Depletion of striatal dopamine by the N-oxide of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

The N-oxide of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is the major metabolite found in vivo and excreted in urine after the parenteral administration of the neurotoxicant, MPTP. In mice (C57BL/6), stereotaxic injection of MPTP N-oxide (15 micrograms) into the neostriatum produced dopamine (DA) depletion similar to that caused by MPTP. The DA depleting effect of MPTP N-oxide was a direct action, whereas the action of MPTP was mediated by the generation of oxidative metabolites. In the mouse striatal synaptosomal preparation, MPTP, MPP+ and MPTP N-oxide all competed with [3H]DA at its uptake site. In addition, MPP+ and MPTP N-oxide promoted [3H]DA release. In contrast to MPTP and MPDP+, MPTP N-oxide did not alter the electrophysiologically recorded field potential in nigro-striatal slices. These observations suggest that MPTP N-oxide can directly cause the chemical depletion of striatal DA without modifying the characteristics of cortico-striate synaptic transmission.     

The effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on presynaptic dopamine uptake sites in the mouse striatum

The effects of the specific dopaminergic neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), were studied on the kinetics of [3H]mazindol binding to striatal membranes of C57 black mice. This radioligand was used to label dopamine uptake sites and when administered in vivo, MPTP caused an irreversible, non-competitive inhibition of mazindol binding, consistent with damage to dopaminergic terminals. This effect was abolished by pretreatment with pargyline, a MAOB inhibitor, suggesting that oxidation of MPTP to the pyridinium moiety, MPP+, is a necessary step for toxicity when mazindol binding is used as an end point. In keeping with these findings, pretreatment of mice with mazindol protected against the dopamine-depleting effects of MPTP in vivo. This data suggests that MPTP exerts its toxic effects via MPP+ which is concentrated intraneuronally via the dopamine uptake system. During this process the neurotoxin irreversibly inactivates the dopamine uptake sites.     

The role of phospholipid methylation in 1-methyl-4-phenyl-pyridinium ion (MPP+)-induced neurotoxicity in PC12 cells

Excessive methylation has been proposed to be involved in the pathogenesis of Parkinson's disease (PD), via mechanisms that involve phospholipid methylation. Meanwhile, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was found to stimulate phospholipid methylation via the oxidized metabolite, 1-methyl-4-phenyl-pyridinium (MPP+), in the rat brain and liver tissues. In the present study, we investigated the effect of MPP+ on phosphatidylethanolamine N-methyltransferases (PENMT) and the potential role of this pathway in MPP(+)-induced neurotoxicity using PC12 cells. The results obtained indicate that MPP+ stimulated phosphatidylethanolamine (PTE) methylation to phosphatidylcholine (PTC) and correspondingly increased the formation of lysophosphatidylcholine (lyso-PTC). Moreover, the addition of S-adenosylmethionine (SAM) to the cell culture medium increases MPP(+)-induced cytotoxicity. The incubation of 1mM MPP+ and various concentrations of SAM (0-4 mM) decreased the viability of PC12 cells from 80% with MPP+ alone to 38% viability with 4 mM SAM for 4 days incubation. The data also revealed that the addition of S-adenosylhomocysteine (SAH), a methylation inhibitor, offered significant protection against MPP(+)-induced cytotoxicity, indicating that methylation plays a role in MPP(+)-induced cytotoxicity. Interestingly, lyso-PTC showed similar actions to MPP+ in causing many cytotoxic changes with at least 10 times higher potency. Lyso-PTC induced dopamine release and inhibited dopamine uptake in PC12 cells. Lyso-PTC also caused the inhibition of mitochondrial potential and increased the formation of reactive oxygen species in PC12 cells. These results indicate that phospholipid methylation pathway might be involved in MPP+ neurotoxicity and lyso-PTC might play a role in MPP(+)-induced neurotoxicity.     

