Dopamine-induced Toxicity Research Articles

Dopamine induces soluble α-synuclein oligomers and nigrostriatal degeneration.

Parkinson’s disease is defined by the loss of dopaminergic neurons in the substantia nigra and formation of Lewy body inclusions containing aggregated α-synuclein. Efforts to explain dopamine neuron vulnerability are hindered by the lack of dopaminergic cell death in α-synuclein transgenic mice. To address this, we manipulated dopamine levels in addition to α-synuclein expression. Nigra-targeted expression of mutant tyrosine hydroxylase with enhanced catalytic activity increased dopamine without damaging neurons in non-transgenic mice. In contrast, raising dopamine in mice expressing human A53T mutant α-synuclein induced progressive nigrostriatal degeneration and reduced locomotion. Dopamine elevation in A53T mice increased levels of potentially toxic α-synuclein oligomers, resulting in conformationally and functionally modified species. Moreover, in genetically tractable C. elegans models expression of α-synuclein mutated at the site of interaction with dopamine prevented dopamine-induced toxicity. The data suggest a unique mechanism linking two cardinal features of Parkinson’s disease, dopaminergic cell death and α-synuclein aggregation.

Gray-matter volume, midbrain dopamine D2/D3 receptors and drug craving in methamphetamine users.

Dysfunction of the mesocorticolimbic system plays a critical role in clinical features of addiction. Despite evidence suggesting that midbrain dopamine receptors influence amphetamine-induced dopamine release and that dopamine is involved in methamphetamine-induced neurotoxicity, associations between dopamine receptors and gray-matter volume have been unexplored in methamphetamine users. Here we used magnetic resonance imaging and [18F]fallypride positron emission tomography, respectively, to measure gray-matter volume (in 58 methamphetamine users) and dopamine D2/D3 receptor availability (binding potential relative to nondisplaceable uptake of the radiotracer, BPnd) (in 31 methamphetamine users and 37 control participants). Relationships between these measures and self-reported drug craving were examined. Although no difference in midbrain D2/D3 BPnd was detected between methamphetamine and control groups, midbrain D2/D3 BPnd was positively correlated with gray-matter volume in the striatum, prefrontal cortex, insula, hippocampus and temporal cortex in methamphetamine users, but not in control participants (group-by-midbrain D2/D3 BPnd interaction, p<0.05 corrected for multiple comparisons). Craving for methamphetamine was negatively associated with gray-matter volume in the insula, prefrontal cortex, amygdala, temporal cortex, occipital cortex, cerebellum, and thalamus (p<0.05 corrected for multiple comparisons). A relationship between midbrain D2/D3 BPnd and methamphetamine craving was not detected. Lower midbrain D2/D3 BPnd may increase vulnerability to deficits in gray-matter volume in mesocorticolimbic circuitry in methamphetamine users, possibly reflecting greater dopamine-induced toxicity. Identifying factors that influence prefrontal and limbic volume, such as midbrain BPnd, may be important for understanding the basis of drug craving, a key factor in the maintenance of substance use disorders.

Curcumin delivery from poly(acrylic acid-co-methyl methacrylate) hollow microparticles prevents dopamine-induced toxicity in rat brain synaptosomes

The potential of poly(methyl methacrylate-co-acrylic acid) (PMMA-AA) copolymers to form hollow particles and their further formulation as curcumin delivery system have been explored. The particles were functionalized by crosslinking the acrylic acid groups via bis-amide formation with either cystamine (CYS) or 3,3′-dithiodipropionic acid dihydrazide (DTP) which simultaneously incorporated reversibility due to the presence of disulfide bonds within the crosslinker. Optical micrographs showed the formation of spherical hollow microparticles with a size ranging from 1 to 7μm. Curcumin was loaded by incubation of its ethanol solution with aqueous dispersions of the cross-linked particles and subsequent evaporation of the ethanol. Higher loading was observed in the microparticles with higher content of hydrophobic PMMA units indicating its influence upon the loading of hydrophobic molecules such as curcumin. The in vitro release studies in a phosphate buffer showed no initial burst effect and sustained release of curcumin that correlated with the swelling of the particles under these conditions. The capacity of encapsulated and free curcumin to protect rat brain synaptosomes against dopamine-induced neurotoxicity was examined. The encapsulated curcumin showed greater protective effects in rat brain synaptosomes as measured by synaptosomal viability and increased intracellular levels of glutathione.

