Synchronous Epileptiform Research Articles

Low concentration dimethyl sulfoxide (DMSO) modulates epileptiform synchronization in the 4-aminopyridine in vitro model

Dimethyl sulfoxide (DMSO) is commonly used to dissolve water-insoluble drugs due to its dipolar and aprotic properties. It also serves as a vehicle in many pharmacological studies. However, it has been reported that DMSO can induce seizures in human patients, lower seizure threshold in vivo, and modulate ion receptors activities in vitro. Therefore, we investigated here the effect of 0.03 % and 0.06 % DMSO, which are 10–50 times lower than what usually employed in previous studies, in the 4-aminopyridine (4AP) model of epileptiform synchronization in male mouse brain slices. We found that 0.03 % and 0.06 % DMSO increase 4AP-induced ictal discharge rate, while 0.06 % DMSO decreases ictal discharge duration. Our results suggest that the effects of DMSO on neuronal excitability deserve further analysis and that investigators need to be aware of its confounding effect as a solvent, even at very low concentrations.

Ictal activity is sustained by the estrogen receptor β during the estrous cycle

Catamenial epilepsy, defined as a periodicity of seizure exacerbation during the menstrual cycle, affects up to 70 % of epileptic women. Seizures in these patients are often non-responsive to medication; however, our understanding of the relation between menstrual cycle and seizure generation (i.e. ictogenesis) remains limited. We employed here field potential recordings in the in vitro 4-aminopyridine model of epileptiform synchronization in female mice (P60–P130) and found that: (i) the estrous phase favors ictal activity in the entorhinal cortex; (ii) these ictal discharges display an onset pattern characterised by the presence of chirps that are thought to mirror synchronous interneuron firing; and (iii) blocking estrogen receptor β-mediated signaling reduces ictal discharge duration. Our findings indicate that the duration of 4AP-induced ictal discharges, in vitro, increases during the estrous phase, which corresponds to the human peri-ovulatory period. We propose that these effects are caused by the presumptive enhancement of interneuron excitability due to increased estrogen receptor β-mediated signaling.

Ligand-gated mechanisms leading to ictogenesis in focal epileptic disorders

We review here the neuronal mechanisms that cause seizures in focal epileptic disorders and, specifically, those involving limbic structures that are known to be implicated in human mesial temporal lobe epilepsy. In both epileptic patients and animal models, the initiation of focal seizures - which are most often characterized by a low-voltage fast onset EEG pattern - is presumably dependent on the synchronous firing of GABA-releasing interneurons that, by activating post-synaptic GABAA receptors, cause large increases in extracellular [K+] through the activation of the co-transporter KCC2. A similar mechanism may contribute to seizure maintenance; accordingly, inhibiting KCC2 activity transforms seizure activity into a continuous pattern of short-lasting epileptiform discharges. It has also been found that interactions between different areas of the limbic system modulate seizure occurrence by controlling extracellular [K+] homeostasis. In line with this view, low-frequency electrical or optogenetic activation of limbic networks restrain seizure generation, an effect that may also involve the activation of GABAB receptors and activity-dependent changes in epileptiform synchronization. Overall, these findings highlight the paradoxical role of GABAA signaling in both focal seizure generation and maintenance, emphasize the efficacy of low-frequency activation in abating seizures, and provide experimental evidence explaining the poor efficacy of antiepileptic drugs designed to augment GABAergic function in controlling seizures in focal epileptic disorders.

Modulation of in vitro epileptiform activity by optogenetic stimulation of parvalbumin-positive interneurons.

