REFRACTORY PERIODS FOLLOWING INTERICTAL SPIKES IN ACUTE EXPERIMENTALLY-INDUCED EPILEPTIC FOCI

Under epileptic conditions, interictal epileptic events are followed b y 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 ep ileptic activity was elicited by application of penicillin onto the mo tor cortex of anesthetized rats. Interictal epileptic discharges were followed by an absolute refractory phase lasting 200-300 msec, in whic h no epileptic event could be elicited by epicortical stimulation. Thi s was followed by a relative refractory period up to 900 msec after on set of the conditioning spike; spikes elicited with intervals between 300 and 900 msec were smaller than those with greater intervals and re quired higher stimulation intensities. This period ends by a sharp dro p 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 pos sibly indicates recurrent neuronal excitations. Stimulations with freq uencies of about 1/sec favoured a transition from a pattern with spike s appearing in an irregular sequence every 2-3 sec, to a discharge pat tern with spikes appearing with regular intervals of about 1 sec. This change of firing pattern was associated with a drop of the spike thre shold. It is concluded that interictal epileptic events are followed b y a refractory period comprising different components. Alterations of the neuronal inhibitions responsible for these refractory phases may b e critical for the activity of the focus and may determine the transit ion from interictal to ictal discharges.