Brainstem stimulation during sleep evokes abnormal rhythmic activity in thalamic neurons in feline penicillin...

Some periods in the sleep-waking cycle are more seizure prone than others. In absence epilepsy, transition periods between nonrapid-eye-movement (nonREM) sleep and waking or REM sleep can be more seizure prone that stable states. One feature of transition periods that is hypothesized to promote seizure activity is the presence of coincident activity in ascending brainstem reticular formation (RF) arousal systems with synchronized thalamo-cortical activity. To evaluate this hypothesis we examined the state-dependent effects of low intensity RF stimulation on thalamic single unit activity in control conditions and following systemic penicillin-G administration to adult cats. In control conditions, RF stimulation during waking and REM sleep typically evoked a short-latency action potential in thalamic neurons. The same stimulation during nonREM sleep commonly evoked a high frequency burst of action potentials followed by a period of suppressed discharge. In 16/26 neurons, a second rebound burst of action potentials followed the period of discharge suppression. The average interval between the initial and rebound bursts was 75.1 +/- 6.0 ms, which was similar to the interburst interval recorded in these same cells during spontaneous EEG spindles. Following administration of penicillin-G, RF stimulation during nonREM sleep evoked high frequency burst firing, followed by 1-2 rebound bursts in 21/22 thalamic neurons. The average evoked interburst interval was 152.5 +/- 7.3 ms, a value comparable to the interburst interval displayed by these same cells during spontaneous spike-wave seizure activity (157.8 +/- 8.7 ms). RF-evoked rhythmic discharges were dependent upon the presence of thalamocortical synchronization, as responses evoked during waking and REM sleep in penicillin treated cats were similar to those observed in control conditions.