Perturbed chloride homeostasis and GABAergic signaling in human temporal lobe epilepsy

Directional spread of activity in synaptic networks of the human lateral amygdala

Spontaneous epileptiform activity has previously been observed in lateral amygdala (LA) slices derived from patients with intractable-temporal lobe epilepsy. The present study aimed to characterize intranuclear LA synaptic connectivity and to test the hypothesis that differences in the spread of flow of neuronal activity may relate to spontaneous epileptiform activity occurrence. Electrical activity was evoked through electrical microstimulation in acute human brain slices containing the LA, signals were recorded as local field potentials combined with fast optical imaging of voltage-sensitive dye fluorescence. Sites of stimulation and recording were systematically varied. Following recordings, slices were anatomically reconstructed using two-dimensional unitary slices as a reference for coronal and parasagittal planes. Local spatial patterns and spread of activity were assessed by incorporating the coordinates of electrical and optical recording sites into the respective unitary slice. A preferential directional spread of evoked electrical signals was observed from ventral to dorsal, rostral to caudal and medial to lateral regions in the LA. No differences in spread of evoked activity were observed between spontaneously and non-spontaneously active LA slices, i.e. basic properties of evoked synaptic responses were similar in the two functional types of LA slices, including input-output relationship, and paired-pulse depression. These results indicate a directed propagation of synaptic signals within the human LA in spontaneously active epileptic slices. We suggest that the lack of differences in local and in systemic information processing has to be found in confined epileptiform circuits within the amygdala likely involving well-known "epileptic neurons".

KCC2 downregulation facilitates epileptic seizures

GABAA receptor-mediated inhibition depends on the maintenance of low level intracellular [Cl-] concentration, which in adult depends on neuron specific K+-Cl- cotransporter-2 (KCC2). Previous studies have shown that KCC2 was downregulated in both epileptic patients and various epileptic animal models. However, the temporal relationship between KCC2 downregulation and seizure induction is unclear yet. In this study, we explored the temporal relationship and the influence of KCC2 downregulation on seizure induction. Significant downregulation of plasma membrane KCC2 was directly associated with severe (Racine Score III and above) behavioral seizures in vivo, and occurred before epileptiform bursting activities in vitro induced by convulsant. Overexpression of KCC2 using KCC2 plasmid effectively enhanced resistance to convulsant-induced epileptiform bursting activities in vitro. Furthermore, suppression of membrane KCC2 expression, using shRNAKCC2 plasmid in vitro and shRNAKCC2 containing lentivirus in vivo, induced spontaneous epileptiform bursting activities in vitro and Racine III seizure behaviors accompanied by epileptic EEG in vivo. Our findings novelly demonstrated that altered expression of KCC2 is not the consequence of seizure occurrence but likely is the contributing factor.

Contribution of Resting Conductance, GABAA-Receptor Mediated Miniature Synaptic Currents and Neurosteroid to Chloride Homeostasis in Central Neurons

Maintenance of a low intraneuronal Cl- concentration, [Cl-]i, is critical for inhibition in the CNS. Here, the contribution of passive, conductive Cl- flux to recovery of [Cl-]i after a high load was analyzed in mature central neurons from rat. A novel method for quantifying the resting Cl- conductance, important for [Cl-]i recovery, was developed and the possible contribution of GABAA and glycine receptors and of ClC-2 channels to this conductance was analyzed. The hypothesis that spontaneous, action potential-independent release of GABA is important for [Cl-]i recovery was tested. [Cl-]i was examined by gramicidin-perforated patch recordings in medial preoptic neurons. Cells were loaded with Cl- by combining GABA or glycine application with a depolarized voltage, and the time course of [Cl-]i was followed by measurements of the Cl- equilibrium potential, as obtained from the current recorded during voltage ramps combined with GABA or glycine application. The results show that passive Cl- flux contributes significantly, in the same order of magnitude as does K+-Cl- cotransporter 2 (KCC2), to [Cl-]i recovery and that Cl- conductance accounts for ∼ 6% of the total resting conductance. A major fraction of this resting Cl- conductance is picrotoxin (PTX)-sensitive and likely due to open GABAA receptors, but ClC-2 channels do not contribute. The results also show that when the decay of GABAA receptor-mediated miniature postsynaptic currents (minis) is slowed by the neurosteroid allopregnanolone, such minis may significantly quicken [Cl-]i recovery, suggesting a possible steroid-regulated role for minis in the control of Cl- homeostasis.

Neonatal Propofol and Etomidate Exposure Enhance Inhibitory Synaptic Transmission in Hippocampal Cornus Ammonis 1 Pyramidal Neurons

Background:

Propofol and etomidate are the most important intravenous general anesthetics in the current clinical use and that mediate gamma-aminobutyric acid's (GABAergic) synaptic transmission. However, their long-term effects on GABAergic synaptic transmission induced by neonatal propofol or etomidate exposure remain unclear. We investigated the long-term GABAergic neurotransmission alterations, following neonatal propofol and etomidate administration.

Methods:

Sprague-Dawley rat pups at postnatal days 4–6 were underwent 6-h-long propofol-induced or 5-h-long etomidate-induced anesthesia. We performed whole-cell patch-clamp recording from pyramidal cells in the cornus ammonis 1 area of acute hippocampal slices of postnatal 80–90 days. Spontaneous and miniature inhibitory GABAergic currents (spontaneous inhibitory postsynaptic currents [sIPSCs] and miniature inhibitory postsynaptic currents [mIPSCs]) and their kinetic characters were measured. The glutamatergic tonic effect on inhibitory transmission and the effect of bumetanide on neonatal propofol exposure were also examined.

Results:

Neonatal propofol exposure significantly increased the frequency of mIPSCs (from 1.87 ± 0.35 Hz to 3.43 ± 0.51 Hz, P < 0.05) and did not affect the amplitude of mIPSCs and sIPSCs. Both propofol and etomidate slowed the decay time of mIPSCs kinetics (168.39 ± 27.91 ms and 267.02 ± 100.08 ms vs. 68.18 ± 12.43 ms; P < 0.05). Bumetanide significantly blocked the frequency increase and reversed the kinetic alteration of mIPSCs induced by neonatal propofol exposure (3.01 ± 0.45 Hz and 94.30 ± 32.56 ms).

Conclusions:

Neonatal propofol and etomidate exposure has long-term effects on inhibitory GABAergic transmission. Propofol might act at pre- and post-synaptic GABA receptor A (GABA_A) receptors within GABAergic synapses and impairs the glutamatergic tonic input to GABAergic synapses; etomidate might act at the postsynaptic site.

Depolarizing γ-aminobutyric acid contributes to glutamatergic network rewiring in epilepsy

OBJECTIVE

Rewiring of excitatory glutamatergic neuronal circuits is a major abnormality in epilepsy. Besides the rewiring of excitatory circuits, an abnormal depolarizing γ-aminobutyric acidergic (GABAergic) drive has been hypothesized to participate in the epileptogenic processes. However, a remaining clinically relevant question is whether early post-status epilepticus (SE) evoked chloride dysregulation is important for the remodeling of aberrant glutamatergic neuronal circuits.

METHODS

Osmotic minipumps were used to infuse intracerebrally a specific inhibitor of depolarizing GABAergic transmission as well as a functionally blocking antibody toward the pan-neurotrophin receptor p75 (p75NTR ). The compounds were infused between 2 and 5 days after pilocarpine-induced SE. Immunohistochemistry for NKCC1, KCC2, and ectopic recurrent mossy fiber (rMF) sprouting as well as telemetric electroencephalographic and electrophysiological recordings were performed at day 5 and 2 months post-SE.

RESULTS

Blockade of NKCC1 after SE with the specific inhibitor bumetanide restored NKCC1 and KCC2 expression, normalized chloride homeostasis, and significantly reduced the glutamatergic rMF sprouting within the dentate gyrus. This mechanism partially involves p75NTR signaling, as bumetanide application reduced SE-induced p75NTR expression and functional blockade of p75NTR decreased rMF sprouting. The early transient (3 days) post-SE infusion of bumetanide reduced rMF sprouting and recurrent seizures in the chronic epileptic phase.

