Glial cells, which comprise the vast majority of brain cells, may play a largely unrecognized role in epilptogenesis. Preliminary data presented with this application show that morphological changes occur in hippocampal astrocytes in too different animal models of temporal lobe epilepsy. Importantly, these changes take place prior to the appearance of clinically observable recurrent, spontaneous seizures and nerve cell loss. In addition, one of the models (measles virus-infected mice) shows a very high activity of the astrocytic enzyme 3-hydroxyanthranilic acid oxygenase (3HA0), which is responsible for the biosynthesis of the excitotoxic brain metabolite quinolinic acid. Like astrocytic hypertrophy, high 3HA0 activity is apparent before seizure onset. Moreover, elevated 3HA0 activity appears to constitute a selective effect since the activity of other astrocytic enzymes is not higher than in control animals. Since intrahippocampal injections of quinolinic acid are known to cause convulsions and selective hippocampal neuropathology in experimental animals, increases in endogenous quinolinate production may be causally involved in the initiation or propagation of seizures. This proposal was therefore designed to further examine the role of astrocytic function in two models of epilepsy, with particular emphasis on quinolinic acid synthesis: i) Rats injected with aininooxyacetic acid into the entorhinal cortex (Specific Aim I); ii) Balb/c mice injected intracerebrally with hamster neurotropic measles virus (Specific Aim 2); In parallel studies performed in Dr. Schwarcz' and Dr. Lothman's laboratory, the precise sequence of chances in the hippocampus will be examined with regard to the following parameters: a) Morphology of astrocytes and neurons; b) Metabolic status of quinolinic acid and a related metabolite, kynurenic acid; c) Spantaneous seizure activity; and d) Electrophysiological response of the hippocampus (rapid kindling, input-output curves of excitation, paired pulse responses). For the morphological studies, silver and Nissl staining will be combined with immunohistochemical techniques. In addition, the selective 3HA0 inhibitor 4-chloro- 3-hydroxyanthranilic acid will be used to examine the role of astrocyte-derived quinolinic acid more directly. Again, parallel histological, biochemical and electrophysiological methods will be used for analysis. In a separate Specific Aim (Aim 3), the morphological and immunohistochemical studies of tissue samples obtained during epilepsy surgery will be continued. These studies of human epileptic brain material will complement the animal work by focussing on the hippocampus/entorhinal cortex region and on kynurenine pathway enzymes (such as 3HA0). Taken together, the resulting data will permit a realistic evaluation of the quinolinate hypothesis of temporal lobe epilepsy, and should help clarify the role of astrocytes in the pathological process.
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