Abstracts

Rationale: Neurogenesis persists throughout life in the adult mammalian dentate gyrus and is strongly potentiated by status epilepticus (SE). Dentate granule cells (DGCs) generated in response to SE can mature and integrate both normally and abnormally into the existing network, and may contribute to the formation of pro-epileptogenic circuitry. Quantitative analysis of synaptic reorganization after SE suggests that patterns of abnormal integration such as mossy fiber sprouting, hilar ectopic migration, and persistent hilar basal dendrites favor an increase in excitatory input onto DGCs. Previous work showing that that ablation of post-SE neurogenesis with an anti-mitotic agent reduces seizure frequency and severity provides evidence that adult-generated neurons contribute to hyperexcitability during epileptogenesis. However, adult-generated DGCs that integrate into the granule cell layer after electrical stimulation-induced SE show reduced excitatory input and increased inhibitory input. To further examine the role of adult-generated DGCs in epileptic hyperexcitability, we use low-dose, focal X-irradiation to transiently suppress neurogenesis after pilocarpine-induced SE. We measured the effect on epileptogenesis with long-term video/EEG monitoring. Methods: SE was induced in young adult (P56) male Sprague Dawley rats by pretreatment with atropine methylbromide (5 mg/kg i.p.) and subsequent injection of pilocarpine hydrochloride (340 mg/kg i.p.). SE was terminated after 90 minutes with diazepam (10 mg/kg i.p.). Animals recovered for four days and then were treated with 6 Gy focal ionizing X-irradiation in divided doses at P60 and P62. Sham irradiated animals were handled identically but not exposed to irradiation. At P70 animals were anesthetized with ketamine/xylezine, placed in a stereotaxic frame and implanted with 4 epidural screw electrodes (bilateral front and parietal lobes) as well as a ground electrode in the nasal sinus and reference in the cerebellum. Continuous video/EEG monitoring (CEEGraph Vision) was performed from two sham irradiated and two irradiated animals for 2 weeks beginning at 6 weeks post SE.Results: Pilot studies using BrdU injection after irradiation confirmed that a 6 Gy divided x-ray dose significantly reduces post-SE cell proliferation acutely, and ongoing work will determine the duration of neurogenesis knock down. Preliminary analysis of video/EEG monitoring data shows that knockdown of post-SE neurogenesis with focal irradiation increases seizure frequency in epileptic animals. Further analysis of the effect of neurogenesis knock-down on spontaneous seizure duration and severity is ongoing. Conclusions: We conclude from our preliminary data that transiently suppressing post-SE neurogenesis does not ameliorate spontaneous seizures in epileptic rodents. Ongoing work is aimed at determining whether the increase in seizure frequency after irradiation is a consistent finding, and how the timing, degree and pattern of neurogenesis recovery after transient knockdown correlates with epileptogenesis.