INTERPRETATION

Our findings show that early post-SE abnormal depolarizing GABA and p75NTR signaling fosters a long-lasting rearrangement of glutamatergic network that contributes to the epileptogenic process. This finding defines promising and novel targets to constrain reactive glutamatergic network rewiring in adult epilepsy. Ann Neurol 2017;81:251-265.

Synaptic Reorganization of the Perisomatic Inhibitory Network in Hippocampi of Temporal Lobe Epileptic Patients

GABAergic inhibition and particularly perisomatic inhibition play a crucial role in controlling the firing properties of large principal cell populations. Furthermore, GABAergic network is a key element in the therapy attempting to reduce epileptic activity. Here, we present a review showing the synaptic changes of perisomatic inhibitory neuronal subtypes in the hippocampus of temporal lobe epileptic patients, including parvalbumin- (PV-) containing and cannabinoid Type 1 (CB1) receptor-expressing (and mainly cholecystokinin-positive) perisomatic inhibitory cells, known to control hippocampal synchronies. We have examined the synaptic input of principal cells in the dentate gyrus and Cornu Ammonis region in human control and epileptic hippocampi. Perisomatic inhibitory terminals establishing symmetric synapses were found to be sprouted in the dentate gyrus. Preservation of perisomatic input was found in the Cornu Ammonis 1 and Cornu Ammonis 2 regions, as long as pyramidal cells are present. Higher density of CB1-immunostained terminals was found in the epileptic hippocampus of sclerotic patients, especially in the dentate gyrus. We concluded that both types of (PV- and GABAergic CB1-containing) perisomatic inhibitory cells are mainly preserved or showed sprouting in epileptic samples. The enhanced perisomatic inhibitory signaling may increase principal cell synchronization and contribute to generation of epileptic seizures and interictal spikes.

Chloride co-transporters as possible therapeutic targets for stroke

Stroke is one of the major causes of death and disability worldwide. The major type of stroke is an ischaemic one, which is caused by a blockage that interrupts blood flow to the brain. There are currently very few pharmacological strategies to reduce the damage and social burden triggered by this pathology. The harm caused by the interruption of blood flow to the brain unfolds in the subsequent hours and days, so it is critical to identify new therapeutic targets that could reduce neuronal death associated with the spread of the damage. Here, we review some of the key molecular mechanisms involved in the progression of neuronal death, focusing on some new and promising studies. In particular, we focus on the potential of the chloride co-transporter (CCC) family of proteins, mediators of the GABAergic response, both during the early and later stages of stroke, to promote neuroprotection and recovery. Different studies of CCCs during the chronic and recovery phases post-stroke reveal the importance of timing when considering CCCs as potential neuroprotective and/or neuromodulator targets. The molecular regulatory mechanisms of the two main neuronal CCCs, NKCC1 and KCC2, are further discussed as an indirect approach for promoting neuroprotection and neurorehabilitation following an ischaemic insult. Finally, we mention the likely importance of combining different strategies in order to achieve more effective therapies.

Neuronal chloride and excitability - the big impact of small changes

Synaptic inhibition is a critical regulator of neuronal excitability, and in the mature brain the majority of synaptic inhibition is mediated by Cl^--permeable GABA_A receptors. Unlike other physiologically relevant ions, Cl^- is dynamically regulated, and alterations in the Cl^- gradient can have significant impact on neuronal excitability. Due to changes in the neuronal Cl^- concentration, GABAergic transmission can bidirectionally regulate the induction of excitatory synaptic plasticity and gate the closing of the critical period for monocular deprivation in visual cortex. GABAergic circuitry can also provide a powerful restraining mechanism for the spread of excitation, however Cl^- extrusion mechanisms can become overwhelmed and GABA can paradoxically contribute to pathological excitation such as the propagation of seizure activity.

The KCC2 Cotransporter and Human Epilepsy: Getting Excited About Inhibition

The cation-Cl- cotransporter KCC2, encoded by SLC12A5, is required for the emergence and maintenance of GABAergic fast synaptic inhibition in organisms across evolution. These findings have suggested that KCC2 deficiency might play a role in the pathogenesis human epilepsy, but this has only recently been substantiated by two lines of genetic evidence. The first is the discovery of heterozygous missense polymorphisms in SLC12A5, causing decreased KCC2-dependent Cl- extrusion capacity, in an Australian family with inherited febrile seizures and in a French-Canadian cohort with severe genetic generalized epilepsy (GGE). The second is the discovery of recessive loss-of-function mutations in SLC12A5 in patients with a severe, early-onset Mendelian disease termed "epilepsy of infancy with migrating focal seizures" (EIMFS). These findings collectively support the paradigm that precisely regulated KCC2 activity is required for synaptic inhibition in humans, and that genetically encoded impairment of KCC2 function, due to effects on gene dosage, intrinsic activity, or extrinsic regulation, can influence epilepsy phenotypes in patients. Accordingly, KCC2 could be a target for a novel antiepileptic strategies that aims to restore GABA inhibition by facilitating Cl- extrusion. Such drugs could have relevance for pharmaco-resistant epilepsies and possibly other diseases characterized by synaptic hyperexcitability, such as the spectrum autism disorders.

Pentylenetetrazole-induced seizures in developing rats prenatally exposed to valproic acid

BACKGROUND

Epidemiological evidence indicates epilepsy is more common in patients with autism spectrum disorders (ASD) (20-25%) than in the general population. The aim of this project was to analyze seizure susceptibility in developing rats prenatally exposed to valproic acid (VPA) as autism model.

METHODS

Pregnant females were injected with VPA during the twelfth embryonic day. Seizures were induced in fourteen-days-old rat pups using two models of convulsions: pentylenetetrazole (PTZ) and lithium-pilocarpine (Li-Pilo).

RESULTS

Two subgroups with different PTZ-induced seizure susceptibility in rats exposed to VPA were found: a high susceptibility (VPA+) (28/42, seizure severity 5) and a low susceptibility (VPA-) (14/42, seizure severity 2). The VPA+ subgroup exhibited an increased duration of the generalized tonic-clonic seizure (GTCS; 45 ± 2.7 min), a higher number of rats showed several GTCS (14/28) and developed status epilepticus (SE) after PTZ injection (19/27) compared with control animals (36.6 ± 1.9 min; 10/39; 15/39, respectively). No differences in seizure severity, latency or duration of SE induced by Li-Pilo were detected between VPA and control animals.

DISCUSSION

Prenatal VPA modifies the susceptibility to PTZ-induced seizures in developing rats, which may be linked to an alteration in the GABAergic transmission. These findings contribute to a better understanding of the comorbidity between autism and epilepsy.

Cl-out is a novel cooperative optogenetic tool for extruding chloride from neurons

Chloride regulation affects brain function in many ways, for instance, by dictating the GABAergic reversal potential, and thereby influencing neuronal excitability and spike timing. Consistent with this, there is increasing evidence implicating chloride in a range of neurological conditions. Investigations about these conditions, though, are made difficult by the limited range of tools available to manipulate chloride levels. In particular, there has been no way to actively remove chloride from neurons; we now describe an optogenetic strategy, ‘Cl-out', to do exactly this. Cl-out achieves its effect by the cooperative action of two different component opsins: the proton pump, Archaerhodopsin and a chloride channel opsin. The removal of chloride happens when both are activated together, using Archaerhodopsin as an optical voltage clamp to provide the driving force for chloride removal through the concurrently opened, chloride channels. We further show that this novel optogenetic strategy can reverse an in vitro epileptogenic phenotype.

Chloride regulation is important for setting GABAergic reversal potential, though tools to manipulate chloride levels are limited. Here, the authors combine Archaerhodopsin with a chloride channel opsin to generate an optogenetic chloride extrusion strategy, ‘Cl-out', which they demonstrate in hippocampal slices.