A model of chronic neurotoxicity: long-term retention of the neurotoxin 1-methyl-4-phenylpyridinium (MPP+) within catecholaminergic neurons

The mechanism by which 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces lesions in the nigrostriatal dopamine system has been extensively studied. MPTP, a lipophilic molecule, enters the brain rapidly where it is converted to the pyridinium metabolite 1-methyl-4-phenylpyridinium (MPP+), by a two-step reaction that requires the enzyme monoamine oxidase. Following this conversion, which occurs primarily in astrocytes, MPP+ is sequestered within monoaminergic neurons by the energy-requiring monoaminergic transporters. Inside the neuron, MPP+ is thought to act as a mitochondrial toxin, slowly sapping the neuron of its energy-producing potential by blocking the action of NADH dehydrogenase. Much attention has been focused on cell death after MPTP administration, but little attention has been paid to the effects of small subtoxic doses of MPTP (i.e., doses that do not produce overt neuropathologic changes), which might occur during environmental exposure to a nigrostriatal toxin. Low doses of MPTP (as little as 1/25th of a toxic dose) produce long-term (greater than 6 weeks) but reversible changes in catecholamine metabolism. These changes are characterized by a decrease in the products of enzymatic oxidative deamination without a concomitant decrease in the amine concentrations (apparent MAO inhibition). Striatal concentrations of MPP+, which is retained in catecholaminergic terminals for similarly long periods, parallel the metabolic changes. Thus, the long-term storage of the MPTP metabolite, MPP+, correlates with altered catecholamine metabolism. The data on the effects of MPTP have been combined into a working model of how MPP+ exerts its effects following subtoxic or toxic doses. The site of this long-term neuronal storage of MPP+ after exposure to subtoxic doses of MPTP is as yet undetermined, but several studies suggest that monoaminergic vesicles may be the primary site, with mitochondria contributing some storage capacity. This vesicular site could represent a potential brain site for the accumulation of toxins during continual or repeated exposure to low levels of MPTP. Induced release from this site might accelerate the toxic interactions with cellular components such as mitochondria.     

Dopamine-releasing action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridine (MPP) in the neostriatum of the rat as demonstrated in vivo by the push-pull perfusion technique: dependence on sodium but not calcium ions

This study examined the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its metabolite, 1-methyl-4-phenylpyridine (MPP+) on the levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in push-pull perfusates of the striatum in chloral hydrate-anaesthetized rats. In control animals the levels of DA and DOPAC remained stable for at least 6 h and responded rapidly to a depolarizing stimulus of 25 mM K+. This K+-induced DA release was Ca2+-dependent since no stimulation was observed when the striatal sites were perfused with high K+ in a Ca2+-free medium containing 2 mM EGTA thus verifying that the striatal sites were functionally active. MPTP (0.025 and 0.05 microgram/microliter) stimulated DA release and inhibited DOPAC output in a dose-related manner. MPP+ (0.01, 0.025 and 0.05 microgram/microliter) produced a more robust dose-dependent increase in DA levels in the perfusates; however, the level of suppression of DOPAC was similar to that in response to MPTP. The effect of MPP+ on DA release was attenuated by 10(-6) M benztropine, the DA re-uptake blocker and completely inhibited by 10 micrograms/kg i.p. benztropine and 10(-4) M ouabain, the Na+, K+-ATPase (Na pump) inhibitor. However, although these substances prevented the MPP+-induced release of DA, the levels of DOPAC in the perfusates did not recover and remained completely suppressed suggesting that MPP+ may inhibit extraneuronal rather than intraneuronal monoamine oxidase (MAO). Perfusion of the striatal sites with a Ca2+-free medium containing 2 mM EGTA did not prevent the MPP+-induced DA release indicating that MPP+ does not release DA from the striatal DA terminals by the Ca2+-dependent process of exocytosis. The responses of DA and DOPAC to 25 mM K+ were markedly suppressed in animals treated with MPTP and MPP+, these effects being most severe with the highest dose of MPP+. Moreover, this suppression of the K+-induced responses persisted in animals perfused with MPP+ in the presence of benztropine or ouabain, thus suggesting that MPP+ may have potent deleterious membrane effects. These studies have provided the first direct in vivo demonstration of the action of MPTP and MPP+ and the neuropharmacological basis of this action on DA metabolism in the rat striatum. The results show that the elevated levels of DA in the striatal perfusates are due to a direct action of MPTP and MPP+ on the nigrostriatal DA terminals and cannot be fully accounted for solely by their inhibition of MAO activity and/or inhibition of DA re-uptake.(ABSTRACT TRUNCATED AT 250 WORDS)     

