Three-dimensional cultured ampullae from rats as a screening tool for vestibulotoxicity: Proof of concept using styrene

Numerous ototoxic drugs, such as some antibiotics and chemotherapeutics, are both cochleotoxic and vestibulotoxic (causing hearing loss and vestibular disorders). However, the impact of some industrial cochleotoxic compounds on the vestibular receptor, if any, remains unknown. As in vivo studies are long and expensive, there is considerable need for predictive and cost-effective in vitro models to test ototoxicity. Here, we present an organotypic model of cultured ampullae harvested from rat neonates. When cultured in a gelatinous matrix, ampulla explants form an enclosed compartment that progressively fills with a high-potassium (K+) endolymph-like fluid. Morphological analyses confirmed the presence of a number of cell types, sensory epithelium, secretory cells, and canalar cells. Treatments with inhibitors of potassium transporters demonstrated that the potassium homeostasis mechanisms were functional. To assess the potential of this model to reveal the toxic effects of chemicals, explants were exposed for either 2 or 72 h to styrene at a range of concentrations (0.5-1 mM). In the 2-h exposure condition, K+ concentration was significantly reduced, but ATP levels remained stable, and no histological damage was visible. After 72 h exposure, variations in K+ concentration were associated with histological damage and decreased ATP levels. This in vitro 3D neonatal rat ampulla model therefore represents a reliable and rapid means to assess the toxic properties of industrial compounds on this vestibular tissue, and can be used to investigate the specific underlying mechanisms.

Elucidating the acid-base mechanisms underlying otolith overgrowth in fish exposed to ocean acidification

Over a decade ago, ocean acidification (OA) exposure was reported to induce otolith overgrowth in teleost fish. This phenomenon was subsequently confirmed in multiple species; however, the underlying physiological causes remain unknown. Here, we report that splitnose rockfish (Sebastes diploproa) exposed to ~1600 μatm pCO₂(pH ~7.5) were able to fully regulated the pH of both blood and endolymph (the fluid that surrounds the otolith within the inner ear). However, while blood was regulated around pH 7.80, the endolymph was regulated around pH ~8.30. These different pH setpoints result in increased pCO₂diffusion into the endolymph, which in turn leads to proportional increases in endolymph [HCO₃^-] and [CO₃^2-]. Endolymph pH regulation despite the increased pCO₂suggests enhanced H⁺removal. However, a lack of differences in inner ear bulk and cell-specific Na⁺/K⁺-ATPase and vacuolar type H⁺-ATPase protein abundance localization pointed out to activation of preexisting ATPases, non-bicarbonate pH buffering, or both, as the mechanism for endolymph pH-regulation. These results provide the first direct evidence showcasing the acid-base chemistry of the endolymph of OA-exposed fish favors otolith overgrowth, and suggests that this phenomenon will be more pronounced in species that count with more robust blood and endolymph pH regulatory mechanisms.

A proteomic analysis of the statocyst endolymph in common cuttlefish (Sepia officinalis): an assessment of acoustic trauma after exposure to sound

Recent studies, both in laboratory and sea conditions, have demonstrated damage after sound exposure in the cephalopod statocyst sensory epithelium, which secretes endolymph protein. Here, the proteomic analysis of the endolymph was performed before and after sound exposure to assess the effects of exposure to low intensity, low frequency sounds on the statocyst endolymph of the Mediterranean common cuttlefish (Sepia officinalis), determining changes in the protein composition of the statocyst endolymph immediately and 24 h after sound exposure. Significant differences in protein expression were observed, especially 24 h after exposure. A total of 37 spots were significantly different in exposed specimens, 17 of which were mostly related to stress and cytoskeletal structure. Among the stress proteins eight spots corresponding to eight hemocyanin isoforms were under-expressed possible due to lower oxygen consumption. In addition, cytoskeletal proteins such as tubulin alpha chain and intermediate filament protein were also down-regulated after exposure. Thus, endolymph analysis in the context of acoustic stress allowed us to establish the effects at the proteome level and identify the proteins that are particularly sensitive to this type of trauma.

Acute blockade of inner ear marginal and dark cell K+ secretion: Effects on gravity receptor function

Specific pharmacological blockade of KCNQ (Kv7) channels with XE991 rapidly (within 20 min) and profoundly alters inner ear gravity receptor responses to head motion (Lee et al., 2017). We hypothesized that these effects were attributable to the suppression of K⁺ secretion following blockade of KCNQ1-KCNE1 channels in vestibular dark cells and marginal cells. To test this hypothesis, K⁺ secretion was independently inhibited by blocking the Na⁺-K⁺-2Cl^- cotransporter (NKCC1, Slc12a2) rather than KCNQ1-KCNE1 channels. Acute blockade of NKCC1 with ethacrynic acid (40 mg/kg) eliminated auditory responses (ABRs) within approximately 70 min of injection, but had no effect on vestibular gravity receptor function (VsEPs) over a period of 2 h in the same animals. These findings show that, vestibular gravity receptors are highly resistant to acute disruption of endolymph secretion unlike the auditory system. Based on this we argue that acute suppression of K⁺ secretion alone does not likely account for the rapid profound effects of XE991 on gravity receptors. Instead the effects of XE991 likely require additional action at KCNQ channels located within the sensory epithelium itself.

The gastric H,K-ATPase in stria vascularis contributes to pH regulation of cochlear endolymph but not to K secretion

BACKGROUND

Disturbance of acid-base balance in the inner ear is known to be associated with hearing loss in a number of conditions including genetic mutations and pharmacologic interventions. Several previous physiologic and immunohistochemical observations lead to proposals of the involvement of acid-base transporters in stria vascularis.

RESULTS

We directly measured acid flux in vitro from the apical side of isolated stria vascularis from adult C57Bl/6 mice with a novel constant-perfusion pH-selective self-referencing probe. Acid efflux that depended on metabolism and ion transport was observed from the apical side of stria vascularis. The acid flux was decreased to about 40 % of control by removal of the metabolic substrate (glucose-free) and by inhibition of the sodium pump (ouabain). The flux was also decreased a) by inhibition of Na,H-exchangers by amiloride, dimethylamiloride (DMA), S3226 and Hoe694, b) by inhibition of Na,2Cl,K-cotransporter (NKCC1) by bumetanide, and c) by the likely inhibition of HCO3/anion exchange by DIDS. By contrast, the acid flux was increased by inhibition of gastric H,K-ATPase (SCH28080) but was not affected by an inhibitor of vH-ATPase (bafilomycin).  K flux from stria vascularis was reduced less than 5 % by SCH28080.

CONCLUSIONS

These observations suggest that stria vascularis may be an important site of control of cochlear acid-base balance and demonstrate a functional role of several acid-base transporters in stria vascularis, including basolateral H,K-ATPase and apical Na,H-exchange. Previous suggestions that H secretion is mediated by an apical vH-ATPase and that basolateral H,K-ATPase contributes importantly to K secretion in stria vascularis are not supported. These results advance our understanding of inner ear acid-base balance and provide a stronger basis to interpret the etiology of genetic and pharmacologic cochlear dysfunctions that are influenced by endolymphatic pH.

A three-dimensional analysis of the endolymph drainage system in Ménière disease

OBJECTIVES/HYPOTHESIS

To measure the volume of the endolymph drainage system in temporal bone specimens with Ménière disease, as compared with specimens with endolymphatic hydrops without vestibular symptoms and with nondiseased specimens STUDY DESIGN: Comparative human temporal bone analysis.

METHODS

We generated three-dimensional models of the vestibular aqueduct, endolymphatic sinus and duct, and intratemporal portion of the endolymphatic sac and calculated the volume of those structures. We also measured the internal and external aperture of the vestibular aqueduct, as well as the opening (if present) of the utriculoendolymphatic (Bast's) valve and compared the measurements in our three study groups.

RESULTS

The volume of the vestibular aqueduct and of the endolymphatic sinus, duct, and intratemporal endolymphatic sac was significantly lower in the Ménière disease group than in the endolymphatic hydrops group (P <.05). The external aperture of the vestibular aqueduct was also smaller in the Ménière disease group. Bast's valve was open only in some specimens in the Ménière disease group.

