Axo-somatic and apical dendritic Kv7/M channels differentially regulate the intrinsic excitability of adult rat CA1 pyramidal cells

Kv7/M channel dysfunction produces hyperexcitability in hippocampal CA1 pyramidal cells of Fmr1 knockout mice

Fragile X syndrome (FXS), the most frequent monogenic form of intellectual disability, is caused by transcriptional silencing of the FMR1 gene that could render neuronal hyperexcitability. Here we show that pyramidal cells (PCs) in the dorsal CA1 region of the hippocampus elicited a larger action potential (AP) number in response to suprathreshold stimulation in juvenile Fmr1 knockout (KO) than wild-type (WT) mice. Because Kv7/M channels modulate CA1 PC excitability in rats, we investigated if their dysfunction produces neuronal hyperexcitability in Fmr1 KO mice. Immunohistochemical and western blot analyses showed no differences in the expression of Kv7.2 and Kv7.3 channel subunits between genotypes; however, the current mediated by Kv7/M channels was reduced in Fmr1 KO mice. In both genotypes, bath application of XE991 (10 μM), a blocker of Kv7/M channels: produced an increased AP number, produced an increased input resistance, produced a decreased AP voltage threshold and shaped AP medium afterhyperpolarization by increasing mean velocities. Retigabine (10 μM), an opener of Kv7/M channels, produced opposite effects to XE991. Both XE991 and retigabine abolished differences in all these parameters found in control conditions between genotypes. Furthermore, a low concentration of retigabine (2.5 μM) normalized CA1 PC excitability of Fmr1 KO mice. Finally, ex vivo seizure-like events evoked by 4-aminopyiridine (200 μM) in the dorsal CA1 region were more frequent in Fmr1 KO mice, and were abolished by retigabine (5-10 μM). We conclude that CA1 PCs of Fmr1 KO mice exhibit hyperexcitability, caused by Kv7/M channel dysfunction, and increased epileptiform activity, which were abolished by retigabine. KEY POINTS: Dorsal pyramidal cells of the hippocampal CA1 region of Fmr1 knockout mice exhibit hyperexcitability. Kv7/M channel activity, but not expression, is reduced in pyramidal cells of the hippocampal CA1 region of Fmr1 knockout mice. Kv7/M channel dysfunction causes hyperexcitability in pyramidal cells of the hippocampal CA1 region of Fmr1 knockout mice by increasing input resistance, decreasing AP voltage threshold and shaping medium afterhyperpolarization. A Kv7/M channel opener normalizes neuronal excitability in pyramidal cells of the hippocampal CA1 region of Fmr1 knockout mice. Ex vivo seizure-like events evoked in the dorsal CA1 region were more frequent in Fmr1 KO mice, and such an epileptiform activity was abolished by a Kv7/M channel opener depending on drug concentration. Kv7/M channels may represent a therapeutic target for treating symptoms associated with hippocampal alterations in fragile X syndrome.

Kv7/KCNQ potassium channels in cortical hyperexcitability and juvenile seizure-related death in Ank2-mutant mice

Autism spectrum disorders (ASD) represent neurodevelopmental disorders characterized by social deficits, repetitive behaviors, and various comorbidities, including epilepsy. ANK2, which encodes a neuronal scaffolding protein, is frequently mutated in ASD, but its in vivo functions and disease-related mechanisms are largely unknown. Here, we report that mice with Ank2 knockout restricted to cortical and hippocampal excitatory neurons (Ank2-cKO mice) show ASD-related behavioral abnormalities and juvenile seizure-related death. Ank2-cKO cortical neurons show abnormally increased excitability and firing rate. These changes accompanied decreases in the total level and function of the Kv7.2/KCNQ2 and Kv7.3/KCNQ3 potassium channels and the density of these channels in the enlengthened axon initial segment. Importantly, the Kv7 agonist, retigabine, rescued neuronal excitability, juvenile seizure-related death, and hyperactivity in Ank2-cKO mice. These results suggest that Ank2 regulates neuronal excitability by regulating the length of and Kv7 density in the AIS and that Kv7 channelopathy is involved in Ank2-related brain dysfunctions.

Neurons: The Interplay between Cytoskeleton, Ion Channels/Transporters and Mitochondria

Neurons are permanent cells whose key feature is information transmission via chemical and electrical signals. Therefore, a finely tuned homeostasis is necessary to maintain function and preserve neuronal lifelong survival. The cytoskeleton, and in particular microtubules, are far from being inert actors in the maintenance of this complex cellular equilibrium, and they participate in the mobilization of molecular cargos and organelles, thus influencing neuronal migration, neuritis growth and synaptic transmission. Notably, alterations of cytoskeletal dynamics have been linked to alterations of neuronal excitability. In this review, we discuss the characteristics of the neuronal cytoskeleton and provide insights into alterations of this component leading to human diseases, addressing how these might affect excitability/synaptic activity, as well as neuronal functioning. We also provide an overview of the microscopic approaches to visualize and assess the cytoskeleton, with a specific focus on mitochondrial trafficking.

The Deletion of GluK2 Alters Cholinergic Control of Neuronal Excitability

Kainate receptors (KARs) are key regulators of synaptic circuits by acting at pre- and postsynaptic sites through either ionotropic or metabotropic actions. KARs can be activated by kainate, a potent neurotoxin, which induces acute convulsions. Here, we report that the acute convulsive effect of kainate mostly depends on GluK2/GluK5 containing KARs. By contrast, the acute convulsive activity of pilocarpine and pentylenetetrazol is not alleviated in the absence of KARs. Unexpectedly, the genetic inactivation of GluK2 rather confers increased susceptibility to acute pilocarpine-induced seizures. The mechanism involves an enhanced excitability of GluK2-/- CA3 pyramidal cells compared with controls upon pilocarpine application. Finally, we uncover that the absence of GluK2 increases pilocarpine modulation of Kv7/M currents. Taken together, our findings reveal that GluK2-containing KARs can control the excitability of hippocampal circuits through interaction with the neuromodulatory cholinergic system.

Time-limited alterations in cortical activity of a knock-in mice model of KCNQ2-related developmental and epileptic encephalopathy

KEY POINTS

The electrophysiological impact of the pathogenic c.821C>T mutation of the KCNQ2 gene (p.T274M variant in Kv7.2 subunit) related to Developmental and Epileptic Encephalopathy has been analyzed both in vivo and ex-vivo in layers II/III and V of motor cortical slice from a knock-in mice model during development at neonatal, post-weaning and juvenile stages. M current density and conductance are decreased and excitability of layers II/III pyramidal cells is increased in slices from neonatal and post-weaning KI mice but not from juvenile KI mice. M current and excitability of layer V pyramidal cells are impacted in KI mice only at post-weaning stage. Spontaneous GABAergic network-driven events are recorded until post-weaning stage and their frequency are increased in layers II/III of the KI mice. KI mice displayed spontaneous seizures preferentially at post-weaning rather than at juvenile stages.

ABSTRACT

De novo missense variants in the KCNQ2 gene encoding the Kv7.2 subunit of the voltage-gated potassium Kv7/M channels are the main cause of Developmental and Epileptic Encephalopathy (DEE) with neonatal onset. While seizures usually resolve during development, cognitive/motor deficits persist. To better understand the cellular mechanisms underlying network dysfunction and their progression over time, we investigated in vivo, using local field potential recordings of freely moving animals, and ex-vivo in layers II/III and V of motor cortical slices, using patch-clamp recordings, the electrophysiological properties of pyramidal cells from a heterozygous knock-in (KI) mouse model carrying the Kv7.2 p.T274M pathogenic variant during neonatal, post-weaning and juvenile developmental stages. We found that KI mice displayed spontaneous seizures preferentially at post-weaning rather than at juvenile stages. At the cellular level, the variant led to a reduction in M ​​current density/conductance and to neuronal hyperexcitability. These alterations were observed during the neonatal period in pyramidal cells of layers II /III and during post-weaning stage in pyramidal cells of layer V. Moreover, there was an increase in the frequency of spontaneous network driven events mediated by GABA receptors suggesting that the excitability of interneurons was also increased. However, all these alterations were no more observed in layers II/III and V of juvenile mice. Thus, our data indicate that the action of the variant is developmentally regulated. This raises the possibility that the age related seizure remission observed in KCNQ2-related DEE patient results from a time limited alteration of Kv7 channels activity and neuronal excitability. Abstract figure legend Knock-in mice harboring the heterozygous pathogenic p.T274M variant in the Kv7.2 subunit (c.821C>T mutation of the KCNQ2 gene) related to Developmental and Epileptic Encephalopathy displayed epileptic seizures preferentially at post-weaning rather than at juvenile developmental stages. At cellular level, in motor cortical slices the variant led to a reduction in M current density, to a hyperexcitability of pyramidal cells and to an increase in the frequency of spontaneous network driven events mediated by GABA receptors. All these alterations are time limited and are observed in pyramidal cells of neonatal mice until post-weaning but not of juvenile mice in which the pyramidal cells have electrophysiological properties similar to those of wild-type mice. This article is protected by copyright. All rights reserved.

