Implication of gamma band in soman-induced seizures.

Soman, an anticholinesterasic neurotoxic drug, induces epileptic seizures during severe intoxication. Their trigger conditions still remain unknown and a great variability between animals is observed. The butterfly model in the catastrophe theory has been used to explain these triggering conditions. We have developed a technique allowing, in freely moving rats, the "in vivo" determination of three sets of neurophysiological data, followed before and during a soman intoxication. For the same rat, we associated cortical acetylcholinesterase (AChE) activity by microdialysis with both the assay of extracellular acetylcholine (ACh) concentrations and electroencephalographic (EEG) recording and power spectrum analysis (gamma band). Data have been analysed to define the critical parameters which lead to the epileptic fit. Although we found thresholds for seizure occurrence, AChE inhibition having to be over 65% and ACh over 200 fold the baseline, these two criteria are not sufficient to predict the appearance of seizures. Only animals with no increase of energy in the gamma band early after soman poisoning will then exhibit an epileptic fit. The butterfly model provides an original interpretation of these results and gives a particular role of the energy in gamma band as a survival attractor.