Metallothionein Inducers Protect Against 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine Neurotoxicity in Mice

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a drug that induces parkinsonism in human and non-human primates. Free radicals are thought to be involved in its mechanism of action. Recently, the participation of metallothionein as scavenger of free radicals has been proposed. In this work, we studied the effect of metallothionein inducers in MPTP neurotoxic action. Male swiss albino mice were pretreated either with cadmium (1 mg/kg) or dexamethasone (5 mg/kg), two well-known inducers of metallothionein synthesis, and 5 hours later with an MPTP administration (30 mg/kg). Treatment schedule was repeated daily for either 3 or 5 consecutive days. All animals were killed 7 days after the last administration, and striatal dopamine and homovanillic acid contents were analyzed as an end-point of MPTP neurotoxicity. Striatal dopamine content of cadmium plus MPTP-treated animals (3-days) increased by 32%, and 48% (5-days) vs MPTP-alone animals. Dexamethasone plus MPTP-treated group also showed increased dopamine levels 28% (3-days) and 43% (5-days). MPTP treatment reduced striatal metallothionein concentration (49% vs control animals). Dexamethasone and cadmium increased metallothionein concentrations in MPTP-treated groups, by 77% and 82% respectively. Results suggest that metallothionein induction provide a significant resistance factor against the deleterious effect of MPTP.     

Potentiation by the Tetraphenylboron Anion of the Effects of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine and Its Pyridinium Metabolite

The 1-methyl-4-phenylpyridinium species (MPP+) is the four-electron oxidation product of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and is widely assumed to be the actual neurotoxic species responsible for the MPTP-induced destruction of dopaminergic neurons. MPTP is oxidized by the enzyme monoamine oxidase-B to a dihydropyridinium intermediate which is oxidized further to MPP+, an effective inhibitor of the oxidation of the Complex I substrates glutamate/malate in isolated mitochondrial preparations. In the present study, the tetraphenylboron anion (TPB) greatly potentiated the inhibitory effects of MPP+ and other selected pyridinium species on glutamate/malate respiration in isolated mouse liver mitochondria. At 10 microM TPB, the potentiation ranged from approximately 50-fold to greater than 1,000-fold for the several pyridinium species tested. In other experiments, TPB greatly enhanced the accumulation of [3H]MPP+ by isolated mitochondrial preparations. This facilitation by TPB of MPP+ accumulation into mitochondria explains, at least in part, the potentiation by TPB of the above-mentioned inhibition of mitochondrial respiration. Moreover, TPB addition increased the amount of lactate formed during the incubation of mouse neostriatal tissue slices with MPTP and other tetrahydropyridines. The administration of TPB also potentiated the dopaminergic neurotoxicity of MPTP in male Swiss-Webster mice. All of these observations, taken together, are consistent with the premise that the inhibitory effect of MPP+ on mitochondrial respiration within dopaminergic neurons is the ultimate mechanism to explain MPTP-induced neurotoxicity.     

Evaluation of the Biological Activity of Several Analogs of the Dopaminergic Neurotoxin 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine

Several analogs of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were synthesized and screened for their capacity to be oxidized by monoamine oxidase (MAO-A or MAO-B) and their capacity to produce nigrostriatal dopaminergic neurotoxicity in mice. All of the compounds were relatively weak substrates for MAO-A but many of the compounds were found to be good substrates for MAO-B. Only three of the compounds, in addition to MPTP itself, were found to be neurotoxic. These were 1-methyl-4-cyclohexyl-1,2,3,6-tetrahydropyridine, 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine and 1-methyl-4-(3'-methoxyphenyl)-1,2,3,6-tetrahydropyridine. All three of these neurotoxic compounds were found to be substrates for MAO-B; in contrast no compound was found to be neurotoxic that was not oxidized by MAO-B. The capacity of the compounds studied to be oxidized by MAO-B appears to be an important aspect of the neurotoxic process.     

