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

Functions of CaBP1 and CaBP2 in the peripheral auditory system

CaBPs are a family of Ca²⁺ binding proteins related to calmodulin. Two CaBP family members, CaBP1 and CaBP2, are highly expressed in the cochlea. Here, we investigated the significance of CaBP1 and CaBP2 for hearing in mice lacking expression of these proteins (CaBP1 KO and CaBP2 KO) using auditory brain responses (ABRs) and distortion product otoacoustic emissions (DPOAEs). In CaBP1 KO mice, ABR wave I was larger in amplitude, and shorter in latency and faster in decay, suggestive of enhanced synchrony of auditory nerve fibers. This interpretation was supported by the greater excitability of CaBP1 KO than WT neurons in whole-cell patch clamp recordings of spiral ganglion neurons in culture, and normal presynaptic function of CaBP1 KO IHCs. DPOAEs and ABR thresholds were normal in 4-week old CaBP1 KO mice, but elevated ABR thresholds became evident at 32 kHz at 9 weeks, and at 8 and 16 kHz by 6 months of age. In contrast, CaBP2 KO mice exhibited significant ABR threshold elevations at 4 weeks of age that became more severe in the mid-frequency range by 9 weeks. Though normal at 4 weeks, DPOAEs in CaBP2 KO mice were significantly reduced in the mid-frequency range by 9 weeks. Our results reveal requirements for CaBP1 and CaBP2 in the peripheral auditory system and highlight the diverse modes by which CaBPs influence sensory processing.

Oncomodulin, an EF-Hand Ca2+ Buffer, Is Critical for Maintaining Cochlear Function in Mice

Oncomodulin (Ocm), a member of the parvalbumin family of calcium binding proteins, is expressed predominantly by cochlear outer hair cells in subcellular regions associated with either mechanoelectric transduction or electromotility. Targeted deletion of Ocm caused progressive cochlear dysfunction. Although sound-evoked responses are normal at 1 month, by 4 months, mutants show only minimal distortion product otoacoustic emissions and 70-80 dB threshold shifts in auditory brainstem responses. Thus, Ocm is not critical for cochlear development but does play an essential role for cochlear function in the adult mouse.

SIGNIFICANCE STATEMENT

Numerous proteins act as buffers, sensors, or pumps to control calcium levels in cochlear hair cells. In the inner ear, EF-hand calcium buffers may play a significant role in hair cell function but have been very difficult to study. Unlike other reports of genetic disruption of EF-hand calcium buffers, deletion of oncomodulin (Ocm), which is predominately found in outer hair cells, leads to a progressive hearing loss after 1 month, suggesting that Ocm critically protects hearing in the mature ear.

Self-Protection against Aminoglycoside Ototoxicity in Guinea Pigs

BACKGROUND: Amikacin is a semisynthetic aminoglycoside. It acts against most of the microbial species. Amikacin limitation of the therapeutic application is the ototoxicity which promotes permanent lesions in the cochlear system. Aminoglycoside antibiotics have ototoxic potential. The target cells are preferentially the outer hair cells in the cochlear basal turns. Amynoglicoside antibiotics can quelate iron forming a complex with oxidate properties and promotes the formation of free radicals. Responsible for production of lesions in the hair cells.

OBJECTIVE: The objective of the present investigation was to determine whether the use of the aminoglycoside amikacin at small doses may lead to the occurrence of some types of resistance to or protection against ototoxicity of the drug by analyzing lesions to the organ of Corti by scanning electron microscopy.

METHODS: The study was conducted on 31 guinea pigs that were divided into 4 groups, amikacin was administered intramuscularly. The groups consisted of: group A = control group: 5 animals (10 cochleae); group B = 5 animals (10 cochleae), amikacin 20 mg/kg/day for 30 days; group c = 7 animals (13 cochleae), amikacin 400 mg/kg/day for 12 days; group d = 14 animals (26 cochleae) amikacin 20 mg/kg/day for 30 days, followed by 400 mg/kg/day for 12 days. Histological studies were performed by scanning electron microscopy. Three cochleae were excluded.