Reduced Efficacy of the KCC2 Cotransporter Promotes Epileptic Oscillations in a Subiculum Network Model

Pharmacoresistant epilepsy is a chronic neurological condition in which a basal brain hyperexcitability results in paroxysmal hypersynchronous neuronal discharges. Human temporal lobe epilepsy has been associated with dysfunction or loss of the potassium-chloride cotransporter KCC2 in a subset of pyramidal cells in the subiculum, a key structure generating epileptic activities. KCC2 regulates intraneuronal chloride and extracellular potassium levels by extruding both ions. Absence of effective KCC2 may alter the dynamics of chloride and potassium levels during repeated activation of GABAergic synapses due to interneuron activity. In turn, such GABAergic stress may itself affect Cl- regulation. Such changes in ionic homeostasis may switch GABAergic signaling from inhibitory to excitatory in affected pyramidal cells and also increase neuronal excitability. Possibly these changes contribute to periodic bursting in pyramidal cells, an essential component in the onset of ictal epileptic events. We tested this hypothesis with a computational model of a subicular network with realistic connectivity. The pyramidal cell model explicitly incorporated the cotransporter KCC2 and its effects on the internal/external chloride and potassium levels. Our network model suggested the loss of KCC2 in a critical number of pyramidal cells increased external potassium and intracellular chloride concentrations leading to seizure-like field potential oscillations. These oscillations included transient discharges leading to ictal-like field events with frequency spectra as in vitro Restoration of KCC2 function suppressed seizure activity and thus may present a useful therapeutic option. These simulations therefore suggest that reduced KCC2 cotransporter activity alone may underlie the generation of ictal discharges.

SIGNIFICANCE STATEMENT

Ion regulation in the brain is a major determinant of neural excitability. Intracellular chloride in neurons, a partial determinant of the resting potential and the inhibitory reversal potentials, is regulated together with extracellular potassium via kation chloride cotransporters. During temporal lobe epilepsy, the homeostatic regulation of intracellular chloride is impaired in pyramidal cells, yet how this dysregulation may lead to seizures has not been explored. Using a realistic neural network model describing ion mechanisms, we show that chloride homeostasis pathology provokes seizure activity analogous to recordings from epileptogenic brain tissue. We show that there is a critical percentage of pathological cells required for seizure initiation. Our model predicts that restoration of the chloride homeostasis in pyramidal cells could be a viable antiepileptic strategy.

Compromised GABAergic inhibition contributes to tumor-associated epilepsy

Glioblastoma Multiforme (GBM) is the most common form of primary brain tumor with 30-50% of patients presenting with epilepsy. These tumor-associated seizures are often resistant to traditional antiepileptic drug treatment and persist after tumor resection. This suggests that changes in the peritumoral tissue underpin epileptogenesis. It is known that glioma cells extrude pathological concentrations of glutamate which is thought to play a role in tumor progression and the development of epilepsy. Given that pathological concentrations of glutamate have been shown to dephosphorylate and downregulate the potassium chloride cotransporter KCC2, we hypothesized that glioma-induced alterations in KCC2 in the peritumoral region may play a role in tumor-associated epilepsy. Consistent with this hypothesis, we observe a decrease in total KCC2 expression and a dephosphorylation of KCC2 at residue Ser940 in a glioma model which exhibits hyperexcitability and the development of spontaneous seizures. To determine whether the reduction of KCC2 could potentially contribute to tumor-associated epilepsy, we generated mice with a focal knockdown of KCC2 by injecting AAV2-Cre-GFP into the cortex of floxed KCC2 mice. The AAV2-Cre-mediated knockdown of KCC2 was sufficient to induce the development of spontaneous seizures. Further, blocking NKCC1 with bumetanide to offset the loss of KCC2 reduced the seizure susceptibility in glioma-implanted mice. These findings support a mechanism of tumor-associated epilepsy involving downregulation of KCC2 in the peritumoral region leading to compromised GABAergic inhibition and suggest that modulating chloride homeostasis may be useful for seizure control.

Compromising KCC2 transporter activity enhances the development of continuous seizure activity

Impaired neuronal inhibition has long been associated with the increased probability of seizure occurrence and heightened seizure severity. Fast synaptic inhibition in the brain is primarily mediated by the type A γ-aminobutyric acid receptors (GABAARs), ligand-gated ion channels that can mediate Cl(-) influx resulting in membrane hyperpolarization and the restriction of neuronal firing. In most adult brain neurons, the K(+)/Cl(-) co-transporter-2 (KCC2) establishes hyperpolarizing GABAergic inhibition by maintaining low [Cl(-)]i. In this study, we sought to understand how decreased KCC2 transport function affects seizure event severity. We impaired KCC2 transport in the 0-Mg(2+) ACSF and 4-aminopyridine in vitro models of epileptiform activity in acute mouse brain slices. Experiments with the selective KCC2 inhibitor VU0463271 demonstrated that reduced KCC2 transport increased the duration of SLEs, resulting in non-terminating discharges of clonic-like activity. We also investigated slices obtained from the KCC2-Ser940Ala (S940A) point-mutant mouse, which has a mutation at a known functional phosphorylation site causing behavioral and cellular deficits under hyperexcitable conditions. We recorded from the entorhinal cortex of S940A mouse brain slices in both 0-Mg(2+) ACSF and 4-aminopyridine, and demonstrated that loss of the S940 residue increased the susceptibility of continuous clonic-like discharges, an in vitro form of status epilepticus. Our experiments revealed KCC2 transport activity is a critical factor in seizure event duration and mechanisms of termination. Our results highlight the need for therapeutic strategies that potentiate KCC2 transport function in order to decrease seizure event severity and prevent the development of status epilepticus.

Expanding Spectrum of Sodium Potassium Chloride Co-transporters in the Pathophysiology of Diseases

Sodium potassium chloride co-transporter (NKCC) belongs to cation-dependent chloride co-transporter family, whose activation allows the entry of Na(+), K(+) and 2Cl(-) inside the cell. It acts in concert with K(+) Cl(-) co-transporter (KCC), which extrudes K(+) and Cl(-) ions from cell. NKCC1 is widely distributed throughout the body, while NKCC2 is exclusively present in kidney. Protein kinase A, protein kinase C, Ste20-related proline-alanine-rich kinase, oxidative stress responsive kinases, With No K=lysine kinase and protein phosphatase type 1 control the phosphorylation/dephosphorylation of key threonine residues of in regulatory domain of NKCC1. The selective inhibitors of NKCC1 including bumetanide and furosemide are conventionally employed as diuretics. However, recent studies have indicated that NKCC1 may be involved in the pathophysiology of anxiety, cerebral ischemia, epilepsy, neuropathic pain, fragile X syndrome, autism and schizophrenia. The inhibitors of NKCC1 are shown to produce anxiolytic effects; attenuate cerebral ischemia-induced neuronal injury; produce antiepileptic effects and attenuate neuropathic pain. In the early developing brain, GABAA activation primarily produces excitatory actions due to high NKCC1/KCC2 ratio. However, as the development progresses, the ratio of NKCC1/KCC2 ratio reverses and there is switch in the polarity of GABAA actions and latter acquires the inhibitory actions. The recapitulation of developmental-like state during pathological state may be associated with increase in the expression and functioning of NKCC1, which decreases the strength of inhibitory GABAergic neurotransmission. The present review describes the expanding role and mechanism of NKCC1 in the pathophysiology of different diseases.

Chloride Regulation: A Dynamic Equilibrium Crucial for Synaptic Inhibition

Fast synaptic inhibition relies on tight regulation of intracellular Cl(-). Chloride dysregulation is implicated in several neurological and psychiatric disorders. Beyond mere disinhibition, the consequences of Cl(-) dysregulation are multifaceted and best understood in terms of a dynamical system involving complex interactions between multiple processes operating on many spatiotemporal scales. This dynamical perspective helps explain many unintuitive manifestations of Cl(-) dysregulation. Here we discuss how taking into account dynamical regulation of intracellular Cl(-) is important for understanding how synaptic inhibition fails, how to best detect that failure, why Cl(-) regulation is energetically so expensive, and the overall consequences for therapeutics.

