Upregulated H-current in hyperexcitable CA1 dendrites after febrile seizures

Electric field effects on neuronal input-output relationship by regulating NMDA spikes

Evidence shows that the dendritic polarization induced by weak electrical field (EF) can affect the neuronal input-output function via modulating dendritic integration of AMPA synapses, indicating that the supralinear dendritic integration of NMDA synapses can also be influenced by dendritic polarization. However, it remains unknown how dendritic polarization affects NMDA-type dendritic integration, and then contributes to neuronal input-output relationship. Here, we used a computational model of pyramidal neuron with inhomogeneous extracellular potentials to characterize the relationship among EF, dendritic integration, and somatic output. Basing on singular perturbation we analyzed the subthreshold dynamics of membrane potentials in response to NMDA synapses, and found that the equilibrium mapping of a fast subsystem can characterize the asymptotic subthreshold input-output (sI/O) relationship for EF-regulated supralinear dendritic integration, allowing us to predict the tendency of EF-regulated dendritic integration by showing the variation of equilibrium mapping under EF stimulation. EF-induced depolarization at distal dendrites receiving synapses plays a crucial role in shifting the steep change of sI/O left by facilitating dendritic NMDA spike generation and in decreasing the plateau of sI/O via reducing driving force. And more effective EF modulation appears at sparsely activated NMDA receptors compared with clustered synaptic inputs. During the action potential (AP) generation, the respective contribution of EF-regulated dendritic integration and EF-induced somatic polarization was identified to show their synergetic or antagonistic effect on AP generation, depending on neuronal excitability. These results provided insight in understanding the modulation effect of EF on neuronal computation, which is important for optimizing noninvasive brain stimulation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11571-022-09922-y.

Concerted suppressive effects of carisbamate, an anti-epileptic alkyl-carbamate drug, on voltage-gated Na+ and hyperpolarization-activated cation currents

Carisbamate (CRS, RWJ-333369) is a new anti-seizure medication. It remains unclear whether and how CRS can perturb the magnitude and/or gating kinetics of membrane ionic currents, despite a few reports demonstrating its ability to suppress voltage-gated Na⁺ currents. In this study, we observed a set of whole-cell current recordings and found that CRS effectively suppressed the voltage-gated Na⁺ (I_Na) and hyperpolarization-activated cation currents (I_h) intrinsically in electrically excitable cells (GH₃ cells). The effective IC₅₀ values of CRS for the differential suppression of transient (I_Na(T)) and late I_Na (I_Na(L)) were 56.4 and 11.4 μM, respectively. However, CRS strongly decreased the strength (i.e., Δarea) of the nonlinear window component of I_Na (I_Na(W)), which was activated by a short ascending ramp voltage (V_ramp); the subsequent addition of deltamethrin (DLT, 10 μM) counteracted the ability of CRS (100 μM, continuous exposure) to suppress I_Na(W). CRS strikingly decreased the decay time constant of I_Na(T) evoked during pulse train stimulation; however, the addition of telmisartan (10 μM) effectively attenuated the CRS (30 μM, continuous exposure)-mediated decrease in the decay time constant of the current. During continued exposure to deltamethrin (10 μM), known to be a pyrethroid insecticide, the addition of CRS resulted in differential suppression of the amplitudes of I_Na(T) and I_Na(L). The amplitude of I_h activated by a 2-s membrane hyperpolarization was diminished by CRS in a concentration-dependent manner, with an IC₅₀ value of 38 μM. For I_h, CRS altered the steady-state I-V relationship and attenuated the strength of voltage-dependent hysteresis (Hys_(V)) activated by an inverted isosceles-triangular V_ramp. Moreover, the addition of oxaliplatin effectively reversed the CRS-mediated suppression of Hys_(V). The predicted docking interaction between CRS and with a model of the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel or between CRS and the hNa_V1.7 channel reflects the ability of CRS to bind to amino acid residues in HCN or hNa_V1.7 channel via hydrogen bonds and hydrophobic interactions. These findings reveal the propensity of CRS to modify I_Na(T) and I_Na(L) differentially and to effectively suppress the magnitude of I_h. I_Na and I_h are thus potential targets of the actions of CRS in terms of modulating cellular excitability.

Degeneracy in epilepsy: multiple routes to hyperexcitable brain circuits and their repair

Due to its complex and multifaceted nature, developing effective treatments for epilepsy is still a major challenge. To deal with this complexity we introduce the concept of degeneracy to the field of epilepsy research: the ability of disparate elements to cause an analogous function or malfunction. Here, we review examples of epilepsy-related degeneracy at multiple levels of brain organisation, ranging from the cellular to the network and systems level. Based on these insights, we outline new multiscale and population modelling approaches to disentangle the complex web of interactions underlying epilepsy and to design personalised multitarget therapies.

Genetic mechanisms in generalized epilepsies

The genetic generalized epilepsies (GGEs) have been proved to generate from genetic impact by twin studies and family studies. The genetic mechanisms of generalized epilepsies are always updating over time. Although the genetics of GGE is complex, there are always new susceptibility genes coming up as well as copy number variations which can lead to important breakthroughs in exploring the problem. At the same time, the development of ClinGen fades out some of the candidate genes. This means we have to figure out what accounts for a reliable gene for GGE, in another word, which gene has sufficient evidence for GGE. This will improve our understanding of the genetic mechanisms of GGE. In this review, important up-to-date genetic mechanisms of GGE were discussed.

The Impact of Altered HCN1 Expression on Brain Function and Its Relationship with Epileptogenesis

Hyperpolarization-activated cyclic nucleotide-gated cation channel 1 (HCN1) is predominantly expressed in neurons from the neocortex and hippocampus, two important regions related to epilepsy. Both animal models for epilepsy and epileptic patients show decreased HCN1 expression and HCN1-mediated Ih current. It has been shown in neuroelectrophysiological experiments that a decreased Ih current can increase neuronal excitability. However, some studies have shown that blocking the Ih current in vivo can exert antiepileptic effects. This paradox raises an important question regarding the causal relationship between HCN1 alteration and epileptogenesis, which to date has not been elucidated. In this review, we summarize the literature related to HCN1 and epilepsy, aiming to find a possible explanation for this paradox, and explore the correlation between HCN1 and the mechanism of epileptogenesis. We analyze the alterations in the expression and distribution of HCN1 and the corresponding impact on brain function in epilepsy. In addition, we also discuss the effect of blocking Ih on epilepsy symptoms. Addressing these issues will help to inspire new strategies to explore the relationship between HCN1 and epileptogenesis, and ultimately promote the development of new targets for epilepsy therapy.