RESULTS: In groups A and B, the cells were normal in all cochleae, in group C there were extensive lesions of the 2 more basal turns, and in group D there was a significant reduction of lesions in the 2 more basal turns compared with group C, which had received the ototoxic dose of amikacin alone.

CONCLUSION: We conclude that the non-ototoxic dose of amikacin administered before the ototoxic dose of the same antibiotic had a statistically significant protective effect on the 2 more basal turns of the guinea pig cochlea.

DPOAE Intensity Increase at Individual Dominant Frequency after Short-Term Auditory Exposure

Previous experiments suggested the possibility of a short-term sound stimulus-evoked and transient increase in DPOAE amplitudes. This phenomenon is possibly due to the complexity of the outer hair cells and their efferent control system and the different time scales of regulatory processes. A total of 100 healthy subjects ranging from 18 to 40 years of age with normal hearing and normal DPOAE values in the range of 781–4000 Hz were recruited in the study. Diagnostic DPOAE measurements were performed after short-term sound exposure. We proposed a 10 sec, 50 dB sound impulse as the most effective stimulus for clinical practice between 40 and 60 sec poststimulus time to detect the aforementioned transient DPOAE increase. We developed a procedure for detection of this transient increase in DPOAE by the application of a short-term sound exposure. The phenomenon was consistent and well detectable. Based on our findings, a new aspect of cochlear adaptation can be established that might be introduced as a routine clinical diagnostic tool. A mathematical model was provided that summarizes various factors that determine electromotility of OHCs and serves as a possible clinical application using this phenomenon for the prediction of individual noise susceptibility.

Cochlear outer hair cell motility

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

High potassium concentrations protect inner and outer hair cells in the newborn rat culture from ischemia-induced damage

Several studies indicate that an increase in the extracellular potassium (K+) concentration is a factor exerting a damaging effect on cochlear hair cells (HCs). The present study was designed to examine the effects of high extracellular K+ concentrations on the HCs under normoxic and ischemic conditions. Organotypic cultures of the organ of Corti of newborn rats were exposed to normoxia and ischemia at K+ concentrations of 5-70 mM in artificial perilymph for 3-4h. The number of IHCs and OHCs in the apical, medial and basal parts of the cochlea were counted 24h later. The work resulted in two main findings: (1) extracellular K+ concentrations of 30-70 mM had no effect on the HCs under normoxic conditions; (2) under ischemic conditions, a clear HC loss, mainly in the medial and basal cochlear parts, was observed at 5 mM K+ as previously reported. In contrast, a high extracellular K+ concentration strongly attenuated the HC loss. This effect nearly completely disappeared by the addition of both eosin, an inhibitor of the plasma membrane calcium ATPase (PMCA), and linopirdine, an inhibitor of the KCNQ4 channel, indicating that a normal activity of the PMCA and the KCNQ4 channels are key factors for HC survival under ischemia and depolarizing conditions.

Protective effect of T-type calcium channel blocker flunarizine on cisplatin-induced death of auditory cells

Changes in intracellular Ca2+ level are involved in a number of intracellular events, including triggering of apoptosis. The role of intracellular calcium mobilization in cisplatin-induced hair cell death, however, is still unknown. In this study, the effect of calcium channel blocker flunarizine (Sibelium), which is used to prescribe for vertigo and tinnitus, on cisplatin-induced hair cell death was investigated in a cochlear organ of Corti-derived cell line, HEI-OC1, and the neonatal (P2) rat organ of Corti explant. Cisplatin induced apoptotic cell death showing nuclear fragmentation, DNA ladder, and TUNEL positive in both HEI-OC1 and primary organ of Corti explant. Flunarizine significantly inhibited the cisplatin-induced apoptosis. Unexpectedly, flunarizine increased the intracellular calcium ([Ca2+]i) levels of HEI-OC1. However, the protective effect of flunarizine against cisplatin was not mediated by modulation of intracellular calcium level. Treatment of cisplatin resulted in ROS generation and lipid peroxidation in HEI-OC1. Flunarizine did not attenuate ROS production but inhibited lipid peroxidation and mitochondrial permeability transition in cisplatin-treated cells. This result suggests that the protective mechanism of flunarizine on cisplatin-induced cytotoxicity is associated with direct inhibition of lipid peroxidation and mitochondrial permeability transition.

