Implication of dopamine transporter system on 1-methyl-4-phenylpyridinium and rotenone effect in striatal synaptosomes

Reprint of: revisiting oxidative stress and mitochondrial dysfunction in the pathogenesis of Parkinson disease-resemblance to the effect of amphetamine drugs of abuse

Parkinson disease (PD) is a chronic and progressive neurological disease associated with a loss of dopaminergic neurons. In most cases the disease is sporadic but genetically inherited cases also exist. One of the major pathological features of PD is the presence of aggregates that localize in neuronal cytoplasm as Lewy bodies, mainly composed of α-synuclein (α-syn) and ubiquitin. The selective degeneration of dopaminergic neurons suggests that dopamine itself may contribute to the neurodegenerative process in PD. Furthermore, mitochondrial dysfunction and oxidative stress constitute key pathogenic events of this disorder. Thus, in this review we give an actual perspective to classical pathways involving these two mechanisms of neurodegeneration, including the role of dopamine in sporadic and familial PD, as well as in the case of abuse of amphetamine-type drugs. Mutations in genes related to familial PD causing autosomal dominant or recessive forms may also have crucial effects on mitochondrial morphology, function, and oxidative stress. Environmental factors, such as MPTP and rotenone, have been reported to induce selective degeneration of the nigrostriatal pathways leading to α-syn-positive inclusions, possibly by inhibiting mitochondrial complex I of the respiratory chain and subsequently increasing oxidative stress. Recently, increased risk for PD was found in amphetamine users. Amphetamine drugs have effects similar to those of other environmental factors for PD, because long-term exposure to these drugs leads to dopamine depletion. Moreover, amphetamine neurotoxicity involves α-syn aggregation, mitochondrial dysfunction, and oxidative stress. Therefore, dopamine and related oxidative stress, as well as mitochondrial dysfunction, seem to be common links between PD and amphetamine neurotoxicity.

Revisiting oxidative stress and mitochondrial dysfunction in the pathogenesis of Parkinson disease--resemblance to the effect of amphetamine drugs of abuse

Parkinson disease (PD) is a chronic and progressive neurological disease associated with a loss of dopaminergic neurons. In most cases the disease is sporadic but genetically inherited cases also exist. One of the major pathological features of PD is the presence of aggregates that localize in neuronal cytoplasm as Lewy bodies, mainly composed of α-synuclein (α-syn) and ubiquitin. The selective degeneration of dopaminergic neurons suggests that dopamine itself may contribute to the neurodegenerative process in PD. Furthermore, mitochondrial dysfunction and oxidative stress constitute key pathogenic events of this disorder. Thus, in this review we give an actual perspective to classical pathways involving these two mechanisms of neurodegeneration, including the role of dopamine in sporadic and familial PD, as well as in the case of abuse of amphetamine-type drugs. Mutations in genes related to familial PD causing autosomal dominant or recessive forms may also have crucial effects on mitochondrial morphology, function, and oxidative stress. Environmental factors, such as MPTP and rotenone, have been reported to induce selective degeneration of the nigrostriatal pathways leading to α-syn-positive inclusions, possibly by inhibiting mitochondrial complex I of the respiratory chain and subsequently increasing oxidative stress. Recently, increased risk for PD was found in amphetamine users. Amphetamine drugs have effects similar to those of other environmental factors for PD, because long-term exposure to these drugs leads to dopamine depletion. Moreover, amphetamine neurotoxicity involves α-syn aggregation, mitochondrial dysfunction, and oxidative stress. Therefore, dopamine and related oxidative stress, as well as mitochondrial dysfunction, seem to be common links between PD and amphetamine neurotoxicity.

Loss of striatal dopaminergic terminals during the early stage in response to MPTP injection in C57BL/6 mice

The molecular mechanisms underlying MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-induced dopaminergic (DAergic) neuronal death in vivo are still not fully understood. To investigate the selective DAergic neurotoxicity, we have developed an immunological technique to isolate DAergic synaptosomes from mouse striatal tissues using an antibody against 20 amino acid residues in the extracellular second loop of dopamine transporter (DAT). The DAT protein level in the isolated DAergic synaptosomes was markedly decreased at 16 h after a single injection of 30 mg/kg MPTP, but not in striatal homogenate and crude synaptosomes fraction. GBR-12909, a dopamine uptake inhibitor, completely reversed the MPTP-induced decrease of DAT protein in the DAergic synaptosomes. These results suggest that the isolated DAergic synaptosomes can be useful to identify mechanisms of loss of the nerve terminals.

