Long-lasting modification of intrinsic discharge properties in subicular neurons following status epilepticus

A single episode of status epilepticus (SE) induces neuropathological changes in the brain that may lead to the development of a permanent epileptic condition. Most studies of this plasticity have focused on the hippocampus, where both synaptic function and intrinsic neuronal excitability have been shown to be persistently modified by SE. However, many other brain structures are activated during SE and may also be involved in the subsequent epileptogenic process. Here we have investigated whether SE, induced in rats with pilocarpine and terminated after 40 min with diazepam, persistently modifies the intrinsic excitability of pyramidal neurons in the subiculum. Subicular slices were prepared from control and SE-experienced rats (2-5 weeks after SE). In the control group, only 4% of the neurons fired bursts in response to intrasomatic, threshold-straddling depolarizing current pulses (low-threshold bursters). The remaining neurons either fired bursts in response to strong (3x threshold) depolarizations (35%; high-threshold bursters) or fired in a completely regular mode (61%; nonbursters). In the SE-experienced group, the fractions of low- and high-threshold bursters markedly increased to 29% and 53%, respectively. This change in firing behaviour was associated with a marked increase in the size of the spike after depolarization, particularly in low-threshold bursters. Experimental suppression of Ca2+ currents selectively blocked low-threshold bursting but did not affect high-threshold bursting, suggesting that a dual Ca2+- dependent and Ca2+- independent mechanism controls bursting in these neurons. The persistent up-regulation of intrinsic bursting in the subiculum, in concert with similar changes in the hippocampus, undoubtedly contributes to epileptogenesis following pilocarpine-induced SE.     

Electrophysiological and morphological diversity of neurons from the rat subicular complex in vitro

We combined whole-cell recordings with Neurobiotin labeling to examine the electrophysiological and morphological properties of neurons from the ventral subicular complex in vitro (including the subicular, presubicular, and parasubicular areas). No a priori morphological sampling criteria were used to select cells. Cells were classified as bursting (IB), regular-spiking (RS), and fast-spiking (FS) according to their firing patterns in response to depolarizing current pulses. A number of cells remained unclassified. We found 54% RS, 26% IB, 11% FS, and 9% unclassified cells out of a total of 131 neurons examined. We also found cells showing intrinsic membrane potential oscillations (MPO) (6%), which represented a subgroup of the unclassified cells. We analyzed several electrophysiological parameters and found that RS and IB cells can be subclassified into two separate subgroups. RS cells were subclassified as tonic and adapting, according to the degree of firing adaptation. Both responded with single spikes to orthodromic stimulation. IB cells were subclassified in two subgroups according to their capacity to fire more than one burst, and showed different responses to orthodromic stimulation. We observed that bursting in these two subgroups appeared to involve both Ca2+ and persistent Na+ components. Both IB and RS cells, as well as MPO neurons, were projecting cells. FS cells were morphologically identified as local circuit interneurons. We also analyzed the spatial distribution of these cell types from the vicinity of CA1 to the parasubicular areas. We conclude that, in contrast to the commonly accepted idea of the subicular complex as a bursting structure, there is a wide electrophysiological variability even within a given cellular group.     

Epileptiform activity induces distance‐dependent alterations of the Ca2+ extrusion mechanism in the apical dendrites of subicular pyramidal neurons

The cellular and molecular mechanisms that underlie acquired changes in Ca²⁺ dynamics of different neuronal compartments are important in the induction and maintenance of epileptiform activity. Simultaneous electrophysiology and Ca²⁺ imaging techniques were used to understand the basic properties of dendritic Ca²⁺ signaling in rat subicular pyramidal neurons during epileptiform activity. Distance‐dependent changes in the Ca²⁺ decay kinetics locked to spontaneous epileptiform discharges and back‐propagating action potentials were observed in the apical dendrites. A decrement in the mean τ value of Ca²⁺ decay was observed in distal parts (95–110 μm) of the apical dendrites compared with proximal segments (30–45 μm) in in‐vitro epileptic conditions but not in control. Pharmacological agents that block Ca²⁺ transporters, i.e. Na⁺/ Ca²⁺ exchangers (Benzamil), plasma membrane Ca²⁺‐ATPase pumps (Calmidazolium) and smooth endoplasmic reticulum Ca²⁺‐ATPase pumps (Thapsigargin), were applied locally to the proximal and distal part of the apical dendrites in both experimental conditions to understand the molecular aspects of the Ca²⁺ extrusion mechanisms. The relative contribution of Na⁺/Ca²⁺ exchangers in Ca²⁺ extrusion was higher in the distal apical dendrites in the in‐vitro epileptic condition and this property modulated the excitability of the neuron in simulation. The Ca²⁺ homeostatic mechanisms that restore normal Ca²⁺ levels could play a major neuroprotective role in the distal dendrites that receive synaptic inputs.     

