Potassium currents in acutely isolated human hippocampal dentate granule cells

Multi-modal characterization and simulation of human epileptic circuitry

Temporal lobe epilepsy is the fourth most common neurological disorder, with about 40% of patients not responding to pharmacological treatment. Increased cellular loss is linked to disease severity and pathological phenotypes such as heightened seizure propensity. While the hippocampus is the target of therapeutic interventions, the impact of the disease at the cellular level remains unclear. Here, we show that hippocampal granule cells change with disease progression as measured in living, resected hippocampal tissue excised from patients with epilepsy. We show that granule cells increase excitability and shorten response latency while also enlarging in cellular volume and spine density. Single-nucleus RNA sequencing combined with simulations ascribes the changes to three conductances: BK, Cav2.2, and Kir2.1. In a network model, we show that these changes related to disease progression bring the circuit into a more excitable state, while reversing them produces a less excitable, "early-disease-like" state.

Low-voltage-activated inward current in murine antral smooth muscle cells is an artifact

Two types of voltage-dependent inward currents were evoked by depolarization in murine antral smooth muscle cells (SMCs) bathed in Ca²⁺-containing physiological solution: high-voltage-activated (HVA) and low-voltage-activated (LVA) inward currents. We examined whether the LVA current was due to: 1) T-type Ca²⁺ channels, 2) Ca²⁺-activated Cl^-channels, 3) nonselective cation channels (NSCC), or 4) voltage-dependent K⁺ channels. Replacement of external Ca²⁺ (2 mM) with equimolar Ba²⁺ increased the amplitude of the HVA current but blocked the LVA current. Nicardipine blocked the HVA current, and in the presence of nicardipine, T-type Ca²⁺ blockers failed to block LVA current. A Cl^- channel antagonist had little effect on LVA current. Cation-free external solution completely abolished both HVA and LVA currents. Addition of Ca²⁺ to the solution restored only HVA currents. Addition of K⁺ (5 mM) to otherwise cation-free solution induced LVA current that reversed at -20 mV. These data suggest that LVA current is not due to T-type Ca²⁺ channels, Ca²⁺-activated Cl^- channels, or NSCC. A-type K⁺ (K_A) currents and delayed rectifying K⁺ (K_DR) currents can be resolved in antral SMCs dialyzed with a solution containing 140 mM K⁺. When cells were exposed to high K⁺ external solution and dialyzed with Cs⁺-rich solution in the presence of nicardipine, LVA current was evoked and reversed at positive potentials. LVA currents were blocked by K⁺ channel blockers, 4-aminopyridine, and tetraethylammonium. In conclusion, LVA inward currents can be generated by K⁺ influx via K_A channels in murine antral SMCs when cells were dialyzed with Cs⁺-rich solution.

Kv4.1, a Key Ion Channel For Low Frequency Firing of Dentate Granule Cells, Is Crucial for Pattern Separation

The dentate gyrus (DG) in the hippocampus may play key roles in remembering distinct episodes through pattern separation, which may be subserved by the sparse firing properties of granule cells (GCs) in the DG. Low intrinsic excitability is characteristic of mature GCs, but ion channel mechanisms are not fully understood. Here, we investigated ionic channel mechanisms for firing frequency regulation in hippocampal GCs using male and female mice, and identified Kv4.1 as a key player. Immunofluorescence analysis showed that Kv4.1 was preferentially expressed in the DG, and its expression level determined by Western blot analysis was higher at 8-week than 3-week-old mice, suggesting a developmental regulation of Kv4.1 expression. With respect to firing frequency, GCs are categorized into two distinctive groups: low-frequency (LF) and high-frequency (HF) firing GCs. Input resistance (R _in) of most LF-GCs is lower than 200 MΩ, suggesting that LF-GCs are fully mature GCs. Kv4.1 channel inhibition by intracellular perfusion of Kv4.1 antibody increased firing rates and gain of the input-output relationship selectively in LF-GCs with no significant effect on resting membrane potential and R _in, but had no effect in HF-GCs. Importantly, mature GCs from mice depleted of Kv4.1 transcripts in the DG showed increased firing frequency, and these mice showed an impairment in contextual discrimination task. Our findings suggest that Kv4.1 expression occurring at late stage of GC maturation is essential for low excitability of DG networks and thereby contributes to pattern separation.SIGNIFICANCE STATEMENT The sparse activity of dentate granule cells (GCs), which is essential for pattern separation, is supported by high inhibitory inputs and low intrinsic excitability of GCs. Low excitability of GCs is thought to be attributable to a high K⁺ conductance at resting membrane potentials, but this study identifies Kv4.1, a depolarization-activated K⁺ channel, as a key ion channel that regulates firing of GCs without affecting resting membrane potentials. Kv4.1 expression is developmentally regulated and Kv4.1 currents are detected only in mature GCs that show low-frequency firing, but not in less mature high-frequency firing GCs. Furthermore, mice depleted of Kv4.1 transcripts in the dentate gyrus show impaired pattern separation, suggesting that Kv4.1 is crucial for sparse coding and pattern separation.

Saikosaponin A modulates remodeling of Kv4.2-mediated A-type voltage-gated potassium currents in rat chronic temporal lobe epilepsy

Background

Chronic temporal lobe epilepsy (cTLE) is the most common intractable epilepsy. Recent studies have shown that saikosaponin A (SSa) could inhibit epileptiform discharges induced by 4 action potentials and selectively increase the transient inactivating K⁺ currents (I_A). However, the mechanisms of SSa on I_A remain unclear. In this study, we comprehensively evaluated the anticonvulsant activities of SSa and explored whether or not it plays an anti-epileptic role in a Li-pilocarpine induced epilepsy rat model via remodeling Kv4.2-mediated A-type voltage-gated potassium currents (Kv4.2-mediated I_A).

Materials and methods

All in vitro spontaneous recurrent seizures (SRS) were recorded with continuous video monitoring. Nissl’s staining was used to evaluate the SSa protection of neurons and immunohistochemistry, Western blot, and quantitative reverse transcription PCR were used to quantify the expression of Kchip1 and Kv4.2 in the hippocampal CA1 field and the adjacent cortex following Li-pilocarpine induced status epilepticus. We used whole-cell current-clamp recordings to evaluate the anticonvulsant activities of SSa in a hippocampal neuronal culture model of cTLE, while whole-cell voltage-clamp recordings were used to evaluate the modulatory effects of SSa on Kv4.2-mediated I_A.

