Reversal of experimental parkinsonism by using selective chemical ablation of the medial globus pallidus

OBJECT

Symptoms from Parkinson's disease improve after surgical ablation of the medial globus pallidus (GPm). Although, in theory, selective chemical ablation of neurons in the GPm could preserve vital structures jeopardized by surgery, the potential of this approach is limited when using traditional techniques of drug delivery. The authors examined the feasibility of convection-enhanced distribution of a neurotoxin by high-flow microinfusion to ablate the neurons of the GPm selectively and reverse experimental Parkinson's disease (akinesia, tremor, and rigidity).

METHODS

Initially, to test the feasibility of this approach, the GPms of two naive rhesus macaques were infused with kainic acid or ibotenic acid through two cannulas that had been placed using the magnetic resonance imaging-guided stereotactic technique. Two weeks later the animals were killed and their brains were examined histologically to determine the presence of neurons in the GPm and the integrity of the optic tract and the internal capsule. To examine the therapeutic potential of this paradigm, unilateral experimental Parkinson's disease was induced in six macaques by intracarotid infusion of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and their behavior was studied for 12 weeks after chemopallidotomy was performed using kainic acid (three animals) or control infusion (three animals).

CONCLUSIONS

Chemopallidotomy using kainic acid permanently reversed the stigmata of MPTP-induced parkinsonism. By contrast, the control animals exhibited a transient recovery following intrapallidal infusion and then relapsed back to their baseline state. The use of high-flow microinfusion of selectively active toxins has the potential for treatment of Parkinson's disease and, by expanding the range of approachable targets to include large nuclei, for broad applications in clinical and experimental neuroscience.

Effect of the dopaminergic neurotoxin MPTP on cocaine-induced locomotor sensitization

The blockade of dopamine (DA) uptake via the dopamine transporter (DAT) in the nucleus accumbens (NAC) and striatum by cocaine has a major role in the reinforcing and psychomotor stimulating effects of the drug. Here we investigated the effect of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on the expression and induction of sensitization to the locomotor stimulating effect of cocaine. MPTP (20 mg/kg x 4) caused 72 and 76% depletion of DAT sites in the NAC and striatum, respectively, in C57BL/6 mice. The magnitude of this depletion 3 and 19 days after MPTP administration was the same. To determine the effect of MPTP on the expression of the sensitized response to cocaine, cocaine-experienced mice (20 mg/kg for 5 days) received MPTP 3 days before a challenge cocaine injection was given on day 15. Cocaine/MPTP mice were significantly more sensitive to the challenge cocaine injection than the cocaine/saline-pretreated mice. To determine whether depletion of NAC and striatal DAT affects the induction of sensitization to cocaine, mice were pretreated with MPTP 3 days before the administration of cocaine (20 mg/kg for 5 days). The magnitude of the sensitized response of MPTP/cocaine-pretreated mice to cocaine challenge was the same as the sensitized response of mice treated with saline/cocaine, while the number of DAT binding sites in the MPTP/cocaine group was significantly lower than the saline/cocaine group. The present study indicates that MPTP exacerbates the expression of locomotor sensitization to cocaine, but it had no effect on the induction of sensitization. We conclude that the expression, but not the induction, of locomotor sensitization to cocaine may be dependent on the level of DAT binding sites.

Creatine and cyclocreatine attenuate MPTP neurotoxicity

Systemic administration of 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) produces parkinsonism in experimental animals by a mechanism involving impaired energy production. MPTP is converted by monoamine oxidase B to 1-methyl-4-phenylpyridinium (MPP+), which blocks complex I of the electron transport chain. Oral supplementation with creatine or cyclocreatine, which are substrates for creatine kinase, may increase phosphocreatine (PCr) or cyclophosphocreatine (PCCr) and buffer against ATP depletion and thereby exert neuroprotective effects. In the present study we found that oral supplementation with either creatine or cyclocreatine produced significant protection against MPTP-induced dopamine depletions in mice. Creatine protected against MPTP-induced loss of Nissl and tyrosine hydroxylase immunostained neurons in the substantia nigra. Creatine and cyclocreatine had no effects on the conversion of MPTP to MPP+ in vivo. These results further implicate metabolic dysfunction in MPTP neurotoxicity and suggest a novel therapeutic approach, which may have applicability for Parkinson's disease.

