Ephaptic Effects Research Articles

Evolution of Hyperventilation-Induced Nystagmus in Acute Unilateral Vestibulopathy-Interpretative Model and Etiopathogenetic Hypotheses.

Hyperventilation induces metabolic changes that can elicit nystagmus (hyperventilation-induced nystagmus, HVIN) in various vestibular disorders, revealing vestibular imbalance and bringing out central or peripheral asymmetries. In acute unilateral vestibulopathy (AUVP, namely vestibular neuritis), hyperventilation can induce different patterns of nystagmus (excitatory, inhibitory, or negative), disclosing or modifying existing static vestibular asymmetries through its ability to invalidate compensation or increase peripheral excitability. In this context, we followed the evolutionary stages of HVIN in AUVP across 35 consecutive patients, with the goal of assessing alterations in the oculomotor pattern caused by hyperventilation over time. In the acute phase, the incidence of the excitatory pattern (and the strongly excitatory one, consisting of a reversal nystagmus evoked by hyperventilation) was significantly higher compared to the inhibitory pattern; then, a progressive reduction in the incidence of the excitatory pattern and a concomitant gradual increase in the incidence of the inhibitory one were observed in the follow-up period. Assuming the role of the ephaptic effect and the transient loss of vestibular compensation as opposing mechanisms, i.e., excitatory and inhibitory, respectively, the oculomotor pattern evoked by hyperventilation is the result of the interaction of these two factors. The data obtained allowed us to hypothesize an interpretative model regarding the pathogenetic aspects of responses evoked by hyperventilation and the etiologies of the disease: according to our hypotheses, the excitatory pattern implies a neuritic (viral) form of AUVP; instead, the inhibitory (and negative) one can be an expression of both the neuritic (viral) and vascular forms of the disease.

Electrophysiological damage to neuronal membrane alters ephaptic entrainment

The brain is commonly understood as a complex network system with a particular organization and topology that can result in specific electrophysiological patterns. Among all the dynamic elements resulting from the circuits of the brain’s network, ephapticity is a cellular communication mechanism that has received little attention. To understand the network’s properties of ephaptic entrainment, we start investigating the ephaptic effect on a single neuron. In this study, we used numerical simulations to examine the relationship between alterations in ephaptic neuronal entrainment and impaired electrophysiological properties of the neuronal membrane, which can occur via spike field coherence (SFC). This change in frequency band amplitude is observed in some neurodegenerative diseases, such as Parkinson’s or Alzheimer’s. To further investigate these phenomena, we proposed a damaged model based on the impairment of both the resistance of the ion channels and the capacitance of the lipid membrane. Therefore, we simulated ephaptic entrainment with the hybrid neural model quadratic integrate-and-fire ephaptic (QIF-E), which mimics an ephaptic entrainment generated by an LFP (simulate a neuronal group). Our results indicate a link between peak entrainment (ephapticity) preference and a shift in frequency band when damage occurs mainly in ion channels. Finally, we discuss possible relationships between ephaptic entrainment and neurodegenerative diseases associated with aging factors.

Role of ephaptic coupling in discordant alternans domain sizes and action potential propagation in the heart.

Discordant alternans, the spatially out-of-phase alternation of the durations of propagating action potentials in the heart, has been linked to the onset of fibrillation, a major cardiac rhythm disorder. The sizes of the regions, or domains, within which these alternations are synchronized are critical in this link. However, computer models employing standard gap junction-based coupling between cells have been unable to reproduce simultaneously the small domain sizes and rapid action potential propagation speeds seen in experiments. Here we use computational methods to show that rapid wave speeds and small domain sizes are possible when a more detailed model of intercellular coupling that accounts for so-called ephaptic effects is used. We provide evidence that the smaller domain sizes are possible, because different coupling strengths can exist on the wavefronts, for which both ephaptic and gap-junction coupling are involved, in contrast to the wavebacks, where only gap-junction coupling plays an active role. The differences in coupling strength are due to the high density of fast-inward (sodium) channels known to localize on the ends of cardiac cells, which are only active (and thus engage ephaptic coupling) during wavefront propagation. Thus, our results suggest that this distribution of fast-inward channels, as well as other factors responsible for the critical involvement of ephaptic coupling in wave propagation, including intercellular cleft spacing, play important roles in increasing the vulnerability of the heart to life-threatening tachyarrhythmias. Our results, combined with the absence of short-wavelength discordant alternans domains in standard gap-junction-dominated coupling models, also provide evidence that both gap-junction and ephaptic coupling are critical in wavefront propagation and waveback dynamics.

