Cellular and homeostatic network mechanisms of posttraumatic epilepsy

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2008-2009Suite aux traumatismes crâniens pénétrants, le cerveau devient graduellement hyperexcitable et génère des activités paroxystiques spontanées. Les mécanismes qui sous-tendent l’épileptogénèse demeurent cependant peu connus. La ligne directrice de nos travaux consiste en l'hypothèse que la diminution de l'activité corticale engendrée par la déafférentation déclenche des mécanismes homéostatiques agissant tant au niveau cellulaire qu’au niveau du réseau cortical, et qui mènent à une excitabilité neuronale accrue culminant en crises d’épilepsie. Nous avons testé cette hypothèse chez des chats adultes, lors de différents états de vigilance ou sous anesthésie, ayant subits une déafférentation partielle du gyrus suprasylvien. Nous avons évalué les effets de la déafférentation corticale aigue et chronique sur la survie des neurones et des cellules gliales et nous avons investigué comment la privation chronique d'afférences neuronales pourrait modifier les propriétés du réseau cortical et déclencher des crises d’épilepsie. Après la déafférentation du gyrus suprasylvien, les neurones situés dans les couches corticales profondes, en particulier les neurones inhibiteurs GABAérgiques, dégénèrent progressivement et parallèlement à une fréquence croissante des activités paroxystiques, notamment pendant le sommeil à ondes lentes. La privation chronique d'afférences neuronales et la perte de neurones activent les mécanismes homéostatiques de plasticité qui favorisent une plus grande connectivité neuronale, une efficacité plus élevée des connexions synaptiques excitatrices et des changements des propriétés neuronales intrinsèques. Ensemble, ces facteurs favorisent une excitation accrue du réseau cortical. L'activité corticale spontanée, mesurée par les taux moyens de décharge, augmente progressivement, en particulier pendant le sommeil à ondes lentes, caractérisé par des périodes silencieuses alternant avec des périodes actives. Ceci soutient, en outre, notre hypothèse concernant la participation des mécanismes de plasticité homéostatique. La dégénération des neurones des couches corticales profondes produit des changements importants dans la distribution laminaire de l'activité neuronale, qui est déplacée vers les couches plus superficielles, dans la partie déafferenté du gyrus. Ce changement dans la distribution de profils de profondeurs de décharges neuronales modifie également le déclenchement de l'activité corticale spontanée. Dans le cortex normal et dans la partie relativement intacte du gyrus suprasylvien, l'activité corticale est générée dans les couches corticales profondes. Pourtant, dans le cortex chroniquement déafferenté, l'oscillation lente et les activités ictales sont générées dans les couches superficielles et puis diffusent vers les couches plus profondes. Le traumatisme cortical induit également une importante gliose réactive et une altération de la fonction normale des cellules gliales, ce qui cause l’enlèvement dysfonctionnel du K+ extracellulaire et qui augmente l'excitabilité des neurones favorisant ainsi la génération d’activités paroxystiques. En conclusion, les mécanismes de plasticité homéostatique déclenchés par le niveau diminué d'activité dans le cortex déafferenté produisent une hyperexcitabilité corticale incontrôlable et génèrent finalement les crises d’épilepsie. Dans ces conditions, l’augmentation de l'activité corticale plutôt que la diminution avec des médicaments antiépileptiques pourrait être salutaire pour empêcher le développement de l'épileptogenèse post-traumatique.After penetrating cortical wounds, the brain becomes gradually hyperexcitable and generates spontaneous paroxysmal activity, but the progressive mechanisms of epileptogenesis remain virtually unknown. The guiding line of our experiments was the hypothesis that the reduced cortical activity following deafferentation triggers homeostatic mechanisms acting at cellular and network levels, leading to an increased neuronal excitability and finally generating paroxysmal activities. We tested this hypothesis either in anesthetized adult cats, or during natural sleep and wake, using the model of partially deafferented suprasylvian gyrus to induce posttraumatic epileptogenesis. We evaluated the effects of acute and chronic cortical deafferentation on the survival of neurons and glial cells and how long-term input deprivation could shape up the properties of neuronal networks and the initiation of spontaneous cortical activity. Following cortical deafferentation of the suprasylvian gyrus, the deeply laying neurons, particularly the inhibitory GABAergic ones, degenerate progressively in parallel with an increased propensity to paroxysmal activity, mainly during slow-wave sleep. The chronic input deprivation and the death of neurons activate homeostatic plasticity mechanisms, which promote a gradual increased neuronal connectivity, higher efficacy of excitatory synaptic connections and changes in intrinsic cellular properties favoring increased excitation. The spontaneous cortical activity quantified by means of firing rate augments also progressively, particularly during slow-wave sleep, characterized by periods of silent states alternating with periods of active states, which supports furthermore our hypothesis regarding the involvement of homeostatic plasticity mechanisms. The degeneration of neurons in the deep cortical layers generates important changes in the laminar distribution of neuronal activity, which is shifted from the deeper layers to the more superficial ones, in the partially deafferented part of the gyrus. This change in the depth profile distribution of firing rates modifies also the initiation of spontaneous cortical activity which, in normal cortex, and in the relatively intact part of the deafferented gyrus, is initiated in the deep cortical layers. Conversely, in late stages of the undercut, both the cortical slow oscillation and the ictal activity are initiated in the more superficial layers and then spread to the deeper ones. Cortical trauma induces also an important reactive gliosis associated with an impaired function of glial cells, responsible for a dysfunctional K+ clearance in the injured cortex, which additionally increases the excitability of neurons, promoting the generation of paroxysmal activity. We conclude, that the homeostatic plasticity mechanisms triggered by the decreased level of activity in the deafferented cortex, generate an uncontrollable cortical hyperexcitability, finally leading to seizures. If this statement is true, augmenting cortical activity rapidly after cortical trauma rather than decreasing it with antiepileptic medication, could prove beneficial in preventing the development of posttraumatic epileptogenesis

