1-Methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'-methyl-MPTP) is less neurotoxic than MPTP in the common marmoset

Four adult marmosets were treated with increasing doses of 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'-methyl-MPTP) in the range 0.23-4.3 mg/kg i.p. to give a cumulative dose of 11.0-11.6 mg/kg over a 6-10 day period. After 4 days of treatment, and as the dosage was gradually increased, the animals exhibited mild motor deficits. These abnormalities slowly declined over the following 1-6 week period. In contrast, similar treatment of common marmosets with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (1-4 mg/kg i.p.) for 3-5 days in a cumulative dose of 6.9-9.2 mg/kg produced gross impairment of motor function which persisted throughout the 5 weeks period of observation. Administration of 2'-methyl-MPTP for 6-10 days caused some decrease in dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC), but not homovanillic acid (HVA) content in the caudate nucleus in animals 5-6 weeks after the start of treatment. There was a small decrease in [3H]dopamine uptake into putamen synaptosomes. This contrasted with the marked decreases in all these parameters observed after MPTP treatment of common marmosets. Histological examination of the substantia nigra from the four animals treated with 2'-methyl-MPTP did not show degeneration or loss of dopamine-containing cell bodies in the zona compacta. In contrast, MPTP caused severe destruction of these pigmented nigral neurones. In the common marmoset 2'-methyl-MPTP does not appear to show the same neurotoxic action as MPTP itself. This contrasts with findings in the mouse where 2'-methyl-MPTP is more toxic to dopamine-containing cells of substantia nigra than MPTP.     

Biochemical mechanism of action of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

The various biochemical mechanisms considered to explain the selective dopaminergic neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) are reviewed. MPTP is metabolized by monoamine oxidase in the brain, ultimately yielding 1-methyl-4-phenylpyridinium cation (MPP+), which is accumulated in dopamine cells by the high-affinity dopamine uptake pump. Cell death appears to reflect a compromise in energy production arising as a result of the Nernstian concentration of MPP+ inside mitochondria and persistent inhibition of Site 1 of the respiratory chain. The structural features underlying each biochemical step involved in the expression of neurotoxicity are described, and the implications of the MPTP phenomenon to efforts aimed at elucidating the pathogenesis of idiopathic parkinsonism are discussed.     

Comparison of Key Steps in 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) Neurotoxicity in Rodents

Abstract: Three steps in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity were compared with the neurodegenerative effects of the toxin in mice and rats. Firstly, we compared the neurotoxicity of MPTP, mediated by monoamine oxidase (MAO)-B, to that of 1-methyl-4-(2′-methylphenyl)-1,2,3,6-tetrahydropyridine (2′-CH₃-MPTP), an analogue oxidized by MAO-A and MAO-B. Both toxins caused degeneration of dopamine terminals in mice but not in rats. In NMRI mice noradrenaline terminals were also affected by both toxins. Pretreatment with deprenyl to prevent MAO-B-mediated oxidation in the capillary endothelium enhanced dopamine toxicity to 2′-CH₃-MPTP in nucleus accumbens but no potentiation was seen in striatum and the olfactory tubercle. Secondly, synaptosomal uptake of the 1-methyl-4-phenylpyridinium ion (MPP⁺) was studied. Uptake in rats was not significantly different from that in the two mice strains. Thirdly, no significant differences were found in MPP⁺-induced lactate production in striatal slices or synaptosomes. We conclude that the lack of effect of MPTP in rats is not due to mechanisms specific for MPTP but probably to the ability of rat catecholamine neurons to cope with, and survive, impaired energy metabolism.     

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.     

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.     

Investigation of the possible dopaminergic toxicity of 1-methyl-3-phenyl-1,2,3,6-tetrahydropyridine, an isomer to the neurotoxin MPTP.

1-Methyl-3-phenyl-1,2,3,6-tetrahydropyridine (M-3-PTP) is an analogue to the Parkinson-producing dopaminergic toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), M-3-PTP, and simple analogues thereof, are versatile intermediates in organic synthesis. The present study was undertaken to investigate the possible dopaminergic toxicity of M-3-PTP. Male albino mice were injected with 50 mg/kg of either MPTP or M-3-PTP and dopamine (DA) and its metabolites were determined 2 hr and 7 days after the administration. Two hr after MPTP profound acute changes in brain DA metabolism were found, i.e. an approximately 50% reduction in the concentration of DA together with a 10-fold increase in the level of 3-methoxytyramine. Seven days after MPTP, DA and metabolites were markedly reduced which is consistent with a degeneration of the dopaminergic neurones. In contrast M-3-PTP produced no acute or long-term alterations in the concentrations of DA and its metabolites in mouse brain. Furthermore, in vitro experiments show that M-3-PTP does not inhibit monoamine oxidase B. Thus, the present data show that M-3-PTP is devoid of dopaminergic toxicity in mouse brain and is not likely to produce Parkinson's disease in humans. The lack of toxicity is probably explained by the low affinity of M-3-PTP for monoamino oxidase B.     

