Flexible N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine analogues: synthesis and monoamine oxidase catalyzed bioactivation

Eighteen analogues of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were synthesized and evaluated as substrates of monoamine oxidase. In general, the flexible analogues, characterized by the presence of a methylene (or ethylene) bridge between the aryl/heteroaryl and tetrahydropyridyl moieties, were better substrates of the enzyme than the conformationally restricted MPTP. It is suggested that the increased oxidative activity of these flexible analogues reflects enhanced binding due to the ability of the C-4-aryl/heteroaryl substituent to gain access to a hydrophobic pocket within the substrate binding site.     

Catecholamine-uptake inhibitors prevent the neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mouse brain

Male albino mice were injected subcutaneously with 50 mg/kg of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Seven days after administration, the striatal levels of dopamine(DA) and its metabolites were markedly reduced, as was the cortical level of noradrenaline (NA). Pretreatment with the selective inhibitor of DA uptake, GBR 13098, selectively and totally prevented the reduction of DA and its metabolites, whereas the selective inhibitor of NA uptake, maprotiline, selectively protected against NA depletion. The unselective inhibitors of DA and NA uptake, mazindol and nomifensine, prevented the MPTP-induced depletion of both DA and NA.     

No apparent difference in the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on the sympathetic system in NMRI and C57 BL/6 mice

The parkinsonism-inducing neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) has been shown to possess marked species as well as strain differences in toxicity on central catecholaminergic systems. In the present study the effects on the peripheral sympathetic nervous system following treatment with MPTP, as well as its metabolite 1-methyl-4-phenylpyridine (MPP+) and the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA) were studied in mice of the NMRI and C57 BL/6 strains, two strains that possess marked difference in MPTP toxicity on central catecholaminergic neurons. No strain differences in the depletions of noradrenaline (NA) in iris and heart auricula and of NA and dopamine (DA) in superior cervical ganglion or in the reduction of the in vitro [3H]NA uptake in iris or heart auricula were found following MPTP treatment (2 X 40 mg/kg s.c., 2 and 7 days). Treatment with the NA uptake blocker desipramine (DMI) did not affect the MPTP-induced NA depletion in either strain. Following treatment with MPP+ (30 mg/kg i.v., 7 days) no differences in the two strains were seen on the reduction of NA levels in iris and heart auricula or decrease in [3H]NA uptake. In addition, no differences were found on NA levels in iris and heart auricula after 6-OHDA treatment (15 mg/kg i.v., 7 days). The data indicate that in the NMRI and C57 BL/6 mice peripheral NA neurons do not possess any notable strain difference in the vulnerability to MPTP or in the mechanism of action of MPTP.     

Effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on striatal dopamine receptors in C57BL/6 mice

The effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a neurotoxin which selectively destroys the nigrostriatal dopaminergic neurons and produces Parkinson's disease-like syndrome, on striatal dopamine receptors was determined in a mouse strain known to be very sensitive to the neurotoxic effect of MPTP. Daily intraperitoneal administration of MPTP (30 mg/kg) for 7 days to male C57BL/6 mice reduced the concentration of striatal dopamine by 90%. This decrease in dopamine concentration was not associated with changes either in the receptor density (B_max) or the apparent dissociation constant (K_d) of [³H]spiroperidol to bind to striatal dopamine receptors. It is concluded that in spite of large decrease in striatal dopamine concentration by MPTP the dopamine receptors labeled with [³H]spiroperidol remain intact.     

Reduction of dopamine uptake and cocaine binding in mouse striatum by N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Administration of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (NMPTP) (daily injections of 8 mg/kg for 5 days via tail vein) reduced [3H]dopamine uptake in striatal synaptosomes by 63% and reduced [3H]cocaine binding to striatal membranes by 61%. [3H]Cocaine binding was not affected in olfactory tubercle, suggesting a selective effect of NMPTP on the nigro-striatal but not on the mesolimbic dopaminergic system. The destruction of dopamine terminals in the striatum did not alter (up-regulate) [3H]spiroperidol binding. The results suggest that NMPTP causes a degenerative destruction of the striatal dopamine pathway and that NMPTP may be useful in developing a rodent model of Parkinson's disease.     

