Short-term changes in bilateral hippocampal coherence precede epileptiform events

Innovative minimally invasive options to treat drug-resistant epilepsies

Despite the regular discovery of new molecules, one-third of epileptic patients are resistant to antiepileptic drugs. Only a few can benefit from resective surgery, the current gold standard. Although effective in 50-70% of cases, this therapy remains risky, costly, and can be associated with long-term cognitive or neurological side effects. In addition, patients are increasingly reluctant to have a craniotomy, emphasizing the need for new less invasive therapies for focal drug-resistant epilepsies. Here, we review different minimally invasive approaches already in use in the clinic or under preclinical development to treat drug-resistant epilepsies. Localized thermolesion of the epileptogenic zone has been developed in the clinic using high-frequency thermo-coagulations or magnetic resonance imaging-guided laser or ultrasounds. Although less invasive, they have not yet significantly improved the outcomes when compared with resective surgery. Radiosurgery techniques have been used in the clinic for the last 20years and have proven efficiency. However, their efficacy is not better than resective surgery, and various side effects have been reported as well as the potential risk of sudden unexpected death associated with epilepsy. Recently, a new strategy of radiosurgery has emerged using synchrotron-generated X-ray microbeams: microbeam radiation therapy (MRT). The low divergence and high-flux of the synchrotron beams and the unique tolerance to MRT by healthy brain tissues, allows a precise targeting of specific brain regions with minimal invasiveness and limited behavioral or functional consequences in animals. Antiepileptic effects over several months have been recorded in animal models, and histological and synaptic tracing analysis suggest a reduction of neuronal connectivity as a mechanism of action. The possibility of transferring this approach to epileptic patients is discussed in this review.

Long-term in vivo application of a potassium channel-based optogenetic silencer in the healthy and epileptic mouse hippocampus

BACKGROUND

Optogenetic tools allow precise manipulation of neuronal activity via genetically encoded light-sensitive proteins. Currently available optogenetic inhibitors are not suitable for prolonged use due to short-lasting photocurrents, tissue heating, and unintended changes in ion distributions, which may interfere with normal neuron physiology. To overcome these limitations, a novel potassium channel-based optogenetic silencer, named PACK, was recently developed. The PACK tool has two components: a photoactivated adenylyl cyclase from Beggiatoa (bPAC) and a cAMP-dependent potassium channel, SthK, which carries a large, long-lasting potassium current in mammalian cells. Previously, it has been shown that activating the PACK silencer with short light pulses led to a significant reduction of neuronal firing in various in vitro and acute in vivo settings. Here, we examined the viability of performing long-term studies in vivo by looking at the inhibitory action and side effects of PACK and its components in healthy and epileptic adult male mice.

RESULTS

We targeted hippocampal cornu ammonis (CA1) pyramidal cells using a viral vector and enabled illumination of these neurons via an implanted optic fiber. Local field potential (LFP) recordings from CA1 of freely moving mice revealed significantly reduced neuronal activity during 50-min intermittent (0.1 Hz) illumination, especially in the gamma frequency range. Adversely, PACK expression in healthy mice induced chronic astrogliosis, dispersion of pyramidal cells, and generalized seizures. These side effects were independent of the light application and were also present in mice expressing bPAC without the potassium channel. Light activation of bPAC alone increased neuronal activity, presumably via enhanced cAMP signaling. Furthermore, we applied bPAC and PACK in the contralateral hippocampus of chronically epileptic mice following a unilateral injection of intrahippocampal kainate. Unexpectedly, the expression of bPAC in the contralateral CA1 area was sufficient to prevent the spread of spontaneous epileptiform activity from the seizure focus to the contralateral hippocampus.

CONCLUSION

Our study highlights the PACK tool as a potent optogenetic inhibitor in vivo. However, further refinement of its light-sensitive domain is required to avoid unexpected physiological changes.

Neurovascular multiparametric MRI defines epileptogenic and seizure propagation regions in experimental mesiotemporal lobe epilepsy

OBJECTIVE

Improving the identification of the epileptogenic zone and associated seizure-spreading regions represents a significant challenge. Innovative brain-imaging modalities tracking neurovascular dynamics during seizures may provide new disease biomarkers.

METHODS

With use of a multi-parametric magnetic resonance imaging (MRI) analysis at 9.4 Tesla, we examined, elaborated, and combined multiple cellular and cerebrovascular MRI read-outs as imaging biomarkers of the epileptogenic and seizure-propagating regions. Analyses were performed in an experimental model of mesial temporal lobe epilepsy (MTLE) generated by unilateral intra-hippocampal injection of kainic acid (KA).

