Hydrogen peroxide induces a long-lasting inhibition of the Ca(2+)-dependent glutamate release in cerebrocortical synaptosomes without interfering with cytosolic Ca2+

Hydrogen Peroxide Affects the Electroretinogram of Isolated Perfused Rat Retina

PURPOSE

To evaluate the effects of H2O2 as an oxidant on the electroretinogram (ERG) in isolated rat retina.

METHODS

Retinas were isolated from rat eyes and perfused with a nutrient solution. ERGs were recorded every 3 min. Once the signal was at a steady state, H2O2 was added to the perfusion solution.

RESULTS

H2O2 caused instantaneous and transient changes in amplitudes and implicit times of the ERG, followed by changes in retinal survival curves. H2O2 0.2 mM produced a rapid increase in b-wave amplitude, followed by a return to the initial value and a survival curve above the control (without H2O2). A slight increase in a-wave was observed, followed by a decrease and a recovery above the control. The slow PIII decreased and then recovered to the initial value. H2O2 0.6 mM induced a small increase in b-wave amplitude, followed by a rapid decrease without recovery. The a-wave and slow PIII decreased rapidly without recovery. The implicit times of the a-wave and b-wave increased moderately with a low dose of H2O2, whereas they significantly increased with a high dose. Whatever the dose, the slow PIII implicit time increased significantly, followed by a return to the initial value. Barium increased the a-wave and b-wave, and then H2O2 reduced the two waves with a stronger effect on the a-wave. Aspartate and barium isolated the fast PIII, which decreased after H2O2 application.

CONCLUSIONS

H2O2 affects retinal function as shown by ERGs in isolated rat retina. The response differs with the dose of H2O2, suggesting that mechanisms underlying the action at low doses might be different from those at high doses. Our results also suggest an effect of H2O2 on ionic currents and/or neurotransmitter releases involved in the generation of the ERG and indicate a more pronounced effect on photoreceptors than on postsynaptic cells.

Sympathetic and angiotensinergic activity in spontaneously hypertensive rats treated with 3-amino-1,2,4-triazole

Previous studies from our laboratory have shown that the pressor response to intracerebroventricular (icv) administered ANG II in normotensive rats or spontaneously hypertensive rats (SHRs) is attenuated by increased central H2O2 concentration, produced either by direct H2O2 icv injection or by increased endogenous H2O2 centrally in response to local catalase inhibition with 3-amino-1,2,4-triazole (ATZ). In the present study, we evaluated the effects of ATZ administered peripherally on arterial pressure and sympathetic and angiotensinergic activity in SHRs. Male SHRs weighing 280-330 g were used. Mean arterial pressure (MAP) and heart rate (HR) were recorded in conscious freely moving SHRs. Acute intravenous injection of ATZ (300 mg/kg of body weight) did not modify MAP and HR during the next 4 h, however, the treatment with ATZ (300 mg/kg of body weight twice per day) for 3 days reduced MAP (144 ± 6, vs. saline, 183 ± 13 mmHg), without changing HR. Intravenous hexamethonium (ganglionic blocker) produced a smaller decrease in MAP 4 h after ATZ (-25 ± 3, vs saline -38 ± 4 mmHg). Losartan (angiotensinergic AT1 receptor blocker) produced a significant depressor response 4 h after ATZ (-22 ± 4, vs. saline: -2 ± 4 mmHg) and in 3-day ATZ treated SHRs (-25 ± 5, vs. saline: -9 ± 4 mmHg). The results suggest that the treatment with ATZ reduces sympathetic activity in SHRs and simultaneously increases angiotensinergic activity.

Chronic administration of catalase inhibitor attenuates hypertension in renovascular hypertensive rats

AIMS

Reactive oxygen species like hydrogen peroxide (H₂O₂) are produced endogenously and may participate in intra- and extracellular signaling, including modulation of angiotensin II responses. In the present study, we investigated the effects of chronic subcutaneous (sc) administration of the catalase inhibitor 3-amino-1,2,4-triazole (ATZ) on arterial pressure, autonomic modulation of arterial pressure, hypothalamic expression of AT1 receptors and neuroinflammatory markers and fluid balance in 2-kidney, 1clip (2K1C) renovascular hypertensive rats.

MATERIALS AND METHODS

Male Holtzman rats with a clip occluding partially the left renal artery and chronic sc injections of ATZ were used.

KEY FINDINGS

Subcutaneous injections of ATZ (600 mg/kg of body weight/day) for 9 days in 2K1C rats reduced arterial pressure (137 ± 8, vs. saline: 182 ± 8 mmHg). ATZ also reduced the sympathetic modulation and enhanced the parasympathetic modulation of pulse interval, reducing the sympatho-vagal balance. Additionally, ATZ reduced mRNA expression for interleukins 6 and IL-1β, tumor necrosis factor-α, AT1 receptor (0.77 ± 0.06, vs. saline: 1.47 ± 0.26 fold change), NOX 2 (0.85 ± 0.13, vs. saline: 1.75 ± 0.15 fold change) and the marker of microglial activation, CD 11 (0.47 ± 0.07, vs. saline, 1.34 ± 0.15 fold change) in the hypothalamus of 2K1C rats. Daily water and food intake and renal excretion were only slightly modified by ATZ.