Periodic bursting activities of locus coeruleus neurons in the rat

Unit activity of locus coeruleus (LC) neurons in rats was investigated. After the animal recovered from anesthesia, the spontaneous activity exhibited periodic bursting discharges at about 15–30 s intervals. The oscillation was observed to last for a long time (1–3 h). It is suggested that many LC neurons exhibited the oscillation synchronously during stress in the awake animal.     

Target-specific output patterns are predicted by the distribution of regular-spiking and bursting pyramidal neurons in the subiculum

Pyramidal neurons in the subiculum project to a variety of cortical and subcortical areas in the brain to convey information processed in the hippocampus. Previous studies have shown that two groups of subicular pyramidal neurons--regular-spiking and bursting neurons--are distributed in an organized fashion along the proximal-distal axis, with more regular-spiking neurons close to CA1 (proximal) and more bursting neurons close to presubiculum (distal). Anatomically, neurons projecting to some targets are located more proximally along this axis, while others are located more distally. However, the relationship between the firing properties and the targets of subicular pyramidal neurons is not known. To study this relationship, we used in vivo injections of retrogradely transported fluorescent beads into each of nine different regions and conducted whole-cell current-clamp recordings from the bead-containing subicular neurons in acute brain slices. We found that subicular projections to each area were composed of a mixture of regular-spiking and bursting neurons. Neurons projecting to amygdala, lateral entorhinal cortex, nucleus accumbens, and medial/ventral orbitofrontal cortex were located primarily in the proximal subiculum and consisted mostly of regular-spiking neurons (～80%). By contrast, neurons projecting to medial EC, presubiculum, retrosplenial cortex, and ventromedial hypothalamus were located primarily in the distal subiculum and consisted mostly of bursting neurons (～80%). Neurons projecting to a thalamic nucleus were located in the middle portion of subiculum, and their probability of bursting was close to 50%. Thus, the fraction of bursting neurons projecting to each target region was consistent with the known distribution of regular-spiking and bursting neurons along the proximal-distal axis of the subiculum. Variation in the distribution of regular-spiking and bursting neurons suggests that different types of information are conveyed from the subiculum to its various targets.     

Persistent sodium current in subicular neurons isolated from patients with temporal lobe epilepsy

The persistent sodium current is a common target of anti-epileptic drugs and contributes to burst firing. Intrinsically burst firing subicular neurons are involved in the generation and spread of epileptic activity. We measured whole-cell sodium currents in pyramidal neurons isolated from the subiculum resected in drug-resistant epileptic patients and in rats. In half of the cells from both patients and rats, the sodium current inactivated within 500 ms at -30 mV. Others displayed a tetrodotoxin-sensitive slowly or non-inactivating sodium current of up to 53% of the total sodium current amplitude. Compared with the transient sodium current in the same cells, this persistent sodium current activated with normal kinetics but its voltage-dependent activation occurred 7 mV more hyperpolarized. Depolarizing voltage steps that lasted 10 s completely inactivated the persistent sodium current. Its voltage dependence did not differ from that of the transient sodium current but its slope was less steep. The voltage dependence and kinetics of the persistent sodium current in cells from patients were not different from that in subicular cells from rats. The current density and the relative amplitude contribution were 3-4 times greater in neurons from drug-resistant epilepsy patients. The abundant presence of persistent sodium current in half of the subicular neurons could lead to a larger number of neurons with intrinsic burst firing. The extraordinarily large amplitude of the persistent sodium current in this subset of subicular neurons might explain why these patients are susceptible to seizures and hard to treat pharmacologically.     