Brain mitochondrial dysfunction in aging, neurodegeneration, and Parkinson's disease

Brain senescence and neurodegeneration occur with a mitochondrial dysfunction characterized by impaired electron transfer and by oxidative damage. Brain mitochondria of old animals show decreased rates of electron transfer in complexes I and IV, decreased membrane potential, increased content of the oxidation products of phospholipids and proteins and increased size and fragility. This impairment, with complex I inactivation and oxidative damage, is named “complex I syndrome” and is recognized as characteristic of mammalian brain aging and of neurodegenerative diseases. Mitochondrial dysfunction is more marked in brain areas as rat hippocampus and frontal cortex, in human cortex in Parkinson's disease and dementia with Lewy bodies, and in substantia nigra in Parkinson's disease. The molecular mechanisms involved in complex I inactivation include the synergistic inactivations produced by ONOO− mediated reactions, by reactions with free radical intermediates of lipid peroxidation and by amine–aldehyde adduction reactions. The accumulation of oxidation products prompts the idea of antioxidant therapies. High doses of vitamin E produce a significant protection of complex I activity and mitochondrial function in rats and mice, and with improvement of neurological functions and increased median life span in mice. Mitochondria-targeted antioxidants, as the Skulachev cations covalently attached to vitamin E, ubiquinone and PBN and the SS tetrapeptides, are negatively charged and accumulate in mitochondria where they exert their antioxidant effects. Activation of the cellular mechanisms that regulate mitochondrial biogenesis is another potential therapeutic strategy, since the process generates organelles devoid of oxidation products and with full enzymatic activity and capacity for ATP production.

Rotenone-induced PC12 cell toxicity is caused by oxidative stress resulting from altered dopamine metabolism

Rotenone is a widely used pesticide. Administration of rotenone can induce biochemical and histological alterations similar to those of Parkinson's disease in rats, leading to the selective loss of dopaminergic neurons in the substantia nigra pars compacta. However, it remains unclear why rotenone seems to affect preferentially dopaminergic cells. To address this question, we studied the effects of rotenone on dopamine distribution and metabolism to determine the role of endogenous dopamine in rotenone-induced PC12 cells toxicity. Results showed that cell viability was decreased and intracellular dopamine concentration was increased with rotenone administration in a dose-dependent manner. Rotenone exposure led to changes of proteins and enzymes associated with dopamine synthesis and transportation in PC12 cells. Tyrosine hydroxylase (TH) and vesicular monoamine transporter 2 (VMAT(2)) were markedly down-regulated, and dopamine transporter (DAT) was up-regulated in the cells. The activity of monoamine oxidase (MAO) was also increased. In addition, rotenone increased ROS formation, which was clearly inhibited by the pretreatment of GSH. Similar inhibitions of ROS formation were also observed in PC12 cells pretreated with the classical dopamine transporter inhibitor of GBR-12909 and the MAO inhibitor L-deprenyl. Moreover, opposite effects were observed in PC12 cells pretreated with the specific VMAT(2) inhibitor reserpine. These results suggest that rotenone administration may interfere with dopamine distribution and metabolism, leading to dopamine accumulated in the cytoplasm of PC12 cells, which may contribute to the ROS formation and cell death. Therefore, the endogenous dopamine resulted from the altered dopamine metabolism and redistribution may play an important role in rotenone toxicity in dopamine neurons.

Dopamine D2 agonists, bromocriptine and quinpirole, increase MPP+ -induced toxicity in PC12 cells

