Neuroprotective effect of arundic acid, an astrocyte-modulating agent, in mouse brain against MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) neurotoxicity

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes the damage of dopaminergic neurons as seen in Parkinson's disease. Oxidative stress has been as one of several pathogenic hypotheses for Parkinson's disease. Here we investigated whether arundic acid, an astrocyte-modulating agent, can protect against alterations of nitric oxide synthase (NOS) and superoxide dismutase (SOD) expression on MPTP neurotoxicity in mice, utilizing an immunohistochemistry. For this purpose, anti-tyrosine hydroxylase (TH) antibody, anti-dopamine transporter (DAT) antibody, anti-Cu/Zn-SOD antibody, anti-Mn-SOD antibody, anti-nNOS antibody, anti-eNOS antibody and anti-iNOS antibody were used. The present study showed that the arundic acid had a protective effect against MPTP-induced neuronal damage in the striatum and substantia nigra of mice. The protective effect may be, at least in part, caused by the reductions of the levels of reactive nitrogen (RNS) and oxygen species (ROS) against MPTP neurotoxicity. These results suggest that the pharmacological modulation of astrocyte may offer a novel therapeutic strategy for the treatment of Parkinson's disease. Furthermore, our results provide further evidence that a combination of nNOS inhibitors, iNOS inhibitors and free radical scavengers may be effective in the treatment of neurodegenerative diseases. Thus our present results provide valuable information for the pathogenesis of degeneration of the nigrostriatal dopaminergic neuronal pathway.     

Behavioural effects of (+)-4-propyl-9-hydroxynaphthoxazine in primates rendered parkinsonian with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Summary Three monkeys received a chronic intravenous course of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) so as to produce a permanent parkinsonian syndrome. One primate was electively commenced on chronic levodopa therapy 6 weeks after the cessation of MPTP treatment. Four months following the termination of MPTP administration, the response to oral doses of the novel D-2 dopamine agonist (+)-4-propyl-9-hydroxynaphthoxazine (PHNO) was assessed in all animals using a clinical rating scale and automatic activity counters.PHNO was found to be a highly potent anti-parkinsonian agent, completely reversing the symptoms of parkinsonism in a dose-dependent manner. Peak-dose dyskinesia was noted in 2 MPTP-treated animals during trials with PHNO, but was more severe in the animal receiving chronic levodopa therapy. Response fluctuations such as end-of-dose deterioration and the on-off henomenon were common to all 3 parkinsonian animals following PHNO. The anti-parkinsonian effect and frequency of treatment-induced side-effects appeared to be similar with PHNO and levodopa. These results confirm the efficacy of PHNO as an anti-parkinsonian drug and link the production of dyskinesia with the D-2 dopamine receptor. Send offprint requests to A. R. Crossman at the above address     

Pharmacological interference with the neurotoxic action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on central catecholamine neurons in the mouse

The effect of pretreatment with various MAO and catecholamine uptake inhibitors on the MPTP-induced reduction of endogenous catecholamine levels and [³H]catecholamine uptake in mouse striatum and cerebral cortex associated with the neurotoxic action of MPTP on dopamine and noradrenaline neurons was investigated. Pargyline and deprenyl almost completely reversed the MPTP-induced reduction of these parameters in both regions while chlorgyline was without effect. Pretreatment with the dopamine uptake inhibitor amfolenic acid preferentially counteracted the depleting effect of MPTP on striatal dopamine levels. The noradrenaline uptake inhibitors desipramine, nortripityline and LY 139603 all antagonized the MPTP-induced reduction of noradrenaline levels in cerebral cortex, while none of these inhibitors affected the action of MPTP on striatal dopamine. The results suggest that MAO-B and the catecholamine uptake system may be critically involved at certain steps in the neurotoxic action of MPTP on catecholamine neurons. The interaction with the uptake mechanism most likely explains the selective neurotoxic action of MPTP on catecholamine neurons.     

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration to C57-black mice leads to parallel decrements in neostriatal dopamine content and tyrosine hydroxylase activity

The administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to C57-black mice (4 doses of 20 mg/kg, at 2 h intervals) led to a large decrement in the neostriatal content of dopamine (92%) and a parallel decrease in tyrosine hydroxylase activity (91%). MPTP administration also caused highly significant but lesser decrements in the neostriatal content of dihydroxyphenylacetic acid (84%) and homovanillic acid (68%). These data are consistent with other observations which suggest that MPTP administration to mice results in a destruction of the dopaminergic nigrostriatal pathway.     

Effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on striatal dopamine receptors in C57BL/6 mice

The effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a neurotoxin which selectively destroys the nigrostriatal dopaminergic neurons and produces Parkinson's disease-like syndrome, on striatal dopamine receptors was determined in a mouse strain known to be very sensitive to the neurotoxic effect of MPTP. Daily intraperitoneal administration of MPTP (30 mg/kg) for 7 days to male C57BL/6 mice reduced the concentration of striatal dopamine by 90%. This decrease in dopamine concentration was not associated with changes either in the receptor density (B_max) or the apparent dissociation constant (K_d) of [³H]spiroperidol to bind to striatal dopamine receptors. It is concluded that in spite of large decrease in striatal dopamine concentration by MPTP the dopamine receptors labeled with [³H]spiroperidol remain intact.     

Neuroprotective mechanism of modafinil on Parkinson disease induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

AIM: To observe the neuroprotective mechanism of modafinil on Parkinson disease (PD) models induced by 1-methyl-4-phenyl- 1, 2, 3, 6-tetrahydropyridine (MPTP). METHODS: The model of PD was induced by intraperitoneally injecting MPTP into C57BL/6J mice for 4 d. Modafinil ( ip, 50 or 100 mg.kg^-1.d^-1 ) was administered at 30min following MPTP for 4 d and for another 10 d continuously. The contents of dopamine (DA), noradrenaline(NA), 5-hydroxytryptamine (5-HT), γ-aminobutyric acid (GABA), glutamine (Glu) in the striatum, and the contents of GABA, Glu, malondialdehyde (MDA), and glutathione (GSH) in the substantia nigra (SN) of model mice were determined. RESULTS: Modafinil (50 and 100 mg/kg) prevented against the decrease of the contents of DA,5-HT, and NA in the striatum and GSH, GABA in the SN induced by MPTP, but reduced the increase of MDA in the SN and GABA in the striatum induced by MPTP. Modafinil preferentially inhibited striatal GABA release, but it did not change the increase of nigrostriatal Glu release induced by MPTP. CONCLUSION: The anti-oxidation and the modulation of nigrostriatal GABA and striatal NA and 5-HT release contributed to the neuroprotective effects of modafinil on PD induced by MPTP.     

A neuroprotective agent, T-817MA (1-{3-[2-(1-benzothiophen-5-yl)ethoxy]propyl} azetidin-3-ol maleate), prevents 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurotoxicity in mice

T-817MA (1-{3-[2-(1-benzothiophen-5-yl)ethoxy]propyl} azetidin-3-ol maleate) is a candidate therapeutic agent for Alzheimer's disease that inhibits oxidative stress and nitric oxide-induced neurotoxicity and acts as a neurotrophic factor. The present study examines the effect of T-817MA on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopaminergic neurotoxicity in C57BL/6J mice. MPTP treatment (10mg/kg, s.c.x4 at 2-h intervals) impaired rotarod performance, and T-817MA improved this deficit. MPTP treatment also decreased dopamine levels and tyrosine hydroxylase immunostaining in the substantia nigra (SNc) and striatum. Pretreatment with T-817MA (10 and 30mg/kg as T-817, p.o.) attenuated these decreases in dopamine levels and tyrosine hydroxylase immunoreactivity, but did not affect brain levels of 1-methyl-4-phenylpyridinium ion, an active metabolite of MPTP. The protective effect was almost complete in the SNc, but only partial in the striatum. MPTP increased levels of the lipid peroxidation product, thiobarbituric acid reactive substance, only in the midbrain, which could be blocked by T-817MA. MPTP caused microglial activation both in the SNc and striatum, but T-817MA did not affect the activation of microglia. These results suggest that T-817MA protects against MPTP-induced neurotoxicity by blocking lipid peroxidation in the SNc, and imply that this compound may be useful for treating neurodegenerative disorders related to oxidative stress, such as Parkinson's disease.     

Neurochemical and behavioral effects of systemic and intranigral administration of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in the rat

At doses of 5-10 mg kg-1, N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (NMPTP) produces in rats acute immobility, retropulsion, straub tail, piloerection, exophthalmos, salivation and clonic movements of the forepaws. It does not produce analgesia as measured by the tail test, nor does it produce permanent motor impairment after chronic or intranigral administration. The acute retropulsion and immobilizing effects can be blocked by methysergide. Administered acutely, NMPTP doubles levels of serotonin in the raphe nucleus and substantia nigra. At the same time, levels of dopamine increase in the caudate nucleus and decrease in the substantia nigra. The NMPTP-induced decrease in dopamine content of the substantia nigra persists in chronically treated rats, but there is no significant decrease in striatal dopamine. After chronic administration of NMPTP, striatal levels of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were decreased by about 50%. Intranigral administrations of NMPTP (10 micrograms daily for 5 days) failed to produce a 6-hydroxydopamine-like lesion in the nigrostriatal system. These results indicate that NMPTP in the rat does not cause selective destruction of dopaminergic neurons, but it does produce acute tryptamine-like effects.     

