Negative potential in scala media during early stage of anoxia

Effects of Furosemide and Ethacrynic Acid on the Endocochlear Direct Current Potential in Normal and Kanamycin Sulfate-Treated Guinea Pigs

The effects of loop diuretics on the endocochlear direct current (DC) potential and the effective electrical resistance of the cochlear partition were studied. The effective electrical resistance was increased, and the endocohlear DC potential was decreased. The decrease in the endocohlear DC potential must not be caused by breaks in the electrical insulation of the cochlear partition. With loop diuretics the endocochlear DC potential decreased less in guinea pigs treated with kanamycin sulfate than it did in the control group. Two interpretations of these phenomena are presented.

Effects of Furosemide and Ethacrynic Acid on The Endocochlear Direct Current Potential in Normal and Kanamycin Sulfate-Treated Guinea Pigs

The effects of loop diuretics on the endocochlear direct current (DC) potential and the effective electrical resistance of the cochlear partition were studied. The effective electrical resistance was increased, and the endocohlear DC potential was decreased. The decrease in the endocohlear DC potential must not be caused by breaks in the electrical insulation of the cochlear partition. With loop diuretics the endocochlear DC potential decreased less in guinea pigs treated with kanamycin sulfate than it did in the control group. Two interpretations of these phenomena are presented.

Computational model of a circulation current that controls electrochemical properties in the mammalian cochlea

Sound-evoked mechanical stimuli permit endolymphatic K(+) to enter sensory hair cells. This transduction is sensitized by an endocochlear potential (EP) of +80 mV in endolymph. After depolarizing the cells, K(+) leaves hair cells in perilymph, and it is then circulated back to endolymph across the lateral cochlear wall. In theory, this process entails a continuous and unidirectional current carried by apical K(+) channels and basolateral K(+) uptake transporters in both the marginal cell and syncytial layers of the lateral wall. The transporters regulate intracellular and extracellular [K(+)], allowing the channels to form K(+) diffusion potentials across each of the two layers. These diffusion potentials govern the EP. What remains uncertain is whether these transport mechanisms accumulating across diverse cell layers make up a continuous circulation current in the lateral wall and how this current might affect the characteristics of the endolymph. To address this question, we developed an electrophysiological model that incorporates channels and transporters of the lateral wall and channels of hair cells that derive a circulation current. The simulation replicated normal experimental EP values and reproduced experimentally measured changes in the EP and intra- and extracellular [K(+)] in the lateral wall when different transporters and channels were blocked. The model predicts that, under these different conditions, the circulation current's contribution to the EP arises from different sources. Finally, our model also accurately simulated EP loss in a mouse model of a chloride channelopathy associated with deafness.

Age-related changes in cochlear endolymphatic potassium and potential in CD-1 and CBA/CaJ mice

The CD-1 mouse strain is known to have early onset of hearing loss that is progressive with aging. We sought to determine whether a disturbance of K+ homeostasis and pathological changes in the cochlear lateral wall were involved in the age-related hearing loss (AHL) of CD-1 as compared to the CBA/CaJ strain which has minimal AHL. In the present study, the endocochlear potential (EP) and endolymphatic K+ concentration ([K+]e) were measured in both strains of mice with double-barrel microelectrodes at "young" (1-2 mo) and "old" (5-9 mo) ages. CBA/CaJ mice displayed no changes with aging in EP and [K+]e of the basal turn. In the apical turn, there was a small positive shift of the EP (10 mV) with aging under both normoxic and acute anoxic conditions (-EP), without any change of [K+]e. Further, there were no obvious pathological changes in the lateral wall of CBA/CaJ mice. By contrast, old CD-1 mice displayed a significantly reduced [K+]e by 30% in both basal and apical turns with no significant changes in normoxic EP. The -EP in the apical turn was significantly reduced in magnitude by 6 mV. A severe loss of cells with aging was observed in the region of type IV fibrocytes of the apical and basal turns and of type II fibrocytes in the basal turn. A complete degeneration of organ of Corti was also observed at the basal turn of old CD-1 mice, as well as a basalward decline of spiral ganglion neuron density. The pathological changes in spiral ligament of CD-1 mice were similar to those of an inbred mouse strain C57BL/6J that expresses an AHL gene (ahl) and might be a primary etiology of AHL of CD-1 mice. These findings have ramifications for our understanding of AHL and for interpretation of genetic mutations in a CD-1 background.

