Anomalous levels of Cl- transporters in the hippocampal subiculum from temporal lobe epilepsy patients make GABA excitatory

Cation-chloride cotransporters and the polarity of GABA signalling in mouse hippocampal parvalbumin interneurons

KEY POINTS

Cation-chloride cotransporters (CCCs) play a critical role in controlling the efficacy and polarity of GABA_A receptor (GABA_A R)-mediated transmission in the brain, yet their expression and function in GABAergic interneurons has been overlooked. We compared the polarity of GABA signalling and the function of CCCs in mouse hippocampal pyramidal neurons and parvalbumin-expressing interneurons. Under resting conditions, GABA_A R activation was mostly depolarizing and yet inhibitory in both cell types. KCC2 blockade further depolarized the reversal potential of GABA_A R-mediated currents often above action potential threshold. However, during repetitive GABA_A R activation, the postsynaptic response declined independently of the ion flux direction or KCC2 function, suggesting intracellular chloride build-up is not responsible for this form of plasticity. Our data demonstrate similar mechanisms of chloride regulation in mouse hippocampal pyramidal neurons and parvalbumin interneurons.

ABSTRACT

Transmembrane chloride gradients govern the efficacy and polarity of GABA signalling in neurons and are usually maintained by the activity of cation-chloride cotransporters, such as KCC2 and NKCC1. Whereas their role is well established in cortical principal neurons, it remains poorly documented in GABAergic interneurons. We used complementary electrophysiological approaches to compare the effects of GABA_A receptor (GABA_A R) activation in adult mouse hippocampal parvalbumin interneurons (PV-INs) and pyramidal cells (PCs). Loose cell-attached, tight-seal and gramicidin-perforated patch recordings all show GABA_A R-mediated transmission is slightly depolarizing and yet inhibitory in both PV-INs and PCs. Focal GABA uncaging in whole-cell recordings reveal that KCC2 and NKCC1 are functional in both PV-INs and PCs but differentially contribute to transmembrane chloride gradients in their soma and dendrites. Blocking KCC2 function depolarizes the reversal potential of GABA_A R-mediated currents in PV-INs and PCs, often beyond firing threshold, showing KCC2 is essential to maintain the inhibitory effect of GABA_A Rs. Finally, we show that repetitive 10 Hz activation of GABA_A Rs in both PV-INs and PCs leads to a progressive decline of the postsynaptic response independently of the ion flux direction or KCC2 function. This suggests intraneuronal chloride build-up may not predominantly contribute to activity-dependent plasticity of GABAergic synapses in this frequency range. Altogether our data demonstrate similar mechanisms of chloride regulation in mouse hippocampal PV-INs and PCs and suggest KCC2 downregulation in the pathology may affect the valence of GABA signalling in both cell types.

Role of NKCC1 and KCC2 in Epilepsy: From Expression to Function

As a main inhibitory neurotransmitter in the central nervous system, γ-aminobutyric acid (GABA) activates chloride-permeable GABAa receptors (GABAa Rs) and induces chloride ion (Cl⁻) flow, which relies on the intracellular chloride concentration ([Cl⁻]_i) of the postsynaptic neuron. The Na-K-2Cl cotransporter isoform 1 (NKCC1) and the K-Cl cotransporter isoform 2 (KCC2) are two main cation-chloride cotransporters (CCCs) that have been implicated in human epilepsy. NKCC1 and KCC2 reset [Cl⁻]_i by accumulating and extruding Cl⁻, respectively. Previous studies have shown that the profile of NKCC1 and KCC2 in neonatal neurons may reappear in mature neurons under some pathophysiological conditions, such as epilepsy. Although increasing studies focusing on the expression of NKCC1 and KCC2 have suggested that impaired chloride plasticity may be closely related to epilepsy, additional neuroelectrophysiological research aimed at studying the functions of NKCC1 and KCC2 are needed to understand the exact mechanism by which they induce epileptogenesis. In this review, we aim to briefly summarize the current researches surrounding the expression and function of NKCC1 and KCC2 in epileptogenesis and its implications on the treatment of epilepsy. We will also explore the potential for NKCC1 and KCC2 to be therapeutic targets for the development of novel antiepileptic drugs.

Impaired regulation of KCC2 phosphorylation leads to neuronal network dysfunction and neurodevelopmental pathology

KCC2 is a vital neuronal K+/Cl- cotransporter that is implicated in the etiology of numerous neurological diseases. In normal cells, KCC2 undergoes developmental dephosphorylation at Thr906 and Thr1007 We engineered mice with heterozygous phosphomimetic mutations T906E and T1007E (KCC2E/+ ) to prevent the normal developmental dephosphorylation of these sites. Immature (postnatal day 15) but not juvenile (postnatal day 30) KCC2E/+ mice exhibited altered GABAergic inhibition, an increased glutamate/GABA synaptic ratio, and greater susceptibility to seizure. KCC2E/+ mice also had abnormal ultrasonic vocalizations at postnatal days 10 to 12 and impaired social behavior at postnatal day 60. Postnatal bumetanide treatment restored network activity by postnatal day 15 but failed to restore social behavior by postnatal day 60. Our data indicate that posttranslational KCC2 regulation controls the GABAergic developmental sequence in vivo, indicating that deregulation of KCC2 could be a risk factor for the emergence of neurological pathology.

Pharmacological enhancement of KCC2 gene expression exerts therapeutic effects on human Rett syndrome neurons and Mecp2 mutant mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the methyl CpG binding protein 2 (MECP2) gene. There are currently no approved treatments for RTT. The expression of K+/Cl- cotransporter 2 (KCC2), a neuron-specific protein, has been found to be reduced in human RTT neurons and in RTT mouse models, suggesting that KCC2 might play a role in the pathophysiology of RTT. To develop neuron-based high-throughput screening (HTS) assays to identify chemical compounds that enhance the expression of the KCC2 gene, we report the generation of a robust high-throughput drug screening platform that allows for the rapid assessment of KCC2 gene expression in genome-edited human reporter neurons. From an unbiased screen of more than 900 small-molecule chemicals, we have identified a group of compounds that enhance KCC2 expression termed KCC2 expression-enhancing compounds (KEECs). The identified KEECs include U.S. Food and Drug Administration-approved drugs that are inhibitors of the fms-like tyrosine kinase 3 (FLT3) or glycogen synthase kinase 3β (GSK3β) pathways and activators of the sirtuin 1 (SIRT1) and transient receptor potential cation channel subfamily V member 1 (TRPV1) pathways. Treatment with hit compounds increased KCC2 expression in human wild-type (WT) and isogenic MECP2 mutant RTT neurons, and rescued electrophysiological and morphological abnormalities of RTT neurons. Injection of KEEC KW-2449 or piperine in Mecp2 mutant mice ameliorated disease-associated respiratory and locomotion phenotypes. The small-molecule compounds described in our study may have therapeutic effects not only in RTT but also in other neurological disorders involving dysregulation of KCC2.