Cellular Size, Gap Junctions, and Sodium Channel Properties Govern Developmental Changes in Cardiac Conduction.

Electrical conduction in cardiac ventricular tissue is regulated via sodium (Na+) channels and gap junctions (GJs). We and others have recently shown that Na+channels preferentially localize at the site of cell-cell junctions, the intercalated disc (ID), in adult cardiac tissue, facilitating coupling via the formation of intercellular Na+nanodomains, also termed ephaptic coupling (EpC). Several properties governing EpC vary with age, including Na+channel and GJ expression and distribution and cell size. Prior work has shown that neonatal cardiomyocytes have immature IDs with Na+channels and GJs diffusively distributed throughout the sarcolemma, while adult cells have mature IDs with preferentially localized Na+channels and GJs. In this study, we perform an in silico investigation of key age-dependent properties to determine developmental regulation of cardiac conduction. Simulations predict that conduction velocity (CV) biphasically depends on cell size, depending on the strength of GJ coupling. Total cell Na+channel conductance is predictive of CV in cardiac tissue with high GJ coupling, but not correlated with CV for low GJ coupling. We find that ephaptic effects are greatest for larger cells with low GJ coupling typically associated with intermediate developmental stages. Finally, simulations illustrate how variability in cellular properties during different developmental stages can result in a range of possible CV values, with a narrow range for both neonatal and adult myocardium but a much wider range for an intermediate developmental stage. Thus, we find that developmental changes predict associated changes in cardiac conduction.

Modeling Depolarization Delay, Sodium Currents, and Electrical Potentials in Cardiac Transverse Tubules

T-tubules are invaginations of the lateral membrane of striated muscle cells that provide a large surface for ion channels and signaling proteins, thereby supporting excitation–contraction coupling. T-tubules are often remodeled in heart failure. To better understand the electrical behavior of T-tubules of cardiac cells in health and disease, this study addresses two largely unanswered questions regarding their electrical properties: (1) the delay of T-tubular membrane depolarization and (2) the effects of T-tubular sodium current on T-tubular potentials. Here, we present an elementary computational model to determine the delay in depolarization of deep T-tubular membrane segments as the narrow T-tubular lumen provides resistance against the extracellular current. We compare healthy tubules to tubules with constrictions and diseased tubules from mouse and human, and conclude that constrictions greatly delay T-tubular depolarization, while diseased T-tubules depolarize faster than healthy ones due to tubule widening. Increasing the tubule length non-linearly delays the depolarization. We moreover model the effect of T-tubular sodium current on intraluminal T-tubular potentials. We observe that extracellular potentials become negative during the sodium current transient (up to −40 mV in constricted T-tubules), which feedbacks on sodium channel function (self-attenuation) in a manner resembling ephaptic effects that have been described for intercalated discs where opposing membranes are very close together. The intraluminal potential and sodium current self-attenuation however greatly depend on sodium current conductance. These results show that (1) the changes in passive electrical properties of remodeled T-tubules cannot explain the excitation–contraction coupling defects in diseased cells; and (2) the sodium current may modulate intraluminal potentials. Such extracellular potentials might also affect excitation–contraction coupling and macroscopic conduction.

Slow moving neural source in the epileptic hippocampus can mimic progression of human seizures

Fast and slow neural waves have been observed to propagate in the human brain during seizures. Yet the nature of these waves is difficult to study in a surgical setting. Here, we report an observation of two different traveling waves propagating in the in-vitro epileptic hippocampus at speeds similar to those in the human brain. A fast traveling spike and a slow moving wave were recorded simultaneously with a genetically encoded voltage sensitive fluorescent protein (VSFP Butterfly 1.2) and a high speed camera. The results of this study indicate that the fast traveling spike is NMDA-sensitive but the slow moving wave is not. Image analysis and model simulation demonstrate that the slow moving wave is moving slowly, generating the fast traveling spike and is, therefore, a moving source of the epileptiform activity. This slow moving wave is associated with a propagating neural calcium wave detected with calcium dye (OGB-1) but is independent of NMDA receptors, not related to ATP release, and much faster than those previously recorded potassium waves. Computer modeling suggests that the slow moving wave can propagate by the ephaptic effect like epileptiform activity. These findings provide an alternative explanation for slow propagation seizure wavefronts associated with fast propagating spikes.