Profiles of resting potentials across the stria vascularis in kanamycin-deafened guinea pigs

The resting potentials of the marginal cells in the stria vascularis of the guinea pig were determined from changes in the combined electrode-tissue resistance of the electrode. The resistance of the electrode was 45.5 +/- 16.0 M omega (n = 20) before penetration of the stria vascularis and 46.7 +/- 17.3 M omega (n = 20) after penetration. The resistance drops across the luminal membrane of the marginal cells were 46.0 +/- 22.6 M omega (n = 12) in kanamycin-deafened guinea pigs and 54.5 +/- 33.1 M omega (n = 9) in normal guinea pigs. The endocochlear potential (EP) and resting potentials in the marginal cells were 90.1 +/- 6.0 mV (n = 14) and 70.4 +/- 11.3 mV (n = 14) in kanamycin-deafened guinea pigs and 84.8 + 5.1 mV (n = 29) and 74.7 +/- 11.7 mV (n = 29) in normal guinea pigs. The resting potentials in the marginal cells decreased gradually and were approximately 0 mV around 20 min after anoxia in both kanamycin-deafened and normal guinea pigs. These changes were comparable to those of EP in kanamycin-deafened guinea pigs during anoxia. The mechanism of the EP in kanamycin-deafened guinea pigs is discussed.

Effect of artificial endolymph injection into the cochlear duct on the endocochlear potential

We investigated the effect of acute endolymphatic hydrops on the positive endocochlear potential (+EP) and negative endocochlear potential (-EP). The +EP was measured in guinea pigs during injection (without outlet) and perfusion (with outlet) of artificial endolymph into the cochlear duct. The -EP was measured during anoxia after the injection or the perfusion had finished. Injection of artificial endolymph produced a slight transient increase in the +EP, and a significant decrease in the magnitude of the -EP. Chronic endolymphatic hydrops produces both +EP and -EP decrease. The +EP decrease in chronic endolymphatic hydrops may cause the chronic change of the inner ear. The +EP increase in acute endolymphatic hydrops may be caused by a shift of the basilar membrane. However, the mechanism of the 'transient' +EP increase is not clear. The -EP decrease was not observed in animals whose cochlear duct was perfused with artificial endolymph. Therefore, the artificial endolymph itself did not cause the decrease in magnitude of the -EP. Dysfunction of the hair cells is a possible explanation for the -EP decrease but the mechanism of such a decrease is not clear in the present study. However, the results of this study support the notion that small increases in endolymphatic pressure below the resolution of recent measurements (DeMott and Salt, 1997) can lead directly to a reduction of the -EP during hydrops. The animal model described here can eliminate the chronic effect of hydrops, therefore, this model is useful for investigations into the effect of hydrops itself on the inner ear and the mechanism of hearing loss in Ménière's disease.

Influence of chronic kanamycin administration on basement membrane anionic sites in the labyrinth

We studied the effect of chronic treatment with kanamycin on the basement membrane (BM) anionic sites in the cochlea and endolymphatic sac using polyethyleneimine (PEI) as a cationic tracer. Albino guinea pigs weighing 250-300 g received kanamycin (400 mg/kg/day, i.m.) for 10 or 17 consecutive days. The number of BM anionic sites as derived from the PEI area was not affected in Reissner's membrane, spiral prominence, basilar membrane or endolymphatic sac, whereas it was significantly decreased in the stria vascularis and spiral limbus, being more marked in the guinea pigs treated for 17 days than in those treated for 10 days. The number of BM anionic sites in these regions did not recover until 6 weeks after kanamycin treatment. These findings suggest that chronically administered kanamycin may selectively and progressively affect the BM anionic sites in the stria vascularis and spiral limbus, resulting in disruption of a barrier function in the cochlea, and that severely impaired BM anionic sites in the cochlea may not recover.

