiments in Ca(2+)-free medium and with pertussis toxin revealed that the main Ca2+ source was intracellular. Pertussis toxin did not affect [Ca2+] increase induced by carbachol. Acetylcholine, administered a few seconds before ATP, did not affect total cytoplasmic [Ca2+] increases. Induced cytoplasmic [Ca2+] increases were high enough (> 500 nM at 50 microM ATP/derivatives) to hyperpolarize the OHC membrane by opening K(+)-channels and decreased little with time.(ABSTRACT TRUNCATED AT 250 WORDS)

Electron-probe analysis of isolated goldfish hair cells: implications for preparing healthy cells

Electron-probe analysis provides an objective criterion for the physiological status of cells: whether they show the high potassium and low sodium that are expected of healthy animal cells. Preparing isolated goldfish hair cells that were healthy by this criterion required several precautions, including: limited exposure to enzymes and to simple salt solutions, a rest period between enzyme treatment and mechanical disruption of the tissue, and presence of bovine albumin in the medium both during the rest period and during mechanical dispersion and plating. Cells prepared with these precautions from the saccule and lagena and kept in an enriched medium had the following elemental composition (mole percentages with respect to phosphorus): K, 103; Na, 18; Cl, 23; S, 13; Mg, 8; Ca, 1.5. These mole percentages were close to these elements' total millimolar concentrations in the cells. If the precautions were not taken, cells with intact surface membranes (as assessed by exclusion and retention of dyes) could be obtained, but the cells had elevated cell sodium and low cell potassium.

Forces involved in length changes of cochlear outer hair cells

Motion or force generation of outer hair cells may contribute to active modulation of cochlear mechanics. In order to determine the force involved in length changes of outer hair cells, a new in vitro method was used. In the first series of experiments, apical and basolateral extracellular spaces of outer hair cells of the guinea-pig cochlea were separated. Changes of the voltage between the two extracellular spaces induced reversible, proportional changes of the cell length of 4.4 nm/mV if the cell had a length of 80 microns. In the second series of experiments, cell elongations in response to negative pressure applied to the basal end of the cells were measured and corrected for frictional effects. From these data, the compliance of the longitudinal axis of the hair cells was calculated. It was 220 +/- 130 m/N (n = 25) and 240 +/- 170 m/N (n = 24) for cells of the third and fourth cochlear turns, respectively, if the water permeability of the cell membrane was neglected. If the water permeability was taken into account, the compliance was probably around 500 m/N [corrected]. Thus, a mechanism that changes the cell length by 1 microm must generate a static force of at least around 2 nN in an outer hair cell of the organ of Corti [corrected]. Electromotility of outer hair cells, induced by changes of the electrical potential difference across the outer hair cell, is a mechanism that generates this force.

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.

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)

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.

C-type potassium channels in the lateral cell membrane of guinea-pig outer hair cells

The basolateral cell membrane of outer hair cells (OHC) from the mammalian cochlea is known to contain K(+)-channels. The prevailing type, a high-conductance K(+)-channel was further characterized in the present study in order to support its classification as C channel. OHC were isolated from the 3rd and 4th turn of the guinea-pig cochlea. Cell-attached and excised inside-out patches of the lateral cell membrane were investigated. The C-type channel had a selectivity for K+ over Na+ of 12:1 to 20:1 and displayed Goldman-type rectification and voltage-dependence of the open probability. The kinetics of both opening and closing could be described by time constants in the range of ms. The channel provides a calcium- and voltage-activated pathway through OHC lateral membranes for passive K+ transport.

