Abstracts

Rationale: Viral infections of the CNS are associated with a significantly increased risk for epileptogenesis. Intracerebral injection of Daniel’s strain of Theiler’s virus (TMEV-DA) leads to acute encephalitis in C57BL/6 (B6) and SJL/J mice. However, during the acute phase of infection >60% of infected B6 mice display Racine Stage 1-5 seizures while SJL/J mice do not. Thus, we hypothesize that TMEV-DA infected B6 mice may provide a novel experimental system to investigate viral-infection induced susceptibility to epilepsy. The purpose of this investigation was to use Fos immunohistochemistry (IHC) to identify the brain structures involved in TMEV-DA induced seizures. Additionally, seizure thresholds were assessed in infected and mock-infected SJL/J mice in order to determine whether alterations in seizure thresholds were specific to B6 mice. Methods: B6 mice (n=70) and SJL/J mice (n=30) were injected intracerebrally with either 2 x 10⁴ PFU TMEV-DA or phosphate buffered saline (PBS). Mice were then monitored for the development of seizures 2 hours per day for 10 days post injection (p.i.). For Fos IHC studies, B6 mice were anesthetized and intracardially perfused with 4% paraformaldehyde at various time points post Stage 4/5 seizure. Brains were sectioned (40μm) and evaluated with standard IHC techniques for Fos. Two months p.i., B6 limbic (6Hz), forebrain (minimal clonic), and hindbrain (maximal tonic hindlimb extension [THE]) seizure thresholds were assessed by corneal electroshock according to the staircase estimation procedure described by Finney (1971). Minimal clonic seizure threshold was also determined in SJL/J mice. Convulsive current curves were generated and statistical significance was defined at p<0.05 using Probit Analysis. Results: Consistent with previous studies, 65% of the TMEV-DA infected B6 mice displayed spontaneous seizures 4-10 days p.i., while no PBS-injected or SJL/J mice exhibited seizures. Fos mapping revealed intense seizure-induced neuronal activation in limbic and forebrain structures including the hippocampus and the thalamus. At 2 months p.i., seized B6 mice had significantly lower minimal clonic (p<0.01) and 6 Hz (p<0.01) seizure thresholds compared to mock-infected B6 mice. Interestingly, non-seized B6 mice had a significantly lower maximal THE seizure threshold (p<0.05) compared to controls while there was no difference between seized and controls. For SJL/J mice, no difference was observed in minimal clonic seizure threshold between infected and control mice. Conclusions: These results indicate that TMEV-DA induced seizures lead to increased seizure susceptibility in forebrain and limbic structures of B6 mice, but may be protective in hindbrain structures. Negative findings in SJL/J mice suggest that this outcome may be strain specific. These findings support the hypothesis that TMEV-DA infection of B6 mice may provide a unique model of infection-induced epilepsy and that genetic background can influence the pathology.