Novel free radical spin traps protect against malonate and MPTP neurotoxicity

Both malonate and 1-methyl-4-phenyl-1,2,5,6 tetrahydropyridine (MPTP) are neurotoxins which cause energy depletion, secondary excitotoxicity, and free radical generation. Malonate is a reversible inhibitor of succinate dehydrogenase, while MPTP is metabolized to 1-methyl-4-phenylpyridinium, an inhibitor of mitochondrial complex I. We examined the effects of pretreatment with the cyclic nitrone free radical spin trap MDL 101,002 on malonate and MPTP neurotoxicity. MDL 101,002 produced dose-dependent neuroprotection against malonate-induced striatal lesions. MDL 101, 002 produced significant protection against MPTP induced depletions of dopamine and its metabolites. MDL 101,002 also significantly attenuated MPTP-induced increases in striatal 3-nitrotyrosine concentrations. The free radical spin trap tempol also produced significant protection against MPTP neurotoxicity. These findings provide further evidence that free radical spin traps produce neuroprotective effects in vivo and suggest that they may be useful in the treatment of neurodegenerative diseases.

Comparison of MPTP-induced changes in spontaneous neuronal discharge in the internal pallidal segment and in the substantia nigra pars reticulata in primates

The basal ganglia are currently viewed as components of segregated corticosubcortical reentrant circuits. One of these circuits, the "motor" circuit, is critically involved in the development of parkinsonian motor signs. Current pathophysiologic models postulate that parkinsonism is associated with increased activity in the basal ganglia output nuclei. The neuronal activity in the motor portion of one of these output nuclei, the internal segment of the globus pallidus (GPi), has been characterized in detail in intact and parkinsonian animals, but the neuronal activity in the second major basal ganglia output nucleus, the substantia nigra pars reticulata (SNr), has received far less attention. This study in primates represents a comparison of the effects of parkinsonism, induced by injections of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), on the neuronal discharge in the GPi and SNr. These electrophysiologic recording experiments were carried out in three African green and two rhesus monkeys. One hundred and twenty-four neurons were recorded in the GPi before treatment with MPTP, and 93 neurons thereafter. In the SNr, 55 cells were recorded before treatment with MPTP, and 41 cells thereafter. MPTP induced a non-significant increase in the average discharge rate and a significant decrease in the median interspike interval length (ISI) in the GPi (by 13%), whereas no changes were detected in either parameter in the SNr. The average ISI distributions were markedly asymmetric in both structures, and could be modeled by a logarithmic normal distribution. With the MPTP treatment, the mode of the ISI distribution fell by 24% in the GPi (P< or =0.01), whereas it did not change significantly in the SNr. An algorithm that detects burst discharges in the raw ISI data (based on the method by Legendy and Salcman) detected a significant increase in the proportion of action potentials that participated in bursts of discharge in both structures (increase by 257% in the GPi, and by 67% in the SNr). Power spectral and autocorrelation analysis revealed that treatment with MPTP increased the proportion of cells with oscillatory burst patterns at 3-8 Hz in both structures (from 0.8% to 27% of all neurons in the GPi, and from none to 10% in the SNr). The results show that neuronal discharge in the SNr is affected in parkinsonism, but that the changes in the SNr are less pronounced then those seen in the GPi.

Recovery of chronic parkinsonian monkeys by autotransplants of carotid body cell aggregates into putamen

We have studied the effect of unilateral autografts of carotid body cell aggregates into the putamen of MPTP-treated monkeys with chronic parkinsonism. Two to four weeks after transplantation, the monkeys initiated a progressive recovery of mobility with reduction of tremor and bradykinesia and restoration of fine motor abilities on the contralateral side. Apomorphine injections induced rotations toward the side of the transplant. Functional recovery was accompanied by the survival of tyrosine hydroxylase-positive (TH-positive) grafted glomus cells. A high density of TH-immunoreactive fibers was seen reinnervating broad regions of the ipsilateral putamen and caudate nucleus. The nongrafted, contralateral striatum remained deafferented. Intrastriatal autografting of carotid body tissue is a feasible technique with beneficial effects on parkinsonian monkeys; thus, this therapeutic approach could also be applied to treat patients with Parkinson's disease.