CONCLUSIONS

In temporal bones with Ménière disease, the volume of the vestibular aqueduct, endolymphatic duct, and intratemporal endolymphatic sac was lower, and the external aperture of the vestibular aqueduct was smaller as compared with bones from donors who had endolymphatic hydrops without vestibular symptoms and with nondiseased bones. The open status of the Bast's valve in the Ménière disease group could be secondary to higher retrograde endolymph pressures caused by smaller drainage systems. These anatomic findings could correlate with the reason that some patients with hydrops develop clinical symptoms, whereas others do not.

LEVEL OF EVIDENCE

N/A Laryngoscope, 127:E170-E175, 2017.

Regulation of the perilymphatic-endolymphatic water shunt in the cochlea by membrane translocation of aquaporin-5

Volume homeostasis of the cochlear endolymph depends on radial and longitudinal endolymph movements (LEMs). LEMs measured in vivo have been exclusively recognized under physiologically challenging conditions, such as experimentally induced alterations of perilymph osmolarity or endolymph volume. The regulatory mechanisms that adjust LEMs to the physiological requirements of endolymph volume homeostasis remain unknown. Here, we describe the formation of an aquaporin (AQP)-based "water shunt" during the postnatal development of the mouse cochlea and its regulation by different triggers. The final complementary expression pattern of AQP5 (apical membrane) and AQP4 (basolateral membrane) in outer sulcus cells (OSCs) of the cochlear apex is acquired at the onset of hearing function (postnatal day (p)8-p12). In vitro, hyperosmolar perfusion of the perilymphatic fluid spaces or the administration of the muscarinic agonist pilocarpine in cochlear explants (p14) induced the translocation of AQP5 channel proteins into the apical membranes of OSCs. AQP5 membrane translocation was blocked by the muscarinic antagonist atropine. The muscarinic M3 acetylcholine (ACh) receptor (M3R) was identified in murine OSCs via mRNA expression, immunolabeling, and in vitro binding studies using an M3R-specific fluorescent ligand. Finally, the water shunt elements AQP4, AQP5, and M3R were also demonstrated in OSCs of the human cochlea. The regulation of the AQP4/AQP5 water shunt in OSCs of the cochlear apex provides a molecular basis for regulated endolymphatic volume homeostasis. Moreover, its dysregulation or disruption may have pathophysiologic implications for clinical conditions related to endolymphatic hydrops, such as Ménière's disease.

Semi-quantitative vs. volumetric determination of endolymphatic space in Menière's disease using endolymphatic hydrops 3T-HR-MRI after intravenous gadolinium injection

Magnetic resonance imaging enhances the clinical diagnosis of Menière's disease. This is accomplished by in vivo detection of endolymphatic hydrops, which are graded using different semi-quantitative grading systems. We evaluated an established, semi-quantitative endolymphatic hydrops score and with a quantitative method for volumetric assessment of the endolymphatic size. 11 patients with Menière's disease and 2 healthy subjects underwent high resolution endolymphatic hydrops 3 Tesla MRI with highly T2 weighted FLAIR and T2DRIVE sequences. The degree of endolymphatic hydrops was rated semi-quantitatively and compared to the results of 3D-volumetry. Moreover, the grade of endolymphatic hydrops was correlated with pure tone audiometry. Semi-quantitative grading and volumetric evaluation of the endolymphatic hydrops are in accordance (r = 0.92) and the grade of endolymphatic hydrops correlates with pure tone audiometry. Patients with a sickness duration of ≥ 30 months showed a significant higher total labyrinth fluid volume (p = 0.03). Fast, semi-quantitative evaluation of endolymphatic hydrops is highly reliable compared to quantitative/volumetric assessment. Endolymphatic space is significantly higher in patients with longer sickness duration.

SLC26A4 targeted to the endolymphatic sac rescues hearing and balance in Slc26a4 mutant mice

Mutations of SLC26A4 are a common cause of human hearing loss associated with enlargement of the vestibular aqueduct. SLC26A4 encodes pendrin, an anion exchanger expressed in a variety of epithelial cells in the cochlea, the vestibular labyrinth and the endolymphatic sac. Slc26a4 (Δ/Δ) mice are devoid of pendrin and develop a severe enlargement of the membranous labyrinth, fail to acquire hearing and balance, and thereby provide a model for the human phenotype. Here, we generated a transgenic mouse line that expresses human SLC26A4 controlled by the promoter of ATP6V1B1. Crossing this transgene into the Slc26a4 (Δ/Δ) line restored protein expression of pendrin in the endolymphatic sac without inducing detectable expression in the cochlea or the vestibular sensory organs. The transgene prevented abnormal enlargement of the membranous labyrinth, restored a normal endocochlear potential, normal pH gradients between endolymph and perilymph in the cochlea, normal otoconia formation in the vestibular labyrinth and normal sensory functions of hearing and balance. Our study demonstrates that restoration of pendrin to the endolymphatic sac is sufficient to restore normal inner ear function. This finding in conjunction with our previous report that pendrin expression is required for embryonic development but not for the maintenance of hearing opens the prospect that a spatially and temporally limited therapy will restore normal hearing in human patients carrying a variety of mutations of SLC26A4.

Blockage pattern of longitudinal flow in Meniere's disease

CONCLUSION

In the present study, classification of the patterns of 3D CT images of the ductus reuniens (reuniting duct) (RD), saccular duct (SD), and endolymphatic sinus (ES) gave more precise information for assessing the pathological condition of Meniere's disease (MD) than our previous study.

OBJECTIVE

This study attempted to provide more detailed information on MD by classifying the patterns of 3D CT images of the RD, SD, and ES in patients with MD.

METHODS

We examined the ears of 62 patients with definitely diagnosed unilateral MD based on the criteria of the Committee on Hearing and Equilibrium of the American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS) using 3D CT. The 3D CT images of bony grooves of RD, SD, and ES (BRD, BSD, and BES) were classified into patterns according to aspects of their patency.

RESULTS

BRD could be classified into six types by assessing their patency defined using the criteria in this study. In the ears on the affected side of patients with MD, the BRD, BSD, and BES lost continuity in 3D CT images along their bony routes and were significantly different from normal healthy ears (p < 0.01). There were no significant differences among each stage of MD in the distributions of BRD and BES except for BSD.

Endolymphatic Na⁺ and K⁺ concentrations during cochlear growth and enlargement in mice lacking Slc26a4/pendrin

Slc26a4 (Δ/Δ) mice are deaf, develop an enlarged membranous labyrinth, and thereby largely resemble the human phenotype where mutations of SLC26A4 cause an enlarged vestibular aqueduct and sensorineural hearing loss. The enlargement is likely caused by abnormal ion and fluid transport during the time of embryonic development, however, neither the mechanisms of ion transport nor the ionic composition of the luminal fluid during this time of development are known. Here we determine the ionic composition of inner ear fluids at the time at which the enlargement develops and the onset of expression of selected ion transporters. Concentrations of Na(+) and K(+) were measured with double-barreled ion-selective electrodes in the cochlea and the endolymphatic sac of Slc26a4 (Δ/+), which develop normal hearing, and of Slc26a4 (Δ/Δ) mice, which fail to develop hearing. The expression of specific ion transporters was examined by quantitative RT-PCR and immunohistochemistry. High Na(+) (∼141 mM) and low K(+) concentrations (∼11 mM) were found at embryonic day (E) 16.5 in cochlear endolymph of Slc26a4 (Δ/+) and Slc26a4 (Δ/Δ) mice. Shortly before birth the K(+) concentration began to rise. Immediately after birth (postnatal day 0), the Na(+) and K(+) concentrations in cochlear endolymph were each ∼80 mM. In Slc26a4 (Δ/Δ) mice, the rise in the K(+) concentration occurred with a ∼3 day delay. K(+) concentrations were also found to be low (∼15 mM) in the embryonic endolymphatic sac. The onset of expression of the K(+) channel KCNQ1 and the Na(+)/2Cl(-)/K(+) cotransporter SLC12A2 occurred in the cochlea at E19.5 in Slc26a4 (Δ/+) and Slc26a4 (Δ/Δ) mice. These data demonstrate that endolymph, at the time at which the enlargement develops, is a Na(+)-rich fluid, which transitions into a K(+)-rich fluid before birth. The data suggest that the endolymphatic enlargement caused by a loss of Slc26a4 is a consequence of disrupted Na(+) transport.

