Hydrogen peroxide opposes the hypoxic depression of evoked synaptic transmission in rat hippocampal slices

Therapeutic potential of targeting hydrogen peroxide metabolism in the treatment of brain ischaemia

For many years after its discovery, hydrogen peroxide (H₂O₂) was viewed as a toxic molecule to human tissues; however, in light of recent findings, it is being recognized as an ubiquitous endogenous molecule of life as its biological role has been better elucidated. Indeed, increasing evidence suggests that H₂O₂ may act as a second messenger with a pro-survival role in several physiological processes. In addition, our group has recently demonstrated neuroprotective effects of H₂O₂ on in vitro and in vivo ischaemic models through a catalase (CAT) enzyme-mediated mechanism. Therefore, the present review summarizes experimental data supporting a neuroprotective potential of H₂O₂ in ischaemic stroke that has been principally achieved by means of pharmacological and genetic strategies that modify either the activity or the expression of the superoxide dismutase (SOD), glutathione peroxidase (GPx) and CAT enzymes, which are key regulators of H₂O₂ metabolism. It also critically discusses a translational impact concerning the role played by H₂O₂ in ischaemic stroke. Based on these data, we hope that further research will be done in order to better understand the mechanisms underlying H₂O₂ functions and to promote successful H₂O₂ signalling based therapy in ischaemic stroke.

The protective role of catalase against cerebral ischemia in vitro and in vivo

The present study aims to assess the protective role of the antioxidant enzyme catalase (CAT) with relation to hydrogen peroxide (H(2)O(2)) degradation in oxygen plus water on electrophysiological and fluorescence changes induced by in vitro ischemia and on brain damage produced by transient in vivo ischemia. Neuroprotective effects of CAT were determined by means of electrophysiological recordings and confocal fluorescence microscopy in the hippocampal slice preparation. Ischemia was simulated in vitro by oxygen/glucose deprivation (OGD). In vivo ischemia was produced by transient middle cerebral artery occlusion (MCAo). A protection of the rat CA1 field excitatory postsynaptic potential (fEPSP) loss caused by a prolonged OGD (40 min) was observed after exogenous CAT (500 U/mL) bath-applied before a combined exposure to OGD and H(2)O(2) (3 mM). Of note, neither H(2)O(2) nor exogenous CAT alone had a protective action when OGD lasted for 40 min. The CAT-induced neuroprotection was confirmed in a transgenic mouse model over-expressing human CAT [Tg(CAT)]. In the presence of H(2)O(2), the hippocampus of Tg(CAT) showed an increased resistance against OGD compared to that of wild-type (WT) animals. Moreover, CAT treatment reduced for about 50 min fEPSP depression evoked by repeated applications of H(2)O(2) in normoxia. A lower sensitivity to H(2)O(2)-induced depression of fEPSPs was also indicated by the rightward shift of concentration-response curve in Tg(CAT) compared to WT mice. Noteworthy, Tg(CAT) mice had a reduced infarct size after MCAo. Our data suggest new strategies to reduce neuronal damage produced by transient brain ischemia through the manipulation of CAT enzyme.

H(2)O(2)-mediated modulation of cytosolic signaling and organelle function in rat hippocampus

Reactive oxygen species (ROS) released from (dys-)functioning mitochondria contribute to normal and pathophysiological cellular signaling by modulating cytosolic redox state and redox-sensitive proteins. To identify putative redox targets involved in such signaling, we exposed hippocampal neurons to hydrogen peroxide (H₂O₂). Redox-sensitive dyes indicated that externally applied H₂O₂ may oxidize intracellular targets in cell cultures and acute tissue slices. In cultured neurons, H₂O₂ (EC₅₀ 118 µM) induced an intracellular Ca²⁺ rise which could still be evoked upon Ca²⁺ withdrawal and mitochondrial uncoupling. It was, however, antagonized by thapsigargin, dantrolene, 2-aminoethoxydiphenyl borate, and high levels of ryanodine, which identifies the endoplasmic reticulum (ER) as the intracellular Ca²⁺ store involved. Intracellular accumulation of endogenously generated H₂O₂—provoked by inhibiting glutathione peroxidase—also released Ca²⁺ from the ER, as did extracellular generation of superoxide. Phospholipase C (PLC)-mediated metabotropic signaling was depressed in the presence of H₂O₂, but cytosolic cyclic adenosine-5′-monophosphate (cAMP) levels were not affected. H₂O₂ (0.2–5 mM) moderately depolarized mitochondria, halted their intracellular trafficking in a Ca²⁺- and cAMP-independent manner, and directly oxidized cellular nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH₂). In part, the mitochondrial depolarization reflects uptake of Ca²⁺ previously released from the ER. We conclude that H₂O₂ releases Ca²⁺ from the ER via both ryanodine and inositol trisphosphate receptors. Mitochondrial function is not markedly impaired even by millimolar concentrations of H₂O₂. Such modulation of Ca²⁺ signaling and organelle interaction by ROS affects the efficacy of PLC-mediated metabotropic signaling and may contribute to the adjustment of neuronal function to redox conditions and metabolic supply.

