Studies on species sensitivity to the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Part 1: Systemic administration

Several parameters necessary for the expression of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced neurotoxicity to dopaminergic neurons were examined in both mice and rats in order to determine if differences in these processes might underlie the marked differences in the sensitivity of the two species to the neurotoxic effects of MPTP. Monoamine oxidase-B activity was greater in brain tissues from rats than from mice. The kinetics of 1-methyl-4-phenylpyridinium (MPP+) uptake into neostriatal synaptosomal preparations from the two species were similar. Brain and neostriatal levels of MPP+ were 2-fold higher in rats after the administration of MPTP at 60 mg/kg and were 10 to 20 times higher in rats than in mice after MPTP treatment which produced similar decrements in the content of neostriatal dopamine. MPP+ concentrations in the extracellular fluid of the neostriatum of the two species were similar after the administration of the same dose of MPTP (40 mg/kg). However, this dose induced a 40-fold increase in neostriatal dopamine efflux in mice, whereas in rats only a 3-fold increase was observed. In addition, pretreatment of rats with guanethidine, a ganglionic blocking agent, permitted the use of high doses of MPTP which resulted in substantial damage to the striatal dopaminergic nerve terminals. It is concluded that nigrostriatal dopaminergic neurons in the rat require exposure to a much higher concentration of MPP+ than do those in mice for the induction of toxicity.

Studies on species sensitivity to the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Part 2: Central administration of 1-methyl-4-phenylpyridinium

There are marked species differences in susceptibility to the neurotoxic effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Mice are sensitive, whereas rats are relatively insensitive to MPTP. In these two species, the effects of peripherally administered MPTP or intrastriatally infused 1-methyl-4-phenylpyridinium (MPP+) were examined to identify potential underlying mechanisms responsible for their difference in susceptibility to MPTP. In vivo intrastriatal microdialysis and an MPP+ 2-day test/challenge paradigm were used to monitor dopamine efflux as an indicator of the neurotoxic effects of MPTP or MPP+. By using this method, the EC50 for neurotoxicity by an intrastriatal infusion of MPP+ in mice was 0.4 mM, whereas it was 10-fold higher in rats (4.3 mM). In addition, by using the traditional postmortem examination, neostriatal dopamine was depleted markedly in mice (> or = 80%), but only depleted marginally in rats in which MPP+ was infused into the neostriatum. These data indicate that rats are relatively insensitive to MPTP as compared to mice, because they are less sensitive to MPP+ whether it is formed in vivo from MPTP administered systemically or administered directly into neostriata. Thus, there appears to be a fundamental difference in the susceptibility of the nigrostriatal systems in these two species to the neurotoxic consequences of MPP+ exposure.

Meige syndrome in the spectrum of Lewy body disease

We report a patient with Meige syndrome (segmental cranial dystonia) who had neuropathologic changes of Parkinson's disease on postmortem examination. Neuropathologic examination showed typical and atypical Lewy bodies in the pigmented nuclei of the brainstem (substantia nigra, locus ceruleus), the nucleus basalis of Meynert, and the nucleus ambiguus. Neurochemical analysis of postmortem brain tissue showed evidence for decreased dopamine turnover in the substantia nigra, striatum, and nucleus accumbens. We propose that some cases of Meige syndrome may be included in the spectrum of Lewy body disease.

1-Methyl-4-(2'-ethylphenyl)-1,2,3,6-tetrahydropyridine-induced toxicity in PC12 cells is enhanced by preventing glycolysis

The effects of 1-methyl-4-(2'-ethylphenyl)-1,2,3,6-tetrahydropyridine (2'Et-MPTP), 1-methyl-4-(2'-ethylphenyl)pyridinium (2'Et-MPP+), and the classic complex 1 inhibitor, rotenone, on toxicity as well as on rates of glucose use and lactate production were studied using the pheochromocytoma PC12 cell line. PC12 cells are neoplastic in nature and have a high rate of glycolysis accompanied by a large production of lactate and a low use of glucose carbon through the Krebs cycle. 1-Methyl-4-phenylpyridinium (MPP+) and analogues such as 2'Et-MPP+ are actively accumulated by mitochondrial preparations in vitro and block NADH dehydrogenase of complex 1. This blockade results in biochemical sequelae that are ultimately cytotoxic. In this study, untreated PC12 cells used glucose and concomitantly accumulated lactate in a time-dependent manner at all concentrations of glucose studied. Treatment with 50 microM 2'Et-MPP+ or 50 nM rotenone increased both rates significantly, indicating a shift toward increased glycolysis. Cell death caused by the neurotoxins was also time and concentration dependent and markedly enhanced by glucose depletion in the medium. The increase in 2'Et-MPTP-induced toxicity in low glucose-supplemented cells was not due to an increase in pyridinium formation from the tetrahydropyridine, but rather to the lack of glucose for glycolysis. Moreover, inhibition of glycolysis with 2-deoxyglucose or iodoacetic acid also enhanced the lethality of the neurotoxins to the cells. The data in this study provide additional support to the hypothesis that 2'Et-MPP+ or related analogues act to kill cells by inhibiting mitochondrial respiration.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopaminergic neurotoxicity of 1-methyl-4-phenylpyridinium analogs in cultured neurons: relationship to the dopamine uptake system and inhibition of mitochondrial respiration

Several analogs of 1-methyl-4-phenylpyridinium (MPP+) were evaluated for their affinity for the dopamine uptake system and their ability to inhibit NADH dehydrogenase (complex I) of the mitochondrial electron-transport chain. Moreover, these compounds were tested for their ability to cause selective dopaminergic neurotoxicity in cultured mesencephalic neurons. Simultaneous [3H]dopamine and gamma-amino-[14C]butyric acid uptake and immunocytochemical techniques were used as indices of neuronal damage in cultured cells. The compounds that were potent and selective dopaminergic neurotoxins had high affinity for the dopamine transport system, as measured by their ability to cause dopamine release, and were similar to MPP+ in inhibiting mitochondrial respiration. One compound (1-methyl-4-phenylpyrimidinium) had high affinity for the dopamine uptake system but was a weak inhibitor of mitochondrial respiration and, accordingly, was not neurotoxic. The 4'-alkylated analogs of MPP+, which were poor substrates for the dopamine uptake system and extremely potent inhibitors of mitochondrial respiration, caused a nonselective damage of neurons in culture. Analogs that were not substrates for the dopamine carrier and not inhibitors of mitochondrial respiration were not neurotoxic. This study describes the neurotoxicity of a number of analogs of MPP+ and highlights the importance of the dopamine uptake system and the ability to inhibit mitochondrial respiration as critical processes in conferring selectivity and neurotoxicity, respectively, to MPP+ and analogs, for dopaminergic neurons in culture.