Effects of M currents on the persistent activity of pyramidal neurons in mouse primary auditory cortex

Neuronal persistent activity (PA) is a common phenomenon observed in many types of neurons. PA can be induced in neurons in the mouse auditory nucleus by activating cholinergic receptors with carbachol (CCh), a dual muscarinic and nicotinic receptor agonist. PA is presumed to be associated with learning-related auditory plasticity at the cellular level. However, the mechanism is not clearly understood. Many studies have reported that muscarinic cholinergic receptor agonists inhibit muscarinic-sensitive potassium channels (M channels). Potassium influx through M channels produces potassium currents, called M currents, which play an essential role in regulating neural excitability and synaptic plasticity. Further study is needed to determine whether M currents affect the PA of auditory central neurons and provide additional analysis of the variations in electrophysiological properties. We used in vitro whole-cell patch-clamp recordings in isolated mouse brain slices to investigate the effects of M currents on the PA in pyramidal neurons in layer V of the primary auditory cortex (AI-L5). We found that blocking M currents with XE991 depolarized the AI-L5 pyramidal neurons, which significantly increased the input resistance. The active threshold and threshold intensity were significantly reduced, indicating that the intrinsic excitability was enhanced. Our results also showed that blocking M currents with XE991 switched the neuronal firing patterns in the AI-L5 pyramidal neurons from regular-spiking to intrinsic-bursting. Blocking M currents facilitated PA by increasing the plateau potential and enhancing intrinsic excitability. Our results suggested that blocking M currents might facilitate the PA in AI-L5 pyramidal neurons, which underlies auditory plasticity.

PIP2-dependent coupling of voltage sensor and pore domains in Kv7.2 channel

Phosphatidylinositol-4,5-bisphosphate (PIP₂) is a signaling lipid which regulates voltage-gated K_v7/KCNQ potassium channels. Altered PIP₂ sensitivity of neuronal K_v7.2 channel is involved in KCNQ2 epileptic encephalopathy. However, the molecular action of PIP₂ on K_v7.2 gating remains largely elusive. Here, we use molecular dynamics simulations and electrophysiology to characterize PIP₂ binding sites in a human K_v7.2 channel. In the closed state, PIP₂ localizes to the periphery of the voltage-sensing domain (VSD). In the open state, PIP₂ binds to 4 distinct interfaces formed by the cytoplasmic ends of the VSD, the gate, intracellular helices A and B and their linkers. PIP₂ binding induces bilayer-interacting conformation of helices A and B and the correlated motion of the VSD and the pore domain, whereas charge-neutralizing mutations block this coupling and reduce PIP₂ sensitivity of K_v7.2 channels by disrupting PIP₂ binding. These findings reveal the allosteric role of PIP₂ in K_v7.2 channel activation.

Pant et al. describe the mechanism by which PIP2 might regulate homomeric Kv7.2 channels. They identify sites important in the binding of the PIP2 lipid to Kv7.2 channels and propose that the PIP2 binding to a specific site results in the coupling between the voltage sensor domain (VSD) and pore domain (PD), which stabilizes the open state of the channel.

Flexible Stoichiometry: Implications for KCNQ2- and KCNQ3-Associated Neurodevelopmental Disorders

KCNQ2 and KCNQ3 pathogenic channel variants have been associated with a spectrum of developmentally regulated diseases that vary in age of onset, severity, and whether it is transient (i.e., benign familial neonatal seizures) or long-lasting (i.e., developmental and epileptic encephalopathy). KCNQ2 and KCNQ3 channels have also emerged as a target for novel antiepileptic drugs as their activation could reduce epileptic activity. Consequently, a great effort has taken place over the last 2 decades to understand the mechanisms that control the assembly, gating, and modulation of KCNQ2 and KCNQ3 channels. The current view that KCNQ2 and KCNQ3 channels assemble as heteromeric channels (KCNQ2/3) forms the basis of our understanding of KCNQ2 and KCNQ3 channelopathies and drug design. Here, we review the evidence that supports the formation of KCNQ2/3 heteromers in neurons. We also highlight functional and transcriptomic studies that suggest channel composition might not be necessarily fixed in the nervous system, but rather is dynamic and flexible, allowing some neurons to express KCNQ2 and KCNQ3 homomers. We propose that to fully understand KCNQ2 and KCNQ3 channelopathies, we need to adopt a more flexible view of KCNQ2 and KCNQ3 channel stoichiometry, which might differ across development, brain regions, cell types, and disease states.

Neural Mechanisms Underlying Repetitive Behaviors in Rodent Models of Autism Spectrum Disorders

Autism spectrum disorder (ASD) is comprised of several conditions characterized by alterations in social interaction, communication, and repetitive behaviors. Genetic and environmental factors contribute to the heterogeneous development of ASD behaviors. Several rodent models display ASD-like phenotypes, including repetitive behaviors. In this review article, we discuss the potential neural mechanisms involved in repetitive behaviors in rodent models of ASD and related neuropsychiatric disorders. We review signaling pathways, neural circuits, and anatomical alterations in rodent models that display robust stereotypic behaviors. Understanding the mechanisms and circuit alterations underlying repetitive behaviors in rodent models of ASD will inform translational research and provide useful insight into therapeutic strategies for the treatment of repetitive behaviors in ASD and other neuropsychiatric disorders.

The Role of Kv7 Channels in Neural Plasticity and Behavior

Activity-dependent persistent changes in neuronal intrinsic excitability and synaptic strength are widely thought to underlie learning and memory. Voltage-gated KCNQ/K_v7 potassium channels have been of great interest as the potential targets for memory disorders due to the beneficial effects of their antagonists in cognition. Importantly, de novo dominant mutations in their neuronal subunits KCNQ2/K_v7.2 and KCNQ3/K_v7.3 are associated with epilepsy and neurodevelopmental disorder characterized by developmental delay and intellectual disability. The role of K_v7 channels in neuronal excitability and epilepsy has been extensively studied. However, their functional significance in neural plasticity, learning, and memory remains largely unknown. Here, we review recent studies that support the emerging roles of K_v7 channels in intrinsic and synaptic plasticity, and their contributions to cognition and behavior.

Input-Output Relationship of CA1 Pyramidal Neurons Reveals Intact Homeostatic Mechanisms in a Mouse Model of Fragile X Syndrome

Cellular hyperexcitability is a salient feature of fragile X syndrome animal models. The cellular basis of hyperexcitability and how it responds to changing activity states is not fully understood. Here, we show increased axon initial segment length in CA1 of the Fmr1^-/y mouse hippocampus, with increased cellular excitability. This change in length does not result from reduced AIS plasticity, as prolonged depolarization induces changes in AIS length independent of genotype. However, depolarization does reduce cellular excitability, the magnitude of which is greater in Fmr1^-/y neurons. Finally, we observe reduced functional inputs from the entorhinal cortex, with no genotypic difference in the firing rates of CA1 pyramidal neurons. This suggests that AIS-dependent hyperexcitability in Fmr1^-/y mice may result from adaptive or homeostatic regulation induced by reduced functional synaptic connectivity. Thus, while AIS length and intrinsic excitability contribute to cellular hyperexcitability, they may reflect a homeostatic mechanism for reduced synaptic input onto CA1 neurons.

