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

A Novel in vitro Model Delineating Hair Cell Regeneration and Neural Reinnervation in Adult Mouse Cochlea

The study of an adult mammalian auditory system, such as regeneration, has been hampered by the lack of an in vitro system in which hypotheses can be tested efficiently. This is primarily due to the fact that the adult inner ear is encased in the toughest bone of the body, whereas its removal leads to the death of the sensory epithelium in culture. We hypothesized that we could take advantage of the integral cochlear structure to maintain the overall inner ear architecture and improve sensory epithelium survival in culture. We showed that by culturing adult mouse cochlea with the (surrounding) bone intact, the supporting cells (SCs) survived and almost all hair cells (HCs) degenerated. To evaluate the utility of the explant culture system, we demonstrated that the overexpression of Atoh1, an HC fate-determining factor, is sufficient to induce transdifferentiation of adult SCs to HC-like cells (HCLCs). Transdifferentiation-derived HCLCs resemble developmentally young HCs and are able to attract adult ganglion neurites. Furthermore, using a damage model, we showed that degenerated adult ganglions respond to regenerated HCLCs by directional neurite outgrowth that leads to HCLC-neuron contacts, strongly supporting the intrinsic properties of the HCLCs in establishing HCLC-neuron connections. The adult whole cochlear explant culture is suitable for diverse studies of the adult inner ear including regeneration, HC-neuron pathways, and inner ear drug screening.

Individualized Treatment of Tinnitus during Sleep Using Combined Tinnitus Signal and Music

INTRODUCTION

Tinnitus is a widely seen otological symptom that interferes with daily activities and causes discomfort. Tinnitus treatments can be classified into 4 main groups: pharmacological treatments, cognitive and behavioral therapy, psychological treatments, and combined treatment approaches made up of at least 2 of these 3 treatment methods.

OBJECTIVE

The aim of this study was to assess whether it would be possible to develop an individualized treatment method of tinnitus by application of a combined tinnitus signal and music during sleep.

METHODS

Forty-three ears of 30 patients who had subjective tinnitus were included. The patients were evaluated using Tinnitus Handicap Inventory, Visual Analogue Scale, and Beck Depression Inventory. The psychoacoustic parameters of tinnitus, such as tinnitus frequency and loudness, and minimal masking levels, were determined. The patients were asked to select musical melodies that they liked. The tinnitus frequency of each patient was taken as the central frequency according to ANSI 2004. All sound files were prepared as stereo channels, with 16-bit resolution and 44,100 Hz sampling rate. The root mean square power value of the music and the band noise's average root mean square power value were equalized with the "Amplification" command, and 70% of the music and 30% of wide/narrow-band noise were mixed as a stereo channel by the "Mix Paste" command. The patients were instructed to listen to that individualized music/narrow-band noise (tinnitus signal) for 2 h during sleep for a duration of 6 months.

RESULTS

Tinnitus frequencies of the patients measured prior to treatment and at the second, fourth, and sixth months of follow-up were not significantly different. A statistically significant decrease was seen in tinnitus loudness, minimal masking levels, and residual inhibition during the follow-up. Tinnitus Handicap Inventory scores decreased significantly during follow-up, and the number of patients who complained of tinnitus decreased (p < 0.05). The Visual Analogue Scale scores significantly decreased during follow-up (p < 0.05). Beck Depression Inventory scores decreased significantly during follow-up (p < 0.05).

CONCLUSION

Stimulation of the auditory and limbic systems during sleep by the tinnitus signal combined with individualized musical melodies seems an alternative, effective, and cheap method in the treatment of tinnitus.