Dopaminergic cell loss in the mesencephalic substantia nigra is the hallmark of Parkinson's disease and may be associated with abnormal oxidative metabolic activity. However, the delicate balance underlying dopamine decline and oxidative stress is still a matter of debate. The aim of this study was to analyze the possible modulation of D2 agonists and antagonists on MPP+ (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridinium ion) -induced cellular death in differentiated and undifferentiated PC12 cells. Using colorimetric assays, western blots and reverse transcriptase-PCR, we demonstrated that two D2 agonists, bromocriptine and quinpirole, consistently increased MPP+ -induced cytotoxicity in both differentiated and undifferentiated PC12 cells, whereas D2 antagonists do not modulate cell death. However, this increase in cellular death was reversed when bromocriptine or quinpirole were used in presence of D2 antagonists. On the other hand, 1-{2-[bis-(4-fluorophenyl)methoxy]ethyl}-4-(3-phenylpropyl)piperazine (GBR 12909), a potent inhibitor of the dopamine transporter, partially reversed MPP+ -induced cellular death and completely abolished the increase of cellular death induced by bromocriptine. Dopamine agonists and antagonists also modulate the expression of the dopamine transporter in PC12 cells; in particular, bromocriptine may alter MPP+ uptake by increasing DAT expression We also show that, in our cellular paradigm, D2 receptor mRNA levels are more abundant that D3 mRNA levels and MPP+ and /or bromocriptine could not modulate D2 gene expression while D3 gene expression clearly decrease after MPP+ and /or bromocriptine treatment.

Potentiating effect of the ATP-sensitive potassium channel blocker glibenclamide on complex I inhibitor neurotoxicity in vitro and in vivo

Previous studies have demonstrated a deficiency in mitochondrial function in Parkinson's disease. We measured the ability of mitochondrial inhibitors of complexes I (rotenone, MPP(+), and HPP(+)), II (amdro), IV (Na cyanide), and an uncoupler (dinoseb) to release preloaded dopamine from murine striatal synaptosomes. These compounds were potent dopamine releasers, and the effect was calcium-dependent. The striatum also contains a significant density of K(ATP)(+) channels, which play a protective role during ATP decline. Blockage of these channels with glibenclamide only potentiated the dopamine release by complex I inhibitors, and a selective potentiating effect of glibenclamide on the toxicity of MPTP was also observed, in vivo, using C57BL/6 mice. Western blots of striatal dopamine transporter (DAT) and tyrosine hydroxylase (TH) proteins demonstrated that 30 mg/kg of glibenclamide alone did not affect the expression of DAT and TH after two weeks of daily treatments, but it significantly enhanced the reduction of DAT and TH by a single dose of 20 mg/kg of MPTP. Amdro or dinoseb alone, or in conjunction with glibenclamide did not alter the expression of DAT and TH. The possible mechanisms underlying dopamine release and the selectivity of glibenclamide were further evaluated, in vitro. (86)Rb efflux assay showed that glibenclamide inhibited rotenone-induced K(+) efflux, but not dinoseb-induced K(+) efflux. Analysis of ATP titers in treated synaptosomes did not support a correlation between mitochondrial inhibition and K(ATP)(+) channel activation. However, assay of reactive oxygen species (ROS) showed that greater amounts of ROS generated by complex I inhibitors was a contributory factor to K(ATP)(+) channel activation and glibenclamide potentiation. Overall, these findings suggest that co-exposure to mitochondrial complex I inhibitors and glibenclamide or a genetic defect in K(ATP)(+) channel function, may increase neurotoxicity in the striatal dopaminergic system.

Expression of dopamine and vesicular monoamine transporters and differential vulnerability of mesostriatal dopaminergic neurons

Numerous studies suggest that the dopamine transporter (DAT), responsible for dopamine reuptake, may act as a vulnerability factor in the pathogenesis of Parkinson's disease (PD) and the vesicular monoamine transporter (VMAT2), responsible for its vesicular storage, as a neuroprotective factor. However, the relevance of each on the differential vulnerability of midbrain DA cells remains unknown. Here we studied the relationship between the expression pattern (mRNA and protein) of both transporters and the differential vulnerability of midbrain DA cells in a model of PD (intracerebroventricular injection of 6-OHDA in rats) and in monkey and human midbrain. Our results revealed that the expression patterns for VMAT2 mRNA and protein and DAT mRNA are similar, with the highest levels in the rostromedial region of substantia nigra (SNrm), followed by the caudoventral region of SN (SNcv), the ventral tegmental area and pigmented parabrabraquial nucleus (VTA/PBP), and finally the linear and interfascicular nuclei (Li/IF). In contrast, the expression of DAT protein in rats, monkeys, and humans followed a caudoventrolateral-to-rostrodorsomedial decreasing gradient (SNcv > SNrm > VTA/PBP > Li/IF), matching the degeneration profile observed after intracerebroventricular injection of 6-OHDA and in PD. In addition, DAT blockade made all midbrain DA cells equally resistant to 6-OHDA. These data indicate that DAT protein levels, but not DAT mRNA levels, are closely related to the differential vulnerability of midbrain DA cells and that this relationship is unaffected by the relative levels of VMAT2. Furthermore, the difference between DAT mRNA and protein profiles suggests internuclear differences in its posttransductional regulation.