Results

SSa treatment significantly reduced the frequency and duration of SRS over the course of eight weeks and increased the production of Kchip1 and Kv4.2. In addition, SSa attenuated spontaneous recurrent epileptiform discharges (SREDs) in the hippocampal neuronal model and up-regulated Kv4.2-mediated I_A.

Conclusions

SSa exerted a disease-modifying effect in our cTLE rat model both in vivo and in vitro; the increase in Kv4.2-mediated I_A may contribute to the anticonvulsant mechanisms of SSa.

Constitutive and Synaptic Activation of GIRK Channels Differentiates Mature and Newborn Dentate Granule Cells

Sparse neural activity in the dentate gyrus is enforced by powerful networks of inhibitory GABAergic interneurons in combination with low intrinsic excitability of the principal neurons, the dentate granule cells (GCs). Although the cellular and circuit properties that dictate synaptic inhibition are well studied, less is known about mechanisms that confer low GC intrinsic excitability. Here we demonstrate that intact G protein-mediated signaling contributes to the characteristic low resting membrane potential that differentiates mature dentate GCs from CA1 pyramidal cells and developing adult-born GCs. In mature GCs from male and female mice, intact G protein signaling robustly reduces intrinsic excitability, whereas deletion of G protein-activated inwardly rectifying potassium channel 2 (GIRK2) increases excitability and blocks the effects of G protein signaling on intrinsic properties. Similarly, pharmacological manipulation of GABA_B receptors (GABA_BRs) or GIRK channels alters intrinsic excitability and GC spiking behavior. However, adult-born new GCs lack functional GIRK activity, with phasic and constitutive GABA_BR-mediated GIRK signaling appearing after several weeks of maturation. Phasic activation is interneuron specific, arising primarily from nNOS-expressing interneurons rather than parvalbumin- or somatostatin-expressing interneurons. Together, these results demonstrate that G protein signaling contributes to the intrinsic excitability that differentiates mature and developing dentate GCs and further suggest that late maturation of GIRK channel activity is poised to convert early developmental functions of GABA_B receptor signaling into GABA_BR-mediated inhibition.SIGNIFICANCE STATEMENT The dentate gyrus exhibits sparse neural activity that is essential for the computational function of pattern separation. Sparse activity is ascribed to strong local inhibitory circuits in combination with low intrinsic excitability of the principal neurons, the granule cells. Here we show that constitutive activity of G protein-coupled inwardly rectifying potassium channels (GIRKs) underlies to the hallmark low resting membrane potential and input resistance of mature dentate neurons. Adult-born neurons initially lack functional GIRK channels, with constitutive and phasic GABA_B receptor-mediated GIRK inhibition developing in tandem after several weeks of maturation. Our results reveal that GABA_B/GIRK activity is an important determinant of low excitability of mature dentate granule cells that may contribute to sparse DG activity in vivo.

SK channels participate in the formation of after burst hyperpolarization and partly inhibit the burst strength of epileptic ictal discharges

Epilepsy is a common disease of the central nervous system. Tetanic spasms and convulsions are the key symptoms exhibited during epileptic seizures. However, the majority of patients have a significant post-seizure silence following a serious seizure; the underlying molecular neural mechanisms in this burst interval are unclear. The aim of the present study was to reveal the effect and role of calcium-activated potassium channels during this seizure interval silence period. Cyclothiazide (CTZ) was used to establish the seizure model in rat hippocampal cultured neurons, then the after-burst hyperpolarization (ABH) activities were recorded using the patch clamp technique. By comparing the amplitude and duration of hyperpolarizations, the present study analyzed the association between epileptiform bursts and ABHs when treated with different concentrations of CTZ. In addition, apamin and iberiotoxin were used for pharmacological tests. An intracranial electroencephalogram (EEG) recording was also performed when the CTZ experiments were repeated on animals. The experimental results revealed that treatment with high levels of CTZ induced larger ABHs and was associated with stronger burst activities, which suggested a positive correlation between ABH and epileptiform burst. Apamin, an antagonist of small conductance calcium-activated potassium (SK) channels, decreased the amplitude of ABH; however, reduced ABH was associated with enhanced burst activity, in burst probability and burst strength. These results revealed an important role of SK channels in the formation of ABH and in the inhibition of burst activity. Iberiotoxin, an antagonist of big conductance calcium-activated potassium (BK) channels, had no significant effect on ABH and burst activity. In addition, a positive correlation was identified between burst duration and ABH parameters. An intracellular calcium chelator impaired the amplitude of ABH; however, it did not affect the burst parameters. The rat cortical EEG recordings also exhibited a similar positive correlation between the duration of epileptic burst and after burst depression. Collectively, the results indicate that ABH may serve in the physiological feedback system to reduce the strength of epileptic hyperexcitation, a process in which SK channels are important.

Interneuronal mechanisms of hippocampal theta oscillations in a full-scale model of the rodent CA1 circuit

The hippocampal theta rhythm plays important roles in information processing; however, the mechanisms of its generation are not well understood. We developed a data-driven, supercomputer-based, full-scale (1:1) model of the rodent CA1 area and studied its interneurons during theta oscillations. Theta rhythm with phase-locked gamma oscillations and phase-preferential discharges of distinct interneuronal types spontaneously emerged from the isolated CA1 circuit without rhythmic inputs. Perturbation experiments identified parvalbumin-expressing interneurons and neurogliaform cells, as well as interneuronal diversity itself, as important factors in theta generation. These simulations reveal new insights into the spatiotemporal organization of the CA1 circuit during theta oscillations.

Interneuronal mechanisms of hippocampal theta oscillations in a full-scale model of the rodent CA1 circuit

The hippocampal theta rhythm plays important roles in information processing; however, the mechanisms of its generation are not well understood. We developed a data-driven, supercomputer-based, full-scale (1:1) model of the rodent CA1 area and studied its interneurons during theta oscillations. Theta rhythm with phase-locked gamma oscillations and phase-preferential discharges of distinct interneuronal types spontaneously emerged from the isolated CA1 circuit without rhythmic inputs. Perturbation experiments identified parvalbumin-expressing interneurons and neurogliaform cells, as well as interneuronal diversity itself, as important factors in theta generation. These simulations reveal new insights into the spatiotemporal organization of the CA1 circuit during theta oscillations.