Enhanced Glutamatergic Currents at Birth in Shank3 KO Mice

Autism spectrum disorders (ASD) are neurodevelopmental disorders induced by genetic and environmental factors. In our recent studies, we showed that the GABA developmental shifts during delivery and the second postnatal week are abolished in two rodent models of ASD. Maternal treatment around birth with bumetanide restored the GABA developmental sequence and attenuated the autism pathogenesis in offspring. Clinical trials conducted in parallel confirmed the usefulness of bumetanide treatment to attenuate the symptoms in children with ASD. Collectively, these observations suggest that an alteration of the GABA developmental sequence is a hallmark of ASD. Here, we investigated whether similar alterations occur in the Shank3 mouse model of ASD. We report that in CA3 pyramidal neurons, the driving force and inhibitory action of GABA are not different in naïve and Shank3-mutant age-matched animals at birth and during the second postnatal week. In contrast, the frequency of spontaneous excitatory postsynaptic currents is already enhanced at birth and persists through postnatal day 15. Therefore, in CA3 pyramidal neurons of Shank3-mutant mice, glutamatergic but not GABAergic activity is affected at early developmental stages, hence reflecting the heterogeneity of mechanisms underlying the pathogenesis of ASD.

Rare Diseases of Neurodevelopment: Maintain the Mystery or Use a Dazzling Tool for Investigation? The Case of Rett Syndrome

The investigation on neurotransmission function during normal and pathologic development is a pivotal component needed to understand the basic mechanisms underlying neurodevelopmental pathologies. To study these diseases, many animal models have been generated which allowed to face the limited availability of human tissues and, as a consequence, most of the electrophysiology has been performed on these models of diseases. On the other hand, the technique of membrane microtransplantation in Xenopus oocytes allows the study of human functional neurotransmitter receptors thanks to the use of tissues from autopsies or surgeries, even in quantities that would not permit other kinds of functional studies. In this short article, we intend to underline how this technique is well-fit for the study of rare diseases by characterizing the electrophysiological properties of GABA_A and AMPA receptors in Rett syndrome. For our purposes, we used both tissues from Rett syndrome patients and Mecp2-null mice, a well validated murine model of the same disease, in order to strengthen the solidity of our results through the comparison of the two. Our findings retrace previous results and, in the light of this, further argue in favor of Prof. Miledi's technique of membrane microtransplantation that proves itself a very useful tool of investigation in the field of neurophysiology. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Biophysical Modeling Suggests Optimal Drug Combinations for Improving the Efficacy of GABA Agonists after Traumatic Brain Injuries

Traumatic brain injuries (TBI) lead to dramatic changes in the surviving brain tissue. Altered ion concentrations, coupled with changes in the expression of membrane-spanning proteins, create a post-TBI brain state that can lead to further neuronal loss caused by secondary excitotoxicity. Several GABA receptor agonists have been tested in the search for neuroprotection immediately after an injury, with paradoxical results. These drugs not only fail to offer neuroprotection, but can also slow down functional recovery after TBI. Here, using computational modeling, we provide a biophysical hypothesis to explain these observations. We show that the accumulation of intracellular chloride ions caused by a transient upregulation of Na+-K+-2Cl- (NKCC1) co-transporters as observed following TBI, causes GABA receptor agonists to lead to excitation and depolarization block, rather than the expected hyperpolarization. The likelihood of prolonged, excitotoxic depolarization block is further exacerbated by the extremely high levels of extracellular potassium seen after TBI. Our modeling results predict that the neuroprotective efficacy of GABA receptor agonists can be substantially enhanced when they are combined with NKCC1 co-transporter inhibitors. This suggests a rational, biophysically principled method for identifying drug combinations for neuroprotection after TBI.

Off-Label Use of Bumetanide for Brain Disorders: An Overview

Bumetanide (BTN or BUM) is a FDA-approved potent loop diuretic (LD) that acts by antagonizing sodium-potassium-chloride (Na-K-Cl) cotransporters, NKCC1 (SLc12a2) and NKCC2. While NKCC1 is expressed both in the CNS and in systemic organs, NKCC2 is kidney-specific. The off-label use of BTN to modulate neuronal transmembrane Cl⁻ gradients by blocking NKCC1 in the CNS has now been tested as an anti-seizure agent and as an intervention for neurological disorders in pre-clinical studies with varying results. BTN safety and efficacy for its off-label use has also been tested in several clinical trials for neonates, children, adolescents, and adults. It failed to meet efficacy criteria for hypoxic-ischemic encephalopathy (HIE) neonatal seizures. In contrast, positive outcomes in temporal lobe epilepsy (TLE), autism, and schizophrenia trials have been attributed to BTN in studies evaluating its off-label use. NKCC1 is an electroneutral neuronal Cl⁻ importer and the dominance of NKCC1 function has been proposed as the common pathology for HIE seizures, TLE, autism, and schizophrenia. Therefore, the use of BTN to antagonize neuronal NKCC1 with the goal to lower internal Cl⁻ levels and promote GABAergic mediated hyperpolarization has been proposed. In this review, we summarize the data and results for pre-clinical and clinical studies that have tested off-label BTN interventions and report variable outcomes. We also compare the data underlying the developmental expression profile of NKCC1 and KCC2, highlight the limitations of BTN’s brain-availability and consider its actions on non-neuronal cells.

