Abstracts

Rationale: Cognitive deficits, memory loss and social withdrawal are common among children with epilepsy. Behavioral difficulties and depressive mood are often exacerbated by seizures and frank post-ictal psychosis can occur. Acute seizures cause both pro-inflammatory cytokine activation and cognitive deficit. However, direct evidence that links seizure-induced brain inflammation and behavioral deficits is lacking. We used CX3cr1GFP/+ transgenic mice to show that behavioral deficits occur in parallel with microglial activation acutely after kainic acid (KA)-induced seizures in young mice. Methods: CX3cr1GFP/+ transgenic mice were generated by replacing fractalkine, with the gene encoding enhanced green fluorescent protein (EGFP) by homologous recombination in embryonic stem cells. All microglia are fluorescently labeled. P25 CX3cr1GFP/+ transgenic mice were tested for exploratory behavior in an open field (152.5 cm x 152.5 cm) for 5 minutes, 1 to 3 days after KA-induced seizures. A spatial memory acquisition was also tested using Barnes maze starting from one day after KA-induced seizures. The mice were trained on four consecutive days with total of 16 trials. To show the effect of activated microglia on exploratory behavior directly, we injected GFP+ microglia (pure microglia culture prepared from CX3cr1GFP/+ transgenic mice) into the hippocampal hilus of the Cx3cr1+/+ wild type mice at two different sites. All mice were tested for exploratory behavior at 24 hours and at 7 days. Results: Compared to control mice, the mice that experienced seizures at P25-P30 showed significantly decreased in exploratory behavior 24 (n=8, P<0.003) at 48 hours (P<0.0001) after KA injections. The exploratory behavior improved on day 3. Similarly, microglia injected into the brain led to significant impairment of exploratory behavior compared to control group (p< 0.05) 24hrs after injection. In a Barnes Maze, all mice learned the spatial task evidenced by decreasing latency to reach the target box across successive sessions. Young mice treated with KA (n=4), however, found the escape box significantly slower than control mice (n=3) (p<0.001). Concomitant to the observed behavioral changes, microglia are activated within 2 days after KA-induced seizures (control 1.67 ± 0.15 vs. KA 3.08 ± 0.28, p<0.005) and remain significantly activated as long as 7 days (area %: 1.64 ± 0.10 PBS vs. 2.35 ± 0.21 KA, p<0.01). Conclusions: Significant reduction of exploratory behavior in an open field and acute impairment in spatial learning and memory are correlated with microglial activation after KA-induced seizures. Taking advantage of transgenic mice in which all microglia are fluorescently labeled, we demonstrated an acute and persistent microgliosis after KA-induced seizures and provided direct evidence that activated microglia can cause behavioral deficit. Our results implicate neuroinflammation in the seizure-related behavior impairment and suggest a potential role for anti-inflammatory therapy to hasten recovery of behavioral changes following prolonged seizures.