DOI:http://dx.doi.org/10.7554/eLife.18566.001

Functional properties of granule cells with hilar basal dendrites in the epileptic dentate gyrus

OBJECTIVE

The maturation of adult-born granule cells and their functional integration into the network is thought to play a key role in the proper functioning of the dentate gyrus. In temporal lobe epilepsy, adult-born granule cells in the dentate gyrus develop abnormally and possess a hilar basal dendrite (HBD). Although morphological studies have shown that these HBDs have synapses, little is known about the functional properties of these HBDs or the intrinsic and network properties of the granule cells that possess these aberrant dendrites.

METHODS

We performed patch-clamp recordings of granule cells within the granule cell layer "normotopic" from sham-control and status epilepticus (SE) animals. Normotopic granule cells from SE animals possessed an HBD (SE⁺ HBD⁺ cells) or not (SE⁺ HBD^- cells). Apical and basal dendrites were stimulated using multiphoton uncaging of glutamate. Two-photon Ca²⁺ imaging was used to measure Ca²⁺ transients associated with back-propagating action potentials (bAPs).

RESULTS

Near-synchronous synaptic input integrated linearly in apical dendrites from sham-control animals and was not significantly different in apical dendrites of SE⁺ HBD^- cells. The majority of HBDs integrated input linearly, similar to apical dendrites. However, 2 of 11 HBDs were capable of supralinear integration mediated by a dendritic spike. Furthermore, the bAP-evoked Ca²⁺ transients were relatively well maintained along HBDs, compared with apical dendrites. This further suggests an enhanced electrogenesis in HBDs. In addition, the output of granule cells from epileptic tissue was enhanced, with both SE⁺ HBD^- and SE⁺ HBD⁺ cells displaying increased high-frequency (>100 Hz) burst-firing. Finally, both SE⁺ HBD^- and SE⁺ HBD⁺ cells received recurrent excitatory input that was capable of generating APs, especially in the absence of feedback inhibition.

SIGNIFICANCE

Taken together, these data suggest that the enhanced excitability of HBDs combined with the altered intrinsic and network properties of granule cells collude to promote excitability and synchrony in the epileptic dentate gyrus.

Potassium Channels in Epilepsy

This review attempts to give a concise and up-to-date overview on the role of potassium channels in epilepsies. Their role can be defined from a genetic perspective, focusing on variants and de novo mutations identified in genetic studies or animal models with targeted, specific mutations in genes coding for a member of the large potassium channel family. In these genetic studies, a demonstrated functional link to hyperexcitability often remains elusive. However, their role can also be defined from a functional perspective, based on dynamic, aggravating, or adaptive transcriptional and posttranslational alterations. In these cases, it often remains elusive whether the alteration is causal or merely incidental. With ∼80 potassium channel types, of which ∼10% are known to be associated with epilepsies (in humans) or a seizure phenotype (in animals), if genetically mutated, a comprehensive review is a challenging endeavor. This goal may seem all the more ambitious once the data on posttranslational alterations, found both in human tissue from epilepsy patients and in chronic or acute animal models, are included. We therefore summarize the literature, and expand only on key findings, particularly regarding functional alterations found in patient brain tissue and chronic animal models.

Complementary functions of SK and Kv7/M potassium channels in excitability control and synaptic integration in rat hippocampal dentate granule cells

The dentate granule cells (DGCs) form the most numerous neuron population of the hippocampal memory system, and its gateway for cortical input. Yet, we have only limited knowledge of the intrinsic membrane properties that shape their responses. Since SK and Kv7/M potassium channels are key mechanisms of neuronal spiking and excitability control, afterhyperpolarizations (AHPs) and synaptic integration, we studied their functions in DGCs. The specific SK channel blockers apamin or scyllatoxin increased spike frequency (excitability), reduced early spike frequency adaptation, fully blocked the medium-duration AHP (mAHP) after a single spike or spike train, and increased postsynaptic EPSP summation after spiking, but had no effect on input resistance (Rinput) or spike threshold. In contrast, blockade of Kv7/M channels by XE991 increased Rinput, lowered the spike threshold, and increased excitability, postsynaptic EPSP summation, and EPSP-spike coupling, but only slightly reduced mAHP after spike trains (and not after single spikes). The SK and Kv7/M channel openers 1-EBIO and retigabine, respectively, had effects opposite to the blockers. Computational modelling reproduced many of these effects. We conclude that SK and Kv7/M channels have complementary roles in DGCs. These mechanisms may be important for the dentate network function, as CA3 neurons can be activated or inhibition recruited depending on DGC firing rate.

Regulation of action potential delays via voltage-gated potassium Kv1.1 channels in dentate granule cells during hippocampal epilepsy

Action potential (AP) responses of dentate gyrus granule (DG) cells have to be tightly regulated to maintain hippocampal function. However, which ion channels control the response delay of DG cells is not known. In some neuron types, spike latency is influenced by a dendrotoxin (DTX)-sensitive delay current (ID) mediated by unidentified combinations of voltage-gated K(+) (Kv) channels of the Kv1 family Kv1.1-6. In DG cells, the ID has not been characterized and its molecular basis is unknown. The response phenotype of mature DG cells is usually considered homogenous but intrinsic plasticity likely occurs in particular in conditions of hyperexcitability, for example during temporal lobe epilepsy (TLE). In this study, we examined response delays of DG cells and underlying ion channel molecules by employing a combination of gramicidin-perforated patch-clamp recordings in acute brain slices and single-cell reverse transcriptase quantitative polymerase chain reaction (SC RT-qPCR) experiments. An in vivo mouse model of TLE consisting of intrahippocampal kainate (KA) injection was used to examine epilepsy-related plasticity. Response delays of DG cells were DTX-sensitive and strongly increased in KA-injected hippocampi; Kv1.1 mRNA was elevated 10-fold, and the response delays correlated with Kv1.1 mRNA abundance on the single cell level. Other Kv1 subunits did not show overt changes in mRNA levels. Kv1.1 immunolabeling was enhanced in KA DG cells. The biophysical properties of ID and a delay heterogeneity within the DG cell population was characterized. Using organotypic hippocampal slice cultures (OHCs), where KA incubation also induced ID upregulation, the homeostatic reversibility and neuroprotective potential for DG cells were tested. In summary, the AP timing of DG cells is effectively controlled via scaling of Kv1.1 subunit transcription. With this antiepileptic mechanism, DG cells delay their responses during hyperexcitation.