Intracarotid 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine Administration: Biochemical and Behavioral Observations in a Primate Model of Hemiparkinsonism

Cynomolgus monkeys received intracarotid injections of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to produce a chronic unilateral model of parkinsonism. Extensive dopamine (DA) depletion was observed in the caudate nucleus and putamen on the side ipsilateral to the injection and this was associated with contralateral tremor, rigidity, and bradykinesia. A dose of 1.25 mg of MPTP caused ipsilateral DA loss of 99.4% in the caudate nucleus, 99.8% in the putamen, and 74.2% in the nucleus accumbens. A dose of 2.5 mg caused ipsilateral DA depletion of 99.3% in the caudate nucleus, 99.5% in putamen, and 90.1% in the nucleus accumbens. The unilateral aspect of the lesion was dose sensitive, with the 2.5-mg dose causing bilateral asymmetric DA depletion. Tissue concentrations of serotonin were not affected by the toxin. These findings confirm that intracarotid injection of MPTP may produce a useful primate model of hemiparkinsonism that can be associated with selective unilateral DA depletion when the appropriate dose of toxin is used.     

Studies on the Neurotoxicity of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine: Inhibition of NAD-Linked Substrate Oxidation by Its Metabolite, 1-Methyl-4-Phenylpyridinium

Abstract: The effects of the parkinsonism-inducing neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its 4-electron oxidation product 1-methyl-4-phenylpyridinium (MPP⁺) were studied in isolated mitochondria and in mouse brain striatal slices. ADP-stimulated oxidation of NAD-linked substrates was inhibited in a time-dependent manner by MPP⁺ (0.1–0.5 m M ), but not MPTP, in mitochondria prepared from rat brain, mouse brain, or rat liver. Under identical conditions, succinate oxidation was relatively unaffected. In neostriatal slices prepared from the mouse, a species susceptible to the dopaminergic neurotoxicity of MPTP, incubation with either MPP⁺ or MPTP caused metabolic changes consistent with inhibition of mitochondnial oxidation, i.e., an increase in the formation of lactate and accumulation of the amino acids glutamate and alanine with concomitant decreases in glutamine and aspartate levels. The changes resulting from incubation with MPTP were prevented by the monoamine oxidase inhibitor pargyline, which blocks formation of MPP⁺ from MPTP. The results suggest that compromise of mitochondrial function and its metabolic sequelae within dopaminergic neurons could be an important factor in the neurotoxicity observed after MPTP administration.     

Preproenkephalin mRNA and Methionine-Enkephalin Increase in Mouse Striatum After 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine Treatment

Dopaminergic neurons that project to the striatum from the substantia nigra are thought to modulate methionine-enkephalin (Met-Enk) metabolism in the striatum. We administered a dose of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) that produces a moderate depletion of dopamine in striatum, about 50%, without overt motor deficits, and found that Met-Enk-like immunoreactivity and preproenkephalin mRNA content increased in the tissue. Pretreatment with the monoamine oxidase B inhibitor deprenyl or the dopamine transport blocker nomifensine prevented these changes, suggesting that the changes were related to the partial loss of dopaminergic neurons rather than to MPTP. Moreover, administering GM1 ganglioside, which partially restores the MPTP-induced dopaminergic deficit, partially corrected the Met-Enk changes in the striatum as well. These findings are consistent with the hypothesis that dopaminergic input to the striatum, in part, modulates Met-Enk metabolism. Moreover, they show that moderate nigrostriatal lesions are sufficient to elevate Met-Enk and preproenkephalin mRNA contents and that restoration of dopaminergic function, as in our studies with GM1 ganglioside, restores the content of Met-Enk.     

Synthesis and Nuclear Magnetic Resonance Spectroscopic, Mass Spectroscopic, and Plasma Amine Oxidase Inhibitory Properties of Analogues of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine

Abstract: Seventeen analogues of l-methyl-4-phenyl-1,2,3,6-tetrahydropyridine were synthesized using three reaction pathways: condensation of phenols with 1-methyl-4-piperidone, reaction of Grignard reagents with 1-methyl-4-piperidone followed by dehydration of the product, and aminomethylation of olefins. The identity of the products of synthesis was established by nuclear magnetic resonance spectroscopy, mass spectroscopy. and elemental analysis. Thirteen analogues were shown to inhibit the oxidation of benzylamine by bovine plasma amine oxidase. Increasing the length of the aliphatic chain of N -substituted analogues resulted in increased inhibition. In 4-phenyl-substituted analogues, both the position and electronic character of the substituent group affected the degree of inhibition.     