Cyclodextrins and Iatrogenic Hearing Loss: New Drugs with Significant Risk

Cyclodextrins are a family of cyclic oligosaccharides with widespread usage in medicine, industry and basic sciences owing to their ability to solubilize and stabilize guest compounds. In medicine, cyclodextrins primarily act as a complexing vehicle and consequently serve as powerful drug delivery agents. Recently, uncomplexed cyclodextrins have emerged as potent therapeutic compounds in their own right, based on their ability to sequester and mobilize cellular lipids. In particular, 2-hydroxypropyl-β-cyclodextrin (HPβCD) has garnered attention because of its cholesterol chelating properties, which appear to treat a rare neurodegenerative disorder and to promote atherosclerosis regression related to stroke and heart disease. Despite the potential health benefits, use of HPβCD has been linked to significant hearing loss in several species, including humans. Evidence in mice supports a rapid onset of hearing loss that is dose-dependent. Ototoxicity can occur following central or peripheral drug delivery, with either route resulting in the preferential loss of cochlear outer hair cells (OHCs) within hours of dosing. Inner hair cells and spiral ganglion cells are spared at doses that cause ~85% OHC loss; additionally, no other major organ systems appear adversely affected. Evidence from a first-to-human phase 1 clinical trial mirrors animal studies to a large extent, indicating rapid onset and involvement of OHCs. All patients in the trial experienced some permanent hearing loss, although a temporary loss of function can be observed acutely following drug delivery. The long-term impact of HPβCD use as a maintenance drug, and the mechanism(s) of ototoxicity, are unknown. β-cyclodextrins preferentially target membrane cholesterol, but other lipid species and proteins may be directly or indirectly involved. Moreover, as cholesterol is ubiquitous in cell membranes, it remains unclear why OHCs are preferentially susceptible to HPβCD. It is possible that HPβCD acts upon several targets—for example, ion channels, tight junctions (TJ), membrane integrity, and bioenergetics—that collectively increase the sensitivity of OHCs over other cell types.

Deletion of Brg1 causes abnormal hair cell planer polarity, hair cell anchorage, and scar formation in mouse cochlea

Hair cells (HCs) are mechanosensors that play crucial roles in perceiving sound, acceleration, and fluid motion. The precise architecture of the auditory epithelium and its repair after HC loss is indispensable to the function of organ of Corti (OC). In this study, we showed that Brg1 was highly expressed in auditory HCs. Specific deletion of Brg1 in postnatal HCs resulted in rapid HC degeneration and profound deafness in mice. Further experiments showed that cell-intrinsic polarity of HCs was abolished, docking of outer hair cells (OHCs) by Deiter’s cells (DCs) failed, and scar formation in the reticular lamina was deficient. We demonstrated that Brg1 ablation disrupted the Gαi/Insc/LGN and aPKC asymmetric distributions, without overt effects on the core planer cell polarity (PCP) pathway. We also demonstrated that Brg1-deficient HCs underwent apoptosis, and that leakage in the reticular lamina caused by deficient scar formation shifted the mode of OHC death from apoptosis to necrosis. Together, these data demonstrated a requirement for Brg1 activity in HC development and suggested a role for Brg1 in the proper cellular structure formation of HCs.

Progressive Hearing Loss in Mice Carrying a Mutation in Usp53

Disordered protein ubiquitination has been linked to neurodegenerative disease, yet its role in inner ear homeostasis and hearing loss is essentially unknown. Here we show that progressive hearing loss in the ethylnitrosourea-generated mambo mouse line is caused by a mutation in Usp53, a member of the deubiquitinating enzyme family. USP53 contains a catalytically inactive ubiquitin-specific protease domain and is expressed in cochlear hair cells and a subset of supporting cells. Although hair cell differentiation is unaffected in mambo mice, outer hair cells degenerate rapidly after the first postnatal week. USP53 colocalizes and interacts with the tight junction scaffolding proteins TJP1 and TJP2 in polarized epithelial cells, suggesting that USP53 is part of the tight junction complex. The barrier properties of tight junctions of the stria vascularis appeared intact in a biotin tracer assay, but the endocochlear potential is reduced in adult mambo mice. Hair cell degeneration in mambo mice precedes endocochlear potential decline and is rescued in cochlear organotypic cultures in low potassium milieu, indicating that hair cell loss is triggered by extracellular factors. Remarkably, heterozygous mambo mice show increased susceptibility to noise injury at high frequencies. We conclude that USP53 is a novel tight junction-associated protein that is essential for the survival of auditory hair cells and normal hearing in mice, possibly by modulating the barrier properties and mechanical stability of tight junctions.

SIGNIFICANCE STATEMENT

Hereditary hearing loss is extremely prevalent in the human population, but many genes linked to hearing loss remain to be discovered. Forward genetics screens in mice have facilitated the identification of genes involved in sensory perception and provided valuable animal models for hearing loss in humans. This involves introducing random mutations in mice, screening the mice for hearing defects, and mapping the causative mutation. Here, we have identified a mutation in the Usp53 gene that causes progressive hearing loss in the mambo mouse line. We demonstrate that USP53 is a catalytically inactive deubiquitinating enzyme and a novel component of tight junctions that is necessary for sensory hair cell survival and inner ear homeostasis.