Systemic aminoglycosides are trafficked via endolymph into cochlear hair cells

Aminoglycoside antibiotics rapidly enter and kill cochlear hair cells via apical mechanoelectrical transduction (MET) channels in vitro. In vivo, it remains unknown whether systemically-administered aminoglycosides cross the blood-labyrinth barrier into endolymph and enter hair cells. Here we show, for the first time, that systemic aminoglycosides are trafficked across the blood-endolymph barrier and preferentially enter hair cells across their apical membranes. This trafficking route is predominant compared to uptake via hair cell basolateral membranes during perilymph infusion.

Failure of fluid absorption in the endolymphatic sac initiates cochlear enlargement that leads to deafness in mice lacking pendrin expression

Mutations of SLC26A4 are among the most prevalent causes of hereditary deafness. Deafness in the corresponding mouse model, Slc26a4^−/−, results from an abnormally enlarged cochlear lumen. The goal of this study was to determine whether the cochlear enlargement originates with defective cochlear fluid transport or with a malfunction of fluid transport in the connected compartments, which are the vestibular labyrinth and the endolymphatic sac. Embryonic inner ears from Slc26a4^+/− and Slc26a4^−/− mice were examined by confocal microscopy ex vivo or after 2 days of organ culture. Culture allowed observations of intact, ligated or partially resected inner ears. Cochlear lumen formation was found to begin at the base of the cochlea between embryonic day (E) 13.5 and 14.5. Enlargement was immediately evident in Slc26a4^−/− compared to Slc26a4^+/− mice. In Slc26a4^+/− and Slc26a4^−/− mice, separation of the cochlea from the vestibular labyrinth by ligation at E14.5 resulted in a reduced cochlear lumen. Resection of the endolymphatic sacs at E14.5 led to an enlarged cochlear lumen in Slc26a4^+/− mice but caused no further enlargement of the already enlarged cochlear lumen in Slc26a4^−/− mice. Ligation or resection performed later, at E17.5, did not alter the cochlea lumen. In conclusion, the data suggest that cochlear lumen formation is initiated by fluid secretion in the vestibular labyrinth and temporarily controlled by fluid absorption in the endolymphatic sac. Failure of fluid absorption in the endolymphatic sac due to lack of Slc26a4 expression appears to initiate cochlear enlargement in mice, and possibly humans, lacking functional Slc26a4 expression.

Distribution of PLGA nanoparticles in chinchilla cochleae

OBJECTIVES

To study the distribution of polylactic/glycolic acid-encapsulated iron oxide nanoparticles (PLGA-NPs) in chinchilla cochleae after application on the round window membrane (RWM).

STUDY DESIGN AND SETTING

Six chinchillas (12 ears) were equally divided into controls (no treatments) and experimentals (PLGA-NP with or without magnetic exposure). After 40 minutes of PLGA-NP placement on the RWM, perilymph was withdrawn from the scala tympani. The RWM and cochleae were fixed with 2.5% glutaraldehyde and processed for transmission electron microscopy.

RESULTS

Nanoparticles were found in cochleae with or without exposure to magnet forces appearing in the RWM, perilymph, endolymph, and multiple locations in the organ of Corti. Electron energy loss spectroscopy confirmed iron elements in nanoparticles.

CONCLUSION

The nanoparticles were distributed throughout the inner ear after application on the chinchilla RWM, with and without magnetic forces.

SIGNIFICANCE

PLGA-NP applied to the RWM may have potential for sustained therapy to the inner ear.

Endothelin-1 inhibits outward potassium currents in mouse outer sulcus cells

The outer sulcus cells are epithelial cells covering the luminal side of spiral sulcus of cochlea. It has been suggested that outer sulcus cells contribute to cation absorption from the lumen of the cochlea. We investigated the electrical properties and the effects of endothelin-1 (ET-1) on the outward potassium currents in mouse outer sulcus cells using a whole-cell patch clamp technique. The cell capacitance was 3.16+/-0.66 pF (n =35) and the resting membrane potential was -98.4+/-1.6 mV (n=6) in extracellular fluid bath solution. The outward K+ currents were activated by depolarizing pulses more positive than -60 mV, and was sensitive to TEA (10 mM). Tail current analysis revealed that it was primarily K+ selective. Application of ET-1 caused a decrease of outward potassium currents within seconds, whereas treatment with BQ123, a competitive inhibitor of the ET type-A receptor, counteracted the inhibitory effect of ET-1. These results suggest that ET-1 inhibits outward potassium currents through the activation of ET type-A receptor. ET-1 may play an important role in maintaining the ionic homeostasis of endolymph.

Estrogen-Related Receptor β/NR3B2 Controls Epithelial Cell Fate and Endolymph Production by the Stria Vascularis

In the mammalian inner ear, endolymph is produced and resorbed by a complex series of epithelia. We show here that estrogen-related receptor beta (ERR-beta; NR3B2), an orphan nuclear receptor, is specifically expressed in and controls the development of the endolymph-producing cells of the inner ear: the strial marginal cells in the cochlea and the vestibular dark cells in the ampulla and utricle. Nr3b2(-/-) strial marginal cells fail to express multiple ion channel and transporter genes, and they show a partial transformation toward the fate of the immediately adjacent Pendrin-expressing epithelial cells. In genetically mosaic mice, Nr3b2(-/-) strial marginal cells produce secondary alterations in gene expression in the underlying intermediate cells and a local loss of strial capillaries. A systematic comparison of transcripts in the WT versus Nr3b2(-/-) stria vascularis has identified a set of genes that is likely to play a role in the development and/or function of endolymph-producing epithelia.

Changes in P2Y4 receptor expression in rat cochlear outer sulcus cells during development

Extracellular adenosine triphosphate (ATP) released from cellular sources plays an important role in variety of the cochlear physiologic processes. The primary purinergic receptor subtype in the cochlea is the P2X2 receptor, which is a subtype of P2X receptor. This receptor appears to mediate a protective decrease in the electrical driving force in response to acoustic overstimulation. Outer sulcus cells (OSCs) in the cochlear lateral wall appear to maintain an adequate K+ concentration in the cochlear endolymph in response to varying intensities of auditory stimulation. However, little is known about developing OSCs. The purpose of this study was to investigate subtypes of purinergic receptors in developing rat OSCs using a voltage-sensitive vibrating probe. Results showed that only two P2 receptors (P2Y4 and P2X2) contributed to the regulation of short circuit currents in neonatal OSCs. ATP increased cation absorption via apical nonselective cation channels after activating P2Y4 receptors in early neonatal OSCs. P2Y4 expression rapidly declined postnatally and reached near adult levels on postnatal day 14. P2X2 was co-expressed with P2Y4 in early neonatal OSCs. Temporal changes in P2Y4 during OSC development might be involved in the establishment of the endolymphatic ion composition needed for normal auditory transduction and/or specific cellular differentiation.

Dynamic expression of FXYD6 in the inner ear suggests a role of the protein in endolymph homeostasis and neuronal activity

A key protein in the production and in the maintenance of the endocochlear potential is the Na,K-ATPase. Previously, we have shown that FXYD6 is a modulator of the Na,K-ATPase expressed in the inner ear (Delprat et al. [2007] J Biol Chem 282:7450-7456). To investigate the potential role of FXYD6 in inner ear function, we studied the developmental expression of FXYD6. Reverse transcriptase-polymerase chain reaction analysis demonstrates that FXYD6 is present as two splice variants. Both variants coimmunoprecipitate with Na,K-ATPase after expression in Xenopus oocytes. Immunohistochemistry of the cochlea (from birth to postnatal day 30) shows that FXYD6 is expressed in several epithelial cells important for endolymph homeostasis. Marked similarities were found in the developmental expression patterns of FXYD6 and Na,K-ATPase, suggesting functional cooperation between the two proteins in the generation and maintenance of the endocochlear potential and ion composition of the endolymph.

[Laser destruction of labyrinthine receptors as a treatment for benign paroxysmal postural vertigo and otolith symptoms]

Outcomes of surgical laser treatment are presented for 15 patients with resistant to conservative therapy benign paroxysmal postural vertigo (BPPV) and otolith-related disorders. Nine patients with Meniere's disease with BPPV (n=3) and otoliths (in all the patients) were exposed to laser impulses on the bony wall of the horizontal semicircular canal. The impulse was directed in the lumen of the canal in 6 patients with BPPV and otolith symptoms. In Meniere's patients vertigo stopped, hearing was at the preoperative level. Neither threy had BPPV nor otolith problems. The same results were obtained in 6 patients with the other diseases. Vestibular excitability of all surgically treated patients decreased to the fifth degree.

