(+)MK-801 prevents the DDC-induced enhancement of MPTP toxicity in mice

In order to reach deeper insight into the mechanism of diethyldithiocarbamate (DDC)-induced enhancement of MPTP toxicity in mice, MK-801, a non-competitive antagonist of NMDA receptors, has been used as a tool to study the role of excitatory amino acids. In agreement with previous reports, (+)MK-801 did not significantly affect either striatal dopamine (DA) or tyrosine-hydroxylase (TH) activity in MPTP-treated animals. On the contrary (+)MK-801, but not (-)MK-801 significantly reduced the DDC + MPTP-induced fall in striatal DA and TH activity. A similar preventing effect on DA metabolites (DOPAC and HVA) and HVA/DA ratio was observed. The number of TH+ neurons in the substantia nigra (SN) of (+)MK-801-pretreated mice was not significantly different from that of control animals, indicating that this treatment specifically antagonized the extensive DDC-induced lesion of dopaminergic cell bodies in this brain area. (+)MK-801 treatment did not affect the DDC-induced changes of striatal MPP+ levels, suggesting that the observed antagonism of MK-801 against DDC is not due to MPP+ kinetic modifications. Pretreatment with the MAO-B inhibitor, L-deprenyl, or with the DA uptake blocker, GBR 12909, completely prevented the marked DA depletion elicited by DDC + MPTP within the striatum. Both treatments also protected from the fall in DA metabolites and TH activity as well. This indicates that DDC-induced potentiation is dependent upon MPP+ production and its uptake by the dopaminergic nerve terminals. All these findings suggest that NMDA receptors play a crucial role in the DDC-induced enhancement of MPTP toxicity.

Locus coeruleus lesions potentiate neurotoxic effects of MPTP in dopaminergic neurons of the substantia nigra

The observation that Parkinson's disease (PD) is associated with locus coeruleus (LC) noradrenergic neuronal degeneration suggests that the LC noradrenergic system may be involved in the pathogenesis and natural progression of the destruction of the substantia nigra (SN) dopaminergic neurons in Parkinson's disease. The relationship of these two systems was examined by injection of subtoxic doses of MPTP into unilateral LC 6-hydroxydopamine (6-OHDA) lesioned mice. A significant loss of dopaminergic cells was only found in the SN on the side of the LC lesions. These results suggest that the LC may have protective effects on SN dopaminergic neurons.

Reduction of MPP(+)-induced hydroxyl radical formation and nigrostriatal MPTP toxicity by inhibiting nitric oxide synthase

N-Methyl, 4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces experimental parkinsonism after oxidation to N-methylpyridinium ion (MPP+), accumulation in dopamine neurons and concentration in mitochondria. Inhibition by MPP+ of mitochondrial electron transport impairs respiratory function, but the molecular mechanisms of cell death are not clear. We tested the hypothesis that locally produced nitric oxide is a key component in MPTP toxicity by providing a necessary intermediate in the production of hydroxyl free radicals. Inhibition of nitric oxide synthase reduced MPP(+)-induced hydroxyl radical formation in striatum and MPTP toxicity to nigrostriatal dopamine terminals, but did not interfere with inhibition of complex-I activity. Nitric oxide appears to be necessary for hydroxyl free radical generation in MPP+ toxicity and may play a role in neuronal degeneration in Parkinson's disease.