Reduction of dopamine uptake and cocaine binding in mouse striatum by N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Administration of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (NMPTP) (daily injections of 8 mg/kg for 5 days via tail vein) reduced [3H]dopamine uptake in striatal synaptosomes by 63% and reduced [3H]cocaine binding to striatal membranes by 61%. [3H]Cocaine binding was not affected in olfactory tubercle, suggesting a selective effect of NMPTP on the nigro-striatal but not on the mesolimbic dopaminergic system. The destruction of dopamine terminals in the striatum did not alter (up-regulate) [3H]spiroperidol binding. The results suggest that NMPTP causes a degenerative destruction of the striatal dopamine pathway and that NMPTP may be useful in developing a rodent model of Parkinson's disease.     

Time-course evaluation of whole blood phagocytosis in mice treated with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Immunological abnormalities have been described in idiopathic Parkinson's disease and in the mouse model of this disorder induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). This investigation was aimed to study the effect of MPTP on inflammatory response of whole blood phagocytes at different time intervals. C57BL male mice were injected intraperitoneally with either MPTP (30 mg/kg) or saline (control group) and the blood samples were collected at 4, 24 and 48 h. 50 l of a 500-fold diluted blood sample was mixed with 150 l of reaction mixture (0.4 mM luminol + 50 g opsonized zymosan + 0.1% gelatin, in Hanks' balanced salt solution) and the chemiluminescence (CL) signal was measured in a luminometer at 37°C. Although the CL response of the whole blood from control and MPTP groups was similar at 4 h, a significant increase in the CL signal was observed in MPTP-treated mice at 24 h post-treatment, which got subsided at 48 h. The findings of this study suggest that in an early stage a pro-inflammatory response following MPTP might trigger a chain of potentially toxic pathways mediated by reactive oxygen species, leading to progressive neuronal damage.     

Prevention of MPTP (N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) dopaminergic neurotoxicity in mice by chronic lithium: involvements of Bcl-2 and Bax

Lithium has been reported to exert neuroprotective activity in several neuronal cell cultures and in vivo models against glutamate toxicity. Since this action was reported to be associated with alterations in the antiapoptotic Bcl-2 family proteins, the effect of chronic lithium diet on the ability of the parkinsonism neurotoxin, N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to deplete striatal dopamine in mice was determined. Mice were fed for with a diet containing 1.1, 2.2, 3.3, and 4.4 g/kg lithium chloride (LiCl) for 4 weeks, during which time serum levels of lithium were monitored. The 3.3 g/kg lithium diet gave serum level value very similar to what is observed in lithium therapy in man and the 4.4 g/kg well above this. At the end of this period the mice received 24 mg/kg MPTP i.p. once daily for 3 days. A direct relation was established with the increase in serum lithium and its ability to prevent MPTP induced depletion of striatal dopamine (DA) and its metabolites DPOAC and HVA. With the diet containing the highest lithium concentration there was an almost complete prevention of striatal dopamine depletion and the reduction in tyrosine hydroxylase activity and protein and it prevented the increase in dopamine turnover (DOPAC+HVA/DA) normally observed in MPTP treatment. Lithium did not interfere with the metabolism of MPTP, or with its brain uptake, since, the level of its monoamine oxidase (MAO) B derived metabolite, MPP⁺, in the striata of lithium and non-lithium treated mice were almost identical. Striatal Bcl-2 was significantly decreased, while Bax was increased in MPTP treated mice. Lithium treatment not only increased striatal Bcl-2 in control mice, but also prevented its reduction as induced by MPTP, and an opposing effect was seen with Bax. The neuroprotective action of lithium in this model of Parkinson’s disease has been attributed to its antiapoptotic activity which among other factors includes induction of Bcl-2 and reduction of Bax.     

