Reactive oxygen species in status epilepticus

Abstract

It has long been recognized that status epilepticus can cause considerable neuronal damage, and this has become one of its defining features. The mechanisms underlying this damage are less clear. Excessive activation of NMDA receptors results in large rises in internal calcium, which eventually lead to neuronal death. Between NMDA receptor activation and neuronal death are a number of intermediary steps, key among which is the generation of free radicals and reactive oxygen and nitrogen species. Although it has long been thought that mitochondria are the primary source for reactive oxygen species, more recent evidence has pointed to a prominent role of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, an enzyme localized in cell membranes. There is burgeoning in vivo and in vitro evidence that therapies that target the production or removal of reactive oxygen species are not only effective neuroprotectants following status epilepticus, but also potently antiepileptogenic. Moreover, combining therapies targeted at inhibiting NADPH oxidase and at increasing endogenous antioxidants seems to offer the greatest benefits.

Keywords: excitotoxicity; free radicals; mitochondria; reactive oxygen species; status epilepticus.

FIGURE 1

Mechanisms leading to neuronal death following activation of NMDA receptors. NMDA receptor activation and calcium entry activate several enzymes including calpains and nitric oxide synthase (NOS). NADPH oxidase (NOX) is activated by calcium entry but also by nonionotropic effects of NMDA receptor activation. Reactive oxygen species and nitric oxide form peroxynitrite, which is toxic to DNA, proteins, and lipids. Calcium from the cytosol is taken up by the mitochondria, and excessive mitochondrial calcium load results in decreased ATP production, energy failure, and failure to maintain cellular ionic gradients. Mitochondrial calcium accumulation and reactive oxygen species contribute to the formation of the mitochondrial permeability transition pore (mPTP), which further disrupts mitochondrial function, but also permits cytochrome c into the cytosol where it can activate programmed necrosis (From [ ¹² ])

FIGURE 2

The mitochondria electron transport chain generates superoxide and hydrogen peroxide. This is a simplified diagram of the electron transport chain on the inner mitochondrial membrane, consisting of four complexes (I–IV), ubiquinone (UQ), and cytochrome C (CytC). The green dotted line shows the direction of electron (e⁻) flow along the chain. Energy from the electron transport is used to pump protons (H⁺) across the mitochondrial membrane. The proton gradient is used by Complex V (ATP synthase) to generate ATP. Leakage of electrons from the complexes I and III generates superoxide (O₂ ^−•), which is then converted by superoxide dismutases (SOD1 and SOD2) to hydrogen peroxide (H₂O₂)

FIGURE 3

Combination of NADPH oxidase (NOX) inhibition and Nrf2 activation suppresses the development of epilepsy following status epilepticus in rats. A, Bar charts of seizure frequency (seizures\week; mean ± SEM) of animals following KA‐induced status epilepticus (2 h), treated immediately after termination of SE (Diazepam 5 mg/kg) with single administration of either vehicle (10% DMSO/saline; n = 10), RTA 408 25 mg/kg (n = 7), AEBSF 50 mg/kg (n = 7) or RTA 408 25 mg/kg, and AEBSF 50 mg/kg (n = 10). F _(3,300) = 8.005, P < 0.001 by generalized log‐linear mixed model followed by sequential Bonferroni post hoc test. *P < 0.05, vs. Vehicle; #P < 0.05, RTA 408 + AEBSF vs. RTA 408; ^P < 0.05, RTA 408 + AEBSF vs. AEBSF. B, Cumulative number of seizures of animals in A. Data are plotted on a logarithmic scale after incrementing each total seizure count by one to avoid zero values. *P < 0.05, **P < 0.01, ***P < 0.001, Student's t‐test (C) Combination therapy increases the latent period, F _(3,24) = 11.197, P < 0.001, by one‐way ANOVA followed by Tukey's post hoc, ***P < 0.001 vs. KA + Vehicle group. D, The pie charts illustrate percentage of animals seizure‐free for the whole study and for the last 5 weeks following KA‐induced status epilepticus treated with either vehicle (left) or with combination of RTA 408 (25 mg/kg) and AEBSF (50 mg/kg) (right) (From Ref. [ ³³ ])