The role of transmembrane channel-like proteins in the operation of hair cell mechanotransducer channels

Sound stimuli elicit movement of the stereocilia that make up the hair bundle of cochlear hair cells, putting tension on the tip links connecting the stereocilia and thereby opening mechanotransducer (MT) channels. Tmc1 and Tmc2, two members of the transmembrane channel–like family, are necessary for mechanotransduction. To assess their precise role, we recorded MT currents elicited by hair bundle deflections in mice with null mutations of Tmc1, Tmc2, or both. During the first postnatal week, we observed a normal MT current in hair cells lacking Tmc1 or Tmc2; however, in the absence of both isoforms, we recorded a large MT current that was phase-shifted 180°, being evoked by displacements of the hair bundle away from its tallest edge rather than toward it as in wild-type hair cells. The anomalous MT current in hair cells lacking Tmc1 and Tmc2 was blocked by FM1-43, dihydrostreptomycin, and extracellular Ca²⁺ at concentrations similar to those that blocked wild type. MT channels in the double knockouts carried Ca²⁺ with a lower permeability than wild-type or single mutants. The MT current in double knockouts persisted during exposure to submicromolar Ca²⁺, even though this treatment destroyed the tip links. We conclude that the Tmc isoforms do not themselves constitute the MT channel but are essential for targeting and interaction with the tip link. Changes in the MT conductance and Ca²⁺ permeability observed in the absence of Tmc1 mutants may stem from loss of interaction with protein partners in the transduction complex.

Marshalin, a microtubule minus-end binding protein, regulates cytoskeletal structure in the organ of Corti

Dramatic structural changes in microtubules (MT) and the assembly of complicated intercellular connections are seen during the development of the cellular matrix of the sense organ for hearing, the organ of Corti. This report examines the expression of marshalin, a minus-end binding protein, during this process of cochlear development. We discovered that marshalin is abundantly expressed in both sensory hair cells and supporting cells. In the adult, prominent marshalin expression is observed in the cuticular plates of hair cells and in the noncentrosomal MT organization centers (MTOC) of Deiters' and pillar cells. Based upon differences in marshalin expression patterns seen in the organ of Corti, we identified eight isoforms ranging from 863 to 1280 amino acids. mRNAs/proteins associated with marshalin's isoforms are detected at different times during development. These isoforms carry various protein-protein interacting domains, including coiled-coil (CC), calponin homology (CH), proline-rich (PR), and MT-binding domains, referred to as CKK. We, therefore, examined membranous organelles and structural changes in the cytoskeleton induced by expressing two of these marshalin isoforms in vitro. Long forms containing CC and PR domains induce thick, spindle-shaped bundles, whereas short isoforms lacking CC and PR induce more slender variants that develop into densely woven networks. Together, these data suggest that marshalin is closely associated with noncentrosomal MTOCs, and may be involved in MT bundle formation in supporting cells. As a scaffolding protein with multiple isoforms, marshalin is capable of modifying cytoskeletal networks, and consequently organelle positioning, through interactions with various protein partners present in different cells.

The composition and role of cross links in mechanoelectrical transduction in vertebrate sensory hair cells

The key components of acousticolateralis systems (lateral line, hearing and balance) are sensory hair cells. At their apex, these cells have a bundle of specialized cellular protrusions, which are modified actin-containing microvilli, connected together by extracellular filaments called cross links. Stereociliary deflections open nonselective cation channels allowing ions from the extracellular environment into the cell, a process called mechanoelectrical transduction. This produces a receptor potential that causes the release of the excitatory neurotransmitter glutamate onto the terminals of the sensory nerve fibres, which connect to the cell base, causing nerve signals to be sent to the brain. Identification of the cellular mechanisms underlying mechanoelectrical transduction and of some of the proteins involved has been assisted by research into the genetics of deafness, molecular biology and mechanical measurements of function. It is thought that one type of cross link, the tip link, is composed of cadherin 23 and protocadherin 15, and gates the transduction channel when the bundle is deflected. Another type of link, called lateral (or horizontal) links, maintains optimal bundle cohesion and stiffness for transduction. This Commentary summarizes the information currently available about the structure, function and composition of the links and how they might be relevant to human hearing impairment.

The development, distribution and density of the plasma membrane calcium ATPase 2 calcium pump in rat cochlear hair cells

Calcium is tightly regulated in cochlear outer hair cells (OHCs). It enters mainly via mechanotransducer (MT) channels and is extruded by the plasma membrane calcium ATPase (PMCA)2 isoform of the PMCA, mutations in which cause hearing loss. To assess how pump expression matches the demands of Ca(2+) homeostasis, the distribution of PMCA2 at different cochlear locations during development was quantified using immunofluorescence and post-embedding immunogold labeling. The PMCA2 isoform was confined to stereociliary bundles, first appearing at the base of the cochlea around post-natal day (P)0 followed by the middle and then the apex by P3, and was unchanged after P8. The developmental appearance matched the maturation of the MT channels in rat OHCs. High-resolution immunogold labeling in adult rats showed that PMCA2 was distributed along the membranes of all three rows of OHC stereocilia at similar densities and at about a quarter of the density in inner hair cell stereocilia. The difference between OHCs and inner hair cells was similar to the ratio of their MT channel resting open probabilities. Gold particle counts revealed no difference in PMCA2 density between low- and high-frequency OHC bundles despite larger MT currents in high-frequency OHCs. The PMCA2 density in OHC stereocilia was determined in low- and high-frequency regions from calibration of immunogold particle counts as 2200/μm(2) from which an extrusion rate of ∼200 ions/s per pump was inferred. The limited ability of PMCA2 to extrude the Ca(2+) load through MT channels may constitute a major cause of OHC vulnerability and high-frequency hearing loss.

Eps8 regulates hair bundle length and functional maturation of mammalian auditory hair cells

Hair cells of the mammalian cochlea are specialized for the dynamic coding of sound stimuli. The transduction of sound waves into electrical signals depends upon mechanosensitive hair bundles that project from the cell's apical surface. Each stereocilium within a hair bundle is composed of uniformly polarized and tightly packed actin filaments. Several stereociliary proteins have been shown to be associated with hair bundle development and function and are known to cause deafness in mice and humans when mutated. The growth of the stereociliar actin core is dynamically regulated at the actin filament barbed ends in the stereociliary tip. We show that Eps8, a protein with actin binding, bundling, and barbed-end capping activities in other systems, is a novel component of the hair bundle. Eps8 is localized predominantly at the tip of the stereocilia and is essential for their normal elongation and function. Moreover, we have found that Eps8 knockout mice are profoundly deaf and that IHCs, but not OHCs, fail to mature into fully functional sensory receptors. We propose that Eps8 directly regulates stereocilia growth in hair cells and also plays a crucial role in the physiological maturation of mammalian cochlear IHCs. Together, our results indicate that Eps8 is critical in coordinating the development and functionality of mammalian auditory hair cells.

