Endocochlear Potential Values Research Articles

Endocochlear Potential and Potassium Concentration Recording in Minipig Cochlea

ObjectiveTo record the endocochlear potential (EP) and calculate potassium concentration [K+] in Minipig cochleae. MethodsWe used multi-barreled electrodes to measure the EP and the potential, [K+]. EP and potassium electrode recording were made in 9 cochleae from 5 minipigs to get normal EP values. ResultsThe average EP value in the cochlea from the minipigs was 77.3 ± 14mV. The average [K+] for the minipigs was 147.1 ± 13mM. ConclusionsThe EP and potential, [K+] in minipigs are lower than in the human and rodents. This may be the reason why porcine ABR thresholds are slightly higher than humans and rodents.

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.

Chronic Reduction of Endocochlear Potential Reduces Auditory Nerve Activity: Further Confirmation of an Animal Model of Metabolic Presbyacusis

Gerbils aged in quiet show a decline of the endocochlear potential (EP) and elevated auditory nerve compound action potential (CAP) thresholds. However, establishing a direct relationship between an age-related reduction in the EP and changes in the activities of primary auditory neurons is difficult owing to the complexity of age-related histological changes in the cochlea. To address this issue, we developed a young gerbil model of "metabolic" presbyacusis that uses an osmotic pump to deliver furosemide into the round window niche for 7 days, resulting in a chronically reduced EP. In this model, the only major histopathologic changes were restricted to the hook region of the cochlea and consisted of loss of strial intermediate cells and massive edema in the lateral wall. The morphological and physiological evidence suggests that the cochlea can adapt to furosemide application over time. The morphology of spiral ganglion cells and hair cells appeared normal throughout the cochlea. CAP responses and EP values in this model are similar to those of quiet-aged ears. The spontaneous activity of single auditory fibers (n = 188) was assessed in 15 young gerbils treated with furosemide for 7 days. The percentage of recorded low-spontaneous rate (SR) fibers at characteristic frequencies (CFs) > or = 6 kHz was significantly lower in furosemide-treated than in control ears. Recovery function tests of CAP responses after prior stimulation also showed a decline in activity of the low-SR population with CFs > or = 6 kHz in the treated cochleas. A similar loss in the activity of low-SR fiber has been previously shown in quiet-aged gerbils. These results suggest that dysfunction of the cochlear lateral wall and subsequent chronic reduction in the EP can directly affect the activity patterns of primary auditory neurons in a manner similar to that seen in aged gerbils.

Effects of gap junction uncoupling in the gerbil cochlea.

To gain insight into molecular and cellular mechanisms regulating cochlear potassium (K+) recycling, including the possible effects of mutations in the gene, which encodes the gap junction protein connexin 26. Intercellular K+ flux was manipulated in vivo by infusion of the gap junction uncoupler proadifen (SKF-525A) into perilymph. Functional and structural alterations induced by gap junction blockade were assessed by electrophysiological and morphologic analysis. Laboratory study using an animal model. Physiological effects of acute and chronic uncoupling of gap junctions in the Mongolian gerbil inner ear were evaluated by measurement of compound action potential (CAP) thresholds and input-output (I/O) functions, distortion product otoacoustic emissions (DPOAE), and the endocochlear potential (EP). Morphologic changes were assessed by electron microscopy. Acute exposures to proadifen resulted in large decreases in EP values, DPOAE magnitudes, and CAP I/O maximum amplitudes and an increase in high-frequency CAP thresholds. These physiological changes were accompanied by vacuolization of type II and type V fibrocytes in the lateral wall of the cochlea. Chronic treatments revealed some recovery in EP values and CAP thresholds, which showed a relatively flat 15- to 20-dB elevation across frequencies. Gap junctions play a significant role in normal cochlear function. In particular they appear to be essential for maintaining the EP, an activity that could be related to their participation in K+ recycling. Thus, hearing losses associated with mutations in the gene that alter the expression or function of connexin 26 may result from a diminished capacity to recycle K+ from perilymph back to the stria vascularis and a consequent decline in the EP.

