Genetic background of mice strongly influences treatment resistance in the 6 Hz seizure model.

The 6 Hz model of focal seizures has been increasingly used to identify anticonvulsant compounds with potential activity against therapy-resistant epilepsy, but the protective response to anticonvulsants in this model could be dependent on experimental conditions and selection of mouse strains. Seizure thresholds in the 6 Hz model were compared in CF-1, NMRI, and C57Bl/6J male mice with two different electrical stimulators (Ugo Basile 5780 and Grass S48). Dose-response curves for phenytoin and levetiracetam were generated in the three strains at 32 and 44 mA current intensities using both devices. Plasma and brain exposure to the two drugs were measured in all three strains. CF-1 mice had the lowest seizure threshold and responded to phenytoin at 32 mA stimulation intensity, but not at 44 mA. NMRI and C57Bl/6J mice had nearly identical threshold values, but NMRI mice responded well to phenytoin at 32 mA and showed limited responsiveness to this drug at 44 mA, whereas C57Bl/6J mice were nearly completely resistant to phenytoin. Furthermore, levetiracetam showed limited efficacy and low potency in CF-1 and C57Bl/6J mice, particularly at 44 mA, whereas in NMRI mice the drug showed much higher potency in all experimental conditions. No obvious difference in the pharmacokinetics of both phenytoin and levetiracetam was detected between the mouse strains that would have explained these unexpected variations in potency. We have also found that the protective effects of both drugs may be influenced by the device type. Collectively these observations clearly indicate that treatment resistance of 6 Hz seizures should be interpreted with strain and experimental conditions in mind. Furthermore, it is important to note that strain differences, much like human genetic differences, may explain why some mice and patients respond to a given treatment and others do not.