Calcium-activated afterhyperpolarizations regulate synchronization and timing of epileptiform bursts in hippocampal CA3 pyramidal neurons

NMDA receptor activity during postnatal development determines intrinsic excitability and mossy fiber long-term potentiation of CA3 pyramidal cells

Experimental manipulations that interfere with the functional expression of N-methyl-D-aspartate receptors (NMDARs) during prenatal neurodevelopment or critical periods of postnatal development are models that mimic behavioral and neurophysiological abnormalities of schizophrenia. Blockade of NMDARs with MK-801 during early postnatal development alters glutamate release and impairs the induction of NMDAR-dependent long-term plasticity at the CA1 area of the hippocampus. However, it remains unknown if other forms of hippocampal plasticity, such as α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated short- and long-term potentiation, are compromised in response to neonatal treatment with MK-801. Consistent with this tenet, short- and long-term potentiation between dentate gyrus axons, the mossy fibers (MF), onto CA3 pyramidal cells (CA3 PCs) are mediated by AMPARs. By combining whole-cell patch clamp and extracellular recordings, we have demonstrated that transient blockade of NMDARs during early postnatal development induces a series of pre- and postsynaptic modifications at the MF-CA3 synapse. We found reduced glutamate release from the mossy boutons, increased paired-pulse ratio, and reduced AMPAR-mediated MF LTP levels. At the postsynaptic level, we found an altered NMDA/AMPA ratio and dysregulation of several potassium conductances that increased the excitability of CA3 PCs. In addition, MK-801-treated animals exhibited impaired spatial memory retrieval in the Barnes maze task. Our data demonstrate that transient hypofunction of NMDARs impacts NMDAR-independent forms of synaptic plasticity of the hippocampus.

Modulation of seizure-like events by the small conductance and ATP-sensitive potassium ion channels

Potassium ion channels are extensively involved in the regulation of epileptic seizures. The small conductance calcium-sensitive potassium channels (SK channels) and ATP-sensitive potassium (K_ATP) channels are activated by calcium ion entry and decrease ATP levels, respectively. These channels can underlie the post-burst afterhyperpolarization and be upregulated during seizures, providing negative feedback during epileptic activity. Using the whole-cell patch-clamp method in rat brain slices, we investigated the effect of SK- and K_ATP-affecting drugs on seizure-like events (SLEs) in the 4-aminopyridine model of epileptic seizures in vitro. We demonstrate that SK and K_ATP channels contribute to sustaining the high-frequency firing of the principal neurons in the deep layers of the entorhinal cortex during injections of depolarizing current and epileptiform discharges. Neither the pharmacological blockade nor the activation of these channels was able to prevent the epileptiform activity in brain slices. However, the blockade of K_ATP channels increases the SLE duration, suggesting that these channels may contribute to the termination of SLEs. Thus, K_ATP channels can be considered a promising target for pharmacological interventions for the treatment of epilepsy.

Double-edged Role of KNa Channels in Brain Tuning: Identifying Epileptogenic Network Micro-Macro Disconnection

Epilepsy is commonly recognized as a disease driven by generalized hyperexcited and hypersynchronous neural activity. Sodium-activated potassium channels (K_Na channels), which are encoded by the Slo 2.2 and Slo 2.1 genes, are widely expressed in the central nervous system and considered as "brakes" to adjust neuronal adaptation through regulating action potential threshold or after-hyperpolarization under physiological condition. However, the variants in K_Na channels, especially gain-of-function variants, have been found in several childhood epileptic conditions. Most previous studies focused on mapping the epileptic network on the macroscopic scale while ignoring the value of microscopic changes. Notably, paradoxical role of K_Na channels working on individual neuron/microcircuit and the macroscopic epileptic expression highlights the importance of understanding epileptogenic network through combining microscopic and macroscopic methods. Here, we first illustrated the molecular and physiological function of K_Na channels on preclinical seizure models and patients with epilepsy. Next, we summarized current hypothesis on the potential role of K_Na channels during seizures to provide essential insight into what emerged as a micro-macro disconnection at different levels. Additionally, we highlighted the potential utility of K_Na channels as therapeutic targets for developing innovative anti-seizure medications.

The Molecular Basis for the Calcium-Dependent Slow Afterhyperpolarization in CA1 Hippocampal Pyramidal Neurons

Neuronal signal transmission depends on the frequency, pattern, and timing of spike output, each of which are shaped by spike afterhyperpolarizations (AHPs). There are classically three post-spike AHPs of increasing duration categorized as fast, medium and slow AHPs that hyperpolarize a cell over a range of 10 ms to 30 s. Intensive early work on CA1 hippocampal pyramidal cells revealed that all three AHPs incorporate activation of calcium-gated potassium channels. The ionic basis for a fAHP was rapidly attributed to the actions of big conductance (BK) and the mAHP to small conductance (SK) or Kv7 potassium channels. In stark contrast, the ionic basis for a prominent slow AHP of up to 30 s duration remained an enigma for over 30 years. Recent advances in pharmacological, molecular, and imaging tools have uncovered the expression of a calcium-gated intermediate conductance potassium channel (IK, KCa3.1) in central neurons that proves to contribute to the slow AHP in CA1 hippocampal pyramidal cells. Together the data show that the sAHP arises in part from a core tripartite complex between Cav1.3 (L-type) calcium channels, ryanodine receptors, and IK channels at endoplasmic reticulum-plasma membrane junctions. Work on the sAHP in CA1 pyramidal neurons has again quickened pace, with identified contributions by both IK channels and the Na-K pump providing answers to several mysteries in the pharmacological properties of the sAHP.

