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

Intravitreal MPTP drives retinal ganglion cell loss with oral nicotinamide treatment providing robust neuroprotection

Neurodegenerative diseases have common underlying pathological mechanisms including progressive neuronal dysfunction, axonal and dendritic retraction, and mitochondrial dysfunction resulting in neuronal death. The retina is often affected in common neurodegenerative diseases such as Parkinson's and Alzheimer's disease. Studies have demonstrated that the retina in patients with Parkinson's disease undergoes changes that parallel the dysfunction in the brain. These changes classically include decreased levels of dopamine, accumulation of alpha-synuclein in the brain and retina, and death of dopaminergic nigral neurons and retinal amacrine cells leading to gross neuronal loss. Exploring this disease's retinal phenotype and vision-related symptoms is an important window for elucidating its pathophysiology and progression, and identifying novel ways to diagnose and treat Parkinson's disease. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is commonly used to model Parkinson's disease in animal models. MPTP is a neurotoxin converted to its toxic form by astrocytes, transported to neurons through the dopamine transporter, where it causes mitochondrial Complex I inhibition and neuron degeneration. Systemic administration of MPTP induces retinal changes in different animal models. In this study, we assessed the effects of MPTP on the retina directly via intravitreal injection in mice (5 mg/mL and 50 mg/mL to 7, 14 and 21 days post-injection). MPTP treatment induced the reduction of retinal ganglion cells-a sensitive neuron in the retina-at all time points investigated. This occurred without a concomitant loss of dopaminergic amacrine cells or neuroinflammation at any of the time points or concentrations tested. The observed neurodegeneration which initially affected retinal ganglion cells indicated that this method of MPTP administration could yield a fast and straightforward model of retinal ganglion cell neurodegeneration. To assess whether this model could be amenable to neuroprotection, mice were treated orally with nicotinamide (a nicotinamide adenine dinucleotide precursor) which has been demonstrated to be neuroprotective in several retinal ganglion cell injury models. Nicotinamide was strongly protective following intravitreal MPTP administration, further supporting intravitreal MPTP use as a model of retinal ganglion cell injury. As such, this model could be utilized for testing neuroprotective treatments in the context of Parkinson's disease and retinal ganglion cell injury.

Biological interactions and attenuation of MPTP-induced toxicity in Drosophila melanogaster by Trans-astaxanthin

Trans-astaxanthin (TA) is a carotenoid with amphipathic chemical structure found in yeast, and aquatic organisms. It is known to possess both antioxidative and anti-inflammatory properties. This study was carried out to investigate the ameliorative action of TA on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced toxicity in Drosophila melanogaster (Fruit fly). The flies were orally treated with TA (2.5 mg/10 g diet) and/or MPTP (500 µM) for 5 days. Thereafter, we evaluated selected biomarkers of locomotor deficits (acetylcholinesterase (AChE) and negative geotaxis), oxidative stress (hydrogen peroxide (H2O2), protein carbonyls (PC)), antioxidants (total thiols (T-SH), non-protein thiols, glutathione-S-transferase (GST) and catalase), and inflammation (nitric oxide (nitrite/nitrate) in the flies. Furthermore, we investigated molecular docking analysis of TA against Kelch-like ECH-associated protein 1 (Keap1)) of Homo sapiens and D. melanogaster. The results indicated that TA increased MPTP-induced decreased activities of AChE, GST, and catalase, as well as levels of non-protein thiols and T-SH compared with MPTP-treated flies (p < 0.05). Furthermore, TA attenuated inflammation, and improved locomotor deficit in the flies. The molecular docking data showed that TA had docking scores for binding both the Human and Drosophila Keap1, nearly closer to or higher than the standard inhibitor. The attenuating effects of TA against MPTP-induced toxicity could arise from its antioxidative and anti-inflammatory properties as well as its chemical structure.