Transition in subicular burst firing neurons from epileptiform activity to suppressed state by feedforward inhibition

The subiculum, a para‐hippocampal structure positioned between the cornu ammonis 1 subfield and the entorhinal cortex, has been implicated in temporal lobe epilepsy in human patients and in animal models of epilepsy. The structure is characterized by the presence of a significant population of burst firing neurons that has been shown previously to lead epileptiform activity locally. Phase transitions in epileptiform activity in neurons following a prolonged challenge with an epileptogenic stimulus has been shown in other brain structures, but not in the subiculum. Considering the importance of the subicular burst firing neurons in the propagation of epileptiform activity to the entorhinal cortex, we have explored the phenomenon of phase transitions in the burst firing neurons of the subiculum in an in vitro rat brain slice model of epileptogenesis. Whole‐cell patch‐clamp and extracellular field recordings revealed a distinct phenomenon in the subiculum wherein an early hyperexcitable state was followed by a late suppressed state upon continuous perfusion with epileptogenic 4‐aminopyridine and magnesium‐free medium. The suppressed state was characterized by inhibitory post‐synaptic potentials in pyramidal excitatory neurons and bursting activity in local fast‐spiking interneurons at a frequency of 0.1–0.8 Hz. The inhibitory post‐synaptic potentials were mediated by GABA_A receptors that coincided with excitatory synaptic inputs to attenuate action potential discharge. These inhibitory post‐synaptic potentials ceased following a cut between the cornu ammonis 1 and subiculum. The suppression of epileptiform activity in the subiculum thus represents a homeostatic response towards the induced hyperexcitability. Our results suggest the importance of feedforward inhibition in exerting this homeostatic control.     

Electrophysiology of regular firing cells in the rat perirhinal cortex.

The electrophysiological properties of neurons in the rat perirhinal cortex were analyzed with intracellular recordings in an in vitro slice preparation. Cells included in this study (n = 59) had resting membrane potential (RMP) = -73.9 +/- 8.5 mV (mean +/- SD), action potential amplitude = 95.5 +/- 10.4 mV, input resistance = 36.1 +/- v 15.7 M omega, and time constant = 13.9 +/- 3.4 ms. When filled with neurobiotin (n = 27) they displayed a pyramidal shape with an apical dendrite and extensive basal dendritic tree. Injection of intracellular current pulses revealed: 1) a tetrodotoxin (TTX, 1 microM)-sensitive, inward rectification in the depolarizing direction (n = 6), and 2) a time- and voltage-dependent hyperpolarizing sag that was blocked by extracellular Cs+ (3 mM, n = 5) application. Prolonged (up to 3 s) depolarizing pulses made perirhinal cells discharge regular firing of fast action potentials that diminished over time in frequency and reached a steady level (i.e., adapted). Repetitive firing was followed by an afterhyperpolarization that was decreased, along with firing adaptation, by the Ca(2+)-channel blocker Co2+ (2 mM, n = 6). Action potential broadening became evident during repetitive firing. This behavior, which was more pronounced when larger pulses of depolarizing current were injected (and thus when repetitive firing attained higher rates), was markedly decreased by Co2+ application. Subthreshold membrane oscillations at 5-12 Hz became apparent when cells were depolarized by 10-20 mV from RMP, and action potential clusters appeared with further depolarization. Application of glutamatergic and GABAA receptor antagonists (n = 4), CO2+ (n = 6), or Cs+ (n = 5) did not prevent the occurrence of these oscillations that were abolished by TTX (n = 6). Our results show that pyramidal-like neurons in the perirhinal cortex are regular firing cells with electrophysiological features resembling those of other cortical pyramidal elements. The ability to generate subthreshold membrane oscillations may play a role in synaptic plasticity and thus in the mnemonic processes typical of this limbic structure.     

Stability of subicular place fields across multiple light and dark transitions

Although hippocampal CA1 place cells can be strongly modulated by visual inputs, the effect of visual modulation on place cells in other areas of the hippocampal formation, such as the subiculum, has been less extensively explored. Here, we investigated the role of visual inputs on the activity of subicular place cells by manipulating ambient light levels while freely‐moving rats foraged for food. Rats were implanted with tetrodes in the dorsal subiculum and units were recorded while the animal performed a pellet‐chasing task during multiple light‐to‐dark and dark‐to‐light transitions. We found that subicular place fields presented a somewhat heterogeneous response to light–dark transitions, with 45% of pyramidal units showing stable locational firing across multiple light–dark–light transitions. These data suggest that visual inputs may participate in spatial information processing by the subiculum. However, as a plurality of units was stable across light–dark transitions, we suggest that the subiculum supports, probably in association with the grid cells of the entorhinal cortex, the neurocognitive processing underlying path integration.     

