L-745,870 reduces L-DOPA-induced dyskinesia in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned macaque model of Parkinson's disease

L-DOPA-induced dyskinesia remains an unmet challenge in the treatment of Parkinson's disease (PD). Here, we investigate the potential antidyskinetic efficacy of 3-([4-(4-chlorophenyl)piperazin-1-yl]methyl)-1H-pyrrolo[2,3-b]pyridine (L-745,870), a potent and selective dopamine D(4) receptor antagonist with a good toxicology profile and an excellent safety and tolerability record in phase I/II clinical studies, for non-PD indications. Six macaques were rendered parkinsonian by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration. After induction of stable and marked dyskinesia, animals were administered acute challenges of L-745,870 in combination with L-DOPA. To guarantee D(4) selectivity at the doses used in the study, we determined the plasma, cerebrospinal fluid, and brain levels of L-745,870. Coadministration of L-745,870 (1 mg/kg) and L-DOPA significantly reduced the severity of dyskinesia, by up to 59%, in comparison with L-DOPA alone (P < 0.01). L-745,870 had no effect on the duration of antiparkinsonian benefit (ON-time) (P > 0.05). However, L-745,870 (1 mg/kg) significantly increased the duration of ON-time without disabling dyskinesia (+204%; P < 0.001) and decreased duration of ON-time with disabling dyskinesia compared with L-DOPA alone (-56%; P < 0.01). Brain levels of L-745,870 (～600 ng/g) were within the range at which L-745,870 provides selective D(4) receptor antagonism. Plasma levels were comparable with those demonstrated to be well tolerated in human studies. These data suggest that selective D(4) receptor antagonists represent a potential therapeutic approach for L-DOPA-induced dyskinesia. It is noteworthy that L-745,870 has already undergone significant clinical development, has an excellent profile for a therapeutic candidate, and could be advanced rapidly to phase IIa clinical studies for dyskinesia in PD.     

Entry of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine into the rat brain

We studied blood-to-brain entry of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 1-methyl-4-phenylpyridinium (MPP+) and butanol in anesthetized rats using the indicator-fractionation method with right atrial bolus injection. Minimal amounts of MPP+, which has low octanol/water partition coefficient, crossed the blood-brain barrier. MPTP and butanol, both of which have high octanol/water partition coefficients, were almost completely extracted by all regions of the brain on the first pass. The main difference between the MPTP and butanol tracers is that butanol rapidly left the brain with an exponential rate constant of 1.24 min-1, whereas MPTP was avidly retained by the brain with a washout rate constant of 0.10 min-1 (mean values for the four brain regions that we studied). Early retention of MPTP by the brain was not due to its rapid metabolism by monoamine oxidase because pargyline pretreatment did not affect this rate constant. However, 30 min after [3H]MPTP injection, brain retention of the 3H tracer was reduced significantly by pargyline treatment, and the ratio of brain MPTP/MPP+ was increased markedly.     

Deprenyl antagonizes acute lethality of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice

In Charles River CFW mice, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) caused lethality with an LD50 of 53.8 mg/kg s.c. In mice pretreated with deprenyl, no lethality occurred with MPTP doses up to 110 mg/kg s.c. MPTP alone at doses of 30 to 90 mg/kg s.c. caused marked salivation, licking and grooming, hyperlocomotion, hyperreactivity and convulsions during the 1st hr, followed by depression, continued salivation and respiratory distress at 2 to 3 hr and at longer times, with death occurring at the higher doses. In deprenyl-pretreated mice, MPTP produced only mild and transient effects. 1-Methyl-4-phenylpyridinium (MPP+) was more potent in causing lethality than was MPTP, and deprenyl did not affect its lethality. MPTP lethality was not antagonized by EXP 561 [4-phenyl-bicyclo-(2,2,2)octan-1-amine hydrochloride monohydrate], an uptake inhibitor that prevented the neurotoxic effects of a lower dose of MPTP on striatal dopamine and cortical norepinephrine neurons. In addition to deprenyl, other monoamine oxidase (MAO) inhibitors effective in inhibiting MAO-B (MD 240928 (R-3-[4-((3-chlorophenyl)methoxy)phenyl]-5-[(methylamino)methyl]-2- oxazolidinone methanesulfonate) and pargyline) protected against MPTP-induced lethality, but LY 51641 (N-[2-(o-chlorophenoxy)ethyl]cyclopropylamine hydrochloride) (a selective inhibitor of MAO-A) did not. The protective effect of deprenyl against MPTP-induced lethality was dose-dependent over a dose range of 0.01 to 10 mg/kg; in this range deprenyl inhibited MAO type B (MAO-B) in brain and liver. A 10-mg/kg i.p. dose of deprenyl antagonized MPTP-induced lethality as long as 14 days.(ABSTRACT TRUNCATED AT 250 WORDS)     