M-Current Inhibition in Hippocampal Excitatory Neurons Triggers Intrinsic and Synaptic Homeostatic Responses at Different Temporal Scales

Persistent alterations in neuronal activity elicit homeostatic plastic changes in synaptic transmission and/or intrinsic excitability. However, it is unknown whether these homeostatic processes operate in concert or at different temporal scales to maintain network activity around a set-point value. Here we show that chronic neuronal hyperactivity, induced by M-channel inhibition, triggered intrinsic and synaptic homeostatic plasticity at different timescales in cultured hippocampal pyramidal neurons from mice of either sex. Homeostatic changes of intrinsic excitability occurred at a fast timescale (1-4 h) and depended on ongoing spiking activity. This fast intrinsic adaptation included plastic changes in the threshold current and a distal relocation of FGF14, a protein physically bridging Na_v1.6 and K_v7.2 channels along the axon initial segment. In contrast, synaptic adaptations occurred at a slower timescale (∼2 d) and involved decreases in miniature EPSC amplitude. To examine how these temporally distinct homeostatic responses influenced hippocampal network activity, we quantified the rate of spontaneous spiking measured by multielectrode arrays at extended timescales. M-Channel blockade triggered slow homeostatic renormalization of the mean firing rate (MFR), concomitantly accompanied by a slow synaptic adaptation. Thus, the fast intrinsic adaptation of excitatory neurons is not sufficient to account for the homeostatic normalization of the MFR. In striking contrast, homeostatic adaptations of intrinsic excitability and spontaneous MFR failed in hippocampal GABAergic inhibitory neurons, which remained hyperexcitable following chronic M-channel blockage. Our results indicate that a single perturbation such as M-channel inhibition triggers multiple homeostatic mechanisms that operate at different timescales to maintain network mean firing rate.SIGNIFICANCE STATEMENT Persistent alterations in synaptic input elicit homeostatic plastic changes in neuronal activity. Here we show that chronic neuronal hyperexcitability, induced by M-type potassium channel inhibition, triggered intrinsic and synaptic homeostatic plasticity at different timescales in hippocampal excitatory neurons. The data indicate that the fast adaptation of intrinsic excitability depends on ongoing spiking activity but is not sufficient to provide homeostasis of the mean firing rate. Our results show that a single perturbation such as M-channel inhibition can trigger multiple homeostatic processes that operate at different timescales to maintain network mean firing rate.

Protein Kinase A-Mediated Suppression of the Slow Afterhyperpolarizing KCa3.1 Current in Temporal Lobe Epilepsy

Brain insults, such as trauma, stroke, anoxia, and status epilepticus (SE), cause multiple changes in synaptic function and intrinsic properties of surviving neurons that may lead to the development of epilepsy. Experimentally, a single SE episode, induced by the convulsant pilocarpine, initiates the development of an epileptic condition resembling human temporal lobe epilepsy (TLE). Principal hippocampal neurons from such epileptic animals display enhanced spike output in response to excitatory stimuli compared with neurons from nonepileptic animals. This enhanced firing is negatively related to the size of the slow afterhyperpolarization (sAHP), which is reduced in the epileptic neurons. The sAHP is an intrinsic neuronal negative feedback mechanism consisting normally of two partially overlapping components produced by disparate mechanisms. One component is generated by activation of Ca²⁺-gated K⁺ (K_Ca) channels, likely KCa3.1, consequent to spike Ca²⁺ influx (the K_Ca-sAHP component). The second component is generated by enhancement of the electrogenic Na⁺/K⁺ ATPase (NKA) by spike Na⁺ influx (NKA-sAHP component). Here we show that the K_Ca-sAHP component is markedly reduced in male rat epileptic neurons, whereas the NKA-sAHP component is not altered. The K_Ca-sAHP reduction is due to the downregulation of KCa3.1 channels, mediated by cAMP-dependent protein kinase A (PKA). This sustained effect can be acutely reversed by applying PKA inhibitors, leading also to normalization of the spike output of epileptic neurons. We propose that the novel "acquired channelopathy" described here, namely, PKA-mediated downregulation of KCa3.1 activity, provides an innovative target for developing new treatments for TLE, hopefully overcoming the pharmacoresistance to traditional drugs.SIGNIFICANCE STATEMENT Epilepsy, a common neurological disorder, often develops following a brain insult. Identifying key molecular and cellular mechanisms underlying acquired epilepsy is critical for developing effective antiepileptic therapies. In an experimental model of acquired epilepsy, we show that principal hippocampal neurons become intrinsically hyperexcitable. This alteration is due predominantly to the downregulation of a ubiquitous class of potassium ion channels, KCa3.1, whose main function is to dampen neuronal excitability. KCa3.1 downregulation is mediated by the cAMP-dependent protein kinase A (PKA) signaling pathway. Most importantly, it can be acutely reversed by PKA inhibitors, leading to recovery of KCa3.1 function and normalization of neuronal excitability. The discovery of this novel epileptogenic mechanism hopefully will facilitate the development of more efficient pharmacotherapy for acquired epilepsy.

Heterozygous loss of epilepsy gene KCNQ2 alters social, repetitive and exploratory behaviors

KCNQ/K_v7 channels conduct voltage‐dependent outward potassium currents that potently decrease neuronal excitability. Heterozygous inherited mutations in their principle subunits K_v7.2/KCNQ2 and K_v7.3/KCNQ3 cause benign familial neonatal epilepsy whereas patients with de novo heterozygous K_v7.2 mutations are associated with early‐onset epileptic encephalopathy and neurodevelopmental disorders characterized by intellectual disability, developmental delay and autism. However, the role of K_v7.2‐containing K_v7 channels in behaviors especially autism‐associated behaviors has not been described. Because pathogenic K_v7.2 mutations in patients are typically heterozygous loss‐of‐function mutations, we investigated the contributions of K_v7.2 to exploratory, social, repetitive and compulsive‐like behaviors by behavioral phenotyping of both male and female KCNQ2^+/− mice that were heterozygous null for the KCNQ2 gene. Compared with their wild‐type littermates, male and female KCNQ2^+/− mice displayed increased locomotor activity in their home cage during the light phase but not the dark phase and showed no difference in motor coordination, suggesting hyperactivity during the inactive light phase. In the dark phase, KCNQ2^+/− group showed enhanced exploratory behaviors, and repetitive grooming but decreased sociability with sex differences in the degree of these behaviors. While male KCNQ2^+/− mice displayed enhanced compulsive‐like behavior and social dominance, female KCNQ2^+/− mice did not. In addition to elevated seizure susceptibility, our findings together indicate that heterozygous loss of K_v7.2 induces behavioral abnormalities including autism‐associated behaviors such as reduced sociability and enhanced repetitive behaviors. Therefore, our study is the first to provide a tangible link between loss‐of‐function K_v7.2 mutations and the behavioral comorbidities of K_v7.2‐associated epilepsy.

The subthreshold-active KV7 current regulates neurotransmission by limiting spike-induced Ca2+ influx in hippocampal mossy fiber synaptic terminals

Little is known about the properties and function of ion channels that affect synaptic terminal-resting properties. One particular subthreshold-active ion channel, the Kv7 potassium channel, is highly localized to axons, but its role in regulating synaptic terminal intrinsic excitability and release is largely unexplored. Using electrophysiological recordings together with computational modeling, we found that the KV7 current was active at rest in adult hippocampal mossy fiber synaptic terminals and enhanced their membrane conductance. The current also restrained action potential-induced Ca2+ influx via N- and P/Q-type Ca2+ channels in boutons. This was associated with a substantial reduction in the spike half-width and afterdepolarization following presynaptic spikes. Further, by constraining spike-induced Ca2+ influx, the presynaptic KV7 current decreased neurotransmission onto CA3 pyramidal neurons and short-term synaptic plasticity at the mossy fiber-CA3 synapse. This is a distinctive mechanism by which KV7 channels influence hippocampal neuronal excitability and synaptic plasticity.

