Abstracts

Rationale: Seizures lead directly to brain injury by affecting the structure and function of dendrites, which may contribute to cognitive deficits and progressive epileptogenesis. Our previous work showed that even brief kainate-induced seizures cause acute dendritic injury in mice, including spine loss and dendritic beading, and the severity of dendritic injury is dependent on seizure duration. However, the long-term consequences of this acute dendritic injury have not been investigated. In this study, we monitored the evolution of this dendritic injury over a 6 week period in vivo, comparing the effects of brief seizures (<5 min) and status epilepticus (30-45 min).Methods: In vivo multiphoton imaging was performed in adult transgenic mice expressing green fluorescent protein (GFP) in neocortical neurons. A cranial window sealed by a cover slip was created over neocortex and screw electrodes were placed adjacent to the cranial window to record EEG. GFP-expressing dendrites from neocortical neurons were imaged with a Zeiss multiphoton microscope. After pre-seizure images were obtained, seizures were induced by i.p. injection of 20 mg/kg kainate. After different cumulative durations of EEG seizures (<5min and 30-45 min), seizures were terminated by isoflurane anesthesia for subsequent post-seizure imaging at 0 hr and 4 hr. The mice were then housed and followed with post-seizure time-lapse imaging for 6 weeks. Control mice received saline injections. Post-hoc image analysis evaluated changes in the number of dendritic spines and the beading of dendrites. The beading was categorized as no beading, mild beading, or severe beading, based on previous criteria.Results: There was no significant spine loss in control mice over a 3 week period, although spine number decreased by <7% at 4 and 6 weeks. Brief seizures caused ~35% spine loss, whereas status epilepticus caused ~80% spine loss immediately following seizure termination. The spine loss from brief seizures partially recovered over 4 hours and fully recovered to control levels within 2 weeks. However, status epilepticus caused more severe spine loss that only partially recovered over 6 weeks of observation. In parallel, the evolution of dendritic beading over 6 weeks showed a similar pattern as spine loss. In control mice, only minimal mild beading was observed over 6 weeks. Brief seizures caused mild beading in 40% of all dendrites and severe beading in 3% of dendrites immediately after termination of seizures; and the dendritic beading recovered almost completely after 6 weeks. Status epilepticus caused beading in all dendrites, with severe beading in most dendrites (72%) immediately after termination of seizure. By 6 weeks, about one-third of dendrites with severe beading recovered completely, but the remaining two-thirds changed to mild beading, which showed no signs of further recovery.Conclusions: Dendritic injury from brief seizures usually recovers within two weeks, whereas status epilepticus-induced injury only partially reverses. These studies demonstrate that seizures of all durations may trigger at least transient neuronal injury.