Biotransformation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in primary cultures of mouse astrocytes

Astrocytes are likely to be a main locus for the metabolic bioactivation of the neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In this study, a detailed analysis of MPTP metabolism was conducted in primary cultures of mouse astrocytes. A constant rate of conversion of 1.22 nmol/mg of protein per hr was observed when astrocyte cultures were incubated in the presence of 250 microM MPTP for 4 days. Three metabolites were detected as products of this conversion: 1-methyl-4-phenyl-2,3-dihydropyridinium ion (MPDP+), 1-methyl-4-phenylpyridinium ion (MPP+) and MPTP N-oxide. Production of MPP+ and MPTP N-oxide occurred at constant rates of 0.68 and 0.43 nmol/mg of protein per hr, respectively, whereas the level of MPDP+ remained quite stable and relatively low throughout the time of incubation. Both clorgyline, an inhibitor of monoamine oxidase (MAO) type A, and deprenyl, a MAO B inhibitor, blocked MPTP conversion to MPDP+ and MPP+; quantitative analysis of the effects of these two inhibitors revealed that MAO A and MAO B contribute to a similar extent to MPP+ production in astrocyte cultures. MAO inhibition did not result in an increased production of MPTP N-oxide and, in fact, the level of this metabolite was reduced markedly in the presence of 100 microM clorgyline. Formation of MPTP N-oxide was probably dependent upon the activity of the flavin-containing monooxygenase because: 1) it was blocked completely by thiobenzamide, a competitive substrate for this microsomal enzyme and 2) it was increased in the presence of n-octylamine, a known positive effector for flavin-containing monooxygenase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fatty acid amide hydrolase (FAAH) inhibition reduces L-3,4-dihydroxyphenylalanine-induced hyperactivity in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned non-human primate model of Parkinson's disease

Dopaminergic therapies remain the most efficacious symptomatic treatments for Parkinson's disease (PD) but are associated with motor complications, including dyskinesia, and nonmotor complications, such as psychosis, impulse control disorders (ICD), and dopamine dysregulation syndrome (DDS). Nondopaminergic neurotransmitter systems, including the endocannabinoid system, are probably critical to the development of these complications. The role of fatty acid amide hydrolase (FAAH) in mediating l-3,4-dihydroxyphenylalanine (L-DOPA)-induced behaviors was explored in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned marmoset model of PD. Pharmacodynamic and locomotor effects of the selective FAAH inhibitor [3-(3-carbamoylphenyl)phenyl] N-cyclohexylcarbamate (URB597) were assessed via bioanalytical (liquid chromatography-tandem mass spectrometry) and behavioral observation approaches. URB597 (3, 10, 30, or 60 mg/kg p.o.) increased plasma levels of the FAAH substrates N-arachidonoyl ethanolamide (anandamide), N-oleoyl ethanolamide, and N-palmitoyl ethanolamide by 10.3 ± 0.3-, 7.8 ± 0.2-, and 1.8 ± 0.1-fold (mean of URB597 groups ± S.E.M.), respectively, compared with vehicle (all p < 0.001) 4 h after administration. Treatment with L-DOPA (20 mg/kg s.c.) alleviated parkinsonism but elicited dyskinesia, psychosis-like-behaviors and hyperactivity, a potential correlate of ICD and DDS. During the 2 to 4 h after L-DOPA, corresponding to 4 to 6 h after URB597 administration, URB597 reduced total L-DOPA-induced activity and the magnitude of hyperactivity by 32 and 52%, respectively, to levels equivalent to those seen in normal animals. Treatment with URB597 (10 mg/kg p.o.) did not modify the antiparkinsonian actions of L-DOPA or L-DOPA-induced dyskinesia and psychosis. URB597 did not alter plasma L-DOPA levels and was without behavioral effects when administered alone. Inhibition of FAAH may represent a novel approach to reducing L-DOPA-induced side effects, such as ICD and DDS, while maintaining the antiparkinsonian benefits of L-DOPA treatment.     