SIGNIFICANCE

The results suggest that the increase of endogenous H₂O₂ availability with chronic treatment with ATZ had an anti-hypertensive effect in 2K1C hypertensive rats. This effect depends on decreased activity of sympathetic pressor mechanisms and mRNA expression of AT1 receptors and neuroinflammatory markers possibly due to reduced angiotensin II action.

Anti-hypertensive effect of hydrogen peroxide acting centrally

Intracerebroventricular (icv) injection of hydrogen peroxide (H₂O₂) or the increase of endogenous H₂O₂ centrally produced by catalase inhibition with 3-amino-1,2,4-triazole (ATZ) injected icv reduces the pressor responses to central angiotensin II (ANG II) in normotensive rats. In the present study, we investigated the changes in the arterial pressure and in the pressor responses to ANG II icv in spontaneously hypertensive rats (SHRs) and 2-kidney, 1-clip (2K1C) hypertensive rats treated with H₂O₂ injected icv or ATZ injected icv or intravenously (iv). Adult male SHRs or Holtzman rats (n = 5-10/group) with stainless steel cannulas implanted in the lateral ventricle were used. In freely moving rats, H₂O₂ (5 μmol/1 μl) or ATZ (5 nmol/1 μl) icv reduced the pressor responses to ANG II (50 ng/1 µl) icv in SHRs (11 ± 3 and 17 ± 4 mmHg, respectively, vs. 35 ± 6 mmHg) and 2K1C hypertensive rats (3 ± 1 and 16 ± 3 mmHg, respectively, vs. 26 ± 2 mmHg). ATZ (3.6 mmol/kg of body weight) iv alone or combined with H₂O₂ icv also reduced icv ANG II-induced pressor response in SHRs and 2K1C hypertensive rats. Baseline arterial pressure was also reduced (-10 to -15 mmHg) in 2K1C hypertensive rats treated with H₂O₂ icv and ATZ iv alone or combined and in SHRs treated with H₂O₂ icv alone or combined with ATZ iv. The results suggest that exogenous or endogenous H₂O₂ acting centrally produces anti-hypertensive effects impairing central pressor mechanisms activated by ANG II in SHRs or 2K1C hypertensive rats.

Catalase blockade reduces the pressor response to central cholinergic activation

Intracerebroventricular (icv) injection of hydrogen peroxide (H₂O₂), a reactive oxygen species, or the blockade of catalase (enzyme that degrades H₂O₂ into H₂O and O₂) with icv injection of 3-amino-1,2,4-triazole (ATZ) reduces the pressor effects of angiotensin II also injected icv. In the present study, we investigated the effects of ATZ injected icv or intravenously (iv) on the pressor responses induced by icv injections of the cholinergic agonist carbachol, which similar to angiotensin II induces pressor responses that depend on sympathoexcitation and vasopressin release. In addition, the effects of H₂O₂ icv on the pressor responses to icv carbachol were also tested to compare with the effects of ATZ. Normotensive non-anesthetized male Holtzman rats (280-300 g, n = 8-9/group) with stainless steel cannulas implanted in the lateral ventricle were used. Previous injection of ATZ (5 nmol/1 μl) or H₂O₂ (5 μmol/1 μl) icv similarly reduced the pressor responses induced by carbachol (4 nmol/1 μl) injected icv (13 ± 4 and 12 ± 4 mmHg, respectively, vs. vehicle + carbachol: 30 ± 5 mmHg). ATZ (3.6 mmol/kg of body weight) injected iv also reduced icv carbachol-induced pressor responses (21 ± 2 mmHg). ATZ icv or iv and H₂O₂ icv injected alone produced no effect on baseline arterial pressure. The treatments also produced no significant change of heart rate. The results show that ATZ icv or iv reduced the pressor responses to icv carbachol, suggesting that endogenous H₂O₂ acting centrally inhibits the pressor mechanisms (sympathoactivation and/or vasopressin release) activated by central cholinergic stimulation.

Endogenous hydrogen peroxide affects antidiuresis to cholinergic activation in the medial septal area

Cholinergic activation of the medial septal area (MSA) with carbachol produces thirst, natriuresis and antidiuresis. Hydrogen peroxide (H₂O₂) injected into the medial septal area (MSA) impairs behavioral and renal responses induced by carbachol at the same site, suggesting the exogenous H₂O₂ may modulate the responses to cholinergic activation in the MSA. In the present study, we investigated if the accumulation of endogenous H₂O₂ in the MSA after the injection of the catalase inhibitor 3-amino-1,2,4-triazole (ATZ) also affects cholinergic responses. In addition, the effects of the combination of ATZ with a non-effective dose of H₂O₂ in the MSA were also tested. Male Holtzman rats (280-320 g) with stainless steel cannulas implanted in the MSA were used. The treatment with ATZ (10 nmol) into the MSA partially reverted the antidiuretic effect of carbachol (10.5 ± 0.7, vs. saline + carbachol: 7.3 ± 0.6 ml/120 min), without changing carbachol-induced water intake (9.5 ± 1.9, vs. saline + carbachol: 10.7 ± 1.6 ml/60 min). The combination of a low dose of ATZ (2.5 nmol) with an ineffective dose of H₂O₂ (0.5 μmol) into the MSA reduced carbachol-induced thirst (7.5 ± 2.0, vs. saline + carbachol: 14.9 ± 1.2 ml/15 min) and reverted the antidiuresis (8.1 ± 1.1, vs. saline + carbachol: 5.3 ± 0.9 ml/120 min). Sodium and potassium excretion were not modified regardless the treatment. Although exogenous H₂O₂ injected in the MSA may affect most of the responses to cholinergic activation of the MSA, the antidiuresis is the response clearly modulated by endogenous H₂O₂.

