Induced Hydroxyl Radical Generation Research Articles

X-ray Induced Hydroxyl Radical Generation by GdYVO4:Eu3+ Nanoparticles in Aqueous Solution: Main Mechanisms

We report on strong X-ray-induced hydroxyl radical (. O H ) generation in an aqueous solution containing UV light pre-treated GdYVO4:Eu3+ nanoparticles (L-GdYVO). The methods of optical spectroscopy were used to detect . O H in the solutions. The complex nature of the mechanism of . O H generation has been revealed and discussed. The experimental data obtained indicate that the mechanism of . O H generation is associated with two main processes: (i) direct . O H generation with the participation of thermalized h+ formed at X-ray irradiation, and (ii) X-ray-facilitated jumps of h+ formed in the nanoparticles’ (NPs’) valence band at UV light pre-treatment and trapped in local levels formed by random scattering potential. At the same time, for GdYVO4:Eu3+ nanoparticles, which were not exposed to UV light before the X-ray irradiation (D-GdYVO), a strong radioprotective effect ascribed to the electron-donation properties of V4+ ions was observed. Thus, depending on the pre-treatment condition, we can change the redox properties of GdYVO4:Eu3+ NPs in an opposite direction, which makes this nanomaterial a unique theranostic agent for radiation therapy (RT) enhancement, allowing the problem of radiation therapy (RT)-resistant hypoxic tumours to be overcome.

Opening ATP-sensitive K+ (KATP) channels increases MPP+-induced hydroxyl radical generation via estrogen action in rat striatum

Opening ATP-sensitive K+ (KATP) channels increases MPP+-induced hydroxyl radical generation via estrogen action in rat striatum

Phosphorylation by okadaic acid enhances MPP+ induced hydroxyl radical generation in rat striatum

Phosphorylation by okadaic acid enhances MPP+ induced hydroxyl radical generation in rat striatum

Fluvastatin, a new inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, resists hydroxyl radical generation in the rat myocardium.

The aim of this study was to determine whether fluvastatin, an inhibitor of low-density lipoprotein (LDL) oxidation, can resist Cu (II)-induced hydroxyl radical generation (*OH) in the extracellular fluid of rat myocardium. Rats were anaesthetized and sodium salicylate in Ringer's solution (0.5 nmol microL(-1) min(-1)) was infused through a microdialysis probe to detect the generation of *OH as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (2,3-DHBA) in the myocardium. When Cu (II) (0, 10, 25 or 50 microM) was administered to 1 mM tyramine-pretreated rats, a marked elevation in the levels of 2,3-DHBA was found, indicating a positive linear correlation between Cu (II) and the increase in *OH formation trapped as 2,3-DHBA in the dialysate (r2 = 0.977). In the presence of fluvastatin (100 microM), a marked decrease in the levels of 2,3-DHBA was found. Corresponding experiments performed with iron (II) (0, 10, 25 or 50 microM), showed a marked elevation in the levels of 2,3-DHBA, indicating a positive linear correlation between iron (II) and the increase in *OH formation trapped as 2,3-DHBA in the dialysate (r2 = 0.986). However, in the presence of fluvastatin (100 microM) a small decrease in the level of 2,3-DHBA was found. The results show that iron (II) against LDL oxidation may be insensitive compared with Cu (II). Cu (II)-induced *OH formation may be reduced by inhibiting LDL with fluvastatin.

Effect of Adenosine A2A Receptor Antagonists on l-DOPA-Induced Hydroxyl Radical Formation in Rat Striatum

