Blockage of cortical spreading depression by picrotoxin foci of paroxysmal activity.

1 per cent Picrotoxin placed on cortex of rat caused paroxysmal ECoG discharges with concomitant increase in [Ke"] from 3 to 6.7 mM with oscillations corresponding to ictal (maximum) and interictal (minimum) spiking. Invasion of the epileptogenic focus by spreading depression was blocked when the amplitude of oscillations of [Ke+] reached 2.6 mM. Epileptogenic activity induced by topical 10 per cent pentazol caused a less marked increase in [Ke+] (4.6 mM) and did not prevent depression from invading the focal area, but did diminish [Ke+] from the normal of 60 to 70 mM to 39 mM. It is concluded that seizure-induced depolarization of neural elements in deep cortical layers, though inadequate to trigger spreading depression, does prevent it from spreading, in part by activating the sodium pump.