Effects of CO2/HCO3− in Perilymph on the Endocochlear Potential in Guinea Pigs

The effect of CO(2)/HCO(3)(-) on the endocochlear potential (EP) was examined by using both ion-selective and conventional microelectrodes and the endolymphatic or perilymphatic perfusion technique. The main findings were as follows: (i) A decrease in the EP from approximately +75 to approximately +35 mV was produced by perilymphatic perfusion with CO(2)/HCO(3)(-)-free solution, which decrease was accompanied by an increase in the endolymphatic pH (DeltapH(e), approximately 0.4). (ii) Perilymphatic perfusion with a solution containing 20 mM NH(4)Cl produced a decrease in the EP (DeltaEP, approximately 20 mV) with an increase in the pH(e) (DeltapH(e), approximately 0.2), whereas switching the perfusion solution from the NH(4)Cl solution to a 5% CO(2)/25 mM HCO(3)(-) solution produced a gradual increase in the EP to the control level with the concomitant recovery of the pH(e). (iii) The perfusion with a solution of high or low HCO(3)(-) with a constant CO(2) level within 10 min produced no significant changes in the EP. (iv) Perfusion of the perilymph with 10 microg/ml nifedipine suppressed the transient asphyxia-induced decrease in EP slightly, but not significantly. (v) By contrast, the administration of 1 microg/ml nifedipine via the endolymph inhibited significantly the reduction in the EP induced by transient asphyxia or perilymphatic perfusion with CO(2)/HCO(3)(-)-free or 20 mM NH(4)Cl solution. These findings suggest that the effect of CO(2) removal from perilymphatic perfusion solution on the EP may be mediated by an increase in cytosolic Ca(2+) concentration induced by an elevation of cytosolic pH in endolymphatic surface cells.

Large Na+ influx and high Na+, K+–ATPase activity in mitochondria-rich epithelial cells of the inner ear endolymphatic sac

Fluid in the mammalian endolymphatic sac (ES) is connected to the endolymph in the cochlea and the vestibule. Since the dominant ion in the ES is Na(+), it has been postulated that Na(+) transport is essential for regulating the endolymph pressure. This study focused on the cellular mechanism of Na(+) transport in ES epithelial cells. To evaluate the Na(+) transport capability of the ES epithelial cells, changes in intracellular Na(+) concentration ([Na(+)](i)) of individual ES cells were measured with sodium-binding benzofurzan isophthalate in a freshly dissected ES sheet and in dissociated ES cells in response to either the K(+)-free or ouabain-containing solution. Analysis of the [Na(+)](i) changes by the Na(+) load and mitochondrial staining with rhodamine 123 showed that the ES cells were classified into two groups; one exhibited an intensive [Na(+)](i) increase, higher Na(+), K(+)-ATPase activity, and intensive mitochondrial staining (mitochondria-rich cells), and the other exhibited a moderate [Na(+)](i) increase, lower Na(+), K(+)-ATPase activity, and moderate mitochondrial staining (filament-rich cells). These results suggest that mitochondria-rich ES epithelial cells (ca. 30% of ES cells) endowed with high Na(+) permeability and Na(+), K(+)-ATPase activity potentially contribute to the transport of Na(+) outside of the endolymphatic sac.

Computational model of vectorial potassium transport by cochlear marginal cells and vestibular dark cells

Cochlear marginal cells and vestibular dark cells transport potassium into the inner ear endolymph, a potassium-rich fluid, the homeostasis of which is essential for hearing and balance. We have formulated an integrated mathematical model of ion transport across these epithelia that incorporates the biophysical properties of the major ion transporters and channels located in the apical and basolateral membranes of the constituent cells. The model is constructed for both open- and short-circuit situations to test the extremes of functional capacity of the epithelium and predicts the steady-state voltages, ion concentrations, and transepithelial currents as a function of various transporter and channel densities. We validate the model by establishing that the cells are capable of vectorial ion transport consistent with several experimental measurements. The model indicates that cochlear marginal cells do not make a significant direct contribution to the endocochlear potential and illustrates how changes to the activity of specific transport proteins lead to reduced K(+) flux across the marginal and dark cell layers. In particular, we investigate the mechanisms of loop diuretic ototoxicity and diseases with hearing loss in which K(+) and Cl(-) transport are compromised, such as Jervell and Lange-Nielsen syndrome and Bartter syndrome, type IV, respectively. Such simulations demonstrate the utility of compartmental modeling in investigating the role of ion homeostasis in inner ear physiology and pathology.

Functional IsK/KvLQT1 potassium channel in a new corticosteroid-sensitive cell line derived from the inner ear

Endolymph, a high K(+)/low Na(+) fluid, participates in mechanoelectrical transduction in inner ear. Molecular mechanisms controlling endolymph ion homeostasis remain elusive, hampered by the lack of appropriate cellular models. We established an inner ear cell line by targeted oncogenesis. The expression of SV40 T antigen was driven by the proximal promoter of the human mineralocorticoid receptor (MR) gene, a receptor expressed in the inner ear. The EC5v cell line, microdissected from the semicircular canal, grew as a monolayer of immortalized epithelial cells forming domes. EC5v cells exhibited on filters of high transepithelial resistance and promoted K(+) secretion and Na(+) absorption. Functional MR and the 11beta-hydroxysteroid dehydrogenase type 2, a key enzyme responsible for MR selectivity were identified. Expression of the epithelial sodium channel and serum glucocorticoid-regulated kinase 1 was shown to be up-regulated by aldosterone, indicating that EC5v represents a novel corticosteroid-sensitive cell line. Ionic measurements and (86)Rb transport assays revealed an apical secretion of K(+) at least in part through the I(sK)/KvLQT1 potassium channel under standard culture conditions. However, when cells were exposed to high apically K(+)/low Na(+) fluid, mimicking endolymph exposure, I(sK)/KvLQT1 actually functioned as a strict apical to basolateral K(+) channel inhibited by clofilium. Quantitative reverse transcriptase-PCR further demonstrated that expression of KvLQT1 but not of I(sK) was down-regulated by high K(+) concentration. This first vestibular cellular model thus constitutes a valuable system to further investigate the molecular mechanisms controlling ionic transports in the inner ear and the pathophysiological consequences of their dysfunctions in vertigo and hearing loss.

Brain and inner-ear fluid homeostasis, cochleovestibular-type tinnitus, and secondary endolymphatic hydrops

Secondary endolymphatic hydrops (SEH) has clinically been found to have a significant incidence of occurrence in patients with subjective idiopathic tinnitus (SIT) of a severe disabling type. The diagnosis is made clinically and has been established by integration in a medical audiological tinnitus patient protocol of the clinical history with results of electrodiagnostic cochleovestibular testing that fulfill the diagnostic criteria of inner-ear disease consistent with Ménière's disease. SEH is hypothesized to be a factor, not an etiology, influencing the clinical course of SIT. Alterations over time (i.e., delay in the homeostatic mechanisms in normnal function of the fluid compartments of the inner-ear perilymph, endolymph, or brain cerebrospinal fluid) result in endolymphatic hydrops and interference in normal function of the inner ear, with resultant inner-ear complaints that can be highlighted by tinnitus rather than by vertigo. The endolymphatic hydrops may be either localized or diffuse within the cochlear or vestibular labyrinth. The etiologies and mechanisms of cochleovestibular-type tinnitus are multiple and are influenced by the SEH. Classically, the tetrad of symptoms--episodic vertigo, fluctuating sensorineural hearing loss, tinnitus, and ear blockage--associated with the histopathological correlate endolymphatic hydrops has been diagnosed as Ménière's disease. Specifically, key etiological agents that have been identified as playing a role in the clinical course of tinnitus (e.g., noise exposure, stress) may serve as "triggers" or stressors (or both), resulting in interference in normal biochemical and physiological function of sensorineural structures in the inner ear or in neural structures in the brain. In both conditions, the alterations over time (i.e., delay) in the clinical manifestation of the tetrad of symptoms of inner-ear dysfunction, when highlighted by SIT rather than vertigo, otherwise fulfill the criteria for diagnosing SEH. The chief complaint of SIT, when presenting as one of the tetrad of inner-ear symptoms and otherwise diagnosed as Ménière's disease, has also been associated clinically with perfusion asymmetries in brain, identified by nuclear medicine brain imaging (single-photon emission computed tomography [SPECT] of brain), and reflects an interference in homeostasis in the blood-brain labyrinth or blood-brain barriers, with a resulting SEH. The medical significance of the SIT in some patients may be a gradual, progressive sensorineural hearing loss. The inclusion of SPECT of brain in SIT patients demonstrates a global approach for improving the accuracy of diagnosing the SIT symptom, for focusing on the contribution of central nervous system dysfunction to the development of SEH, and for understanding and influencing the clinical course of SIT.

