Abstracts

RATIONALE: The midline thalamic nuclei have strong reciprocal connections with many of the limbic regions associated with temporal lobe epilepsy, including the hippocampus. There is evidence that pharmacological manipulation of this region affects the behavioral expression of limbic seizures in several models. We wished to determine whether pharmacological manipulation of the dorsal midline thalamus can alter seizure activity, and whether limbic seizures could be elicited by direct midline thalamic stimulation. METHODS: Under urethane anesthesia, tetanic (15 sec) stimuli were given every 5 minutes to the hippocampus until the electrographic afterdischarge stabilized in the contralateral hippocampus and the midline thalamus, and the effect of the drug infusion on subsequent afterdischarges was examined. In awake rats kindling stimuli (10 sec, 20Hz, up to 800 A) were given to either the hippocampus or the thalamus hourly, and the behavioral and electrographic responses were recorded. RESULTS: Lidocaine infusion significantly shortened the afterdischarge duration whereas bicuculline significantly lengthened the duration. These effects resolved following the cessation of the infusion. Afterdischarges could only rarely be elicited by thalamic stimulation, and these seizures were significantly shorter than those elicitied by hippocampal stimulation. The behavioral effects of thalamic stimulation were limited to the duration of the stimulation, and were not typical for limbic seizures brought on by hippocampal stimulation. CONCLUSIONS: These findings suggest that the midline thalamic region has a powerful modulatory effect on seizures induced by hippocampal stimulation. However, the difficulty in eliciting seizure activity by direct thalamic stimulation suggests that this region can regulate but not induce seizures.