Fast spiking interneuron control of seizure propagation in a cortical slice model of focal epilepsy

Feedforward inhibition ahead of ictal wavefronts is provided by both parvalbumin- and somatostatin-expressing interneurons

Key points

There is a rapid interneuronal response to focal activity in cortex, which restrains laterally propagating activity, including spreading epileptiform activity.

The interneuronal response involves intense activation of both parvalbumin‐ and somatostatin‐expressing interneurons.

Interneuronal bursting is time‐locked to glutamatergic barrages in the pre‐ictal period.

Ca²⁺ imaging using conditional expression of GCaMP6f provides an accurate readout of the evolving firing patterns in both types of interneuron.

The activation profiles of the two interneuronal classes are temporally offset, with the parvalbumin population being activated first, and typically, at higher rates.

Abstract

Previous work has described powerful restraints on laterally spreading activity in cortical networks, arising from a rapid feedforward interneuronal response to focal activity. This response is particularly prominent ahead of an ictal wavefront. Parvalbumin‐positive interneurons are considered to be critically involved in this feedforward inhibition, but it is not known what role, if any, is provided by somatostatin‐expressing interneurons, which target the distal dendrites of pyramidal cells. We used a combination of electrophysiology and cell class‐specific Ca²⁺ imaging in mouse brain slices bathed in 0 Mg²⁺ medium to characterize the activity profiles of pyramidal cells and parvalbumin‐ and somatostatin‐expressing interneurons during epileptiform activation. The GCaMP6f signal strongly correlates with the level of activity for both interneuronal classes. Both interneuronal classes participate in the feedfoward inhibition. This contrasts starkly with the pattern of pyramidal recruitment, which is greatly delayed. During these barrages, both sets of interneurons show intense bursting, at rates up to 300Hz, which is time‐locked to the glutamatergic barrages. The activity of parvalbumin‐expressing interneurons appears to peak early in the pre‐ictal period, and can display depolarizing block during the ictal event. In contrast, somatostatin‐expressing interneuronal activity peaks significantly later, and firing persists throughout the ictal events. Interictal events appear to be very similar to the pre‐ictal period, albeit with slightly lower firing rates. Thus, the inhibitory restraint arises from a coordinated pattern of activity in the two main classes of cortical interneurons.

There is a rapid interneuronal response to focal activity in cortex, which restrains laterally propagating activity, including spreading epileptiform activity.

The interneuronal response involves intense activation of both parvalbumin‐ and somatostatin‐expressing interneurons.

Interneuronal bursting is time‐locked to glutamatergic barrages in the pre‐ictal period.

Ca²⁺ imaging using conditional expression of GCaMP6f provides an accurate readout of the evolving firing patterns in both types of interneuron.

The activation profiles of the two interneuronal classes are temporally offset, with the parvalbumin population being activated first, and typically, at higher rates.

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.

Gamma Oscillations in the Basolateral Amygdala: Biophysical Mechanisms and Computational Consequences

The basolateral nucleus of the amygdala (BL) is thought to support numerous emotional behaviors through specific microcircuits. These are often thought to be comprised of feedforward networks of principal cells (PNs) and interneurons. Neither well-understood nor often considered are recurrent and feedback connections, which likely engender oscillatory dynamics within BL. Indeed, oscillations in the gamma frequency range (40 − 100 Hz) are known to occur in the BL, and yet their origin and effect on local circuits remains unknown. To address this, we constructed a biophysically and anatomically detailed model of the rat BL and its local field potential (LFP) based on the physiological and anatomical literature, along with in vivo and in vitro data we collected on the activities of neurons within the rat BL. Remarkably, the model produced intermittent gamma oscillations (∼50 − 70 Hz) whose properties matched those recorded in vivo, including their entrainment of spiking. BL gamma-band oscillations were generated by the intrinsic circuitry, depending upon reciprocal interactions between PNs and fast-spiking interneurons (FSIs), while connections within these cell types affected the rhythm’s frequency. The model allowed us to conduct experimentally impossible tests to characterize the synaptic and spatial properties of gamma. The entrainment of individual neurons to gamma depended on the number of afferent connections they received, and gamma bursts were spatially restricted in the BL. Importantly, the gamma rhythm synchronized PNs and mediated competition between ensembles. Together, these results indicate that the recurrent connectivity of BL expands its computational and communication repertoire.

Flexible Nanopipettes for Minimally Invasive Intracellular Electrophysiology In Vivo

Intracellular recordings in vivo remains the best technique to link single-neuron electrical properties to network function. Yet existing methods are limited in accuracy, throughput, and duration, primarily via washout, membrane damage, and movement-induced failure. Here, we introduce flexible quartz nanopipettes (inner diameters of 10-25 nm and spring constant of ∼0.08 N/m) as nanoscale analogs of traditional glass microelectrodes. Nanopipettes enable stable intracellular recordings (seal resistances of 500 to ∼800 MΩ, 5 to ∼10 cells/nanopipette, and duration of ∼1 hr) in anaesthetized and awake head-restrained mice, exhibit minimal diffusional flux, and facilitate precise recording and stimulation. When combined with quantum-dot labels and microprisms, nanopipettes enable two-photon targeted electrophysiology from both somata and dendrites, and even paired recordings from neighboring neurons, while permitting simultaneous population imaging across cortical layers. We demonstrate the versatility of this method by recording from parvalbumin-positive (Pv) interneurons while imaging seizure propagation, and we find that Pv depolarization block coincides with epileptic spread. Flexible nanopipettes present a simple method to procure stable intracellular recordings in vivo.

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.

GABAergic Interneurons in Seizures: Investigating Causality With Optogenetics

Seizures are complex pathological network events characterized by excessive and hypersynchronized activity of neurons, including a highly diverse population of GABAergic interneurons. Although the primary function of inhibitory interneurons under normal conditions is to restrain excitation in the brain, this system appears to fail intermittently, allowing runaway excitation. Recent developments in optogenetics, combined with genetic tools and advanced electrophysiological and imaging techniques, allow us for the first time to assess the causal roles of identified cell-types in network dynamics. While these methods have greatly increased our understanding of cortical microcircuits in epilepsy, the roles played by individual GABAergic cell-types in controlling ictogenesis remain incompletely resolved. Indeed, the ability of interneurons to suppress epileptic discharges varies across different subtypes, and an accumulating body of evidence paradoxically implicates some interneuron subtypes in the initiation and maintenance of epileptiform activity. Here, we bring together findings from this growing field and discuss what can be inferred regarding the causal role of different GABAergic cell-types in seizures.

Input-specific maturation of NMDAR-mediated transmission onto parvalbumin-expressing interneurons in layers 2/3 of the visual cortex

Parvalbumin-expressing (PV) GABAergic interneurons regulate local circuit dynamics. In terms of the excitation driving PV interneuron activity, the N-methyl-d-aspartate receptor (NMDAR)-mediated component onto PV interneurons tends to be smaller than that onto pyramidal neurons but makes a significant contribution to their physiology and development. In the visual cortex, PV interneurons mature during the critical period. We hypothesize that during the critical period, the NMDAR-mediated signaling and functional properties of glutamatergic synapses onto PV interneurons are developmentally regulated. We therefore compared the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)- and NMDAR-mediated synaptic responses before (postnatal days 15-20, P15-P20), during (P25-P40), and after (P50-P60) the visual critical period. AMPAR miniature excitatory postsynaptic currents (mEPSCs) showed a developmental decrease in frequency, whereas NMDAR mEPSCs were absent or showed extremely low frequencies throughout development. For evoked responses, we consistently saw a NMDAR-mediated component, suggesting pre- or postsynaptic differences between evoked and spontaneous neurotransmission. Evoked responses showed input-specific developmental changes. For intralaminar inputs, the NMDAR-mediated component significantly decreased with development. This resulted in adult intralaminar inputs almost exclusively mediated by AMPARs, suited for the computation of synaptic inputs with precise timing, and likely having NMDAR-independent forms of plasticity. In contrast, interlaminar inputs maintained a stable NMDAR-mediated component throughout development but had a shift in the AMPAR paired-pulse ratio from depression to facilitation. Adult interlaminar inputs with facilitating AMPAR responses and a substantial NMDAR component would favor temporal integration of synaptic responses and could be modulated by NMDAR-dependent forms of plasticity. NEW & NOTEWORTHY We show for the first time input-specific developmental changes in the N-methyl-d-aspartate receptor component and short-term plasticity of the excitatory drive onto layers 2/3 parvalbumin-expressing (PV) interneurons in the visual cortex during the critical period. These developmental changes would lead to functionally distinct adult intralaminar and interlaminar glutamatergic inputs that would engage PV interneuron-mediated inhibition differently.

