Abstracts

RATIONALE: Several rat models have examined morphological and physiological changes in the dentate gyrus associated with the development of temporal lobe epilepsy (TLE), but analogous studies in mice have been few. Systemic kainate does not induce these changes in some murine strains, but pilocarpine injection leads to TLE and mossy fiber sprouting in C57BL/6 and CD1 mice. Since these mice are commonly used as a background for genetic mutation studies, the physiological consequences of TLE development was examined in the dentate gyrus of these strains.
METHODS: Systemic pilocarpine injection (280-290 mg/kg) was used to induce status epilepticus (SE) in adult male ICR (CD1) and C57BL/6Nhsd mice. Seizure behavior was monitored for the next two months. Transverse slices of the ventral hippocampus were made from pilocarpine-treated and untreated mice, and extracellular field potentials were recorded in the granule cell layer of the dentate gyrus. Recording solutions were nominally magnesium-free and contained bicuculline methiodide. Population activity was recorded after electrical stimulation of the mossy fibers in the hilus and in response to photoactivation of glutamate within the granule cell layer. Slices were subsequently processed for Timm and Nissl histochemistry.
RESULTS: Data were obtained from 12 mice that survived pilocarpine-induced SE, 8 mice that were injected with pilocarpine, but did not undergo SE, and 6 control mice. Most SE survivors, but not other mice, had spontaneous seizures in the weeks after treatment. Electrical stimulation of the hilus resulted in a single population spike in the dentate gyrus of slices from control mice and animals that did not experience SE. In SE survivors, similar stimulation resulted in a population spike followed by a DC shift of variable latency, which was often accompanied by repetitive afterdischarges lasting 3-60 s. Afterdischarges were blocked by glutamate receptor antagonists. Uncaging glutamate at the recording pipette tip resulted in a negative DC shift in most slices. Negative shifts and population spikes were also elicited by glutamate uncaged in the granule cell layer at sites distant from the recording pipette in slices from SE survivors, but not other groups. Timm staining revealed robust mossy fiber sprouting in the inner molecular layer of slices from SE survivors, but not other groups.
CONCLUSIONS: These data confirm that SE leads to development of spontaneous seizures and mossy fiber sprouting in CD1 and C57BL/6 mice. They further indicate that pilocarpine-induced SE and consequent mossy fiber sprouting results in formation of recurrent excitatory circuits between granule cells of the dentate gyrus in these murine strains. The dentate gyrus of pilocarpine-treated mice shares significant physiological and morphological characteristics with rat models. These data support the hypothesis that SE-induced mossy fiber sprouting and synaptic reorganization are relevant characteristics of TLE. The murine model proposed may be a useful means of examining the genetic regulation of the cascade of events leading to this circuit remodeling.
[Supported by: Louisiana Board of Regents (LEQSF-RD-A-35) and the American Heart Association (SDG-0030284N).]