Correlation between the neostriatal content of the 1-methyl-4-phenylpyridinium species and dopaminergic neurotoxicity following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration to several strains of mice

In the present study we observed pronounced differences in the capacity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to induce dopaminergic neurotoxicity in several strains of mice. For example, there was no MPTP-induced decrement in neostriatal dopamine content in Ace Swiss-Webster mice and a 92% decrement in Taconic Farms C57 bl mice. Several parameters which could possibly explain this differential sensitivity to MPTP were studied. These include: 1) neostriatal monoamine oxidase-B (MAO-B) activity; 2) the capacity of neostriatal synaptosomes prepared from the mouse strains to accumulate 1-methyl-4-phenylpyridinium (MPP+), the major metabolite of MPTP formed via oxidation by MAO-B; and 3) the neostriatal MPP+ content after MPTP administration to the mice. There were no significant differences in the Km values for MAO-B in the neostriatum among the strains of mice examined. Neostriatal Vmax values for MAO-B differed somewhat among the strains, with a low of 2915 +/- 172 nmol/g of tissue per hr (CD-1 mice from Charles River) and a high of 3884 +/- 203 nmol/g of tissue per hr (C57 bl mice from Taconic Farms). However, Vmax values for MAO-B in the mouse strains did not correlate significantly with the relative sensitivity of the strains to MPTP. There were no significant differences in the capacity of neostriatal synaptosomes prepared from the mouse strains to accumulate MPP+. Studies on the metabolism of MPTP after peripheral administration revealed that there was a significant (P less than .01) positive correlation between the relative sensitivity of the mouse strains to MPTP and their neostriatal MPP+ content after MPTP administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Competitive and noncompetitive antagonists at N-methyl-D-aspartate receptors protect against methamphetamine-induced dopaminergic damage in mice

The administration of methamphetamine (METH) to experimental animals results in damage to nigrostriatal dopaminergic neurons. We have demonstrated previously that the excitatory amino acids may be involved in this neurotoxicity. For example, several compounds which bind to the phenyclidine site within the ion channel linked to the N-methyl-D-aspartate (NMDA) receptor protected mice from the METH-induced loss of neostriatal tyrosine hydroxylase activity and dopamine content. The present study was conducted to characterize further the role of the excitatory amino acids in mediating the neurotoxic effects of METH. The administration of three or four injections of METH (10 mg/kg) every 2 hr to mice produced large decrements in neostriatal dopamine content (80-84%) and in tyrosine hydroxylase activity (65-74%). A dose-dependent protection against these METH-induced decreases was seen with two noncompetitive NMDA antagonists, ifenprodil and SL 82.0715 (25-50 mg/kg/injection), both of which are thought to bind to a polyamine or sigma site associated with the NMDA receptor complex, and with two competitive NMDA antagonists, CGS 19755 (25-50 mg/kg/injection) and NPC 12626 (150-300 mg/kg/injection). Moreover, an intrastriatal infusion of NMDA (0.1 mumol) produced a slight but significant loss of neostriatal dopamine which was potentiated in mice that also received a systemic injection of METH. The results of these studies strengthen the hypothesis that the excitatory amino acids play a critical role in the nigrostriatal dopaminergic damage induced by METH.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine- and 1-methyl-4-(2'-ethylphenyl)-1,2,3,6-tetrahydropyridine-induced toxicity in PC12 cells: role of monoamine oxidase A

The toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 1-methyl-4-(2'-ethylphenyl)-1,2,3,6-tetrahydropyridine (2'Et-MPTP), and their corresponding pyridinium species was studied in the rat pheochromocytoma PC12 cell line. MPTP and its analogues are known to be metabolized by monoamine oxidase (MAO) to dihydropyridinium intermediates which are further transformed, either enzymatically or spontaneously, into pyridinium species. MAO activity in PC12 cells is almost exclusively of the A form, and 2'Et-MPTP is a good substrate for both MAO-A and MAO-B. In contrast, MPTP is a poor substrate for MAO-A, but a good substrate for MAO-B. 2'Et-MPTP caused considerably more cell death than MPTP in the PC12 cells. However, 1-methyl-4-(2'-ethylphenyl)pyridinium and 1-methyl-4-phenylpyridinium, the corresponding pyridinium species formed from 2'Et-MPTP and MPTP, respectively, were equipotent as toxins. The toxic effects of the tetrahydropyridines and their corresponding pyridiniums were both concentration- and time-dependent. Measurements of the levels of the pyridinium species formed and the remaining tetrahydropyridine in the media indicated that 2'Et-MPTP was converted about five to seven times more readily into its toxic pyridinium species than was MPTP. There was, moreover, an excellent correlation between amount of pyridinium formed and cell death. There was also a parallel between the capacity of clorgyline and pargyline, irreversible MAO inhibitors, to decrease the formation of the pyridinium species and their capacity to protect against the toxic actions of the tetrahydropyridines. These data are consistent with the concept that the MAO-A-dependent formation of the pyridinium species from the tetrahydropyridine is a prerequisite for toxicity in PC12 cells.