Toxication of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and analogs by monoamine oxidase. A structure-reactivity relationship study

MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) elicits motor deficits similar to those observed in Parkinson's disease. Before exerting its neurotoxic action, MPTP must be activated by brain monoamine oxidase (MAO) to the neurotoxic metabolite MPP+ (1-methyl-4-phenylpyridinium). MPTP derivatives differ in their reactivity as MAO substrates and in their neurotoxicity. A structure-reactivity relationship study based on literature data was undertaken in order to determine the key features in the structure of MPTP and analogs that are responsible for the reactivity towards MAO. Thirty-three MPTP derivatives (including MPTP itself) were included in the study. To explain the reactivity towards MAO of the 33 MPTP analogs, different statistical methods (principal component analysis, multiple linear regression analysis) as well as the CoMFA (Comparative Molecular Field Analysis) approach, a new tool in structure-activity correlations, were used. Linear regression analysis failed to yield any predictive model, but suggested some trends. In contrast, the CoMFA approach was successful in correlating structural features and MAO reactivity. Coefficient contour maps showed where differences in the steric field (van der Waals' interactions) are most highly associated with differences in MAO reactivity. Several positive (in the ortho- and meta-position of the phenyl group) and negative (in the para-position of the phenyl group; beyond the N-methyl group) interaction regions were identified. Some structural features of the MAO active site could be postulated. First, the N-methyl group has the ideal size and elicits ideal interactions within the MAO active pocket, while smaller or larger groups are less favorable; second, para-substituent on the phenyl ring produce steric hindrances and are unfavorable to reactivity; third, ortho- and meta-substituents may have stabilizing interactions within the active pocket and are favorable to the reactivity. Moreover the model derived by CoMFA allowed us to make successful predictions of reactivity towards MAO for several additional tetrahydropyridines.     

Effect of nicotine and amphetamine on the neurotoxicity of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice

The present results show the potentiating effect of amphetamine on the ability of MPTP to destroy dopaminergic neurons in striatum of the mouse. A single injection of MPTP (8 mg/kg, retro-orbital) reduced the binding of [3H]mazindol, a marker for dopamine terminals, by 24%. When D-amphetamine (10 mg/kg, s.c.) was given 20 min prior to MPTP, the binding of [3H]mazindol, measured 3-5 days later, was reduced by 58%. It is proposed that the mechanism of this potentiation primarily involves an increased release of dopamine by D-amphetamine, and free radical-mediated processes. Although nicotine also releases dopamine from the striatum, no effect was observed when it was administered prior to MPTP. The lack of effect is probably related to short duration of action of nicotine and the modest effect on release of dopamine as compared to that of amphetamine.     

Regional brain uptake of 2-deoxyglucose in N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism in the macaque monkey

This preliminary report describes application of the 2-deoxyglucose (2DG) autoradiographic technique to study regional changes in brain metabolism in experimental parkinsonism, induced in the monkey by administration of the neurotoxin MPTP. In one monkey, rendered severely parkinsonian by MPTP, there was a marked increase in the uptake of 2DG in the globus pallidus (both medial and lateral segments) and in the ventral anterior and ventral lateral thalamic nuclei, in comparison to non-parkinsonian animals. Increased uptake of 2DG in the globus pallidus may reflect increased activity of striatopallidal synapses secondary to loss of nigrostriatal dopaminergic neurones. The findings are in sharp contrast to our observations on regional brain metabolism in experimental choreiform dyskinesia in the monkey.     

Disposition of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice before and after monoamine oxidase and catecholamine reuptake inhibition

The distribution of radioactivity in pigmented mice after a single intravenous injection of 1-(3H) methyl-4-phenyl-1,2,3,6-tetrahydropyridine (3H-MPTP) was studied by quantitative whole body autoradiography and liquid scintillation counting. Pretreatment with the monoamine oxidase inhibitors clorgyline, pargyline and deprenyl, or the catecholamine reuptake inhibitor nomifensine was performed 30 min. prior to the 3H-MPTP administration. A high uptake of radioactivity was observed in the striatum, nucleus accumbens, midbrain area and locus coeruleus, and also in the adrenal medulla. This uptake was inhibited by deprenyl or pargyline pretreatment, but not after clorgyline or nomifensine pretreatment. An extensive uptake which was not influenced by deprenyl or pargyline treatment was found in the melanin-containing tissues of the eye. This accumulation is due to the melanin affinity of MPTP and its metabolites. A comparatively rapid elimination from the brain of MPTP and its metabolites was observed, which may be due to the lack of neuromelanin in mice.     