Parkinsonism Treatment: Part III—Update

OBJECTIVE:

The purpose of this review is to update clinicians with recent advances in the management of parkinsonism, including drug therapy, transplantation, and diet.

DATA SOURCES:

Pertinent articles were obtained from an English-language literature search using MEDLINE (1970–1991), Index Medicus (1987–1991), Current Contents (1990), and bibliographic reviews of review articles. Index terms included parkinsonism, selegiline, pergolide, vitamin E, and transplantation. Fifty-five articles (representing 85 percent of the complete literature search) were selected by multiple reviewers for their contribution to the stated purpose. Emphasis was placed on double-blind, placebo-controlled, and randomized studies. Data from cited articles were examined by multiple reviewers for support of their stated hypothesis and were included as background for justification of major points in this article; critical studies were abstracted in more detail.

RESULTS:

New therapeutic measures have been added to the treatment of parkinsonism. Selegiline, a monoamine oxidase inhibitor type B, has shown beneficial results, especially in early stages. Pergolide, a dopamine agonist, may be an efficacious alternative to bromocriptine resistance or intolerable adverse effects. Vitamin E may have protective antioxidant properties, but very few clinical data are available. Fetal tissue transplantation needs continued research and remains very controversial. Diet modification may maximize the results of therapy with exogenous dopamine therapy.

CONCLUSIONS:

Clinicians should familiarize themselves with new alternatives for the management of parkinsonism in order to be reliable consultants for both professional and lay persons.

2'-NH(2)-MPTP [1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine] depletes serotonin and norepinephrine in rats: a comparison with 2'-CH(3)-MPTP [1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine]

The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) analog, 1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH(2)-MPTP), depletes brain serotonin and norepinephrine in mice without affecting striatal dopamine. The present study was conducted to determine whether 2'-NH(2)-MPTP would be similarly neurotoxic to rats. Four injections of 20 mg/kg 2'-NH(2)-MPTP caused 80 to 90% depletions in serotonin and norepinephrine in frontal cortex and hippocampus in rats 1 week post-treatment. A lower dose of 2'-NH(2)-MPTP (4 x 15 mg/kg) also produced large decrements in serotonin and norepinephrine levels and in serotonin transporter density measured 3 weeks after neurotoxin administration. Furthermore, this lower dose of 2'-NH(2)-MPTP altered functional serotonin neurotransmission as evidenced by a 2-fold potentiation of 1-(3-chlorophenyl)-piperazine.2HCl-induced hyperthermia, an index of serotonergic denervation supersensitivity. At both doses, 2'-NH(2)-MPTP was without effect on striatal dopamine. For comparison, additional rats were treated with a second 2'-substituted analog of MPTP, 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'-CH(3)-MPTP), at 2 x 20 mg/kg. This dosing regimen causes substantial striatal dopamine depletion in mice. 2'-CH(3)-MPTP had no effect on brain levels of serotonin, norepinephrine, or dopamine in rats. Together, these results demonstrate that rats are sensitive to the toxic effects of 2'-NH(2)-MPTP but not to 2'-CH(3)-MPTP at doses known to cause neurotoxicity in mice. Moreover, this study clearly shows that 2'-NH(2)-MPTP can be utilized in rats as a tool to study the serotonergic and noradrenergic neurotransmitter systems.

Clorgyline and deprenyl attenuate striatal malonate and 3-nitropropionic acid lesions

We have previously shown that dopamine depletion reduces striatal damage elicited by the mitochondrial neurotoxins malonate and 3-nitropropionic acid (3NP). Metabolism of dopamine by monoamine oxidase results in the formation of hydrogen peroxide, which may mediate dopamine toxicity. In this study, administration of the monoamine oxidase inhibitors clorgyline and deprenyl resulted in a 42% and 75% reduction in lesion volumes in malonate- and 3NP-treated animals, respectively, compared to controls.