A(2A) adenosine receptor antagonists have been proposed as a new therapy for Parkinson's disease (PD). Since oxidative stress plays an important role in the pathogenesis of PD, we studied the effect of the selective A(2A) adenosine receptor antagonists 8-(3-chlorostyryl)caffeine (CSC) and 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) on L: -3,4-dihydroxyphenylalanine (L: -DOPA)-induced hydroxyl radical generation using in vivo microdialysis in the striatum of freely moving rats. L: -DOPA (100 mg/kg; in the presence of benserazide, 50 mg/kg) given acutely or repeatedly for 14 days generated a high level of hydroxyl radicals, measured by HPLC with electrochemical detection, as the product of their reaction with p-hydroxybenzoic acid (PBA). CSC (1 mg/kg) and ZM 241385 (3 mg/kg) decreased haloperidol (0.5 mg/kg)-induced catalepsy, while at low doses of 0.1 and 0.3 mg/kg, respectively, they did not display an effect. CSC (1 and 5 mg/kg) and ZM 241385 (3 and 9 mg/kg) given acutely, or CSC (1 mg/kg) and ZM 241385 (3 mg/kg) given repeatedly, increased the production of hydroxyl radicals in dialysates from rat striatum. Both acute and repeated administration of CSC (0.1 and 1 mg/kg) and ZM 241385 (3 mg/kg) decreased L: -DOPA-induced generation of hydroxyl radicals. However, a high single dose of either CSC (5 mg/kg) and ZM 241385 (9 mg/kg) markedly potentiated the effect of L: -DOPA on hydroxyl radical production. The increase in hydroxyl radical production by acute and chronic injection of CSC and ZM 241385 may be related to the increased release of dopamine (DA) and its metabolism in striatal dialysates. Similarly, increased DA release following a single high dose of CSC or ZM 241385 appears to be responsible for augmentation of L: -DOPA-induced hydroxyl radical formation. Conversely, the inhibition of L: -DOPA-induced production of hydroxyl radical by single and repeated low doses of CSC or repeated low doses of ZM 241385 may be related to reduced DA metabolism. Summing up, A(2A) antagonists, used as a supplement of L: -DOPA therapy, depending on the dose used, may have a beneficial or adverse effect on ongoing neurodegenerative processes and accompanying oxidative stress.

The protective effect of fluvastatin on hydroxyl radical generation by inhibiting low-density lipoprotein (LDL) oxidation in the rat myocardium

The present study examined the effect of fluvastatin on Cu 2+-induced hydroxyl radical generation ( OH) in the extracellular fluid of rat myocardium using microdialysis technique (O system). Fluvastatin, an inhibitor of low-density lipoprotein (LDL) oxidation, was administered at a dose of 5.0 mg/kg/day i.p. for 4 weeks. Rats were anesthetized and sodium salicylate in Ringer's solution (0.5 nmol/μl/min) was infused through a microdialysis probe to detect the generation of OH as reflected by the nonenzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the myocardium. When CuSO 4 was infused through the microdialysis probe, CuSO 4 clearly produced an increase in OH formation trapped as 2,3-DHBA ( R 2 = 0.983). However, when corresponding experiments were performed with fluvastatin (5.0 mg/kg/day i.p. for 4 weeks) pretreated animals, small increases in the level of 2,3-DHBA products were observed. When LDL is oxidized by Cu 2+, Cu 2+ can be reduced to Cu 1+ by LDL. Fenton-type reactions in the presence of Cu 1+ yields highly cytotoxic OH. These results suggest that Cu 2+-induced OH generation may be reduced by inhibiting LDL oxidation with fluvastatin.

Effect of desferrioxamine, a strong iron (III) chelator, on 1-methyl-4-phenylpyridinium ion (MPP +)-induced hydroxyl radical generation in the rat striatum

The present study was examined that the desferrioxamine, a strong iron (III) chelator, enhanced 1 methyl-4-phenylpyridinium ion (MPP +)-induced hydroxyl radical ( OH) generation in the extracellular fluid of caudate nucleus anesthetized rats. Rats were anesthetized, and sodium salicylate in Ringer's solution (0.5 nmol/μl/min) was infused through a microdialysis probe to detect the generation of OH as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the striatum. Induction of desferrioxamine (50 μM) drastically increased the formation of OH trapped as 2,3-DHBA by the action of MPP +, as compared with MPP +-only-treated animals. Although desferrioxamine did not change the levels of MPP +-induced dopamine, a marked elevation of OH formation trapped as 2,3-DHBA was observed. When corresponding experiments were performed with reserpinized animals, the level of dopamine and 2,3-DHBA drastically decreased. However, the level of dopamine did not change, but desferrioxamine significantly increased the level of 2,3-DHBA in reserpinized animals. Iron (III) decreased MPP +-induced 2,3-DHBA formations in the presence of dopamine (10 μM). Moreover, when iron (II) was administered to desferrioxamine-treated animals, a marked elevation of 2,3-DHBA was observed, compared with MPP +-only-treated animals, that showed a positive linear correlation between iron (II) and OH formation trapped as 2,3-DHBA ( R 2 = 0.981) in the dialysate. The present study indicates that the suppression of MPP +-induced OH formation by iron (III) may play a key role in protective effect of iron (III) on the rat brain.

