Altered expression of voltage-gated potassium channel 4.2 and voltage-gated potassium channel 4-interacting protein, and changes in intracellular calcium levels following lithium-pilocarpine-induced status epilepticus

Effects of L-Type Voltage-Gated Calcium Channel (LTCC) Inhibition on Hippocampal Neuronal Death after Pilocarpine-Induced Seizure

Epilepsy, marked by abnormal and excessive brain neuronal activity, is linked to the activation of L-type voltage-gated calcium channels (LTCCs) in neuronal membranes. LTCCs facilitate the entry of calcium (Ca2+) and other metal ions, such as zinc (Zn2+) and magnesium (Mg2+), into the cytosol. This Ca2+ influx at the presynaptic terminal triggers the release of Zn2+ and glutamate to the postsynaptic terminal. Zn2+ is then transported to the postsynaptic neuron via LTCCs. The resulting Zn2+ accumulation in neurons significantly increases the expression of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunits, contributing to reactive oxygen species (ROS) generation and neuronal death. Amlodipine (AML), typically used for hypertension and coronary artery disease, works by inhibiting LTCCs. We explored whether AML could mitigate Zn2+ translocation and accumulation in neurons, potentially offering protection against seizure-induced hippocampal neuronal death. We tested this by establishing a rat epilepsy model with pilocarpine and administering AML (10 mg/kg, orally, daily for 7 days) post-epilepsy onset. We assessed cognitive function through behavioral tests and conducted histological analyses for Zn2+ accumulation, oxidative stress, and neuronal death. Our findings show that AML's LTCC inhibition decreased excessive Zn2+ accumulation, reactive oxygen species (ROS) production, and hippocampal neuronal death following seizures. These results suggest amlodipine's potential as a therapeutic agent in seizure management and mitigating seizures' detrimental effects.

Glycogen Synthase Kinase 3: Ion Channels, Plasticity, and Diseases

Glycogen synthase kinase 3β (GSK3) is a multifaceted serine/threonine (S/T) kinase expressed in all eukaryotic cells. GSK3β is highly enriched in neurons in the central nervous system where it acts as a central hub for intracellular signaling downstream of receptors critical for neuronal function. Unlike other kinases, GSK3β is constitutively active, and its modulation mainly involves inhibition via upstream regulatory pathways rather than increased activation. Through an intricate converging signaling system, a fine-tuned balance of active and inactive GSK3β acts as a central point for the phosphorylation of numerous primed and unprimed substrates. Although the full range of molecular targets is still unknown, recent results show that voltage-gated ion channels are among the downstream targets of GSK3β. Here, we discuss the direct and indirect mechanisms by which GSK3β phosphorylates voltage-gated Na⁺ channels (Na_v1.2 and Na_v1.6) and voltage-gated K⁺ channels (K_v4 and K_v7) and their physiological effects on intrinsic excitability, neuronal plasticity, and behavior. We also present evidence for how unbalanced GSK3β activity can lead to maladaptive plasticity that ultimately renders neuronal circuitry more vulnerable, increasing the risk for developing neuropsychiatric disorders. In conclusion, GSK3β-dependent modulation of voltage-gated ion channels may serve as an important pharmacological target for neurotherapeutic development.

Involvement of TRPV4 in changes in rapidly inactivating potassium channels in the early stage of pilocarpine-induced status epilepticus in mice

The rapidly inactivating potassium current (I_A ) is important in controlling neuronal action potentials. Altered I_A function and K⁺ channel expression have been found in epilepsy, and activation of the transient receptor potential vanilloid 4 (TRPV4) channel is involved in epilepsy pathogenesis. This study examined whether TRPV4 affects Kv4.2 and K⁺ channel interacting protein (KCHIP) expression and I_A changes following pilocarpine-induced status epilepticus (PISE) in mice. Herein, hippocampal protein levels of Kv4.2 and KCHIP2 increased 3 h-3 d and decreased 7-30 d; that of KCHIP1 increased 3-24 h and decreased 3-30 d post-PISE. The TRPV4 antagonist HC-067047 attenuated the increased protein levels of Kv4.2 and KCHIP2 but not that of KCHIP1 post-PISE. The TRPV4 agonist GSK1016790A increased hippocampal protein levels of Kv4.2 and KCHIP2 but had no effect on KCHIP1 expression. HC-067047 attenuated the increased I_A in hippocampal pyramidal neurons 24 h and 3 d post-PISE. GSK1016790A increased I_A in hippocampal pyramidal neurons, shifting the voltage-dependent inactivation curve toward depolarization. The GSK1016790A-induced increase of I_A was blocked by protein kinase A and calcium/calmodulin-dependent kinase II antagonists but was unaffected by protein kinase C antagonists. We conclude that TRPV4 activation may be responsible for the increases of Kv4.2 and KCHIP2 expression in hippocampi and I_A in hippocampal pyramidal neurons in PISE mice, which are likely compensatory measures for hyperexcitability at the early stage of epilepsy.

