Expression of glutamine synthetase and glutamate dehydrogenase in the latent phase and chronic phase in the kainate model of temporal lobe epilepsy

It has been suggested that astrocytic glutamate release or perturbed glutamate metabolism contributes to the proneness to epileptic seizures. Here we investigated whether astrocytic contents of the major glutamate degrading enzymes glutamine synthetase (GS) and glutamate dehydrogenase (GDH) decreases on moving from the latent phase (prior to seizures) to the chronic phase (after onset of seizures) in the kainate (KA) model of temporal lobe epilepsy. Western blotting and immunogold analysis of hippocampal formation indicated similar levels of GDH in the latent and chronic phases of KA injected rats and in corresponding controls. In contrast, the level of GS was increased in the latent phase compared with controls, as assessed by Western blots of whole hippocampal formation and subregions. The increase in GS paralleled that of glial fibrillary acidic protein (GFAP). Compared with the latent phase, the chronic phase revealed a lower level of GS (approaching control levels) but an unchanged GFAP content. The decrease in GS from latent to chronic phase was significant in whole hippocampal formation, dentate gyrus and CA3. It is concluded that kainate treated rats show an initial increase in GS, pari passu with the increase in GFAP, and a secondary decrease in GS that is not accompanied by a similar loss of GFAP. In a situation where glutamate catabolism is in high demand the secondary reduction in GS level may be sufficient to contribute to the seizure proneness that develops between the latent and chronic phases.