Switching between persistent firing and depolarization block in individual rat CA1 pyramidal neurons

The hippocampal formation plays a role in mnemonic tasks and epileptic discharges in vivo. In vitro, these functions and malfunctions may relate to persistent firing (PF) and depolarization block (DB), respectively. Pyramidal neurons of the CA1 field have previously been reported to engage in either PF or DB during cholinergic stimulation. However, it is unknown whether these cells constitute disparate populations of neurons. Furthermore, it is unclear which cell-specific peculiarities may mediate their diverse response properties. However, it has not been shown whether individual CA1 pyramidal neurons can switch between PF and DB states. Here, we used whole cell patch clamp in the current clamp mode on in vitro CA1 pyramidal neurons from acutely sliced rat tissue to test various intrinsic properties which may provoke individual cells to switch between PF and DB. We found that individual cells could switch from PF to DB, in a cholinergic agonist concentration dependent manner and depending on the parameters of stimulation. We also demonstrate involvement of TRPC and potassium channels in this switching. Finally, we report that the probability for DB was more pronounced in the proximal than in the distal half of CA1. These findings offer a potential mechanism for the stronger spatial modulation in proximal, compared to distal CA1, as place field formation was shown to be affected by DB. Taken together, our results suggest that PF and DB are not mutually exclusive response properties of individual neurons. Rather, a cell's response mode depends on a variety of intrinsic properties, and modulation of these properties enables switching between PF and DB.

Genetics of neonatal-onset epilepsies

Although the majority of seizures in neonates are related to acute brain injury, a substantial minority are the first symptom of a neonatal-onset epilepsy often linked to a pathogenic genetic variant. Historically, studies on neonatal seizures including treatment response and long-term consequences have lumped all etiologies together. However, etiology has been consistently shown to be the most important determinant of outcome. In the past few years, an increasing number of monogenic disorders have been described and might explain up to a third of neonatal-onset epilepsy syndromes previously included under the umbrella of Ohtahara syndrome and early myoclonic encephalopathy. In this chapter, we define the concept of genetic epilepsy and review the classification. Then, we review the most relevant monogenic neonatal-onset epilepsies, detail their underlying pathophysiologic mechanisms, and present their electroclinical phenotypes. We highlight that, in some cases, such as neonates with KCNQ2 or KCNT1 gene mutations, the early recognition of the electroclinical phenotype can lead to targeted diagnostic testing and precision medicine treatment, enabling the possibility of improved outcome.

Gq-Coupled Muscarinic Receptor Enhancement of KCNQ2/3 Channels and Activation of TRPC Channels in Multimodal Control of Excitability in Dentate Gyrus Granule Cells

KCNQ (Kv7, "M-type") K⁺ channels and TRPC (transient receptor potential, "canonical") cation channels are coupled to neuronal discharge properties and are regulated via G_q/11-protein-mediated signals. Stimulation of G_q/11-coupled receptors both consumes phosphatidylinositol 4,5-bisphosphate (PIP₂) via phosphalipase Cβ hydrolysis and stimulates PIP₂ synthesis via rises in Ca²⁺ _i and other signals. Using brain-slice electrophysiology and Ca²⁺ imaging from male and female mice, we characterized threshold K⁺ currents in dentate gyrus granule cells (DGGCs) and CA1 pyramidal cells, the effects of G_q/11-coupled muscarinic M₁ acetylcholine (M₁R) stimulation on M current and on neuronal discharge properties, and elucidated the intracellular signaling mechanisms involved. We observed disparate signaling cascades between DGGCs and CA1 neurons. DGGCs displayed M₁R enhancement of M-current, rather than suppression, due to stimulation of PIP₂ synthesis, which was paralleled by increased PIP₂-gated G-protein coupled inwardly rectifying K⁺ currents as well. Deficiency of KCNQ2-containing M-channels ablated the M₁R-induced enhancement of M-current in DGGCs. Simultaneously, M₁R stimulation in DGGCs induced robust increases in [Ca²⁺]_i, mostly due to TRPC currents, consistent with, and contributing to, neuronal depolarization and hyperexcitability. CA1 neurons did not display such multimodal signaling, but rather M current was suppressed by M₁R stimulation in these cells, similar to the previously described actions of M₁R stimulation on M-current in peripheral ganglia that mostly involves PIP₂ depletion. Therefore, these results point to a pleiotropic network of cholinergic signals that direct cell-type-specific, precise control of hippocampal function with strong implications for hyperexcitability and epilepsy.SIGNIFICANCE STATEMENT At the neuronal membrane, protein signaling cascades consisting of ion channels and metabotropic receptors govern the electrical properties and neurotransmission of neuronal networks. Muscarinic acetylcholine receptors are G-protein-coupled metabotropic receptors that control the excitability of neurons through regulating ion channels, intracellular Ca²⁺ signals, and other second-messenger cascades. We have illuminated previously unknown actions of muscarinic stimulation on the excitability of hippocampal principal neurons that include M channels, TRPC (transient receptor potential, "canonical") cation channels, and powerful regulation of lipid metabolism. Our results show that these signaling pathways, and mechanisms of excitability, are starkly distinct between peripheral ganglia and brain, and even between different principal neurons in the hippocampus.

Reduced axonal surface expression and phosphoinositide sensitivity in Kv7 channels disrupts their function to inhibit neuronal excitability in Kcnq2 epileptic encephalopathy

Neuronal K_v7/KCNQ channels are voltage-gated potassium channels composed of K_v7.2/KCNQ2 and K_v7.3/KCNQ3 subunits. Enriched at the axonal membrane, they potently suppress neuronal excitability. De novo and inherited dominant mutations in K_v7.2 cause early onset epileptic encephalopathy characterized by drug resistant seizures and profound psychomotor delay. However, their precise pathogenic mechanisms remain elusive. Here, we investigated selected epileptic encephalopathy causing mutations in calmodulin (CaM)-binding helices A and B of K_v7.2. We discovered that R333W, K526N, and R532W mutations located peripheral to CaM contact sites decreased axonal surface expression of heteromeric channels although only R333W mutation reduced CaM binding to K_v7.2. These mutations also altered gating modulation by phosphatidylinositol 4,5-bisphosphate (PIP₂), revealing novel PIP₂ binding residues. While these mutations disrupted K_v7 function to suppress excitability, hyperexcitability was observed in neurons expressing K_v7.2-R532W that displayed severe impairment in voltage-dependent activation. The M518 V mutation at the CaM contact site in helix B caused most defects in K_v7 channels by severely reducing their CaM binding, K⁺ currents, and axonal surface expression. Interestingly, the M518 V mutation induced ubiquitination and accelerated proteasome-dependent degradation of K_v7.2, whereas the presence of K_v7.3 blocked this degradation. Furthermore, expression of K_v7.2-M518V increased neuronal death. Together, our results demonstrate that epileptic encephalopathy mutations in helices A and B of K_v7.2 cause abnormal K_v7 expression and function by disrupting K_v7.2 binding to CaM and/or modulation by PIP₂. We propose that such multiple K_v7 channel defects could exert more severe impacts on neuronal excitability and health, and thus serve as pathogenic mechanisms underlying Kcnq2 epileptic encephalopathy.

Inhibition of striatal cholinergic interneuron activity by the Kv7 opener retigabine and the nonsteroidal anti-inflammatory drug diclofenac

Striatal cholinergic interneurons provide modulation to striatal circuits involved in voluntary motor control and goal-directed behaviors through their autonomous tonic discharge and their firing "pause" responses to novel and rewarding environmental events. Striatal cholinergic interneuron hyperactivity was linked to the motor deficits associated with Parkinson's disease and the adverse effects of chronic antiparkinsonian therapy like l-DOPA-induced dyskinesia. Here we addressed whether Kv7 channels, which provide negative feedback to excitation in other neuron types, are involved in the control of striatal cholinergic interneuron tonic activity and response to excitatory inputs. We found that autonomous firing of striatal cholinergic interneurons is not regulated by Kv7 channels. In contrast, Kv7 channels limit the summation of excitatory postsynaptic potentials in cholinergic interneurons through a postsynaptic mechanism. Striatal cholinergic interneurons have a high reserve of Kv7 channels, as their opening using pharmacological tools completely silenced the tonic firing and markedly reduced their intrinsic excitability. A strong inhibition of striatal cholinergic interneurons was also observed in response to the anti-inflammatory drugs diclofenac and meclofenamic acid, however, this effect was independent of Kv7 channels. These data bring attention to new potential molecular targets and pharmacological tools to control striatal cholinergic interneuron activity in pathological conditions where they are believed to be hyperactive, including Parkinson's disease.