Regional and temporal expression of the peripheral benzodiazepine receptor in MPTP neurotoxicity

We used the dopaminergic neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to evaluate the sensitivity and specificity of the peripheral benzodiazepine receptor (PBR) as a biomarker of chemical-induced neurotoxicity. Receptor autoradiography of [3H]-PK11195, a PBR selective ligand, indicated dose-dependent increases throughout the nigrostriatal dopaminergic system as early as 24 h after MPTP administration (10-80 mg/kg), which persisted for at least 21 days. The binding of [3H]-PK11195 was increased as much as 98% in the corpus striatum and 114% in the substantia nigra, following MPTP exposure. The integrity of nigrostriatal dopaminergic terminals in the corpus striatum was assessed by measuring high affinity dopamine transporter (DAT) levels and dopamine content. DAT levels were measured by [3H]-WIN 35,428 autoradiography, and dopamine content decreased with increasing MPTP dose. Reductions of both indices of dopaminergic terminal integrity correlated with increased levels of [3H]-PK11195-binding in the striatum (r2 = 0.84 for DAT and 0.93 for dopamine content). Tyrosine hydroxylase (TH) immunohistochemistry demonstrated dose-dependent reductions of dopaminergic neurons in the substantia nigra pars compacta, with a 67% loss measured 7 days after treatment with 80 mg/kg MPTP. The loss of TH-positive neurons was correlated (r2 = 0.95) with increased levels of [3H]-PK11195 binding in the substantia nigra. These findings demonstrate that the PBR is both sensitive and specific for identifying brain regions involved in MPTP neurotoxicity.

MPTP-induced Parkinsonism in minipigs: A behavioral, biochemical, and histological study

Fourteen male Göttingen minipigs were used in this study. Nine were administered N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) at a dosage of 1 mg/kg/day, SC, for 6 days, the last five pigs received saline injections for 6 days. All MPTP-treated animals developed Parkinson symptoms, i.e., muscle rigidity, hypokinesia, and impaired coordination within 5 days. The brain levels of dopamine (DA), and its major metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were determined in caudatum and putamen 2, 14, and 93 days (n = 3/time point) after the last drug administration. In eight of the MPTP-treated animals, striatal DA, DOPAC, and HVA concentrations were reduced from 50 to 95% compared to control animals at all time intervals. Animals with the lowest striatal DA concentrations showed the most severe signs of Parkinsonism. The number of cells in substantia nigra (SN) showed a decline only 3 months after MPTP treatment. The minipigs represent a nonprimate model of MPTP-induced parkinsonism syndromes lasting at least months.

Diethyldithiocarbamate causes nigral cell loss and dopamine depletion with nontoxic doses of MPTP

Although nontoxic when administered alone, diethyldithiocarbamate (DDC) is known to enhance the dopamine-depleting effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in the mouse striatum. The purpose of the present study was twofold: (i) to carefully characterize the effects of DDC on MPTP-induced degeneration of dopaminergic neurons in substantia nigra pars compacta using unbiased, stereological cell counting techniques and (ii) to determine whether or not DDC can convert a nontoxic dose of MPTP into one which is clearly toxic on dopaminergic neurons in the substantia nigra. A single low dose of MPTP (15 mg/kg intraperitoneally (ip)) was used for these studies, which failed to induce any neurochemical or histological effects on the nigrostriatal system of C57BL/6 mice when administered alone. However, when animals were pretreated with DDC (400 mg/kg ip), the same dose of MPTP resulted in a 50% loss of neurons in the substantia nigra pars compacta, as well as a 70% reduction in striatal dopamine (DA). A 31% reduction of DA in the ventral mesencephalon was also seen. This combined regimen of DDC and MPTP was not significantly different from a maximally tolerated "toxic" dose of MPTP alone (15 mg/kg x 4, 1 h apart, ip). As expected, animals receiving DDC alone did not show any dopamine depletion nor nigral neuronal loss. The present study confirms previous work suggesting that DDC enhances MPTP-induced nigral cell loss and shows for the first time that DDC can "unmask" MPTP toxicity. These observations could have implications for theories on the cause of Parkinson's disease.

