Kv7 and Kir6 Channels Shape the Slow AHP in Mouse Dentate Gyrus Granule Cells and Control Burst-like Firing Behavior.

The slow afterhyperpolarizing potential (sAHP) can silence a neuron for hundreds of milliseconds. Thereby, the sAHP determines the discharge behavior of many types of neurons. In dentate granule cells (DGCs), serving as a filter into the hippocampal network, mostly tonic or adapting discharge properties have been described. As under standard whole-cell recording conditions the sAHP is inhibited, we reevaluated the intrinsic functional phenotype of DGCs and the conductances underlying the sAHP, using gramicidine-perforated patch-clamp technique. We found that in 97/113 (86%) of the DGCs, a burst of action potentials (APs) to excitation ended by a large sAHP, despite continued depolarization. This result suggests that burst-like firing is the default functional phenotype of DGCs and that sAHPs are important for it. Indeed, burst-like firing DGCs showed a significantly higher sAHP-current (IsAHP) amplitude compared to spike-frequency adapting cells (16/113 = 14%). The IsAHP was mediated by Kv7 and Kir6 channels by pharmacological inhibition using XE991 and tolbutamide, although heterogeneously among DGCs. The percent inhibition of IsAHP by these compounds also correlated with the AP number and AP burst length. Application of 100 µM nickel after XE991 and tolbutamide detected a third conductance contributing to burst-like firing and the sAHP, most likely mediated by T-type calcium channels. Lastly, medial perforant path-dentate gyrus long-term potentiation was amplified by XE991 and tolbutamide. In conclusion, the sAHP shapes intrinsic burst-like firing which, under physiological circumstances, could be controlled via cholinergic afferents and ATP metabolism.

Correlation between Kir4.1 expression and barium-sensitive currents in rat and human glioma cell lines.

Gliomas are the most common primary brain tumors and often become apparent through symptomatic epileptic seizures. Glial cells express the inwardly rectifying K+ channel Kir4.1 playing a major role in K+ buffering, and are presumably involved in facilitating epileptic hyperexcitability. We therefore aimed to investigate the molecular and functional expression of Kir4.1 channels in cultured rat and human glioma cells. Quantitative PCR showed reduced expression of Kir4.1 in rat C6 and F98 cells as compared to control. In human U-87MG cells and in patient-derived low-passage glioblastoma cultures, Kir4.1 expression was also reduced as compared to autopsy controls. Testing Kir4.1 function using whole-cell patch-clamp experiments on rat C6 and two human low-passage glioblastoma cell lines (HROG38 and HROG05), we found a significantly depolarized resting membrane potential (RMP) in HROG05 (-29 ± 2 mV, n = 11) compared to C6 (-71 ± 1 mV, n = 12, P < 0.05) and HROG38 (-60 ± 2 mV, n = 12, P < 0.05). Sustained K+ inward or outward currents were sensitive to Ba2+ added to the bath solution in HROG38 and C6 cells, but not in HROG05 cells, consistent with RMP depolarization. While immunocytochemistry confirmed Kir4.1 in all three cell lines including HROG05, we found that aquaporin-4 and Kir5.1 were also significantly reduced suggesting that the Ba2+-sensitive K+ current is generally impaired in glioma tissue. In summary, we demonstrated that glioma cells differentially express functional inwardly rectifying K+ channels suggesting that impaired K+ buffering in cells lacking functional Ba2+-sensitive K+ currents may be a risk factor for increased excitability and thereby contribute to the differential epileptogenicity of gliomas.

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.

Brain Endothelial- and Epithelial-Specific Interferon Receptor Chain 1 Drives Virus-Induced Sickness Behavior and Cognitive Impairment.

Sickness behavior and cognitive dysfunction occur frequently by unknown mechanisms in virus-infected individuals with malignancies treated with type I interferons (IFNs) and in patients with autoimmune disorders. We found that during sickness behavior, single-stranded RNA viruses, double-stranded RNA ligands, and IFNs shared pathways involving engagement of melanoma differentiation-associated protein 5 (MDA5), retinoic acid-inducible gene 1 (RIG-I), and mitochondrial antiviral signaling protein (MAVS), and subsequently induced IFN responses specifically in brain endothelia and epithelia of mice. Behavioral alterations were specifically dependent on brain endothelial and epithelial IFN receptor chain 1 (IFNAR). Using gene profiling, we identified that the endothelia-derived chemokine ligand CXCL10 mediated behavioral changes through impairment of synaptic plasticity. These results identified brain endothelial and epithelial cells as natural gatekeepers for virus-induced sickness behavior, demonstrated tissue specific IFNAR engagement, and established the CXCL10-CXCR3 axis as target for the treatment of behavioral changes during virus infection and type I IFN therapy.