4'-alkylated analogs of 1-methyl-4-phenylpyridinium ion are potent inhibitors of mitochondrial respiration

1-Methyl-4-phenylpyridinium ion, a major brain metabolite of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, is an inhibitor of Complex I of the mitochondrial respiratory chain. We have synthesized several analogs of 1-methyl-4-phenylpyridinium ion containing various alkyl groups in the 4' position of the phenyl ring and have tested them for their abilities to inhibit the oxidation of NADH-linked substrates by intact mouse liver mitochondria. These compounds are considerably more potent inhibitors than MPP+ itself, with potency increasing as the length of the alkyl chain increases. The most potent inhibitor, 1-methyl-4-(4'heptylphenyl)pyridinium ion, was about 200 times as effective as MPP+. These analogs should prove to be useful tools for studying the nature of the process whereby MPP+ and its pyridinium analogs interact with Complex I to inhibit mitochondrial respiration.

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.

Steady plasma levodopa concentrations required for good clinical response to CR-4 in patients with 'on-off'

Ten patients with Parkinson's disease and severe motor fluctuations were given Sinemet (25/100) for 4 weeks followed by 4 weeks of Sinemet (CR-4). After each drug preparation was optimized, patients were rated by neurological examination and plasma levodopa (LD) measured at hourly intervals (9 a.m.-4 p.m.). For the group as a whole, variations throughout the day of plasma LD and clinical state were no different on the 2 formulations. Three patients whose fluctuations responded well to CR-4 had either much less variable plasma LD levels on CR-4 or were able to maintain plasma LD above a minimum threshold. In severe fluctuators, a major benefit from CR-4 can be expected only in those patients who can maintain steady plasma LD levels above the threshold for achieving the 'on' state.

Monoamine oxidase and the bioactivation of MPTP and related neurotoxins: relevance to DATATOP

The DATATOP study is a clinical trial in which deprenyl, a selective inhibitor of monoamine oxidase-B (MAO-B), is being given to newly diagnosed Parkinsonian patients in an attempt to halt the progression of their disorder. In part, this is being done because of the working hypothesis than an MPTP-like molecule may be the cause of Parkinsonism, and deprenyl is known to protect against MPTP-induced dopaminergic neurotoxicity in experimental animals. In the present study we point out that several analogs of MPTP are good substrates not only for MAO-B but also for MAO-A. In addition, we point out that with long-term administration to rodents, deprenyl loses its selectivity as an inhibitor of MAO-B and also inhibits MAO-A. We believe that these observations have relevance for the DATATOP study.

Oxidation of analogs of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine by monoamine oxidases A and B and the inhibition of monoamine oxidases by the oxidation products

Twenty analogs of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were tested for their capacity to be oxidized by pure monoamine oxidase-A (MAO-A) prepared from human placenta and pure monoamine oxidase-B (MAO-B) prepared from beef liver. Several of the MPTP analogs were very good substrates for MAO-A, for MAO-B, or for both and had low Km values and high turnover numbers. These values were similar to or even better than those of kynuramine and benzylamine, good substrates for MAO-A and MAO-B, respectively. MPTP had relatively low Km values for oxidation by both MAO-A and MAO-B. In contrast, the turnover number for MPTP oxidation by MAO-B was considerably higher than the value for MAO-A. The corresponding pyridinium species of MPTP and several of the MPTP analogs inhibited MAO-A competitively with Ki values at micromolar concentrations; in contrast the pyridinium species inhibited MAO-B competitively at considerably higher concentrations (i.e., 100 microM or greater Ki values). The data provide information concerning the structural requirements for the oxidation of tetrahydropyridines by MAO-A and MAO-B and the inhibition of these enzymes by pyridiniums.

Structural dependence of the inhibition of mitochondrial respiration and of NADH oxidase by 1-methyl-4-phenylpyridinium (MPP+) analogs and their energized accumulation by mitochondria

Nineteen structural analogs of 1-methyl-4-phenylpyridinium (MPP+) were studied for their capacity to inhibit the mitochondrial oxidation of NAD+-linked substrates and the aerobic oxidation of NADH in inner membrane preparations from cardiac mitochondria. In the majority of cases, a good correlation was found between the two inhibition effects monitored. A few compounds were effective inhibitors of NADH oxidase but had only marginal effects on mitochondrial respiration. From studies of their accumulation by mitochondria, it appears likely that the latter compounds are not effectively concentrated by intact mitochondria by the electrical gradient and, in part for this reason, cannot reach sufficiently high concentrations at the appropriate binding site of NADH dehydrogenase. In addition, evidence is presented that the penetration of pyridinium analogs to the inhibition site in the NADH dehydrogenase complex may also be rate limiting. The data support the thesis that, for a substituted tetrahydropyridine to be acutely neurotoxic, its pyridinium oxidation product must be actively accumulated in the mitochondria and must inhibit NADH-ubiquinone oxidoreductase in its membrane environment.

Experience with continuous enteral levodopa infusions in the treatment of 9 patients with advanced Parkinson's disease

Nine patients with advanced Parkinson's disease were started on continuous enteral levodopa infusions during the past 3 years. Six have remained on the infusion system for 1 to 28 months. All patients experienced immediate amelioration of motor fluctuations, and 5 patients continue to obtain relief. One patient found that his ability to achieve the "on" state without unacceptable dyskinesia waned. Experience thus far indicates that continuous long-term levodopa infusions are a practical but complex form of therapy for patients failing more conventional treatment.