GABAA signaling is surprisingly involved in the initiation of epileptiform activity since increased interneuron firing, presumably leading to excessive GABA release, often precedes ictal discharges. Field potential theta (4-12 Hz) oscillations, which are thought to mirror the synchronization of interneuron networks, also lead to ictogenesis. However, the exact role of parvalbumin-positive (PV) interneurons in generating theta oscillations linked to epileptiform discharges remains unexplored. We analyzed here the field responses recorded in the CA3, entorhinal cortex (EC), and dentate gyrus (DG) during 8-Hz optogenetic stimulation of PV-positive interneurons in brain slices obtained from PV-ChR2 mice during 4-aminopyridine (4AP) application. This optogenetic protocol triggered similar field oscillations in both control conditions and during 4AP application. However, in the presence of 4AP, optogenetic stimuli also induced: 1) interictal discharges that were associated in all regions with 8-Hz field oscillations and 2) low-voltage fast onset ictal discharges. Interictal and ictal events occurred more frequently during optogenetic activation than during periods of no stimulation. 4AP also increased synchronicity during PV-interneuron activation in all three regions. In opsin-negative mice, optogenetic stimulation did not change the rate of both types of epileptiform activity. Our findings suggest that PV-interneuron recruitment at theta (8 Hz) frequency contributes to epileptiform synchronization in limbic structures in the in vitro 4AP model.NEW & NOTEWORTHY Previous studies have identified contradictory roles of PV-interneurons in ictogenesis and the link between theta oscillations and epileptiform activity remains unexplored. Here, we investigated in vitro the effect of PV-interneuron optogenetic stimulation under 4AP in temporal lobe regions obtained from PV-ChR2 transgenic mice. Under theta (8 Hz) optogenetic stimulation and 4AP application, interictal spikes and low-voltage fast onset ictal discharges were triggered, suggesting that the activation of PV-interneurons favors synchronization and ictogenesis.

WS1.4. Generalized Epilep tiform Patterns – Interictal and Ictal: Defining the Clinical Syndrome

Generalised epileptiform patterns (GEPs) are bilateral and synchronous epileptiform discharges that appear over corresponding parts of both hemispheres, or the whole brain. Their morphology, amplitude and timing should be the same or similar over both hemispheres. The distinction between ictal and interictal GEPs is generally less clear compared with focal patterns. Ictal patterns often represent longer repetitions of interictal patterns. Clinical signs are more likely to be apparent during GEPs if the bursts are longer, the spike component is more persistent, and tests of responsiveness are adequately and repeatedly tested by the EEG technician. GEPs include 3Hz spike-wave discharges, polyspike-wave discharges, slow spike- wave discharges, generalised paroxysmal fast activity, electrodecremental response, photosensitive responses and ictal patterns. 3Hz spike and wave complexes are characteristic of idiopathic (genetic) generalised epilepsy, which includes childhood absence, juvenile absence and juvenile myoclonic epilepsy as well as epilepsy with generalised tonic-clonic seizures only. In children with 3Hz spike and wave, spontaneous remission is more likely if there is co-existent OIRDA and less likely if there is co-existent photoparoxysmal responses. Polyspike and wave complexes, either single or repetitive, can be seen in both idiopathic (typically in JME) and symptomatic generalised epilepsies. Slow spike and wave complexes occur at a frequency of 1.5–2.5 Hz and usually arise from an abnormal background; they, along with generalised paroxysmal fast activity (GPFA-bilaterally synchronous10–25 Hz activity) are typically seen in symptomatic and cryptogenic generalised epilepsies, notably Lennox-Gastaut syndrome. GPFA is typically seen in tonic seizures, but may also accompany atonic and generalised tonic-clonic seizures. Electrodecremental responses are seen in the atonic and tonic seizures of symptomatic and cryptogenic generalised epilepsies. Photoparoxysmal responses are bursts of generalised spike or polyspike and wave discharges that appear at certain flash frequencies and may outlast the stimulus.

CDDO-Me Attenuates Astroglial Autophagy via Nrf2-, ERK1/2-SP1- and Src-CK2-PTEN-PI3K/AKT-Mediated Signaling Pathways in the Hippocampus of Chronic Epilepsy Rats.

Clasmatodendrosis is an autophagic astroglial death showing extensive swollen cell bodies with vacuoles and disintegrated/beaded processes. This astroglial degeneration is closely relevant to the synchronous epileptiform discharges. However, the underlying molecular mechanisms and the roles of clasmatodendrosis in spontaneous seizure activity are still unknown. The 2-cyano-3,12-dioxo-oleana-1,9(11)-dien-28-oic acid methyl ester (CDDO-Me; RTA 402) is one of the activators for nuclear factor-erythroid 2-related factor 2 (Nrf2) that is a redox-sensitive transcription factor. In the present study, we explored the effects of CDDO-Me on clasmatodendrosis in chronic epilepsy rats, which could prevent epilepsy-related complications. In the present study, clasmatodendritic astrocytes showed reduced Nrf2 expression and its nuclear accumulation, which were restored by CDDO-Me. CDDO-Me also abrogated heat shock protein 25 (HSP25) upregulation in clasmatodendritic astrocytes by regulating extracellular signal-related kinases 1/2 (ERK1/2)-specificity protein 1 (SP1)- and Src-casein kinase 2 (CK2)-phosphatase and tensin homolog deleted on chromosome 10 (PTEN)-phosphatidylinositol-3-kinase (PI3K)-AKT-glycogen synthase kinase 3β (GSK3β)-bax-interacting factor 1 (Bif-1)-mediated signaling pathways in chronic epilepsy rats. In addition, CDDO-Me ameliorated spontaneous seizure duration, but not seizure frequency and behavioral seizure severity. Therefore, our findings suggest that clasmatodendrosis may affect seizure duration in chronic epilepsy rats, and that CDDO-Me may attenuate autophagic astroglial degeneration by regulating various signaling pathways.