Impaired neuronal KCC2 function by biallelic SLC12A5 mutations in migrating focal seizures and severe developmental delay

Epilepsy of infancy with migrating focal seizures (EIMFS) is one of the early-onset epileptic syndromes characterized by migrating polymorphous focal seizures. Whole exome sequencing (WES) in ten sporadic and one familial case of EIMFS revealed compound heterozygous SLC12A5 (encoding the neuronal K(+)-Cl(-) co-transporter KCC2) mutations in two families: c.279 + 1G > C causing skipping of exon 3 in the transcript (p.E50_Q93del) and c.572 C >T (p.A191V) in individuals 1 and 2, and c.967T > C (p.S323P) and c.1243 A > G (p.M415V) in individual 3. Another patient (individual 4) with migrating multifocal seizures and compound heterozygous mutations [c.953G > C (p.W318S) and c.2242_2244del (p.S748del)] was identified by searching WES data from 526 patients and SLC12A5-targeted resequencing data from 141 patients with infantile epilepsy. Gramicidin-perforated patch-clamp analysis demonstrated strongly suppressed Cl(-) extrusion function of E50_Q93del and M415V mutants, with mildly impaired function of A191V and S323P mutants. Cell surface expression levels of these KCC2 mutants were similar to wildtype KCC2. Heterologous expression of two KCC2 mutants, mimicking the patient status, produced a significantly greater intracellular Cl(-) level than with wildtype KCC2, but less than without KCC2. These data clearly demonstrated that partially disrupted neuronal Cl(-) extrusion, mediated by two types of differentially impaired KCC2 mutant in an individual, causes EIMFS.

Optogenetic study of networks in epilepsy

Currently, approximately 30% of patients with epilepsy do not have adequate seizure control. A greater understanding of the underlying mechanisms by which seizures start or propagate could lead to new therapeutic strategies. The recent development of optogenetics, because of its unprecedented precision for controlling activity within distinct neuronal populations, has revolutionized neuroscience, including epilepsy research. This Review discusses recent breakthroughs made with optogenetics in epilepsy research. These breakthroughs include new insights into the key roles that different cell types play in mediating seizures as well as in the development of epilepsy. Subsequently, we discuss how targeting different brain regions and cell populations has opened up the possibility of highly specific therapies that can stop seizures on demand. Finally, we illustrate how combining newly available neuroscience tools with whole-brain imaging techniques will allow researchers to understand better the spread of seizures on a network level. © 2016 The Authors. Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Reducing premature KCC2 expression rescues seizure susceptibility and spine morphology in atypical febrile seizures

Atypical febrile seizures are considered a risk factor for epilepsy onset and cognitive impairments later in life. Patients with temporal lobe epilepsy and a history of atypical febrile seizures often carry a cortical malformation. This association has led to the hypothesis that the presence of a cortical dysplasia exacerbates febrile seizures in infancy, in turn increasing the risk for neurological sequelae. The mechanisms linking these events are currently poorly understood. Potassium-chloride cotransporter KCC2 affects several aspects of neuronal circuit development and function, by modulating GABAergic transmission and excitatory synapse formation. Recent data suggest that KCC2 downregulation contributes to seizure generation in the epileptic adult brain, but its role in the developing brain is still controversial. In a rodent model of atypical febrile seizures, combining a cortical dysplasia and hyperthermia-induced seizures (LHS rats), we found a premature and sustained increase in KCC2 protein levels, accompanied by a negative shift of the reversal potential of GABA. In parallel, we observed a significant reduction in dendritic spine size and mEPSC amplitude in CA1 pyramidal neurons, accompanied by spatial memory deficits. To investigate whether KCC2 premature overexpression plays a role in seizure susceptibility and synaptic alterations, we reduced KCC2 expression selectively in hippocampal pyramidal neurons by in utero electroporation of shRNA. Remarkably, KCC2 shRNA-electroporated LHS rats show reduced hyperthermia-induced seizure susceptibility, while dendritic spine size deficits were rescued. Our findings demonstrate that KCC2 overexpression in a compromised developing brain increases febrile seizure susceptibility and contribute to dendritic spine alterations.

Antiepileptic drugs in development pipeline: A recent update

Epilepsy is the most common neurological disorder which significantly affects the quality of life and poses a health as well as economic burden on society. Epilepsy affects approximately 70 million people in the world. The present article reviews the scientific rationale, brief pathophysiology of epilepsy and newer antiepileptic drugs which are presently under clinical development. We have searched the investigational drugs using the key words ‘antiepileptic drugs,’ ‘epilepsy,’ ‘Phase I,’ ‘Phase II’ and ‘Phase III’ in American clinical trial registers (clinicaltrials.gov), the relevant published articles using National Library of Medicine's PubMed database, company websites and supplemented results with a manual search of cross-references and conference abstracts. This review provides a brief description about the antiepileptic drugs which are targeting different mechanisms and the clinical development status of these drugs. Besides the presence of old as well as new AEDs, still there is a need of new drugs or the modified version of old drugs in order to make affected people free of seizures. An optimistic approach should be used to translate the success of preclinical testing to clinical practice. There is an urgent need to improve animal models and to explore new targets with better understanding in order to develop the novel drugs with more efficacy and safety.

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This review primarily focused on antiepileptic drugs under clinical development.

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The more realistic approach is needed to discover and develop the novel antiepileptic drugs.

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Modification of conventional drugs or search of newer targets can lead to development of promising antiepileptic drugs.

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To develop more efficacious and safe drugs for treatment of epilepsy and refractory seizures

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There are a number of novel antiepileptic compounds which are under various stages of drug development.

Brain Derived Neurotrophic Factor Modification of Epileptiform Burst Discharges in a Temporal Lobe Epilepsy Model

INTRODUCTION

Transforming Growth Factor-Beta 1 (TGF-β1) is a pleiotropic cytokine with potent anti-inflammatory property, which has been considered as an essential risk factor in the inflammatory process of Ischemic Stroke (IS), by involving in the pathophysiological progression of hypertension, atherosclerosis, and lipid metabolisms. -509C/T TGF-β1 gene polymorphism has been found to be associated with the risk of IS. The aim of this meta-analysis was to provide a relatively comprehensive account of the relation between -509C/T gene polymorphisms of TGF-β1 and susceptibility to IS.

METHODS

Male Wistar rats were divided into sham (receiving phosphate buffered saline within dorsal hippocampus), pilocarpine (epileptic model of TLE), single injection BDNF (epileptic rats which received single high dose of BDBF within dorsal hippocampus), and multiple injections BDNF (epileptic rats which received BDNF in days 10, 11, 12, and 13 after induction of TLE) groups. Their electrocorticogram was recorded and amplitude, frequency, and duration of spikes were evaluated.

RESULTS

Amplitude and frequency of epileptiform burst discharges were significantly decreased in animals treated with BDNF compared to pilocarpine group.

CONCLUSION

Our findings suggested that BDNF may modulate the epileptic activity in the animal model of TLE. In addition, it may have therapeutic effect for epilepsy. More studies are necessary to clarify the exact mechanisms of BDNF effects.

Extracellular Potassium and Seizures: Excitation, Inhibition and the Role of Ih

Seizure activity leads to increases in extracellular potassium concentration ([K[Formula: see text]]o), which can result in changes in neuronal passive and active membrane properties as well as in population activities. In this study, we examined how extracellular potassium modulates seizure activities using an acute 4-AP induced seizure model in the neocortex, both in vivo and in vitro. Moderately elevated [K[Formula: see text]]o up to 9[Formula: see text]mM prolonged seizure durations and shortened interictal intervals as well as depolarized the neuronal resting membrane potential (RMP). However, when [K[Formula: see text]]o reached higher than 9[Formula: see text]mM, seizure like events (SLEs) were blocked and neurons went into a depolarization-blocked state. Spreading depression was never observed as the blockade of ictal events could be reversed within 1-2[Formula: see text]min after the raised [K[Formula: see text]]o was changed back to control levels. This concentration-dependent dual effect of [K[Formula: see text]]o was observed using in vivo and in vitro mouse brain preparations as well as in human neocortical tissue resected during epilepsy surgery. Blocking the Ih current, mediated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, modulated the elevated [K[Formula: see text]]o influence on SLEs by promoting the high [K[Formula: see text]]o inhibitory actions. These results demonstrate biphasic actions of raised [K[Formula: see text]]o on neuronal excitability and seizure activity.