Voltage-Dependent Calcium Channels, Calcium Binding Proteins, and Their Interaction in the Pathological Process of Epilepsy

As an important second messenger, the calcium ion (Ca²⁺) plays a vital role in normal brain function and in the pathophysiological process of different neurodegenerative diseases including Alzheimer’s disease (AD), Parkinson’s disease (PD), and epilepsy. Ca²⁺ takes part in the regulation of neuronal excitability, and the imbalance of intracellular Ca²⁺ is a trigger factor for the occurrence of epilepsy. Several anti-epileptic drugs target voltage-dependent calcium channels (VDCCs). Intracellular Ca²⁺ levels are mainly controlled by VDCCs located in the plasma membrane, the calcium-binding proteins (CBPs) inside the cytoplasm, calcium channels located on the intracellular calcium store (particular the endoplasmic reticulum/sarcoplasmic reticulum), and the Ca²⁺-pumps located in the plasma membrane and intracellular calcium store. So far, while many studies have established the relationship between calcium control factors and epilepsy, the mechanism of various Ca²⁺ regulatory factors in epileptogenesis is still unknown. In this paper, we reviewed the function, distribution, and alteration of VDCCs and CBPs in the central nervous system in the pathological process of epilepsy. The interaction of VDCCs with CBPs in the pathological process of epilepsy was also summarized. We hope this review can provide some clues for better understanding the mechanism of epileptogenesis, and for the development of new anti-epileptic drugs targeting on VDCCs and CBPs.

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.

Differential Roles of Mitochondrial Translocation of Active Caspase-3 and HMGB1 in Neuronal Death Induced by Status Epilepticus

Under pathophysiological conditions, aberrant mitochondrial dynamics lead to the different types of neuronal death: excessive mitochondrial fission provokes apoptosis and abnormal mitochondrial elongation induces necrosis. However, the underlying mechanisms how the different mitochondrial dynamics result in the distinct neuronal death patterns have been elusive. In the present study, status epilepticus (SE) evoked excessive mitochondrial fission in parvalbumin (PV) cells (one of GABAergic interneurons) and abnormal mitochondrial elongation in CA1 neurons in the rat hippocampus. These impaired mitochondrial dynamics were accompanied by mitochondrial translocations of active caspase-3 and high mobility group box 1 (HMGB1) in PV cells and CA1 neurons, respectively. WY14643 (an activator of mitochondrial fission) aggravated SE-induced PV cell loss by enhancing active caspase-3 induction and its mitochondrial translocation, which were attenuated by Mdivi-1 (an inhibitor of mitochondrial fission). Mitochondrial HMGB1 import was not observed in PV cell. In contrast to PV cells, Mdivi-1 deteriorated SE-induced CA1 neuronal death concomitant with mitochondrial HMGB1 translocation, which was abrogated by WY14643. These findings suggest that SE-induced aberrant mitochondrial dynamics may be involved in translocation of active caspase-3 and HMGB1 into mitochondria, which regulate neuronal apoptosis and necrosis, respectively.

Pyramidal cell activity levels affect the polarity of activity-induced gene transcription changes in interneurons

Changes in gene expression are an important mechanism by which activity levels are regulated in the nervous system. It is not known, however, how network activity influences gene expression in interneurons; since they themselves provide negative feedback in the form of synaptic inhibition, there exists a potential conflict between their cellular homeostatic tendencies and those of the network. We present a means of examining this issue, utilizing simple in vitro models showing different patterns of intense network activity. We found that the degree of concurrent pyramidal activation changed the polarity of the induced gene transcription. When pyramidal cells were quiescent, interneuronal activation led to an upregulation of glutamate decarboxylase 1 ( GAD1) and parvalbumin ( Pvalb) gene transcriptions, mediated by activation of the Ras/extracellular signal-related kinase mitogen-activated protein kinase (Ras/ERK MAPK) pathway. In contrast, coactivation of pyramidal cells led to an ionotropic glutamate receptor N-methyl-d-aspartate 2B-dependent decrease in transcription. Our results demonstrate a hitherto unrecognized complexity in how activity-dependent gene expression changes are manifest in cortical networks. NEW & NOTEWORTHY We demonstrate a novel feedback mechanism in cortical networks, by which glutamatergic drive, mediated through the Ras/ERK MAPK pathway, regulates gene transcription in interneurons. Using a unique feature of certain in vitro epilepsy models, we show that without this glutamatergic feedback, intense activation of interneurons causes parvalbumin and glutamate decarboxylase 1 mRNA expression to increase. If, on the other hand, pyramidal cells are coactivated with interneurons, this leads to a downregulation of these genes.

A Blind Module Identification Approach for Predicting Effective Connectivity Within Brain Dynamical Subnetworks

Model-based network discovery measures, such as the brain effective connectivity, require fitting of generative process models to measurements obtained from key areas across the network. For distributed dynamic phenomena, such as generalized seizures and slow-wave sleep, studying effective connectivity from real-time recordings is significantly complicated since (i) outputs from only a subnetwork can be practically measured, and (ii) exogenous subnetwork inputs are unobservable. Model fitting, therefore, constitutes a challenging blind module identification or model inversion problem for finding both the parameters and the many unknown inputs of the subnetwork. We herein propose a novel estimation framework for identifying nonlinear dynamic subnetworks in the case of slowly-varying, otherwise unknown local inputs. Starting with approximate predictions obtained using Cubature Kalman filtering, residuals of local output predictions are utilized to improve upon local input estimates. The algorithm performance is tested on both simulated and clinical EEG of induced seizures under electroconvulsive therapy (ECT). For the simulated network, the algorithm significantly boosted the estimation accuracy for inputs and connections from noisy EEG. For the clinical data, the algorithm predicted increased subnetwork inputs during the pre-stimulus anesthesia condition. Importantly, it predicted an increased frontocentral connectivity during the generalized seizure that is commensurate with electrode placement and that corroborates the clinical hypothesis of increased frontal focality of therapeutic ECT seizures. The proposed framework can be extended to account for several input configurations and can in principle be applied to study effective connectivity within brain subnetworks defined at the microscale (cortical lamina interaction) or at the macroscale (sensory integration).

Specificity, Versatility, and Continual Development: The Power of Optogenetics for Epilepsy Research

Optogenetics is a powerful and rapidly expanding set of techniques that use genetically encoded light sensitive proteins such as opsins. Through the selective expression of these exogenous light-sensitive proteins, researchers gain the ability to modulate neuronal activity, intracellular signaling pathways, or gene expression with spatial, directional, temporal, and cell-type specificity. Optogenetics provides a versatile toolbox and has significantly advanced a variety of neuroscience fields. In this review, using recent epilepsy research as a focal point, we highlight how the specificity, versatility, and continual development of new optogenetic related tools advances our understanding of neuronal circuits and neurological disorders. We additionally provide a brief overview of some currently available optogenetic tools including for the selective expression of opsins.