The Functional Role of Spontaneously Opening GABAA Receptors in Neural Transmission

Ionotropic type of γ-aminobutyric acid receptors (GABA_ARs) produce two forms of inhibitory signaling: phasic inhibition generated by rapid efflux of neurotransmitter GABA into the synaptic cleft with subsequent binding to GABA_ARs, and tonic inhibition generated by persistent activation of extrasynaptic and/or perisynaptic GABA_ARs by GABA continuously present in the extracellular space. It is widely accepted that phasic and tonic GABAergic inhibition is mediated by receptor groups of distinct subunit composition and modulated by different cytoplasmic mechanisms. Recently, however, it has been demonstrated that spontaneously opening GABA_ARs (s-GABA_ARs), which do not need GABA binding to enter an active state, make a significant input into tonic inhibitory signaling. Due to GABA-independent action mode, s-GABA_ARs promise new safer options for therapy of neural disorders (such as epilepsy) devoid of side effects connected to abnormal fluctuations of GABA concentration in the brain. However, despite the potentially important role of s-GABA_ARs in neural signaling, they still remain out of focus of neuroscience studies, to a large extent due to technical difficulties in their experimental research. Here, we summarize present data on s-GABA_ARs functional properties and experimental approaches that allow isolation of s-GABA_ARs effects from those of conventional (GABA-dependent) GABA_ARs.

KCC2 overexpression prevents the paradoxical seizure-promoting action of somatic inhibition

Although cortical interneurons are apparently well-placed to suppress seizures, several recent reports have highlighted a paradoxical role of perisomatic-targeting parvalbumin-positive (PV+) interneurons in ictogenesis. Here, we use an acute in vivo model of focal cortical seizures in awake behaving mice, together with closed-loop optogenetic manipulation of PV+ interneurons, to investigate their function during seizures. We show that photo-depolarization of PV+ interneurons rapidly switches from an anti-ictal to a pro-ictal effect within a few seconds of seizure initiation. The pro-ictal effect of delayed photostimulation of PV+ interneurons was not shared with dendrite-targeting somatostatin-positive (SOM+) interneurons. We also show that this switch can be prevented by overexpression of the neuronal potassium-chloride co-transporter KCC2 in principal cortical neurons. These results suggest that strategies aimed at improving the ability of principal neurons to maintain a trans-membrane chloride gradient in the face of excessive network activity can prevent interneurons from contributing to seizure perpetuation.

A Preliminary Study Evaluating the Safety and Efficacy of Bumetanide, an NKCC1 Inhibitor, in Patients with Drug-Resistant Epilepsy

BACKGROUND

Dysregulation of cation-chloride cotransporters NKCC1 and KCC2 expression was shown to be related to drug-resistant epilepsy. Previous studies suggested that bumetanide, an inhibitor of NKCC1, might have antiepileptic effects.

OBJECTIVE

The aim of this study was to investigate the safety and efficacy of bumetanide add-on therapy in patients with drug-resistant epilepsy and its relation to cation-chloride cotransporters NKCC1 and KCC2.

METHODS

We conducted an open-label, single-arm clinical trial in drug-resistant temporal lobe epilepsy (TLE) patients. This study consisted of three phases: pretreatment (3 months), titration (3 weeks), and active treatment (6 months). During the pretreatment phase, the dose of antiepileptic drugs was stabilized, and bumetanide was then added at an initial dose of 0.5 mg/day, increasing by 0.5 mg/week until a target dose of 2 mg/day was achieved. Bumetanide treatment was then continued for 6 months. Seizure frequency and adverse events were assessed at every monthly visit. Blood samples were collected from patients and 12 healthy controls were used for polymerase chain reaction and Western blot analyses. Primary clinical outcomes were drug safety and change in seizure frequency. Changes in NKCC1 and KCC2 expression were the non-clinical endpoints.

RESULTS

A total of 30 patients were enrolled, 27 of whom completed the study. The mean duration of epilepsy was 16.5 years. Median seizure frequency per month was 9 [interquartile range (IQR) 7-14.5] at baseline, 3.67 (IQR 1.84-6.17) at the first 3 months, and 2 (IQR 0.84-4.34) at the last 3 months (p < 0.001). Five adverse events were detected in six patients. The reported adverse events were anorexia in four patients, nausea and vomiting in two patients, and agitation, headache and increased seizure frequency in one patient each. The level of NKCC1 and KCC2 gene transcripts and KCC2 protein did not change significantly following treatment (p > 0.05); however, we observed a significant reduction in NKCC1 protein levels (p = 0.042).

CONCLUSIONS

Bumetanide might be an effective and relatively tolerable drug in patients with drug-resistant TLE. Downregulation of NKCC1 protein following bumetanide treatment may be responsible for its antiepileptic effects.

IRANIAN REGISTRY OF CLINICAL TRIALS IDENTIFIER

IRCT 201012115368N1.

Methylation of cation-chloride cotransporters NKCC1 and KCC2 in patients with juvenile myoclonic epilepsy

The etiology of juvenile myoclonic epilepsy (JME) is still unknown and the process of elaboration of multiple genetic mechanisms is ongoing. The aim of this study was to investigate the potential role of NKCC1 (SCL12A2) and KCC2 (SCL12A5) in JME by comparing their DNA methylation status in patients with JME versus healthy controls. Forty-nine patients with JME and 39 healthy individuals were compared for DNA methylation at the 5CpG islands. A total of 71 (81%) samples were found to have methylation in the NKCC1 gene, 36 (73%) from patients and 35 (90%) from healthy individuals. Out of the KCC2 samples, 50 (57%) were found to have methylation, 33 (67%) from patients and 17 (44%) from healthy individuals. In patients with JME, methylation of NKCC1 (73%) was lower than its methylation in the controls (90%) (p = 0.047). On the other hand, methylation of KCC2 in patients with JME (67%) was greater than the methylation in the controls (44%) (p = 0.022). Twenty-eight patients were treated with VPA and ongoing medications were not found to be associated with methylation (p > 0.05). In the present study, we determined significantly lower NKCC1 DNA methylation and significantly higher KCC2 DNA methylation levels in patients with JME compared with the healthy controls. This implies that NKCC1 expression can be higher and KCC2 expression can be reduced in affected people. Further studies that investigate the potential effect of DNA methylation mechanisms regulating gene expression on seizure activity and how they change JME network activity will be helpful.