1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine Metabolism and l-Methyl-4-Phenylpyridinium Uptake in Dissociated Cell Cultures from the Embryonic Mesencephalon

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a contaminant found in a synthetic illicit drug, can elicit in humans and monkeys a severe extrapyramidal syndrome similar to Parkinson's disease. It also induces alterations of the dopamine (DA) pathways in rodents. MPTP neurotoxicity requires its enzymatic transformation into 1-methyl-4-phenylpyridinium (MPP+) by monoamine oxidase followed by its concentration into target cells, the DA neurons. Here, we show that mesencephalic glial cells from the mouse embryo can take up MPTP in vitro, transform it into MPP+, and release it into the culture medium. MPTP is not taken up by neurons from either the mesencephalon or the striatum in vitro (8 days in serum-free conditions). However, mesencephalic neurons in culture revealed a high-affinity uptake mechanism for the metabolite MPP+, similar to that for DA. The affinity (Km) for DA uptake is fivefold higher than that for MPP+ (0.2 and 1.1 microM, respectively), whereas the number of uptake sites for MPP+ is double (Vmax = 25 and 55 pmol/mg of protein/min for DA and MPP+, respectively). Mazindol, a DA uptake inhibitor, blocks the uptake of DA and MPP+ equally well under these conditions. Moreover, by competition experiments, the two molecules appear to use the same carrier(s) to enter DA neurons. Small concentrations of MPP+ are also taken up by striatal neurons in vitro. The amount taken up represented less than 10% of the MPP+ uptake in mesencephalic neurons. Depolarization induced by veratridine released comparable proportions of labeled DA and MPP+ from mesencephalic cultures.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine and 1-Methyl-4-Phenylpyridinium Ion on Activities of the Enzymes in the Electron Transport System in Mouse Brain

The effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium ion (MPP+) on activities of enzyme complexes in the electron transport system were studied using isolated mitochondrial preparations from C57BL/6J mouse brains. Both MPTP and MPP+ dose-dependently inhibited activity of NADH-ubiquinone oxidoreductase (EC 1.6.5.3). The inhibition was reversible. Preincubation of freeze-thawed mitochondria with MPTP or MPP+ had no effect on the inhibition; however, when nonfrozen mitochondria were used, NADH-ubiquinone oxidoreductase activity was reduced to 46% of that in the nonincubated sample after a 5-min preincubation with MPTP and to 77% of that in the nonincubated sample after a 5-min preincubation with MPP+. Kinetic analyses revealed that inhibition of MPTP was noncompetitive and that of MPP+ uncompetitive with respect to NADH. On the other hand, inhibition of MPTP was uncompetitive and that of MPP+ noncompetitive with respect to ubiquinone. Succinate-ubiquinone oxidoreductase (complex II), dihydroubiquinone-cytochrome c oxidoreductase (complex III), and ferrocytochrome c-oxygen oxidoreductase (EC 1.9.3.1) activities were either slightly inhibited or not inhibited by MPTP or MPP+. The significance of these findings is discussed in relation to the mechanism of MPTP-induced neuronal degeneration.     

Oxidation Products Arising from the Action of Monoamine Oxidase B on 1-Methyl-4-Benzyl-1,2,3,6-Tetrahydropyridine, a Nonneurotoxic Analogue of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine

Abstract: 1-Methyl-4-benzyl-1,2,3,6-tetrahydropyridine (MBzTP), an analogue of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, despite its rapid oxidation by monoamine oxidase B (MAOB), is not neurotoxic. The pyridinium expected to arise from the four-electron oxidation of MBzTP inhibits mitochondrial respiration and the oxidation of NADH in inner membranes and is only moderately less inhibitory than 1-methyl-4-phenylpyridinium. It is also a competitive inhibitor of dopamine uptake by the dopamine transporter and hence likely to be taken up into neurons, despite its relatively high K₁ value (K₁= 21 μM). Incubation of MBzTP with purified MAO B yields first the dihydropyridinium form, then a mixture of the pyridinium form and another unidentified product, in proportions that depend on the concentrations of MAO B and oxygen. At low MAO B concentration and moderate oxygen concentration, nonenzymatic formation, of the unidentified product predominates. The lack of neurotoxicity of MBzTP appears to be due to the oxidative destruction of the dihydropyridine and consequent failure of accumulation of 1-methyl-4-behzylpyridinium.     