Mechanisms of increased hippocampal excitability in the Mashl+/- mouse model of Na+ /K+ -ATPase dysfunction

OBJECTIVE

Na⁺ /K⁺ -ATPase dysfunction, primary (mutation) or secondary (energy crisis, neurodegenerative disease) increases neuronal excitability in the brain. To evaluate the mechanisms underlying such increased excitability we studied mice carrying the D801N mutation, the most common mutation causing human disease, specifically alternating hemiplegia of childhood (AHC) including epilepsy. Because the gene is expressed in all neurons, particularly γ-aminobutyric acid (GABA)ergic interneurons, we hypothesized that the pathophysiology would involve both pyramidal cells and interneurons and that fast-spiking interneurons, which have increased firing rates, would be most vulnerable.

METHODS

We performed extracellular recordings, as well as whole-cell patch clamp recordings from pyramidal cells and interneurons, in the CA1 region on hippocampal slices. We also performed immunohistochemistry from hippocampal sections to count CA1 pyramidal cells as well as parvalbumin-positive interneurons. In addition, we performed video-electroencephalography (EEG) recordings from the dorsal hippocampal CA1 region.

RESULTS

We observed that juvenile knock-in mice carrying the above mutation reproduce the human phenotype of AHC. We then demonstrated in the CA1 region of these mice the following findings as compared to wild type: (1) Increased number of spikes evoked by electrical stimulation of Schaffer collaterals; (2) equalization by bicuculline of the number of spikes induced by Schaffer collateral stimulation; (3) reduced miniature, spontaneous, and evoked inhibitory postsynaptic currents, but no change in excitatory postsynaptic currents; (4) robust action potential frequency adaptation in response to depolarizing current injection in CA1 fast-spiking interneurons; and (5) no change in the number of pyramidal cells, but reduced number of parvalbumin positive interneurons.

SIGNIFICANCE

Our data indicate that, in our genetic model of Atp1α3 mutation, there is increased excitability and marked dysfunction in GABAergic inhibition. This supports the performance of further investigations to determine if selective expression of the mutation in GABAergic and or glutamatergic neurons is necessary and sufficient to result in the behavioral phenotype.

Initiation and slow propagation of epileptiform activity from ventral to dorsal medial entorhinal cortex is constrained by an inhibitory gradient

KEY POINTS

The medial entorhinal cortex (mEC) has an important role in initiation and propagation of seizure activity. Several anatomical relationships exist in neurophysiological properties of mEC neurons; however, in the context of hyperexcitability, previous studies often considered it as a homogeneous structure. Using multi-site extracellular recording techniques, ictal-like activity was observed along the dorso-ventral axis of the mEC in vitro in response to various ictogenic stimuli. This originated predominantly from ventral areas, spreading to dorsal mEC with a surprisingly slow velocity. Modulation of inhibitory tone was capable of changing the slope of ictal initiation, suggesting seizure propagation behaviours are highly dependent on levels of GABAergic function in this region. A distinct disinhibition model also showed, in the absence of inhibition, a prevalence for interictal-like initiation in ventral mEC, reflecting the intrinsic differences in mEC neurons. These findings suggest the ventral mEC is more prone to hyperexcitable discharge than the dorsal mEC, which may be relevant under pathological conditions.

ABSTRACT

The medial entorhinal cortex (mEC) has an important role in the generation and propagation of seizure activity. The organization of the mEC is such that a number of dorso-ventral relationships exist in neurophysiological properties of neurons. These range from intrinsic and synaptic properties to density of inhibitory connectivity. We examined the influence of these gradients on generation and propagation of epileptiform activity in the mEC. Using a 16-shank silicon probe array to record along the dorso-ventral axis of the mEC in vitro, we found 4-aminopyridine application produces ictal-like activity originating predominantly in ventral areas. This activity spreads to dorsal mEC at a surprisingly slow velocity (138 μm s^-1 ), while cross-site interictal-like activity appeared relatively synchronous. We propose that ictal propagation is constrained by differential levels of GABAergic control since increasing (diazepam) or decreasing (Ro19-4603) GABA_A receptor activation, respectively, reduced or increased the slope of ictal initiation. The observation that ictal activity is predominately generated in ventral mEC was replicated using a separate 0-Mg²⁺ model of epileptiform activity in vitro. By using a distinct disinhibition model (co-application of kainate and picrotoxin) we show that additional physiological features (for example intrinsic properties of mEC neurons) still produce a prevalence for interictal-like initiation in ventral mEC. These findings suggest that the ventral mEC is more likely to initiate hyperexcitable discharges than the dorsal mEC, and that seizure propagation is highly dependent on levels of GABAergic expression across the mEC.

Reliable and Elastic Propagation of Cortical Seizures In Vivo

Mapping the fine-scale neural activity that underlies epilepsy is key to identifying potential control targets of this frequently intractable disease. Yet, the detailed in vivo dynamics of seizure progression in cortical microcircuits remain poorly understood. We combine fast (30-Hz) two-photon calcium imaging with local field potential (LFP) recordings to map, cell by cell, the spread of locally induced (4-AP or picrotoxin) seizures in anesthetized and awake mice. Using single-layer and microprism-assisted multilayer imaging in different cortical areas, we uncover reliable recruitment of local neural populations within and across cortical layers, and we find layer-specific temporal delays, suggesting an initial supra-granular invasion followed by deep-layer recruitment during lateral seizure spread. Intriguingly, despite consistent progression pathways, successive seizures show pronounced temporal variability that critically depends on GABAergic inhibition. We propose an epilepsy circuit model resembling an elastic meshwork, wherein ictal progression faithfully follows preexistent pathways but varies flexibly in time, depending on the local inhibitory restraint.

Simultaneous profiling of activity patterns in multiple neuronal subclasses

BACKGROUND

Neuronal networks typically comprise heterogeneous populations of neurons. A core objective when seeking to understand such networks, therefore, is to identify what roles these different neuronal classes play. Acquiring single cell electrophysiology data for multiple cell classes can prove to be a large and daunting task. Alternatively, Ca²⁺ network imaging provides activity profiles of large numbers of neurons simultaneously, but without distinguishing between cell classes.

NEW METHOD

We therefore developed a strategy for combining cellular electrophysiology, Ca²⁺ network imaging, and immunohistochemistry to provide activity profiles for multiple cell classes at once. This involves cross-referencing easily identifiable landmarks between imaging of the live and fixed tissue, and then using custom MATLAB functions to realign the two imaging data sets, to correct for distortions of the tissue introduced by the fixation or immunohistochemical processing.

RESULTS

We illustrate the methodology for analyses of activity profiles during epileptiform events recorded in mouse brain slices. We further demonstrate the activity profile of a population of parvalbumin-positive interneurons prior, during, and following a seizure-like event.

COMPARISON WITH EXISTING METHODS

Current approaches to Ca²⁺ network imaging analyses are severely limited in their ability to subclassify neurons, and often rely on transgenic approaches to identify cell classes. In contrast, our methodology is a generic, affordable, and flexible technique to characterize neuronal behaviour with respect to classification based on morphological and neurochemical identity.

CONCLUSIONS

We present a new approach for analysing Ca²⁺ network imaging datasets, and use this to explore the parvalbumin-positive interneuron activity during epileptiform events.

Increased neuronal synchrony prepares mesial temporal networks for seizures of neocortical origin

OBJECTIVE

To gain understanding of the neuronal mechanisms underlying regional seizure spread, the impact of regional synchrony between seizure focus and downstream networks on neuronal activity during the transition to seizure in those downstream networks was assessed.