Decreases in mitochondrial reactive oxygen species initiate GABA(A) receptor-mediated electrical suppression in anoxia-tolerant turtle neurons

Anoxia induces hyper-excitability and cell death in mammalian brain but in the anoxia-tolerant western painted turtle (Chrysemys picta bellii) neuronal electrical activity is suppressed (i.e. spike arrest), adenosine triphosphate (ATP) consumption is reduced, and cell death does not occur. Electrical suppression is primarily the result of enhanced γ-aminobutyric acid (GABA) transmission; however, the underlying mechanism responsible for initiating oxygen-sensitive GABAergic spike arrest is unknown. In turtle cortical pyramidal neurons there are three types of GABA(A) receptor-mediated currents: spontaneous inhibitory postsynaptic currents (IPSCs), giant IPSCs and tonic currents. The aim of this study was to assess the effects of reactive oxygen species (ROS) scavenging on these three currents since ROS levels naturally decrease with anoxia and may serve as a redox signal to initiate spike arrest. We found that anoxia, pharmacological ROS scavenging, or inhibition of mitochondrial ROS generation enhanced all three types of GABA currents, with tonic currents comprising ∼50% of the total current. Application of hydrogen peroxide inhibited all three GABA currents, demonstrating a reversible redox-sensitive signalling mechanism. We conclude that anoxia-mediated decreases in mitochondrial ROS production are sufficient to initiate a redox-sensitive inhibitory GABA signalling cascade that suppresses electrical activity when oxygen is limited. This unique strategy for reducing neuronal ATP consumption during anoxia represents a natural mechanism in which to explore therapies to protect mammalian brain from low-oxygen insults.

Hydrogen peroxide attenuates the dipsogenic, renal and pressor responses induced by cholinergic activation of the medial septal area

Cholinergic activation of the medial septal area (MSA) with carbachol produces thirst, natriuresis, antidiuresis and pressor response. In the brain, hydrogen peroxide (H2O2) modulates autonomic and behavioral responses. In the present study, we investigated the effects of the combination of carbachol and H2O2 injected into the MSA on water intake, renal excretion, cardiovascular responses and the activity of vasopressinergic and oxytocinergic neurons in the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. Furthermore, the possible modulation of carbachol responses by H2O2 acting through K+ATP channels was also investigated. Male Holtzman rats (280-320 g) with stainless steel cannulas implanted in the MSA were used. The pre-treatment with H2O2 in the MSA reduced carbachol-induced thirst (7.9±1.0, vs. carbachol: 13.2±2.0 ml/60 min), antidiuresis (9.6±0.5, vs. carbachol: 7.0±0.8 ml/120 min,), natriuresis (385±36, vs. carbachol: 528±46 μEq/120 min) and pressor response (33±5, vs. carbachol: 47±3 mmHg). Combining H2O2 and carbachol into the MSA also reduced the number of vasopressinergic neurons expressing c-Fos in the PVN (46.4±11.2, vs. carbachol: 98.5±5.9 c-Fos/AVP cells) and oxytocinergic neurons expressing c-Fos in the PVN (38.5±16.1, vs. carbachol: 75.1±8.5 c-Fos/OT cells) and in the SON (57.8±10.2, vs. carbachol: 102.7±7.4 c-Fos/OT cells). Glibenclamide (K+ATP channel blocker) into the MSA partially reversed H2O2 inhibitory responses. These results suggest that H2O2 acting through K+ATP channels in the MSA attenuates responses induced by cholinergic activation in the same area.

Endogenous hydrogen peroxide in the hypothalamic paraventricular nucleus regulates sympathetic nerve activity responses to L-glutamate