A characteristic occipital epileptiform EEG pattern in ADCK3-related mitochondrial disease.

ADCK3-related disease is a mitochondrial disorder associated with an abnormality of coenzyme Q10 metabolism. Ataxia and epilepsy are common, and the phenotype overlaps with other mitochondrial encephalopathies, particularly POLG-related disease. CoQ10 supplementation may be beneficial. We have noted a remarkable epileptiform pattern in ADCK3-related encephalopathy, and since EEG studies in this rare condition are limited, we wished to assess the evolution of EEG characteristics in patients with this disorder. All EEG recordings of the four known patients from Mid-Norway were systematically reviewed. EEG graphoelements were classified according to the standardized computer-based organized reporting of EEG (SCORE) and international glossary terms. The evolution of EEG features was assessed. A total of 96 recordings spanning over 15-32 years were available, with a mean of 24 per patient (range: 17-28). Altogether, 50 digital recordings were reviewed, including four long-term and 46 selected paper segments. In three patients, EEG showed prominent bilateral asynchronous and synchronous epileptiform discharges in occipital and posterior-temporal regions. This intense activity included multiple epileptiform graphoelements, which occurred continuously, nearly continuously or in prolonged runs. The findings remained stable over many years. Although the number of patients is small, we suggest that interictal EEG findings of continuous/nearly continuous bi-occipital spike-waves may serve as a biomarker for this potentially treatable condition. This peculiar EEG pattern might help to differentiate ADCK3-related disease from the more common POLG-related disease, which is usually characterized by lateralized or focal slowing with more sporadic epileptiform elements of similar topography.

Mecamylamine inhibits seizure-like activity in CA1-CA3 hippocampus through antagonism to nicotinic receptors.

Cholinergic modulation of hippocampal network function is implicated in multiple behavioral and cognitive states. Activation of nicotinic and muscarinic acetylcholine receptors affects neuronal excitability, synaptic transmission and rhythmic oscillations in the hippocampus. In this work, we studied the ability of the cholinergic system to sustain hippocampal epileptiform activity independently from glutamate and GABA transmission. Simultaneous CA3 and CA1 field potential recordings were obtained during the perfusion of hippocampal slices with the aCSF containing AMPA, NMDA and GABA receptor antagonists. Under these conditions, spontaneous epileptiform discharges synchronous between CA3 and CA1 were recorded. Epileptiform discharges were blocked by addition of the calcium-channel blocker Cd2+ and disappeared in CA1 after a surgical cut between CA3 and CA1. Cholinergic antagonist mecamylamine abolished CA3-CA1 synchronous epileptiform discharges, while antagonists of α7 and α4β2 nAChRs, MLA and DhβE, had no effect. Our results suggest that activation of nicotinic acetylcholine receptors can sustain CA3-CA1 synchronous epileptiform activity independently from AMPA, NMDA and GABA transmission. In addition, mecamylamine, but not α7 and α4β2 nAChRs antagonists, reduced bicuculline-induced seizure-like activity. The ability of mecamylamine to decrease hippocampal network synchronization might be associated with its therapeutic effects in a wide variety of CNS disorders including addiction, depression and anxiety.

Neurosteroids and Focal Epileptic Disorders.