Evaluation of Intracochlear Trauma Caused by Insertion of Cochlear Implant Electrode Arrays through Different Quadrants of the Round Window

Hypothesis. This study aimed to evaluate whether there is a difference in the degree of intracochlear trauma when the cochlear implant electrode arrays is inserted through different quadrants of the round window membrane. Background. The benefits of residual hearing preservation in cochlear implant recipients have promoted the development of atraumatic surgeries. Minimal trauma during electrode insertion is crucial for residual hearing preservation. Methods. In total, 25 fresh human temporal bones were subjected to mastoidectomy and posterior tympanotomy. The cochlear implant electrode array was inserted through the anterosuperior quadrant of the round window membrane in 50% of the bones and through the anteroinferior quadrant in the remaining 50%. The temporal bones were dehydrated, embedded in epoxy, serially polished, stained, viewed through a stereomicroscope, and photographed with the electrode arrays in situ. The resulting images were analyzed for signs of intracochlear trauma. Results. Histological examinations revealed varying degrees of damage to the intracochlear structures, although the incidence and severity of intracochlear trauma were not influenced by the quadrant of insertion. Conclusions. The incidence and severity of intracochlear trauma were similar in all samples, irrespective of electrode array insertion through the anterosuperior or anteroinferior quadrant of the round window membrane.

Tricellulin deficiency affects tight junction architecture and cochlear hair cells

The two compositionally distinct extracellular cochlear fluids, endolymph and perilymph, are separated by tight junctions that outline the scala media and reticular lamina. Mutations in TRIC (also known as MARVELD2), which encodes a tricellular tight junction protein known as tricellulin, lead to nonsyndromic hearing loss (DFNB49). We generated a knockin mouse that carries a mutation orthologous to the TRIC coding mutation linked to DFNB49 hearing loss in humans. Tricellulin was absent from the tricellular junctions in the inner ear epithelia of the mutant animals, which developed rapidly progressing hearing loss accompanied by loss of mechanosensory cochlear hair cells, while the endocochlear potential and paracellular permeability of a biotin-based tracer in the stria vascularis were unaltered. Freeze-fracture electron microscopy revealed disruption of the strands of intramembrane particles connecting bicellular and tricellular junctions in the inner ear epithelia of tricellulin-deficient mice. These ultrastructural changes may selectively affect the paracellular permeability of ions or small molecules, resulting in a toxic microenvironment for cochlear hair cells. Consistent with this hypothesis, hair cell loss was rescued in tricellulin-deficient mice when generation of normal endolymph was inhibited by a concomitant deletion of the transcription factor, Pou3f4. Finally, comprehensive phenotypic screening showed a broader pathological phenotype in the mutant mice, which highlights the non-redundant roles played by tricellulin.

Tight junctions and the modulation of barrier function in disease

Tight junctions create a paracellular barrier in epithelial and endothelial cells protecting them from the external environment. Two different classes of integral membrane proteins constitute the tight junction strands in epithelial cells and endothelial cells, occludin and members of the claudin protein family. In addition, cytoplasmic scaffolding molecules associated with these junctions regulate diverse physiological processes like proliferation, cell polarity and regulated diffusion. In many diseases, disruption of this regulated barrier occurs. This review will briefly describe the molecular composition of the tight junctions and then present evidence of the link between tight junction dysfunction and disease.

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.

TRPV1 regulators mediate gentamicin penetration of cultured kidney cells

Transient receptor potential (TRP) receptors are, typically, calcium-permeant cation channels that transduce environmental stimuli. Both kidney epithelial and inner ear sensory cells express TRPV1, are mechanosensors and accumulate the aminoglycoside antibiotic gentamicin. Recently, we showed that Texas Red-conjugated gentamicin (GTTR) enters kidney cells via an endosome-independent pathway. Here, we used GTTR to investigate this non-endocytotic mechanism of gentamicin uptake. In serum-free buffers, GTTR penetrated MDCK cells within 30 s and uptake was modulated by extracellular, multivalent cations (Ca2+, La3+, Gd3+) or protons. We verified the La3+ modulation of GTTR uptake using immunocytochemical detection of unconjugated gentamicin. Membrane depolarization, induced by high extracellular K+ or valinomycin, also reduced GTTR uptake, suggesting electrophoretic permeation through ion channels. GTTR uptake was enhanced by the TRPV1 agonists, resiniferatoxin and anandamide, in Ca2+-free media. Competitive antagonists of the TRPV1 cation current, iodo-resiniferatoxin and SB366791, also enhanced GTTR uptake independently of Ca2+, reinforcing these antagonists' potential as latent agonists in specific situations. Ruthenium Red blocked GTTR uptake in the presence or absence of these TRPV1-agonists and antagonists. In addition, GTTR uptake was blocked by RTX in the presence of more physiological levels (2 mM) of Ca2+. Thus gentamicin enters cells via cation channels, and gentamicin uptake can be modulated by regulators of the TRPV1 channel.