The hypothalamic paraventricular nucleus (PVN) is important for maintenance of sympathetic nerve activity (SNA) and cardiovascular function. PVN-mediated increases of SNA often involve the excitatory amino acid L-glutamate (L-glu), whose actions can be positively and negatively modulated by a variety of factors, including reactive oxygen species. Here, we determined modulatory effects of the highly diffusible reactive oxygen species hydrogen peroxide (H(2)O(2)) on responses to PVN L-glu. Renal SNA (RSNA), arterial blood pressure, and heart rate were recorded in anesthetized rats. L-Glu (0.2 nmol in 100 nl) microinjected unilaterally into PVN increased RSNA (P < 0.05), without affecting mean arterial blood pressure or heart rate. Effects of endogenously generated H(2)O(2) were determined by comparing responses to PVN L-glu before and after PVN injection of the catalase inhibitor 3-amino-1,2,4-triazole (ATZ; 100 nmol/200 nl, n = 5). ATZ alone was without effect on recorded variables, but attenuated the increase of RSNA elicited by PVN L-glu (P < 0.05). PVN injection of exogenous H(2)O(2) (5 nmol in 100 nl, n = 4) and vehicle (artificial cerebrospinal fluid) were without affect, but H(2)O(2), like ATZ, attenuated the increase of RSNA to PVN L-glu (P < 0.05). Tonic effects of endogenous H(2)O(2) were determined by PVN injection of polyethylene glycol-catalase (1.0 IU in 200 nl, n = 5). Whereas polyethylene glycol-catalase alone was without effect, increases of RSNA to subsequent PVN injection of L-glu were increased (P < 0.05). From these data, we conclude that PVN H(2)O(2) tonically, but submaximally, suppresses RSNA responses to L-glu, supporting the idea that a change of H(2)O(2) availability within PVN could influence SNA regulation under physiological and/or disease conditions.

SNAP-25 contains non-acylated thiol pairs that can form intrachain disulfide bonds: possible sites for redox modulation of neurotransmission

Intrachain disulfide bond formation among the cysteine thiols of SNAP-25, a component of the SNARE protein complex required for neurotransmitter release, has been hypothesized to link oxidative stress and inhibition of synaptic transmission. However, neither the availability in vivo of SNAP-25 thiols, which are known targets of S-palmitoylation, nor the tendency of these thiols to form intrachain disulfide bonds is known. We have examined, in rat brain extracts, both the availability of closely spaced, or vicinal, thiol pairs in SNAP-25 and the propensity of these dithiols toward disulfide bond formation using a method improved by us recently that exploits the high chemoselectivity of phenylarsine oxide (PAO) for vicinal thiols. The results show for the first time that a substantial fraction of soluble and, to a lesser extent, particulate SNAP-25 contain non-acylated PAO-binding thiol pairs and that these thiols in soluble SNAP-25 in particular have a high propensity toward disulfide bond formation. Indeed, disulfide bonds were detected in a small fraction of soluble SNAP-25 even under conditions designed to prevent or greatly limit protein thiol oxidation during experimental procedures. These results provide direct experimental support for the availability, in a subpopulation of SNAP-25, of vicinal thiols that may confer on one or more isoforms of this family of proteins a sensitivity to oxidative stress.

Inhibition of central angiotensin II-induced pressor responses by hydrogen peroxide

Hydrogen peroxide (H(2)O(2)), important reactive oxygen species produced endogenously, may have different physiological actions. The superoxide anion (O(2)(·-)) is suggested to be part of the signaling mechanisms activated by angiotensin II (ANG II) and central virus-mediated overexpression of the enzyme superoxide dismutase (that dismutates O(2)(·-) to H(2)O(2)) reduces pressor and dipsogenic responses to central ANG II. Whether this result might reflect elevation of H(2)O(2) rather than depletion of O(2)(·-) has not been addressed. Here we investigated the effects of H(2)O(2) injected intracerebroventricularly (i.c.v.) or ATZ (3-amino-1,2,4-triazole, a catalase inhibitor) injected intravenously (i.v.) or i.c.v. on the pressor responses induced by i.c.v. injections of ANG II. Normotensive male Holtzman rats (280-320 g, n=5-13/group) with stainless steel cannulas implanted in the lateral ventricle were used. Prior injection of H(2)O(2) (5 μmol/1 μl) or ATZ (5 nmol/1 μl) i.c.v. almost abolished the pressor responses induced by ANG II (50 ng/1 μl) also injected i.c.v. (7 ± 3 and 5 ± 3 mm Hg, respectively, vs. control: 19 ± 4 mm Hg). Injection of ATZ (3.6 mmol/kg b.wt.) i.v. also reduced central ANG II-induced pressor responses. Injections of H(2)O(2) i.c.v. and ATZ i.c.v. or i.v. alone produced no effect on baseline arterial pressure. Central ANG II, H(2)O(2) or ATZ did not affect heart rate. The results show that central injections of H(2)O(2) and central or peripheral injections of ATZ reduced the pressor responses induced by i.c.v. ANG II, suggesting that exogenous or endogenous H(2)O(2) may inhibit central pressor mechanisms activated by ANG II.

Cardiovascular responses to hydrogen peroxide into the nucleus tractus solitarius

