Dopamine- and L-beta-3,4-dihydroxyphenylalanine hydrochloride (L-Dopa)-induced cytotoxicity towards catecholaminergic neuroblastoma SH-SY5Y cells. Effects of oxidative stress and antioxidative factors

Levodopa is toxic to dopamine neurons in an in vitro but not an in vivo model of oxidative stress

Levodopa is the "gold standard" for the symptomatic treatment of Parkinson's disease (PD). There is a theoretical concern, however, that levodopa might accelerate the rate of nigral degeneration, because it undergoes oxidative metabolism and is toxic to cultured dopaminergic neurons. Most in vivo studies do not show evidence of levodopa toxicity; levodopa is not toxic to normal rodents, nonhuman primates, or humans and is not toxic to dopamine neurons in dopamine-lesioned rodents or nonhuman primates in most studies. However, the potential for levodopa to be toxic in vivo has not been tested under conditions of oxidative stress such as exist in PD. To assess whether levodopa is toxic under these circumstances, we have examined the effects of levodopa on dopamine neurons in mesencephalic cultures and rat pups in which glutathione synthesis has been inhibited by L-buthionine sulfoximine. Levodopa toxicity to cultured dopaminergic neurons was enhanced by glutathione depletion and diminished by antioxidants. In contrast, treatment of neonatal rats with levodopa, administered either alone or in combination with glutathione depletion, did not cause damage to the dopamine neurons of the substantia nigra or changes in striatal levels of dopamine and its metabolites. This study provides further evidence to support the notion that although levodopa can be toxic to dopamine neurons in vitro, it is not likely to be toxic to dopamine neurons in vivo and specifically in conditions such as PD.

Apoptosis-inducing neurotoxicity of dopamine and its metabolites via reactive quinone generation in neuroblastoma cells

Neurotoxic properties of L-dopa and dopamine (DA)-related compounds were assessed in human neuroblastoma SH-SY5Y cells with reference to their structural relationship. L-Dopa and its metabolites containing two free hydroxyl residues on their benzene ring showed toxicity in the cell, which was prevented by superoxide dismutase (SOD) and reduced glutathione (GSH), but not by catalase. Furthermore, a synthetic derivative of DA, 3-hydroxy-4-methoxyphenethylamine (HMPE) containing methoxy residue at position 4 in the benzene ring, exerted partial cytotoxicity, which was not prevented by SOD, GSH or catalase. However, the metabolites containing methoxy residue at position 3 failed to show a toxic effect in the SH-SY5Y cells. Moreover, DA induced apoptotic cell death, which was observed by nuclear and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) staining and measurement of caspase-3 activity; this compound up-regulated apoptotic factor p53 while down-regulating anti-apoptotic factor Bcl-2. In the cell-free in vitro electron spin resonance (ESR) spectrometry, DA possessing two hydroxyl groups showed generation of DA-semiquinone radicals, which were markedly prevented by addition of SOD or GSH but not by catalase. On the other hand, methylation of one of the hydroxyl residues on the benzene ring of DA converted DA to an unoxidizable compound (3-MT or HMPE), and caused it to lose the property to produce semiquinone radicals. It has been previously reported that SOD acting as a superoxide:semiquinone oxidoreductase prevents quinone formation, and that reduced GSH through forming a complex with DA-quinone prevents quinone binding to the thiol group of the intact protein. Therefore, the present results suggest that DA and its metabolites containing two hydroxyl residues exert cytotoxicity mainly due to generation of highly reactive quinones.

Dopamine- or L-DOPA-induced neurotoxicity: the role of dopamine quinone formation and tyrosinase in a model of Parkinson's disease

Dopamine (DA)- or L-dihydroxyphenylalanine-(L-DOPA-) induced neurotoxicity is thought to be involved not only in adverse reactions induced by long-term L-DOPA therapy but also in the pathogenesis of Parkinson's disease. Numerous in vitro and in vivo studies concerning DA- or L-DOPA-induced neurotoxicity have been reported in recent decades. The reactive oxygen or nitrogen species generated in the enzymatical oxidation or auto-oxidation of an excess amount of DA induce neuronal damage and/or apoptotic or non-apoptotic cell death; the DA-induced damage is prevented by various intrinsic and extrinsic antioxidants. DA and its metabolites containing two hydroxyl residues exert cytotoxicity in dopaminergic neuronal cells mainly due to the generation of highly reactive DA and DOPA quinones which are dopaminergic neuron-specific cytotoxic molecules. DA and DOPA quinones may irreversibly alter protein function through the formation of 5-cysteinyl-catechols on the proteins. For example, the formation of DA quinone-alpha-synuclein consequently increases cytotoxic protofibrils and the covalent modification of tyrosine hydroxylase by DA quinones. The melanin-synthetic enzyme tyrosinase in the brain may rapidly oxidize excess amounts of cytosolic DA and L-DOPA, thereby preventing slowly progressive cell damage by auto-oxidation of DA, thus maintainng DA levels. Since tyrosinase also possesses catecholamine-synthesizing activity in the absence of tyrosine hydroxylase (TH), the double-edged synthesizing and oxidizing functions of tyrosinase in the dopaminergic system suggest its potential for application in the synthesis of DA, instead of TH in the degeneration of dopaminergic neurons, and in the normalization of abnormal DA turnover in the long-term L-DOPA-treated Parkinson's disease patients.

Terminally differentiated SH-SY5Y cells provide a model system for studying neuroprotective effects of dopamine agonists

We characterized undifferentiated (UN) and three differentiation conditions of the SH-SY5Y neuroblastoma cell line for phenotypic markers of dopaminergic cells, sensitivity to the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridinium ion (MPP+), the requirement to utilize the dopamine (DA) transporter (DAT) for MPP+ toxicity, and the neuroprotective effects of pramipexole. Cells were differentiated with retinoic acid (RA), 12-O-tetradecanoyl-phorbol-13-acetate (TPA), and RA followed by TPA (RA/TPA). RA/TPA treated cells exhibited the highest levels of tyrosine hydroxylase and DAT but lower levels of vesicular monoamine transporter. The kinetics of [3H]DA uptake and [3H]MPP+ uptake to DAT in RA/TPA differentiated cells were similar to that of rat and mouse caudate-putamen synaptosomes. RA/TPA differentiated cells evidenced high sensitivity to the neurotoxic effects of MPP+ (0.03 to 3.0 mM), and the neurotoxic effects of MPP+ were blocked with the DAT inhibitor 1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenylpropyl)piperazine (GBR 12909). DA-induced cell death was not more sensitive in RA vs RA/TPA differentiated cells and was not inhibited by transporter inhibitors. RA/TPA differentiated cells exhibited 3-fold and 6-fold higher levels, respectively, of DA D2 and D3 receptors than UN or RA differentiated cells. Pretreatment with pramipexole was protective against MPP+ in the RA/TPA differentiated cells but not in undifferentiated or RA differentiated cells. The neuroprotective effect of pramipexole was concentration-dependent and dopamine D2/D3 receptor dependent. In contrast, protection by pramipexole against DA was not DA receptor dependent. Further characterization of the neuroprotective effects of DA agonists in this model system can provide unique information about DA receptor dependent and independent mechanisms of neuroprotection.

TGF-beta1 increases tyrosine hydroxylase expression by a mechanism blocked by BMP-2 in human neuroblastoma SH-SY5Y cells

Human neuroblastoma SH-SY5Y cells were used to study the effects of transforming growth factor beta1 (TGF-beta1) and bone morphogenetic protein 2 (BMP-2) on neuronal differentiation and acquisition of a catecholaminergic phenotype. SH-SY5Y cells express the intracellular factors activated through the receptors of the TGFbeta superfamily members, Smad1 and Smad4, as in basal conditions or after differentiation with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) or retinoic acid (RA). TGF-beta1 and BMP-2 induce differentiation in SH-SY5Y cells by different pathways: the effect of TGF-beta1 is potentiated by TPA and the effect of BMP-2 is blocked by RA. Cell differentiation due to TGF-beta1 treatment is accompanied by an increase in tyrosine hydroxylase (TH) expression, more pronounced in the presence of TPA or RA and counteracted by BMP-2. BMP-2 and RA both induce noncatecholaminergic cell differentiation, and together they may induce choline acetyltransferase expression in serum-cultured cells. In conclusion, our results suggest that TGF-beta1 and BMP-2 may contribute, in opposite ways, to regulation of the neuronal catecholaminergic phenotype.