Differential Control of Axonal and Somatic Resting Potential by Voltage-Dependent Conductances in Cortical Layer 5 Pyramidal Neurons

Voltage-dependent conductances not only drive action potentials but also help regulate neuronal resting potential. We found differential regulation of resting potential in the proximal axon of layer 5 pyramidal neurons compared to the soma. Axonal resting potential was more negative than the soma, reflecting differential control by multiple voltage-dependent channels, including sodium channels, Cav3 channels, Kv7 channels, and HCN channels. Kv7 current is highly localized to the axon and HCN current to the soma and dendrite. Because of impedance asymmetry between the soma and axon, axonal Kv7 current has little effect on somatic resting potential, while somatodendritic HCN current strongly influences the proximal axon. In fact, depolarizing somatodendritic HCN current is critical for resting activation of all the other voltage-dependent conductances, including Kv7 in the axon. These experiments reveal complex interactions among voltage-dependent conductances to control region-specific resting potential, with somatodendritic HCN channels playing a critical enabling role.

A computational model for how the fast afterhyperpolarization paradoxically increases gain in regularly firing neurons

The gain of a neuron, the number and frequency of action potentials triggered in response to a given amount of depolarizing injection, is an important behavior underlying a neuron's function. Variations in action potential waveform can influence neuronal discharges by the differential activation of voltage- and ion-gated channels long after the end of a spike. One component of the action potential waveform, the afterhyperpolarization (AHP), is generally considered an inhibitory mechanism for limiting firing rates. In dentate gyrus granule cells (DGCs) expressing fast-gated BK channels, large fast AHPs (fAHP) are paradoxically associated with increased gain. In this article, we describe a mechanism for this behavior using a computational model. Hyperpolarization provided by the fAHP enhances activation of a dendritic inward current (a T-type Ca²⁺ channel is suggested) that, in turn, boosts rebound depolarization at the soma. The model suggests that the fAHP may both reduce Ca²⁺ channel inactivation and, counterintuitively, enhance its activation. The magnitude of the rebound depolarization, in turn, determines the activation of a subsequent, slower inward current (a persistent Na⁺ current is suggested) limiting the interspike interval. Simulations also show that the effect of AHP on gain is also effective for physiologically relevant stimulation; varying AHP amplitude affects interspike interval across a range of "noisy" stimulus frequency and amplitudes. The mechanism proposed suggests that small fAHPs in DGCs may contribute to their limited excitability. NEW & NOTEWORTHY The afterhyperpolarization (AHP) is canonically viewed as a major factor underlying the refractory period, serving to limit neuronal firing rate. We recently reported that enhancing the amplitude of the fast AHP (fAHP) in a relatively slowly firing neuron (vs. fast spiking neurons) expressing fast-gated BK channels augments neuronal excitability. In this computational study, we present a novel, quantitative hypothesis for how varying the amplitude of the fAHP can, paradoxically, influence a subsequent spike tens of milliseconds later.

CA1 pyramidal cells have diverse biophysical properties, affected by development, experience, and aging

Neuron types (e.g., pyramidal cells) within one area of the brain are often considered homogeneous, despite variability in their biophysical properties. Here we review literature demonstrating variability in the electrical activity of CA1 hippocampal pyramidal cells (PCs), including responses to somatic current injection, synaptic stimulation, and spontaneous network-related activity. In addition, we describe how responses of CA1 PCs vary with development, experience, and aging, and some of the underlying ionic currents responsible. Finally, we suggest directions that may be the most impactful in expanding this knowledge, including the use of text and data mining to systematically study cellular heterogeneity in more depth; dynamical systems theory to understand and potentially classify neuron firing patterns; and mathematical modeling to study the interaction between cellular properties and network output. Our goals are to provide a synthesis of the literature for experimentalists studying CA1 PCs, to give theorists an idea of the rich diversity of behaviors models may need to reproduce to accurately represent these cells, and to provide suggestions for future research.

The Role of Kv7/M Potassium Channels in Controlling Ectopic Firing in Nociceptors

Peripheral nociceptive neurons encode and convey injury-inducing stimuli toward the central nervous system. In normal conditions, tight control of nociceptive resting potential prevents their spontaneous activation. However, in many pathological conditions the control of membrane potential is disrupted, leading to ectopic, stimulus-unrelated firing of nociceptive neurons, which is correlated to spontaneous pain. We have investigated the role of K_V7/M channels in stabilizing membrane potential and impeding spontaneous firing of nociceptive neurons. These channels generate low voltage-activating, noninactivating M-type K⁺ currents (M-current, I_M ), which control neuronal excitability. Using perforated-patch recordings from cultured, rat nociceptor-like dorsal root ganglion neurons, we show that inhibition of M-current leads to depolarization of nociceptive neurons and generation of repetitive firing. To assess to what extent the M-current, acting at the nociceptive terminals, is able to stabilize terminals' membrane potential, thus preventing their ectopic activation, in normal and pathological conditions, we built a multi-compartment computational model of a pseudo-unipolar unmyelinated nociceptive neuron with a realistic terminal tree. The modeled terminal tree was based on the in vivo structure of nociceptive peripheral terminal, which we assessed by in vivo multiphoton imaging of GFP-expressing nociceptive neuronal terminals innervating mice hind paw. By modifying the conductance of the K_V7/M channels at the modeled terminal tree (terminal g_KV7/M) we have found that 40% of the terminal g_KV7/M conductance is sufficient to prevent spontaneous firing, while ~75% of terminal g_KV7/M is sufficient to inhibit stimulus induced activation of nociceptive neurons. Moreover, we showed that terminal M-current reduces susceptibility of nociceptive neurons to a small fluctuations of membrane potentials. Furthermore, we simulated how the interaction between terminal persistent sodium current and M-current affects the excitability of the neurons. We demonstrated that terminal M-current in nociceptive neurons impeded spontaneous firing even when terminal Na_(V)1.9 channels conductance was substantially increased. On the other hand, when terminal g_KV7/M was decreased, nociceptive neurons fire spontaneously after slight increase in terminal Na_(V)1.9 conductance. Our results emphasize the pivotal role of M-current in stabilizing membrane potential and hereby in controlling nociceptive spontaneous firing, in normal and pathological conditions.

Kv7 channels in the nucleus accumbens are altered by chronic drinking and are targets for reducing alcohol consumption

Alcohol use disorders (AUDs) are a major public health issue and produce enormous societal and economic burdens. Current Food and Drug Administration (FDA)-approved pharmacotherapies for treating AUDs suffer from deleterious side effects and are only effective in a subset of individuals. It is therefore essential to find improved medications for the management of AUDs. Emerging evidence suggests that anticonvulsants are a promising class of drugs for treating individuals with AUDs. In these studies, we used integrative functional genomics to demonstrate that genes that encode Kv7 channels (i.e. Kcnq2/3) are related to alcohol (ethanol) consumption, preference and acceptance in rodents. We then tested the ability of the FDA-approved anticonvulsant retigabine, a Kv7 channel opener, to reduce voluntary ethanol consumption of Wistar rats in a two-bottle choice intermittent alcohol access paradigm. Systemic administration and microinjections of retigabine into the nucleus accumbens significantly reduced alcohol drinking, and retigabine was more effective at reducing intake in high- versus low-drinking populations of Wistar rats. Prolonged voluntary drinking increased the sensitivity to the proconvulsant effects of pharmacological blockade of Kv7 channels and altered surface trafficking and SUMOylation patterns of Kv7.2 channels in the nucleus accumbens. These data implicate Kcnq2/3 in the regulation of ethanol drinking and demonstrate that long-term drinking produces neuroadaptations in Kv7 channels. In addition, these results have identified retigabine as a potential pharmacotherapy for treating AUDs and Kv7 channels as a novel therapeutic target for reducing heavy drinking.

