Cation-exchange high-performance liquid chromatography assay for the nigrostriatal toxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and its monoamine oxidase B generated metabolites in brain tissues

Evaluation of the oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to toxic pyridinium cations by monoamine oxidase (MAO) enzymes and its use to search for new MAO inhibitors and protective agents

Monoamine oxidase (MAO) enzymes catalyze the oxidative deamination of amines and neurotransmitters and inhibitors of MAO are useful as neuroprotectants. This work evaluates the human MAO-catalyzed oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a dopaminergic neurotoxin, to the directly-acting neurotoxic metabolites, 1-methyl-4-phenyl-2,3-dihydropyridinium (MPDP(+)) and 1-methyl-4-phenylpyridinium (MPP(+)) measured by High-Performance Liquid Chromatography (HPLC), and this approach is subsequently used as a new method for screening of MAO inhibitors and protective agents. Oxidation of MPTP by human MAO-B was more efficient than by MAO-A. R-Deprenyl, a known neuroprotectant, norharman (β-carboline), 5-nitroindazole and menadione (vitamin K3) inhibited MAO-B and reduced the formation of toxic pyridinium cations. Clorgyline and the β-carbolines, harman and norharman, inhibited the oxidation of MPTP by MAO-A. Cigarette smoke, as well as the naturally occurring β-carbolines (norharman and harman) isolated from smoke and coffee inhibited the oxidation of MPTP by MAO-B and/or MAO-A, suggesting protective effects against MPTP. The results show the suitability of the approach used to search for new MAO inhibitors with eventual neuroprotective activity.

Susceptibility to a parkinsonian toxin varies during primate development

Symptoms of Parkinson's disease typically emerge later in life when loss of nigrostriatal dopamine neuron function exceeds the threshold of compensatory mechanisms in the basal ganglia. Although nigrostriatal dopamine neurons are lost during aging, in Parkinson's disease other detrimental factors must play a role to produce greater than normal loss of these neurons. Early development has been hypothesized to be a potentially vulnerable period when environmental or genetic abnormalities may compromise central dopamine neurons. This study uses a specific parkinsonian neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), to probe the relative vulnerability of nigrostriatal dopamine neurons at different stages of primate development. Measures of dopamine, homovanillic acid, 1-methyl-pyridinium concentrations and tyrosine hydroxylase immunoreactive neurons indicated that at mid-gestation dopamine neurons are relatively vulnerable to MPTP, whereas later in development or in the young primate these neurons are resistant to the neurotoxin. These studies highlight a potentially greater risk to the fetus of exposure during mid-gestation to environmental agents that cause oxidative stress. In addition, the data suggest that uncoupling protein-2 may be a target for retarding the progressive loss of nigrostriatal dopamine neurons that occurs in Parkinson's disease and aging.

Protocol for the MPTP mouse model of Parkinson's disease

This protocol describes our method of producing a reliable mouse model of Parkinson's disease (PD) using the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We discuss the particulars of the model, provide key references and outline what investigators need to know to develop the MPTP mouse model of PD safely and successfully. Completion of this protocol depends on the regimen of MPTP used and on the actual planned studies, which often range from 7 to 30 d. This protocol calls for implementation of safety measures and for the acquisition of several pieces of equipment, which are a one-time investment worth making if one elects to use this model on a regular basis.

Disruption of lateral olivocochlear neurons via a dopaminergic neurotoxin depresses sound-evoked auditory nerve activity

We applied the dopaminergic (DA) neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to the guinea pig cochlear perilymph. Immunolabeling of lateral olivocochlear (LOC) neurons using antibodies against synaptophysin was reduced after the MPTP treatment. In contrast, labeling of the medial olivocochlear innervation remained intact. As after brainstem lesions of the lateral superior olive (LSO), the site of origin of the LOC neurons, the main effect of disrupting LOC innervation of the cochlea via MPTP was a depression of the amplitude of the compound action potential (CAP). CAP amplitude depression was similar to that produced by LSO lesions. Latency of the N1 component of the CAP, and distortion product otoacoustic emission amplitude and adaptation were unchanged by the MPTP treatment. This technique for selectively lesioning descending LOC efferents provides a new opportunity for examining LOC modulation of afferent activity and behavioral measures of perception.

