Abstracts

Rationale: Mechanism of seizure spread and activation of specific nuclei and cortical regions during the prolonged seizure activity of SE are poorly understood. We hypothesize that GluA1 AMPA receptor plasticity is critical for seizure spread during SE. Methods: We used transgenic mice in which targeted recombination in active populations of neurons (TRAP) leads to expression of the reporter protein (tdTomato) in the neurons activated in a small window following activity-inducing stimulus. We used passive tissue clearing along with advanced microscopy to visualize tdTomato-expressing neurons at various points in the SE induced by continuous hippocampal stimulation (CHS). We also determined patterns of neuronal activation in mice lacking the expression of GluA1 subunit of AMPARs (GluA1-/-) using endogenous cfos labeling. Results: CHS for 60 min triggered continuous self-sustaining seizures that generally lasted for 3 hrs. Initially, tdTomato-expressing neurons were mainly present around the electrodes. After an hour of continuous seizure activity, hippocampal DGCs, CA3, CA1, and subicular neurons expressed tdTomato. Maximum activation of neurons was observed at 2 and 3 hrs of SE; at these times neurons in the lateral entorhinal cortex, lateral septum, and olfactory cortex also expressed tdTomato. The afterdischarges became irregular after 3 hr and tdTomato-expressing neurons were restricted to CA1, subiculum, lateral entorhinal cortex, and lateral septum in animals that experienced 4 hr of SE. The number of activated DGCs was highest during the 2nd and 3rd hour of SE and their activation was sparse during the 4th hour of SE. The endogenous cfos expression in GluA1+/+ mice followed a pattern similar to that of tdTomato expression seen in TRAP mice. Only a small fraction of GluA1-/- mice experienced SE; in these animals the cfos labeling was restricted to hippocampus. Activation of neurons in lateral septum, lateral entorhinal cortex, and olfactory cortex was not observed. The SE was also of a significantly shorter duration in GluA1-/- mice. A prior study has found that GluA1 subunit expression is mainly localized to hippocampus, lateral entorhinal cortex, lateral septum, and olfactory cortex. Conclusions: We have mapped the seizure spread during SE at a single cell level. These studies revealed continuous activity of hippocampus throughout SE. Seizures spread from hippocampus to structures known to express GluA1 subunit and then receded. We hypothesize that seizure spread during SE occurs along neuronal pathways expressing GluA1 subunit-containing AMPARs. Funding: NIH RO1