The border between the central and the peripheral nervous system in the cat cochlear nerve: a light and scanning electron microscopical study

The transition between the central (CNS) and peripheral nervous system (PNS) in cranial and spinal nerve roots, referred to here as the CNS-PNS border, is of relevance to nerve root disorders and factors that affect peripheral-central regeneration. Here, this border is described in the cat cochlear nerve using light microscopical sections, and scanning electron microscopy of the CNS-PNS interfaces exposed by fracture of the nerve either prior to or following critical point drying. The CNS-PNS border represents an abrupt change in type of myelin, supporting elements, and vascularization. Because central myelin is formed by oligodendrocytes and peripheral myelin by Schwann cells, the myelinated fibers are as a rule equipped with a node of Ranvier at the border passage. The border is shallower and smoother in cat cochlear nerve than expected from other nerves, and the borderline nodes are largely in register. The loose endoneurial connective tissue of the PNS compartment is closed at the border by a compact glial membrane, the mantle zone, of the CNS compartment. The mantle zone is penetrated by the nerve fibers, but is otherwise composed of astrocytes and their interwoven processes like the external limiting membrane of the brain surface with which it is continuous. The distal surface of the mantle zone is covered by a fenestrated basal lamina. Only occasional vessels traverse the border. From an anatomical point of view, the border might be expected to be a weak point along the cochlear nerve and thus vulnerable to trauma. In mature animals, the CNS-PNS border presents a barrier to regrowth of regenerating nerve fibers and to invasion of the CNS by Schwann cells. An understanding of this region in the cochlear nerve is therefore relevant to head injuries that lead to hearing loss, to surgery on acoustic Schwannomas, and to the possibility of cochlear nerve regeneration.

The ultrastructural distribution of prestin in outer hair cells: a post-embedding immunogold investigation of low-frequency and high-frequency regions of the rat cochlea

Outer hair cells (OHCs) of the mammalian cochlea besides being sensory receptors also generate force to amplify sound-induced displacements of the basilar membrane thus enhancing auditory sensitivity and frequency selectivity. This force generation is attributable to the voltage-dependent contractility of the OHCs underpinned by the motile protein, prestin. Prestin is located in the basolateral wall of OHCs and is thought to alter its conformation in response to changes in membrane potential. The precise ultrastructural distribution of prestin was determined using post-embedding immunogold labelling and the density of the labelling was compared in low-frequency and high-frequency regions of the cochlea. The labelling was confined to the basolateral plasma membrane in hearing rats but declined towards the base of the cells below the nucleus. In pre-hearing animals, prestin labelling was lower in the membrane and also occurred in the cytoplasm, presumably reflecting its production during development. The densities of labelling in low-frequency and high-frequency regions of the cochlea were similar. Non-linear capacitance, thought to reflect charge movements during conformational changes in prestin, was measured in OHCs in isolated cochlear coils of hearing animals. The OHC non-linear capacitance in the same regions assayed in the immunolabelling was also similar in both the apex and base, with charge densities of 10,000/microm(2) expressed relative to the lateral membrane area. The results suggest that prestin density, and by implication force production, is similar in low-frequency and high-frequency OHCs.

WITHDRAWN: The origin of the cochlear amplifier

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Localisation of the mechanotransducer channels in mammalian cochlear hair cells provides clues to their gating

Our sense of hearing and balance relies on the very rapid gating of mechanotransducer channels known to be located close to the tops of the hair cell stereocilia within the stereociliary bundle. The molecular identity of the channels is unknown but functional aspects such as permeation, block and sensitivity to bundle displacement are well known. The channel has high calcium permeability and this feature has been used in conjunction with fast confocal calcium imaging to unambiguously localise the channels at the top of the two shorter rows of stereocilia in mammalian cochlear hair cells. The data suggest that they are completely absent from the tallest row. It is thought that the structures connecting stereocilia in adjacent rows, the tip links, are either directly responsible for the channel's mechanical gating, or are closely associated with the gating process. The channels must therefore be associated with the bottom part of the tip links and not the top. This feature has important implications for both the channel's gating mechanism and its regulatory adaptation mechanism. The tip link remains an attractive candidate for mechanical coupling between the bundle and the channel or an accessory protein. The localisation of the mechanotransducer channels to the lower end of the tip link represents an important milestone in the journey towards eventual identification of the channel and its gating mechanism.

The dimensions and structural attachments of tip links in mammalian cochlear hair cells and the effects of exposure to different levels of extracellular calcium

The tip links between stereocilia of acousticolateral hair cells have been suggested to contain cadherin 23 (CDH23) comprising an upper branched portion that is bound to a lower portion composed of protocadherin 15 (PCDH15). The molecular conformation of CDH23, its binding to PCDH15, the tip links, and mechanoelectrical transduction have all been shown previously to be sensitive to exposure to low levels of calcium. The aim of this study was to compare the characteristics of tip links in guinea-pig cochlear hair cells with reported features of the CDH23-PCDH15 complex. Tip links were examined using field emission scanning electron microscopy and transmission electron microscopy in conventional preparations and after treatment with the detergent Triton-X-100 or varying calcium concentrations in the extracellular solution. The results showed that tip links have a twisted double-stranded appearance with a branched upper region. They survived demembranation of the stereocilia by detergent suggesting that they have transmembrane domains at both ends. Their lengths, when fixed in the presence of 2 mM extracellular calcium, were approximately 150 nm. With prior exposure to 1 mM calcium their lengths were approximately 164 nm. The lengths in 50 microM calcium are similar ( approximately 185 nm) to those reported for CDH23-PCDH15 complexes in 100 microM calcium ( approximately 180 nm). Exposure to 1 microM calcium caused loss of tip links and an increased distance between the residual attachment sites. The data indicate that extracellular calcium concentration affects tip-link length. One model compatible with the recently proposed tip-link structure is that the CDH23 double strand undergoes calcium-dependent unfolding, changing the length of the links. The bundle may also tilt in the direction of the tallest row of stereocilia as the tip link lengthens and then is lost. Overall, our data are consistent with a tip link composed of complexes of CDH23 and PCDH15 but do not rule out other possibilities.

A large-conductance calcium-selective mechanotransducer channel in mammalian cochlear hair cells

Sound stimuli are detected in the cochlea by opening of hair cell mechanotransducer (MT) channels, one of the few ion channels not yet conclusively identified at a molecular level. To define their performance in situ, we measured MT channel properties in inner hair cells (IHCs) and outer hair cells (OHCs) at two locations in the rat cochlea tuned to different characteristic frequencies (CFs). The conductance (in 0.02 mM calcium) of MT channels from IHCs was estimated as 260 pS at both low-frequency and mid-frequency positions, whereas that from OHCs increased with CFs from 145 to 210 pS. The combination of MT channel conductance and tip link number, assayed from scanning electron micrographs, accounts for variation in whole-cell current amplitude for OHCs and its invariance for IHCs. Channels from apical IHCs and OHCs having a twofold difference in unitary conductance were both highly calcium selective but were distinguishable by a small but significant difference in calcium permeability and in their response to lowering ionic strength. The results imply that the MT channel has properties possessed by few known candidates, and its diversity suggests expression of multiple isoforms.

