The spatiotemporal dynamics of human focal seizures

Spontaneous human focal seizures can present with a plethora of behavioral manifestations that vary according to the affected cortical regions; however, several key features have been consistently observed. During my doctoral studies, I applied both theoretical and experimental methods to study mechanisms underpinning these consistently seen dynamics. I first analyzed human intracranial EEG recordings, describing statistical methods for measuring their electrophysiological signatures. I next proposed several neurophysiological hypotheses that could explain seizure dynamics and verified them in rodent seizure models. Finally, a computational model was developed, successfully explaining how the complex spatiotemporal evolution of focal seizures emerges from simple neurophysiological principles. In Chapter 1, the long-standing behavioral manifestations and the most up-to-date electrophysiology findings are reviewed. This section details the inspiration for the studies reported in the subsequent chapters. In Chapter 2, I describe several statistical methods for estimating traveling wave velocities. I show most ictal discharges can be described as traveling waves whose velocities contain rich information about the stages of seizure evolution. I compare performance of various statistical methods and propose a robust approach to boost the quality of each method’s estimation results. In Chapter 3, I show how inhibition modulates seizure propagation patterns. Surround inhibition spatially restrains focal seizures and masks excitatory projections of ictal activities. When compromised, two patterns of seizure propagation emerge according to the position of inhibition defects relative to the ictal focus. I show that two distant ictal foci can communicate via physiological connectivity without any chronic rewiring processes – confirming the existence of long-range propagation pathways that could lead to epileptic network formation. In Chapter 4, I show that thalamic inputs might be necessary for interictal epileptiform discharges (IEDs). The relative positions between IEDs and ictal foci indicate that surround inhibition, shown in the previous chapter, can be exhausted by repetitive exposure to ictal projections. In Chapter 5, I propose a neural network model that can explain both long-standing behavioral observations of seizures and account for the most up-to-date electrophysiological recordings of spontaneous human focal seizures. The model relies on few assumptions, all of which are proved or supported in earlier chapters of this thesis. The model explains phasic evolution of seizure dynamics – how the commonly observed patterns arise from simple neurophysiological principles, as well as seizure onset subtypes, traveling wave directions and speeds. The model also predicts how spontaneous seizures might arise from synaptic plasticity. The chapter ends with a discussion of the model’s implications and future work. The thesis is organized in a way that each chapter can be read independently, with Chapter 5 summarizing the central theory spanning the whole study. Each chapter is also tightly linked to a clinically relevant question. In sum, the dissertation’s goal is to provide an in-principle understanding of focal seizure dynamics. With rapid advancement of clinical and experimental tools, I believe this work provides a roadmap for future therapies for epilepsy patients

Incessant transitions between active and silent states in cortico-thalamic circuits and altered neuronal excitability lead to epilepsy