Potent Neurotoxic Fluorinated 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine Analogs as Potential Probes in Models of Parkinson Disease

We synthesized a number of fluorinated analogs of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and tested their suitability as substrates for monoamine oxidase B in vitro and their dopaminergic neurotoxicity in vivo. Two of the compounds tested, 2'-F-MPTP and 2'-CF3-MPTP, were better enzyme substrates and possessed more potent neurotoxicity for nigrostriatal dopamine neurons than MPTP, especially 2'-F-MPTP. The results of the in vivo neurotoxicity experiments correlated well with the suitability of the compounds as substrates for monoamine oxidase. These findings could serve as a basis for the use of 18F-labeled analogs of MPTP for positron emission tomography studies of nonhuman primates for better understanding of the factors underlying MPTP toxicity. Furthermore, the discovery of two MPTP analogs with enhanced selective neurotoxicity to dopaminergic neurons may be an important clue in the continuing efforts to define the chemical structure-activity factors governing MPTP neurotoxic activation mechanisms.     

Mechanisms of Toxicity and Cellular Resistance to 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine and 1-Methyl-4-Phenylpyridinium in Adrenomedullary Chromaffin Cell Cultures

Bovine adrenomedullary chromaffin (BAMC) cells, cultured in a defined medium, were used to study the mechanisms of toxicity and cellular resistance to the catecholamine neuron toxicants 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium (MPP+). The viability of the cells was assessed biochemically [cellular catecholamine content and the catalytic activities of tyrosine hydroxylase (TH) and lactate dehydrogenase (LDH)] and anatomically (by electron microscopy). When cultures of BAMC cells were exposed to MPTP or MPP+ for 3 days, a marked loss of cellular catecholamines and TH activity was observed. The addition of an inhibitor of monoamine oxidase (MAO) B (Ro 19-6327), but not MAO A (clorgyline), prevented the toxicity of MPTP but not that of MPP+. In addition, the cellular toxicity of MPP+, but not MPTP, was antagonized by desmethylimipramine, an inhibitor of cellular catecholamine uptake. The toxicity of MPP+ was time dependent, with losses of TH and the release of cellular LDH occurring after 48 h in culture. Catecholamine depletion occurred somewhat sooner, being evident after 24 h of exposure to MPP+. The cellular toxicity of MPP+ was concentration dependent and significantly enhanced by inhibitors of catecholamine vesicular uptake (reserpine, tetrabenazine, or Ro 4-1284). Electron microscopic examination of cells treated with either MPP+, tetrabenazine, or their combination revealed that MPP+ damaged BAMC cells and that this damage was markedly potentiated by the inhibition of vesicular uptake by tetrabenazine. The concentration of glucose in the culture media of untreated cells slowly decreased as a function of time. The rate of glucose consumption was markedly accelerated by MPP+ treatment and the losses in cell TH and the release of LDH into the media were preceded by a 99% depletion of glucose from the media. In cultures not treated with MPP+, lactate accumulated in the media as a function of time. Addition of MPP+ to the media increased the formation of lactate, in a concentration-dependent manner. Reserpine pretreatment further enhanced the production of lactate in response to MPP+. Culturing cells in glucose-free medium greatly potentiated the effects of MPP+ on cellular TH and catecholamines. The toxicity observed after 3 days' exposure of BAMC cells to MPP+ could be prevented when the medium was replaced with fresh medium every 24 h. The effects of glucose deprivation and reserpine were observed to be additive. The ability of MPP+ to affect mitochondrial function is determined by the capacity of the storage vesicle to sequester the pyridinium, acting as a cytosolic "buffer."(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of 1-Methyl-4-Phenyl- 1,2,3,6-Tetrahydropyridine and 1 -Methyl-4-Phenylpyridinium Ion on ATP Levels of Mouse Brain Synaptosomes