New insights into the role of pendrin (SLC26A4) in inner ear fluid homeostasis

For over 100 years after the first description of the disorder, the molecular pathology underlying the deafness and thyroid pathology in Pendred syndrome (PS) remained unknown. In 1997, early progress towards understanding the molecular basis of the disorder was made when we identified the PS gene and found it to belong to the SLC26 family of anion transporters. The realization that an anion transporter was responsible for these clinical features soon highlighted a potential role for pendrin in thyroid hormone biosynthesis. The role of pendrin in deafness, however, remained unclear. Our determination of its expression pattern in the inner ear along with the development of a mouse with a targeted disruption of the Slc26a4 gene has revealed that Slc26a4 is expressed in areas of the endolymphatic compartment known to play a role in endolymph reabsorption and that absence of this protein leads to a profound prenatal endolymphatic hydrops and destruction of many of the epithelial cells surrounding the scala media. The precise mechanisms underlying endolymph reabsorption in the inner ear are not yet known; these studies, however, provide some of the groundwork for allowing the future delineation of these processes.

Contribution of Endolymphatic Fluid Shift to Caloric Response in Plugged Semicircular Canals

The purpose of this study was to clarify the role of endolymphatic fluid shift in caloric response, using frog posterior semicircular canals (PSCs). PSCs were sutured using 10-0 nylon thread and were used as a model of canal plugging. Compound action potentials (CAPs) of the PSC nerve evoked by a cooling stimulus were recorded. The CAPs after suturing the PSCs were found to be greater than those before suturing. This indicates that the fluid shift effect increases after canal suturing. Additionally, we present a clinical case in which caloric nystagmus was observed after lateral canal plugging. In this case MRI revealed the fluid space from the plugged portion toward the ampulla to be intact. There was another case with lateral canal plugging that showed the same findings on MRI. The above findings support the hypothesis that fluid shift is responsible for the caloric response without the convective flow of endolymph in the plugged canal.

Effects of lithium on endolymph homeostasis and experimentally induced endolymphatic hydrops

There is evidence to suggest that water homeostasis in the inner ear is regulated via the vasopressin (VP)-aquaporin 2 (AQP2) system in the same fashion as in the kidney. The VP-AQP2 system in the kidney is well known to be inhibited by lithium, resulting in polyuria due to a decrease in reabsorption of water in the collecting duct of the kidney. Therefore, lithium is also likely to inhibit the VP-AQP2 system in the inner ear, and consequently exert some influence on inner ear fluid homeostasis. In this study, we investigated the effects of lithium on AQP2 expression in the rat inner ear, and on the cochlear fluid volume in hydropic ears of guinea pigs. A quantitative PCR study revealed that lithium reduced AQP2 mRNA expression in the cochlea and endolymphatic sac. Lithium application also decreased the immunoreactivity of AQP2 in the cochlea and endolymphatic sac. In a morphological study, lithium intake significantly reduced endolymphatic hydrops dose-dependently. These results indicate that lithium acts on the VP-AQP2 system in the inner ear, consequently producing a dehydratic effect on the endolymphatic compartment.

Endolymphatic potassium of the chicken vestibule during embryonic development

The endolymph fills the lumen of the inner ear membranous labyrinth. Its ionic composition is unique in vertebrates as an extracellular fluid for its high-K(+)/low-Na(+) concentration. The endolymph is actively secreted by specialized cells located in the vestibular and cochlear epithelia. We have investigated the early phases of endolymph secretion by measuring the endolymphatic K(+) concentration in the chicken vestibular system during pre-hatching development. Measurements were done by inserting K(+)-selective microelectrodes in chicken embryo ampullae dissected at different developmental stages from embryonic day 9 up to embryonic day 21 (day of hatching). We found that the K(+) concentration is low (<10mM/L) up to embryonic day 11, afterward it increases steeply to reach a plateau level of about 140 mM/L at embryonic day 19--21. We have developed a short-term in vitro model of endolymph secretion by culturing vestibular ampullae dissected from embryonic day 11 chicken embryos for a few days. The preparation reproduced a double compartment system where the luminal K(+) concentration increased along with the days of culturing. This model could be important for (1) investigating the development of cellular mechanisms contributing to endolymph homeostasis and (2) testing compounds that influence those mechanisms.

Low endolymph calcium concentrations in deafwaddler2J mice suggest that PMCA2 contributes to endolymph calcium maintenance

In vertebrates, transduction of sound into an electrochemical signal is carried out by hair cells that rely on calcium to perform specialized functions. The apical surfaces of hair cells are surrounded by endolymphatic fluid containing calcium at concentrations that must be maintained by active transport. The mechanism of this transport is unknown, but an ATP-dependent pump is believed to participate. Mutation of the Atp2b2 gene that encodes plasma membrane calcium ATPase type 2 (PMCA2) produces the deaf, ataxic mouse: deafwaddler2J (dfw2J). We hypothesized that PMCA2 might transport calcium into the endolymph and that dfw2J mice would have low endolymph calcium concentrations, possibly contributing to their deafness and ataxia. First, using immunocytochemistry, we demonstrated that PMCA2 is present in control mice inner and outer hair cell stereocilia where it could pump calcium into the endolymph and that PMCA2 is absent in dfw2J stereocilia. Second, using an aspirating microelectrode and calcium-sensitive fluorescent dye, we found that dfw2J mice endolymph calcium concentrations are significantly lower than those of control mice. These findings suggest that PMCA2, located in hair cell stereocilia, contributes significantly to endolymph calcium maintenance.

Acute endolymphatic hydrops generated by exposure of the ear to nontraumatic low-frequency tones

Low-frequency sounds presented at high nontraumatizing levels induce temporary hyperacusis in humans and animals. One explanation of this finding is that the basilar membrane operating point may be disturbed by an endolymph volume change. This possibility was investigated using volume and flow markers iontophoresed into the endolymphatic space of guinea pigs. Marker concentrations were measured with ion-selective microelectrodes placed apically and basally to the iontophoresis site during exposure of the ear to low-frequency tones. Concentration changes were interpreted quantitatively using a finite-element model of the endolymphatic space that allowed changes of endolymph cross-sectional area and flow to be derived. Stimulation with a 200 Hz tone at 115 dB SPL for 3 min produced marker concentration changes consistent with the induction of transient endolymphatic hydrops and a basally directed displacement of endolymph. Endocochlear potentials were greater than normal after the exposure when hydrops was present. During identical tone exposures of animals without marker, we found that action potential (AP) threshold changes and endolymph potassium changes associated with the hydropic state were small. Marker concentration changes were compared with changes in endocochlear potential and AP thresholds for a range of exposure frequencies and levels. AP hypersensitivity occurred with 200 Hz exposure levels below those inducing endolymph volume disturbances. Endolymph volume changes are thought to be the result of, rather than the cause of, changes in operating point of the cochlear transducer. The observations that auditory threshold and endolymph potassium changes are minimal under conditions where substantial endolymphatic hydrops is present is relevant to our understanding of the hearing loss in patients with Meniere's disease.

Effect of Gravity on the Caloric Stimulation of the Inner Ear

Robert Barany won the 1914 Nobel Prize in medicine for his convection hypothesis for caloric stimulation. Microgravity caloric tests aboard the 1983 SpaceLab 1 mission produced nystagmus results that contradicted the basic premise of Barany's convection theory. In this paper, we present a fluid structural analysis of the caloric stimulation of the lateral semicircular canal. Direct numerical simulations indicate that on earth, natural convection is the dominant mechanism for endolymphatic flow. However, in the microgravity environment of orbiting spacecraft, where buoyancy effects are mitigated, an expansive convection becomes the sole mechanism for producing endolymph motion and cupular displacement. Transient 1 g and microgravity case studies are presented to delineate the different dynamic behaviors of the 1 g and microgravity endolymphatic flows. The associated fluid-structural interactions are also analyzed based on the time evolution of cupular displacements.