Imaging and serum biomarkers reflecting the functional efficacy of extended erythropoietin treatment in rats following infantile traumatic brain injury

OBJECTIVE Traumatic brain injury (TBI) is a leading cause of death and severe morbidity for otherwise healthy full-term infants around the world. Currently, the primary treatment for infant TBI is supportive, as no targeted therapies exist to actively promote recovery. The developing infant brain, in particular, has a unique response to injury and the potential for repair, both of which vary with maturation. Targeted interventions and objective measures of therapeutic efficacy are needed in this special population. The authors hypothesized that MRI and serum biomarkers can be used to quantify outcomes following infantile TBI in a preclinical rat model and that the potential efficacy of the neuro-reparative agent erythropoietin (EPO) in promoting recovery can be tested using these biomarkers as surrogates for functional outcomes. METHODS With institutional approval, a controlled cortical impact (CCI) was delivered to postnatal Day (P)12 rats of both sexes (76 rats). On postinjury Day (PID)1, the 49 CCI rats designated for chronic studies were randomized to EPO (3000 U/kg/dose, CCI-EPO, 24 rats) or vehicle (CCI-veh, 25 rats) administered intraperitoneally on PID1-4, 6, and 8. Acute injury (PID3) was evaluated with an immunoassay of injured cortex and serum, and chronic injury (PID13-28) was evaluated with digitized gait analyses, MRI, and serum immunoassay. The CCI-veh and CCI-EPO rats were compared with shams (49 rats) primarily using 2-way ANOVA with Bonferroni post hoc correction. RESULTS Following CCI, there was 4.8% mortality and 55% of injured rats exhibited convulsions. Of the injured rats designated for chronic analyses, 8.1% developed leptomeningeal cyst-like lesions verified with MRI and were excluded from further study. On PID3, Western blot showed that EPO receptor expression was increased in the injured cortex (p = 0.008). These Western blots also showed elevated ipsilateral cortex calpain degradation products for αII-spectrin (αII-SDPs; p < 0.001), potassium chloride cotransporter 2 (KCC2-DPs; p = 0.037), and glial fibrillary acidic protein (GFAP-DPs; p = 0.002), as well as serum GFAP (serum GFAP-DPs; p = 0.001). In injured rats multiplex electrochemiluminescence analyses on PID3 revealed elevated serum tumor necrosis factor alpha (TNFα p = 0.01) and chemokine (CXC) ligand 1 (CXCL1). Chronically, that is, in PID13-16 CCI-veh rats, as compared with sham rats, gait deficits were demonstrated (p = 0.033) but then were reversed (p = 0.022) with EPO treatment. Diffusion tensor MRI of the ipsilateral and contralateral cortex and white matter in PID16-23 CCI-veh rats showed widespread injury and significant abnormalities of functional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD); MD, AD, and RD improved after EPO treatment. Chronically, P13-P28 CCI-veh rats also had elevated serum CXCL1 levels, which normalized in CCI-EPO rats. CONCLUSIONS Efficient translation of emerging neuro-reparative interventions dictates the use of age-appropriate preclinical models with human clinical trial-compatible biomarkers. In the present study, the authors showed that CCI produced chronic gait deficits in P12 rats that resolved with EPO treatment and that chronic imaging and serum biomarkers correlated with this improvement.

Developmental pharmacology of benzodiazepines under normal and pathological conditions

Benzodiazepines are allosteric agonists of GABAA receptors (GABAAR), pentameric ligand-gated Cl(-) channels, which serve both an important neurodevelopmental role but are also the principal inhibitory system in the brain. However, their subunit composition, channel properties, and function, as well as their region-specific expression patterns, change through development. These processes have been extensively studied in rodents and to some extent confirmed in higher species. Specifically, GABAARs acquire faster kinetics with age and their pharmacology changes rendering them more sensitive to drugs that have higher affinity for α1 subunit-containing GABAARs, such as benzodiazepines, but also, their inhibitory function becomes more potent as they shift from having depolarising to hyperpolarising responses due to a shift in Cl(-) gradient and cation chloride cotransporter expression. Concerns have been raised about possible pro-apoptotic and paradoxical effects of benzodiazepines in the neonatal normal rat brain, although it is unclear, as yet, whether this extends to brains exposed to seizures. Growing evidence indicates that the pharmacology and physiology of GABAARs may be altered in the brain of rats or humans with seizures or epilepsy, or different aetiologies that predispose to epilepsy. These changes follow different paths, depending on sex, age, region, cell type, aetiology, or time-point specific factors. Identification of dynamic biomarkers that could enable these changes in vivo to be monitored would greatly facilitate the selection of more effective agonists with fewer side effects.

Dynamic changes of depolarizing GABA in a computational model of epileptogenic brain: Insight for Dravet syndrome

Abnormal reemergence of depolarizing GABAA current during postnatal brain maturation may play a major role in paediatric epilepsies, Dravet syndrome (DS) being among the most severe. To study the impact of depolarizing GABA onto distinct patterns of EEG activity, we extended a neural mass model as follows: one sub-population of pyramidal cells was added as well as two sub-populations of interacting interneurons, perisomatic-projecting interneurons (basket-like) with fast synaptic kinetics GABAA (fast, I1) and dendritic-projecting interneurons with slow synaptic kinetics GABAA (slow, I2). Basket-like cells were interconnected to reproduce mutual inhibition mechanisms (I1➔I1). The firing rate of interneurons was adapted to mimic the genetic alteration of voltage gated sodium channels found in DS patients, SCN1A(+/-). We implemented the "dynamic depolarizing GABAA" mediated post-synaptic potential in the model, as some studies reported that the chloride reversal potential can switch from negative to more positive value depending on interneuron activity. The "shunting inhibition" promoted by GABAA receptor activation was also implemented. We found that increasing the proportion of depolarizing GABAA mediated IPSP (I1➔I1 and I1➔P) only (i.e., other parameters left unchanged) was sufficient to sequentially switch the EEG activity from background to (1) interictal isolated polymorphic epileptic spikes, (2) fast onset activity, (3) seizure like activity and (4) seizure termination. The interictal and ictal EEG patterns observed in 4 DS patients were reproduced by the model via tuning the amount of depolarizing GABAA postsynaptic potential. Finally, we implemented the modes of action of benzodiazepines and stiripentol, two drugs recommended in DS. Both drugs blocked seizure-like activity, partially and dose-dependently when applied separately, completely and with a synergic effect when combined, as has been observed in DS patients. This computational modeling study constitutes an innovative approach to better define the role of depolarizing GABA in infantile onset epilepsy and opens the way for new therapeutic hypotheses, especially in Dravet syndrome.

Antiepileptic drug treatment strategies in neonatal epilepsy

The highest risk of seizures across the lifespan is in the neonatal period. The enhanced excitability of the immature brain compared to the mature brain is related to the sequential development and expression of essential neurotransmitter signaling pathways. During the neonatal period there is an overabundance of excitatory receptors, and γ-amino-butyric acid (GABA) is potentially depolarizing, as opposed to hyperpolarizing in the older brain. While this enhanced excitability is required for regulation of activity-dependent synapse formation and refining of synaptic connections that are necessary for normal brain development, enhanced excitability predisposes the immature brain to seizures. In addition to being common, neonatal seizures are very difficult to treat; antiepileptic drugs used in older children and adults are less efficacious, and possibly detrimental to brain development. In an effort to target the unique features of neurotransmission in the neonate, bumetanide, an NKCC1 inhibitor which reduces intraneuronal Cl(-) and induces a significant shift of EGABA toward more hyperpolarized values in vitro, has been used to treat neonatal seizures. As the understanding of the pathophysiology of genetic forms of neonatal epilepsy has evolved there have been a few successful attempts to pharmacologically target the mutated protein. This approach, while promising, is challenging due to the findings that the genetic syndromes presenting in infancy demonstrate genetic heterogeneity in regard to both the mutated gene and its function.