Endocochlear potentials and compound action potential recovery: functions in the C57BL/6J mouse

The C57BL/6J mouse suffers from cochlear degeneration beginning at an early age and has been used as a model of age-related hearing loss (presbyacusis). Here, the endocochlear potential (EP) and compound action potential (CAP) responses were determined in one-, four-, 12- and 24-month-old C57BL/6J mice. CAP measures included thresholds to tone pips, input/output (I/O) functions, and recovery functions to conditioning tones. EP values among the four age groups did not differ significantly ( P>0.05) in either the basal or apical turns. CAP thresholds were increased significantly by 10 to 30 dB in the four-month group compared to the one-month controls at 11.3, 16, 20, and 22.6 kHz. CAP I/O functions were shallower in the four-month group compared to controls at all frequencies. In the 12- and 24-month-old mice, CAP responses were absent, despite normal EP values in these animals. Recovery functions after conditioning tones were obtained at 8, 16, 20 and 22.6 kHz; the functions had fast and slow components at all frequencies tested in both the one- and four-month-old groups. The corresponding recovery curves were identical for both age groups, even with significant threshold shifts in the older group. The two component recovery curves provide the first physiological evidence that different spontaneous rate (SR) classes of auditory neurons exist in the C57BL/6J mouse. Moreover, the unchanged recovery functions in the older group suggest that there was no loss of activity of the low-SR fiber population with age under conditions where the EP remains stable, in contrast to the gerbil model of presbyacusis where there is a loss of low-SR fiber activity and EP does decline with age.

Long-Term Changes in Off-Lesion Endocochlear Potential after Induction of Localized Lesions in the Lateral Wall

Localized lesions were produced in various turns of the guinea pig cochlea by means of a photochemical reaction between systemically administered rose bengal dye and green light illumination. The endocochlear potential (EP) was measured at various off-lesion sites, and a morphological examination was performed. In a previous study, this same investigation was done at 3 days, at which time all sites apical to the lesion showed significant EP depression, and damage to the stria vascularis at the lesion was ongoing. In the present 2-week study, the apical EP values were not different from the basal values, and all experimental values were essentially the same as the EP values found in control animals. Morphological examination revealed that the previously damaged structures were greatly repaired. Localized damage and early apical EP depression followed by damage repair and eventual EP recovery could account for the clinical course of certain cases of idiopathic sudden hearing loss involving low-tone deafness.

か牛血流障害と突発性難聴

Focal microcirculation lesion in the lateral wall of the cochlea was made by photochemical reaction in guinea pigs. The damage was made by systemic injection of rose bengal and green light illumination to the bony cochlear wall. The diameter of the lesion was 0.9-1mm. The effects of the lesion were evaluated using a non-contact laser blood flowmeter, the endocochlear potential (EP) and the compound action potential (CAP). The cochlear blood flow gradually decreased during the 30min observation period parallel to the gradual decline of the EP. The average CAP threshold was significantly increased 15min and thereafter. Mid-lesion EP measurements were obtained at various intervals up to 3 weeks. EP reached the minimal value after 3 days and recovered to almost normal after 2 weeks. Off-lesion EP values 3 days after illumination were depressed at all measured sites apical to the lesion but remained unchanged at sites basal to the lesion. Two weeks after illumination, decreased apical EP was found to have returned to normal. Focal damage in the lower cochlear turn may have a depressant effect on EP measured apical to the lesion. This reversible depressant effect of a focal lesion on apical EP could account for the clinical course in certain cases of idiopathic sudden hearing loss.

Endocochlear potential in focal lesions of the guinea pig cochlea

To estimate the correlation between the cochlear lateral wall and endocochlear potential (EP), the EP was measured at different time intervals up to 3 weeks at cochlear focal lesions made in the guinea pig. Lesions were produced by a photochemical reaction between systemically administered Rose Bengal and green light illumination in the second cochlear turn. Focal strial lesions (mean diameter 975 μm) became apparent under a scanning electron microscope 60 min after illumination, and degeneration of the organ of Corti was recognized at 3 days. The EP was measured continuously for 1 h following onset of the reaction in eight ears. In the other ears, EP measurements were done after various intervals of time up to 3 weeks. The EP showed two declines from pre-illumination level. The first started soon after illumination, reached a minimum value at 25 min, and then slightly recovered by 1 h. The second decline appeared at 3 days after illumination. In contrast to evident focal morphological degeneration 2 weeks post illumination, the EP values had recovered to almost normal. We suggest that the sequential changes in the EP were produced depending on the degeneration and then repairing processes of the endolymphatic boundary tissues.

