Cellular and homeostatic network mechanisms of posttraumatic epilepsy

Authors: Avramescu, Sinziana
Advisor: Timofeev, Igor
Abstract: 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.
Document Type: Thèse de doctorat
Issue Date: 2008
Open Access Date: 20 April 2018
Permalink: http://hdl.handle.net/20.500.11794/25651
Grantor: Université Laval
Collection:Thèses et mémoires

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