Neurotoxicity of the meperidine analogue N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on brain catecholamine neurons in the mouse

The effect of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (NMPTP) on central monoamine neurons in mice was investigated using histo- and biochemical techniques. NMPTP (2 X 10 mg/kg i.v.) produced a rapid and long-lasting reduction (-30%) of striatal dopamine, while the dopamine levels were only transiently reduced in mesencephalon and frontal cortex. HVA and DOPAC were initially markedly reduced (-50 to -70%) in striatum while a marked recovery was found in the chronic stage. NMPTP also induced a long-term reduction of noradrenaline in striatum and frontal cortex while 5-hydroxytryptamine and 5-HIAA levels were essentially unaltered. The data indicate a neurotoxic action of NMPTP on both dopamine and noradrenaline nerve terminals in mouse brain.     

Biochemical and immunohistological changes in the brain of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mouse.

We investigated neurochemically and neuropathologically the utility of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice as a model of Parkinson's disease. The changes in dopamine D1 and D2 receptors and dopamine uptake sites were determined by quantitative autoradiography using [3H]SCH23390, [3H]raclopride and [3H]mazindol, respectively. Dopamine and 3,4-dihydroxyphenyl acetic acid (DOPAC) contents in the striatum were measured by high-performance liquid chromatography. The distribution of nigral neurons and reactive astrocytes was determined by immunohistochemical staining with antibody against tyrosine hydroxylase (TH) and glial fibrillary acidic protein (GFAP). The mice received four intraperitoneal injections of MPTP (10 mg/kg) at 1-h intervals and then the brains were analyzed at 3 and 7 days after the treatments. No significant change in dopamine D1 receptors was observed in the striatum and substantia nigra after acute treatment with MPTP. Dopamine D2 receptors were reduced significantly in the substantia nigra only 7 days after the MPTP treatment, whereas striatum showed no significant change in the binding throughout the experiments. In contrast, dopamine uptake sites were reduced markedly in the striatum and substantia nigra 3 and 7 days after the MPTP treatment. Dopamine and DOPAC content were also reduced in the striatum 3 and 7 days after the MPTP treatment. An immunohistochemical study indicated a loss of the number of TH-positive neurons in the substantia nigra 7 days after the MPTP treatment. In contrast, numerous GFAP-positive astrocytes were evident in the striatum 7 days after the MPTP treatment. These results provide valuable information for the pathogenesis of acute stage of Parkinson's disease.     

The effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on striatal and limbic catecholamine neurones in white and black mice. Antagonism by monoamine oxidase inhibitors

Albino mice and pigmented mice were treated for 6 days with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) at the maximum tolerated doses (2 days at 30 mg/kg i.p., 2 days at 40 mg/kg i.p. and 2 days at 50 mg/kg i.p. in white mice, 6 days at 30 mg/kg i.p. in pigmented mice) and the effects of simultaneous treatment with the monoamine oxidase inhibitors, deprenyl (1 mg/kg, i.p.), MDL 72145 (0.5 mg/kg, i.p.) and clorgyline (5 mg/kg, i.p.), determined behaviourally (daily for 6 days and for 4 days after withdrawal) and biochemically (92 hr after withdrawal of drug). In albino mice MPTP caused depletions of dopamine (90%), dihydroxyphenylacetic acid (DOPAC; 82%) and homovanillic acid (HVA; 65%) in the striatum and in dopamine (54%), DOPAC (51%) and HVA (53%) in the nigra. However, MPTP was not selective in its action since the levels of dopamine and its metabolites were also reduced in limbic tissue. Further, MPTP affected the function of noradrenaline, with reduced levels in tissues of the striatum (74%) and nigra (46%). Pigmented mice were as susceptible as albino mice to the actions of MPTP to reduce the levels of dopamine and metabolites in the striatum. However, the limbic areas and substantia nigra of the pigmented mouse were more resistant to the actions of MPTP. Treatment with deprenyl and MDL 72145 (but not clorgyline) could be shown to reduce the biochemical and behavioural consequences of the action of MPTP (although behavioural changes, development of severe motor incapacitation and prostrate appearance, appeared to be non-specific).(ABSTRACT TRUNCATED AT 250 WORDS)     

