Abstracts

Glutamate and GABA, the main excitatory and inhibitory neurotransmitters in the CNS, are known to play a central role in the physiopathological mechanisms underlying seizure activity. We previously observed that intrahippocampal administration of the muscarinic receptor agonist pilocarpine, which produces limbic motor seizures, also leads to increases in hippocampal extracellular (EC) glutamate and GABA. However, the link between these changes and seizure activity has been disputed in literature. In this study, we investigated the behavioural, electrographic and neurochemical changes that occur after intrahippocampal administration of picrotoxin, a GABA[sub]A[/sub] receptor antagonist, and compared these changes to those observed following intrahippocampal pilocarpine administration.
Seizures were evoked in conscious male albino Wistar rats (270-300 g) by perfusing pilocarpine (10 mM, 40 min at 2 [micro]l/min) or picrotoxin (100 [micro]M, 40 min at 2 [micro]l/min) via a microdialysis probe into the hippocampus. Seizure activity was monitored by electrocorticography. Dialysate hippocampal glutamate and GABA levels were determined by microbore liquid chromatography.
Intrahippocampal administration of both pilocarpine and picrotoxin produced significant increases in EC concentrations of glutamate (265% of basal value [p[lt]0.01] for pilocarpine and 499% of basal value [p[lt]0.001] for picrotoxin) and GABA (211% of basal value [p[lt]0.05] for pilocarpine and 253% of basal value [p[lt]0.01] for picrotoxin) in the hippocampus. Increases in glutamate were significantly higher and lasted longer in the picrotoxin group, while increases in GABA were of similar extent but of shorter duration compared to the pilocarpine group. We hypothesise that picrotoxin induces seizures through disinhibition of hippocampal glutamatergic neurons, by inhibiting postsynaptic GABA[sub]A[/sub] receptors. Pilocarpine, on the other hand, is thought to produce seizures through a K⁺-channel mediated increase in hippocampal excitability. This could explain why increases in EC glutamate are higher and last longer in the picrotoxin group. We also suggest that the sustained increases in EC GABA after pilocarpine administration are the result of both direct muscarinic stimulation of hippocampal GABA-ergic interneurons and activation of local inhibitory feedback mechanisms.
Our data strengthen the hypothesis that increases in hippocampal EC concentrations of glutamate and GABA are directly related to seizure activity, and can serve as a marker for epilepsy. Relative differences in the extent and duration of the increases in hippocampal EC glutamate and GABA between the pilocarpine and the picrotoxin group can be explained by the fact that picrotoxin causes seizures through disinhibition of hippocampal glutamatergic neurons, while pilocarpine increases hippocampal excitability.