General review of tinnitus: prevalence, mechanisms, effects, and management

Tinnitus is an increasing health concern across all strata of the general population. Although an abundant amount of literature has addressed the many facets of tinnitus, wide-ranging differences in professional beliefs and attitudes persist concerning its clinical management. These differences are detrimental to tinnitus patients because the management they receive is based primarily on individual opinion (which can be biased) rather than on medical consensus. It is thus vitally important for the tinnitus professional community to work together to achieve consensus. To that end, this article provides a broad-based review of what is presently known about tinnitus, including prevalence, associated factors, theories of pathophysiology, psychological effects, effects on disability and handicap, workers' compensation issues, clinical assessment, and various forms of treatment. This summary of fundamental information has relevance to both clinical and research arenas.

Developmental expression of two-pore domain K+ channels, TASK-1 and TREK-1, in the rat cochlea

Developmental expression of two-pore domain potassium (2P K) channels, TASK-1 and TREK-1, was investigated in the rat cochlea at onset of hearing and after maturity using RT-PCR and immunocytochemistry. TASK-1 and TREK-1 mRNAs were detected by RT-PCR at postnatal day (P) 9-12. TASK-1 like immunoreactivity (LIR) in the P13 cochlea was observed in Deiters', pillar, Claudius' and outer sulcus cells, spiral limbus fibrocytes, and neuroglia. At P13, TREK-1-LIR was more wide-spread, and included sensory and supporting cells of the organ of Corti, spiral ganglion, stria vascularis, Reissner's membrane, inner and outer sulcus cells, connective and support tissues surrounding modiolus. By P105 the pattern of TASK-1- and TREK-1-LIR became limited to a subset of the above structures, suggesting developmental regulation. During postnatal development, TASK-1 may be important in the onset (around P11) and maturation (by P22) of endocochlear potential and hearing. The distribution of TASK-1 and TREK-1 suggest a role in K cycling and homeostasis. As TASK-1 and TREK-1 are inhibited by local anesthetics at doses used to treat tinnitus, 2P K channels may also be important in cochlear dysfunction.

Riluzole rescues cochlear sensory cells from acoustic trauma in the guinea-pig

Acoustic trauma is the major cause of hearing loss in industrialised nations. We show in guinea-pigs that sound exposure (6 kHz, 120 dB sound pressure level for 30 min) leads to sensory cell death and subsequent permanent hearing loss. Ultrastructural analysis reveals that degeneration of the noise-damaged hair cells involved different mechanisms, including typical apoptosis, autolysis and, to a lesser extent, necrosis. Whatever the mechanisms, a common feature of noise damage to hair cells was mitochondrial alteration. Riluzole (2-amino-6-trifluoromethoxy benzothiazole) is a neuroprotective agent that prevents apoptosis- and necrosis-induced cell death. Perfusion of riluzole into the cochlea via an osmotic minipump prevents mitochondrial damage and subsequent translocation of cytochrome c, DNA fragmentation, and hair cell degeneration. This was confirmed by functional tests showing a clear dose-dependent reduction (ED(50)=16.8 microM) of permanent hearing loss and complete protection at 100 microM. Although less efficient than intracochlear perfusion, intraperitoneal injection of riluzole rescues the cochlea within a therapeutic window of 24 h after acoustic trauma.These results show that riluzole is able to prevent and rescue the cochlea from acoustic trauma. It may thus be an interesting molecule for the treatment of inner ear injuries.

Low-frequency masking for detection of endolymphatic hydrops in patients with glaucoma

The coincidence of various eye and ear abnormalities has been described in the literature. Some authors discuss the possible existence of endolymphatic hydrops in patients with glaucoma. Whereas the current diagnostic tests for glaucoma are well-defined and evident, those for endolymphatic hydrops are not so reliable. This has made it difficult to accurately study the coincidence of endolymphatic hydrops and glaucoma. For better detection of endolymphatic hydrops, we performed low-frequency masking tests in 23 patients with primary open-angle glaucoma without signs of Ménìere's disease. The phase dependent sensitivity of the organ of Corti to a short test stimulus can be measured by applying a low-frequency masker tone to determine the modulation depth. Whereas the modulation depth in the normal hearing population is around 20-35 dB, the modulation depth in patients with Ménière's disease may be significantly decreased (5-10 dB), depending on the stage of disease. A decreased modulation depth was found in at least one ear in 19 of our 23 patients with glaucoma. Correlations between homeostatic mechanisms and their histological characteristics, e.g. melanocytes and their hormonal and enzymatic regulation, will be discussed. If the common pathogenesis of eye and ear lesions is better understood, it may be possible to develop new and more effective strategies for prevention and therapy.

