Single-unit discharges in the dorsolateral thalamus of behaving cats: ictal activity.

A new model of experimental epilepsy was designed to study in undrugged, behaving cats the involvement of the thalamus during a cortical epileptiform after-discharge (EAD). EADs were induced by electrical repetitive stimulation applied to the posterior region of an isolated suprasylvian gyrus and allowed to invade the thalamus through a pedicle of white matter connecting areas 5 and 7 of the gyrus to their thalamic target, the dorsolateral nuclei. Extracellular recordings were obtained from 49 dorsolateral cells. Projection to the thalamus of the slowly propagating cortical EAD resulted in a characteristic sequence of events in 88% of the cells. First there occurred an arrest of cellular discharges upon invasion of the anterior suprasylvian cortex (phase I). During phase II, the cell fired slow-wave sleep (SWS)-like bursts strictly timed with surface events. Later (phase III), the cell generated long trains of action potentials on a one-to-one basis with cortical waves; in most cells, partial spike inactivation was seen within each burst. A statistical study indicated that the mean intraburst interspike intervals of SWS and phase II were not significantly different. Surprisingly, a significant increase (P < 0.005) in the mean intraburst interspike interval was found between SWS (4.4 ± 0.4 ms) and phase III (6.0 ± 0.6 ms). Arguments are presented which suggest that the long intraburst interspike intervals are orthodromically generated.