Oxidative and non-oxidative mechanisms of neuronal cell death and apoptosis by L-3,4-dihydroxyphenylalanine (L-DOPA) and dopamine

1. The present study was designed to evaluate the nature of intervening agents in L-DOPA- and dopamine-induced neurotoxicity in Neuro-2A cells. 2. In the absence of cells and in conditions of light protection, at 37 degrees C, L-DOPA or dopamine (1 mM) in culture medium degraded spontaneously in a time-dependent manner, this being prevented by ascorbic acid (200 microM) and other antioxidants, namely glutathione (1 mM), N-acetyl-L-cysteine (1 mM), sodium metabisulphite (200 microM), but not N-ter-butyl-alpha-phenylnitrone (1 mM) and deferoxamine (100 microM). 3. The viability of Neuro-2A cells declined following treatment with L-DOPA or dopamine in a concentration- and time-dependent manner. The decrease in cell viability by L-DOPA (10+/-4% of control) or dopamine (15+/-4% of control) was markedly attenuated by antioxidants (ascorbic acid, glutathione, N-acetyl-L-cysteine and sodium metabisulphite). Autoxidation of L-DOPA or dopamine was accompanied by the formation of H(2)O(2) in a time-dependent manner, this being completely prevented by ascorbic acid at 24 h or markedly reduced at 48 h. 4. Protective effects of 100 U ml(-1) catalase (40+/-1% of control) against L-DOPA-induced cell death were lower than those conferred by 200 microM ascorbic acid (70+/-3% of control). Catalase-induced protection (59+/-5% of control) against dopamine-induced cell death was similar to that conferred by 200 microM ascorbic acid (57+/-4% of control). L-DOPA-induced neuronal cell death was also accompanied by increases in caspase-3 activity, this being insensitive to ascorbic acid. Dopamine-induced increase in caspase-3 activity occurred only when autoxidation of the amine was prevented by ascorbic acid. 5. It is suggested that in addition to generation of H(2)O(2) and quinone formation, L-DOPA- and dopamine-induced cell death may result from induction of apoptosis, as evidenced by increases in caspase-3 activity. Dopamine per se induces apoptosis by a mechanism independent of oxidative stress, as evidenced by the fact that increases in caspase-3 activity occurred only when autoxidation of the amine was prevented.

Levodopa modulating effects of inducible nitric oxide synthase and reactive oxygen species in glioma cells

Neurological injury and Parkinson disease (PD) are often associated with the increase of nitric oxide (NO) and free radicals from resident glial cells in the brain. In vitro, exposure to L-3-4-dihydroxyphenylalanine (L-DOPA), one of the main therapeutic agents for the treatment of PD, can lead to neurotoxicity. In this study, lipopolysaccharide (LPS) and interferon-gamma (IFN-g) were used to stimulate C6 glioma cells in the presence of varying concentrations of L-DOPA (1 microM-1 mM). The results indicated a slight augmentation of NO(2)(-) production at low concentrations of L-DOPA (<100 microM) and complete inhibition of NO(2)(-) at higher concentrations (500 microM, 1 mM), (p < 0.001). Western blot analysis corroborated that L-DOPA effects on iNOS was at the level of its protein expression. Total reactive oxygen species (ROS) were detected using 2', 7'-dichlorofluorescein diacetate fluorescence dye (2', 7'-DCFC) and there was an increase of intensity with the increasing concentrations of L-DOPA. Furthermore, large amounts of superoxide (O(2)(-)) and hydrogen peroxide (H(2)O(2)) were generated from the autoxidation of L-DOPA. C6 cells contain high levels of catalase, with inadequate levels of superoxide dismutase (SOD); therefore, there was an accumulation of O(2)(-), tantamount to elevation in 2'7'-DCFC intensity. Simultaneous accumulation of O(2)(-) and NO(2)(-) would propel formation of peroxynitrite (ONOO-). SOD completely attenuated the autoxidation of L-DOPA and significantly reversed the inhibitory effects on iNOS at high concentrations. The data obtained confirmed that the observed effects on iNOS were not due to the activation of the D(1) or beta1 adrenergic receptors by L-DOPA. It was concluded from this study that L-DOPA contributed to the modulation of iNOS and to the increase of O(2)(-) production in the stimulated glioma cells in vitro.

Behavioral effects of aminochrome and dopachrome injected in the rat substantia nigra

The exact mechanism of cell death in neurodegenerative diseases remains obscure, although there is evidence that their pathogenesis may involve the formation of free radicals originating from the oxidative metabolism of catecholamines. The purpose of this study was to evaluate the degree of neurodegenerative changes and behavioral impairments induced by unilateral injection into the rat substantia nigra of cyclized o-quinones, aminochrome and dopachrome, derived from oxidizing dopamine and L-DOPA, respectively, with Mn(3+)-pyrophosphate complex. The behavioral changes were compared with those induced after selective lesions of dopaminergic neurons with 6-hydroxydopamine (6-OHDA). Intranigral injection of aminochrome and dopachrome produced impairment in motor and cognitive behaviors. The behavioral impairment was also revealed by apomorphine-induced rotational asymmetry. Apomorphine (0.5 mg/kg sc) significantly increased rotational behavior in rats injected with aminochrome and dopachrome. These rats presented a clear motor bias showing a significant contralateral rotation activity, similar but less vigorous that in rats injected with 6-OHDA. The avoidance conditioning was seriously impaired in rats injected with aminochrome and dopachrome although only dopachrome-injected rats showed a similar hypomotility to 6-OHDA-injected rats. The behavioral effects were correlated to the extent of striatal tyrosine hydroxylase (TH)-positive fiber loss. Rats receiving unilateral intranigral aminochrome and dopachrome injections exhibited a 47.9+/-5.1% and a 39.7+/-4.4% reduction in nigrostriatal TH-positive fiber density. In conclusion, this study provided evidence that oxidizing DA and L-DOPA to cytotoxic quinones, aminochrome and dopachrome appears to be an important mediator of oxidative damage in vivo.

Endogenous released ascorbic acid suppresses ethanol-induced hydroxyl radical production in rat striatum

Previous studies have shown that acute systemic administration of ethanol induced ascorbic acid release in the striatum. However, the pharmacological implications of ethanol-induced striatal ascorbic acid release are unclear. In the present study, ethanol-induced extracellular changes of ascorbic acid and hydroxyl radical levels were detected in rat striatum by using brain microdialysis coupled to high-performance liquid chromatography with electrochemical detection. It was found that both in male and female rats, ethanol (3.0 g/kg, i.p.) increased striatal ascorbic acid release in the first 60 min after ethanol administration. Meanwhile, the extracellular hydroxyl radical levels, detected as 2,3- and 2,5-DHBA, were significantly decreased. However, when the ascorbic acid levels returned to the baseline, hydroxyl radical levels rebounded. Administration of DL-fenfluramine (20 mg/kg, i.p.) had no effect on the basal levels of ascorbic acid and hydroxyl radical, but significantly blocked ethanol-induced ascorbic acid release and increased hydroxyl radical levels significantly. Exogenous administration of ascorbic acid (20 mg/kg, s.c.) increased the extracellular levels of ascorbic acid in the striatum, and inhibited the increase of 2,3- and 2,5-DHBA in DL-fenfluramine plus ethanol group. These results provide first evidence that release of endogenous ascorbic acid in the striatum plays an important role in preventing oxidative stress by trapping hydroxyl radical in the central nervous system.

