Extracellular unit analysis of the hippocampal penicillin focus.

Abstract Extracellular recordings were made of unit activity in penicillin foci in the cat hippocampus, and the location of selected cells was ascertained by methyl blue marks. Most units demonstrated firing patterns consistent with previously reported intracellular data, consisting of a short burst of spikes followed by a period of prolonged inhibition during the inter-ictal discharge. Two additional firing patterns, suggestive of inter-neuron activity, were noted. The first of these showed prolonged trains of high frequency spikes during inter-ictal discharges occuring at low frequencies (approximately every 10 sec). As the inter-ictal discharges were evoked more frequently the spike trains became disrupted and eventually dropped out before a seizure developed. The second firing pattern consisted of short bursts of spikes coincident with the surface paroxysm, which became very long trains as the discharges were evoked slightly more frequently. Cells showing all three firing patterns were located in all layers of the hippocampus. Some cells showing all three patterns could be activated antidromically, indicating that they were pyramidal cells. When studied for several seizure periods, cells were seen to convert from one firing pattern to another. Inter-ictal firing pattern was not correlated with firing pattern during seizure. It is concluded that anatomic cell types in the hippocampus cannot be identified by their firing pattern during epileptic events. Furthermore, it appears that even during the very stereotyped inter-ictal discharges which occur in hippocampal penicillin foci, individual cells may significantly vary their firing pattern during the paroxysms.