METHODS

Seven patients undergoing diagnostic intracranial electroencephalographic studies for surgical resection of epileptogenic regions were implanted with subdural clinical electrodes into the cortex (site of seizure initiation) and mesial temporal lobe (MTL) structures (downstream) as well as microwires into MTL. Neural activity was recorded (24/7) in parallel with the clinical intracranial electroencephalogram recordings for the duration of the patient's diagnostic stay. Changes in (1) regional synchrony (ie, coherence) between the presumptive neocortical seizure focus and MTL, (2) local synchrony between MTL neurons and their local field potential, and (3) neuronal firing rates within MTL in the time leading up to seizure were examined to study the mechanisms underlying seizure spread.

RESULTS

In seizures of neocortical origin, an increase in regional synchrony preceded the spread of seizures into MTL (predominantly hippocampal). Within frequencies similar to those of regional synchrony, MTL networks showed an increase in unit-field coherence and a decrease in neuronal firing rate, specifically for inhibitory interneuron populations but not pyramidal cell populations.

SIGNIFICANCE

These results suggest a mechanism of spreading seizures whereby the seizure focus first synchronizes local field potentials in downstream networks to the seizure activity. This change in local field coherence modifies the activity of interneuron populations in these downstream networks, which leads to the attenuation of interneuronal firing rate, effectively shutting down local interneuron populations prior to the spread of seizure. Therefore, regional synchrony may influence the failure of downstream interneurons to prevent the spread of the seizures during generalization.

TRPC6-mediated ERK1/2 Activation Regulates Neuronal Excitability via Subcellular Kv4.3 Localization in the Rat Hippocampus

Recently, we have reported that transient receptor potential channel-6 (TRPC6) plays an important role in the regulation of neuronal excitability and synchronization of spiking activity in the dentate granule cells (DGC). However, the underlying mechanisms of TRPC6 in these phenomena have been still unclear. In the present study, we investigated the role of TRPC6 in subcellular localization of Kv4.3 and its relevance to neuronal excitability in the rat hippocampus. TRPC6 knockdown increased excitability and inhibitory transmission in the DGC and the CA1 neurons in response to a paired-pulse stimulus. However, TRPC6 knockdown impaired γ-aminobutyric acid (GABA)ergic inhibition in the hippocampus during and after high-frequency stimulation (HFS). TRPC6 knockdown reduced the Kv4.3 clusters in membrane fractions and its dendritic localization on DGC and GABAergic interneurons. TRPC6 knockdown also decreased extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation and the efficacy of 4-aminopyridine (4-AP) in neuronal excitability. An ERK1/2 inhibitor generated multiple population spikes in response to a paired-pulse stimulus, concomitant with reduced membrane Kv4.3 translocation. A TRPC6 activator (hyperforin) reversed the effects of TRPC knockdown, except paired-pulse inhibition. These findings provide valuable clues indicating that TRPC6-mediated ERK1/2 activation may regulate subcellular Kv4.3 localization in DGC and interneurons, which is cause-effect relationship between neuronal excitability and seizure susceptibility.

Persistent seizure control in epileptic mice transplanted with gamma-aminobutyric acid progenitors

OBJECTIVE

A significant proportion of the more than 50 million people worldwide currently suffering with epilepsy are resistant to antiepileptic drugs (AEDs). As an alternative to AEDs, novel therapies based on cell transplantation offer an opportunity for long-lasting modification of epileptic circuits. To develop such a treatment requires careful preclinical studies in a chronic epilepsy model featuring unprovoked seizures, hippocampal histopathology, and behavioral comorbidities.

METHODS

Transplantation of progenitor cells from embryonic medial or caudal ganglionic eminence (MGE, CGE) were made in a well-characterized mouse model of status epilepticus-induced epilepsy (systemic pilocarpine). Behavioral testing (handling and open field), continuous video-electroencephalographic (vEEG) monitoring, and slice electrophysiology outcomes were obtained up to 270 days after transplantation (DAT). Post-hoc immunohistochemistry was used to confirm cell identity.

RESULTS

MGE progenitors transplanted into the hippocampus of epileptic mice rescued handling and open field deficits starting at 60 DAT. In these same mice, an 84% to 88% reduction in seizure activity was observed between 180 and 210 DAT. Inhibitory postsynaptic current frequency, measured on pyramidal neurons in acute hippocampal slices at 270 DAT, was reduced in epileptic mice but restored to naïve levels in epileptic mice receiving MGE transplants. No reduction in seizure activity was observed in epileptic mice receiving intrahippocampal CGE progenitors.

INTERPRETATION

Our findings demonstrate that transplanted MGE progenitors enhance functional GABA-mediated inhibition, reduce spontaneous seizure frequency, and rescue behavioral deficits in a chronic epileptic animal model more than 6 months after treatment. Ann Neurol 2017;82:530-542.

Interneuronal Network Activity at the Onset of Seizure-Like Events in Entorhinal Cortex Slices

The onset of focal seizures in humans and in different animal models of focal epilepsy correlates with reduction of neuronal firing and enhanced interneuronal network activity. Whether this phenomenon contributes to seizure generation is still unclear. We used the in vitro entorhinal cortex slices bathed in 4-aminopirydine (4-AP) as an experimental paradigm model to evaluate the correlation between interneuronal GABAergic network activity and seizure-like events. Epileptiform discharges were recorded in layer V-VI pyramidal neurons and fast-spiking interneurons in slices from male and female mice and in the isolated female guinea pig brain preparation during perfusion with 4-AP. We observed that 90% of seizure-like events recorded in principal cells were preceded by outward currents coupled with extracellular potassium shifts, abolished by pharmacological blockade of GABA_A receptors. Potassium elevations associated to GABA_A receptor-mediated population events were confirmed in the entorhinal cortex of the in vitro isolated whole guinea pig brain. Fast-rising and sustained extracellular potassium increases associated to interneuronal network activity consistently preceded the initiation of seizure-like events. We conclude that in the 4-AP seizure model, interneuronal network activity occurs before 4-AP-induced seizures and therefore supports a role of interneuron activity in focal seizure generation.SIGNIFICANCE STATEMENT The paper focuses on the mechanisms of ictogenesis, a topic that requires a step beyond the simplistic view that seizures, and epilepsy, are due to an increase of excitatory network activity. Focal temporal lobe seizures in humans and in several experimental epilepsies likely correlate with a prevalent activation of interneurons. The potassium channel blocker 4-aminopyridine reliably induces seizure-like events in temporal lobe structures. Herein, we show that a majority of seizures in the entorhinal cortex starts with interneuronal network activity accompanied by a fast and sustained increase in extracellular potassium. Our new findings reinforce and add a new piece of evidence to the proposal that limbic seizures can be supported by GABAergic hyperactivity.

p47Phox/CDK5/DRP1-Mediated Mitochondrial Fission Evokes PV Cell Degeneration in the Rat Dentate Gyrus Following Status Epilepticus

Parvalbumin (PV) is one of the calcium-binding proteins, which plays an important role in the responsiveness of inhibitory neurons to an adaptation to repetitive spikes. Furthermore, PV neurons are highly vulnerable to status epilepticus (SE, prolonged seizure activity), although the underlining mechanism remains to be clarified. In the present study, we found that p47Phox expression was transiently and selectively increased in PV neurons 6 h after SE. This up-regulated p47Phox expression was accompanied by excessive mitochondrial fission. In this time point, CDK5-tyrosine 15 and dynamin-related protein 1 (DRP1)-serine 616 phosphorylations were also increased in PV cells. Apocynin (a p47Phox inhibitor) effectively mitigated PV cell loss via inhibition of CDK5/DRP1 phosphorylations and mitochondrial fragmentation induced by SE. Roscovitine (a CDK5 inhibitor) and Mdivi-1 (a DRP1 inhibitor) attenuated SE-induced PV cell loss by inhibiting aberrant mitochondrial fission. These findings suggest that p47Phox/CDK5/DRP1 may be one of the important upstream signaling pathways in PV cell degeneration induced by SE via excessive mitochondrial fragmentation.