Spines slow down dendritic chloride diffusion and affect short-term ionic plasticity of GABAergic inhibition

Cl⁻ plays a crucial role in neuronal function and synaptic inhibition. However, the impact of neuronal morphology on the diffusion and redistribution of intracellular Cl⁻ is not well understood. The role of spines in Cl⁻ diffusion along dendritic trees has not been addressed so far. Because measuring fast and spatially restricted Cl⁻ changes within dendrites is not yet technically possible, we used computational approaches to predict the effects of spines on Cl⁻ dynamics in morphologically complex dendrites. In all morphologies tested, including dendrites imaged by super-resolution STED microscopy in live brain tissue, spines slowed down longitudinal Cl⁻ diffusion along dendrites. This effect was robust and could be observed in both deterministic as well as stochastic simulations. Cl⁻ extrusion altered Cl⁻ diffusion to a much lesser extent than the presence of spines. The spine-dependent slowing of Cl⁻ diffusion affected the amount and spatial spread of changes in the GABA reversal potential thereby altering homosynaptic as well as heterosynaptic short-term ionic plasticity at GABAergic synapses in dendrites. Altogether, our results suggest a fundamental role of dendritic spines in shaping Cl⁻ diffusion, which could be of relevance in the context of pathological conditions where spine densities and neural excitability are perturbed.

Conundrums of high-frequency oscillations (80-800 Hz) in the epileptic brain

Pathological high-frequency oscillations (HFOs) (80-800 Hz) are considered biomarkers of epileptogenic tissue, but the underlying complex neuronal events are not well understood. Here, we identify and discuss several outstanding issues or conundrums in regards to the recording, analysis, and interpretation of HFOs in the epileptic brain to critically highlight what is known and what is not about these enigmatic events. High-frequency oscillations reflect a range of neuronal processes contributing to overlapping frequencies from the lower 80 Hz to the very fast spectral frequency bands. Given their complex neuronal nature, HFOs are extremely sensitive to recording conditions and analytical approaches. We provide a list of recommendations that could help to obtain comparable HFO signals in clinical and basic epilepsy research. Adopting basic standards will facilitate data sharing and interpretation that collectively will aid in understanding the role of HFOs in health and disease for translational purpose.

Effectiveness of ketogenic diet in pentylenetetrazol-induced and kindling rats as well as its potential mechanisms

The effects and mechanisms of ketogenic diets (KD) are unclear. In this study, we aimed to reveal electrographic and behavioral thresholds in responses to the KD in pentylenetetrazol (PTZ)-induced seizures, as well as its antiepileptogenic effects on PTZ-kindling rats. Additionally, we investigated the potential link between KD and expression levels of two cation chloride co-transporters: K(+)-Cl(-) co-transporter 2 (KCC2) and Na(+)-K(+)-Cl(-) co-transporter 1 (NKCC1). The KD group had significantly higher electrographic thresholds than the control (ND) group for the first spike-and-wave, subcontinuous spike-and-wave, high amplitude spike-and-wave, and polyspikes both in the cortex and hippocampus. Compared to the ND group, the KD group had higher behavioral thresholds for behavioral absence, first jerk, first overt myoclonia, and generalized seizures. In the PTZ-kindling model, KD not only prolonged the latency of myoclonic and clonic convulsions, but shortened clonic and generalized duration. In addition, KD rats had higher KCC2 protein expression before kindling, during myoclonic jerks, and GTCS compared with ND rats. There were no significant differences in NKCC1 protein levels between both groups following the four-week dietary intervention without PTZ exposure (before kindling). Moreover, KD inhibited the upregulation of NKCC1 expression induced by kindling in myoclonic jerks and GTCS. Therefore, our findings demonstrated that KD had antiepileptic features in elevating thresholds to most electrographic and behavioral seizure patterns in PTZ-induced rats, as well as delaying the progression and alleviating the severity of seizure in PTZ-kindling model. The antiepileptogenic effects of KD may be attributed to its regulatory properties on KCC2 and NKCC1 protein expression.

Mild KCC2 Hypofunction Causes Inconspicuous Chloride Dysregulation that Degrades Neural Coding

Disinhibition caused by Cl⁻ dysregulation is implicated in several neurological disorders. This form of disinhibition, which stems primarily from impaired Cl⁻ extrusion through the co-transporter KCC2, is typically identified by a depolarizing shift in GABA reversal potential (E_GABA). Here we show, using computer simulations, that intracellular [Cl⁻] exhibits exaggerated fluctuations during transient Cl⁻ loads and recovers more slowly to baseline when KCC2 level is even modestly reduced. Using information theory and signal detection theory, we show that increased Cl⁻ lability and settling time degrade neural coding. Importantly, these deleterious effects manifest after less KCC2 reduction than needed to produce the gross changes in E_GABA required for detection by most experiments, which assess KCC2 function under weak Cl⁻ load conditions. By demonstrating the existence and functional consequences of “occult” Cl⁻ dysregulation, these results suggest that modest KCC2 hypofunction plays a greater role in neurological disorders than previously believed.

GABAergic Synchronization in Epilepsy

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the cerebral cortex. GABAergic inhibition enables synchronization of activity in cortical networks, and contributes to generation of variety of brain activity patterns. In relation to epilepsy, GABAergic inhibition has been traditionally viewed as the main mechanism counterbalancing glutamatergic excitation and preventing hypersynchronous neuronal discharges. Indeed, deficits in GABAergic functions most commonly result in a hyperexcitable epileptic state, and many of the currently used antiepileptic drugs act through enhancement of GABAergic functions. However, a number of observations show that some epileptiform activity patterns involve synchronization by GABAergic mechanisms. These include two main categories that will be reviewed here: (1) synchronization of epileptiform oscillations based on GABAergic inhibition, and (2) epileptiform events driven by depolarizing and excitatory GABA. The conclusion is reached that GABAergic control of spike timing, either through inhibition or excitation under certain conditions, may work as a powerful synchronizing mechanism during epilepsy.

Dynamic Changes from Depolarizing to Hyperpolarizing GABAergic Actions during Giant Depolarizing Potentials in the Neonatal Rat Hippocampus

During development, GABA exerts depolarizing action on immature neurons and, acting in synergy with glutamate, drives giant depolarizing potentials (GDPs) in the hippocampal network. Yet, blockade of the GABA(A) receptors transforms GDPs to epileptiform discharges suggesting dual, both excitatory and inhibitory, actions of GABA in the immature hippocampal network. However, the nature of this dualism in early GABA actions is poorly understood. Here we characterized the dynamics of synaptic currents mediated by GABA(A) and glutamate receptors through an estimation of the changes in their conductance and driving forces in neonatal rat CA3 pyramidal cells during GDPs. We found that depolarizing GABAergic and glutamatergic currents act in synergy at the GDPs' onset. However, during the peak of the population discharge, the inward synaptic current was essentially mediated by glutamate receptors whereas GABA currents transiently switched their direction from depolarizing to hyperpolarizing as a result of neuronal depolarization above the GABA(A) reversal potential. Thus, the action of GABA on CA3 pyramidal cells dynamically changes during GDPs from excitatory at the GDPs' onset to inhibitory at the GDPs' peak. We propose that the dynamic changes in GABA actions occurring during GDPs enable GABAergic interneurons not only to initiate the discharge of pyramidal cells but also to control excitation in the recurrent CA3 network preventing epileptiform synchronization.