Oxidative stress in stroke pathophysiology validation of hydrogen peroxide metabolism as a pharmacological target to afford neuroprotection

Reactive oxygen species (ROS) accumulation has been described in the brain following an ischemic insult. Superoxide anion is converted by superoxide dismutase into hydrogen peroxide (H2O2), and the latter is then transformed into the toxic hydroxyl radical, through the Haber-Weiss reaction, converted to water by glutathione peroxidase (GPx) or dismuted to water and oxygen through catalase. Accumulation of H2O2 has been suggested to exert neurotoxic effects, although recent in vitro studies have demonstrated either physiological or protective roles of this molecule in the brain. In particular, oxidative stress is critically involved in brain damage induced by transient cerebral ischemia. Here, we demonstrate that inhibition of GPx by systemic (i.p.) administration of mercaptosuccinate (MS, 1.5-150 mg/kg) dose-dependently reduces brain infarct damage produced by transient (2 h) middle cerebral artery occlusion (MCAo) in rat. Neuroprotection was observed when the drug was administered 15 min before the ischemic insult, whereas no effect was detected when the drug was injected 1h before MCAo or upon reperfusion. Furthermore, application of MS (1 mM) to corticostriatal slices limited the irreversible functional derangement of field potentials caused by a prolonged (12 min) oxygen-glucose deprivation. This effect was reverted by concomitant bath application of the catalase inhibitor 3-aminotriazole (20mM), suggesting the involvement of catalase in mediating the neuroprotective effects of MS. Thus, our findings demonstrate that MS is neuroprotective in both in vivo and in vitro ischemic conditions, through a mechanism which may involve increased endogenous levels of H2O2 and its consequent conversion to molecular oxygen by catalase.

Neuroprotective effect of hydrogen peroxide on an in vitro model of brain ischaemia

BACKGROUND AND PURPOSE

Reactive oxygen species (ROS) have been postulated to play a crucial role in the pathogenesis of ischaemia-reperfusion injury. Among these, hydrogen peroxide (H(2)O(2)) is known to be a toxic compound responsible for free-radical-dependent neuronal damage. In recent years, however, the 'bad reputation' of H(2)O(2) and other ROS molecules has changed. The aim of this study was to assess the protective role of H(2)O(2) and modification in its endogenous production on the electrophysiological and morphological changes induced by oxygen/glucose deprivation (OGD) on CA1 hippocampal neurons.

EXPERIMENTAL APPROACH

Neuroprotective effects of exogenous and endogenous H(2)O(2) were determined using extracellular electrophysiological recordings of field excitatory post synaptic potentials (fEPSPs) and morphological studies in a hippocampal slice preparation. In vitro OGD was delivered by switching to an artificial cerebrospinal fluid solution with no glucose and with oxygen replaced by nitrogen.

KEY RESULTS

Neuroprotection against in vitro OGD was observed in slices treated with H(2)O(2) (3 mM). The rescuing action of H(2)O(2) was mediated by catalase as pre-treatment with the catalase inhibitor 3-amino-1,2,4-triazole blocked this effect. More interestingly, we showed that an increase of the endogenous levels of H(2)O(2), due to a combination of an inhibitor of the glutathione peroxidase enzyme and addition of Cu,Zn-superoxide dismutase in the tissue bath, prevented the OGD-induced irreversible depression of fEPSPs.

CONCLUSIONS AND IMPLICATIONS

Taken together, our results suggest new possible strategies to lessen the damage produced by a transient brain ischaemia by increasing the endogenous tissue level of H(2)O(2).

