Blockade of cortical spreading depression in electrically and chemically stimulated areas of cerebral cortex in rats.

Penetration of cortical spreading depression (SD) into epileptic foci established in the cerebral cortex by penicillin or Metrazol and into electrically stimulated cortical regions was studied in anaesthetized rats. SD suppressed the activity of penicillin foci with low rates of interictal discharge (0.3 Hz) but did not invade more active foci (1 Hz) or foci triggered by electrical stimulation (1-3 Hz). Metrazol foci did not block SD propagation unless stimulated at 6-10 Hz. Repetitive direct cortical responses elicited by 0.05-0.1 msec pulses blocked SD propagation when applied at 6-10 Hz for 5-20 min. The SD blockade covered an area 3-5 mm in diameter around the bipolar stimulating electrodes. The block outlasted the stimulation for several minutes but was fully reversible. New stimulation reinstated the SD blockade after a shorter latency and at lower stimulus intensities and rates. Interaction of the blocked cortical area and SD resulted in anomalous SD propagation, characterized by reentry or circle waves, returning through or around the stimulated region to the recovered cortex. The dynamics of the onset and offset of blocking suggests that SD propagation is prevented by enhanced K+ reabsorption which rapidly removes the K+ ions penetrating the stimulated area from the SD wave front. The interactive phenomena, particularly SD circulation around an epileptic focus, may account for periodic changes of ictal and interictal activity found in some types of focal epilepsy.