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.

Intrinsic rescaling of granule cells restores pattern separation ability of a dentate gyrus network model during epileptic hyperexcitability.

The dentate gyrus (DG) is thought to enable efficient hippocampal memory acquisition via pattern separation. With patterns defined as spatiotemporally distributed action potential sequences, the principal DG output neurons (granule cells, GCs), presumably sparsen and separate similar input patterns from the perforant path (PP). In electrophysiological experiments, we have demonstrated that during temporal lobe epilepsy (TLE), GCs downscale their excitability by transcriptional upregulation of "leak" channels. Here we studied whether this cell type-specific intrinsic plasticity is in a position to homeostatically adjust DG network function. We modified an established conductance-based computer model of the DG network such that it realizes a spatiotemporal pattern separation task, and quantified its performance with and without the experimentally constrained leaky GC phenotype. Two proposed TLE seizure mechanisms were implemented in various degrees and combinations: recurrent GC excitation via mossy fiber sprouting and increased PP input. While increasing PP strength degraded pattern separation only gradually, already the slight elevation of sprouting drastically (non-linearly) impaired pattern separation. In most tested hyperexcitable networks, leaky GCs ameliorated pattern separation. However, in some sprouting situations with all-or-none seizure behavior, pattern separation was disabled with and without leaky GCs. In the mild sprouting (and PP increase) region of non-linear impairment, leaky GCs were particularly effective in restoring pattern separation performance. These results are compatible with the hypothesis that the experimentally observed intrinsic rescaling of GCs serves to maintain the physiological function of the DG network.

Presynaptic mechanisms of neuronal plasticity and their role in epilepsy.

Synaptic communication requires constant adjustments of pre- and postsynaptic efficacies. In addition to synaptic long term plasticity, the presynaptic machinery underlies homeostatic regulations which prevent out of range transmitter release. In this minireview we will discuss the relevance of selected presynaptic mechanisms to epilepsy including voltage- and ligand-gated ion channels as well as cannabinoid and adenosine receptor signaling.

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.

Septotemporal position in the hippocampal formation determines epileptic and neurogenic activity in temporal lobe epilepsy.

It is a matter of ongoing debate whether newly generated granule cells contribute to epileptic activity in the hippocampus. To address this question, we investigated neurogenesis and epileptiform activity (EA) along the hippocampal septotemporal axis in the intrahippocampal kainate (KA) mouse model for temporal lobe epilepsy. Multisite intrahippocampal in vivo recordings and immunolabeling for c-Fos showed that the KA-induced status epilepticus (SE) extended along the septotemporal axis of both hippocampi with stronger intensity at ipsilateral temporal and contralateral sites. Accordingly, we found a position-dependent increase in proliferation (incorporation of bromodeoxyuridine) and neurogenesis (immunolabeling for doublecortin): Both were selectively increased in the ipsilateral temporal and entire contralateral subgranular zone, sparing the septal region close to the injection site. The newborn neurons were hyperexcitable and functionally integrated into the hippocampal network as revealed by patch-clamp recordings. Analysis of chronic EA also showed a differential intensity pattern along the hippocampal axis: EA was low in the septal portion with prominent sclerosis and granule cell dispersion but most pronounced in the transition zone where neurogenesis reappeared. In conclusion, SE stimulates neurogenesis in a position-dependent manner and coincidence of neurogenesis and stronger EA distal to the injection site suggests a proepileptogenic effect of increased neurogenesis.

Adaptive intrinsic plasticity in human dentate gyrus granule cells during temporal lobe epilepsy.