Attenuation by dopamine uptake blockers of the inhibitory effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and some of its analogs on NADH-linked metabolism in mouse neostriatal slices

In previous studies it was shown that the pyridinium species formed by the monoamine-oxidase-catalyzed conversion of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-(2'methylphenyl)-1,2,3,6-tetrahydropyridine (2'Me-MPTP) inhibited mitochondrial electron transport at Complex I. In addition, these substances, when incubated with mouse neostriatal slices, caused an increased lactate accumulation. However, it was not clear whether this inhibition could occur within dopamine nerve terminals. In the present study we investigated if dopamine uptake blockers, shown previously to protect against tetrahydropyridine-induced dopaminergic neurotoxicity, would attenuate MPTP- and 2'Me-MPTP-promoted lactate formation in neostriatal slices. 2'Me-MPTP-induced lactate accumulation in neostriatal slices from mice or rats with a lesion of the nigrostriatal pathway was also studied. The dopamine uptake blocker, Win 35,428 [8-azabicyclo[3.2.1]octane-2-carboxylic acid, 3-(4-fluorophenyl)-8-methyl-, methylester [1R-(exo, exol)]), attenuated the increased lactate formation caused by MPTP and 2'Me-MPTP. At low concentrations of 2'Me-MPTP (5 microM), the increased lactate production was inhibited completely by Win 35,428. These latter data suggest that at low concentrations of 2'Me-MPTP, the increased lactate accumulation was associated primarily with dopaminergic nerve terminals. Two other dopamine uptake inhibitors, McN 5908 [trans-4-(1,2,3,5,6,10b-hexahydropyrrolo[2,1-alpha]isoquinolin+ ++-6-yl) benzenamine hydrobromide methanolate (4:4:1)] and mazindol [5-(4-chlorophenyl)-2,5-dihydro-3H-imidazo- [2,1-alpha]isoindol-5-ol], also attenuated tetrahydropyridine-induced lactate formation. At concentrations selective for their respective uptake systems, norepinephrine- and serotonin-uptake inhibitors did not attenuate 2'Me-MPTP-induced lactate accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

1-Methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'CH3-MPTP)-induced degeneration of mesostriatal dopaminergic neurons in the mouse: biochemical and neuroanatomical studies

The neurotoxic effects of 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'CH3-MPTP), a substituted analog of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, were studied in BALB/cJ mice. Moderate doses of 2'CH3-MPTP produced a greater depletion of dopamine (DA) in the striatum (45%) than in the nucleus accumbens (23%), and in these same animals, there was a 35% loss of midbrain DA neurons. The greatest loss of DA cells occurred within the substantia nigra (43%), and there was also a significant loss of cells within the ventral tegmental area (28%). Higher doses of 2'CH3-MPTP decreased levels of DA more in the axon terminal/forebrain region (72%) than in the cell body/midbrain region (25%). Similar forebrain/midbrain DA depletion ratios were also found in mice that received an electrolytic lesion of the midbrain DA neurons; there was a greater Da depletion in the forebrain (29%) than in the midbrain (8%). In both 2'CH3-MPTP and electrolytically lesioned animals there was a significant increase in DA turnover in the forebrain region, as measured by the homovanillic acid/DA ratio. These data indicate that 2'CH3-MPTP: (1) destroys DA neurons within two midbrain regions containing cells which project to the striatum (i.e. mesostriatal DA neurons), rather than just nigrostriatal DA neurons; (2) produces a greater loss of DA in the axon terminal region than in the cell body region; and (3) influences the mesostriatal DA neurons in the same way as does a lesion to the cell bodies. These data are discussed with regard to the pathophysiology of 2'CH3-MPTP.

Continuous levodopa infusions to treat complex dystonia in Parkinson's disease

We report the clinical spectrum of 3 patients with Parkinson's disease who experienced complex patterns of levodopa-related dystonia. Dystonia was unrelieved by multiple medication regimens but responded well to continuous, duodenal levodopa infusions. Patients were able to remain mobile without severe dystonia despite a very narrow window of benefit between the levodopa concentration necessary to achieve the "on" state and that which caused the onset of dystonic spasms.

Structure-activity study of the mechanism of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced neurotoxicity. I. Evaluation of the biological activity of MPTP analogs

Several analogs of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were synthesized and compared to MPTP for their ability to be oxidized by monoamine oxidase (MAO) and for their ability to cause nigrostriatal dopaminergic neurotoxicity in mice. Most of the compounds were oxidized by mouse brain MAO, either predominantly by the B-form or by both the A- and B-forms. The MAO-catalyzed oxidation of all of the MAO substrates resulted in the formation of dihydropyridinium intermediates which, in turn, except for the dihydropyridinium of 1-methyl-4-benzyl-1,2,3,6-tetrahydropyridine, formed pyridinium species as the final oxidation product. Nine analogs were found to be neurotoxic; all were oxidized by MAO to pyridinium compounds. However, some non-neurotoxic MPTP analogs were also oxidized by MAO. Neither 1-methyl-4-benzyl-1,2,3,6-tetrahydropyridine nor the compounds which were not substrates for MAO were neurotoxic. Also, the neurotoxicity of all of the compounds tested was blocked by inhibiting either MAO-B, MAO-A or both MAO-B and MAO-A together, indicating that MAO activity was necessary for the neurotoxicity of the compounds to be manifested. The capacity of an MPTP analog to be oxidized by MAO to a pyridinium appears to be a necessary, but not sufficient, parameter in determining the neurotoxic potential of the compound.

Structure-activity study of the mechanism of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced neurotoxicity. II. Evaluation of the biological activity of the pyridinium metabolites formed from the monoamine oxidase-catalyzed oxidation of MPTP analogs

In the accompanying paper, several tetrahydropyridine analogs of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were screened for their abilities to be oxidized by monoamine oxidase (MAO) to pyridiniums and to produce neurotoxicity in mice. We reported that most of the analogs were oxidized by MAO to pyridiniums and some of the analogs were neurotoxic. We concluded that the capacity of a tetrahydropyridine MPTP analog to be oxidized by MAO to a pyridinium was a necessary, but not sufficient, condition for the compound to be a neurotoxin. In the present paper we attempt to explain further the neurotoxicity or lack of neurotoxicity of these analogs by evaluating the abilities of the pyridinium compounds to serve as substrates for the neostriatal dopamine (DA) transport system and as inhibitors of mitochondrial respiration. We now report that all of the neurotoxic MPTP analogs are oxidized to pyridiniums that are good substrates for the neostriatal DA carrier and good inhibitors of mitochondrial respiration. The results are consistent with an important role for both uptake of the pyridiniums by the DA carrier and inhibition by the pyridiniums of mitochondrial respiration in the neurotoxicity induced by MPTP and its analogs.