The Kappa Opioid Receptor System in Temporal Lobe Epilepsy

Temporal lobe epilepsy is considered to be one of the most common and severe forms of focal epilepsies. Patients frequently develop cognitive deficits and emotional blunting along progression of the disease. The high incidence of refractoriness to antiepileptic drugs and a frequent lack of admissibility to surgery pose an unmet medical challenge. In the urgent quest for novel treatment strategies, neuropeptides and their receptors are interesting candidates. However, their therapeutic potential has not yet been fully exploited. This chapter focuses on the functional role of the dynorphins (Dyns) and the kappa opioid receptor (KOR) system in temporal lobe epilepsy and the hippocampus.Genetic polymorphisms in the prepro-dynorphin (pDyn) gene causing lower levels of Dyns in humans and pDyn gene knockout in mice increase the risk to develop epilepsy. This suggests a role of Dyns and KOR as modulators of neuronal excitability. Indeed, KOR agonists induce inhibition of presynaptic neurotransmitter release, as well as postsynaptic hyperpolarization in glutamatergic neurons, both producing anticonvulsant effects.The development of new approaches to modulate the complex KOR signalling cascade (e.g. biased agonism and gene therapy) opens up new exciting therapeutic opportunities with regard to seizure control and epilepsy. Potential adverse side effects of KOR agonists may be minimized through functional selectivity or locally restricted treatment. Preclinical data suggest a high potential of such approaches to control seizures.

Insertion of Calcium-Permeable AMPA Receptors during Epileptiform Activity In Vitro Modulates Excitability of Principal Neurons in the Rat Entorhinal Cortex

Epileptic activity leads to rapid insertion of calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (CP-AMPARs) into the synapses of cortical and hippocampal glutamatergic neurons, which generally do not express them. The physiological significance of this process is not yet fully understood; however, it is usually assumed to be a pathological process that augments epileptic activity. Using whole-cell patch-clamp recordings in rat entorhinal cortex slices, we demonstrate that the timing of epileptiform discharges, induced by 4-aminopyridine and gabazine, is determined by the shunting effect of Ca2+-dependent slow conductance, mediated predominantly by K+-channels. The blockade of CP-AMPARs by IEM-1460 eliminates this extra conductance and consequently increases the rate of discharge generation. The blockade of NMDARs reduced the additional conductance to a lesser extent than the blockade of CP-AMPARs, indicating that CP-AMPARs are a more significant source of intracellular Ca2+. The study's main findings were implemented in a mathematical model, which reproduces the shunting effect of activity-dependent conductance on the generation of discharges. The obtained results suggest that the expression of CP-AMPARs in principal neurons reduces the discharge generation rate and may be considered as a protective mechanism.

Physiology and Therapeutic Potential of SK, H, and M Medium AfterHyperPolarization Ion Channels

SK, HCN, and M channels are medium afterhyperpolarization (mAHP)-mediating ion channels. The three channels co-express in various brain regions, and their collective action strongly influences cellular excitability. However, significant diversity exists in the expression of channel isoforms in distinct brain regions and various subcellular compartments, which contributes to an equally diverse set of specific neuronal functions. The current review emphasizes the collective behavior of the three classes of mAHP channels and discusses how these channels function together although they play specialized roles. We discuss the biophysical properties of these channels, signaling pathways that influence the activity of the three mAHP channels, various chemical modulators that alter channel activity and their therapeutic potential in treating various neurological anomalies. Additionally, we discuss the role of mAHP channels in the pathophysiology of various neurological diseases and how their modulation can alleviate some of the symptoms.

The Theta Rhythm of the Hippocampus: From Neuronal and Circuit Mechanisms to Behavior

This review focuses on the neuronal and circuit mechanisms involved in the generation of the theta (θ) rhythm and of its participation in behavior. Data have accumulated indicating that θ arises from interactions between medial septum-diagonal band of Broca (MS-DbB) and intra-hippocampal circuits. The intrinsic properties of MS-DbB and hippocampal neurons have also been shown to play a key role in θ generation. A growing number of studies suggest that θ may represent a timing mechanism to temporally organize movement sequences, memory encoding, or planned trajectories for spatial navigation. To accomplish those functions, θ and gamma (γ) oscillations interact during the awake state and REM sleep, which are considered to be critical for learning and memory processes. Further, we discuss that the loss of this interaction is at the base of various neurophatological conditions.

Spreading Depolarization Facilitates the Transition of Neuronal Burst Firing from Interictal to Ictal State

The transition of neuronal burst firing from the interictal to ictal state contributes to seizure initiation in human temporal lobe epilepsy. The low-Mg²⁺ model of seizure is characterized by initial spontaneous interictal bursting events, which later developed into ictaform discharges. Both experimental and clinical studies point to a complex link between spreading depolarization (SD) and epileptiform field potentials (EFP), including SD-induced epileptic seizures. To investigate the mechanism of SD and EFP interactions, the effect of SD on the transition of interictal to ictal state in low-Mg²⁺ model of seizure was studied in the rat hippocampus in vitro. After the appearance of interictal activities, SD was elicited by local application of KCl. SD significantly increased the amplitude and duration of action potentials and after-hyperpolarization, and hyperpolarized the membrane potential. Furthermore, SD significantly increased the duration of interictal activities and the threshold potentials of interictal activities. In addition, SD significantly accelerated the transition from interictal to ictal state compared to the control tissues. Ictal activities after induction of SD exhibited a significantly longer duration. This study revealed that SD accelerates interictal-to-ictal transitions and facilitates development of ictaform discharges, possibly via the enhancement of neural synchronization, and points to the potential role of SD in seizure initiation.