Protection against methamphetamine-induced neurotoxicity to neostriatal dopaminergic neurons by adenosine receptor activation

Methamphetamine (METH)-induced neurotoxicity to nigrostriatal dopaminergic neurons in experimental animals appears to have a glutamatergic component because blockade of N-methyl-D-aspartate receptors prevents the neuropathologic consequences. Because adenosine affords neuroprotection against various forms of glutamate-mediated neuronal damage, the present studies were performed to investigate whether adenosine plays a protective role in METH-induced toxicity. METH-induced decrements in neostriatal dopamine content and tyrosine hydroxylase activity in mice were potentiated by concurrent treatment with caffeine, a nonselective adenosine antagonist that blocks both A1 and A2 adenosine receptors. In contrast, chronic treatment of mice with caffeine through their drinking water for 4 weeks, which increased the number of adenosine A1 receptors in the neostriatum and frontal cortex, followed by drug washout, prevented the neurochemical changes produced by the treatment of mice with METH treatment. In contrast, this treatment did not prevent 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine-induced dopaminergic neurotoxicity. Furthermore, concurrent administration of cyclopentyladenosine, an adenosine A1 receptor agonist, attenuated the METH-induced neurochemical changes. This protection by cyclopentyladenosine was blocked by cyclopentyltheophylline, an A1 receptor antagonist. These results indicate that activation of A1 receptors can protect against METH-induced neurotoxicity in mice.

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)

In vivo generation of hydroxyl radicals and MPTP-induced dopaminergic toxicity in the basal ganglia

The in vivo generation of .OH free radicals in specific brain regions can be measured by intracerebral microdialysis perfusion of salicylate, avoiding many of the pitfalls inherent in systemic administration of salicylate. Direct infusion of salicylate into the brain can minimize the hepatic hydroxylation of salicylate and its contribution to brain levels of 2,5-DHBA. Levels of 2,5-DHBA detected in the brain dialysate may reflect the .OH adduct plus some enzymatic hydroxylation of salicylate in the brain. After minimizing the contribution of enzyme and/or blood-borne 2,5-DHBA, the present data demonstrate the validity of the use of 2,3-DHBA and apparently 2,5-DHBA as indices of .OH formation in the brain. Therefore, intracranial microdialysis of salicylic acid and measurement of 2,3-DHBA appears to be a useful .OH trapping procedure for monitoring the time course of .OH generation in the extracellular fluid of the brain. These results indicate that nonenzymatic and/or enzymatic oxidation of the dopamine released by MPTP analogues in the extracellular fluid may play a key role in the generation of .OH free radicals in the iron-rich basal ganglia. Moreover, a site-specific generation of cytotoxic .OH free radicals and quinone/semiquinone radicals in the striatum may cause the observed lipid peroxidation, calcium overload, and retrograde degeneration of nigrostriatal neurons. This free-radical-induced nigral injury can be suppressed by antioxidants (i.e., U-78517F, DMSO, and deprenyl) and possibly hypothermia as well. In the future, this in vivo detection of .OH generation may be useful in answering some of the fundamental questions concerning the relevance of oxidants and antioxidants in neurodegenerative disorders during aging. It could also pave the way for the research and development of novel neuroprotective antioxidants and strategies for the early or preventive treatment of neurodegenerative disorders, such as Parkinson's disease (Wu et al., this issue), amyotrophic lateral sclerosis, head trauma, and possibly Alzheimer's cognitive dysfunction as well. In conclusion, this in vivo free-radical trapping procedure provides evidence to support a current working hypothesis that a site-specific formation of cytotoxic .OH free radicals in the basal ganglia may be one of the neurotoxic mechanisms underlying nigrostriatal degeneration and Parkinsonism caused by the dopaminergic neurotoxin MPTP. Addendum added in proof: The controversy concerning possible neurotoxic and/or neuroprotective roles of NO. in cell cultures was discussed and debated at the symposium (Wink et al., this issue; Dawson et al., this issue; Lipton et al., this issue).(ABSTRACT TRUNCATED AT 400 WORDS)

Chronic MPTP treatment reproduces in baboons the differential vulnerability of mesencephalic dopaminergic neurons observed in Parkinson's disease

