Ictogenesis and epileptogenesis in EL mice

In EL mice, ictogenesis is established at age approximately 10 weeks, whereas epileptogenesis is induced through an experience of repetitive seizures during development. An "abnormal neural plasticity" has been suggested to be involved in these pathologic processes. It also is known that two isoforms of nitric oxide (NO) synthetase (nNOS and eNOS) are essential for the long-term potentiation (LTP), a plastic response of neurons. It appears, therefore, that these NO synthetases might play a major role in the establishment of abnormal neural plasticity. The purpose of the present study was to investigate ictogenesis and epileptogenesis by observing alterations of NO synthetases as well as immediate early gene (IEG) expressions and gamma-aminobutyric acid (GABA) distributions in the brain during development and with respect to seizure history.

IEG (c-fos and zif) expression in EL mice were analyzed by in situ hybridization with 35S. Distribution of GABA concentrations and glutamic acid decarboxylase (GAD) activities in the parietal cortex of EL mice was quantitatively determined using ultramicroenzymatic chemistry (Lowry, 1978). Three isoforms of NOS were assayed by immunoblotting analysis for hippocampal tissues of EL mice and their control animals, DDY mice. DNA fragmentation was detected with the TUNEL method.

In EL mouse brains, IEG expression was related to the seizure history, seizure threshold, and age. Even in the interictal period, the animals expressed IEG continuously when their seizure thresholds were very low. Among various IEG expression sites in the brain, hippocampal CA1 was the most remarkable. These IEG expression sites were almost identical to the brain regions of EL mice where GABA concentrations and GAD activities were altered. Unexpectedly, the eNOS content of EL was very small, although eNOS appears to be responsible for NO that mediates an increase in local cerebral blood flow during focal seizures. nNOS, iNOS, and to a lesser extent, eNOS were essential to establish both ictogenicity and epileptogenicity. DNA fragmentation was observed in the hippocampus of EL mice in the interictal period.

Continuous IEG expression and abnormal GABAergic function are involved in the epileptogenesis of EL mice. Transiently expressed IEG, on the other hand, is associated with the ictogenesis. It is conceivable that an excess amount of iNOS (and subsequent increase in harmful antimicrobial NO) and a lesser amount of eNOS (and subsequent decrease in NO or endothelium-derived relaxing factor, EDRF) may work together to contribute to a focus complex and ictogenesis. Drugs that suppress iNOS and/or potentiate eNOS may be promising candidates for a new type of antiepileptic agent.