Protective role of hydrogen peroxide in oxygen-deprived dopaminergic neurones of the rat substantia nigra

Hydrogen peroxide (H2O2) is a reactive oxygen species, responsible for cytotoxic damage through the formation of hydroxyl radicals. Dopamine (DA) neurones of the substantia nigra pars compacta (SNc) are highly sensitive to metabolic stress, and they typically respond to energy deprivation with membrane hyperpolarization, mainly through opening of ATP-dependent K+ channels. Accordingly, H2O2 (3 mM) induced a tolbutamide-sensitive outward current in DA neurones. Conversely, in a hypoxic medium, H2O2 reverted membrane hyperpolarization, which is associated with oxygen deprivation in DA neurones, restored their action potential firing, and reduced the hypoxia-mediated outward current in a concentration-dependent manner, between 0.1 and 3 mM (IC50 0.6+/-0.1 mM). Notably, H2O2 did not counteract membrane hyperpolarization associated with hypoglycaemia, moreover, when catalase was inhibited with 3-amino-1,2,4-triazole (3-AT; 30 mM), H2O2 did not reduce hypoxia-mediated outward current. The counteracting action of H2O2 on hypoxia-mediated effects was further confirmed by single-unit extracellular recordings of presumed DA neurones in acute midbrain slices preparations, using a planar multi-electrode array device. Whilst a prolonged period of hypoxia (40 min) caused firing suppression, which did not recover after perfusion in normoxic conditions, the presence of H2O2 (3 mM) during this prolonged hypoxic period rescued most of the neurones from irreversible firing inhibition. Accordingly, morphological studies showed that H2O2 counteracts the cytochrome c release provoked by prolonged hypoxic treatment. Taken together, our data suggest that H2O2 prevents the metabolic stress of DA neurones induced by hypoxia by serving as a supplementary source of molecular oxygen, through its degradation by catalase.

Hydrogen peroxide increases the activity of rat sympathetic preganglionic neurons in vivo and in vitro

Reactive oxygen species (ROS) have been shown to modulate neuronal synaptic transmission and have also been implicated in cardiovascular diseases such as hypertension. The hypothesis that H(2)O(2) acting on sympathetic preganglionic neurons (SPNs) affects spinal sympathetic outflow was tested in the present study. H(2)O(2) was applied intrathecally via an implanted cannula to the T7-T9 segments of urethane-anesthetized rats. Blood pressure and heart rate were used as indices to evaluate the spinal sympathetic effects of H(2)O(2) in vivo. Intrathecal H(2)O(2) (100-1000 nmol) dose-dependently increased both the mean arterial pressure and heart rate. Reproducible pressor effects of H(2)O(2) (1000 nmol) applied consecutively at intervals of 30 min were observed. The pressor effects of intrathecal H(2)O(2) (1000 nmol) were attenuated by pretreatment with intrathecal administration of catalase (500 units), or N-acetyl-cysteine (1000 nmol). The pressor effects of intrathecal H(2)O(2) (1000 nmol) were also antagonized dose-dependently by prior intrathecal injection of AP-5 (DL-2-amino-5- phosphonovaleric acid, 10 and 30 nmol), or 6-cyano-7- nitroquinoxaline-2,3-dione, 10 and 30 nmol. In vitro electrophysiological study in spinal cord slices showed that superfusion of 1 mM H(2)O(2) for 3 min, which had no effect on membrane potential, caused an increase in amplitude of excitatory postsynaptic potentials in SPNs, but had little effect on that of inhibitory postsynaptic potentials. Taken together, these results demonstrated that oxidative stress in spinal cord may cause an increase in spinal sympathetic tone by acting on SPNs, which may contribute to ROS-induced cardiovascular dysfunction.