Activation of small conductance calcium-activated potassium channels suppresses seizure susceptibility in the genetically epilepsy-prone rats

Small conductance calcium-activated potassium (SK) channels dampen neuronal excitability by contributing to slow afterhyperpolarization (AHP) that follows a series of action potentials, and therefore may represent an intrinsic inhibitory mechanism to prevent seizures. We have previously reported that susceptibility to acoustically evoked seizures was associated with downregulation of SK1 and SK3 subtypes of SK channels in the inferior colliculus of the moderated seizure severity strain of the genetically epilepsy-prone rats (GEPR-3s). Here, we evaluated the effects of 1-ethyl-2-benzimidazolinone (1-EBIO), a potent activator of SK channels, on acoustically evoked seizures in both male and female adult GEPR-3s at various time points post-treatment. Systemic administration of 1-EBIO at various tested doses suppressed seizure susceptibility in both male and female GEPR-3s; however, the complete seizure suppression was only observed following administration of relatively higher doses of 1-EBIO in females. These findings indicate that activation of SK channels results in anticonvulsive action against generalized tonic-clonic seizures in both male and female GEPR-3s, with males exhibiting higher sensitivity than females.

Oral administration of the casein kinase 2 inhibitor TBB leads to persistent KCa2.2 channel up-regulation in the epileptic CA1 area and cortex, but lacks anti-seizure efficacy in the pilocarpine epilepsy model

Temporal lobe epilepsy (TLE) is the most common epileptic syndrome in adults and often presents with seizures that prove intractable with currently available anticonvulsants. Thus, there is still a need for new anti-seizure drugs in this condition. Recently, we found that the casein kinase 2 inhibitor 4,5,6,7-tetrabromotriazole (TBB) prevented the emergence of spontaneous epileptic discharges in an acute in vitro epilepsy model. This prompted us to study the anti-seizure effects of TBB in the pilocarpine model of chronic epilepsy in vivo. To this end, we performed long-term video-EEG monitoring lasting 78-167 days of nine chronically epileptic rats and obtained a baseline seizure rate of 3.3 ± 1.3 per day (baseline of 27-80 days). We found a significant age effect with more pronounced seizure rates in older animals as compared to younger ones. However, the seizure rate increased to 6.3 ± 2.2 per day during the oral TBB administration (treatment period of 21-50 days), and following discontinuation of TBB, this rate remained stable with 5.2 ± 1.4 seizures per day (follow-up of 30-55 days). After completing the video-EEG during the follow-up the hippocampal tissue was prepared and studied for the expression of the Ca²⁺-activated K⁺ channel K_Ca2.2. We found a significant up-regulation of K_Ca2.2 in the epileptic CA1 region and in the neocortex, but in no other hippocampal subfield. Hence, our findings indicate that oral administration of TBB leads to persistent up-regulation of K_Ca2.2 in the epileptic CA1 subfield and in the neocortex, but lacks anti-seizure efficacy in the pilocarpine epilepsy model.

Potassium Channel Gain of Function in Epilepsy: An Unresolved Paradox

Exome and targeted sequencing have revolutionized clinical diagnosis. This has been particularly striking in epilepsy and neurodevelopmental disorders, for which new genes or new variants of preexisting candidate genes are being continuously identified at increasing rates every year. A surprising finding of these efforts is the recognition that gain of function potassium channel variants are actually associated with certain types of epilepsy, such as malignant migrating partial seizures of infancy or early-onset epileptic encephalopathy. This development has been difficult to understand as traditionally potassium channel loss-of-function, not gain-of-function, has been associated with hyperexcitability disorders. In this article, we describe the current state of the field regarding the gain-of-function potassium channel variants associated with epilepsy (KCNA2, KCNB1, KCND2, KCNH1, KCNH5, KCNJ10, KCNMA1, KCNQ2, KCNQ3, and KCNT1) and speculate on the possible cellular mechanisms behind the development of seizures and epilepsy in these patients. Understanding how potassium channel gain-of-function leads to epilepsy will provide new insights into the inner working of neural circuits and aid in developing new therapies.

Echinacoside, an active constituent of Herba Cistanche, suppresses epileptiform activity in hippocampal CA3 pyramidal neurons

Echinacoside, an active compound in the herb Herba Cistanche, has been reported to inhibit glutamate release. In this study, we investigated the effects of echinacoside on spontaneous excitatory synaptic transmission changes induced by 4-aminopyridine (4-AP), by using the in vitro rat hippocampal slice technique and whole-cell patch clamp recordings from CA3 pyramidal neurons. Perfusion with echinacoside significantly suppressed the 4-AP-induced epileptiform activity in a concentration-dependent manner. Echinacoside reduced 4-AP-induced increase in frequency of spontaneous excitatory postsynaptic currents (sEPSCs) but it did not affect the amplitude of sEPSCs or glutamate-activated currents, implicating a presynaptic mechanism of action. Echinacoside also potently blocked sustained repetitive firing, which is a basic mechanism of antiepileptic drugs. These results suggest that echinacoside exerts an antiepileptic effect on hippocampal CA3 pyramidal neurons by simultaneously decreasing glutamate release and blocking abnormal firing synchronization. Accordingly, our study provides experimental evidence that echinacoside may represent an effective pharmacological agent for treating epilepsy.