KV 7/M channels as targets for lipopolysaccharide-induced inflammatory neuronal hyperexcitability

KEY POINTS

Neuroinflammation associated with CNS insults leads to neuronal hyperexcitability, which may culminate in epileptiform discharges. Application of the endotoxin lipopolysaccharide (LPS) to brain tissue initiates a neuroinflammatory cascade, providing an experimental model to study the mechanisms of neuroinflammatory neuronal hyperexcitability. Here we show that LPS application to hippocampal slices markedly enhances the excitability of CA1 pyramidal cells by inhibiting a specific potassium current, the M-current, generated by K_V 7/M channels, which controls the excitability of almost every neuron in the CNS. The LPS-induced M-current inhibition is triggered by sequential activation of microglia, astrocytes and pyramidal cells, mediated by metabotropic purinergic and glutamatergic transmission, leading to blockade of K_V 7/M channels by calcium released from intracellular stores. The identification of the downstream molecular target of neuroinflammation, namely the K_V 7/M channel, potentially has far reaching implications for the understanding and treatment of many acute and chronic brain disorders.

ABSTRACT

Acute brain insults and many chronic brain diseases manifest an innate inflammatory response. The hallmark of this response is glia activation, which promotes repair of damaged tissue, but also induces structural and functional changes that may lead to an increase in neuronal excitability. We have investigated the mechanisms involved in the modulation of neuronal activity by acute inflammation. Initiating inflammatory responses in hippocampal tissue rapidly led to neuronal depolarization and repetitive firing even in the absence of active synaptic transmission. This action was mediated by a complex metabotropic purinergic and glutamatergic glia-to-neuron signalling cascade, leading to the blockade of neuronal K_V 7/M channels by Ca²⁺ released from internal stores. These channels generate the low voltage-activating, non-inactivating M-type K⁺ current (M-current) that controls intrinsic neuronal excitability, and its inhibition was the predominant cause of the inflammation-induced hyperexcitability. Our discovery that the ubiquitous K_V 7/M channels are the downstream target of the inflammation-induced cascade, has far reaching implications for the understanding and treatment of many acute and chronic brain disorders.

Modulation of Kv7 channels and excitability in the brain

Neuronal Kv7 channels underlie a voltage-gated non-inactivating potassium current known as the M-current. Due to its particular characteristics, Kv7 channels show pronounced control over the excitability of neurons. We will discuss various factors that have been shown to drastically alter the activity of this channel such as protein and phospholipid interactions, phosphorylation, calcium, and numerous neurotransmitters. Kv7 channels locate to key areas for the control of action potential initiation and propagation. Moreover, we will explore the dynamic surface expression of the channel modulated by neurotransmitters and neural activity. We will also focus on known principle functions of neural Kv7 channels: control of resting membrane potential and spiking threshold, setting the firing frequency, afterhyperpolarization after burst firing, theta resonance, and transient hyperexcitability from neurotransmitter-induced suppression of the M-current. Finally, we will discuss the contribution of altered Kv7 activity to pathologies such as epilepsy and cognitive deficits.

Mechanisms of Action of Antiseizure Drugs and the Ketogenic Diet

Antiseizure drugs (ASDs), also termed antiepileptic drugs, are the main form of symptomatic treatment for people with epilepsy, but not all patients become free of seizures. The ketogenic diet is one treatment option for drug-resistant patients. Both types of therapy exert their clinical effects through interactions with one or more of a diverse set of molecular targets in the brain. ASDs act by modulation of voltage-gated ion channels, including sodium, calcium, and potassium channels; by enhancement of γ-aminobutyric acid (GABA)-mediated inhibition through effects on GABAA receptors, the GABA transporter 1 (GAT1) GABA uptake transporter, or GABA transaminase; through interactions with elements of the synaptic release machinery, including synaptic vesicle 2A (SV2A) and α2δ; or by blockade of ionotropic glutamate receptors, including α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors. The ketogenic diet leads to increases in circulating ketones, which may contribute to the efficacy in treating pharmacoresistant seizures. Production in the brain of inhibitory mediators, such as adenosine, or ion channel modulators, such as polyunsaturated fatty acids, may also play a role. Metabolic effects, including diversion from glycolysis, are a further postulated mechanism. For some ASDs and the ketogenic diet, effects on multiple targets may contribute to activity. Better understanding of the ketogenic diet will inform the development of improved drug therapies to treat refractory seizures.

Exposure to Gulf War Illness chemicals induces functional muscarinic receptor maladaptations in muscle nociceptors

Chronic pain is a component of the multisymptom disease known as Gulf War Illness (GWI). There is evidence that pain symptoms could have been a consequence of prolonged and/or excessive exposure to anticholinesterases and other GW chemicals. We previously reported that rats exposed, for 8 weeks, to a mixture of anticholinesterases (pyridostigmine bromide, chlorpyrifos) and a Nav (voltage activated Na(+) channel) deactivation-inhibiting pyrethroid, permethrin, exhibited a behavior pattern that was consistent with a delayed myalgia. This myalgia-like behavior was accompanied by persistent changes to Kv (voltage activated K(+)) channel physiology in muscle nociceptors (Kv7, KDR). In the present study, we examined how exposure to the above agents altered the reactivity of Kv channels to a muscarinic receptor (mAChR) agonist (oxotremorine-M). Comparisons between muscle nociceptors harvested from vehicle and GW chemical-exposed rats revealed that mAChR suppression of Kv7 activity was enhanced in exposed rats. Yet in these same muscle nociceptors, a Stromatoxin-insensitive component of the KDR (voltage activated delayed rectifier K(+) channel) exhibited decreased sensitivity to activation of mAChR. We have previously shown that a unique mAChR-induced depolarization and burst discharge (MDBD) was exaggerated in muscle nociceptors of rats exposed to GW chemicals. We now provide evidence that both muscle and vascular nociceptors of naïve rats exhibit MDBD. Examination of the molecular basis of the MDBD in naïve animals revealed that while the mAChR depolarization was independent of Kv7, the action potential burst was modulated by Kv7 status. mAChR depolarizations were shown to be dependent, in part, on TRPA1. We argue that dysfunction of the MDBD could be a functional convergence point for maladapted ion channels and receptors consequent to exposure to GW chemicals.

Study on dynamic characteristics' change of hippocampal neuron reduced models caused by the Alzheimer's disease

In the paper, based on the electrophysiological experimental data, the Hippocampal neuron reduced model under the pathology condition of Alzheimer's disease (AD) has been built by modifying parameters' values. The reduced neuron model's dynamic characteristics under effect of AD are comparatively studied. Under direct current stimulation, compared with the normal neuron model, the AD neuron model's dynamic characteristics have obviously been changed. The neuron model under the AD condition undergoes supercritical Andronov-Hopf bifurcation from the rest state to the continuous discharge state. It is different from the neuron model under the normal condition, which undergoes saddle-node bifurcation. So, the neuron model changes into a resonator with monostable state from an integrator with bistable state under AD's action. The research reveals the neuron model's dynamic characteristics' changing under effect of AD, and provides some theoretic basis for AD research by neurodynamics theory.

M-type potassium conductance controls the emergence of neural phase codes: a combined experimental and neuron modelling study

Rate and phase codes are believed to be important in neural information processing. Hippocampal place cells provide a good example where both coding schemes coexist during spatial information processing. Spike rate increases in the place field, whereas spike phase precesses relative to the ongoing theta oscillation. However, what intrinsic mechanism allows for a single neuron to generate spike output patterns that contain both neural codes is unknown. Using dynamic clamp, we simulate an in vivo-like subthreshold dynamics of place cells to in vitro CA1 pyramidal neurons to establish an in vitro model of spike phase precession. Using this in vitro model, we show that membrane potential oscillation (MPO) dynamics is important in the emergence of spike phase codes: blocking the slowly activating, non-inactivating K+ current (IM), which is known to control subthreshold MPO, disrupts MPO and abolishes spike phase precession. We verify the importance of adaptive IM in the generation of phase codes using both an adaptive integrate-and-fire and a Hodgkin-Huxley (HH) neuron model. Especially, using the HH model, we further show that it is the perisomatically located IM with slow activation kinetics that is crucial for the generation of phase codes. These results suggest an important functional role of IM in single neuron computation, where IM serves as an intrinsic mechanism allowing for dual rate and phase coding in single neurons.