KCND2 variants associated with global developmental delay differentially impair Kv4.2 channel gating

Here, we report on six unrelated individuals, all presenting with early-onset global developmental delay, associated with impaired motor, speech and cognitive development, partly with developmental epileptic encephalopathy and physical dysmorphisms. All individuals carry heterozygous missense variants of KCND2, which encodes the voltage-gated potassium (Kv) channel α-subunit Kv4.2. The amino acid substitutions associated with the variants, p.(Glu323Lys) (E323K), p.(Pro403Ala) (P403A), p.(Val404Leu) (V404L) and p.(Val404Met) (V404M), affect sites known to be critical for channel gating. To unravel their likely pathogenicity, recombinant mutant channels were studied in the absence and presence of auxiliary β-subunits under two-electrode voltage-clamp in Xenopus oocytes. All channel mutants exhibited slowed and incomplete macroscopic inactivation, and the P403A variant in addition slowed activation. Co-expression of KChIP2 or DPP6 augmented the functional expression of both wild-type and mutant channels, however, the auxiliary β-subunit-mediated gating modifications differed from wild-type and among mutants. To simulate the putative setting in the affected individuals, heteromeric Kv4.2 channels (wild-type + mutant) were studied as ternary complexes (containing both KChIP2 and DPP6). In the heteromeric ternary configuration, the E323K variant exhibited only marginal functional alterations compared to homomeric wild-type ternary, compatible with mild loss-of-function. By contrast, the P403A, V404L and V404M variants displayed strong gating impairment in the heteromeric ternary configuration, compatible with loss or gain-of-function. Our results support the etiological involvement of Kv4.2 channel gating impairment in early-onset monogenic global developmental delay. In addition, they suggest that gain-of-function mechanisms associated with a substitution of V404 increase epileptic seizure susceptibility.

Transient receptor potential vanilloid 4 activation inhibits the delayed rectifier potassium channels in hippocampal pyramidal neurons: An implication in pathological changes following pilocarpine-induced status epilepticus

Activation of transient receptor potential vanilloid 4 (TRPV4) can increase hippocampal neuronal excitability. TRPV4 has been reported to be involved in the pathogenesis of epilepsy. Voltage-gated potassium channels (VGPCs) play an important role in regulating neuronal excitability and abnormal VGPCs expression or function is related to epilepsy. Here, we examined the effect of TRPV4 activation on the delayed rectifier potassium current (I_K ) in hippocampal pyramidal neurons and on the Kv subunits expression in male mice. We also explored the role of TRPV4 in changes in Kv subunits expression in male mice following pilocarpine-induced status epilepticus (PISE). Application of TRPV4 agonists, GSK1016790A and 5,6-EET, markedly reduced I_K in hippocampal pyramidal neurons and shifted the voltage-dependent inactivation curve to the hyperpolarizing direction. GSK1016790A- and 5,6-EET-induced inhibition of I_K was blocked by TRPV4 specific antagonists, HC-067047 and RN1734. GSK1016790A-induced inhibition of I_K was markedly attenuated by calcium/calmodulin-dependent kinase II (CaMKII) antagonist. Application of GSK1016790A for up to 1 hr did not change the hippocampal protein levels of Kv1.1, Kv1.2, or Kv2.1. Intracerebroventricular injection of GSK1016790A for 3 d reduced the hippocampal protein levels of Kv1.2 and Kv2.1, leaving that of Kv1.1 unchanged. Kv1.2 and Kv2.1 protein levels as well as I_K reduced markedly in hippocampi on day 3 post PISE, which was significantly reversed by HC-067047. We conclude that activation of TRPV4 inhibits I_K in hippocampal pyramidal neurons, possibly by activating CaMKII. TRPV4-induced decrease in Kv1.2 and Kv2.1 expression and I_K may be involved in the pathological changes following PISE.

Modulation of Kv4.2/KChIP3 interaction by the ceroid lipofuscinosis neuronal 3 protein CLN3