MPTP causes a non-reversible depression of synaptic transmission in mouse neostriatal brain slice

MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) causes a Parkinson's disease-like syndrome. The mechanism of MPTP's neurotoxicity is unknown; however, one hypothesis is that MPP+ (1-methyl-4-phenylpyridinium), a product of MPTP's oxidation, is the neurotoxic agent. Using a mouse brain slice preparation we studied the effects of MPTP and MPP+ on synaptic transmission. We found MPTP caused a decrease in amplitude of an excitatory synaptic response not reversed by washing. This non-reversible action of MPTP was prevented by GBR-32 and pargyline. MPP+s caused a decrease in synaptic transmission, but this decrease was reversed by washing. The results suggest that the toxic effect of MPTP on synaptic transmission is not accounted for by the action of MPP+.     

Production and disposition of 1-methyl-4-phenylpyridinium in primary cultures of mouse astrocytes.

Dopaminergic neurons are a primary target for 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity. However, the conversion of MPTP to its neurotoxic 1-methyl-4-phenylpyridinium metabolite (MPP+) is likely to occur in astrocytes via the monoamine oxidase (MAO)-dependent formation of the 1-methyl-4-phenyl-2,3-dihydropyridinium intermediate (MPDP+). The main purpose of this study was to characterize the molecular mechanism(s) by which MPP+, once generated by astrocytes, may reach the extracellular space to become available for the active accumulation into dopaminergic neurons. Primary cultures of mouse astrocytes were used as an in vitro model system. After the addition of MPTP, levels of MPP+ were found to increase at constant rates both intracellularly and extracellularly at time points when no sign of cytotoxicity was evident. In contrast, MPDP+ levels remained quite stable during 4 days of incubation in the presence of MPTP. Finally, when astrocytes were allowed to accumulate MPP+ by pretreatment with either MPTP or MPP+ and then were incubated in fresh medium not containing MPTP or MPP+, intracellular levels of MPP+ rapidly declined and corresponding amounts of this compound were found in the incubation medium. Results of this study are compatible with the following conclusions: 1) the MPP+ accumulated in the extracellular compartment during incubations with MPTP is not released from astrocytes as a consequence of its own cytotoxic effects; 2) MPP+ can be formed extracellularly presumably via autoxidation of MPDP+ after this latter compound has been generated within astrocytes and has crossed astrocyte membranes; and 3) despite its charged chemical structure, MPP+ can cross the plasma membrane toward the extracellular space after being formed within astrocytes.     

Acetaldehyde directly enhances MPP+ neurotoxicity and delays its elimination from the striatum

We have previously shown that ethanol and acetaldehyde (ACE) potentiate MPTP toxicity in mice, selectively enhancing dopamine (DA) depletion in the striatum and markedly increasing loss of DA neurons in the substantia nigra. Several months after these combined treatments there is no evidence of any recovery. In the present study, we measured the accumulation of the MPTP toxic metabolite 1-methyl-4-phenylpyridinium ion (MPP+) in both striatum and whole brain, after MPTP alone or after combined treatments with ethanol or acetaldehyde, in order to determine whether this enhancement of toxicity is caused by changes in the MPTP metabolism. We also investigated whether acetaldehyde interfered with the conversion of MPTP to MPP+ by glial cells in vitro and studied its effects on the MPP+ uptake and spontaneous release from mesencephalic DA neurons or striatal astrocytes in primary cell cultures from E13 mouse embryos. The results from the in vivo experiments indicated that relatively low doses of ethanol or acetaldehyde potentiate directly MPP+ toxicity, apparently without interfering with its pharmacokinetics. However when higher doses of these drugs were administered, they also decreased MPP+ clearance from the striatum. ACE also increased initial MPTP accumulation in the whole brain but failed to enhance MPP+ levels, thus indicating that ACE effect is not related to MPTP metabolism. In vitro studies confirmed that ACE does not modify MPTP metabolism in striatal or mesencephalic astrocytes in culture. In mesencephalic neuronal cultures ACE does not change the levels of MPP+ uptake (MPP+ is accumulated in putative DA neurons in vitro with a mechanism similar to that of the DA high affinity uptake) nor its spontaneous release. These results indicate that the slower MPP+ clearance from the stratum after ACE is not related to a direct effect of ACE on DA neurons or astrocytes.     