Mutagenicity of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and its metabolites

The by-product from a "synthetic heroin" is 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a chemical contaminant found to produce neurotoxicity similar to Parkinsonism in susceptible animals. MPTP and its oxidative metabolites were tested in the Salmonella mutagenicity test. Strains of Salmonella typhimurium that carry a nonsense mutation at the site of reversion detect a variety of naturally occurring and direct-acting mutagens. TA 100 is reverted by MPDP+ (1-methyl-4-phenyl-2,3-dihydropyridinium species). This strain is more sensitive to MPDP+ mutagenesis than any other available strain of Salmonella and MPDP+ is considerably more mutagenic than MPTP and other oxidative metabolites including MPTP N-oxide or MPP+ (1-methyl-4-phenylpyridinium ion). Mutagenicity studies of metabolic incubates of MPTP with monoamine oxidase (MAO) suggest the involvement of metabolic bioactivation in the mutagenicity of MPTP. Since MPDP+ is generated from MPTP by MAO and since iminium ions are generated during cellular metabolism, they may make a contribution to the risk of human cancer.     

Attenuation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity by the novel selective dopamine D3-receptor partial agonist FAUC 329 predominantly in the nucleus accumbens of mice

We previously synthesised a novel dopamine (DA) partial agonist FAUC 329 with high affinity and selectivity for the DA D(3) receptor. This is the first in vivo study to investigate the protective effects of FAUC 329 in a MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) mouse model of Parkinson's disease. Adult male C57bl/6 mice were injected with FAUC 329 (0, 0.1, 0.5, 0.75, or 1 mg/kg) 30 min before MPTP (2 x 30 mg/kg, 4 hr apart). One week later, accumbal and striatal tissue was processed for DA and metabolite HPLC determination as well as immunohistochemical analysis of DA transporter positive neurons in the substantia nigra pars compacta and ventral tegmental area was carried out. FAUC 329 showed a significant attenuation of MPTP-induced DA reduction in the nucleus accumbens (0.5, 0.75 and 1 mg/kg) in a dose-dependent manner. FAUC 329 (0.75 mg/kg) partly protected against DA depletion in the dorsal striatum as well as protected against loss of DA transporter immunoreactivity in the substantia nigra pars compacta. The highest dose of FAUC 329 (1 mg/kg), however, showed a non-significant tendency to augment the MPTP-induced striatal DA reduction. The protective effect of FAUC 329 against MPTP-induced DA depletion was most pronounced in the nucleus accumbens and appears to be linked to the preferential abundance of D(3) receptors in this region. Targeting the mesolimbic DA system may have implications for improvement of impaired motor behaviour and particularly non-motor functions related to the nucleus accumbens.     

Placental toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a well-known model substance for inducing in humans and monkeys a severe extrapyramidal syndrome similar to Parkinson's disease. The neurotoxic action of MPTP can be exerted not only in adult animals but also during fetal development by diaplacental passage. Here we show that, during the gestation period of mice, the placenta is another important target organ of MPTP cytotoxicity. Pregnant NMRI mice on gestation day 15 received a single intraperitoneal dose of 20, 40, or 60 mg/kg MPTP. Developmental parameters of the fetuses and the placentas were determined on gestation day 18. Placental weight was consistently reduced in all experimental groups. Histology showed conspicuous alterations of the labyrinth layer; at 20 mg/kg MPTP there was already a significant reduction of the trabecular diameters and from 40 mg/kg onwards, severe necrosis of the syncytial trophoblast cells. In addition, there were necrotic alterations of the cells of the visceral yolk sac. The toxic effects are confined to the placenta at the doses used in the present experiments, leading at just 60 mg/kg to a marked placental insufficiency syndrome.     

Catecholamine-uptake inhibitors prevent the neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mouse brain

Male albino mice were injected subcutaneously with 50 mg/kg of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Seven days after administration, the striatal levels of dopamine(DA) and its metabolites were markedly reduced, as was the cortical level of noradrenaline (NA). Pretreatment with the selective inhibitor of DA uptake, GBR 13098, selectively and totally prevented the reduction of DA and its metabolites, whereas the selective inhibitor of NA uptake, maprotiline, selectively protected against NA depletion. The unselective inhibitors of DA and NA uptake, mazindol and nomifensine, prevented the MPTP-induced depletion of both DA and NA.     