Chronic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to baboons was shown previously to result in a motor syndrome and a pattern of striatal dopaminergic fibre loss similar to those observed in idiopathic Parkinson's disease. In the present study, tyrosine hydroxylase-immunoreactive neurons were quantified in the mesencephalon of control (n = 4) and chronically MPTP-treated (n = 3) baboons. MPTP induced a significant reduction in neuronal cell density in the substantia nigra (63.8% reduction) and the ventral tegmental area (53.1%). Within the substantia nigra, obvious mediolateral and dorsoventral gradients of neuronal cell loss were observed. First, the pars lateralis was more affected than the lateral divisions of the pars compacta (89.6% vs 73.8% cell loss), which in turn were more depleted than the medial divisions (60.1% reduction). Second, the ventral regions of the pars compacta were more degenerated than the dorsal parts (82.4 vs 51.5% decrease). This regional pattern is strikingly similar to that observed in Parkinson's disease and indicates that two subpopulations of dopaminergic neurons are distinguishable on the basis of their differential vulnerability to MPTP. Finally, the present study confirms that chronic mitochondrial complex I inhibition using MPTP in primates is sufficient to reproduce the typical dopaminergic cell loss and striatal fibre depletion observed in Parkinson's disease.

Antiparkinsonian activity of talipexole in MPTP-treated monkeys: in combination with L-dopa and as chronic treatment

We examined whether or not the antiparkinsonian activity of talipexole (B-HT 920, 6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]-azepine) could be optimised by combination with L-3,4-dihydroxyphenylalanine (L-dopa). Additionally, the effects of chronic treatment with talipexole on motor behavior were investigated using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated and normal common marmosets. Administration of MPTP (0.5 mg/animal i.v. once or twice) to marmosets induced persistent parkinsonian motor deficits. The antiparkinsonian activity of talipexole (40 micrograms/kg s.c.) was significantly enhanced by its combination with L-dopa (30 mg/kg i.p.). This may further support the postulated postsynaptic dopamine D2 receptor agonist properties of talipexole. Chronic treatment with talipexole (a daily dose of 40 micrograms/kg s.c. for 21 days) did not lead to tolerance to the antiparkinsonian activity in MPTP-treated animals. No obvious dyskinesia was seen throughout the chronic treatment. In contrast, in normal marmosets, talipexole at a dose of 80 micrograms/kg which is a dose sufficient to induce hyperactivity did not increase motor activity during the treatment repeated for 21 days. These results suggest that talipexole is a selective dopamine D2 receptor agonist drug of potential use in the treatment of Parkinson's disease.

Studies on species sensitivity to the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Part 1: Systemic administration

Several parameters necessary for the expression of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced neurotoxicity to dopaminergic neurons were examined in both mice and rats in order to determine if differences in these processes might underlie the marked differences in the sensitivity of the two species to the neurotoxic effects of MPTP. Monoamine oxidase-B activity was greater in brain tissues from rats than from mice. The kinetics of 1-methyl-4-phenylpyridinium (MPP+) uptake into neostriatal synaptosomal preparations from the two species were similar. Brain and neostriatal levels of MPP+ were 2-fold higher in rats after the administration of MPTP at 60 mg/kg and were 10 to 20 times higher in rats than in mice after MPTP treatment which produced similar decrements in the content of neostriatal dopamine. MPP+ concentrations in the extracellular fluid of the neostriatum of the two species were similar after the administration of the same dose of MPTP (40 mg/kg). However, this dose induced a 40-fold increase in neostriatal dopamine efflux in mice, whereas in rats only a 3-fold increase was observed. In addition, pretreatment of rats with guanethidine, a ganglionic blocking agent, permitted the use of high doses of MPTP which resulted in substantial damage to the striatal dopaminergic nerve terminals. It is concluded that nigrostriatal dopaminergic neurons in the rat require exposure to a much higher concentration of MPP+ than do those in mice for the induction of toxicity.