Cell to network computational model of the epileptic human hippocampus suggests specific roles of network and channel dysfunctions in the ictal and interictal oscillations

The mechanisms underlying the generation of hippocampal epileptic seizures and interictal events and their interactions with the sleep-wake cycle are not yet fully understood. Indeed, medial temporal lobe epilepsy is associated with hippocampal abnormalities both at the neuronal (channelopathies, impaired potassium and chloride dynamics) and network level (neuronal and axonal loss, mossy fiber sprouting), with more frequent seizures during wakefulness compared with slow-wave sleep. In this article, starting from our previous computational modeling work of the hippocampal formation based on realistic topology and synaptic connectivity, we study the role of micro- and mesoscale pathological conditions of the epileptic hippocampus in the generation and maintenance of seizure-like theta and interictal oscillations. We show, through the simulations of hippocampal activity during slow-wave sleep and wakefulness that: (i) both mossy fiber sprouting and sclerosis account for seizure-like theta activity, (ii) but they have antagonist effects (seizure-like activity occurrence increases with sprouting but decreases with sclerosis), (iii) though impaired potassium and chloride dynamics have little influence on the generation of seizure-like activity, they do play a role on the generation of interictal patterns, and (iv) seizure-like activity and fast ripples are more likely to occur during wakefulness and interictal spikes during sleep.

Emergence of stochastic resonance in a two-compartment hippocampal pyramidal neuron model

In vitro studies have shown that hippocampal pyramidal neurons employ a mechanism similar to stochastic resonance (SR) to enhance the detection and transmission of weak stimuli generated at distal synapses. To support the experimental findings from the perspective of multicompartment model analysis, this paper aimed to elucidate the phenomenon of SR in a noisy two-compartment hippocampal pyramidal neuron model, which was a variant of the Pinsky-Rinzel neuron model with smooth activation functions and a hyperpolarization-activated cation current. With a bifurcation analysis of the model, we demonstrated the underlying dynamical structure responsible for the occurrence of SR. Furthermore, using a stochastically generated biphasic pulse train and broadband noise generated by the Orenstein-Uhlenbeck process as noise perturbation, both SR and suprathreshold SR were observed and quantified. Spectral analysis revealed that the distribution of spectral power under noise perturbations, in addition to inherent neurodynamics, is the main factor affecting SR behavior. The research results suggested that noise enhances the transmission of weak stimuli associated with elongated dendritic structures of hippocampal pyramidal neurons, thereby providing support for related laboratory findings.

Conventional measures of intrinsic excitability are poor estimators of neuronal activity under realistic synaptic inputs

Activity-dependent regulation of intrinsic excitability has been shown to greatly contribute to the overall plasticity of neuronal circuits. Such neuroadaptations are commonly investigated in patch clamp experiments using current step stimulation and the resulting input-output functions are analyzed to quantify alterations in intrinsic excitability. However, it is rarely addressed, how such changes translate to the function of neurons when they operate under natural synaptic inputs. Still, it is reasonable to expect that a strong correlation and near proportional relationship exist between static firing responses and those evoked by synaptic drive. We challenge this view by performing a high-yield electrophysiological analysis of cultured mouse hippocampal neurons using both standard protocols and simulated synaptic inputs via dynamic clamp. We find that under these conditions the neurons exhibit vastly different firing responses with surprisingly weak correlation between static and dynamic firing intensities. These contrasting responses are regulated by two intrinsic K-currents mediated by Kv1 and Kir channels, respectively. Pharmacological manipulation of the K-currents produces differential regulation of the firing output of neurons. Static firing responses are greatly increased in stuttering type neurons under blocking their Kv1 channels, while the synaptic responses of the same neurons are less affected. Pharmacological blocking of Kir-channels in delayed firing type neurons, on the other hand, exhibit the opposite effects. Our subsequent computational model simulations confirm the findings in the electrophysiological experiments and also show that adaptive changes in the kinetic properties of such currents can even produce paradoxical regulation of the firing output.

Acute, Low-Dose Neutron Exposures Adversely Impact Central Nervous System Function

A recognized risk of long-duration space travel arises from the elevated exposure astronauts face from galactic cosmic radiation (GCR), which is composed of a diverse array of energetic particles. There is now abundant evidence that exposures to many different charged particle GCR components within acute time frames are sufficient to induce central nervous system deficits that span from the molecular to the whole animal behavioral scale. Enhanced spacecraft shielding can lessen exposures to charged particle GCR components, but may conversely elevate neutron radiation levels. We previously observed that space-relevant neutron radiation doses, chronically delivered at dose-rates expected during planned human exploratory missions, can disrupt hippocampal neuronal excitability, perturb network long-term potentiation and negatively impact cognitive behavior. We have now determined that acute exposures to similar low doses (18 cGy) of neutron radiation can also lead to suppressed hippocampal synaptic signaling, as well as decreased learning and memory performance in male mice. Our results demonstrate that similar nervous system hazards arise from neutron irradiation regardless of the exposure time course. While not always in an identical manner, neutron irradiation disrupts many of the same central nervous system elements as acute charged particle GCR exposures. The risks arising from neutron irradiation are therefore important to consider when determining the overall hazards astronauts will face from the space radiation environment.

Homeostatic plasticity and burst activity are mediated by hyperpolarization-activated cation currents and T-type calcium channels in neuronal cultures

Homeostatic plasticity stabilizes neuronal networks by adjusting the responsiveness of neurons according to their global activity and the intensity of the synaptic inputs. We investigated the homeostatic regulation of hyperpolarization-activated cyclic nucleotide-gated (HCN) and T-type calcium (Ca_V3) channels in dissociated and organotypic slice cultures. After 48 h blocking of neuronal activity by tetrodotoxin (TTX), our patch-clamp experiments revealed an increase in the depolarizing voltage sag and post-inhibitory rebound mediated by HCN and Ca_V3 channels, respectively. All HCN subunits (HCN1 to 4) and T-type Ca-channel subunits (Ca_V3.1, 3.2 and 3.3) were expressed in both control and activity-deprived hippocampal cultures. Elevated expression levels of Ca_V3.1 mRNA and a selective increase in the expression of TRIP8b exon 4 isoforms, known to regulate HCN channel localization, were also detected in TTX-treated cultured hippocampal neurons. Immunohistochemical staining in TTX-treated organotypic slices verified a more proximal translocation of HCN1 channels in CA1 pyramidal neurons. Computational modeling also implied that HCN and T-type calcium channels have important role in the regulation of synchronized bursting evoked by previous activity-deprivation. Thus, our findings indicate that HCN and T-type Ca-channels contribute to the homeostatic regulation of excitability and integrative properties of hippocampal neurons.

Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels as Drug Targets for Neurological Disorders

The hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are voltage-gated ion channels that critically modulate neuronal activity. Four HCN subunits (HCN1-4) have been cloned, each having a unique expression profile and distinctive effects on neuronal excitability within the brain. Consistent with this, the expression and function of these subunits are altered in diverse ways in neurological disorders. Here, we review current knowledge on the structure and distribution of the individual HCN channel isoforms, their effects on neuronal activity under physiological conditions, and how their expression and function are altered in neurological disorders, particularly epilepsy, neuropathic pain, and affective disorders. We discuss the suitability of HCN channels as therapeutic targets and how drugs might be strategically designed to specifically act on particular isoforms. We conclude that medicines that target individual HCN isoforms and/or their auxiliary subunit, TRIP8b, may provide valuable means of treating distinct neurological conditions.

Homeostatic Plasticity in Epilepsy

In the healthy brain, neuronal excitability and synaptic strength are homeostatically regulated to keep neuronal network activity within physiological boundaries. Epilepsy is characterized by episodes of highly synchronized firing across in widespread neuronal populations, due to a failure in regulation of network activity. Here we consider epilepsy as a failure of homeostatic plasticity or as a maladaptive response to perturbations in the activity. How homeostatic compensation is involved in epileptogenic processes or in the chronic phase of epilepsy, is still debated. Although several theories have been proposed, there is relatively little experimental evidence to evaluate them. In this perspective, we will discuss recent results that shed light on the potential role of homeostatic plasticity in epilepsy. First, we will present some recent insights on how homeostatic compensations are probably active before and during epileptogenesis and how their actions are temporally regulated and closely dependent on the progression of pathology. Then, we will consider the dual role of transcriptional regulation during epileptogenesis, and finally, we will underline the importance of homeostatic plasticity in the context of therapeutic interventions for epilepsy. While classic pharmacological interventions may be counteracted by the epileptic brain to maintain its potentially dysfunctional set point, novel therapeutic approaches may provide the neuronal network with the tools necessary to restore its physiological balance.

Downregulation of hyperpolarization-activated cyclic nucleotide-gated channels (HCN) in the hippocampus of patients with medial temporal lobe epilepsy and hippocampal sclerosis (MTLE-HS)

Changes in the expression of HCN ion channels leading to changes in I_h function and neuronal excitability are considered to be possible mechanisms involved in epileptogenesis in kinds of human epilepsy. In previous animal studies of febrile seizures and temporal lobe epilepsy, changes in the expression of HCN1 and HCN2 channels at different time points and in different parts of the brain were not consistent, suggesting that transcriptional disorders involving HCNs play a crucial role in the epileptogenic process. Therefore, we aimed to assess the transcriptional regulation of HCN channels in Medial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS) patients. This study included eight nonhippocampal sclerosis patients and 40 MTLE-HS patients. The mRNA expression of HCN channels was evaluated by qRT-PCR, while the protein expression was quantitatively analyzed by Western blotting. The subcellular localization of HCN channels in the hippocampus was explored by immunofluorescence. We demonstrated that the mRNA and protein expression of HCN1 and HCN2 are downregulated in controls compared to that in MTLE-HS patients. In the hippocampal CA1/CA4 subregion and GCL, in addition to a large decrease in neurons, the expression of HCN1 and HCN2 on neuronal cell membranes was also downregulated in MTLE-HS patients. These findings suggest that the expression of HCN channels are downregulated in MTLE-HS, which indicates that the decline in HCN channels in the hippocampus during chronic epilepsy in MTLE-HS patients leads to the downregulation of I_h current density and function, thereby reducing the inhibitory effect and increasing neuronal excitability and eventually causing disturbances in the electrical activity of neurons.

Ca2+ Channels Involvement in Low-Frequency Stimulation-Mediated Suppression of Intrinsic Excitability of Hippocampal CA1 Pyramidal Cells in a Rat Amygdala Kindling Model

Low-frequency stimulation has demonstrated promising seizure suppression in animal models of epilepsy, while the mechanism of the effect is still debated. Changes in intrinsic properties have been recognized as a prominent pathophysiologically relevant feature of numerous neurological disorders including epilepsy. Here, it was evaluated whether LFS can preserve the intrinsic neuronal electrophysiological properties in a rat model of epilepsy, focusing on the possible involvement of voltage-gated Ca²⁺ channels. The amygdala kindling model was induced by 3 s monophasic square wave pulses (50 Hz, 1 ms duration, 12times/day at 5 min intervals). Both LFS alone and kindled plus LFS (KLFS) groups received four packages of LFS (each consisting of 200 monophasic square pulses, 0.1 ms pulse duration at 1 Hz with the after discharge threshold intensity), which in KLFS rats was applied immediately after kindling induction. Whole-cell patch-clamp recordings were made in the presence of fast synaptic blockers 24 h after the last kindling stimulations or following kindling stimulations plus LFS application. In the KLFS group, both the rebound excitation and kindling-induced intrinsic hyperexcitability were decreased, associated with a regular intrinsic firing as indicated by a lower coefficient of variation. The amplitude of afterdepolarization (ADP) and its area under the curve were both decreased in the KLFS group compared to the kindled group. LFS prevented the increasing effect of kindling on Ca²⁺ currents in the KLFS group. Findings provided evidence for a novel form of epileptiform activity suppression by LFS in the presence of synaptic blockade possibly by decreasing Ca²⁺ currents.