Changes in cochlear function after double-membrane rupture in the guinea pig

We measured the transiently evoked otoacoustic emissions (TEOAEs), compound action potentials (CAPs) and cochlear microphonics (CMs) in guinea pigs after rupture of the round window membrane alone (n = 5) or of the round window membrane with localized cochlear damage (n = 10). The localized cochlear damage entailed rupture of Reissner's membrane with damage to the stria vascularis. We determined the time course of changes in the total echo power (TEP) in TEOAEs and the minimal detectable levels of CAPs and CMs. The endocochlear potential (EP) was measured in the cochlea with localized damage. There were no changes in TEOAEs, CAPs or CMs in the guinea pigs subjected to round window membrane rupture alone, but the minimal detectable levels of CAPs and CMs were increased in all the guinea pigs in which TEOAEs were absent after rupture of the round window membrane with localized cochlear damage. Our results suggest that double-membrane rupture (rupture of the round window membrane with localized cochlear damage) produces acute sensorineural hearing loss. The hearing loss appeared to be related to damage to the cochlea, which may be induced by influx of potassium-rich endolymph into the perilymph, and by morphological damage to the scala media.

Distribution of endothelin-1-like activity in the cochlea of normal guinea pigs

Distribution of endothelin-1 (ET-1) in the cochlea of normal guinea pigs was determined by immunohistochemistry. ET-1 activity was identified with the mouse anti-human ET-1 IgG1 monoclonal antibody. ET-1 activity was distributed in the modiolus, spiral ligament, stria vascularis, spiral prominence. Reissner's membrane, supporting cells of the organ of Corti and spiral ganglion cells. These findings suggest that ET-1 may be involved in the regulation of fluid volume and ions of the cochlea.

Possible roles of outer hair cell d.c. movements in the cochlea

Outer hair cells are expected to change the position of the cochlear partition in response to sound stimulation. This may contribute to the extended dynamic range of the cochlea. This paper explores the possible underlying mechanisms and the consequences for models of cochlear function of such position changes.

Effect of lymph composition on an in vitro preparation of the alligator lizard cochlea

The effects of different artificial lymphs on the cochlear duct of the alligator lizard were studied in an in vitro preparation. The duct was dissected and cemented to the glass floor of a chamber that had been filled with an artificial lymph. The vestibular membrane was removed and latex beads (1-5 microns in diameter) were allowed to settle on the endolymphatic surface of the duct. During perfusion with an artificial lymph solution, the positions of beads were measured and video images of the duct were obtained. Artificial lymphs were isosmotic and included artificial endolymph (AE), artificial perilymph (AP), Leibovitz's L-15 culture medium, an AE solution whose calcium concentration was the same as that of AP, and AE and AP solutions in which gluconate was substituted for chloride ions. Results obtained in AE were consistently different from those in other lymphs. The displacements of beads, the projected area of the papilla, the occurrence of blebs, and direct observation of cells in the duct all indicated that the tissue swelled in AE (with or without 2 mmol/l Ca) but showed no consistent shrinking or swelling in any of the other artificial lymphs. Thus for the solutions we used, the presence of both potassium and chloride was required to elicit the swelling response to isosmotic artificial lymphs. There were some regional differences in the swelling response: the swelling of the endolymphatic surface of the tissue in a direction orthogonal to the basilar membrane surface was smaller on the free-standing region of the basilar papilla than either on the tectorial membrane or on the hyaline epithelial cells. The preparation was osmotically stable in AP and in both AE and AP solutions in which gluconate was substituted for chloride ions. After exposure to these solutions for as much as 300 min, the preparation showed no gross signs of deterioration visible with the light microscope, and continued to exhibit a highly specific osmotic response to the composition of the bathing medium.

