Ionic environment of cochlear hair cells

Calcium signaling and genetic rare diseases: An auditory perspective

Deafness is a highly heterogeneous disorder which stems, for 50%, from genetic origins. Sensory transduction relies mainly on sensory hair cells of the cochlea, in the inner ear. Calcium is key for the function of these cells and acts as a fundamental signal transduction. Its homeostasis depends on three factors: the calcium influx, through the mechanotransduction channel at the apical pole of the hair cell as well as the voltage-gated calcium channel at the base of the cells; the calcium buffering via Ca²⁺-binding proteins in the cytoplasm, but also in organelles such as mitochondria and the reticulum endoplasmic mitochondria-associated membranes with specialized proteins; and the calcium extrusion through the Ca-ATPase pump, located all over the plasma membrane. In addition, the synaptic transmission to the central nervous system is also controlled by calcium. Genetic studies of inherited deafness have tremendously helped understand the underlying molecular pathways of calcium signaling. In this review, we discuss these different factors in light of the associated genetic diseases (syndromic and non-syndromic deafness) and the causative genes.

Potentials and limitations of pharmaceutical and pharmacological applications of bile acids in hearing loss treatment

Hearing loss is a worldwide epidemic, with approximately 1.5 billion people currently struggling with hearing-related conditions. Currently, the most wildly used and effective treatments for hearing loss are primarily focus on the use of hearing aids and cochlear implants. However, these have many limitations, highlighting the importance of developing a pharmacological solution that may be used to overcome barriers associated with such devices. Due to the challenges of delivering therapeutic agents to the inner ear, bile acids are being explored as potential drug excipients and permeation enhancers. This review, therefore, aims to explore the pathophysiology of hearing loss, the challenges in treatment and the manners in which bile acids could potentially aid in overcoming these challenges.

Medical-Grade Silicone Rubber-Hydrogel-Composites for Modiolar Hugging Cochlear Implants

The gold standard for the partial restoration of sensorineural hearing loss is cochlear implant surgery, which restores patients’ speech comprehension. The remaining limitations, e.g., music perception, are partly due to a gap between cochlear implant electrodes and the auditory nerve cells in the modiolus of the inner ear. Reducing this gap will most likely lead to improved cochlear implant performance. To achieve this, a bending or curling mechanism in the electrode array is discussed. We propose a silicone rubber–hydrogel actuator where the hydrogel forms a percolating network in the dorsal silicone rubber compartment of the electrode array to exert bending forces at low volume swelling ratios. A material study of suitable polymers (medical-grade PDMS and hydrogels), including parametrized bending curvature measurements, is presented. The curvature radii measured meet the anatomical needs for positioning electrodes very closely to the modiolus. Besides stage-one biocompatibility according to ISO 10993-5, we also developed and validated a simplified mathematical model for designing hydrogel-actuated CI with modiolar hugging functionality.

17β-Estradiol promotes angiogenesis of stria vascular in cochlea of C57BL/6J mice

It is widely accepted that the stria vascularis (SV) in cochlea plays a critical role in the generation of endocochlear potential (EP) and the secretion of the endolymph. 17β-estradiol (E2) is the most potent and abundant endogenous estrogen during the premenopausal period, thus, considered as the reference estrogen. This study aimd to investigate the protective effect of E2 by promoting the expression of vascular endothelial growth factor (VEGF) and thus promoting the vascular regeneration of the SV in elderly mice. After being treated with E2 either in vivo or in vitro, the hearing threshold changes of C57BL/6J elder mice continuously reduced, endothelial cell morphology improved, the number of endothelial cells (ECs) tubular nodes increased significantly, the ability of tubular formation enhanced significantly and the expression of VEGF increased. In vitro, cell model in conjunction with in vivo ovariectomized model was established to demonstrate for the first time that E2 promotes angiogenesis by promoting the secretion of VEGF through the phosphatidylinositol 3-kinase (PI3K)/AKT pathway (PI3K/AKT). In conclusion, E2 demonstrated potent angiogenesis properties with significant protection against Age-Related Hearing Loss (ARHL), which provides a new idea for the improvement of ARHL.

Single-Cell RNA-Seq of Cisplatin-Treated Adult Stria Vascularis Identifies Cell Type-Specific Regulatory Networks and Novel Therapeutic Gene Targets

The endocochlear potential (EP) generated by the stria vascularis (SV) is necessary for hair cell mechanotransduction in the mammalian cochlea. We sought to create a model of EP dysfunction for the purposes of transcriptional analysis and treatment testing. By administering a single dose of cisplatin, a commonly prescribed cancer treatment drug with ototoxic side effects, to the adult mouse, we acutely disrupt EP generation. By combining these data with single cell RNA-sequencing findings, we identify transcriptional changes induced by cisplatin exposure, and by extension transcriptional changes accompanying EP reduction, in the major cell types of the SV. We use these data to identify gene regulatory networks unique to cisplatin treated SV, as well as the differentially expressed and druggable gene targets within those networks. Our results reconstruct transcriptional responses that occur in gene expression on the cellular level while identifying possible targets for interventions not only in cisplatin ototoxicity but also in EP dysfunction.

A Review on Recent Advancement on Age-Related Hearing Loss: The Applications of Nanotechnology, Drug Pharmacology, and Biotechnology

Aging is considered a contributing factor to many diseases such as cardiovascular disease, Alzheimer’s disease, and hearing loss. Age-related hearing loss, also termed presbycusis, is one of the most common sensory impairments worldwide, affecting one in five people over 50 years of age, and this prevalence is growing annually. Associations have emerged between presbycusis and detrimental health outcomes, including social isolation and mental health. It remains largely untreatable apart from hearing aids, and with no globally established prevention strategies in the clinical setting. Hence, this review aims to explore the pathophysiology of presbycusis and potential therapies, based on a recent advancement in bile acid-based bio-nanotechnologies. A comprehensive online search was carried out using the following keywords: presbycusis, drugs, hearing loss, bile acids, nanotechnology, and more than 150 publications were considered directly relevant. Evidence of the multifaceted oxidative stress and chronic inflammation involvement in cellular damage and apoptosis that is associated with a loss of hair cells, damaged and inflamed stria vascularis, and neuronal signalling loss and apoptosis continues to emerge. New robust and effective therapies require drug delivery deeper into the various layers of the cochlea. Bile acid-based nanotechnology has gained wide interest in its permeation-enhancing ability and potential for numerous applications in treating presbycusis.