β-Secretase BACE1 regulates hippocampal and reconstituted M-currents in a β-subunit-like fashion

The β-secretase BACE1 is widely known for its pivotal role in the amyloidogenic pathway leading to Alzheimer's disease, but how its action on transmembrane proteins other than the amyloid precursor protein affects the nervous system is only beginning to be understood. We report here that BACE1 regulates neuronal excitability through an unorthodox, nonenzymatic interaction with members of the KCNQ (Kv7) family that give rise to the M-current, a noninactivating potassium current with slow kinetics. In hippocampal neurons from BACE1(-/-) mice, loss of M-current enhanced neuronal excitability. We relate the diminished M-current to the previously reported epileptic phenotype of BACE1-deficient mice. In HEK293T cells, BACE1 amplified reconstituted M-currents, altered their voltage dependence, accelerated activation, and slowed deactivation. Biochemical evidence strongly suggested that BACE1 physically associates with channel proteins in a β-subunit-like fashion. Our results establish BACE1 as a physiologically essential constituent of regular M-current function and elucidate a striking new feature of how BACE1 impacts on neuronal activity in the intact and diseased brain.

KV7 Channels Regulate Firing during Synaptic Integration in GABAergic Striatal Neurons

Striatal projection neurons (SPNs) process motor and cognitive information. Their activity is affected by Parkinson's disease, in which dopamine concentration is decreased and acetylcholine concentration is increased. Acetylcholine activates muscarinic receptors in SPNs. Its main source is the cholinergic interneuron that responds with a briefer latency than SPNs during a cortical command. Therefore, an important question is whether muscarinic G-protein coupled receptors and their signaling cascades are fast enough to intervene during synaptic responses to regulate synaptic integration and firing. One of the most known voltage dependent channels regulated by muscarinic receptors is the KV7/KCNQ channel. It is not known whether these channels regulate the integration of suprathreshold corticostriatal responses. Here, we study the impact of cholinergic muscarinic modulation on the synaptic response of SPNs by regulating KV7 channels. We found that KV7 channels regulate corticostriatal synaptic integration and that this modulation occurs in the dendritic/spines compartment. In contrast, it is negligible in the somatic compartment. This modulation occurs on sub- and suprathreshold responses and lasts during the whole duration of the responses, hundreds of milliseconds, greatly altering SPNs firing properties. This modulation affected the behavior of the striatal microcircuit.

A recurrent KCNQ2 pore mutation causing early onset epileptic encephalopathy has a moderate effect on M current but alters subcellular localization of Kv7 channels

Mutations in the KCNQ2 gene encoding the voltage-dependent potassium M channel Kv7.2 subunit cause either benign epilepsy or early onset epileptic encephalopathy (EOEE). It has been proposed that the disease severity rests on the inhibitory impact of mutations on M current density. Here, we have analyzed the phenotype of 7 patients carrying the p.A294V mutation located on the S6 segment of the Kv7.2 pore domain (Kv7.2(A294V)). We investigated the functional and subcellular consequences of this mutation and compared it to another mutation (Kv7.2(A294G)) associated with a benign epilepsy and affecting the same residue. We report that all the patients carrying the p.A294V mutation presented the clinical and EEG characteristics of EOEE. In CHO cells, the total expression of Kv7.2(A294V) alone, assessed by western blotting, was only 20% compared to wild-type. No measurable current was recorded in CHO cells expressing Kv7.2(A294V) channel alone. Although the total Kv7.2(A294V) expression was rescued to wild-type levels in cells co-expressing the Kv7.3 subunit, the global current density was still reduced by 83% compared to wild-type heteromeric channel. In a configuration mimicking the patients' heterozygous genotype i.e., Kv7.2(A294V)/Kv7.2/Kv7.3, the global current density was reduced by 30%. In contrast to Kv7.2(A294V), the current density of homomeric Kv7.2(A294G) was not significantly changed compared to wild-type Kv7.2. However, the current density of Kv7.2(A294G)/Kv7.2/Kv7.3 and Kv7.2(A294G)/Kv7.3 channels were reduced by 30% and 50% respectively, compared to wild-type Kv7.2/Kv7.3. In neurons, the p.A294V mutation induced a mislocalization of heteromeric mutant channels to the somato-dendritic compartment, while the p.A294G mutation did not affect the localization of the heteromeric channels to the axon initial segment. We conclude that this position is a hotspot of mutation that can give rise to a severe or a benign epilepsy. The p.A294V mutation does not exert a dominant-negative effect on wild-type subunits but alters the preferential axonal targeting of heteromeric Kv7 channels. Our data suggest that the disease severity is not necessarily a consequence of a strong inhibition of M current and that additional mechanisms such as abnormal subcellular distribution of Kv7 channels could be determinant.

Modulation of BK channels contributes to activity-dependent increase of excitability through MTORC1 activity in CA1 pyramidal cells of mouse hippocampus

Memory acquisition and synaptic plasticity are accompanied by changes in the intrinsic excitability of CA1 pyramidal neurons. These activity-dependent changes in excitability are mediated by modulation of intrinsic currents which alters the responsiveness of the cell to synaptic inputs. The afterhyperpolarization (AHP), a major contributor to the regulation of neuronal excitability, is reduced in animals that have acquired several types of hippocampus-dependent memory tasks and also following synaptic potentiation by high frequency stimulation. BK channels underlie the fast AHP and contribute to spike repolarization, and this AHP is reduced in animals that successfully acquired trace-eyeblink conditioning. This suggests that BK channel function is activity-dependent, but the mechanisms are unknown. In this study, we found that blockade of BK channels with paxilline (10 μM) decreased I_AHP amplitude and increased spike half-width and instantaneous frequency in response to a +100 pA depolarization. In addition, induction of long term potentiation (LTP) by theta burst stimulation (TBS) in CA1 pyramidal neurons reduced BK channel’s contribution to I_AHP, spike repolarization, and instantaneous frequency. This result indicates that BK channel activity is decreased following synaptic potentiation. Interestingly, blockade of mammalian target of rapamycin (MTORC1) with rapamycin (400 nM) following synaptic potentiation restored BK channel function, suggesting a role for protein translation in signaling events which decreased postsynaptic BK channel activity following synaptic potentiation.

Neurophysiological modification of CA1 pyramidal neurons in a transgenic mouse expressing a truncated form of disrupted-in-schizophrenia 1

A t(1;11) balanced chromosomal translocation transects the Disc1 gene in a large Scottish family and produces genome-wide linkage to schizophrenia and recurrent major depressive disorder. This study describes our in vitro investigations into neurophysiological function in hippocampal area CA1 of a transgenic mouse (DISC1_tr) that expresses a truncated version of DISC1 designed to reproduce aspects of the genetic situation in the Scottish t(1;11) pedigree. We employed both patch-clamp and extracellular recording methods in vitro to compare intrinsic properties and synaptic function and plasticity between DISC1_tr animals and wild-type littermates. Patch-clamp analysis of CA1 pyramidal neurons (CA1-PNs) revealed no genotype dependence in multiple subthreshold parameters, including resting potential, input resistance, hyperpolarization-activated ‘sag’ and resonance properties. Suprathreshold stimuli revealed no alteration to action potential (AP) waveform, although the initial rate of AP production was higher in DISC1_tr mice. No difference was observed in afterhyperpolarizing potentials following trains of 5–25 APs at 50 Hz. Patch-clamp analysis of synaptic responses in the Schaffer collateral commissural (SC) pathway indicated no genotype-dependence of paired pulse facilitation, excitatory postsynaptic potential summation or AMPA/NMDA ratio. Extracellular recordings also revealed an absence of changes to SC synaptic responses and indicated input–output and short-term plasticity were also unaltered in the temporoammonic (TA) input. However, in DISC1_tr mice theta burst-induced long-term potentiation was enhanced in the SC pathway but completely lost in the TA pathway. These data demonstrate that expressing a truncated form of DISC1 affects intrinsic properties of CA1-PNs and produces pathway-specific effects on long-term synaptic plasticity.