Developmental expression of Ca(v)1.3 (alpha1d) calcium channels in the mouse inner ear

Voltage-gated calcium channels are important for neurotransmission at the level of inner hair cells (IHCs) and outer hair cells (OHCs). These channels open when mechanical stimulation depolarises the hair cell membrane and the resulting calcium influx triggers neurotransmitter release. Voltage-gated calcium channels expressed in hair cells are known to be of the L-type with a predominance of the Ca(v)1.3 subunit. The present study describes the developmental expression of the Ca(v)1.3 protein in the cochlea and the vestibular system using immunohistochemical technique. In the adult organ of Corti (OC), Ca(v)1.3 was localized in both sensory and non-sensory cells with a more intense expression in IHCs and Deiters cells when compared to OHCs. In both hair cell types, immunoreactivity was observed in the apical pole, basolateral membrane and at the basal pole (synaptic zone). Similar results were obtained in the vestibular organs. During development, Ca(v)1.3 immunoreactivity was observed in the cochlea as early as embryonic day 15, with expression increasing at birth. At these early stages of cochlear development, Ca(v)1.3 was expressed in all cell types surrounding the scala media. In the OC, the labeling was observed in IHCs, OHCs and supporting cells. The Ca(v)1.3 expression reached an adult-like pattern by the end of the second postnatal week. The present findings suggested that, in addition to their implication in hair cells synaptic transmission, Ca(v)1.3 calcium channels also play an important role in vesicle recycling and transport, as suggested by their extrasynaptic location at the apical pole of the hair cells. The Ca(v)1.3 channels in Deiters cells could participate in active calcium-induced changes in micromechanics of these supporting cells. An early expression during development suggested that these calcium channels are in addition important in the development of the cochlear and vestibular sensory epithelium.

Identification of differentially expressed cDNA clones from gerbil cochlear outer hair cells

In order to identify genes that are associated with outer hair cell(OHC)-specific function, a plasmid library enriched with OHC-specific gene products was constructed using single cell-type-specific complementary DNA (cDNA) and a PCR subtractive hybridization strategy. As a first step, we created separate OHC and inner hair cell (IHC) cDNA pools from individually collected cells using a nonspecific reverse transcription polymerase chain reaction. Next, the OHC cDNA was subtracted against IHC cDNA using a PCR-based subtractive technique. IHCs and OHCs share many common features, making IHC cDNA an ideal 'driver' to 'subtract away' common hair cell gene products and enrich differentially expressed cDNAs, including OHC-specific genes. The subtracted OHC cDNAs were then cloned to generate an OHC - IHC subtracted cDNA plasmid library. Finally, a differential screening procedure was performed, resulting in 477 differentially positive clones. After analysis of these 477 clones, 50 known genes were identified, including two previously known OHC-specific proteins: oncomodulin and the recently described motor protein prestin. An additional 84 novel clones were also found. As this library of cDNA fragments represents differentially expressed genes in OHCs, it can be used as starting material for isolation and characterization of a complete set of OHC gene products, an important step in investigating normal and abnormal cochlear function.

Otoproteção das células ciliadas auditivas contra a ototoxicidade da amicacina

Objetivo: Este estudo teve por objetivo verificar a ocorrência de otoproteção contra a ototoxicidade da amicacina em cobaias. Forma de estudo: experimental. Material e método: Foram estudadas as células ciliadas externas do órgão de Corti de 31 animais por meio de microscopia eletrônica de varredura (JEOL-JMS 5200). Foram estudados 4 grupos: A, controle; B, amicacina 20 mg/kg/dia por 30 dias; C, amicacina 400 mg/Kg/ dia 12 dias; grupo D, amicacina 20 mg/kg/dia por 30 dias seguido de 400 mg/kg/dia por 12 dias. Resultados: Nos grupos A e B, as células estavam normais em todas as cócleas; no grupo C houve lesões extensas nas duas espiras mais basais; no grupo D houve redução significante das lesões nas duas mais basais em comparação com o grupo C que recebeu a dose ototóxica de amicacina isoladamente. Conclusão: Conclui-se que a dose não ototóxica de amicacina administrada previamente à dose ototóxica do mesmo antibiótico tem efeito protetor, estatisticamente significante, nas duas espiras mais basais das cócleas de cobaias.