La ligne directrice de nos expériences a été l'hypothèse que l'apparition et/ou la persistance des fluctuations de longue durée entre les états silencieux et actifs dans les réseaux néocorticaux et une excitabilité neuronale modifiée sont les facteurs principaux de l'épileptogenèse, menant aux crises d’épilepsie avec expression comportementale. Nous avons testé cette hypothèse dans deux modèles expérimentaux différents. La déafférentation corticale chronique a essayé de répliquer la déafférentation physiologique du neocortex observée pendant le sommeil à ondes lentes. Dans ces conditions, caractérisées par une diminution de la pression synaptique et par une incidence augmentée de périodes silencieuses dans le système cortico-thalamique, le processus de plasticité homéostatique augmente l’excitabilité neuronale. Par conséquent, le cortex a oscillé entre des périodes actives et silencieuses et, également, a développé des activités hyper-synchrones, s'étendant de l’hyperexcitabilité cellulaire à l'épileptogenèse focale et à des crises épileptiques généralisées. Le modèle de stimulation sous-liminale chronique (« kindling ») du cortex cérébral a été employé afin d'imposer au réseau cortical une charge synaptique supérieure à celle existante pendant les états actifs naturels - état de veille ou sommeil paradoxal (REM). Dans ces conditions un mécanisme différent de plasticité qui s’est exprimé dans le système thalamo-corticale a imposé pour des longues périodes de temps des oscillations continuelles entre les époques actives et silencieuses, que nous avons appelées des activités paroxysmiques persistantes. Indépendamment du mécanisme sous-jacent de l'épileptogenèse les crises d’épilepsie ont montré certaines caractéristiques similaires : une altération dans l’excitabilité neuronale mise en évidence par une incidence accrue des décharges neuronales de type bouffée, une tendance constante vers la généralisation, une propagation de plus en plus rapide, une synchronie augmentée au cours du temps, et une modulation par les états de vigilance (facilitation pendant le sommeil à ondes lentes et barrage pendant le sommeil REM). Les états silencieux, hyper-polarisés, de neurones corticaux favorisent l'apparition des bouffées de potentiels d’action en réponse aux événements synaptiques, et l'influence post-synaptique d'une bouffée de potentiels d’action est beaucoup plus importante par rapport à l’impacte d’un seul potentiel d’action. Nous avons également apporté des évidences que les neurones néocorticaux de type FRB sont capables à répondre avec des bouffées de potentiels d’action pendant les phases hyper-polarisées de l'oscillation lente, propriété qui peut jouer un rôle très important dans l’analyse de l’information dans le cerveau normal et dans l'épileptogenèse. Finalement, nous avons rapporté un troisième mécanisme de plasticité dans les réseaux corticaux après les crises d’épilepsie - une diminution d’amplitude des potentiels post-synaptiques excitatrices évoquées par la stimulation corticale après les crises - qui peut être un des facteurs responsables des déficits comportementaux observés chez les patients épileptiques. Nous concluons que la transition incessante entre des états actifs et silencieux dans les circuits cortico-thalamiques induits par disfacilitation (sommeil à ondes lentes), déafférentation corticale (épisodes ictales à 4-Hz) ou par une stimulation sous-liminale chronique (activités paroxysmiques persistantes) crée des circonstances favorables pour le développement de l'épileptogenèse. En plus, l'augmentation de l’incidence des bouffées de potentiels d’actions induisant une excitation post-synaptique anormalement forte, change l'équilibre entre l'excitation et l'inhibition vers une supra-excitation menant a l’apparition des crises d’épilepsie.The guiding line in our experiments was the hypothesis that the occurrence and / or the persistence of long-lasting fluctuations between silent and active states in the neocortical networks, together with a modified neuronal excitability are the key factors of epileptogenesis, leading to behavioral seizures. We addressed this hypothesis in two different experimental models. The chronic cortical deafferentation replicated the physiological deafferentation of the neocortex observed during slow-wave sleep (SWS). Under these conditions of decreased synaptic input and increased incidence of silent periods in the corticothalamic system the process of homeostatic plasticity up-regulated cortical cellular and network mechanisms and leaded to an increased excitability. Therefore, the deafferented cortex was able to oscillate between active and silent epochs for long periods of time and, furthermore, to develop highly synchronized activities, ranging from cellular hyperexcitability to focal epileptogenesis and generalized seizures. The kindling model was used in order to impose to the cortical network a synaptic drive superior to the one naturally occurring during the active states - wake or rapid eye movements (REM) sleep. Under these conditions a different plasticity mechanism occurring in the thalamo-cortical system imposed long-lasting oscillatory pattern between active and silent epochs, which we called outlasting activities. Independently of the mechanism of epileptogenesis seizures showed some analogous characteristics: alteration of the neuronal firing pattern with increased bursts probability, a constant tendency toward generalization, faster propagation and increased synchrony over the time, and modulation by the state of vigilance (overt during SWS and completely abolished during REM sleep). Silent, hyperpolarized, states of cortical neurons favor the induction of burst firing in response to depolarizing inputs, and the postsynaptic influence of a burst is much stronger as compared to a single spike. Furthermore, we brought evidences that a particular type of neocortical neurons - fast rhythmic bursting (FRB) class - is capable to consistently respond with bursts during the hyperpolarized phase of the slow oscillation, fact that may play a very important role in both normal brain processing and in epileptogenesis. Finally, we reported a third plastic mechanism in the cortical network following seizures - a decreasing amplitude of cortically evoked excitatory post-synaptic potentials (EPSP) following seizures - which may be one of the factors responsible for the behavioral deficits observed in patients with epilepsy. We conclude that incessant transitions between active and silent states in cortico-thalamic circuits induced either by disfacilitation (sleep), cortical deafferentation (4-Hz ictal episodes) and by kindling (outlasting activities) create favorable circumstances for epileptogenesis. The increase in burst-firing, which further induce abnormally strong postsynaptic excitation, shifts the balance of excitation and inhibition toward overexcitation leading to the onset of seizures

