Effects of a focal penicillin lesion on responses of rabbit cortical neurones to putative neurotransmitters.

1. Epileptogenic foci were generated in rabbit cerebral cortex by the topical application of penicillin.2. Responses to iontophoretically applied putative neurotransmitters were compared in cortical neurones firing spontaneously with those driven by applied excitant substances, both before and after establishing the penicillin focus.3. In the presence of the spiking focus, currents of gamma-aminobutyric acid, which normally produced 100% depression of neuronal firing, were ineffective.4. In the same situation, currents of (+/-)-homocysteic acid, glutamate and acetylcholine produced predominantly depolarization block responses, and hence depression of firing.5. It is concluded that an epileptogenic focus can alter the responses of rabbit cortical neurones to microiontophoretically applied neurotransmitter substances. Possible mechanisms for the spread of seizure activity are discussed.