Voltage-gated potassium (Kv) channels of the Kv4 subfamily associate with Kv channel-interacting proteins (KChIPs), which leads to enhanced surface expression and shapes the inactivation gating of these channels. KChIP3 has been reported to also interact with the late endosomal/lysosomal membrane glycoprotein CLN3 (ceroid lipofuscinosis neuronal 3), which is modified because of gene mutation in juvenile neuronal ceroid lipofuscinosis (JNCL). The present study was undertaken to find out whether and how CLN3, by its interaction with KChIP3, may indirectly modulate Kv4.2 channel expression and function. To this end, we expressed KChIP3 and CLN3, either individually or simultaneously, together with Kv4.2 in HEK 293 cells. We performed co-immunoprecipitation experiments and found a lower amount of KChIP3 bound to Kv4.2 in the presence of CLN3. In whole-cell patch-clamp experiments, we examined the effects of CLN3 co-expression on the KChIP3-mediated modulation of Kv4.2 channels. Simultaneous co-expression of CLN3 and KChIP3 with Kv4.2 resulted in a suppression of the typical KChIP3-mediated modulation; i.e. we observed less increase in current density, less slowing of macroscopic current decay, less acceleration of recovery from inactivation, and a less positively shifted voltage dependence of steady-state inactivation. The suppression of the KChIP3-mediated modulation of Kv4.2 channels was weaker for the JNCL-related missense mutant CLN3R334C and for a JNCL-related C-terminal deletion mutant (CLN3ΔC). Our data support the notion that CLN3 is involved in Kv4.2/KChIP3 somatodendritic A-type channel formation, trafficking, and function, a feature that may be lost in JNCL.

Control of neuronal excitability by GSK-3beta: Epilepsy and beyond

Glycogen synthase kinase 3beta (GSK-3β) is an enzyme with a variety of cellular functions in addition to the regulation of glycogen metabolism. In the central nervous system, different intracellular signaling pathways converge on GSK-3β through a cascade of phosphorylation events that ultimately control a broad range of neuronal functions in the development and adulthood. In mice, genetically removing or increasing GSK-3β cause distinct functional and structural neuronal phenotypes and consequently affect cognition. Precise control of GSK-3β activity is important for such processes as neuronal migration, development of neuronal morphology, synaptic plasticity, excitability, and gene expression. Altered GSK-3β activity contributes to aberrant plasticity within neuronal circuits leading to neurological, psychiatric disorders, and neurodegenerative diseases. Therapeutically targeting GSK-3β can restore the aberrant plasticity of neuronal networks at least in animal models of these diseases. Although the complete repertoire of GSK-3β neuronal substrates has not been defined, emerging evidence shows that different ion channels and their accessory proteins controlling excitability, neurotransmitter release, and synaptic transmission are regulated by GSK-3β, thereby supporting mechanisms of synaptic plasticity in cognition. Dysregulation of ion channel function by defective GSK-3β activity sustains abnormal excitability in the development of epilepsy and other GSK-3β-linked human diseases.

MicroRNA inhibition upregulates hippocampal A-type potassium current and reduces seizure frequency in a mouse model of epilepsy

Epilepsy is often associated with altered expression or function of ion channels. One example of such a channelopathy is the reduction of A-type potassium currents in the hippocampal CA1 region. The underlying mechanisms of reduced A-type channel function in epilepsy are unclear. Here, we show that inhibiting a single microRNA, miR-324-5p, which targets the pore-forming A-type potassium channel subunit Kv4.2, selectively increased A-type potassium currents in hippocampal CA1 pyramidal neurons in mice. Resting membrane potential, input resistance and other potassium currents were not altered. In a mouse model of acquired chronic epilepsy, inhibition of miR-324-5p reduced the frequency of spontaneous seizures and interictal epileptiform spikes supporting the physiological relevance of miR-324-5p-mediated control of A-type currents in regulating neuronal excitability. Mechanistic analyses demonstrated that microRNA-induced silencing of Kv4.2 mRNA is increased in epileptic mice leading to reduced Kv4.2 protein levels, which is mitigated by miR-324-5p inhibition. By contrast, other targets of miR-324-5p were unchanged. These results suggest a selective miR-324-5p-dependent mechanism in epilepsy regulating potassium channel function, hyperexcitability and seizures.

Transient Potassium Channels: Therapeutic Targets for Brain Disorders

Transient potassium current channels (I_A channels), which are expressed in most brain areas, have a central role in modulating feedforward and feedback inhibition along the dendroaxonic axis. Loss of the modulatory channels is tightly associated with a number of brain diseases such as Alzheimer’s disease, epilepsy, fragile X syndrome (FXS), Parkinson’s disease, chronic pain, tinnitus, and ataxia. However, the functional significance of I_A channels in these diseases has so far been underestimated. In this review, we discuss the distribution and function of I_A channels. Particularly, we posit that downregulation of I_A channels results in neuronal (mostly dendritic) hyperexcitability accompanied by the imbalanced excitation and inhibition ratio in the brain’s networks, eventually causing the brain diseases. Finally, we propose a potential therapeutic target: the enhanced action of I_A channels to counteract Ca²⁺-permeable channels including NMDA receptors could be harnessed to restore dendritic excitability, leading to a balanced neuronal state.

