PROLONGED FIELD POTENTIALS-EVOKED BY 1-HZ STIMULATION IN THE DENTATE GYRUS OF TEMPORAL-LOBE EPILEPTIC HUMAN BRAIN-SLICES

An abnormal electrophysiological response in brain slices of the denta te gyrus from biopsy material from patients surgicially treated for in tractable epilepsy (46/57), exhibited characteristics similar to the p hysiological hallmark of epilepsy, the paroxysmal discharge, a prolong ed (30-600 ms) and often large amplitude field potential. The most str iking feature of the prolonged response to a single perforant path sti mulus was a predominantly biphasic field potential (23/46 cases). The biphasic response was characterized by a negative field potential of s ubstantial duration exceeding 180 ms which followed an initial shorter duration positive field potential. Multiple population spikes occurre d during both phases of the response. During a 1 Hz stimulus train app lied to tile perforant path, the magnitude and duration of the negativ e component of the field response was significantly increased. Approxi mately hall of the cases (Group I; 30/57) exhibited potentiation of th e biphasic response, while the remaining cases (Group 2: 27/57) exhibi ted no negative field component during 1 Hz stimulation trains. This r epetitive stimulation, in general increased the area of the field resp onse in a large majority of cases (44/57) regardless of the sign of th e field potential. The number of population spikes following in both g roups, although the increase was greater for those in Group 1 than in Group 2. Paired pulse depression (20 ms ISI) was reduced in cases that exhibited potentiated biphasic responses during 1 Hz stimulation (Gro up 1) in comparison to cases that exhibited no negative field potentia ls (Group 2). Paired pulse depression at a 200 ms ISI was not signific antly different between the groups, During a single stimulus, bicucull ine disinhibition (20 mu M) resulted in either a prolonged positive or biphasic field potential. Intracellularly recorded responses to singl e perforant path stimuli also exhibited prolonged and large depolariza tions that were comparable in time course to the duration of field pot entials recorded in the same area whether generated in the absence or presence of bicuculline. The prolonged field potential after bicuculli ne was reduced by APV (20 mu M). We suggest that the prolonged field r esponse, whether biphasic or monophasic when generated by either 1 Hz stimulation or bicuculline disinhibition, may be clue directly or indi rectly to an increase in membrane depolarization mediated by activatio n of the NMDA, receptor.