Toward a unified theory of focal penicillin epileptogenesis: an intracortical evoked potential investigation.

Microinjection of picoliter quantities of isotonic sodium penicillin through one barrel of a double micropipet into cortical area 17 of cats induced discrete and temporary foci of epileptogenic alteration in evoked potentials. These were elicited by punctate visual stimuli, recorded through the second barrel and from a second micropipet located in surrounding cortex. Penicillin induced (1) an initial graded increase in the primary latency, "physiologic" response and subsequently (2) a new "late" response which evolved into (3) an "interictal" spike potential. The enhanced physiologic response appears to represent a direct effect of penicillin upon neuronal excitability. The epileptic "late response" possessed distinctive properties that suggest that additional involvement of local circuit interactions within the neuronal population affected by the penicillin. We conclude that penicillin epileptogenesis in striate neocortex is dependent upon both intrinsic neuronal and population interaction abnormalities. Relationships of this unified theory to those of the "epileptic neuron" and "epileptic aggregate" are discussed.