Altered A-type potassium channel function in the nucleus tractus solitarii in acquired temporal lobe epilepsy

Sudden unexpected death in epilepsy (SUDEP) is among the leading causes of death in people with epilepsy. Individuals with temporal lobe epilepsy (TLE) have a high risk for SUDEP because the seizures are often medically intractable. Neurons in the nucleus tractus solitarii (NTS) have been implicated in mouse models of SUDEP and play a critical role in modulating cardiorespiratory and autonomic output. Increased neuronal excitability of inhibitory, GABAergic neurons in the NTS develops during epileptogenesis, and NTS dysfunction has been implicated in mouse models of SUDEP. In this study we used the pilocarpine-induced status epilepticus model of TLE (i.e., pilo-SE mice) to investigate the A-type voltage-gated K⁺ channel as a potential contributor to increased excitability in GABAergic NTS neurons during epileptogenesis. Compared with age-matched control mice, pilo-SE mice displayed an increase in spontaneous action potential frequency and half-width 9-12 wk after treatment. Activity of GABAergic NTS neurons from pilo-SE mice showed less sensitivity to 4-aminopyridine. Correspondingly, reduced A-type K⁺ current amplitude was detected in these neurons, with no change in activation or inactivation kinetics. No changes were observed in K_v4.1, K_v4.2, K_v4.3, KChIP1, KChIP3, or KChIP4 mRNA expression. These changes contribute to the increased excitability in GABAergic NTS neurons that develops in TLE and may provide insight into potential mechanisms contributing to the increased risk for cardiorespiratory collapse and SUDEP in this model. NEW & NOTEWORTHY Sudden unexpected death in epilepsy (SUDEP) is a leading cause of death in epilepsy, and dysfunction in central autonomic neurons may play a role. In a mouse model of acquired epilepsy, GABAergic neurons in the nucleus tractus solitarii developed a reduced amplitude of the A-type current, which contributes to the increased excitability seen in these neurons during epileptogenesis. Neuronal excitability changes in inhibitory central vagal circuitry may increase the risk for cardiorespiratory collapse and SUDEP.

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.

Epileptic pilocarpine-treated rats exhibit aberrant hippocampal EPSP-spike potentiation but retain long-term potentiation

Hippocampal neuron plasticity is strongly associated with learning, memory, and cognition. In addition to modification of synaptic function and connectivity, the capacity of hippocampal neurons to undergo plasticity involves the ability to change nonsynaptic excitability. This includes altering the probability that EPSPs will generate action potentials (E‐S plasticity). Epilepsy is a prevalent neurological disorder commonly associated with neuronal hyperexcitability and cognitive dysfunction. We examined E‐S plasticity in chronically epileptic Sprague–Dawley rats 3–10 weeks after pilocarpine‐induced status epilepticus. CA1 neurons in hippocampal slices were assayed by whole‐cell current clamp to measure EPSPs evoked by Schaffer collateral stimulation. Using a weak spike‐timing‐dependent protocol to induce plasticity, we found robust E‐S potentiation in conjunction with weak long‐term potentiation (LTP) in saline‐treated rats. In pilocarpine‐treated rats, a similar degree of LTP was found, but E‐S potentiation was reduced. Additionally, the degree of E‐S potentiation was not correlated with the degree of LTP for either group, suggesting that they independently contribute to neuronal plasticity. E‐S potentiation also differed from LTP in that E‐S plasticity could be induced solely from action potentials generated by postsynaptic current injection. The calcium chelating agent BAPTA in the intracellular solution blocked LTP and E‐S potentiation, revealing the calcium dependence of both processes. These findings suggest that LTP and E‐S potentiation have overlapping but nonidentical mechanisms of inducing neuronal plasticity that may independently contribute to cognitive disruptions observed in the chronic epileptic state.

Somatodendritic surface expression of epitope-tagged and KChIP binding-deficient Kv4.2 channels in hippocampal neurons

Kv4.2 channels mediate a subthreshold-activating somatodendritic A-type current (ISA) in hippocampal neurons. We examined the role of accessory Kv channel interacting protein (KChIP) binding in somatodendritic surface expression and activity-dependent decrease in the availability of Kv4.2 channels. For this purpose we transfected cultured hippocampal neurons with cDNA coding for Kv4.2 wild-type (wt) or KChIP binding-deficient Kv4.2 mutants. All channels were equipped with an externally accessible hemagglutinin (HA)-tag and an EGFP-tag, which was attached to the C-terminal end. Combined analyses of EGFP self-fluorescence, surface HA immunostaining and patch-clamp recordings demonstrated similar dendritic trafficking and functional surface expression for Kv4.2[wt]HA,EGFP and the KChIP binding-deficient Kv4.2[A14K]HA,EGFP. Coexpression of exogenous KChIP2 augmented the surface expression of Kv4.2[wt]HA,EGFP but not Kv4.2[A14K]HA,EGFP. Notably, activity-dependent decrease in availability was more pronounced in Kv4.2[wt]HA,EGFP + KChIP2 coexpressing than in Kv4.2[A14K]HA,EGFP + KChIP2 coexpressing neurons. Our results do not support the notion that accessory KChIP binding is a prerequisite for dendritic trafficking and functional surface expression of Kv4.2 channels, however, accessory KChIP binding may play a potential role in Kv4.2 modulation during intrinsic plasticity processes.