Understanding the Generation of Network Bursts by Adaptive Oscillatory Neurons

Experimental and numerical studies have revealed that isolated populations of oscillatory neurons can spontaneously synchronize and generate periodic bursts involving the whole network. Such a behavior has notably been observed for cultured neurons in rodent's cortex or hippocampus. We show here that a sufficient condition for this network bursting is the presence of an excitatory population of oscillatory neurons which displays spike-driven adaptation. We provide an analytic model to analyze network bursts generated by coupled adaptive exponential integrate-and-fire neurons. We show that, for strong synaptic coupling, intrinsically tonic spiking neurons evolve to reach a synchronized intermittent bursting state. The presence of inhibitory neurons or plastic synapses can then modulate this dynamics in many ways but is not necessary for its appearance. Thanks to a simple self-consistent equation, our model gives an intuitive and semi-quantitative tool to understand the bursting behavior. Furthermore, it suggests that after-hyperpolarization currents are sufficient to explain bursting termination. Through a thorough mapping between the theoretical parameters and ion-channel properties, we discuss the biological mechanisms that could be involved and the relevance of the explored parameter-space. Such an insight enables us to propose experimentally-testable predictions regarding how blocking fast, medium or slow after-hyperpolarization channels would affect the firing rate and burst duration, as well as the interburst interval.

Acetylcholine Facilitates a Depolarization-Induced Enhancement of Inhibition in Rat CA1 Pyramidal Neurons

Cholinergic mechanisms in the hippocampus regulate forms of synaptic plasticity linked with cognition and spatial navigation, but the underlying mechanisms remain largely unknown. Here, in rat hippocampal CA1 pyramidal cells under blockade of ionotropic glutamate receptors, we report that a single acetylcholine pulse and repeated depolarization activated a robust and enduring postsynaptic depolarization-induced enhancement of inhibition (DEI) that masked a presynaptic depolarization-induced suppression of inhibition (DSI). Increased cytosolic Ca2+ and M1-muscarinic receptor activation caused the rise in voltage-sensitive α5βγ2-containing γ-aminobutyric acid type-A receptors that generated DEI. In summary, this muscarinic-mediated activity-dependent plasticity rapidly transfers depolarization effects on inhibition from presynaptic suppression or DSI to postsynaptic enhancement or DEI, a change potentially relevant in behavior.

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

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

Potassium Channels in Epilepsy

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

A new rapid kindling variant for induction of cortical epileptogenesis in freely moving rats

Kindling, one of the most used models of experimental epilepsy is based on daily electrical stimulation in several brain structures. Unlike the classic or slow kindling protocols (SK), the rapid kindling types (RK) described until now require continuous stimulation at suprathreshold intensities applied directly to the same brain structure used for subsequent electrophysiological and immunohistochemical studies, usually the hippocampus. However, the cellular changes observed in these rapid protocols, such as astrogliosis and neuronal loss, could be due to experimental manipulation more than to epileptogenesis-related alterations. Here, we developed a new RK protocol in order to generate an improved model of temporal lobe epilepsy (TLE) which allows gradual progression of the epilepsy as well as obtaining an epileptic hippocampus, thus avoiding direct surgical manipulation and electric stimulation over this structure. This new protocol consists of basolateral amygdala (BLA) stimulation with 10 trains of biphasic pulses (10 s; 50 Hz) per day with 20 min-intervals, during 3 consecutive days, using a subconvulsive and subthreshold intensity, which guarantees tissue integrity. The progression of epileptic activity was evaluated in freely moving rats through electroencephalographic (EEG) recordings from cortex and amygdala, accompanied with synchronized video recordings. Moreover, we assessed the effectiveness of RK protocol and the establishment of epilepsy by evaluating cellular alterations of hippocampal slices from kindled rats. RK protocol induced convulsive states similar to SK protocols but in 3 days, with persistently lowered threshold to seizure induction and epileptogenic-dependent cellular changes in amygdala projection areas. We concluded that this novel RK protocol introduces a new variant of the chronic epileptogenesis models in freely moving rats, which is faster, highly reproducible and causes minimum cell damage with respect to that observed in other experimental models of epilepsy.

A small conductance calcium-activated K+ channel in C. elegans, KCNL-2, plays a role in the regulation of the rate of egg-laying

In the nervous system of mice, small conductance calcium-activated potassium (SK) channels function to regulate neuronal excitability through the generation of a component of the medium afterhyperpolarization that follows action potentials. In humans, irregular action potential firing frequency underlies diseases such as ataxia, epilepsy, schizophrenia and Parkinson's disease. Due to the complexity of studying protein function in the mammalian nervous system, we sought to characterize an SK channel homologue, KCNL-2, in C. elegans, a genetically tractable system in which the lineage of individual neurons was mapped from their early developmental stages. Sequence analysis of the KCNL-2 protein reveals that the six transmembrane domains, the potassium-selective pore and the calmodulin binding domain are highly conserved with the mammalian homologues. We used widefield and confocal fluorescent imaging to show that a fusion construct of KCNL-2 with GFP in transgenic lines is expressed in the nervous system of C. elegans. We also show that a KCNL-2 null strain, kcnl-2(tm1885), demonstrates a mild egg-laying defective phenotype, a phenotype that is rescued in a KCNL-2-dependent manner. Conversely, we show that transgenic lines that overexpress KCNL-2 demonstrate a hyperactive egg-laying phenotype. In this study, we show that the vulva of transgenic hermaphrodites is highly innervated by neuronal processes and by the VC4 and VC5 neurons that express GFP-tagged KCNL-2. We propose that KCNL-2 functions in the nervous system of C. elegans to regulate the rate of egg-laying.