Sound preprocessing by ac and dc movements of cochlear outer hair cells

In inner and outer hair cells, a sound event results mechano-electrically in a receptor potential from the hair cells by the functioning of apical and lateral K(+)-channels. However, after this point, the signal transfer is divided. Inner hair cells (IHC) release an unknown afferent transmitter. By contrast, outer hair cells (OHC) are proposed to produce mechanical ac and dc responses. In our model, the ac components of the sound signal, the carrier frequencies, determine the response of the OHC. Usually, they respond by ac and dc movements. The rapid ac movements of OHC, for which the underlying mechanism is unknown, may respond cycle-by-cycle to and interfere with the carrier frequency of the traveling wave. Near hearing threshold, they could drastically amplify the traveling wave thus contributing to the postulated cochlear amplifier. Active dc movements of the cytoskeleton of the cell body, as well as of the cuticular plate with the sensory hairs, are proposed to respond to millisecond changes of the sound stimulus over time. Such changes could be a modulation of the amplitude (AM), i.e., an increase or decrease of the sound pressure level (SPL), which is reflected in the envelope of the traveling wave. The active mechanical dc response of OHC to the amplitude (AM) and frequency modulation (FM) pattern is then expected to result in dc position changes of the reticular lamina (RL). These should control the operation point of the stereocilia, thus influencing their transfer function and sensitivity. In addition, experimental data suggest that there are modulations of the compliance of the organ of Corti (OC) and changes of its geometry. This dc modulation of micromechanical properties and geometry of the OC by active force generation of OHCs might contribute to automatic gain control, adaptation, TTS, as well as to the homeostasis of the basilar membrane location. In particular, the motile mechanism may protect the vulnerable cochlear partition against high sound pressure levels. Moreover, according to this model, changes of the sound signal with time are expected to be encoded in the actively produced dc movements of the RL. As the signal changes may carry important information (e.g., complex sound signal modulations such as formant transitions in speech), this is extracted and mechanically encoded by the proposed active dc mechanism. It cannot be excluded that the information-carrying dc signal is transferred to inner hair cells contributing to their adequate stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of endocochlear potential suppression upon susceptibility to acoustic trauma

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

Low-frequency modulation of compound action potential in experimental perilymphatic fistula and endolymphatic hydrops

We have tested the hypothesis that the cause of cochlear dysfunction associated with perilymphatic fistula (PLF) is closely related to endolymphatic hydrops (ELH). Using guinea pigs, we studied the tone-burst elicited compound action potential (CAP) and its modulation as caused by a 50 Hz biasing tone in experimental PLF. We compared these results with those of experimental ELH. Following perilymph aspiration through the perforated round window membrane, mild but significant elevations of CAP thresholds at tested frequencies were found. A reduction in the amplitude of cochlear microphonics (CM) for a 50 Hz sine wave appeared to correlate with these CAP threshold changes. However, there were no significant changes in the modulation effect of the 50 Hz biasing tone on the CAP elicited by an 8 kHz tone burst. This finding differed from that in ears with experimental ELH, in which significant reductions of both 50 Hz CM and the degree of CAP modulation were consistently observed. We concluded that it is unlikely that the underlying mechanisms of a modification to the low frequency response of the base of the cochlea following perilymph aspiration is linked to that of experimental ELH.

A non-invasive, objective test of endolymphatic hydrops

Our previous work has shown that auditory brainstem responses (ABRs) can be used to measure the basilar membrane travelling wave velocity (TWV). Based upon the hypothesis that increased pressure in the scala media will increase the stiffness of the basilar membrane and, hence, increase the speed of the travelling wave. ABRs have been used to provide a means of objectively testing for endolymphatic hydrops. Ten normally hearing subjects and 20 Meniere's patients were tested. The data for the Meniere's group show that the TWV was within normal limits at all frequencies tested, except 5.7 kHz. At this frequency, the TWV greatly exceeded that of the normal group. A short duration (15-30 min) clinical test has been developed which involves the recording of two masked brainstem responses and the measurement of the difference in wave V latencies. Clinical verification that the technique is valid was obtained by monitoring patients undergoing glycerol dehydration. The additional hardware to enable this test to be carried out with a standard evoked response system has been constructed and three, parallel, clinical trials are in hand. Currently, this technique is being applied to longitudinal studies of Meniere's patients and to evaluating the action and efficacy of various drugs. A double-blind trial using a placebo and two different drugs is underway and preliminary results are presented here.

