Abstracts

Rationale: Mapping mouse seizure susceptibility loci provides a framework for identifying novel candidate genes for complex human disorders like epilepsy. Using C57BL/6J (B6, seizure-resistant) X A/J (seizure-susceptible) chromosome substitution strains (CSS), we previously identified loci conferring susceptibility to pilocarpine-induced limbic seizures on mouse chromosomes 1, 2, 5 and 10. CSS have a single chromosome from a donor strain (A/J) transferred onto a host strain (B6) by backcrossing. Here we examine response to 6Hz ECT, an electrical model of limbic seizures, in A/J, B6, and CSS mice as a foundation for identification of genes underlying strain susceptibility. ECT has been used to map loci contributing to the maximum electroshock threshold; 6Hz ECT susceptibility has not been mapped. We focus on pilocarpine-susceptible CSS to identify model-specific and broadly-acting loci.Methods: Mice were obtained from Jackson Laboratories (JAX) and bred at Columbia University. 10-11 week-old animals were tested with transcorneal electrodes and a Grass stimulator (Model S48), at 6Hz, 0.2-ms pulse width, 3.0-s duration, and constant voltage. Mice received 0.5% tetracaine to each eye prior to stimulus. Severity was recorded as 0 = no seizure, 1 = stunned, 2 = partial seizure (jaw chomping, limb clonus, tail/body tremors), 3 = generalized seizure (hyperexcitability, rearing, falling). A/J and B6 mice were tested over a range of currents (6, 9, 12, and 16 mA) to identify strain-specific response patterns and to choose the best current for mapping, determined to be 6mA (see results). Subsequently A/J, B6 and CSS 1, 2, 5 and 10 mice were tested at 6 mA.Results: A/J had significantly more severe seizures than B6 mice at almost every current (Fig 1). The 6mA current best distinguished the two strains while allowing a range of severity. At 6 mA, a greater percentage of A/J and CSS animals seized compared with B6 (B6: 55%; A/J: 91%; CSS1: 92%; CSS2: 100%; CSS5: 88%; CSS10: 86%). A/J, CSS1, and CSS2 were also significantly more likely to have severe seizures than B6 animals (Fig 2). There was a trend for CSS5 and CSS10 but numbers were small.Conclusions: In response to 6 Hz ECT, a higher proportion of A/J mice seize and have more severe seizures compared to B6 mice at every current except 9 mA. Convergence of A/J and B6 at 9 mA may result because susceptible strains do not prevent seizure spread at 6 mA, whereas at 9 mA, inhibitory drive may be increased and spread diminished. Above 9 mA, when excitatory drive predominates, the seizure may generalize in susceptible strains, producing a classic U-shaped response curve. Pilocarpine-susceptible CSS are also ECT-susceptible, suggesting that these CSS contain QTLs influencing response to both modes of seizure induction. Although our results are at the chromosomal level, they suggest that underlying loci contribute to a common pathway for neuronal hyperexcitability. Breeding susceptible CSS to F2 or N2 generations will enable us to fine map these loci. These loci and genes can be tested in functional studies and human epilepsy populations.