Effects of projected cortical epileptiform discharges on neuronal activities in ventrobasal thalamus of the cat: Ictal discharge

Extracellular recordings were made from neurons in nucleus ventralis posterolateralis during projected ictal discharges from penicillin foci in cat posterior sigmoid cortex. During the initial phase of the electrographic seizure, spike generation in the thalamic nucleus was suppressed. Excitatory effects became prominent during the tonic phase of the ictal episode when rhythmic firing of ventralis posterolateralis neurons occurred. During the clonic phase of the seizure, thalamic cells generated prolonged high frequency bursts of spikes which might begin before the onset of the surface paroxysm. Analysis of interval histograms and autocorrelograms of spike discharges in some thalamocortical relay cells during the clonic phase of the seizure suggested that both orthodromic and antidromic spike generation occurred. The findings together with those from previous studies suggest that the excitability of intracortical axons of thalamocortical relay cells increases as the ictal episode develops and spontaneous bursts originate in these axons and antidromically invade the cell body. Burst generation in thalamocortical relay cells during ictal episodes, and shifts from inhibitory to excitatory behavior in these units during a seizure would provide a powerful positive feedback to the epileptogenic focus. Propagation of bursts throughout the arborizations of thalamocortical fibers to presynaptic terminals would result in a marked increase in excitatory synaptic impingement, and a synchronization of neuronal activity which characterizes the ictal episode.