The sensory and motor roles of auditory hair cells

Cochlear hair cells respond with phenomenal speed and sensitivity to sound vibrations that cause submicron deflections of their hair bundle. Outer hair cells are not only detectors, but also generate force to augment auditory sensitivity and frequency selectivity. Two mechanisms of force production have been proposed: contractions of the cell body or active motion of the hair bundle. Here, we describe recently identified proteins involved in the sensory and motor functions of auditory hair cells and present evidence for each force generator. Both motor mechanisms are probably needed to provide the high sensitivity and frequency discrimination of the mammalian cochlea.

The concentrations of calcium buffering proteins in mammalian cochlear hair cells

Calcium buffers are important for shaping and localizing cytoplasmic Ca2+ transients in neurons. We measured the concentrations of the four main calcium-buffering proteins (calbindin-D28k, calretinin, parvalbumin-alpha, and parvalbumin-beta) in rat cochlear hair cells in which Ca2+ signaling is a central element of fast transduction and synaptic transmission. The proteins were quantified by calibrating immunogold tissue counts against gels containing known amounts of each protein, and the method was verified by application to Purkinje cells in which independent estimates exist for some of the protein concentrations. The results showed that, in animals with fully developed hearing, inner hair cells had 110 of the proteinaceous calcium buffer of outer hair cells in which the cell body contained parvalbumin-beta (oncomodulin) and calbindin-D28k at levels equivalent to 5 mm calcium-binding sites. Both proteins were partially excluded from the hair bundles, which may permit fast unbuffered Ca2+ regulation of the mechanotransducer channels. The sum of the calcium buffer concentrations decreased in inner hair cells and increased in outer hair cells as the cells developed their adult properties during cochlear maturation. The results suggest that Ca2+ has distinct roles in the two types of hair cell, reflecting their different functions in auditory transduction. Ca2+ is used in inner hair cells primarily for fast phase-locked synaptic transmission, whereas Ca2+ may be involved in regulating the motor capability underlying cochlear amplification of the outer hair cell. The high concentration of calcium buffer in outer hair cells, similar only to skeletal muscle, may protect against deleterious consequences of Ca2+ loading after acoustic overstimulation.

Differential distribution of beta- and gamma-actin in guinea-pig cochlear sensory and supporting cells

Sensory and supporting cells of the mammalian organ of Corti have cytoskeletons containing beta- and gamma-actin isoforms which have been described as having differing intracellular distributions in chick cochlear hair cells. Here, we have used post-embedding immunogold labelling for beta- and gamma-actin to investigate semiquantitatively how they are distributed in the guinea-pig cochlea and to compare different frequency locations. Amounts of beta-actin decrease and gamma-actin increase in the order, outer pillar cells, inner pillar cells, Deiters' cells and hair cells. There is also more beta-actin and less gamma-actin in outer pillar cells in higher than lower frequency regions. In hair cells, beta-actin is present in the cuticular plate but is more concentrated in the stereocilia, especially in the rootlets and towards the periphery of their shafts; labelling densities for gamma-actin differ less between these locations and it is the predominant isoform of the hair-cell lateral wall. Alignments of immunogold particles suggest beta-actin and gamma-actin form homomeric filaments. These data confirm differential distribution of these actin isoforms in the mammalian cochlea and reveal systematic differences between sensory and supporting cells. Increased expression of beta-actin in outer pillar cells towards the cochlear base may contribute to the greater stiffness of this region.

An immunogold investigation of the distribution of GABA and glycine in nerve terminals on the somata of spherical bushy cells in the anteroventral cochlear nucleus of guinea pig

Spherical bushy neurons in the anteroventral cochlear nucleus receive glutamatergic primary terminals from the cochlear nerve and terminals of noncochlear (i.e. nonprimary) origin, many of which colocalize gamma-aminobutyric acid (GABA) and glycine. Here the relationship between GABA and glycine in these terminals has been investigated using postembedding immunogold labelling. A significant negative correlation was found between the density of terminal labelling for GABA and for glycine in four guinea pigs. Terminals could be divided into three categories, GABA-only, glycine-only, or colocalizing depending on whether they had a significantly higher labelling density for either amino acid than the primary terminals. The overall labelling density in all four animals was significantly greater for GABA in GABA-only terminals than colocalizing ones but similar for glycine in both. Within the terminals, the labelling density over synaptic vesicles, nonvesicular regions of cytoplasm and mitochondria was also investigated. No significant difference was detected in the labelling density of vesicles compared with nonvesicular regions for either amino acid. However, a significant difference was found between the overall labelling density over mitochondria and nonvesicular regions for both. There was also significantly more mitochondrial GABA labelling in GABA-only terminals compared to colocalizing terminals but mitochondrial glycine labelling was similar in glycine-only and colocalizing terminals. Thus the level of GABA is higher in single than in colocalizing terminals, particularly in the mitochondria, but similar for glycine in both. It is possible therefore that the presence of glycine affects the level of GABA in the nonprimary terminals but that the presence of GABA does not affect the level of glycine.

The distribution of calcium buffering proteins in the turtle cochlea

Hair cells of the inner ear contain high concentrations of calcium-binding proteins that limit calcium signals and prevent cross talk between different signaling pathways during auditory transduction. Using light microscope immunofluorescence and post-embedding immunogold labeling in the electron microscope, we characterized the distribution of three calcium-buffering proteins in the turtle cochlea. Both calbindin-D28k and parvalbumin-beta were confined to hair cells in which they showed a similar distribution, whereas calretinin was present mainly in hair-cell nuclei but also occurred in supporting cells and nerve fibers. The hair-cell concentration of calbindin-D28k but not of parvalbumin-beta increased from the low- to high-frequency end of the cochlea. Calibration against standards containing known amounts of calcium-buffering protein processed in the same fluid drop as the cochlear sections gave cytoplasmic concentrations of calbindin-D28k as 0.13-0.63 mm and parvalbumin-beta as approximately 0.25 mm, but calretinin was an order of magnitude less. Total amount of Ca 2+-binding sites on the proteins is at least 1.0 mm in low-frequency hair cells and 3.0 mm in high-frequency cells. Reverse transcription-PCR showed that mRNA for all three proteins was expressed in turtle hair cells. We suggest that calbindin-D28k and parvalbumin-beta may serve as endogenous mobile calcium buffers, but the predominantly nuclear location of calretinin argues for another role in calcium signaling. The results support conclusions from electrophysiological measurements that millimolar concentrations of endogenous calcium buffers are present in turtle hair cells. Parvalbumin-beta was also found in both inner and outer hair cells of the guinea pig cochlea.