Clozapine-induced paroxysmal discharges

PhD ThesisThe atypical antipsychotic clozapine is a widely prescribed and effective treatment for the positive and negative symptoms of schizophrenia, but reports of side effects are common. In one study EEG abnormalities were observed in 53% of patients treated with clozapine, and the absence or presence of EEG abnormalities correlated with the plasma clozapine concentration. Here, epileptiform activity was present in conventional EEG recordings from a 32 year old male patient with psychiatric illness taking clozapine for 3 weeks. Brief (ca.100ms), transient epileptiform spikes occurred at a frequency of approximately 2 per h and originated primarily in parietal cortex. One month after withdrawal of clozapine, epileptiform spikes were no longer present. An in vitro model was developed using the equivalent region of association cortex, namely 2⁰ somatosensory cortex, in normal rat brain slices to probe such activity with increased spatial and temporal resolution, and to investigate mechanisms underlying its generation. Wide band in vitro recordings revealed that clozapine (10-20µM) induced regular, frequent very fast oscillations (VFO, > 70Hz) in this region. These VFO comprised short transient high frequency discharges and were maximal in patches along layer V. The atypical antipsychotic olanzapine, but not the classical antipsychotic haloperidol, also induced prominent VFO in this region. Sharp electrode intracellular recordings revealed that there was almost no correlation between the somatic activity of layer V regular spiking (RS) pyramidal cells and field VFO, but layer V intrinsically bursting (IB) cells did correlate to some extent with the local field. Interestingly, IB cell spikelets were also weakly correlated with field VFO suggesting a role for axonal hyperexcitability in this cell type in the mechanism. Clozapine-induced VFO persisted following blockade of AMPA, NMDA, and GABAA chemical synaptic receptors, and the gap junction blockers carbenoxolone and quinine also failed to significantly attenuate the power of this activity. Although octanol abolished clozapine-induced VFO, it was not clear that this effect resulted from blockade of gap junctions as this drug also blocks spikes. In addition to VFO events, clozapine (10-20µM) also induced occasional, spontaneous transient paroxysmal discharges, similar to the EEG phenomena, in 33% (11/33 slices) of slices in vitro. Sharp electrode intracellular recordings revealed that clozapine- induced full paroxysmal discharges were associated with spikes, EPSPs and IPSPs in layer V RS and IB cells, suggesting that these events were mediated via chemical synaptic transmission in both of these cell types. Multi-electrode array recordings of local field potentials and units suggested that clozapine-induced paroxysmal events started superficially in association cortex, moved deeper and then propagated horizontally along these deep layers. The onset of clozapine-induced VFO was accompanied by a significant elevation in parvalbumin immunoreactivity, particularly in layer II-IV, where there was a greater than twofold increase in the signal, and this may be relevant to the therapeutic action of the drug