Achievements and Challenges in Transplantation of Mesenchymal Stem Cells in Otorhinolaryngology

Otorhinolaryngology enrolls head and neck surgery in various tissues such as ear, nose, and throat (ENT) that govern different activities such as hearing, breathing, smelling, production of vocal sounds, the balance, deglutition, facial animation, air filtration and humidification, and articulation during speech, while absence of these functions can lead to high morbidity and even mortality. Conventional therapies for head and neck damaged tissues include grafts, transplants, and artificial materials, but grafts have limited availability and cause morbidity in the donor site. To improve these limitations, regenerative medicine, as a novel and rapidly growing field, has opened a new therapeutic window in otorhinolaryngology by using cell transplantation to target the healing and replacement of injured tissues. There is a high risk of rejection and tumor formation for transplantation of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs); mesenchymal stem cells (MSCs) lack these drawbacks. They have easy expansion and antiapoptotic properties with a wide range of healing and aesthetic functions that make them a novel candidate in otorhinolaryngology for craniofacial defects and diseases and hold immense promise for bone tissue healing; even the tissue sources and types of MSCs, the method of cell introduction and their preparation quality can influence the final outcome in the injured tissue. In this review, we demonstrated the anti-inflammatory and immunomodulatory properties of MSCs, from different sources, to be safely used for cell-based therapies in otorhinolaryngology, while their achievements and challenges have been described too.

Stem Cell-Based Therapeutic Approaches to Restore Sensorineural Hearing Loss in Mammals

The hair cells that reside in the cochlear sensory epithelium are the fundamental sensory structures responsible for understanding the mechanical sound waves evoked in the environment. The intense damage to these sensory structures may result in permanent hearing loss. The present strategies to rehabilitate the hearing function include either hearing aids or cochlear implants that may recover the hearing capability of deaf patients to a limited extent. Therefore, much attention has been paid on developing regenerative therapies to regenerate/replace the lost hair cells to treat the damaged cochlear sensory epithelium. The stem cell therapy is a promising approach to develop the functional hair cells and neuronal cells from endogenous and exogenous stem cell pool to recover hearing loss. In this review, we specifically discuss the potential of different kinds of stem cells that hold the potential to restore sensorineural hearing loss in mammals and comprehensively explain the current therapeutic applications of stem cells in both the human and mouse inner ear to regenerate/replace the lost hair cells and spiral ganglion neurons.

Pou3f4-expressing otic mesenchyme cells promote spiral ganglion neuron survival in the postnatal mouse cochlea

During inner ear development, primary auditory neurons named spiral ganglion neurons (SGNs) are surrounded by otic mesenchyme cells, which express the transcription factor Pou3f4. Mutations in Pou3f4 are associated with DFNX2, the most common form of X-linked deafness and typically include developmental malformations of the middle ear and inner ear. It is known that interactions between Pou3f4-expressing mesenchyme cells and SGNs are important for proper axon bundling during development. However, Pou3f4 continues to be expressed through later phases of development, and potential interactions between Pou3f4 and SGNs during this period had not been explored. To address this, we documented Pou3f4 protein expression in the early postnatal mouse cochlea and compared SGNs in Pou3f4 knockout mice and littermate controls. In Pou3f4^y/- mice, SGN density begins to decline by the end of the first postnatal week, with approximately 25% of SGNs ultimately lost. This period of SGN loss in Pou3f4^y/- cochleae coincides with significant elevations in SGN apoptosis. Interestingly, this period also coincides with the presence of a transient population of Pou3f4-expressing cells around and within the spiral ganglion. To determine if Pou3f4 is normally required for SGN peripheral axon extension into the sensory domain, we used a genetic sparse labeling approach to track SGNs and found no differences compared with controls. We also found that Pou3f4 loss did not lead to changes in the proportions of Type I SGN subtypes. Overall, these data suggest that otic mesenchyme cells may play a role in maintaining SGN populations during the early postnatal period.

Acoustic Trauma Modulates Cochlear Blood Flow and Vasoactive Factors in a Rodent Model of Noise-Induced Hearing Loss

Noise exposure affects the organ of Corti and the lateral wall of the cochlea, including the stria vascularis and spiral ligament. Although the inner ear vasculature and spiral ligament fibrocytes in the lateral wall consist of a significant proportion of cells in the cochlea, relatively little is known regarding their functional significance. In this study, 6-week-old male C57BL/6 mice were exposed to noise trauma to induce transient hearing threshold shift (TTS) or permanent hearing threshold shift (PTS). Compared to mice with TTS, mice with PTS exhibited lower cochlear blood flow and lower vessel diameter in the stria vascularis, accompanied by reduced expression levels of genes involved in vasodilation and increased expression levels of genes related to vasoconstriction. Ultrastructural analyses by transmission electron microscopy revealed that the stria vascularis and spiral ligament fibrocytes were more damaged by PTS than by TTS. Moreover, mice with PTS expressed significantly higher levels of proinflammatory cytokines in the cochlea (e.g., IL-1β, IL-6, and TNF-α). Overall, our findings suggest that cochlear microcirculation and lateral wall pathologies are differentially modulated by the severity of acoustic trauma and are associated with changes in vasoactive factors and inflammatory responses in the cochlea.

PMCA2 pump mutations and hereditary deafness

Hair cells of the inner ear detect sound stimuli, inertial or gravitational forces by deflection of their apical stereocilia. A small number of stereociliary cation-selective mechanotransduction (MET) channels admit K⁺ and Ca²⁺ ions into the cytoplasm promoting hair cell membrane depolarization and, consequently, neurotransmitter release at the cell basolateral pole. Ca²⁺ influx into the stereocilia compartment is counteracted by the unusual w/a splicing variant of plasma-membrane calcium-pump isoform 2 (PMCA2) which, unlike other PMCA2 variants, increases only marginally its activity in response to a rapid variation of the cytoplasmic free Ca²⁺ concentration ([Ca²⁺]_c). Missense mutations of PMCA2w/a cause deafness and loss of balance in humans. Mouse models in which the pump is genetically ablated or mutated show hearing and balance impairment, which correlates with defects in homeostatic regulation of stereociliary [Ca²⁺]_c, decreased sensitivity of mechanotransduction channels to hair bundle displacement and progressive degeneration of the organ of Corti. These results highlight a critical role played by the PMCA2w/a pump in the control of hair cell function and survival, and provide mechanistic insight into the etiology of deafness and vestibular disorders.

Potential of Gene and Cell Therapy for Inner Ear Hair Cells

Sensorineural hearing loss is caused by the loss of sensory hair cells (HCs) or a damaged afferent nerve pathway to the auditory cortex. The most common option for the treatment of sensorineural hearing loss is hearing rehabilitation using hearing devices. Various kinds of hearing devices are available but, despite recent advancements, their perceived sound quality does not mimic that of the "naïve" cochlea. Damage to crucial cochlear structures is mostly irreversible and results in permanent hearing loss. Cochlear HC regeneration has long been an important goal in the field of hearing research. However, it remains challenging because, thus far, no medical treatment has successfully regenerated cochlear HCs. Recent advances in genetic modulation and developmental techniques have led to novel approaches to generating HCs or protecting against HC loss, to preserve hearing. In this review, we present and review the current status of two different approaches to restoring or protecting hearing, gene therapy, including the newly introduced CRISPR/Cas9 genome editing, and stem cell therapy, and suggest the future direction.