Deafness in Claudin 11-null mice reveals the critical contribution of basal cell tight junctions to stria vascularis function

Generation of a strong electrical potential in the cochlea is uniquely mammalian and may reflect recent evolutionary advances in cellular voltage-dependent amplifiers. This endocochlear potential is hypothesized to dramatically improve hearing sensitivity, a concept that is difficult to explore experimentally, because manipulating cochlear function frequently causes rapid degenerative changes early in development. Here, we examine the deafness phenotype in adult Claudin 11-null mice, which lack the basal cell tight junctions that give rise to the intrastrial compartment and find little evidence of cochlear pathology. Potassium ion recycling is normal in these mutants, but endocochlear potentials were below 30 mV and hearing thresholds were elevated 50 dB sound pressure level across the frequency spectrum. Together, these data demonstrate the central importance of basal cell tight junctions in the stria vascularis and directly verify the two-cell hypothesis for generation of endocochlear potential. Furthermore, these data indicate that endocochlear potential is an essential component of the power source for the mammalian cochlear amplifier.

Endolymphatic calcium supply for fish otolith growth takes place via the proximal portion of the otocyst

The presence of calcium within the utricle of larval cichlid fish Oreochromis mossambicus was analysed by means of energy-filtering transmission electron microscopy. Electron-spectroscopic imaging and electron energy loss spectra revealed discrete calcium precipitations that were more numerous in the proximal endolymph than in the distal endolymph, clearly indicating a decreasing proximo-distal gradient. This decreasing proximo-distal gradient was also present within the proximal endolymph between the sensory epithelium and the otolith. Further calcium particles covered the peripheral proteinaceous layer of the otolith. They were especially pronounced at the proximal surface of the otolith indicating that otolithic calcium incorporation takes place here. Other calcium precipitates accumulated at the macular junctions clearly supporting an earlier assumption according to which the endolymph is supplied with calcium via a paracellular pathway. The present results clearly show that the apical region of the macular epithelium is involved in the release of calcium and that the calcium supply of the otoliths takes place via the proximal endolymph.

Expression of an inwardly rectifying K+ channel, Kir5.1, in specific types of fibrocytes in the cochlear lateral wall suggests its functional importance in the establishment of endocochlear potential

Cochlear endolymph contains 150 mm K+ and has a highly positive potential of approximately +80 mV. The specialized ionic composition and high potential in endolymph are essential for hearing and maintained by circulation of K+ from perilymph to endolymph through the cochlear lateral wall. Various types of K+ channel such as Kir4.1 and KCNQ1/KCNE1 are expressed in stria vascularis of the lateral wall and play essential roles in K+ circulation. In this study, we examined a distribution of another K+ channel, Kir5.1, and found it specifically expressed in the spiral ligament of the cochlear lateral wall. Specific immunoreactivity for Kir5.1 was detected in type II, IV and V fibrocytes of the ligament and spiral limbus, all of which are directly involved in K+ circulation. Kir5.1 was not found in either type I or III fibrocytes. Although Kir5.1 assembles with Kir4.1 to form a functional Kir channel in renal epithelia and retinal Müller cells, double-immunolabelling revealed that they were expressed in distinct regions in the cochlea lateral wall, i.e. Kir4.1 only in stria vascularis vs. Kir5.1 in spiral ligament. During development, the expression of Kir5.1 subunits started significantly later than Kir4.1 and was correlated with the 'rapid' phase of the elevation of endocochlear potential (EP). Kir5.1 and Kir4.1 channel-subunits may therefore play distinct functional roles in K+ circulation in the cochlear lateral wall.

Calcium-tracers disclose the site of biomineralization in inner ear otoliths of fish

Since changing gravity (concerning direction and amplitude) strongly affects inner ear otolith growth and otolithic calcium incorporation in developing fish, it was the aim of the present study to locate the site of mineralization in order to gain cues and insights into the provenance of the otoliths inorganic compounds. Therefore, larval cichlid fish (Oreochromis mossambicus) were incubated in the calcium-tracer alizarin complexone (AC; red fluorescence). After maintenance in aquarium water for various periods (1, 2, 3, 6, 9 and 12 h; 1, 2, 3, 5, 6, 7, 15, 29, 36 and 87 d), the animals were incubated in the calcium-tracer calcein (CAL; green fluorescence). AC thus labeled calcium being incorporated at the beginning of the experiment and would subsequently accompany calcium in the course of a possible dislocation, whereas CAL visualized calcium being deposited right at the end of the test. Subsequently, the otoliths were analyzed using a laser scanning microscope and it was shown that the initial site of calcium incorporation was located directly adjacent to the sensory epithelium and the otolithic membrane. Later, calcium deposits were also found on further regions of the otoliths' surface area, where they had been shifted to in the course of dislocation. This finding strongly indicates that the sensory epithelium plays a prominent role in otolithic biomineralization, which is in full agreement with an own electron microscopical study [ELGRA News 23 (2003) 63].

[Quantification of endolymphatic hydrops in experimental animal model]

OBJECTIVE

To investigate a method of quantification of the endolymphatic hydrops in experimental animal model.

METHODS

Thirty guinea pigs were divided into three groups at random. In control group, there were ten guinea pigs without operation on both ears. Endolymphatic hydrops was induced by endolymphatic sac obliteration through extradural posterior cranial fossa approach in right ear, including 4-week postoperative group(n = 10) and 8-week postoperative group(n = 10). The area of scala vestibuli (SV) and scala media(SM) of each turn on both cochlear midmodiolar sections was measured, respectively, using auto computer aided design(AutoCAD R14) software combined with digital camera, and then the maximum scala media area(SMA) ratio was calculated and compared.

RESULTS

No endolymphatic hydrops was observed in all non-operated ears, however, variety degree of hydrops was present in all operated ears. The average maximum SMA ratio in the 4-week group (2.2231 +/- 0.1996) was greater than that in the control group (1.0971 +/- 0.0644). The average maximum SMA ratio of the 8-week group (4.0142 +/- 0.5218) was greater than that in the 4-weekgroup. There was significant difference between the two groups(P < 0.05).

CONCLUSION

It is convenience and reliable to be used to quantify the experimental endolymphatic hydrops with the present method. This study provides a reliable methodological base for the experimental study of Meniere's disease.

Time Course of Dehydrating Effects of Isosorbide on Experimentally Induced Endolymphatic Hydrops in Guinea Pigs

Osmotic diuretics are therapeutic agents used to reduce endolymphatic hydrops. However, glycerol-induced change in endolymph volume is followed by a rebound phenomenon. In this study, we investigated the rebound phenomenon occurring with isosorbide, an osmotic diuretic used as a therapeutic agent for Ménière's disease in Japan. Forty guinea pigs underwent surgical obliteration of the endolymphatic sac. Thirty received isosorbide orally 1 month after surgery. These animals were sacrificed 3, 6, or 12 h after isosorbide intake. The remaining 10 animals served as controls. Quantitative assessment of changes in the endolymphatic space was performed light-microscopically. Isosorbide reduced cochlear endolymph volume, with a peak reduction 6 h after intake. Thereafter, no prominent rebound phenomenon was noted. Clinically, since isosorbide is orally administered every 8 h, rebound phenomenon need not be considered in the treatment with isosorbide.

Calcium gradients in the fish inner ear sensory epithelium and otolithic membrane visualized by energy filtering transmission electron microscopy (EFTEM)

Inner ear otolith formation in fish is supposed to be performed by the molecular release of proteinacious precursor material from the sensory epithelia, followed by an undirected and diffuse precipitation of calcium carbonate (which is mainly responsible for the functionally important weight of otoliths). The pathway of calcium into the endolymph, however, still remains obscure. Therefore, the presence of calcium within the utricle of larval cichlid fish Oreochromis mossambicus was analyzed by means of energy filtering transmission electron microscopy (EFTEM). Electron spectroscopic imaging (ESI) and electron energy loss spectra (EELS) revealed discrete calcium precipitations, which were especially numerous in the proximal endolymph as compared to the distal endolymph. A decreasing proximo-distal gradient was also present within the proximal endolymph between the sensory epithelium and the otolith. Further calcium particles covered the peripheral proteinacious layer of the otolith. They were especially pronounced at the proximal surface of the otolith. Other calcium precipitates were found to be accumulated at the macular junctions. These results strongly suggest that the apical region of the macular epithelium is involved in the release of calcium and that calcium supply of the otoliths takes place in the proximal endolymph.