Functional PET Evaluation of the Photosensitive Baboon

The baboon provides a unique, natural model of epilepsy in nonhuman primates. Additionally, photosensitivity of the epileptic baboon provides an important window into the mechanism of human idiopathic generalized epilepsies. In order to better understand the networks underlying this model, our group utilized functional positron emission tomography (PET) to compare cerebral blood flow (CBF) changes occurring during intermittent light stimulation (ILS) and rest between baboons photosensitive, epileptic (PS) and asymptomatic, control (CTL) animals. Our studies utilized subtraction and covariance analyses to evaluate CBF changes occurring during ILS across activation and resting states, but also evaluated CBF correlations with ketamine doses and interictal epileptic discharge (IED) rate during the resting state. Furthermore, our group also assessed the CBF responses related to variation of ILS in PS and CTL animals. CBF changes in the subtraction and covariance analyses reveal the physiological response and visual connectivity in CTL animals and pathophysiological networks underlying responses associated with the activation of ictal and interictal epileptic discharges in PS animals. The correlation with ketamine dose is essential to understanding differences in CBF responses between both groups, and correlations with IED rate provides an insight into an epileptic network independent of visual activation. Finally, the ILS frequency dependent changes can help develop a framework to study not only spatial connectivity but also the temporal sequence of regional activations and deactivations related to ILS. The maps generated by the CBF analyses will be used to target specific nodes in the epileptic network for electrophysiological evaluation using intracranial electrodes

Investigating epileptiform activity associated with slow wave sleep

PhD ThesisThe characteristic EEG trait of patients with nocturnal idiopathic epilepsies during childhood is the spike and wave discharge. Cognitive dysfunction is prevalent among these patients and is thought to be linked to disturbances in memory consolidation processes that normally occur during slow wave sleep. Several genetic mutations of nicotinic receptor subunits have been linked to these disorders. However, there is little known about the underlying mechanisms or the spatiotemporal characteristics of this epileptiform activity within the neocortex. This thesis presents a rat in vitro model of the epileptiform activity synonymous with nocturnal childhood epilepsies, that allows for pharmacological manipulation of receptor subunits linked to these disorders. The application of DTC [10 M], a non-selective, competitive nicotinic acetylcholine receptor antagonist, to an in vitro model of the cortical delta rhythm induced two individual forms of paroxysm events - wave discharges and the conventional spike and wave discharges. Pharmacological manipulation of this model suggest that the epileptiform activity is mediated by excitatory currents which is consistent with the use of glutamate antagonists as anticonvulsants. A blanket blockade of inhibition by a GABAA antagonist resulted in severe discharges, hence hugely increasing excitatory response. Only partial disinhibition is suggested to be required to generate epileptiform activity as nicotinic acetylcholine receptors and 5-HT3 receptors are located on dendrite targeting interneurons. Mapping of unit activity revealed the di erence between the two paroxysm events was recruitment of super cial layers with simultaneous paroxysm events in delta frequency-generating Layer V pyramidal cells. It is proposed that the hyperexcitability responsible for the generation of the spike component of a spike and wave discharge is mediated by the lack of excitatory tone in 5-HT3 and nicotinic acetylecholine receptor expressing inhibitory interneuron subtypes. The disinhibition, spike generation and disruption of interplay between deep and super cial layers of the neocortex is thought to be associated with synaptic plastic changes