Differential effect of catecholamines and MPP(+) on membrane permeability in brain mitochondria and cell viability in PC12 cells

The present study examined the effect of dopamine, 6-hydroxydopamine (6-OHDA), and MPP(+) on the membrane permeability transition in brain mitochondria and on viability in PC12 cells. Dopamine and 6-hydroxydopamine induced the swelling and membrane potential change in mitochondria, which was inhibited by addition of antioxidant enzymes, SOD and catalase. In contrast, antioxidant enzymes did not reduce the effect of MPP(+) on mitochondrial swelling and membrane potential. Catecholamines enhanced the Ca(2+) uptake and release by mitochondria, and the addition of MPP(+) induced Ca(2+) release. Catecholamines induced a thiol oxidation in mitochondria that was decreased by antioxidant enzymes. MPP(+) showed a little effect on the cytochrome c release from mitochondria and did not induce thiol oxidation. Catecholamines and MPP(+) induced a cell death, including apoptosis, in PC12 cells that was inhibited by addition of antioxidant enzymes. The result suggests that the oxidation of dopamine and 6-hydroxydopamine could modulate the membrane permeability in brain mitochondria and induce PC12 cell death, which may be ascribed to oxidative stress. MPP(+) appears to exert a toxic effect on neuronal cells by the action, which is different from catecholamines.

Cytoprotective role of taurine in a renal epithelial cell culture model

Taurine (TAU) is a sulfur-containing amino acid that has been shown to decrease during aging and is believed to be important for cytoprotection. A decrease in TAU could exacerbate the accumulation of free radical-induced damage that may lead to cell death during the aging process. We have shown previously that TAU directly inhibits dopamine (DA) and (-)-3-(3,4-dihydroxyphenyl)-L-alanine (L-dopa) oxidation. Experiments were conducted to establish a cytoprotective role for TAU. Porcine renal epithelial cells were treated for 1 hr with iron and catecholamines (L-dopa and DA) to produce cytotoxicity by a free radical and quinone mechanism in the absence and presence of 10 or 20mM TAU. Viability assays, protein, and DNA measurements were performed after a 24hr recovery period. In some experiments, cells were extracted immediately after the insult for DA and TAU content measurements using high performance liquid chromatography with electrochemical detection. Catecholamine-induced cytotoxicity caused a 50% loss in cell viability, and 10 or 20mM TAU provided significant protection from cytotoxicity and maintained the functional integrity of the cells. Photomicrographs showed attenuation in cell loss and swelling in the presence of TAU. Pretreatment with 1mM TAU followed by exposure to iron and L-dopa in the presence of 1mM TAU caused a moderate but non-significant increase in cell survival. These data conclusively show that TAU can play a cytoprotective role in the LLC-PK(1) cell culture model.

Effect of R-(-)-deprenyl and harmaline on dopamine- and peroxynitrite-induced membrane permeability transition in brain mitochondria

The present study examined the effect of MAO inhibitors, deprenyl and harmaline, on the membrane permeability transition in brain mitochondria. Deprenyl, harmaline, and antioxidant enzymes (SOD and catalase) attenuated alteration of the swelling, membrane potential, cytochrome c release, and Ca2+ transport in mitochondria treated with dopamine. In contrast, deprenyl and harmaline did not reduce the peroxynitrite-induced change in membrane permeability. Deprenyl and harmaline inhibited the decrease in thioredoxin reductase activity and the thiol oxidation in mitochondria treated with dopamine but did not decrease the effect of peroxynitrite. Deprenyl and harmaline significantly decreased the formation of melanin from dopamine. The results suggest that deprenyl and harmaline may protect brain mitochondria against the toxic action of dopamine oxidation by the maintenance of thioredoxin reductase activity, inhibition of thiol oxidation, and inhibition of dopamine oxidation product formation. In contrast, MAO inhibitors may not defend brain mitochondria against damaging action of peroxynitrite.

Protective effect of boldine on dopamine-induced membrane permeability transition in brain mitochondria and viability loss in PC12 cells

Boldine ([S]-2,9-dihydroxy-1,10-dimethoxyaporphine) has been shown to exert antioxidant and anti-inflammatory effects. The present study elucidated the protective effect of boldine on catecholamine-induced membrane permeability transition in brain mitochondria and viability loss in PC12 cells. Dopamine (200 microM) and 6-hydroxydopamine (6-OHDA, 100 microM) attenuated Ca(2+) and succinate-induced mitochondrial swelling and membrane potential formation. Boldine (10-100 microM) and 10 microg/mL of superoxide dismutase (SOD) or catalase reduced the effect of catecholamine oxidation on brain mitochondria. Boldine, SOD, and catalase decreased catecholamine-induced mitochondrial cytochrome c release. Antioxidant enzymes attenuated the depressant effect of catecholamines on mitochondrial electron flow, whereas boldine did not reduce it. Boldine inhibited the catecholamine-induced decrease in thioredoxin reductase activity and the increase in thiol oxidation in mitochondria. It also showed a scavenging action on hydrogen peroxide and hydroxyl radicals and decreased the formation of melanin from dopamine. Boldine and antioxidant enzymes decreased the dopamine-induced cell death, including apoptosis, in PC12 cells. The results suggest that boldine may attenuate the catecholamine oxidation-induced brain mitochondrial dysfunction and decrease the dopamine-induced death of PC12 cells through a scavenging action on reactive oxygen species and inhibition of melanin formation and thiol oxidation.

Oxidation of proteinaceous cysteine residues by dopamine-derived H2O2 in PC12 cells

Cellular metabolism of dopamine (DA) generates H2O2, which is further reduced to hydroxyl radicals in the presence of iron. Cellular damage inflicted by DA-derived hydroxyl radicals is thought to contribute to Parkinson's disease. We have previously developed procedures for detecting proteins that contain H2O2-sensitive cysteine (or selenocysteine) residues. Using these procedures, we identified ERP72 and ERP60, two members of the protein disulfide isomerase family, creatine kinase, glyceraldehyde-3-phosphate dehydrogenase, phospholipase C-gamma1, and thioredoxin reductase as the targets of DA-derived H2O2. Experiments with purified enzymes identified the essential Cys residues of creatine kinase and glyceraldehyde-3-phosphate dehydrogenase, that are specifically oxidized by H2O2. Although the identified proteins represent only a fraction of the targets of DA-derived H2O2, functional impairment of these proteins has previously been associated with cell death. The oxidation of proteins that contain reactive Cys residues by DA-derived H2O2 is therefore proposed both to be largely responsible for DA-induced apoptosis in neuronal cells and to play an important role in the pathogenesis of Parkinson's disease.

Dendrodendritic inhibition through reversal of dopamine transport

Synapses in the central nervous system are usually defined by presynaptic exocytotic release sites and postsynaptic differentiations. We report here a demonstration of dendrodendritic inhibition that does not engage a conventional synapse. Using amperometric and patch-clamp recordings in rat brain slices of the substantia nigra, we found that blockade of the dopamine transporter abolished the dendritic release of dopamine and the resulting self-inhibition. These findings demonstrate that dendrodendritic autoinhibition entails the carrier-mediated release of dopamine rather than conventional exocytosis. This suggests that some widely used antidepressants that inhibit the dopamine transporter may benefit patients in the early stages of Parkinson's disease.