Endogenous cholinergic tone modulates spontaneous network level neuronal activity in primary cortical cultures grown on multi-electrode arrays

BACKGROUND

Cortical cultures grown long-term on multi-electrode arrays (MEAs) are frequently and extensively used as models of cortical networks in studies of neuronal firing activity, neuropharmacology, toxicology and mechanisms underlying synaptic plasticity. However, in contrast to the predominantly asynchronous neuronal firing activity exhibited by intact cortex, electrophysiological activity of mature cortical cultures is dominated by spontaneous epileptiform-like global burst events which hinders their effective use in network-level studies, particularly for neurally-controlled animat ('artificial animal') applications. Thus, the identification of culture features that can be exploited to produce neuronal activity more representative of that seen in vivo could increase the utility and relevance of studies that employ these preparations. Acetylcholine has a recognised neuromodulatory role affecting excitability, rhythmicity, plasticity and information flow in vivo although its endogenous production by cortical cultures and subsequent functional influence upon neuronal excitability remains unknown.

RESULTS

Consequently, using MEA electrophysiological recording supported by immunohistochemical and RT-qPCR methods, we demonstrate for the first time, the presence of intrinsic cholinergic neurons and significant, endogenous cholinergic tone in cortical cultures with a characterisation of the muscarinic and nicotinic components that underlie modulation of spontaneous neuronal activity. We found that tonic muscarinic ACh receptor (mAChR) activation affects global excitability and burst event regularity in a culture age-dependent manner whilst, in contrast, tonic nicotinic ACh receptor (nAChR) activation can modulate burst duration and the proportion of spikes occurring within bursts in a spatio-temporal fashion.

CONCLUSIONS

We suggest that the presence of significant endogenous cholinergic tone in cortical cultures and the comparability of its modulatory effects to those seen in intact brain tissues support emerging, exploitable commonalities between in vivo and in vitro preparations. We conclude that experimental manipulation of endogenous cholinergic tone could offer a novel opportunity to improve the use of cortical cultures for studies of network-level mechanisms in a manner that remains largely consistent with its functional role.

Small-conductance Ca2+-activated K+ channels modulate action potential-induced Ca2+ transients in hippocampal neurons

In hippocampal pyramidal neurons, voltage-gated Ca(2+) channels open in response to action potentials. This results in elevations in the intracellular concentration of Ca(2+) that are maximal in the proximal apical dendrites and decrease rapidly with distance from the soma. The control of these action potential-evoked Ca(2+) elevations is critical for the regulation of hippocampal neuronal activity. As part of Ca(2+) signaling microdomains, small-conductance Ca(2+)-activated K(+) (SK) channels have been shown to modulate the amplitude and duration of intracellular Ca(2+) signals by feedback regulation of synaptically activated Ca(2+) sources in small distal dendrites and dendritic spines, thus affecting synaptic plasticity in the hippocampus. In this study, we investigated the effect of the activation of SK channels on Ca(2+) transients specifically induced by action potentials in the proximal processes of hippocampal pyramidal neurons. Our results, obtained by using selective SK channel blockers and enhancers, show that SK channels act in a feedback loop, in which their activation by Ca(2+) entering mainly through L-type voltage-gated Ca(2+) channels leads to a reduction in the subsequent dendritic influx of Ca(2+). This underscores a new role of SK channels in the proximal apical dendrite of hippocampal pyramidal neurons.

Hippocalcin and KCNQ channels contribute to the kinetics of the slow afterhyperpolarization

The calcium-activated slow afterhyperpolarization (sAHP) is a potassium conductance implicated in many physiological functions of the brain including memory, aging, and epilepsy. In large part, the sAHP's importance stems from its exceedingly long-lasting time-course, which integrates action potential-induced calcium signals and allows the sAHP to control neuronal excitability and prevent runaway firing. Despite its role in neuronal physiology, the molecular mechanisms that give rise to its unique kinetics are, to our knowledge, still unknown. Recently, we identified KCNQ channels as a candidate potassium channel family that can contribute to the sAHP. Here, we test whether KCNQ channels shape the sAHP rise and decay kinetics in wild-type mice and mice lacking Hippocalcin, the putative sAHP calcium sensor. Application of retigabine to speed KCNQ channel activation accelerated the rise of the CA3 pyramidal neuron sAHP current in both wild-type and Hippocalcin knockout mice, indicating that the gating of KCNQ channels limits the sAHP activation. Interestingly, we found that the decay of the sAHP was prolonged in Hippocalcin knockout mice, and that the decay was sensitive to retigabine modulation, unlike in wild-type mice. Together, our results demonstrate that sAHP activation in CA3 pyramidal neurons is critically dependent on KCNQ channel kinetics whereas the identity of the sAHP calcium sensor determines whether KCNQ channel kinetics also limit the sAHP decay.

