Bilateral maximal dentate activation is critical for the appearance of an afterdischarge in the dentate gyrus

Recently, a phenomenon has been described in the dentate gyrus termed maximal dentate activation, which is defined by the appearance of bursts of large amplitude population spikes associated with a negative shift of the d.c. potential and a secondary rise of the extracellular potassium level. Previous work has linked maximal dentate activation to kindling of afterdischarges, either when they are elicited in the hippocampus or outside of the hippocampus in the amygdala. Recording bilaterally in the dentate gyrus, it was found that maximal dentate activation occurred on both sides, with the side ipsilateral to the stimulus (either CA3 or angular bundle) being activated first. An afterdischarge did not appear unless bilateral maximal dentate activation had occurred. With repeated stimulation, the time to onset of maximal dentate activation on the two sides of the brain became nearly equal. This was associated with the appearance of afterdischarges. However, complete synchronization of the onset of maximal dentate activation was not necessary for afterdischarge production. Maximal dentate activation and afterdischarges could be readily elicited in rats in which the hippocampal commissures had been cut. It appears that, in the intact brain, the lack of maximal dentate activation on one side of the brain can function as a “brake” for epileptic activity, preventing afterdischarges. Once this brake is removed, by cutting the hippocampal commissures or by initiating maximal dentate activation, the dentate gyrus readily expresses afterdischarges.