Extracellular K + and Ca 2+ changes during epileptiform discharges in the immature rat neocortex

Abstract

Picrotoxin-induced epileptiform activity was examined in neocortical slices prepared from 8- to 15-day-old rats. This activity consisted of spontaneous bursts of 3–5 discharges that resembled interictal spikes and were interspersed with ictal-like paroxysms lasting 10–30 s. Measurements of extracellular potassium ([K +] o) and calcium ([Ca 2+] o) were made during these spontaneous epileptiform events, using ion-sensitive electrodes. Individual interictal spikes were associated with [Ca 2+] o decreases of 0.1–0.2 mM, whereas sustained ictal-like discharges were accompanied by decreases of 0.3–0.4 mM. Measurement of [K +] o showed that individual interictal spikes were associated with increases in [K +] o up to 12 mM, whereas increases to more than 20 mM accompanied long-lasting ictal-like discharges. Maximum increases in [K +] o were observed ca. 600 μm below the pial surface. [K +] o increases were followed by undershoots of the resting [K +] o level. The unusually high [K +] o levels associated with epileptiform discharges in the immature neocortex suggests that disturbances in [K +] o regulation may contribute to the generation of the picrotoxin-induced, spontaneous, prolonged ictal-like discharges observed in the 8- to 15-day age group.