New evidences of inflammatory mediators in absence epilepsy

L'epilessia dell'assenza appartiene alle forme di epilessia di tipo generalizzato. Crisi di assenza sono caratterizzate a livello elettroencefalografico dall'insorgenza di onde-punta caratteristiche (SWDs) dovute ad una patologica attivit\ue0 sincronizzata del circuito talamo-corticale. Gli studi sono stati effettuati sul modello animale (Wag/Rij), ratti che dal sesto mese d'et\ue0 presentano crisi di epilessia dell'assenza ricorrenti e spontanee. Ratti Wag/rij di quattro mesi d\u2019et\ue0 non ancora epilettici sono stati utilizzati come controllo. La pi\uf9 recente teoria volta a spiegare le cause dell'insorgenza di questa patologia mette in luce il ruolo primario della corteccia cerebrale ed identifica nella regione peri-orale della corteccia somato-sensoriale il focus di inizio delle crisi. Al fine di avvalorare tale teoria, sono stati svolti studi di risonanza magnetica strutturale (T2 e DWI) e funzionale (rCBV e rCBF), esperimenti di spettroscopia di risonanza magnetica (MRS) e analisi del profilo di espressione genica nelle aree cerebrali maggiormente coinvolte in questa patologia. Inoltre, la registrazione e l'analisi di tracciati EEG in seguito al trattamento con IL-1\u3b2 e TNF-\u3b1 hanno messo in luce il ruolo di tali citochine infiammatorie sull'insorgenza e la modulazione delle crisi d'assenza.Absence epilepsy is the most pure form of generalized epilepsy characterized in the EEG by widespread bilaterally synchronous spike-wave discharges (SWDs) caused by thalamo-cortical oscillations. Latest Cortical Focus theory suggests a consistent cortical \u201cfocus\u201d within the peri-oral region of the somatosensory cortex. In order to raise the evidence of a focal cortical theory, structural and functional MRI data were collected in all the cerebral areas involved in spindles generation and propagation in a genetic model of absence epilepsy. Four-months-old WAG/Rij rats were used as control (no SWDs) whereas nine-months-old rats (daily SWDs) were referred as experimental group. In order to exacerbate SWDs episodes, rats were treated with Vigabatrin. Moreover to provides chemical information about different brain regions in Wag/Rij rats, we performed MRS analysis for the measure of the levels of different metabolites which reflects specific cellular and biochemical processes. It has become increasingly obvious during recent decades that genetic factors play a main role in the idiopathic generalized epilepsies, including absence epilepsy. In this view, gene-array analysis for cell signaling pathways involved in SWDs in different cerebral areas, and qPCR data were performed and correlated with EEG data. The actions of proinflammatory cytokines in the CNS are only partially discovered. Some cytokines have been recently shown to affect neurotransmitters or are required to preserve the synaptic strength at excitatory synapses, or affect the expression of various neuropeptides and neurotrophic factors in several brain regions. Changes in the immune system may change the excitability of the CNS and alter the susceptibility of exogenous induced or genetically determined types of epilepsy. Here we investigate the role of two cytokines, IL-1\u3b2 and TNF-\u3b1 in WAG/Rij rats. Our hypothesis is that cerebral blood flow alterations and cytokines/chemokines release can modulate the occurrence of SWDs. Both controls and WAG/Rij rats were injected i.p. by TNF-\u3b1 (2\ub5g/kg) and IL-1\u3b2 (2\ub5g/kg) and EEG was recordered for 72h after the treatment. Furthermore, we analyzed the blood serum by ELISA method for TNF-\u3b1 and IL-1\u3b2 in control and epileptic animals

Mechanisms of altered cortical excitability in photosensitive epilepsy

Despite the multiplicity of approaches and techniques so far applied for identifying the pathophysiological mechanisms of photosensitive epilepsy, a generally agreed explanation of the phenomenon is still lacking. The present thesis reports on three interlinked original experimental studies conducted to explore the neurophysiological correlates and the phatophysiological mechanism of photosensitive epilepsy. In the first study I assessed the role of the habituation of the Visual Evoked Response test as a possible biomarker of epileptic visual sensitivity. The two subsequent studies were designed to address specific research questions emerging from the results of the first study. The findings of the three intertwined studies performed provide experimental evidence that photosensitivity is associated with changes in a number of electrophysiological measures suggestive of altered balance between excitatory and inhibitory cortical processes. Although a strong clinical association does exist between specific epileptic syndromes and visual sensitivity, results from this research indicate that photosensitivity trait seems to be the expression of specific pathophysiological mechanisms quite distinct from the “epileptic” phenotype. The habituation of Pattern Reversal Visual Evoked Potential (PR-VEP) appears as a reliable candidate endo-phenotype of visual sensitivity. Interpreting the findings of this study in the context of the broader literature on visual habituation we can hypothesise the existence of a shared neurophysiological background between photosensitive epilepsy and migraine. Future studies to elucidate the relationship between the proposed indices of cortical excitability and specific polymorphisms of excitatroy and inhibitory neurotransmission will need to be conducted to assess their potential role as biomarkers of photosensitivity