Protective effect of fluvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, on copper-induced hydroxyl radical generation in the rat heart

The present study was examined the effect of fluvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, on Cu(II)-induced hydroxyl radical generation ( OH) in the extracellular fluid of rat myocardium. Rats were anesthetized and sodium salicylate in Ringer's solution (0.5 nmol/μl/min) was infused through a microdialysis probe to detect the generation of OH as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the myocardium. When Cu(II) was infused through the microdialysis probe, Cu(II) increased in OH formation trapped as 2,3-DHBA in the dialysate. When fluvastatin (100 μM) was administered to Cu(II) (50 μM)-pretreated animals, the levels of 2,3-DHBA at 300 min after administration of fluvastatin significantly decreased. In cumulative dose dependent experiments, three concentrations of Cu(II), 10, 25 and 50 μM, were infused through the microdialysis probe in the rat myocardium. A positive linear correlation between Cu(II) and the formation of 2,3-DHBA ( R 2 = 0.980) was observed. However, when corresponding experiments were performed with fluvastatin (100 μM) pretreated animals, the level of 2,3-DHBA decreased. These results suggest that blocking LDL oxidation by fluvastatin may attenuate Cu(II)-induced OH formation in the rat heart.

Nitric oxide and MPP+-induced hydroxyl radical generation

Although neuroprotective effect of nitric oxide (NO) is discussed, NO has a role of pathogenesis of cellular injury. NO is synthesized from L-arginine by NO synthase (NOS). NO contributes to the extracellular potassium-ion concentration ([K(+)](o))-induced hydroxyl radical ((*)OH) generation. Cytotoxic free radicals such as peroxinitrite (ONOO(-)) and (*)OH may also be implicated in NO-mediated cell injury. NO activation was induced by K(+) depolarization. NO may react with superoxide anion (O(2) (-)) to form ONOO(-) and its decomposition generates (*)OH. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) metabolite 1-methyl-4-phenylpyridinium ion (MPP(+)) involve toxicity induced by NO. Intraneuronal Ca(2+) triggered by MPP(+) may be detrimental to the functioning of dopaminergic nerve terminals in the striatum. Although the [K(+)](o)-induced depolarization enhances the formation of (*)OH product due to MPP(+), the (*)OH generation via NOS activation may be unrelated the dopamine (DA)-induced (*)OH generation. Depolarization enhances the MPP(+)-induced (*)OH formation via NOS activation. NOS inhibition is associated with a protective effect due to suppression of depolarization-induced (*)OH generation. ONOO(-) has been implicated as a causative factor under conditions in which DA neurons are damaged. These findings may be useful in elucidating the actual mechanism of free radical formation in the pathogenesis of neurodegenerative brain disorders, including Parkinson's disease and traumatic brain injuries.

Imidaprilat suppresses nonylphenol and 1-methyl-4-phenylpyridinium ion (MPP +)-induced hydroxyl radical generation in rat striatum

