INHIBITION IN EPILEPTIC NEURONS

The hyperpolarizations which follow depolarization shifts (DS) in neurons of penicillin epileptogenic foci were investigated using techniques of intracellular recording and stimulation. The hyperpolarization following DS could be inverted by hyperpolarizing current pulses or by injection of chloride ions through the micropipette. These data support the conclusion that the hyperpolarizations represent inhibitory postsynaptic potentials (IPSP). The IPSP following DS were of higher amplitude and longer duration than other inhibitory potentials in the same cells. It is postulated that the high frequency spike discharges which occur in the majority of the cells of the focus during DS activate inhibitory circuits which in turn inhibit cells in the focus and surrounding cortex. The IPSP following DS were more effective in limiting the frequency of unit spike discharges than the frequency of orthodromically triggered DS. Full-sized DS could often be evoked during the IPSP following DS at times when suprathreshold depolarizing pulses failed to fire the cell because of membrane resistance changes. This evidence suggests that the DS is generated on dentrites remote from the sites of IPSP generation. Widespread IPSP generation in and around the focus could limit the frequency of DS by affecting presynaptic elements which provide the excitatory drive required for DS generation. © 1968.