Amygdala stimulation produces two types of hippocampal afterdischarges in the urethane-anesthetized rat

How seizures spread during epileptiform events has been the subject of intensive investigation over the years. The present study explored the relationship between the amygdala and the hippocampus during afterdischarges. Stimulating electrodes were placed in the amygdala and CA3 regions on the left side in urethane-anesthetized rats. Recording microelectrodes were placed in the dentate gyrus on the left and the CA1 cell layer on the right. Afterdischarges were elicited by stimulus trains between 10 and 50 Hz to the amygdala. Most of the afterdischarges consisted of broad positive potentials in dentate gyrus and no shift of the DC potential. When the stimulus trains were repeated, the afterdischarge evolved, first by spreading to the contralateral side and then by the appearance of maximal dentate activation. The onset of maximal dentate activation was indicated by the appearance of bursts of large amplitude population spikes and a negative shift of the DC potential. These data demonstrate that two types of afterdischarges can be produced in the hippocampus of the anesthetized rat after amygdala stimulation. The observations support the hypothesis that maximal dentate activation represents synchronized reverberatory activity throughout the hippocampal-parahippocampal circuit and indicate that amygdala stimulation can access this circuit in the anesthetized animal.