Granule cells in the dentate gyrus are only sparsely active in vivo and survive hippocampal sclerosis (HS) during temporal lobe epilepsy better than neighboring cells. This phenomenon could be related to intrinsic properties specifically adapted to counteract excitation. We studied the mechanisms underlying the excitability of human granule cells using acute hippocampal slices obtained during epilepsy surgery. Patch-clamp recordings were combined with pharmacology, immunocytochemistry, and computer simulations. The input resistance of granule cells correlated negatively with the duration of epilepsy and the degree of HS. Hyperpolarization-activated, ZD7288-sensitive cation (I(H), HCN) currents and highly Ba(2+)-sensitive, inwardly rectifying K(+) (Kir) currents (and HCN1 and Kir2.2 protein) were present somatodendritically and further enhanced in patients with severe HS versus mild HS. The properties and function of I(H) were characterized in granule cells. Although I(H) depolarized the membrane, it strongly reduced the input resistance and shifted the current-frequency function to higher input values. The shunting influence of HCN and Kir was similar and these conductances correlated. Resonance was not observed. Simulations suggest that the combined upregulation of Kir and HCN conductances attenuates excitatory synaptic input, while stabilizing the membrane potential and responsiveness. Thus, granule cells homeostatically downscale their input-output transfer function during epilepsy.

Upregulation of inward rectifier K+ (Kir2) channels in dentate gyrus granule cells in temporal lobe epilepsy.

In humans, temporal lobe epilepsy (TLE) is often associated with Ammon's horn sclerosis (AHS) characterized by hippocampal cell death, gliosis and granule cell dispersion (GCD) in the dentate gyrus. Granule cells surviving TLE have been proposed to be hyperexcitable and to play an important role in seizure generation. However, it is unclear whether this applies to conditions of AHS. We studied granule cells using the intrahippocampal kainate injection mouse model of TLE, brain slice patch-clamp recordings, morphological reconstructions and immunocytochemistry. With progressing AHS and GCD, 'epileptic' granule cells of the injected hippocampus displayed a decreased input resistance, a decreased membrane time constant and an increased rheobase. The resting leak conductance was doubled in epileptic granule cells and roughly 70-80% of this difference were sensitive to K(+) replacement. Of the increased K(+) leak, about 50% were sensitive to 1 mm Ba(2+). Approximately 20-30% of the pathological leak was mediated by a bicuculline-sensitive GABA(A) conductance. Epileptic granule cells had strongly enlarged inwardly rectifying currents with a low micromolar Ba(2+) IC(50), reminiscent of classic inward rectifier K(+) channels (Irk/Kir2). Indeed, protein expression of Kir2 subunits (Kir2.1, Kir2.2, Kir2.3, Kir2.4) was upregulated in epileptic granule cells. Immunolabelling for two-pore weak inward rectifier K(+) channels (Twik1/K2P1.1, Twik2/K2P6.1) was also increased. We conclude that the excitability of granule cells in the sclerotic focus of TLE is reduced due to an increased resting conductance mainly due to upregulated K(+) channel expression. These results point to a local adaptive mechanism that could counterbalance hyperexcitability in epilepsy.

Increased leak conductance in dentate gyrus granule cells of temporal lobe epilepsy patients with Ammon's horn sclerosis.

PURPOSE: Temporal lobe epilepsy (TLE) is often associated with Ammon's horn sclerosis (AHS) characterized by hippocampal cell death and dentate gyrus granule cell dispersion (GCD). Granule cells survive AHS and have been proposed to be hyperexcitable in TLE. Here we studied whether the passive excitability of granule cells correlates with the severity of AHS. METHODS: We analyzed the passive membrane properties of identified granule cells using patch-clamp recordings in acute tissue slices obtained from TLE surgery. Independent Wyler grading and GCD measurements were used to assess the severity of AHS. RESULTS: The input resistances and membrane time constants of granule cells were reduced in high-grade versus low-grade AHS samples and negatively correlated with the degree of GCD. Granule cells possessed large Ba(2+)-sensitive, inwardly rectifying K(+) conductances. DISCUSSION: The increased leak conductance, likely mediated by K(+) channels, does not argue for an increased excitability of granule cells but rather points to a neuroprotective mechanism in the sclerotic focus in TLE.

Synaptic background activity controls spike transfer from thalamus to cortex.

Characterizing the responsiveness of thalamic neurons is crucial to understanding the flow of sensory information. Typically, thalamocortical neurons possess two distinct firing modes. At depolarized membrane potentials, thalamic cells fire single action potentials and faithfully relay synaptic inputs to the cortex. At hyperpolarized potentials, the activation of T-type calcium channels promotes burst firing, and the transfer is less accurate. Our results suggest that this duality no longer holds if synaptic background activity is taken into account. By injecting stochastic conductances into guinea-pig thalamocortical neurons in slices, we show that the transfer function of these neurons is strongly influenced by conductance noise. The combination of synaptic noise with intrinsic properties gives a global responsiveness that is more linear, mixing single-spike and burst responses at all membrane potentials. Because in thalamic neurons, background synaptic input originates mainly from cortex, these results support a determinant role of corticothalamic feedback during sensory information processing.