The nucleus tractus solitarius (NTS), a major hindbrain area involved in cardiovascular regulation, receives primary afferent fibers from peripheral baroreceptors and chemoreceptors. Hydrogen peroxide (H(2)O(2)) is a relatively stable and diffusible reactive oxygen species (ROS), which acting centrally, may affect neural mechanisms. In the present study, we investigated effects of H(2)O(2) alone or combined with the glutamatergic antagonist kynurenate into the NTS on mean arterial pressure (MAP) and heart rate (HR). Conscious or anesthetized (urethane and alpha-chloralose) male Holtzman rats (280-320 g) were used. Injections of H(2)O(2) (125 to 1500 pmol/40 nl) into the intermediate NTS of anesthetized rats evoked dose-dependent and transient hypotension (-18 +/- 3 to -55 +/- 11 mmHg) and bradycardia (-16 +/- 5 to -116 +/- 40 bpm). Injection of the catalase inhibitor 3-amino-1,2,4-triazole (100 nmol/40 nl) into the NTS also produced hypotension and bradycardia. Previous injection of the ionotropic L-glutamate receptor antagonist kynurenate (7 nmol/40 nl) attenuated by 48% the bradycardic response, without changing the hypotension evoked by H(2)O(2) (500 pmol/40 nl) in anesthetized rats. The antioxidant L-ascorbate (600 pmol/80 nl) injected into the NTS attenuated the bradycardic (42%) and hypotensive (67%) responses to H(2)O(2) (500 pmol/40 nl) into the NTS. In conscious rats, injection of H(2)O(2) (50 nmol/100 nl) into the NTS also evoked intense bradycardia (-207 +/- 8 bpm) and hypotension (-54 +/- 6 mmHg) that were abolished by prior injection of kynurenate (7 nmol/100 nl). The results show that H(2)O(2) into the NTS induces hypotension and bradycardia probably due to activation of glutamatergic mechanisms.

Pharmacological consequences of oxidative stress in ocular tissues

The eye is a unique organ because of its constant exposure to radiation, atmospheric oxygen, environmental chemicals and physical abrasion. That oxidative stress mechanisms in ocular tissues have been hypothesized to play a role in diseases such as glaucoma, cataract, uveitis, retrolental fibroplasias, age-related macular degeneration and various forms of retinopathy provides an opportunity for new approaches to their prevention and treatment, In the anterior uvea, both H2O2 and synthetic peroxides exert pharmacological/toxicological actions tissues of the anterior uvea especially on the sympathetic nerves and smooth muscles of the iris-ciliary bodies of several mammalian species. Effects produced by peroxides require the presence of trace amounts of extracellular calcium and the functional integrity of mitochondrial calcium stores. Arachidonic acid metabolites appear to be involved in both the excitatory action of peroxides on sympathetic neurotransmission and their inhibitory effect on contractility of the iris smooth muscle to muscarinic receptor activation. In addition to the peroxides, isoprostanes (products of free radical catalyzed peroxidation of arachidonic acid independent of the cyclo-oxygenase enzyme) can also alter sympathetic neurotransmission in anterior uveal tissues. In the retina, both H2O2 and synthetic peroxides produced an inhibitory action on potassium depolarization induced release of [3H] D-aspartate, in vitro and on the endogenous glutamate and glycine concentrations in vivo. Effects caused by peroxides in the retina are mediated, at least in part, by second messengers such as nitric oxide, prostaglandins and isoprostanes. The ability of H2O2 to alter the integrity of neurotransmitter pools from sympathetic nerves in the anterior uvea and glutaminergic nerves in the retina could underlie its role in the etiology of glaucoma.

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.

Endogenous glutamate contributes to the maintenance of glutathione level under oxidative stress in isolated nerve terminals

Exposure of isolated nerve terminals to hydrogen peroxide (25-500 microM) for 10 min produced a partially reversible decrease in the total and reduced glutathione level. No release and resynthesis of glutathione by the oxidant was involved in this effect. Loss of reduced glutathione was associated with elimination of H(2)O(2), which was very quick with >70% of the oxidant eliminated within 5 min. Recovery of both total and reduced glutathione was pronounced after 10 min when the majority of H(2)O(2) was eliminated. Previously we have reported that glutamate metabolism under oxidative stress contributes to the operation of the Krebs cycle, thus to the production of NAD(P)H [J. Neurosci. 20 (2000) 8972]. In the present study we addressed whether metabolism of endogenous glutamate plays a role in the maintenance of glutathione level in nerve terminals. Glutamine and beta-hydroxybutyrate (5mM), alternative metabolites in synaptosomes, were able to decrease the loss of total and reduced glutathione induced by hydrogen peroxide. Metabolic consumption of glutamate was reduced at the same time. In addition an increased demand on the glutathione system by the catalase inhibitor aminotriazole augmented the metabolic consumption of glutamate. It is concluded that under oxidative stress glutamate metabolism contributes to the maintenance of glutathione level, thus to the antioxidant capacity of nerve terminals.

Brain antioxidant regulation in mammals and anoxia-tolerant reptiles: balanced for neuroprotection and neuromodulation

Reactive oxygen species (ROS) generated by mitochondrial respiration and other processes are often viewed as hazardous substances. Indeed, oxidative stress, defined as an imbalance between oxidant production and antioxidant protection, has been linked to several neurological disorders, including cerebral ischemia-reperfusion and Parkinson's disease. Consequently, cells and organisms have evolved specialized antioxidant defenses to balance ROS production and prevent oxidative damage. Research in our laboratory has shown that neuronal levels of ascorbate, a low molecular weight antioxidant, are ten-fold higher than those in much less metabolically active glial cells. Ascorbate levels are also selectively elevated in the CNS of anoxia-tolerant reptiles compared to mammals; moreover, plasma and CSF ascorbate concentrations increase markedly in cold-adapted turtles and in hibernating squirrels. Levels of the related antioxidant, glutathione, vary much less between neurons and glia or among species. An added dimension to the role of the antioxidant network comes from recent evidence that ROS can act as neuromodulators. One example is modulation of dopamine release by endogenous hydrogen peroxide, which we describe here for several mammalian species. Together, these data indicate adaptations that prevent oxidative stress and suggest a particularly important role for ascorbate. Moreover, they show that the antioxidant network must be balanced precisely to provide functional levels of ROS, as well as neuroprotection.