Effets de la stimulation cérébrale profonde dans l'épilepsie focale motrice

Epileptic seizures arise from pathological synchronization of neuronal ensemble.Seizures originating from primary motor cortex are often pharmacoresistant, and many times unsuitable for respective surgery because of location of epileptic focus in eloquent area. Basal ganglia play important role in seizure propagation. Micro electrode recordings performed during previous studies indicated that input structures of basal ganglia such as GPe, Putamen and Subthalamic nucleus (STN) are strongly modified during seizures. For example the mean firing rate of neurons of the STN and Putamen increased and the percentage of oscillatory neurons synchronized with the ictal EEG was higher during seizures as compared to interictal periods. Pilot studies in humans have shown the possible beneficial effect of chronic DBS applied to STN in treatment of pharmacoresistant motor seizures. Our study was aimed at studying the therapeutic effect of electrical stimulation of input structures of basal ganglia . We first developed a stable, predictable primate model of focal motor epilepsy by intracortical injection of penicillin and we documented it's pharmacoresistence. We then stereotactically implanted DBS electrodes in the STN and Putamen. The stimulator was embedded at the back of the animals. Subthreshold electrical stimulations at 130 Hz were applied to STN. Stimulator was turned ON when penicillin was injected. Sham stimulation at 0 volt was used as a control situation, each monkey being its own control. The time course, number and duration of seizures occurring in each epochs of 1 h were compared during ON and sham stimulation periods. Each experimental session lasted uptoo 6 hours,We also studied preventive high frequency stimulation of STN and subthershold low frequency stimulation of Putamen with 5 Hz and 20 Hz in the same model .Finally we studied combined effects of high frequency STN and low frequency Putamen stimulation in one monkey Results: Data was analysed from 1572 seizures in 30 experiments in three monkeys for chronic STN stimulation , 454 seizures in 10 experiments in one moneky during preventive STN stimulation ,289 seizures from 14 experiments in two monkeys during LFS putamen stimulation and 477 seizures from 10 sessions during combined STN and Putamen stimulation in one monkey The best results were observed during chronic STN stimulation The occurrence of first seizure was significantly delayed as compared to sham situation. Total time spent in focal seizures was significantly reduced by ≥69% on an average (p ≤0.05) after STN stimulation, due to a significant decrease in the number of seizures especially so during the first 3 hours after stimulation. The duration of individual seizures reduced moderately. Bipolar and monopolar stimulation modes were equally effective Preventive HFS STN (in one specimen) was not found to be superior to acute stimulation. LFS Putamen alone was effective but mainly in first two hours of stimulation .In a combined HFS STN and LFS Putamen stimulation the effect of stimulation in terms of seizure control was modest and poor compared to HFS STN alone or LFS Putamen alone. This study provides original data in primates showing the potential therapeutic effect of chronic HFS-STN DBS to treat focal motor seizures . A discussion explaining these results and comparison with STN DBS in human motor seizures as well as future translational perspective in human therapeutics is provided.Les crises d'épilepsie proviennent d'une synchronisation pathologique de réseaux neuronaux du cortex. Les crises motrices, générées à partir du cortex moteur primaire, sont souvent pharmaco-résistantes. La résection neurochirurgicale du foyer épileptique est rarement l'option thérapeutique de choix au regard