Double-edged GABAergic synaptic transmission in seizures: The importance of chloride plasticity

GABAergic synaptic inhibition, which is a critical regulator of neuronal excitability, is closely involved in epilepsy. Interestingly, fast GABAergic transmission mediated by Cl- permeable GABAA receptors can bi-directionally exert both seizure-suppressing and seizure-promoting actions. Accumulating evidence suggests that chloride plasticity, the driving force of GABAA receptor-mediated synaptic transmission, contributes to the double-edged role of GABAergic synapses in seizures. Large amounts of Cl- influx can overwhelm Cl- extrusion during seizures not only in healthy tissue in a short-term "activity-dependent" manner, but also in chronic epilepsy in a long-term, irreversible "pathology-dependent" manner related to the dysfunction of two chloride transporters: the chloride importer NKCC1 and the chloride exporter KCC2. In this review, we address the importance of chloride plasticity for the "activity-dependent" and "pathology-dependent" mechanisms underlying epileptic events and provide possible directions for further research, which may be clinically important for the design of GABAergic synapse-targeted precise therapeutic interventions for epilepsy.

Insights into GABAAergic system alteration in Huntington's disease

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disease that is characterized by a triad of motor, psychiatric and cognitive impairments. There is still no effective therapy to delay or halt the disease progress. The striatum and cortex are two particularly affected brain regions that exhibit dense reciprocal excitatory glutamate and inhibitory gamma-amino butyric acid (GABA) connections. Imbalance between excitatory and inhibitory signalling is known to greatly affect motor and cognitive processes. Emerging evidence supports the hypothesis that disrupted GABAergic circuits underlie HD pathogenesis. In the present review, we focused on the multiple defects recently found in the GABAergic inhibitory system, including altered GABA level and synthesis, abnormal subunit composition and distribution of GABA_A receptors and aberrant GABA_A receptor-mediated signalling. In particular, the important role of cation–chloride cotransporters (i.e. NKCC1 and KCC2) is discussed. Recent studies also suggest that neuroinflammation contributes significantly to the abnormal GABAergic inhibition in HD. Thus, GABA_A receptors and cation–chloride cotransporters are potential therapeutic targets for HD. Given the limited availability of therapeutic treatments for HD, a better understanding of GABAergic dysfunction in HD could provide novel therapeutic opportunities.

Altered hippocampal interneuron activity precedes ictal onset

Although failure of GABAergic inhibition is a commonly hypothesized mechanism underlying seizure disorders, the series of events that precipitate a rapid shift from healthy to ictal activity remain unclear. Furthermore, the diversity of inhibitory interneuron populations poses a challenge for understanding local circuit interactions during seizure initiation. Using a combined optogenetic and electrophysiological approach, we examined the activity of identified mouse hippocampal interneuron classes during chemoconvulsant seizure induction in vivo. Surprisingly, synaptic inhibition from parvalbumin- (PV) and somatostatin-expressing (SST) interneurons remained intact throughout the preictal period and early ictal phase. However, these two sources of inhibition exhibited cell-type-specific differences in their preictal firing patterns and sensitivity to input. Our findings suggest that the onset of ictal activity is not associated with loss of firing by these interneurons or a failure of synaptic inhibition but is instead linked with disruptions of the respective roles these interneurons play in the hippocampal circuit.

Effect of Co-administration of Bumetanide and Phenobarbital on Seizure Attacks in Temporal Lobe Epilepsy

Introduction:

The resistance of temporal lobe epilepsy to classic drugs is thought to be due to disruption in the excitation/inhibition of this pathway. Two chloride transporters, NKCC1 and KCC2, are expressed differently for the excitatory state of Gamma-Amino Butyric Acid (GABA). The present study explored the effect of bumetanide as a selective NKCC1 inhibitor either alone or in combination with the phenobarbital in the pilocarpine model of epilepsy.

Methods:

An animal model of Status Epilepticus (SE) was induced with pilocarpine in Wistar male rats followed by phenobarbital and or bumetanide or saline administration for 45 days after the induction of SE by Intraperitoneal (IP) injection. The rats were monitored, their behavior was recorded, and after 24 hours they were sacrificed to study the expression of NKCC1 and KCC2 using real time PCR.

Results:

The data showed that the effects of a combination of bumetanide with phenobarbital on frequency rate and duration of seizure attack were more than those of the phenobarbital alone. In addition, in the bumetanide and combined treatment groups, NKCC1 expression decreased significantly, compared with untreated epileptic animals. A delayed decrement in NKCC1/KCC2 expression ratio after bumetanide application was also observed.

Conclusion:

The combination of bumetanide with phenobarbital increases the inhibition of SE and maximizes the potential of GABA signaling pathway, and can be considered as an effective therapeutic strategy in patients with epilepsy.

Potentiating KCC2 activity is sufficient to limit the onset and severity of seizures

The type 2 K+/Cl- cotransporter (KCC2) allows neurons to maintain low intracellular levels of Cl-, a prerequisite for efficient synaptic inhibition. Reductions in KCC2 activity are evident in epilepsy; however, whether these deficits directly contribute to the underlying pathophysiology remains controversial. To address this issue, we created knock-in mice in which threonines 906 and 1007 within KCC2 have been mutated to alanines (KCC2-T906A/T1007A), which prevents its phospho-dependent inactivation. The respective mice appeared normal and did not show any overt phenotypes, and basal neuronal excitability was unaffected. KCC2-T906A/T1007A mice exhibited increased basal neuronal Cl- extrusion, without altering total or plasma membrane accumulation of KCC2. Critically, activity-induced deficits in synaptic inhibition were reduced in the mutant mice. Consistent with this, enhanced KCC2 was sufficient to limit chemoconvulsant-induced epileptiform activity. Furthermore, this increase in KCC2 function mitigated induction of aberrant high-frequency activity during seizures, highlighting depolarizing GABA as a key contributor to the pathological neuronal synchronization seen in epilepsy. Thus, our results demonstrate that potentiating KCC2 represents a therapeutic strategy to alleviate seizures.