Isolation of Guinea pig inner hair cells using manual microsurgical dissection

Cochlear inner hair cells (IHCs) and outer hair cells (OHCs) show distinctive morphological features that are usually sufficient to distinguish these two species in vitro. However, OHCs may sometimes resemble IHCs when they are mechanically distorted or begin swelling at their basal end. As a result, accurately discriminating the cells based on morphology was thought to be problematic. An objective method that allows us to clearly and unambiguously distinguish these two cell types is therefore of continued interest. We describe a protocol in which solitary IHCs were harvested from guinea pig cochlea using a manual microsurgical dissection.

Return of potassium ion channels in regenerated hair cells: possible pathways and the role of intracellular calcium signaling

Recent electrophysiological studies in pigeon have demonstrated that potassium channels are completely functional in regenerated type II hair cells at 21 days post-treatment (PT) with ototoxic doses of streptomycin. The currents return in the order they appear during development. The mixture of ionic currents in a regenerated type II hair cell in a particular region of the neuroepithelium is the same as in its ancestor in that region. The return of currents in regenerated type I hair cells is more complicated. The dominant conductance gKI is not present until after 70 days PT. Before 70 days, the ionic currents in type I hair cells resemble those of regenerated type II hair cells, suggesting that the ionic currents in type II hair cells might be precursors of the ionic currents in regenerated type I hair cells. New data show that at one year PT, the kinetics and drug sensitivity of the dominant K+ conductance in type I hair cells are identical to gKI. Supporting cells, believed to be the precursors of regenerated type II hair cells, have effectively no voltage-gated outward potassium channels, suggesting that regenerated type II hair cells must develop these channels de novo. The next step is to understand the mechanisms by which the potassium channel protein is synthesized, migrates through the cytosol, and is inserted into the plasmalemma of regenerating hair cells. These mechanisms are unknown. We propose that intracellular calcium is involved in this process, as well as in the differentiation, proliferation, and gene regulation of precursor cells fated to become hair cells.

Dynamic studies of ototoxicity in mature avian auditory epithelium

Hearing loss induced by ototoxicity is a worldwide problem despite the development of newer antibiotics and chemotherapy agents. The cellular mechanisms responsible for aminoglycoside-induced hearing loss are still poorly understood. We have developed two different methods of studying the dynamic cellular and subcellular changes in the chick auditory sensory epithelium that occur during hair cell death. The first study was performed in mature chicks after a single, high dose injection of gentamicin, which results in the rapid loss of all hair cells in the basal third of the cochlea. Chicks were sacrificed at discrete time points after drug treatment, and transmission electron microscopy was performed to study the ultrastructural changes in basal hair cells during the course of cell death. We noted various changes in the cell morphology including accumulation of cytoplasmic inclusion bodies, dispersion of the cytoplasmic polyribosomes, mitochondrial swelling, and cellular extrusion by 24 h after injection. The next two studies were performed using tissue cultures from mature avian auditory sensory epithelium. Cultured cells were labeled using vital fluorescent markers, and levels of intracellular calcium and reactive oxygen species within hair cells were studied following aminoglycoside exposure. We identified a dose-dependent increase in the levels of intracellular calcium, which was blocked by an inhibitor of voltage-gated calcium channels. We also found that levels of reactive oxygen species in hair cells greatly increased after exposure to gentamicin, and this response was blocked by two different antioxidants. These studies serve to identify key cellular and molecular changes in hair cells in response to ototoxic drugs. Further study of these processes may lead to a better understanding of how ototoxicity is induced and to potential preventative interventions.

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.