Rescue of mesencephalic dopaminergic neurons in culture by low-level stimulation of voltage-gated sodium channels.

We used a model system in which dopaminergic (DA) neurons from embryonic rat mesencephalon undergo spontaneous and selective degeneration as they develop in culture. Here, we show that DA cell loss can be prevented efficiently by low concentrations of the Na+ channel agonist veratridine. The survival promoting effect of veratridine was reproduced by, but independent of, glial cell line-derived neurotrophic factor. Neuroprotection by veratridine was exquisitely specific to DA neurons, short-lived after withdrawal, and abolished by tetrodotoxin, indicating that activation of voltage-gated Na+ channels was crucially involved. Calcium measurements showed that veratridine-induced Na+ influx was necessary to maintain intracellular Ca2+ within a neuroprotective range through the stimulation of low-voltage activated T-type calcium channels, a mechanism that was distinct from that elicited by high K+-evoked depolarization. Interestingly, increasing neuronal excitability by treatment with apamin, an inhibitor of Ca2+-activated K+ channels, or with ouabain, a blocker of the Na+/K+-ATPase pump, was also neuroprotective by a mechanism involving T-type calcium channel activation. These results support the idea that mesencephalic DA neurons depend primarily on excitatory input for their survival during development.

Exploring spike transfer through the thalamus using hybrid artificial-biological neuronal networks.

We use dynamic clamp to construct "hybrid" thalamic circuits by connecting a biological neuron in situ to silicon- or software-generated "neurons" through artificial synapses. The purpose is to explore cellular sensory gating mechanisms that regulate the transfer efficiency of signals during different sleep-wake states. Hybrid technology is applied in vitro to different paradigms such as: (1) simulating interactions between biological thalamocortical neurons, artificial reticular thalamic inhibitory interneurons and a simulated sensory input, (2) grafting an artificial sensory input to a wholly biological thalamic network that generates spontaneous sleep-like oscillations, (3) injecting in thalamocortical neurons a background synaptic bombardment mimicking the activity of corticothalamic inputs. We show that the graded control of the strength of intrathalamic inhibition, combined with the membrane polarization and the fluctuating synaptic noise in thalamocortical neurons, is able to govern functional shifts between different input/output transmission states of the thalamic gate.

Selective coupling of T-type calcium channels to SK potassium channels prevents intrinsic bursting in dopaminergic midbrain neurons.

Dopaminergic midbrain (DA) neurons display two principal activity patterns in vivo, single-spike and burst firing, the latter coding for reward-related events. We have shown recently that the small-conductance calcium-activated potassium channel SK3 controls pacemaker frequency and precision in DA neurons of the substantia nigra (SN), and previous studies have implicated SK channels in the transition to burst firing. To identify the upstream calcium sources for SK channel activation in DA SN neurons, we studied the sensitivity of SK channel-mediated afterhyperpolarization (AHP) currents to inhibitors of different types of voltage-gated calcium channels in perforated patch-clamp recordings. Cobalt-sensitive AHP currents were not affected by L-type and P/Q-type calcium channel inhibitors and were reduced slightly (26%) by the N-type channel inhibitor omega-conotoxin-GVIA. In contrast, AHP currents were blocked substantially (85-94%) by micromolar concentrations of nickel (IC50, 33.75 microm) and mibefradil (IC50, 4.83 microm), indistinguishable from the nickel and mibefradil sensitivities of T-type calcium currents (IC50 values, 33.86 and 4.59 microm, respectively). These results indicate that SK channels are activated selectively via T-type calcium channels in DA SN neurons. Consequently, SK currents displayed use-dependent inactivation with similar time constants when compared with those of T-type calcium currents and generated a transient rebound inhibition. Both SK and T-type channels were essential for the stability of spontaneous pacemaker activity, and, in some DA SN neurons, T-type channel inhibition was sufficient to induce intrinsic burst firing. The functional coupling of SK to T-type channels has important implications for the temporal integration of synaptic input and might help to understand how DA neurons switch between pacemaker and burst-firing modes in vivo.