Two approaches to double post-embedding immunogold labeling of freeze-substituted tissue embedded in low temperature Lowicryl HM20 resin

Double labeling is used for localizing two antigens simultaneously in the same tissue. We have used two approaches to post-embedding immunogold labeling to investigate whether nerve terminals in the guinea-pig anteroventral cochlear nucleus (AVCN) that contain gamma-aminobutyric acid (GABA) or glycine are capable of retrieving the other amino acid as part of an investigation of colocalization of these putative neurotransmitters. For this, vibroslices of perfusion-fixed brain stem were freeze-substituted and embedded in the low temperature resin, Lowicryl HM20. Simultaneous labeling of ultrathin sections was then performed with a mixture of a rabbit primary antibody to GABA and a guinea-pig primary antibody to the glycine transporter, GLYT2, followed by labeling with a mixture of secondary antibodies (goat anti-rabbit IgG-30 nm gold, goat anti-guinea pig IgG-15 nm gold). This approach indicated that GLYT2 occurs in the plasma membrane of some terminals that contain GABA. The other approach involved sequential labeling of ultrathin sections with a rabbit primary antibody to the GABA transporter, GAT1, followed by an anti-rabbit secondary antibody conjugated to 15-nm gold particles. Sections were then treated with paraformaldehyde vapor to denature any free anti-IgG binding sites on the first antibody, and labeled with a primary antibody to glycine also raised in rabbit followed by an anti-rabbit secondary antibody conjugated to 30-nm gold particles. This approach indicated that GAT1 occurs in the plasma membrane of some terminals that contain glycine. Thus, these techniques can be used to localize heat-labile multiple antigens in the same tissue.

An immunogold investigation of the distribution of calmodulin in the apex of cochlear hair cells

Calmodulin is found in the mechanosensitive stereociliary bundle of hair cells where it plays a role in various calcium-sensitive events associated with mechanoelectrical transduction. In this study, we have investigated the ultrastructural distribution of calmodulin in the apex of guinea-pig cochlear hair cells, using post-embedding immunogold labelling, in order to determine in more detail where calmodulin-dependent processes may be occurring. Labelling was found in the cuticular plate as well as the hair bundle, the rootlets of the stereocilia being more densely labelled than the surrounding filamentous matrix. In the bundle, labelling was found almost exclusively at the periphery rather than over the centre of the actin core of the stereocilia, and was clearly associated with the attachments of the lateral links that connect them to their nearest neighbours. It was also found to be denser towards the tips of stereocilia compared to other stereociliary regions and occurred consistently at either end of the tip link that connects stereocilia of adjacent rows. The contact region between stereocilia that is found just below the tip link was also clearly labelled. These concentrations of labelling in the bundle are likely to indicate sites where calmodulin is associated with calcium/calmodulin-sensitive proteins such as the various myosin isoforms and the plasma membrane ATPase (PMCA2a) that are known to occur there, and possibly with the transduction channels themselves. At least one of the myosin isoforms, myosin 1c, is thought to be associated with slow adaptation, and PMCA2a with control of calcium levels in the bundle. The concentration of calmodulin in the contact region further supports the suggestion that this is a functionally distinct region rather than a simple geometrical association between adjacent stereocilia.

Distribution of the glutamate/aspartate transporter GLAST in relation to the afferent synapses of outer hair cells in the guinea pig cochlea

The glutamate/aspartate transporter GLAST is known to occur in the plasma membrane of supporting cells and their glialike processes around the synaptic region of inner hair cells of the mammalian cochlea. Its function there is presumably to take up glutamate following the release of this putative amino acid neurotransmitter from the inner hair cells. In this study, we have investigated whether GLAST is also associated with the outer hair cells using postembedding immunogold labeling. This is interesting because it is uncertain whether the outer hair cells have a functional synapse at which glutamate may be released. However, earlier ultrastructural studies of the afferent synapses in outer hair cells in several mammalian species have shown features normally associated with synaptic activity. These observations are confirmed and extended here in guinea pig where these afferent synapses have presynaptic bodies, putative synaptic vesicles, and coated pits associated with them. Immunoreactivity for GLAST was found along the plasma membranes of Deiters' cells, especially around the synaptic region of the hair cell, on processes wrapped around approaching nerve fibers. Semiquantitative analysis of the distribution of immunogold labeling of Deiters' cells confirmed that it was densest in the region adjacent to the synapses. There was also more labeling in apical than in basal regions of the cochlea in three of the four animals examined, suggesting an association with the number of afferent synapses, which are more numerous in apical regions. Interestingly, labeling also occurred in other regions of the cell membrane away from the afferent terminals. This suggests that glutamate uptake is also required away from the immediate vicinity of synapses, perhaps as a consequence of glutamate dispersal resulting from the mechanical displacement of the cochlear partition during stimulation. Nonetheless, the particular association of GLAST with the synaptic region of the outer hair cell implies that the latter have active afferent synapses at which glutamate is released.

Functional effects of a monoclonal antibody on mechanoelectrical transduction in outer hair cells

The functional effect of a monoclonal antibody, RA6.3, on mechanoelectrical transduction (MET) of outer hair cells (OHCs) isolated from the adult guinea-pig cochlea was investigated. This antibody was raised by an antiidiotypic approach against amiloride binding sites. RA6.3 irreversibly reduced the receptor current, independent of membrane potential. The time course of the functional block was independent of dilution (1:100, 50 and 10), beginning 1.2+/-0.5 min after the start of application and decreasing exponentially with a time constant of 0.29+/-0.18 min to 53+/-8% of the control current. The residual current was reversibly blocked by amiloride (300 microM), mainly at negative membrane potentials. Block of control current by amiloride was competitively inhibited by simultaneous application of RA6.3. These results suggest that RA6.3 binds to or in close proximity to amiloride receptor sites associated with the MET channels. Irreversibility, incompleteness, independence of membrane potential and independence of antibody dilution of the functional block can all be explained by irreversible binding of one antibody molecule to a receptor site, yielding a non-blocked state, followed by a relatively slow, reversible transition to a blocked state. It is proposed that the reversible transition might represent intramolecular binding of the second antibody combining site to the second receptor site.

Clues to the cochlear amplifier from the turtle ear

Sound stimuli are detected in the cochlea by vibration of hair bundles on sensory hair cells, which activates mechanotransducer ion channels and generates an electrical signal. Remarkably, the process can also work in reverse with additional force being produced by the ion channels as they open and close, evoking active movements of the hair bundle. These movements could supplement the energy of the sound stimuli but to be effective they would need to be very fast. New measurements in the turtle ear have shown that such active bundle movements occur with delays of less than a millisecond, and are triggered by the entry of Ca(2+) into the cell via the mechanotransducer channel. Furthermore, their speed depends on the frequency to which the hair cell is most sensitive, suggesting that such movements could be important in cochlear amplification and frequency discrimination.