RESULTS

In the ipsilateral epileptogenic hippocampi, tissue T₁ and blood-brain barrier (BBB) permeability to gadolinium were increased 48-72 hours post-KA, as compared to sham and contralateral hippocampi. BBB permeability endured during spontaneous focal seizures (4-6 weeks), along with a significant increase of apparent diffusion coefficient (ADC) and blood volume fraction (BVf). Simultaneously, ADC and BVf were augmented in the contralateral hippocampus, a region characterized by electroencephalographic seizure spreading, discrete histological neurovascular cell modifications, and no tissue sclerosis. We next asked whether combining all the acquired MRI parameters could deliver criteria to classify the epileptogenic from the seizure-spreading and sham hippocampi in these experimental conditions and over time. To differentiate sham from epileptogenic areas, the automatic multi-parametric classification provided a maximum accuracy of 97.5% (32 regions) 48-72 hours post-KA and of 100% (60 regions) at spontaneous seizures stage. To differentiate sham, epileptogenic, and seizure-spreading areas, the accuracies of the automatic classification were 93.1% (42 regions) 48-72 hours post-KA and 95% (80 regions) at spontaneous seizure stage.

SIGNIFICANCE

Combining multi-parametric MRI acquisition and machine-learning analyses delivers specific imaging identifiers to segregate the epileptogenic from the contralateral seizure-spreading hippocampi in experimental MTLE. The potential clinical value of our findings is critically discussed.

Hippocampal low-frequency stimulation prevents seizure generation in a mouse model of mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is the most common form of focal, pharmacoresistant epilepsy in adults and is often associated with hippocampal sclerosis. Here, we established the efficacy of optogenetic and electrical low-frequency stimulation (LFS) in interfering with seizure generation in a mouse model of MTLE. Specifically, we applied LFS in the sclerotic hippocampus to study the effects on spontaneous subclinical and evoked generalized seizures. We found that stimulation at 1 Hz for 1 hr resulted in an almost complete suppression of spontaneous seizures in both hippocampi. This seizure-suppressive action during daily stimulation remained stable over several weeks. Furthermore, LFS for 30 min before a pro-convulsive stimulus successfully prevented seizure generalization. Finally, acute slice experiments revealed a reduced efficacy of perforant path transmission onto granule cells upon LFS. Taken together, our results suggest that hippocampal LFS constitutes a promising approach for seizure control in MTLE.

Glial responses during epileptogenesis in Mus musculus point to potential therapeutic targets

The Mesio-Temporal Lobe Epilepsy syndrome is the most common form of intractable epilepsy. It is characterized by recurrence of focal seizures and is often associated with hippocampal sclerosis and drug resistance. We aimed to characterize the molecular changes occurring during the initial stages of epileptogenesis in search of new therapeutic targets for Mesio-Temporal Lobe Epilepsy. We used a mouse model obtained by intra-hippocampal microinjection of kainate and performed hippocampal whole genome expression analysis at 6h, 12h and 24h post-injection, followed by multilevel bioinformatics analysis. We report significant changes in immune and inflammatory responses, neuronal network reorganization processes and glial functions, predominantly initiated during status epilepticus at 12h and persistent after the end of status epilepticus at 24h post-kainate. Upstream regulator analysis highlighted Cyba, Cybb and Vim as central regulators of multiple overexpressed genes implicated in glial responses at 24h. In silico microRNA analysis indicated that miR-9, miR-19b, miR-129, and miR-223 may regulate the expression of glial-associated genes at 24h. Our data support the hypothesis that glial-mediated inflammatory response holds a key role during epileptogenesis, and that microglial cells may participate in the initial process of epileptogenesis through increased ROS production via the NOX complex.

Seizure progression and inflammatory mediators promote pericytosis and pericyte-microglia clustering at the cerebrovasculature

BACKGROUND

Cerebrovascular dysfunction and inflammation occur in epilepsy. Here we asked whether pericytes, a pivotal cellular component of brain capillaries, undergo pathological modifications during experimental epileptogenesis and in human epilepsy. We evaluated whether pro-inflammatory cytokines, present in the brain during seizures, contribute to pericyte morphological modifications.