Reynosin protects against neuronal toxicity in dopamine-induced SH-SY5Y cells and 6-hydroxydopamine-lesioned rats as models of Parkinson's disease: Reciprocal up-regulation of E6-AP and down-regulation of α-synuclein

Aggregation of α-synuclein (ASYN) is considered a major determinant of neuronal loss in Parkinson's disease (PD). E6-associated protein (E6-AP), an E3 ubiquitin protein ligase, has been known to promote the degradation of α-synuclein. The aim of this study was to assess the effects of the sesquiterpene lactone reynosin on dopamine (DA)-induced neuronal toxicity and regulation of E6-associated protein and α-synuclein proteins in both in vitro and in vivo models of Parkinson's disease. Usi“ng flow cytometry and western blot analysis, we determined that reynosin significantly protected both against cell death from dopamine-induced toxicity in human neuroblastoma SH-SY5Y cells and against the loss of tyrosine hydroxylase (TH)-positive cells in 6-hydroxydopamine (6-OHDA)-lesioned rats (a rodent Parkinson's disease model system). In addition, reynosin made up-regulation of E6-associated protein expression and down-regulation of the over-expression of α-synuclein protein in both dopamine-treated SH-SY5Y cells and 6-hydroxydopamine-lesioned rats. These results suggest that the protective effect of reynosin against dopamine-induced neuronal cell death may be due to the reciprocal up-regulation of E6-associated protein and down-regulation of α-synuclein protein expression.

(2S)-5, 2′, 5′-Trihydroxy-7-Methoxyflavanone, a Natural Product from Abacopteris penangiana, Presents Neuroprotective Effects In Vitro and In Vivo

The aim of this study was to investigate the neuroprotective effects of (2S)-5, 2', 5'-trihydroxy-7-methoxyflavanone (TMF), a natural product from Abacopteris penangiana (Hook.) Ching, in oxidative stress-induced neurodegeneration models in vitro and in vivo. In PC12 cells, preincubation of TMF (3-20μM) for 24h decreased the dopamine-induced toxicity and attenuated the redox imbalance in PC12 cells through regulating the ratio of reduced glutathione/oxidized glutathione (GSH/GSSG), which is a sensitive marker of oxidative stress. Additionally, long-term intraperitoneal (i.p.) injection of TMF (4 or 8mg/kg/day) for 2weeks significantly improved the behavioral performance of D-galactose (D-gal) treated mice in a Morris water maze test. Biochemical analysis revealed that TMF inhibited the activation of AP-1 (activator protein-1) and upregulated the level of BDNF (brain derived neurophicfactor) as well as the ratio of GSH/GSSG in the hippocampus of D-gal treated mice. Furthermore, western blotting analysis indicated that TMF increased phosphorylation of cAMP-response element-binding protein (CREB). Therefore, the natural product TMF possessed a potential for the treatment of neurodegenerative diseases.

Mitochondrial neuronal uncoupling proteins: a target for potential disease-modification in Parkinson's disease

This review gives a brief insight into the role of mitochondrial dysfunction and oxidative stress in the converging pathogenic processes involved in Parkinson's disease (PD). Mitochondria provide cellular energy in the form of ATP via oxidative phosphorylation, but as an integral part of this process, superoxides and other reactive oxygen species are also produced. Excessive free radical production contributes to oxidative stress. Cells have evolved to handle such stress via various endogenous anti-oxidant proteins. One such family of proteins is the mitochondrial uncoupling proteins (UCPs), which are anion carriers located in the mitochondrial inner membrane. There are five known homologues (UCP1 to 5), of which UCP4 and 5 are predominantly expressed in neural cells. In a series of previous publications, we have shown how these neuronal UCPs respond to 1-methyl-4-phenylpyridinium (MPP+; toxic metabolite of MPTP) and dopamine-induced toxicity to alleviate neuronal cell death by preserving ATP levels and mitochondrial membrane potential, and reducing oxidative stress. We also showed how their expression can be influenced by nuclear factor kappa-B (NF-κB) signaling pathway specifically in UCP4. Furthermore, we previously reported an interesting link between PD and metabolic processes through the protective effects of leptin (hormone produced by adipocytes) acting via UCP2 against MPP+-induced toxicity. There is increasing evidence that these endogenous neuronal UCPs can play a vital role to protect neurons against various pathogenic stresses including those associated with PD. Their expression, which can be induced, may well be a potential therapeutic target for various drugs to alleviate the harmful effects of pathogenic processes in PD and hence modify the progression of this disease.