Ion Channels in Genetic Epilepsy: From Genes and Mechanisms to Disease-Targeted Therapies

Epilepsy is a common and serious neurologic disease with a strong genetic component. Genetic studies have identified an increasing collection of disease-causing genes. The impact of these genetic discoveries is wide reaching-from precise diagnosis and classification of syndromes to the discovery and validation of new drug targets and the development of disease-targeted therapeutic strategies. About 25% of genes identified in epilepsy encode ion channels. Much of our understanding of disease mechanisms comes from work focused on this class of protein. In this study, we review the genetic, molecular, and physiologic evidence supporting the pathogenic role of a number of different voltage- and ligand-activated ion channels in genetic epilepsy. We also review proposed disease mechanisms for each ion channel and highlight targeted therapeutic strategies.

Frequency-dependent regulation of intrinsic excitability by voltage-activated membrane conductances, computational modeling and dynamic clamp

As one of the most unique properties of nerve cells, their intrinsic excitability allows them to transform synaptic inputs into action potentials. This process reflects a complex interplay between the synaptic inputs and the voltage-dependent membrane currents of the postsynaptic neuron. While neurons in natural conditions mostly fire under the action of intense synaptic bombardment and receive fluctuating patterns of excitation and inhibition, conventional techniques to characterize intrinsic excitability mainly utilize static means of stimulation. Recently, we have shown that voltage-gated membrane currents regulate the firing responses under current step stimulation and under physiologically more realistic inputs in a differential manner. At the same time, a multitude of neuron types have been shown to exhibit some form of subthreshold resonance that potentially allows them to respond to synaptic inputs in a frequency-selective manner. In this study, we performed virtual experiments in computational models of neurons to examine how specific voltage-gated currents regulate their excitability under simulated frequency-modulated synaptic inputs. The model simulations and subsequent dynamic clamp experiments on mouse hippocampal pyramidal neurons revealed that the impact of voltage-gated currents in regulating the firing output is strongly frequency-dependent and mostly affecting the synaptic integration at theta frequencies. Notably, robust frequency-dependent regulation of intrinsic excitability was observed even when conventional analysis of membrane impedance suggested no such tendency. Consequently, plastic or homeostatic regulation of intrinsic membrane properties can tune the frequency selectivity of neuron populations in a way that is not readily expected from subthreshold impedance measurements.

The pathophysiology of cardiac dysfunction in epilepsy

Alterations in cardiac electrophysiology are an established consequence of long-standing drug resistant epilepsy. Patients with chronic epilepsy display abnormalities in both sinoatrial node pacemaker current as well as ventricular repolarizing current that places them at a greater risk of developing life-threatening cardiac arrhythmias. The development of cardiac arrhythmias secondary to drug resistant epilepsy is believed to be a key mechanism underlying the phenomenon of Sudden Unexpected Death in EPilepsy (SUDEP). Though an increasing amount of studies examining both animal models and human patients have provided evidence that chronic epilepsy can detrimentally affect cardiac function, the underlying pathophysiology remains unclear. Recent work has shown the expression of several key cardiac ion channels to be altered in animal models of genetic and acquired epilepsies. This has led to the currently held paradigm that cardiac ion channel expression may be secondarily altered as a consequence of seizure activity-resulting in electrophysiological cardiac dysfunction. Furthermore, cortical autonomic dysfunction - resulting from seizure activity-has also been suggested to play a role, whereby seizure activity may indirectly influence cardiac function via altering centrally-mediated autonomic output to the heart. In this review, we discuss various cardiac dysrhythmias associated with seizure events-including tachycardia, bradycardia and QT prolongation, both ictally and inter-ictally, as well as the role of the autonomic nervous system. We further discuss key ion channels expressed in both the heart and the brain that have been shown to be altered in epilepsy and may be responsible for the development of cardiac dysrhythmias secondary to chronic epilepsy.

Temporal Coordination of Hippocampal Neurons Reflects Cognitive Outcome Post-febrile Status Epilepticus

The coordination of dynamic neural activity within and between neural networks is believed to underlie normal cognitive processes. Conversely, cognitive deficits that occur following neurological insults may result from network discoordination. We hypothesized that cognitive outcome following febrile status epilepticus (FSE) depends on network efficacy within and between fields CA1 and CA3 to dynamically organize cell activity by theta phase. Control and FSE rats were trained to forage or perform an active avoidance spatial task. FSE rats were sorted by those that were able to reach task criterion (FSE-L) and those that could not (FSE-NL). FSE-NL CA1 place cells did not exhibit phase preference in either context and exhibited poor cross-theta interaction between CA1 and CA3. FSE-L and control CA1 place cells exhibited phase preference at peak theta that shifted during active avoidance to the same static phase preference observed in CA3. Temporal coordination of neuronal activity by theta phase may therefore explain variability in cognitive outcome following neurological insults in early development.

Calcium regulation of HCN channels supports persistent activity in a multiscale model of neocortex

Neuronal persistent activity has been primarily assessed in terms of electrical mechanisms, without attention to the complex array of molecular events that also control cell excitability. We developed a multiscale neocortical model proceeding from the molecular to the network level to assess the contributions of calcium (Ca(2+)) regulation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in providing additional and complementary support of continuing activation in the network. The network contained 776 compartmental neurons arranged in the cortical layers, connected using synapses containing AMPA/NMDA/GABAA/GABAB receptors. Metabotropic glutamate receptors (mGluR) produced inositol triphosphate (IP3) which caused the release of Ca(2+) from endoplasmic reticulum (ER) stores, with reuptake by sarco/ER Ca(2+)-ATP-ase pumps (SERCA), and influence on HCN channels. Stimulus-induced depolarization led to Ca(2+) influx via NMDA and voltage-gated Ca(2+) channels (VGCCs). After a delay, mGluR activation led to ER Ca(2+) release via IP3 receptors. These factors increased HCN channel conductance and produced firing lasting for ∼1min. The model displayed inter-scale synergies among synaptic weights, excitation/inhibition balance, firing rates, membrane depolarization, Ca(2+) levels, regulation of HCN channels, and induction of persistent activity. The interaction between inhibition and Ca(2+) at the HCN channel nexus determined a limited range of inhibition strengths for which intracellular Ca(2+) could prepare population-specific persistent activity. Interactions between metabotropic and ionotropic inputs to the neuron demonstrated how multiple pathways could contribute in a complementary manner to persistent activity. Such redundancy and complementarity via multiple pathways is a critical feature of biological systems. Mediation of activation at different time scales, and through different pathways, would be expected to protect against disruption, in this case providing stability for persistent activity.