Involvement of oxidative stress in ascorbate-induced proapoptotic death of PC12 cells

Ascorbate is a reducing agent, but it is also known to oxidize cellular components under specific conditions. The mechanism of this oxidative action, however, is not well established. Ascorbate treatment increased lipid peroxide content in PC12 cells, but did not increase quantities of lipid peroxide when homogenates of PC12 cells were treated with ascorbate, suggesting that cellular integrity is required for ascorbate to generate lipid peroxidation. However, dehydroascorbate increased lipid peroxide production in both intact PC12 cells and the cell homogenates. These differential effects of ascorbate and dehydroascorbate on intact cells versus homogenates suggest that the dehydroascorbate in cytosol induces an oxidative stress. Ascorbate in culture medium is rapidly oxidized to dehydroascorbate, which is transported into cells by a glucose transporter (GLUT). The GLUT antagonists wortmannin and cytochalasin B, or a high concentration of glucose, blocked (14)C uptake (from ascorbate) in a time-dependent manner and suppressed lipid peroxide production in PC12 cells. These observations support the concept that ascorbate is oxidized to dehydroascorbate, which is transported into cells via GLUT. The dehydroascorbate induces oxidative stress. The oxidative stress triggered apoptosis according to ceramide production, caspase-3 activation, and TUNEL. We have concluded that ascorbate is taken up after oxidation to dehydroascorbate via a "dehydroascorbate transporter" (GLUT), and the dehydroascorbate generates an oxidative stress which triggers apoptosis. These studies have significant implications for conditions under which a high concentration of ascorbate in a tissue is released during a period of hypoxia (e.g., stroke) and taken up during a reperfusion period as dehydroascorbate. Inhibiting uptake of dehydroascorbate may offer novel therapeutic strategies to alleviate brain damage during a reperfusion period.

The transcription factor E2F1 promotes dopamine-evoked neuronal apoptosis by a mechanism independent of transcriptional activation

The E2F1 transcription factor plays an important role in promoting neuronal apoptosis; however, it is not clear how E2F1 does this. Here we show that E2F1 is involved in dopamine (DA)-evoked apoptosis in cerebellar granule neurons (CGNs). E2F1 -/- CGNs and CGNs expressing an antisense E2F1 cDNA were significantly protected from DA-toxicity relative to controls. The neuronal protection was accompanied by significantly reduced caspase 3 activity. E2F1-mediated neuronal apoptosis did not require activation of gene transcription because: (1) ectopic expression of E2F1 or its mutants lacking the transactivation domain induced neuronal apoptosis, whereas an E2F1 mutant lacking the DNA-binding domain did not; (2) under all of these conditions, known E2F1 target genes including cyclin A, cdc2 and p19(ARF) were not induced; and (3) DA-evoked neuronal apoptosis was associated with up-regulated E2F1, but not transcription of its target genes. Finally, E2F1-mediated neuronal apoptosis was associated with reduced nuclear factor (NF)-kappaB DNA-binding activity. Taken together, these data suggest that E2F1 promotes DA-evoked caspase 3-dependent neuronal apoptosis by a mechanism independent of gene transactivation, and this may possibly occur through inhibition of anti-apoptotic genes including NF-kappaB.

Protective effect of harmaline and harmalol against dopamine- and 6-hydroxydopamine-induced oxidative damage of brain mitochondria and synaptosomes, and viability loss of PC12 cells

The present study elucidated the protective effect of beta-carbolines (harmaline, harmalol and harmine) against oxidative damage of brain mitochondria, synaptosomes and PC12 cells induced by either dopamine or 6-hydroxydopamine. Harmaline, harmalol and antioxidant enzymes (superoxide dismutase/SOD and catalase) decreased the alteration of mitochondrial swelling and membrane potential induced by 200 microM dopamine or 100 microM 6-hydroxydopamine. Deprenyl attenuated the dopamine-induced mitochondrial dysfunction but did not reduce the effect of 6-hydroxydopamine. While beta-carbolines inhibited the electron flow in mitochondria, they did not enhance the depressant effect of catecholamines. beta-Carbolines and antioxidant enzymes reversed the depression of synaptosomal Ca2+ uptake induced by 10 microM catecholamines. The compounds inhibited the catecholamine-induced thioredoxin reductase inhibition, thiol oxidation and carbonyl formation in mitochondria and synaptosomes. beta-Carbolines decreased the reactive species-induced deoxyribose degradation. Harmaline and harmalol reduced the catecholamine-induced loss of the transmembrane potential and of cell viability in PC12 cells. beta-Carbolines alone did not show a significant cytotoxic effect on PC12 cells. The results suggest that beta-carbolines may attenuate the dopamine- or 6-hydroxydopamine-induced alteration of brain mitochondrial and synaptosomal functions, and viability loss in PC12 cells, by a scavenging action on reactive oxygen species and inhibition of thiol oxidation.

Protection of intracellular dopamine cytotoxicity by dopamine disposition and metabolism factors

Dopamine has been hypothesized as a contributing factor for the selective degeneration of dopaminergic neurons in Parkinson's disease. However, the cytotoxic mechanisms of dopamine and its metabolites remain poorly understood. Using a stable aromatic amino acid decarboxylase (AADC) expressing a fibroblast cell line, we previously demonstrated a novel, non-oxidative cytotoxicity of intracellular dopamine. In this study, we further investigate the roles of dopamine metabolism and disposition proteins against intracellular dopamine cytotoxicity by co-expressing these factors in AADC-expressing cells. Our results indicate that overexpression of the vesicular monoamine transporter and monoamine oxidase A-induced protection against intracellular dopamine toxicity, and conversely that pharmacological inhibition of these pathways potentiated L-DOPA toxicity in catecholaminergic PC12 cells. Macrophage migration inhibitory factor and glutathione S-transferase (GST), factors that have recently been shown to be involved in dopamine metabolism, also exhibited a strong protective role against intracellular dopamine cytotoxicity. Our results support a potential role for non-oxidative cytoplasmic dopamine toxicity, and imply that disruption in dopamine disposition and/or metabolism could underlie the progressive degeneration of dopaminergic neurons in Parkinson's disease.

Anti-apoptotic effect of trans-resveratrol on paclitaxel-induced apoptosis in the human neuroblastoma SH-SY5Y cell line

Paclitaxel, an anticancer drug, induces apoptosis in human neuroblastoma cell line SH-SY5Y. The addition of trans-resveratrol, a natural antioxidant present in grapes and red wine, to SH-SY5Y cultures exposed to paclitaxel significantly reduces cellular death. The neuroprotective action of trans-resveratrol is due neither to its antioxidant capacity nor to interference with the polymerization of tubulin induced by paclitaxel. However, trans-resveratrol is able to inhibit the activation of caspase 7 and degradation of poly-(ADP-ribose)-polymerase which occur in SH-SY5Y exposed to paclitaxel. Resveratrol, therefore, exerts its anti-apoptotic effect by modulating the signal pathways that commit these neuronal-like cells to apoptosis.

Evidence for non-oxidative dopamine cytotoxicity: potent activation of NF-kappa B and lack of protection by anti-oxidants

A stable aromatic acid decarboxylase expressing the Chinese hamster ovary cell line was developed to study the cytotoxic properties of intracellular and extracellular dopamine. The relative impermeability of cells to dopamine, but not to L-DOPA, allows the differentiation of extracellular and intracellular dopamine cytotoxicity. In contrast to extracellular dopamine, intracellular dopamine toxicity was resistant to antioxidant protection, and did not require melanin formation for its toxicity. Furthermore, we demonstrated a rapid and potent activation of the stress-inducible transcription factor NF-kappa B by intracellular dopamine, which was also largely insensitive to antioxidant inhibition. A distinctly slower and less potent NF-kappa B activation by extracellular dopamine was blocked by antioxidants and acetylsalicylic acid. Our results indicate the existence of a non-oxidative mechanism of dopamine cytotoxicity. Mitigating intracellular dopamine toxicity could be a novel strategy of slowing the progressive degeneration of dopaminergic neurons in Parkinson's disease.