Quinoprotein Adducts Accumulate in the Substantia Nigra of Aged Rats and Correlate with Dopamine-Induced Toxicity in SH-SY5Y Cells

Parkinson's disease (PD) is an age-dependent neurodegenerative disorder characterized by dopaminergic neuron loss in substantia nigra. Previous studies have implicated a role of dopamine oxidation in PD. Dopamine oxidation leads to the formation of dopamine quinone, which generates reactive oxygen species and covalently modifies cysteinyl proteins to form quinoprotein adduct. We compared quinoprotein adduct formation and lipid peroxidation in different brain regions of young and old rats. We found a prominent age-dependent accumulation of quinoprotein adducts in the substantia nigra, while no significant change of lipid peroxidation was detected in any brain regions of 2- to 15-month old rats. To determine whether quinoprotein adduct formation correlates with dopamine-induced cytotoxicity, we analyzed dopamine treated SH-SY5Y cells and found a strong correlation between quinoprotein adduct formation and cytotoxicity. Together, our results indicate that quinoprotein adduct formation may play a role in the age-dependent selective vulnerability of dopaminergic neurons in PD.

ChemInform Abstract: Regulation of Dopaminergic Neuronal Death by Endogenous Dopamine and Proteasome Activity

AbstractReview: 16 refs.

Screening of Therapeutic Strategies for Huntington's Disease in YAC128 Transgenic Mice

Huntington’s disease (HD) is a hereditary neurodegenerative disorder caused by an unstable expansion of cytosine-adenine-guanine (CAG) repeats in the HD gene. The symptoms include cognitive dysfunction and severe motor impairment with loss of voluntary movement coordination that is later replaced by bradykinesia and rigidity. The neuropathology is characterized by neuronal loss mainly in the striatum and cortex, and the appearance of neuronal intranuclear inclusions of mutant huntingtin. The mechanisms responsible for neurodegeneration are still not fully understood although excitotoxicity and a consequent increase in intracellular calcium concentration as well as the activation of caspases and calapins are known to play a key role. There is currently no satisfactory treatment or cure for this disease. The YAC128 transgenic mice express the full-length human HD gene with 128 CAG repeats and constitute a unique model for the study of HD as they replicate the slow and biphasic progression of behavioral deficits characteristic of the human condition and show striatal neuronal loss. As such, these transgenic mice have been an invaluable model not only for the elucidation of the neurodegenerative pathways in HD, but also for the screening and development of new therapeutic approaches. Here, I will review the unique characteristics of this transgenic HD model and will provide a summary of the therapies that have been tested in these mice, namely: potentiation of the protective roles of wild-type huntingtin and mutant huntingtin aggregation, transglutaminase inhibition, inhibition of glutamate- and dopamine-induced toxicity, apoptosis inhibition, use of essential fatty acids, and the novel approach of intrabody gene therapy. The insights obtained from these and future studies will help identify potential candidates for clinical trials and will ultimately contribute to the discovery of a successful treatment for this devastating neurodegenerative disorder.

Regulation of Dopaminergic Neuronal Death by Endogenous Dopamine and Proteasome Activity

Parkinson disease is one of the most common neurodegenerative disorders and is characterized by the selective loss of dopaminergic neurons in the substantia nigra. Although endogenous dopamine itself could serve as a vulnerability factor for dopaminergic neurons, the mechanism by which dopamine contributes to dopaminergic neuronal death remains unknown. In addition, although a decrease in proteasome activity was found in patients with sporadic Parkinson disease, the relationship between the ubiquitin-proteasome system and dopaminergic neuronal death remains to be elucidated. Here we provide an overview of the roles of endogenous dopamine and proteasome activity in dopaminergic cell death. Treatment of catecholaminergic PC12 cells with the herbicide paraquat, a potential risk factor for the development of Parkinson disease, induced an increase in dopamine content, and depletion of intracellular dopamine suppressed paraquat-induced cytotoxicity. Although glutathione, which scavenged dopamine oxidation intermediate, provided almost complete protection against dopamine-mediated toxicity, catalase provided only partial protection against cell death caused by dopamine. These data suggest that the generation of dopamine oxidation intermediate, rather than that of reactive oxygen species, plays a pivotal role in dopamine-induced toxicity. Moreover, treatment with paraquat induced a decrease in proteasome activity, and proteasome inhibition suppressed dopamine-mediated cytotoxicity. Suppression of proteasome activity stimulated the NF-E2-related factor 2 (Nrf2)-antioxidant response element (ARE) pathway, and elevated γ-glutamylcysteine synthetase mRNA and glutathione content. Furthermore, suppression of the paraquat-induced increase in gluthathione content exacerbated paraquat toxicity. These results suggest that the reduction of proteasome activity may be involved in cellular defense mechanisms against dopamine-mediated paraquat toxicity.