Role of small conductance Ca²⁺-activated K⁺ channels in controlling CA1 pyramidal cell excitability

Small-conductance Ca(2+)-activated K(+) (SK or K(Ca)2) channels are widely expressed in the CNS. In several types of neurons, these channels were shown to become activated during repetitive firing, causing early spike frequency adaptation. In CA1 pyramidal cells, SK channels in dendritic spines were shown to regulate synaptic transmission. However, the presence of functional SK channels in the somata and their role in controlling the intrinsic firing of these neurons has been controversial. Using whole-cell voltage-clamp and current-clamp recordings in acute hippocampal slices and focal applications of irreversible and reversible SK channel blockers, we provide evidence that functional SK channels are expressed in the somata and proximal dendrites of adult rat CA1 pyramidal cells. Although these channels can generate a medium duration afterhyperpolarizing current, they play only an auxiliary role in controlling the intrinsic excitability of these neurons, secondary to the low voltage-activating, noninactivating K(V)7/M channels. As long as K(V)7/M channels are operative, activation of SK channels during repetitive firing does not notably affect the spike output of CA1 pyramidal cells. However, when K(V)7/M channel activity is compromised, SK channel activation significantly and uniquely reduces spike output of these neurons. Therefore, proximal SK channels provide a "second line of defense" against intrinsic hyperexcitability, which may play a role in multiple conditions in which K(V)7/M channels activity is compromised, such as hyposmolarity.

Polarized axonal surface expression of neuronal KCNQ potassium channels is regulated by calmodulin interaction with KCNQ2 subunit

KCNQ potassium channels composed of KCNQ2 and KCNQ3 subunits give rise to the M-current, a slow-activating and non-inactivating voltage-dependent potassium current that limits repetitive firing of action potentials. KCNQ channels are enriched at the surface of axons and axonal initial segments, the sites for action potential generation and modulation. Their enrichment at the axonal surface is impaired by mutations in KCNQ2 carboxy-terminal tail that cause benign familial neonatal convulsion and myokymia, suggesting that their correct surface distribution and density at the axon is crucial for control of neuronal excitability. However, the molecular mechanisms responsible for regulating enrichment of KCNQ channels at the neuronal axon remain elusive. Here, we show that enrichment of KCNQ channels at the axonal surface of dissociated rat hippocampal cultured neurons is regulated by ubiquitous calcium sensor calmodulin. Using immunocytochemistry and the cluster of differentiation 4 (CD4) membrane protein as a trafficking reporter, we demonstrate that fusion of KCNQ2 carboxy-terminal tail is sufficient to target CD4 protein to the axonal surface whereas inhibition of calmodulin binding to KCNQ2 abolishes axonal surface expression of CD4 fusion proteins by retaining them in the endoplasmic reticulum. Disruption of calmodulin binding to KCNQ2 also impairs enrichment of heteromeric KCNQ2/KCNQ3 channels at the axonal surface by blocking their trafficking from the endoplasmic reticulum to the axon. Consistently, hippocampal neuronal excitability is dampened by transient expression of wild-type KCNQ2 but not mutant KCNQ2 deficient in calmodulin binding. Furthermore, coexpression of mutant calmodulin, which can interact with KCNQ2/KCNQ3 channels but not calcium, reduces but does not abolish their enrichment at the axonal surface, suggesting that apo calmodulin but not calcium-bound calmodulin is necessary for their preferential targeting to the axonal surface. These findings collectively reveal calmodulin as a critical player that modulates trafficking and enrichment of KCNQ channels at the neuronal axon.

Conditional deletions of epilepsy-associated KCNQ2 and KCNQ3 channels from cerebral cortex cause differential effects on neuronal excitability

KCNQ2 and KCNQ3 potassium channels have emerged as central regulators of pyramidal neuron excitability and spiking behavior. However, despite an abundance of evidence demonstrating that KCNQ2/3 heteromers underlie critical potassium conductances, it is unknown whether KCNQ2, KCNQ3, or both are obligatory for maintaining normal pyramidal neuron excitability. Here, we demonstrate that conditional deletion of Kcnq2 from cerebral cortical pyramidal neurons in mice results in abnormal electrocorticogram activity and early death, whereas similar deletion of Kcnq3 does not. At the cellular level, Kcnq2-null, but not Kcnq3-null, CA1 pyramidal neurons show increased excitability manifested as a decreased medium afterhyperpolarization and a longer-lasting afterdepolarization. As a result, these Kcnq2-deficient neurons are hyperexcitable, responding to current injections with an increased number and frequency of action potentials. Biochemically, the Kcnq2 deficiency secondarily results in a substantial loss of KCNQ3 and KCNQ5 protein levels, whereas loss of Kcnq3 only leads to a modest reduction of other KCNQ channels. Consistent with this finding, KCNQ allosteric activators can still markedly dampen neuronal excitability in Kcnq3-null pyramidal neurons, but have only weak effects in Kcnq2-null pyramidal neurons. Together, our data reveal the indispensable function of KCNQ2 channels at both the cellular and systems levels, and demonstrate that pyramidal neurons have near normal excitability in the absence of KCNQ3 channels.

Heteromeric Kv7.2/7.3 channels differentially regulate action potential initiation and conduction in neocortical myelinated axons

Rapid energy-efficient signaling along vertebrate axons is achieved through intricate subcellular arrangements of voltage-gated ion channels and myelination. One recently appreciated example is the tight colocalization of K(v)7 potassium channels and voltage-gated sodium (Na(v)) channels in the axonal initial segment and nodes of Ranvier. The local biophysical properties of these K(v)7 channels and the functional impact of colocalization with Na(v) channels remain poorly understood. Here, we quantitatively examined K(v)7 channels in myelinated axons of rat neocortical pyramidal neurons using high-resolution confocal imaging and patch-clamp recording. K(v)7.2 and 7.3 immunoreactivity steeply increased within the distal two-thirds of the axon initial segment and was mirrored by the conductance density estimates, which increased from ~12 (proximal) to 150 pS μm(-2) (distal). The axonal initial segment and nodal M-currents were similar in voltage dependence and kinetics, carried by K(v)7.2/7.3 heterotetramers, 4% activated at the resting membrane potential and rapidly activated with single-exponential time constants (~15 ms at 28 mV). Experiments and computational modeling showed that while somatodendritic K(v)7 channels are strongly activated by the backpropagating action potential to attenuate the afterdepolarization and repetitive firing, axonal K(v)7 channels are minimally recruited by the forward-propagating action potential. Instead, in nodal domains K(v)7.2/7.3 channels were found to increase Na(v) channel availability and action potential amplitude by stabilizing the resting membrane potential. Thus, K(v)7 clustering near axonal Na(v) channels serves specific and context-dependent roles, both restraining initiation and enhancing conduction of the action potential.

What are the arguments for and against rational therapy for epilepsy?

Although more than a dozen new anti-seizure drugs (ASDs) have entered the market since 1993, a substantial proportion of patients (~30 %) remain refractory to current treatments. Thus, a concerted effort to identify and develop new therapies that will help these patients continues. Until this effort succeeds, it is reasonable to re-assess the use of currently available therapies and to consider how these therapies might be utilized in a more efficacious manner. This applies to the selection of monotherapies in newly-diagnosed epilepsy, but perhaps, more importantly, to the choice of combination treatments in otherwise drug-refractory epilepsy. Rational polytherapy is a concept that is predicated on the combination of drugs with complementary mechanisms of action (MoAs) that work synergistically to maximize efficacy and minimize the potential for adverse events. Furthermore, rational polytherapy requires a detailed understanding of the MoA subclasses amongst available ASDs and an appreciation of the empirical evidence that supports the use of specific combinations. The majority of ASDs can be loosely categorized into those that target neurotransmission and network hyperexcitability, modulate intrinsic neuronal properties through ion channels, or possess broad-spectrum efficacy as a result of multiple mechanisms. Within each of these categories, there are discrete pharmacological profiles that differentiate individual ASDs. This chapter will consider how knowledge of MoA can help guide therapy in a rational manner, both in the selection of monotherapies for specific seizure types and syndromes, but also in the choice of drug combinations for patients whose epilepsy is not optimally controlled with a single ASD.

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

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

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'.