Homeostatic regulation of h-conductance controls intrinsic excitability and stabilizes the threshold for synaptic modification in CA1 neurons

KEY POINTS

We determined the contribution of the hyperpolarization-activated cationic (h) current (Ih ) to the homeostatic regulation of CA1 pyramidal cells in vitro using chronic treatments (48 h) that either increase (picrotoxin) or decrease (kynurenate) neuronal activity. The h-conductance was found to be up- or down-regulated following chronic activity enhancement or activity deprivation, respectively. This bidirectional plasticity of Ih was found to subsequently alter both apparent input resistance and intrinsic neuronal excitability. Bidirectional homeostatic plasticity of Ih also determined EPSP waveform and EPSP summation tested at 5-30 Hz. Long-term synaptic modification induced by repetitive stimulation of the Schaffer collaterals was found to be constant across treatments in the presence of Ih but not when Ih was blocked pharmacologically. Thus, bidirectional homeostatic regulation of Ih stabilizes induction of long-term synaptic modification in CA1 pyramidal neurons that depends on EPSP summation.

ABSTRACT

The hyperpolarization-activated cationic (h) current is a voltage-shock absorber, highly expressed in the dendrites of CA1 pyramidal neurons. Up-regulation of Ih has been reported following episodes of intense network activity but the effect of activity deprivation on Ih and the functional consequence of homeostatic regulation of Ih remain unclear. We determined here the contribution of Ih to the homeostatic regulation of CA1 pyramidal cell excitability. Intrinsic neuronal excitability was decreased in neurons treated for 2-3 days with the GABAA channel blocker picrotoxin (PiTx) but increased in neurons treated (2-3 days) with the glutamate receptor antagonist kynurenate (Kyn). Membrane capacitance remained unchanged after treatment but the apparent input resistance was reduced for PiTx-treated neurons and enhanced for Kyn-treated neurons. Maximal Ih conductance was up-regulated after chronic hyperactivity but down-regulated following chronic hypoactivity. Up-regulation of Ih in PiTx-treated cultures was found to accelerate EPSP kinetics and reduce temporal summation of EPSPs whereas opposite effects were observed in Kyn-treated cultures, indicating that homeostatic regulation of Ih may control the induction of synaptic modification depending on EPSP summation. In fact, stimulation of the Schaffer collaterals at 3-10 Hz induced differential levels of plasticity in PiTx-treated and Kyn-treated neurons when Ih was blocked pharmacologically but not in control conditions. These data indicate that homeostatic regulation of Ih normalizes the threshold for long-term synaptic modification that depends on EPSP summation. In conclusion, bidirectional homeostatic regulation of Ih not only controls spiking activity but also stabilizes the threshold for long-term potentiation induced in CA1 pyramidal neurons by repetitive stimulation.

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.

Epilepsies associated with hippocampal sclerosis

Hippocampal sclerosis (HS) is considered the most frequent neuropathological finding in patients with mesial temporal lobe epilepsy (MTLE). Hippocampal specimens of pharmacoresistant MTLE patients that underwent epilepsy surgery for seizure control reveal the characteristic pattern of segmental neuronal cell loss and concomitant astrogliosis. However, classification issues of hippocampal lesion patterns have been a matter of intense debate. International consensus classification has only recently provided significant progress for comparisons of neurosurgical and clinic-pathological series between different centers. The respective four-tiered classification system of the International League Against Epilepsy subdivides HS into three types and includes a term of "gliosis only, no-HS". Future studies will be necessary to investigate whether each of these subtypes of HS may be related to different etiological factors or with postoperative memory and seizure outcome. Molecular studies have provided potential deeper insights into the pathogenesis of HS and MTLE on the basis of epilepsy-surgical hippocampal specimens and corresponding animal models. These include channelopathies, activation of NMDA receptors, and other conditions related to Ca(2+) influx into neurons, the imbalance of Ca(2+)-binding proteins, acquired channelopathies that increase neuronal excitability, paraneoplastic and non-paraneoplastic inflammatory events, and epigenetic regulation promoting or facilitating hippocampal epileptogenesis. Genetic predisposition for HS is clearly suggested by the high incidence of family history in patients with HS, and by familial MTLE with HS. So far, it is clear that HS is multifactorial and there is no individual pathogenic factor either necessary or sufficient to generate this intriguing histopathological condition. The obvious variety of pathogenetic combinations underlying HS may explain the multitude of clinical presentations, different responses to clinical and surgical treatment. We believe that the stratification of neuropathological patterns can help to characterize specific clinic-pathological entities and predict the postsurgical seizure control in an improved fashion.

Cortical HCN channels: function, trafficking and plasticity

The hyperpolarization-activated cyclic nucleotide-gated (HCN) channels belong to the superfamily of voltage-gated potassium ion channels. They are, however, activated by hyperpolarizing potentials and are permeable to cations. Four HCN subunits have been cloned, of which HCN1 and HCN2 subunits are predominantly expressed in the cortex. These subunits are principally located in pyramidal cell dendrites, although they are also found at lower concentrations in the somata of pyramidal neurons as well as other neuron subtypes. HCN channels are actively trafficked to dendrites by binding to the chaperone protein TRIP8b. Somato-dendritic HCN channels in pyramidal neurons modulate spike firing and synaptic potential integration by influencing the membrane resistance and resting membrane potential. Intriguingly, HCN channels are present in certain cortical axons and synaptic terminals too. Here, they regulate synaptic transmission but the underlying mechanisms appear to vary considerably amongst different synaptic terminals. In conclusion, HCN channels are expressed in multiple neuronal subcellular compartments in the cortex, where they have a diverse and complex effect on neuronal excitability.