6-Hydroxydopamine injections into the nigrostriatal pathway attenuate striatal malonate and 3-nitropropionic acid lesions

The mitochondrial inhibitors malonate and 3-nitropropionic (3NP) acid are potent neurotoxins in vivo. Administration of these compounds results in neuronal loss similar to that seen in Huntington's disease. Although the mechanism of cell death produced by these compounds likely involves activation of N-methyl-D-aspartate receptors, it remains unclear why the striatum demonstrates regional susceptibility to the toxicity of these and other mitochondrial poisons. We hypothesized that dopamine, a weak neurotoxin that occurs in high concentrations in the striatum, may contribute to the neuronal damage caused by mitochondrial inhibition. We investigated whether depletion of striatal dopamine using the catecholaminergic toxin 6-hydroxydopamine would attenuate lesions induced by mitochondrial inhibition. We found that dopamine depletion reduced significantly the extent of histological damage in the striatum elicited by both intraparenchymal injections of 0.8 micromol malonate and 20 mg/kg systemic administration of 3NP. These data suggest that dopamine or one of its metabolites may contribute to mitochondrial toxin-induced cell death.

Deprenyl in the treatment of Parkinson's disease: clinical effects and speculations on mechanism of action

Selegiline is a relatively selective inhibitor of monoamine oxidase type B that has been used in Parkinson's disease as an adjunct to levodopa and as putative neuroprotective therapy. Clinical trials demonstrate that selegiline slows the rate of disease progression and delays the appearance of disability necessitating levodopa. However, confounding symptomatic effects have made it difficult to ascertain the presence of any direct neuroprotective effect. Laboratory studies demonstrate that selegiline protects dopaminergic neurons through a mechanism that does not involve MAO-B inhibition. Recent studies suggest that neuroprotection in laboratory models may be related to the capacity of selegiline to up-regulate a series of anti-oxidant and anti-apoptotic molecules which promote cell survival. Further delineation of the precise mechanism whereby selegiline induces this effect may permit for the development of enhanced neuroprotective benefits in PD patients.

Differences in the disposition and toxicity of 1-methyl-4-phenylpyridinium in cultured rat and mouse astrocytes

Species difference in the susceptibility to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was investigated in cultured rat and mouse astrocytes, where 1-methyl-4-phenylpyridinium (MPP+), the toxic mediator of MPTP, is formed. Type A monoamine oxidase (MAO) predominated in both rat and mouse astrocytes, while its activity was slightly higher in mouse cells compared to rat cells; MAO-B activity, on the other hand, was significantly lower in mouse astrocytes than in rat astrocytes. Because both types of MAO have been reported to make similar contributions to MPP+ production in astrocytes, their total activity was examined and results indicated that there was no significant difference between these two species. In addition, MPP+ caused a dose-dependent loss of cell viability as judged by the amount of lactate dehydrogenase released into the incubation medium. The toxicity of MPP+ on astrocytes started to be seen after a 2 day incubation period. Mouse astrocytes were more vulnerable to MPP+ than rat astrocytes. The threshold values for MPP+ toxicity in mouse and rat cultures were 10 microM and 70 microM, respectively. After addition of [3H] MPP+ to the medium, intracellular [3H] MPP+ was found to increase in both cultures. Mouse astrocytes accumulated more MPP+ than rat astrocytes (150 pmol/mg protein vs. 65 pmol/mg protein). When astrocytes were allowed to accumulate [3H] MPP+ and then incubated in fresh medium not containing [3H] MPP+, intracellular levels of [3H] MPP+ in both cells rapidly declined (110 pmol/protein in mouse vs. 40 pmol/mg protein in rat of MPP+ been released).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a selective MAO-A inhibitor (Ro 41-1049) on striatal L-dopa and dopamine metabolism: an in vivo study

We administered Ro 41-1049, an inhibitor of the enzyme monoamine oxidase type A (MAO-A) to rats and monitored extracellular catecholamine levels in the corpus striatum before and after the intraperitoneal (IP) administration of a bolus of L-dopa. Acute administration of Ro 41-1049 (1-50 mg/kg IP) produced a dose-dependent decrease in basal levels of the dopamine metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) and an increase in basal levels of dopamine. In rats treated with Ro 41-1049 (20 mg/kg IP), L-dopa administration (100 mg/kg IP) produced a greater increase in striatal levels of dopamine than it did in controls, while DOPAC and HVA formation was attenuated. We conclude that inhibition of central MAO-A activity promotes synaptic accumulation of dopamine following administration of pharmacological doses of L-dopa.