The effects of pyridinium salts, structurally related compounds of 1-methyl-4-phenylpyridinium ion (MPP), on tyrosine hydroxylation in rat striatal tissue slices

We had previously reported that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which produces Parkinson's disease in humans and animals, inhibited tyrosine hydroxylation, the rate-limiting step of dopamine synthesis, in striatal tissue slices after its conversion to 1-methyl-4-phenylpyridinium ion by monoamine oxidase. In this report, structurally related compounds of 1-methyl-4-phenylpyridinium ion (MPP+) were synthesized and tested for their ability to inhibit tyrosine hydroxylation in rat striatal tissue slices. The following pyridinium salts showed inhibitory effect on tyrosine hydroxylation: pyridinium salts that substituted the alkyl group for the methyl group of MPP+ (1-ethyl-, 1-propyl-, 1-isopropyl-4-phenylpyridinium ions); pyridinium salts that changed the position of the phenyl group (1-methyl-2-phenyl-, 1-methyl-3-phenylpyridinium ions); pyridinium salts that modified the phenyl ring at 4 position (1-methyl-4-tolylpyridinium ion, 1-methyl-4-(4'-methoxyphenyl)pyridinium ion); and N-methylisoquinolinium ion. In contrast, pyridinium salts in which the phenyl group was replaced with hydrogen, methyl or methoxycarbonyl group, paraquat (1,1'-dimethyl-4,4'-dipyridinium chloride, one of bipyridinium compounds and a widely used herbicide), and N-methylquinolinium ion, showed no inhibitory effect. Nomifensine, an inhibitor of dopamine uptake, prevented the inhibition caused by 1-methyl-2-phenylpyridinium ion. The result suggests that the effective pyridinium salts are taken up into dopaminergic neurons likewise MPP+ by the dopamine transport system and inhibit tyrosine hydroxylation in striatal tissue slices. N-methylisoquinolinium ion could be one of the candidates of endogenous or environmental factors that produce Parkinson's disease.     

The effect of glia-conditioned medium on dopamine neurons in culture. Modulation of apoptosis, tyrosine hydroxylase expression and 1-methyl-4-phenylpyridinium toxicity

Recent experiments have shown that glia-conditioned medium (GCM) protects against L-3,4-Dihydroxyphenylalanine (L-DOPA) toxicity for dopamine neurons in culture. In this study we have investigated the effect of GCM on the number of tyrosine hydroxylase (TH) immunoreactive neurons, levels of dopamine, number of high affinity dopamine uptake sites, and percentage of apoptotic cells in midbrain neuronal cultures, before and after exposure to 1-methyl-4-phenylpyridinium (MPP+). Fetal midbrain neuronal cultures were treated with vehicle, MPP+, 10(-5) M, mesencephalic GCM, or MPP+ plus GCM. GCM was administered a) simultaneously, b) 24 hours before MPP+, and c) 24 and d) 72 hours after MPP+, respectively. In the absence of GCM, MPP+ reduced the number of TH immunoreactive neurons and increased apoptosis. GCM increased the number of TH+ neurons and the levels of dopamine and decreased apoptosis. In the cultures treated with GCM and MPP+, GCM counteracted the effects of MPP+ and increased the length and arborization of TH+ neurites. The protective effect of GCM was maximal in cultures co-treated with GCM and MPP+ simultaneously, but it also restored dopamine parameters in cultures receiving GCM 1 or 3 days after MPP+. The protective effect of GCM was negligible in cultures pretreated with GCM and receiving MPP+ 24 hours later. In neuronal cultures, grown for 8 days in vitro untreated with MPP+, short term exposure to GCM reversed the effect of aging and restored the number of TH+ neurons to levels higher than those observed at the time of seeding. Therefore, GCM does not only protect against MPP+ but does also induce de novo expression of dopamine phenotype in midbrain cultures.     

