Abstracts

Rationale: Mesial temporal lobe epilepsy (MTLE), the most common form of medically refractory focal epilepsy, arises from the hippocampus and amygdala, structures known to be affected by early life stress (ELS). Stress, particularly in early life, may be a causal factor for MTLE: ELS is already strongly implicated in the causation of mental disorders and studies have revealed stress effects on seizures and epileptogenesis. We previously demonstrated that i) maternal separation stress (MSS) in early life and ii) exogenous corticosterone (CORT) supplementation decreased the threshold for seizures, and increased the rate of progression of amygdala kindling in adult rats. As CORT is known to be released briefly following seizures and MSS rats have been shown to exhibit enhanced CORT responses to stress in later life, we hypothesized that seizure-related CORT release would be augmented in MSS rats and that this might be relevant to limbic epileptogenesis. Methods: Wistar rat pups underwent the following interventions: MSS for 180 min/day (n=11), or brief early handling and separation for 15 min/day (EH, n=15) on postnatal days 2-14. Rapid amygdala kindling (electrical stimulations via an implanted electrode every 15 mins, 24 times/day) commenced at 8 weeks of age. Serum CORT levels were measured before kindling, immediately following the 12th and 24th stimulations (during kindling on day 1) and after a Class V seizure in the fully kindled state (two weeks after reaching the fully kindled state). Unbiased stereological pyramidal cell counts in the ipsilateral hippocampal CA3c region were performed on kindled brains of MSS and EH rats that were killed two weeks after the fully kindled state. All data are presented as mean ± SEM. Results: MSS rats had enhanced rates of kindling epileptogenesis requiring fewer stimulations to reach the fully-kindled state (two-way ANOVA, 45.70 ±5.36 vs 73.53±6.46, p<0.01). CORT levels were higher in MSS than EH exposed rats post-seizure with the 24th kindling stimulation (474.22 ± 54.58 vs. 266.95 ±30.50, n=11-15) and in fully kindled rats (534.21 ±70.64 vs. 333.08±48.17, n= 8-12) (over all effect of two-way ANOVA repeated measures with gender as a co-variant; F (3,28) =13.91, p<0.0001). Stereological cell counts revealed less CA3c pyramidal cells in MSS versus EH rats (total neurons: 40076 ±3008 vs 48526 ± 3433, p=0.04, Student t-test, n= 11-12). Conclusions: Our two findings in MSS kindled rats give important leads to potential mechanisms: (i) cell loss in CA3c, a hippocampal subregion considered crucial to MTLE causation, suggests that ELS may increase vulnerability to seizure induced excitotoxicity and cell death; and (ii) larger pulses of CORT release following seizures implicate HPA hyper-responsiveness in MTLE pathogenesis, given the reported effects of elevated CORT on epileptogenesis and the hippocampus.Thus, the hyper-reactive HPA function might be culprit in pathogenesis of human TLE.