Abstracts

Rationale: Axon sprouting by granule cells in the dentate gyrus (mossy fiber sprouting) is a common abnormality found in patients and models of temporal lobe epilepsy. Mossy fiber sprouting produces an aberrant positive-feedback network among granule cells, but its role in epileptogenesis remains unclear and controversial. It would be useful experimentally, and perhaps therapeutically, to block mossy fiber sprouting after epileptogenic injuries. The present study evaluated whether mossy fiber sprouting could be suppressed by prolonged, focal infusion of rapamycin. Rapamycin inhibits a protein kinase in a signaling pathway that regulates cell growth.Methods: Pilocarpine (380 mg/kg) was administered to rats (male, Sprague-Dawley, 37 d old, n=5) to induce status epilepticus, which was treated with diazepam after 2 h. Later the same day, an osmotic pump and cannula directed toward the left, dorsal dentate gyrus was implanted to deliver 1 mM rapamycin at 0.25 μl/h. After 28 d of continuous infusion, rats were perfused with sulfide solution and fixative. Infused (left) and non-infused (right) hippocampi were isolated, straightened, and sectioned transversely. A 1-in-6 series of sections from the entire septotemporal length of each hippocampus was processed for Timm stain. A Neurolucida system was used to outline and measure the area of the granule cell layer + molecular layer and the Timm-positive part of the granule cell layer + molecular layer. Results: The Timm stain revealed mossy fiber sprouting into the granule cell layer and molecular layer in both rapamycin-infused and non-infused hippocampi. However, in the region around the infusion site (± 600 μm along the septotemporal axis) the percent of the granule cell layer + molecular layer that was Timm-positive was reduced to 74% of that of the corresponding septotemporal region in non-infused hippocampi: 12.1 ± 1.6% (mean ± SEM) in the non-infused side versus 8.9 ± 1.0% in the infused side (p<0.03, paired t test).Conclusions: Aberrant Timm staining was reduced in the rapamycin-treated region of the dentate gyrus in this model of temporal lobe epilepsy. This finding suggests that rapamycin may suppress mossy fiber sprouting. Supported by NINDS & NCRR of NIH.