Recent studies have sought to identify the genes involved in excitotoxic neurodegeneration. Here we report that certain strains of mice, including strains that are used for gene targeting studies, do not exhibit excitotoxic cell death after kainic acid seizures. Kainic acid produced excitotoxic cell death in the CA3 and CA1 subfields of the hippocampus in 129͞SvEMS and FVB͞N mice, in the same pattern as described in rats. C57BL͞6 and BALB͞c mice exhibited excitotoxic cell death only at very high doses of kainate, and then only in a very restricted area, although they exhibited comparable seizures. Hybrids of 129͞SvEMS ؋ C57BL͞6 mice created using embryonic stem cells from 129͞SvEMS mice also did not exhibit excitotoxic cell death. These results demonstrate that C57BL͞6 and BALB͞c strains carry gene(s) that convey protection from glutamate-induced excitotoxicity. This differential susceptibility to excitotoxicity represents a potential complication for gene targeting studies.

SUMMARYPurpose: Clinically, perturbations in the semaphorin signaling system have been associated with autism and epilepsy. The semaphorins have been implicated in guidance, migration, differentiation, and synaptic plasticity of neurons. The semaphorin 3F (Sema3F) ligand and its receptor, neuropilin 2 (NPN2) are highly expressed within limbic areas. NPN2 signaling may intimately direct the apposition of presynaptic and postsynaptic locations, facilitating the development and maturity of hippocampal synaptic function. To further understand the role of NPN2 signaling in central nevous system (CNS) plasticity, structural and functional alterations were assessed in NPN2 deficient mice. Methods: In NPN2 deficient mice, we measured seizure susceptibility after kainic acid or pentylenetetrazol, neuronal excitability and synaptic throughput in slice preparations, principal and interneuron cell counts with immunocytochemical protocols, synaptosomal protein levels with immunoblots, and dendritic morphology with Golgi-staining.Results: NPN2 deficient mice had shorter seizure latencies, increased vulnerability to seizurerelated death, were more likely to develop spontaneous recurrent seizure activity after chemical challenge, and had an increased slope on input/ output curves. Principal cell counts were unchanged, but GABA, parvalbumin, and neuropeptide Y interneuron cell counts were significantly reduced. Synaptosomal NPN2 protein levels and total number of GABAergic synapses were decreased in a gene dose-dependent fashion. CA1 pyramidal cells showed reduced dendritic length and complexity, as well as an increased number of dendritic spines. Discussion: These data suggest the novel hypothesis that the Sema 3F signaling system's role in appropriate placement of subsets of hippocampal interneurons has critical downstream consequences for hippocampal function, resulting in a more seizure susceptible phenotype.

No abstract

Host genetic factors are likely to contribute to differences in individual susceptibility to seizure-induced excitotoxic neuronal damage. Similarly, inbred strains of mice differ in their susceptibility to the kainic acid (KA) model of seizure-induced cell death, but the genes responsible for the differences are not known. Here, we define the inheritance patterns of susceptibility to KA-induced neurodegeneration in the hippocampus by assessing 331 back-cross (N2) progeny of two inbred mouse strains, C57BL/6 and FVB/N, previously shown to display resistance and sensitivity to KA-induced cell death, respectively. Results of phenotypic analysis suggest that the difference in susceptibility between these two strains is conferred by a single dominant gene. Therefore, we used an N2 back-cross between the inbred C57BL/6 and FVB/N strains for a genome-wide search for quantitative trait loci (QTLs), which are chromosomal sites containing genes influencing the magnitude of susceptibility. Genome-wide interval mapping in N2 progeny identified a locus on distal chromosome (Chr) 18 with a peak LOD score of 4.9 localized between D18Mit186 and D18Mit4 as having the strongest and most significant effect in this model. QTLs of minor effect were detected on Chr 15 (D15Mit174-D15Mit156) and Chr 4 (D4Mit264-D4Mit91), with peak LOD scores of 3.02 and 2.46, respectively. The three significant QTLs (Chrs 4, 15, 18) together account for nearly 25% of the trait variance for both genders combined. Reduced KA-induced cell death susceptibility was observed in a congenic strain in which the highly susceptible FVB/N strain carried putative resistance alleles from the C57BL/6 strain on Chr 18.

Mouse strains differ from one another in their susceptibility to seizure-induced excitotoxic cell death. Previously, we have demonstrated that mature inbred strains of mice show remarkable genetic differences in susceptibility to the neuropathological consequences of seizures in the kainate model of status epilepticus. At present, while the cellular mechanisms underlying strain-dependent differences in susceptibility remain unclear, some of this variation is assumed to have a genetic basis. However, it remains unclear whether strain differences in susceptibility to seizure-induced cell death observed following kainate administration are observed following systemic administration of other chemoconvulsants. In rodents, the cholinomimetic convulsant pilocarpine is widely used to induce status epilepticus (SE), followed by hippocampal damage and spontaneous recurrent seizures, resembling temporal lobe epilepsy. This model has initially been described in rats, but is increasingly used in mice. We characterized neuronal pathologies after pilocarpine-induced status epilepticus (SE) in eight inbred strains of mice focusing on the hippocampus. A ramping-up dose protocol for pilocarpine was used and behavior was monitored for 4–5 hours. While we did not observe any significant differences in seizure latency or duration to pilocarpine among the inbred strains, we did observe a significant difference in susceptibility to the neuropathological consequences of pilocarpine-induced SE. Of the eight genetically diverse mouse strains screened for pilocarpine-induced status, BALB/cJ and BALB/cByJ were the only two strains that were resistant to the neuropathological consequences of seizure-induced cell death. Additional studies of these murine strains may be useful for investigating genetic influences on pilocarpine-induced status epilepticus.