SIGNIFICANCE STATEMENT

During development GABA exerts a depolarizing action on immature neurons. However, at the network level the effects of GABA are complex involving both excitatory and inhibitory actions. Here we show that GABA actions critically depend on the network state. Although GABA depolarizes neurons at rest and at the onset of population bursts, it transiently becomes hyperpolarizing at the peak of the population bursts. These dynamic changes in GABA actions enable GABAergic interneurons not only to initiate the network discharge but also to control excitation to prevent epileptiform synchronization.

Epilepsy and brain tumors

Seizures are common in patients with brain tumors, and epilepsy can significantly impact patient quality of life. Therefore, a thorough understanding of rates and predictors of seizures, and the likelihood of seizure freedom after resection, is critical in the treatment of brain tumors. Among all tumor types, seizures are most common with glioneuronal tumors (70-80%), particularly in patients with frontotemporal or insular lesions. Seizures are also common in individuals with glioma, with the highest rates of epilepsy (60-75%) observed in patients with low-grade gliomas located in superficial cortical or insular regions. Approximately 20-50% of patients with meningioma and 20-35% of those with brain metastases also suffer from seizures. After tumor resection, approximately 60-90% are rendered seizure-free, with most favorable seizure outcomes seen in individuals with glioneuronal tumors. Gross total resection, earlier surgical therapy, and a lack of generalized seizures are common predictors of a favorable seizure outcome. With regard to anticonvulsant medication selection, evidence-based guidelines for the treatment of focal epilepsy should be followed, and individual patient factors should also be considered, including patient age, sex, organ dysfunction, comorbidity, or cotherapy. As concomitant chemotherapy commonly forms an essential part of glioma treatment, enzyme-inducing anticonvulsants should be avoided when possible. Seizure freedom is the ultimate goal in the treatment of brain tumor patients with epilepsy, given the adverse effects of seizures on quality of life.

Bumetanide reduces seizure progression and the development of pharmacoresistant status epilepticus

OBJECTIVE

We investigated the role of chloride homeostasis in seizure progression and development of pharmacoresistant status epilepticus (SE) by pharmacologically targeting the Na-K-Cl cotransporter (NKCC1) with bumetanide. We also investigated the ability of bumetanide to restore the efficacy of diazepam following SE.

METHODS

Kainic acid (KA)-induced SE in vivo and 0-Mg(2+) -induced seizure-like events (SLEs) in vitro were monitored using electroencephalography (EEG) recordings in freely moving adult male mice and extracellular field potential recordings in acute entorhinal cortex-hippocampus slices, respectively. The ability of bumetanide to decrease epileptiform activity and prevent the development of pharmacoresistance to diazepam following SE was evaluated.

RESULTS

Bumetanide treatment significantly reduced KA-induced ictal activity in vivo and SLEs in vitro. In addition, bumetanide restored the efficacy of diazepam in decreasing ictal activity following SE in both the in vivo and in vitro models.

SIGNIFICANCE

Our data demonstrate an anticonvulsant effect of bumetanide on KA-induced seizures in adult mice, suggesting a role for chloride plasticity in seizure progression. These data also demonstrate that the erosion of inhibition during seizure progression could underlie the development of pharmacoresistant SE and implicate a role for chloride plasticity in this process.

Treatment during a vulnerable developmental period rescues a genetic epilepsy

The nervous system is vulnerable to perturbations during specific developmental periods. Insults during such susceptible time windows can have long-term consequences, including the development of neurological diseases such as epilepsy. Here we report that a pharmacological intervention timed during a vulnerable neonatal period of cortical development prevents pathology in a genetic epilepsy model. By using mice with dysfunctional Kv7 voltage-gated K(+) channels, which are mutated in human neonatal epilepsy syndromes, we demonstrate the safety and efficacy of the sodium-potassium-chloride cotransporter NKCC1 antagonist bumetanide, which was administered during the first two postnatal weeks. In Kv7 current-deficient mice, which normally display epilepsy, hyperactivity and stereotypies as adults, transient bumetanide treatment normalized neonatal in vivo cortical network and hippocampal neuronal activity, prevented structural damage in the hippocampus and restored wild-type adult behavioral phenotypes. Furthermore, bumetanide treatment did not adversely affect control mice. These results suggest that in individuals with disease susceptibility, timing prophylactically safe interventions to specific windows during development may prevent or arrest disease progression.

On the contribution of KCC2 and carbonic anhydrase to two types of in vitro interictal discharge

GABAA receptor-mediated inhibition--which is due to Cl(-) and HCO3 (-) currents controlled by KCC2 and carbonic anhydrase activity, respectively--contributes to short- and long-lasting interictal events recorded from the CA3 region of hippocampus during application of 4-aminopyridine (4AP, 50 μM). Here, we employed field potential recordings in an in vitro brain slice preparation to establish the effects induced by the KCC2 blockers VU0240551 (10 μM) or bumetanide (50 μM) and by the carbonic anhydrase inhibitor acetazolamide (10 μM) on the two types of interictal events. We found that blocking KCC2 activity decreased the amplitude of the short-lasting events. In addition, this pharmacological procedure increased the interval of occurrence of the long-lasting events and reduced their amplitude. Blocking carbonic anhydrase activity with acetazolamide reduced the interval of occurrence and the duration of the short-lasting events while increasing their amplitude; acetazolamide also reduced the duration and amplitude of the long-lasting events. Finally, blocking either KCC2 or carbonic anhydrase activity increased the interval of occurrence of pharmacologically isolated synchronous GABAergic events and decreased their duration and amplitude. These data substantiate further the role of GABAA receptor-mediated signaling in driving neuronal populations toward hypersynchronous states presumably by increasing extracellular [K(+)].

Detrimental effect of post Status Epilepticus treatment with ROCK inhibitor Y-27632 in a pilocarpine model of temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is the most common type of epilepsy in adults where 20-30% of the patients are refractory to currently available anti-epileptic drugs. The RhoA/Rho-kinase signaling pathway activation has been involved in inflammatory responses, neurite outgrowth and neuronal death under pathological conditions such as epileptic insults. Acute preventive administration of ROCK inhibitor has been reported to have beneficial outcomes in Status Epilepticus (SE) epilepsy. In the present study, we evaluate the effect of chronic post SE treatment with the ROCK inhibitor Y-27632 in a rat pilocarpine model of TLE. We used chronic i.p. injections of Y-27632 for 5 days in 6 week old control rats or rats subjected to pilocarpine treatment as a model of TLE. Surprisingly, our findings demonstrate that a systemic administration of Y-27632 in pilocarpine-treated rats increases neuronal death in the CA3 region and ectopic recurrent mossy fiber sprouting (rMFS) in the dentate gyrus of the hippocampal formation. Interestingly, we found that chronic treatment with Y-27632 exacerbates the down-regulation and pathological distribution of the K(+)-Cl(-) cotransporter KCC2, thus providing a putative mechanism for post SE induced neuronal death. The involvement of astrogliosis in this mechanism appears to be intricate as ROCK inhibition reduces reactive astrogliosis in pilocarpine rats. Conversely, in control rats, chronic Y-27632 treatment increases astrogliosis. Together, our findings suggest that Y-27632 has a detrimental effect when chronically used post SE in a rat pilocarpine model of TLE.

Human brain slices for epilepsy research: Pitfalls, solutions and future challenges

Increasingly, neuroscientists are taking the opportunity to use live human tissue obtained from elective neurosurgical procedures for electrophysiological studies in vitro. Access to this valuable resource permits unique studies into the network dynamics that contribute to the generation of pathological electrical activity in the human epileptic brain. Whilst this approach has provided insights into the mechanistic features of electrophysiological patterns associated with human epilepsy, it is not without technical and methodological challenges. This review outlines the main difficulties associated with working with epileptic human brain slices from the point of collection, through the stages of preparation, storage and recording. Moreover, it outlines the limitations, in terms of the nature of epileptic activity that can be observed in such tissue, in particular, the rarity of spontaneous ictal discharges, we discuss manipulations that can be utilised to induce such activity. In addition to discussing conventional electrophysiological techniques that are routinely employed in epileptic human brain slices, we review how imaging and multielectrode array recordings could provide novel insights into the network dynamics of human epileptogenesis. Acute studies in human brain slices are ultimately limited by the lifetime of the tissue so overcoming this issue provides increased opportunity for information gain. We review the literature with respect to organotypic culture techniques that may hold the key to prolonging the viability of this material. A combination of long-term culture techniques, viral transduction approaches and electrophysiology in human brain slices promotes the possibility of large scale monitoring and manipulation of neuronal activity in epileptic microcircuits.

