The role of superoxide dismutase 1 in the selected vulnerability of hippocampal subfield after status epilepticus

Abstract

Selected vulnerability of hippocampal neuron after epileptic insults is a well-known phenomenon. Pyramidal neurons in the Cornus Ammonis (CA) 1 and 3 are known as particularly vulnerable; however, the granule cells in the dentate gyrus (DG) as well as neurons in the CA2 are known to be resistant to the various insults including status epilepticus (SE) in the animal models or even in the human temporal lobe epilepsy. The molecular basis for this different susceptibility remains controversial, but different defense mechanisms against exogenous insults have been suggested. Excessive reactive oxygen species (ROS) production has been reported as a responsible process for neural cell death in the various neurological disorders such as ischemic stroke, Alzheimer’s disease, or Huntington disease. In addition, the pathological overproduction of ROS has been suggested as a crucial process in the neuronal death by SE. However, the role of superoxide anion (O2？-), one of the most potent ROS, in the seizure-induced neuronal death has remained unclear. I hypothesized that the pathological excessive production of O2？- may contribute to hippocampal neuronal death after SE, and different activation of superoxide dismutase 1 (SOD1), the selective inhibitor of O2？-, in the hippocampal subfields might be responsible for this selected vulnerability after SE. Adult male C57BL/6J mice were given injections of pilocarpine to induce SE, which was confirmed by visual inspection and electroencephalography. Hippocampal neuronal death was assessed by both cresyl-violet and TUNEL staining. The temporal and spatial production of O2？- in each hippocampal subfield was investigated using in situ detection of oxidized hydroethidine (HEt). Western blot, activity assay, and immunohistochemical staining of SOD1 in the each hippocampal subfield were performed to investigate the role of specific defense system of O2？-. Lamotrigine (LTG), a neuroprotectant in the various animal models by modulating ROS, was treated to see its effect on SOD1. Inhibitor of SOD1, diethyldithiocarbamate (DDC) was treated to identify the effect of SOD1 inhibition. Neuronal cell death and TUNEL-positive cells increased significantly in the hippocampal CA1 and CA3 compared to DG after SE. O2？- measured by oxidized HEt significantly increased after pilocarpine-induced SE, especially in the CA1 and CA3 compared to DG. In the normal control, SOD1 expression in the DG was significantly higher than those of CA1 and CA3. The expression and activity of SOD1 significantly increased in the DG compared to CA1 and CA3 as early as 12 hours after SE. Treatment of LTG significantly increased SOD1 activity, expression, and decreased O2？- production, thus ameliorated the selective neuronal death in the CA1 and CA3. DDC treatment resulted in marked decrease of SOD1 activity, increased production of O2？-, and subsequent extensive neuronal cell death in the DG after SE. This study confirmed that the excessive production of O2？- after SE resulted in different neuronal cell death according to the hippocampal subfields, to less degree in the DG compared to CA1 and CA3. Increased expression and activity of SOD1 in the DG and marked neuronal death by SOD1 inhibition were confirmed. The difference defense mechanism against O2？- by SOD1 in each hippocampal subfield may have a pivotal role in the selected vulnerability of hippocampal subfields after SE.