TGFβ signaling is associated with changes in inflammatory gene expression and perineuronal net degradation around inhibitory neurons following various neurological insults

Brain damage due to stroke or traumatic brain injury (TBI), both leading causes of serious long-term disability, often leads to the development of epilepsy. Patients who develop post-injury epilepsy tend to have poor functional outcomes. Emerging evidence highlights a potential role for blood-brain barrier (BBB) dysfunction in the development of post-injury epilepsy. However, common mechanisms underlying the pathological hyperexcitability are largely unknown. Here, we show that comparative transcriptome analyses predict remodeling of extracellular matrix (ECM) as a common response to different types of injuries. ECM-related transcriptional changes were induced by the serum protein albumin via TGFβ signaling in primary astrocytes. In accordance with transcriptional responses, we found persistent degradation of protective ECM structures called perineuronal nets (PNNs) around fast-spiking inhibitory interneurons, in a rat model of TBI as well as in brains of human epileptic patients. Exposure of a naïve brain to albumin was sufficient to induce the transcriptional and translational upregulation of molecules related to ECM remodeling and the persistent breakdown of PNNs around fast-spiking inhibitory interneurons, which was contingent on TGFβ signaling activation. Our findings provide insights on how albumin extravasation that occurs upon BBB dysfunction in various brain injuries can predispose neural circuitry to the development of chronic inhibition deficits.

Selective Silencing of Hippocampal Parvalbumin Interneurons Induces Development of Recurrent Spontaneous Limbic Seizures in Mice

Temporal lobe epilepsy (TLE) is the most frequent form of focal epilepsies and is generally associated with malfunctioning of the hippocampal formation. Recently, a preferential loss of parvalbumin (PV) neurons has been observed in the subiculum of TLE patients and in animal models of TLE. To demonstrate a possible causative role of defunct PV neurons in the generation of TLE, we permanently inhibited GABA release selectively from PV neurons of the ventral subiculum by injecting a viral vector expressing tetanus toxin light chain in male mice. Subsequently, mice were subjected to telemetric EEG recording and video monitoring. Eighty-eight percent of the mice presented clusters of spike-wave discharges (C-SWDs; 40.0 ± 9.07/month), and 64% showed spontaneous recurrent seizures (SRSs; 5.3 ± 0.83/month). Mice injected with a control vector presented with neither C-SWDs nor SRSs. No neurodegeneration was observed due to vector injection or SRS. Interestingly, mice that presented with only C-SWDs but no SRSs, developed SRSs upon injection of a subconvulsive dose of pentylenetetrazole after 6 weeks. The initial frequency of SRSs declined by ∼30% after 5 weeks. In contrast to permanent silencing of PV neurons, transient inhibition of GABA release from PV neurons through the designer receptor hM4Di selectively expressed in PV-containing neurons transiently reduced the seizure threshold of the mice but induced neither acute nor recurrent seizures. Our data demonstrate a critical role for perisomatic inhibition mediated by PV-containing interneurons, suggesting that their sustained silencing could be causally involved in the development of TLE.SIGNIFICANCE STATEMENT Development of temporal lobe epilepsy (TLE) generally takes years after an initial insult during which maladaptation of hippocampal circuitries takes place. In human TLE and in animal models of TLE, parvalbumin neurons are selectively lost in the subiculum, the major output area of the hippocampus. The present experiments demonstrate that specific and sustained inhibition of GABA release from parvalbumin-expressing interneurons (mostly basket cells) in sector CA1/subiculum is sufficient to induce hyperexcitability and spontaneous recurrent seizures in mice. As in patients with nonlesional TLE, these mice developed epilepsy without signs of neurodegeneration. The experiments highlight the importance of the potent inhibitory action mediated by parvalbumin cells in the hippocampus and identify a potential mechanism in the development of TLE.

Hypocretin (orexin) is critical in sustaining theta/gamma-rich waking behaviors that drive sleep need

Hcrt gene inactivation in mice leads to behavioral state instability, abnormal transitions to paradoxical sleep, and cataplexy, hallmarks of narcolepsy. Sleep homeostasis is, however, considered unimpaired in patients and narcoleptic mice. We find that whereas Hcrt^ko/ko mice respond to 6-h sleep deprivation (SD) with a slow-wave sleep (SWS) EEG δ (1.0 to 4.0 Hz) power rebound like WT littermates, spontaneous waking fails to induce a δ power reflecting prior waking duration. This correlates with impaired θ (6.0 to 9.5 Hz) and fast-γ (55 to 80 Hz) activity in prior waking. We algorithmically identify a theta-dominated wakefulness (TDW) substate underlying motivated behaviors and typically preceding cataplexy in Hcrt^ko/ko mice. Hcrt^ko/ko mice fully implement TDW when waking is enforced, but spontaneous TDW episode duration is greatly reduced. A reformulation of the classic sleep homeostasis model, where homeostatic pressure rises exclusively in TDW rather than all waking, predicts δ power dynamics both in Hcrt^ko/ko and WT mouse baseline and recovery SWS. The low homeostatic impact of Hcrt^ko/ko mouse spontaneous waking correlates with decreased cortical expression of neuronal activity-related genes (notably Bdnf, Egr1/Zif268, and Per2). Thus, spontaneous TDW stability relies on Hcrt to sustain θ/fast-γ network activity and associated plasticity, whereas other arousal circuits sustain TDW during SD. We propose that TDW identifies a discrete global brain activity mode that is regulated by context-dependent neuromodulators and acts as a major driver of sleep homeostasis. Hcrt loss in Hcrt^ko/ko mice causes impaired TDW maintenance in baseline wake and blunted δ power in SWS, reproducing, respectively, narcolepsy excessive daytime sleepiness and poor sleep quality.

The influence of depolarization block on seizure-like activity in networks of excitatory and inhibitory neurons

The inhibitory restraint necessary to suppress aberrant activity can fail when inhibitory neurons cease to generate action potentials as they enter depolarization block. We investigate possible bifurcation structures that arise at the onset of seizure-like activity resulting from depolarization block in inhibitory neurons. Networks of conductance-based excitatory and inhibitory neurons are simulated to characterize different types of transitions to the seizure state, and a mean field model is developed to verify the generality of the observed phenomena of excitatory-inhibitory dynamics. Specifically, the inhibitory population's activation function in the Wilson-Cowan model is modified to be non-monotonic to reflect that inhibitory neurons enter depolarization block given strong input. We find that a physiological state and a seizure state can coexist, where the seizure state is characterized by high excitatory and low inhibitory firing rate. Bifurcation analysis of the mean field model reveals that a transition to the seizure state may occur via a saddle-node bifurcation or a homoclinic bifurcation. We explain the hysteresis observed in network simulations using these two bifurcation types. We also demonstrate that extracellular potassium concentration affects the depolarization block threshold; the consequent changes in bifurcation structure enable the network to produce the tonic to clonic phase transition observed in biological epileptic networks.