des risques de deficits moteurs potentiellement induits par la résection. Les ganglions de la base ont un rôle important dans la propagation des crises. Des enregistrements par micro-électrode réalisés dans une précédente étude ont montré que les activités des structures d'entrée des ganglions de la base telles que le Putamen, le noyau caudé et le noyau sous-thalamique (NST) sont fortement modifiées pendant des crises motrices. Le taux de décharge moyen des neurones du NST et du Putamen augmente et le pourcentage de neurones oscillants synchronisés avec l'EEG durant la période ictale est plus élevé durant les crises que pendant la période inter-ictale. Des études pilotes chez l'humain ont montré un effet bénéfique potentiel de la stimulation cérébrale profonde (SCP) chronique du NST pour traiter les crises motrices pharmaco-résistantes. Le but de notre étude est d'évaluer les effets thérapeutiques de la SCP des structures d'entrée des ganglions de la base. Nous avons dans un premier temps développé un modèle primate de crise d'épilepsie motrice focale stable et reproductible par injection intra-corticale de pénicilline. Nous avons ensuite caractérisé la pharmaco-résistance du modèle. Nous avons implanté stéréotactiquement des électrodes de SCP dans le NST et le Putamen. Le stimulateur a été placé sous la peau dans le dos de l'animal. Un protocole de stimulation à 130 Hz à un voltage inférieur à l'apparition d'effets secondaires a été réalisé dans le NST. Le stimulateur était mis en marche au moment de l'injection de la pénicilline. Un protocole de stimulation à 0 volt a été réalisé comme condition contrôle. Chaque primate étant son propre contrôle. L'apparition des crises, leur nombre et leur durée ont été comparés par période de 1 heure entre la condition stimulée et non stimulée. Chaque session expérimentale a été menée sur une durée de plus de six heures. Nous avons évalué l'effet préventif de la SCP à haute fréquence (130 Hz) du NST sur les crises motrices. Nous avons également étudié l'effet préventif de la SCP à basse fréquence (5-20 Hz) du Putamen sur ce même modèle. Enfin, sur un autre primate, nous avons étudié l'effet combiné de la SCP du NST à haute fréquence et du Putamen à basse fréquence sur les crises motrices. Résultats : Les effets de la SCP chronique du NST à haute fréquence ont été analysés à partir de 1572 crises apparues au cours de 30 sessions expérimentales chez 3 primates. Les effets de la SCP préventive du NST ont été évalués sur 454 crises motrices durant 10 sessions expérimentales chez un primate. L'effet de la SCP du Putamen à basse fréquence a été analysé sur 289 crises durant 14 sessions chez 2 primates. Enfin l'effet combiné de la SCP du NST et du Putamen a été évalué sur 477 crises durant 10 sessions. Les meilleurs résultats ont été obtenus par SCP chronique du NST. L'apparition de la première crise était significativement retardée lorsque le primate était stimulé. Le temps total passé en situation de crise motrice était diminué en moyenne d'environ 69 % (p ≤0.05) par rapport à la condition non-stimulé au regard de la diminution significative du nombre de crises particulièrement durant les 3 heures après le début de la stimulation. La durée de chaque crise était modérément réduite. Les modes de stimulation mono-polaire ou bi-polaire avaient une efficacité similaire. La SCP préventive du NST n'a pas eu d'effet supérieur à la stimulation chronique du NST. La SCP chronique du Putamen à basse fréquence avait un effet positif mais principalement durant les deux premières heures de stimulation. L'effet combiné de la SCP du NST et du Putamen était inférieur à la SCP chronique du NST ou du Putamen

An investigation into dysfunctional feed-forward inhibition within the cortico-thalamocortical network on absence seizure generation using DREADD technology