[The orientation and its significance of aquaporin-2 in the rat inner ear]

OBJECTIVE

To study the distribution of aquaporin-2(water channel protein; Aqp-2) in various structures of the SD rat inner ear.

METHOD

Fifteen SD rat were used in this experiment and were cardically perfused. The temporal bones were taken out, fixed in 4% paraformaldehyde, processed and sectioned by paraffin-embedded technique. The expression and distribution of Aqp-2 in the rat inner ear were examined by SP immunohistochemical technique.

RESULT

Aqp-2 were observed mainly in endolymphatic sac, stria vascularis and spiral ganglion. In addition, Aqp-2 were also labeled in Cortis organ, basilar membrane, vestibular lip and tympanic lip of spiral limus, tectorial membrane and spiral prominence.

CONCLUSION

Aqp-2 distributed in the rat inner ear tissues closely related with endolymph metabolism: the endolymphatic sac and stris vascularis. Its expression in the Corti's organ gave another explanation for the handicap hearing along with endolymphatic hydrops, and the distribution of Aqp-2 in the spiral ganglion indicated its possible role in the maintenance of hearing.

Endolymphatic sodium homeostasis by Reissner's membrane

Cochlear sensory transduction depends on active extrusion of sodium ion (Na(+)) from the luminal fluid, endolymph. Reissner's membrane epithelium forms much of the barrier between cochlear endolymph and perilymph and we hypothesized that Reissner's membrane might be responsible for this function. We found that Reissner's membrane isolated from gerbil produced a short circuit current (I(sc)) directed into the apical side, consistent with cation absorption and/or anion secretion. I(sc) was inhibited by amiloride analogs in the potency sequence benzamil>amiloride>>ethylisopropylamiloride, consistent with Na(+) absorption through an epithelial sodium channel in the apical cell membrane. I(sc) was also inhibited by an inhibitor of Na(+),K(+)-ATPase, ouabain, and by the K(+) channel blockers Ba(2+), 4-aminopyridine and quinine but not tetraethylammonium nor glibenclamide, consistent with the presence of a voltage-activated K(+) channel. Bumetanide, an inhibitor of the Na(+),2Cl(-),K(+)-cotransporter, had no effect on I(sc). Contrary to previous hypotheses, no evidence was found for electrogenic secretion of Cl(-) under control of cAMP since neither forskolin nor genistein affected I(sc) when Na(+) absorption was blocked. These results provide the first direct evidence that Reissner's membrane contributes to normal cochlear function by absorption of Na(+) from endolymph.

Composition of biomineral organic matrices with special emphasis on turbot (Psetta maxima) otolith and endolymph

The soluble organic matrix (OM) of various biominerals (red coral skeleton, oyster shell, sea urchin test, turbot otolith, chicken eggshell) was extracted after demineralization with acetic acid. The protein content of the OM varies strongly from 0.02 to 1.6 microg/mg biomineral whereas proteoglycans present less variations (from 0.7 to 1.4 microg/mg biomineral). Electrophoresis of biominerals OM shows differences in their protein pattern although several bands are present in all matrices. OM of all biominerals shows carbonic anhydrase activity but no activity was detectable in the endolymph. OM of all biominerals also displays an anticalcifying activity. After separation of the OM extracts by chloroform-methanol, 80% of the anticalcifying activity was found in the methanol phase except in the urchin test. After OM precipitation with trichloracetic acid, 70% of the activities was found in the supernatants. Partial biochemical characterization suggests that the anticalcifying factor is a polyanionic and water-soluble molecule, which could be proteoglycans. The endolymph surrounding the otolith also displays an anticalcifying activity although its inhibitous activity was 50 times lower than that of the otolith OM. However, the anticalcifying activity of the endolymph is assumed by a proteic structure (80% activity precipitated with TCA treatment). Our results suggest that both carbonic anhydrase and anticalcifying activities are widespread and play a significant role in the regulation of biomineral formation. Results are discussed in relation to the calcification process that takes place at the fluid-mineral interface.

Comparison of the protective efficacy of neurotrophins and antioxidants for vibration-induced trauma

BACKGROUND

Patients undergoing temporal bone surgery or subjects working with vibrating tools may develop vibration-induced hearing loss (VHL). The aim of this study was to characterize the effects of pretreatment with N-acetylcysteine (NAC) or the neurotrophic factors, brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor (CNTF), on VHL in an animal model.

METHODS

Trauma to the cochlea was created with a vibrating probe placed on the bone of the external ear canal. BDNF and CNTF(Ax1) were delivered into the cochlea with mini-osmotic pumps. NAC was delivered into the cochlea by round window membrane (RWM) injection, by RWM permeation, or by oral administration. Hearing was evaluated with electrocochleography (ECoG).

RESULTS

For control animals, vibration resulted in an average immediate threshold shift of 42 +/- 26 dB. NAC provided no protective benefit in animals subjected to VHL, regardless of the delivery method, with average threshold shifts varying from 38 to 56 dB across groups. NAC injection through the round window membrane was toxic, causing a ECoG threshold shift of >25 dB. In BDNF+CNTF(Ax1)-treated animals, immediate hearing loss was similar to that in control animals. There was a trend of threshold recovery by 1 day after vibration; however, the improvement was not statistically significant, nor was there a significant difference in 1-day thresholds across groups.

CONCLUSIONS

Local infusion of BDNF and CNTF(Ax1) may enhance the rate of recovery from VHL, compared to control animals. In contrast, NAC had no effect on VHL, and when delivered by RWM injection, was actually toxic to the inner ear.

Prophylactic effect of Ca2+ -deficient artificial perilymph perfusion on noise-induced hearing loss

OBJECTIVE

To investigate the prophylactic effect of low calcium concentration perilymph on noise-induced hearing loss.

METHODS

Forty guinea pigs with normal hearing weighing 250-350 g were assigned to five groups (8 in each group): (1) Ca(2+)-deficient perilymph perfusion (CDP) for 2 h; (2) white noise (120 dB SPL) exposure (WNE) only for 1 h, (3) combination of calcium-deficient perilymph perfusion and white noise (120 dB SPL) exposure (WNE + CDP); (4) normal artificial perilymph (NAP) perfusion for 2 h; and (5) white noise exposure + normal artificial perilymph perfusion (WNE + NAP) for 2 h. Compound action potentials (CAP) evoked by click was recorded from round window every 15 min. The cochleae from 5 animals in each group were examined with scanning electron microscope.

RESULTS

The CAP for group 1 experienced a threshold shift (TS) of 15-26 dB, while group 2 yielded a 46-59 dB TS and group 3 a 37-45 dB TS; no threshold shift occurred in group 4. The CAP TS in group 5 was 33-64 dB. The CAP TS of group 3 was less than that of group 2. After one hour of noise exposure, the CAP TS of group 3 were 45.92 +/- 2.90 dB and 59.30 +/- 3.95 dB in group 2. There were significant differences (P < 0.05) between groups 3 and 2. The CAP TS of group 3 was less than that of group 5 at the points of 1, 1.5 and 2 h after noise exposure. There was a significant difference between groups 3 and 5 (P < 0.01). Stereocilia of 89 OHC(3) were in disarray in five cochleae after noise exposure in group 2. The cuticular plates of 8 OHC(2),3 sank and the stereocilia became fused in only one animal cochlea after noise exposure in group 3 combined with low calcium perilymph perfusion.

CONCLUSIONS

Low calcium concentration appears to participate in preventing noise-induced hearing loss and the rising of calcium concentrations in inner hair cells after noise exposure, which may have been due to the opening of calcium channels in inner hair cells during noise exposure. The mechanism of the prophylactic effect might be caused by a lower calcium concentration in inner hair cells in the cochlea attenuating the influence of noise exposure on hearing loss; calcium deficient perilymph perfusion prevented calcium accumulation in inner hair cells of the cochlea. The motility of the OHCs might be partially inhibited by low calcium concentration that reduced noise-induced hearing loss in turn.