Non-autonomous Cellular Responses to Ototoxic Drug-Induced Stress and Death

The first major recognition of drug-induced hearing loss can be traced back more than seven decades to the development of streptomycin as an antimicrobial agent. Since then at least 130 therapeutic drugs have been recognized as having ototoxic side-effects. Two important classes of ototoxic drugs are the aminoglycoside antibiotics and the platinum-based antineoplastic agents. These drugs save the lives of millions of people worldwide, but they also cause irreparable hearing loss. In the inner ear, sensory hair cells (HCs) and spiral ganglion neurons (SGNs) are important cellular targets of these drugs, and most mechanistic studies have focused on the cell-autonomous responses of these cell types in response to ototoxic stress. Despite several decades of studies on ototoxicity, important unanswered questions remain, including the cellular and molecular mechanisms that determine whether HCs and SGNs will live or die when confronted with ototoxic challenge. Emerging evidence indicates that other cell types in the inner ear can act as mediators of survival or death of sensory cells and SGNs. For example, glia-like supporting cells (SCs) can promote survival of both HCs and SGNs. Alternatively, SCs can act to promote HC death and inhibit neural fiber expansion. Similarly, tissue resident macrophages activate either pro-survival or pro-death signaling that can influence HC survival after exposure to ototoxic agents. Together these data indicate that autonomous responses that occur within a stressed HC or SGN are not the only (and possibly not the primary) determinants of whether the stressed cell ultimately lives or dies. Instead non-cell-autonomous responses are emerging as significant determinants of HC and SGN survival vs. death in the face of ototoxic stress. The goal of this review is to summarize the current evidence on non-cell-autonomous responses to ototoxic stress and to discuss ways in which this knowledge may advance the development of therapies to reduce hearing loss caused by these drugs.

Ca2+ signaling, apoptosis and autophagy in the developing cochlea: Milestones to hearing acquisition

In mammals, the sense of hearing arises through a complex sequence of morphogenetic events that drive the sculpting of the auditory sensory epithelium into its terminally functional three-dimensional shape. While the majority of the underlying mechanisms remain unknown, it has become increasingly clear that Ca²⁺ signaling is at center stage and plays numerous fundamental roles both in the sensory hair cells and in the matrix of non-sensory, epithelial and supporting cells, which embed them and are tightly interconnected by a dense network of gap junctions formed by connexin 26 (Cx26) and connexin 30 (Cx30) protein subunits. In this review, we discuss the intricate interplay between Ca²⁺ signaling, connexin expression and function, apoptosis and autophagy in the crucial steps that lead to hearing acquisition.

Critical role of ATP-induced ATP release for Ca2+ signaling in nonsensory cell networks of the developing cochlea

Spatially and temporally coordinated variations of the cytosolic free calcium concentration ([Ca²⁺]_c) play a crucial role in a variety of tissues. In the developing sensory epithelium of the mammalian cochlea, elevation of extracellular adenosine trisphosphate concentration ([ATP]_e) triggers [Ca²⁺]_c oscillations and propagation of intercellular inositol 1,4,5-trisphosphate (IP₃)-dependent Ca²⁺ waves. What remains uncertain is the relative contribution of gap junction channels and connexin hemichannels to these fundamental mechanisms, defects in which impair hearing acquisition. Another related open question is whether [Ca²⁺]_c oscillations require oscillations of the cytosolic IP₃ concentration ([IP₃]_c) in this system. To address these issues, we performed Ca²⁺ imaging experiments in the lesser epithelial ridge of the mouse cochlea around postnatal day 5 and constructed a computational model in quantitative adherence to experimental data. Our results indicate that [Ca²⁺]_c oscillations are governed by Hopf-type bifurcations within the experimental range of [ATP]_e and do not require [IP₃]_c oscillations. The model replicates accurately the spatial extent and propagation speed of intercellular Ca²⁺ waves and predicts that ATP-induced ATP release is the primary mechanism underlying intercellular propagation of Ca²⁺ signals. The model also uncovers a discontinuous transition from propagating regimes (intercellular Ca²⁺ wave speed > 11 μm⋅s^-1) to propagation failure (speed = 0), which occurs upon lowering the maximal ATP release rate below a minimal threshold value. The approach presented here overcomes major limitations due to lack of specific connexin channel inhibitors and can be extended to other coupled cellular systems.

Sodium channel diversity in the vestibular ganglion: NaV1.5, NaV1.8, and tetrodotoxin-sensitive currents

Firing patterns differ between subpopulations of vestibular primary afferent neurons. The role of sodium (NaV) channels in this diversity has not been investigated because NaV currents in rodent vestibular ganglion neurons (VGNs) were reported to be homogeneous, with the voltage dependence and tetrodotoxin (TTX) sensitivity of most neuronal NaV channels. RT-PCR experiments, however, indicated expression of diverse NaV channel subunits in the vestibular ganglion, motivating a closer look. Whole cell recordings from acutely dissociated postnatal VGNs confirmed that nearly all neurons expressed NaV currents that are TTX-sensitive and have activation midpoints between -30 and -40 mV. In addition, however, many VGNs expressed one of two other NaV currents. Some VGNs had a small current with properties consistent with NaV1.5 channels: low TTX sensitivity, sensitivity to divalent cation block, and a relatively negative voltage range, and some VGNs showed NaV1.5-like immunoreactivity. Other VGNs had a current with the properties of NaV1.8 channels: high TTX resistance, slow time course, and a relatively depolarized voltage range. In two NaV1.8 reporter lines, subsets of VGNs were labeled. VGNs with NaV1.8-like TTX-resistant current also differed from other VGNs in the voltage dependence of their TTX-sensitive currents and in the voltage threshold for spiking and action potential shape. Regulated expression of NaV channels in primary afferent neurons is likely to selectively affect firing properties that contribute to the encoding of vestibular stimuli.

