Abstracts

Rationale: Interictal spikes (IS) are briefs, high amplitude potentials observed in epileptic patient EEGs in periods between seizures. They are a key feature in the diagnostic of epileptic disorders and the location of their occurrence is often used to identify the structure involved in seizures. However, little is known about the dynamics of their generation and propagation. Here we analyze the parallel evolution of field potentials in hippocampus and connected structures after localized infusion of pilocarpine and during the following period leading to chronic epilepsy. Methods: Sprague Dawley rats were implanted with an injection canula in the right ventral hippocampus and an electrode assembly with leads aimed at the Right and left dorsal hippocampi, the right prefrontal cortex (PFC), right subiculum (Sub) and Right medial entorhinal cortex (MEC). The last leads were modified in order to record independently the superficial and deep layers of the entorhinal cortex. A weak after surgery, pilocarpine (600mg/mL, approx 2ul) was infused progressively in the ventral hippocampus until electrophysiological and behavioral (stage 4 to 5) seizures were observed. Five days after, the depth field potentials were recorded daily for an hour and monitored for behavioral seizures for 12 hours. Results: Interictal spikes were observed immediately after the seizure stopped in both hippocampi and in MEC and Sub. They disappeared 48 to 72 hours after Status Epilepticus induction. Ten to fifteen days after induction, spontaneous IS appeared in all regions. Shortly after (2-3 days), they were followed with spontaneous seizures. Interestingly, the most common and prominent IS were observed in the MEC. IS appeared either independently or synchronously in all recorded areas, including PFC and the deep and superficial layers of the MEC. The most common pattern of IS propagation started from the superficial layers of the MEC, propagated to the hippocampus and deep layers simultaneously and spread to the subiculum. In this case, only a large evoked potential was observed in the PFC. No seizure was witnessed to initiate in another structure than the hippocampus. Conclusions: Independant IS could be observed in other, interconnected regions to the primary epileptic focus, suggesting either the apparition of an secondary epileptic focus or an independance of the IS with regard to the epileptic focus.