Role for excitatory amino acids in methamphetamine-induced nigrostriatal dopaminergic toxicity

The systemic administration of either methamphetamine or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to experimental animals produces degenerative changes in nigrostriatal dopaminergic neurons or their axon terminals. This study was conducted to determine if excitatory amino acids, which appear to be involved in various neurodegenerative disorders, might also contribute to the dopaminergic neurotoxicity produced in mice by either methamphetamine or MPTP. MK-801, phencyclidine, and ketamine, noncompetitive antagonists of one subtype of excitatory amino acid receptor, the N-methyl-D-aspartate receptor, provided substantial protection against neurotoxicity produced by methamphetamine but not that produced by MPTP. These findings indicate that excitatory amino acids play an important role in the nigrostriatal dopaminergic damage induced by methamphetamine.

Interactions of D1 and D2 dopamine receptors on the ipsilateral vs. contralateral side in rats with unilateral lesions of the dopaminergic nigrostriatal pathway

In rats with a unilateral lesion of the nigrostriatal dopaminergic pathway, the ipsilateral rotation produced by the enhanced actions of endogenous dopamine (DA) on the nonlesioned side, induced by either the DA-releasing drug amphetamine or the DA uptake inhibitor GBR 13069, was blocked effectively by pretreatment with either the selective D1 DA receptor antagonist, SCH 23390, or the D2 selective antagonist, haloperidol. In contrast, contralateral rotation produced by apomorphine or I-dihydroxyphenylalanine, which lead to the preferential activation of D1 and D2 receptors on the lesioned side, was effectively prevented only when both receptor subtypes were inhibited. The results of these experiments demonstrate that the interaction between D1 and D2 receptors in the lesioned side differs from that in the nonlesioned side. Whereas the simultaneous stimulation of both DA receptor subtypes in the normally innervated basal ganglia is required for the production of turning behavior, the stimulation of either subtype alone in the dopaminergic denervated side can produce rotation. However, the concurrent administration of the D1 agonist, SKF 38393, with the D2 agonist, LY 171555, produced a synergistic effect on contralateral rotation. These results suggest that there is preservation of at least some functional interaction between D1 and D2 receptors in the lesioned basal ganglia but that there may be in addition a mechanism by which the two receptor subtypes can function independently of each other. The unilaterally lesioned rat appears to be a very good model in which to study the interaction between D1 and D2 receptors under conditions of both normal innervation and of DA denervation.(ABSTRACT TRUNCATED AT 250 WORDS)

Importance of monoamine oxidase A in the bioactivation of neurotoxic analogs of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a potent dopaminergic neurotoxin that causes biochemical, pharmacological, and pathological deficits in experimental animals similar to those seen in human parkinsonian patients. All of the deficits can be prevented by treating mice with selective inhibitors of monoamine oxidase B (MAO-B), including deprenyl, prior to MPTP administration. We now report that the dopaminergic neurotoxicity of two potent MPTP analogs, namely the 2'-methyl and 2'-ethyl derivatives (2'-MeMPTP and 2'-EtMPTP), cannot be prevented by deprenyl pretreatment. However, the neurotoxicity of these two analogs can be prevented by pretreatment with a combination of deprenyl and the selective MAO-A inhibitor clorgyline at doses that are sufficient to almost completely inhibit both MAO-B and MAO-A activities. Moreover, the neurotoxicity of 2'-EtMPTP (but not of 2'-MeMPTP and MPTP) can be significantly attenuated by clorgyline alone. There was a parallel between the capacity of the MAO inhibitors to decrease the brain content of the pyridinium species after administration of the tetrahydropyridines and the capacity of the MAO inhibitors to protect against the neurotoxic action of the tetrahydropyridines. The data support the conclusion that both 2'-MeMPTP and 2'-EtMPTP are bioactivated to pyridinium species to a significant extent by MAO-A. Further, it appears that the formation of the pyridinium species plays an important role in the neurotoxic process.

Isolated jejunal pouches for levodopa delivery in parkinsonian patients with "on-off". Successful experimental model in dogs

In eight dogs, a 20 cm section of isolated jejunum with intact blood supply was externalized to the abdominal wall and used as a device for levodopa (LD) administration. Overall, Sinemet tablets and LD suspension produced similar plasma levodopa concentrations with oral and pouch administration. The most ideal plasma concentration curves were obtained for CR-3, a sustained release Sinemet preparation, given through the jejunal pouches. Plasma LD concentrations rose within the first hour after administration of CR-3 and remained constant for the next 3 h, before falling slowly. Isolated jejunal pouches may therefore be an effective, simple means of maintaining constant plasma LD concentrations in parkinsonian patients with motor fluctuations and may diminish the deleterious effects of erratic gastric emptying and competition with food-derived amino acids at the gut/blood transport system.

Studies with the neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and several of its analogs

The nigrostriatal dopaminergic neurotoxicity of MPTP was prevented in mice in a dose-dependent manner by the monoamine oxidase-B (MAO-B) inhibitor deprenyl. This finding, combined with other observations, points out the important role of MAO-B in the bioactivation of MPTP. In the present study, some comparisons between MPTP and several of its structural analogs will be presented.

