Abstracts

Rationale: Previous studies in C57Bl/6J (B6, seizure resistant) and DBA/2J (D2, susceptible) strains of mice mapped a QTL (Szs1) for susceptibility to seizures induced by 3 different chemoconvulsants and electroshock to a common region of distal mouse Chromosome 1 (CH1) suggesting the presence of a gene(s) with fundamental influence on neuronal excitability. Subsequent refined mapping led to nomination of Kcnj10 as the gene underlying Szs1. More recently, we identified contributions to pilocarpine-induced (limbic) seizure susceptibility from mouse CH1 using B6 X A/J chromosome substitution strains (CSS). To explore this finding on CH1, and the range of effect of the Szs1 locus on limbic seizure models, we performed a bioinformatic analysis of B6, A/J, and D2 genomes in the distal CH1 locus and Kcnj10 gene, and examined susceptibility of the B6.D2-Szs1 congenic and Kcnj10-BAC transgenic mice to acute pilocarpine-induced seizures. Methods: We tested 10-11 week old CSS1 (N=43) and B6 (N=194) animals with pilocarpine (250-300 mg/kg, i.p.) pretreated with atropine (1 mg/kg, i.p.) and observed seizure behaviors for 2.5 hours. We used the Jax Phenome Project site to compare B6, A/J and D2 genomes for the distal Ch1 locus and for Kcnj10. To examine the effects of Szs1 genotype on pilocarpine susceptibility, we tested B6.D2-Szs1 congenic (C) and BAC transgenic (TG) mice with pilocarpine (200-350 mg/kg, ip). The Kaplan-Meier method was used to estimate median survival for both genotypes at each dosage. The stratified log-rank statistic was computed for the effect of genotype, stratified on dosage. Results: In the CSS screen, 3% of B6 mice reached the highest seizure stages (Stage 4: rearing/hyperactivity or tonic/clonic seizures with loss of posture; Stage 5: continuous tonic/clonic seizures), compared with 48% of CSS1 animals (p< 0.0001). At 300 mg/kg 30% of B6 mice reached stage 4 or 5 vs. 100% of CSS1 mice. Bioinformatic sequence analysis showed that at the Ch1 locus and in Kcnj10, B6 mice differ dramatically from both D2 and A/J while D2 and A/J differ very little from one another. This suggests that the QTL identified using B6 X A/J CSS may be the same as that identified using B6 and D2 strains in electroshock, PTZ, DMCM, and kainate studies, and that kcnj10 may be the underlying susceptibility gene. To examine the effect of Szs1 genotype on limbic seizure susceptibility, we tested 44 C and 61 TG mice with pilocarpine. C mice progressed through seizure stages more quickly than TG mice; this difference was most pronounced for latency to partial seizure.