A nicotinic acetylcholine receptor-like alpha-bungarotoxin-binding site on outer hair cells

Acetylcholine (ACh) appears to be the major neurotransmitter liberated from olivocochlear efferents terminating on outer hair cells (OHC). Recently, cholinergic receptor epitopes were visualized at the basal pole of the OHCs. To evaluate the ACh receptor type at OHC we performed binding studies with [125I]-labelled alpha-bungarotoxin (alpha-bgtx), a close to irreversibly acting blocker of the nicotinic acetylcholine receptor (nAChR) of skeletal muscle and of electrocytes of Torpedo and Electrophorus. An irreversible and saturable binding (80 nM) of the radiolabelled compound to OHCs was observed. The number of alpha-bgtx sensitive binding sites present on each OHC was calculated to be about 2 X 10(-17) mol/OHC, which would amount to about 10(7) binding sites/cell. Preincubation with the reversibly acting cholinergic ligands, carbamylcholine (1 mM), nicotine (0.1 mM) and d-tubocurarine (1-100 microM) was found to inhibit alpha-bgtx binding to a varying degree. Atropine (0.05 mM), a muscarinic antagonist, had no influence on the binding of alpha-bgtx to OHCs. [3H]-QNB, a specific marker and antagonist for muscarinic AChR, and [125I]-kappa-toxin, known to react with neuronal and ganglionic nAChR, showed no specific binding to OHCs. The data indicate that a peripheral type nAChR is present on OHCs mediating ACh-induced modulation of the biomechanics of the cochlea by influencing OHC motility.

Cell potential and motility of isolated mammalian vestibular sensory cells

Vestibular hair cells (VHCs) were isolated from the guinea pig inner ear. Using the whole cell variant of the patch clamp recording technique a zero current cell potential of -63.1 +/- 9.9 mV was measured in macular hair cells. Depolarization and repolarization were accompanied by mechanical responses of the solitary VHCs. In addition to the evoked motile events spontaneous shape changes of VHCs were observed. Implications for vestibular micromechanics of the observed evoked force generation in VHCs are discussed.

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.

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.

Calcium distribution and mobilization during depolarization in single cochlear hair cells. Imaging microscopy and fura-2

Intracellular distribution of cytoplasm-free Ca2+ concentrations ((Ca2+)i) and dynamic changes during stimulation of viable hair cells were studied using digital imaging microscopy and the Ca2(+)-sensitive dye fura-2. (Ca2+)i was visualized on pseudo-colour images and three-dimensional computer graphics. In the resting state, the intra-cellular distribution of (Ca2+)i in both the outer and inner hair cells was heterogeneous, and the amount of (Ca2+)i in most of the peripheral cytoplasm just beneath the plasma membrane was greater than that throughout the entire cytoplasm. Cell depolarization, induced by elevated K+, led to an increase in (Ca2+)i in the outer hair cells. The increase in (Ca2+)i was not observed under conditions of depolarization in Ca2(+)-free medium. These observations are interpreted to mean that the increase in (Ca2+)i is induced by depolarization with the result that there is an influx of extracellular Ca2+ into the cytoplasm. When Mn2+ was applied during depolarization, a fluorescence quenching occurred. By such means the site of Ca2+ channels was elucidated.

Visualization and functional testing of acetylcholine receptor-like molecules in cochlear outer hair cells

The efferent nerve endings at outer hair cells (OHCs) have been suggested to regulate active mechanical processes in the cochlea. The discovery of acetylcholine (ACh)-producing and -degrading enzymes in these synapses gave rise to the speculation that ACh might be one of the efferent transmitters. However, there has as yet been no identification and characterization of any corresponding receptor in OHCs which is required for further clarification of this question. In the present paper existence, location and first characterization of acetylcholine receptors (AChRs) in OHCs are reported. Using two anti-AChR monoclonal antibodies, AChR epitopes were found forming a cup at the basal end of the OHCs opposite to the efferent nerve endings. Furthermore, the studied molecules could be shown to extend through the cell membrane. In addition, the denervated OHC AChR-epitopes seem to move by lateral diffusion. Application of Carbachol and ACh to the basal pole of OHCs induced a weak, reversible cell contraction. Pharmacological controls revealed, that hte motile responses were mediated by the AChRs.

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.