In vivo protection of striatal dopaminergic system against 1-methyl-4-phenylpyridinium neurotoxicity by phenobarbital

We have studied the effect of a semichronic and acute treatment of phenobarbital on in vivo 1-methyl-4-phenylpyridinium ion- (MPP+)-induced neurotoxicity. A group of rats were intraperitoneally injected for 12 days with phenobarbital (80 mg/Kg of body weight, semichronic treatment) in order to induce cytochrome P450 levels in brain. At day 10 of treatment, rats received unilateral left striatal injections of 1 or 2 μg of MPP+. Two days after the injection of the toxin a dose-dependent loss of dopamine uptake along with a concomitant decrease of dopamine levels and its metabolites was produced in control rats. In phenobarbital treated animals striatal injection of 1 μg of MPP+ did not produce any effect on dopaminergic parameters but injection of 2 μg of MPP+ caused losses of dopamine levels and dopamine transporter although smaller than in control rats. TH immunohistochemistry in semichronic phenobarbital treated rats also demonstrated the protective effect of this drug against MPP+ toxicity. Dopamine uptake in synaptosomes from semichronic phenobarbital treated rats did not change with respect to the controls, thereby diminished MPP+ toxicity in phenobarbital treated rats is not due to an alterated uptake of the toxin. Neuroprotection found by intraperitoneal injection of phenobarbital 30 min before MPP+ intrastriatal injection (acute treatment) could discard the induction of cytochrome P450 as responsible for this suppressed neurotoxicity of MPP+. The neuroprotective effect of phenobarbital could be produced by its action as an excitatory amino acid antagonist or as a GABA agonist. J. Neurosci. Res. 49:301–308, 1997. © 1997 Wiley-Liss, Inc.     

Acute mitochondrial and chronic toxicological effects of 1-methyl-4-phenylpyridinium in human neuroblastoma cells

At low micromolar concentrations, 1-methyl-4-phenylpyridinium (MPP+), the toxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) selectively kills nigrostriatal dopaminergic neurons by mechanisms believed to involve impairment of mitochondrial complex I. A human neuroblastoma cell line expressing the dopamine transporter (DAT) was utilized to examine the effects of MPP+ on acute physiologic responses and subsequent cell death. Acute responses were measured by microphysiometry and by monitoring mitochondrial membrane potential with [3H]tetraphenylphosphonium (TPP+) uptake. MPP+ (10 microM) increased extracellular proton excretion in DAT-expressing cells within 2-3 min, but had no effect in untransfected cells. The lipophilic complex I inhibitor, rotenone, increased proton excretion in both cell lines. In DAT-expressing cells, mitochondrial membrane potential was reduced within I h of 10 microM MPP+ exposure. Rotenone reduced mitochondrial membrane potential in both cell lines. MPP+ caused apoptotic death of DAT-transfected cells 2-3 days after drug application, but did not kill untransfected cells. Thus, MPP+ produces immediate mitochondrial impairment only in cells that express DAT, and these changes occur days before overt cellular toxicity. The magnitude, time course and nature of these changes were similar to those produced by rotenone, confirming the site of action of MPP+ as mitochondrial complex I. These immediate mitochondrial effects appear to be an accurate predictor of subsequent cell death.     

1—甲基—4—苯基吡啶离子诱导MES23．5细胞线粒体功能异常的实验研究

目的：探讨1－甲基－4－苯基吡啶离子（MPP^ ）诱导多巴胺（DA）能神经元死亡的线粒体功能异常机制。方法：不同浓度的1－甲基－4－苯基－1,2,3,6－四氢吡啶（MPTP）、MPP 及还原型谷胱甘肽（GSH）与MES23．5细胞共同培养,采用MTT比色法及流式细胞术检测细胞线粒体膜电势（△Ψm）和氧自由基（ROS）的变.MES23\5在力在MPP^ 组显著减低,且与浓度呈正相关,GSH＋MPP^ 组轻度减低,而PTP组不同降低,此外,用MPP^ 处理后MES23．5细胞线粒体△Ψm明显下降和ROS生成显著增加,结论：线粒体△Ψm下降及ROS生成增多可能与了MPP^ 诱导黑质DNA能神经元死亡的机制。     

Transgenic mice with increased Cu/Zn-superoxide dismutase activity are resistant to N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurotoxicity.