银杏叶提取物对1—甲基—4—苯基—1，2，3，6—四氢吡啶所致帕金森症的保护作用及机制

目的:观察银杏叶提取物(EGb)对1-甲基-4-苯基-1,2,3,6-四氢吡啶(MPTP)及其离子1-甲基-4-苯基吡啶(MPP~ )诱导的帕金森症(PD)模型的保护作用。方法:用脑立体定位仪向黑质(AP-5.4mm,-2.2mm,H8.3mm)内注射MPTP诱导大鼠旋转。在注射MPTP 24h后将大鼠处死,硫代巴比妥法测定黑质中丙二醛(MDA),羟胺法(即改进的黄嘌呤氧化酶法)测定黑质中超氧化物歧化酶(SOD),荧光分光光度法(激发波长310nm,发射波长390nm)测定纹状体中多巴胺(DA)的含量。MPP~ 诱导PC12细胞凋亡,HE染色,光镜下观察凋亡细胞;吖啶橙/溴乙锭(AO/EB)染色,荧光显微镜记数凋亡细胞,观察不同浓度EGb(25,50,100mg/L)在6h,12h,24h对细胞凋亡率的影响。结果:EGb 100mg/kg组可减少模型鼠的旋转次数及旋转持续时间(n=10,P<0.05);与MPTP组比较,EGb 50mg/kg和100mg/kg组MDA相对降低,SOD及DA相对增高(n=10,P<0.05和P<0.01)。MPP~ 10μmol/L可诱导PC12细胞凋亡,EGb 50和100mg/L组在6h,12h,24h可降低细胞凋亡率(P<0.05和P<0.01,n=3)。结论:EGb对MPTP诱导的PD动物模型及其离子MPP~ 诱导的PD细胞模型均有保护作用,其保护机制与清除自由基及抑制神经元凋亡有关。     

Contribution of N-oxygenation to the metabolism of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) by various liver preparations

Liver microsomes from uninduced mice and rats catalyze NADPH- and oxygen-dependent N-oxygenation of the neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine). The N-oxide is the principal product and accounts for 95-96% of the total MPTP metabolized by microsomes. Demethylation of MPTP is detectable but the rate of nor-MPTP formation was never more than 4-6% of the rate of N-oxygenation. Studies on the biochemical mechanisms for N-oxygenation of MPTP suggest that this reaction is catalyzed exclusively by the flavin-containing monooxygenase. This conclusion is based on the effects of selective cytochrome P-450 inhibitors, positive effectors, and alternate substrates for the flavin-containing monooxygenase as well as on studies with the purified hog liver enzyme. MPTP is an excellent substrate for this monooxygenase with a Km of 30-33 microM. Limited studies with human liver whole homogenates suggest that N-oxygenation is also a major route for the metabolism of MPTP in man and the rate of N-oxide formation is approximately equal to the rate of mitochondrial monoamine oxidase-dependent MPDP+ (1-methyl-4-phenyl-2,3-dihydropyridinium species) production.     

Obligatory role for complex I inhibition in the dopaminergic neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).

Administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to mice and nonhuman primates causes a parkinsonian disorder characterized by a loss of dopamine-producing neurons in the substantia nigra and corresponding motor deficits. MPTP has been proposed to exert its neurotoxic effects through a variety of mechanisms, including inhibition of complex I of the mitochondrial respiratory chain, displacement of dopamine from vesicular stores, and formation of reactive oxygen species from mitochondrial or cytosolic sources. However, the mechanism of MPTP-induced neurotoxicity is still a matter of debate. Recently, we reported that the yeast single-subunit nicotinamide adenine dinucleotide (reduced) dehydrogenase (NDI1) is resistant to rotenone, a complex I inhibitor that produces a parkinsonian syndrome in rats, and that overexpression of NDI1 in SK-N-MC cells prevents the toxicity of rotenone. In this study, we used viral-mediated overexpression of NDI1 in SK-N-MC cells and animals to determine the relative contribution of complex I inhibition in the toxicity of MPTP. In cell culture, NDI1 overexpression abolished the toxicity of 1-methyl-4-phenylpyridinium, the active metabolite of MPTP. Overexpression of NDI1 through stereotactic administration of a viral vector harboring the NDI1 gene into the substantia nigra protected mice from both the neurochemical and behavioral deficits elicited by MPTP. These data identify inhibition of complex I as a requirement for dopaminergic neurodegeneration and subsequent behavioral deficits produced by MPTP. Furthermore, combined with reports of a complex I defect in Parkinson's disease (PD) patients, the present study affirms the utility of MPTP in understanding the molecular mechanisms underlying dopaminergic neurodegeneration in PD.     

Effect of amphetamine on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity in mice

Amphetamine has been shown to either potentiate or protect against MPTP neurotoxicity. The time course of changes in dopamine and its metabolites was examined after MPTP, amphetamine, or MPTP plus amphetamine administration. Results suggest that under conditions of granular depletion and release of dopamine by 10 mg/kg amphetamine, increased MPTP neurotoxicity occurs. Amphetamine injections at 2-5 mg/kg prevents the decline in dopamine possibly by blockade of the uptake of MPP+, rather than by an inhibition of monoamine oxidase.