Neurosteroids are a family of compounds that are synthesized in principal excitatory neurons and glial cells, and derive from the transformation of cholesterol into pregnenolone. The most studied neurosteroids—allopregnanolone and allotetrahydrodeoxycorticosterone (THDOC)—are known to modulate GABAA receptor-mediated transmission, thus playing a role in controlling neuronal network excitability. Given the role of GABAA signaling in epileptic disorders, neurosteroids have profound effects on seizure generation and play a role in the development of chronic epileptic conditions (i.e., epileptogenesis). We review here studies showing the effects induced by neurosteroids on epileptiform synchronization in in vitro brain slices, on epileptic activity in in vivo models, i.e., in animals that were made epileptic with chemoconvulsant treatment, and in epileptic patients. These studies reveal that neurosteroids can modulate ictogenesis and the occurrence of pathological network activity such as interictal spikes and high-frequency oscillations (80–500 Hz). Moreover, they can delay the onset of spontaneous seizures in animal models of mesial temporal lobe epilepsy. Overall, this evidence suggests that neurosteroids represent a new target for the treatment of focal epileptic disorders.

The subiculum and its role in focal epileptic disorders.

The subicular complex (hereafter referred as subiculum), which is reciprocally connected with the hippocampus and rhinal cortices, exerts a major control on hippocampal outputs. Over the last three decades, several studies have revealed that the subiculum plays a pivotal role in learning and memory but also in pathological conditions such as mesial temporal lobe epilepsy (MTLE). Indeed, subicular networks actively contribute to seizure generation and this structure is relatively spared from the cell loss encountered in this focal epileptic disorder. In this review, we will address: (i)the functional properties of subicular principal cells under normal and pathological conditions; (ii) the subiculum role in sustaining seizures in in vivo models of MTLE and in in vitro models of epileptiform synchronization; (iii) its presumptive role in human MTLE; and (iv) evidence underscoring the relationship between subiculum and antiepileptic drug effects. The studies reviewed here reinforce the view that the subiculum represents a limbic area with relevant, as yet unexplored, roles in focal epilepsy.

The intrinsic cell type-specific excitatory connectivity of the developing mouse subiculum is sufficient to generate synchronous epileptiform activity.

The activity of local excitatory circuits of the subiculum has been suggested to be involved in the initiation of pathological activity in epileptic patients and experimental animal models of temporal lobe epilepsy. We have taken advantage of multimodal techniques to classify subicular cells in distinct subclasses and have investigated their morphofunctional properties and connectivity in vitro. Our results indicate that local subicular excitatory circuits are connected in a cell type-specific fashion and that synapses are preferentially established on basal vs. apical dendrites. We show that local excitatory circuits, isolated from extrasubicular inputs and pharmacologically disinhibited, are sufficient to initiate synchronous epileptiform activity in vitro. In conclusion, this work provides a high-resolution description of local excitatory circuits of the subiculum and highlights their mechanistic involvement in the generation of pathological activity. The subiculum has been suggested to be involved in the initiation of pathological discharges in human patients and animal models of temporal lobe epilepsy. Although converging evidence has revealed the existence of functional diversity within its principal neurons, much less attention has been devoted to its intrinsic connectivity and whether its local excitatory circuits are sufficient to generate epileptiform activity. Here, we have directly addressed these two key points in mouse subicular slices. First, using multivariate techniques, we have distinguished two groups of principal cells, which we have termed type1 and type2. These subgroups roughly overlap with what were classically indicated as regular and bursting cells, and showed differences in the extension and complexity of their apical dendrites. Functional connectivity was found both between similar (homotypic) and different (heterotypic) types of cells, with a marked asymmetry within heterotypic pairs. Unitary excitatory postsynaptic potentials (uEPSPs) revealed a high degree of variability both in amplitude, failure rate, rise time and half-width. Post hoc analysis of functionally connected pairs suggested that the observed uEPSPs were mediated by few contact sites, predominantly located on the basal dendrites. When surgically isolated from extrasubicular excitatory afferents, pharmacologically disinhibited subicular slices generated hyper-synchronous discharges. Thus, we conclude that local subicular excitatory circuits, connected according to cell type-specific rules, are sufficient to promote epileptiform activity. This conclusion fits well with a previous suggestion that local subicular events, purely mediated by excitatory connections, may underlie the pre-ictal discharges that govern interictal-ictal transitions.

Evolving Mechanistic Concepts of Epileptiform Synchronization and their Relevance in Curing Focal Epileptic Disorders.