Expression and localization of voltage dependent potassium channel Kv4.2 in epilepsy associated focal lesions

An increasing number of observations suggest an important role for voltage-gated potassium (Kv) channels in epilepsy. We studied the cell-specific distribution of Kv4.2, phosphorylated (p) Kv4.2 and the Kv4.2 interacting protein NCS-1 using immunocytochemistry in different epilepsy-associated focal lesions. In hippocampal sclerosis (HS), Kv4.2 and pKv4.2 immunoreactivity (IR) was reduced in the neuropil in regions with prominent neuronal cell loss. In both HS and malformations of cortical development (MCD), intense labeling was found in neuronal somata, but not in dendrites. Strong NCS-1 IR was observed in neurons in all lesion types. Western blot analysis demonstrated an increase of total Kv4.2 in all lesions and activation of the ERK pathway in HS and ganglioglioma. These findings indicate that Kv4.2 is expressed in both neuronal and glial cells and its regulation may involve potassium channel interacting proteins, alterations in the subcellular localization of the channel, as well as phosphorylation-mediated posttranslational modifications.

Weighing the evidence for a ternary protein complex mediating A-type K+ currents in neurons

The subthreshold-operating A-type K(+) current in neurons (I(SA)) has important roles in the regulation of neuronal excitability, the timing of action potential firing and synaptic integration and plasticity. The channels mediating this current (Kv4 channels) have been implicated in epilepsy, the control of dopamine release, and the regulation of pain plasticity. It has been proposed that Kv4 channels in neurons are ternary complexes of three types of protein: pore forming subunits of the Kv4 subfamily and two types of auxiliary subunits, the Ca(2+) binding proteins KChIPs and the dipeptidyl peptidase-like proteins (DPPLs) DPP6 (also known as DPPX) and DPP10 (4 molecules of each per channel for a total of 12 proteins in the complex). Here we consider the evidence supporting this hypothesis. Kv4 channels in many neurons are likely to be ternary complexes of these three types of protein. KChIPs and DPPLs are required to efficiently traffic Kv4 channels to the plasma membrane and regulate the functional properties of the channels. These proteins may also be important in determining the localization of the channels to specific neuronal compartments, their dynamics, and their response to neuromodulators. A surprisingly large number of additional proteins have been shown to modify Kv4 channels in heterologous expression systems, but their association with native Kv4 channels in neurons has not been properly validated. A critical consideration of the evidence suggests that it is unlikely that association of Kv4 channels with these additional proteins is widespread in the CNS. However, we cannot exclude that some of these proteins may associate with the channels transiently or in specific neurons or neuronal compartments, or that they may associate with the channels in other tissues.

A Kv4.2 truncation mutation in a patient with temporal lobe epilepsy

Temporal lobe epilepsy (TLE) has a multifactorial etiology involving developmental, environmental, and genetic components. Here, we report a voltage-gated potassium channel gene mutation found in a TLE patient, namely a Kv4.2 truncation mutation. Kv4.2 channels, encoded by the KCND2 gene, mediate A currents in the brain. The identified mutation corresponds to an N587fsX1 amino acid change, predicted to produce a truncated Kv4.2 protein lacking the last 44 amino acids in the carboxyl terminal. Electrophysiological analysis indicates attenuated K+ current density in cells expressing this Kv4.2-N587fsX1 mutant channel, which is consistent with a model of aberrant neuronal excitability characteristic of TLE. Our observations, together with other lines of evidence, raise the intriguing possibility of a role for KCND2 in the etiology of TLE.

Methodological approaches to exploring epileptic disorders in the human brain in vitro

Brain surgery, and in particular epilepsy surgery, offers the unique opportunity to study viable human central nervous tissue in vitro. This does not only open a window to address the basic mechanisms underlying human disease, such as epilepsy, but it allows to venture into investigating neurophysiological functions per se. In the present paper, we describe the most commonly used methods in the electrophysiological (and, at least to some extent, also histochemical and molecular) analysis of human tissue in vitro. In addition, we consider the pitfalls and limitations of such studies, in particular regarding the issue of tissue sampling procedures and control experiments.

Cellular and molecular mechanisms of epilepsy in the human brain

Animal models have provided invaluable data for identifying the pathogenesis of epileptic disorders. Clearly, the relevance of these experimental findings would be strengthened by the demonstration that similar fundamental mechanisms are at work in the human epileptic brain. Epilepsy surgery has indeed opened the possibility to directly study the functional properties of human brain tissue in vitro, and to analyze the mechanisms underlying seizures and epileptogenesis. Here, we summarize the findings obtained over the last 40 years from electrophysiological, histochemical and molecular experiments made with the human brain tissue. In particular, this review will focus on (i) the synaptic and non-synaptic properties of neocortical neurons along with their ability to produce synchronous activity; (ii) the anatomical and functional alterations that characterize limbic structures in patients presenting with mesial temporal lobe epilepsy; (iii) the issue of antiepileptic drug action and resistance; and (iv) the pathophysiology of seizure genesis in Taylor's type focal cortical dysplasia. Finally, we will address some of the problems that are inherent to this type of experimental approach, in particular the lack of proper controls and possible strategies to obviate this limitation.

Pituitary adenylate cyclase activating polypeptide reduces expression of Kv1.4 and Kv4.2 subunits underlying A-type K(+) current in adult mouse olfactory neuroepithelia

A-type K(+) currents (I(A)) in olfactory receptor neurons have been characterized electrophysiologically but the molecular identities of the underlying channel subunits have not been determined. Using RT-PCR, immunoblot and immunohistochemistry, we found that the two candidate channel families underlying I(A), shaker and shal, are expressed in olfactory epithelia of Swiss Webster mice. Specifically, Kv1.4, the only I(A) candidate from the shaker family, and Kv4.2 and Kv4.3 from the shal family were expressed, but Kv4.1 mRNA was not amplified from the olfactory epithelia. Immunoblot and immunohistochemical studies confirmed the existence of Kv1.4 and Kv4.2/3 subunits. Furthermore, quantitative RT-PCR showed that pituitary adenylate cyclase activating polypeptide (PACAP) reduced the expression of Kv1.4 and Kv4.2 but did not reduce the already low expression of Kv4.3. The PACAP-induced reduction of Kv4.1 and Kv4.2 expression was completely blocked by inhibiting the phospholipase C (PLC) pathway but was still significantly downregulated by PACAP when the cyclic AMP pathway was inhibited. In addition, downstream of the PLC pathway, calcium mediated the reduction of both Kv1.4 and Kv4.2 expression and I(A) current density. Phosphokinase C (PKC) activation did not affect Kv1.4 and Kv4.2 mRNA expression, even though PKC reduced I(A) current density. Together with our previous studies, our data suggest that A-type K(+) currents in olfactory receptor neurons are composed of multiple K(+) channel subunits, among which Kv1.4 and Kv4.2 are subject to transcriptional modulation by PACAP. We also found that PACAP predominately uses a PLC-calcium pathway to modulate Kv4.1 and Kv4.2 expression. Modulation of A-type K(+) current expression may contribute to the previously observed neuroprotective effects of PACAP on olfactory receptor neurons.