Asphyxia and diuretic-induced changes in the Ca2+ concentration of endolymph

Using Ca2+ -selective microelectrodes based on the neutral carrier, ETH-1001 with polyvinyl chloride (PVC), we have measured changes in the free Ca2+ concentration of guinea pig cochlear endolymph ([Ca](e)) after transient asphyxia or intravenous administration of diuretics. Under the control conditions, the endocochlear potential (EP) was +80 mV, and the [Ca](e) was in the range 1.4 x 10(-7)-2.4 x 10(-6) M (n = 16). Transient asphyxia (1-1.5 min) produced an increase in the [Ca](e) with a fall in the EP, whereas the cessation of the asphyxia led to a quick recovery of both [Ca](e) and EP to their control levels. Intravenous administration of furosemide (60 mg/kg) or bumetanide (30 mg/kg) also caused an increase in the [Ca](e) with a fall in the EP, followed by a gradual recovery of both [Ca](e) and EP. From these results, we obtained a significant correlation between EP and p[Ca](e) (= -log[Ca](e)), and conclude that (1) the [Ca](e) is extremely low, around 10(-6) M or less, under normal conditions and (2) the [Ca](e) is directly correlated with EP under physiological conditions.

Analysis of gentamicin kinetics in fluids of the inner ear with round window administration

HYPOTHESIS

That a theoretical basis for quantifying drug distribution in the inner ear with local applications can be established.

BACKGROUND

As methods of local drug delivery to the inner ear gain wider clinical acceptance it becomes important to establish how drugs are distributed in the ear as a function of time and for different delivery methods.

METHODS

The time course of gentamicin concentration in the inner ear fluids was simulated with a program that considered general pharmacokinetic principles and incorporated inner ear dimensions and drug dispersal processes, including diffusion, clearance, and intercompartmental exchange.

RESULTS

Cochlear fluid space dimensions of the chinchilla were derived from three-dimensional magnetic resonance images and were incorporated into the simulator. The published time course of gentamicin in vestibular perilymph of chinchillas was closely approximated by the adjustment of parameters defining round window membrane permeability, clearance, and interscala exchange. To simulate the time course, it was necessary for drug entry into the vestibule to be dominated by interscala exchange rather than longitudinal spread through the helicotrema. The effects of different round window delivery methods were also calculated. Perilymph drug levels and spatial distribution in the ear were shown to be markedly influenced by the time the applied drug remained in the middle ear.

CONCLUSION

The development of local inner ear drug application strategies requires consideration of inner ear pharmacokinetic characteristics, delivery methods, and therapeutic range of the drug.

Otolith growth in trout Oncorhynchus mykiss: supply of Ca2+ and Sr2+ to the saccular endolymph

Kinetic and pharmacological characteristics of Ca2+ fluxes across the saccular epithelium of trout were studied using a perfused isolated inner ear. 45Ca2+ influx from the Ringer solution to the endolymph was 3-4 nmoles h-1μl-1 endolymph, which corresponds to a global turnover rate of the endolymph calcium of 200 %h-1. Ca2+ entry into the proximal endolymph was faster than into the distal fluid. Net Ca2+ movement across the saccular epithelium depended on the direction and intensity of the chemical gradient of calcium between the Ringer solution and the endolymph. Increasing the calcium concentration in the Ringer solution up to 4.4 mmol l-1 provoked an accumulation of Ca2+ in both proximal and distal endolymphs, and equilibrium was reached about 30 min after the beginning of perfusion. Perfusion with calcium-free Ringer partially emptied the proximal compartment of calcium, whereas the calcium levels in the distal endolymph did not vary during 70 min of perfusion. Verapamil (10-5 mol l-1) and cyanide (CN, 10-3 mol l-1) did not modify the accumulation of Ca2+ within the endolymph in the presence of a favourable calcium chemical gradient. Furthermore the relationship between Ca2+ net fluxes and the chemical calcium gradient across the saccular epithelium was linear, indicating a passive diffusional mechanism via a paracellular pathway. Similar relationships were found for Sr2+ fluxes across the saccular epithelium in the presence of positive chemical gradients (1, 2 and 4 mmol l-1 Sr2+). In vivo experiments in which trout were intraperitoneously injected with CaCl2 solution confirmed the tight relationship between the calcium levels in plasma and endolymph (both proximal and distal). Sampling proximal and distal endolymphs in trout and turbot saccules revealed a decreasing proximo—distal calcium gradient in endolymph of both fish species. The present results strongly suggest that the endolymph is supplied with Ca2+ and Sr2+via a paracellular pathway located in the proximal area of the saccular epithelium.

Adrenergic and muscarinic control of cochlear endolymph production

The transduction of sound into nerve impulses in the cochlea is dependent on the stria vascularis. It is a multilayered epithelium, which is part of the epithelial barrier between endolymph and perilymph. The current model designed to explain the generation of the endocochlear potential assumes that the molecular mechanism for the generation of the endocochlear potential is the K(IR)4.1 K+ channel localized in the intermediate cells and that strial marginal cells play an indirect role in the generation of the endocochlear potential. This role is limited to the maintenance of a low K+ concentration in the intrastrial space by absorbing K+ from this space and secreting it into the endolymph. The molecular mechanisms for K+ secretion by strial marginal cells are well established. Strial marginal cells absorb K+ from the intrastrial space via the Na+-K+-ATPase and the Na+2Cl-K+ cotransporter and secrete it across the apical membrane via the IsK/KvLQT1 K+ channel. K+ secretion by strial marginal cells is not only required for the maintenance of the endocochlear potential and to provide the charge carrier for the transduction mechanism, but also to maintain a constant volume of endolymph. Thus, the presence of multiple control mechanisms regulating the rate of K+ secretion is likely. Recent observations suggest that the rate of K+ secretion in strial marginal cells is stimulated by beta1-adrenergic receptors and inhibited by M3 and/or M4 muscarinic receptors.

Aquaporin-2 regulation by vasopressin in the rat inner ear

Our previous studies have suggested a close relationship between vasopressin and endolymphatic hydrops, or the increased volume of endolymph in the inner ear. Endolymphatic hydrops is also thought to occur in Ménière's disease patients. In the kidney collecting duct, vasopressin induces the expression of aquaporin-2 (AQP2), resulting in increased water reabsorption. We explored the possibility, using a quantitative PCR method, that vasopressin regulates the expression of AQP2 mRNA in the rat inner ear, as it does in the kidney. The levels of AQP2 mRNA in the cochlea and endolymphatic sac were significantly higher in rats treated with vasopressin than the levels in control animals. We speculate that over-expression of AQP2 may be involved in the formation of endolymphatic hydrops.

Ultrastructure and blood supply of the tegmentum vasculosum in the cochlea of the duckling

The tegmentum vasculosum of the duckling consists of a highly folded epithelium which extends over the dorsal and lateral walls of the cochlear duct, separating the scala media from the scala vestibuli. This epithelium consists of two distinct cell types, dark cells and light cells, and is well vascularized. The surface of the epithelium is formed by a mosaic of alternating dark and light cells. The goblet-shaped dark cells have an electron-dense, organelle-rich cytoplasm, and are expanded basally by extensive basolateral plasma membrane infoldings, within which are numerous mitochondria. Dark cells are isolated from each other and from the capillaries within the epithelium by intervening light cells. In contrast, columnar light cells exhibit an electron-lucent, organelle-poor cytoplasm and may extend from the underlying capillaries to the endolymphatic surface. Light cells contain abundant, coated endocytic vesicles on their apical surfaces and are bound, apically, to other light cells or to dark cells by tight junctions and desmosomes. Laterally, light cells are linked to each other either by complex, fluid-filled membrane interdigitations or by extensive gap junctions. Plasma membrane interdigitations and obvious, fluid-filled intercellular spaces characterize the lateral borders between light and dark cells. Vascular corrosion casting reveals the three-dimensional anatomy of the cochlear vasculature. A continuous arteriolar loop fed by anterior and posterior cochlear arterioles encircles the cochlear duct. The rich capillary beds of the tegmentum vasculosum are supplied by arching arterioles arising from this loop. These capillaries are the continuous type and are situated primarily within the core of the epithelium or along its border with the scala vestibuli. The structure and blood supply of the tegmentum vasculosum are characteristic of an epithelium involved in active transport.