Immunochemical analysis of vesicular monoamine transporter (VMAT2) protein in Parkinson's disease

The vesicular monoamine transporter (VMAT2) has been suggested to be an excellent marker of presynaptic dopaminergic nerve terminals in the striatum of Parkinson's disease patients based on its high level of expression and insensitivity to drugs used to treat the disease. Previous in vivo imaging and postmortem binding studies have detected a loss in striatal VMAT2 binding in Parkinson's diseased (PD) brain; however, these techniques have poor spatial resolution and may suffer from nonspecific binding of some ligands. In this study, we use novel polyclonal antibodies to distinct regions of human VMAT2 to quantify and localize the protein. Western blot analysis demonstrated marked reductions in VMAT2 immunoreactivity in putamen, caudate, and nucleus accumbens of PD brain compared to control cases. Immunohistochemistry revealed VMAT2 immunoreactive fibers and puncta that were dense throughout the striatum of control brains, but which were drastically reduced in putamen of PD brains. In PD brains the caudate showed a significant degree of sparing along the border of the lateral ventricle and the nucleus accumbens was relatively preserved. The distribution of VMAT2 in striatum and its loss in PD paralleled that of the dopamine transporter (DAT), a phenotypic marker of dopamine neurons. Thus, immunochemical analysis of VMAT2 protein provides novel and sensitive means for localizing and quantifying VMAT2 protein and nigrostriatal dopamine terminals in PD. Furthermore, the relative expression of VMAT2 compared to that of DAT may predict the differential vulnerability of dopamine neurons in PD.

The inflammatory reaction following 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine intoxication in mouse

In degenerative disorders of the CNS an immune system involvement in the pathological process is postulated. The MPTP model of Parkinson's disease seem to be a good model for studying an inflammation following toxic neurodegeneration. In this model, microglial and astroglial reactions were previously found around impaired neurons. In the present work we showed an immune reaction, including lymphocytic infiltration of CD4+ and CD8+ T cells in the substantia nigra and striatum and elevated MHC class I and II antigens expression on microglia. Many activated lymphocytes were present, showing increased LFA-1 and CD44 antigen expression. We found also that ICAM-1 expression increased on the endothelium and appeared on microglia in the injured regions. Treatment with dexamethasone inhibited T-cell infiltration and MHC class II expression, lessened the glial reaction, and also diminished neuronal impairment. These findings suggest that an immune mechanism may contribute to the neuronal damage following MPTP administration.

Transgenic mice expressing a dominant negative mutant interleukin-1beta converting enzyme show resistance to MPTP neurotoxicity

Increasing evidence implicates apoptosis as a major mechanism of cell death in neurodegenerative diseases. Recent evidence has demonstrated that chronic administration of MPTP can lead to apoptotic cell death. In the present study we examined whether transgenic mice expressing a dominant negative inhibitor of interleukin-1beta convertase enzyme (ICE) are resistant to MPTP induced neurotoxicity. MPTP resulted in a significant depletion of dopamine, DOPAC and HVA in littermate control mice which were completely inhibited in the mutant interleukin-1beta converting enzyme mice. There was also significant protection against MPTP-induced depletion of tyrosine hydroxylase-immunoreactive neurons. There was no alteration in MPTP uptake or metabolism. These results provide further evidence that apoptotic cell death as well as ICE may play an important role in the neurotoxicity of MPTP.