METHODS

In vivo, unilateral intra-hippocampal kainic acid (KA) injections were performed in NG2DsRed/C57BL6 mice to induce status epilepticus (SE), epileptogenesis, and spontaneous recurrent seizures (SRS). NG2DsRed mice were used to visualize pericytes during seizure progression. The effect triggered by recombinant IL-1β, TNFα, or IL-6 on pericytes was evaluated in NG2DsRed hippocampal slices and in human-derived cell culture. Human brain specimens obtained from temporal lobe epilepsy (TLE) with or without sclerosis (HS) and focal cortical dysplasia (FCD-IIb) were evaluated for pericyte-microglial cerebrovascular assembly.

RESULTS

A disarray of NG2DsRed⁺ pericyte soma and ramifications was found 72 h post-SE and 1 week post-SE (epileptogenesis) in the hippocampus. Pericyte modifications topographically overlapped with IBA1⁺ microglia clustering around the capillaries with cases of pericytes lodged within the microglial cells. Microglial clustering around the NG2DsRed pericytes lingered at SRS. Pericyte proliferation (Ki67⁺) occurred 72 h post-SE and during epileptogenesis and returned towards control levels at SRS. Human epileptic brain tissues showed pericyte-microglia assemblies with IBA1/HLA microglial cells outlining the capillary wall in TLE-HS and FCD-IIb specimens. Inflammatory mediators contributed to pericyte modifications, in particular IL-1β elicited pericyte morphological changes and pericyte-microglia clustering in NG2DsRed hippocampal slices. Modifications also occurred when pro-inflammatory cytokines were added to an in vitro culture of pericytes.

CONCLUSIONS

These results indicate the occurrence of pericytosis during seizures and introduce a pericyte-microglial mediated mechanism of blood-brain barrier dysfunction in epilepsy.

Seizure development in the acute intrahippocampal epileptic focus

Currently, an epileptic seizure is considered to involve a temporary network that exists for a finite period of time. Formation of this network evolves through spread of epileptiform activity from a seizure onset zone (SOZ). Propagation of seizures evoked by kainic acid injection in hippocampus to different brain areas was analyzed at macro- and micro-intervals. The mean latency of seizure occurrence in different brain areas varied between 0.5 sec and 85 sec (mean 14.9 ± 14.5 (SD)), and it increased after each consecutive seizure in areas located contralateral to the area of injection, but not in the ipsilateral sites. We have shown that only 41% of epileptic individual events in target brain areas were driven by epileptic events generated in the SOZ once the seizure began. Fifty-nine percent of epileptiform events in target areas occurred one millisecond before or after events in the SOZ. These data illustrate that during seizure maintenance, only some individual epileptiform events in areas outside of SOZ could be consistently triggered by the SOZ; and the majority must be triggered by a driver located outside the SOZ or brain areas involved in ictal activity could be coupled to each other via an unknown mechanism such as stochastic resonance.

Assessing directionality and strength of coupling through symbolic analysis: an application to epilepsy patients

Inferring strength and direction of interactions from electroencephalographic (EEG) recordings is of crucial importance to improve our understanding of dynamical interdependencies underlying various physiological and pathophysiological conditions in the human epileptic brain. We here use approaches from symbolic analysis to investigate--in a time-resolved manner--weighted and directed, short- to long-ranged interactions between various brain regions constituting the epileptic network. Our observations point to complex spatial-temporal interdependencies underlying the epileptic process and their role in the generation of epileptic seizures, despite the massive reduction of the complex information content of multi-day, multi-channel EEG recordings through symbolization. We discuss limitations and potential future improvements of this approach.

Identification of pre-spike network in patients with mesial temporal lobe epilepsy

Background: Seizures and interictal spikes in mesial temporal lobe epilepsy (MTLE) affect a network of brain regions rather than a single epileptic focus. Simultaneous electroencephalography and functional magnetic resonance imaging (EEG-fMRI) studies have demonstrated a functional network in which hemodynamic changes are time-locked to spikes. However, whether this reflects the propagation of neuronal activity from a focus, or conversely the activation of a network linked to spike generation remains unknown. The functional connectivity (FC) changes prior to spikes may provide information about the connectivity changes that lead to the generation of spikes. We used EEG-fMRI to investigate FC changes immediately prior to the appearance of interictal spikes on EEG in patients with MTLE.

Methods/principal findings: Fifteen patients with MTLE underwent continuous EEG-fMRI during rest. Spikes were identified on EEG and three 10 s epochs were defined relative to spike onset: spike (0–10 s), pre-spike (−10 to 0 s), and rest (−20 to −10 s, with no previous spikes in the preceding 45s). Significant spike-related activation in the hippocampus ipsilateral to the seizure focus was found compared to the pre-spike and rest epochs. The peak voxel within the hippocampus ipsilateral to the seizure focus was used as a seed region for FC analysis in the three conditions. A significant change in FC patterns was observed before the appearance of electrographic spikes. Specifically, there was significant loss of coherence between both hippocampi during the pre-spike period compared to spike and rest states.