Distribution of cardiac sodium channels in clusters potentiates ephaptic interactions in the intercalated disc.

It has been proposed that ephaptic conduction, relying on interactions between the sodium (Na+ ) current and the extracellular potential in intercalated discs, might contribute to cardiac conduction when gap junctional coupling is reduced, but this mechanism is still controversial. In intercalated discs, Na+ channels form clusters near gap junction plaques, but the functional significance of these clusters has never been evaluated. In HEK cells expressing cardiac Na+ channels, we show that restricting the extracellular space modulates the Na+ current, as predicted by corresponding simulations accounting for ephaptic effects. In a high-resolution model of the intercalated disc, clusters of Na+ channels that face each other across the intercellular cleft facilitate ephaptic impulse transmission when gap junctional coupling is reduced. Thus, our simulations reveal a functional role for the clustering of Na+ channels in intercalated discs, and suggest that rearrangement of these clusters in disease may influence cardiac conduction. It has been proposed that ephaptic interactions in intercalated discs, mediated by extracellular potentials, contribute to cardiac impulse propagation when gap junctional coupling is reduced. However, experiments demonstrating ephaptic effects on the cardiac Na+ current (INa ) are scarce. Furthermore, Na+ channels form clusters around gap junction plaques, but the electrophysiological significance of these clusters has never been investigated. In patch clamp experiments with HEK cells stably expressing human Nav 1.5 channels, we examined how restricting the extracellular space modulates INa elicited by an activation protocol. In parallel, we developed a high-resolution computer model of the intercalated disc to investigate how the distribution of Na+ channels influences ephaptic interactions. Approaching the HEK cells to a non-conducting obstacle always increased peak INa at step potentials near the threshold of INa activation and decreased peak INa at step potentials far above threshold (7 cells, P=0.0156, Wilcoxon signed rank test). These effects were consistent with corresponding control simulations with a uniform Na+ channel distribution. In the intercalated disc computer model, redistributing the Na+ channels into a central cluster of the disc potentiated ephaptic effects. Moreover, ephaptic impulse transmission from one cell to another was facilitated by clusters of Na+ channels facing each other across the intercellular cleft when gap junctional coupling was reduced. In conclusion, our proof-of-principle experiments demonstrate that confining the extracellular space modulates cardiac INa , and our simulations reveal the functional role of the aggregation of Na+ channels in the perinexus. These findings highlight novel concepts in the physiology of cardiac excitation.

Ephaptic Effects Potentiate the Threshold Behavior of the Cardiac Sodium Current in a High Resolution Mathematical Model of a Narrow Intercellular Cleft

Cardiac arrhythmias frequently result from disorders of impulse propagation. Recent studies proposed that electrical potentials occurring in intercalated discs may contribute to cardiac conduction in addition to gap junctional coupling (ephaptic conduction). Furthermore, Na+ channels are found in clusters near gap junctions, suggesting that Na+ channel distribution in intercalated discs may modulate ephaptic intercellular interactions. Our aim was to investigate computationally how cardiac Na+ current (INa) dynamics are influenced in a narrow extracellular cleft by the extracellular potential (Ve) and by the distribution of Na+ channels. The model consisted of a disc-shaped cleft discretized at a high resolution separating either one cell membrane containing Na+ channels and a non-conductive obstacle, or two excitable membranes representing an intercalated disc. INa was modeled using the Hodgkin-Huxley formalism. The intracellular domain of the first cell was subjected to a voltage-clamp activation protocol while the intracellular potential of the second cell was clamped to the resting potential. In the model with one membrane facing a non-conducting obstacle, decreasing cleft width from 1000 to 20 nm resulted in decreasing INa peak intensity at voltage steps ≥-30 mV. This decrease was caused by the negative Ve in the cleft, which decreased the driving force for INa. However, at voltage steps just above threshold, INa was activated in a ring-shaped peripheral region. This resulted in larger INa in simulations with clefts 10-200 nm wide. When total INa was considered, the negative Ve resulted in a cleft-dependent shift of the steady state activation curve to more negative potentials and a steepening of the curve. Narrowing the cleft thus lowered and accentuated the threshold of INa. These effects were more prominent in larger discs and when Na+ channels were redistributed in the center of the disc. In the two-cell model, the negative Ve caused by INa in the first cell resulted in activation of INa in the second cell after a ∼1 ms delay for clefts <70 nm. Therefore, the Ve caused by INa in a narrow restricted extracellular space exerts a major feedback on Na+ channel behavior, modulating the voltage-dependence of activation and the threshold behavior of INa, and thus cellular excitability. These effects are strongly influenced by the spatial distribution of Na+ channels. These findings are relevant for a comprehensive understanding of cardiac excitation.

