Characteristics of plateau activity during the latent period prior to epileptiform discharges in slices from rat piriform cortex

Characteristics of plateau activity during the latent period prior to epile ptiform discharges in slices from rat piriform cortex. J. Neurophysiol. 83: 1088-1098, 2000. The deep piriform region has an unusually high seizure su sceptibility. Voltage imaging previously located the sites of epileptiform discharge onset in slices of rat piriform cortex and revealed the spatiotem poral pattern of development of two types of electrical activity during the latent period prior to discharge onset. A ramplike depolarization (onset a ctivity) appears at the site of discharge onset. Onset activity is preceded by a sustained low-amplitude depolarization (plateau activity) at another site, which shows little if any overlap with the site of onset. Because syn aptic blockade at either of these two sites blocks discharges, it was propo sed that both forms of latent period activity are necessary for the generat ion of epileptiform discharges and that the onset and plateau sites work to gether in the amplification of electrical activity. The capacity for amplif ication was examined here by studying subthreshold responses in slices of p iriform cortex using two different in vitro models of epilepsy. Under some conditions electrically evoked responses showed a nonlinear dependence on s timulus current, suggesting amplification by strong polysynaptic excitatory responses. The sites of plateau and onset activity were mapped for differe nt in vitro models of epilepsy and different sites of stimulation. These ex periments showed that the site of plateau activity expanded into deep layer s of neighboring neocortex in parallel with expansions of the onset site in to neocortex. These results provide further evidence that interactions betw een the sites of onset and plateau activity play an important role in the i nitiation of epileptiform discharges. The site of plateau activity showed l ittle variation with different stimulation sires in the piriform cortex, bu t when stimulation was applied in the endopiriform nucleus (in the sites of onset of plateau activity), plateau activity had a lower amplitude and bec ame distributed over a much wider area. These results indicate that in the initiation of epileptiform discharges, the location of the circuit that gen erates plateau activity is not rigidly defined but can exhibit flexibility.