The present study examined the antioxidant effects of angiotensin-converting enzyme inhibitor (ACE), imidaprilat, on para-nonylphenol and 1-methyl-4-phenylpyridinium ion (MPP +)-induced hydroxyl radical ( OH) formation and dopamine (DA) efflux in extracellular fluid of rat striatum, using a microdialysis technique. para-Nonylphenol clearly enhanced OH formation and DA efflux induced by MPP +. When imidaprilat was infused in para-nonylphenol and MPP +-treated rats, DA efflux and OH formation significantly decreased, as compared with that in the para-nonylphenol and MPP +-treated control. Imidaprilat was able to scavenge OH and DA efflux induced by para-nonylphenol and MPP +. When iron(II) was administered to para-nonylphenol-pretreated animals, iron(II) clearly produced a dose-dependent increase in the levels of 2,3-dihydroxybenzoic acid (2,3-DHBA), as compared with MPP +-only-treated rats. A positive linear correlation was observed between iron(II) and 2,3-DHBA ( R 2 = 0.985) in the dialysate. However, in the presence of imidaprilat, a small increase in the levels of 2,3-DHBA products was observed. The results suggest that imidaprilat may protect against para-nonylphenol and MPP +-induced OH formation via suppressing DA efflux in the rat striatum.

Allopurinol suppresses 2-bromoethylamine and 1-methyl-4-phenylpyridinium ion (MPP +)-induced hydroxyl radical generation in rat striatum

The present study was examined whether or not 2-bromoethyamine, a semicarbazide-sensitive amine oxidase (SSAO, EC; 1.4.3.6) inhibitor, would increase an active dopaminergic neurotoxin, 1-methyl-4-phenylpyridinium ion (MPP +)-induced hydroxyl radical ( OH) generation in the rat striatum. Rats were anesthetized, and sodium salicylate (0.5 mM or 0.5 nmol/μl/min) was infused through a microdialysis probe to detect the generation of OH as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the striatum. Infusion of 2-bromoethylamine (100 μM or 100 pmol/μl/min) into the striatum drastically increased the formation of OH products, trapped as DHBA by the action of MPP +. Further, I studied the effect of allopurinol, a xanthine oxidase inhibitor, an 2-bromoethylamine and MPP +-induced OH generation. Allopurinol (10 μM or 10 pmol/μl/min) significantly suppressed 2-bromoethyamine and MPP +-induced OH. These results suggest that a definite mechanism is not clear at the moment, after inhibition of tissue-bound and/or blood plasma SSAO activity, with consequent increases in bioactive amine levels, enhances the formation of OH products of efflux/oxidation due to MPP +.

Measurement of Ascorbic Acid (Vitamin C) Induced Hydroxyl Radical Generation in Household Drinking Water

Ascorbic acid (vitamin C) induced hydroxyl radical formation was measured in household drinking water samples using the hydroxyl radical sensitive probe coumarin-3-carboxylic acid. Vitamin C, a reducing agent that is commonly used as a food additive, triggered a significant hydroxyl radical generating reaction when added to the tap-water samples tested. The capacity of ascorbic acid to trigger hydroxyl radical formation in the tap-water samples was dependent on the flushing time before the samples were taken indicating that the water in the copper piping had been contaminated by copper ions. In line with this, high concentrations of copper were measured in the hydroxyl radical generating first-draw samples. Moreover, a strong correlation was found between the hydroxyl radical generation capacity seen in the coumarin-3-carboxylic acid based microplate assay and the DNA damage seen in an agarose gel assay using the pBluescript plasmid. In the water samples showing high capacity to hydroxylate coumarin-3-carboxylic acid, a rapid formation of the open circular form of the plasmid could also be seen indicating a copper assisted hydroxyl radical attack on the DNA. In conclusion, our results show that addition of vitamin C to household tap water that is contaminated with copper ions, results in Fenton type reactions that continuously generate harmful and reactive hydroxyl radicals.

Protective effect of tamoxifen on 1-methyl-4-phenylpyridine-induced hydroxyl radical generation in the rat striatum

We examined whether tamoxifen could suppress 1-methyl-4-phenylpyridine (MPP +)-induced hydroxyl radical generation in the extracellular fluid of rat striatum, using in vivo microdialysis system. MPP + (5 mM) enhanced generation of hydroxyl radicals with concomitant increased efflux of dopamine. Tamoxifen (1–100 μM) dose-dependently suppressed the hydroxyl radical formation induced by MPP +. Tamoxifen (100 μM) significantly attenuated dopamine efflux induced by MPP +. The result in the present study is the first demonstration showing the protective effect of tamoxifen on hydroxyl radical generation induced by MPP + by suppressing dopamine efflux.

