Characterization and development of a stroke-induced model of acquired epilepsy in organotypic hippocampal slice cultures: role of the cannabinoid CB1 receptors in modulation of neuronal excitation and inhibition

Stroke is the most common cause of acquired epilepsy in persons 35 and older. The massive increase in extracellular glutamate during stroke causes a cascade of intracellular events that can lead to cell death or the molecular changes that initiate the development of epilepsy. In addition, many studies point to a modulatory role of the endocannabinoid system in controlling seizures. Animal models of stroke induced acquired epilepsy have been difficult to develop. Therefore, this dissertation was initiated to develop an organotypic hippocampal slice culture model of acquired epilepsy and examine the changes in distribution and function of the endogenous CB1 receptor system. We utilized 4-aminopyridine and glutamate to induce separate excitotoxic injuries to slice cultures. Both injuries produced significant cell death acutely following the injury. After a latency period, we observed a significant increase in the number of slice cultures that displayed electrographic seizures in both injury models. Western blot analysis demonstrated that the cannabinoid CB1 receptor protein was significantly upregulated following injury with glutamate. Immnohistochemical studies demonstrated that this receptor upregulation was likely specific to the glutamatergic terminals. Electrophysiological experiments were performed to study endocannabinoid modulation of inhibitory and excitatory signaling in the CA3 pyramidal cells. We demonstrated that depolarization induced suppression of excitation (DSE) was enhanced in slice cultures that had undergone glutamate injury. This indicated that the upregulation of CB1 receptors following glutamate injury was physiologically functional, as it enhanced cannabinoid control of the excitatory signaling. These studies support the hypothesis that there is a functional alteration of CB1 receptors in the epileptic state that acts to suppress seizures. The development of an organotypic hippocampal slice culture model of stroke acquired epilepsy provides a unique tool to study the neuronal plasticity changes associated with epileptogenesis. It also provides a practical model to study pharmacological agents that may be useful in preventing or treating epilepsy