Activation of cortical circuits during interictal spikes

Penicillin, bicuculline, and strychnine were used as topical convulsants in unanesthetized rats to study aspects of interictal spiking. Bicuculline (0.1 mM) was more potent than penicillin (3.0 mM) or strychnine (1.5 mM) in initiating simple discharges, and at only slightly higher doses caused complex spikes and afterdischarges with motor convulsions. Penicillin caused similar types of discharges at much higher concentrations (> 50 mM), but strychnine failed to produce either complex spikes or afterdischarges. Penicillin and bicuculline were similar in causing an increase in utilization of carbon 14‐deoxyglucose in cortical layers I to III and V. Strychnine increased metabolism in layer V, with individual columns running from this base to the surface. Areas of decreased metabolism highlighted the strychnine focus and are probably related to a “pericolumnar inhibition” mediated by γ‐aminobutyric acid. Such inhibition would be blocked in the penicillin and bicuculline focus‐an effect which might be responsible for the genesis of complex spikes and afterdischarges and a more uniform activation of cortical metabolism.