Functional Neuroplasticity in the Nucleus Tractus Solitarius and Increased Risk of Sudden Death in Mice with Acquired Temporal Lobe Epilepsy

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in individuals with refractory acquired epilepsy. Cardiorespiratory failure is the most likely cause in most cases, and central autonomic dysfunction has been implicated as a contributing factor to SUDEP. Neurons of the nucleus tractus solitarius (NTS) in the brainstem vagal complex receive and integrate vagally mediated information regarding cardiorespiratory and other autonomic functions, and GABAergic inhibitory NTS neurons play an essential role in modulating autonomic output. We assessed the activity of GABAergic NTS neurons as a function of epilepsy development in the pilocarpine-induced status epilepticus (SE) model of temporal lobe epilepsy (TLE). Compared with age-matched controls, mice that survived SE had significantly lower survival rates by 150 d post-SE. GABAergic NTS neurons from mice that survived SE displayed a glutamate-dependent increase in spontaneous action potential firing rate by 12 wks post-SE. Increased spontaneous EPSC frequency was also detected, but vagal afferent synaptic release properties were unaltered, suggesting that an increase in glutamate release from central neurons developed in the NTS after SE. Our results indicate that long-term changes in glutamate release and activity of GABAergic neurons emerge in the NTS in association with epileptogenesis. These changes might contribute to increased risk of cardiorespiratory dysfunction and sudden death in this model of TLE.

The Segregated Expression of Voltage-Gated Potassium and Sodium Channels in Neuronal Membranes: Functional Implications and Regulatory Mechanisms

Neurons are highly polarized cells with apparent functional and morphological differences between dendrites and axon. A critical determinant for the molecular and functional identity of axonal and dendritic segments is the restricted expression of voltage-gated ion channels (VGCs). Several studies show an uneven distribution of ion channels and their differential regulation within dendrites and axons, which is a prerequisite for an appropriate integration of synaptic inputs and the generation of adequate action potential (AP) firing patterns. This review article will focus on the signaling pathways leading to segmented expression of voltage-gated potassium and sodium ion channels at the neuronal plasma membrane and the regulatory mechanisms ensuring segregated functions. We will also discuss the relevance of proper ion channel targeting for neuronal physiology and how alterations in polarized distribution contribute to neuronal pathology.

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.

The Shaping of Two Distinct Dendritic Spikes by A-Type Voltage-Gated K(+) Channels

Dendritic ion channels have been a subject of intense research in neuroscience because active ion channels in dendrites shape input signals. Ca²⁺-permeable channels including NMDA receptors (NMDARs) have been implicated in supralinear dendritic integration, and the I_A conductance in sublinear integration. Despite their essential roles in dendritic integration, it has remained uncertain whether these conductance coordinate with, or counteract, each other in the process of dendritic integration. To address this question, experiments were designed in hippocampal CA1 neurons with a recent 3D digital holography system that has shown excellent performance for spatial photoactivation. The results demonstrated a role of I_A as a key modulator for two distinct dendritic spikes, low- and high-threshold Ca²⁺ spikes, through a preferential action of I_A on Ca²⁺-permeable channel-mediated currents, over fast AMPAR-mediated currents. It is likely that the rapid kinetics of I_A provides feed-forward inhibition to counteract the regenerative Ca²⁺ channel-mediated dendritic excitability. This research reveals one dynamic ionic mechanism of dendritic integration, and may contribute to a new understanding of neuronal hyperexcitability embedded in several neural diseases such as epilepsy, fragile X syndrome and Alzheimer’s disease.

Homeostasis or channelopathy? Acquired cell type-specific ion channel changes in temporal lobe epilepsy and their antiepileptic potential

Neurons continuously adapt the expression and functionality of their ion channels. For example, exposed to chronic excitotoxicity, neurons homeostatically downscale their intrinsic excitability. In contrast, the “acquired channelopathy” hypothesis suggests that proepileptic channel characteristics develop during epilepsy. We review cell type-specific channel alterations under different epileptic conditions and discuss the potential of channels that undergo homeostatic adaptations, as targets for antiepileptic drugs (AEDs). Most of the relevant studies have been performed on temporal lobe epilepsy (TLE), a widespread AED-refractory, focal epilepsy. The TLE patients, who undergo epilepsy surgery, frequently display hippocampal sclerosis (HS), which is associated with degeneration of cornu ammonis subfield 1 pyramidal cells (CA1 PCs). Although the resected human tissue offers insights, controlled data largely stem from animal models simulating different aspects of TLE and other epilepsies. Most of the cell type-specific information is available for CA1 PCs and dentate gyrus granule cells (DG GCs). Between these two cell types, a dichotomy can be observed: while DG GCs acquire properties decreasing the intrinsic excitability (in TLE models and patients with HS), CA1 PCs develop channel characteristics increasing intrinsic excitability (in TLE models without HS only). However, thorough examination of data on these and other cell types reveals the coexistence of protective and permissive intrinsic plasticity within neurons. These mechanisms appear differentially regulated, depending on the cell type and seizure condition. Interestingly, the same channel molecules that are upregulated in DG GCs during HS-related TLE, appear as promising targets for future AEDs and gene therapies. Hence, GCs provide an example of homeostatic ion channel adaptation which can serve as a primer when designing novel anti-epileptic strategies.