Cell Volume Control in Healthy Brain and Neuropathologies

Regulation of cellular volume is a critical homeostatic process that is intimately linked to ionic and osmotic balance in the brain tissue. Because the brain is encased in the rigid skull and has a very complex cellular architecture, even minute changes in the volume of extracellular and intracellular compartments have a very strong impact on tissue excitability and function. The failure of cell volume control is a major feature of several neuropathologies, such as hyponatremia, stroke, epilepsy, hyperammonemia, and others. There is strong evidence that such dysregulation, especially uncontrolled cell swelling, plays a major role in adverse pathological outcomes. To protect themselves, brain cells utilize a variety of mechanisms to maintain their optimal volume, primarily by releasing or taking in ions and small organic molecules through diverse volume-sensitive ion channels and transporters. In principle, the mechanisms of cell volume regulation are not unique to the brain and share many commonalities with other tissues. However, because ions and some organic osmolytes (e.g., major amino acid neurotransmitters) have a strong impact on neuronal excitability, cell volume regulation in the brain is a surprisingly treacherous process, which may cause more harm than good. This topical review covers the established and emerging information in this rapidly developing area of physiology.

Spontaneously opening GABAA receptors play a significant role in neuronal signal filtering and integration

Continuous (tonic) charge transfer through ionotropic receptors of γ-aminobutyric acid (GABAARs) is an important mechanism of inhibitory signalling in the brain. The conventional view has been that tonic GABA-ergic inhibitory currents are mediated by low concentrations of ambient GABA. Recently, however, it was shown that the GABA-independent, spontaneously opening GABAARs (s-GABAARs), may contribute significantly to the tonic GABAAR current. One of the common approaches to temporal lobe epilepsy (TLE) therapy is an increase of GABA concentration in the cerebrospinal fluid to augment tonic current through GABAARs. Such an increase, however, generates multiple side effects, which impose significant limitations on the use of correspondent drugs. In contrast, activation/deactivation of s-GABAARs in a GABA-independent manner may provide a mechanism of regulation of tonic conductance without modification of extracellular GABA concentration, thus avoiding connected side effects. Although s-GABAARs have been detected in our earlier work, it is unclear whether they modulate neural signalling, or, due to their independence from the neurotransmitter, they provide just a stable background effect without much impact on neural crosstalk dynamics. Here, we focused on the causal relationship between s-GABAAR activity and signal integration in the rat's dentate gyrus granule cells to find that s-GABAARs play an important role in neural signal transduction. s-GABAARs shape the dynamics of phasic inhibitory responses, regulate the action potential generation machinery and control the coincidence detection window pertinent to excitatory input summation. Our results demonstrate that tonic inhibition delivered by s-GABAARs contributes to the key mechanisms that ensure implementation of neural signal filtering and integration, in a GABA-independent manner. This makes s-GABAAR a new and important actor in the regulation of long-term neural plasticity and a perspective target for TLE therapy.

Locally Reducing KCC2 Activity in the Hippocampus is Sufficient to Induce Temporal Lobe Epilepsy

Mesial temporal lobe epilepsy (mTLE) is the most common form of epilepsy, believed to arise in part from compromised GABAergic inhibition. The neuronal specific K+/Cl- co-transporter 2 (KCC2) is a critical determinant of the efficacy of GABAergic inhibition and deficits in its activity are observed in mTLE patients and animal models of epilepsy. To test if reductions of KCC2 activity directly contribute to the pathophysiology of mTLE, we locally ablated KCC2 expression in a subset of principal neurons within the adult hippocampus. Deletion of KCC2 resulted in compromised GABAergic inhibition and the development of spontaneous, recurrent generalized seizures. Moreover, local ablation of KCC2 activity resulted in hippocampal sclerosis, a key pathological change seen in mTLE. Collectively, our results demonstrate that local deficits in KCC2 activity within the hippocampus are sufficient to precipitate mTLE.

Different patterns of epileptiform-like activity are generated in the sclerotic hippocampus from patients with drug-resistant temporal lobe epilepsy

Human hippocampal slice preparations from patients with temporal lobe epilepsy (TLE) associated with hippocampal sclerosis (HS) are excellent material for the characterization of epileptiform-like activity. However, it is still unknown if hippocampal regions as cornu Ammonis (CA) 1, CA3 and CA4, generate population epileptiform-like activity. Here, we investigated epileptiform activities of the subiculum, CA1, CA2, CA3, CA4 (induced by elevation of extracellular potassium concentration) and the dentate gyrus (induced with hilar stimulation and elevation of potassium concentration) from sclerotic hippocampi of patients with drug-resistant TLE. Five types of epileptiform-like activity were observed: interictal-like events; periodic ictal spiking; seizure-like events; spreading depression-like events; tonic seizure-like events and no activity. Different susceptibilities to generate epileptiform activity among hippocampal regions were observed; the dentate gyrus was the most susceptible region followed by the subiculum, CA4, CA1, CA2 and CA3. The incidence of epileptiform activity pattern was associated with specific regions of the hippocampal formation. Moreover, it was observed that each region of the hippocampal formation exhibits frequency-specific ranges in each subfield of the sclerotic human tissue. In conclusion, this study demonstrates that epileptiform-like activity may be induced in different regions of the hippocampal formation, including regions that are severely affected by neuronal loss.