Molecular cloning, localization and characterization of a 40-kDa catecholamine-regulated protein

We have previously described catecholamine-regulated proteins of molecular masses 47, 40 and 26 kDa (CRP47/40/26). In mammals, these proteins are detected only in brain and have been implicated as playing a role in dopaminergic neurotransmission. In this report, we have cloned the cDNA encoding CRP40 from bovine brain. Analysis of the predicted amino acid sequence revealed that the CRP40 product contains an hsp70 motif and shares homology with heat-shock protein hsp70. Immunolocalization studies using mAbs to dopamine show that it colocalizes with CRP40 in the vesicles of dopaminergic neuroblastoma SH-SY5Y cells. The constitutive expression of CRP40 was increased by exposure to heat shock similar to inducible heat-shock protein hsp70 in SH-SY5Y cells. Dopamine significantly modulated the levels of CRP40, whereas, the expression of hsp70 remained unchanged upon dopamine treatment of these cells. Moreover, CRP40 is able to prevent the thermal aggregation of luciferase in vitro, similar to hsp70, suggesting that CRP40 encodes a dopamine-inducible protein with properties similar to heat-shock proteins. The immunofluorescence analyses show that in SH-SY5Y cells, CRP40 translocates to the nucleus during dopamine-induced apoptosis. These results suggest that CRP40 could play a protective role against the harmful effects of catecholamine metabolites.

Dopamine induces ERK activation in renal epithelial cells through H2O2 produced by monoamine oxidase

BACKGROUND

The rat renal proximal tubule cells contain a large amount of monoamine oxidase, which catalyzes the oxidative deamination of catecholamines such as dopamine (DA). The aim of this study is to investigate the potential role of hydrogen peroxide (H2O2) produced by monoamine oxidase (MAO) isoform on regulation of cell signaling and function.

METHODS

Primary rat proximal tubular cells, which contain almost exclusively MAO-A, and human embryonic kidney 293 (HEK 293) cells stably transfected with human MAO-B cDNA were treated with DA or tyramine in the presence or the absence of some inhibitors. Then, Shc protein tyrosine phosphorylation and extracellular-regulated kinase (ERK) activation were evaluated by immunoprecipitation/immunoblot analysis and cell proliferation by [3H]thymidine incorporation or cell counting.

RESULTS

In rat proximal tubule cells, DA induced tyrosine phosphorylation of Shc, ERK activation, and a significant increase in DNA synthesis. The involvement of MAO-dependent H2O2 generation induced by DA (5 micromol/L) was supported by the demonstration that the DA effects were (1) fully prevented by cell pretreatment with the MAO inhibitor pargyline, the antioxydant N-acetylcysteine (NAC), and the DA uptake inhibitor GBR 12909; (2) not abrogated by the D1 and D2 receptor antagonists; (3) observed in HEK 293 MAO-B cells but not in HEK 293 wild-type cells, which do not express MAO; and (4) similar to those induced by another MAO substrate, tyramine.

CONCLUSIONS

Taken together, these results show that in addition to the effects related to receptor stimulation, DA, and probably the other catecholamines, may induce some of its effects through the MAO-dependent H2O2 production.

Toxic effects of apomorphine on rat cultured neurons and glial C6 cells, and protection with antioxidants

Many catechol derivatives are currently used as drugs, even if they produce reactive oxygen species that may cause tissue damage. Among them, apomorphine, a potent dopamine agonist, displays efficient anti-parkinsonian properties, but the consequences of its oxidant and toxic properties have been poorly investigated on in vitro models. In the present work, we investigated apomorphine cytotoxicity by incubating cultures of rat glioma C6 cells and primary cultures of neurons with different concentrations of the drug. Apomorphine-promoted cell death was proportional to its concentration and was time-dependent. The ED(50) of apomorphine on C6 cell death after 48 hr was about 200 microM. The cytotoxic effects induced by apomorphine were correlated to its autoxidation, which leads to the formation of reactive oxygen species, semiquinones, quinones, and a melanin-like pigment. C6 cells that underwent treatment with 400 microM apomorphine for 6 hr displayed features of necrosis, including loss of membrane integrity, degeneration of mitochondria, and DNA fragmentation. Thiols, such as cysteine, N-acetyl-L-cysteine, and glutathione, significantly protected cultured neurons and C6 cells against apomorphine-induced cytotoxicity. Thiols also inhibited apomorphine autoxidation. These data strongly suggest that apomorphine cytotoxicity towards neurons and C6 cells results from an intracellular oxidative stress.

Inhibition of tyrosinase reduces cell viability in catecholaminergic neuronal cells

The biosynthesis of dopamine (DA) in catecholaminergic neurons is regulated by tyrosine hydroxylase, which converts tyrosine into 3, 4-dihydroxyphenylalanine (L-DOPA). In melanocytes, tyrosinase catalyzes both the hydroxylation of tyrosine and the consequent oxidation of L-DOPA to form melanin. Although it has been demonstrated that tyrosinase is also expressed in the brain, the physiological role of tyrosinase in the brain is still obscure. In this study, to investigate the role of tyrosinase in catecholaminergic neuronal cells, we examined the effects of tyrosinase inhibition on the viability of CATH.a and SH-SY5Y cells using tyrosinase inhibitors-specifically, phenylthiourea (PTU) and 5-hydroxyindole (5-HI)-and the transfection of antisense tyrosinase cDNA. Both inhibitors significantly reduced the cell viability of CATH.a cells in a dose-dependent manner. PTU also specifically enhanced DA-induced cell death, but 5-HI did not. This discrepancy in cell death is probably due to the inhibitors' different mechanism of action: 5-HI inhibits the hydroxylation of tyrosine as a competitor for the substrate to induce cell death that may be due to depletion of DA, whereas PTU mainly inhibits the enzymatic oxidation of L-DOPA and DA rather than tyrosine hydroxylation to increase consequently autooxidation of DA. Indeed, the intracellular DA content in CATH.a cells was enhanced by PTU exposure. In contrast, PTU showed no enhancing effects on DA-induced cell death of SH-SY5Y cells, which express little tyrosinase. Furthermore, transfection with antisense tyrosinase cDNA into CATH.a cells dramatically reduced cell viability and significantly enhanced DA-induced cell death. These results suggest that tyrosinase controls the intracellular DA content by biosynthesis or enzymatic oxidation of DA, and the dysfunction of this activity induces cell death by elevation of intracellular DA level and consequent gradual autooxidation of DA to generate reactive oxygen species.

Effects of dopamine and L-DOPA on survival of PC12 cells

The effects of dopamine and L-DOPA on survival were examined in differentiated PC12 cells. Addition of dopamine to the culture medium at 3-30 microM prevented cell death induced by depletion of serum and nerve growth factor (NGF). At 100 microM, dopamine induced cell death. The cell-protective effect of dopamine was not affected by nomifensine, an inhibitor of dopamine uptake, or pargyline, an inhibitor of monoamine oxidase, suggesting that dopamine is working outside the cell. The cell-protective effect of dopamine was blunted by SCH-23390, a D(1) antagonist, but not sulpiride, a D(2) antagonist, indicating that the cell protective effect of dopamine is mediated by D(1) receptors in PC12 cells. L-DOPA also protected PC12 cells from cell death induced by depletion of serum and NGF at low concentrations and showed toxicity at high concentration. The effect of L-DOPA was unchanged after inhibition of conversion of L-DOPA to dopamine by m-hydroxybenzylhydrazine (NSD-1015), an inhibitor of DOPA decarboxylase, suggesting that L-DOPA itself is working for cell protection. Intracellular Ca(2+) concentration and mitogen-activated protein (MAP) kinase activity were increased by both dopamine and L-DOPA. The effects of dopamine and L-DOPA on cell survival were blunted by nicardipine, a Ca(2+) channel blocker, and PD-98059, an inhibitor of MAP kinase kinase (MEK). These results taken together raised the possibility that dopamine and L-DOPA protect PC12 cells from cell death at low concentrations by activating MAP kinase activity via elevation of intracellular Ca(2+) concentration.