Conclusion/significance: In keeping with previous findings of abnormal inter-hemispheric hippocampal connectivity in MTLE, our findings specifically link reduced connectivity to the period immediately before spikes. This brief decoupling is consistent with a deficit in mutual (inter-hemispheric) hippocampal inhibition that may predispose to spike generation.

Differential vulnerability of interneurons in the epileptic hippocampus

The loss of hippocampal interneurons has been considered as one reason for the onset of temporal lobe epilepsy (TLE) by shifting the excitation-inhibition balance. Yet, there are many different interneuron types which show differential vulnerability in the context of an epileptogenic insult. We used the intrahippocampal kainate (KA) mouse model for TLE in which a focal, unilateral KA injection induces status epilepticus (SE) followed by development of granule cell dispersion (GCD) and hippocampal sclerosis surrounding the injection site but not in the intermediate and temporal hippocampus. In this study, we characterized the loss of interneurons with respect to septotemporal position and to differential vulnerability of interneuron populations. To this end, we performed intrahippocampal recordings of the initial SE, in situ hybridization for glutamic acid decarboxylase 67 (GAD67) mRNA and immunohistochemistry for parvalbumin (PV) and neuropeptide Y (NPY) in the early phase of epileptogenesis at 2 days and at 21 days after KA injection, when recurrent epileptic activity and GCD have fully developed. We show that SE extended along the entire septotemporal axis of both hippocampi, but was stronger at distant sites than at the injection site. There was an almost complete loss of interneurons surrounding the injection site and expanding to the intermediate hippocampus already at 2 days but increasing until 21 days after KA. Furthermore, we observed differential vulnerability of PV- and NPY-expressing cells: while the latter were lost at the injection site but preserved at intermediate sites, PV-expressing cells were gone even at sites more temporal than GCD. In addition, we found upregulation of GAD67 mRNA expression in dispersed granule cells and of NPY staining in ipsilateral granule cells and ipsi- and contralateral mossy fibers. Our data thus indicate differential survival capacity of interneurons in the epileptic hippocampus and compensatory plasticity mechanisms depending on the hippocampal position.

Fiber tract stimulation can reduce epileptiform activity in an in-vitro bilateral hippocampal slice preparation

Mesial temporal lobe epilepsy (MTLE) is a common medically refractory neurological disease that has been treated with electrical stimulation of gray matter with limited success. However, stimulation of a white matter tract connecting the hippocampi could maximize treatment efficacy and extent. We tested low-frequency stimulation (LFS) of a novel target that enables simultaneous targeting of bilateral hippocampi: the ventral hippocampal commissure (VHC) with a novel in-vitro slice preparation containing bilateral hippocampi connected by the VHC. The goal of this study is to understand the role of hippocampal interplay in seizure propagation and reduction by commissural fiber tract stimulation. LFS is applied to the VHC as extracellular and intracellular recording techniques are combined with signal processing to estimate several metrics of epilepsy including: (1) total time occupied by seizure activity (%); (2) seizure duration (s); (3) seizures per minute (#); and (4) power in the ictal (V(2)Hz(-1)); as well as (5) interictal spectra (V(2)Hz(-1)). Bilateral epileptiform activity in this preparation is highly correlated between hippocampi. Application of LFS to the VHC reduces all metrics of epilepsy during treatment in an amplitude and frequency dependent manner. This study lends several insights into the mechanisms of bilateral seizure reduction by LFS of the VHC, including that depolarization blocking, LTD/LTP and GABA(A) are not involved. Importantly, enhanced post-stimulation 1-Hz spiking correlates with long-lasting seizure reduction and both are heightened by targeting bilateral hippocampi via the VHC. Therefore, stimulating bilateral hippocampi via a single electrode in the VHC may provide an effective MTLE treatment.

Stereologic estimation of hippocampal GluR2/3- and calretinin-immunoreactive hilar neurons (presumptive mossy cells) in two mouse models of temporal lobe epilepsy

PURPOSE

Hippocampal mossy cells receive dense innervation from dentate granule cells and, in turn, mossy cells innervate both granule cells and interneurons. Mossy cell loss is thought to trigger granule cell mossy fiber sprouting, which may affect granule cell excitability. The aim of this study was to quantify mossy cell loss in two animal models of temporal lobe epilepsy, and determine whether there exists a relationship between mossy cell loss, mossy fiber sprouting, and granule cell dispersion.