Role for monoamine oxidase-A (MAO-A) in the bioactivation and nigrostriatal dopaminergic neurotoxicity of the MPTP analog, 2'Me-MPTP

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration leads to the selective destruction of the dopaminergic neurons of the nigrostriatal pathway in experimental animals including monkeys and mice. The neurotoxicity of MPTP is dependent upon its monoamine oxidase-B (MAO-B)-catalyzed conversion to the 1-methyl-4-phenylpyridinium species (MPP+). A methylated analog of MPTP. A methylated analog of MPTP, namely 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'Me-MPTP), is a more potent dopaminergic neurotoxin than MPTP in mice. Although the selective inhibition of MAO-B is sufficient to protect mice against MPTP-induced neurotoxicity, it is reported here that complete inhibition of MAO-B failed to prevent 2'Me-MPTP-induced dopaminergic neurotoxicity. However, the neurotoxicity of 2'Me-MPTP was completely prevented and 2'Me-MPP+ formation was markedly attenuated in mice in which both MAO-A and MAO-B were almost totally inhibited. This information about the role of MAO-A in the bioactivation of 2'Me-MPTP may be of relevance to those who speculate that the MAO-B catalyzed bioactivation of MPTP or a similar compound may be the cause of idiopathic Parkinson's disease.

Continuous duodenal infusions of levodopa: plasma concentrations and motor fluctuations in Parkinson's disease

Four parkinsonian patients received continuous duodenal infusions of levodopa (LD) for severe "on-off" phenomena associated with Sinemet (carbidopa/levodopa) therapy. All patients had marked decreases in motor fluctuations on the infusions compared with tablets. Two factors that appear related to this improvement are the ability of duodenal infusions to produce steadier plasma LD concentrations and/or to maintain LD concentrations above a minimum threshold needed to achieve the "on" state.

Neurotoxic effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and methamphetamine in several strains of mice

We previously reported that C57 black mice were more sensitive than several other strains of mice to the neurotoxic actions of MPTP. We now report that a significantly higher amount of the 1-methyl-4-phenylpyridinium species, a very toxic metabolite of MPTP, was found in the brains of C57 black mice than in the brains of CF-W or CD-1 mice, two strains of mice which are less sensitive to MPTP. In addition, we also found that there were strain differences in the response of mice to the neurotoxic actions of methamphetamine with CF-W mice being more sensitive to methamphetamine than C57 black mice.

Different effects of monoamine oxidase inhibition on MPTP depletion of heart and brain catecholamines in mice

Pargyline, an inhibitor of monoamine oxidase type B (MAO-B), did not prevent the depletion of heart norepinephrine 24 hr after a single dose of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in mice. In mice killed 24 hr after the last of 4 daily doses of MPTP, the depletion of dopamine in the striatum and of norepinephrine in the frontal cortex was completely prevented by pargyline, but the depletion of heart norepinephrine was not prevented. These results with pargyline are the same as results obtained earlier with deprenyl, another selective inhibitor of MAO-B. The doses of pargyline and of deprenyl that were used resulted in almost complete inhibition of MAO-B activity (phenylethylamine as substrate) in brain, heart and liver of mice. Deprenyl did not inhibit MAO-A activity (serotonin as substrate) in brain, but pargyline caused some inhibition of MAO-A in brain. In heart and liver, serotonin was oxidized only at about 1/10 the rate of phenylethylamine oxidation, suggesting that MAO-B predominates in these tissues. Both pargyline and deprenyl caused some inhibition of serotonin deamination in heart and liver, suggesting that the oxidation may have been due partly to MAO-B. Experiments with selective MAO inhibitors in vitro showed that only about 20% of the oxidation of serotonin was occurring via MAO-B in heart and liver. The in vitro oxidation of MPTP by MAO in mouse brain, heart and liver was almost completely inhibited by pretreatment with either pargyline or deprenyl. Neither pargyline nor deprenyl had any significant effect on the concentrations of MPTP in brain or heart one-half hr after injection of MPTP into mice. The concentrations of the metabolite, MPP+ (1-methyl-4-phenyl-pyridinium), were markedly reduced in brain and in heart by pretreatment with either pargyline or deprenyl. The data suggest that MPP+ formation, which is necessary for the depletion of brain catecholamines after MPTP injection, may not be necessary for depletion of norepinephrine in heart. Since the oxidation of MPTP in vitro was inhibited more by pargyline or deprenyl pretreatment than was the appearance of MPP+ in vivo, the possibility exists that some MPP+ formation might occur by an enzyme other than MAO.

Mitochondrial and metabolic toxicity of 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine

In previous studies and in the accompanying paper, 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'CH3-MPTP) was found to be more potent than MPTP in producing dopaminergic neurotoxicity in mice. One purpose of the present study was to determine whether 1-methyl-4-(2'-methylphenyl)pyridinium (2'CH3-MPP+), the primary oxidation product of 2'CH3-MPTP both in vivo and in vitro, inhibits mitochondrial respiration as does 1-methyl-4-phenylpyridinium (MPP+), the primary oxidation product of MPTP. Another aim was to determine whether treatments which modify MPTP- and 2'CH3-MPTP-induced neurotoxicity in vivo also cause parallel changes in the metabolic toxicity of these compounds. It was found that 2'CH3-MPP+, like MPP+, inhibited the oxidation of NAD(H)-linked substrates by isolated brain mitochondria in a concentration- and time-dependent manner, whereas succinate oxidation was not affected. Thus, the effect was on Complex I in the electron transport chain. Furthermore, 2'CH3-MPP+, like MPP+, enhanced lactate formation by neostriatal tissue slices as would be expected if Complex respiration were inhibited. MPP+ was slightly more potent than 2'CH3-MPP+ in both of these studies. However, 2'CH3-MPTP was several-fold more potent than MPTP in increasing lactate accumulation by the neostriatal slices. This difference in potency correlated with the differing capacities of 2'CH3-MPTP and MPTP to be oxidized by monoamine oxidase (MAO).(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine-induced neurotoxicity in the mouse