Optogenetic Low-Frequency Stimulation of Specific Neuronal Populations Abates Ictogenesis

Despite many advances made in understanding the pathophysiology of epileptic disorders, seizures remain poorly controlled in approximately one-third of patients with mesial temporal lobe epilepsy. Here, we established the efficacy of cell type-specific low-frequency stimulation (LFS) in controlling ictogenesis in the mouse entorhinal cortex (EC) in an in vitro brain slice preparation. Specifically, we used 1 Hz optogenetic stimulation of calcium/calmodulin-dependent protein kinase II-positive principal cells as well as of parvalbumin- or somatostatin-positive interneurons to study the effects of such repetitive activation on epileptiform discharges induced by 4-aminopyridine. We found that 1 Hz stimulation of any of these cell types reduced the frequency and duration of ictal discharges in some trials, while completely blocking them in others. The field responses evoked by the stimulation of each cell type revealed that their duration and amplitude were higher when principal cells were targeted. Furthermore, following a short period of silence ranging from 67 to 135 s, ictal discharges were re-established with similar duration and frequency as before stimulation; however, this period of silence was longer following principal cell stimulation compared with parvalbumin- or somatostatin-positive interneuron stimulation. Our results show that LFS of either excitatory or inhibitory cell networks in EC are effective in controlling ictogenesis. Although optogenetic stimulation of either cell type significantly reduced the occurrence of ictal discharges, principal cell stimulation resulted in a more prolonged suppression of ictogenesis, and, thus, it may constitute a better approach for controlling seizures.SIGNIFICANCE STATEMENT Epilepsy is a neurological disorder characterized by an imbalance between excitation and inhibition leading to seizures. Many epileptic patients do not achieve adequate seizure control using antiepileptic drugs. Low-frequency stimulation (LFS) is an alternative tool for controlling epileptiform activity in these patients. However, despite the temporal and spatial control offered by LFS, such a procedure lacks cell specificity, which may limit its efficacy. Using an optogenetic approach, we report here that LFS of two interneuron subtypes and, even more so, of principal cells can reliably shorten or abolish seizures in vitro Our work suggests that targeted LFS may constitute a reliable means for controlling seizures in patients presenting with focal seizures.

α4β2∗ nicotinic receptors stimulate GABA release onto fast-spiking cells in layer V of mouse prefrontal (Fr2) cortex

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α4β2^∗ nAChRs stimulate IPSCs in FS interneurons, in layer V of the mouse PFC (Fr2).

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In P16–P63 mice, nicotine increased both IPSC and mIPSC frequencies.

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GABAergic terminals adjacent to PV+ cells expressed α4 nAChR.

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The percentage of FS cells with somatic α4β2^∗ currents decreased with age.

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Hence, nAChRs may be able to induce local circuit disinhibition in Fr2 PFC.

Nicotinic acetylcholine receptors (nAChRs) produce widespread and complex effects on neocortex excitability. We studied how heteromeric nAChRs regulate inhibitory post-synaptic currents (IPSCs), in fast-spiking (FS) layer V neurons of the mouse frontal area 2 (Fr2). In the presence of blockers of ionotropic glutamate receptors, tonic application of 10 μM nicotine augmented the spontaneous IPSC frequency, with minor alterations of amplitudes and kinetics. These effects were studied since the 3rd postnatal week, and persisted throughout the first two months of postnatal life. The action of nicotine was blocked by 1 μM dihydro-β-erythroidine (DHβE; specific for α4^∗ nAChRs), but not 10 nM methyllycaconitine (MLA; specific for α7^∗ nAChRs). It was mimicked by 10 nM 5-iodo-3-[2(S)-azetidinylmethoxy]pyridine (5-IA; which activates β2^∗ nAChRs). Similar results were obtained on miniature IPSCs (mIPSCs). Moreover, during the first five postnatal weeks, approximately 50% of FS cells displayed DHβE-sensitive whole-cell nicotinic currents. This percentage decreased to ∼5% in mice older than P45. By confocal microscopy, the α4 nAChR subunit was immunocytochemically identified on interneurons expressing either parvalbumin (PV), which mainly labels FS cells, or somatostatin (SOM), which labels the other major interneuron population in layer V. GABAergic terminals expressing α4 were observed to be juxtaposed to PV-positive (PV+) cells. A fraction of these terminals displayed PV immunoreactivity. We conclude that α4β2^∗ nAChRs can produce sustained regulation of FS cells in Fr2 layer V. The effect presents a presynaptic component, whereas the somatic regulation decreases with age. These mechanisms may contribute to the nAChR-dependent stimulation of excitability during cognitive tasks as well as to the hyperexcitability caused by hyperfunctional heteromeric nAChRs in sleep-related epilepsy.

Interaction between synaptic inhibition and glial-potassium dynamics leads to diverse seizure transition modes in biophysical models of human focal seizures

How focal seizures initiate and evolve in human neocortex remains a fundamental problem in neuroscience. Here, we use biophysical neuronal network models of neocortical patches to study how the interaction between inhibition and extracellular potassium ([K (+)] o ) dynamics may contribute to different types of focal seizures. Three main types of propagated focal seizures observed in recent intracortical microelectrode recordings in humans were modelled: seizures characterized by sustained (∼30-60 Hz) gamma local field potential (LFP) oscillations; seizures where the onset in the propagated site consisted of LFP spikes that later evolved into rhythmic (∼2-3 Hz) spike-wave complexes (SWCs); and seizures where a brief stage of low-amplitude fast-oscillation (∼10-20 Hz) LFPs preceded the SWC activity. Our findings are fourfold: (1) The interaction between elevated [K (+)] o (due to abnormal potassium buffering by glial cells) and the strength of synaptic inhibition plays a predominant role in shaping these three types of seizures. (2) Strengthening of inhibition leads to the onset of sustained narrowband gamma seizures. (3) Transition into SWC seizures is obtained either by the weakening of inhibitory synapses, or by a transient strengthening followed by an inhibitory breakdown (e.g. GABA depletion). This reduction or breakdown of inhibition among fast-spiking (FS) inhibitory interneurons increases their spiking activity and leads them eventually into depolarization block. Ictal spike-wave discharges in the model are then sustained solely by pyramidal neurons. (4) FS cell dynamics are also critical for seizures where the evolution into SWC activity is preceded by low-amplitude fast oscillations. Different levels of elevated [K (+)] o were important for transitions into and maintenance of sustained gamma oscillations and SWC discharges. Overall, our modelling study predicts that the interaction between inhibitory interneurons and [K (+)] o glial buffering under abnormal conditions may explain different types of ictal transitions and dynamics during propagated seizures in human focal epilepsy.

Do Cortical Circuits Need Protecting from Themselves?

All hippocampal and neocortical networks can be driven to seize quite easily. This can be done using drugs, by altering the ionic constituency of the bathing medium [cerebrospinal fluid (CSF)], or by electrical stimulation (both experimentally and clinically, as in electroconvulsive therapy). It is worth asking why this is so, because this will both tell us more about potentially devastating neurological disorders and extend our understanding of cortical function and architecture. Here I review work examining the features of cortical networks that bias activity towards and away from hyperexcitability. I suggest that several cellular- and circuit-level features of rapidly responsive interneuron networks tip the balance away from seizure in the healthy brain.

Synchronous Bioimaging of Intracellular pH and Chloride Based on LSS Fluorescent Protein

Ion homeostasis regulates critical physiological processes in the living cell. Intracellular chloride concentration not only contributes in setting the membrane potential of quiescent cells but it also plays a role in modulating the dynamic voltage changes during network activity. Dynamic chloride imaging demands new tools, allowing faster acquisition rates and correct accounting of concomitant pH changes. Joining a long-Stokes-shift red-fluorescent protein to a GFP variant with high sensitivity to pH and chloride, we obtained LSSmClopHensor, a genetically encoded fluorescent biosensor optimized for the simultaneous chloride and pH imaging and requiring only two excitation wavelengths (458 and 488 nm). LSSmClopHensor allowed us to monitor the dynamic changes of intracellular pH and chloride concentration during seizure like discharges in neocortical brain slices. Only cells with tightly controlled resting potential revealed a narrow distribution of chloride concentration peaking at about 5 and 8 mM, in neocortical neurons and SK-N-SH cells, respectively. We thus showed that LSSmClopHensor represents a new versatile tool for studying the dynamics of chloride and proton concentration in living systems.