Novel HCN2 mutation contributes to febrile seizures by shifting the channel's kinetics in a temperature-dependent manner

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channel-mediated currents, known as I h, are involved in the control of rhythmic activity in neuronal circuits and in determining neuronal properties including the resting membrane potential. Recent studies have shown that HCN channels play a role in seizure susceptibility and in absence and limbic epilepsy including temporal lobe epilepsy following long febrile seizures (FS). This study focused on the potential contributions of abnormalities in the HCN2 isoform and their role in FS. A novel heterozygous missense mutation in HCN2 exon 1 leading to p.S126L was identified in two unrelated patients with FS. The mutation was inherited from the mother who had suffered from FS in a pedigree. To determine the effect of this substitution we conducted whole-cell patch clamp electrophysiology. We found that mutant channels had elevated sensitivity to temperature. More specifically, they displayed faster kinetics at higher temperature. Kinetic shift by change of temperature sensitivity rather than the shift of voltage dependence led to increased availability of I h in conditions promoting FS. Responses to cyclic AMP did not differ between wildtype and mutant channels. Thus, mutant HCN2 channels cause significant cAMP-independent enhanced availability of I h during high temperatures, which may contribute to hyperthermia-induced neuronal hyperexcitability in some individuals with FS.

Ih tunes theta/gamma oscillations and cross-frequency coupling in an in silico CA3 model

channels are uniquely positioned to act as neuromodulatory control points for tuning hippocampal theta (4–12 Hz) and gamma (25 Hz) oscillations, oscillations which are thought to have importance for organization of information flow. contributes to neuronal membrane resonance and resting membrane potential, and is modulated by second messengers. We investigated oscillatory control using a multiscale computer model of hippocampal CA3, where each cell class (pyramidal, basket, and oriens-lacunosum moleculare cells), contained type-appropriate isoforms of . Our model demonstrated that modulation of pyramidal and basket allows tuning theta and gamma oscillation frequency and amplitude. Pyramidal also controlled cross-frequency coupling (CFC) and allowed shifting gamma generation towards particular phases of the theta cycle, effected via 's ability to set pyramidal excitability. Our model predicts that in vivo neuromodulatory control of allows flexibly controlling CFC and the timing of gamma discharges at particular theta phases.

HCN and KV7 (M-) channels as targets for epilepsy treatment

Voltage-gated ion channels are important determinants of cellular excitability. The Hyperpolarization-activated Cyclic Nucleotide-gated (HCN) and KV7 (M-) channels are voltage-gated ion channels. Both channels are activated at sub-threshold potentials and have biophysical properties that mirror each other. KV7 channels inhibit neuronal excitability. Thus, mutations in KV7 channels that are associated with Benign Familial Neonatal Convulsions (BFNC) are likely to be epileptogenic. Mutations in HCN channels have also been associated with idiopathic epilepsies such as GEFS+. In addition, HCN channel expression and function are modulated during symptomatic epilepsies such as temporal lobe epilepsy. It is, though, unclear as to whether the changes in HCN channel expression and function associated with the various forms of epilepsy promote epileptogenesis or are adaptive. In this review, we discuss this as well as the potential for KV7 and HCN channels as drug targets for the treatment of epilepsy. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

Reliability of spike and burst firing in thalamocortical relay cells

The reliability and precision of the timing of spikes in a spike train is an important aspect of neuronal coding. We investigated reliability in thalamocortical relay (TCR) cells in the acute slice and also in a Morris-Lecar model with several extensions. A frozen Gaussian noise current, superimposed on a DC current, was injected into the TCR cell soma. The neuron responded with spike trains that showed trial-to-trial variability, due to amongst others slow changes in its internal state and the experimental setup. The DC current allowed to bring the neuron in different states, characterized by a well defined membrane voltage (between -80 and -50 mV) and by a specific firing regime that on depolarization gradually shifted from a predominantly bursting regime to a tonic spiking regime. The filtered frozen white noise generated a spike pattern output with a broad spike interval distribution. The coincidence factor and the Hunter and Milton measure were used as reliability measures of the output spike train. In the experimental TCR cell as well as the Morris-Lecar model cell the reliability depends on the shape (steepness) of the current input versus spike frequency output curve. The model also allowed to study the contribution of three relevant ionic membrane currents to reliability: a T-type calcium current, a cation selective h-current and a calcium dependent potassium current in order to allow bursting, investigate the consequences of a more complex current-frequency relation and produce realistic firing rates. The reliability of the output of the TCR cell increases with depolarization. In hyperpolarized states bursts are more reliable than single spikes. The analytically derived relations were capable to predict several of the experimentally recorded spike features.

Dendritic ion channelopathy in acquired epilepsy

Ion channel dysfunction or "channelopathy" is a proven cause of epilepsy in the relatively uncommon genetic epilepsies with Mendelian inheritance. But numerous examples of acquired channelopathy in experimental animal models of epilepsy following brain injury have also been demonstrated. Our understanding of channelopathy has grown due to advances in electrophysiology techniques that have allowed the study of ion channels in the dendrites of pyramidal neurons in cortex and hippocampus. The apical dendrites of pyramidal neurons comprise the vast majority of neuronal surface membrane area, and thus the majority of the neuronal ion channel population. Investigation of dendritic ion channels has demonstrated remarkable plasticity in ion channel localization and biophysical properties in epilepsy, many of which produce hyperexcitability and may contribute to the development and maintenance of the epileptic state. Herein we review recent advances in dendritic physiology and cell biology, and their relevance to epilepsy.

Ion channels in genetic and acquired forms of epilepsy

Genetic mutations causing dysfunction of both voltage- and ligand-gated ion channels make a major contribution to the cause of many different types of familial epilepsy. Key mechanisms comprise defective Na(+) channels of inhibitory neurons, or GABA(A) receptors affecting pre- or postsynaptic GABAergic inhibition, or a dysfunction of different types of channels at axon initial segments. Many of these ion channel mutations have been modelled in mice, which has largely contributed to the understanding of where and how the ion channel defects lead to neuronal hyperexcitability. Animal models of febrile seizures or mesial temporal epilepsy have shown that dendritic K(+) channels, hyperpolarization-activated cation channels and T-type Ca(2+) channels play important roles in the generation of seizures. For the latter, it has been shown that suppression of their function by pharmacological mechanisms or in knock-out mice can antagonize epileptogenesis. Defects of ion channel function are also associated with forms of acquired epilepsy. Autoantibodies directed against ion channels or associated proteins, such as K(+) channels, LGI1 or NMDA receptors, have been identified in epileptic disorders that can largely be included under the term limbic encephalitis which includes limbic seizures, status epilepticus and psychiatric symptoms. We conclude that ion channels and associated proteins are important players in different types of genetic and acquired epilepsies. Nevertheless, the molecular bases for most common forms of epilepsy are not yet clear, and evidence to be discussed indicates just how much more we need to understand about the complex mechanisms that underlie epileptogenesis.