Metabolomic profiling in inner ear fluid by gas chromatography/mass spectrometry in guinea pig cochlea

The composition and homeostasis of inner ear fluids are important in hearing function. The purpose of this study was to perform metabolomic analysis of the inner ear fluid in guinea pig cochlea, which has not been previously reported in literature, using gas chromatography/mass spectrometry (GC/MS). Seventy-seven kinds of metabolites were detected in the inner ear fluid. Six metabolites, ascorbic acid, fructose, galactosamine, inositol, pyruvate+oxaloacetic acid, and meso-erythritol, were significantly more abundant, and nine metabolites, phosphate, valine, glycine, glycerol, ornithine, glucose, citric acid+isocitric acid, mannose, and trans-4-hydroxy-L-proline, were less abundant in the inner ear fluid than in plasma. The levels of ten metabolites, 3-hydroxy-butyrate, glycerol, fumaric acid, galactosamine, pyruvate+oxaloacetic acid, phosphate, meso-erythritol, citric acid+isocitric acid, mannose, and inositol, in the inner ear fluid significantly changed after loud noise exposure. These observations may help to elucidate various clinical conditions of sensorineural hearing loss, including noise-induced hearing loss.

Recent advances in local drug delivery to the inner ear

Inner ear diseases are not adequately treated by systemic drug administration mainly because of the blood-perilymph barrier that reduces exchanges between plasma and inner ear fluids. Local drug delivery methods including intratympanic and intracochlear administrations are currently developed to treat inner ear disorders more efficiently. Intratympanic administration is minimally invasive but relies on diffusion through middle ear barriers for drug entry into the cochlea, whereas intracochlear administration offers direct access to the colchlea but is rather invasive. A wide range of drug delivery systems or devices were evaluated in research and clinic over the last decade for inner ear applications. In this review, different strategies including medical devices, hydrogels and nanoparticulate systems for intratympanic administration, and cochlear implant coating or advanced medical devices for intracoclear administration were explored with special attention to in vivo studies. This review highlights the promising systems for future clinical applications as well as the current hurdles that remain to be overcome for efficient inner ear therapy.

Spontaneous activity in the developing auditory system

Spontaneous electrical activity is a common feature of sensory systems during early development. This sensory-independent neuronal activity has been implicated in promoting their survival and maturation, as well as growth and refinement of their projections to yield circuits that can rapidly extract information about the external world. Periodic bursts of action potentials occur in auditory neurons of mammals before hearing onset. This activity is induced by inner hair cells (IHCs) within the developing cochlea, which establish functional connections with spiral ganglion neurons (SGNs) several weeks before they are capable of detecting external sounds. During this pre-hearing period, IHCs fire periodic bursts of Ca(2+) action potentials that excite SGNs, triggering brief but intense periods of activity that pass through auditory centers of the brain. Although spontaneous activity requires input from IHCs, there is ongoing debate about whether IHCs are intrinsically active and their firing periodically interrupted by external inhibitory input (IHC-inhibition model), or are intrinsically silent and their firing periodically promoted by an external excitatory stimulus (IHC-excitation model). There is accumulating evidence that inner supporting cells in Kölliker's organ spontaneously release ATP during this time, which can induce bursts of Ca(2+) spikes in IHCs that recapitulate many features of auditory neuron activity observed in vivo. Nevertheless, the role of supporting cells in this process remains to be established in vivo. A greater understanding of the molecular mechanisms responsible for generating IHC activity in the developing cochlea will help reveal how these events contribute to the maturation of nascent auditory circuits.

Hearing molecules, mechanism and transportation: modeled in Drosophila melanogaster

Mechanosensory transduction underlies the perception of touch, sound and acceleration. The mechanical signals exist in the environment are resensed by the specialized mechanosensory cells, which convert the external forces into the electrical signals. Hearing is a magnificent example that relies on the mechanotransduction mediated by the auditory cells, for example the inner-ear hair cells in vertebrates and the Johnston's organ (JO) in fly. Previous studies have shown the fundamental physiological processes in the fly and vertebrate auditory organs are similar, suggesting that there might be a set of similar molecules underlying these processes. The molecular studies of the fly JO have been shown to be remarkably successful in discovering the developmental and functional genes that provided further implications in vertebrates. Several evolutionarily conserved molecules and signaling pathways have been shown to govern the development of the auditory organs in both vertebrates and invertebrates. The current review describes the similarities and differences between the vertebrate and fly auditory organs at developmental, structural, molecular, and transportation levels.

Exome sequencing and linkage analysis identified tenascin-C (TNC) as a novel causative gene in nonsyndromic hearing loss

In this study, a five-generation Chinese family (family F013) with progressive autosomal dominant hearing loss was mapped to a critical region spanning 28.54 Mb on chromosome 9q31.3-q34.3 by linkage analysis, which was a novel DFNA locus, assigned as DFNA56. In this interval, there were 398 annotated genes. Then, whole exome sequencing was applied in three patients and one normal individual from this family. Six single nucleotide variants and two indels were found co-segregated with the phenotypes. Then using mass spectrum (Sequenom, Inc.) to rank the eight sites, we found only the TNC gene be co-segregated with hearing loss in 53 subjects of F013. And this missense mutation (c.5317G>A, p.V1773M ) of TNC located exactly in the critical linked interval. Further screening to the coding region of this gene in 587 subjects with nonsyndromic hearing loss (NSHL) found a second missense mutation, c.5368A>T (p. T1796S), co-segregating with phenotype in the other family. These two mutations located in the conserved region of TNC and were absent in the 387 normal hearing individuals of matched geographical ancestry. Functional effects of the two mutations were predicted using SIFT and both mutations were deleterious. All these results supported that TNC may be the causal gene for the hearing loss inherited in these families. TNC encodes tenascin-C, a member of the extracellular matrix (ECM), is present in the basilar membrane (BM), and the osseous spiral lamina of the cochlea. It plays an important role in cochlear development. The up-regulated expression of TNC gene in tissue repair and neural regeneration was seen in human and zebrafish, and in sensory receptor recovery in the vestibular organ after ototoxic injury in birds. Then the absence of normal tenascin-C was supposed to cause irreversible injuries in cochlea and caused hearing loss.

Mutations in ap1b1 cause mistargeting of the Na(+)/K(+)-ATPase pump in sensory hair cells

The hair cells of the inner ear are polarized epithelial cells with a specialized structure at the apical surface, the mechanosensitive hair bundle. Mechanotransduction occurs within the hair bundle, whereas synaptic transmission takes place at the basolateral membrane. The molecular basis of the development and maintenance of the apical and basal compartments in sensory hair cells is poorly understood. Here we describe auditory/vestibular mutants isolated from forward genetic screens in zebrafish with lesions in the adaptor protein 1 beta subunit 1 (ap1b1) gene. Ap1b1 is a subunit of the adaptor complex AP-1, which has been implicated in the targeting of basolateral membrane proteins. In ap1b1 mutants we observed that although the overall development of the inner ear and lateral-line organ appeared normal, the sensory epithelium showed progressive signs of degeneration. Mechanically-evoked calcium transients were reduced in mutant hair cells, indicating that mechanotransduction was also compromised. To gain insight into the cellular and molecular defects in ap1b1 mutants, we examined the localization of basolateral membrane proteins in hair cells. We observed that the Na(+)/K(+)-ATPase pump (NKA) was less abundant in the basolateral membrane and was mislocalized to apical bundles in ap1b1 mutant hair cells. Accordingly, intracellular Na(+) levels were increased in ap1b1 mutant hair cells. Our results suggest that Ap1b1 is essential for maintaining integrity and ion homeostasis in hair cells.