An immunogold investigation of the relationship between the amino acids GABA and glycine and their transporters in terminals in the guinea-pig anteroventral cochlear nucleus

The majority of terminals contacting spherical bushy cell bodies in the guinea-pig anteroventral cochlear nucleus contain GABA, glycine or both (colocalizing). Double labeling with antibodies to each amino acid and the plasma membrane transporter for the other was performed using different sizes of gold particles. The transporter for GABA occurs in the plasma membranes of some terminals containing glycine and vice versa suggesting that colocalizing terminals can retrieve both amino acids.

The distribution of the calcium buffer calbindin in the cochlea of the guinea-pig

INTRODUCTION: Ca2+ is involved in many important functions in the inner ear and its intracellular levels are controlled by extrusion mechanisms and stores, and also by buffers such as calbindin. The latter has been demonstrated previously in hair cells of the organ of Corti using immunofluorescence at the light microscopic level, although its precise intracellular distribution has yet to be determined. One postulated role for calbindin in the brain is protection against hypoxic cell injury and degeneration, in which Ca2+ overload has been implicated. METHODS: We have therefore performed postembedding immunogold labelling for electron microscopy on ultrathin sections of apical and basal turns of the guinea-pig cochlea with anticalbindin (Swant), followed by appropriated gold conjugated secondary antibodies. This technique allows quantitative analysis of the distribution of labelling by counting gold particle densities. RESULTS: Outer hair cells in the apical turn were significantly more heavily labelled than those in the base (P < 0. 001). In addition, at the apex there was a progressive reduction across the three rows of outer hair cells towards the modiolus (P < 0.005) although at the base no radial gradient was detected. CONCLUSIONS: These results confirm previous reports of gradients of calbindin distribution in the organ of Corti. This distribution corresponds with the pattern of hair cell loss that occurs, e.g. after some forms of ototoxicity and it may therefore contribute to the diffential susceptibility of hair cells to damage. ACKNOWLEDGEMENT: This work was supported by the Wellcome Trust.

Regeneration of the mammalian vestibular sensory epithelium following gentamicin-induced damage

OBJECTIVES

The aims of this study are (1) to investigate if significant long-term recovery of mature hair bundle (MHB) numbers takes place following gentamicin-induced damage to the mammalian vestibular sensory epithelium and (2) to assess if the different MHB types in the vestibular sensory epithelium have a different susceptibility to ototoxic damage.

METHODS

Gentamicin (8 mg in 0.1-mL sterile water) was injected transtympanically into one ear of guinea pigs, the contralateral ear acting as a control. The animals were killed at 4 days, 4 weeks, and 3 and 10 months post-treatment and the utricles (n = 38) were extracted from both ears. Mature hair bundle and immature-looking hair bundle (IHB) densities on the surface of the utricle were determined using scanning electron microscopy.

RESULTS

The MHB density showed a significant decline between 4 days and 4 weeks post-treatment. There was greater loss of type I MHBs (tallest stereocilia comparable in height to the kinocilium) than type II MHBs (kinocilium taller than the tallest stereocilia). A significant increase in IHB density was seen at 4 weeks post-treatment, after which it declined rapidly. A significant but incomplete recovery in MHB density (to 66% of control value) was seen in the striolar region at 10 months post-treatment, and these were composed mainly of type II MHBs.

CONCLUSIONS

It would appear that the mature mammalian vestibular sensory epithelium does have the capacity for long-term recovery of MHB numbers following gentamicin-induced damage, but this is limited and does not result in complete restoration of the epithelium. Type I MHBs are more susceptible to ototoxic damage than type II MHBs. Sommaire

Localization of the glutamate-aspartate transporter, GLAST, in rat taste buds

A number of putative neurotransmitter substances have been found in vertebrate taste buds. Amongst these glutamate has been localized in fibres innervating the buds and uptake of glutamate has been shown to occur into receptor cells. It is therefore possible that, in common with other sensory systems, glutamate is a neurotransmitter in taste buds. In the inner ear and retina of mammals, the membranes of supporting cells have been shown to contain the glial glutamate transporter GLAST. In the brain, this protein is involved in glutamate re-uptake into glial cells where the glutamate is converted into glutamine for recycling into glutamatergic terminals. In this study, the presence of GLAST has been investigated in taste buds in the rat vallate papilla and its distribution compared with that of glutamine to determine whether there are cells in this system that play a glia-like role in glutamate handling. Immunofluorescent labelling showed that a subset of cells in the taste bud contains GLAST. Immunogold labelling indicated that it occurs in the plasma membranes of supporting cells, especially on the fine cytoplasmic processes of dark cells towards the basal region of the bud. A protein of molecular mass similar to that of cerebellar GLAST was detected in immunoblots of excised papillae. Double labelling and semiquantitative analysis of glutamine and GLAST immunoreactivity showed that the GLAST-positive cells have a higher level of cytoplasmic glutamine than the adjacent cells. It is proposed that these GLAST-positive cells play a glia-like role in the uptake of glutamate following its release at synapses within the taste bud although the precise location of the latter remains uncertain. The GLAST-positive cells may also be involved in its subsequent conversion to glutamine in a glutamate/glutamine cycle similar to that described in the brain.

Gene disruption of p27(Kip1) allows cell proliferation in the postnatal and adult organ of corti

Hearing loss is most often the result of hair-cell degeneration due to genetic abnormalities or ototoxic and traumatic insults. In the postembryonic and adult mammalian auditory sensory epithelium, the organ of Corti, no hair-cell regeneration has ever been observed. However, nonmammalian hair-cell epithelia are capable of regenerating sensory hair cells as a consequence of nonsensory supporting-cell proliferation. The supporting cells of the organ of Corti are highly specialized, terminally differentiated cell types that apparently are incapable of proliferation. At the molecular level terminally differentiated cells have been shown to express high levels of cell-cycle inhibitors, in particular, cyclin-dependent kinase inhibitors [Parker, S. B., et al. (1995) Science 267, 1024-1027], which are thought to be responsible for preventing these cells from reentering the cell cycle. Here we report that the cyclin-dependent kinase inhibitor p27(Kip1) is selectively expressed in the supporting-cell population of the organ of Corti. Effects of p27(Kip1)-gene disruption include ongoing cell proliferation in postnatal and adult mouse organ of Corti at time points well after mitosis normally has ceased during embryonic development. This suggests that release from p27(Kip1)-induced cell-cycle arrest is sufficient to allow supporting-cell proliferation to occur. This finding may provide an important pathway for inducing hair-cell regeneration in the mammalian hearing organ.

