Abstracts

RATIONALE: Synaptic reorganization after neural injury may form the basis of recurrent excitatory networks. Axon guidance cues including the semaphorins (sema) provide targeting information to axons along predetermined pathways during development. It is unclear which of these axon guidance cues participate in the formation of adaptive and maladaptive circuitry after neural injury. Class III sema ligands which bind to neuropilin 2 (NPN2) containing sema receptors are selectively down-regulated in CA1 pyramidal cells of hippocampus in mature rat brain during the epoch of synaptic reorganization after kainic acid status epilepticus. This suggested the hypothesis that loss of the tonic activation of NPN2 containing sema receptors may lead to axonal sprouting from CA1 pryamidal cells and thereby promote epileptogenesis. To further investigate this hypothesis, we have studied of genetically defined strains of mice, which may lead to innovative approaches to the understanding of the molecular basis of synaptic organization and epileptogenesis.
METHODS: FVB/N mice but not C57Bl/6 mice have kainic acid (KA) induced cell death of CA3/Hilar regions and synaptic reorganization (mossy fiber sprouting) despite equivalent behavioral seizures and comparable metabolic activation of hippocampal neurons (Schauwecker et al, Exp. Neurology 2000). Using serial videotape recording sessions (60 hours & six months post KA-SE), we determined rates of epileptogenesis (epilepsy defined as 2 or more observed seizures) in these strains after KA-SE. To determine whether axon guidance gene expression is altered after KA-SE, in situ hybridization, immunoblot analysis, and imunnocytochemistry of the semaphorin ligands and receptors (neuropilins) were performed in mouse hippocampus.
RESULTS: Despite equivalent duration (FVB: 198 min vs. C57Bl/6: 191 min) and severity (average seizure class: IV) of KA-SE, death of CA3b neurons was evident in FVB but not C57Bl/6 mice (FVB: 150 +/- 15 cells/[mu]m2 vs 85 +/- 17 cells/[mu]m2; C57Bl/6 160 +/- 25 cells/[mu]m2 vs 167 +/- 22 cells/[mu]m2) without any detectable CA1 pyramidal cell loss 7 days after KA-SE. Five of nine FVB mice (55 %) were observed to have a chronic epilepsy starting 4-8 weeks after KA-SE, whereas no C57Bl/6 mice (N=9) so far have had any recorded spontaneous behavioral seizures. In concert with this finding, Sema 3F, NPN2, and NPN1 protein/mRNA content were decreased an average of 75% (p[lt]0.001) in CA1 neurons of FVB but not C57Bl/6 mice 7 days after KA-SE. In contrast, sema 3A, sema 3C, and sema 4C mRNA content were unaltered in all hippocampal subregions post KA-SE irregardless of the strain.
CONCLUSIONS: These data demonstrate that status epilepticus is necessary but not sufficent to regulate epileptogenesis and selected axon guidance genes in mouse brain after KA-SE. Transcriptional regulatory complexes unique to the hippocampal neurons of the FVB/N mouse but not the C57Bl/6 mouse strain may underlie [define] its susceptibility to KA-SE induced neural injury and subsequent generation of epileptic circuitry.
[Supported by: NINDS, AES, and Exelixis Pharmaceuticals.]