Differential neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in Swiss-Webster mice from different sources

Male Swiss-Webster mice obtained from three different commercial suppliers were treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Mice from one of the three suppliers were considerably more sensitive to the neurotoxic effects of MPTP than mice from the others. For example the MPTP-induced decrements in the neostriatal levels of dopamine and its metabolites, which are a reflection of the neurotoxic process, were considerably greater in the sensitive strain than in the less sensitive strains.     

Depletion of cardiac norepinephrine in rats and mice by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a commercially available chemical reagent. Although little has been known about its biological effects, recently MPTP has been reported to cause irreversible Parkinson's disease-like symptoms in humans and in monkeys. We describe here another pharmacologic effect of MPTP, the ability to deplete cardiac norepinephrine in rats and mice. In mice, cardiac norepinephrine concentration decreased within 1 hr, was maximally depleted at 24 hr, and recovered by 4-7 days after i.p. injection of a 32 mg/kg dose of MPTP. The depletion was antagonized by desipramine pretreatment, as was norepinephrine depletion by tyramine. In rats, cardiac norepinephrine depletion by 10-30 mg/kg, i.p., doses of MPTP was accompanied by depletion of cardiac dopamine and of norepinephrine in the mesenteric artery. In rats and in mice, norepinephrine in brain was affected to a smaller degree than was norepinephrine in heart, and dopamine in brain was depleted very little if at all. In spontaneously hypertensive rats, the depletion of cardiac norepinephrine was associated with a marked antihypertensive effect. The p-hydroxy analog of MPTP did not deplete cardiac norepinephrine in rats, indicating that its possible formation as a metabolite of MPTP was not involved in the depletion of cardiac norepinephrine. These findings extend the spectrum of known pharmacologic effects of MPTP.     

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.     

Depression of the hepatic cytochrome P-450 monooxygenase system by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

The effects of the neurotoxic compound, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on the hepatic cytochrome P-450 monooxygenase system were assessed using C57 BL/6J mice. Treatment with MPTP caused a marked depression of hepatic cytochrome P-450 content, ethoxyresorufin O-dealkylase and NADPH cytochrome C reductase activities. This effect was maximal 3 to 6 hours after treatment and was dependent on the dose of MPTP administered. Depression of spectrophotometrically measured cytochrome P-450 content was associated with increase in cytochrome P-420 content and lipid peroxidation. In vitro studies showed the formation of a metabolic-intermediate complex with cytochrome P-450 which may partially explain the depression of cytochrome P-450 content and activity by MPTP.     

Neuroprotective effects of phenylethanoid glycosides from Cistanches salsa against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopaminergic toxicity in C57 mice

The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) has been employed to create a Parkinson's disease-like model in both rodents and primates based primarily on its ability to create a striatal dopamine deficit due to the loss of dopaminergic neurons in the substantia nigra compacta. The present study was carried out to determine the possible effects of phenylethanoid glycosides (PhGs) from Cistanches salsa (C. A. MEY, G. BECK) on attenuating the serious behavioral disorder and increasing dopamine (DA) levels in the striata of MPTP-lesioned C57 mice. MPTP (30 mg/kg i.p. for 4 d) induced serious behavioral disorders and significantly reduced striatal DA levels in C57 mice. In spontaneous motor activity and rotarod tests, obvious behavioral differences were seen between control and model groups. PhGs (10, 50 mg/kg) significantly increased the spontaneous movement number and latent period of mice on the rotating rod (p<0.01). Injections of MPTP 30 mg/kg for 4 d caused a significant reduction in DA, 3,4-dihydroxyphenyl acetic acid, and homovanillic acid in striata analyzed by HPLC-electrochemistry (p<0.01). The neurotoxic effects of MPTP were attenuated by pretreatment with PhGs (10, 50 mg/kg) in a dose-dependent fashion. The apparent neuroprotective effects of PhGs on nigral dopaminergic neurons were also confirmed by the results of immunohistochemical staining. The present in vivo data clearly demonstrate that PhGs can protect dopaminergic neurons against dopamine neurotoxicity induced by MPTP, as suggested by an earlier in vitro study. The neuroprotective effects of PhGs were the first reported for a natural product.     

