Interictal afterdischarge in focal penicillin epilepsy: thalamocortical unit activity.

The involvement of thalamic versus cortical structures for the initiation and maintenance of brief interictal afterdischarge was evaluated by recording extracellularly units and field potentials from different subcortical and cortical sites. Afterdischarge oscillations at 16 to 22/s that followed interictal spikes with a delay of 170 to 220 ms usually appeared 10 to 30 min after the topical application of penicillin to the cat's precruciate cortex. Units in the ventrolateral nucleus of the thalamus fired in burst discharges during cortical afterdischarge and less reliably during the cortical interictal spike. In contrast, units recorded at the cortical penicillin focus and homologous contralateral site remained silent during afterdischarge but had a typical burst discharge during the interictal spike. Although these data support a thalamic basis for the rhythm, the lack of an afterdischarge-like oscillation in the thalamic field potential and the independent appearance of afterdischarge and cortical recruiting waves elicited by stimulation of the nucleus reticularis would favor its cortical origin. In accordance with its frequency characteristics and data gained from earlier cooling studies we suggest a cortical mechanism requiring thalamic triggering for the generation of afterdischarge.