Functional changes of the basal ganglia circuitry in Parkinson's disease

The basal ganglia circuitry processes the signals that flow from the cortex, allowing the correct execution of voluntary movements. In Parkinson's disease, the degeneration of dopaminergic neurons of the substantia nigra pars compacta triggers a cascade of functional changes affecting the whole basal ganglia network. The most relevant alterations affect the output nuclei of the circuit, the medial globus pallidus and substantia nigra pars reticulata, which become hyperactive. Such hyperactivity is sustained by the enhanced glutamatergic inputs that the output nuclei receive from the subthalamic nucleus. The mechanisms leading to the subthalamic disinhibition are still poorly understood. According to the current model of basal ganglia organization, the phenomenon is due to a decrease in the inhibitory control exerted over the subthalamic nucleus by the lateral globus pallidus. Recent data, however, suggest that additional if not alternative mechanisms may underlie subthalamic hyperactivity. In particular, given the reciprocal innervation of the substantia nigra pars compacta and the subthalamic nucleus, the dopaminergic deficit might influence the subthalamic activity, directly. In addition, the increased excitatory drive to the dopaminergic nigral neurons originating from the hyperactive subthalamic nucleus might sustain the progression of the degenerative process. The identification of the role of the subthalamic nucleus and, more in general, of the glutamatergic mechanisms in the pathophysiology of Parkinson's disease might lead to a new approach in the pharmacological treatment of the disease. Current therapeutic strategies rely on the use of L-DOPA and/or dopamine agonists to correct the dopaminergic deficit. Drugs capable of antagonizing the effects of glutamate might represent, in the next future, a valuable tool for the development of new symptomatic and neuroprotective strategies for therapy of Parkinson's disease.

Protective effect of harmalol and harmaline on MPTP neurotoxicity in the mouse and dopamine-induced damage of brain mitochondria and PC12 cells

The present study elucidated the protective effect of beta-carbolines (harmaline, harmalol, and harmine) on oxidative neuronal damage. MPTP treatment increased activities of total superoxide dismutase, catalase, and glutathione peroxidase and levels of malondialdehyde and carbonyls in the basal ganglia, diencephalon plus midbrain of brain compared with control mouse brain. Coadministration of harmalol (48 mg/kg) attenuated the MPTP effect on the enzyme activities and formation of tissue peroxidation products. Harmaline, harmalol, and harmine attenuated both the 500 microM MPP(+)-induced inhibition of electron flow and membrane potential formation and the 100 microM dopamine-induced thiol oxidation and carbonyl formation in mitochondria. The scavenging action of beta-carbolines on hydroxyl radicals was represented by inhibition of 2-deoxy-D-ribose degradation. Harmaline and harmalol (100 microM) attenuated 200 microM dopamine-induced viability loss in PC12 cells. The beta-carbolines (50 microM) attenuated 50 microM dopamine-induced apoptosis in PC12 cells. The compounds alone did not exhibit significant cytotoxic effects. The results indicate that beta-carbolines attenuate brain damage in mice treated with MPTP and MPP(+)-induced mitochondrial damage. The compounds may prevent dopamine-induced mitochondrial damage and PC12 cell death through a scavenging action on reactive oxygen species and inhibition of monoamine oxidase and thiol oxidation.

Manganese increases L-DOPA auto-oxidation in the striatum of the freely moving rat: potential implications to L-DOPA long-term therapy of Parkinson's disease

We have previously shown that manganese enhances L-dihydroxyphenylanine (L-DOPA) toxicity to PC12 cells in vitro. The supposed mechanism of manganese enhancing effect [an increase in L-DOPA and dopamine (DA) auto-oxidation] was studied using microdialysis in the striatum of freely moving rats. Systemic L-DOPA [25 mg kg(-1) intraperitoneally (i.p.) twice in a 12 h interval] significantly increased baseline dialysate concentrations of L-DOPA, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and uric acid, compared to controls. Conversely, DA and ascorbic acid concentrations were significantly decreased. A L-DOPA oxidation product, presumptively identified as L-DOPA semiquinone, was detected in the dialysate. The L-DOPA semiquinone was detected also following intrastriatal infusion of L-DOPA. In rats given L-DOPA i.p. , intrastriatal infusion of N-acetylcysteine (NAC) significantly increased DA and L-DOPA dialysate concentrations and lowered those of L-DOPA semiquinone; in addition, NAC decreased DOPAC+HVA and uric acid dialysate concentrations. In rats given L-DOPA either systemically or intrastriatally, intrastriatal infusion of manganese decreased L-DOPA dialysate concentrations and greatly increased those of L-DOPA semiquinone. These changes were inhibited by NAC infusion. These findings demonstrate that auto-oxidation of exogenous L-DOPA occurs in vivo in the rat striatum. The consequent reactive oxygen species generation may account for the decrease in dialysate DA and ascorbic acid concentrations and increase in enzymatic oxidation of xanthine and DA. L-DOPA auto-oxidation is inhibited by NAC and enhanced by manganese. These results may be of relevance to the L-DOPA long-term therapy of Parkinson's disease.

Role of dopamine in malignant tumor growth

The regulatory role of dopamine, a monoamine neurotransmitter and/or a neurohormone in controlling the secretion of several anterior pituitary hormones, cardiovascular, and renal functions, has already been extensively used by clinicians for therapeutic purposes. In addition to these important functions of dopamine, some recent reports also indicate its novel role in regulating malignant cell proliferation and controlling immune functions in tumor-bearing animals. Therefore, in this article, we discuss all the relevant information correlating dopamine and malignant tumor growth in order to understand the host-tumor relationship at the level of a neurotransmitter and/or a neurohormone.

Dopamine agonists and analogues have an antiproliferative effect on CHO-K1 cells

Epidemiological studies have shown a reduced incidence of cancer in Parkinson's disease. Since nearly all parkinsonian patients with clinical impairment are treated with L-beta-3,4-dihydroxyphenylalanine (L-DOPA) and dopamine (DA)ergic agonists, a possibility exists that these therapeutic agents can influence the risk of cancer. We studied the antiproliferative effect of these therapeutic agents (and substances structurally correlated) on Chinese hamster ovary (CHO)-K1 cell growth. Among the compounds tested, apomorphine proved to be the most potent inhibitor of CHO-K1 cell growth, with an EC(50) of 3.35 +/- 0.12 micro M. The apomorphine analogues, apocodeine and hydroxyethylnorapomorphine, were less active as inhibitors of CHO-K1 cell growth. The activity of DA, 6-hydroxydopamine (6-OHDA), phenylethylamine (PEA), L-DOPA and bromocriptine as antiproliferative was one order of magnitude lower than that of apomorphine while pergolide was ineffective. To test whether or not the oxidative potential of these compounds was important for their antiproliferative effect, several antioxidants were assayed. Among them glutathione (GSH) and dithiothreitol (DTT) were effective in reversing the anti-proliferative effect of apomorphine, DA, 6-OHDA and PEA, conversely they did not work with bromocriptine. GSH and DTT are sulphydryl-reducing agents; while their effect could explain the efficacy against apomorphine, DA and 6-OHDA, it is difficult to understand why they should have any effect on PEA as this substance does not react with sulphydryl groups. The oxidative potential as a mechanism of action was also questioned by the results obtained with dihydrorhodamine 123, a probe that changes its fluorescent emission wave when oxidized. None of the compounds, with the exception of 6-OHDA, had any effect on the fluorescent emission wave of the probe at the maximal concentrations used to inhibit CHO-K1 cell growth. At concentrations five times higher, apomorphine and DA generated reactive oxygen species but PEA and bromocriptine did not. These data demonstrate that the antiproliferative effect of these compounds is not due to their oxidative potential, but another mechanism must be postulated.