Acetylcholine, carbachol, and GABA induce no detectable change in the length of isolated outer hair cells

The mechanical and electrical properties of cochlear outer hair cells (OHCs) are suggested to modulate transduction by inner hair cells. These properties of OHCs are presumably regulated by efferent neurons which use several transmitters including acetylcholine (Ach) and gamma aminobutyric acid (GABA). Since it had been suggested that Ach causes isolated OHCs to shorten visibly, this study was designed to investigate whether GABA also alters the length of OHCs. OHCs were isolated from the guinea pig cochlea by mechanical dispersion after collagenase treatment. Cells were initially selected by strict morphological criteria. In addition they were only included in further studies if they attained a constant length during 10 min of superfusion with buffer solution. Neither GABA (20 microM: 100 microM), Ach (5 mM; 10 microM with 10 microM eserine) or carbachol (10 microM; 100 microM) altered OHC length when applied in iso-osmotic Hank's balanced salt solution (total number of cells tested, 72). If a change in length occurred it must have been smaller than 0.3 microns, our detection ability. In contrast, high potassium and variations in osmolarity changed hair cell length by 3-10% in agreement with other reports.

Phantom auditory perception (tinnitus): mechanisms of generation and perception

Phantom auditory perception--tinnitus--is a symptom of many pathologies. Although there are a number of theories postulating certain mechanisms of its generation, none have been proven yet. This paper analyses the phenomenon of tinnitus from the point of view of general neurophysiology. Existing theories and their extrapolation are presented, together with some new potential mechanisms of tinnitus generation, encompassing the involvement of calcium and calcium channels in cochlear function, with implications for malfunction and aging of the auditory and vestibular systems. It is hypothesized that most tinnitus results from the perception of abnormal activity, defined as activity which cannot be induced by any combination of external sounds. Moreover, it is hypothesized that signal recognition and classification circuits, working on holographic or neuronal network-like representation, are involved in the perception of tinnitus and are subject to plastic modification. Furthermore, it is proposed that all levels of the nervous system, to varying degrees, are involved in tinnitus manifestation. These concepts are used to unravel the inexplicable, unique features of tinnitus and its masking. Some clinical implications of these theories are suggested.

Potassium-induced release of an endogenous toxic activity for outer hair cells and auditory neurons in the cochlea: a new pathophysiological mechanism in Menière's disease?

In Menière's disease, the increase of extracellular potassium concentration in the perilymph is thought to play a key role in determining the progressive loss of cochlear hair cells. In this paper, we describe a serum-free culture preparation of hair cells from 5 day-old rat and report the release by the cochlea, in response to an increase of extracellular potassium concentration, of a cytotoxic activity active on hair cells and auditory neurons. The toxic activity is associated with low molecular weight (less than 10,000 Dalton) molecule(s) as revealed by ultrafiltration. Morphological studies performed on the organ of Corti incubated during 24 h in the presence of the cochlea-derived toxic activity (CTA), show that this factor is toxic for hair cells and not for supporting or surrounding cells. The release of CTA occurs both in the spiral ganglion and in the organ of Corti. We suggest that this cochlea-derived toxic activity may play an important role in the pathophysiology of the hearing loss that occurs during the progression of Menière's disease.

Fatal contractions: ultrastructural and electromechanical changes in outer hair cells following transmembraneous electrical stimulation

A form of outer hair cell vulnerability was examined as related to cellular structure. Electrically-induced somatic displacements on the order of 500 nm (peak-to-peak) were accompanied by a superimposed slow tonic contraction in cell length. Shortening by as much as 22% of the initial cell length was seen to occur at rates of 100 nm/s. Such changes often resulted in increased detection thresholds as well as a reduction in asymmetry of the frequency-following motile response component. The tonic contractions were non-reversible. Solitary cells were recovered following transmembraneous electrical stimulation in order to examine possible structural changes associated with the contractions. In a number of well-preserved controls, multiple flattened non-fenestrated cisternae were found lining the cytoplasmic compartment. In contrast, cells which underwent tonic shortening invariably possessed fragmented membraneous cisternae resembling those typically seen in whole-organ preparations. The degree of fenestration corresponded to the extent of cell shortening. These findings suggest that the so-called 'fenestrated' subsurface cisternae, previously thought to be a normal anatomical component of the outer hair cell, may be the remnant of a highly organized structure in vivo. The potential role of the cisternal system as a force generating substrate is discussed.