Mitochondrial UCP5 is neuroprotective by preserving mitochondrial membrane potential, ATP levels, and reducing oxidative stress in MPP + and dopamine toxicity

We explored the protective mechanisms of human neuronal mitochondrial uncoupling protein-5 (UCP5) in MPP +- and dopamine-induced toxicity after its stable overexpression in SH-SY5Y cells. We raised specific polyclonal antibodies. Overexpressed UCP5 localized in mitochondria but not in cytosol. UCP5 overexpression increased proton leak, decreased mitochondrial membrane potential (MMP), reduced ATP production, and increased overall oxygen consumption (demonstrating uncoupling activity). UCP5 overexpression did not affect other neuronal UCP expression (UCP2 and UCP4). Overexpressing UCP5 is protective against MPP +- and dopamine-induced toxicity. MPP + and dopamine exposure for 6 h reduced MMP and increased superoxide levels. ATP levels in UCP5-overexpressing cells were preserved under MPP + and dopamine toxicity, comparable to levels in untreated vector controls. At 24 h, UCP5 overexpression preserved MMP, ATP levels, and cell survival; attenuated superoxide generation; and maintained oxidative phosphorylation as indicated by lower lactate levels. MPP + and dopamine exposure induced UCP5 mRNA transcription but did not decrease transcript degradation, as inhibition of transcription by actinomycin-D abolished induction by either toxin. Compared with our previous studies on UCP4, we observed functional differences between UCP4 and UCP5 in enhancing mitochondrial efficiency. These neuronal UCP homologues may work synergistically to maintain oxidative balance (through uncoupling activities) and ATP production (by modifying MMP).

Overexpression of NQO1 protects human SK-N-MC neuroblastoma cells against dopamine-induced cell death

NAD(P)H quinone oxidoreductase 1 (NQO1) can metabolize dopamine-derived quinones (DAQ) and absence of NQO1 due to the NQO1*2 polymorphism has been suggested to be a risk factor for Parkinson's disease. In order to define whether NQO1 plays a protective role in dopamine toxicity, we have examined the potential role of NQO1 in the SK-N-MC human neuroblastoma cell line. SK-N-MC cells were stably transfected with NQO1 to generate stable clones with NQO1 enzymatic activity of 245 nmol/mg min while vector control and parental cells had NQO1 activities of less than 12 nmol/mg min. Incubation of dopamine for 24 h in both parental and vector control SK-N-MC cells resulted in 85% and 72% cell death as assessed by annexin-V/propidium iodide analysis. In agreement, 88% and 84% of parental and vector control cells, respectively underwent loss of mitochondrial membrane potential (MMP) assessed by tetramethylrhodamine ethyl ester. In contrast, NQO1-transfected cells were resistant to dopamine toxicity and both cell death and loss of MMP were markedly abrogated in NQO1-transfected SK-N-MC cells. When dopamine was added to medium, oxygen uptake could be detected indicating autoxidation with concomitant formation of oxygen radicals and quinones. However, dopamine-induced cell death was not affected by the inclusion of either superoxide dismutase or catalase suggesting that superoxide and hydrogen peroxide were not involved in toxicity. Quinones formed in medium may exert toxicity extracellularly or intracellularly but the protective role of NQO1 argues for an intracellular mechanism. In summary, transfection of SK-N-MC cells with NQO1 protects against dopamine-induced toxicity.