The synchronized activity of neuronal networks under physiological conditions is mirrored by specific oscillatory patterns of the EEG that are associated with different behavioral states and cognitive functions. Excessive synchronization can, however, lead to focal epileptiform activity characterized by interictal and ictal discharges in epileptic patients and animal models. This review focusses on studies that have addressed epileptiform synchronization in temporal lobe regions by em-ploying in vitro and in vivo recording techniques. First, we consider the role of ionotropic and metabotropic excitatory glu-tamatergic transmission in seizure generation as well as the paradoxical role of GABAA signaling in initiating and perhaps maintaining focal seizure activity. Second, we address non-synaptic mechanisms (which include voltage-gated ionic currents and gap junctions) in the generation of epileptiform synchronization. For each mechanism, we discuss the actions of anti-epileptic drugs that are presumably modulating excitatory or inhibitory signaling and voltage-gated currents to prevent sei-zures in epileptic patients. These findings provide insights into the mechanisms of seizure initiation and maintenance, thus leading to the development of specific pharmacological treatments for focal epileptic disorders.

A hybrid Local Binary Pattern and wavelets based approach for EEG classification for diagnosing epilepsy

Epilepsy is one of the grave neurological ailments affecting approximately 70 million people globally. Detection of epileptic attack is commonly carried out by viewing and analysing long-duration multi-channel EEG records. To counter this time-consuming process, a hybrid Local Binary Pattern-Wavelet based approach, classifying EEG in epileptic patients, is adopted in this research. Epilepsy is characterized by multiple ictal patterns in the form of synchronous epileptiform discharge transients. This work attempts to classify seizure from normal EEG recordings using the low-frequency activity. In order to perform this classification, the EEG signal is filtered and then transformed using Local Binary Pattern (LBP) into a new signal. Discrete Wavelet Transform (DWT) is employed to decompose the obtained signal. Wavelet coefficients are calculated to 5 levels of decomposition. A combination of univariate and bivariate features forms the feature set for seizure detection. This feature set extracted from low-frequency band coefficients helps in bringing out the dispersion, symmetry, and peakedness present in the EEG signal. A novel LBP based Spatio-temporal analysis of the continuous EEG signal for epilepsy detection is carried out on 105 seizures from 14 randomly selected subjects of CHB-MIT EEG database. A sensitivity of 100% is achieved on the CHB-MIT database while long term EEG is being tested with Linear Discriminant Analysis (LDA) classifier. The algorithm works well to obtain a false detection rate (FP/Hour) of 0.59. The specificity of 99.8% is attained with a mean accuracy of 99.6% when tested on 498.9 h of EEG data.

KCC2 antagonism increases neuronal network excitability but disrupts ictogenesis in vitro.

The potassium-chloride cotransporter 2 (KCC2) plays a role in epileptiform synchronization, but it remains unclear how it influences such a process. Here, we used tetrode recordings in the in vitro rat entorhinal cortex (EC) to analyze the effects of the KCC2 antagonist VU0463271 on 4-aminopyridine (4AP)-induced ictal and interictal activity. During 4AP application, ictal events were associated with significant increases in interneurons and principal cells activities. VU0463271 application transformed ictal discharges to shorter ictal-like events that were not accompanied by significant increases in interneuron or principal cell firing. Interictal events persisted during VU0463271 application at an accelerated frequency of occurrence with significant increases in interneuron and principal cell activity. Further analysis revealed that interneuron and principal cell firing rate during 4AP-induced interictal events were increased after VU0463271 application without changes in synchronicity. Overall, our results demonstrate that in the EC, KCC2 antagonism enhances both interneuron and principal cell excitability, while paradoxically decreasing the ability of neuronal networks to generate structured ictal events.NEW & NOTEWORTHY We are the first to use tetrode recordings in the entorhinal cortex to demonstrate that antagonizing potassium-chloride cotransporter 2 (KCC2) function abolishes ictal discharges and the associated, dynamic changes in single-unit firing in the in vitro 4-aminopyrine model of epileptiform synchronization. Interictal discharges were, however, shorter and more frequent during KCC2 antagonism, while the associated single-unit activity increased, suggesting augmented neuronal excitability. Our findings highlight the complex role of KCC2 in disease pathology.

Chemogenetic Recruitment of Specific Interneurons Suppresses Seizure Activity.