L-DOPA-induced neurotoxic and apoptotic changes on cultured chromaffin cells

Adrenal chromaffin cell (ACC) transplantation has been considered as one of the therapeutic strategies for Parkinson disease (PD). This strategy involves the administration of L-DOPA, although in reduced doses, to ACC-transplanted patients. Using cytochemical and morphological methods, we examined the effects of clinically applicable concentrations of L-DOPA on cultured chromaffin cells. We found an increase of cell death in both necrotic and apoptotic patterns. These data suggest that therapeutic preventive measures during ACC transplantation processes for PD should be taken.

Review of apoptosis vs. necrosis of substantia nigra pars compacta in Parkinson's disease

The discovery that melanized neurons of the pars compacta of substantia nigra (pcSN) degenerate in the midbrain of human Parkinsonians is nearly a century old, but only in this decade have we gained insights into mechanisms underlying this neuronal loss. Although it had long been assumed that pcSN neurons underwent necrosis, recent (1) in vitro studies on isolated neurons, (2) in vivo studies in animals treated with neurotoxins, and (3) postmortem study of human Parkinsonian brain provide strong evidence that pcSN cells may be lost more from apoptosis (i.e., cell suicide) than from necrosis. This paper gives some historical perspective, but focuses primarily on mechanisms involved in both necrosis and apoptosis of neurons, primarily dopaminergic, and reviews the recent literature relating to apoptosis and apoptotic factors now identified in neurons undergoing neurotoxin-induced death and in postmortem human Parkinsonian brain. The weight of evidence in favor of apoptosis and apoptotic factors in these neurons, provides us with tools needed to develop anti-apoptotic factors that can be targeted to proteins on genes, so that it may be possible to decelerate or prevent the progressive neuronal cell loss in human Parkinsonians or in humans with other neurodegenerative disorders.

Taurine inhibition of metal-stimulated catecholamine oxidation

Taurine is an abundant amino acid found in mammalian tissues and it has been suggested to have cytoprotective functions. The aim of the present study was to determine if taurine had the potential to reduce oxidative stress associated with metal-stimulated catecholamine oxidation. Taurine and structural analogs of taurine were tested for their ability to inhibit metal-stimulated quinone formation from dopamine or L-dopa. Oxidative damage to proteins and lipids were also assessed in vitro and the effects of taurine were determined. Taurine (20 mM) was found to decrease significantly ferric iron (50-500 microM)- and manganese (10 microM)-stimulated L-dopa or dopamine oxidation. Taurine had no effect on zinc-induced dopamine oxidation and slightly potentiated copper- and NaIO(4)-stimulated quinone formation. Ferric iron-stimulated lipid peroxidation was not affected by taurine (1-20 mM). Protein carbonyl formation induced by ferric iron (500 microM) and L-dopa (500 microM) was significantly reduced by 10 mM taurine. The cytotoxicity of L-dopa (250 microM) and ferric chloride (75 microM) to LLC-PK(1) cells was attenuated by 10 mM taurine or hypotaurine. Homotaurine alone stimulated L-dopa oxidation and potentiated the cytotoxic effects of ferric iron. Homotaurine was found to be cytotoxic when combined with L-dopa or L-dopa/iron. In contrast, hypotaurine inhibited quinone formation and protected LLC-PK(1) cells. These studies suggest that taurine may exhibit cytoprotective effects against the oxidation products of catecholamines by acting as a scavenger for free radicals and cytotoxic quinones.

Modulation of brain mitochondrial membrane permeability and synaptosomal Ca2+ transport by dopamine oxidation

Effects of dopamine on the membrane permeability transition, thioredoxin reductase activity, production of free radicals and oxidation of sulfhydryl groups in brain mitochondria and the Ca2+ uptake by Na+-Ca2+ exchange and sulfhydryl oxidation in brain synaptosomes were examined. The brain mitochondrial swelling and the fall of transmembrane potential were altered by pretreatment of dopamine in a dose dependent manner. Depressive effect of dopamine on mitochondrial swelling was reversed by 10 microg/ml catalase, and 10 mM DMSO. The activities of thioredoxin reductase in intact or disrupted mitochondria were decreased by dopamine (1-100 microM), 25 microM Zn2+ and 50 microM Mn2+. Dopamine-inhibited enzyme activity was reversed by 10 microg/ml SOD and 10 microg/ml catalase. Pretreatment of dopamine decreased Ca2+ transport in synaptosomes, which was restored by 10 microg/ml SOD and 10 mM DMSO. Dopamine (1-100 microM) in the medium containing mitochondria produced superoxide anion and hydrogen peroxide, while its effect on nitrite production was very weak. The oxidation of sulfhydryl groups in mitochondria and synaptosomes were enhanced by dopamine with increasing incubation times. Results suggest that dopamine could modulate membrane permeability in mitochondria and calcium transport at nerve terminals, which may be ascribed to the action of free radicals and the loss of reduced sulfhydryl groups.

Contrasting effects of catecholic and O-methylated tetrahydroisoquinolines on hydroxyl radical production

Tetrahydroisoquinolines (TIQs) are intraneuronal, catecholamine-derived alkaloids that have been implicated in the etiology of Parkinson's disease and in alcohol related disorders. The in vitro production of the cytotoxic hydroxyl radical (*OH) was recorded during the autoxidation of salsolinol (SAL) and salsolinol-1-carboxylic acid (SAL-1C), but not when these two catecholic TIQs were oxidized by tyrosinase. Significantly higher levels of the radical were produced when these catecholic TIQs were incubated with *OH generating complexes, or with chelated iron. In contrast, mono-O-methylated TIQs such as salsoline (SLN) and salsoline-1-carboxylic acid (SLN-1C) did not generate *OH during autoxidation or when incubated with chelated iron or tyrosinase. Radical production by *OH-generating complexes was reduced in the presence of O-methylated TIQs. The neurotoxicity of TIQs may result from their propensity to autoxidize and generate reactive quinoids and ensuing oxygen radicals. The functional significance of the replacement of a hydroxyl group attached to C-7 of SAL or SAL-1C with a methoxyl group remains to be determined. This single structural modification may prevent mono-O-methylated TIQs from participating in catalytic redox cycling reactions that would otherwise augment *OH production. If true, then O-methylation and other cellular mechanisms that circumvent the autoxidation of catecholamine-derived TIQs may reduce the likelihood of these substances forming cytotoxic quinoids and influencing endogenous *OH-generating reactions.

Determination of hydroxyl free radical formation in human platelets using high-performance liquid chromatography with electrochemical detection

The formation of the hydroxyl free radical (HFR) can be quantified indirectly, by measuring two products of the hydroxylation of salicylic acid, 2,3-dihydroxybenzoate (2,3-DHB) and 2,5-dihydroxybenzoate (2,5-DHB). In this study, we used reversed-phase high-performance liquid chromatography with electrochemical (coulometric) detection to measure 2,3-and 2,5-DHB levels in human platelets. The limits of detection of the method were 10 and 5 fmol on column for 2,3-DHB and 2,5-DHB, respectively. We tested the technique by measuring increases in dihydroxybenzoate levels after exposure of platelets to experimentally induced oxidative stress. Then, we measured platelet levels of 2,3- and 2,5-DHB in patients with Parkinson's disease, under therapy with L-DOPA, and in normal subjects. We also measured platelet concentrations of L-DOPA and its major metabolite, 3-O-methyldopa (3-OMD). Parkinsonian patients showed increased levels of both 2,3- and 2,5-DHB. Platelet levels of 2,3-DHB were positively correlated with platelet levels of L-DOPA and 3-OMD. The technique we describe proved simple and extremely sensitive and may represent a useful tool for the study of oxidative stress in humans.