Effects of iptakalim on rotenone-induced cytotoxicity and dopamine release from PC12 cells

Parkinson's disease is characterized by an extensive loss of dopaminergic neurons in the substantia nigra pars compacta. The final common pathway in the demise of these cells may involve dopamine-dependent oxidative stress. Previous studies revealed a new neuronal protective role of ATP-sensitive potassium channel openers. But the exact mechanism is still unknown. In the present study, the neuroprotective effect of iptakalim, a novel ATP-sensitive potassium channel opener, was studied against rotenone-induced cytotoxicity in rat dopaminergic PC12 cells. Rotenone decreased cell viability significantly after 48 h exposure and induced dopamine release from PC12 cells concentration-dependently. Iptakalim significantly enhanced dopamine uptake and alleviated rotenone-induced PC12 cells death and reduced dopamine release induced by rotenone or GBR-12909, a classical dopamine transporter inhibitor. These results suggest that iptakalim may open mitochondrial K(ATP) channels to modulate dopamine transporter and reduce extracellular dopamine levels, thereby it protecting PC12 cells against rotenone-induced injury.

Rotenone increases glutamate-induced dopamine release but does not affect hydroxyl-free radical formation in rat striatum

Impairment of the mitochondrial complex I has been found in Parkinson's disease and recently long-term treatment with the complex I inhibitor rotenone led to neurodegeneration and Lewy body-like inclusions in rats. To investigate the relationship of free radical formation, complex I inhibition, and dopamine release, rotenone (15 mg/kg s.c.) was injected in male Sprague Dawley rats. Complex I inhibition was measured in the striatum and substantia nigra using the lactate accumulation assay. Dopamine release and free radical formation was determined using striatal microdialysis in combination with the salicylate hydroxylation assay. In a second experiment, glutamate (10 mM) stimulation via the microdialysis probe was used to provoke hydroxyl radical formation and dopamine release 60 min after rotenone or vehicle pretreatment. Rotenone significantly increased striatal and nigral lactate levels. However, rotenone did not produce a significant increase in hydroxyl radical formation and dopamine release, but led to a pronounced hypokinesia. In contrast, rotenone in comparison to vehicle pretreatment produced a significant augmentation of glutamate-induced dopamine release (67-fold and 31-fold increase, respectively) and did not affect the glutamate-induced hydroxyl free radical formation (23-fold and 21-fold increase, respectively). The present study demonstrates that a single systemic rotenone administration does not lead to neurotoxicity, but rather to enhanced glutamate-induced dopamine release with no further increase of hydroxyl free radical formation. Thus, acute complex I inhibition in the presence or absence of high extracellular dopamine and glutamate levels is not critically involved in the formation of hydroxyl free radicals.

Parkinson's disease and exposure to infectious agents and pesticides and the occurrence of brain injuries: role of neuroinflammation

Idiopathic Parkinson's disease (PD) is a devastating movement disorder characterized by selective degeneration of the nigrostriatal dopaminergic pathway. Neurodegeneration usually starts in the fifth decade of life and progresses over 5-10 years before reaching the fully symptomatic disease state. Despite decades of intense research, the etiology of sporadic PD and the mechanism underlying the selective neuronal loss remain unknown. However, the late onset and slow-progressing nature of the disease has prompted the consideration of environmental exposure to agrochemicals, including pesticides, as a risk factor. Moreover, increasing evidence suggests that early-life occurrence of inflammation in the brain, as a consequence of either brain injury or exposure to infectious agents, may play a role in the pathogenesis of PD. Most important, there may be a self-propelling cycle of inflammatory process involving brain immune cells (microglia and astrocytes) that drives the slow yet progressive neurodegenerative process. Deciphering the molecular and cellular mechanisms governing those intricate interactions would significantly advance our understanding of the etiology and pathogenesis of PD and aid the development of therapeutic strategies for the treatment of the disease.