METHODS

Representative hippocampal sections from p35 knockout mice and mice with unilateral intrahippocampal kainate injection were immunolabeled for GluR2/3, two subunits of the amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor and calretinin to identify mossy cells. Mossy fibers were immunostained against synaptoporin.

KEY FINDINGS

p35 Knockout mice showed no hilar cell death, but moderate mossy fiber sprouting and granule cell dispersion. In the kainate-injected hippocampus, there was an 80% and 85% reduction of GluR2/3- and GluR2/3/calretinin-positive hilar neurons, respectively, and dense mossy fiber sprouting and significant granule cell dispersion. In the contralateral hippocampus there was a 52% loss of GluR2/3-, but only a 20% loss of GluR2/3-calretinin-immunoreactive presumptive mossy cells, and granule cell dispersion; no mossy fiber sprouting was observed.

SIGNIFICANCE

These results indicate a probable lack of causality between mossy cell death and mossy fiber sprouting.

Septotemporal position in the hippocampal formation determines epileptic and neurogenic activity in temporal lobe epilepsy

It is a matter of ongoing debate whether newly generated granule cells contribute to epileptic activity in the hippocampus. To address this question, we investigated neurogenesis and epileptiform activity (EA) along the hippocampal septotemporal axis in the intrahippocampal kainate (KA) mouse model for temporal lobe epilepsy. Multisite intrahippocampal in vivo recordings and immunolabeling for c-Fos showed that the KA-induced status epilepticus (SE) extended along the septotemporal axis of both hippocampi with stronger intensity at ipsilateral temporal and contralateral sites. Accordingly, we found a position-dependent increase in proliferation (incorporation of bromodeoxyuridine) and neurogenesis (immunolabeling for doublecortin): Both were selectively increased in the ipsilateral temporal and entire contralateral subgranular zone, sparing the septal region close to the injection site. The newborn neurons were hyperexcitable and functionally integrated into the hippocampal network as revealed by patch-clamp recordings. Analysis of chronic EA also showed a differential intensity pattern along the hippocampal axis: EA was low in the septal portion with prominent sclerosis and granule cell dispersion but most pronounced in the transition zone where neurogenesis reappeared. In conclusion, SE stimulates neurogenesis in a position-dependent manner and coincidence of neurogenesis and stronger EA distal to the injection site suggests a proepileptogenic effect of increased neurogenesis.

Changes in hippocampal neuronal activity during and after unilateral selective hippocampal ischemia in vivo

The hippocampal formation is one of the brain regions most sensitive to ischemic damage. However, there are no studies about changes in hippocampal neuronal activity during and after a selective unilateral hippocampal ischemia. We developed a novel unilateral cerebrovascular ischemia model in mice that selectively shuts down blood supply to the ipsilateral hippocampal formation. Using a modified version of the photothrombotic method, we stereotaxically targeted the initial ascending part of the longitudinal hippocampal artery in urethane anesthetized and rose bengal-injected mice. To block blood flow in the targeted artery, we photoactivated the rose bengal by illuminating the longitudinal hippocampal artery through an optical fiber inserted into the brain. In vivo field potential recordings in the CA1 region of the hippocampus before, during and after the induction of ischemia demonstrated a high-frequency discharge (HFD) reaching frequencies of >300 Hz and lasting 7-24 s during the illumination consistent with a massive synchronous neuronal activity. The HFD was invariably followed by a DC voltage shift and a decreased activity at both low (30-57 Hz)- and high (63-119 Hz)-gamma frequencies. This decrease in gamma activity lasted for the entire duration of the recordings (∼160 min) following ischemia. The contralateral hippocampus displayed HFDs but with different frequency spectra and without DC voltage shifts or long-lasting decreases in gamma oscillations. Our findings reveal for the first time the acute effects of unilateral hippocampal ischemia on ensemble hippocampal neuronal activities.

