The antioxidative and neuroprotective effects of neurosteroids in pilocarpine-induced status epilepticus mouse model

Abstract

Seizures are defined as symptom complexes precipitated by abnormal and excessive neuronal discharges, whereas epilepsy is defined as a condition of enduring predisposition of seizure recurrences. Mechanisms of either seizure occurrence (ictogenesis) or inducing epilepsy (epileptogenesis) are still unknown. However, it has been well documented that a brain insult precipitates acute neuronal death followed by a complex process of brain recovery to establish altered hyperexcitable neuronal networks, which are associated with gliosis, synaptic reorganization, inflammation, altered neurotransmitters and receptors, etc., Status epilepticus (SE) precipitated by either chemicals or electrical stimulation in mice are the most widely used animal models of acquired temporal lobe epilepsy. SE induces acute cell loss, in which excessive formation of ROS caused by excessive neuronal hyperactivities play a major role. In normal physiological condition, cells constantly produce ROS but, at the same time, they have proper antioxidant defense system operating in balance to prevent any excessive oxidative stresses. The antioxidant defense system consists of various free radical scavenging enzymes, such as catalase, superoxide dismutase (SOD), glutathione peroxidase (GP), as well as numerous non-enzymatic antioxidants such as glutathione. The balance between ROS and its scavenging systems are disturbed in various pathologic conditions, which may include both acute cerebral injuries and chronic conditions like neurodegenration. In SE-mouse models, production of ROS is rapidly increased by repetitive ictal discharges in the hippocampal region, which may precipitate acute neuronal death in this structure. However, the degree of neuronal injury may be determined by the balance between ROS and its defense mechanism in individual subregions of hippocampus, which may explain the phenomenon of selective vulnerability of hippocampal subregions. In the pilocarpine-induced SE mouse model, neuropathological investigations clearly demonstrated severe neuronal death associated with the increased lipid peroxidation and free radical formation and the decreased glutathione content,1-3 which was most pronounced in the hippocampus.4 Especially, pyramidal neurons in CA1 and CA3 regions of hippocampus are highly vulnerable to damage whereas CA2 and DG regions escape from severe neuronal injury.5,6 Neurosteroid is a family of steroid being synthesized and metabolized by reductase in the central nervous system, which has a wide range of potential clinical applications ranging from a sedative drug to the treatment of epilepsy and traumatic brain injury.7,8 It was found that progesterone and deoxycorticosterone (DOC) carry anticonvulsant effect,9,10 which is mediated by their metabolites, allopregnanolone and tetrahydro-DOC (THDOC). Neurosteriods including allopregnanolone bind to GABAA receptor to enhance inhibitory effects on brain activity. Allopregnanolone has powerful anticonvulsive effect shown in various rodent seizure models.11 They may also exert effects on gene expression by intracellular steroid hormone receptors. However, neuroprotective effect of allopregnanolone in relation with ROS-mediated acute neuronal death has not been investigated yet.In this study, the neuroprotective effect of allopregnanolone treatment in relationship with the expression of superoxide anion and its scavenger enzyme, superoxide dismutase (SOD) in the hippocampus was investigated in a pilocarpine–induced SE mouse model.