Hippocampal evoked field potentials and interictal spikes in hippocampally kindled cats

Kindling-related changes of the hippocampal evoked field potentials and patterns of the spontaneous interictal spikes were investigated in 10 hippocampally kindled cats. A complex potential waveform was recorded by macroelectrodes placed in the CA3 region of the hippocampal gyrus and hilus of the gyrus dentatus, close to the granule cell layer, after stimulation of the entorhinal cortex. After high intensity repetitive (10/s) stimulation a late component could be recorded with the latency of about 30-40 ms, in addition to the early response originating in the gyrus dentatus. Probably this component developed during kindling into a delayed, high amplitude spike. After application of the double shock test, post-stimulus facilitation of the spike response was observed within time limits of 20-100 ms. Another observation was a widespread, ipsilateral and bilateral long-term enhancement of the amplitudes of field potentials evoked by entorhinal and intrahippocampal stimulation. It was the most common effect observed during kindling. Widespread synchronized discharges of hippocampal spikes and localized clusters of brief irregular spikes were the most significant features of spontaneous interictal spikes. The paroxysmal discharges of spikes could be evoked by ipsi or by contralateral stimulation of the afferent pathways projecting to the kindled hippocampus, rather than by direct electrical stimulation of the kindled hippocampal gyrus.