An experimental study of ototoxicity induced by deferoxamine mesilate

We investigated the possibility of ototoxicity induced by deferoxamine mesilate (DFO). DFO was administered to guinea pigs intraperitoneally at 200 or 400 mg/kg body weight/day, 6 days/week for 5 weeks, or 600 mg/kg body weight/day, 6 days/week for 4 weeks. Physiological saline instead of DFO was given to animals as a control. After the course of administration was completed, we measured the threshold of the compound action potential (CAP) and the endocochlear potential (EP). We also examined the cochlear histology by scanning electron microscopy. The CAP threshold of animals treated with 200 or 400 mg of DFO per kg was not elevated. In the animals treated with 600 mg/kg , however, the CAP threshold was clearly elevated when compared with that of control animals. The EP values of animals treated with 200 or 400 mg of DFO per kg were comparable to those of control animals. In some animals treated with 600 mg of DFO per kg, however, the EP values were decreased. Histological examination revealed that the outer hair cells of the cochlea were missing. DFO presumably acts on hair cells, resulting in elevation of the CAP threshold. We concluded that hair cell damage is one of the contributing factors to CAP threshold elevation.

Effects of aging on potassium homeostasis and the endocochlear potential in the gerbil cochlea

Previous work has shown that the endocochlear potential (EP) decreases with age in the gerbil. Concomitant with the EP decrease is an age-related loss of activity of Na,K-ATPase in the lateral wall and stria vascularis. We hypothesized that the EP decrease is associated with a similar decrease in the endolymphatic potassium concentration [K e +]. This hypothesis was tested using double-barrelled, K +-selective electrodes introduced into scala media through the round window in young and quiet-aged gerbils. Results show that the means (±S.D.) of the [K e +] in young and aged gerbils were not significantly different (178.2 ± 14.2 mM and 171.2 ± 34.4 mM, respectively), although the intersubject variability was much greater in the aged animals than in the young. These values of [K e +] are slightly higher than those found for other mammals and may reflect the higher plasma osmolarity found in the gerbil. The concentration of perilymphatic potassium [K p +] in scala tympani at the round window was also similar for the young and aged groups (3.57 ± 1.17 mM and 4.18 ± 2.03 mM, respectively). On the other hand, mean EP values in the young and aged gerbils were 92.0 ± 5.7 mV and 64.8 ± 15.8 mV, respectively and were statistically different ( P < 0.001). Overall, EP and [K e +] showed little correlation ( R 2 = 0.23), except that when [K e +] fell below 150 mM, the EP was always less than 60 mV. An analysis of the chemical potential for K e + with respect to K p + shows that it was similar for young and aged gerbils (overall mean of 103.1 ± 13.7 mV) and remained constant with respect to the EP, in spite of an overall electrochemical potential of K e + that varied from 120 to 210 mV. Thus, the system maintains K e + homeostasis at the expense of the EP, even when the EP is on the verge of collapse.

Distortion-product otoacoustic emissions in Mongolian gerbils with resistance to noise-induced hearing loss.

Distortion-product otoacoustic emissions (DPOAEs) and the endocochlear potential (EP) were recorded in adult Mongolian gerbils exposed to noise for either 1 or 12 days. The exposure was an octave band of noise centered at 4 kHz at 80 dB SPL with a duty cycle of 6 h on, 18 h off. A previous study showed that a single such exposure causes 20-50 dB of temporary threshold shift (TTS) in the neural response at 4-8 kHz, but that the TTS is reduced to less than 10 dB following 12 daily exposures [Boettcher, J. Acoust. Soc. Am. 94, 3207-3214 (1993)]. This reduction in TTS is commonly referred to as resistance to noise-induced hearing loss (NIHL). To further analyze whether resistance to NIHL is caused by changes in the outer hair cell (OHC) system or the lateral wall system (or both), DPOAEs and EPs were measured in the exposed ears. The amplitudes of DPOAEs were significantly reduced in the frequency region from 4 to 10 kHz in subjects exposed to noise for 1 day, but were relatively normal in subjects exposed for 12 days. DPOAE amplitudes from frequency regions below the spectrum of the exposure were similar across the exposure and control groups except at the low-frequency edge of the noise where DPOAE amplitudes were consistently higher than normal in the exposed animals. The EP values in both exposure groups were not reduced from normal, unexposed levels. Thus there was no causal relationship between changes in the EP and the reduction of the DPOAE amplitudes. These data suggest that the development of resistance to noise is related to an initial depression of OHC activity followed by a recovery of activity to a stable level, despite an ongoing exposure.

