Abstracts

Rationale: Chemoconvulsant-induced status epilepticus (SE) is often used to initiate pathological changes leading to the occurrence of spontaneous recurrent seizures (SRSs). However, unlike models that employ systemic injections of kainate (KA) and lead to bilateral and widespread damage, Intra-Amygdala KA (IAK) microinjection in mice has been reported to cause limited unilateral hippocampal damage similar to human temporal lobe epilepsy (Mouri et al. Brain Research 2018; 1213:140-151). Furthermore, IAK mouse models also offer several advantages that make them particularly well suited for assessing the efficacy of antiseizure therapies (e.g., SRSs are reported to manifest after a short latent period and occur with a relatively regular and stable frequency: JimenezPacheco et al. J. Neurosci. 2016; 36:5920-5932). However, the sensitivity of SRSs in these models to available antiseizure drugs (ASDs) are largely unknown. Therefore, an IAK microinjection model of temporal lobe epilepsy is being developed under the auspices of the Epilepsy Therapy Screening Program (ETSP), and SRSs in this model were evaluated in an initial study for their sensitivity to ASDs from several mechanistic classes. Methods: Male C57Bl/6J mice were surgically implanted with a guide cannula for microinjection into the right hemisphere basolateral amygdala and a bipolar depth EEG electrode into the ipsilateral CA1 region of the hippocampus. After recovery, mice were microinjected with KA (0.39 µg in 0.26 µL) to induce SE. Three days following SE, mice were connected via a tether and video/EEG monitored 24 hours per day, 7 days per week, to assess frequency of SRSs. Separate cohorts of mice were used to evaluate each drug. After 3-4 weeks, a cohort of mice experiencing SRSs were randomly divided into two groups and blindly received i.p. injections of the test drug or vehicle over 5 days. Following a 2-day washout, groups were crossed over and treated again for 5 days, with mice that were previously treated with vehicle now receiving drug, and vice versa. SRS data was analyzed manually and drug effects on SRS frequency and seizure freedom were statistically compared to vehicle. Drugs/doses tested were phenytoin (20 mg/kg, b.i.d), carbamazepine (30 mg/kg, t.i.d.), valproate (240 mg/kg, t.i.d), and diazepam (0.4 mg/kg, b.i.d). Dosing parameters were based on other internal studies.  Results: 192 mice were used in this study. Within 14 days post KA-induced SE, 105 mice (55%) developed SRSs, 44 mice (23%) did not develop SRSs, and 43 mice (22%) died. 80 of the mice that developed SRSs were used to assess ASDs. Both valproate (N=13) and diazepam (N=24) significantly attenuated SRS frequency relative to vehicle controls (p<0.01, Mann-Whitney Test) at doses devoid of observable adverse behavioral effects. Only diazepam significantly increased seizure freedom (p<0.0001, Fisher's exact test). Neither phenytoin (N=21) nor carbamazepine (N=22) significantly altered SRS frequency or freedom under these experimental conditions. Conclusions: The data presented here demonstrates that SRSs in this IAK model of temporal lobe epilepsy may be pharmacoresistant to two representative sodium channel-inhibiting ASDs (phenytoin and carbamazepine) but not to a GABA receptor modulating ASD (diazepam) or a mixed-mechanism ASD (valproate). Further work with this experimental paradigm will evaluate its suitability for inclusion in the ETSP pharmacoresistance screening platform. Funding: NINDS HHSN271201600048C