Abstracts

RATIONALE: Kainic acid (KA)-induced seizures are a model of the mesial temporal lobe epilepsy syndrome. KA causes a prolonged acute seizure followed by a latent phase of a few weeks followed by chronic recurrent seizures. The mechanisms involved in epileptogenesis during the latent phase remain unclear. In adult animals, pyramidal cell loss and synaptic reorganization are thought to underlie this process, but in immature animals this is not consistently seen despite more severe acute seizures. We have observed a selective vulnerability of CA1 oriens/alveus (OA) interneurons to KA-induced seizures in both groups. The goal of the present study is to further delineate the subgroups of interneurons susceptible to KA-induced seizures and assess if this selective vulnerability is due to differential acute effects of kainate.
METHODS: Prolonged seizures were induced with two intraperitoneal injections of KA (6.75 mg/kg) at one-hour interval in post-natal day 20 (P20) and P30 rats. Control rats received a saline solution. Animals were observed for 40 days for recurrent seizures. We then performed immuno-histochemical studies with glutamic acid decarboxylase (GAD), parvalbumin (PV) and somatostatin (SS) staining.
A second group of naive animals were sacrificed at P30 for electrophysiological studies. Transverse brain slices were prepared. Responses of CA1 interneurons were monitored with whole-cell patch-clamp recordings in current-clamp mode. We recorded membrane potential and cell input resistance before, during and after a 5uM KA application for a period of 10 to 15 minutes. In some experiments, we blocked Na+ channels with TTX 1uM, AMPA receptors with GYKI52466 30uM and non-NMDA receptors with CNQX 20uM. In experiments where calcium imaging was performed simultaneously, cells were dialyzed with intracellular solution containing the fluorescent cell-impermeable calcium indicator: Oregon Green Bapta-1 (50uM).
RESULTS: At P20, we observed a significant and selective loss of CA1 GAD positive interneurons (60%) but an even more severe loss of PV (88%) and SS (90%) positive cells. In P30 animals, we observed a reduction of 55, 65, and 67% respectively. Cell loss was present in OA layers but none was seen in pyramidale or R-LM layers. Electrophysiological studies comparing interneurons in OA (vulnerable) and in R-LM (resistant) showed that both populations responded to KA with similar levels of depolarization and input resistance decrease. These responses were also equally blocked by GYKI52466 and CNQX, suggesting an implication of AMPA receptors. Similarly, in calcium imaging experiments, Ca2+ elevations were not significantly different in both cell groups following KA application.
CONCLUSIONS: The vulnerability of CA1 cells in the KA model involves similar subgroups of interneurons in juvenile and mature animals, which supports a key role for the loss of interneurons in epileptogenesis. Furthermore, this selective vulnerability may not be due to a differential sensitivity to KA involving direct acute effects on membrane potential, input resistance or intracellular Ca2+ elevation.
[Supported by: Savoy Foundation for Epilepsy]