Abstracts

Mossy fiber sprouting into the granule cell layer and molecular layer of the dentate gyrus is a common abnormality found in patients and models of temporal lobe epilepsy. The role of mossy fiber sprouting in epileptogenesis is unclear, and its blockade would be useful experimentally and perhaps therapeutically. Results of previous attempts to block mossy fiber sprouting after epileptogenic treatments have been disappointing or controversial. In other brain regions some forms of axon sprouting and post-trauma epileptogenesis have been blocked by chronic application of the sodium channel blocker tetrodotoxin (TTX). The present study tested the hypothesis that chronic, focal infusion of TTX would block mossy fiber sprouting after pilocarpine-induced status epilepticus.
Adult rats were treated systemically with pilocarpine. Ten experienced at least 2 h of status epilepticus and 5 (controls) did not. Several hours to 3 d after pilocarpine treatment a pump with a cannula directed into the left dentate gyrus was surgically implanted to continuously deliver vehicle (artificial cerebrospinal fluid) (n=5) or 10 [micro]M TTX (n=5). A previous study showed that this method blocked local EEG activity (Galvan et al, 2000, J Neurosci 20:2904). Rats were videotaped 40 h/wk for motor seizure activity. After 28 d rats were perfused and their isolated hippocampi were sectioned transversely and processed for Nissl-staining, Timms-staining, and TTX-immunocytochemistry. A 1/6 series of Timm-stained sections through the entire septotemporal length of the hippocampus was analyzed using a Neurolucida system to measure the areas of the total granule cell layer + molecular layer and the Timms-positive part of the granule cell layer + molecular layer. The Cavalieri method was used to estimate the volumes of the total and Timms-positive granule cell layer + molecular layer.
Seizures were observed almost twice as frequently in vehicle-infused compared to TTX-infused post-status epilepticus rats, but the difference did not reach statistical significance. Nissl-staining revealed hilar neuron loss in post-status epilepticus rats, but not in controls. In TTX-infused but not vehicle-infused rats TTX-immunoreactivity was evident in sections from the septal part of the left hippocampus where the cannula had been placed. The Timms-positive proportion of the granule cell layer + molecular layer was 8% in controls, 27% in vehicle-infused post-status epilepticus rats, and 27% in TTX-infused post-status epilepticus rats. A plot of the Timms-positive proportion of the granule cell layer + molecular layer versus section position along the septotemporal axis revealed similar levels of aberrant Timm-staining in the region of infusion of both TTX and vehicle.
Chronic, focal infusion of TTX into the dentate gyrus might have been anticonvulsant but it was not antiepileptogenic and it did not block mossy fiber sprouting. These findings suggest that sodium channel mediated neuronal activity is not necessary for mossy fiber sprouting.
[Supported by: NIH]