Estimating Neural Background Input with Controlled and Fast Perturbations: A Bandwidth Comparison between Inhibitory Opsins and Neural Circuits.

To test the importance of a certain cell type or brain area it is common to make a “lack of function” experiment in which the neuronal population of interest is inhibited. Here we review physiological and methodological constraints for making controlled perturbations using the corticothalamic circuit as an example. The brain with its many types of cells and rich interconnectivity offers many paths through which a perturbation can spread within a short time. To understand the side effects of the perturbation one should record from those paths. We find that ephaptic effects, gap-junctions, and fast chemical synapses are so fast that they can react to the perturbation during the few milliseconds it takes for an opsin to change the membrane potential. The slow chemical synapses, astrocytes, extracellular ions and vascular signals, will continue to give their physiological input for around 20 ms before they also react to the perturbation. Although we show that some pathways can react within milliseconds the strength/speed reported in this review should be seen as an upper bound since we have omitted how polysynaptic signals are attenuated. Thus the number of additional recordings that has to be made to control for the perturbation side effects is expected to be fewer than proposed here. To summarize, the reviewed literature not only suggests that it is possible to make controlled “lack of function” experiments, but, it also suggests that such a “lack of function” experiment can be used to measure the context of local neural computations.

Can Neural Activity Propagate by Endogenous Electrical Field?

It is widely accepted that synaptic transmissions and gap junctions are the major governing mechanisms for signal traveling in the neural system. Yet, a group of neural waves, either physiological or pathological, share the same speed of ∼0.1 m/s without synaptic transmission or gap junctions, and this speed is not consistent with axonal conduction or ionic diffusion. The only explanation left is an electrical field effect. We tested the hypothesis that endogenous electric fields are sufficient to explain the propagation with in silico and in vitro experiments. Simulation results show that field effects alone can indeed mediate propagation across layers of neurons with speeds of 0.12 ± 0.09 m/s with pathological kinetics, and 0.11 ± 0.03 m/s with physiologic kinetics, both generating weak field amplitudes of ∼2-6 mV/mm. Further, the model predicted that propagation speed values are inversely proportional to the cell-to-cell distances, but do not significantly change with extracellular resistivity, membrane capacitance, or membrane resistance. In vitro recordings in mice hippocampi produced similar speeds (0.10 ± 0.03 m/s) and field amplitudes (2.5-5 mV/mm), and by applying a blocking field, the propagation speed was greatly reduced. Finally, osmolarity experiments confirmed the model's prediction that cell-to-cell distance inversely affects propagation speed. Together, these results show that despite their weak amplitude, electric fields can be solely responsible for spike propagation at ∼0.1 m/s. This phenomenon could be important to explain the slow propagation of epileptic activity and other normal propagations at similar speeds.

Microdomain Effects on Transverse Cardiac Propagation

The effect of gap junctional coupling, sodium ion channel distribution, and extracellular conductivity on transverse conduction in cardiac tissue is explored using a microdomain model that incorporates aspects of the inhomogeneous cellular structure. The propagation velocities found in our model are compared to those in the classic bidomain model and indicate a strong ephaptic microdomain contribution to conduction depending on the parameter regime. We show that ephaptic effects can be quite significant in the junctional spaces between cells, and that the cell activation sequence is modified substantially by these effects. Further, we find that transverse propagation can be maintained by ephaptic effects, even in the absence of gap junctional coupling. The mechanism by which this occurs is found to be cablelike in that the junctional regions act like inverted cables. Our results provide insight into several recent experimental studies that indirectly indicate a mode of action potential propagation that does not rely exclusively on gap junctions.

Does Ephaptic Coupling Contribute to Propagation in Cardiac Tissue?

Does Ephaptic Coupling Contribute to Propagation in Cardiac Tissue?