Studies on species sensitivity to the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Part 2: Central administration of 1-methyl-4-phenylpyridinium

There are marked species differences in susceptibility to the neurotoxic effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Mice are sensitive, whereas rats are relatively insensitive to MPTP. In these two species, the effects of peripherally administered MPTP or intrastriatally infused 1-methyl-4-phenylpyridinium (MPP+) were examined to identify potential underlying mechanisms responsible for their difference in susceptibility to MPTP. In vivo intrastriatal microdialysis and an MPP+ 2-day test/challenge paradigm were used to monitor dopamine efflux as an indicator of the neurotoxic effects of MPTP or MPP+. By using this method, the EC50 for neurotoxicity by an intrastriatal infusion of MPP+ in mice was 0.4 mM, whereas it was 10-fold higher in rats (4.3 mM). In addition, by using the traditional postmortem examination, neostriatal dopamine was depleted markedly in mice (> or = 80%), but only depleted marginally in rats in which MPP+ was infused into the neostriatum. These data indicate that rats are relatively insensitive to MPTP as compared to mice, because they are less sensitive to MPP+ whether it is formed in vivo from MPTP administered systemically or administered directly into neostriata. Thus, there appears to be a fundamental difference in the susceptibility of the nigrostriatal systems in these two species to the neurotoxic consequences of MPP+ exposure.

MPTP-induced oxidative stress and neurotoxicity are age-dependent: evidence from measures of reactive oxygen species and striatal dopamine levels

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes marked depletion of dopamine (DA) levels and reduction in the activity of tyrosine hydroxylase (TH) in the nigrostriatal DA pathway. In the brain, the enzyme monoamine oxidase B converts MPTP to 1-methyl-4-phenylpyridinium (MPP+) which enters DA terminals via DA uptake sites. Within the DA terminals, MPP+ blocks the mitochondrial complex I and causes ATP depletion. This is thought to be the main cause of MPTP-induced terminal degeneration. In addition, reactive oxygen species (ROS) generated after blockade of the complex I as well as those generated due to DA oxidation may participate in MPTP-induced dopaminotoxicity. The present study sought to determine if a single injection of a large dose of MPTP generates ROS. We also sought to determine if these changes as well as changes in DA levels were correlated and age-dependent. Toward that end, we have used C57/B6N male mice that were 22 days or 12 months old. These animals were injected with a single dose of MPTP (40 mg/kg, ip). Animals were sacrificed at various times after drug administration. MPTP produced no significant increase in ROS nor decreases in DA or HVA concentrations in the striatum of the younger mice. However, DOPAC concentrations were significantly decreased from 15-120 min after drug administration. In the older mice, MPTP caused significant increases in ROS from the beginning to the end of the study period. DA concentrations were decreased from 60 min onward. DOPAC concentrations were decreased significantly after 15-120 min while HVA concentrations were significantly increased after 60 and 120 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation by D2 dopamine receptors of in vivo dopamine synthesis in striata of rats and mice with experimental parkinsonism

The effects of D2 dopamine (DA) receptor antagonism or stimulation by systemic haloperidol or quinpirole, respectively, on in vivo DA synthesis in 6-hydroxydopamine (6-OHDA)-lesioned rats and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated black mice were assessed by measuring the rate of dihydroxyphenylalanine (DOPA) accumulation following acute inhibition of L-aromatic amino acid decarboxylase with NSD-1015. 6-OHDA and MPTP caused partial lesions of nigrostriatal input to the striatum. Dopamine synthetic capacity was preserved relative to the severity of nigrostriatal lesion over a broad range of DA depletions. An exponential increase in fractional DA synthesis (the ratio DOPA/DA) was observed with increasing DA depletion, suggesting an elevation of the DA synthetic capacity per surviving DA terminal. In both lesioned rats and mice, haloperidol caused a significant increase in fractional DA synthesis above that induced by the lesion alone, while quinpirole significantly depressed fractional DA synthesis. Our results provide evidence that nigrostriatal terminals acquire increased DA synthetic capacity as nigrostriatal lesions exceed 90%, but that the increase in fractional DA synthesis observed in partially lesioned animals is not due to a loss of autoreceptor function. Pharmacological strategies to stimulate DA synthesis and release in moderately advanced Parkinson's disease should be pursued.