CTP (Cochlin-tomoprotein) detection in the profuse fluid leakage (gusher) from cochleostomy

CONCLUSIONS

By testing 125 samples, we confirmed that Cochlin-tomoprotein (CTP) is present in the perilymph, not in cerebrospinal fluid (CSF). Perilymph and CSF exist in two distinct compartments, even in the case of a malformed inner ear with a bony defect in the lamina cribrosa, as described here. Cochleostomy might have suddenly decreased the perilymph pressure, allowing the influx of CSF into the inner ear resulting in profuse fluid leakage, first perilymph then CSF.

OBJECTIVES

The first purpose of this study was to further confirm the specificity of the perilymph-specific protein CTP that we reported recently. Secondly, we assessed the nature of the fluid leakage from the cochleostomy using the CTP detection test.

METHODS

A standardized CTP detection test was performed on 65 perilymph and 60 CSF samples. Samples of profuse fluid leakage collected from cochleostomy during cochlear implantation surgery of one patient with branchio-oto-renal (BOR) syndrome were also tested by the CTP detection test.

RESULTS

CTP was detected in 60 of 65 perilymph samples but not in any of the CSF samples. The leaked fluid was shown to contain CTP, i.e. perilymph, at the outset, and then the CTP detection signals gradually disappeared as time elapsed.

Developmental regulation of spontaneous activity in the Mammalian cochlea

Neurons in the developing auditory system fire bursts of action potentials before the onset of hearing. This spontaneous activity promotes the survival and maturation of auditory neurons and the refinement of synaptic connections in auditory nuclei; however, the mechanisms responsible for initiating this activity remain uncertain. Previous studies indicate that inner supporting cells (ISCs) in the developing cochlea periodically release ATP, which depolarizes inner hair cells (IHCs), leading to bursts of action potentials in postsynaptic spiral ganglion neurons (SGNs). To determine when purinergic signaling appears in the developing cochlea and whether it is responsible for initiating auditory neuron activity throughout the prehearing period, we examined spontaneous activity from ISCs, IHCs, and SGNs in cochleae acutely isolated from rats during the first three postnatal weeks. We found that ATP was released from ISCs within the cochlea from birth until the onset of hearing, which led to periodic inward currents, Ca(2+) transients, and morphological changes in these supporting cells. This spontaneous release of ATP also depolarized IHCs and triggered bursts of action potentials in SGNs for most of the postnatal prehearing period, beginning a few days after birth as IHCs became responsive to ATP, until the onset of hearing when ATP was no longer released from ISCs. When IHCs were not subject to purinergic excitation, SGNs exhibited little or no activity. These results suggest that supporting cells in the cochlea provide the primary excitatory stimulus responsible for initiating bursts of action potentials in auditory nerve fibers before the onset of hearing.

ATP-induced morphological changes in supporting cells of the developing cochlea

The developing cochlea of mammals contains a large group of columnar-shaped cells, which together form a structure known as Kölliker's organ. Prior to the onset of hearing, these inner supporting cells periodically release adenosine 5'-triphosphate (ATP), which activates purinergic receptors in surrounding supporting cells, inner hair cells and the dendrites of primary auditory neurons. Recent studies indicate that purinergic signaling between inner supporting cells and inner hair cells initiates bursts of action potentials in auditory nerve fibers before the onset of hearing. ATP also induces prominent effects in inner supporting cells, including an increase in membrane conductance, a rise in intracellular Ca(2+), and dramatic changes in cell shape, although the importance of ATP signaling in non-sensory cells of the developing cochlea remains unknown. Here, we review current knowledge pertaining to purinergic signaling in supporting cells of Kölliker's organ and focus on the mechanisms by which ATP induces changes in their morphology. We show that these changes in cell shape are preceded by increases in cytoplasmic Ca(2+), and provide new evidence indicating that elevation of intracellular Ca(2+) and IP(3) are sufficient to initiate shape changes. In addition, we discuss the possibility that these ATP-mediated morphological changes reflect crenation following the activation of Ca(2+)-activated Cl(-) channels, and speculate about the possible functions of these changes in cell morphology for maturation of the cochlea.

Aminoglycoside-induced phosphatidylserine externalization in sensory hair cells is regionally restricted, rapid, and reversible

The aminophospholipid phosphatidylserine (PS) is normally restricted to the inner leaflet of the plasma membrane. During certain cellular processes, including apoptosis, PS translocates to the outer leaflet and can be labeled with externally applied annexin V, a calcium-dependent PS-binding protein. In mouse cochlear cultures, annexin V labeling reveals that the aminoglycoside antibiotic neomycin induces rapid PS externalization, specifically on the apical surface of hair cells. PS externalization is observed within approximately 75 s of neomycin perfusion, first on the hair bundle and then on membrane blebs forming around the apical surface. Whole-cell capacitance also increases significantly within minutes of neomycin application, indicating that blebbing is accompanied by membrane addition to the hair cell surface. PS externalization and membrane blebbing can, nonetheless, occur independently. Pretreating hair cells with calcium chelators, a procedure that blocks mechanotransduction, or overexpressing a phosphatidylinositol 4,5-biphosphate (PIP2)-binding pleckstrin homology domain, can reduce neomycin-induced PS externalization, suggesting that neomycin enters hair cells via transduction channels, clusters PIP2, and thereby activates lipid scrambling. The effects of short-term neomycin treatment are reversible. After neomycin washout, PS is no longer detected on the apical surface, apical membrane blebs disappear, and surface-bound annexin V is internalized, distributing throughout the supranuclear cytoplasm of the hair cell. Hair cells can therefore repair, and recover from, neomycin-induced surface damage. Hair cells lacking myosin VI, a minus-end directed actin-based motor implicated in endocytosis, can also recover from brief neomycin treatment. Internalized annexin V, however, remains below the apical surface, thereby pinpointing a critical role for myosin VI in the transport of endocytosed material away from the periphery of the hair cell.