A scientific rationale for protective therapy in Parkinson's disease

The desire to introduce neuroprotective therapy for Parkinson's disease has begun to focus attention on pathogenetic mechanisms responsible for cell death. Considerable theory and some evidence have now accumulated to suggest that factors related to oxidative stress, mitochondrial bioenergetic defects, excitatory neurotoxicity, calcium cytotoxicity, and trophic factor deficiencies acting either singularly or in combination may contribute to the development of cell death in Parkinson's disease. A better understanding of the specific pathogenetic mechanism involved in cell degeneration might provide a scientific basis for testing a putative neuroprotective therapy. This chapter reviews the theory and evidence in support of these different mechanisms and possible strategies that might provide neuroprotection and interfere with the natural progression of Parkinson's disease.

Sustained depletion of cortical and hippocampal serotonin and norepinephrine but not striatal dopamine by 1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH2-MPTP): a comparative study with 2'-CH3-MPTP and MPTP

Unlike 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which produces consistent decreases in levels of striatal dopamine (DA) with considerably smaller and more variable effects on mouse brain levels of serotonin (5-HT) and norepinephrine (NE), a novel amine-substituted MPTP analogue, 1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH2-MPTP), administered in a standard mouse dosing paradigm for MPTP (20 mg/kg x 4) did not affect striatal DA but led to marked reductions (60-70%) in levels of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), and NE measured in frontal cortex and hippocampus 1 week after treatment. Another 2'-substituted MPTP analogue, 1-methyl-4-(2'-methylphenyl)-1,2,3,6- tetrahydropyridine, affected cortical and hippocampal 5-HT, 5-HIAA, and NE only minimally, while markedly reducing the DA content in striatum (90%), thus indicating that the substituent (-NH2 versus -CH3) at the 2' position is important for the differential effects of these MPTP analogues. In a replication study with a 3-week end point, hippocampal and cortical 5-HT, 5-HIAA,, and NE levels remained depressed with no indication of recovery. These results suggest that 2'-NH2-MPTP may be a novel, regionally selective neurotoxin for serotonergic and noradrenergic nerve terminals.

A rationale for monoamine oxidase inhibition as neuroprotective therapy for Parkinson's disease

Neurons in the substantia nigra may be vulnerable to oxidant stress because (a) the metabolism of dopamine generates peroxides, which, in the presence of iron, can lead to the formation of the highly reactive hydroxyl free radical; and (b) neuromelanin within nigral neurons can bind metals such as iron and aluminum and thereby promote the site-specific formation of free radicals. Postmortem studies show increased iron, decreased glutathione, and increased lipid peroxidation in the substantia nigra of patients with Parkinson's disease (PD). Recent studies also report iron and aluminum accumulation within neuromelanin granules of patients with PD. These findings suggest that the substantia nigra in the patient with PD is in a state of oxidant stress and that antioxidant therapy might protect residual dopamine neurons and slow the natural progression of PD. Selective inhibitors of monoamine oxidase type B (MAO-B) have been chosen for study because of their capacity to interfere with the oxidative metabolism of dopamine and so diminish the likelihood that free radicals will be formed. Initial studies demonstrate that the MAO-B inhibitor L-deprenyl (selegiline) delays the development of disability in otherwise untreated patients with early Parkinson's disease. Although the mechanism responsible for these observations remains unclear, these results are consistent with the possibility that L-deprenyl provides neuroprotective effects.

