Abstracts

Rationale: Vagus nerve stimulation (VNS) has been shown to be progressively effective in the reduction of epileptic seizures in patients who have medically refractory epilepsy. The effect of long-term VNS has never been demonstrated in an animal model of chronic epilepsy. We found a reduction of seizures in the lithium/pilocarpine rat model of chronic epilepsy and studied the effect of VNS on gene expression to understand the mechanism of the VNS effect. Methods: Status epilepticus was induced in 10 week old male Sprague-Dawley rats by LiCl (3 mmol/kg, i.p.) 20-24 hours prior to pilocarpine (30 mg/kg, s.c.) to produce chronic epilepsy. After a stable seizure frequency was established, epidural recording electrodes were implanted and the EEG was monitored continuously to establish a 4 week baseline seizure frequency. Model 103 VNS stimulators (Cyberonics) were then implanted and the rats were monitored for 12 weeks to determine the effect of chronic VNS on seizure frequency. In some animals, the VNS devices were not turned on. At the end of the monitoring period, all rats were sacrificed and brains were immediately frozen on dry ice and stored at -80^oC until sectioned. Laser Capture Microdissection was used to select homogeneous cell populations from various brain nuclei. RNA was extracted from 200-250 cells from each targeted nucleus, amplified and analyzed on whole rat genome microarrays (Lumina). The results were analyzed using Gene Spring v.7. Results: Mean seizure frequency across all animals during the 4 week baseline period was 22.8/week (range 0.75 to 31.5 seizures/week). During the 12 week stimulation period there was a progressive improvement in seizure control. A total of 3 out of 10 rats achieved at least a 50% reduction of seizure frequency, but 7 of 10 rats achieved a statistically significant reduction of seizure frequency compared with baseline. The results of the gene expression studies will be presented. Conclusions: Our observations, using stimulation parameters equivalent to those used in humans, were: 1) similar responder rates to those observed in human trials and 2) a progressive improvement of seizure control during long-term VNS. These findings suggest that this model may be a valuable tool with which to systematically study optimal stimulation parameters. Furthermore, the model should be useful in understanding mechanisms of VNS efficacy in chronic epilepsy using approaches such as the gene expression studies that will be presented here.