Comparative study of the neuroprotective effect of dehydroepiandrosterone and 17beta-estradiol against 1-methyl-4-phenylpyridium toxicity on rat striatum

The effects of dehydroepiandrosterone, estradiol and testosterone on 1-methyl-4-phenylpyridium (MPP+)-induced neurotoxicity of the nigrostriatal dopaminergic system were examined in rat. They were subjected to a unilateral intrastriatal infusion of the following treatment conditions: MPP+ alone or co-injection of MPP+ plus each hormone. Four days after injection, concentrations of dopamine and their metabolites were determined from the corpus striatum. To corroborate the neurochemical data an immunohistochemical analysis of tyrosine hydroxylase-immunoreactive fibers and acetylcholinesterase histochemistry in the striatum was performed. Moreover, we performed a dose-response study of the three hormones on the high-affinity dopamine transport system in rat striatal synaptosomes. Rats co-injected within the striatum with MPP+ and either dehydroepiandrosterone or estradiol had significantly greater concentrations of dopamine and less tyrosine hydroxylase-immunoreactive fibers and acetylcholinesterase fiber density loss compared with their respective controls. In addition, 4 days after injection, the brain was fixed and cut into coronal sections, and was immunostained with major histocompatibility complex class II antigens for activated microglia, and glial fibrillary acidic protein for activated astrocytes. Dehydroepiandrosterone also attenuated microglial cell activation. In contrast, testosterone showed reductions in dopamine concentrations similar to those obtained by MPP+. The protective effect of dehydroepiandrosterone against the MPP+ neurotoxic dopaminergic system may be produced by its partial prevention of MPP+ inhibition of NADH oxidase activity, whereas the estradiol may function as a neuroprotectant by reducing the uptake of MPP+ into dopaminergic neurons. Our findings we suggest indicate that dehydroepiandrosterone and estradiol by a non-genomic effect may have an important modulatory action, capable of attenuating degeneration within the striatum, and in this way serve as neuroprotectants of the nigrostriatal dopaminergic system.

Mechanisms of MPP(+) incorporation into cerebellar granule cells

Exposure of cerebellar granule cells to 1-methyl-4-phenylpiridinium (MPP(+)) results in cell death. We have studied the implication of various membrane transporter systems on MPP(+) neurotoxicity, including the dopamine transporter system (DAT) and cationic amino acid transporters (CAT). We have showed a partial protection against MPP(+) toxicity when the dopamine transporter is inhibited by 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]4-(3-phenylpropyl)piperazinedihydrochloride (GBR-12909). However, almost full protection is only achieved by the simultaneous addition of GBR-12909 and cationic amino acids. These results suggest two ways system of MPP(+) entrance into cerebellar granule cells: the DAT with high activity and the CAT with low activity. We also demonstrated that 5,7-dichlorokynurenic acid (MK-801) failed to protect against MPP(+) exposure, evidencing that N-methyl-D-aspartate (NMDA) receptor is not involved in the MPP(+)-induced cell death.

The endogenous amine 1-methyl-1,2,3,4- tetrahydroisoquinoline prevents the inhibition of complex I of the respiratory chain produced by MPP(+)

The endogenous monoamine 1-methyl-1,2,3,4-tetrahydroisoquinoline has been shown to prevent the neurotoxic effect of MPP(+) and other endogenous neurotoxins, which produce a parkinsonian-like syndrome in humans. We have tested its potential protective effect in vivo by measuring the protection of 1-methyl-1,2,3,4-tetrahydroisoquinoline in the neurotoxicity elicited by MPP(+) in rat striatum by tyrosine hydroxylase immunocytochemistry. Because we know that cellular damage caused by MPP(+) is primarily the result of mitochondrial respiratory inhibition at the complex I level, we have extended the study further to understand this protective mechanism. We found that the inhibitory effect on the mitochondrial respiration rate induced by MPP(+) in isolated rat liver mitochondria and striatal synaptosomes was prevented by addition of 1-methyl-1,2,3,4-tetrahydroisoquinoline. This compound has no antioxidant capacity; therefore, this property is not involved in its protective effect. Thus, we postulate that the preventive effect that 1-methyl-1,2,3,4-tetrahydroisoquinoline has on mitochondrial inhibition for MPP(+) could be due to a "shielding effect," protecting the energetic machinery, thus preventing energetic failure. These results suggest that this endogenous amine may protect against the effect of several parkinsonism-inducing compounds that are associated with progressive impairment of the mitochondrial function.