Gene expression analysis of the emergence of epileptiform activity after focal injection of kainic acid into mouse hippocampus

We report gene profiling data on genomic processes underlying the progression towards recurrent seizures after injection of kainic acid (KA) into the mouse hippocampus. Focal injection enabled us to separate the effects of proepileptic stimuli initiated by KA injection. Both the injected and contralateral hippocampus participated in the status epilepticus. However, neuronal death induced by KA treatment was restricted to the injected hippocampus, although there was some contralateral axonal degeneration. We profiled gene expression changes in dorsal and ventral regions of both the injected and contralateral hippocampus. Changes were detected in the expression of 1526 transcripts in samples from three time-points: (i) during the KA-induced status epilepticus, (ii) at 2 weeks, before recurrent seizures emerged, and (iii) at 6 months after seizures emerged. Grouping genes with similar spatio-temporal changes revealed an early transcriptional response, strong immune, cell death and growth responses at 2 weeks and an activation of immune and extracellular matrix genes persisting at 6 months. Immunostaining for proteins coded by genes identified from array studies provided evidence for gliogenesis and suggested that the proteoglycan biglycan is synthesized by astrocytes and contributes to a glial scar. Gene changes at 6 months after KA injection were largely restricted to tissue from the injection site. This suggests that either recurrent seizures might depend on maintained processes including immune responses and changes in extracellular matrix proteins near the injection site or alternatively might result from processes, such as growth, distant from the injection site and terminated while seizures are maintained.

Increase in BDNF-mediated TrkB signaling promotes epileptogenesis in a mouse model of mesial temporal lobe epilepsy

Mesio-temporal lobe epilepsy (MTLE), the most common drug-resistant epilepsy syndrome, is characterized by the recurrence of spontaneous focal seizures after a latent period that follows, in most patients, an initial insult during early childhood. Many of the mechanisms that have been associated with the pathophysiology of MTLE are known to be regulated by brain-derived neurotrophic factor (BDNF) in the healthy brain and an excess of this neurotrophin could therefore play a critical role in MTLE development. However, such a function remains controversial as other studies revealed that BDNF could, on the contrary, exert protective effects regarding epilepsy development. In the present study, we further addressed the role of increased BDNF/TrkB signaling on the progressive development of hippocampal seizures in the mouse model of MTLE obtained by intrahippocampal injection of kainate. We show that hippocampal seizures progressively developed in the injected hippocampus during the first two weeks following kainate treatment, within the same time-frame as a long-lasting and significant increase of BDNF expression in dentate granule cells. To determine whether such a BDNF increase could influence hippocampal epileptogenesis via its TrkB receptors, we examined the consequences of (i) increased or (ii) decreased TrkB signaling on epileptogenesis, in transgenic mice overexpressing the (i) TrkB full-length or (ii) truncated TrkB-T1 receptors of BDNF. Epileptogenesis was significantly facilitated in mice with increased TrkB signaling but delayed in mutants with reduced TrkB signaling. In contrast, TrkB signaling did not influence granule cell dispersion, an important feature of this mouse model which is also observed in most MTLE patients. These results suggest that an increase in TrkB signaling, mediated by a long-lasting BDNF overexpression in the hippocampus, promotes epileptogenesis in MTLE.

Involvement of the thalamic parafascicular nucleus in mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is characterized by focal seizures, associated with hippocampal sclerosis, and often resistance to antiepileptic drugs. The parafascicular nucleus (PF) of the thalamus is involved in the generation of physiological oscillatory rhythms. It receives excitatory inputs from the cortex and inhibitory inputs from the basal ganglia, a system implicated in the control of epileptic seizures. The aim of this study was to examine the involvement of the PF in the occurrence of hippocampal paroxysmal discharges (HPDs) in a chronic animal model of MTLE in male mice. We recorded the local field potential (LFP) and the extracellular and intracellular activity of hippocampal and PF neurons during spontaneous HPDs in vivo. The end of the HPDs was concomitant with a slow repolarization in hippocampal neurons leading to an electrical silence. In contrast, it was associated in the PF with a transient increase in the power of the 10-20 Hz band in LFPs and a depolarization of PF neurons resulting in a sustained firing. We tested the role of the PF in the control of HPDs by single 130 Hz electrical stimulation of this nucleus and bilateral intra-PF injection of NMDA and GABA(A) antagonist and agonist. High-frequency PF stimulation interrupted ongoing HPDs at an intensity devoid of behavioral effects. NMDA antagonist and GABA(A) agonist suppressed hippocampal discharges in a dose-dependent way, whereas NMDA agonist and GABA(A) antagonist increased HPDs. Altogether, these data suggest that the PF nucleus plays a role in the modulation of MTLE seizures.