Enhanced MPTP neurotoxicity after treatment with isoflurophate or cholinergic agonists

Administration of the irreversible cholinesterase inhibitor isoflurophate (diisopropylfluorophosphate, DFP) before 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) enhanced the loss in tyrosine hydroxylase activity and dopamine and 3,4-dihydroxy-phenylacetic acid content in the striatum of mice in a dose-dependent manner. The effect of DFP on the MPTP-induced changes of dopaminergic markers was evident 30 days after initiating treatment, suggesting augmented neurotoxicity. Neurotoxicity was also enhanced by prior treatment with nicotine, carbachol or oxotremorine. We conclude that activation of either muscarinic or nicotinic receptors enhances the neurotoxicity of MPTP.

MPTP-induced parkinsonism in primates: pattern of striatal dopamine loss following acute and chronic administration

We analyzed the effect of two different schedules of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment on dopaminergic systems in the striatum of cynomolgus monkeys. Acute MPTP treatment produced a marked dopamine (DA) depletion, more severe in the caudate nucleus than in the putamen. Chronic MPTP induced a more pronounced reduction in DA levels, the putamen being slightly more affected than the caudate nucleus, in accord with immunohistochemical findings that showed a higher loss of tyrosine-hydroxylase positive neurons in ventral subpopulations of the substantia nigra pars compacta. A striking increment in the quotient DOPAC+HVA/DA was also observed in chronically but not in acutely treated monkeys, especially in the putamen. In chronically treated animals there was a nearly complete loss of DA in all subdivisions of the putamen. In the caudate nucleus, a rostrocaudal gradient of DA depletion was found, with a greater decrease in DA concentration in the rostral parts, especially in the dorsolateral portions. The pattern of striatal DA loss characteristic of Parkinson's disease can be reproduced to a certain extent in MPTP-intoxicated primates.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) affects the actin cytoskeleton and calcium level of Swiss 3T3 mouse fibroblasts

Organization of the actin cytoskeleton, the cytosolic free-calcium concentrations and ATP levels were analyzed in 3T3 mouse fibroblasts treated with 0.75 or 1.5 mM MPTP. In the presence of the drug actin filaments were time- and dose-dependently disorganized, ATP level was unaffected and intracellular calcium increased within 5 s. The correlation between MPTP cytotoxicity and [Ca2+]i level emerging from these results, suggests that the primary effect of the molecule itself is on the plasma membrane's integrity for calcium ion regulation.

MRI detects acute degeneration of the nigrostriatal dopamine system after MPTP exposure in hemiparkinsonian monkeys

Exposure to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) can cause an acute chemical toxicity resulting in a parkinsonian state in humans and nonhuman primates. We wished to assess whether the toxicity from MPTP is associated with changes on magnetic resonance images of brain structures containing dopamine neuronal processes or with disrupture of the blood-brain barrier. Normal rhesus monkeys and monkeys at various times after being subjected to unilateral intracarotid injection of MPTP (0.4 mg/kg) were studied with magnetic resonance imaging using T1- and T2-weighted spin-echo and gradient-echo sequences. Disrupture of the blood-brain barrier was assessed also with magnetic resonance imaging after administration of gadolinium-diethylenetriamine pentaacetic acid. Parkinsonian symptoms contralateral to the infused carotid usually appeared within 1 day after MPTP exposure, reaching their peak severity by 7 days, when all monkeys showed clear clinical abnormalities. Magnetic resonance imaging changes developed in concomitance with the clinical signs and were characterized by increased signal intensity on T2-weighted images as well as decreased intensity on T1-weighted images of the ipsilateral caudate and putamen. T2 hyperintensity was also present just dorsal to the pars compacta of the substantia nigra, in the region of the proximal nigrostriatal tract. All magnetic resonance imaging changes dissipated in the next 2 weeks. There were no abnormalities at any time in the globus pallidus, nucleus accumbens, and other structures innervated by the mesocorticolimbic dopamine system. After MPTP exposure, there was no evidence of blood-brain barrier disrupture, suggesting that vasogenic edema was an unlikely factor in the production of the observed abnormalities.(ABSTRACT TRUNCATED AT 250 WORDS)