On the mechanisms underlying 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine neurotoxicity: the effect of perinigral infusion of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, its metabolite and their analogs in the rat

The discovery that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes parkinsonism in humans and other primates by selective destruction of substantia nigra dopaminergic neurons has spurred research to define the mechanisms underlying its toxicity. To avoid variables such as tissue distribution, extracerebral metabolism and blood-brain barrier permeability, the authors studied the neurochemical and morphologic effects of direct perinigral infusions of various concentrations of MPTP, its metabolites and analogs in the rat. MPTP, in the highest dose used, 1000 nmol, decreased dopamine and its metabolites in ipsilateral striatum by approximately 75%, whereas 3,3-dimethyl-MPTP (which is oxidized to 1,3,3-trimethyl-4-phenyl-2,3-dihydropyridinium cation but not to a pyridinium species) had no effect. The 2,2 and 3,3-dimethyl analogs of 1-methyl-4-phenyl-2,3-dihydropyridinium cation which also cannot be oxidized to pyridinium species, reduced striatal dopamine, suggesting that these compounds are toxic in their own right. 1-Methyl-4-phenylpyridinium cation (MPP+) and its 4-(4-fluorophenyl) and 4-(2-pyridyl) analogs that have less negative reduction potentials than MPP+, were most potent in decreasing striatal dopamine and metabolites, with MPP+ being 5 to 10 times more effective than its two analogs and approximately 100 times more potent than MPTP and the two dimethyl 1-methyl-4-phenyl-2,3-dihydropyridinium cation analogs. These findings suggest that MPP+ is ultimately responsible for MPTP toxicity but does not act via oxidant stress mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)     

1,2,3,6-tetrahydro-1-methyl-4-(methylpyrrol-2-yl)pyridine: studies on the mechanism of action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Two analogs of the nigrostriatal neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were examined for their deleterious effects on nigrostriatal neurons. 1,2,3,6-Tetrahydro-1-methyl-4-(methylpyrrol-2-yl)pyridine (TMMP), but not 1-methyl-4-(1-methylpyrrol-2-yl)-4-piperidinol, caused persistent depletion of striatal dopamine and induced histologic evidence of nerve terminal degeneration in mice. These findings differ diametrically from results previously reported for the two analogs. TMMP produced larger dopamine depletions than MPTP when the two drugs were given in equivalent doses. Further experiments demonstrated that TMMP is preferentially oxidized by mouse brain monoamine oxidase B to a water-soluble compound, most likely the pyridinium ion species. Prior treatment of mice with either the monoamine oxidase inhibitor pargyline or the dopamine reuptake inhibitor bupropion blocked the ability of TMMP to deplete striatal dopamine. Thus, the pharmacologic profile of TMMP closely resembles that of MPTP. That TMMP and MPTP induce dopamine depletions by a similar mechanism tends to support the proposed neurochemical sequence of events thought to lead to the expression of MPTP-induced neurotoxicity. The authors' observations provide further evidence that MPTP is not unique in its capability to damage nigrostriatal neurons.     

Toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in primary cultures of mouse astrocytes.

The conversion of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to its toxic 1-methyl-4-phenylpyridinium (MPP+) metabolite catalyzed by monoamine oxidase (MAO) type B is likely to occur within glial cells in the central nervous system. In this study, primary cultures of mouse astrocytes were used to assess the biochemical and toxic consequences of exposure to MPTP. MPTP caused a concentration-dependent loss of cell viability. This effect was probably due to the intracellular generation of MPP+, because cytotoxicity was prevented by preincubation of astrocytes in the presence of MAO inhibitors. After addition of 250 microM MPTP, loss of cell viability was preceded by an increased rate of glucose utilization and lactate accumulation, and by depletion of ATP. The ratio between the rates of lactate production (0.37 mM/hr) and glucose consumption (0.2 mM/hr) was 1.85, indicating that most of the glucose present in the medium was stoichiometrically converted to lactate via glycolysis. A remarkable correlation was found between ATP depletion and cytotoxicity caused by MPTP, and, when astrocytes were incubated in glucose-free medium, both ATP depletion and loss of viability occurred more rapidly. Finally, even after exposure for several days, astrocyte death could be prevented by washing MPTP from the incubation medium, suggesting that MPP(+)-induced mitochondrial damage may be reversible. We conclude that prolonged exposure of astrocytes to MPTP may result in loss of viability via the MAO-dependent generation of MPP+ and the ability of this toxic metabolite to impair mitochondrial function.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sympathetic innervation in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine primate model of Parkinson's disease