Current anti-epileptic medications that boost synaptic inhibition are effective in reducing several types of epileptic seizure activity. Nevertheless, these drugs can generate significant side-effects and even paradoxical responses due to the broad nature of their action. Recently developed chemogenetic techniques provide the opportunity to pharmacologically recruit endogenous inhibitory mechanisms in a selective and circuit-specific manner. Here, we use chemogenetics to assess the potential of suppressing epileptiform activity by enhancing the synaptic output from three major interneuron populations in the rodent hippocampus: parvalbumin (PV), somatostatin (SST), and vasoactive intestinal peptide (VIP) expressing interneurons. To target different neuronal populations, promoter-specific cre-recombinase mice were combined with viral-mediated delivery of chemogenetic constructs. Targeted electrophysiological recordings were then conducted in an in vitro model of chronic, drug-resistant epilepsy. In addition, behavioral video-scoring was performed in an in vivo model of acutely triggered seizure activity. Pre-synaptic and post-synaptic whole cell recordings in brain slices revealed that each of the three interneuron types increase their firing rate and synaptic output following chemogenetic activation. However, the interneuron populations exhibited different effects on epileptiform discharges. Recruiting VIP interneurons did not change the total duration of epileptiform discharges. In contrast, recruiting SST or PV interneurons produced robust suppression of epileptiform synchronization. PV interneurons exhibited the strongest effect per cell, eliciting at least a fivefold greater reduction in epileptiform activity than the other cell types. Consistent with this, we found that in vivo chemogenetic recruitment of PV interneurons suppressed convulsive behaviors by more than 80%. Our findings support the idea that selective chemogenetic enhancement of inhibitory synaptic pathways offers potential as an anti-seizure strategy.

F01. Paroxysmal fast activity and genetic generalized epilepsies

Paroxysmal fast activity is the hallmark of “symptomatic generalized” epilepsy of the Lennox Gastaut type, often occurring with slow spike-wave complexes. Generalized, synchronous epileptiform discharges are considered the typical abnormality on EEG in genetic generalized epilepsy (GGE). Recent literature suggests that generalized paroxysmal fast activity (GPFA) may occur in the context of GGE, and may contribute to clinical heterogeneity. Here, we present three patients with generalized fast spike-wave discharges and polyspikes with concurrent GPFA. Institutional databases of EEG tracings and the electronic medical records were reviewed for cases of GPFA with an electro-clinical diagnosis of GGE. All three patients had onset of their seizures in childhood from age 8–13. Two of the three had mild cognitive impairment. MRI of all three patients were unremarkable. Their seizures were all medically intractable, requiring at least 3–5 broad spectrum antiepileptic drugs, and all 3 were recommended to pursue or chose to have a vagal nerve stimulator implanted. Seizure subtypes included generalized tonic clonic (most frequent) followed by automotor, myoclonic, and astatic seizures. All three EEGs showed brief generalized spike-wave complexes at 3–6 Hz lasting 1–4 s, and generalized paroxysmal fast activity (12–20 Hz) lasting 5–20 s. One tracing showed evolution of GPFA to polyspikes that correlated to a clinical myoclonic seizure. GPFA is classically associated with symptomatic generalized epilepsies of the Lennox-Gastaut type, but these three cases demonstrate GPFA can be seen in GGE. Such patients may be more refractory than the usual GGE, and may belong on a continuum between GGE and LGS.

Seizure Dynamics of Coupled Oscillators with Epileptor Field Model

The focus of this paper is to investigate the dynamics of seizure activities by using the Epileptor coupled model. Based on the coexistence of seizure-like event (SLE), refractory status epilepticus (RSE), depolarization block (DB), and normal state, we first study the dynamical behaviors of two coupled oscillators in different activity states with Epileptor model by linking them with slow permittivity coupling. Our research has found that when one oscillator in normal states is coupled with any oscillator in SLE, RSE or DB states, these two oscillators can both evolve into SLE states under appropriate coupling strength. And then these two SLE oscillators can perform epileptiform synchronization or epileptiform anti-synchronization. Meanwhile, SLE can be depressed when considering the fast electrical or chemical coupling in Epileptor model. Additionally, a two-dimensional reduced model is also given to show the effect of coupling number on seizures. Those results can help to understand the dynamical mechanism of the initiation, maintenance, propagation and termination of seizures in focal epilepsy.