Glutamate release by an Na+ load and oxidative stress in nerve terminals: relevance to ischemia/reperfusion

Previously we have reported that oxidative stress induced by hydrogen peroxide exacerbates the effect of an Na+ load in isolated nerve terminals, with a consequence of an ATP depletion, [Ca2+]i and [Na+]i deregulation, and collapse of mitochondrial membrane potential. In the present study, the release of glutamate in response to a combined effect of an [Na+] load and oxidative stress was measured in isolated nerve terminals over an incubation for 15 min. Exposure to hydrogen peroxide (100 micro m) had no effect on the release of glutamate, but significantly enhanced the Ca2+-independent glutamate release induced by a small [Na+] load achieved with 10 micro m veratridine. The effect of a larger Na+ load induced by 40 micro m veratridine was not further increased by hydrogen peroxide; in contrast the external Ca2+-dependent glutamate release was completely eliminated by the oxidant under this condition. The effects of oxidative stress superimposed on a Na+ load are consistent with at least two factors: (i) a relatively modest Na+ load induced by veratridine is augmented by H2O2 giving rise to an increased Ca2+-independent release of glutamate (ii) oxidative stress in combination with a larger Na+ load causes severe ATP depletion limiting the Ca2+-dependent vesicular glutamate release. Given the concurrent presence of an Na+ load and oxidative stress in ischemia/reperfusion these results indicate that the extent of the Na+ load developing during the ischemic period could determine the release of glutamate induced by an oxidative stress during reperfusion.

Exposure to ebselen changes glutamate uptake and release by rat brain synaptosomes

We investigated effects of Ebselen, diphenyl diselenide (PhSe)2 and diphenyl ditelluride (PhTe)2 on [3H]glutamate uptake and release by brain synaptosomes. Ebselen after acute exposure inhibited K+-stimulated [3H]glutamate release by brain synaptosomes. (PhSe)2 and (PhTe)2 did not change [3H]glutamate release by brain synaptosomes. Ebselen, (PhSe)2 and (PhTe)2 had no significantly effects on [3H]glutamate uptake after acute exposure. In vitro, Ebselen (100 microM) inhibited [3H]glutamate release and uptake. (PhSe)2 had no significant effect, while (PhTe)2 (100 microM) inhibited [3H]glutamate uptake by brain synaptosomes. In vitro, (PhSe)2, (PhTe)2 and Ebselen caused a significant inhibition of [3H]glutamate uptake by brain synaptic vesicles in vitro. The results demonstrated that organochalcogenides have a rather complex effect on glutamate homeostasis depending on the compound and the schedule of exposition. We propose that the neuroprotective action of Ebselen can be related, in addition to its glutathione peroxidase-like and antilipoperoxidative activity, to a direct interaction with the glutamatergic system by reducing K+-evoked glutamate release.

Adenosine inhibits glutamate exocytosis largely without interfering with Ca2+ influx in rat cerebrocortical synaptosomes

Adenosine is an inhibitor of glutamate release in synaptosomes. The inhibition is removed by the A(1) adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). We monitored the variations of cytoplasmic free Ca(2+) concentrations ([Ca(2+)](i)) in KCl or 4-aminopyridine-stimulated synaptosomes, in the presence of adenosine or adenosine plus DPCPX. The increment of [Ca(2+)](i) upon stimulation was unmodified by adenosine (up to 400-500 microM) while it was strongly decreased when exocytosis was decreased to a similar extent by lowering KCl or 4-aminopyridine. Adenosine also inhibited glutamate release induced by the Ca(2+) ionophore ionomycin. Increasing adenosine to 1.5 mM resulted in a decrease of the stimulus-induced increase of [Ca(2+)](i) and in the further potentiation of the adenosine inhibition of exocytosis from 41+/-3 to 51+/-4%. We conclude that adenosine affects glutamate exocytosis mostly in a Ca(2+) independent mode.