A subpopulation of outer hair cells possessing GABA receptors with tonotopic organization

The olivocochlear innervation has been postulated to regulate active mechanical processes in the mammalian cochlea. Histochemical studies led to the suggestion that a subpopulation of these efferent nerves, which predominantly terminate on outer hair cells (OHCs), are gamma-aminobutyric acid (GABA)-ergic. By means of two monoclonal antibodies, we were able to visualize GABAA-receptor immunoreactivity at the basal pole of isolated sensory cells. Both subunits of the GABAA receptor, the alpha- and beta-subunit, are known to form the transmembranous GABA/benzodiazepine-receptor complex and were present on OHCs. In addition, these inhibitory receptors were more numerous in the apical turns of the cochlea, indicating another criterion for distinguishing the apical from basal turns of the cochlea. These results support the concept that a subpopulation of axosomatic synapses at the basal pole of OHCs liberate the inhibitory neurotransmitter GABA into the synaptic cleft. Binding of the transmitter to these newly observed subsynaptic receptors is possibly followed by a change in OHC motility and a subsequent modulation of the movement of the basilar membrane.

Actin-associated proteins and fibronectin in the fetal human inner ear

The distribution of alpha-actinin, vinculin, alpha-spectrin, beta-spectrin and fibronectin was analyzed in 14- to 21-week-old fetal human inner ears using immunofluorescence microscopy. Staining for alpha-actinin was fairly evenly distributed at the epithelial surfaces of all five vestibular organs, whereas in the cochlea it was mainly at the surface of the receding greater epithelial ridge and in some foci apically at the lesser epithelial ridge. Fluorescence for vinculin was observed mainly at the surface of vestibular organs, but was lacking in the LER. Intense fluorescence for alpha-spectrin was found at the apical surface of individual cells of the cristae and maculae. Antibodies against beta-spectrin mainly stained the endothelial cells of blood vessels, but faint staining of the epithelial cell surfaces of the vestibular organs was also detected. The fluorescence pattern of the actin-associated proteins is indicative of structural differences between cochlear and vestibular hair cells. Fibronectin was identified only between mesenchymal cells and its functional importance in the mature inner ear epithelia can be discounted.

Whole cell currents and mechanical responses of isolated outer hair cells

Outer hair cells (OHCs) exhibit electrically induced cell movements which are considered to enhance the frequency selectivity and sensitivity of basilar membrane vibration. Using simultaneous whole cell voltage clamp and video analysis, we demonstrate that the mechanical response of OHCs is not altered by agents which alter membrane currents under voltage clamp. Thus the underlying mechanism of OHC movements appears to be dependent upon membrane potential, rather than transmembrane currents.

Volume and length changes in outer hair cells of the guinea pig after potassium-induced shortening

Measurements of volume and length were made on isolated guinea pig cochlear outer hair cells (OHCs) in an attempt to establish whether OHC shortening was accompanied by changes in cell volume. A sustained shortening in response to an isotonic 100 mM KCl solution was found to be accompanied by a significant increase in OHC volume. The application of hypotonic solutions had a similar effect. When testing solutions with unchanged or reduced [K+] [Cl-] products in order to avoid loading the cells with chloride ions and producing concomitant water influx, the most frequent response was a rapid shortening followed by an elongation beyond the original cell length. These findings of a sustained and a spontaneously reversible shortening suggest that the potassium-induced response may consist of two components: a rapid one, which may be reversible in the presence of the stimulus, and a second, slower, component resulting in sustained shortening.