Neocortical Post-Traumatic Epileptogenesis Is Associated with Loss of GABAergic Neurons

The subtle mechanisms of post-traumatic epileptogenesis remain unknown, although the incidence of chronic epilepsy after penetrating cortical wounds is high. Here, we investigated whether the increased frequency of seizures occurring within 6 weeks following partial deafferentation of the suprasylvian gyrus in cats is accompanied with a change in the ratio between the number of excitatory and inhibitory neurons. Immuno-histochemical labeling of all neurons with neuronal-specific nuclear protein (NeuN) antibody, and of the GABAergic inhibitory neurons with either gamma-aminobutyric acid (GABA) or glutamic acid decarboxylase (GAD 65&67) antibodies, was performed on sections obtained from control and epileptic animals with chronically deafferented suprasylvian gyrus. Quantification of the labeled neurons was performed in control animals and at 2, 4, and 6 weeks following cortical deafferentation, in the suprasylvian and marginal gyri, both ipsi- and contra-lateral to the cortical trauma. In all epileptic animals, the neuronal loss was circumscribed to the deafferented suprasylvian gyrus. Inhibitory GABAergic neurons were particularly more sensitive to cortical deafferentation than excitatory ones, leading to a progressively increasing ratio between excitation and inhibition towards excitation, potentially explaining the increased propensity to seizures in chronic undercut cortex

Memory Deficits Induced by Inflammation Are Regulated by α5-Subunit-Containing GABAA Receptors

Systemic inflammation causes learning and memory deficits through mechanisms that remain poorly understood. Here, we studied the pathogenesis of memory loss associated with inflammation and found that we could reverse memory deficits by pharmacologically inhibiting α5-subunit-containing γ-aminobutyric acid type A (α5GABAA) receptors and deleting the gene associated with the α5 subunit. Acute inflammation reduces long-term potentiation, a synaptic correlate of memory, in hippocampal slices from wild-type mice, and this reduction was reversed by inhibition of α5GABAA receptor function. A tonic inhibitory current generated by α5GABAA receptors in hippocampal neurons was increased by the key proinflammatory cytokine interleukin-1β through a p38 mitogen-activated protein kinase signaling pathway. Interleukin-1β also increased the surface expression of α5GABAA receptors in the hippocampus. Collectively, these results show that α5GABAA receptor activity increases during inflammation and that this increase is critical for inflammation-induced memory deficits