Refractory periods following interictal spikes in acute experimentally induced epileptic foci

Under epileptic conditions, interictal epileptic events are followed by large inhibitions which prevent the transition to ictal discharges. In the present experiments the refractory period following interictal epileptic spikes was investigated in animal experiments. Interictal epileptic activity was elicited by application of penicillin onto the motor cortex of anesthetized rats. Interictal epileptic discharges were followed by an absolute refractory phase lasting 200-300 msec, in which no epileptic event could be elicited by epicortical stimulation. This was followed by a relative refractory period up to 900 msec after onset of the conditioning spike; spikes elicited with intervals between 300 and 900 msec were smaller than those with greater intervals and required higher stimulation intensities. This period ends by a sharp drop of threshold. In two-thirds of the experiments, spikes were favoured in intervals of 300-500 msec due to a sag of the threshold, which possibly indicates recurrent neuronal excitations. Stimulations with frequencies of about 1/sec favoured a transition from a pattern with spikes appearing in an irregular sequence every 2-3 sec, to a discharge pattern with spikes appearing with regular intervals of about 1 sec. This change of firing pattern was associated with a drop of the spike threshold. It is concluded that interictal epileptic events are followed by a refractory period comprising different components. Alterations of the neuronal inhibitions responsible for these refractory phases may be critical for the activity of the focus and may determine the transition from interictal to ictal discharges.