Enhanced limbic network excitation in the pilocarpine animal model of temporal lobe epilepsy

Through the use of chronic experimental animal models, the majority of in vitro investigations of temporal lobe epilepsy have demonstrated enhanced network activity within the subdivisions of the hippocampal formation. However, clinical evidence in combination with in vivo and in vitro studies indicates that structures external to the hippocampus contribute to the genesis of seizure activity. To address the effects of limbic network excitation, I have utilized combined hippocampal---entorhinal cortex brain slices from pilocarpine-treated rats that display chronic seizures.My investigations have focused upon three structures, the subiculum, entorhinal cortex and the insular cortex. The experiments in the pilocarpine-treated subiculum demonstrated increased network excitability that was attributed to a more positive GABAA receptor mediated inhibitory post-synaptic potential (IPSP) reversal point coupled with a reduced IPSP peak conductance. Utilizing RT-PCR analysis and immunohistochemical staining we observed a decline in K+-Cl- cotransporter mRNA expression and a reduced number of parvalbumin-positive, presumptive inhibitory interneurons. My second project assessed the network hyperexcitability in layer V of the lateral entorhinal cortex. This is the first study to report spontaneous bursting, in the absence of epileptogenic agents, in the epileptic entorhinal cortex. We attributed this level of network excitation to reduced GABAA receptor mediated inhibition and increased synaptic sprouting. In the final project, we extended our slice preparation to include the insular cortex, a structure external to the temporal lobe. Our investigations identified a mechanism of NMDA receptor dependent synaptic bursting that masked GABA A receptor mediated conductances