This proposal is intended to extend observations made during the current funding period. The goal of our research is to obtain information on the basic mechanisms underlying epileptogenesis. As part of a multifaceted effort, this proposal will concentrate on demonstrating the role that activation of specific excitatory amino acid (EAA) receptors play in afterdischarge generation and the transition to ictal activity. We will use transverse rat hippocampal slices taken during different developmental stages. Epileptiform activity will be generated using the disinhibited hippocampal slice model. Data will be gathered using recording techniques that include intracellular, whole-cell-patch voltage-clamp, field and ion-sensitive microelectrode recordings. EAA receptor involvement will be assessed using iontophoretic and bath application of a range of agonists and antagonists. We have demonstrated a developmental window, Post-natal-days (PND) 9-19, during which the CA3 region of a rat hippocampus exhibits a pronounced capacity for the generation of electrographic seizures. In this period there is a greater degree of functional connectivity within the recurrent excitatory microcircuits. We also find a slow depolarization arising in the proximal portion of the basilar dendrites that is separate from the current sinks generated by the Depolarization Shift in both pyramidal cell dendritic arbors. It is also a separate physiological process from the rapid repetitive depolarizations of the afterdischarge. This EAA receptor mediated synaptic current is a critical component in seizure generation. Factors that produce and influence this current could then play a significant role in the appearance and spread of ictal activity. We believe one such factor is the presence of a unique isoform of the NMDA receptor. During PND 9-19 decreases in extracellular calcium, such as occur during ictal activity, have a unique ability to increase both electrographic seizure activity and the NMDA evoked responses of CA3 pyramidal cells. The voltage dependence of the NMDA response has also been seen to be less sensitive to magnesium and more sensitive to calcium than the same response evoked in mature tissue. A developmental change in the efficacy of competitive NMDA antagonists has also been observed. Therefore this proposal concentrates on the role played by changes in the NMDA receptor-channel complex in epileptogenesis.
