Kindling-induced changes in the EEG recorded during stimulation from the site of stimulation. II. Comparison between spontaneous and evoked potentials.

We previously described how the EEG activity recorded during kindling-inducing tetanic stimulation, from the site of stimulation, consists of three major components. Here, we investigate the nature and role of component 1, an initial evoked potential, seen at the beginning of tetanus, and we compare it with the potential evoked by single-pulse stimulation and with the spontaneous interictal discharges seen after kindling in both the amygdala and hippocampus. The amplitude of component 1 increased during kindling, and the waveform, latency, and stimulus-response curve of component 1 were all similar to those of potentials evoked by single-pulse stimulation. Furthermore, in acute experiments, in which the amygdala was kindled by tetanic stimulation at 1-h intervals, the evoked potentials in the amygdala and pyriform cortex were markedly potentiated, whereas no obvious potential was recorded at the same latency from the ventral or dorsal hippocampus, subiculum, nor the prepyriform region. Thus, we conclude that, in the case of amygdala kindling, the nature of component 1 is a local direct and monosynaptic response involving the amygdala and pyriform cortex, and that the increased amplitude of component 1 after kindling may be related to long-term potentiation. Although the single-pulse stimulation produced late burst-like components in the amygdala and hippocampus after kindling, it produced neither EEG suppression, well-formed rhythmic synchronous bursts, nor epileptiform discharge, suggesting that component 1 alone is not sufficient for the subsequent events that may be essential to the seizure-triggering mechanism.