1-Methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'CH3-MPTP) was shown previously to be a more potent neurotoxicant than 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice. The present investigation was conducted to determine possible reasons for the greater potency of 2'CH3-MPTP and to determine if its neurotoxic action might be similar to that of MPTP. 2'CH3-MPTP was a much better substrate for monoamine oxidase than was MPTP (Km values of 66 and 114 microM and Vmax values of 3433 and 1389 nmol/g of tissue per hr for 2'CH3-MPTP and MPTP, respectively) and it is likely that this is an important feature which contributes to its greater potency. In addition, its pyridinium metabolite, 1-methyl-4-(2'-methylphenyl)pyridinium was found to be an excellent substrate for the dopamine carrier with Km and Vmax values (513 nM and 4.1 nmol/g of tissue per min, respectively) similar to those of 1-methyl-4-phenylpyridinium (872 nM and 5.2 nmol/g of tissue per min, respectively). In vivo, 2'CH3-MPTP-induced neurotoxicity, like MPTP-induced neurotoxicity, was attenuated by the pretreatment of mice with a dopamine uptake inhibitor (mazindol or GBR 13069). However, selective doses of the monoamine oxidase (MAO)-B inhibitors, deprenyl or MDL 72145, failed to prevent in vivo neurotoxicity induced by 2'CH3-MPTP although these doses effectively blocked MPTP-induced neurotoxicity. Protection against 2'CH3-MPTP-induced neurotoxicity was observed only at a nonselective dose of MDL 72145 which blocked both MAO-B and MAO-A activities.(ABSTRACT TRUNCATED AT 250 WORDS)

1-Methyl-4-cyclohexyl-1,2,3,6-tetrahydropyridine (MCTP): an alicyclic MPTP-like neurotoxin

1-Methyl-4-cyclohexyl-1,2,3,6-tetrahydropyridine (MCTP), an analog of MPTP, was found to be an MPTP-like neurotoxin. MCTP administration caused extensive losses of neostriatal dopamine and its major metabolites in male Swiss-Webster mice. Under similar experimental conditions, MCTP was approximately as potent as MPTP. Like MPTP, MCTP was a good substrate for monoamine oxidase-B (MAO-B) and its neurotoxicity was prevented in mice by AGN-1135, a selective inhibitor of MAO-B. The neurotoxicity of MCTP and of MPTP was also prevented by the dopamine uptake inhibitor mazindol. 1-Methyl-4-cyclohexylpyridinium ion (MCP+), the 4-electron oxidation product of MCTP, caused release of previously accumulated [3H]dopamine from mouse neostriatal synaptosomes. This release was blocked by mazindol, which indicates that MCP+, like 1-methyl-4-phenylpyridinium ion (MPP+), the 4-electron oxidation product of MPTP, is a substrate for the dopamine transport system. Like MPP+, MCP+ was found to inhibit the mitochondrial oxidation of NADH-linked substrates. It appears that conjugation between the tetrahydropyridine ring and a 4-substituent is not a requirement for an MPTP analog to possess neurotoxicity.

The use of the MPTP-treated mouse as an animal model of parkinsonism

The MPTP-treated mouse has proven to be a valuable model of parkinsonism. For example, C57 black mice treated with MPTP exhibit a large decrement in the neostriatal content of dopamine and its metabolites, a marked reduction in the capacity of neostriatal synaptosomal preparations to accumulate [3H]dopamine, a large decrease in neostriatal tyrosine hydroxylase activity, a marked loss of nerve cells in the zona compacta of the substantia nigra, and pronounced behavioral deficits. These biochemical, pathological and behavioral deficits are similarly observed in MPTP-treated primates and in humans with idiopathic parkinsonism. A great deal of our current knowledge concerning MPTP has come from experimentation carried out in the mouse.

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.

MPTP, MPP+ and mitochondrial function

1-Methyl-4-phenylpyridinium (MPP+), the putative toxic metabolite of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), inhibited NAD(H)-linked mitochondrial oxidation at the level of Complex I of the electron transport system. MPTP and MPP+ inhibited aerobic glycolysis in mouse striatal slices, as measured by increased lactate production; MPTP-induced effects were prevented by inhibition of monoamine oxidase B activity. Several neurotoxic analogs of MPTP also form pyridinium metabolites via MAO; these MPP+ analogs were all inhibitors of NAD(H)-linked oxidation by isolated mitochondria. 2'-Methyl-MPTP, a more potent neurotoxin in mice than MPTP, was also more potent than MPTP in inducing lactate accumulation in mouse brain striatal slices. Overall, the studies support the hypothesis that compromise of mitochondrial oxidative capacity is an important factor in the mechanisms underlying the toxicity of MPTP and similar compounds.

1-Methyl-4-phenylpyridine (MPP+): regional dopamine neuron uptake, toxicity, and novel rotational behavior following dopamine receptor proliferation

The regional uptake and subsequent dopaminergic toxicity, receptor proliferation, and rotational behavior pharmacology following intracerebral 1-methyl-4-phenylpyridine (MPP+) administration was determined in the rat. [3H]MPP+ was transported by the high-affinity dopamine uptake system equally in the caudate-putamen (CP), nucleus accumbens (NA) and olfactory tubercle (OT), and to a lesser extent in the substantia nigra. Consistent with the equivalent uptake of [3H]MPP+ by mesostriatal and mesolimbic dopamine neurons, dopamine concentrations of the ipsilateral CP and NA were decreased equally (83-98%) following a 10, 17.5 or 25 microgram injection of MPP+ along the left medial forebrain bundle (MFB). At four weeks after a 25 microgram injection of MPP+ into the MFB, the concentration (Bmax) of D2 receptors in the left CP was increased by 42% compared with the intact hemisphere. D2 receptors did not proliferate in the denervated nucleus accumbens. The affinity (Kd) of D2 receptors was not affected in either the CP or NA. The MPP+ injection, which was restricted to the region of striatonigral efferent fibers, also produced a 60% decrease in the GABA content of the substantia nigra. Ipsiversive rotational behavior was induced in MPP+-treated rats by systemic injections of d-amphetamine. Systemic injections of neither the dopamine agonist apomorphine nor agonist prodrug formulation of 1-DOPA and carbidopa induced contraversive rotation. These behavioral and neurochemical results are identical to those observed following concomitant destruction of striatonigral GABA and mesostriatal dopamine projections, and indicate that MPP+ may be toxic to GABAergic as well as to A10 and A9 dopaminergic neurons.