Cardiac sympathetic denervation occurs commonly in Parkinson's disease. This study explored whether analogous denervation occurs in primates with Parkinsonism from systemic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). 6-[18F]Fluorodopamine positron emission tomographic scanning and plasma levels of catecholamines and their deaminated metabolites were used to assess sympathetic and adrenomedullary function in rhesus monkeys, in the untreated state (n = 3), 2 weeks after a series of four MPTP injections, before establishment of Parkinsonism (acute phase, n = 1); a month later, after four more MPTP doses, associated with severe Parkinsonism (subacute phase, n = 1); or more than 2 years from the last dose (remote phase, n = 3), with persistent severe Parkinsonism. A positive control received i.v. 6-hydroxydopamine 1 week before 6-[18F]fluorodopamine scanning. Acute MPTP treatment increased cardiac 6-[18F]fluorodopamine-derived radioactivity, whereas 6-hydroxydopamine markedly decreased cardiac radioactivity, despite similarly low plasma levels of catecholamines and metabolites after either treatment. Subacutely, plasma catecholamines remained decreased, but now with myocardial 6-[18F]fluorodopamine-derived radioactivity also decreased. Remotely, MPTP-treated monkeys had lower plasma catecholamines and higher myocardial 6-[18F]fluorodopamine-derived radioactivity than did untreated animals. The results indicate that in nonhuman primates, systemic MPTP administration produces multiphasic effects on peripheral catecholamine systems, with nearly complete recovery by 2 years. MPTP- and 6-hydroxydopamine-induced changes differ markedly, probably from ganglionic or preganglionic neurotoxicity with the former and more severe cardiac sympathetic neurotoxicity with the latter. Because of multiphasic sympathetic and adrenomedullary effects, without cardioselective sympathetic denervation at any time, the primate MPTP model does not mimic the changes in peripheral catecholamine systems that characterize the human disease.     

Neurochemical and toxic effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and 1-methyl-4-phenylpyridine to rat serotonin neurons in dissociated cell cultures

Dissociated cell cultures from the pontine area of embryonic rat brain were used to study the sensitivity of serotonin (5-hydroxy-tryptamine (5-HT)) neurons to the neurotoxins 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridine (MPP+). Treatment with MPTP (up to 100 microM) for 7 days did not cause degeneration of 5-HT neurons. A 50% inhibition of [3H]5-HT uptake caused by 100 microM MPTP was a direct effect on the 5-HT uptake carrier, reversed by washing for 7 days. Incubation of cultures with MPTP increased the intraneuronal levels of 5-HT and reduced the levels of 5-hydroxyindoleacetic acid, suggesting a reduction in 5-HT metabolism. MPTP reduced monoamine oxidase activity in the cultures, which probably led to the reduction in 5-HT metabolism. Exposure to MPP+ (0.5-10 microM) for 4 to 7 days decreased [3H]5-HT uptake and induced loss of neurons stained with antibodies against 5-HT. Comparison between 5-HT and dopamine (DA) neurons indicated a differential sensitivity to MPP+ toxicity with DA neurons being more susceptible. Analysis of the competition of MPP+ with the natural substrates for uptake sites of 5-HT and DA neurons demonstrated higher affinity of MPP+ for DA compared to 5-HT neurons. The lower affinity of MPP+ for 5-HT neurons could be responsible for the accumulation of lower MPP+ levels observed in pontine cultures and explain the resistance of 5-HT neurons to this toxin.     