HCN channelopathies: pathophysiology in genetic epilepsy and therapeutic implications

Hyperpolarization-activated cyclic nucleotide-gated channels (HCN) can act as pacemakers in the brain making them strong candidates for driving aberrant hypersynchronous network activity seen in epilepsy. Transcriptional changes in HCN channels occur in several animal models of epilepsy. However, only recently have genetic studies demonstrated sequence variation in HCN1 and HCN2 genes associated with human epilepsy. These include a triple proline deletion in HCN2 that increases channel function and occurs more often in patients with febrile seizure syndromes. Other HCNx gene variants have been described in idiopathic generalized epilepsy although the functional consequence of these remains unclear. In this review we explore potential cellular and network mechanisms involving HCN channels in the genetic epilepsies. We suggest how new genetic sequencing technology, medium-throughput functional assays and the ability to develop syndrome-specific animal models will provide a more comprehensive understanding of how I(h) contributes to pathogenic mechanisms underlying human genetic epilepsy. We also discuss what is known about the pharmacological manipulation of HCN channels in the context of epilepsy and how this may help future efforts in developing HCN-channel-based therapy.

Know your current I(h): interaction with a shunting current explains the puzzling effects of its pharmacological or pathological modulations

The non-specific, hyperpolarization activated, I(h) current is particularly involved in epilepsy and it exhibits an excitatory or inhibitory action on synaptic integration in an apparently inconsistent way. It has been suggested that most of the inconsistencies could be reconciled invoking an indirect interaction with the M-type K(+) current, another current involved in epilepsy. However, here we show that the original experiments, and the simplified model used to explain and support them, cannot explain in a conclusive way the puzzling I(h) actions observed in different experimental preparations. Using a realistic model, we show instead how and why a shunting current, such as that carried by TASK-like channels, and dependent on I(h) channel is able to explain virtually all experimental findings on I(h) up- or down-regulation by modulators or pathological conditions. The model results suggest several experimentally testable predictions to characterize in more details this elusive and peculiar interaction, which may be of fundamental importance in the development of new treatments for all those pathological and cognitive dysfunctions caused, mediated, or affected by I(h).

Towards an integrated view of HCN channel role in epilepsy

Epilepsy is the third most common brain disorder and affects millions of people. Epilepsy is characterized by the occurrence of spontaneous seizures, that is, bursts of synchronous firing of large populations of neurons. These are believed to result from abnormal regulation of neuronal excitability that favors hypersynchrony. Among the intrinsic conductances that govern neuronal excitability, the hyperpolarization-activated current (I(h)) plays complex and important roles in the fine-tuning of both cellular and network activity. Not surprisingly, dysregulation of I(h) and/or of its conducting ion-channels (HCN) has been strongly implicated in various experimental models of epilepsy, as well as in human epilepsy. Here we provide an overview of recent findings on the distinct physiological roles played by I(h) in specific contexts, and the cellular mechanisms that underlie these functions, including the subunit make-up of the channels. We further discuss current knowledge of dysregulation of I(h) and HCN channels in epilepsy in light of the multifaceted functions of I(h) in the brain.

Encoding of spatio-temporal input characteristics by a CA1 pyramidal neuron model

The in vivo activity of CA1 pyramidal neurons alternates between regular spiking and bursting, but how these changes affect information processing remains unclear. Using a detailed CA1 pyramidal neuron model, we investigate how timing and spatial arrangement variations in synaptic inputs to the distal and proximal dendritic layers influence the information content of model responses. We find that the temporal delay between activation of the two layers acts as a switch between excitability modes: short delays induce bursting while long delays decrease firing. For long delays, the average firing frequency of the model response discriminates spatially clustered from diffused inputs to the distal dendritic tree. For short delays, the onset latency and inter-spike-interval succession of model responses can accurately classify input signals as temporally close or distant and spatially clustered or diffused across different stimulation protocols. These findings suggest that a CA1 pyramidal neuron may be capable of encoding and transmitting presynaptic spatiotemporal information about the activity of the entorhinal cortex-hippocampal network to higher brain regions via the selective use of either a temporal or a rate code.

Pyramidal neurons in the hippocampus are crucially involved in learning and memory functions, but the ways in which they contribute to the processing of sensory inputs and their internal representation remain mostly unclear. The principal neurons of the CA1 region of the hippocampus are surrounded by at least 21 different types of interneurons. This feature, together with the fact that CA1 pyramidal dendrites associate two major glutamatergic inputs arriving from the entorhinal cortex, makes it laborious to track the ‘how’ and ‘what’ of synaptic integration. The present study tries to shed light on the ‘what’, that is, the information content of the CA1 discharge pattern. Using a detailed biophysical CA1 neuron model, we show that the output of the model neuron contains spatial and temporal features of the incoming synaptic input. This information lies in the temporal pattern of the inter-spike intervals produced during the bursting activity which is induced by the temporal coincidence of the two activated synaptic streams. Our findings suggest that CA1 pyramidal neurons may be capable of capturing features of the ongoing network activity via the use of a temporal code for information transfer.

HCN channels in behavior and neurological disease: too hyper or not active enough?

The roles of cells within the nervous system are based on their properties of excitability, which are in part governed by voltage-gated ion channels. HCN channels underlie the hyperpolarization-activated current, I(h), an important regulator of excitability and rhythmicity through control of basic membrane properties. I(h) is present in multiple neuronal types and regions of the central nervous system, and changes in I(h) alter cellular input-output properties and neuronal circuitry important for behavior such as learning and memory. Furthermore, the pathophysiology of neurological diseases of both the central and peripheral nervous system involves defects in excitability, rhythmicity, and signaling, and animal models of many of these disorders have implicated changes in HCN channels and I(h) as critical for pathogenesis. In this review, we focus on recent research elucidating the role of HCN channels and I(h) in behavior and disease. These studies have utilized knockout mice as well as animal models of disease to examine how I(h) may be important in regulating learning and memory, sleep, and consciousness, as well as how misregulation of I(h) may contribute to epilepsy, chronic pain, and other neurological disorders. This review will help guide future studies aimed at further understanding the function of this unique conductance in both health and disease of the mammalian brain.