Sulfhydryl oxidation reduces hippocampal susceptibility to hypoxia-induced spreading depression by activating BK channels

The cytosolic redox status modulates ion channels and receptors by oxidizing/reducing their sulfhydryl (SH) groups. We therefore analyzed to what degree SH modulation affects hippocampal susceptibility to hypoxia. In rat hippocampal slices, severe hypoxia caused a massive depolarization of CA1 neurons and a negative shift of the extracellular DC potential, the characteristic sign of hypoxia-induced spreading depression (HSD). Oxidizing SH groups by 5,5'-dithiobis 2-nitrobenzoic acid (DTNB, 2 mM) postponed HSD by 30%, whereas their reduction by 1,4-dithio-dl-threitol (DTT, 2 mM) or alkylation by N-ethylmaleimide (500 microM) hastened HSD onset. The DTNB-induced postponement of HSD was not affected by tolbutamide (200 microM), dl-2-amino-5-phosphonovaleric acid (150 microM), or 6-cyano-7-nitroquinoxaline-2,3-dione (25 microM). It was abolished, however, by Ni2+ (2 mM), withdrawal of extracellular Ca2+, charybdotoxin (25 nM), and iberiotoxin (50 nM). In CA1 neurons DTNB induced a moderate hyperpolarization, blocked spontaneous spike discharges and postponed the massive hypoxic depolarization. DTT induced burst firing, depolarized glial cells, and hastened the onset of the massive hypoxic depolarization. Schaffer-collateral/CA1 synapses were blocked by DTT but not by DTNB; axonal conduction remained intact. Mitochondria did not markedly respond to DTNB or DTT. While the targets of DTT are less clear, the postponement of HSD by DTNB indicates that sulfhydryl oxidation increases the tolerance of hippocampal tissue slices against hypoxia. We identified as the underlying mechanism the activation of BK channels in a Ca(2+)-sensitive manner. Accordingly, ionic disregulation and the loss of membrane potential occur later or might even be prevented during short-term insults. Therefore well-directed oxidation of SH groups could mediate neuroprotection.

Cloning and characterization of SK2 channel from chicken short hair cells

In the inner ear of birds, as in mammals, reptiles and amphibians, acetylcholine released from efferent neurons inhibits hair cells via activation of an apamin-sensitive, calcium-dependent potassium current. The particular potassium channel involved in avian hair cell inhibition is unknown. In this study, we cloned a small-conductance, calcium-sensitive potassium channel (gSK2) from a chicken cochlear library. Using RT-PCR, we demonstrated the presence of gSK2 mRNA in cochlear hair cells. Electrophysiological studies on transfected HEK293 cells showed that gSK2 channels have a conductance of approximately 16 pS and a half-maximal calcium activation concentration of 0.74+/-0.17 microM. The expressed channels were blocked by apamin (IC(50)=73.3+/-5.0 pM) and d-tubocurarine (IC(50)=7.6+/-1.0 microM), but were insensitive to charybdotoxin. These characteristics are consistent with those reported for acetylcholine-induced potassium currents of isolated chicken hair cells, suggesting that gSK2 is involved in efferent inhibition of chicken inner ear. These findings imply that the molecular mechanisms of inhibition are conserved in hair cells of all vertebrates.

Physiological and morphological characterization of dentate granule cells in the p35 knock-out mouse hippocampus: evidence for an epileptic circuit

There is a high correlation between pediatric epilepsies and neuronal migration disorders. What remains unclear is whether there are intrinsic features of the individual dysplastic cells that give rise to heightened seizure susceptibility, or whether these dysplastic cells contribute to seizure activity by establishing abnormal circuits that alter the balance of inhibition and excitation. Mice lacking a functional p35 gene provide an ideal model in which to address these questions, because these knock-out animals not only exhibit aberrant neuronal migration but also demonstrate spontaneous seizures. Extracellular field recordings from hippocampal slices, characterizing the input-output relationship in the dentate, revealed little difference between wild-type and knock-out mice under both normal and elevated extracellular potassium conditions. However, in the presence of the GABA(A) antagonist bicuculline, p35 knock-out slices, but not wild-type slices, exhibited prolonged depolarizations in response to stimulation of the perforant path. There were no significant differences in the intrinsic properties of dentate granule cells (i.e., input resistance, time constant, action potential generation) from wild-type versus knock-out mice. However, antidromic activation (mossy fiber stimulation) evoked an excitatory synaptic response in over 65% of granule cells from p35 knock-out slices that was never observed in wild-type slices. Ultrastructural analyses identified morphological substrates for this aberrant excitation: recurrent axon collaterals, abnormal basal dendrites, and mossy fiber terminals forming synapses onto the spines of neighboring granule cells. These studies suggest that granule cells in p35 knock-out mice contribute to seizure activity by forming an abnormal excitatory feedback circuit.

Structure and function of Kv4-family transient potassium channels

Shal-type (Kv4.x) K(+) channels are expressed in a variety of tissue, with particularly high levels in the brain and heart. These channels are the primary subunits that contribute to transient, voltage-dependent K(+) currents in the nervous system (A currents) and the heart (transient outward current). Recent studies have revealed an enormous degree of complexity in the regulation of these channels. In this review, we describe the surprisingly large number of ancillary subunits and scaffolding proteins that can interact with the primary subunits, resulting in alterations in channel trafficking and kinetic properties. Furthermore, we discuss posttranslational modification of Kv4.x channel function with an emphasis on the role of kinase modulation of these channels in regulating membrane properties. This concept is especially intriguing as Kv4.2 channels may integrate a variety of intracellular signaling cascades into a coordinated output that dynamically modulates membrane excitability. Finally, the pathophysiology that may arise from dysregulation of these channels is also reviewed.