Neurobiological Correlates of Alpha-Tocopherol Antiepileptogenic Effects and MicroRNA Expression Modulation in a Rat Model of Kainate-Induced Seizures

Seizure-triggered maladaptive neural plasticity and neuroinflammation occur during the latent period as a key underlying event in epilepsy chronicization. Previously, we showed that α-tocopherol (α-T) reduces hippocampal neuroglial activation and neurodegeneration in the rat model of kainic acid (KA)-induced status epilepticus (SE). These findings allowed us to postulate an antiepileptogenic potential for α-T in hippocampal excitotoxicity, in line with clinical evidence showing that α-T improves seizure control in drug-resistant patients. To explore neurobiological correlates of the α-T antiepileptogenic role, rats were injected with such vitamin during the latent period starting right after KA-induced SE, and the effects on circuitry excitability, neuroinflammation, neuronal death, and microRNA (miRNA) expression were investigated in the hippocampus. Results show that in α-T-treated epileptic rats, (1) the number of population spikes elicited by pyramidal neurons, as well as the latency to the onset of epileptiform-like network activity recover to control levels; (2) neuronal death is almost prevented; (3) down-regulation of claudin, a blood-brain barrier protein, is fully reversed; (4) neuroinflammation processes are quenched (as indicated by the decrease of TNF-α, IL-1β, GFAP, IBA-1, and increase of IL-6); (5) miR-146a, miR-124, and miR-126 expression is coherently modulated in hippocampus and serum by α-T. These findings support the potential of a timely intervention with α-T in clinical management of SE to reduce epileptogenesis, thus preventing chronic epilepsy development. In addition, we suggest that the analysis of miRNA levels in serum could provide clinicians with a tool to evaluate disease evolution and the efficacy of α-T therapy in SE.

Restoring GABAergic inhibition rescues memory deficits in a Huntington's disease mouse model

Huntington's disease (HD) is classically characterized as a movement disorder, however cognitive impairments precede the motor symptoms by ∼15 y. Based on proteomic and bioinformatic data linking the Huntingtin protein (Htt) and KCC2, which is required for hyperpolarizing GABAergic inhibition, and the important role of inhibition in learning and memory, we hypothesized that aberrant KCC2 function contributes to the hippocampal-associated learning and memory deficits in HD. We discovered that Htt and KCC2 interact in the hippocampi of wild-type and R6/2-HD mice, with a decrease in KCC2 expression in the hippocampus of R6/2 and YAC128 mice. The reduced expression of the Cl--extruding cotransporter KCC2 is accompanied by an increase in the Cl--importing cotransporter NKCC1, which together result in excitatory GABA in the hippocampi of HD mice. NKCC1 inhibition by the FDA-approved NKCC1 inhibitor bumetanide abolished the excitatory action of GABA and rescued the performance of R6/2 mice on hippocampal-associated behavioral tests.

Inhibitory or excitatory? Optogenetic interrogation of the functional roles of GABAergic interneurons in epileptogenesis

Alteration in the excitatory/inhibitory neuronal balance is believed to be the underlying mechanism of epileptogenesis. Based on this theory, GABAergic interneurons are regarded as the primary inhibitory neurons, whose failure of action permits hyperactivity in the epileptic circuitry. As a consequence, optogenetic excitation of GABAergic interneurons is widely used for seizure suppression. However, recent evidence argues for the context-dependent, possibly “excitatory” roles that GABAergic cells play in epileptic circuitry. We reviewed current optogenetic approaches that target the “inhibitory” roles of GABAergic interneurons for seizure control. We also reviewed interesting evidence that supports the “excitatory” roles of GABAergic interneurons in epileptogenesis. GABAergic interneurons can provide excitatory effects to the epileptic circuits via several distinct neurological mechanisms. (1) GABAergic interneurons can excite postsynaptic neurons, due to the raised reversal potential of GABA receptors in the postsynaptic cells. (2) Continuous activity in GABAergic interneurons could lead to transient GABA depletion, which prevents their inhibitory effect on pyramidal cells. (3) GABAergic interneurons can synchronize network activity during seizure. (4) Some GABAergic interneurons inhibit other interneurons, causing disinhibition of pyramidal neurons and network hyperexcitability. The dynamic, context-dependent role that GABAergic interneurons play in seizure requires further investigation of their functions at single cell and circuitry level. New optogenetic protocols that target GABAergic inhibition should be explored for seizure suppression.

Seizing Control of KCC2: A New Therapeutic Target for Epilepsy

Deficits in GABAergic inhibition result in the abnormal neuronal activation and synchronization that underlies seizures. However, the molecular mechanisms responsible for transforming a normal brain into an epileptic one remain largely unknown. Hyperpolarizing inhibition mediated by type A GABA (GABAA) receptors is dependent on chloride extrusion by the neuron-specific type 2K+-Cl- cotransporter (KCC2). Loss-of-function mutations in KCC2 are a known cause of infantile epilepsy in humans and KCC2 dysfunction is present in patients with both idiopathic and acquired epilepsy. Here we discuss the growing evidence that KCC2 dysfunction has a central role in the development and severity of the epilepsies.

NKCC1 Chloride Importer Antagonists Attenuate Many Neurological and Psychiatric Disorders

In physiological conditions, adult neurons have low intracellular Cl^- [(Cl^-)_I] levels underlying the γ-aminobutyric acid (GABA)ergic inhibitory drive. In contrast, neurons have high (Cl^-)_I levels and excitatory GABA actions in a wide range of pathological conditions including spinal cord lesions, chronic pain, brain trauma, cerebrovascular infarcts, autism, Rett and Down syndrome, various types of epilepsies, and other genetic or environmental insults. The diuretic highly specific NKCC1 chloride importer antagonist bumetanide (PubChem CID: 2461) efficiently restores low (Cl^-)_I levels and attenuates many disorders in experimental conditions and in some clinical trials. Here, I review the mechanisms of action, therapeutic effects, promises, and pitfalls of bumetanide.

WONOEP appraisal: Development of epilepsy biomarkers-What we can learn from our patients?

OBJECTIVE

Current medications for patients with epilepsy work in only two of three patients. For those medications that do work, they only suppress seizures. They treat the symptoms, but do not modify the underlying disease, forcing patients to take these drugs with significant side effects, often for the rest of their lives. A major limitation in our ability to advance new therapeutics that permanently prevent, reduce the frequency of, or cure epilepsy comes from a lack of understanding of the disease coupled with a lack of reliable biomarkers that can predict who has or who will get epilepsy.

METHODS

The main goal of this report is to present a number of approaches for identifying reliable biomarkers from observing patients with brain disorders that have a high probability of producing epilepsy.

RESULTS

A given biomarker, or more likely a profile of biomarkers, will have both a quantity and a time course during epileptogenesis that can be used to predict who will get the disease, to confirm epilepsy as a diagnosis, to identify coexisting pathologies, and to monitor the course of treatments.

SIGNIFICANCE

Additional studies in patients and animal models could identify common and clinically valuable biomarkers to successfully translate animal studies into new and effective clinical trials.