Ultrastructural localisation of spectrin in sensory and supporting cells of guinea-pig organ of Corti

Spectrin is a cytoskeletal protein found in the cortex of many cell types. It is known to occur in cochlear outer hair cells (OHCs) with previous immunoelectron microscopical studies showing that it is located in the cuticular plate and the cortical lattice. The latter is a network of filaments associated with the lateral plasma membrane that is thought to play a role in OHC motility. Spectrin has also been found in inner hair cells (IHCs) and supporting cells using immunofluorescent techniques, but its ultrastructural distribution in these cells has not yet been described. This has, therefore, been investigated using a monoclonal antibody to alpha-spectrin in conjunction with pre- and post-embedding immunogold labelling for transmission electron microscopy. Labelling was found in a meshwork of filaments beneath the plasma membranes of both IHCs and supporting cells and, in pillar cells, close to microtubule/microfilament arrays. It was also found in association with the stereocilia of OHCs and IHCs and, as expected, in the cortical lattice and cuticular plate of OHCs. Thus, spectrin is a general component of cytoskeletal structures involved in maintaining the specialised cell shapes in the organ of Corti and may contribute to the mechanical properties of all the cell types examined.

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

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

The effect of explantation and neomycin on hair cells and supporting cells in organotypic cultures of the adult guinea-pig utricle

Recent reports suggest that immature hair bundles are observed following aminoglycoside-induced hair-cell loss in the mammalian utricle in vitro as well as in vivo. It is therefore important to document the initial morphological changes associated with both culturing and aminoglycoside application so that degeneration can be clearly distinguished from regeneration. In this study, utricles from adult guinea pigs were maintained in culture for either 3 or 8 days, half being exposed to neomycin for days 2 and 3. They were then processed for microscopical examination and compared with control utricles from animals of the same age. The numbers of hair-cell and supporting-cell nuclei were counted and hair-cell morphology assessed. Bundles were classified as having either stepped (SHB) or unstepped (UHB) stereocilia, and their density determined. The numbers of hair-cell, but not supporting-cell, nuclei declined significantly compared with controls in both untreated and treated explants, the greatest reduction occurring 5 days after neomycin administration. The density of SHBs also declined but there was no significant change in UHB density, resulting in a residual population of hair bundles of more immature appearance in both untreated and treated utricles in vitro than in vivo. Although degenerative events such as hair-cell ejection from, or retraction into, the sensory epithelium were observed, no evidence of regeneration was found.

Ultrastructural localization of cadherin in the adult guinea-pig organ of Corti

The apices of the majority of cells of the organ of Corti are connected together by junctional complexes to form the reticular lamina, a barrier that prevents the mixing of endolymph and perilymph. These complexes include tight junctions, adherens junctions and desmosomes. Further information is required about the identity and distribution of the molecules involved in these connections if the function and organization of the reticular lamina are to be well understood. One major category of molecules occurring in adherens junctions and desmosomes, and involved in the maintenance of tissue integrity, is the cadherins. However, although cadherin has been identified in junctions between supporting cells in the adult mammalian organ of Corti at the light microscopic level, its ultrastructural distribution has not so far been described. A post-embedding immunogold labelling technique has therefore been used in conjunction with a monoclonal antibody to cadherin to investigate its ultrastructural distribution in the adult guinea-pig reticular lamina. Immunolabelling is observed in hair cell-supporting cell junctions and in supporting cell-supporting cell junctions. In addition, there is more labelling associated with inner hair cell-supporting cell junctions than with outer hair cell-supporting cell junctions. This may indicate that the junctions associated with the two types of hair cell have different functional properties.

Kinematic analysis of shear displacement as a means for operating mechanotransduction channels in the contact region between adjacent stereocilia of mammalian cochlear hair cells

In sensory hair cells of the cochlea, deflection of the stereociliary bundle results in direct mechanical gating of mechanoelectrical transduction channels, a function generally attributed to the tip link running between the tips of short stereocilia and the sides of adjacent taller ones. However, immunocytochemical experiments indicate that the channels may not be associated with the tip link but occur just below it in a region of contact between the stereocilia. To determine whether transduction channels in this location could be operated during physiologically appropriate deflections as effectively by shear displacement as if they were associated with the tip link, a two dimensional kinematic analysis of relative motion between stereocilia has been performed assuming contact between stereocilia is maintained during deflection. Bundle geometry and dimensions were determined from transmission electron micrographs of hair cells from several frequency locations between 0.27 and 13.00 kHz in the guinea-pig cochlea. The analysis indicates that for a 10 nm deflection of the tallest stereocilia of both inner and outer hair cells, i.e. within the range of the maximum sensitivity of mammalian hair bundles, the average shear displacement in the contact region would be 1.6 nm, but that it increases systematically towards higher frequency regions for outer hair cells. This displacement is comparable in magnitude to tip-link elongation for individual stereociliary pairs.

Putative immunolocalization of the mechanoelectrical transduction channels in mammalian cochlear hair cells

Hair cells bear an apical bundle of stereocilia arranged in serried rows. Deflection of the bundle controls the opening and closing of mechanoelectrical transduction channels, thereby altering the conductance across the apical plasma membrane. Two locations for these channels have been proposed in the bundle, either near the bases of the stereocilia or towards their tips. One hypothesis that is consistent with the latter possibility suggests that fine extracellular filaments, which run between the tips of the shorter stereocilia and the sides of the taller stereocilia behind, operate the channels. Determining the precise position of the channels is essential to test this hypothesis. We have therefore attempted to localize them immunocytochemically. Because hair-cell transduction is amiloride sensitive, the channels may have an amiloride-binding site associated with them. We have therefore used a polyclonal antibody raised against another amiloride-sensitive ion channel to hunt for them. This antibody recognizes a 62-64 kDa band in immunoblots of cochlear tissue, and produces discrete labelling in the hair bundle. This is most concentrated just below the tips of the shorter stereocilia, coinciding with a region of specialization in the closely apposed membranes of the short and tall stereocilia but not with either end of the tip link.

Immunoreactivity of sensory hair bundles of the guinea-pig cochlea to antibodies against elastin and keratan sulphate

The stereociliary bundles of hair cells contain cross-linking extracellular filaments which have been suggested to play a role in mechanoelectrical transduction. To investigate the composition of these filaments, antibodies to the extracellular matrix molecules elastin and keratan sulphate have been used for light- and electron-microscopic immunocytochemistry of the guinea-pig organ of Corti. With the antibody to elastin, no immunoreactivity was found in hair bundles. This implies either that the epitope recognised by this antibody is not present in the links or that it is obscured. The antibody to keratan sulphate labelled the stereociliary bundles of both inner and outer hair cells but not supporting cells. The tips of the tallest stereocilia, especially on outer hair cells, the tips of the shorter stereocilia where the tip links attach to the stereociliary membrane, and the attachments of the lateral links, were labelled. This suggests that the links contain keratan sulphate proteoglycans, molecules which in other tissues are known to maintain structural integrity and fibrillar spacing, and to influence the microenvironment of the cell surface.