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

It has been suggested that active motor capacities may explain the high sensitivity and sharp tuning of auditory transduction in the mammalian cochlea. Recently, slow and fast motile responses of isolated outer hair cells (OHCs) have been demonstrated. These consist of reversible length changes in the cylindrical cell body. Here we show that isolated OHCs are also capable of slow and fast movements of the cuticular plate (CP) which cause passive displacements of the stereocilia. The molecular mechanisms that underlie the slow movements appear to reside in the highly organized actomyosin cytoskeleton in the CP. Additional, fast movements of the CP were observed in OHCs which were exposed to alternating electrical fields. These rapid mechanical events followed the electrical stimulus cycle-by-cycle, and unlike the slow movements, did not depend on the actomyosin skeleton in the CP. The fast motility of the CP and the attached stereocilia suggest an additional active mechanism which may contribute to the sensitivity and the sharp tuning of the cochlea near threshold. By controlling the operating point of the reticular lamina and of the hair bundles, the slow CP and stereociliary movements may influence basilar membrane homeostasis and temporary threshold shift at high sound pressure levels.

Outer hair cells as fast and slow cochlear amplifiers with a bidirectional transduction cycle

Cochlear hair cells fulfil the key function in hearing. However, possible differences in functions of inner and outer hair cells are the subject of a current scientific discussion. In the present paper, we present steps of the sensory transduction in living mammalian hair cells. The transduction is a mechano-electrical process based upon ionic channels in the outer cell membrane. These mechanisms are suggested to be similar in inner and outer hair cells, allowing inner hair cells the actual sound perception. By contrast, the outer hair cells seem to control actively the micromechanics of the inner ear on the basis of fast and slow motor properties. Our results revealed that in these cells, electro-mechanical mechanisms can induce fast and slow motile events. Together with the above-mentioned mechano-electrical transduction, the electro-mechanical mechanisms are assumed to establish a bidirectional transduction cycle in outer hair cells. These active motor properties of outer hair cells are obviously disturbed in acute attacks of Meniére's disease or in aminoglycoside intoxication, and may thus partly explain the resulting deafness.

Evidence that phosphoinositides mediate motility in cochlear outer hair cells

Cochlear outer hair cells are postulated to modulate auditory transduction by their mechanical properties which presumably are controlled by efferent neurons and regulated by the levels of intracellular calcium. In a number of biological systems these calcium levels are controlled by inositol trisphosphate (InsP3), the second messenger of the phosphoinositide cascade. We have investigated whether the phosphoinositides function in the signal transfer in mammalian auditory sensory cells. Live isolated outer hair cells synthesize the lipids of the phosphoinositide cycle, phosphatidic acid (Ptd), phosphatidylinositol (PtdIns), phosphatidylinositol 4-phosphate (PtdInsP) and phosphatidylinositol 4,5-bisphosphate (PtdInsP2). After one hour of incubation with [32P]orthophosphate, Ptd constituted 21 +/- 7% of labeled lipids, PtdIns 19 +/- 5%, PtdInsP 28 +/- 7% and PtdInsP2 33 +/- 7%. Contractions were elicited in isolated cells after their membrane was permeabilized with Triton X-100. Addition of calcium and ATP resulted in contractions (average, 7.2% of initial cell length) in 81% of the cells studied; the omission of ATP or calcium (i.e. the presence of the calcium-chelator EGTA) reduced the incidence of contractions to 0% and 17%, respectively. In the absence of calcium, the putative second messenger, InsP3, caused contractions in 63% of the observed cells while the physiologically inactive agents inositol and inositol bisphosphate failed to elicit responses. The results are compatible with the suggestion that InsP3 mediates contractility in outer hair cells.

Motility in auditory sensory cells

Three different procedures were employed to induce contraction of isolated cochlear outer hair cells. When cells were permeabilized and exposed to calcium and ATP both a shortening of cell length (14%) and a decrease in cell diameter (12-16%) was observed indicating a longitudinal and a circumferential contraction. When intact cells were treated with positively charged ferritin a shortening of about 10% was observed together with an increase in cell diameter. Application of solutions containing high potassium concentrations gave similar but more rapid results as compared with positively charged ferritin. The observed responses of inner ear sensory cells together with their morphological and immunohistochemical characteristics further support the idea that the hearing organ acts as a motile system.