Administration of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to mammals causes damage to the nigrostriatal dopaminergic pathway similar to that observed in Parkinson's disease. It has been suggested that the mechanism by which MPTP kills dopamine (DA) neurons involves an energy crisis due to the inhibition of mitochondrial complex I. In addition, superoxide radicals (O2-), generated subsequent to the blockade of mitochondrial complex I, may also be involved in MPTP-induced neurotoxicity. Superoxide dismutase (SOD) is a scavenger enzyme that protects cells from the hazard of O2- radicals. To evaluate further the role of O2- radical in MPTP-induced toxicity, we tested the effects of MPTP in transgenic mice with increased SOD activity. In nontransgenic littermates with normal SOD activity, MPTP injection causes a marked reduction in striatal levels of DA and its metabolites as well as in striatal and nigral 3H-DA uptake; these findings are consistent with a loss in dopaminergic neurons. In contrast, in transgenic mice with increased SOD activity, MPTP injection does not cause any significant changes either in levels of DA and metabolites or in 3H-DA uptake. We show that this lack of toxicity is not due to a lower delivery of MPTP to the brain following its intraperitoneal injection, to reduced brain biotransformation of MPTP to N-methyl-4-phenylpyridinium ion (MPP+), to diminished striatal mitochondrial monoamine oxidase B activity, to decreased synaptosomal uptake of MPP+, to lower potency of MPP+ to inhibit the complex I of the mitochondrial electron transport chain, or to faster brain elimination of MPP+. These results suggest that increased SOD activity is, most likely, the protective factor that confers resistance to transgenic mice against MPTP-induced neurotoxicity. Thus, this study provides further evidence that some of the deleterious effects of MPTP may be mediated by O2- radicals. The similarity between the MPTP model and Parkinson's disease further raises the possibility that oxy-radicals may play a significant role in the etiology of this neurodegenerative disorder.     

Mechanisms of MPTP (1-methyl-4-phenyl-l, 2, 3, 6-tetrahydropyridine) neurotoxicity to striatal dopamine neurons in mice

1. MPTP given to mice in 4 daily doses (20 mg/kg s.c.) resulted in 56–70% depletion of striatal dopamine 1 week after the last dose.

2. Pretreatment with deprenyl or MD 240928, selective inhibitors of monoamine oxidase type B, or with amfonelic acid or nomifensine, selective inhibitors of dopamine uptake, prevented the depletion of striatal dopamine. In contrast, pretreatment with -methyltyrosine, Ro 4-1284 or haloperidol did not prevent the depletion of striatal dopamine by MPTP.

3. The results are compatible with the view that dopamine itself is not involved in the neurotoxic effect of MPTP but that MPP+, a metabolite of MPTP formed by MAO-B and accumulated by the dopamine uptake carrier, is responsible for the neurotoxicity.     

Effect of ascorbic acid on the synaptosomal uptake of [3H]MPP+, [3H]dopamine, and [14C]GABA

The effects of ascorbic acid on the synaptosomal uptake of [3H]MPP+, [3H]dopamine, and [14C]GABA were examined in attempts to understand the mechanism of ascorbic acid attenuation of MPTP neurotoxicity. [3H]Dopamine uptake was increased at lower levels (0.01 and 0.1 mM) and decreased at higher levels (10 mM) of ascorbic acid. Ascorbic acid inhibited [3H]MPP+ uptake (IC50 = 0.1 mM) and [14C]GABA uptake (IC50 = 10 mM). Washout of ascorbic acid restored uptake of [3H]dopamine and [3H]MPP+, suggesting that ascorbic-acid-induced lipid peroxidation was not involved in the effect on uptake. In addition to the possible involvement of antioxidant mechanisms in the in vivo attenuation of the neurotoxicity of MPTP by ascorbic acid, the present results indicate a direct effect of ascorbic acid in inhibiting the uptake of MPP+ into dopaminergic nerve terminals.     

Selective decrease in extracellular DOPAC concentrations in rat striatum following in vivo dialysis with low concentrations of MPP+.