Neurotoxic relationship between dopamine and iron in the striatal dopaminergic nerve terminals

The neurotoxic effect of dopamine (DA) and iron(III) on DAergic terminals in striatum has been studied by intracerebral microdialysis technique. Twenty-four hours after surgery (day 1), DA and/or iron(III) with and without DA reuptake inhibitor, nomifensine, were perfused for 1 h. Forty-eight hours after surgery (day 2), MPP(+) 1 mM was perfused for 15 min and the output of DA was measured, its amount being directly proportional to the remaining striatal DAergic terminals, supported by tyrosine hydroxylase immunohistochemistry technique. Perfusion of exogenous DA, as well as iron(III) 10 and 100 microM, did not produce any neurotoxic effect. However, perfusion of iron(III) (333 and 1000 microM) produced a concentration-dependent toxic effect. Co-perfusion of iron(III) at non-toxic concentration (100 microM) with DA (15 microM) produced a toxic effect. Elevation of the endogenous extracellular levels of DA by inhibiting its uptake with nomifensine increased the neurotoxic effect of iron(III) in a dose-dependent manner. The use of tetrodotoxin after elevation of DA with nomifensine partially prevented the neurotoxic effect of its co-perfusion with iron(III) (100 microM). These results suggest that DAergic system could be synergistically damaged by DA and iron(III). Thus, alterations in the clearance of DA from extracellular space along with an increase of iron may have significant consequences for DAergic system toxicity.

Structural significance of azaheterocyclic amines related to Parkinson's disease for dopamine transporter

We have evaluated the neuronal uptake of 12 neutral and quaternary azaheterocyclic amines that are possible candidates for idiopathic Parkinson's disease via dopamine transporter of striatal synaptosomes. The double-reciprocal plots for dopamine transporter obtained from Wistar rat and C57BL/6 mouse synaptosomes with N-methyl-4-phenylpyridinium cation (MPP+) as a substrate were identical to each other. Neutral beta-carbolines and tetrahydroisoquinolines were unfavorable substrates for dopamine transporter. The quarternization of these compounds strikingly increased the affinity for dopamine transporter with 2-10 times greater Km and 10 times smaller Vmax values than MPP+. Although catechol tetrahydroisoquinolines were weak substrates, their quarternization reduced their original properties as substrates for dopamine transporter. These results provide both topographic and electrogenic information of azaheterocyclic amines for the dopamine transporter-mediated influx. The intramolecular distance between the N-atom and the centroid of the benzene ring could be an important factor for the recognition of binding site of dopamine transporter, and an adequate net charge similar to dopamine would be further required for translocation into the cells.

In situ detection of apoptotic nuclei in the substantia nigra compacta of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice using terminal deoxynucleotidyl transferase labelling and acridine orange staining

The neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was used to generate a dose-dependent cell death of dopaminergic nigral neurons in the C57B1 mouse. Mice were injected with a total cumulative dose of 150 mg/kg of MPTP delivered over five days and killed at different time points both during and after the toxin injections. Two independent histological methods were used to determine whether the dopaminergic nigral neurons died via an apoptotic mechanism. In situ end-labelling with terminal deoxynucleotidyl transferase was used on paraformaldehyde-fixed, serial, frozen sections to identity cells with double-stranded DNA breaks. Apoptotic cell death was found to be initiated within 72 h of the first injection of the neurotoxin and peaked 24 h after the final MPTP injection. The metachromatic fluorochrome, Acridine Orange, was used on alternate sections to provide structural confirmation of the nuclear chromatin "clumping" considered to be representative of apoptosis. Confocal laser imaging combined with deconvolution techniques was used to resolve the fluorescent signal emitted by the in situ Acridine Orange-DNA complexes. The number of Acridine Orange-stained nuclei demonstrating chromatin clumping was identical to that of the positive in situ end-labelled nuclei observed over a 25 day period. Based upon these two independent methods of assessing apoptosis in situ, we conclude that a 150 mg/kg dose of MPTP can elicit apoptotic cell death in nigral dopaminergic neurons of the C57B1 mouse.