Enhancing effect of manganese on L-DOPA-induced apoptosis in PC12 cells: role of oxidative stress

L-DOPA and manganese both induce oxidative stress-mediated apoptosis in catecholaminergic PC12 cells. In this study, exposure of PC12 cells to 0.2 mM MnCl2 or 10-20 microM L-DOPA neither affected cell viability, determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, nor induced apoptosis, tested by flow cytometry, fluorescence microscopy, and the TUNEL technique. L-DOPA (50 microM) induced decreases in both cell viability and apoptosis. When 0.2 mM MnCl2 was associated with 10, 20, or 50 microM L-DOPA, a concentration-dependent decrease in cell viability was observed. Apoptotic cell death also occurred. In addition, manganese inhibited L-DOPA effects on dopamine (DA) metabolism (i.e., increases in DA and its acidic metabolite levels in both cell lysate and incubation medium). The antioxidant N-acetyl-L-cysteine significantly inhibited decreases in cell viability, apoptosis, and changes in DA metabolism induced by the manganese association with L-DOPA. An increase in autoxidation of L-DOPA and of newly formed DA is suggested as a mechanism of manganese action. These data show that agents that induce oxidative stress-mediated apoptosis in catecholaminergic cells may act synergistically.

Peripheral markers of oxidative stress in Parkinson's disease. The role of L-DOPA

Oxidative stress plays a central role in the pathogenesis of Parkinson's disease (PD). L-DOPA, the gold standard in PD therapy, may paradoxically contribute to the progression of the disease because of its pro-oxidant properties. The issue, however, is controversial. In this study, we evaluated peripheral markers of oxidative stress in normal subjects, untreated PD patients and PD patients treated only with L-DOPA. We also measured platelet and plasma levels of L-DOPA, 3-O-methyldopa (the long-lasting metabolite of the drug), and dopamine. We found that isolated platelets of treated PD patients form higher amounts of 2,3-dihydroxybenzoate, an index of hydroxyl radical generation, than platelets of controls or untreated patients. In treated patients, platelet levels of 2,3-dihydroxybenzoate were positively correlated with platelet levels of L-DOPA, 3-O-methyldopa, and with the score of disease severity. Disease severity was correlated with platelet and plasma levels of L-DOPA, as well as with the daily intake of the drug. No significant differences in platelet levels of cytosolic and mitochondrial isoforms of the antioxidant enzyme superoxide dismutase were found between PD patients, either treated or untreated, and controls. Our findings lend further support to the hypothesis that L-DOPA might promote free radical formation in PD patients.

Neuroprotection by pramipexole against dopamine- and levodopa-induced cytotoxicity

Pramipexole, a novel non-ergoline dopamine (DA) agonist, has been applied successfully for treatment of Parkinson's disease (PD). We report here that pramipexole can protect dopaminergic cell line Mes23.5 against dopamine- and levodopa-induced cytotoxicity possibly through a mechanism related to antioxidant activity. In the MES 23.5 cultures, DA and L-DOPA induce a dose- and time-dependent cytotoxicity, as determined by tetrazolium salt and trypan blue assays. Furthermore, an in situ terminal deoxynucleotidyl transferase assay demonstrates that DA-induced cell death is apoptotic. Pretreatment with pramipexole in a concentration range (4-100 microM) significantly attenuates DA- or L-DOPA-induced cytotoxicity and apoptosis, an action which is not blocked by D3 antagonist U-99194 A or D2 antagonist raclopride. Pramipexole also protects MES 23.5 cells from hydrogen peroxide-induced cytotoxicity in a dose-dependent manner. In cell-free system, pramipexole can effectively inhibit the formation of melanin, an end product resulting from DA or L-DOPA oxidation. These results indicate that pramipexole exerts its neuroprotective effect possibly through a mechanism, which is independent of DA receptors but related to antioxidation or scavenging of free radicals (e.g. hydrogen peroxide). As a direct DA agonist and potentially neuroprotective agent, pramipexole remains attractive in the treatment of PD.

Mechanisms of apomorphine cytoxicity towards rat glioma C6 cells: protection by bovine serum albumin and formation of apomorphine-protein conjugates

Apomorphine cytotoxicity towards rat glioma C6 cells was recently demonstrated to be time- and concentration-dependent. In the present work, the mechanism of cytotoxicity of apomorphine was further studied in the C6 cell line. We showed that bovine serum albumin partially protects C6 cells against apomorphine cytotoxicity. However, serum albumin did not prevent apomorphine autoxidation and melanin formation, suggesting that this protein scavenges apomorphine reactive products formed during its oxidation. The use of radioactive tracers, fluorimetry and protein electrophoresis showed that apomorphine autoxidation products covalently and nonspecifically bind to serum albumin and to rat liver microsomes. L-Cysteine, which is a thiol reagent that inhibits apomorphine autoxidation also prevented the formation of apomorphine-serum albumin adducts. These results suggest that quinone derivatives formation and oxidative stress should be responsible for apomorphine cytotoxicity.

Protection of PC12 cells glutathione peroxidase in L-DOPA induced cytotoxicity

L-DOPA may cause side-effects during the treatment of Parkinson's disease. We investigated the role of glutathione peroxidase (GSHPx) in cellular defense against L-DOPA cytotoxicity. A line of PC12 cells overexpressing GSHPx with plasmid pRc/CMV-GSHPx was established and stable transfectants overexpressing GSHPx were used for this study. GSHPx activity was found to be 1.5-fold higher in GSHPx-transfectants than in mock-controlled transfectants. Transfectants over expressing GSHPx were also significantly more resistant to exposure to either L-DOPA or t-butyl hydroperoxide than mock-transfected cells. Results suggested that L-DOPA may cause neuronal cell death by an oxidative pathway and GSHPx may play an important role in cellular defense against oxidative stress.

5-HT1A receptors mediate inhibition of ethanol-induced ascorbic acid release in rat striatum studied by microdialysis

Our previous study showed that the serotonergic system was involved in the ethanol-induced striatal ascorbic acid release in rat. In the present study, the 5-HT1A agonists and antagonists were used to analyze the possible mechanism of ethanol-induced ascorbic acid release in rat striatum. The results showed that ethanol (3.0 g/kg, i.p.) significantly increased striatal ascorbic acid release. Buspirone (5.0 mg/kg, s.c.), a partial agonist of 5-HT1A receptors, and 8-OH-DPAT (0.5 mg/kg, s.c.), a selective agonist of 5-HT1A receptors, showed no effect on basal ascorbic acid release in striatum, but both drugs significantly antagonized the ascorbic acid release induced by ethanol in striatum. WAY 100635 (0.5 mg/kg, s.c.), a selective antagonist of 5-HT1A receptors, affecting neither the basal nor the ethanol-induced ascorbic acid release per se, antagonized the suppressing effect of 8-OH-DPAT on ethanol-induced ascorbic acid release in striatum. This study gives the first evidence that activation of 5-HT1A receptors is involved in ethanol-induced ascorbic acid release in rat striatum.

Metabolic activation of dopamine o-quinones to o-semiquinones by NADPH cytochrome P450 reductase may play an important role in oxidative stress and apoptotic effects

In this study, it is shown that considerable evidence for the possible pathway by which dopamine o-quinone, o-quinone and aminochrome can be activated metabolically by NADPH cytochrome P450 reductase to high reactive semiquinones. These findings were discussed from a mechanistic standpoint as well as in terms of potential physiological implications of dopamine o-quinones and o-semiquinones' concerted action in oxidative stress and apoptotic events.

pH drift of "physiological buffers" and culture media used for cell incubation during in vitro studies

In pharmacological or toxicological studies performed at room atmosphere comparison of various media used for cell incubation revealed discrepancies among results due to pH instability when these media contain bicarbonate. With the classically used protocols, a relatively fast and notable rise of the pH of such media has been observed, and values higher than 8.5 could be reached after 1 h of incubation. A less important rise in pH was also observed for media containing low amounts of sodium bicarbonate, e.g., Hank's formula-derived media. Because Hepes-buffered media or media with abnormal osmolarity cannot always be used for such studies, our choice of media is limited.