Mechanism of lack of development of negative endocochlear potential in guinea pigs with hair cell loss

The endocochlear potential (EP), and the concentration of K+, Na+ and Cl- were measured simultaneously in endolymph of guinea pigs. The EP was 85.6 +/- 0.8 mV in normal guinea pigs, 90.7 +/- 0.8 mV in the kanamycin-treated animals, and 91.6 +/- 1.2 mV in those treated with nitrogen mustard-N-oxide (NMNO). Thirty minutes after the onset of anoxia, the EP (negative EP) was -29.3 +/- 1.0 mV in the normal group, -0.2 +/- 1.0 mV in the kanamycin-treated group, and -1.9 +/- 1.3 mV in the NMNO-treated group. The permeability coefficients of K+ (Pk), Na+ (Pna) and Cl- (Pcl) across the endolymph-perilymph barrier during the period of 20-30 min after the onset of anoxia in the normal group were (341.6 +/- 38.2) x 10(-9) cm3 sec-1, (53.0 +/- 8.1) x 10(-9) cm3 sec-1 and (111.8 +/- 27.2) x 10(-9) cm3 sec-1, respectively. Pk was decreased in the kanamycin- and NMNO-treated groups. Pna did not differ between the normal and treated groups. Pcl was increased in the kanamycin- and NMNO-treated groups. The K+:Na+:Cl- permeability ratio was 1:0.16:0.32 in the normal group, 1:1.12:11.6 in the kanamycin-treated group, and 1:0.44:5.60 in the NMNO-treated group. The results indicate that the lack of development of a negative EP in the kanamycin- and NMNO-treated guinea pigs was attributable to the increased Pcl and the decreased Pk across the endolymph-perilymph barrier, probably the organ of Corti, during anoxia.

Effects of K(+)-channel blockers on cochlear potentials in the guinea pig

The effects of different K+ channel blockers, 4-aminopyridine (4-AP), tetraethylammonium (TEA) and quinine, on the various cochlear potentials were observed by the means of perilymph infusion. Each of the three blockers depressed the compound action potential. However, they exerted quite different effects on other cochlear potentials, especially comparing 4-AP, a fast K(+)-channel blocker, with two other blockers. 4-AP induced a significant increase in the magnitude of summating potential, while TEA and quinine decreased it; 4-AP showed no effect on the general endocochlear potential (G-EP, the EP value recorded directly from the scala media, SM) and the negative EP component (N-EP), while TEA and Quinine increased G-EP and decreased the absolute value of N-EP. They also exerted different effects on the EP changes induced by exposure to intense noise. The results indicate the different roles of different K(+)-channels in the generation of cochlear potentials. The relationship of the two components of EP (positive and negative) and the G-EP was discussed.

Origin of the endolymphatic DC potential in the cochlea and ampulla of the guinea pig

In order to determine the origin of the endolymphatic DC potential, potentials in the cochlea (CEP) and ampulla of the posterior semicircular canal (AEP) were recorded simultaneously in normal control guinea pigs and guinea pigs treated with aminoglycoside antibiotics. There were no significant differences in the resting levels of the CEP and AEP between the two groups. In the normal control guinea pigs, the endolymphatic potential of each animal decreased differently during anoxia and after an injection of furosemide. In kanamycin-treated guinea pigs whose cochlear hair cells were damaged, the CEP response to 150s of anoxia was quite different from that of the control animals: it decreased more slowly and did not reach a zero level. In contrast, the AEP had a normal response to anoxia. In streptomycin-treated guinea pigs with damage to the ampullary hair cells, the situation was reversed: the AEP responded normally while the CEP responded abnormally. These findings indicate that the AEP is most likely independent of the CEP and is generated within the ampulla. They also indicate that sensory epithelial in the cochlea and ampulla play an important role in the maintenance of the CEP and AEP, respectively.

Effects of nitrogen mustard-N-oxide on ionic activities of inner ear fluid and ionic permeabilities of the cochlear partition in the guinea pig

The effect of nitrogen mustard-N-oxide (NMO) on the endocochlear potential (EP) was investigated from the aspect of the ion concentrations and permeabilities in the cochlea. Compared with the untreated animals, in NMO-treated animals 20 to 30 hours after administration, the EP was decreased (30.8 +/- 3.5 mV in NMO versus 82.4 +/- 1.6 mV in control), the K+ concentration in perilymph of the scala tympani was increased (8.2 +/- 1.0 mM versus 5.3 +/- 0.7 mM), the K+ concentration in endolymph was decreased (128.5 +/- 10.6 mM versus 157.9 +/- 7.9 mM), and the Na+ concentration in endolymph was increased (9.6 +/- 3.6 mM versus 2.5 +/- 0.4 mM). The permeability coefficient for Na+ of the cochlear partition in the NMO-treated animals significantly decreased, while that for Cl- significantly increased. The negative EP, which presumably exists in the normal state, diminished further (-2.7 mV versus -27.8 mV), and the calculated electrogenic potential of the EP was depressed remarkably (33.5 mV versus 110.2 mV). The results suggest that the effects of NMO involved changes in ion permeabilities of the partition and the inhibition of electrogenic transport processes in the cochlea.