Iron accelerates the conversion of dopamine-oxidized intermediates into melanin and provides protection in SH-SY5Y cells

Parkinson's disease (PD) is characterized by the selective loss of dopaminergic neurons in the substantia nigra (SN), and it has been suggested that dopamine is one of the main endogenous toxins in the genesis of PD. We demonstrated that thiol antioxidants (the reduced form of glutathione, N-acetyl-L-cysteine, and L-cysteine), which conjugate with one dopamine oxidation intermediate, o-quinone, provided almost complete protection from dopamine-mediated toxicity in SH-SY5Y, a human neuroblastoma cell line. In contrast, catalase partially provided protection against cell death caused by dopamine. These data suggest that the generation of dopamine oxidation intermediates, rather than hydrogen peroxide, plays a pivotal role in dopamine-induced toxicity. Iron accumulated in the SN of patients with PD can cause dopaminergic neuronal degeneration by enhancing oxidative stress. However, we found that iron reduced the total amounts of dopamine oxidation intermediates and enhanced the formation of melanin, a final product of dopamine oxidation. Also, addition of iron inhibited dopamine-induced cytotoxicity. These results suggest that iron can provide protection when it accelerates the conversion of dopamine oxidation intermediates.

Induction of GADD45 and GADD153 in Neuroblastoma Cells by Dopamine-Induced Toxicity

Dopamine (DA) metabolism and oxidation produce both reactive oxygen species (ROS) and reactive quinones. These chemical species are implicated in dopamine neurotoxicity and neurodegeneration. In the present studies, human neuroblastoma (SK-N-SH) cells were exposed to toxic concentrations of dopamine (333 μM) in order to investigate molecular pathways involved in dopamine toxicity. cDNA hybridization array (microarray) technology demonstrated that GADD45 and GADD153 (growth arrest and DNA-damage inducible) gene expression was increased in dopamine-treated cells (333 μM for 18 h). Subsequent Northern and Western blot analysis confirmed these changes in GADD45 and GADD153 gene expression. The antioxidant, ascorbic acid, significantly reduced the increase in GADD45 gene expression but did not significantly reduce GADD153 gene expression. Currently, the precise function of the GADD gene products is not known. It is known, however, that these genes are upregulated in response to stress to allow cells time to repair macromolecular damage. In the present case, GADD gene expression (manifested as increased mRNA and protein levels) preceded dopamine-induced cytotoxicity. It appears that dopamine, through the formation of reactive oxygen species and quinones, may damage cellular macromolecules to the point of inducing GADD gene expression. Other genes that displayed changes, but that have not been subjected to post-hoc confirmation, include clusterin (increased), ubiquitin (increased), CD27 ligand (increased), CD27BP (increased), and rac-PK-beta (decreased).

Glutathione protects PC12 cells from ascorbate- and dopamine-induced apoptosis.

The role of reduced glutathione (GSH) on ascorbate- and dopamine-induced apoptosis in PC12 cells was investigated. Ascorbate is a potent reducing agent and is thus expected to protect against dopamine-induced apoptosis. However, we found that both ascorbate and dopamine killed PC12 cells and ascorbate enhanced dopamine-induced toxicity. The EC50 of cell toxicity induced by ascorbate, dopamine and dopamine plus 0.1 mM ascorbate during 24-h treatment were 0.93+/-0.15 mM, 0.18+/-0.05 mM and 0.13+/-0.04 mM, respectively. When the medium contained 10 mM GSH, the EC50 increased approximately three- and sevenfold for ascorbate and dopamine, respectively. With increased treatment duration, no further toxic effects of ascorbate or dopamine were observed. The GSH synthesis inhibitor, DL-buthionine-(S,R)-sulfoximine (BSO), induced cell toxicity and potentiated the toxic effects of ascorbate and dopamine, suggesting that endogenous GSH participates in protecting against basal oxidative stress. We conclude that both ascorbate and dopamine induce apoptosis in PC12 cells and further that GSH protects them from apoptosis. This study indicates that the toxic effects of ascorbate are potentially due to an oxidative mechanism, similar to that induced by dopamine.

Mechanisms of Dopamine-Induced Cell Death in Cultured Rat Forebrain Neurons: Interactions with and Differences from Glutamate-Induced Cell Death