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.

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.

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.

Elevated basal glutamate and unchanged glutamine and GABA in refractory epilepsy: Microdialysis study of 79 patients at the yale epilepsy surgery program

OBJECTIVE

Aberrant glutamate and γ-aminobutyric acid (GABA) neurotransmission contribute to seizure generation and the epileptic state. However, whether levels of these neurochemicals are abnormal in epileptic patients is unknown. Here, we report on interictal levels of glutamate, glutamine, and GABA in epilepsy patients at seizure onset and nonepileptic sites, cortical lesions, and from patients with poorly localized neocortical epilepsies.

METHODS

Subjects (n = 79) were medically refractory epilepsy patients undergoing intracranial electroencephalogram evaluation. Microdialysis probes (n = 125) coupled to depth electrodes were implanted within suspected seizure onset sites and microdialysis samples were obtained during interictal periods. Glutamate, glutamine, and GABA were measured using high-performance liquid chromatography. Probe locations were subsequently classified by consensus of expert epileptologists.

RESULTS

Glutamate levels were elevated in epileptogenic (p = 0.03; n = 7), nonlocalized (p < 0.001), and lesional cortical sites (p < 0.001) when compared to nonepileptogenic cortex. Glutamate was also elevated in epileptogenic (p < 0.001) compared to nonepileptogenic hippocampus. There were no statistical differences in GABA or glutamine, although GABA levels showed high variability across patients and groups.

INTERPRETATION

Our findings indicate that chronically elevated extracellular glutamate is a common pathological feature among epilepsies with different etiology. Contrary to our predictions, GABA and glutamine levels were not decreased in any of the measured areas. Whereas variability in GABA levels may in part be attributed to the use of GABAergic antiepileptic drugs, the stability in glutamine across patient groups indicate that extracellular glutamine levels are under tighter metabolic regulation than previously thought. Ann Neurol 2016;80:35-45.

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.

Chloride Homeostasis Critically Regulates Synaptic NMDA Receptor Activity in Neuropathic Pain

Chronic neuropathic pain is a debilitating condition that remains difficult to treat. Diminished synaptic inhibition by GABA and glycine and increased NMDA receptor (NMDAR) activity in the spinal dorsal horn are key mechanisms underlying neuropathic pain. However, the reciprocal relationship between synaptic inhibition and excitation in neuropathic pain is unclear. Here, we show that intrathecal delivery of K(+)-Cl(-) cotransporter-2 (KCC2) using lentiviral vectors produces a complete and long-lasting reversal of pain hypersensitivity induced by nerve injury. KCC2 gene transfer restores Cl(-) homeostasis disrupted by nerve injury in both spinal dorsal horn and primary sensory neurons. Remarkably, restoring Cl(-) homeostasis normalizes both presynaptic and postsynaptic NMDAR activity increased by nerve injury in the spinal dorsal horn. Our findings indicate that nerve injury recruits NMDAR-mediated signaling pathways through the disruption of Cl(-) homeostasis in spinal dorsal horn and primary sensory neurons. Lentiviral vector-mediated KCC2 expression is a promising gene therapy for the treatment of neuropathic pain.

The K(+)-Cl(-) Cotransporter KCC2 and Chloride Homeostasis: Potential Therapeutic Target in Acute Central Nervous System Injury

The K(+)-Cl(-) cotransporter-2 (KCC2) is a well-known member of the electroneutral cation-chloride cotransporters with a restricted expression pattern to neurons. This transmembrane protein mediates the efflux of Cl(-) out of neurons and exerts a critical role in inhibitory γ-aminobutyric acidergic (GABAergic) and glycinergic neurotransmission. Moreover, KCC2 participates in the regulation of various physiological processes of neurons, including cell migration, dendritic outgrowth, spine morphology, and dendritic synaptogenesis. It is important to note that down-regulation of KCC2 is associated with the pathogenesis of multiple neurological diseases, which is of particular relevance to acute central nervous system (CNS) injury. In this review, we aim to survey the pathogenic significance of KCC2 down-regulation under the condition of acute CNS injuries. We propose that further elucidation of the molecular mechanisms regarding KCC2 down-regulation after acute CNS injuries is necessary because of potential promising avenues for prevention and treatment of acute CNS injury.

Exploring novel AEDs from drugs used for treatment of non-epileptic disorders

Epilepsy is a chronic neurological disease. Although many anti-epileptic drugs (AEDs) have been developed for clinical use, they have no effect on 20-30% of patients and do not generally prevent epileptogenesis. Because of the long development cycle for new AEDs and the high cost, increasing efforts are being made to find anti-epileptic effects among drugs that are already listed for the treatment of other diseases and repurpose them as potential anti-epileptic treatments. Here, we review the progress that has been made in this field as a result of animal and clinical trials of drugs such as rapamycin, everolimus, losartan, celecoxib, bumetanide and other non-epileptic drugs. These drugs can prevent the epileptogenesis, reduce the epileptic pathological changes, and even be used to treat intractable epilepsy. Their mechanisms of action are completely different from those of existing AEDs, prompting researchers to change their perspectives in the search for new AEDs.

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.

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.

Homeostasis or channelopathy? Acquired cell type-specific ion channel changes in temporal lobe epilepsy and their antiepileptic potential

Neurons continuously adapt the expression and functionality of their ion channels. For example, exposed to chronic excitotoxicity, neurons homeostatically downscale their intrinsic excitability. In contrast, the “acquired channelopathy” hypothesis suggests that proepileptic channel characteristics develop during epilepsy. We review cell type-specific channel alterations under different epileptic conditions and discuss the potential of channels that undergo homeostatic adaptations, as targets for antiepileptic drugs (AEDs). Most of the relevant studies have been performed on temporal lobe epilepsy (TLE), a widespread AED-refractory, focal epilepsy. The TLE patients, who undergo epilepsy surgery, frequently display hippocampal sclerosis (HS), which is associated with degeneration of cornu ammonis subfield 1 pyramidal cells (CA1 PCs). Although the resected human tissue offers insights, controlled data largely stem from animal models simulating different aspects of TLE and other epilepsies. Most of the cell type-specific information is available for CA1 PCs and dentate gyrus granule cells (DG GCs). Between these two cell types, a dichotomy can be observed: while DG GCs acquire properties decreasing the intrinsic excitability (in TLE models and patients with HS), CA1 PCs develop channel characteristics increasing intrinsic excitability (in TLE models without HS only). However, thorough examination of data on these and other cell types reveals the coexistence of protective and permissive intrinsic plasticity within neurons. These mechanisms appear differentially regulated, depending on the cell type and seizure condition. Interestingly, the same channel molecules that are upregulated in DG GCs during HS-related TLE, appear as promising targets for future AEDs and gene therapies. Hence, GCs provide an example of homeostatic ion channel adaptation which can serve as a primer when designing novel anti-epileptic strategies.

