Epileptiform burst discharges in hippocampal CA3 neurons of young but not mature Noda epileptic rats (NER)

Abstract

Noda epileptic rat (NER), originally found in a colony of Crj; Wistar rats, shows spontaneous tonic–clonic convulsion characterized by the appearance of high voltage polyspikes in cortical and hippocampal EEG once every 2–3 days after 2–4 months of age. Electrophysiological studies using hippocampal slice preparations of NER were performed to determine whether hippocampal neurons have abnormal excitability. When a single stimulus (1–25 V) was delivered to the mossy fibers of NER at 4–6 weeks old before they showed any seizures, a long-lasting depolarization shift (DS) accompanied by repetitive firings and after-hyperpolarization following the abnormal firing was observed in seven of 14 hippocampal CA3 neurons. A lower stimulation intensity evoked DS and abnormal firing in three of nine CA3 neurons of NER at 10–15 weeks old which had already showed seizures at 10–15 weeks of age. However, the abnormal firing was not observed in any 10 neurons of the animals at more than 20 weeks old nor in Wistar rats. The input impedances of CA3 neurons in NER with abnormal firing were lower than those without abnormal firing and those in Wistar rats. The abnormal excitability obtained in NER at an age when it did not display any seizures suggests that the hippocampus may play a role in epileptogenicity in NER.