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.     

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.     

The role of membrane and vesicular monoamine transporters in the neurotoxic and hypothermic effects of 1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH(2)-MPTP)

The neurotoxin 1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH(2)-MPTP) damages forebrain serotonin (5-HT) and norepinephrine (NE) nerve terminals while sparing striatal dopaminergic innervation. Previous studies suggest that 2'-NH(2)-MPTP acts by a mechanism that involves uptake by the plasma membrane 5-HT and NE transporters. The present investigation further explores the molecular mechanism of 2'-NH(2)-MPTP with regard to cellular transport and effects on body temperature. Mice with genetically controlled decreases in serotonin transporter (SERT) expression were studied to corroborate pharmacologic evidence implicating SERT in 2'-NH(2)-MPTP-induced serotonin neurotoxicity. To investigate whether sequestration by the intracellular vesicular monoamine transporter type 2 (VMAT2) occurs, mice with reduced VMAT2 expression or mice receiving the VMAT2 inhibitor Ro 4-1284 (2-hydroxy-2-ethyl-3-isobutyl-9,10-dimethoxy-1,2,3,4,6,7-hexahydrobenzo[alpha]chinolizin hydrochloride) were treated with 2'-NH(2)-MPTP. Body temperature was measured as a function of reduced SERT or VMAT2 expression. 2'-NH(2)-MPTP caused a 2 degrees C drop in temperature that was attenuated by decreased SERT but not VMAT2. In addition, complete loss of SERT attenuated cortical and hippocampal depletions in 5-HT but not NE. In contrast, mice with a 50% reduction in VMAT2 exhibited similar 5-HT and NE toxicity when compared with wild-type mice at higher doses of 2'-NH(2)-MPTP, whereas a slight potentiation of toxicity was observed at very low doses of 2'-NH(2)-MPTP. Pharmacologic inhibition of VMAT2 caused minimal potentiation of neurotransmitter depletions in response to moderate doses of 2'-NH(2)-MPTP. Thus, 2'-NH(2)-MPTP seems to be similar to MPTP in its requirement for selective plasma membrane transport and the expression of acute hypothermia; however, unlike MPTP, VMAT2 does not appear to play a major role in the toxic mechanism of 2'-NH(2)-MPTP.     

The potentiating effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on paraquat-induced neurochemical and behavioral changes in mice

Although the etiology of Parkinson's disease (PD) is not fully understood, there are numerous studies that have linked the increased risk for developing PD to pesticides exposure including paraquat (PQ). Moreover, the exposure to a combination of compounds or chemical mixtures has been suggested to further increase this risk. In the current study, the effects of PQ on the nigrostriatal dopaminergic system in male C57BL6 mice exposed to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were examined to assess the impact of toxic substance mixtures exposure on neurochemical and behavioral changes. In this study, a low non-toxic dose of MPTP (10mg/kg) was injected once a day for 5 days and was followed by PQ (7 mg/kg) once a day for 6 days (subacute protocol) or once a week for 10 weeks (chronic protocol). The results from the subacute protocol showed that PQ reduced the turnover of dopamine (DA) as indicated by a 21% and a 22.3% decrease in dihydroxyphenyl acetic acid (DOPAC), homovanillic acid and increased S-adenosyl methionine/S-adenosyl homocysteine index (SAM/SAH) by 100%. However, the administration of PQ to MPTP primed mice resulted in the decrease of DOPAC, HVA, DA, by 35.8%, 35.2% and 22.1%, respectively. In addition, PQ decreased the total number of movements (TM) by 28% but MPTP plus PQ decreased TM by 41%. The SAM/SAH index showed that MPTP increased methylation by 33.3%, but MPTP plus PQ increased methylation by 81%. In the chronic protocol, the data showed that MPTP administration did not affect DA, DOPAC, and HVA levels. The administration of PQ led to significant decrease in DOPAC, HVA, and TD by 31.6%, 19.9%, and 21.2% respectively with no effect on DA levels. The MPTP plus PQ group showed reduced DA, DOPAC, HVA, and total distance traveled by 58.4%, 82.8%, 55.8%, and 83.9%, respectively. Meanwhile, PQ administration caused a reduction in tyrosine hydroxylase immunoreactivity in the substantia nigra, and this effect was more pronounced in MPTP pretreated mice. It was concluded from this study that prior treatment with MPTP potentiated the effects of PQ in reducing DA, DOPAC, HVA, TH immunoreactivity, locomotor activity, and increasing the methylation index. The enhanced effects of PQ following MPTP administration further support the role of toxic substance mixtures in causing Parkinson's disease.