Quinolinic acid but not MK-801 protects the dopaminergic system from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced toxicity in goldfish retina

The toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, intravitreally injected in goldfish eye, involves interplexiform retinal neurons and depletes tyrosine hydroxylase immunoreactivity and dopamine levels. This induced neurotoxicity was prevented by the concomitant administration in non-toxic doses (10 micrograms) of quinolinic acid, an endogenous structural analogue of N-methyl D-aspartate with excitotoxic properties. Quinolinic acid is ineffective on the retinal degeneration induced by 1-methyl-4-phenylpyridinium ion. This fact suggests that quinolinic acid inhibits the MAO-B oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. MK-801, a noncompetitive antagonist of glutamate NMDA-receptors, exerts partial protective effects on MPTP-induced delayed toxicity in mammals. In the goldfish eye, MK-801, injected in low concentration, and in conjunction with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine or 1-methyl-4-phenylpyridinium ion, did not prevent retinal neurodegeneration. Ten micrograms of MK-801 alone did not affect retinal neurons, while a higher concentration (20 micrograms) causes the chromatolysis of some photoreceptor nuclei.

Mitochondrial abnormalities in nigral neurons of the marmoset and Macaca fascicularis treated with MPTP

In a previous study we observed the processes determining the cellular death in nigral neurons of Macaca fascicularis and common Marmoset treated with MPTP. The purpose of this study is to consider, in substantia nigra neurons of the same animals, mitochondrial abnormalities caused by neurotoxin. Three Macaca fascicularis and 5 common Marmosets, respectively treated with 3 and 4 mg/kg of MPTP dissolved in saline + 10% ethanol, were suppressed by means of an i.v. injection phenobarbital. The substantia nigra, isolated from the mesencephalon, has been examined under Electron Microscope. Different mitochondrial abnormalities have been observed in nigral neurons: crystal abnormalities occur more frequently than other alterations. They are correlated with various degrees of mitochondrial enlargement. Dissolution of the matrix induces formation of spherical highly electron-dense inclusions of various volume and in different number; they are probably lipoprotein. Laminar bodies and myelin-like figures are determined by alterations of the external membrane. Particular roundish organules, placed near normal mitochondria, are limited by fragments of membrane with a granular material, occasionally flowing in the cytoplasm. The biochemical hypothesis concerning the genesis of the observed abnormalities is discussed in the present study.

MPTP-induced behavioural and biochemical deficits: a parametric analysis

Two experiments were performed to study the parametric effects of long-term administration of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), as a functional model of parkinsonism in mice. The behavioural deficits induced by different doses of MPTP (5, 10, 20, 30 or 40 mg/kg, s.c., each injected on two occasions) at a 3-week or a 3-month treatment-testing interval were evidenced by significant reductions of spontaneous motor activity, from the 10 mg/kg dosages upwards at the 3-week interval and from 30-40 mg/kg at the 3-month interval. Significant dopamine (DA) reductions in the mouse striatum were obtained at these dose levels and intervals. The behavioural deficit of the 40 mg/kg dose (injected on two occasions) and tested at the 3-, 6-, 12-, 24- and 40-week intervals (separate as well as repeated testing groups) indicated marked and relatively comparable reductions of all three parameters of motor activity, locomotion, rearing and total activity. DA depletions were severe at all five test intervals. These results offer functional and neurochemical evidence that MPTP treatment produces permanent damage to the nigrostriatal motor system in mice.