Neuroprotective or neurotoxic effects of 4-aminopyridine mediated by KChIP1 regulation through adjustment of Kv 4.3 potassium channels expression and GABA-mediated transmission in primary hippocampal cells

4-Aminopyridine (4-AP) is a potassium channel blocker used for the treatment of neuromuscular disorders. Otherwise, it has been described to produce a large number of adverse effects among them cell death mediated mainly by blockage of K(+) channels. However, a protective effect against cell death has also been described. On the other hand, Kv channel interacting protein 1 (KChIP1) is a neuronal calcium sensor protein that is predominantly expressed at GABAergic synapses and it has been related with modulation of K(+) channels, GABAergic transmission and cell death. According to this KChIP1 could play a key role in the protective or toxic effects induced by 4-AP. We evaluated, in wild type and KChIP1 silenced primary hippocampal neurons, the effect of 4-AP (0.25μM to 2mM) with or without semicarbazide (0.3M) co-treatment after 24h and after 14 days 4-AP alone exposure on cell viability, the effect of 4-AP (0.25μM to 2mM) on KChIP1 and Kv 4.3 potassium channels gene expression and GABAergic transmission after 24h treatment or after 14 days exposure to 4-AP (0.25μM to1μM). 4-AP induced cell death after 24h (from 1mM) and after 14 days treatment. We observed that 4-AP modulates KChIP1 which regulate Kv 4.3 channels expression and GABAergic transmission. Our study suggests that KChIP1 is a key gene that has a protective effect up to certain concentration after short-term treatment with 4-AP against induced cell injury; but this protection is erased after long term exposure, due to KChIP1 down-regulation predisposing cell to 4-AP induced damages. These data might help to explain protective and toxic effects observed after overdose and long term exposure.

Are alterations in transmitter receptor and ion channel expression responsible for epilepsies?

Neuronal voltage-gated ion channels and ligand-gated synaptic receptors play a critical role in maintaining the delicate balance between neuronal excitation and inhibition within neuronal networks in the brain. Changes in expression of voltage-gated ion channels, in particular sodium, hyperpolarization-activated cyclic nucleotide-gated (HCN) and calcium channels, and ligand-gated synaptic receptors, in particular GABA and glutamate receptors, have been reported in many types of both genetic and acquired epilepsies, in animal models and in humans. In this chapter we review these and discuss the potential pathogenic role they may play in the epilepsies.

The loss of interneuron functional diversity in the piriform cortex after induction of experimental epilepsy

Interneuronal functional diversity is thought to be an important factor in the control of neural network oscillations in many brain regions. Specifically, interneuron action potential firing patterns are thought to modulate brain rhythms. In neurological disorders such as epilepsy where brain rhythms are significantly disturbed interneuron function is largely unexplored. Thus the purpose of this study was to examine the functional diversity of piriform cortex interneurons (PC; an area of the brain that easily supports seizures) before and after kindling-induced epilepsy. Using cluster analysis, we found five control firing behaviors. These groups were termed: non-adapting very high frequency (NAvHF), adapting high frequency (AHF), adapting low frequency (ALF), strongly adapting low frequency (sALF), and weakly adapting low frequency (wALF). A morphological analysis showed these spiking patterns were not associated with any specific interneuronal morphology although we found that most of the cells displaying NAvHF firing pattern were multipolar. After kindling about 40% of interneuronal firing pattern changed, and neither the NAvHF nor the wALF phenotypes were found. We also found that in multipolar interneurons a long-lasting potassium current was increased. A qPCR analysis indicated Kv1.6 subtype was up-regulated after kindling. An immunocytochemical analysis showed that Kv1.6 protein expression on parvalbumin (multipolar) interneurons increased by greater than 400%. We also examined whether these changes could be due to the selective death of a subset of interneurons but found that there was no change in cell number. These data show an important loss of the functional diversity of interneurons in the PC. Our data suggest that under pathophysiological condition interneurons are plastic resulting in the attenuation of high frequency network oscillations in favor of low frequency network activity. This may be an important new mechanism by which network synchrony is disturbed in epileptic seizures.

