Abstracts

Rationale: The mechanisms underlying temporal lobe epileptogenesis are not well understood. Excess corticosterone (CORT) exposure enhances neuronal excitability and can provoke seizures in animal models of epilepsy (Kumar, 2007). Similarly, excess CORT decreases neurogenesis and cell survival in the hippocampal dentate gyrus (Cameron, 1994). Recent studies indicate that status epilepticus (SE) persistently elevates basal CORT (Mazarati, 2009), suggesting that glucocorticoids may play a role in the pathogenesis of epilepsy. Therefore, we explored whether blockade of glucocorticoid signaling might mitigate pathological changes normally present in the hippocampus after SE, such as reactive neurogenesis or ectopic granule cell accumulation. Methods: Male C57B/6 mice (n=32) received 420mg/kg of pilocarpine to induce three hours of SE. Subsequently, mice were treated with mifepristone (MF, 20mg/kg) or propylene glycol (vehicle VE) once per day for 8 days. Groups were as follows: 1) SE-VE (n=8) 2) SE-MF (n=8), 3) C-VE (n=8) and C-MF (n=8). On days 4 through 7, animals were injected with BrdU (50mg/kg). Blood samples for CORT analysis were collected in the morning 1, 4 and 7 days post-SE. Mice were sacrificed the morning of the 8^th day. Statistical comparisons were done via 2-way ANOVA. Results: Four days after SE, CORT levels in SE-VE mice [115.6±22.6ng/ml] were significantly higher than controls [C-vehicle:31.3±2.2, C-MF:33.9±4.6, p<0.01]. MF treatment blocked this increase, in that SE-MF mice were indistinguishable from controls [58.4±10.3 p= 0.09]. By contrast, mifepristone did not block the well-established SE-induced increase in cell proliferation in the dentate granule cell layer, as demonstrated by labeling with the proliferative markers Ki-67 [SE-MF:24.7±5.8; SE-VE:18.4±6.2; C-MF:2.9±0.5; C-VE:2.9±0.2 cells/dentate] and BrdU (SE-MF:31.5±9.4; SE-VE:29.75±8.5; C-MF:8.8±1.1; C-VE:8.3±0.8] Interestingly, however, MF did decrease the number of BrdU positive cells in the dentate hilus (SE-MF:9.8±2.6; SE-VE:21.3±5.6 p<0.05) and it increased the number of cells expressing the immature neuronal marker calretinin in the dentate granule cell layer [SE-MF:8.5±1.8; SE-VE:3.4±1.2; C-MF:3.8±0.5; C-VE:2.5±0.4]. Conclusions: Our data shows that the SE-induced increase in basal CORT can be normalized by a four-day treatment with MF. The fact that SE+MF treatment increases calretinin positive cells without affecting Ki-67 cell number is intriguing, as calretinin is transiently expressed by immature granule cells after Ki-67 expression is downregulated. This suggests either that the treatment rescues immature granule cells born at an earlier time point that would otherwise die, or it accelerates cell maturation. MF treatment also reduced the number of BrdU labeled cells in the hilus, suggesting that it might mitigate the accumulation of ectopic granule cells in this region. Future studies will determine the mechanisms and functional significance of these changes in cell proliferation and integration.