Enhanced expression of potassium-chloride cotransporter KCC2 in human temporal lobe epilepsy

Synaptic reorganization in the epileptic hippocampus involves altered excitatory and inhibitory transmission besides the rearrangement of dendritic spines, resulting in altered excitability, ion homeostasis, and cell swelling. The potassium-chloride cotransporter-2 (KCC2) is the main chloride extruder in neurons and hence will play a prominent role in determining the polarity of GABAA receptor-mediated chloride currents. In addition, KCC2 also interacts with the actin cytoskeleton which is critical for dendritic spine morphogenesis, and for the maintenance of glutamatergic synapses and cell volume. Using immunocytochemistry, we examined the cellular and subcellular levels of KCC2 in surgically removed hippocampi of temporal lobe epilepsy (TLE) patients and compared them to control human tissue. We also studied the distribution of KCC2 in a pilocarpine mouse model of epilepsy. An overall increase in KCC2-expression was found in epilepsy and confirmed by Western blots. The cellular and subcellular distributions in control mouse and human samples were largely similar; moreover, changes affecting KCC2-expression were also alike in chronic epileptic human and mouse hippocampi. At the subcellular level, we determined the neuronal elements exhibiting enhanced KCC2 expression. In epileptic tissue, staining became more intense in the immunopositive elements detected in control tissue, and profiles with subthreshold expression of KCC2 in control samples became labelled. Positive interneuron somata and dendrites were more numerous in epileptic hippocampi, despite severe interneuron loss. Whether the elevation of KCC2-expression is ultimately a pro- or anticonvulsive change, or both-behaving differently during ictal and interictal states in a context-dependent manner-remains to be established.

Hippocampal sharp wave-ripple: A cognitive biomarker for episodic memory and planning

Sharp wave ripples (SPW-Rs) represent the most synchronous population pattern in the mammalian brain. Their excitatory output affects a wide area of the cortex and several subcortical nuclei. SPW-Rs occur during "off-line" states of the brain, associated with consummatory behaviors and non-REM sleep, and are influenced by numerous neurotransmitters and neuromodulators. They arise from the excitatory recurrent system of the CA3 region and the SPW-induced excitation brings about a fast network oscillation (ripple) in CA1. The spike content of SPW-Rs is temporally and spatially coordinated by a consortium of interneurons to replay fragments of waking neuronal sequences in a compressed format. SPW-Rs assist in transferring this compressed hippocampal representation to distributed circuits to support memory consolidation; selective disruption of SPW-Rs interferes with memory. Recently acquired and pre-existing information are combined during SPW-R replay to influence decisions, plan actions and, potentially, allow for creative thoughts. In addition to the widely studied contribution to memory, SPW-Rs may also affect endocrine function via activation of hypothalamic circuits. Alteration of the physiological mechanisms supporting SPW-Rs leads to their pathological conversion, "p-ripples," which are a marker of epileptogenic tissue and can be observed in rodent models of schizophrenia and Alzheimer's Disease. Mechanisms for SPW-R genesis and function are discussed in this review.

Neto2-null mice have impaired GABAergic inhibition and are susceptible to seizures

Neto2 is a transmembrane protein that interacts with the neuron-specific K⁺-Cl⁻ cotransporter (KCC2) in the central nervous system (CNS). Efficient KCC2 transport is essential for setting the neuronal Cl⁻ gradient, which is required for fast GABAergic inhibition. Neto2 is required to maintain the normal abundance of KCC2 in neurons, and increases KCC2 function by binding to the active oligomeric form of this cotransporter. In the present study, we characterized GABAergic inhibition and KCC2-mediated neuronal chloride homeostasis in pyramidal neurons from adult hippocampal slices. Using gramicidin perforated patch clamp recordings we found that the reversal potential for GABA (E_GABA) was significantly depolarized. We also observed that surface levels of KCC2 and phosphorylation of KCC2 serine 940 (Ser940) were reduced in Neto2^−/− neurons compared to wild-type controls. To examine GABAergic inhibition we recorded spontaneous inhibitory postsynaptic currents (sIPSCs) and found that Neto2^−/− neurons had significant reductions in both their amplitude and frequency. Based on the critical role of Neto2 in regulating GABAergic inhibition we rationalized that Neto2-null mice would be prone to seizure activity. We found that Neto2-null mice demonstrated a decrease in the latency to pentylenetetrazole (PTZ)-induced seizures and an increase in seizure severity.

Prenatal Hypoxia-Ischemia Induces Abnormalities in CA3 Microstructure, Potassium Chloride Co-Transporter 2 Expression and Inhibitory Tone

Infants who suffer perinatal brain injury, including those with encephalopathy of prematurity, are prone to chronic neurological deficits, including epilepsy, cognitive impairment, and behavioral problems, such as anxiety, inattention, and poor social interaction. These deficits, especially in combination, pose the greatest hindrance to these children becoming independent adults. Cerebral function depends on adequate development of essential inhibitory neural circuits and the appropriate amount of excitation and inhibition at specific stages of maturation. Early neuronal synaptic responses to γ-amino butyric acid (GABA) are initially excitatory. During the early postnatal period, GABA_AR responses switch to inhibitory with the upregulation of potassium-chloride co-transporter KCC2. With extrusion of chloride by KCC2, the Cl⁻ reversal potential shifts and GABA and glycine responses become inhibitory. We hypothesized that prenatal hypoxic–ischemic brain injury chronically impairs the developmental upregulation of KCC2 that is essential for cerebral circuit formation. Following late gestation hypoxia–ischemia (HI), diffusion tensor imaging in juvenile rats shows poor microstructural integrity in the hippocampal CA3 subfield, with reduced fractional anisotropy and elevated radial diffusivity. The loss of microstructure correlates with early reduced KCC2 expression on NeuN-positive pyramidal neurons, and decreased monomeric and oligomeric KCC2 protein expression in the CA3 subfield. Together with decreased inhibitory post-synaptic currents during a critical window of development, we document for the first time that prenatal transient systemic HI in rats impairs hippocampal CA3 inhibitory tone. Failure of timely development of inhibitory tone likely contributes to a lower seizure threshold and impaired cognitive function in children who suffer perinatal brain injury.

Homeostatic regulation of KCC2 activity by the zinc receptor mZnR/GPR39 during seizures

The aim of this study was to investigate the role of the synaptic metabotropic zinc receptor mZnR/GPR39 in physiological adaptation to epileptic seizures. We previously demonstrated that synaptic activation of mZnR/GPR39 enhances inhibitory drive in the hippocampus by upregulating neuronal K(+)/Cl(-) co-transporter 2 (KCC2) activity. Here, we first show that mZnR/GPR39 knockout (KO) adult mice have dramatically enhanced susceptibility to seizures triggered by a single intraperitoneal injection of kainic acid, when compared to wild type (WT) littermates. Kainate also substantially enhances seizure-associated gamma oscillatory activity in juvenile mZnR/GPR39 KO hippocampal slices, a phenomenon that can be reproduced in WT tissue by extracellular Zn(2+) chelation. Importantly, kainate-induced synaptic Zn(2+) release enhances surface expression and transport activity of KCC2 in WT, but not mZnR/GPR39 KO hippocampal neurons. Kainate-dependent upregulation of KCC2 requires mZnR/GPR39 activation of the Gαq/phospholipase C/extracellular regulated kinase (ERK1/2) signaling cascade. We suggest that mZnR/GPR39-dependent upregulation of KCC2 activity provides homeostatic adaptation to an excitotoxic stimulus by increasing inhibition. As such, mZnR/GPR39 may provide a novel pharmacological target for dampening epileptic seizure activity.