Experimental studies on different thresholds for epileptiform activity

This investigation was carried out to clarify the mechanisms for production of different seizure threshold. To study this problem changes of a single cell activity were examined by means of penicillin iontophoretical application to a single cell of both low seizure threshold hippocampus and the high seizure threshold olfactory bulb, and also topical application to the surface of hippocampus and olfactory bulb. Penicillin was applied iontophoretically using five barrel glass micropipettes. Extracellular recordings from 58 hippocampal pyramidal cells and 68 olfactory bulb mitral cells were analyzed. Penicillin increased the firing rate in most of both hippocampal pyramidal and olfactory bulb mitral cells. This suggests that penicillin finally had an excitatory effect on both cells. The results from the experiments of intermittent and continuous GABA application on a cell during penicillin iontophoresis revealed that GABA inhibitory action was reduced by penicillin in both cells. A progressively shortened duration of the post-discharge inhibition during penicillin iontophoresis also suggested that penicillin may reflect a decrease of GABA inhibitory action. No obvious seizure activity and/or no change of firing pattern were observed even after long penicillin iontophoresis. There was no significant different responses to penicillin iontophoresis in the hippocampus and the olfactory bulb single cell. Extracellular activity from 64 hippocampal pyramidal cells and 32 olfactory bulb mitral cells was recorded. The effects of penicillin topical application on single cell function were examined. From hippocampus surface, paroxysmal activity begun to appear within 6 min after topical application, and became more regular and stereotyped in form. The firing rate initially decreased in the majority of cells, with no increase prior to interictal or ictal events. The unit responses to both intermittent and continuous GABA iontophoresis was suppression in unit firing incidence, except for the burst patterns associated with the surface interictal or ictal discharges. Post-discharge inhibition was not significantly different either before or after the onset of the surface interictal discharges. These results indicate the firing rate and inhibitory function of the hippocampal pyramidal cells were not significantly changed during and following the development of interictal and/or ictal epileptiform activity. Olfactory bulb surface activity was not significantly changed and no epileptiform activity was observed even several hours after topical penicillin application.(ABSTRACT TRUNCATED AT 400 WORDS)