TRPML3 mutations cause impaired mechano-electrical transduction and depolarization by an inward-rectifier cation current in auditory hair cells of varitint-waddler mice

TRPML3 (mucolipin-3) belongs to one of the transient-receptor-potential (TRP) ion channel families. Mutations in the Trpml3 gene cause disorganization of the stereociliary hair bundle, structural aberrations in outer and inner hair cells and stria vascularis defects, leading to deafness in the varitint-waddler (Va) mouse. Here we refined the stereociliary localization of TRPML3 and investigated cochlear hair cell function in varitint-waddler (Va(J)) mice carrying the TRPML3<I362T/A419P> mutations. Using a TRPML3-specific antibody we detected a approximately 68 kDa protein with near-equal expression levels in cochlea and vestibule of wild-type and Va(J) mutants. At postnatal days 3 and 5, we observed abundant localization of TRPML3 at the base of stereocilia near the position of the ankle links. This stereociliary localization domain was absent in Va(J) heterozygotes and homozygotes. Electrophysiological recordings revealed reduced mechano-electrical transducer currents in hair cells from Va(J)/+ and Va(J)/Va(J) mice. Furthermore, FM1-43 uptake and [(3)H]gentamicin accumulation were decreased in hair cells in cultured organs of Corti from Va(J)/+ and Va(J)/Va(J) mice. We propose that TRPML3 plays a critical role at the ankle-link region during hair-bundle growth and that an adverse effect of mutant TRPML3 on bundle development and mechano-electrical transduction is the main cause of hearing loss in Va(J)/+ mutant mice. Outer hair cells of Va(J)/Va(J) mice additionally had depolarized resting potentials due to an inwardly rectifying leak conductance formed by the mutant channels, leading over time to hair-cell degeneration and contributing to their deafness. Our findings argue against TRPML3 being a component of the hair-cell transducer channel.

Cadherins and mechanotransduction by hair cells

Mechanotransduction, the conversion of a mechanical stimulus into an electrical signal is crucial for our ability to hear and to maintain balance. Recent findings indicate that two members of the cadherin superfamily are components of the mechanotransduction machinery in sensory hair cells of the vertebrate inner ear. These studies show that cadherin 23 (CDH23) and protocadherin 15 (PCDH15) form several of the extracellular filaments that connect the stereocilia and kinocilium of a hair cell into a bundle. One of these filaments is the tip link that has been proposed to gate the mechanotransduction channel in hair cells. The extracellular domains of CDH23 and PCDH15 differ in their structure from classical cadherins and their cytoplasmic domains bind to distinct effectors, suggesting that evolutionary pressures have shaped the two cadherins for their function in mechanotransduction.

Cadherin 23 and protocadherin 15 interact to form tip-link filaments in sensory hair cells

Hair cells of the inner ear are mechanosensors that transduce mechanical forces arising from sound waves and head movement into electrochemical signals to provide our sense of hearing and balance. Each hair cell contains at the apical surface a bundle of stereocilia. Mechanoelectrical transduction takes place close to the tips of stereocilia in proximity to extracellular tip-link filaments that connect the stereocilia and are thought to gate the mechanoelectrical transduction channel. Recent reports on the composition, properties and function of tip links are conflicting. Here we demonstrate that two cadherins that are linked to inherited forms of deafness in humans interact to form tip links. Immunohistochemical studies using rodent hair cells show that cadherin 23 (CDH23) and protocadherin 15 (PCDH15) localize to the upper and lower part of tip links, respectively. The amino termini of the two cadherins co-localize on tip-link filaments. Biochemical experiments show that CDH23 homodimers interact in trans with PCDH15 homodimers to form a filament with structural similarity to tip links. Ions that affect tip-link integrity and a mutation in PCDH15 that causes a recessive form of deafness disrupt interactions between CDH23 and PCDH15. Our studies define the molecular composition of tip links and provide a conceptual base for exploring the mechanisms of sensory impairment associated with mutations in CDH23 and PCDH15.

A functional study of plasma-membrane calcium-pump isoform 2 mutants causing digenic deafness

Ca2+ enters the stereocilia of hair cells through mechanoelectrical transduction channels opened by the deflection of the hair bundle and is exported back to endolymph by an unusual splicing isoform (w/a) of plasma-membrane calcium-pump isoform 2 (PMCA2). Ablation or missense mutations of the pump cause deafness, as described for the G283S mutation in the deafwaddler (dfw) mouse. A deafness-inducing missense mutation of PMCA2 (G293S) has been identified in a human family. The family also was screened for mutations in cadherin 23, which accentuated hearing loss in a previously described human family with a PMCA2 mutation. A T1999S substitution was detected in the cadherin 23 gene of the healthy father and affected son but not in that of the unaffected mother, who presented instead the PMCA2 mutation. The w/a isoform was overexpressed in CHO cells. At variance with the other PMCA2 isoforms, it became activated only marginally when exposed to a Ca2+ pulse. The G293S and G283S mutations delayed the dissipation of Ca2+ transients induced in CHO cells by InsP3. In organotypic cultures, Ca2+ imaging of vestibular hair cells showed that the dissipation of stereociliary Ca2+ transients induced by Ca2+ uncaging was compromised in the dfw and PMCA2 knockout mice, as was the sensitivity of the mechanoelectrical transduction channels to hair bundle displacement in cochlear hair cells.

Purinergic signalling and intercellular Ca2+ wave propagation in the organ of Corti

Extracellular ATP is a key neuromodulator of visual and auditory sensory epithelia. In the rat cochlea, pharmacological dissection indicates that ATP, acting through a highly sensitive purinergic/IP(3)-mediated signaling pathway with (little or) no involvement of ryanodine receptors, is the principal paracrine mediator implicated in the propagation of calcium waves through supporting and epithelial cells. Measurement of sensitivity to UTP and other purinergic agonists implicate P2Y(2) and P2Y(4) as the main P2Y receptor isoforms involved in these responses. Ca2+ waves, elicited under highly reproducible conditions by carefully controlling dose (1 microM) and timing of focal agonist application (0.2s), extended over radial distance greater than 160 microm from the source, identical to those activated by damaging single outer hair cells. Altogether, these results indicate that intercellular calcium waves are a robust phenomenon that confers a significant ability for cell-cell communication in the mammalian cochlea. Further ongoing research will reveal the roles that such Ca2+ waves play in the inner ear.

Prenatal cocaine exposure accelerates morphological changes and transient expression of tyrosine hydroxylase in the cochlea of developing rats

Prenatal cocaine exposure causes alterations in auditory brainstem response in children and experimental animals and has adverse effects on auditory information processing and language skills in children. These effects may result from lesions in the cochlea since this organ is particularly sensitive to chemical insults during the development. We have thus studied here the effect of prenatal cocaine exposure on the maturation of the rat cochlea using the transient non-catecholaminergic expression of tyrosine hydroxylase in spiral ganglion neurons as an index of cochlear maturation and morphometry to evaluate the maturation of primary auditory neurons and the organ of Corti. We showed that prenatal cocaine exposure accelerated the cochlear maturation. In the basal coil of cochleas from PND8 cocaine-treated pups, the Kölliker's organ had disappeared, the tunnel of Corti was opened, and the stria vascularis no longer contained undifferentiated marginal cells. The maximum expression of tyrosine hydroxylase in type I primary auditory neurons occurred at PND8 instead of PND12 in pair-fed controls. On the other hand, the prenatal cocaine exposure had no effect on the width and height of the organ of Corti, spiral ganglion volume and number and size of primary auditory neurons. In conclusion, our data suggest that prenatal cocaine exposure, though not lethal to primary auditory neurons, accelerates aspects of the cochlear sensorineural maturation. This accelerated cochlear maturation in cocaine-treated rat pups could cause auditory dysfunctions by desynchronizing the development of the whole auditory pathway.