1-甲基-4-苯基-1,2,3,6-四氢吡啶诱导的帕金森病小鼠模型多巴胺能神经元的缺失方式

目的:观察1-甲基-4-苯基-1,2,3,6-四氢吡啶(1-methyl-4-phenyl-1,2,3,6-tetrahydropyritine,MPTP)诱导的帕金森病小鼠急性模型与亚急性模型多巴胺能神经元的缺失方式。方法:实验于2004-11/2005-02在华北煤炭医学院解剖教研室实验室完成。①C57bl小鼠30只,随机分为3组:对照组、急性模型组、亚急性模型组,每组10只。②给予模型组小鼠腹腔注射MPTP,其中亚急性组30mg/(kg·d),连用5d;急性组一天内给药4次,每次20mg/kg,间隔时间一两个小时;对照组给予等量生理盐水。③最后一次注射后24h内取脑,制备石蜡切片。应用原位末端标记法、酪氨酸羟化酶、原癌基因蛋白Bcl-2和细胞色素C蛋白免疫组化染色观察多巴胺能神经元凋亡和改变。结果:30只小鼠均进入结果分析。经免疫组化法发现酪氨酸羟化酶、原癌基因蛋白Bcl-2、细胞色素C蛋白染色阳性细胞数在急性模型组与亚急性模型组之间差别具有显著意义[(22.40±2.06),(68.20±3.61)个/切片;(20.60±2.41),(66.80±2.12)个/切片;(93.0±2.24),(23.2±2.58)个/切片,P<0.001]。④应用原位末端标记法在急性模型组未发现凋亡,而亚急性模型组存在凋亡现象。结论:MPTP诱导的帕金森病小鼠急性模型多巴胺能神经元的缺失可能通过坏死实现,而亚急性模型则是通过凋亡来实现的。     

The neurotoxins 1-methyl-4-phenylpyridinium and 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine are substrates for the organic cation transporter in renal brush border membrane vesicles

We examined the effects of the neurotoxins 1-methyl-4-phenyl-pyridinium (MPP+) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on the transport of the prototypic organic cation, N1-[3H]methylnicotinamide ([3H]NMN), in canine renal brush border membrane vesicles. A dose-response curve for MPTP and its oxidized metabolite, MPP+, revealed IC50 values of 160 and 16 microM, respectively. MPTP (5 mM) and MPP+ (5 mM), maximally inhibited H+-driven NMN (50 microM) uptake by 86.0 and 86.6%, respectively. Additionally, serotonin (0.5 mM) and harmaline (0.5 mM) inhibited NMN transport by 65.8 and 87.1%, respectively. Mepiperphenidol (Darstine) (0.5 mM), a classical organic cation competitor, inhibited NMN transport by 80.6%. However, dopamine (0.5 mM) was not as effective and resulted in only 24.9% inhibition. The cationic specificity was demonstrated by showing that MPP+ and MPTP had no effect on the transport of the organic anion, p-aminohippurate. Additionally, the effect of MPP+ on NMN was not due to either a voltage effect or vesicle disruption. In countertransport studies, MPTP acted as an elicitor of NMN counterflow and produced trans stimulation whereas MPP+ did not and resulted in trans inhibition. Inasmuch as transport involves both binding and translocation we speculate that MPTP binds and is translocated; in contrast MPP+ binds and is not translocated. These reagents may serve as a basis for elucidating the structure-activity requirements of the organic cation/H+ antiporter.     

Edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one), a radical scavenger, prevents 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurotoxicity in the substantia nigra but not the striatum

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes nigrostriatal dopaminergic neurotoxicity and behavioral impairment in rodents, and previous studies suggest that nitric oxide and reactive oxygen species are involved in MPTP-induced neurotoxicity. The present study examines the effect of edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one), a radical scavenger, on MPTP-induced neurotoxicity in the striatum and substantia nigra pars compacta (SNc) of C57BL/6J mice. MPTP treatment (10 mg/kg s.c. x 4 with 2-h intervals) decreased dopamine levels and tyrosine hydroxylase immunostaining in the striatum and SNc. Pretreatment with edaravone (1 and 3 mg/kg i.p.) significantly reduced the neurotoxicity in the SNc but not striatum. An immunohistochemical study showed that MPTP caused microglial activation both in the striatum and SNc, whereas it increased 3-nitrotyrosine immunoreactivity, an in vivo biomarker of peroxynitrite production, in the SNc but not the striatum. Furthermore, MPTP increased lipid peroxidation product thiobarbituric acid reactive substance in the midbrain, but not the striatum. Edaravone inhibited activation of the microglia and the increased 3-nitrotyrosine immunoreactivity in the SNc but not the striatum, and it also inhibited thiobarbituric acid reactive substance levels in the midbrain. Behavioral analyses showed that edaravone improved MPTP-induced impairment of locomotion and Rotorod performance. These results suggest that edaravone protects against MPTP-induced neurotoxicity in the SNc by blocking the production of reactive oxygen species or peroxynitrite and imply that dopaminergic degeneration in the SNc may play an important role in MPTP-induced motor dysfunction of mice.     