Characterization of a fast transient outward current in neocortical neurons from epilepsy patients

A-type currents powerfully modulate discharge behavior and have been described in a large number of different species and cell types. However, data on A-type currents in human brain tissue are scarce. Here we have examined the properties of a fast transient outward current in acutely dissociated human neocortical neurons from the temporal lobe of epilepsy patients by using the whole-cell voltage-clamp technique. The A-type current was isolated with a subtraction protocol. In addition, delayed potassium currents were reduced pharmacologically with 10 mM tetraethylammonium chloride. The current displayed an activation threshold of about -70 mV. The voltage-dependent activation was fitted with a Boltzmann function, with a half-maximal conductance at -14.8 +/- 1.8 mV (n = 5) and a slope factor of 17.0 +/- 0.5 mV (n = 5). The voltage of half-maximal steady-state inactivation was -98.9 +/- 8.3 mV (n = 5), with a slope factor of -6.6 +/- 1.9 mV (n = 5). Recovery from inactivation could be fitted monoexponentially with a time constant of 18.2 +/- 7.5 msec (n = 5). At a command potential of +30 mV, application of 5 mM 4-aminopyridine or 100 microM flecainide resulted in a reduction of A-type current amplitude by 35% or 22%, respectively. In addition, flecainide markedly accelerated inactivation. Current amplitude was reduced by 31% with application of 500 microM cadmium. All drug effects were reversible. In conclusion, neocortical neurons from epilepsy patients express an A-type current with properties similar to those described for animal tissues.

Vigabatrin reduces epileptiform activity in brain slices from pharmacoresistant epilepsy patients

Human neocortical temporal lobe tissue resected for treatment of pharmacoresistant epilepsy was investigated. In slices prepared from this tissue, epileptiform field potentials (EFP) were induced by omission of magnesium from the artificial cerebrospinal fluid (ACSF). The effects of the gamma-aminobutyric acid transaminase inhibitor vigabatrin on EFP were tested. Vigabatrin exerted a dose-dependent reduction of the repetition rate of EFP: after 3 h of administration of vigabatrin in concentrations of 100 and 200 micromol/l, the repetition rate of EFP was reduced to 35% and 18% of the initial values, respectively. This effect was not reversible. In control experiments with neocortical slices from rats, vigabatrin reduced EFP in a comparable range. The results demonstrate a strong antiepileptic effect of vigabatrin on EFP in tissues from pharmacoresistant epilepsy patients.

Surviving granule cells of the sclerotic human hippocampus have reduced Ca(2+) influx because of a loss of calbindin-D(28k) in temporal lobe epilepsy

In mesial temporal lobe epilepsy (mTLE), the predominant form of epilepsy in adults, and in animal models of the disease, there is a conspicuous loss of the intracellular Ca(2+)-binding protein calbindin-D(28k) (CB) from granule cells (GCs) of the dentate gyrus. The role of this protein in nerve cell function is controversial, but here we provide evidence for its role in controlling Ca(2+) influx into human neurons. In patients with Ammon's horn sclerosis (AHS), the loss of CB from GCs markedly increased the Ca(2+)-dependent inactivation of voltage-dependent Ca(2+) currents (I(Ca)), thereby diminishing Ca(2+) influx during repetitive neuronal firing. Introducing purified CB into GCs restored Ca(2+) current inactivation to levels observed in cells with normal CB content harvested from mTLE patients without AHS. Our data are consistent with the possibility of neuroprotection secondary to the CB loss. By limiting Ca(2+) influx through an enhanced Ca(2+)-dependent inactivation of voltage-dependent Ca(2+) channels during prolonged neuronal discharges, the loss of CB may contribute to the resistance of surviving human granule cells in AHS.

Surviving granule cells of the sclerotic human hippocampus have reduced Ca(2+) influx because of a loss of calbindin-D(28k) in temporal lobe epilepsy

In mesial temporal lobe epilepsy (mTLE), the predominant form of epilepsy in adults, and in animal models of the disease, there is a conspicuous loss of the intracellular Ca(2+)-binding protein calbindin-D(28k) (CB) from granule cells (GCs) of the dentate gyrus. The role of this protein in nerve cell function is controversial, but here we provide evidence for its role in controlling Ca(2+) influx into human neurons. In patients with Ammon's horn sclerosis (AHS), the loss of CB from GCs markedly increased the Ca(2+)-dependent inactivation of voltage-dependent Ca(2+) currents (I(Ca)), thereby diminishing Ca(2+) influx during repetitive neuronal firing. Introducing purified CB into GCs restored Ca(2+) current inactivation to levels observed in cells with normal CB content harvested from mTLE patients without AHS. Our data are consistent with the possibility of neuroprotection secondary to the CB loss. By limiting Ca(2+) influx through an enhanced Ca(2+)-dependent inactivation of voltage-dependent Ca(2+) channels during prolonged neuronal discharges, the loss of CB may contribute to the resistance of surviving human granule cells in AHS.

Effects of retigabine (D-23129) on different patterns of epileptiform activity induced by low magnesium in rat entorhinal cortex hippocampal slices

PURPOSE

The objective of this study was to evaluate the effect of a new antiseizure drug, retigabine (D-23129; N-(2-amino-4-[fluorobenzylamino]-phenyl) carbamic acid ethyl ester) on low-Mg2+-induced epileptiform discharges in rat in vitro.

METHODS

Three types of epileptiform discharges (recurrent short discharges in the hippocampus, seizure-like events, and late recurrent discharges in the entorhinal cortex) were elicited in rat combined entorhinal cortex-hippocampal slices by perfusion with low-Mg2+-artificial cerebrospinal fluid (ACSF). The antiepileptic properties of retigabine were evaluated as effect on the frequency and amplitude of the epileptiform activities as well as time of onset of the effect in the entorhinal cortex (EC) and in hippocampal area CA1 (CA1) by using extracellular recording techniques.

RESULTS

Retigabine (20 microM) reversibly suppressed the recurrent short discharges otherwise sensitive only to high doses of valproate (VPA) but insensitive to standard antiepileptic drugs (AEDs) in CA1, whereas 10 microM reduced the frequency of discharges by 34+/-18.8%, with no significant effect on the amplitude. In EC, retigabine (50 microM) reversibly suppressed the seizure-like events, whereas 20 microM blocked seizure-like events in 71.5% of the slices. The seizure-like events were also sensitive to standard AEDs. Late recurrent discharges in EC that are not blocked by standard AEDs were reversibly suppressed by retigabine (100 microM), whereas 50 microM reduced the frequency of the discharges by 94.4+/-7.7%, and 20 microM, by 74.2+/-18.0%, with no significant effect on the amplitude.