Nitric oxide enhances MPP +-induced hydroxyl radical generation via depolarization activated nitric oxide synthase in rat striatum

We examined the effect of N G-nitro- l-arginine methyl ester ( l-NAME), a nitric oxide synthase (NOS) inhibitor, on extracellular potassium ion concentration ([K +] o)-enhanced hydroxyl radical ( ·OH) generation due to 1-methyl-4-phenylpyridinium ion (MPP +) was examined in the rat striatum. Rats were anesthetized, and sodium salicylate in Ringer’s solution (0.5 nmol/μl per min) was infused through a microdialysis probe to detect the generation of ·OH as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the striatum. Induction of KCl (20, 70 and 140 mM) increased MPP +-induced ·OH formation trapped as 2,3-dihydroxybenzoic acid (DHBA) in a concentration dependent manner. However, the application of l-NAME (5 mg/kg i.v.) abolished the [K +] o depolarization-induced ·OH formation with MPP +. Dopamine (DA; 10 μM) also increased the levels of DHBA due to MPP +. However, the effect of DA after application of l-NAME did not change the levels of DHBA. On the other hand, the application of allopurinol (20 mg/kg i.v., 30 min prior to study), a xanthine oxidase (XO) inhibitor was abolished the both [K +] o- and DA-induced ·OH generation. Moreover, when iron(II) was administered to MPP + then [K +] o (70 mM)-pretreated animals, a marked increase in the level of DHBA. However, when corresponding experiments were performed with l-NAME-pretreated animals, the same results were obtained. Therefore, NOS activation may be no relation to Fenton-type reaction via [K +] o depolarization-induced ·OH generation. The present results suggest that [K +] o-induced depolarization augmented MPP +-induced ·OH formation by enhancing NO synthesis.

Allopurinol suppresses para-nonylphenol and 1-methyl-4-phenylpyridinium ion (MPP +)-induced hydroxyl radical generation in rat striatum

We recently demonstrated that para-nonylphenol, an environmental estrogen-like chemical, enhances hydroxyl radical ( OH) generation in the rat striatum. In the present study we have examined whether para-nonylphenol enhanced 1-methyl-4-phenylpyridinium ion (MPP +)-induced OH generation in the rat striatum using a microdialysis technique. Para-nonylphenol significantly enhanced MPP +-induced OH generation. Further, we studied the effect of allopurinol, a xanthine oxidase inhibitor, on para-nonylphenol and MPP +-induced OH generation. Allopurinol significantly suppressed para-nonylphenol and MPP +-induced OH generation. The results indicate that para-nonylphenol enhanced OH generation based on superoxide anion production, and allopurinol may have preventive effect on para-nonylphenol and MPP +-induced OH generation.

Cu-Metallothioneins (Cu(I)8-MTs) in LEC Rat Livers 13 Weeks after Birth Still Act as Antioxidants

Redox properties of metallothioneins (MTs) and Cu in the cytosol from Long–Evans Cinnamon (LEC) rat livers 13 weeks after birth were investigated. MTs from LEC rat livers contain 8 g atoms of Cu and 1 g atom of Zn per mole of protein (Cu(I)8-MTs). Titration of Cu(I)8-MTs with CuCl2 indicates that Cu(I)8-MTs were able to reduce further 2-g atoms of cupric ions per mole MTs as bound form. Hg2+-induced hydroxyl radical generation from Cu(I)8-MTs was demonstrated by ESR using the spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The intensity of DMPO-OH signal from Cu-loaded MTs was increased with the increasing number of Cu in MTs. The used cytosol fraction contained 1.37 mM total Cu and 5 mM DTNB titrable-SH groups has a potential to reduce 2 mM CuCl2. No ESR signal due to Cu2+ was also detected with LEC rat liver cytosol, whereas strong Cu2+ signal appeared by the addition of HgCl2. The rate constants for the reaction of Cu(I)8-MTs with superoxide and hydroxyl radicals were estimated to be 2 × 106 and ≥1012 M−1s−1, respectively, from competition kinetics. Cu2+-catalyzed oxidation of DNA was strongly inhibited both in the presence of Cu-unsaturated MTs and GSH. The results suggest that Cu(I)8-MTs from LEC rat livers just before hepatitis still act as antioxidants.