Elevation of antioxidant potency in the brain of mice by low-dose gamma-ray irradiation and its effect on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced brain damage

The elevation of endogenous thiol-related antioxidants and free radical scavenging enzymes in the brain of C57BL/6 female mice after low-dose gamma-ray irradiation and its inhibitory effect on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced brain damage were investigated. The brain level of the reduced form of glutathione (GSH) increased soon after irradiation with 50 cGy of gamma-rays, reached a maximum at 3 h post-treatment, and remained elevated until 12 h. Thioredoxin (TRX) was also transiently increased after irradiation. The activities of free radical scavenging enzymes, including Cu/Zn-superoxide dismutase, catalase and glutathione peroxidase, were significantly induced after irradiation as well. Cerebral malondialdehyde was remarkably elevated by MPTP treatment, and this elevation was suppressed by pre-irradiation (50 cGy). The contents of GSH and TRX were significantly decreased by MPTP treatment in comparison with those of the control group. These reductions both seemed to be attenuated by pre-irradiation with gamma-rays. These results suggest that low-dose gamma-ray irradiation induces endogenous antioxidative potency in the brain of mice and might be effective for the prevention and/or therapy of various reactive oxygen species-related neurodegenerative disorders, such as Parkinson's disease and Alzheimer's disease.

CEP-1347/KT-7515, an inhibitor of c-jun N-terminal kinase activation, attenuates the 1-methyl-4-phenyl tetrahydropyridine-mediated loss of nigrostriatal dopaminergic neurons In vivo

We have identified a bis-ethylthiomethyl analog of K-252a, CEP-1347/KT-7515, that promotes neuronal survival in culture and in vivo. The neuronal survival properties of CEP-1347/KT-7515 may be related to its ability to inhibit the activation of c-jun N-terminal kinase, a key kinase in some forms of stress-induced neuronal death and perhaps apoptosis. There is evidence that the selective nigrostriatal dopaminergic neurotoxin, MPTP, produces neuronal apoptosis in culture and in adult mice. Thus, our studies were designed to determine if CEP-1347/KT-7515 could protect dopaminergic neurons from MPTP-mediated neurotoxicity. CEP-1347/KT-7515 was assessed for neuroprotective activity in a low dose MPTP model (20 mg/kg) where there was a 50% loss of striatal dopaminergic terminals in the absence of substantia nigra neuronal loss, and a high dose (40 mg/kg) MPTP model where there was a complete loss of dopaminergic terminals and 80% loss of dopaminergic cell bodies. In the low dose MPTP model, CEP-1347/KT-7515 (0.3 mg/kg/day) attenuated the MPTP-mediated loss of striatal dopaminergic terminals by 50%. In the high dose model, CEP-1347/KT-7515 ameliorated the loss of dopaminergic cell bodies by 50% and partially preserved striatal dopaminergic terminals. CEP-1347/KT-7515 did not inhibit monoamine oxidase B or the dopamine transporter, suggesting that the neuroprotective effects of CEP-1347/KT-7515 occur downstream of the metabolic conversion of MPTP to MPP+ and accumulation of MPP+ into dopaminergic neurons. These data implicate a c-jun N-terminal kinase signaling system in MPTP-mediated dopaminergic degeneration and suggest that CEP-1347/KT-7515 may have potential as a treatment for Parkinson's disease.

Absence of MPTP-induced neuronal death in mice lacking the dopamine transporter

MPTP has been shown to induce parkinsonism both in human and in nonhuman primates. The precise mechanism of dopaminergic cell death induced following MPTP treatment is still subject to intense debate. MPP+, which is the oxidation product of MPTP, is actively transported into presynaptic dopaminergic nerve terminals through the plasma membrane dopamine transporter (DAT). In this study, we used mice lacking the DAT by homologous recombination and demonstrated that the MPTP-induced dopaminergic cell loss is dependent on the presence of the DAT. For this we have used tyrosine hydroxylase immunoreactivity (TH-IR) labeling of dopamine cells of the substantia nigra compacta in wild-type, heterozygote, and homozygote mice that were given either saline or MPTP treatments (two ip injections of 30 mg/kg, 10 h apart). Our results show a significant loss of TH-IR in wild type (34.4%), less loss in heterozygotes (22.5%), and no loss in homozygote animals. Thus dopamine cell loss is related to levels of the DAT. These results shed light on the degenerative process of dopamine neurons and suggest that individual differences in developing Parkinson's disease in human may be related to differences of uptake through the DAT of a yet unidentified neurotoxin.