Reversing nerve cell pathology by optimizing modulatory action on target ion channels

In diseases of the brain, the distribution and properties of ion channels display deviations from healthy control subjects. We studied three cases of ion channel alteration related to epileptogenesis. The first case of ion channel alteration represents an enhanced sodium current, the second case addresses the downregulation of the transient potassium current K(A), and the third case relates to kinetic properties of K(A) in a patient with temporal lobe epilepsy. Using computational modeling and optimization, we aimed at reversing the pathological characteristics and restoring normal neural function by altering ion channel properties. We identified two key aspects of neural dysfunction in epileptogenesis: an enhanced response to synaptic input in general and to highly synchronized synaptic input in particular. In previous studies, we showed that the potassium channel K(A) played a major role in neural responses to highly synchronized input. It was therefore selected as the target upon which modulators would act. In biophysical simulations, five experimentally characterized endogenous modulations on the K(A) channel were included. Relative concentrations of these modulators were controlled by a numerical optimizer that compared model output to predefined neural output, which represented a normal physiological response. Several solutions that restored the neuron function were found. In particular, distinct subtype compositions of the auxiliary proteins Kv channel-interacting proteins 1 and dipeptidyl aminopeptidase-like protein 6 were able to restore changes imposed by the enhanced sodium conductance or suppressed K(A) conductance. Moreover, particular combinations of protein kinese C, calmodulin-dependent protein kinase II, and arachidonic acid were also able to restore these changes as well as the channel pathology found in a patient with temporal lobe epilepsy. The solutions were further analyzed for sensitivity and robustness. We suggest that the optimization procedure can be used not only for neurons, but also for other organs with excitable cells, such as the heart and pancreas where channelopathies are found.

Acute alterations of somatodendritic action potential dynamics in hippocampal CA1 pyramidal cells after kainate-induced status epilepticus in mice

Pathophysiological remodeling processes at an early stage of an acquired epilepsy are critical but not well understood. Therefore, we examined acute changes in action potential (AP) dynamics immediately following status epilepticus (SE) in mice. SE was induced by intraperitoneal (i.p.) injection of kainate, and behavioral manifestation of SE was monitored for 3-4 h. After this time interval CA1 pyramidal cells were studied ex vivo with whole-cell current-clamp and Ca(2+) imaging techniques in a hippocampal slice preparation. Following acute SE both resting potential and firing threshold were modestly depolarized (2-5 mV). No changes were seen in input resistance or membrane time constant, but AP latency was prolonged and AP upstroke velocity reduced following acute SE. All cells showed an increase in AP halfwidth and regular (rather than burst) firing, and in a fraction of cells the notch, typically preceding spike afterdepolarization (ADP), was absent following acute SE. Notably, the typical attenuation of backpropagating action potential (b-AP)-induced Ca(2+) signals along the apical dendrite was strengthened following acute SE. The effects of acute SE on the retrograde spread of excitation were mimicked by applying the Kv4 current potentiating drug NS5806. Our data unveil a reduced somatodendritic excitability in hippocampal CA1 pyramidal cells immediately after acute SE with a possible involvement of both Na(+) and K(+) current components.

Expression and distribution of Kv4 potassium channel subunits and potassium channel interacting proteins in subpopulations of interneurons in the basolateral amygdala

The Kv4 potassium channel α subunits, Kv4.1, Kv4.2, and Kv4.3, determine some of the fundamental physiological properties of neurons in the CNS. Kv4 subunits are associated with auxiliary β-subunits, such as the potassium channel interacting proteins (KChIP1 - 4), which are thought to regulate the trafficking and gating of native Kv4 potassium channels. Intriguingly, KChIP1 is thought to show cell type-selective expression in GABA-ergic inhibitory interneurons, while other β-subunits (KChIP2-4) are associated with principal glutamatergic neurons. However, nothing is known about the expression of Kv4 family α- and β-subunits in specific interneurons populations in the BLA. Here, we have used immunofluorescence, co-immunoprecipitation, and Western Blotting to determine the relative expression of KChIP1 in the different interneuron subtypes within the BLA, and its co-localization with one or more of the Kv4 α subunits. We show that all three α-subunits of Kv4 potassium channel are found in rat BLA neurons, and that the immunoreactivity of KChIP1 closely resembles that of Kv4.3. Indeed, Kv4.3 showed almost complete co-localization with KChIP1 in the soma and dendrites of a distinct subpopulation of BLA neurons. Dual-immunofluorescence studies revealed this to be in BLA interneurons immunoreactive for parvalbumin, cholecystokin-8, and somatostatin. Finally, co-immunoprecipitation studies showed that KChIP1 was associated with all three Kv4 α subunits. Together our results suggest that KChIP1 is selectively expressed in BLA interneurons where it may function to regulate the activity of A-type potassium channels. Hence, KChIP1 might be considered as a cell type-specific regulator of GABAergic inhibitory circuits in the BLA.