Mutagenicity of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and its metabolites

The by-product from a "synthetic heroin" is 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a chemical contaminant found to produce neurotoxicity similar to Parkinsonism in susceptible animals. MPTP and its oxidative metabolites were tested in the Salmonella mutagenicity test. Strains of Salmonella typhimurium that carry a nonsense mutation at the site of reversion detect a variety of naturally occurring and direct-acting mutagens. TA 100 is reverted by MPDP+ (1-methyl-4-phenyl-2,3-dihydropyridinium species). This strain is more sensitive to MPDP+ mutagenesis than any other available strain of Salmonella and MPDP+ is considerably more mutagenic than MPTP and other oxidative metabolites including MPTP N-oxide or MPP+ (1-methyl-4-phenylpyridinium ion). Mutagenicity studies of metabolic incubates of MPTP with monoamine oxidase (MAO) suggest the involvement of metabolic bioactivation in the mutagenicity of MPTP. Since MPDP+ is generated from MPTP by MAO and since iminium ions are generated during cellular metabolism, they may make a contribution to the risk of human cancer.     

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

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

Inhibition of human benzylamine oxidase (BzAO) by analogues of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

A sensitive assay for human plasma BzAO, involving the conversion of 14C-benzylamine to 14C-benzaldehyde, was developed. MPTP and several of its analogues were found to be competitive inhibitors of the enzyme. Ki values for the MPTP analogues in the presence of human plasma BzAO were determined. The analogues had a different rank order of inhibition of human plasma BzAO compared with the rank order of inhibition of bovine plasma BzAO found previously. MPTP and 1-methyl-4-(2-methylphenyl)-1,2,3,6-tetrahydropyridine (2'-CH3-MPTP), which are potent nigrostriatal toxins, were weak inhibitors of human plasma BzAO.     

Oxidation by rat brain of N-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) coupled to tetrazolium nitro blue

The present study shows that the dehydrogenation of N-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) can be coupled to tetrazolium nitro blue (TNB), a substance used as electron acceptor in many dehydrogenase reactions. The MPTP-TNB reductase system has an apparent Km value and inhibitor sensitivity similar to those of monoamine oxidase (MAO) type B. MPTP oxidation in the brain may very well be mediated by MAO-B.     

Comparison of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium (MPP+) effects on mouse heart norepinephrine

MPP+ (1-methyl-4-phenylpyridinium) mimicked MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in producing marked, dose-related depletion of cardiac norepinephrine after a single oral or subcutaneous dose in mice. MPP+ was approximately 4-fold more potent than MPTP in depleting norepinephrine, but the onset of depletion was not faster for MPP+ than for MPTP. The time courses of the effects of both compounds were similar to that for 6-hydroxydopamine, with maximum depletion occurring at 1 day, partial recovery at 2 and 4 days, and full recovery of norepinephrine concentrations at 1 week. Desipramine, over a dose range that completely prevented the depletion of cardiac norepinephrine by 6-hydroxydopamine at 24 hr, did not prevent cardiac norepinephrine depletion by either MPP+ or MPTP. In a short duration experiment, one or two doses of desipramine also failed to prevent heart norepinephrine depletion by MPP+ or by MPTP, although a slight antagonism was found. EXP 561 (4-phenylbicyclo[2,2,2]octan-1-amine hydrochloride monohydrate), another uptake inhibitor with possibly longer duration of action, also did not protect against norepinephrine depletion by a single dose of MPP+ or MPTP at a dose that prevented norepinephrine depletion by 6-hydroxydopamine. In mice given four daily doses of MPTP, EXP 561 prevented the depletion of norepinephrine in the frontal cortex and of dopamine in the striatum but not the depletion of norepinephrine in heart or spleen. Thus, both MPTP and MPP+ deplete norepinephrine in mouse heart, and this effect of the two compounds is resistant to antagonism by uptake inhibitors that antagonize the effects of MPTP on brain catecholamines.