Injury to the brain, whether by ischemia or trauma, results in the uncontrolled release of many neurotransmitters, including glutamate and dopamine. Both of these neurotransmitters are neurotoxic in high concentrations, and the oxidative stress caused by reactive oxygen species generation has been implicated in the mechanism of neurotoxicity. In this study, we used cultured rat forebrain neurons to characterize cell death caused by exposure to dopamine and/or glutamate and to investigate potential acute mechanisms of toxicity. Dopamine exposure (250 μMfor 2 h) reduced cell viability to 34.3 ± 5.5% of untreated control 20 h later and increased the number of neurons with apoptotic morphology. The antioxidantN-acetylcysteine (100 μM) inhibited dopamine-induced toxicity and prevented the covalent binding of dopamine quinones to protein. In contrast, glutamate toxicity lacked the hallmark characteristics of apoptosis. When neurons were exposed successively to sublethal concentrations of dopamine and glutamate, cell viability at 20 h was reduced to 62.3 ± 5.2% of untreated control. Apoptosis was not evident, andN-acetylcysteine blocked the potentiating effect of dopamine on glutamate-induced toxicity. We used single-cell fluorescence assays to measure changes in intraneuronal glutathione, intraneuronal Ca2+, mitochondrial membrane potential, and DNA integrity as potential acute inducers of neuronal injury. While changes in these parameters could be demonstrated, none were identified as the sole acute inducer of cell death caused by dopamine. In summary, we have characterized a number of neuronal responses to lethal dopamine injury. Also, we have demonstrated that dopamine and glutamate can interactin vitroto potentiate cell death and that the potentiation appears to be induced by oxidative stress.

R(−)-Deprenyl Potentiates Dopamine-Induced Cytotoxicity toward Catecholaminergic Neuroblastoma SH-SY5Y Cells

Autoxidation of dopamine orl-DOPA (3,4-dihydroxyphenylalanine) generates reactive oxygen species (ROS), i.e., hydrogen peroxide, superoxide, and hydroxyl radical, which are potentially cytotoxic. Increased formation of ROS has been proposed to be involved in the pathogenesis of many human diseases, including Parkinson's disease. Several reports suggest that R(−)-deprenyl (an MAO-B inhibitor and anti-Parkinsonian drug) may directly or indirectly exert antioxidant effects and thus protect neurons. We have assessed the toxic effects of dopamine andl-DOPA toward catecholaminergic neuroblastoma SH-SY5Y cells and whether R(−)-deprenyl and several structurally related compounds possess antioxidant effects in this system. The results show that both dopamine andl-DOPA are quite cytotoxic toward SH-SY5Y cells. R(−)-deprenyl rather than reducing this dopamine-induced toxicity actually enhances it. Structural analogues of R(−)-deprenyl, such as 4-methyldeprenyl, (−)-methylamphetamine, and clorgyline, exhibited similar effects. Some different MAO-B inhibitors, namely, the aliphaticN-methylpropargylamines, e.g., (±)-M-2-PP [N-(2-pentyl)-N-methylpropargylamine] andN-[2-hexyl]-N-methylpropargylamine, which can also protect and rescue neurons in severalin vivoandin vitromodels, did not exacerbate the cytotoxicity of dopamine. Neither R(−)-deprenyl nor (±)-M-2-PP affected thel-DOPA-induced cytotoxicity toward SH-SY5Y cells.

Dopamine neurotoxicity: inhibition of mitochondrial respiration.

Dopamine, due to metabolism by monoamine oxidase or autoxidation, can generate toxic products such as hydrogen peroxide, oxygen-derived radicals, semiquinones, and quinones and thus exert its neurotoxic effects. Intracerebroventricular injection of dopamine into rats pretreated with the monoamine oxidase nonselective inhibitor pargyline caused mortality in a dose-dependent manner with LD50 = 90 micrograms. Norepinephrine was less effective with LD50 = 141 micrograms. The iron chelator desferrioxamine completely protected against dopamine-induced mortality. In the absence of pargyline more rats survived, indicating that the products of dopamine enzymatic metabolism are not the main contributors to dopamine-induced toxicity. Biochemical analysis of frontal cortex and striatum from rats that received a lethal dose of dopamine did not show any difference from control rats in lipid and protein peroxidation and glutathione reductase and peroxidase activities. Moreover, dopamine significantly reduced the formation of iron-induced malondialdehyde in vitro, thus suggesting that earlier events in cell damage are involved in dopamine toxicity. Indeed, dopamine inhibited mitochondrial NADH dehydrogenase activity with IC50 = 8 microM, and that of norepinephrine was twice as much (IC50 = 15 microM). Dopamine-induced inhibition of NADH dehydrogenase activity was only partially reversed by desferrioxamine, which had no effect on norepinephrine-induced inhibition. These results suggest that catecholamines can cause toxicity not only by inducing an oxidative stress state but also possibly through direct interaction with the mitochondrial electron transport system. The latter was further supported by the ability of ADP to reverse dopamine-induced inhibition of NADH dehydrogenase activity in a dose-dependent manner.