Using the technique of in vivo dialysis, 1-methyl-4-phenylpyridinium (MPP+), the neurotoxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), was applied to the rat striatum and the effects of this treatment on the efflux of striatal dopamine (DA) and metabolites were monitored. The inclusion of low concentrations of MPP+ (1 and 10 microM) in the dialysis solution caused a progressive decrease in the efflux of dihydroxyphenylacetic acid (DOPAC), the major deamination product of DA, while homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) remained unchanged. Unlike the effects of dialysis with millimolar concentrations of MPP+, a large increase in the efflux of striatal DA was not observed. The effect of dialysis with 1 microM MPP+ was blocked if 1 microM GBR 12909, a specific DA reuptake blocker, was included in the dialysis fluid, suggesting uptake of MPP+ into striatal DA terminals mediated this effect.     

MPDP causes a non-reversible decrease in neostriatal synaptic transmission in mouse brain slice

MPTP causes a Parkinson's disease-like syndrome in man and certain other animals. The toxic effect occurs if monoamine oxidase B is available, indicating that an MPTP metabolite may cause the toxic effect. We tested the effect of MPDP+, the first product of MPTP oxidation, and found that it, like MPTP, caused a non-reversible decrease in synaptic transmission in the mouse brain slice preparation. As the second oxidation product, MPP+ had been shown not to cause a similar, non-reversible decrease in synaptic transmission, MPDP+ is a better candidate for the role of toxic substance.     

Significance of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine for the etiology and therapy of idiopathic Parkinson disease

Exposure of drug addicts to MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) has caused a Parkinsonian syndrome accompanied by a selective destruction of dopamine containing neurones in the pars compacta of the substantia nigra. MPTP in the human causes a severe irreversible state that very closely resembles idiopathic Parkinson's disease both in its clinical features and response to pharmacological treatment. Interest in potential environmental agents that might play a role in the aetiology of idiopathic Parkinson's disease is likely to increase as the result of the discovery of the relatively simple molecule MPTP which is highly toxic to the substantia nigra. Until the discovery of the neurotoxicity of MPTP there was no effective animal model of Parkinson's disease. Administration of PTP to monkeys induces persistent parkinsonism which responds to classical antiparkinsonian therapy. The morphological and biochemical changes in the brains of the animals are more limited and selective than those seen in idiopathic Parkinson's disease. The model of MPTP-treated monkeys appears to provide a useful testbed for the evaluation of future treatments for the disease. The precise mechanism of MPTP toxicity has yet to be determined and may provide the clue to the mechanism of neuronal death in Parkinson's disease. After entering the brain MPTP is oxidized to MPP+ (1-methyl-4-phenylpyridine) at an extraneuronal site.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects on dopamine metabolism of MPTP and MPP+ infused through a push-pull cannula into the caudate nucleus of awake adult male rats

Our previous data demonstrated that both 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium ion (MPP+) exerted potent inhibition on endogenous 3,4-dihydroxyphenylacetic acid (DOPAC) output and potent stimulation on endogenous dopamine (DA) release from the rat corpus striatum superfused in vitro. In this report, using a push-pull perfusion technique, we examined in vivo the acute effects of MPTP and MPP+ on DA metabolism in the rat caudate nucleus (CN). MPTP or MPP+ in modified Krebs-Ringer phosphate buffer at concentrations of 10(-6), 10(-5) and 10(-4) M was administered directly into the CN for 15 min, each 90 min apart. Thirty minutes after the infusion of 10(-6) M MPP+, DOPAC output was reduced to a significantly lower value and subsequent infusions of high concentrations of MPP+ further decreased DOPAC output. Homovanillic acid (HVA) output was also decreased by MPP+ infusions, however, at higher concentrations. In respect to DA release, 1 of 10, 4 of 10 and 7 of 10 animals responded with significant increases to 10(-6), 10(-5) and 10(-4) M MPP+, respectively. On the other hand, MPTP was effective in reducing DOPAC output only at 10(-4) M and ineffective in altering DA and HVA output at all doses tested. In addition, neither drugs had a significant effect on 5-hydroxyindoleacetic acid. Accompanying the dramatic changes in DA metabolism caused by MPP+, two uncommon behavioral syndromes were also observed; tremor-body twist and body shaking.(ABSTRACT TRUNCATED AT 250 WORDS)