Diversity in mechanisms of substrate oxidation by cytochrome P450 2D6. Lack of an allosteric role of NADPH-cytochrome P450 reductase in catalytic regioselectivity

Cytochrome P450 (P450) 2D6 was first identified as the polymorphic human debrisoquine hydroxylase and subsequently shown to catalyze the oxidation of a variety of drugs containing a basic nitrogen. Differences in the regioselectivity of oxidation products formed in systems containing NADPH-P450 reductase/NADPH and the model oxidant cumene hydroperoxide have been proposed by others to be due to an allosteric influence of the reductase on P450 2D6 (Modi, S., Gilham, D. E., Sutcliffe, M. J., Lian, L.-Y., Primrose, W. U., Wolf, C. R., and Roberts, G. C. K. (1997) Biochemistry 36, 4461-4470). We examined the differences in the formation of oxidation products of N-methyl-4-phenyl-1,2,5,6-tetrahydropyridine, metoprolol, and bufuralol between reductase-, cumene hydroperoxide-, and iodosylbenzene-supported systems. Catalytic regioselectivity was not influenced by the presence of the reductase in any of the systems supported by model oxidants, ruling out allosteric influences. The presence of the reductase had little effect on the affinity of P450 2D6 for any of these three substrates. The addition of the reaction remnants of the model oxidants (cumyl alcohol and iodobenzene) to the reductase-supported system did not affect reaction patterns, arguing against steric influences of these products on catalytic regioselectivity. Label from H(2)18O was quantitatively incorporated into 1'-hydroxybufuralol in the iodosylbenzene- but not in the reductase- or cumene hydroperoxide-supported reactions. We conclude that the P450 systems utilizing NADPH-P450 reductase, cumene hydroperoxide, and iodosylbenzene use similar but distinct chemical mechanisms. These differences are the basis for the variable product distributions, not an allosteric influence of the reductase.

2-deoxyglucose enhances 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced ATP loss in the mouse brain

The effects of 2-deoxyglucose (2-DG), an inhibitor of the uptake and use of glucose, on ATP loss caused by the neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were determined in the mouse brain. 2-DG alone had no effect on brain ATP levels, but when administered 30 min before MPTP exposure, 2-DG significantly enhanced MPTP-induced ATP reduction. This was reflected as an increase in ATP loss in the striatum (from 15 to 27%) as well as a significant decrease in ATP in the cerebellar cortex, an area of the brain that was not affected after exposure to MPTP alone. In mice pretreated with 2-DG, striatal ATP levels remained significantly decreased for > 8 h after MPTP administration. In contrast, ATP levels in the cerebellar cortex returned to normal values within 4 h from MPTP exposure. Mazindol, a catecholamine uptake blocker, completely protected against MPTP-induced loss of striatal ATP in the absence of 2-DG, but it only partially prevented striatal ATP decrease after administration of both 2-DG and MPTP; mazindol was also ineffective in protecting against ATP loss caused by 2-DG and MPTP in the cerebellar cortex. 2-DG/MPTP-induced ATP loss appeared to be associated with the presence of the 1-methyl-4-phenylpyridinium (MPP+) metabolite because (1) the pattern of ATP recovery in the striatum and cerebellar cortex appeared to reflect the pattern of MPP+ clearance from these areas of the brain (i.e., significant MPP+ levels persisted longer in the striatum than in the cerebellar cortex), and (2) ATP decrease was completely prevented by blocking the conversion of MPTP to MPP+ with the monoamine oxidase B inhibitor deprenyl.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

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

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

MPTP mechanisms of neurotoxicity and their implications for Parkinson's disease

Systemic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) gives rise to motor deficits in humans and other primates which closely resemble those seen in patients with Parkinson's disease. These deficits are associated with a relatively selective loss of cells in the pars compacta of the substantia nigra and severe reductions in the concentrations of dopamine, noradrenaline and serotonin in the striatum. Similarly, in mice of various different strains the administration of MPTP also induces a marked loss of dopaminergic cells with severe depletion of biogenic amines, but higher doses of MPTP are required to produce these effects in mice than in primates. This review summarises advances made in understanding the biochemical events which underlie the remarkable neurotoxic action of MPTP. Major steps in the expression of neurotoxicity involve the conversion of MPTP to the toxic agent 1-methyl-4-phenylpyridinium ion (MPP+) by type B monoamine oxidase (MAO-B) in the glia, specific uptake of MPP+ into the nigro-striatal dopaminergic neurones, the intraneuronal accumulation of MPP+, and the neurotoxic action of MPP+. This is exerted mainly through the inhibition of the enzymes of the respiratory chain (Complex I), the disturbance of Ca2+ homeostasis, and possibly by the formation of free radicals. The relevance of the MPTP model to idiopathic Parkinson's disease is discussed.

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.

