Frings Audiogenic Seizure-Susceptible Mouse Model

Brief Description

Frings AGS-susceptible mice are genetically susceptible to sound-induced reflex seizures [1, 2]. Their seizure phenotype is characterized by wild running, loss of righting reflex, tonic flexion, and tonic extension in response to high-intensity sound stimulation. In contrast to other seizure models, the Frings AGS-susceptible mouse, like the DBA2J AGS-susceptible mouse, is non-discriminatory with respect to clinical categories of anticonvulsant drugs [3], and thus, does not offer high predictive value in screening investigational compounds. For example, the prototypical anticonvulsants phenytoin and ethosuximide display widely divergent clinical spectrums in humans, but are both active against sound-induced seizures in mice [4]. Yet, the preclinical utility of this animal model rests in the useful information that can be obtained within a genetically susceptible model of seizures. Further, efficacy in this model provides proof of concept of brain bioavailability following systemic administration. For this reason, the Frings AGS-susceptible is used to screen novel investigational compounds at the ETSP.

Introduction

The Frings AGS-susceptible mouse has a well-validated epilepsy phenotype that makes it particularly useful as a screening model. Beginning at about 21 days of age, Frings AGS-susceptible mice display prominent seizure activity in response to a high-intensity sound stimulus. They then remain susceptible to sound throughout their life. This is in stark contrast to the DBA2J AGS-susceptible mouse, which is the other common model of sound-induced seizures, as DBA2J mice are only susceptible to sound-induced seizures during a narrow developmental window (post-natal days 18-30). Thus, the Frings AGS-susceptible mouse seizures respond to a wide range of CNS-active drugs and display sound-induced seizures throughout their lifetime. In this regard, Frings AGS-susceptible mice are a highly useful screening model for the identification and characterization of compounds potentially effective against inherited epilepsy.

Methods

Male and female Frings audiogenic seizure-susceptible mice (18-25 g) are maintained in an in-house colony at the University of Utah. For each screening test, groups of 8 mice each are treated i.p. with varying doses of the investigational compound. At the time of peak effect as determined in the MES test (Test 4 in CF1 mice), individual mice are placed in a round plexiglass jar (diameter, 15 cm; height, 18 cm) and exposed to a sound stimulus of 110 decibels (11 KHz) delivered for 20 sec. Mice are observed for 25 sec for the presence or absence of hind limb tonic extension. Mice not displaying hind limb tonic extension are considered protected. The severity of a seizure may also be quantitated by assigning a numerical score to the observed response, e.g. no response - 0; wild running for <10 sec - 1; wild running for >10 sec - 2; clonic seizure - 3; forelimb extension/hind limb flexion - 4; tonic seizure - 5.

Results

The Frings AGS-susceptible mouse can be used to identify compounds that protect against sound-induced tonic extension and can also be used to quantify the protection in the presence of the investigational compound. The ability of a test substance to block audiogenic seizures can be quantitated by results collected from different doses with protection between 0% and 100% used to calculate an ED50. The anticonvulsant activity of those test substances that afford protection in this model is quantitated and the ED50 and the 95% confidence interval calculated by probit analysis.

Discussion

Genetically susceptible models of epilepsy are of great utility for the early identification of potentially novel anticonvulsant compounds at the ETSP. Frings AGS-susceptible mice show a broad response profile to many currently approved anticonvulsant drugs and are thus not a useful tool to differentiate investigational compounds or identify compounds effective in difficult to treat partial seizures [4]. However, the Frings AGS-susceptible mouse still affords the ETSP a key screening model for the early identification of potentially novel anticonvulsant compounds. The Frings AGS-susceptible mouse phenotype arises due to a mutation in the monogenic audiogenic seizure susceptible (MASS1) gene, which is linked to mouse chromosome 13 [5, 6]. The MASS1 mutation is a unique seizure susceptibility gene in that it is only one of two such mutations not associated with an ion channel [5, 7]. Interestingly, a recent report demonstrated a nonsense loss-of-function mutation in the MASS1 gene in a human family with febrile and afebrile seizures [8]. Thus, the Frings AGS-susceptible mouse model affords the ETSP a useful preclinical genetic model of seizure susceptibility.