Dissociation of spontaneous seizures and brainstem seizure thresholds in mice exposed to eight flurothyl‐induced generalized seizures

Abstract

Summary Objective C57BL/6J mice exposed to eight flurothyl-induced generalized clonic seizures exhibit a change in seizure phenotype following a 28-day incubation period and subsequent flurothyl rechallenge. Mice now develop a complex seizure semiology originating in the forebrain and propagating into the brainstem seizure network (a forebrain→brainstem seizure). In contrast, this phenotype change does not occur in seizure-sensitive DBA/2J mice. The underlying mechanism(s) was the focus of these studies. Methods DBA2/J mice were exposed to eight flurothyl-induced seizures (1/day) followed by 24-hour video-electroencephalographic recordings for 28-days. Forebrain and brainstem seizure thresholds were determined in C57BL/6J and DBA/2J mice following one or eight flurothyl-induced seizures, or after eight flurothyl-induced seizures, a 28-day incubation period, and final flurothyl rechallenge. Results Similar to C57BL/6J mice, DBA2/J mice expressed spontaneous seizures. However, unlike C57BL/6J mice, DBA2/J mice continued to have spontaneous seizures without remission. Because DBA2/J mice do not express forebrain→brainstem seizures following flurothyl rechallenge after a 28-day incubation period, this indicated that spontaneous seizures were not sufficient for the evolution of forebrain→brainstem seizures. Therefore, we determined whether brainstem seizure thresholds were changing during this repeated-flurothyl model and whether this could account for the expression of forebrain→brainstem seizures. Brainstem seizure thresholds were not different between C57BL/6J and DBA/2J mice on day one or on the last induction seizure trial (day eight). However, brainstem seizure thresholds did differ significantly on flurothyl rechallenge (day 28) with DBA/2J mice showing no lowering of their brainstem seizure thresholds. Significance These results demonstrated that DBA/2J mice exposed to the repeated-flurothyl model develop spontaneous seizures without evidence of seizure remission and provide a new model of epileptogenesis. Moreover, these findings indicated that the transition of forebrain ictal discharge into the brainstem seizure network occurs due to changes in brainstem seizure thresholds that are independent of spontaneous seizure expression.

The relevance of inter- and intrastrain differences in mice and rats and their implications for models of seizures and epilepsy

“…Together, these data suggest that mouse strain rather than sex explains our findings on strain differences in the latent period of the intrahippocampal kainate model [101]. Lastly, further support for a lack of a latent period in mice comes from studies utilizing 8 repeated flurothyl-induced seizures, in which both B6J and DBA2/J mice develop SRS with no latent period [110, 111]. …”

“…Recently,the repeated flurothyl seizure model has been shown to produce SRS in both B6J and DBA2/J mice without an obvious latent period and without hippocampal cell death [110,111]. These SRS could be blocked with valproate, but not with phenytoin [110].…”

“…These SRS could be blocked with valproate, but not with phenytoin [110]. Interestingly, the spontaneous seizures observed in B6J mice following flurothyl kindling remitted without any treatment after 1 month, but did not remit in DBA/2J mice; however B6J mice tended to have more spontaneous seizures than DBA/2J mice [110,111]. Not only does this implicate a genetic influence on whether seizures remit with flurothyl kindling, but also allows for the possibility that differing inbred strains of mice have differential spontaneous seizure characteristics that are amenable to genetic mapping.…”

The Repeated Flurothyl Seizure Model in Mice

“…However, these seizures begin to remit over the 28-day incubation phase (Kadiyala et al , 2016). Interestingly, DBA/2J mice similarly treated also develop spontaneous seizures with fewer seizure occurrences than C57BL6/J mice, but the spontaneous seizures observed in DBA/2J mice do not appear to remit (Kadiyala and Ferland, 2017). …”

“…We refer to these seizures as forebrain→brainstem seizures denoting the ictal progression from the forebrain seizure network to the brainstem seizure network (Papandrea et al ., 2009; Kadiyala et al ., 2015). Lastly, C57BL6/J mice exposed to the repeated flurothyl seizure model rapidly develop spontaneous seizures that appear to remit without treatment following 1 month (Kadiyala et al ., 2016), in contrast to DBA2/J mice which also rapidly develop spontaneous seizures that do not remit (Kadiyala and Ferland, 2017). Here, we describe the methods for assaying mice in the repeated flurothyl seizure model, which was originally described 20 years ago (Applegate et al ., 1997; Samoriski and Applegate, 1997) and continues to be characterized.…”