Differential effects of cortical speading depression on epileptic foci induced by various convulsants.

Interaction between epileptic foci and spreading depression (SD) was studied in the cerebral cortex of rats anesthetized with Nembutal. At comparable discharge rates, picrotoxin and penicillin caused complete and partial SD blockade respectively, strychnine and Metrazol were ineffective and Aldactone facilitated SD. Conversely, the duration of SD-induced blockade of epileptic activity was maximal for Aldactone and minimal for picrotoxin. Treatment of the picrotoxin focus with tetrodotoxin (10(-4)M) reduced the discharge rate and reinstated SD propagation into the focus. [K+]e measured with ion-selective K+ electrodes 1 mm below the cortical surface increased to 8 mM in penicillin foci blocking SD and remained below 5 mM in Aldactone foci. It is concluded that the differential effect of various convulsants on SD propagation depends on the potassium concentration in the depth of the focus rather than on the discharge rate or on the mechanism of the the epileptogenic effect.