Abstracts

Rationale: The role of granule cell axon (mossy fiber) sprouting in temporal lobe epileptogenesis is unclear and controversial. Rapamycin suppresses mossy fiber sprouting, but its reported effects on seizure frequency are mixed. The present study used high-dose rapamycin to more completely block mossy fiber sprouting and measure the effect on seizure frequency.Methods: Mice were treated with pilocarpine to induce status epilepticus. Beginning 24 h later and continuing for 2 months, vehicle or rapamycin (10 mg/kg/d) was administered. Starting 1 month after status epilepticus, mice were video-monitored 9 h/d every day for 1 month to measure the frequency of spontaneous motor seizures. At the end of seizure monitoring, a subset of mice was prepared for anatomical analysis. Mossy fiber sprouting was measured as the proportion of the granule cell layer and molecular layer that displayed black labeling in Timm-stained sections.Results: Extensive mossy fiber sprouting developed in mice that experienced status epilepticus and were treated with vehicle (22.0 1.6% of the granule cell layer plus molecular layer displayed black Timm-staining, n=12). In rapamycin-treated mice, mossy fiber sprouting was blocked almost to the level of na ve controls (4.9 0.4%, n=25 and 3.3 0.2%, n=6, respectively, both significantly less than vehicle-treated mice, p<0.05, ANOVA). More than 29,000 mouse-hours of seizure monitoring was evaluated, and 3881 seizures ( Racine scale 3) were identified. Seizure frequency was similar in vehicle- (0.137 0.013 seizures/h, n=64) and rapamycin-treated mice (0.133 0.12 seizures/h, n=64). Rapamycin treatment suppressed dentate gyrus hypertrophy but not hilar neuron loss.Conclusions: These findings suggest mossy fiber sprouting is not necessary for epileptogenesis in the mouse pilocarpine model. They also reveal that rapamycin does not have anti-seizure or anti-epileptogenic effects in this model.