Time-frequency characterization of electrocorticographic recordings of epileptic patients using frequency-entropy similarity: a comparison to other bi-variate measures

Expert evaluation of electrocorticographic (ECoG) recordings forms the linchpin of seizure onset zone localization in the evaluation of epileptic patients for surgical resection. Numerous methods have been developed to analyze these complex recordings, including uni-variate (characterizing single channels), bi-variate (comparing channel pairs) and multivariate measures. Developing reliable algorithms may be helpful in clinical tasks such as localization of epileptogenic zones and seizure anticipation, as well as enabling better understanding of neuronal function and dynamics. Recently we have developed the frequency-entropy (F-E) similarity measure, and have tested its capability in mapping the epileptogenic zones. The F-E similarity measure compares time-frequency characterizations of two recordings. In this study, we examine the method's principles and utility and compare it to previously described bi-variate correspondence measures such as correlation, coherence, mean phase coherence and spectral comparison methods. Specially designed synthetic signals were used for illuminating theoretical differences between the measures. Intracranial recordings of four epileptic patients were then used for the measures' comparative analysis by creating a mean inter-electrode matrix for each of the correspondence measures and comparing the structure of these matrices during the inter-ictal and ictal periods. We found that the F-E similarity measure is able to discover spectral and temporal features in data which are hidden for the other measures and are important for foci localization.

Network dynamics during development of pharmacologically induced epileptic seizures in rats in vivo

In epilepsy, the cortical network fluctuates between the asymptomatic interictal state and the symptomatic ictal state of seizures. Despite their importance, the network dynamics responsible for the transition between the interictal and ictal states are largely unknown. Here we used multielectrode single-unit recordings from the hippocampus to investigate the network dynamics during the development of seizures evoked by various chemoconvulsants in vivo. In these experiments, we detected a typical network dynamics signature that preceded seizure initiation. The preictal state preceding pilocarpine-, kainate-, and picrotoxin-induced seizures was characterized by biphasic network dynamics composed of an early desynchronization phase in which the tendency of neurons to fire correlated action potentials decreased, followed by a late resynchronization phase in which the activity and synchronization of the network gradually increased. This biphasic network dynamics preceded the initiation both of the initial seizure and of recurrent spontaneous seizures that followed. During seizures, firing of individual neurons and interneuronal synchronization further increased. These findings advance our understanding of the network dynamics leading to seizure initiation and may in future help in the development of novel seizure prediction algorithms.

Attention enhances the retrieval and stability of visuospatial and olfactory representations in the dorsal hippocampus

Attention enhances the encoding and retrieval of olfactory and visuospatial representations by modulating place field stability, firing rate, and neuronal synchronization of pyramidal cells in the hippocampus.

A key question in the analysis of hippocampal memory relates to how attention modulates the encoding and long-term retrieval of spatial and nonspatial representations in this region. To address this question, we recorded from single cells over a period of 5 days in the CA1 region of the dorsal hippocampus while mice acquired one of two goal-oriented tasks. These tasks required the animals to find a hidden food reward by attending to either the visuospatial environment or a particular odor presented in shifting spatial locations. Attention to the visuospatial environment increased the stability of visuospatial representations and phase locking to gamma oscillations—a form of neuronal synchronization thought to underlie the attentional mechanism necessary for processing task-relevant information. Attention to a spatially shifting olfactory cue compromised the stability of place fields and increased the stability of reward-associated odor representations, which were most consistently retrieved during periods of sniffing and digging when animals were restricted to the cup locations. Together, these results suggest that attention selectively modulates the encoding and retrieval of hippocampal representations by enhancing physiological responses to task-relevant information.

Attention modulates the encoding and retrieval of memories, but the physiological basis of this interaction has largely been unexplored. The formation of memories which depend on the hippocampus involves the conscious recall of events that occur in specific spatial contexts, a form of memory known as episodic. To investigate the physiological consequences of the interaction between attention and memory in the hippocampus, we recorded single-cell activity and local field potentials — the local rhythmic oscillatory activity of neurons — from the same cells over several days while animals learned one of two goal-oriented tasks. In the visuospatial version of the task, mice had to associate a specific spatial location with a reward, independent of an odor cue. In the nonspatial, olfactory version, mice had to associate a specific odor with the food reward, independent of spatial location. We found that, during periods of navigation, only neurons in the visuospatially trained animals displayed long-term stable representations of space, and neuronal synchronization to so-called gamma oscillations, a mechanism of signal amplification that has been proposed to underlie attentional processes. Conversely, when animals were sniffing the odors in fixed spatial locations, only neurons in the olfactory-trained group displayed a stable increase in firing rate in response to the reward-associated odor. Our data suggest that attention modulates what is encoded and retrieved by hippocampal cells and that neuronal synchronization to gamma oscillations may underlie the mechanism whereby attention leads to stable spatial memory retrieval during navigation.