Effects of nimodipine on noise-induced hearing loss

The effects of nimodipine, a calcium channel blocker, on noise-induced hearing loss were examined in gerbils. Animals were implanted subcutaneously with a timed-release pellet containing either nimodipine (approximately 10 mg/kg/day) or placebo and exposed to either 102 or 107 dBA noise. Serum levels were tested in two subjects and were in the range known to protect humans from cerebral artery vasospasm and ischemia-related neurologic deficits. Nimodipine and control groups had similar amounts of noise-induced (a) permanent threshold shift; (b) reductions in distortion product otoacoustic emissions; (c) reductions in tuning and suppression of the compound action potential; and (d) loss of outer hair cells. The results suggest that nimodipine, at a dose which results in clinically relevant serum levels, does not provide protection from the effects of moderately intense noise exposures.

Immunolocalization of peptide 19 and other calcium-binding proteins in the guinea pig cochlea

Calcium ions are known to play critical roles in a variety of cochlear functions. The distributions of a number of calcium binding proteins that regulate calcium ion levels within the cochlea have previously been described. In this report we extend and refine previous reports of the distribution of immunostaining for calmodulin, calbindin, and calretinin and show for the first time the distribution for peptide 19. There were longitudinal and radial gradients of immunostaining for peptide 19 within outer hair cells that appeared to match previously described gradients of efferent innervation of these cells. Gradients of immunostaining for calbindin within outer hair cells were in the opposite directions, which suggests that levels of this protein are correlated with afferent innervation density and perhaps the abundance of subsurface cisternae. No gradients were seen in the distributions of cells stained for calmodulin and calretinin, which included sensory cells and supporting cells respectively. All ganglion cells were stained for calmodulin but the other proteins appeared to be present in limited ganglion cell subpopulations. In addition to staining of sensorineural elements, antisera to all compounds but peptide 19 showed immunostaining of cells within the lateral wall and the spiral limbus. The results suggest that the proteins under study are involved in a wide variety of calcium-regulated functions within the cochlea. Knowledge of the unique distribution of each of the compounds should facilitate further studies of their roles in cochlear function.

Cholinergic responses in developing outer hair cells of the rat cochlea

Acetylcholine-evoked currents were investigated using the conventional whole-cell patch-clamp recording technique in developing outer hair cells (OHCs). The cells were isolated from the rat cochlea at different stages of postnatal development ranging from day 4 (P4) to P30. Acetylcholine-evoked currents could be recorded at P6 and P8. At this developmental stage, the majority of OHCs displayed inward nicotinic-like currents near the resting membrane potential. These cholinergic currents zeroed near 0 mV, as expected for a non-selective cation current, and could be reversibly blocked by d-tubocurarine. At P12 and adult stage, the cholinergic response of OHCs switched to an outward current reversing near EK and displaying a bell shape peaking between -40 and -30 mV. This change in polarity of the acetylcholine response during postnatal development might be explained by progressive functional coupling between acetylcholine ionotropic receptors permeable to Ca2+ and nearby Ca(2+)-activated K+ channels at the synaptic pole of OHCs.

Structural and ultrastructural aspects of isolated immature cochlear outer hair cells maintained in short-term culture

Immature outer hair cells (OHCs), isolated from developing rat cochlea without using proteolytic enzymes, were maintained in short-term culture in a clot of coagulated plasma. Cell viability was assessed by a laser scanning image cytometer, using double-fluorescent labeling. Light and transmission electron microscopy was used to study the morphology of isolated cells. Ten to 60 healthy OHCs were obtained from one cochlea, either as single isolated cells or clusters containing 2-10 cells from the same row. Although dead cells were observed only 1 h after dissociation, there were still viable cells after 6 h. Isolated OHCs were not perfectly cylindrical, due to the immaturity of their cortical structures. One hour after dissociation the ultrastructural organization of the isolated cells was generally well preserved, but this was followed by dilatation of the Golgi apparatus and endoplasmic reticulum. Specific changes in isolated OHCs were also observed at the subsurface cisternae and cuticular plate. Although degenerating OHCs generally showed a classic pattern of necrosis, certain morphological features reminiscent of apoptosis were also observed. This study emphasises the difficulty involved in investigating isolated immature OHCs in vitro and provides a basis for future research into the physiological requirements of isolated immature OHCs.