Autonomic and cellular mechanisms mediating detrimental cardiac effects of status epilepticus

Prolonged seizure activity (status epilepticus; SE) can result in increased susceptibility to lethal ventricular arrhythmias for an extended period of time following seizure termination. SE is accompanied by acute, intense activation of the sympathetic nervous system (SymNS) and results in myocyte myofilament damage, arrhythmogenic alterations in cardiac electrical activity, and increased susceptibility to ventricular arrhythmias. However, the mechanisms mediating the changes in cardiac function, and the specific arrhythmogenic substrate produced during SE are unknown. To determine if detrimental cardiac effects of SE are mediated by SymNS stimulation of the heart, we examined the effects of B-adrenergic blockade (atenolol) during seizure activity on blood pressure, heart rate, myocyte myofilament injury (cardiac troponin I, cTnI), electrocardiographic activity, and susceptibility to arrhythmias. Furthermore, we determined if SE was associated with altered expression of the Kv4.x potassium channels, which are critical for action potential repolarization and thereby contribute significantly to normal cardiac electrical activity. Lithium-pilocarpine induced SE was associated with acute tachycardia, hypertension, and cardiomyocyte damage. Arrhythmogenic alterations in cardiac electrical activity accompanied by increased susceptibility to experimentally induced arrhythmias were evident during the first 2 weeks following SE. Both were prevented by atenolol treatment during seizures. Furthermore, one and two weeks after SE, myocyte ion channel remodeling, characterized by a decreased expression of cardiac Kv4.2 potassium channels, was evident. These data suggest that the cardiac effects of prolonged and intense SymNS activation during SE induce myofilament damage and downregulation of Kv4.2 channels, which alter cardiac electrical activity and increase susceptibility to lethal arrhythmias.

Roles of KChIP1 in the regulation of GABA-mediated transmission and behavioral anxiety

K⁺ channel interacting protein 1 (KChIP1) is a neuronal calcium sensor (NCS) protein that interacts with multiple intracellular molecules. Its physiological function, however, remains largely unknown. We report that KChIP1 is predominantly expressed at GABAergic synapses of a subset of parvalbumin-positive neurons in the brain. Forced expression of KChIP1 in cultured hippocampal neurons increased the frequency of miniature inhibitory postsynaptic currents (mIPSCs), reduced paired pulse facilitation of autaptic IPSCs, and decreases potassium current density. Furthermore, genetic ablation of KChIP1 potentiated potassium current density in neurons and caused a robust enhancement of anxiety-like behavior in mice. Our study suggests that KChIP1 is a synaptic protein that regulates behavioral anxiety by modulating inhibitory synaptic transmission, and drugs that act on KChIP1 may help to treat patients with mood disorders including anxiety.

Intrinsic plasticity in acquired epilepsy: too much of a good thing?

Mechanisms of plasticity are important to the astounding capacity of the brain to adapt and learn. Ion channels are significant contributors to neuronal plasticity, but their dysfunction has been implicated in several nervous system diseases from movement disorders to epilepsy. Although many inherited ion channel mutations have been associated with these disorders, it has been recently recognized that channelopathies can also include aberrant ion channel function that is acquired after an insult or injury to the brain. These acquired alterations are being investigated in animal models of temporal lobe epilepsy, where studies have shown functional changes in voltage-gated ion channels that lead to increases in excitability. Studies of these hyperexcitable neurons have included recordings in the hippocampus, entorhinal cortex, and thalamus and support the existence of an extended seizure network with several nodes of altered activity that are established during epileptogenesis. A better understanding of the key ion channels and brain regions that are responsible for the development of this hyperexcitability, along with the molecular mechanisms involved, may provide novel treatment strategies for epilepsy.

KChIP1: a potential modulator to GABAergic system

Compelling evidences from transgenic mice, immunoprecipitation data, gene expression analysis, and functional heterologous expression studies supported the role of Kv channel interacting proteins (KChIPs) as modulators of Kv4 (Shal) channels underlying the cardiac transient outward current and neuronal A-type current. Till now, there are four members (KChIP1-4) identified in this family. KChIP1 is expressed predominantly in brain, with relative abundance in Purkinje cells of cerebellum, the reticular thalamic nuclei, the medial habenular nuclei, the hippocampus, and striatum. Our results from in situ hybridization and immunostaining assay revealed that KChIP1 was expressed in a subpopulation of parvalbumin-positive neurons suggesting its functional relationship with the GABAergic inhibitory neurons. Moreover, results obtained from KChIP1-deficient mice showed that KChIP1 mutation did not impair survival or alter the overall brain architecture, arguing against its essential function in brain development. However, the mice bearing KChIP1 deletion showed increased susceptibility to anti-GABAergic convulsive drug pentylenetetrazole-induced seizure, indicating that KChIP1 might play pivotal roles in the GABAergic inhibitory system.