Ephaptic Coupling in Cardiac Myocytes

While it is widely believed that conduction in cardiac tissue is regulated by gap junctions, recent experimental evidence suggests that the extracellular space may play a significant role in action potential propagation. Cardiac tissue with low gap junctional coupling still exhibits conduction, with conflicting degrees of slowing that may be due to variations in the extracellular space. Inhomogeneities in the extracellular space caused by the complex cellular structure in cardiac tissue can lead to ephaptic, or field effect, coupling. Here, we present data from simulations of a cylindrical strand of cells in which we see the dramatic effect highly resistant extracellular spaces have on propagation velocity. We find that ephaptic effects occur in all areas of small extracellular spaces and are not restricted to the junctional cleft between cells. This previously unrecognized type of field coupling, which we call lateral coupling, can allow conduction in the absence of gap junctions. We compare our results with the classically used cable theory, demonstrating the quantitative difference in propagation velocity arising from the cellular geometry. Ephaptic effects are shown to be highly dependent upon parameter values, frequently enhancing, but sometimes decreasing propagation speed. Our mathematical analysis incorporates the inhomogeneities in the extracellular microdomains that cannot be directly measured by experimental techniques and will aid in optimizing cardiac treatments that require manipulation of the cellular geometry and understanding heart functionality.

Simulation of the Ephaptic Effect in the Cone--Horizontal Cell Synapse of the Retina

The drift-diffusion (Poisson--Nernst--Planck) model---including a numerical model for cell membranes that resolves surface-charge boundary layers---is applied to the cone--horizontal cell synapse in the outer plexiform layer of the retina. Numerical simulations reproduce the experimental calcium current-voltage (IV) curves for the goldfish retina in response to a bright spot, with and without an illuminated background. The ephaptic (electrical) effect is demonstrated by computing the shift in the IV curve for background off versus background on for increasingly narrower openings between the sides of the cone and the horizontal cell.

Non-synaptic inhibition between grouped neurons in an olfactory circuit

SummaryDiverse sensory organs, including mammalian taste buds and insect chemosensory sensilla, show a striking compartmentalization of receptor cells. However, the functional impact of this organization remains unclear. Here we show that compartmentalized Drosophila olfactory receptor neurons (ORNs) communicate with each other directly. The sustained response of one ORN is inhibited by the transient activation of a neighboring ORN. Mechanistically, such lateral inhibition does not depend on synapses and is likely mediated by ephaptic coupling. Moreover, lateral inhibition in the periphery can modulate olfactory behavior. Together, the results show that integration of olfactory information can occur via lateral interactions between ORNs. Inhibition of a sustained response by a transient response may provide a means of encoding salience. Finally, a CO2-sensitive ORN in the malaria mosquito Anopheles can also be inhibited by excitation of an adjacent ORN, suggesting a broad occurrence of lateral inhibition in insects and possible applications in insect control.

Intimate neuronal whispers

It's a touching story of cohabitation and meaningful communication. Two neighbouring fruitfly neurons talk to each other not by means of synaptic junctions but by interactions through the surrounding electrical field. See Article p.66 It has been suggested that the electrical activity of one neuron can influence that of its unlinked neighbour, but this so-called ephaptic effect is widely assumed to be weak and has been relatively little studied. Here, John Carlson and colleagues demonstrate that olfactory receptor neurons (ORNs) of fruitflies communicate with one another through large ephaptic signals with significant impact on olfactory behaviour. The sustained response of one ORN is inhibited by the transient activation of its neighbour. The authors also demonstrate that a CO2-sensitive ORN in the malaria mosquito Anopheles can be inhibited by excitation of an adjacent ORN and that lateral inhibition of a CO2 neuron in Drosophila can reduce a behavioural response to CO2. As CO2 is a key cue used by mosquitoes to find human hosts, this work suggests a novel means of insect control. These findings suggest that ephaptic effects may be more widespread than previously appreciated, particularly among neurons housed in confined peripheral sensory compartments such as taste buds.

Modeling electrical activity of myocardial cells incorporating the effects of ephaptic coupling.