Characterization of hepatic microsomal metabolism as an in vivo detoxication pathway of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a selective neurotoxin to the nigrostriatal dopaminergic neurons, has been shown to be metabolized by microsomal flavin-containing monooxygenase (FMO) to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine N-oxide and by cytochrome (cyt.) P-450 to 4-phenyl-1,2,3,6-tetrahydropyridine in the liver. The present study was conducted to determine whether and to what extent each of these metabolic processes would function as the detoxication pathway(s) of MPTP. Administration of either MPTP metabolites, MPTP N-oxide or 4-phenyl-1,2,3,6-tetrahydropyridine, to mice resulted in no significant reductions in striatal dopamine and its metabolites. Pretreatment of mice with alternate substrate of FMO, N-methylmercaptoimidazole or thiobenzamide significantly (P less than .001 to .05) enhanced MPTP-induced reductions in striatal dopamine and its metabolites. In contrast, neither pretreatments with cyt. P-450 inhibitors, SKF-525A (2-diethylaminoethyl-2,2-diphenyl-valerate), quinidine and cimetidine nor with the inducers, phenobarbital and 3-methyl-cholanthrene altered the neurotoxic effects of MPTP. The rate of clearance (Vmax/Km) of N-oxygenation by hepatic microsomes from intact mice was 32 times greater than that of N-demethylation. Although phenobarbital treatment increased Vmax/Km of N-demethylation by 100%, it was still 10 times lower than that of N-oxygenation. Thus, the different responsivenesses of MPTP-treated mice to alternate substrates of FMO and cyt. P-450 modulators appear to come from the difference in the rate of metabolism between N-oxygenation and N-demethylation of MPTP.(ABSTRACT TRUNCATED AT 250 WORDS)     

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)     

Dopaminergic neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in the mouse: an in vivo electrochemical study

The long-term (i.e., 4-5 months) effects of large doses (3 X 50 mg/kg) of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on striatal dopamine-containing afferents were studied in the NMRI strain of mice. Recently improved in vivo electrochemical methods were first used to examine the magnitude, spatial distribution and temporal dynamics of monoamine release initiated via local application of potassium in various regions of the mouse striatum. Immunohistochemical localization of tyrosine hydroxylase and computer-based image analysis were also used to quantitate regional catecholamine-containing nerve fiber densities in the caudate nucleus. The in vivo electrochemical studies showed a statistically significant decrease in the average potassium-evoked release of electroactive species from the MPTP-treated mouse caudate nucleus vs. control. Greater decreases in release were seen in dorsal than in ventral striatum (55% vs. 33%). The average rise time of potassium-evoked release was also significantly prolonged (greater than 50%) after MPTP pretreatment. Histochemical studies showed an overall reduction in the density of dopamine-containing terminals in the drug-treated mice, with a greater loss observed in the more dorsal regions of the caudate nucleus. The experimental data thus support a long-term selective destruction of dorsal vs. ventral dopamine-containing afferents to the striatum by the neurotoxin MPTP in mice.     

Dopamine,but not norepinephrine or serotonin,reuptake inhibition reverses motor deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated primates