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

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

Verification of a new test of endolymphatic hydrops

A new objective test of endolymphatic hydrops has been described previously. The test uses auditory brain-stem response (ABR) techniques to estimate the basilar membrane travelling wave velocity (TWV). The underlying hypothesis is that raised pressure in the scala media will lead to an increase in the stiffness of the basilar membrane and that this will give rise to a travelling wave velocity that is greater than normal. It was considered that verification of the technique could be obtained by carrying out the new test before and during a glycerol dehydration procedure. Patients who required this procedure for verification of the diagnosis or as a prognostic indicator for sacculotomy, underwent the following tests. Prior to dehydration an audiogram, the ABR TWV test and a blood sample (for plasma osmolality) were taken. The patient was then given the appropriate amount of glycerol mixed with lemon juice and laid on a bed in a test room. The audiogram and a blood sample were repeated every hour and the ABR TWV test was repeatedly carried out every twenty minutes between the other tests. Six of these procedures have been carried out. In five of them the 0.5 and 1 kHz average threshold improved by at least 10dB and in all those cases the ABR TWV test showed an abnormal pre-dehydration result which moved towards the normal value following dehydration. One patient gave ABR TWV results that were within normal limits before and during the procedure and gave an audiometric change of only 5dB.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional role of the olivo-cochlear bundle: a motor unit control system in the mammalian cochlea

A fiber optic lever is applied to the measurement of the motion of the basilar membrane motion in guinea pigs. In response to intense tones from either ear, the motion includes a substantial summating shift in the mean position in addition to a travelling wave originally described by von Békésy. His stroboscopic technique and most techniques used since have been concentrated upon measuring vibrations of the basilar membrane synchronous with the stimulus and have been insensitive to variations in the baseline position such as a summating component of motion analogous to the extracellular summating potential. In addition to the role of the outer hair cells in providing normal hearing sensitivity, they evidently play a role in regulating the mean position of the basilar membrane. For a fixed frequency, the polarity of the mean position varies systematically with sound level and place and summates with time since onset. Since these cells are the target cells for the olivocochlear bundle, homeostasis in the cochlea would appear to be linked efferent function and involve cochlear mechanics. The negative damping hypothesis asserts that hair cell activity is necessary for low thresholds. The results presented here demonstrate that OHC activity exists independent of neural thresholds. The discussion develops the concept that threshold losses are due to a mismatch of opposing tonic forces which normally maintain the mean position of the basilar membrane. Structure is examined in relation to function and the group of outer hair cells innervated by a single medial efferent neuron is identified as a motor unit. Implications of central control of individual motor units include peripheral involvement in selective attention tasks.

Transtympanic administration of aminoglycosides in patients with Menière's disease

Transtympanic administration of gentamicin was used to treat patients with unilateral Menière's disease and was stopped at the first signs of damage to cochlear and/or vestibular function. Twenty-eight patients were treated between 1974 and 1981 and 34 patients were managed between 1981 and 1987. In the two groups of patients, objective vertigo disappeared in 85%, hearing improved in 26%, aural fullness disappeared in 78%, while tinnitus ceased altogether in 51% and became less intense in 24%. Our findings show that this kind of treatment can be used after unsuccessful medical therapy and prior to any labyrinthine or endolymphatic sac surgery.

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

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

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

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

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

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

Fast motility of isolated mammalian auditory sensory cells

Auditory sensory cells (hair cells) are responsible for sound transduction in the cochlea of the inner ear. In the presence of a longitudinal a.c. field isolated living outer hair cells showed reversible motile responses. They followed the stimulus up to at least 1 kHz. Control experiments in the presence of cytochalasin B, phalloidin and dinitrophenol excluded actomyosin as a molecular basis of the high frequency motility. The results suggest, that outer hair cells might amplify sound-induced oscillations in the inner ear and thus increase sensitivity and frequency selectivity of hearing.

Osmotically induced motility of outer hair cells: implications for Menière's disease

Osmolarity changes in inner ear fluids have long been considered to be contributing factors to Menière's disease. Our present study demonstrates that small changes in the osmolarity of a surrounding in vitro medium induce fast contractions (hypo-osmotic solution) or elongations (hyperosmotic solution) in isolated outer hair cells of the guinea pig. These changes were reversible upon returning cells to iso-osmotic conditions. Up to five cycles of shape change could be sustained by these cells without obvious detriment to their morphology. These findings suggest that fluctuant changes in osmolarity of inner ear fluids can result in similar fluctuant changes of hair cell shape. Since the outer hair cells may control cochlear micromechanics and function by their active motility, osmotically induced abnormalities of cell dimensions and motility may contribute to the audiological manifestations of fluctuant hearing loss.

Motile responses in outer hair cells

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