β-Adrenergic modulation of spontaneous spatiotemporal activity patterns and synchrony in hyperexcitable hippocampal circuits

A description of healthy and pathological brain dynamics requires an understanding of spatiotemporal patterns of neural activity and characteristics of its propagation between interconnected circuits. However, the structure and modulation of the neural activation maps underlying these patterns and their propagation remain elusive. We investigated effects of β-adrenergic receptor (β-AR) stimulation on the spatiotemporal characteristics of emergent activity in rat hippocampal circuits. Synchronized epileptiform-like activity, such as interictal bursts (IBs) and ictal-like events (ILEs), were evoked by 4-aminopyridine (4-AP), and their dynamics were studied using a combination of electrophysiology and fast voltage-sensitive dye imaging. Dynamic characterization of the spontaneous IBs showed that they originated in dentate gyrus/CA3 border and propagated toward CA1. To determine how β-AR modulates spatiotemporal characteristics of the emergent IBs, we used the β-AR agonist isoproterenol (ISO). ISO significantly reduced the spatiotemporal extent and propagation velocity of the IBs and significantly altered network activity in the 1- to 20-Hz range. Dual whole cell recordings of the IBs in CA3/CA1 pyramidal cells and optical analysis of those regions showed that ISO application reduced interpyramidal and interregional synchrony during the IBs. In addition, ISO significantly reduced duration not only of the shorter duration IBs but also the prolonged ILEs in 4-AP. To test whether the decrease in ILE duration was model dependent, we used a different hyperexcitability model, zero magnesium (0 Mg(2+)). Prolonged ILEs were readily formed in 0 Mg(2+), and addition of ISO significantly reduced their durations. Taken together, these novel results provide evidence that β-AR activation dynamically reshapes the spatiotemporal activity patterns in hyperexcitable circuits by altering network rhythmogenesis, propagation velocity, and intercellular/regional synchronization.

Endoplasmic reticulum potassium–hydrogen exchanger and small conductance calcium-activated potassium channel activities are essential for ER calcium uptake in neurons and cardiomyocytes

Calcium pumping into the endoplasmic reticulum (ER) lumen is thought to be coupled to a countertransport of protons through sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) and the members of the ClC family of chloride channels. However, pH in the ER lumen remains neutral, which suggests a mechanism responsible for proton re-entry. We studied whether cation–proton exchangers could act as routes for such a re-entry. ER Ca2+ uptake was measured in permeabilized immortalized hypothalamic neurons, primary rat cortical neurons and mouse cardiac fibers. Replacement of K+ in the uptake solution with Na+ or tetraethylammonium led to a strong inhibition of Ca2+ uptake in neurons and cardiomyocytes. Furthermore, inhibitors of the potassium–proton exchanger (quinine or propranolol) but not of the sodium–proton exchanger reduced ER Ca2+ uptake by 56–82%. Externally added nigericin, a potassium–proton exchanger, attenuated the inhibitory effect of propranolol. Inhibitors of small conductance calcium-sensitive K+ (SKCa) channels (UCL 1684, dequalinium) blocked the uptake of Ca2+ by the ER in all preparations by 48–94%, whereas inhibitors of other K+ channels (IKCa, BKCa and KATP) had no effect. Fluorescence microscopy and western blot analysis revealed the presence of both SKCa channels and the potassium–proton exchanger leucine zipper-EF-hand-containing transmembrane protein 1 (LETM1) in ER in situ and in the purified ER fraction. The data obtained demonstrate that SKCa channels and LETM1 reside in the ER membrane and that their activity is essential for ER Ca2+ uptake.

High firing rate of neonatal hippocampal interneurons is caused by attenuation of afterhyperpolarizing potassium currents by tonically active kainate receptors

In the neonatal hippocampus, the activity of interneurons shapes early network bursts that are important for the establishment of neuronal connectivity. However, mechanisms controlling the firing of immature interneurons remain elusive. We now show that the spontaneous firing rate of CA3 stratum lucidum interneurons markedly decreases during early postnatal development because of changes in the properties of GluK1 (formerly known as GluR5) subunit-containing kainate receptors (KARs). In the neonate, activation of KARs by ambient glutamate exerts a tonic inhibition of the medium-duration afterhyperpolarization (mAHP) by a G-protein-dependent mechanism, permitting a high interneuronal firing rate. During development, the amplitude of the apamine-sensitive K+ currents responsible for the mAHP increases dramatically because of decoupling between KAR activation and mAHP modulation, leading to decreased interneuronal firing. The developmental shift in the KAR function and its consequences on interneuronal activity are likely to have a fundamental role in the maturation of the synchronous neuronal oscillations typical for adult hippocampal circuitry.

Altered expression and function of small-conductance (SK) Ca(2+)-activated K+ channels in pilocarpine-treated epileptic rats