Somatostatin depresses the excitability of subicular bursting cells: Roles of inward rectifier K+ channels,KCNQ channels and Epac

The hippocampus is a crucial component for cognitive and emotional processing. The subiculum provides much of the output for this structure but the modulation and function of this region is surprisingly under‐studied. The neuromodulator somatostatin (SST) interacts with five subtypes of SST receptors (sst₁ to sst₅) and each of these SST receptor subtypes is coupled to Gi proteins resulting in inhibition of adenylyl cyclase (AC) and decreased level of intracellular cAMP. SST modulates many physiological functions including cognition, emotion, autonomic responses and locomotion. Whereas SST has been shown to depress neuronal excitability in the subiculum, the underlying cellular and molecular mechanisms have not yet been determined. Here, we show that SST hyperpolarized two classes of subicular neurons with a calculated EC₅₀ of 0.1 μM. Application of SST (1 μM) induced outward holding currents by primarily activating K⁺ channels including the G‐protein‐activated inwardly‐rectifying potassium channels (GIRK) and KCNQ (M) channels, although inhibition of cation channels in some cells may also be implicated. SST‐elicited hyperpolarization was mediated by activation of sst₂ receptors and required the function of G proteins. The SST‐induced hyperpolarization resulted from decreased activity of AC and reduced levels of cAMP but did not require the activity of either PKA or PKC. Inhibition of Epac2, a guanine nucleotide exchange factor, partially blocked SST‐mediated hyperpolarization of subicular neurons. Furthermore, application of SST resulted in a robust depression of subicular action potential firing and the SST‐induced hyperpolarization was responsible for its inhibitory action on LTP at the CA1‐subicilum synapses. Our results provide a novel cellular and molecular mechanism that may explain the roles of SST in modulation of subicular function and be relevant to SST‐related physiological functions.     

Responses of prepositus hypoglossi neurons to optokinetic and vestibular stimulations in the rat

The responses of 47 nucleus prepositus hypoglossi neurons to vestibular optokinetic stimulations in the horizontal plane were recorded in immobilized, pigmented rats. During sinusoidal vestibular stimulation in the dark, type II (62%) and type I (38%) responses were recorded. In addition to the sinusoidal modulation of firing rate, units often showed fast rhythmic increases or decreases in firing (nystagmic modulation). The mean phase of the response relative acceleration measured at 0.025 and 0.2 Hz were 19 and 84 deg., respectively. Some units (25%) showed larger phase-lags. The sensitivities of unit responses at 0.025 and 0.2 Hz were 1.6 and 0.5 spikes × s⁻¹/deg × s⁻², respectively.The responses of NPH neurons to binocular optokinetic stimulation were divided in 2 classes: (i) neurons with unidirectional responses (18%) were excited by stimuli moving towards the side of recording and showed no change in firing on oppositely directed stimulation; all of them showed a type II pattern during vestibular stimulation; (ii) bidirectional responses showed an increase in one direction and a decrease in firing for stimulation in the opposite direction. In every case the optokinetic responses were synergistic with the vestibular responses, which consisted of both type I and type II units.On the basis of the directionality of their optokinetic response, the value of their time constants and the shape of their velocity tuning curves, it is suggested that unidirectional type II NPH neurons could serve as relays in the optokinetic pathways between NRTP (or PT) and vestibular neurons. Some other neurons, having time constants particularly long and different for the rising and falling of the response, probably serve other functions.     

In vitro electrophysiology of neurons in the lateral dorsal tegmental nucleus

The lateral dorsal tegmental nucleus (LDT) provides ascending cholinergic projections to forebrain structures such as prefrontal cortex, septum, habenula, and thalamus, but relatively little is known of the physiology of LDT neurons. Intracellular recordings from LDT neurons in guinea pig brain slices found that most neurons fired action potentials either tonically or in bursts. The voltage dependent characteristics of the neurons suggest that a prolonged afterhyperpolarization due to an outward potassium current and a low-threshold calcium conductance contributed to these two modes of firing. Intracellular injections of Lucifer Yellow and subsequent staining for NADPH-diaphorase activity permitted positive identification of cholinergic neurons.     