Effects of N-methylmercaptoimidazole on the disposition of MPTP and its metabolites in mice

The effects of N-methylmercaptoimidazole, an alternative substrate for flavin-containing monooxygenase, on the disposition of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its metabolites, 1-methyl-4-phenyl-1,3-dihydropyridine (MPDP+) and 1-methyl-4-phenylpyridine (MPP+) in plasma and in brain tissues were studied in mice. Pretreatment of mice with N-methylmercaptoimidazole caused a significant (P less than 0.01) increase in the plasma concentration of MPTP, whereas there was no significant change in the plasma concentrations of MPDP+ and MPP+. N-Methylmercaptoimidazole caused a significant (P less than 0.01) increase in the levels of MPTP, MPDP+ and MPP+ in both the striatum and cortex. The conversion rates of MPTP to MPDP+ to MPP+ in whole brain homogenates were not affected by N-methylmercaptoimidazole. These results suggest that N-methylmercaptoimidazole increases the amount of MPTP delivered from the peripheral to the central nervous system, presumably by inhibiting MPTP N-oxygenation via the hepatic microsomal flavin-containing monooxygenase. An increased level of striatal MPP+, caused by the oxidation of more MPTP by brain monoamine oxidase, appears to be the mechanism by which the neurotoxicity of MPTP is enhanced by N-methylmercaptoimidazole.

Biochemistry and genetics of monoamine oxidase

This chapter reviews the two mitochondrial flavin containing isozymes of monoamine oxidase. Section 1, "Biochemistry" discusses assays, substrates and inhibitors, phylogenic and tissue distribution, interactions with lipids, nutritional studies, protein structure, kinetic and chemical mechanistic proposals, and biosynthesis. Section 2, "Inheritance" discusses possible genes involved in expression, genetic studies of platelet MAO-B and fibroblast MAO-A, and chromosomal location. Section 3, "Molecular Genetics" reviews the cloning of their cDNAs, their intra- and interspecies homology and structural inferences made from deduced amino acid sequences. Section 4, "Regulation" gives an overview of levels in development and aging, and effect of drugs. The final section 5, "Role in Human Disease" discusses physiological function and effects of altered levels in humans and animal models including complete absence due to a submicroscopic chromosomal deletion in several human patients.

Depletion of cardiac norepinephrine but not brain catecholamines by MPTP-N-oxide in mice

After a single dose in mice, MPTP-N-oxide caused a dose-dependent depletion of cardiac norepinephrine which was similar although less pronounced than that caused by MPTP itself. After repeated daily doses, MPTP-N-oxide depleted cardiac norepinephrine, but did not deplete norepinephrine in the frontal cortex or dopamine in the striatum of mice. MPTP-N-oxide differed from MPTP, which depleted cardiac norepinephrine, cortical norepinephrine and striatal dopamine after repeated daily doses, but was similar to MPP+, another metabolite of MPTP, which depleted only cardiac norepinephrine. These data suggest that MPTP-N-oxide may contribute to the peripheral catecholamine-depleting effects after MPTP injection.

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.

Metabolism of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) by bovine brain microvessel endothelial cells

Incubation of bovine brain microvessel endothelial cell monolayers with the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine led to the monoamine oxidase (MAO)-mediated formation of the oxidative metabolites 1-methyl-4-phenyl-2,3-dihydropyridine and 1-methyl-4-phenylpyridine (MPP+). The flux of low nanomolar concentrations of [3H]MPP+ across endothelial cell monolayers appeared to be restricted, probably due to the oxidation by MAO. [3H]MPP+ did not significantly cross endothelial cell monolayers.

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.

A sheep model for MPTP induced Parkinson-like symptoms

Administration of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) causes behaviors reminiscent of idiopathic Parkinson's disease in man and other primates, but development of such symptomology has not been reported to date in other species. We now report a sheep model which responds to administration of low levels of the compound with well defined, apparently permanent symptomology very similar to that seen in primates. Histological examination indicates drug dependent destruction of the substantia nigra which, in sheep, lacks the high levels of neuromelanin present in primates. Following infusion of either MPTP or MPP+, only the metabolite MPP+ was detected in serum with this metabolite demonstrating a very long half life. The rapid disappearance of MPTP suggests that its potency will be directly related to a function of body size and inversely related to heart rate.

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.

Chemical oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its in vivo metabolism in rat brain and liver

We investigated in vivo the metabolism of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in the brain and liver of rats 45 min after the systemic administration of 50 mg/kg of the neurotoxin. The metabolites present in brain and liver extracts were identified through multiple analytical methods by comparison to authentic compounds obtained from a number of chemical oxidations of MPTP. Our results indicate the presence of approximately 15% unreacted MPTP and relatively large amounts of both 1-methyl-4-phenylpyridinium (MPP+) and a mixture of three nonpolar lactams: 1-methyl-4-phenyl-5,6-dihydro-2(1H)-pyridinone, 1-methyl-4-phenyl-2(1H)-pyridinone, and a previously unreported metabolite 1-methyl-4-phenyl-2-piperidinone. Whereas MPP+ was more prevalent in the brain than in the liver, the lactam metabolites were more predominant in the liver. The amounts of the N-oxide and N-demethylated metabolites of MPTP were minimal.