GDNF improves dopamine function in the substantia nigra but not the putamen of unilateral MPTP-lesioned rhesus monkeys

Microdialysis measurements of dopamine (DA) and DA metabolites were carried out in the putamen and substantia nigra of unilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned rhesus monkeys that received intraventricular injections of vehicle or glial-derived neurotrophic factor (GDNF, 300 microg) 3 weeks prior to the microdialysis studies. Following behavioral measures in the MPTP-lesioned monkeys, they were anesthetized with isoflurane and placed in a stereotaxic apparatus. Magnetic resonance imaging (MRI)-guided sterile stereotaxic procedures were used for implantations of the microdialysis probes. Basal extracellular levels of DA and the DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were found to be decreased by >95% in the right putamen of the MPTP-lesioned monkeys as compared to normal animals. In contrast, basal DA levels were not significantly decreased, and DOPAC and HVA levels were decreased by only 65% and 30%, respectively, in the MPTP-lesioned substantia nigra. Significant reductions in d-amphetamine-evoked DA release were also observed in the MPTP-lesioned substantia nigra and putamen of the monkeys as compared to normal animals. A single intraventricular administration of GDNF into one group of MPTP-lesioned monkeys elicited improvements in the parkinsonian symptoms in these animals at 2-3 weeks post-administration. In addition, d-amphetamine-evoked overflow of DA was significantly increased in the substantia nigra but not the putamen of MPTP-lesioned monkeys that had received GDNF. Moreover, post-mortem brain tissue studies showed increases in whole tissue levels of DA and DA metabolite levels primarily within the substantia nigra in MPTP-lesioned monkeys that had received GDNF. Taken together, these data support that single ventricular infusions of GDNF produce improvements in motoric behavior in MPTP-lesioned monkeys that correlate with increases in DA neuronal function that are localized to the substantia nigra and not the putamen.

Alteration of neurotensin receptors in MPTP-treated mice

We examined the sequential changes in neurotensin receptors in the striatum and substantia nigra of mouse brains lesioned with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) by receptor autoradiography, in comparison with the alterations in dopamine uptake sites. The mice received four intraperitoneal injections of MPTP (10 mg/kg) at 1-h intervals and then the brains were analyzed at 6 h and 1, 3, 7, and 21 days after the treatments. [3H]Neurotensin and [3H]mazindol were used to label neurotensin receptors and dopamine uptake sites, respectively. [3H]Neurotensin binding was significantly decreased in the striatum from 6 h to 21 days after MPTP treatment. In the substantia nigra, pars reticulata also showed a significant decrease in [3H]neurotensin binding from 3 to 21 days post-MPTP treatment. However, no significant change in [3H]neurotensin binding was observed in the pars compacta even after 21 days. On the other hand, [3H]mazindol binding was markedly decreased in the striatum and substantia nigra from 6 h to 21 days after MPTP treatment. These results indicate that neurotoxin MPTP can produce a severe decrease in neurotensin receptors and dopamine uptake sites in the striatum and substantia nigra of mice. Thus, our findings provide evidence that the dysfunction in neurotensin receptors may be involved in the degenerative processes causing Parkinson's disease.

Enzyme-catalyzed bioactivation of cyclic tertiary amines to form potential neurotoxins

The pyridinium metabolites formed in the MAO-B catalyzed oxidation of 1-methyl-4-substituted-1,2,3,6-tetrahydropyridinyl derivatives, such as the parkinsonian inducing agent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), cause the selective degeneration of nigrostriatal neurons, presumably by inhibition of mitochondrial respiration and depletion of ATP stores. The possibility that other partially oxidized piperidinyl derivatives also may be biotransformed to toxic pyridinium metabolites has led us to examine the metabolic fate of the neuroleptic agent haloperidol (HP) and its tetrahydropyridinyl dehydration product 4-(4-chlorophenyl)-1[4-(4-fluorophenyl)-4-oxobutyl]- 1,2,3,6-tetrahydropyridine (HPTP). In vitro metabolic studies employing tissue preparations isolated from rodents, baboons and humans have documented that cytochrome P4503A enzymes catalyze the biotransformation of both HP and HPTP to yield the corresponding pyridinium metabolite HPP+. An analogous biotransformation profile has been observed with "reduced haloperidol" (RHP), an abundant, circulating metabolite of HP formed by the stereospecific reduction of the benzoyl carbonyl group of HP. In vivo studies also have documented these pathways in humans, baboons and rodents. Although both HPP+ and RHPP+ are found in the urine and plasma of HP treated patients and HP or HPTP treated baboons, attempts to identify an MPTP-type lesion in baboons following long-term treatment with HPTP have failed. On the other hand, evidence for a lesion of the nucleus basalis of Meynert has been obtained. Additionally, the urinary excretion of abnormal organic acids and acylcarnitine conjugates suggests that HP and/or metabolites derived from HP interfere with energy production pathways.