M‐type channels selectively control bursting in rat dopaminergic neurons

Midbrain dopaminergic neurons in the substantia nigra, pars compacta and ventral tegmental area are critically important in many physiological functions. These neurons exhibit firing patterns that include tonic slow pacemaking, irregular firing and bursting, and the amount of dopamine that is present in the synaptic cleft is much increased during bursting. The mechanisms responsible for the switch between these spiking patterns remain unclear. Using both in‐vivo recordings combined with microiontophoretic or intraperitoneal drug applications and in‐vitro experiments, we have found that M‐type channels, which are present in midbrain dopaminergic cells, modulate the firing during bursting without affecting the background low‐frequency pacemaker firing. Thus, a selective blocker of these channels, 10,10‐bis(4‐pyridinylmethyl)‐9(10H)‐anthracenone dihydrochloride, specifically potentiated burst firing. Computer modeling of the dopamine neuron confirmed the possibility of a differential influence of M‐type channels on excitability during various firing patterns. Therefore, these channels may provide a novel target for the treatment of dopamine‐related diseases, including Parkinson’s disease and drug addiction. Moreover, our results demonstrate that the influence of M‐type channels on the excitability of these slow pacemaker neurons is conditional upon their firing pattern.     

Topographical and laminar organization of subicular projections to the parahippocampal region of the rat

In this study, we analyzed in detail the topographic organization of the subiculoparahippocampal projection in the rat. The anterograde tracers Phaseolus vulgaris leucoagglutinin-L and biotinylated dextran amine were injected into the subiculum at different septotemporal and transverse levels. Deep layers of the ento-, peri-, and postrhinal cortices are the main recipients of subicular projections, but in all cases we noted that a small fraction of the projections also terminates in the superficial layers II and III. Analysis of the fiber patterns in the parahippocampal region revealed a topographic organization, depending on the location of the cells of origin along both the transverse and the septotemporal axes of the subiculum. Projections originating from subicular cells close to CA1, i.e., proximal part of subiculum, terminate exclusively in the lateral entorhinal cortex and in the perirhinal cortex. In contrast, projections from cells closer to the subiculum-presubiculum border, i.e., distal part of subiculum, terminate in the medial entorhinal cortex and in the postrhinal cortex. In addition, cells in septal portions of the subiculum project to a lateral band of entorhinal cortex parallel to the rhinal sulcus and to peri- or postrhinal cortices, whereas cells in more temporal portions project to more medial parts of the entorhinal cortex. These results indicate that subicular projections to the parahippocampal region precisely reciprocate the known inputs from this region to the hippocampal formation. We thus suggest that the reciprocal connectivity between the subiculum and the parahippocampal region is organized as parallel pathways that serve to segregate information flow and thus maintain the identity of processed information. Although this parallel organization is comparable to that of the CA1-parahippocampal projections, differences exist with respect to the degree of collateralization.     

Dopamine depresses polysynaptic inhibition in rat subicular neurons

Schizophrenia is considered to be associated with a hyperfunction of the dopaminergic system and with abnormalities in hippocampal information processing. To clarify whether an enhanced dopaminergic activity alters the hippocampal output, the effect of dopamine (DA) on inhibitory postsynaptic responses (IPSPs) in subicular neurons was examined. DA (200 microM) induced a small and inconsistent hyperpolarization that was accompanied by a reduction of membrane resistance. DA decreased polysynaptic IPSPs which was paralleled by a depression of isolated AMPA/kainate and NMDA receptor-mediated excitatory postsynaptic responses (EPSPs). In contrast, DA had no effect on isolated monosynaptic GABA(A) and GABA(B) receptor-mediated IPSP/Cs. We conclude that in addition to membrane effects, DA decreases polysynaptic IPSPs by attenuating the glutamatergic drive onto subicular interneurons.     