Mouse brain synaptosomes, essentially devoid of mitochondrial contamination, were used as a model to study the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its toxic metabolite 1-methyl-4-phenylpyridinium ion (MPP+) on the levels of ATP of neuronal terminals. Similar to known inhibitors of ATP synthesis, both MPTP and MPP+ caused a dramatic depletion of synaptosomal ATP. This depletion was dose dependent and occurred as a relatively early biochemical event in the absence of any apparent damage to synaptosomal membranes. MPP+ was more effective than its parent compound in decreasing ATP; it induced a significant loss at concentrations (10-100 microM) similar to those it reaches in the brain in vivo. MPTP-induced ATP depletion was completely prevented by the monoamine oxidase B inhibitor deprenyl, which, on the contrary, was ineffective against MPP+. As expected in view of the heterogeneous population of nerve terminals present in our synaptosomal preparations, the catecholamine uptake blocker mazindol did not significantly affect the ATP loss caused by both compounds. Data indicate that (1) administration of MPTP may cause a depletion of ATP within neuronal terminals resulting from the generation of MPP+, and (2) exposure to the levels of MPP+ reached in vivo may cause biochemical changes that are nonselective for dopaminergic terminals.     

MK-801 Prevents 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine-Induced Parkinsonism in Primates

In cynomologus monkeys, systemic administration of MK-801, a noncompetitive antagonist for the N-methyl-D-aspartate receptor, prevented the development of the parkinsonian syndrome induced by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MK-801 also attenuated dopamine depletion in the caudate and putamen and protected dopaminergic neurons in the substantia nigra from the degeneration induced by the neurotoxin. Nevertheless, 7 days after MPTP administration in the caudate and putamen of monkeys also receiving MK-801, the levels of toxic 1-methyl-4-phenylpyridinium were even higher than those measured in monkeys receiving MPTP alone. This indicates that the protective action of MK-801 is not related to MPTP metabolism and strongly suggests that, in primates, the excitatory amino acids could play a crucial role in the mechanism of the selective neuronal death induced by MPTP.     

Clonidine Suppresses 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine-Induced Reductions of Striatal Dopamine and Tyrosine Hydroxylase Activity in Mice

Abstract: Recent findings have shown that excitatory amino acid may be involved in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity. At the same time, evidence is accumulating that the endogenous nor-adrenergic system plays a protective role in MPTP-induced striatal dopamine (DA) depletion and nigral dopaminergic cell death. Recently, α₂-adrenoceptors located on glutamatergic axons have been shown to inhibit glutamate overflow. In this study, we evaluated the effects of an α₂-agonist (clonidine) and an α₂-antagonist (yohimbine) on MPTP-induced striatal DA depletion and tyrosine hydroxylase activity reduction. We show that clonidine is able to prevent the neurotoxicity of MPTP in mice. To exert this effect, clonidine (0.5 mg/kg) must be administered at least twice (30 min before and 30 min after MPTP). Administration of another α₂-agonist (detomidine, 0.3 mg/kg) attenuated the neurotoxicity induced by MPTP. We provide evidence that the protective effect obtained with clonidine was not due to decreased striatal content of 1-methyl-4-phenylpyridinium (MPP⁺). We also show that yohimbine, which is a classic α₂-adrenoceptor antagonist with low affinity for imidazoline receptors, produced by itself an enhancement of MPTP toxicity and was able to block the protective effect of clonidine. These data raise the possibility that α₂-adrenoceptor may modulate the susceptibility of the nigrostriatal dopaminergic pathway to neurotoxicity.     

Immunity of Fetal Mice to Prenatal Administration of the Dopaminergic Neurotoxin 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine

Abstract: Subcutaneous injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) HC1 (25 mg/kg) in pregnant female mice at the 17th day of gestation markedly depleted striatal dopamine (DA) concentrations in the mothers 24 h later and at 24 h and 28 days after delivery. By contrast, in the offspring of the female mice exposed to MPTP during pregnancy, fetal brain DA concentrations at 24 h after injection and at 24 h after birth and striatal DA levels at 14 and 28 days postnatally were unaffected and identical to those in age-matched controls. The postnatal ontogenesis of striatal DA levels was identical in offspring of control vehicle- and MPTP-treated pregnant mice. Also, prenatal challenge with MPTP did not make nigrostriatal DA neurons more vulnerable to a second postnatal treatment with the toxin. Striatal DA depletions were identical in 6-week-old mice given MPTP, whether they were exposed to MPTP or to vehicle in utero. Monoamine oxidase (EC 1.4.3.4; MAO) type B activity was extremely low in the fetal brain and, relatively, much lower than that of MAO-A. Prenatal MPTP administration reduced maternal striatal and also embryonal brain MAO-B activity at 24 h post treatment but did not alter the normal postnatal development of striatal MAO-A and -B activities in the offspring. Study suggests that resistance of fetal DA neurons to the DA-depleting effect of MPTP may be due, at least in part, to an absence in the embryonal brain of adequately developed MAO-B activity required for the conversion of MPTP to its toxic metabolite, 1-methyl-4-phenylpyridinium ion.     