Monoamine reuptake inhibitors that do not discriminate between the transporters for dopamine (DA), norepinephrine (NE), or 5-hydroxytryptamine (5-HT, serotonin) can reverse locomotor deficits and motor disability in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated common marmosets. DA reuptake inhibition is presumed to be primarily responsible, but the role played by inhibition of NE and 5-HT reuptake is unknown. We now evaluate the efficacy of a range of monoamine reuptake inhibitors either alone or in combination in MPTP-treated common marmosets to determine the actions required for effective antiparkinsonian activity. Monoamine reuptake inhibitors not discriminating between the DA, NE, and 5-HT transporters [1-[1-(3,4-dichlororphenyl)cyclobutyl]-2-(3-diaminethylaminopropylthio)ethanone monocitrate (BTS 74 398) and nomifensine] reversed locomotor deficits and motor disability in MPTP-treated marmosets but bupropion was without effect. The selective DA reuptake inhibitor 1-(2-(bis-(4-fluorophenyl)-methoxy)ethyl)-4-(3-phenylpropyl) piperazine) dihydrochloride (GBR 12909) also reversed these motor deficits. The relative efficacy of the compounds (BTS 74 398 > GBR 12909 > nomifensine > bupropion) paralleled their potency in inhibiting DA uptake in vitro and in vivo. In contrast, the selective NE reuptake inhibitor nisoxetine and the 5-HT reuptake inhibitor sertraline administered alone failed to improve motor function and tended to worsen the deficits. Coadministration of nisoxetine attenuated the improvement in motor deficits produced by GBR 12909. Coadministration of sertraline also abolished the reversal of motor deficits produced by GBR 12909. Coadministration of both sertraline and nisoxetine similarly abolished the improvement of motor deficits produced by GBR 12909. Molecules possessing potent DA reuptake inhibitory activity may be useful in the treatment of the motor symptoms of Parkinson's disease. In contrast, there seems to be no role for NE or 5-HT reuptake inhibitors, and they may impair antiparkinsonian activity mediated through dopaminergic mechanisms.     

A reactive metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine is formed in rat brain in vitro by type B monoamine oxidase

The formation of a reactive intermediate in the oxidative metabolism of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) that can covalently bind to monoamine oxidase or other cellular macromolecules has been postulated by several authors. We report here direct in vitro evidence that MPTP is converted by monoamine oxidase, predominantly type B, to a reactive metabolite, which binds irreversibly to proteins in rat brain. Rat brain homogenates were incubated at 37 degrees C with 1-[methyl-3H]MPTP and the perchloric acid precipitates were washed exhaustively with organic solvents and counted for radioactivity. The amount of recovered radioactivity was enzyme-related: it was time- and temperature-dependent and did not occur with preboiled tissue. This metabolic activity required oxygen; it was concentrated in the crude mitochondrial fraction and varied in different brain regions. Pargyline and deprenyl prevented the radioactivity binding, whereas clorgyline was less potent, indicating that monoamine oxidase, predominantly of type B, is the enzyme responsible for the production of the reactive metabolite. Glutathione and, to a lesser extent, cysteine and dithiothreitol, but not ascorbic acid, inhibited the irreversible protein binding, suggesting that sulfhydryl groups may react with the metabolite possibly leading to SH-conjugates. 1-Methyl-4-phenyl-2,3-dihydropyridinium increased the irreversible protein binding, indicating that the reactive metabolite of MPTP may not be identified as the dihydropyridinium compound. This chemically reactive intermediate might play a role in MPTP neurotoxicity.     

Effect of D1 and D2 agonists and antagonists on dyskinesia produced by L-dopa in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys.

A group of five cynomolgus monkeys was rendered parkinsonian by the toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and then treated daily with levodopa until all animals developed evident dyskinesia in the limbs after each dose. At this point, levodopa was replaced by selective D2 and D1 dopamine receptor agonists. All D2 agonists including quinpirole, (+)-4-propyl-9-hydroxynaphthoxazine, bromocriptine, terguride and (-)-3-(3-hydroxyphenyl)-N-n-propylpiperone reproduced the same dyskinesia, the intensity and duration of which was dose-dependent and paralleled the therapeutic effect. One D1 agonist, SFK-38393, was without effect (antiparkinsonian and dyskinetic) whereas another, CY-208243, induced antiparkinsonian at a low dose but also a dyskinetic effect at a higher dose. The effect of the selective agonists were antagonized completely by their corresponding antagonist but partially by the noncorresponding one. The effect of either D1 and D2 agonist was totally suppressed by the dopamine depleting agent alpha-methyl-p-tyrosine, but the effect was restored by a small subthreshold dose of the other (complementary) agonist. Our results thus indicate that dyskinesia cannot be ascribed solely to the D2 or the D1 receptor and that some cooperation between the two receptors appears necessary for their manifestation.