Efferent synaptic connections of dopaminergic neurons grafted into the caudate nucleus of experimentally induced parkinsonian monkeys are different from those of control animals

This study investigated the question of whether grafted dopamine cells in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys form synapses and, if they do, whether their postsynaptic targets were the same as those in control monkeys or in previous studies in rats. Electron-microscopic single immunostaining was performed for tyrosine hydroxylase on vibratome sections prepared from the head of the caudate nucleus of controls and MPTP-treated African green monkeys (Cercopithecus aethiops sabaeus) that received a graft. Furthermore, correlated light- and electron-microscopic double immunostaining was carried out for tyrosine hydroxylase and calbindin in the same brain area of MPTP-treated plus grafted animals. In control monkeys, the majority (97%) of dopamine boutons terminate on spines that were also synaptic targets of immunonegative boutons forming asymmetric synaptic contacts: synaptic triads. In MPTP-treated, grafted animals, the majority of transplanted dopamine cells terminate on dendritic shafts (67%) and somata (32%), and only a few (1.33%) form axospine synapses. The results of the double immunostaining experiments indicated that these newly formed axosomatic and axodendritic synapses are associated with calbindin-immunoreactive, medium-sized, spiny striatonigral projection neurons. These observations indicate that: (1) dopamine from transplanted embryonic tissue acts via synaptic contacts on host neurons; (2) the primary synaptic targets of transplanted dopamine cells are not spines but dendrites and somata of host neurons; (3) these target neurons are the same as in control animals; and (4) comparing these observations with results of control and grafted rats, there are major species differences between rats and monkeys in the dopamine innervation of both control and transplanted animals.

Cytotoxicity of 1-amino-4-phenyl-1,2,3,6-tetrahydropyridine and 1-amino-4-phenylpyridinium ion, 1-amino analogues of MPTP and MPP+, to clonal pheochromocytoma PC12 cells

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces parkinsonism in humans after its oxidation into 1-methyl-4-phenylpyridinium ion (MPP+) by type B monoamine oxidase. The 1-amino analogues of MPTP and MPP+, 1-amino-4-phenyl-1,2,3, 6-tetrahydropyridine (APTP) and 1-amino-4-phenylpyridinium ion (APP+), were synthesized, and their cytotoxicity to clonal pheochromocytoma PC12 cells was examined using a tetrazolium formazan assay. After incubation for 48 and 72 h, both APP+ and APTP were found to be cytotoxic to PC12 cells, whereas with the N-methyl analogues, only MPP+, but not MPTP, was cytotoxic. The cytotoxicity of APTP increased with incubation time and equaled that of MPP+ after 72 h. It was found that APTP was oxidized to APP+ by type A monoamine oxidase in PC12 cells, suggesting that APP+ itself may damage the cells. In addition to APTP and APP+, N-amino analogues of N-methylisoquinolines and related derivatives were also synthesized and examined for their cytotoxicity to PC12 cells.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyride neurotoxicity is attenuated in mice overexpressing Bcl-2

The proto-oncogene Bcl-2 rescues cells from a wide variety of insults. Recent evidence suggests that Bcl-2 protects against free radicals and that it increases mitochondrial calcium-buffering capacity. The neurotoxicity of 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyride (MPTP) is thought to involve both mitochondrial dysfunction and free radical generation. We therefore investigated MPTP neurotoxicity in both Bcl-2 overexpressing mice and littermate controls. MPTP-induced depletion of dopamine and loss of [3H]mazindol binding were significantly attenuated in Bcl-2 overexpressing mice. Protection was more profound with an acute dosing regimen than with daily MPTP administration over 5 d. 1-Methyl-4-phenylpyridinium (MPP+) levels after MPTP administration were similar in Bcl-2 overexpressing mice and littermates. Bcl-2 blocked MPP+-induced activation of caspases. MPTP-induced increases in free 3-nitrotyrosine levels were blocked in Bcl-2 overexpressing mice. These results indicate that Bcl-2 overexpression protects against MPTP neurotoxicity by mechanisms that may involve both antioxidant activity and inhibition of apoptotic pathways.