Existing models of electrical activity in myocardial tissue are unable to easily capture the effects of ephaptic coupling. Homogenized models do not account for cellular geometry, while detailed spatial models are too complicated to simulate in three dimensions. Here we propose a unique model that accurately captures the geometric effects while being computationally efficient. We use this model to provide an initial study of the effects of changes in extracellular geometry, gap junctional coupling, and sodium ion channel distribution on propagation velocity in a single 1D strand of cells. In agreement with previous studies, we find that ephaptic coupling increases propagation velocity at low gap junctional conductivity while it decreases propagation at higher conductivities. We also find that conduction velocity is relatively insensitive to gap junctional coupling when sodium ion channels are located entirely on the cell ends and cleft space is small. The numerical efficiency of this model, verified by comparison with more detailed simulations, allows a thorough study in parameter variation and shows that cellular structure and geometry has a nontrivial impact on propagation velocity. This model can be relatively easily extended to higher dimensions while maintaining numerical efficiency and incorporating ephaptic effects through modeling of complex, irregular cellular geometry.

Mixture Interactions in Moth Olfactory Physiology: Examining the Effects of Odorant Mixture, Concentration, Distal Stimulation, and Antennal Nerve Transection on Sensillar Responses

The insect olfactory system is challenged to decipher valid signals from among an assortment of chemical cues present in the airborne environment. In the moth, Heliothis virescens, males rely upon detection and discrimination of a unique blend of components in the female sex pheromone to locate mates. The effect of variable odor mixtures was used to examine physiological responses from neurons within sensilla on the moth antenna sensitive to female sex pheromone components. Increasing concentrations of heliothine sex pheromone components applied in concert with the cognate stimulus for each neuronal type resulted in mixture suppression of activity, except for one odorant combination where mixture enhancement was apparent. Olfactory receptor neuron (ORN) responses were compared between moths with intact and transected antennal nerves to determine whether specific instances of suppression might be influenced by central mechanisms. Type A sensilla showed little variation in response between transected and intact preparations; however, recordings from type B sensilla with transected antennal nerves exhibited reduced mixture suppression. Testing by parallel stimulation of distal antennal segments while recording and stimulating proximal segments dismissed the possibility of interneuronal or ephaptic effects upon sensillar responses. The results indicate that increasing concentrations of "noncognate" odorants in an odor mixture or antennal nerve transection can produce variation in the intensity and temporal dynamics of physiological recordings from H. virescens ORNs.

Initial results from a model of ephaptic excitation in the electrically excited peripheral auditory nervous system

This article reports on a study performed to investigate the occurrence and effect of ephaptic excitation in electrical stimulation of the auditory system. The objective of the study was to quantify the influence of ephaptic excitation on nerve stimulation and determine whether it is a necessary factor in neuromodeling. It was shown with a simple model that ephaptic excitation could be important at stimulus intensities close to threshold. The results show that the contribution of ephaptic excitation is significant up to at least 6–7 dB above threshold. Cochlear implant patients normally have a small dynamic range (average of 7 dB), indicating that the ephaptic effect might be important in models of the implanted cochlea.

Proton-Mediated Feedback Inhibition of Presynaptic Calcium Channels at the Cone Photoreceptor Synapse

Generation of center-surround antagonistic receptive fields in the outer retina occurs via inhibitory feedback modulation of presynaptic voltage-gated calcium channels in cone photoreceptor synaptic terminals. Both conventional and unconventional neurotransmitters, as well as an ephaptic effect, have been proposed, but the intercellular messaging that mediates the inhibitory feedback signal from postsynaptic horizontal cells (HCs) to cones remains unknown. We examined the possibility that proton concentration in the synaptic cleft is regulated by HCs and that it carries the feedback signal to cones. In isolated, dark-adapted goldfish retina, we assessed feedback in the responses of HCs to light and found that strengthened pH buffering reduced both rollback and the depolarization to red light. In zebrafish retinal slices loaded with Fluo-4, depolarization with elevated K(+) increased Ca signals in the synaptic terminals of cone photoreceptors. Kainic acid, which depolarizes HCs but has no direct effect on cones, depressed the K(+)-induced Ca signal, whereas CNQX, which hyperpolarizes HCs, increased the Ca signals, suggesting that polarization of HCs alters inhibitory feedback to cones. We found that these feedback signals were blocked by elevated extracellular pH buffering, as well as amiloride and divalent cations. Voltage clamp of isolated HCs revealed an amiloride-sensitive conductance that could mediate modulation of cleft pH dependent on the membrane potential of these postsynaptic cells.