Convulsive status epilepticus duration as determinant for epileptogenesis and interictal discharge generation in the rat limbic system

Abstract

We analyzed with EEG-video monitoring the epileptic activity recorded during the latent and chronic periods in rats undergoing 30 or 120 min pilocarpine-induced convulsive status epilepticus (SE). Interictal discharges frequency in the entorhinal cortex (EC) of animals exposed to 120 min SE was significantly higher in the chronic than in the latent period. Following seizure appearance, interictal spikes diminished in duration in the CA3 of the 120 min SE group, and occurred at higher rates in the amygdala in all animals. Rats exposed to 120 min SE generated shorter seizures but presented twice as many non-convulsive seizures per day as the 30 min group. Finally, seizures most frequently initiated in CA3 in the 120 min SE group but had similar onset in CA3 and EC in the 30 min group. These findings indicate that convulsive SE duration influences the development of interictal and ictal activity, and that interictal discharges undergo structure-specific changes after seizure appearance.

Fig. 1

A. EEG recordings illustrating interictal activity in a rat that survived 30 min SE induced by pilocarpine injection. A similar EEG pattern was observed in animals that experienced 120 min SE. a. EEG obtained from the neocortex, EC, CA3 and amygdala showing interictal discharges during the latent period. b. Expanded traces of the region marked by the rectangles showing monophasic and biphasic interictal events. Vertical dashed lines show that interictal activity starts at the same time in all recorded brain areas. B. Histograms representing the distribution of interictal spikes in CA3 in a. rat exposed to 30 min and b. 120 min SE; the distribution suggests two types of interictal discharges that were observed in all recorded brain areas. C. Bar graph representing interictal activity frequency during the latent period. Results are expressed as mean±SEM. D. Duration of interictal discharges measured in EC, CA3 and amygdala for rats experiencing 30 min (n=6) and 120 min (n=7) SE. Results are expressed as a mean±SEM.

Fig. 2

A. EEG recordings obtained from the chronic period representing interictal activity in a rat that experienced 30 min SE. The same EEG pattern was recorded in animals experiencing 120 min SE. a. EEG traces obtained from the neocortex, EC, CA3 and amygdala showing that interictal discharges occur at irregular intervals. b. Expanded traces of the region marked by the rectangles representing monophasic and biphasic interictal events. Note that vertical dashed lines show that interictal events have the same onset time in all brain areas. B. Histograms showing interictal discharge distributions in CA3 in rat exposed to a. 30 min and to b. 120 min SE. The distribution suggests two types of interictal events that were observed in all recorded brain areas and in all animals. C. The frequency of interictal discharges from rats in the 30 and 120 min SE group during the chronic period. Note that interictal events are more frequent in EC in animals that survived longer SE. Results are expressed as a mean±SEM, *p<0.05, LSD test after significant two-way ANOVA. D. Bar graph representing interictal discharges duration for rats that experienced 30 min (n=6) and 120 min (n=7) SE. Note that there are no statistically significant differences between the two groups.

Fig. 3

A spontaneous non-convulsive seizure (stage 2 of Racine’s scale) after pilocarpine treatment in a rat that experienced a 30 min SE. A. EEG recordings representing the electrographic seizure in the neocortex, EC, CA3 and amygdala. Arrows indicate the end of ictal discharges. B. Expanded traces of an ictal discharge showing a. and b. seizure initiation marked in each recorded brain area by a triangle and c. the first behavioral sign of stage 2 pointed by an asterisk.

Fig. 4

EEG traces obtained from a rat that experienced 30 min SE showing a spontaneous convulsive seizure at stage 5 of Racine’s scale. A. EEG recordings representing the pattern of a seizure in the neocortex, EC, CA3 and amygdala. The end of the seizure is marked by arrows in each brain structure. B. Expanded traces of the ictal discharge in all recorded brain areas showing a. seizure initiation (the onset of the seizure is marked with triangles) and b. the first convulsive manifestation of stage 5, marked with an asterisk, that appears right before the high-amplitude events.

Fig. 5

A. a. Non-convulsive and convulsive seizure frequency for rats experiencing 30 and 120 min SE. For each group n=10 and *p<0.05, **p<0.01, Student’s t-test. Insert: bar graph showing the number of seizures per day and night periods. Note that animals that survived longer SE exhibited much more seizures per any time period. b. Percentage of convulsive and non-convulsive seizures for rats that experienced 30 and 120 min SE. Significance with *p<0.05 for non-convulsive seizures between two groups of rats (Student’s t-test). B. Histogram showing the average seizure duration per day, night and 24 h for rats that experienced 30 min (n=10) and 120 min SE (n=10) induced by pilocarpine injection. Results are expressed as a mean±SEM; **p<0.01. C. First day of the chronic period. Bar graphs showing a. seizure number and b. seizure duration in animals exposed to 30 min (n=10) and 120 min SE (n=10) induced by pilocarpine injection. Values represent mean±SEM; *p<0.05, **p<0.01. D. Bar graph showing for both groups of rats the total time spent in seizures during the entire recording period; *p<0.05. E. Histogram showing the number of seizures per rat per day. Note that the number of seizures tends to decrease over time for animals exposed to 30 and 120 min SE. First day on x-axis represents the first day of the chronic period.

Fig. 6

A. a. Compressed spectral array display showing non-convulsive seizure at stage 2 in all recorded brain areas. The first behavioral sign observed in animal experiencing stage 2 is pointed with an asterisk. b. Expanded spectral array display showing onset of the non-convulsive seizure marked with triangles. B. a. Compressed spectral array display representing convulsive seizure at stage 5 in all recorded brain structures. The first convulsive manifestation of stage 5 is pointed with an asterisk. b. Expanded spectral array display showing initiation of convulsive seizure. Note that the seizure onset is marked with triangles C. Onset time of electrographic seizures. Values refer to the percentage of seizures that have origin in EC, CA3 or amygdala in rats that experienced 30 min (n=8) and 120 min (n=8) SE. Note that for rats in the 30 min SE group seizures start in EC and CA3, while for the 120 min SE group seizures were first observed in CA3. Data represent mean±SEM; ^p<0.05 between amygdala vs. CA3 in rats experiencing 120 min SE, ⁺p<0.05 between amygdala vs. EC in animals that survived 30 min SE; Fisher (LSD) test.

Fig. 7

Number of spontaneous seizures per day during the chronic period in animals treated with pilocarpine. ‘1’ on x-axis represents the first day of the appearance of electrographic seizures. The same percentage of rats experiencing 30 and 120 min SE exhibited A. a cyclic pattern of seizures, B. the first cluster of seizures up to the ninth day and then any ictal discharge up to the third week of recordings and C. the gradual increase of seizures up to the seventeenth day of the chronic period. Each bar graph represents data obtained from one experiment.