Reduced MPTP neurotoxicity in striatum of the mutant mouse tottering

The effects of MPTP treatment (4 x 10 mg/kg, 2-h intervals) on in vivo striatal binding of (+)-alpha-[3H]dihydrotetrabenazine ((+)-[3H]DTBZ) to the vesicular monoamine transporter type 2 (VMAT2) were examined in wild type (+,+) and tottering (tg/tg) mice of the C57BL/6J strain. The tottering mutant has been previously characterized as having hyperinnervation of noradrenergic terminals in the brain, with increased concentrations of norepinephrine and increased numbers of VMAT2 binding sites. In wild-type mice, MPTP caused a significant decrease in specific striatal (+)-[3H]DTBZ binding in both males (-71%) and females (-57%), consistent with dopaminergic terminal losses. In the tottering mice, the neurotoxic effects of MPTP were diminished, with smaller losses of (+)-[3H]DTBZ binding observed both in males (-45%) and females (-26%). These results are consistent with the hypothesis that vesicular storage (as a result of hyperinnervation) offers neuroprotection toward MPTP toxicity, although the confounding effects of increases in norepinephrine concentrations or changes in calcium ion channel function (both also characteristics of the tottering mutant) cannot be ruled out. The tottering mutant does, however, offer another animal model to examine the biochemical features responsible for MPTP toxicity.

[Metabolic activation of azaheterocyclics induced dopaminergic toxicity: possible candidate neurotoxins underlying idiopathic Parkinson's disease]

In 1983, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a contaminant of "synthetic heroin", has been reported to induce parkinsonian symptoms in humans, who were responsive to L-DOPA therapy, as a result of the degeneration of nigrostriatal neurons. The "MPTP story" hypothesizes that Parkinson's disease may be initiated or percipitated by environmental and/or endogenous toxins by a mechanism similar to that of MPTP in genetically-predisposed individuals. Several classes of heterocyclic molecules structurally related to MPTP have been advanced as possible neurotoxicant precursors underlying the nigrostriatal degeneration in Parkinson's disease. Indoleamine-related beta-carbolines (beta Cs), a class of heterocyclics which are basically plant alkaloids, are proposed as the most promising natural MPTP-like toxicants or protoxicants. In this article, beta Cs and N-methylated beta C cations are reviewed with regards to their formation, bioactivation, toxicity and presence in the human central nervous system. The enzymes in mammalian brain particulate fractions methylate beta Cs, sequentially forming 2-mono-[N]-methylated (2-Me beta C+s) and neurotoxic 2,9-di-[N, N']-methylated (2,9-Me2 beta C+s) beta-carbolinium cations. These beta C+s are structural analogs of 1-methyl-4-phenylpyridinium ion (MPP+), an active metabolite of MPTP, with a nitrogen bridge. The beta C+s not only inhibit DA reuptake and tyrosine hydroxylase, but also function as NADH-linked respiratory inhibitors in isolated mitochondria. The quarternization of beta C strikingly increased the affinity for dopamine transporter with 2-10 times greater Km and 10 times smaller Vmax values than MPP+. Furthermore, we have found higher concentrations of beta C+s localized in the nigra than in the cortex, and observed the S-adenosyl-L-methionine-dependent methylation of 2[beta]- and 9[indole]-nitrogens of beta Cs in non-parkinsonian human brains. Moreover, the cerebrospinal fluid levels of these beta C+s are higher in parkinsonian than non-parkinsonian patients. Simple beta-carboline induced parkinsonian-like symptoms in mice via N-methylation. These results indicated that beta C is a selective dopaminergic toxin precursor, that is sequentially methylated to form 2,9-Me2 beta C+ that could be an underlying factor in idiopathic Parkinson's disease.