Direct measurements of Ca(2+)-activated K+ currents in inner hair cells of the guinea-pig cochlea using photolabile Ca2+ chelators

Intracellular photorelease of Ca2+ from caged Ca2+ (DM-nitrophen or nitr5) and the patch-clamp technique in the whole-cell configuration were used to investigate Ca(2+)-activated currents in inner hair cells (IHCs) of the mammalian cochlea. Photoliberation of intracellular Ca2+ activated outward currents with a mean amplitude of 260 +/- 110 pA when IHCs were voltage-clamped, near the resting membrane potential, at -50 mV. The photoactivated currents were reversibly blocked by extracellular application of tetraethylammonium (TEA, 10 mM), neomycin (1 mM) and charybdotoxin (1 microM), but not by apamin. The voltage dependence of membrane currents activated by photolysis of DM-nitrophen demonstrated a reversal potential near the K+ equilibrium potential (Ek) and saturation near 0 mV. The presence of Ca(2+)-activated currents was further confirmed by the effects of extracellular adenosine 5'-triphosphate (ATP, 10 microM) and the Ca2+ ionophore ionomycin (10 microM). Both agents raised intracellular Ca2+ and simultaneously activated outward currents when IHCs were voltage-clamped near the resting membrane potential. In experiments where currents were activated by depolarizing voltage steps, nifedipine (50 microM) and Cd2+ (1 mM) reduced significantly (20-50%) the whole-cell outward currents, suggesting the presence of L-type Ca2+ currents activating K+ currents. These results are the first direct evidence for Ca(2+)-activated K+ currents in mammalian IHCs, these currents being potentially important for cell repolarization during sound-induced depolarization and synaptic transmission.

Intra- and extracellular calcium modulates stereocilia stiffness on chick cochlear hair cells

Segments of the chick basilar papilla were isolated and maintained in culture medium. The sensory hair bundle of individual hair cells was observed with light microscopy and stimulated with a water microjet at 600 Hz. Hair bundle motion was slowed by illuminating the microscope with stroboscopic light, and water jet intensity was systematically varied in decibel (dB) steps until a visual detection level (VDL) threshold of hair bundle motion was achieved. The VDL threshold of many hair cells was measured in each isolated papilla. However, only one of eight extracellular calcium concentrations (0.0, 0.0001, 0.001, 0.01, 0.1, 1.25, 6.0, and 12.0 mM) was used with each papilla. In a second series, a calcium ionophore (ionomycin) was added to the culture medium, and VDL thresholds were again measured at seven of these extracellular calcium concentrations. With extracellular calcium alone, the stimulus level needed to achieve threshold was reduced by 2.73 dB between 0.1 and 0.01 mM. This change in threshold represented a 1.37-fold decrease in hair bundle stiffness. When ionomycin was added to the culture medium, a progressively greater stimulus intensity was needed to achieve threshold as calcium concentration increased. The 11.7-dB increase in threshold, with the addition of ionomycin, between 0.0001 and 6.0 mM extracellular calcium was equivalent to a 3.85-fold increase in bundle stiffness. These large changes in hair-bundle stiffness, as a function of the extra- or intracellular calcium environment, may play an important role in the micromechanical behavior of the hair cell during sound simulation.

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

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

Membrane stains as an objective means to distinguish isolated inner and outer hair cells

The use of isolated cochlear outer and inner hair cells has become widespread. While the morphological features of these two cell types in general are sufficiently different to allow discrimination, there are situations where confusion can arise. Small outer hair cells, particularly when they are swollen or distorted, can take on an appearance suggestive of inner hair cells. We describe here two fluorescent membrane stains, 3,3'-dihexyloxacarbocyanine iodide and rhodamine B hexyl ester, as an objective means to distinguish between cochlear hair cell types. Both stains mark the subsurface cisternae of outer hair cells thereby delineating the cell outline, and the interior of the cell shows discrete structure. On the other hand, in inner hair cells, the outline of the cell is not resolved while the interior is diffusely fluorescent. Since the two probes have different excitation and emission wavelengths (fluorescein- and rhodamine-like, respectively), this staining procedure can even be used in the presence of another fluorescent marker (for example, a calcium-indicating dye) by appropriate choice of the membrane stain.

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.

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

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

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

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