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.