Involvement of cortical fast-spiking parvalbumin-positive basket cells in epilepsy

GABAergic interneurons of the parvalbumin-positive fast-spiking basket cells subtype (PV INs) are important regulators of cortical network excitability and of gamma oscillations, involved in signal processing and cognition. Impaired development or function of PV INs has been associated with epilepsy in various animal models of epilepsy, as well as in some genetic forms of epilepsy in humans. In this review, we provide an overview of some of the experimental data linking PV INs dysfunction with epilepsy, focusing on disorders of the specification, migration, maturation, synaptic function, or connectivity of PV INs. Furthermore, we reflect on the potential therapeutic use of cell-type specific stimulation of PV INs within active networks and on the transplantation of PV INs precursors in the treatment of epilepsy and its comorbidities.

A potential role for glia-derived extracellular matrix remodeling in postinjury epilepsy

Head trauma and vascular injuries are known risk factors for acquired epilepsy. The sequence of events that lead from the initial injury to the development of epilepsy involves complex plastic changes and circuit rewiring. In-depth, comprehensive understanding of the epileptogenic process is critical for the identification of disease-modifying targets. Here we review the complex interactions of cellular and extracellular components that may promote epileptogenesis, with an emphasis on the role of astrocytes. Emerging evidence demonstrates that astrocytes promptly respond to brain damage and play a critical role in the development of postinjury epilepsy. Astrocytes have been shown to regulate extracellular matrix (ECM) remodeling, which can affect plasticity and stability of synapses and, in turn, contribute to the epileptogenic process. From these separate lines of evidence, we present a hypothesis suggesting a possible role for astrocyte-regulated remodeling of ECM and perineuronal nets, a specialized ECM structure around fast-spiking inhibitory interneurons, in the development and progression of posttraumatic epilepsies. © 2016 Wiley Periodicals, Inc.

GABAergic networks jump-start focal seizures

Abnormally enhanced glutamatergic excitation is commonly believed to mark the onset of a focal seizure. This notion, however, is not supported by firm evidence, and it will be challenged here. A general reduction of unit firing has been indeed observed in association with low-voltage fast activity at the onset of seizures recorded during presurgical intracranial monitoring in patients with focal, drug-resistant epilepsies. Moreover, focal seizures in animal models start with increased γ-aminobutyric acid (GABA)ergic interneuronal activity that silences principal cells. In vitro studies have shown that synchronous activation of GABAA receptors occurs at seizure onset and causes sizeable elevations in extracellular potassium, thus facilitating neuronal recruitment and seizure progression. A paradoxical involvement of GABAergic networks is required for the initiation of focal seizures characterized by low-voltage fast activity, which represents the most common seizure-onset pattern in focal epilepsies.

The role of inhibition in epileptic networks

Inhibition plays many roles in cortical circuits, including coordination of network activity in different brain rhythms and neuronal clusters, gating of activity, gain control, and dictating the manner in which activity flows through the network. This latter is particularly relevant to epileptic states, when extreme hypersynchronous discharges can spread across cortical territories. We review these different physiological and pathological roles and discuss how inhibition can be compromised and why this predisposes the network to seizures.

Unaltered Network Activity and Interneuronal Firing During Spontaneous Cortical Dynamics In Vivo in a Mouse Model of Severe Myoclonic Epilepsy of Infancy

Severe myoclonic epilepsy of infancy (SMEI) is associated with loss of function of the SCN1A gene encoding the Na_V1.1 sodium channel isoform. Previous studies in Scn1a^−/+ mice during the pre-epileptic period reported selective reduction in interneuron excitability and proposed this as the main pathological mechanism underlying SMEI. Yet, the functional consequences of this interneuronal dysfunction at the circuit level in vivo are unknown. Here, we investigated whether Scn1a^−/+ mice showed alterations in cortical network function. We found that various forms of spontaneous network activity were similar in Scn1a^−/+ during the pre-epileptic period compared with wild-type (WT) in vivo. Importantly, in brain slices from Scn1a^−/+ mice, the excitability of parvalbumin (PV) and somatostatin (SST) interneurons was reduced, epileptiform activity propagated more rapidly, and complex synaptic changes were observed. However, in vivo, optogenetic reduction of firing in PV or SST cells in WT mice modified ongoing network activities, and juxtasomal recordings from identified PV and SST interneurons showed unaffected interneuronal firing during spontaneous cortical dynamics in Scn1a^−/+ compared with WT. These results demonstrate that interneuronal hypoexcitability is not observed in Scn1a^−/+ mice during spontaneous activities in vivo and suggest that additional mechanisms may contribute to homeostatic rearrangements and the pathogenesis of SMEI.

Parvalbumin-Positive Inhibitory Interneurons Oppose Propagation But Favor Generation of Focal Epileptiform Activity

Parvalbumin (Pv)-positive inhibitory interneurons effectively control network excitability, and their optogenetic activation has been reported to block epileptic seizures. An intense activity in GABAergic interneurons, including Pv interneurons, before seizures has been described in different experimental models of epilepsy, raising the hypothesis that an increased GABAergic inhibitory signal may, under certain conditions, initiate seizures. It is therefore unclear whether the activity of Pv interneurons enhances or opposes epileptiform activities. Here we use a mouse cortical slice model of focal epilepsy in which the epileptogenic focus can be identified and the role of Pv interneurons in the generation and propagation of seizure-like ictal events is accurately analyzed by a combination of optogenetic, electrophysiological, and imaging techniques. We found that a selective activation of Pv interneurons at the focus failed to block ictal generation and induced postinhibitory rebound spiking in pyramidal neurons, enhancing neuronal synchrony and promoting ictal generation. In contrast, a selective activation of Pv interneurons distant from the focus blocked ictal propagation and shortened ictal duration at the focus. We revealed that the reduced ictal duration was a direct consequence of the ictal propagation block, probably by preventing newly generated afterdischarges to travel backwards to the original focus of ictal initiation. Similar results were obtained upon individual Pv interneuron activation by intracellular depolarizing current pulses. The functional dichotomy of Pv interneurons here described opens new perspectives to our understanding of how local inhibitory circuits govern generation and spread of focal epileptiform activities.

Slow Spatial Recruitment of Neocortex during Secondarily Generalized Seizures and Its Relation to Surgical Outcome

Understanding the spatiotemporal dynamics of brain activity is crucial for inferring the underlying synaptic and nonsynaptic mechanisms of brain dysfunction. Focal seizures with secondary generalization are traditionally considered to begin in a limited spatial region and spread to connected areas, which can include both pathological and normal brain tissue. The mechanisms underlying this spread are important to our understanding of seizures and to improve therapies for surgical intervention. Here we study the properties of seizure recruitment-how electrical brain activity transitions to large voltage fluctuations characteristic of spike-and-wave seizures. We do so using invasive subdural electrode arrays from a population of 16 patients with pharmacoresistant epilepsy. We find an average delay of ∼30 s for a broad area of cortex (8 × 8 cm) to be recruited into the seizure, at an estimated speed of ∼4 mm/s. The spatiotemporal characteristics of recruitment reveal two categories of patients: one in which seizure recruitment of neighboring cortical regions follows a spatially organized pattern consistent from seizure to seizure, and a second group without consistent spatial organization of activity during recruitment. The consistent, organized recruitment correlates with a more regular, compared with small-world, connectivity pattern in simulation and successful surgical treatment of epilepsy. We propose that an improved understanding of how the seizure recruits brain regions into large amplitude voltage fluctuations provides novel information to improve surgical treatment of epilepsy and highlights the slow spread of massive local activity across a vast extent of cortex during seizure.