H(2)O(2) is a novel, endogenous modulator of synaptic dopamine release

Recent evidence suggests that reactive oxygen species (ROS) might act as modulators of neuronal processes, including synaptic transmission. Here we report that synaptic dopamine (DA) release can be modulated by an endogenous ROS, H(2)O(2). Electrically stimulated DA release was monitored in guinea pig striatal slices using carbon-fiber microelectrodes with fast-scan cyclic voltammetry. Exogenously applied H(2)O(2) reversibly inhibited evoked release in the presence of 1.5 mM Ca(2+). The effectiveness of exogenous H(2)O(2), however, was abolished or decreased by conditions that enhance Ca(2+) entry, including increased extracellular Ca(2+) concentration ([Ca(2+)](o); to 2.4 mM), brief, high-frequency stimulation, and blockade of inhibitory D(2) autoreceptors. To test whether DA release could be modulated by endogenous H(2)O(2), release was evoked in the presence of the H(2)O(2)-scavenging enzyme, catalase. In the presence of catalase, evoked [DA](o) was 60% higher than after catalase washout, demonstrating that endogenously generated H(2)O(2) can also inhibit DA release. Importantly, the Ca(2+) dependence of the catalase-mediated effect was opposite to that of H(2)O(2): catalase had a greater enhancing effect in 2.4 mM Ca(2+) than in 1.5 mM, consistent with enhanced H(2)O(2) generation in higher [Ca(2+)](o). Together these data suggest that H(2)O(2) production is Ca(2+) dependent and that the inhibitory mechanism can be saturated, thus preventing further effects from exogenous H(2)O(2). These findings show for the first time that endogenous H(2)O(2) can modulate vesicular neurotransmitter release, thus revealing an important new signaling role for ROS in synaptic transmission.

Ebselen prevents excitotoxicity provoked by glutamate in rat cerebellar granule neurons

Ebselen is a selenium compound that have glutathione peroxidase-like activity which is neuroprotective in acute stroke ischemia. The efficacy of ebselen to prevent excitotoxicity provoked by glutamate in cerebellar granule neurons was investigated at various time points and concentrations. Simultaneous addition of ebselen with glutamate decreased neuronal death and was completely reversed by 3 microM of ebselen (3 (4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) and propidium iodide assays). However, when 1 microM of ebselen was added with glutamate and remained in the culture medium until 24 or 48 h, the neuronal survival increased to the control. The mechanism proposed for neuroprotection was the ability of ebselen to prevent lipoperoxidation provoked by glutamate. The present findings propose to amplify the use of ebselen in others neurodegenerative disorders involving glutamatergic system.

Reversible depolarization of in situ mitochondria by oxidative stress parallels a decrease in NAD(P)H level in nerve terminals

We have reported recently (Chinopoulos et al., 1999 J. Neurochem. 73, 220 228) that mitochondrial membrane potential (delta(psi)m) in isolated nerve terminals is markedly reduced by H2O2 in the absence of F0F1-ATPase working as a proton pump. Here we demonstrate that delta(psi)m reduced by H2O2 (0.5 mM) in the presence of oligomycin (10 mM), an inhibitor of the F0F1-ATPase, was able to recover by the addition of catalase (2000 U). Similarly, a decrease in the NAD(P)H level due to H2O2 can be reversed by catalase. In addition, H2O2 decreased the ATP level and the [ATP]:[ADP] ratio measured in the presence of oligomycin reflecting an inhibition of glycolysis by H2O2, but this effect was not reversible. The effect of H2O2 on delta(psi)m in the presence of the complex I inhibitor, rotenone, was also unaltered by addition of catalase. These results provide circumstantial evidence for a relationship between the decreased NAD(P)H level and the inability of mitochondria to maintain delta(psi)m during oxidative stress.

Exacerbated responses to oxidative stress by an Na(+) load in isolated nerve terminals: the role of ATP depletion and rise of [Ca(2+)](i)

We have explored the consequences of a [Na(+)](i) load and oxidative stress in isolated nerve terminals. The Na(+) load was achieved by veratridine (5-40 microM), which allows Na(+) entry via a voltage-operated Na(+) channel, and oxidative stress was induced by hydrogen peroxide (0.1-0.5 mM). Remarkably, neither the [Na(+)](i) load nor exposure to H(2)O(2) had any major effect on [Ca(2+)](i), mitochondrial membrane potential (Deltapsim), or ATP level. However, the combination of an Na(+) load and oxidative stress caused ATP depletion, a collapse of Deltapsim, and a progressive deregulation of [Ca(2+)](i) and [Na(+)](i) homeostasis. The decrease in the ATP level was unrelated to an increase in [Ca(2+)](i) and paralleled the rise in [Na(+)](i). The loss of Deltapsim was prevented in the absence of Ca(2+) but unaltered in the presence of cyclosporin A. We conclude that the increased ATP consumption by the Na,K-ATPase that results from a modest [Na(+)](i) load places an additional demand on mitochondria metabolically compromised by an oxidative stress, which are unable to produce a sufficient amount of ATP to fuel the ATP-driven ion pumps. This results in a deregulation of [Na(+)](i) and [Ca(2+)](i), and as a result of the latter, collapse of Deltapsim. The vicious cycle generated in the combined presence of Na(+) load and oxidative stress could be an important factor in the neuronal injury produced by ischemia or excitotoxicity, in which the oxidative insult is superimposed on a disturbed Na(+) homeostasis.

