Decreased brainstem seizure thresholds and facilitated seizure propagation in mice exposed to repeated flurothyl-induced generalized forebrain seizures

Abstract

We recently have described a novel model of epileptogenesis utilizing the inhalant chemoconvulsant, flurothyl (Applegate et al., 1997; Samoriski and Applegate, 1997). The hallmark feature of this model is a change in behavioral seizure phenotype from a forebrain seizure, observed during the initial flurothyl exposures, to a brainstem seizure, elicited by flurothyl, after a 28-day stimulation free incubation period. In this study, we sought to establish the basis for this change in behavioral seizure response. To this end, we examined the effects of exposure to this paradigm on the generalized brainstem seizure threshold and on the propagation of forebrain seizures onto the brainstem seizure substrate. Ten mice were given flurothyl-induced generalized forebrain seizures on 8 consecutive days (induction phase). The other ten mice were not exposed to the flurothyl induction paradigm and served as controls. Minimal corneal electroconvulsive shock (mECS--20 mA) was used to assay whether there was any change in the animals' generalized brainstem seizure thresholds at 3, 14 and 28 days following the last flurothyl seizure trial. Mice that were exposed to flurothyl exhibited a progressive increase in the percentage of animals having a mECS-induced brainstem seizure when tested at 3 (40%), 14 (70%) and 28 (90%) days following the last flurothyl seizure. Control mice rarely had a brainstem seizure at any of the three time points tested, mostly forebrain seizures were observed. These results suggest that there is a significant progressive lowering of the brainstem seizure threshold, during the incubation phase of the flurothyl paradigm, which is coincident with the previously reported time course of change in the behavioral seizure phenotype observed using this flurothyl model (Applegate et al., 1997; Samoriski and Applegate, 1997). Following mECS testing, mice were implanted with bipolar electrodes and kindled from the olfactory bulb (OB). Mice exposed to the flurothyl paradigm demonstrated significantly faster kindling rates, longer afterdischarge durations. and longer durations of and latencies to stage 5 seizures compared to controls. Furthermore, animals exposed to the flurothyl protocol demonstrated an increase in the expression of brainstem seizures after focally-elicited OB afterdischarges. These results suggest that there is an increased interaction between the forebrain and brainstem seizure systems after exposure to this model of epileptogenesis. Together, results indicate that the change in behavioral seizure phenotype observed following exposure to our flurothyl paradigm are promoted by both decreases in brainstem seizure thresholds and facilitated forebrain seizure propagation onto the brainstem seizure system.