Rapid ATP Loss Caused by 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine in Mouse Brain

The effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on ATP levels in different areas of mouse brain were studied after rapid fixation of cerebral tissue in situ by microwave irradiation. ATP levels in the striatum, ventral mesencephalon, and cerebellum of untreated C57BL/6 mice killed by microwave irradiation were 2-3 times greater than values measured in the brains of animals killed by cervical dislocation. In microwaved mice, administration of MPTP (40 mg/kg s.c.) caused a 10-20% decrease in ATP concentrations as compared to control animals injected with saline. This decrease was relatively rapid and selective because it occurred in both the striatum and ventral mesencephalon, but not in the cerebellar and frontal cortex, at 30, 60, 120, and 240 min after MPTP exposure. Furthermore, ATP loss in the striatum was prevented by mazindol, a catecholamine uptake blocker, indicating a rather selective effect of MPTP on the ATP content of dopaminergic terminals. Results of this study are consistent with mitochondrial damage in the MPTP model of parkinsonism and provide the first direct experimental evidence in vivo that a decrease in ATP may play a role in MPTP-induced neurotoxicity.     

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

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

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

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

Accumulation of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine in Cultured Cerebellar Astrocytes

Cultured cerebellar astrocytes rapidly accumulate 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) from the incubation medium, reaching a plateau within 10 min, whereas within that time negligible amounts of 1-methyl-4-phenylpyridinium (MPP+) have entered the astrocytes. MPTP accumulation is essentially independent of temperature and is proportional to extracellular concentration at steady state: The steady-state concentration achieved within these cells is about 50-fold higher at relatively low extracellular concentrations. MPTP appears to accumulate intracellularly within lysosomes, because lysosomotropic agents such as ammonium chloride and chloroquine markedly diminish the accumulation. Moreover, a proton gradient is required, because MPTP accumulation is abolished by the hydrogen ion antiporter monensin. Over an interval of several days, MPTP is converted to MPP+ intracellularly, with a concomitant decrease in medium MPTP and increase in medium MPP+. A constant, small but significant amount of MPP+ is retained intracellularly over a 72-h interval. Increasing the medium MPTP concentrations results in increased conversion of MPTP and enhanced intracellular retention of MPTP and MPP+. Neither MPTP nor MPP+ is neurotoxic to cultured cerebellar astrocytes as determined by cell counts and rate of conversion of MPTP to MPP+.     

Neurochemical and Histochemical Characterization of Neurotoxic Effects of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine on Brain Catecholamine Neurones in the Mouse

Systemic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) caused a rapid and long-lasting reduction of both 3,4-dihydroxyphenylalanine (dopamine, DA) and noradrenaline (NA) in mouse brain, as observed histo- and neurochemically. The depleting effects were more pronounced after repeated MPTP administration and the most marked reductions were observed after 2 X 50 mg MPTP/kg s.c., when DA in striatum and NA in frontal cortex were reduced by greater than 90% 1 week after MPTP. Mice with such catecholamine depletions were markedly sedated and almost completely immobilized. The behavioural syndrome after MPTP resembled that seen after reserpine, a monoamine-depleting drug. MPTP also caused a long-lasting reduction of catecholamine uptake in striatal DA and cortical NA nerve terminals and reduced tyrosine hydroxylase activity in these regions. There was no evidence that MPTP caused any marked DA and NA cell body death. MPTP given acutely transiently elevated serotonin levels. The results are compatible with a neurotoxic action of MPTP on both DA and NA nerve terminals. The nigro-striatal DA and the locus coeruleus NA neurone systems appeared to be most susceptible. Synthesis and utilization of residual striatal DA and cortical NA were increased, as often observed in partially denervated monoamine-innervated brain regions. Both DA and NA showed a gradual recovery, which took months to become complete and may have been related to a regrowth of catecholamine nerve terminals.     

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

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

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

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

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

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

1 -Methyl-4-Phenyl- 1,2,3,6-Tetrahydropyridine: Correspondence of Its Binding Sites to Monoamine Oxidase in Rat Brain, and Inhibition of Dopamine Oxidative Deamination In Vivo and In Vitro

A saturable, specific, high-affinity binding site for [3H]1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine was found in rat brain homogenates. The CNS regional distribution, the subcellular fractionation, and the displacement by pargyline, clorgyline, and deprenyl suggest that this binding site may correspond to monoamine oxidase. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine inhibited the oxidative deamination of dopamine, both in vivo and in vitro. Striatal levels of 3,4-dihydroxyphenylacetic acid were significantly reduced shortly after intravenous administration, and returned to normal values after a few hours. The in vitro formation of 3,4-dihydroxyphenylacetic acid from dopamine was inhibited by concentrations of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine comparable to those of pargyline.     

1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine- and 1-Methyl-4-(2''-Ethylphenyl)-1,2,3,6-Tetrahydropyridine-Induced Toxicity in PC 12 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.     

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

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

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

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

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

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

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.     

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

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

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

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

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

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

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

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

Oxidation of Analogs of l-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine by Monoamine Oxidases A and B and the Inhibition of Monoamine Oxidases by the Oxidation Products

Intraterminal free Ca2+ concentration modulates the subsequent release of neurotransmitters. Depolarization of synaptosomes with 29 mM K+ augments cytosolic free Ca2+ concentration, which is triphasic, the peak times being at 10, 60, and 180 s. We examined the characteristics of each elevation of cytosolic free Ca2+ concentration in rat brain synaptosomes which had been preincubated for 3 min with a Ca2+-channel blocker, such as La3+, diltiazem, nifedipine, or verapamil, and under conditions of hypoxia or acidosis. The concentration of free Ca2+ in the quin-2-loaded rat brain synaptosomes was detected fluorometrically. All these elevations were suppressed in the presence of 200 microM EGTA or 100 microM La3+. At the first phase, the elevation of cytosolic free Ca2+ concentration with high K+ stimuli was significantly inhibited by La3+ (20 microM) or by acidosis (pH 6.7). On the other hand, diltiazem, which is a more potent blocker of the release of Ca2+ from the mitochondria, inhibited the increasing cytosolic free Ca2+ concentration at the third phase in a concentration-dependent manner. Hypoxia also showed inhibition at the third phase. These results suggest that the augmentation of high K+-evoked cytosolic free Ca2+ concentration may be due to the influx of extracellular Ca2+. The increase in cytosolic free Ca2+ concentration at the third phase is no doubt linked to the mitochondrial function.     

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

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