Epileptiform activities in slices of hippocampus from mice after intra-hippocampal injection of kainic acid

Intra-hippocampal kainate injection induces the emergence of recurrent seizures after a delay of 3-4 weeks. We examined the cellular and synaptic basis of this activity in vitro using extracellular and intracellular records from longitudinal hippocampal slices. These slices permitted recordings from the dentate gyrus, the CA3 and CA1 regions and the subiculum of both the injected and the contralateral non-injected hippocampus. A sclerotic zone was evident in dorsal regions of slices from the injected hippocampus, while ventral regions and tissue from the contralateral hippocampus were not sclerotic. Interictal field potentials of duration 50-200 ms were generated spontaneously in both ipsilateral and contralateral hippocampal slices, but not in the sclerotic region, at 3-12 months after injection. They were initiated in the CA1 and CA3 regions and the subiculum. They were blocked by antagonists at glutamatergic receptors and were transformed into prolonged epileptiform events by GABAergic receptor antagonists. The membrane potential and the reversal potential of GABAergic synaptic events were more depolarized in CA1 pyramidal cells from kainate-treated animals than in control animals. Ictal-like events of duration 8-80 s were induced by tetanic stimulation (50 Hz, 0.2-1 s) preferentially in dorsal contralateral and ventral ipsilateral slices. Similar events were initiated by focal application of a combination of high K(+) and GABA. These data show that both interictal and ictal-like activities can be induced in slices of both ipsilateral and contralateral hippocampus from kainate-treated animals and suggest that changes in cellular excitability and inhibitory synaptic signalling may contribute to their generation.

[Animal models to develop surgery of focal epilepsies?]

A model is a simplified preparation that reproduces only the most critical features of a disease. To be considered as a validated animal model, such an experimental preparation must fulfill three criteria: isomorphism or similarities of the symptoms; predictivity or identical pharmacological reactivity; homology or etiological similarity. In epilepsy, the use of animal models helps our understanding of physiological and pathological networks involved in the genesis, maintenance, and propagation of seizures. The animal models of epilepsy are also useful in designing and testing new surgical therapeutical strategies, in particular using deconnection or neuromodulation in drug-resistant focal epilepsies. Here we describe three animal models of focal epilepsy, adapted to addressing experimental surgery issues. Kindling consists in the regular liminar stimulation of a given brain structure in the rodent to develop a focal discharge that is secondarily generalized. The local application of epileptogenic agents such as cobalt, iron, or penicillin leads to focal discharges that do not generalize in the rodent or the primate. It is a model of focal neocortical epilepsy without secondary generalization. The focal application of kainate, an excitotoxic glutamate agonist, in the dorsal hippocampus of the adult mouse results, after a latent period, in spontaneous and recurrent focal discharges, behavioral interictal troubles, drug resistance, and histological anomalies reminiscent of hippocampal sclerosis. This constitutes a model of mesial-temporal epilepsy. Better knowledge, in these models, of the neural networks generating, propagating, and/or controlling the seizures should make it possible to design innovative surgical approaches for the treatment of drug-resistant epilepsies.

Botulinum neurotoxin E (BoNT/E) reduces CA1 neuron loss and granule cell dispersion, with no effects on chronic seizures, in a mouse model of temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is often the result of an early insult that induces a reorganization in hippocampal circuitry leading, after a latent period, to chronic epilepsy. Hippocampal rearrangements during the latent phase include neuronal loss, axonal and dendritic plasticity, neurogenesis, and cell repositioning, but the role of these changes in epilepsy development is unclear. Here we have tested whether administration of the synaptic blocker botulinum neurotoxin E (BoNT/E) interferes with development of spontaneous seizures and histopathological changes following an episode of status epilepticus (SE). SE was induced by unilateral intrahippocampal injection of kainic acid in mice and BoNT/E was delivered to the same hippocampus 3 h later. We found that treatment with BoNT/E prolonged the duration of the latent period but did not block the occurrence of spontaneous seizures. At the histopathological level, BoNT/E reduced loss of CA1 pyramidal neurons and dispersion of dentate granule cells. Downregulation of reelin expression along the hippocampal fissure was also suppressed by BoNT/E treatment. Our findings indicate that administration of BoNT/E after SE inhibits specific morphological changes in hippocampal circuitry but not the development of spontaneous seizures. This indicates a dissociation between certain anatomical modifications and establishment of chronic epilepsy in MTLE.