Differences in the reserpine-sensitive storage in vivo of 1-methyl-4-phenylpyridinium in rats and mice may explain differences in catecholamine toxicity to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Administration of reserpine, an inhibitor of vesicular catecholamine storage, differentially reduced the accumulation of MPP+ formed from MPTP in rats and mice. The effects were most pronounced in the adrenal gland for either species. In rats, reserpine decreased striatal and hippocampal MPP+ levels while in mice reserpine did not affect the disposition of MPP+ in the striatum but decreased hippocampal MPP+. The data suggest that mice may be more sensitive to the toxicant because less striatal MPP+ appears to be stored in the reserpine-sensitive storage vesicle.

Medium latency stretch reflexes in young-onset Parkinson's disease and MPTP-induced parkinsonism

It is still unclear why components of the phasic stretch reflex are increased in Parkinson's disease (PD). To study the role of aging, we assessed medium latency (ML) stretch reflexes in 19 Parkinson patients (8 with young-onset PD, 11 with late-onset PD) and 23 normal subjects (10 young, 13 old). To assess the contribution of supraspinal dopaminergic influences, we also studied 5 young parkinsonian patients with a selective central dopamine deficiency induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). ML responses were recorded from the medial gastrocnemius muscle which was stretched by 4 degrees toe-up rotations of a platform upon which subjects were standing. ML responses were significantly enhanced in late-onset patients compared with older controls. In contrast, ML responses did not differ between young-onset patients and young controls. This observation could be attributed to the significantly different influence of age on ML-amplitudes in patients and controls. Thus, ML-amplitudes declined with age in controls, whereas they increased with age in PD possibly because older patients were more severely affected than younger patients. Thus, the difference between young-onset and late-onset PD seems to be related to the opposite effects of increasing age and disease severity in patients and controls. ML-amplitudes did not differ between patients with MPTP-induced parkinsonism, patients with young-onset PD and controls, suggesting that supraspinal dopaminergic systems are not critically involved in control of ML responses. This result also indicates that the severity effect in PD may be related to disruption of non-dopaminergic pathways which occurs in late-onset, but not young-onset patients.(ABSTRACT TRUNCATED AT 250 WORDS)

A study of the biological pharmacology of IFO, a new selective and reversible monoamine oxidase-B inhibitor

3-[4-[3-(1H-Imidazol-1-yl)propoxy]phenyl]-5-trifluoromethyl-1,2,4- oxadiazole (IFO), designed to be a novel selective inhibitor of monoamine oxidase (MAO), showed highly selective inhibition for type-B (MAO-B); its IC50 was approximately > 200 microM and 30 nM for type-A (MAO-A) and MAO-B, respectively, in the standard assay using mitochondrial preparations from rat brain or liver. The in vitro MAO-B inhibition by IFO was time-independent, non-competitive and tight-binding; and furthermore, in the presence of sodium cholate its inhibition was not tight-binding and was reversible. Oral administration of IFO (0.5-100 mg/kg) produced a dose-dependent MAO-B inhibition in mouse brain; its ED50 (p.o., 1 hr) was 1.6 mg/kg, while L-deprenyl inhibited the enzyme with the ED50 of approximately 8.0 mg/kg. The ED50 for MAO-A was > 100 mg/kg for either IFO and L-deprenyl. The MAO inhibitive effect of IFO in mouse liver was the same as that in the brain, but that of L-deprenyl in mouse liver was different from that in the brain as shown by the ED50 values of 35 mg/kg and 0.6 mg/kg for MAO-A and MAO-B, respectively. In mice, IFO increased the striatal concentrations of 2-phenylethylamine (2-PEA) and showed almost the same protective efficacy as L-deprenyl against the lethality and striatal dopamine (DA) depletion induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). These results indicate that IFO appears to be a potent inhibitor of MAO-B in mouse brain.