Childhood absence epilepsy (CAE) is one of the most prevalent paediatric epilepsies, accounting for between 10-17% of all diagnosed cases of epilepsies seen in school-aged children. Absence seizures are characterized by behavioural arrest/loss of awareness and electrographic signature of spike-wave discharges (SWDs) measuring 2.5-4 Hz on an electroencephalogram (EEG). These brief episodes of impaired consciousness can occur hundreds of times a day and might increase the chance of physical injury when undertaking activities like swimming and cycling. Current treatment options are not sufficient and up to 30% of patients are pharmaco-resistant. ~60% of children with CAE have severe neuropsychiatric comorbid conditions including attention deficits, mood disorders, impairments in memory and cognition. Ethosuximide (ETX), an anti-absence epileptic drug which was first introduced almost six decades ago remains the first choice for initial monotherapy for the treatment of CAE. Large-scale clinical trials suggested that efficacy of ethosuximide is considerably lower than previous findings. Thus, safe, effective and patient specific treatment approach is imperative. For this, it is crucial first to understand the precise cellular and molecular mechanisms of absence seizures which may enable the development of novel therapeutic targets and discovery of new anti-epileptic drugs (AEDs). EEG and functional imaging evidence suggest that absence seizures are likely due to aberrant activity within the cortico-thalamocortical (CTC) network. Studies involving the genetic rodent models have shown that the cortex is the driving source for the origin of SWDs but is not capable of maintaining discharges on its own, nor is the thalamus. General consensus is that, within the CTC network, a cortical focus initiates rhythmic epileptic discharges, however, once the rhythmic oscillations are established, both the cortex and thalamus form an integrated network. Rhythmic absence-SWDs are sustained via the cortex and thalamus driving each other. Within the CTC network, feed-forward inhibition (FFI) is essential to prevent runaway excitation. FFI is mediated by fast spiking parvalbumin expressing (PV+) inhibitory interneurons in the somatosensory cortex (SScortex) and the reticular thalamic nucleus (RTN). Studies conducted in well-established stargazer mouse model of absence epilepsy with a genetic deficit in stargazin i.e. TARP [a transmembrane α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor regulatory protein] have shown reduced expression of GluA4-AMPARs at excitatory synapses in feed-forward inhibitory (PV+) interneurons in the SScortex and RTN thalamus of the CTC network. However, the extent of this deficit in AMPARs expression impacting FFI and possibly contributing towards generation of absence-SWDs is not established via functional studies. Hence, this thesis was aimed at investigating the impact of dysfunctional feed-forward inhibitory PV+ interneurons within CTC network on absence seizure generation and behaviour. For this purpose, inhibitory and excitatory Designer Receptors Exclusively Activated by Designer Drug (DREADD) approach was utilized to silence/excite feed-forward inhibitory PV+ interneurons within the CTC network. DREADD mediated regional silencing of PV+ interneurons within the CTC network generated ETX-sensitive absence-like SWDs. Activating PV+ interneurons either prevented or suppressed pentylenetetrazole (PTZ)-induced absence-SWDs. Finally, impact of impaired FFI in γ-aminobutyric acid (GABA) levels by affecting its synthesizing enzymes (GADs) and transporter proteins (GATs) in stargazer animal model of absence epilepsy and CNO treated inhibitory Gi-DREADD animals was determined. Results indicate that upregulation of GAD65 in the SScortex of epileptic stargazers may be a consequence of absence seizures or this may have contribution in absence seizure generation. The work presented in this thesis provide an electrophysiological insight into the possible mechanism underlying the absence seizure generation. This work provides convincing evidence that dysfunctional feed-forward inhibitory PV+ interneurons within the CTC network is likely to be involved in altered excitation/inhibition balance resulting SWDs as activating these interneurons dramatically protected animals from PTZ induced absence seizures. The clinical relevance of this study is that it potentially uncovers the possibility of focally targeting PV+ interneurons within the CTC network to control absence seizures in human patients