The binding site on cochlear stereocilia for antisera raised against renal Na+ channels is blocked by amiloride and dihydrostreptomycin

The mechanoelectrical transduction channels on hair cells have been suggested to be operated by tip links that are stretched when the hair bundle is deflected in the direction of the tallest row of stereocilia. Localising these channels is therefore an important test of this hypothesis. The transduction channels are known to be amiloride-sensitive and immunogold labelling with antibodies raised against the amiloride-sensitive epithelial Na+ channel from kidney (alpha NaCh), has suggested that sites with similar characteristics are located in the region where the tips of the shorter stereocilia appear to come into contact with the sides of the adjacent taller stereocilia rather than being associated directly with the tip links. Now, further immunocytochemical experiments have been performed to determine if amiloride and dihydrostreptomycin, both of which can block transduction, can affect this labelling. Immunofluorescent labelling of the stereocilia is obtained when surface preparations of the organ of Corti are fixed and incubated with alpha NaCh followed by an appropriate secondary antibody. This labelling is abolished by trypsinization prior to fixation but retained if the tissue is pretreated with amiloride and then trypsinized in its presence. Because amiloride is known to protect amiloride-binding sites from degradation by trypsin, these results suggest that alpha NaCh is revealing amiloride-binding sites on the stereocilia. Similarly, immunofluorescent labelling of the stereocilia is abolished if cochlear tissue is pretreated with dihydrostreptomycin (DHS) and fixed in its presence prior to incubation with alpha NaCh. Quantitative analysis of colloidal gold labelling using transmission electron microscopy shows that DHS treatment produces a significant reduction in the number of gold particles on stereocilia, especially in the region of contact between them. These results suggest that anti-Na+ recognises a site with characteristics similar to the mechanoelectrical transduction channels.

Immunocytochemical evidence that glutamate is a neurotransmitter in the cochlear nerve: a quantitative study in the guinea-pig anteroventral cochlear nucleus

The large so-called type I afferents of the cochlear nerve carry the majority of the auditory input from the cochlea to the cochlear nuclei in the brainstem. These fibres are excitatory and previous studies have suggested they may use glutamate as their neurotransmitter. In the present investigation therefore, antibodies to glutamate and to the glutamate precursor, glutamine, were applied to resin sections of perfusion-fixed brains and of in vitro brain slices subjected to depolarizing levels of potassium before fixation to study glutamate handling and synaptic release. Ultrathin sections were labelled by the immunogold technique, and the immunoreactivity was quantified by recording the density of gold particles over the various tissue profiles. Non-primary, presumably inhibitory, terminals and glial processes were used as reference structures. The cochlear primary terminals proved to be strongly immunoreactive for glutamate. The density of glutamate labelling was higher in primary terminals than in non-primary ones, and lowest in glial processes. The ratio between the mean glutamate and glutamine labelling densities was also higher in primary terminals than in non-primary ones, and lowest in glial processes in each case. In the primary terminals, the glutamate immunoreactivity was higher over vesicle-containing regions than over vesicle-free regions, whilst glutamine was evenly distributed throughout. The in vitro brain slices showed a potassium-induced, partly calcium-dependent depletion of glutamate from the primary terminals but not from the non-primary ones. These observations strongly support the conclusion that glutamate is a neurotransmitter of type I cochlear afferents.

Aluminium, beta-amyloid and non-enzymatic glycosylation

The non-enzymatic glycosylation of beta-amyloid is implicated in the aetiology of Alzheimer's disease. However, controversy surrounds the nature of any involvement and a potential mechanism has not been fully elucidated. We present evidence of an aluminium-induced aggregation of the A beta P(25-35) peptide and speculate that the mechanism of formation of our ordered beta-amyloid aggregates might involve non-enzymatic glycosylation and/or site-specific crosslinking of beta-amyloid fibrils by atomic aluminium.

Mechanotransduction in vertebrate hair cells: structure and function of the stereociliary bundle

The mechanosensitive hair cells of the vertebrate acousticolateralis system have an apical bundle of stereocilia, deflections of which control the opening of mechano-electrical transduction channels and thus generate receptor potentials in the cell below. This review describes current theories of hair cell function in the light of recent immunocytochemical and ultrastructural findings; in particular, the location and operation of the transduction channels are considered. The most widely accepted hypothesis of mechanotransduction by hair cells is that fine extracellular links that run between the tips of shorter stereocilia and the sides of taller ones operate the transduction channels. However, the fact that the transduction channels are amiloride sensitive has led to labeling experiments using antibodies to the amiloride-sensitive epithelial Na+ channel from kidney which suggest that the mechanotransduction channels may not be directly associated with the tip links. Instead, they appear to be located near a junctionlike structure at the point of contact between the shorter and taller stereocilia. The implications of these findings for the tip link hypothesis are discussed.

The functional morphology of stereociliary bundles on turtle cochlear hair cells

The stereociliary bundles of hair cells from the basilar papilla of the red-eared turtle were examined with transmission and high resolution scanning electron microscopy to provide a description of their morphology, orientation and inter-ciliary connections for comparison with physiological observations. Bundles on hair cells in the basilar membrane region are of a uniform shape and orientation, but bundles on the apical and basal limbus are distinct in having elongated kinocilia more than twice the length of the tallest stereocilia. Bundles in the basilar membrane region show a roughly two-fold increase in height from 5 to 9 microns from base to apex. Electrical recordings from isolated hair cells indicate that the bundle height is inversely proportional to the cell's characteristic frequency. It is argued that the change in dimensions is insufficient to contribute significantly to the cochlea's frequency selectivity. The cytoplasm adjacent to the kinocilium is filled with microtubules and large vesicles, and there are coated pits in the apical membrane which, it is suggested, may be indicative of rapid turnover of the membrane in this region.

Microslicing of the resin-embedded cochlea in comparison with the surface preparation technique for analysis of hair cell number and morphology

The surface preparation technique for hair cell counting is frequently used for the analysis of cochlear pathology in relation to hearing impairment. To overcome problems encountered with standard surface preparation methods, a new combination of resin embedding and microslicing has been applied to the mammalian cochlea, to permit hair cell counting to be followed by electron microscopic analysis. Partial removal of the cochlear shell is followed by resin infiltration under vacuum prior to polymerization. The cochlea is then subdivided into hemicoils using a mounted annular diamond blade. Embedding before microslicing ensures that both apical and basal regions of the cochlea are preserved equally well with an evenly distributed and therefore predictable 10% hair cell loss. By comparison, the standard surface preparation method often produces distortion of remaining organ of Corti and unpredictable losses of hair cells, for example, up to 17%. The damage tends to be greatest towards the base of the cochlea, an area which shows particular susceptibility to ototoxic agents such as the aminoglycoside antibiotics. Thus, for assessment of pathology caused by ototoxic agents, this method has considerable advantages over the surface preparation technique.