Spontaneous recovery of MPTP-damaged catecholamine systems in goldfish brain areas

In goldfish, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administered for 3 consecutive days (10 mg/kg), produced a marked decrease in dopamine (DA) and noradrenaline (NA) levels in telencephalon, diencephalon and medulla oblongata, without affecting the serotonin (5-HT) content. Furthermore the neurotoxin decreased either [3H]DA high affinity uptake or K(+)-stimulated DA release from synaptosomal (P2) preparations, with concomitant up-regulation of D2 postsynaptic receptors as well. No significant changes of choline acetyltransferase and glutamic acid decarboxylase activity or [3H]glutamate uptake were observed. Moreover the pretreatment with deprenyl (1 mg/kg) or mazindol (10 mg/kg) but not with clorgyline (5 mg/kg) prevented catecholamine depletion. Added in vitro to synaptosomal preparations both MPTP and more potently MPP+, in a concentration-dependent manner, inhibited [3H]DA uptake. Time course study revealed that MPTP-induced alteration of neurochemical parameters in goldfish brain areas were almost completely reversed within 6 weeks, suggesting that catecholamine systems in goldfish brain show a remarkable power of recovery after MPTP lesion.

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)

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.

1-Methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'-methyl-MPTP) is less neurotoxic than MPTP in the common marmoset

Four adult marmosets were treated with increasing doses of 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'-methyl-MPTP) in the range 0.23-4.3 mg/kg i.p. to give a cumulative dose of 11.0-11.6 mg/kg over a 6-10 day period. After 4 days of treatment, and as the dosage was gradually increased, the animals exhibited mild motor deficits. These abnormalities slowly declined over the following 1-6 week period. In contrast, similar treatment of common marmosets with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (1-4 mg/kg i.p.) for 3-5 days in a cumulative dose of 6.9-9.2 mg/kg produced gross impairment of motor function which persisted throughout the 5 weeks period of observation. Administration of 2'-methyl-MPTP for 6-10 days caused some decrease in dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC), but not homovanillic acid (HVA) content in the caudate nucleus in animals 5-6 weeks after the start of treatment. There was a small decrease in [3H]dopamine uptake into putamen synaptosomes. This contrasted with the marked decreases in all these parameters observed after MPTP treatment of common marmosets. Histological examination of the substantia nigra from the four animals treated with 2'-methyl-MPTP did not show degeneration or loss of dopamine-containing cell bodies in the zona compacta. In contrast, MPTP caused severe destruction of these pigmented nigral neurones. In the common marmoset 2'-methyl-MPTP does not appear to show the same neurotoxic action as MPTP itself. This contrasts with findings in the mouse where 2'-methyl-MPTP is more toxic to dopamine-containing cells of substantia nigra than MPTP.

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

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

Monoamine oxidase and oxidative stress at dopaminergic synapses

Increased oxidation of dopamine by monoamine oxidase (MAO) in the striatum is associated with an oxidant stress, expressed as a rise in the level of oxidized glutathione. Oxidation of glutathione is suppressed by MAO inhibitors, such as deprenyl and clorgyline. These observations related to Parkinson's disease and to the clinical trial of deprenyl as an agent that may retard progression of the disease.

Deprenyl suppresses the oxidant stress associated with increased dopamine turnover

Tissue glutathione disulfide (GSSG) was studied as an index of changes in redox state in the striatum. When increased turnover of dopamine was provoked in mice by injection of haloperidol (1 mg/kg), the concentration of GSSG in the striatum tripled. Deprenyl (2.5 mg/kg) suppressed the rise in GSSG by 71.9%. These results indicate that deprenyl suppresses an oxidant stress associated with increased dopamine turnover.

