Developmental and seizure-related regional differences in immediate early gene expression and GABAergic abnormalities...

To examine the hypothesized role of the immediate early gene (IEG) response in synaptic plasticity and in epileptogenesis, we studied the spatial specificity of the expression of IEG in EL mice, a well known mutant model of epilepsy. Also to examine the 'GABA hypothesis' in epilepsy, GABA concentration and GAD activity was determined in micro brain regions (10-300 ng) of EL mice related to the focus in the parietal cortex and the hippocampus. We found that the IEG expression after seizures is not related to the seizure pattern, but to the seizure history, seizure threshold and development of EL[s]. Even in the interictal period, EL mice with long seizure histories and very low seizure thresholds demonstrate IEG expression continuously. This is probably strengthened by repeated seizures. The IEG expression site is however located in the hippocampal CA1, which is the final terminal of various inputs from other areas of the limbic system. It is proposed that the continuous expression of IEGs might play a different role from that of transiently expressed IEGs. Developmentally, the site of IEG expression shifted from one site to another in a very similar manner as in the IEG expression with propagation of paroxysmal discharges in each seizure, and the three-dimensional expression area was gradually expanded, suggesting a change in the regional active site during epileptogenesis. These lines of evidence suggest that during development as well as repetitive seizures, frequent expressions of IEGs and syntheses of Fos and Zif proteins might facilitate synaptic conductivity involved in epileptogenesis. The sites of abnormal GABA concentrations and GAD activities were almost the same in the parietal cortex, around Sidman atlas coronal section No. 300 and in the hippocampal CA1 pyramidal cells as the spatio-temporal specific IEG expression sites. These findings strongly suggest that IEG expression and abnormal GABAergic functions are involved in epileptogenesis in EL mice.