The interneuron energy hypothesis: Implications for brain disease

Fast-spiking, inhibitory interneurons - prototype is the parvalbumin-positive (PV+) basket cell - generate action potentials at high frequency and synchronize the activity of numerous excitatory principal neurons, such as pyramidal cells, during fast network oscillations by rhythmic inhibition. For this purpose, fast-spiking, PV+ interneurons have unique electrophysiological characteristics regarding action potential kinetics and ion conductances, which are associated with high energy expenditure. This is reflected in the neural ultrastructure by enrichment with mitochondria and cytochrome c oxidase, indicating the dependence on oxidative phosphorylation for adenosine-5'-triphosphate (ATP) generation. The high energy expenditure is most likely required for membrane ion transport in dendrites and the extensive axon arbor as well as for presynaptic release of neurotransmitter, gamma-aminobutyric acid (GABA). Fast-spiking, PV+ interneurons are central for the emergence of gamma oscillations (30-100Hz) that provide a fundamental mechanism of complex information processing during sensory perception, motor behavior and memory formation in networks of the hippocampus and the neocortex. Conversely, shortage in glucose and oxygen supply (metabolic stress) and/or excessive formation of reactive oxygen and nitrogen species (oxidative stress) may render these interneurons to be a vulnerable target. Dysfunction in fast-spiking, PV+ interneurons might set a low threshold for impairment of fast network oscillations and thus higher brain functions. This pathophysiological mechanism might be highly relevant for cerebral aging as well as various acute and chronic brain diseases, such as stroke, vascular cognitive impairment, epilepsy, Alzheimer's disease and schizophrenia.

The contribution of raised intraneuronal chloride to epileptic network activity

Altered inhibitory function is an important facet of epileptic pathology. A key concept is that GABAergic activity can become excitatory if intraneuronal chloride rises. However, it has proved difficult to separate the role of raised chloride from other contributory factors in complex network phenomena, such as epileptic pathology. Therefore, we asked what patterns of activity are associated with chloride dysregulation by making novel use of Halorhodopsin to load clusters of mouse pyramidal cells artificially with Cl(-). Brief (1-10 s) activation of Halorhodopsin caused substantial positive shifts in the GABAergic reversal potential that were proportional to the charge transfer during the illumination and in adult neocortical pyramidal neurons decayed with a time constant of τ = 8.0 ± 2.8s. At the network level, these positive shifts in EGABA produced a transient rise in network excitability, with many distinctive features of epileptic foci, including high-frequency oscillations with evidence of out-of-phase firing (Ibarz et al., 2010). We show how such firing patterns can arise from quite small shifts in the mean intracellular Cl(-) level, within heterogeneous neuronal populations. Notably, however, chloride loading by itself did not trigger full ictal events, even with additional electrical stimulation to the underlying white matter. In contrast, when performed in combination with low, subepileptic levels of 4-aminopyridine, Halorhodopsin activation rapidly induced full ictal activity. These results suggest that chloride loading has at most an adjunctive role in ictogenesis. Our simulations also show how chloride loading can affect the jitter of action potential timing associated with imminent recruitment to an ictal event (Netoff and Schiff, 2002).

Single unit action potentials in humans and the effect of seizure activity

Spike-sorting algorithms have been used to identify the firing patterns of isolated neurons ('single units') from implanted electrode recordings in patients undergoing assessment for epilepsy surgery, but we do not know their potential for providing helpful clinical information. It is important therefore to characterize both the stability of these recordings and also their context. A critical consideration is where the units are located with respect to the focus of the pathology. Recent analyses of neuronal spiking activity, recorded over extended spatial areas using microelectrode arrays, have demonstrated the importance of considering seizure activity in terms of two distinct spatial territories: the ictal core and penumbral territories. The pathological information in these two areas, however, is likely to be very different. We investigated, therefore, whether units could be followed reliably over prolonged periods of times in these two areas, including during seizure epochs. We isolated unit recordings from several hundred neurons from four patients undergoing video-telemetry monitoring for surgical evaluation of focal neocortical epilepsies. Unit stability could last in excess of 40 h, and across multiple seizures. A key finding was that in the penumbra, spike stereotypy was maintained even during the seizure. There was a net tendency towards increased penumbral firing during the seizure, although only a minority of units (10-20%) showed significant changes over the baseline period, and notably, these also included neurons showing significant reductions in firing. In contrast, within the ictal core territories, regions characterized by intense hypersynchronous multi-unit firing, our spike sorting algorithms failed as the units were incorporated into the seizure activity. No spike sorting was possible from that moment until the end of the seizure, but recovery of the spike shape was rapid following seizure termination: some units reappeared within tens of seconds of the end of the seizure, and over 80% reappeared within 3 min (τrecov = 104 ± 22 s). The recovery of the mean firing rate was close to pre-ictal levels also within this time frame, suggesting that the more protracted post-ictal state cannot be explained by persistent cellular neurophysiological dysfunction in either the penumbral or the core territories. These studies lay the foundation for future investigations of how these recordings may inform clinical practice.See Kimchi and Cash (doi:10.1093/awv264) for a scientific commentary on this article.

Unit Activity of Hippocampal Interneurons before Spontaneous Seizures in an Animal Model of Temporal Lobe Epilepsy

Mechanisms of seizure initiation are unclear. To evaluate the possible roles of inhibitory neurons, unit recordings were obtained in the dentate gyrus, CA3, CA1, and subiculum of epileptic pilocarpine-treated rats as they experienced spontaneous seizures. Most interneurons in the dentate gyrus, CA1, and subiculum increased their firing rate before seizures, and did so with significant consistency from seizure to seizure. Identification of CA1 interneuron subtypes based on firing characteristics during theta and sharp waves suggested that a parvalbumin-positive basket cell and putative bistratified cells, but not oriens lacunosum moleculare cells, were activated preictally. Preictal changes occurred much earlier than those described by most previous in vitro studies. Preictal activation of interneurons began earliest (>4 min before seizure onset), increased most, was most prevalent in the subiculum, and was minimal in CA3. Preictal inactivation of interneurons was most common in CA1 (27% of interneurons) and included a putative ivy cell and parvalbumin-positive basket cell. Increased or decreased preictal activity correlated with whether interneurons fired faster or slower, respectively, during theta activity. Theta waves were more likely to occur before seizure onset, and increased preictal firing of subicular interneurons correlated with theta activity. Preictal changes by other hippocampal interneurons were largely independent of theta waves. Within seconds of seizure onset, many interneurons displayed a brief pause in firing and a later, longer drop that was associated with reduced action potential amplitude. These findings suggest that many interneurons inactivate during seizures, most increase their activity preictally, but some fail to do so at the critical time before seizure onset.

Initiation, Propagation, and Termination of Partial (Focal) Seizures

The neurophysiological patterns that correlate with partial (focal) seizures are well defined in humans by standard electroencephalogram (EEG) and presurgical depth electrode recordings. Seizure patterns with similar features are reproduced in animal models of partial seizures and epilepsy. However, the network determinants that support interictal spikes, as well as the initiation, progression, and termination of seizures, are still elusive. Recent findings show that inhibitory networks are prominently involved at the onset of these seizures, and that extracellular changes in potassium contribute to initiate and sustain seizure progression. The end of a partial seizure correlates with an increase in network synchronization, which possibly involves both excitatory and inhibitory mechanisms.