Effect of focal cochlear vascular lesion on endocochlear potential in guinea pigs

The alteration endocochlear potential (EP) in response to total cochlear ischemia induced by various experimental manipulations has been studied. However, the effect of restricted areal damage to the microvessels (restricted to small area in the lateral wall of a cochlear turn) on the EP value is still unknown. In the current investigation we adopted a photochemical method to produce a focal (i.e., restricted area) microvessel injury in the lateral wall of the guinea pig cochlea and examined the effect of these insults on EP recorded in the same region. The small area of the microvessel lesion (small fenestra: approximately 0.2 × 0.4 mm 2) induced by photoactivation did not yield significant EP changes, suggesting that damage to such a small area of microcirculation in the lateral wall of the cochlea has no statistically significant effects on EP values. In subjects with a large area of the microvessel lesion (large fenestra: ≈ 0.2 × 0.8 mm 2), a decrease in the EP value (mean ± SEM 7.9 ± 0.8 mV) was noted. However, the control group animals with a large fenestra but without microvessel lesion also displayed a decrease (8.6 ± 0.8 mV) in EP. In the current study we were unable to differentiate whether the EP changes in animals with the large fenestra microvessel lesions were caused by the cochlear blood flow decrease or by the surgical preparation. However, the results of this study indicated if the EP value was affected by the large area of the microvessel lesion, the level of decrease would not be large. That is, the EP decrease was less than the change in the control group (mean: 8.6 mV). Considering the dependence of EP on blood flow, the data of this study suggest that compensatory mechanisms in the cochlea may maintain the EP following a focal lesion in the lateral wall of the cochlea. This study also indicates that the photochemical method provides a reliable approach to produce the animal model with the focal microvessel lesion in the lateral wall of the cochlea.

Expression of α and β subunit isoforms of Na,K-ATPase in the mouse inner ear and changes with mutations at the W v or Sl d loci

Mice homozygous for mutations at the viable dominant spotting (W v) and Steel-dickie (Sl d) loci exhibit a similar phenotype which includes deafness. The auditory dysfunction derives from failure of the stria vascularis to develop normally and to generate a high positive endocochlear potential (EP). Because strial function is driven by Na,K-ATPase its expression was investigated in inner ears of W v W v and Sl d Sl d mice and their wild-type littermates by immunostaining with antisera against four of the enzyme's subunit isoforms. Wild-type mice from two different genetic backgrounds showed an identical distribution of subunit isoforms among inner ear transport cells. Several epithelial cell types coexpressed the α1 and β1 subunits. Vestibular dark cells showed no reactivity for β1 but expressed abundant β2, whereas, strial marginal cells stained strongly for both β isoforms. The only qualitative difference between mutant and wild-type mice was the absence of β1 subunit in marginal cells of the mutant's stria. However, it is unlikely that this difference accounts for failure of mutants to generate a high EP because the β1 subunit is not present in the stria vascularis of either rats or gerbils with normal EP values. Strong immunostaining for Na,K-ATPase in lateral wall fibrocytes of normal mice along with diminished immunoreactivity in the mutants supports the concept that these strategically located transport fibrocytes actively resorb K + leaked across Reissner's membrane into scala vestibuli or effluxed from hair cells and nerves into scala tympani. It is further speculated that the resorbed K + normally is siphoned down its concentration gradient into the intrastrial space through gap junctions between fibrocytes and strial basal and intermediate cells where it is recycled back to endolymph via marginal cells. Thus, failure of mutants to generate a positive EP could be explained by the absence of intermediate cells which may form the final link in the conduit for moving K + from perilymph to the intrastrial compartment.

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 centred 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.