An atlas of glycine- and GABA-like immunoreactivity and colocalization in the cochlear nuclear complex of the guinea pig

The distribution and colocalization of gamma-aminobutyric acid (GABA)- and glycine-like immunoreactivity in the cochlear nuclear complex of the guinea pig have been studied to produce a light microscopic atlas. The method used was based on post-embedding immunocytochemistry in pairs of 0.5-micron-thick plastic sections treated with polyclonal antibodies against conjugated GABA and glycine respectively. Immunoreactive cells, presumably short axon neurones, predominated in the dorsal cochlear nucleus, with mostly single-GABA-labelled cells in the superficial layer, double-labelled in the middle, and single-glycine-labelled in the deep layers. A few large single-glycine-labelled cells, interpreted as commissural neurons, occurred in the ventral nucleus. Scattered double-labelled cells, probably Golgi cells, were seen in the granule cell domain. Immunolabeled puncta of all three staining categories occurred in large numbers throughout the complex, apposed to somata and in the neuropil, showing a differential distribution onto different types of neuron. Three immunolabeled tracts were noted: the tuberculoventral tract, the commissural acoustic stria, and the trapezoidal descending fibres. Most of the fibres in these tracts were single-labelled for glycine, although in the last mentioned tract single-GABA- and double-labelled fibres were also found. Some of the immunolabeled cell types described here are proposed as the origins of the similarly labelled puncta and fibres on the basis of known intrinsic connections.

Behavioural evidence for recovery of auditory function in guinea pigs following kanamycin administration

Deterioration followed by recovery of behavioural absolute threshold and frequency selectivity has been observed in guinea pigs following kanamycin administration of 200 mg/kg body weight daily for 16 days. Deterioration in function consistently follows a high-to-low frequency pattern and recovery generally occurs at the lowest of the high (8-32 kHz) frequencies affected. The degree of recovery is related to the magnitude of the threshold elevation; where large (40-45 dB) elevations occur initially, the process appears to be partial since threshold recovers only to within 5-12 dB of pre-administration levels. In instances where smaller threshold elevations (5-20 dB) take place initially, recovery can sometimes be complete. However, when threshold elevations of over 50 dB occur, no recovery is apparent. Recovery is relatively slow, taking place over periods of up to 100 days post-kanamycin administration. Hair cell counts have established that the threshold elevation which remains in instances of partial recovery is not related to a reduction in hair cell numbers at the light microscope level.

Localisation of putative mechanoelectrical transducer channels in cochlear hair cells by immunoelectron microscopy

Displacement of the apical stereociliary bundle of cochlear hair cells mechanically gates transducer channels. Knowing the position of the channels with regard to the apical structures of the hair cell could indicate how this mechanism operates. At present, there is conflicting evidence regarding their precise location; the channels have been suggested to be located either towards the base of the stereocilia or at the tips where they could be operated by extracellular links running from the top of shorter stereocilia to the sides of adjacent taller ones. The channels have been shown to be reversibly blocked by amiloride. This has prompted us to use a polyclonal antibody raised against another amiloride-sensitive channel to search for them using immunolabelling. The location of the primary antibody has been revealed using pre-embedding labelling with a colloidal gold-conjugated secondary antibody followed by scanning transmission electron microscopy of semi-thin sections. In this way, more complete information on the relationship of the labelling to the three-dimensional organisation of the stereociliary bundle has been obtained in comparison with previous immunofluorescence and transmission electron microscopic results. Labelling occurs in discrete areas towards the tips of the stereocilia, one of the possible sites for the transducer channels, predominantly between the membranes of shorter and taller stereocilia.

Anatomy of the cochlear nuclear complex of guinea pig

The cyto- and fibre-architecture of the cochlear nuclear complex of the guinea-pig has been studied in serial sections using Nissl, Golgi and combined cell-myelin staining of normal material, and a silver degeneration method after cochlear ablation. The nuclear subdivisions and major cell types can be recognised on the basis of those found in the cat, but there are some differences between the two species in the precise distribution and morphology of the neurons. The rostrodorsal part of the anteroventral cochlear nucleus (AVCN) contains predominantly spherical bushy cells, but these cannot be readily divided into large and small types as in the cat. Globular bushy cells are seen in the caudal region of the AVCN, but the majority occur in the posteroventral cochlear nucleus (PVCN), in an area extending from the nerve root right up to the boundary of the dorsal cochlear nucleus (DCN). The octopus cells constitute a distinct region in the most dorsomedial part of the PVCN underneath the DCN. Giant cells are seen scattered around the nerve root region. Multipolar and small cells are seen throughout the non-granular regions of the ventral cochlear nucleus (VCN) except for the octopus cell area, but occur mainly in the more rostral regions of the PVCN. Small cells occur in greatest abundance in the thin cap area at the dorsal edge of the VCN below a superficial granule cell layer. The latter covers the dorsolateral surface of the VCN, and a lamina of granule cells partially separates the PVCN from the DCN. The DCN can be divided into four layers. The outermost molecular layer (layer 1) is separated from the deeper regions by a prominent layer of granule cells (layer 2) which also contains the pyramidal cells. Molecular layer stellate cells are seen in layer 1 and a staggered row of cartwheel neurons is found at the boundary between layers 1 and 2. Layer 3 contains the basal dendrites of the pyramidal cells and some small (vertical) cells, and is innervated by the descending branches of the cochlear nerve. The deepest layer 4, which contains multipolar cells and giant cells, does not appear to receive this direct cochlear input.

Organization of microtubules in cochlear hair cells

The organization of microtubules in hair cells of the guinea-pig cochlea has been investigated using transmission electron microscopy and correlated with the location of tubulin-associated immunofluorescence in surface preparations of the organ of Corti. Results from both techniques reveal consistent distributions of microtubules in inner and outer hair cells. In the inner hair cells, microtubules are most concentrated in the apex. Reconstruction from serial sections shows three main groups: firstly, in channels through the cuticular plate and in a discontinuous belt around its upper perimeter; secondly, forming a ring inside a rim extending down from the lower perimeter of the plate; and thirdly, in a meshwork underlying the main body of the plate. In the cell body, microtubules line the inner face of the subsurface cistern and extend longitudinally through a tubulo-vesicular track between the apex and base. In outer hair cells, the pattern of microtubules associated with the cuticular plate is similar, although there are fewer present than in inner hair cells. In outer hair cells from the apex of the cochlea, microtubules occur around an infracuticular protrusion of cuticular plate material. In the cell body, many more microtubules occur in the region below the nucleus compared with inner hair cells. The possible functions of microtubules in hair cells are discussed by comparison with those found in other systems. These include morphogenesis and maintenance of cell shape; intracellular transport, e.g., of neurotransmitter vesicles; providing a possible substrate for motility; mechanical support of structures associated with sensory transduction.

Comparative ultrastructure of subsurface cisternae in inner and outer hair cells of the guinea pig cochlea

The subsurface cisternal systems of outer hair cells (OHCs) from different cochlear regions have been compared with the subsurface cisternal system of inner hair cells. Three main observations have been made: (1) the number of cisternal layers, when there is more than one present, is reduced along the length of an individual outer hair cell; (2) basal outer hair cells may have only one fenestrated cisternal layer; and (3) the inner hair cells possess a lateral cistern and associated pillar and filament complexes which are very similar to those of some basal OHCs. These observations are discussed in relation to hypotheses regarding the role of these structures in hair cell motility.

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

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

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

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