Sustained and accelerating activity at two discrete sites generate epileptiform discharges in slices of piriform cortex

When near-threshold electrical stimulation is used to evoke epileptiform di scharges in brain slices, a latent period of up to 150 msec elapses before the discharge begins. During this period most neurons are silent, and abnor mal electrical activity is difficult to detect with microelectrodes. A fund amental question about epileptiform activity concerns how synchronous disch arges arise abruptly in a relatively quiescent slice. This issue was addres sed here by using voltage imaging techniques to study epileptiform discharg es in rat piriform cortex slices. These experiments revealed two distinct f orms of electrical activity during the latent period. (1) A steeply increas ing depolarization, referred to here as onset activity, has been described previously and occurs at the site of discharge onset. (2) A sustained depol arization that precedes onset activity, referred to here as plateau activit y, has not been described previously. Plateau and onset activity occurred i n different subregions of the endopiriform nucleus (a region of high seizur e susceptibility). When cobalt or kynurenic acid was applied focally to inh ibit electrical activity at the site of plateau activity, discharges were b locked. However, application of these agents to other nearby sites (except the site of onset) failed to block discharges. Plateau activity represents a novel form of electrical activity that precedes and is necessary for epil eptiform discharges. Discharges thus are generated in a sequential process by two spatially distinct neuronal circuits, The first circuit amplifies an d sustains activity initiated by the stimulus, and the second generates the actual discharge in response to an excitatory drive from the first.