Temporal firing characteristics and the strategic role of subicular neurons in short-term memory

The role of subicular neurons is explored with respect to their participation in short-term memory during performance of a spatial Delayed-Nonmatch-to-Sample (DNMS) task by well-trained rats. Subicular and CA1 neuron firing was examined in the same animals in relation to the encoding of task-relevant events during the DNMS trial. The results indicate that subicular neurons have completely different firing signatures than well-characterized hippocampal neurons in this task. Firing patterns of subicular neurons consisted of five different categories spanning all three phases of the DNMS trial, but concentrated mostly within the Sample and early portion of the Delay period. Unlike hippocampal neurons, subicular cells did not exhibit conjunctive firing correlates with respect to particular combinations of task events; rather, subicular cell firing was differentiated primarily on the basis of temporal specificity within the trial. Only two of the five subicular cell types fired differentially on correct versus error trials; however, one cell type exhibited such differential firing as an inverse function of duration of delay interval. Experiments employing gamma-aminobutyric acid GABA(B) receptor agonists and antagonists showed that both behavioral performance as well as subicular cell firing were disrupted significantly by baclofen at short delays, while performance at long delays and hippocampal cell firing were relatively immune to this effect. The relevance of subicular cell firing in the task with respect to its temporal relation to delay-dependent hippocampal neuronal activity suggests that the structures have complementary roles in the encoding and representation of items in short-term memory.     

Complementary subicular pathways to the anterior thalamic nuclei and mammillary bodies in the rat and macaque monkey brain

The origins of the hippocampal (subicular) projections to the anterior thalamic nuclei and mammillary bodies were compared in rats and macaque monkeys using retrograde tracers. These projections form core components of the Papez circuit, which is vital for normal memory. The study revealed a complex pattern of subicular efferents, consistent with the presence of different, parallel information streams, whose segregation appears more marked in the rat brain. In both species, the cells projecting to the mammillary bodies and anterior thalamic nuclei showed laminar separation but also differed along other hippocampal axes. In the rat, these diencephalic inputs showed complementary topographies in the proximal–distal (columnar) plane, consistent with differential involvement in object‐based (proximal subiculum) and context‐based (distal subiculum) information. The medial mammillary inputs, which arose along the anterior–posterior extent of the rat subiculum, favoured the central subiculum (septal hippocampus) and the more proximal subiculum (temporal hippocampus). In contrast, anterior thalamic inputs were largely confined to the dorsal (i.e. septal and intermediate) subiculum, where projections to the anteromedial nucleus favoured the proximal subiculum while those to the anteroventral nucleus predominantly arose in the distal subiculum. In the macaque, the corresponding diencephalic inputs were again distinguished by anterior–posterior topographies, as subicular inputs to the medial mammillary bodies predominantly arose from the posterior hippocampus while subicular inputs to the anteromedial thalamic nucleus predominantly arose from the anterior hippocampus. Unlike the rat, there was no clear evidence of proximal–distal separation as all of these medial diencephalic projections preferentially arose from the more distal subiculum.     

Sources of axonal calcium loading during in vitro ischemia of rat dorsal roots

A detailed understanding of injury mechanisms in peripheral nerve fibers will help guide successful design of therapies for peripheral neuropathies. This study was therefore undertaken to examine the ionic mechanisms of Ca2+ overload in peripheral myelinated fibers subjected to chemical inhibition of energy metabolism. Myelinated axons from rat dorsal roots were co-loaded with Ca2+-sensitive (Oregon Green BAPTA-1) and Ca2+-insensitive (Alexa Fluor 594) dextran-conjugated fluorophores and imaged using confocal laser scanning microscopy. Axoplasmic regions were clearly outlined by the Ca2+-insensitive dye, from which axonal Ca2+-dependent fluorescence changes (FCa.ax) were measured. Block of Na+-K+ ATPase (ouabain), opening of Na+ channels (veratridine), and inhibiting energy metabolism (iodoacetate + NaN3) caused a rapid rise in FCa.ax to 96% above control after 30 min. Chemical ischemia (iodoacetate + NaN3) caused a more gradual increase in FCa.ax (54%), which was almost completely dependent on bath Ca2+, indicating that most of the Ca2+ accumulation occurred via influx across the axolemma. Na+ channel block (tetrodotoxin) reduced ischemic FCa.ax rise (14%); however, inhibition of L-type Ca2+ channels (nimodipine) had no effect (60%). In contrast, Na+-Ca2+ exchange inhibition (KB-R7943) significantly reduced ischemic FCa.ax rise (18%). Together our results indicate that the bulk of Ca2+ overload in injured peripheral myelinated axons occurs via reverse Na+-Ca2+ exchange, driven by axonal Na+ accumulation through voltage-gated tetrodotoxin-sensitive Na+ channels. This mechanism may represent a viable therapeutic target for peripheral neuropathies.