The influence of dose and dosing interval on MPTP-induced dopaminergic neurotoxicity in mice

The effects of different dosing paradigms for the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were investigated in C57-black and CF-W albino mice. Several groups of mice received a total dose of 80 mg/kg of MPTP administered at different doses per injection and/or at different time intervals. In C57-black mice, the effects of MPTP administration on the neostriatal content of dopamine ranged from a 91% depletion (4 injections of 20 mg/kg per injection at 1 h intervals) to a non-significant effect (4 injections of 10 mg/kg per injection at 2 h intervals on each of 2 successive days). There was also considerable influence of the MPTP dose per injection and the dosing interval in CF-W mice, although the extent of neostriatal dopamine depletion in CF-W mice was not as great as that observed in C57-black mice. In addition, MPTP produced variable effects on neostriatal dopamine levels in different strains of mice as well as in Swiss-Webster mice obtained from different sources. Some of the strains were affected to a great extent while others were only marginally affected.

Prevention of the nigrostriatal toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine by inhibitors of 3,4-dihydroxyphenylethylamine transport

The 3,4-dihydroxyphenylethylamine (DA, dopamine) uptake inhibitors GBR 13,069, amfonelic acid, WIN-35,065-2, WIN-35,428, nomifensine, mazindol, cocaine, McN-5908, McN-5847, and McN-5292 were effective in preventing [3H]DA and [3H]1-methyl-4-phenylpyridinium (MPP+) uptake in rat and mouse neostriatal tissue slices. These DA uptake inhibitors also were effective in attenuating the MPP+-induced release of [3H]DA in vitro. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration to mice (6 X 25 mg/kg i.p.) resulted in a large (70-80%) decrement in neostriatal DA. WIN-35,428 (5 mg/kg), GBR 13,069 (10 mg/kg), McN-5292 (5 mg/kg), McN-5908 (2 mg/kg), and amfonelic acid (2 mg/kg), when administered intraperitoneally 30 min prior to each MPTP injection, fully protected against MPTP-induced neostriatal damage. Other DA uptake inhibitors showed partial protection in vivo at the doses selected. Desmethylimipramine did not prevent [3H]MPP+ uptake or MPP+-induced release of [3H]DA in vitro, and did not protect against MPTP neurotoxicity in vivo. These results support the hypothesis put forth previously by others that the active uptake of MPP+ by dopaminergic neurons is necessary for toxicity.

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

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 mM), 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 mitochondrial 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.

Prevention of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced dopaminergic toxicity in mice by MDL 72145, a selective inhibitor of MAO-B

Pretreatment of mice with the potent and selective monoamine oxidase B (MAO-B) inhibitor MDL 72145 ((E)-2-(3',4'-dimethoxyphenyl)-3-fluoroallylamine) protected against the dopaminergic neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Mice treated with MDL 72145 prior to MPTP did not exhibit the decrement in the neostriatal content of dopamine and its metabolites normally seen after MPTP administration. This observation adds further support to the concept that the oxidation of MPTP by MAO-B to its corresponding pyridinium analog, 1-methyl-4-phenylpyridinium (MPP+), is an important feature of the neurotoxic process.

1-Methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'-CH3-MPTP) is a more potent dopaminergic neurotoxin than MPTP in mice

The administration to mice of 1-methyl-4-(2'-methylphenyl)-1,2,3, 6-tetrahydropyridine (2'-CH3-MPTP), a substituted analog of the dopaminergic neurotoxin MPTP caused even more dopaminergic toxicity than MPTP itself. Under conditions in which MPTP was relatively ineffective (i.e. two injections per day of 0.113 mmol/kg at an interval of 6 h for one or two days), 2'-CH3-MPTP caused a very large decrement in the neostriatal content of dopamine and its metabolites and a corresponding decrement in the capacity of a neostriatal synaptosomal preparation to take up [3H]dopamine. Moreover, 2'-CH3-MPTP administration (as few as four injections) caused a virtually complete loss of nerve cells in the zona compacta of the substantia nigra. This compound, like MPTP, may prove to be a valuable research tool.

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.

Dopaminergic toxicity of rotenone and the 1-methyl-4-phenylpyridinium ion after their stereotaxic administration to rats: implication for the mechanism of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity

The 1-methyl-4-phenyl-pyridinium ion (MPP+) is the four electron oxidation product of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6 -tetrahydropyridine (MPTP). MPP+ can be formed by the oxidation of MPTP by monoamine oxidase B to the intermediate dihydropyridinium species, MPDP+, which is spontaneously transformed to MPP+. In the present study, MPP+, like the mitochondrial toxin rotenone, inhibited pyruvate-malate respiration in isolated mitochondrial preparations. Moreover, the stereotaxic administration of both MPP+ and rotenone caused damage to the dopaminergic nigrostriatal pathway. These data clearly demonstrate that a mitochondrial toxin, administered stereotaxically, is extremely neurotoxic. The data lend support to the concept that MPTP-induced neurotoxicity may be due to the detrimental actions of enzymatically formed MPP+ on mitochondrial function.

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.

Prevention of MPTP-induced neurotoxicity by AGN-1133 and AGN-1135, selective inhibitors of monoamine oxidase-B

Two selective and potent inhibitors of monoamine oxidase (MAO) type B, namely AGN-1133 (N-methyl-N-propynyl-1-indanamine) and AGN-1135 (N-propynyl-1-indanamine), given to mice prior to the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) protected against the neurotoxic effects of MPTP. For example, mice treated with these agents prior to MPTP, did not exhibit the decrement in the neostriatal content of dopamine and its metabolites normally seen after MPTP administration. These data lend further support to the concept that the oxidation of MPTP by MAO-B to its corresponding pyridinium analog, 1-methyl-4-phenyl-pyridinium (MPP+) is an important feature of the neurotoxic process.