Effects of 1-Methyl-4-Phenyl-1,2,5,6-Tetrahydropyridine and Its Metabolite 1-Methyl-4-Phenylpyridine on Acetylcholine Synthesis in Synaptosomes from Rat Forebrain

1-Methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) and its metabolite, 1-methyl-4-phenylpyridine (MPP+), have been shown to cause a number of lesions in dopaminergic pathways of the nigro-striatal region of the brain. However, data on the effects of these neurotoxins on other aspects of brain metabolism are scarce. The data presented here show that MPTP and MPP+ inhibit glucose oxidation via the tricarboxylic acid cycle, and acetylcholine synthesis in synaptosomal preparations from rat forebrain. Monoamine oxidase B inhibitors (e.g., pargyline, MDL 72145) relieve the inhibition caused by MPTP but not MPP+. The inhibitory effects of MPP+ on glucose oxidation and acetylcholine synthesis are a consequence of the decreased glucose metabolism in synaptosomes and are consistent with its role as an inhibitor of the Complex I (NADH-CoQ reductase) of the mitochondrial respiratory chain.     

Lack of Change in Basal Ganglia Neuropeptide Content Following Subacute 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine Treatment of the Common Marmoset

Treatment of common marmosets with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 1-4 mg/kg for up to 4 days) caused a profound parkinsonian state. Ten days from the start of MPTP treatment, all animals showed marked motor impairment, consisting of bradykinesia and akinesia, limb rigidity, postural abnormalities, loss of vocalisation and blink reflex, and, on occasions, postural tremor. Measurement of caudate-putamen monoamine content at this time showed a profound loss in 3,4-dihydroxyphenylethylamine, homovanillic acid, and 3,4-dihydroxyphenylacetic acid concentrations. Measurement of neuropeptide concentrations in the caudate-putamen, internal and external segments of the globus pallidus, nucleus accumbens, substantia nigra, frontal cortex, and hippocampus showed met-enkephalin, leu-enkephalin, and cholecystokinin (CCK-8) concentrations to be unaffected by MPTP treatment. There was a small decrease in the substance P content of frontal cortex, but otherwise the content of this neuropeptide was unaltered. Parkinsonism in the marmoset, induced by MPTP treatment 10 days earlier, does not alter neuropeptide concentrations in the manner observed in Parkinson's disease.     

Studies on the Oxidation of the Dopaminergic Neurotoxin 1-Methyl-4-Phenyl-1,2,5,6-Tetrahydropyridine by Monoamine Oxidase B

1-Methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) is a chemical that, after injection into experimental animals, including mice and monkeys, causes a degeneration of the nigrostriatal pathway. We carried out experiments designed to study the in vitro oxidation of MPTP by mouse brain mitochondrial preparations. MPTP was actively oxidized by the mitochondrial preparations, with Km and Vmax values very similar to those of benzylamine, a typical substrate for MAO-B. MPTP was oxidized considerably better than many of its analogs, even those with relatively minor structural changes. Several monoamine oxidase inhibitors (MAOI) were potent inhibitors of MPTP oxidation, and there was a highly significant correlation between the capacity of the MAOI tested to inhibit MPTP oxidation and benzylamine oxidation. There was no correlation between the capacity of the MAOI to inhibit MPTP oxidation and their capacity to inhibit the oxidation of tryptamine, a substrate for MAO-A. In other experiments, MPTP was an excellent substrate for pure MAO-B, prepared from bovine liver. All of these data, combined with the fact that MAO-B inhibitors can protect against MPTP-induced dopaminergic neurotoxicity in vivo, point to an important role for MAO-B in MPTP metabolism in vivo.