A possible site of production of the negative endocochlear DC potential

The scala vestibuli or the scala tympani of guinea pigs was perfused with artificial perilymph containing 1, 5, 10, 20, 30, 40 and 50 mM of potassium chloride in a total concentration of 150 mM with the background composed of sodium chloride. With the perfusion of the scala vestibuli, each concentration failed to alter the magnitude of the negative endocochlear DC potential produced by anoxia or the intravenous injection of 100 mg/kg of body weight of furosemide. With the perfusion of the scala tympani, the negative endocochlear DC potential disappeared precipitously and the maximum output of the cochlear microphonic was severely depressed with concentrations of potassium chloride of 30 mM or greater. The magnitude of the negative endocochlear DC potential appears to be closely related to the maximum output of the cochlear microphonic. These results suggest that the site of production of the negative EP is in the hair cells of the organ of Corti.

Support of cochlear metabolic and ion transport processes solely by perilymphatic perfusion

Morphologic considerations would seem to suggest that the cochlear duct could not be maintained in a fully functional state in the absence of a blood supply. We found, however, that perilymphatic perfusion could be used as a substitute for the normal vascular circulation. The criteria used to determine cochlear function included (1) normal endocochlear potential, (2) normal net secretory flux of rubidium (as a tracer for K), and (3) normal levels of ATP in both the organ of Corti and the stria vascularis. All criteria were satisfied by our perfusion regimen.

Longitudinal distribution of cochlear potentials and the K+ concentration in the endolymph after acoustic trauma

Guinea pigs were exposed to 142 dB third-octave band of noise control at 1 kHz for 1 h. At different times after exposure the endocochlear potential (EP), the anoxic negative endocochlear potential (-EP), the concentration of K+ (K+e) and microphonic potentials were recorded in scala media in four cochlear turns. The remaining hair cells were counted in each animal. Immediately after the exposure, the EP and K+e decreased evenly in all four cochlear turns and gradually returned to normal physiological values in 5-20 days. When measured 20 days after the exposure, essentially normal EP and K+e values were observed, with an apicalwards decline, which was similar to that found along the cochlea in nonexposed animals. Abnormal increased EP was observed in some animals 20 days after the exposure in the first and second turns. In contrast to positive EP and K+e values, the anoxic negative EP attained less negative values in the second turn of exposed animals, i.e., in the turn where the narrow band noise exerted the major destructive effect. An almost normal distribution of hair cells and most negative EP values were found in the fourth turn. The distribution of persistent hair cells correlated positively with the values of the anoxic negative EP and amplitudes of the microphonic potentials. It is assumed that, in addition to the difference in K+ concentration between endolymph and perilymph, the anoxic negative EP is dependent upon the functional state of the organ of Corti.

Recovery of the endocochlear potential and the K+ concentrations in the cochlear fluids after acoustic trauma

Intense noise stimulation (142 dB, 1/3-octave-band noise centred at 1 kHz for 1 h) causes damage mainly in the second turn of the cochlea. Several hours (3-5) after the noise exposure, the endocochlear potential (EP) was found to be very low (5.7 +/- 6.0 mV). Similarly, the K+ concentration in the endolymph (Ke+) had decreased to low values (18.9 +/- 9.5 mM). The return of EP and Ke+ to normal values took 5-20 days. In contrast to the Ke+ changes, the perilymph K+ concentration (Kp+) increased slightly after the noise exposure to 4.5 +/- 1.7 mM and returned to normal values one day after the exposure. Differences were found in the time course of the EP, Ke+ and Kp+ changes after the arrest of ventilat ion when animals with acoustic trauma were com,ared with normal healthy individuals. The anoxic EP in noise-exposed animals never decreased to values more negative than -20 mV. The results imply that the inner ear mechanisms maintaining positive EP, Ke+ and Kp+ are severely damaged after acoustic trauma and that their function is restored in 5-20 days. With respect to some parameters (decrease of the EP during anoxia, the value of anoxic negative EP, EP overshoot after reventilation) the inner ear mechanisms are, however, still abnormal.

