Brief seizures cause dendritic injury
Highlights
► Seizures cause acute dendritic beading and loss of dendritic spines of cortical neurons in mice. ► The severity of dendritic injury is dependent on the duration of the seizures. ► Even brief (< 5 min) seizures cause some moderate dendritic injury. ► Dendritic injury from brief seizures recovers over 2 weeks, but status epileptus may cause irreversible dendritic damage.
Introduction
Seizures may directly induce brain injury and potentially contribute to neurological and cognitive deficits that frequently occur in epilepsy patients. While seizures can cause neuronal death in some situations, they may also have deleterious effects on neuronal structure and function via a variety of “non-lethal” mechanisms. Dendritic spines represent the main anatomic sites of contact for excitatory, glutamatergic synaptic inputs onto cortical neurons and are strongly implicated in mechanisms of synaptic plasticity and learning. A loss of dendritic spines in neocortex or hippocampus have been observed in pathological specimens from human epilepsy patients (Isokawa and Levesque, 1991, Multani et al., 1994, Scheibel et al., 1974) and animal seizure models (Drakew et al., 1996, Isokawa, 1998, Jiang et al., 1998, Muller et al., 1993, Olney et al., 1983), suggesting that dendritic spine loss could represent a pathological substrate of memory deficits and other cognitive dysfunction in epilepsy (Swann et al., 2000, Wong, 2005). However, it is difficult to determine the direct contribution of seizures to dendritic injury, as well as the time course of these changes, based on conventional pathological studies of fixed tissue alone. Recently modern cellular imaging techniques have assessed the direct effects of seizures (Mizrahi et al., 2004, Rensing et al., 2005, Zeng et al., 2007) and ischemia (Risher et al., 2010, Zhang et al., 2005) on dendritic structure in living mice on a very rapid time scale. In particular, kainate-induced status epilepticus can trigger an acute beading of dendrites and loss of dendritic spines of neocortical neurons within minutes (Zeng et al., 2007). However, whether brief seizures produce similar dendritic changes is unknown, and the long-term consequences of this acute dendritic injury have not been investigated.
In this study, we utilized in vivo multiphoton imaging to examine the dependence of dendritic injury on seizure duration and follow the long-term evolution of seizure-induced dendritic changes over several weeks. Interestingly, even brief seizures, lasting less than 5 min, caused some degree of dendritic spine loss, although this was reversible over time. In contrast, status epilepticus produced severe dendritic injury that persisted for several weeks through the duration of the study.
Section snippets
Animals
Two- to three-month-old transgenic mice with a C57BL/6 background expressing enhanced green fluorescent protein (GFP) under a thy1 promoter (line GFP-M) were used for all experiments (Feng et al., 2000). In neocortex, GFP-M mice exhibit expression of GFP in a subpopulation of pyramidal neurons, primarily in cortical layer 5 and, to a lesser extent, layer 2/3. The C57BL/6 genetic background was advantageous for these studies, as this strain of mice is resistant to kainate-induced neuronal death
Acute dendritic injury occurs following brief seizures and depends on seizure duration
To determine the effect of seizure duration on dendritic injury, we first examined acute dendritic changes between 0 and 4 h after kainate-induced seizures of various cumulative durations (< 5 min, 5–10 min, 10–20 min, 20–30 min and 30–45 min) based on EEG recordings. Following kainate injection, seizures exhibit a typical pattern of development and progression, usually starting with brief, intermittent individual seizures (Fig. 1A) and then quickly transitioning into longer, more frequent seizures
Discussion
Recent studies utilizing in vivo multiphoton imaging have demonstrated very rapid structural changes in dendrites, including dendritic beading and spine loss, which occur immediately following status epilepticus and in the absence of neuronal death (Zeng et al., 2007). However, the effects of shorter seizures and the long-term evolution of this dendritic injury are not known. In the present study, we demonstrated the surprising and important finding that even brief seizures, less than 5 min in
Acknowledgments
This work was supported by the National Institutes of Health (R01 NS056872 to MW; NIH Neuroscience Blueprint Core Grant NS057105 to Washington University), Citizens United for Research in Epilepsy, and the Alafi Family Foundation.
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