Utility of magnetic resonance spectroscopic imaging for human epilepsy

This review discusses the potential utility of broad based use of magnetic resonance (MR) spectroscopic imaging for human epilepsy and seizure localization. The clinical challenges are well known to the epilepsy community, intrinsic in the variability of location, volumetric size and network extent of epileptogenic tissue in individual patients. The technical challenges are also evident, with high performance requirements in multiple steps, including magnet homogeneity, detector performance, sequence design, speed of acquisition in addition to large territory spectral processing. We consider how MR spectroscopy and spectroscopic imaging has been informative for epilepsy thus far, with specific attention to what is measured, the interpretation of such measurements and technical performance challenges. Examples are shown from medial temporal lobe and neocortical epilepsies are considered from 4T, 7T and most recently 3T.

Beyond the CA1 subfield: Local hippocampal shape changes in MRI-negative temporal lobe epilepsy

OBJECTIVE

Hippocampal atrophy in temporal lobe epilepsy (TLE) can indicate mesial temporal sclerosis and predict surgical success. Yet many patients with TLE do not have significant atrophy (magnetic resonance imaging (MRI) negative), which presents a diagnostic challenge. We used a new variant of high-dimensional large-deformation mapping to assess whether patients with apparently normal hippocampi have local shape changes that mirror those of patients with significant hippocampal atrophy.

METHODS

Forty-seven patients with unilateral TLE and 32 controls underwent structural brain MRI. High-dimensional large-deformation mapping provided hippocampal surface and volume estimates for each participant, dividing patients into low versus high hippocampal atrophy groups. A vertex-level generalized linear model compared local shape changes between groups.

RESULTS

Patients with low-atrophy TLE (MRI negative) had significant local hippocampal shape changes compared to controls, similar to those in the contralateral hippocampus of high-atrophy patients. These changes primarily involved the subicular and hilar/dentate regions, instead of the classically affected CA1 region. Disease duration instead co-varied with lateral hippocampal atrophy, co-localizing with the CA1 subfield.

SIGNIFICANCE

These findings show that patients with "MRI-negative" TLE have regions of hippocampal atrophy that cluster medially, sparing the lateral regions (CA1) involved in high-atrophy patients. This suggests an overall effect of temporal lobe seizures manifesting as bilateral medial hippocampal atrophy, and a more selective effect of hippocampal seizures leading to disease-proportional CA1 atrophy, potentially reflecting epileptogenesis.

Age-dependent changes in intrinsic neuronal excitability in subiculum after status epilepticus

Kainic acid-induced status epilepticus (KA-SE) in mature rats results in the development of spontaneous recurrent seizures and a pattern of cell death resembling hippocampal sclerosis in patients with temporal lobe epilepsy. In contrast, KA-SE in young animals before postnatal day (P) 18 is less likely to cause cell death or epilepsy. To investigate whether changes in neuronal excitability occur in the subiculum after KA-SE, we examined the age-dependent effects of SE on the bursting neurons of subiculum, the major output region of the hippocampus. Patch-clamp recordings were used to monitor bursting in pyramidal neurons in the subiculum of rat hippocampal slices. Neurons were studied either one or 2-3 weeks following injection of KA or saline (control) in immature (P15) or more mature (P30) rats, which differ in their sensitivity to KA as well as the long-term sequelae of the KA-SE. A significantly greater proportion of subicular pyramidal neurons from P15 rats were strong-bursting neurons and showed increased frequency-dependent bursting compared to P30 animals. Frequency-dependent burst firing was enhanced in P30, but not in P15 rats following KA-SE. The enhancement of bursting induced by KA-SE in more mature rats suggests that the frequency-dependent limitation of repetitive burst firing, which normally occurs in the subiculum, is compromised following SE. These changes could facilitate the initiation of spontaneous recurrent seizures or their spread from the hippocampus to other parts of the brain.

Evidence for an excitatory GABAA response in human motor cortex in idiopathic generalised epilepsy

PURPOSE

Impaired GABAergic inhibition has been implicated in the pathophysiology of epilepsy. The possibility of a paradoxical excitatory effect of GABA in epilepsy has been suggested, but has not been investigated in vivo. We investigated pre- and post-synaptic GABAergic mechanisms in patients with idiopathic generalised epilepsy (IGE).

METHOD

In 10 patients and 12 control subjects we explored short- and long-interval intracortical inhibition (SICI, LICI; post-synaptic GABAA and GABAB-mediated respectively) and long-interval intracortical facilitation (LICF; pre-synaptic disinhibition) using transcranial magnetic stimulation.

RESULTS

While post-synaptic GABAB-mediated inhibition was unchanged in IGE (p=0.09), LICF was reduced compared to controls (controls: 141±17% of baseline; untreated patients: 107±12%, p=0.2; treated patients: 79±10%, p=0.003). GABAA-mediated inhibition was reduced in untreated patients (response amplitude 56±4% of baseline vs. 26±6% in controls, p=0.004) and normalised with treatment (37±12%, p=0.5 vs. controls). When measured during LICI, GABAA-mediated inhibition became excitatory in untreated IGE (response amplitude 120±10% of baseline, p=0.017), but not in treated patients.

CONCLUSION

Pre- and post-synaptic GABA-mediated inhibitory mechanisms are altered in IGE. The findings lend in vivo support to evidence from experimental models and in vitro studies of human epileptic brain tissue that GABA may have a paradoxical excitatory role in ictogenesis.