Effects of noise on cochlear potentials and endolymph potassium concentration recorded with potassium-selective electrodes

Guinea pig cochleas were exposed to either broad-band noise at intensities between 95 and 115 dBA or octave-band noise centered at 380 Hz or 4.2 kHz at intensities between 115 and 125 dB SPL. Cochlear microphonics (CM), summating potentials (SP) and action potentials (AP) were recorded from differential electrodes in the perilymphatic scalae between successive 20-min periods of noise exposure. The endocochlear potential (EP) and endolymph potassium concentration [Kendo+] were recorded continuously from scala media using double-barreled potassium-sensitive electrodes. It was found that the initial exposure to noise at 115 dBA produced considerable suppression of the CM and AP, while the EP and [Kendo+] were elevated above their normal values. When animals previously treated with kanamycin were subjected to the same level of noise exposure no systematic increase in either EP ro [Kendo+] was observed. After prolonged exposure to 380 Hz octave-band noise at 125 dB SPL, a slow decline of EP and [Kendo+] was observed. The relationships between the changes in EP, [Kendo+] and CM are discussed.

Some observations on negative endocochlear potential during anoxia

The effects of anoxia on the endocochlear potential (EP) and +K and +Na concentrations in the endolymph were studied in three groups of guinea pigs: kanamycin-treated guinea pigs, waltzing guinea pigs and normal guinea pigs. The magnitude of the EP in kanamycin-treated guinea pigs and waltzing guinea pigs did not show marked deviation from that observed in normal animals. The +K and +Na concentrations in the endolymph in those animals with severe suppression of sound-evoked cochlear potentials were also within the normal range. The changes in +K and +Na concentrations in the endolymph in anoxic condition were similar in the three groups of animals. However, the rate of decline of the EP was slower in kanamycin-treated guinea pigs and old waltzing guinea pigs. In young waltzing guinea pigs showing moderate suppression on the cochlear microphonics, the decline of the EP during anoxia was comparable to that observed in normal guinea pigs. The results indicate that anoxia decreases +K and increases +Na concentrations in the endolymph in a similar fashion in kanamycin-treated guinea pigs, waltzing guinea pigs and normal guinea pigs. It is suggested that the decline of the EP during anoxia is correlated with the +K conductance of the organ of Corti.

Effects of local application of ototoxic antibiotics on cochlear potentials in guinea pigs

The effects of neomycin, kanamycin and dihydrostreptomycin on the cochlear microphonic, the action potential of the auditory nerve and the endocochlear potential were studied in guinea pigs in which these drugs were locally administered by perfusion. These drugs suppressed the cochlear responses markedly when applied to the endolymph but were less effective when applied to the perilymph. The mechanisms of action of antibiotics on the hair cells of the organ of Corti are discussed.

Noxious effects upon cochlear metabolism

The influence of various toxic substances and of drugs with ototoxic side effects upon energy generation, energy utilization, and membrane processes of the cochlea were studied. None of the drugs tested interfered with energy generation to as great an extent as did anoxia or cyanide and 2,4-dinitrophenol. Ouabain produced a pronounced interference with energy utilization of the stria vascularis. The "loop" diuretics ethacrynic acid and furosemide produced a reduction of energy utilization of a lesser degree than did ouabain. The "loop" diuretics do not seem to exert their toxic action upon strial Na+K+-ATPase, but may act by interfering with strial adenylate cyclase. Aminoglycoside antibiotics and diuretic and nondiuretic mercurials seem to exert their primary noxious action upon cochlear function by interfering with membrane processes of the structures bounding the cochlear duct.