Abnormal pyramidal cell morphology and HCN channel expression in cortical dysplasia

Cortical dysplasia is often associated with intractable seizures. Studies in animal models have described changes in inhibitory and excitatory synaptic function that contribute to hyperexcitability. The role of changes in intrinsic excitability and abnormal dendritic properties has received less attention. Changes in hyperpolarization-activated nonselective cation (HCN) channels have been implicated in several models of epilepsy. Herein we review evidence for alterations in HCN channels and dendritic morphology in the rat freeze-lesion model of cortical dysplasia. Immunocytochemical HCN1 staining, typically seen in the apical dendrites of layer V pyramidal cells in normal cortex, was greatly reduced in the region adjacent to the freeze-induced microgyrus. Although staining was preserved in layer I, fewer dendrites were stained in upper cortical layers. Deeper cortical layers were virtually devoid of immunoreactivity. Examination of biocytin-labeled pyramidal cells revealed markedly altered dendritic trees in the lesioned animals. In addition, resting membrane properties were altered and a subpopulation of neurons with abnormal dendritic arbors was present. These changes are likely to interact with the previously reported synaptic changes in this model of cortical dysplasia. HCN channel alterations are a potentially important cellular mechanism underlying hyperexcitability in cortical dysplasia.

Novel etiological and therapeutic strategies for neurodiseases: mechanisms and consequences of febrile seizures: lessons from animal models

Febrile seizures (FS) are the most common type of convulsive events in infancy and childhood. Genetic and environmental elements have been suggested to contribute to FS. FS can be divided into simple and complex types, the former being benign, whereas it is controversial whether complex FS have an association with the development of temporal lobe epilepsy (TLE) in later life. In the hippocampus of TLE patients, several structural and functional alterations take place that render the region an epileptic foci. Thus, it is important to clarify the cellular and molecular changes in the hippocampus after FS and to determine whether they are epileptogenic. To achieve this goal, human studies are too limited because the sample tissues are only available from adult patients in the advanced and drug-resistant stages of the disease, masking the underlying etiology. These facts have inspired researchers to take advantage of well-established animal models of FS to answer the following questions: 1) How does hyperthermia induce FS? 2) Do FS induce neuroanatomical changes? 3) Do FS induce neurophysiological changes? 4) Do FS affect the behavior in later life? Here we introduce and discuss accumulating reports to answer these questions.

Understanding effects on excitability of simulated I (h) modulation in simple neuronal models

The hyperpolarization-activated, inward, mixed cation current, I (h), appears in a wide variety of cells in the nervous system, contributes to diverse neuronal properties, and is up-regulated by a number of important neurotransmitters. Up-regulation of I (h) is usually associated with an excitability-enhancing depolarization of resting membrane potential and an excitability-depressing shunting effect caused by a decrease in input resistance. In order to gain a better understanding of the interaction of these effects and their influence on excitability with I (h) modulation, we systematically analyze changes in neuronal properties associated with excitability during I (h) modulation in simplified, yet, biophysical neuron models based on a hippocampal pyramidal neuron. We simulate I (h) modulation by varying both its maximal conductance and its half-activation voltage, mimicking the effects of cAMP-linked neurotransmitters, through ranges of physiologically realistic parameter regimes. Of particular interest is the contribution of the different effects of I (h) up-regulation when membrane potentials are held at common levels and neuronal excitability is probed. Our modeling results suggest that, although holding potentials at common levels may compensate for changes in resting membrane potentials, this protocol may exaggerate the excitability-depressing influences of changes in input resistances with I (h) up-regulation.

Loss of dendritic HCN1 subunits enhances cortical excitability and epileptogenesis

Hyperpolarization-activated cation nonselective 1 (HCN1) plasticity in entorhinal cortical (EC) and hippocampal pyramidal cell dendrites is a salient feature of temporal lobe epilepsy. However, the significance remains undetermined. We demonstrate that adult HCN1 null mice are more susceptible to kainic acid-induced seizures. After termination of these with an anticonvulsant, the mice also developed spontaneous behavioral seizures at a significantly more rapid rate than their wild-type littermates. This greater seizure susceptibility was accompanied by increased spontaneous activity in HCN1(-/-) EC layer III neurons. Dendritic Ih in these neurons was ablated, too. Consequentially, HCN1(-/-) dendrites were more excitable, despite having significantly more hyperpolarized resting membrane potentials (RMPs). In addition, the integration of EPSPs was enhanced considerably such that, at normal RMP, a 50 Hz train of EPSPs produced action potentials in HCN1(-/-) neurons. As a result of this enhanced pyramidal cell excitability, spontaneous EPSC frequency onto HCN1(-/-) neurons was considerably greater than that onto wild types, causing an imbalance between normal excitatory and inhibitory synaptic activity. These results suggest that dendritic HCN channels are likely to play a critical role in regulating cortical pyramidal cell excitability. Furthermore, these findings suggest that the reduction in dendritic HCN1 subunit expression during epileptogenesis is likely to facilitate the disorder.

Double Trouble? Potential for Hyperexcitability Following Both Channelopathic up- and Downregulation of I(h) in Epilepsy

Studies of pathological ion channel regulation as an underlying mechanism of epilepsy have revealed alterations in the h-current in several animal models. While earlier reports indicate that downregulation of the h-current is pro-excitatory on the single neuron level, we found an upregulation of I(h) in hyperexcitable CA1 pyramidal neuron dendrites following experimental febrile seizures. In addition, in several CA1 pyramidal neuron computational models of different complexity, h-current upregulation has been shown to lead to pro-excitable effects. This focused review examines the complex impact of altered h-current on neuronal resting membrane potential (RMP) and input resistance (R(in)), as well as reported interactions with other ionic conductances.

Hyperpolarization activated cyclic-nucleotide gated (HCN) channels in developing neuronal networks

Developing neuronal networks evolve continuously, requiring that neurons modulate both their intrinsic properties and their responses to incoming synaptic signals. Emerging evidence supports roles for the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in this neuronal plasticity. HCN channels seem particularly suited for fine-tuning neuronal properties and responses because of their remarkably large and variable repertoire of functions, enabling integration of a wide range of cellular signals. Here, we discuss the involvement of HCN channels in cortical and hippocampal network maturation, and consider potential roles of developmental HCN channel dysregulation in brain disorders such as epilepsy.