Ultrastructure of the normal human organ of corti. new anatomical findings in surgical specimens

CONCLUSION

A thorough scanning electron microscopy (SEM) investigation of immediately fixed human adult cochleae obtained during surgery for petro-clival meningiomas conveyed new information about morphology.

OBJECTIVE

To investigate the ultrastructure of human adult cochleae using SEM.

MATERIAL AND METHODS

Two human cochleae were decalcified, fixed with glutaraldehyde and osmium and prepared for SEM.

RESULTS

The excellently preserved morphology showed the pathways of nerve fibres through the organ of Corti. Undulating lateral cell membranes of Hensen and Claudius cells created an enlarged surface that may be important for homoeostasis. The distal attachment of the tectorial membrane to the reticular lamina was present in the shape of a marginal net, which was extended through marginal pillars. Stereocilia imprints extended as far as the distal end of the marginal pillars. The presence of an irregularly distributed fourth row of outer hair cells attached to the marginal pillars raises questions about differences in the excitation of the last row of outer hair cells as a result of differences in the composition of the tectorial membrane.

High resolution scanning electron microscopy of the human organ of Corti

Scanning electron microscopy on immediately fixed human cochleae obtained during surgery for life-threatening petro-clival meningioma showed excellently preserved morphology. We compared the morphological findings with those from transmission electron microscopic sections of well preserved human and animal tissue. The characteristics of neural innervation, the pathways of the nerves through the organ of Corti and the intimate relation of nerves to supporting cells along their route could be studied in detail. The lateral membranes of Hensen and Claudius cells were folded creating a surface enlargement. Marginal pillars extended the distal end of the tectorial membrane and correspond to the marginal net or "randfasernetz" described earlier. Stereocilia imprints at the undersurface of the tectorial membrane go as far as to the distal end of the marginal pillars. The presence of an irregularly distributed fourth row of outer hair cell, attached to the marginal pillars, raises questions about differences in the excitation of the last row of outer hair cells. The complex nature of many supporting cells, stria vascularis and Reissner's membrane, intracellular complexities as well as surface features are described. Supernumerary inner hair cells were observed and the different arrangement of outer spiral fibres in contrast to findings in animals and variations of nerve fibres within the organ of Corti between apex and base are discussed.

Global genetic analysis

The introduction of molecular markers in genetic analysis has revolutionized medicine. These molecular markers are genetic variations associated with a predisposition to common diseases and individual variations in drug responses. Identification and genotyping a vast number of genetic polymorphisms in large populations are increasingly important for disease gene identification, pharmacogenetics and population-based studies. Among variations being analyzed, single nucleotide polymorphisms seem to be most useful in large-scale genetic analysis. This review discusses approaches for genetic analysis, use of different markers, and emerging technologies for large-scale genetic analysis where millions of genotyping need to be performed.

Transient receptor potential ion channels and animal sensation: lessons from Drosophila functional research

Ion channels of the transient receptor potential (TRP) superfamily are non-selective cationic channels with six transmembrane domains. The TRP channel made its first debut as a light-gated Ca2+ channel in Drosophila. Recently, research on animal sensation in Drosophila disclosed other members of the TRP family that are required for touch sensation and hearing as well as the sensation of painful stimuli.

Quantitative Analysis of Nucleic Acids - the Last Few Years of Progress

DNA and RNA quantifications are widely used in biological and biomedical research. In the last ten years, many technologies have been developed to enable automated and high-throughput analyses. In this review, we first give a brief overview of how DNA and RNA quantifications are carried out. Then, five technologies (microarrays, SAGE, differential display, real time PCR and real competitive PCR) are introduced, with an emphasis on how these technologies can be applied and what their limitations are. The technologies are also evaluated in terms of a few key aspects of nucleic acids quantification such as accuracy, sensitivity, specificity, cost and throughput.

Meningitis-associated hearing loss: Protection by adjunctive antioxidant therapy

Hearing loss is the most frequent long-term complication of pneumococcal meningitis, affecting up to 40% of survivors. Unfortunately, adjuvant therapy with dexamethasone has failed to satisfactorily reduce its incidence. Therefore, we evaluated the use of antioxidants for the adjunctive therapy of meningitis-associated deafness. Eighteen hours after intracisternal injection of 7.5 x 10(5) colony-forming units of Streptococcus pneumoniae, rats were treated systemically either with ceftriaxone and the antioxidants and peroxynitrite scavengers Mn(III)tetrakis(4-benzoic acid)-porphyrin (MnTBAP) or N-acetyl-L-cysteine (NAC) or placebo (1 ml phosphate-buffered saline) for 4 days. Hearing was assessed by auditory brainstem response audiometry. Adjunctive antioxidant therapy significantly reduced the long-term hearing loss (14 days after infection) for square wave impulses (mean hearing loss +/- SD: ceftriaxone and placebo, 45+/-26 dB; ceftriaxone and MnTBAP, 9+/-23 dB; ceftriaxone and NAC, 19+/-30 dB) as well as 1 kHz (ceftriaxone and placebo, 28+/-19 dB; ceftriaxone and MnTBAP, 10+/-16 dB; ceftriaxone and NAC, 10+/-17 dB), and 10 kHz tone bursts (ceftriaxone and placebo, 62+/-27 dB; ceftriaxone and MnTBAP, 16+/-13 dB; ceftriaxone and NAC, 25+/-26 dB). Furthermore, both antioxidants attenuated the morphological correlates of meningogenic hearing loss, namely, long-term blood-labyrinth barrier disruption, spiral ganglion neuronal loss, and fibrous obliteration of the perilymphatic spaces. Adjuvant antioxidant therapy is highly otoprotective in meningitis and therefore is a promising future treatment option.