Modulation of glutamate exocytosis by redox changes of superficial thiol groups in rat cerebrocortical synaptosomes

The treatment of cerebral cortex synaptosomes with the membrane impermeable thiol reagent 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) induces a long-lasting partial inhibition (about 40%) of the KCl-stimulated Ca2+-dependent exocytosis of glutamate. Synaptosomes are not damaged by the treatment. The increase of cytoplasmic free Ca2+ concentration ([Ca2+]i) upon depolarization is not affected by DTNB. The inhibition is observed also if exocytosis is induced with the Ca2+-ionophore ionomycin. In all cases the inhibition is reversed by the impermeable reductant glutathione (GSH). Similarly the inhibition of exocytosis by H2O2 (Zoccarato, F., Valente, M. and Alexandre, A., Hydrogen peroxide induces a long-lasting inhibition of the Ca2+-dependent glutamate release in cerebrocortical synaptosomes without interfering with cytosolic Ca2+. J. Neurochem., 64 (1995) 2552-2558.) is reversed by GSH. It is concluded that redox changes (possibly thiol-disulfide transitions) of superficial groups modulate the exocytotic apparatus directly. In an attempt to identify the protein(s) involved in this novel type of control, we evidenced DTNB (H2O2) reactive bands at 35 and at 85-150 kDa which can be labeled with a monobromotrimethylammoniobimane bromide (qBBr) derivatization.

Redox mechanisms at the glutamate synapse and their significance: a review

This paper reviews what is currently known about the redox state of the glutamate synapse and its possible role in modulating synaptic plasticity and thus learning and neurocomputation. The hypothesis is presented that the growth or pruning of the synaptic spine is controlled in part by the balance in the synapse between neurodestructive pro-oxidants (e.g., nitric acid radical and hydrogen peroxide) and neuroprotective antioxidants (e.g., ascorbate and carnosine). In addition, there may be a role for catecholamines, in particular dopamine, related to its role in reinforcement signalling. Activation of the dopamine D2 receptor induces the synthesis of an antioxidant enzyme, possibly catalase. Dopamine may also affect the redox balance in the glutamate synapse directly by diffusion from the adjacent dopaminergic bouton-en-passage. Catecholamines are powerful antioxidants, scavengers of free radicals and iron chelators. Catecholamine-iron complexes are potent dismuters of superoxide ions. Additional agents participating in spine pruning may be neurotoxic catecholamine o-quinones present in the brain. This system may be at fault in schizophrenia and Parkinson's disease. Experiments to test the hypothesis are suggested.

The pH-sensitive dye acridine orange as a tool to monitor exocytosis/endocytosis in synaptosomes

We introduce the use of the pH-sensitive dye acridine orange (AO) to monitor exo/endocytosis of acidic neurotransmitter-containing vesicles in synaptosomes. AO is accumulated exclusively in acidic v-ATPase-dependent bafilomycin (Baf)-sensitive compartments. A fraction of the accumulated AO is rapidly released (fluorescence increase) upon depolarization with KCl in the presence of Ca2+. The release (completed in 5-6 s) is followed by reuptake to values below the predepolarization baseline. The reuptake, but not the release, is inhibited by Baf added 5 s prior to KCl. In a similar protocol, Baf does not affect the initial fast phase of glutamate release measured enzymatically, but it abolishes the subsequent slow phase. Thus, the fast AO release corresponds to the rapid phase of glutamate release and the slow phase depends on vesicle cycling. AO reuptake depends in part on the progressive accumulation of acid-loaded vesicles during cycling. Stopping exocytosis at selected times after KCl by Ca2+ removal with EGTA evidences endocytosis: Its T(1/2) was 12 +/- 0.6 s. The K(A)+, channel inhibitors 4-aminopyridine (100 microM) and alpha-dendrotoxin (10-100 nM) are known to induce glutamate release by inducing the firing of Na+ channels; their action is potentiated by the activation of protein kinase C. Also these agents promote a Ca2+-dependent AO release, which is prevented by the Na+ channel inhibitor tetrodotoxin and potentiated by 4beta-phorbol 12-myristate 13-acetate (PMA). With alpha-dendrotoxin, endocytosis was monitored by stopping exocytosis at selected times with EGTA or alternatively with Cd2+ or tetrodotoxin. The T(1/2) of endocytosis, which was unaffected by PMA, was 12 +/- 0.4 s with EGTA and Cd2+ and 9.5 +/- 0.5 s with tetrodotoxin. Protein kinase C activation appeared to facilitate vesicle turnover.

Biphasic effect of hydrogen peroxide on field potentials in rat hippocampal slices

In the CA1 region of rat hippocampal slices, H2O2 (0.294-2.94 mM) caused initial augmentation, and subsequent long-lasting depression, of population spikes and excitatory postsynaptic potentials. The effect of H2O2 may not be mediated by its degradation product, hydroxyl radicals, because an iron chelator deferoxamine did not block the effect. A catalase inhibitor 3-amino-1,2,4-triazole only modestly attenuated the initial augmentation, suggesting that the effect of H2O2 is not attributable to catalase-dependent O2 generation, either. An N-methyl-D-aspartate receptor antagonist DL-2-amino-5-phosphonovaleric acid had no influence on the effect of H2O2, whereas a gamma-aminobutyric acid type A receptor channel blocker picrotoxin attenuated long-lasting depression, indicating that gamma-aminobutyric acid-mediated inhibition is altered during the depression phase. The initial augmentation but not subsequent depression was attenuated by a phospholipase A2/C inhibitor 4-bromophenacyl bromide, suggesting the involvement of lipid signaling molecule(s) in the enhancement of excitatory synaptic transmission. These results suggest that H2O2 regulates hippocampal synaptic transmission via multiple mechanisms.