Effects of perilymphatic perfusion with neomycin on the cochlear microphonic potential in the guinea pig

The effects of three concentrations of neomycin, administered by a method of acute perilymphatic perfusion of the guinea pig cochlea, on the cochlear microphonic potential (CM) at 4 kHz and 500 Hz are described. A concentration-dependent reduction in CM occured during the 60 minute perfusion period. Neomycin at 10-4 M did not change the CM magnitude, while at 10-3 and 102 M it caused 4 kHz (and 500 Hz) CM reductions which began within 24 (for both frequencies) minutes and 10 (and 12) minutes of drug application respectively. CM reduction proceeded at a higher rate for greater neomycin concentration. The perfusion technique, the implication of the frequency indifference, and the potential of the perfusion technique for inner ear biochemical analysis are discussed.

Physiological basis of cochlear transduction and sensitivity

Experiments were conducted in the guinea pig cochlea and the Xenopus laevis lateral line organ to obtain information about the electrical impedance properties of the cochlea, the sources and characteristics of the cochlear potentials, and to determine if hair cells are electrically excitable. The significance of the resting and evoked cochlear potentials in the transduction process leading to excitation of the acoustic nerve fibers was evaluated by comparing the results of electrophysiological experiments with the prediction of a model of the cochlea designed after the mechanoelectric theory of hair cell function. The results of these experiments 1) were compatible with the prediction of the mechoelectric theory of cochlear function; 2) made it possible to estimate and compare the energy dissipated in generating and maintaining the cochlear potentials with the stores of biochemical energy available in the cochlea; and 3) showed that the hair cells are electrically excitable, from which it was concluded that interaction between the hair cells is feasible.

Effects of anoxia and ethacrynic acid upon ampullar endolymphatic potential and upon high energy phosphates in ampullar wall

The ampullar endolymphatic potential (AEP) was studied in the guinea pig during ischemia and asphyxia and following systemic application of ethacrynic acid. In addition the specialized and nonspecialized portions of the ampullar wall were analyzed for ATP and P-creatine at different conditions of metabolic interference. Under control conditions the AEP amounted to + 4.6 +/- 1.2 mV. In both types of hypoxia the decline of the AEP proceeded on a much slower time scale than that of the cochlear endolymphatic potential (CEP), and the maximum negativity reached was considerably less. Quantitative analysis of both types of ampullar wall tissue indicated a much slower decline in hypoxia of ATP levels than in the stria vascularis. Changes in P-creatine levels were considerably more rapid. The AEP became reduced and changed polarity also by intoxication with ethacrynic acid (EA), but higher dosages (above 70 mg/kg) were necessary than for effects upon the CEP and much longer time periods were required for attainment of maximum negativity. The maximum negativity of the AEP was significantly greater at a dosage of 100 mg/kg of EA than during ischemia. At the point of maximum depression of the AEP P-creatine levels in both types of ampullar tissue were unchanged, but ATP levels were significantly reduced in the specialized portions of ampullar wall.

Electroanatomy of the lateral wall of the cochlea

Metabolically, the most active membrane in the stria vascularis is the marginal cell membrane adjacent to the intermediate cells. It is probably at this site that K+ions are actively transported in and Cl minus ions are actively transported out by the Na+ in equilibrium K+, K+in equilibrium H+, and Cl minus ion pumps. Support for this hypothesis was derived from the work of other authors, from our own earlier results and the experimental results presented in this paper. Our experiments indicate that the marginal cells of the stria vascularis facing the endolymphatic space have a positive intracellular potential similar to the potential of the scala media, Therefore, the major portion of the active ion transport takes place on the intermediate cell side of the marginal cells. The pumping mechanisms, localized in this membrane, are responsible for the generation of the positive endocochlear potential (EP).

The peripheral auditory apparatus

This review of the peripheral auditory apparatus represents an attempt to analyse critically recent developments in the field. The coverage is not exhaustive, the emphasis is on functional aspects and no attempt is made to review the anatomy of the ear. Particular emphasis is placed on three broad sections: the physiology of the middle ear, basilar membrane mechanics and the electrophysiology of the cochlea. It is in these areas that recent technical advances have led to experiments which throw doubt on traditionally held concepts. Such an advance is the application of the Mössbauer technique to the problem of middle- and inner-ear mechanics. Because of its novelty, this technique is discussed in detail. Other new methods such as laser holography are just starting to be used, as are the ion selective microelectrodes for measuring dynamic changes in endolymph concentration. After many years of slow progress there is a sudden spark of enthusiasm for peripheral auditory research, and the coming decade promises to be most exciting indeed.