Comparison of isolated outer hair cells from five mammalian species

Live outer hair cells were isolated from guinea pig, chinchilla, rat, mouse, and gerbil. The organ of Corti from selected turns of the cochlea was briefly incubated with collagenase and outer hair cells were separated from the tissue by micromanipulation under microscopic observation. Morphological criteria for cell viability were: cylindrical cell shape without swelling or distortion of the membrane; location of the nucleus in its normal position near the base of the cell; cytoplasm devoid of Brownian motion and granulation. Both yield and quality (as judged by these morphological criteria) of isolated hair cells varied with the species and the turn from which the isolation was attempted. Consistently high yields and cells of good morphology were obtained from guinea pigs and chinchillas. Fewer cells were obtained from rats and mice, and their quality was less consistent. Gerbils gave the poorest yield and quality of outer hair cells. In all species, the preparation was more successful from the apical than from the basal turn. The length of apical hair cells varied almost 4-fold from 60 to 80 microns in guinea pig and chinchilla to 20 to 40 microns in the other species, while their diameter only varied 1.5-fold from 7 (mouse) to 10 microns (chinchilla). Outer hair cells could be maintained in vitro in good condition for several hours. Typical early signs of degeneration were increased Brownian motion and granulation in the cytoplasm, upward movement of the nucleus, or distortion of cell shape. Degeneration was always accompanied by a shortening along the long axis.

K+-induced motility and depolarization of cochlear hair cells. Direct evidence for a new pathophysiological mechanism in Ménière's disease

In addition to an endolymphatic hydrops in various models of Ménière's disease, ruptures of the membranes lining the endolymphatic spaces or massive changes in their biochemical permeability are believed to allow large amounts of potassium-rich endolymph to deluge the normally low [K+] perilymphatic fluid. Our clinic has shown that exposure of the lateral and basal membrane parts of living isolated outer hair cells to increasing concentrations of potassium ions resulted in a sustained reversible cellular depolarization. Furthermore, potassium intoxication was accompanied by a strictly longitudinal contraction of the hair cells present. This was followed by hair cell relaxation in the presence of artificial perilymph. These findings suggest a supplementary hypothesis for the clinical manifestations of Ménière's disease: the reversible hair cell depolarization can explain the sudden tinnitus and parts of the hearing loss incurred as well as presumably the vertigo experienced; the reversible longitudinal hair cell contraction induces an abnormal change of cochlear micromechanics, resulting in concomitant attacks of deafness and also possibly contributing to the tinnitus perceived.

Motile responses in outer hair cells

Motile responses of cochlear hair cells open new perspectives for the understanding of cochlear hearing mechanisms and hearing disorders located in hair cells. Direct visualization of hair cell motility was achieved by a method for the study of living isolated mammalian outer hair cells (OHCs) which has overcome some of the complexities in dealing with the heterogeneous organ of Corti. Electrophysiological giga-seal whole-cell recordings of single OHC prepared by this approach had revealed negative cell potentials ranging from -32 mV to -70 mV (Gitter et al. (1986) Oto-Rhino-Laryngol. in press). Elucidation of HC motility has come from two lines of experiments. One follows from the observation that exposure of the lateral and basal membrane parts of living OHCs to increasing bath K+ concentrations resulted in a sustained reversible depolarization of the cell. Here, we report that by depolarization of the cell membrane in the presence of 25-125 mM K+/Cl- a sustained contraction of OHC was induced. This was followed by relaxation in the presence of artificial perilymph containing 5.4 mM K+/Cl-. By alternating these procedures OHCs were made to undergo as many as five cycles of contraction and relaxation. External Ca2+ was not required for the initial contraction but was essential for relaxation. Following repeated contraction/relaxation cycles the cytoplasm of individual OHCs exhibited a filamentous network, correlating with a new infracuticular anti-actin binding capacity. The second series of experiments originates in the observation that permeabilized OHCs contracted in the presence of ATP. No response was seen in the presence of control nucleotides.(ABSTRACT TRUNCATED AT 250 WORDS)