Small conductance calcium (Ca(2+)) activated SK channels are critical regulators of neuronal excitability in hippocampus. Accordingly, these channels are thought to play a key role in controlling neuronal activity in acute models of epilepsy. In this study, we investigate the expression and function of SK channels in the pilocarpine model of mesial temporal lobe epilepsy. For this purpose, protein expression was assessed using western blotting assays and gene expression was analyzed using TaqMan-based probes and the quantitative real-time polymerase chain reaction (qPCR) comparative method delta-delta cycle threshold ( big up tri, open big up tri, openCT) in samples extracted from control and epileptic rats. In addition, the effect of SK channel antagonist UCL1684 and agonist NS309 on CA1 evoked population spikes was studied in hippocampal slices. Western blotting analysis showed a significant reduction in the expression of SK1 and SK2 channels at 10days following status epilepticus (SE), but levels recovered at 1month and at more than 2months after SE. In contrast, a significant down-regulation of SK3 channels was detected after 10days of SE. Analysis of gene expression by qPCR revealed a significant reduction of transcripts for SK2 (Kcnn1) and SK3 (Kcnn3) channels as early as 10days following pilocarpine-induced SE and during the chronic phase of the pilocarpine model. Moreover, bath application of UCL1684 (100nM for 15min) induced a significant increase of the population spike amplitude and number of spikes in the hippocampal CA1 area of slices obtained control and chronic epileptic rats. This effect was obliterated by co-administration of UCL1684 with SK channel agonist NS309 (1microM). Application of NS309 failed to modify population spikes in the CA1 area of slices taken from control and epileptic rats. These data indicate an abnormal expression of SK channels and a possible dysfunction of these channels in experimental MTLE.

The KCNQ5 potassium channel mediates a component of the afterhyperpolarization current in mouse hippocampus

Mutations in KCNQ2 and KCNQ3 voltage-gated potassium channels lead to neonatal epilepsy as a consequence of their key role in regulating neuronal excitability. Previous studies in the brain have focused primarily on these KCNQ family members, which contribute to M-currents and afterhyperpolarization conductances in multiple brain areas. In contrast, the function of KCNQ5 (Kv7.5), which also displays widespread expression in the brain, is entirely unknown. Here, we developed mice that carry a dominant negative mutation in the KCNQ5 pore to probe whether it has a similar function as other KCNQ channels. This mutation renders KCNQ5(dn)-containing homomeric and heteromeric channels nonfunctional. We find that Kcnq5(dn/dn) mice are viable and have normal brain morphology. Furthermore, expression and neuronal localization of KCNQ2 and KCNQ3 subunits are unchanged. However, in the CA3 area of hippocampus, a region that highly expresses KCNQ5 channels, the medium and slow afterhyperpolarization currents are significantly reduced. In contrast, neither current is affected in the CA1 area of the hippocampus, a region with low KCNQ5 expression. Our results demonstrate that KCNQ5 channels contribute to the afterhyperpolarization currents in hippocampus in a cell type-specific manner.

Ictal activity induced by group I metabotropic glutamate receptor activation and loss of afterhyperpolarizations

Exposure to the group I metabotropic glutamate receptor (mGluR) agonist dihydroxyphenylglycine (DHPG) produces long-lasting changes in network excitability and epileptiform activity in the CA3 region of rat hippocampal slices that continues in the absence of the agonist and includes both interictal and more prolonged ictal-like activity. We evaluated the afterhyperpolarization (AHP) that follows repetitive neuronal firing in neurons exposed to DHPG and related the change in the AHP to the pattern of epileptiform activity. In contrast to neurons from control slices that had a robust AHP following neuronal depolarization and action potential generation, neurons that had been exposed to DHPG displayed a minimal AHP following depolarization. Whole-cell voltage-clamp recordings showed a small outward or transient inward current following a depolarizing pulse in neurons from slices that had been exposed to DHPG while control neurons had a long-lasting outward current. In slices that demonstrated ictal patterns after exposure to DHPG, bath application of 1-ethyl-2-benzimidazolinone (1-EBIO, 1 mM) or 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one (DCEBIO, 100 microM) which enhance the AHP, suppressed ictal discharges. Whole-cell voltage-clamp recordings demonstrated the return of the medium and slow AHP current in neurons that had transiently been exposed to DHPG when 1-EBIO or DCEBIO was bath-applied. Co-application of either 1-EBIO or DCEBIO with DHPG blocked the induction of epileptiform activity. Transient DHPG exposure caused a long-term suppression of the AHP and ictal patterns of epileptiform activity. 1-EBIO or DCEBIO which re-established both the medium and slow AHP suppressed ictal discharges. These results support the hypothesis that the loss of the AHP contributes to the generation of ictal activity after transient DHPG exposure.

Mutation in the myelin proteolipid protein gene alters BK and SK channel function in the caudal medulla

Proteolipid protein (Plp) gene mutation in rodents causes severe CNS dysmyelination, early death, and lethal hypoxic ventilatory depression (Miller et al., 2004). To determine if Plp mutation alters neuronal function critical for control of breathing, the nucleus tractus solitarii (nTS) of four rodent strains were studied: myelin deficient rats (MD), myelin synthesis deficient (Plp(msd)), and Plp(null) mice, as well as shiverer (Mbp(shi)) mice, a myelin basic protein mutant. Current-voltage relationships were analyzed using whole-cell patch-clamp in 300 microm brainstem slices. Voltage steps were applied, and inward and outward currents quantified. MD, Plp(msd), and Plp(null), but not Mbp(shi) neurons exhibited reduced outward current in nTS at P21. Apamin blockade of SK calcium-dependent currents and iberiotoxin blockade of BK calcium-dependent currents in the P21 MD rat demonstrated reduced outward current due to dysfunction of these channels. These results provide evidence that Plp mutation specifically alters neuronal excitability through calcium-dependent potassium channels in nTS.