Na,K-ATPase expression in the mouse cochlea is not dependent on the mineralocorticoid receptor

This study was performed in order to test the hypothesis that the mineralocorticoid hormone stimulates the expression of Na,K-ATPase in the cochlea of the mouse. Immunohistochemistry was used to investigate the distribution of the mineralocorticoid receptor (MR) in the cochlea of the C57Bl/J6 mouse at different ages between gestational day 19 and postnatal day 30, and the occurrence and distribution of Na,K-ATPase in the inner ear of a mouse with a null mutation of the MR. Adult patterns of staining for MR were found as early as on gestational day 19 in the cochlea, with small changes thereafter. MR was detected in the same structures in the cochlea as Na,K-ATPase in earlier studies, where the amount of Na,K-ATPase increased after postnatal day 4. Thus there is latency between the increase of MR and the increase of Na,K-ATPase. In the cochlea of the MR deficient mouse, antibody labelling of Na,K-ATPase showed no significant difference as compared to the control wild type mouse. The hypothesis that mineralocorticoid hormone alone via MR stimulates the formation of Na,K-ATPase in the inner ear could not be confirmed by this study, and other regulating mechanisms must be considered.

Alteration of the calcium content in inner hair cells of the cochlea of the guinea pig after acute noise trauma with and without application of the organic calcium channel blocker diltiazem

Calcium ions are known to be important to the process of signal transduction across the apical and basal sides of the inner hair cells. Calcium channel antagonists have been demonstrated by light microscopy to provide protection against acoustic trauma. To evaluate the protective effect of calcium channel blocker on the inner ear cells to noise exposure, the amount of the histochemical reaction products formed in the cytoplasm of the inner hair cells of the guinea pig after application of pyroantimonate was measured by an image processing system connected to an energy-filtering transmission electron microscope (EFTEM). Compared to untreated control specimens (experimental animal group I) the amount of precipitable calcium had clearly increased in the inner hair cells of noise-exposed cochleae 60 h after an acute noise trauma (group III). In addition, small electron- lucent areas could be identified in the cytoplasm of the hair cells probably representing damage to the cellular fine structure. When the calcium channel blocker diltiazem was administered without any additional noise exposure the calcium content was drastically reduced in the inner hair cells (group II), but when the antagonist was given before and after acute noise trauma (group IV), the calcium content in the inner hair cells was nearly compared to the amount determined in the untreated group of animals. The role of diltiazem is discussed in respect of tissue protection and to the maintenance of the calcium-dependent physiological processes in the inner hair cells during signal transduction.

Distribution of Na,K-ATPase is normal in the inner ear of a mouse with a null mutation of the glucocorticoid receptor

This study was performed in order to test the hypothesis that the glucocorticoid hormone stimulates the formation of Na,K-ATPase in the inner ear of the mouse. An immunohistochemical study with respect to the presence and distribution of glucocorticoid receptors (GR) and Na,K-ATPase in the vestibular and cochlear regions of the inner ear was performed on a C57BL mouse with a null mutation of the glucocorticoid receptor (GR mutant mouse). The wild type C57BL mouse and the CBA mouse served as normal controls. As expected, the homozygous GR mutant mouse showed no specific staining for GR in the inner ear. The heterozygous GR mutant mouse showed faint staining of GR in the spiral limbus, the spiral ganglion, the organ of Corti and the utricle. This staining was markedly less than in the wild type C57BL mouse. Antibody labelling of Na,K-ATPase in the inner ear showed no significant difference between the homozygous and the heterozygous GR mutant mouse as compared to the control wild type C57BL mouse or the CBA mouse. Although earlier studies have shown a positive correlation between levels of glucocorticoid hormone in serum and the concentration of Na,K-ATPase in the inner ear, the hypothesis that glucocorticoid hormones alone stimulate the formation of Na,K-ATPase in the inner ear could not be confirmed by this study. Thus other regulating substances must be considered.

4-aminopyridine in scala media reversibly alters the cochlear potentials and suppresses electrically evoked oto-acoustic emissions

Iontophoresis of 4-aminopyridine into scala media of the guinea pig cochlea caused elevation of the thresholds of the compound action potential of the auditory nerve, loss of amplitude of the extracellular cochlear microphonic response (CM), increase in the endocochlear potential (EP) and reduction in the amplitude of electrically evoked oto-acoustic emissions (EEOAEs). These changes were reversible over 10-20 min. The reciprocity of the changes in the CM and the EP was consistent with an interruption of both DC and AC currents through outer hair cells (OHCs), probably by blockade of mechano-electrical transduction (MET) channels in OHCs. Reductions in EEOAEs were consistent with the extrinsically applied generating current entering the OHC via the MET channels. Implications for the activation of OHC electromotility in vivo are discussed.

The selectivity of the hair cell's mechanoelectrical-transduction channel promotes Ca2+ flux at low Ca2+ concentrations

The mechanoelectrical-transduction channel of the hair cell is permeable to both monovalent and divalent cations. Because Ca2+ entering through the transduction channel serves as a feedback signal in the adaptation process that sets the channel's open probability, an understanding of adaptation requires estimation of the magnitude of Ca2+ influx. To determine the Ca2+ current through the transduction channel, we measured extracellular receptor currents with transepithelial voltage-clamp recordings while the apical surface of a saccular macula was bathed with solutions containing various concentrations of K+, Na+, or Ca2+. For modest concentrations of a single permeant cation, Ca2+ carried much more receptor current than did either K+ or Na+. For higher cation concentrations, however, the flux of Na+ or K+ through the transduction channel exceeded that of Ca2+. For mixtures of Ca2+ and monovalent cations, the receptor current displayed an anomalous mole-fraction effect, which indicates that ions interact while traversing the channel's pore. These results demonstrate not only that the hair cell's transduction channel is selective for Ca2+ over monovalent cations but also that Ca2+ carries substantial current even at low Ca2+ concentrations. At physiological cation concentrations, Ca2+ flux through transduction channels can change the local Ca2+ concentration in stereocilia in a range relevant for the control of adaptation.

Alteration of loosely bound calcium in the guinea pig organ of Corti after treatment with diltiazem as calcium channel blocker

After oral administration of the organic calcium channel blocker diltiazem to guinea pigs for 7 days, calcium ions were precipitated with potassium antimonate in the cochleae. The spatial distribution of the precipitates was studied by energy-filtering transmission electron microscopy and the amount of the ultrastructural reaction products formed was determined semiquantitatively by an image processing system. Compared with untreated control ears, the number of the formed precipitates was reduced drastically in the inner hair cells after diltiazem treatment. In addition, electron microscopic analysis revealed that the number of calcium precipitates attached at the basolateral membrane of the outer hair cells was clearly reduced when compared with untreated control specimens. A large number of histochemical reaction products could be identified in the basilar membrane and were also observed in the untreated control specimens. The spatial distribution of the calcium precipitates in the lamina reticularis was not affected by diltiazem treatment and calcium precipitates could be identified within different cell membranes. The techniques used was considered to be helpful for identifying calcium channels ultrastructurally in intact undissected tissues and to support light microscopic analyses and patch-clamp electrophysiological measurements.