Protective effect of fluvastatin, a new inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, on MPP +-induced hydroxyl radical in the rat striatum

We examined whether fluvastatin, an inhibitor of low-density lipoprotein (LDL) oxidation, can resist 1-methyl-4-phenylpyridine (MPP +)-induced hydroxyl radical generation (·OH) in the extracellular fluid of rat striatum. Rats were anesthetized and sodium salicylate in Ringer's solution (0.5 nmol/μl/min) was infused through a microdialysis probe to detect the generation of ·OH as reflected by the nonenzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the striatum. MPP + (5 mM; total dose 75 nmol) clearly produced an increase in ·OH formation. However, fluvastatin (100 μM) reduced the ·OH formation by the action of MPP +. These results indicated that fluvastatin, a potent inhibitor of LDL oxidation, may resist the formation of ·OH products of MPP +.

Potassium chloride depolarization enhances MPP +-induced hydroxyl radical generation in the rat striatum

We determined that extracellular potassium ion concentration, [K +] o-induced depolarization, enhances 1-methyl-4-phenylpyridinium ion (MPP +)-induced hydroxyl radical (⋅OH) generation in the rat striatum. Rats were anesthetized, and sodium salicylate in Ringer's solution (0.5 nmol/μl/min) was infused through a microdialysis probe to detect the generation of ⋅OH as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the striatum. Induction of high concentration KCl (70 mM) drastically increased formation of ⋅OH trapped as DHBA by the action of MPP +. When dopamine (DA) was administered to the high KCl-treated animals, a marked elevation of DHBA was observed, compared with MPP +-only-treated animals, that showed a positive linear correlation between DA and ⋅OH formation trapped as DHBA ( R 2=0.979) in the dialysate. When corresponding experiments were performed with iron (II), the same results were obtained: a positive linear correlation between the release of iron (II) and DHBA ( R 2=0.988) in the dialysate. These results suggest that [K +] o-induced depolarization enhances the formation of ⋅OH products of efflux/oxidation due to MPP +.

Protective effect of histidine on iron (II)-induced hydroxyl radical generation in rat hearts

We investigated the efficacy of histidine on iron (II)-induced hydroxyl radical ( . OH) generation in extracellular fluid of the rat myocardium using a flexibly mounted microdialysis technique (O system). Rats were anesthetized and a microdialysis probe was implanted in the left ventricular, followed by infusion of sodium salicylate in Ringer's solution (0.5 nmol/μL/min) to detect the generation . OH as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA). Iron (II) clearly produced a concentration-dependent increase in . OH formation. A positive linear correlation between iron (II) and the formation of 2,3-DHBA (R 2 = 0.987) was observed. However, histidine (25 mM) was infused through a microdialysis probe; iron (II) failed to increase the 2,3-DHBA formation obtained. To examine the effect of histidine on ischemia-reperfusion of the myocardium, the heart was subjected to myocardial ischemia for 15 min by occlusion of the left anterior descending coronary artery (LAD). When the heart was reperfused, a marked elevation of the levels of 2,3-DHBA was observed in the heart dialysate. When corresponding experiments were performed with histidine (25 mM)-pretreated animals, histidine prevented the ischemia-reperfusion induced . OH generation trapped as 2,3-DHBA. These results indicate that histidine protects the myocardium against ischemia-reperfusion damage by . OH generation.

Protective effect of histidine on MPP+-induced hydroxyl radical generation in rat striatum

We investigated the efficacy of histidine on MPP+-induced hydroxyl radical (⋅OH) formation in extracellular fluid of rat striatum. Rats were anesthetized and sodium salicylate in Ringer's solution (0.5 nmol μl−1 min−1) was infused through a microdialysis probe to detect the generation of ⋅OH as reflected by the nonenzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the striatum. MPP+ (5 mM) clearly produced an increase in ⋅OH formation. However, histidine (25 mM) reduced the ⋅OH formation by the action of MPP+. These results indicate that histidine protects MPP+-induced ⋅OH formation in rat striatum.