Mesencephalic dopamine neurons become less sensitive to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity during development in vitro

The in vitro development of monoamine oxidase (MAO) activity and [3H]dopamine (DA) uptake capacity of dissociated cell cultures from rat embryo mesencephalon were correlated with the potency of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridine (MPP+) neurotoxicity. Specific activities of both MAO-A and MAO-B increased during in vitro development of the cultures, with MAO-B activity increasing 20-fold between the first and fourth week. Similarly, [3H]DA accumulation increased 2.6-fold between the first and third week in vitro, when it reached a plateau. Unexpectedly, the toxicities of MPTP and MPP+ were substantially decreased in the older cultures. Exposure to MPTP reduced [3H]DA accumulation per culture by 77% in 1-week-old cultures and by 36% in 4-week-old cultures. Similarly, damage caused by MPPT was reduced from 84% of control in the first week to 34% of control in the fourth week. The attenuation of neurotoxicity was not due to an increase in storage of MPP+ in the synaptic vesicles of DA neurons, nor to a change in the distribution of MPP+ between dopaminergic and other cellular components of the cultures. The damage to DA neurons caused by the mitochondrial toxin, rotenone, also showed a similar reduction in the older cultures. These observations coupled with an increase in lactate formation and glucose consumption during the in vitro development of the cultures suggest a shift toward increased glycolysis and decreased dependence on aerobic metabolism. This would render the cells more resistant to the inhibition of mitochondrial function by MPP+.

Dopamine turnover and glutathione oxidation: implications for Parkinson disease

Parkinson disease is characterized by a major loss (approximately 80% or more) of dopaminergic nigrostriatal neurons and by an increased turnover of neurotransmitter by surviving neurons of the nigrostriatal tract. In theory, increased turnover of dopamine should be associated with an oxidative stress derived from increased production of hydrogen peroxide. The peroxide is formed during the oxidative deamination of dopamine by monoamine oxidase. In experiments with mice, increased presynaptic turnover of dopamine was evoked by injection of reserpine, which interferes with the storage of dopamine in synaptic vesicles. Loss of dopamine and formation of deaminated metabolites were accompanied by a significant rise (87.8%) in the level of oxidized glutathione in brain. This change was observed in the striatum, which is richly innervated by dopamine terminals, but not in the frontal cortex, which receives a much sparser innervation by catecholamine nerve terminals. The rise in oxidized glutathione was seen even though dopamine terminals constitute only 1% or less of the mass of the striatum. Clorgyline, an inhibitor of monoamine oxidase type A, blocked the formation of oxidized glutathione. These observations confirm that a selective increase in neurotransmitter turnover within nigrostriatal nerve terminals can evoke a change in cellular redox status. We suggest that an oxidative stress may play a role in the natural history of Parkinson disease.

Oxidation of N-methyl-1,2,3,4-tetrahydroisoquinoline into the N-methyl-isoquinolinium ion by monoamine oxidase

N-Methyl-1,2,3,4-tetrahydroisoquinoline (NMTIQ) was found to be oxidized by monoamine oxidase (MAO) into N-methylisoquinolinium ion, which was proved to inhibit enzymes related to the metabolism of catecholamines, such as tyrosine hydroxylase, aromatic-L-amino acid decarboxylase, and MAO. NMTIQ was oxidized by both types A and B MAO in human brain synaptosomal mitochondria. Oxidation was dependent on the amount of MAO sample and the reaction time. Enzyme activity with respect to NMTIQ reached optimum at a pH of approximately 7.25, as was the case with other substrates. Type A MAO had higher activity for this substrate than type B. The Km and Vmax values of the oxidation by types A and B MAO were 571 +/- 25 microM and 0.29 +/- 0.06 pmol/min/mg protein, and 463 +/- 43 microM and 0.16 +/- 0.03 pmol/min/mg protein, respectively. The Vmax values of types A and B MAO for NMTIQ were much smaller than those for other substrates such as kynuramine. NMTIQ was the first tetrahydroisoquinoline shown to be oxidized into the isoquinolinium ion by MAO in the brain.