Abstracts

Rationale: MRI studies have demonstrated that an acute and transient T2 hypersignal usually appears in the limbic cortex of rats after status epilepticus induced by pilocarpine. The histopathologic meaning of that post-status epilepticus T2 hypersignal (pSE-T2-Hs) is not fully known. We assessed correlations between the kinetic of evolution of the pSE-T2-Hs and some basic histopathological phenomena such as microglial activation, astrocytic activation and neuronal degeneration, in the amygdala (AMG) during the first 2 weeks following the status epilepticus (SE) induced by pilocarpine in rats. Methods: Fifteen Sprague-Dawley male rats experienced 2 h of SE induced by Pilocarpine hydrochloride (350 mg/kg), stopped by Diazepam (10mg/kg), and then were separated into three groups: D1 (n = 6), D3 (n = 5) and D15 (n = 4) which respectively had a T2-MRI on day 1, day 3 and day 15 after the SE. Two naive control rats (no-SE) had received saline instead of pilocarpine, followed by T2-MRI 15 days later. For each group of rats, the average volume of pSE-T2-Hs in the AMG was calculated using software (Créatools). Rats from each group were deeply anesthetized immediately after the MRI. Brains were fixed, cryoprotected, frozen and stored at −80°C. Immunohistochemical detections in the AMG cortex slides were performed for each rat using GFAP for astrocytic activation, CD11b for microglial activation, and Fluoro-Jade B (Chemicon) to stain degenerating neurons. Results: A pSE-T2-Hs was identified in the limbic cortex of all the rats treated by pilocarpine. No T2-Hs was found in cortex of the two naïve control rats (no-SE). The average volume of pSE-T2-Hs in the AMG cortex was maximal on day 3 post-SE then declined to a minimal volume on day 15. Astrocytic and microglial activations constantly increased from day 1 to day 15, with a maximal peak at day 15. Ongoing acute neuronal degeneration revealed by Fluoro-Jade B staining took place mainly from day 1 to day 3. Very few Fluoro-Jade B stained cells could still be seen at day 15. No significant astrocytic activation, microglial activation, or degenerating neurons were found in the control rat’s AMG cortex. Conclusions: There is no parallel correlation between the kinetics of evolution of the pSE-T2-Hs and those of astrocytic or microglial activations. In contrast, the pSE-T2-Hs had a temporal evolution pattern similar to that of Fluoro-Jade B staining which is a marker of ongoing neuronal degeneration. These findings suggest that the limbic pSE-T2-Hs may be underpinned by the acute ongoing neuronal degeneration but not the neuronal inflammation. If these results are confirmed by other studies, T2-MRI could then be used for in vivo assessment of acute neuronal degeneration in the pilocarpine model of epilepsy.