CONCLUSIONS

Retigabine is an effective AED with suppressive effects on recurrent short discharges and on late recurrent discharges normally insensitive to standard AEDs.

Modulation of a slowly inactivating potassium current, I(D), by metabotropic glutamate receptor activation in cultured hippocampal pyramidal neurons

I(D) is a slowly inactivating 4-aminopyridine (4-AP)-sensitive potassium current of hippocampal pyramidal neurons and other CNS neurons. Although I(D) exerts multifaceted influence on CNS excitability, whether I(D) is subject to modulation by neurotransmitters or neurohormones has not been clear. We report here that one prominent effect of metabotropic glutamate receptor (mGluR) activation by short (3 min) exposure to 1S, 3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) (100 microM) is suppression of I(D) by acceleration of its inactivation. I(D) was identified as a target of mGluR-mediated modulation because inactivation of a component of outward current sensitive to 100-200 microM 4-AP was accelerated by 1S,3R-ACPD, and because 4-AP occluded any further actions of 1S,3R-ACPD. Enhancement of I(D) inactivation was induced by the group I-preferring agonist RS-3, 5-dihydroxyphenylglycine (3,5-DHPG) and the group II-preferring agonist 2S,2'R,3'R)-2-(2',3'dicarboxycyclopropyl)-glycine (DCG-IV), but not by the group III-preferring agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4); it was blocked by the broadly acting mGluR antagonist S-alpha-methyl-4-carboxyphenylglycine (S-MCPG). Furthermore, inactivation of I(D) was enhanced by inclusion of GTPgammaS in the internal solution and blocked by inclusion of GDPbetaS. Metabotropic GluR-induced suppression of I(D) was manifest in three aspects of excitability previously linked to I(D) by their sensitivity to 4-AP: reduction in input conductance and enhanced excitability at voltages just positive to the resting potential, reduced delay to action potential firing during depolarizing current injections, and delayed action potential repolarization. We suggest that mGluR-induced suppression of I(D) could contribute to enhancement of hippocampal neuron excitability and synaptic connections.

Modulation of a slowly inactivating potassium current, I(D), by metabotropic glutamate receptor activation in cultured hippocampal pyramidal neurons

I(D) is a slowly inactivating 4-aminopyridine (4-AP)-sensitive potassium current of hippocampal pyramidal neurons and other CNS neurons. Although I(D) exerts multifaceted influence on CNS excitability, whether I(D) is subject to modulation by neurotransmitters or neurohormones has not been clear. We report here that one prominent effect of metabotropic glutamate receptor (mGluR) activation by short (3 min) exposure to 1S, 3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) (100 microM) is suppression of I(D) by acceleration of its inactivation. I(D) was identified as a target of mGluR-mediated modulation because inactivation of a component of outward current sensitive to 100-200 microM 4-AP was accelerated by 1S,3R-ACPD, and because 4-AP occluded any further actions of 1S,3R-ACPD. Enhancement of I(D) inactivation was induced by the group I-preferring agonist RS-3, 5-dihydroxyphenylglycine (3,5-DHPG) and the group II-preferring agonist 2S,2'R,3'R)-2-(2',3'dicarboxycyclopropyl)-glycine (DCG-IV), but not by the group III-preferring agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4); it was blocked by the broadly acting mGluR antagonist S-alpha-methyl-4-carboxyphenylglycine (S-MCPG). Furthermore, inactivation of I(D) was enhanced by inclusion of GTPgammaS in the internal solution and blocked by inclusion of GDPbetaS. Metabotropic GluR-induced suppression of I(D) was manifest in three aspects of excitability previously linked to I(D) by their sensitivity to 4-AP: reduction in input conductance and enhanced excitability at voltages just positive to the resting potential, reduced delay to action potential firing during depolarizing current injections, and delayed action potential repolarization. We suggest that mGluR-induced suppression of I(D) could contribute to enhancement of hippocampal neuron excitability and synaptic connections.

Ca(2+)-dependent inactivation of high-threshold Ca(2+) currents in hippocampal granule cells of patients with chronic temporal lobe epilepsy

Intracellular Ca(2+) represents an important trigger for various second-messenger mediated effects. Therefore a stringent control of the intracellular Ca(2+) concentration is necessary to avoid excessive activation of Ca(2+)-dependent processes. Ca(2+)-dependent inactivation of voltage-dependent calcium currents (VCCs) represents an important negative feedback mechanism to limit the influx of Ca(2+) that has been shown to be altered in the kindling model of epilepsy. We therefore investigated the Ca(2+)-dependent inactivation of high-threshold VCCs in dentate granule cells (DGCs) isolated from the hippocampus of patients with drug-refractory temporal lobe epilepsy (TLE) using the patch-clamp method. Ca(2+) currents showed pronounced time-dependent inactivation when no extrinsic Ca(2+) buffer was present in the patch pipette. In addition, in double-pulse experiments, Ca(2+) entry during conditioning prepulses caused a reduction of VCC amplitudes elicited during a subsequent test pulse. Recovery from Ca(2+)-dependent inactivation was slow and only complete after 1 s. Ca(2+)-dependent inactivation could be blocked either by using Ba(2+) as a charge carrier or by including bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) or EGTA in the intracellular solution. The influence of the cytoskeleton on Ca(2+)-dependent inactivation was investigated with agents that stabilize and destabilize microfilaments or microtubules, respectively. From these experiments, we conclude that Ca(2+)-dependent inactivation in human DGCs involves Ca(2+)-dependent destabilization of both microfilaments and microtubules. In addition, the microtubule-dependent pathway is modulated by the intracellular concentration of GTP, with lower concentrations of guanosine triphosphate (GTP) causing increased Ca(2+)-dependent inactivation. Under low-GTP conditions, the amount of Ca(2+)-dependent inactivation was similar to that observed in the kindling model. In summary, Ca(2+)-dependent inactivation was present in patients with TLE and Ammon's horn sclerosis (AHS) and is mediated by the cytoskeleton similar to rat pyramidal neurons. The similarity to the kindling model of epilepsy may suggest the possibility of altered Ca(2+)-dependent inactivation in patients with AHS.

