Abstracts

Seizures may injure the brain and contribute to neurological and cognitive deficits of epilepsy patients. Although it is well-established that seizures may cause neuronal death under some conditions, seizures may also induce more subtle [quot]non-lethal[quot] pathophysiological changes in neuronal structure and function, including abnormalities in synaptic transmission. In particular, seizures have been shown to cause morphological changes in dendrites and dendritic spines, but the molecular mechanisms mediating these structural changes are poorly understood. Actin represents a major structural protein of dendrites and actin filaments (F-actin) can be depolymerized by the regulatory molecule, cofilin, leading to structural changes in dendrites. Thus, we examined changes in F-actin and cofilin in hippocampus due to 4-aminopyridine (4-AP)-induced seizure activity [italic]in vivo[/italic] and [italic]in vitro.[/italic], Seizures were induced with intrahippocampal injections of 4-AP in mice [italic]in vivo[/italic]. Epileptiform activity was elicited by superfusion of 4-AP over hippocampal slices [italic]in vitro[/italic]. The gross morphology of dendrites and the number of dendritic spines following one hour of seizure activity was assessed by confocal imaging of fixed hippocampal sections from mice with [italic]in vivo[/italic] seizures or by time-lapse multiphoton imaging of living hippocampal slices during [italic]in vitro[/italic] epileptiform activity. Changes in F-actin levels in hippocampus following seizures was assessed by the phalloidin-rhodamine immunolabeling technique. Total actin, total cofilin, and phosphorylated (inactive) cofilin (p-cofilin) were measured following seizures by Western blotting., Following one hour of [italic]in vivo[/italic] or [italic]in vitro[/italic] seizure activity, there was no significant difference in the gross morphological properties of dendrites and number of dendritic spines in hippocampus. However, hippocampal seizures led to a significant decrease (30%) in F-actin levels, indicating a depolymerization of actin filaments. Correspondingly, seizure activity caused a significant decrease (64%) in p-cofilin, but no change in total cofilin levels, implying an increase in unphosporylated (active) cofilin, which should depolymerize F-actin. By comparison, elevated extracellular potassium (50 mM KCl) induced a rapid swelling and loss of spines in hippocampal slices and a strong decrease (93%) in p-cofilin., Seizures may induce activation of cofilin and depolymerization of F-actin. Although no gross changes in dendritic structure or spine number were observed with seizures over a short time period, these changes in actin dynamics with seizures could be related to dendritic spine loss over a longer time period, more subtle changes in spine morphology, or non-structural roles of actin in synaptic transmission. Additional studies are required to define the functional and clinical relevance of changes in actin polymerization in seizure-induced brain injury., (Supported by NIH K02 NS045583 (MW).)