Pre- and Postsynaptic Activation of GABAB Receptors Modulates Principal Cell Excitation in the Piriform Cortex

The piriform cortex (PC), like other cortical regions, normally operates in a state of dynamic equilibrium between excitation and inhibition. Here we examined the roles played by pre- and postsynaptic GABAB receptors in maintaining this equilibrium in the PC. Using whole-cell recordings in brain slices from the anterior PC of mice, we found that synaptic activation of postsynaptic GABAB receptors hyperpolarized the two major classes of layer 2 principal neurons and reduced the intrinsic electrical excitability of these neurons. Presynaptic GABAB receptors are expressed on the terminals of associational (intracortical) glutamatergic axons in the PC. Heterosynaptic activation of these receptors reduced excitatory associational inputs onto principal cells. Presynaptic GABAB receptors are also expressed on the axons of GABAergic interneurons in the PC, and blockade of these autoreceptors enhanced inhibitory inputs onto principal cells. Hence, presynaptic GABAB autoreceptors produce disinhibition of principal cells. To study the functional consequences of GABAB activation in vivo, we used 2-photon calcium imaging to simultaneously monitor the activity of ~200 layer 2 neurons. Superfusion of the GABAB agonist baclofen reduced spontaneous random firing but also promoted synchronous epileptiform activity. These findings suggest that, while GABAB activation can dampen excitability by engaging pre- and postsynaptic GABAB heteroreceptors on glutamatergic neurons, it can also promote excitability by disinhibiting principal cells by activating presynaptic GABAB autoreceptors on interneurons. Thus, depending on the dynamic balance of hetero- and autoinhibition, GABAB receptors can function as variable modulators of circuit excitability in the PC.

Role of KCC2-dependent potassium efflux in 4-Aminopyridine-induced Epileptiform synchronization

A balance between excitation and inhibition is necessary to maintain stable brain network dynamics. Traditionally, seizure activity is believed to arise from the breakdown of this delicate balance in favor of excitation with loss of inhibition. Surprisingly, recent experimental evidence suggests that this conventional view may be limited, and that inhibition plays a prominent role in the development of epileptiform synchronization. Here, we explored the role of the KCC2 co-transporter in the onset of inhibitory network-induced seizures. Our experiments in acute mouse brain slices, of either sex, revealed that optogenetic stimulation of either parvalbumin- or somatostatin-expressing interneurons induced ictal discharges in rodent entorhinal cortex during 4-aminopyridine application. These data point to a proconvulsive role of GABAA receptor signaling that is independent of the inhibitory input location (i.e., dendritic vs. somatic). We developed a biophysically realistic network model implementing dynamics of ion concentrations to explore the mechanisms leading to inhibitory network-induced seizures. In agreement with experimental results, we found that stimulation of the inhibitory interneurons induced seizure-like activity in a network with reduced potassium A-current. Our model predicts that interneuron stimulation triggered an increase of interneuron firing, which was accompanied by an increase in the intracellular chloride concentration and a subsequent KCC2-dependent gradual accumulation of the extracellular potassium promoting epileptiform ictal activity. When the KCC2 activity was reduced, stimulation of the interneurons was no longer able to induce ictal events. Overall, our study provides evidence for a proconvulsive role of GABAA receptor signaling that depends on the involvement of the KCC2 co-transporter.

Spatiotemporal changes in regularity of gamma oscillations contribute to focal ictogenesis

In focal ictogenesis, gamma oscillations (30–70 Hz) recorded by electroencephalography (EEG) are related to the epileptiform synchronization of interneurons that links the seizure onset zone (SOZ) to the surrounding epileptogenic zone. We hypothesized that the synchronization of interneurons could be detected as changes in the regularity of gamma oscillation rhythmicity. We used multiscale entropy (MSE) analysis, which can quantify the regularity of EEG rhythmicity, to investigate how the regularity of gamma oscillations changes over the course of a seizure event. We analyzed intracranial EEG data from 13 pediatric patients with focal cortical dysplasia. The MSE analysis revealed the following characteristic changes of MSE score (gamma oscillations): (1) during the interictal periods, the lowest MSE score (the most regular gamma oscillations) was always found in the SOZ; (2) during the preictal periods, the SOZ became more similar to the epileptogenic zone as the MSE score increased in the SOZ (gamma oscillations became less regular in the SOZ); and (3) during the ictal periods, a decreasing MSE score (highly regular gamma oscillations) propagated over the epileptogenic zone. These spatiotemporal changes in regularity of gamma oscillations constitute an important demonstration that focal ictogenesis is caused by dynamic changes in interneuron synchronization.