NMDA receptor activation mediates hydrogen peroxide-induced pathophysiology in rat hippocampal slices

Endogenous reactive oxygen species (ROS) can act as modulators of neuronal activity, including synaptic transmission. Inherent in this process, however, is the potential for oxidative damage if the balance between ROS production and regulation becomes disrupted. Here we report that inhibition of synaptic transmission in rat hippocampal slices by H2O2 can be followed by electrical hyperexcitability when transmission returns during H2O2 washout. As in previous studies, H2O2 exposure (15 min) reversibly depressed the extracellular population spike (PS) evoked by Schaffer collateral stimulation. Recovery of PS amplitude, however, was typically accompanied by mild epileptiform activity. Inclusion of ascorbate (400 microM) during H2O2 washout prevented this pathophysiology. No protection was seen with isoascorbate, which is a poor substrate for the stereoselective ascorbate transporter and thus remains primarily extracellular. Epileptiform activity was also prevented by the N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonopentanoic acid (AP5) during H2O2 washout. Once hyperexcitability was induced, however, AP5 did not reverse it. When present during H2O2 exposure, AP5 did not alter PS depression by H2O2 but did inhibit the recovery of PS amplitude seen during pulse-train stimulation (10 Hz, 5 s) in H2O2. Inhibition of glutamate uptake by l-trans-2,4-pyrrolidine dicarboxylate (PDC; 50 microM) during H2O2 washout markedly enhanced epileptiform activity; coapplication of ascorbate with PDC prevented this. These data indicate that H2O2 exposure can cause activation of normally silent NMDA receptors, possibly via inhibition of redox-sensitive glutamate uptake. When synaptic transmission returns during H2O2 washout, enhanced NMDA receptor activity leads to ROS generation and consequent oxidative damage. These data reveal a pathological cycle that could contribute to progressive degeneration in neurological disorders that involve oxidative stress, including cerebral ischemia.

Mechanisms underlying H(2)O(2)-mediated inhibition of synaptic transmission in rat hippocampal slices

Hydrogen peroxide (H(2)O(2)) inhibits the population spike (PS) evoked by Schaffer collateral stimulation in hippocampal slices. Proposed mechanisms underlying this effect include generation of hydroxyl radicals (.OH) and inhibition of presynaptic Ca(2+) entry. We have examined these possible mechanisms in rat hippocampal slices. Inhibition of the evoked PS by H(2)O(2) was sharply concentration-dependent: 1.2 mM H(2)O(2) had no effect, whereas 1.5 and 2.0 mM H(2)O(2) reversibly depressed PS amplitude by roughly 80%. The iron chelator, deferoxamine (1 mM), and the endogenous.OH scavenger, ascorbate (400 microM), prevented PS inhibition, confirming.OH involvement. Isoascorbate (400 microM), which unlike ascorbate is not taken up by brain cells, also prevented PS inhibition, indicating an extracellular site of.OH generation or action. We then investigated whether H(2)O(2)-induced PS depression could be overcome by prolonged stimulation, which enhances Ca(2+) entry. During 5-s, 10-Hz trains under control conditions, PS amplitude increased to over 200% during the first three-four pulses, then stabilized. In the presence of H(2)O(2), PS amplitude was initially depressed, but began to recover after 2.5 s of stimulation, finally reaching 80% of the control maximum. In companion experiments, we assessed the effect of H(2)O(2) on presynaptic Ca(2+) entry by monitoring extracellular Ca(2+) concentration ([Ca(2+)](o)) during train stimulation in the presence of postsynaptic receptor blockers. Evoked [Ca(2+)](o) shifts were apparently unaltered by H(2)O(2), suggesting a lack of effect on Ca(2+) entry. Taken together, these findings suggest new ways in which reactive oxygen species (ROS) might act as signaling agents, specifically as modulators of synaptic transmission.

Acute peroxide treatment of rat hippocampal slices induces adenosine-mediated inhibition of excitatory transmission in area CA1

Brief exposure to conditions that generate free radicals inhibits synaptic transmission in hippocampal slices, most likely via a presynaptic mechanism. Because other physiologically stressful conditions that generate free radicals, such as hypoxia or ischemia, stimulate the release of adenosine from brain slices, we determined whether increases in extracellular adenosine mediate the presynaptic inhibition of excitatory transmission induced by peroxide treatment. Simultaneous addition of hydrogen peroxide (0.01%) and ferrous sulfate (100 microM) resulted in a >80% decrease in synaptic potentials recorded in the CA1 region of hippocampal slices of adult male rats. Treatment with theophylline (200 microM), a non-selective adenosine receptor antagonist, or 8-cyclopentyl-1,3-dipropylxanthine (100 nM), a selective adenosine A1 receptor antagonist, prior to and during hydrogen peroxide superfusion prevented the inhibition. These results demonstrate that acute exposure to hydrogen peroxide induces an adenosine-mediated decrease in synaptic transmission in hippocampal slices.