Role of multiple calcium and calcium-dependent conductances in regulation of hippocampal dentate granule cell excitability

We have constructed a detailed model of a hippocampal dentate granule (DG) cell that includes nine different channel types. Channel densities and distributions were chosen to reproduce reported physiological responses observed in normal solution and when blockers were applied. The model was used to explore the contribution of each channel type to spiking behavior with particular emphasis on the mechanisms underlying postspike events. T-type calcium current in more distal dendrites contributed prominently to the appearance of the depolarizing after-potential, and its effect was controlled by activation of BK-type calcium-dependent potassium channels. Coactivation and interaction of N-, and/or L-type calcium and AHP currents present in somatic and proximal dendritic regions contributed to the adaptive properties of the model DG cell in response to long-lasting current injection. The model was used to predict changes in channel densities that could lead to epileptogenic burst discharges and to predict the effect of altered buffering capacity on firing behavior. We conclude that the clustered spatial distributions of calcium related channels, the presence of slow delayed rectifier potassium currents in dendrites, and calcium buffering properties, together, might explain the resistance of DG cells to the development of epileptogenic burst discharges.

Voltage-dependent Ca2+ currents in epilepsy

Voltage-dependent Ca2+ channels (VCCs) represent one of the main routes of Ca2+ entry into neuronal cells. Changes in intracellular Ca2+ dynamics and homeostasis can cause long-lasting cellular changes via activation of different Ca2+ dependent signalling pathways. We have investigated the properties of VCCs in human hippocampal dentate granule cells (DGCs) using the whole-cell configuration of the patch-clamp method. Classical high-threshold Ca2+ currents were composed mainly of omega-CgTx-sensitive N-type and nifedipine-sensitive L-type currents that were present in similar proportions. In addition, a Ca2+ current component that was sensitive to low concentrations of Ni2+, but not to nifedipine or omega-conotoxin GVIA (omega-CgTx GVIA) was present. This latter component showed a half-maximal inactivation at more hyperpolarized potentials than high-threshold currents and a more rapid time-dependent inactivation. This current was termed T-type Ca2+ current. Current components with similar pharmacological and kinetic characteristics could be elicited in acutely isolated control rat DGCs. The current density of high threshold and T-type Ca2+ components was significantly larger in human DGCs and in the kainate model compared to DGCs isolated from adult control rats. These differences in current density were not accompanied by parallel differences in the voltage-dependence of VCCs. Taken together, these data suggest that an up-regulation of Ca2+ current density may occur in hippocampal epileptogenesis without consistent changes in Ca2+ current properties.

Electrophysiological characterization of Na+ currents in acutely isolated human hippocampal dentate granule cells

1. Properties of voltage-dependent Na+ currents were investigated in forty-two dentate granule cells (DGCs) acutely isolated from the resected hippocampus of twenty patients with therapy-refractory temporal lobe epilepsy (TLE) using the whole-cell patch-clamp technique. 2. Depolarizing voltage commands elicited large, rapidly activating and inactivating Na+ currents (140 pS microm-2; 163 mM extracellular Na+) that were reduced in amplitude by lowering the Na+ gradient (43 mM extracellular Na+). At low temperatures (8-12 C), the time course of Na+ currents slowed and could be well described by the model of Hodgkin & Huxley. 3. Na+ currents were reversibly blocked by tetrodotoxin (TTX) and saxitoxin (STX) with a half-maximal block of 4.7 and 2.6 nM, respectively. In order to reduce series resistance errors, the Na+ current was partially blocked by low toxin concentrations (10-15 nM) in the experiments described below. Under these conditions, Na+ currents showed a threshold of activation of about -50 mV, and the voltages of half-maximal activation and inactivation were -29 and -55 mV, respectively. 4. The time course of recovery from inactivation could be described with a double-exponential function (time constants, 3-20 and 60-200 ms). The rapid and slow time constants showed a distinct voltage dependence with maximal values around -55 and -80 mV, respectively. These properties contributed to a reduction of the Na+ currents during repetitive stimulation that was more pronounced with higher stimulation frequencies and also showed a dependence on the holding potential. 5. In summary, the most striking features of DGC Na+ currents were the large current density and the presence of a current component showing a slow recovery from inactivation. Our data provide a basis for comparison with properties of Na+ currents in animal models of epilepsy, and for the study of drug actions in therapy-refractory epilepsy.

Calcium-dependent inactivation of high-threshold calcium currents in human dentate gyrus granule cells

1. Dentate gyrus granule cells acutely dissociated from hippocampal slices obtained from chronic temporal lobe epilepsy (TLE) patients displayed a high-voltage activated (HVA) Ca2+ conductance with a pronounced Ca2+-dependent inactivation. 2. Inactivation time constants and peak HVA Ca2+ current (ICa) amplitudes did not differ between perforated patch and whole-cell recordings without added exogenous Ca2+ buffers, indicating that the Ca2+-dependent characteristics of ICa inactivation were well preserved in whole-cell recordings. 3. Inactivation time constants correlated with whole-cell ICa, and were increased when Ca2+ was replaced with Ba2+ in the external solution or 5 mM BAPTA was added to the pipette solution. 4. In recordings without added exogenous Ca2+ buffers, the time course of ICa inactivation was comparable between human TLE and kindled rat granule cells. Conversely, the time course of ICa in human TLE granule cells loaded with 5 mM intracellular BAPTA resembled that observed in buffer-free recordings from control rat neurones. 5. The loss of a putative intraneuronal Ca2+ buffer, the Ca2+-binding protein calbindin (CB), from human granule cells during TLE may result in the pronounced Ca2+-dependent ICa inactivation. This process could serve a neuroprotective role by significantly decreasing Ca2+ entry during prolonged trains of action potentials known to occur during seizures.

Ion channels in epilepsy

There are specific alterations in the structure or function of ion channels in the epileptic brain. Some of these alterations may promote hyperexcitability, whereas others may protect neurons from the deleterious effects of epileptic discharges. With the use of human tissue resected from epilepsy patients and the comparison of cellular properties to those found in well-defined experimental models, we will continue to gain insight into the specific ion channel changes associated with epilepsies. Further genetic studies will help to elucidate the altered molecular mechanisms underlying ion channel changes in this devastating neurological disorder (Noebels, 1996). Whether it is a change in structure, function, or both, the study of ion channels in epilepsies will soon reveal specific characteristics of ion channels found only in epileptic tissue. If the altered properties of such ion channels cannot be found in control (nonepileptic) neurons, these channels might be called "epileptic" ion channels. An understanding of the specific structure, function, and pharmacology of these "epileptic" channels will yield important clues for future therapeutical approaches aimed at preventing epileptogenesis, and insight into the processes whereby ion channels become "epileptic" may finally open the way to prophylactic treatments of the epilepsies.