Lack of kainic acid-induced gamma oscillations predicts subsequent CA1 excitotoxic cell death

Gamma oscillations are a prominent feature of hippocampal network activity, but their functional role remains debated, ranging from mere epiphenomena to being crucial for information processing. Similarly, persistent gamma oscillations sometimes appear prior to epileptic discharges in patients with mesial temporal sclerosis. However, the significance of this activity in hippocampal excitotoxicity is unclear. We assessed the relationship between kainic acid (KA)-induced gamma oscillations and excitotoxicity in genetically engineered mice in which N-methyl-D-aspartic acid receptor deletion was confined to CA3 pyramidal cells. Mutants showed reduced CA3 pyramidal cell firing and augmented sharp wave-ripple activity, resulting in higher susceptibility to KA-induced seizures, and leading to strikingly selective neurodegeneration in the CA1 subfield. Interestingly, the increase in KA-induced gamma-aminobutyric acid (GABA) levels, and the persistent 30-50-Hz gamma oscillations, both of which were observed in control mice prior to the first seizure discharge, were abolished in the mutants. Consequently, on subsequent days, mutants manifested prolonged epileptiform activity and massive neurodegeneration of CA1 cells, including local GABAergic neurons. Remarkably, pretreatment with the potassium channel blocker alpha-dendrotoxin increased GABA levels, restored gamma oscillations, and prevented CA1 degeneration in the mutants. These results demonstrate that the emergence of low-frequency gamma oscillations predicts increased resistance to KA-induced excitotoxicity, raising the possibility that gamma oscillations may have potential prognostic value in the treatment of epilepsy.

Molecular and cellular basis of small--and intermediate-conductance, calcium-activated potassium channel function in the brain

Small conductance calcium-activated potassium (SK or K_Ca2) channels link intracellular calcium transients to membrane potential changes. SK channel subtypes present different pharmacology and distribution in the nervous system. The selective blocker apamin, SK enhancers and mice lacking specific SK channel subunits have revealed multifaceted functions of these channels in neurons, glia and cerebral blood vessels. SK channels regulate neuronal firing by contributing to the afterhyperpolarization following action potentials and mediating I_AHP, and partake in a calcium-mediated feedback loop with NMDA receptors, controlling the threshold for induction of hippocampal long-term potentiation. The function of distinct SK channel subtypes in different neurons often results from their specific coupling to different calcium sources. The prominent role of SK channels in the modulation of excitability and synaptic function of limbic, dopaminergic and cerebellar neurons hints at their possible involvement in neuronal dysfunction, either as part of the causal mechanism or as potential therapeutic targets.

Signaling mechanisms underlying group I mGluR-induced persistent AHP suppression in CA3 hippocampal neurons

Activation of group I metabotropic glutamate receptors (mGluRs) leads to a concerted modulation of spike afterpotentials in guinea pig hippocampal neurons including a suppression of both medium and slow afterhyperpolarizations (AHPs). Suppression of AHPs may be long-lasting, in that it persists after washout of the agonist. Here, we show that persistent AHP suppression differs from short-term, transient suppression in that distinct and additional signaling processes are required to render the suppression persistent. Persistent AHP suppression followed DHPG application for 30 min, but not DHPG application for 5 min. Persistent AHP suppression was temperature dependent, occurring at 30-31 degrees C, but not at 25-26 degrees C. Preincubation of slices in inhibitors of protein synthesis (cycloheximide or anisomycin) prevented the persistent suppression of AHPs by DHPG. Similarly, preincubation of slices in an inhibitor of p38 MAP kinase (SB 203580) prevented persistent AHP suppression. In contrast, a blocker of p42/44 MAP kinase activation (PD 98059) had no effect on persistent AHP suppression. Additionally, we show that the mGluR5 antagonist MPEP, but not the mGluR1 antagonist LY 367385, prevented DHPG-induced persistent AHP suppression. Thus persistent AHP suppression by DHPG in hippocampal neurons requires activation of mGluR5. In addition, activation of p38 MAP kinase signaling and protein synthesis are required to impart persistence to the DHPG-activated AHP suppression.

The slow afterhyperpolarization modulates high pH-induced changes in the excitability of rat CA1 pyramidal neurons

Extra- and intracellular recordings from the CA1 region of rat hippocampal slices were employed to examine the role of the slow afterhyperpolarization (sAHP) in modulating the increases in neuronal excitability observed on increasing extracellular pH (pHo) from 7.4 to 7.7. In the majority of experiments, an antidromic conditioning stimulus applied in the presence of D(-)-2-amino-5-phosphonopentanoic acid (D-APV), 6-cyano-7-nitroquinoxaline-2,3-dione disodium salt (CNQX) and bicuculline was employed to elicit a sAHP, and an antidromic test stimulus was applied during the sAHP. At pHo 7.4, a single conditioning stimulus elicited an action potential followed by a sAHP, which in turn inhibited the response to the test stimulus compared with the conditioning stimulus. Increasing the number of action potentials in the conditioning stimulus augmented the sAHP and further inhibited the test response, whereas isoproterenol inhibited the sAHP and prevented the relative inhibition of the test response. At pHo 7.7, a single conditioning stimulus elicited a burst of action potentials followed by a large sAHP, which in turn prevented the test stimulus from eliciting a burst of action potentials and, in extracellular recordings, further increased the inhibition of the test response. The latter effect did not solely reflect a high pHo-induced increase in the conditioning response (and, thus, the subsequent sAHP), but rather involved a more direct effect of high pHo to augment the sAHP. The results indicate that increasing pHo increases the excitability of CA1 neurons to an initial stimulus; however, a high pHo-dependent increase in the sAHP evoked by the initial stimulus limits the response to subsequent stimuli.