GABA-RESPONSES OF CA3-NEURONS AT EPILEPTOGENIC THRESHOLD CONCENTRATIONS OF CONVULSANTS

EPILEPTOGENic actions of convulsants are often attributed to weakened inhibitory synaptic mechanisms. This assumption was tested by studying GABA-induced postsynaptic membrane potential (MP) changes of CA3 neurones (guinea-pig) before and during exposure to bicuculline methoiodide (BMI), pentylenetetrazol (PTZ), penicillin (PEN) and caffeine (CAF). Under control conditions GABA ejections elicited polyphasic MP fluctuations (components I-III). After adding BMI, PTZ, PEN or CAF, early hyperpolarizations (component I) did not change at epileptogenic threshold concentrations. These convulsants, however, exerted differential effects on the depolarizing component II, but only threshold concentrations of penicillin strongly reduced the amplitude of this component. Simultaneously, component III was slightly accentuated. These findings indicate that changes of GABA responses are not an essential prerequisite for the generation of paroxysmal depolarizations.