Dopamine transporter mRNA levels are high in midbrain neurons vulnerable to MPTP

α-Synuclein-Dependent Calcium Entry Underlies Differential Sensitivity of Cultured SN and VTA Dopaminergic Neurons to a Parkinsonian Neurotoxin

Parkinson's disease (PD) is a debilitating neurodegenerative disease characterized by a loss of dopaminergic neurons in the substantia nigra (SN). Although mitochondrial dysfunction and dysregulated α-synuclein (aSyn) expression are postulated to play a role in PD pathogenesis, it is still debated why neurons of the SN are targeted while neighboring dopaminergic neurons of the ventral tegmental area (VTA) are spared. Using electrochemical and imaging approaches, we investigated metabolic changes in cultured primary mouse midbrain dopaminergic neurons exposed to a parkinsonian neurotoxin, 1-methyl-4-phenylpyridinium (MPP+). We demonstrate that the higher level of neurotoxicity in SN than VTA neurons was due to SN neuron-specific toxin-induced increase in cytosolic dopamine (DA) and Ca2+, followed by an elevation of mitochondrial Ca2+, activation of nitric oxide synthase (NOS), and mitochondrial oxidation. The increase in cytosolic Ca2+ was not caused by MPP+-induced oxidative stress, but rather depended on the activity of both L-type calcium channels and aSyn expression, suggesting that these two established pathogenic factors in PD act in concert.

Chronic MPTP administration regimen in monkeys: a model of dopaminergic and non-dopaminergic cell loss in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder clinically characterized by cardinal motor deficits including bradykinesia, tremor, rigidity and postural instability. Over the past decades, it has become clear that PD symptoms extend far beyond motor signs to include cognitive, autonomic and psychiatric impairments, most likely resulting from cortical and subcortical lesions of non-dopaminergic systems. In addition to nigrostriatal dopaminergic degeneration, pathological examination of PD brains, indeed, reveals widespread distribution of intracytoplasmic inclusions (Lewy bodies) and death of non-dopaminergic neurons in the brainstem and thalamus. For that past three decades, the MPTP-treated monkey has been recognized as the gold standard PD model because it displays some of the key behavioral and pathophysiological changes seen in PD patients. However, a common criticism raised by some authors about this model, and other neurotoxin-based models of PD, is the lack of neuronal loss beyond the nigrostriatal dopaminergic system. In this review, we argue that this assumption is largely incorrect and solely based on data from monkeys intoxicated with acute administration of MPTP. Work achieved in our laboratory and others strongly suggest that long-term chronic administration of MPTP leads to brain pathology beyond the dopaminergic system that displays close similarities to that seen in PD patients. This review critically examines these data and suggests that the chronically MPTP-treated nonhuman primate model may be suitable to study the pathophysiology and therapeutics of some non-motor features of PD.

Increased expression of the dopamine transporter leads to loss of dopamine neurons, oxidative stress and l-DOPA reversible motor deficits

The dopamine transporter is a key protein responsible for regulating dopamine homeostasis. Its function is to transport dopamine from the extracellular space into the presynaptic neuron. Studies have suggested that accumulation of dopamine in the cytosol can trigger oxidative stress and neurotoxicity. Previously, ectopic expression of the dopamine transporter was shown to cause damage in non-dopaminergic neurons due to their inability to handle cytosolic dopamine. However, it is unknown whether increasing dopamine transporter activity will be detrimental to dopamine neurons that are inherently capable of storing and degrading dopamine. To address this issue, we characterized transgenic mice that over-express the dopamine transporter selectively in dopamine neurons. We report that dopamine transporter over-expressing (DAT-tg) mice display spontaneous loss of midbrain dopamine neurons that is accompanied by increases in oxidative stress markers, 5-S-cysteinyl-dopamine and 5-S-cysteinyl-DOPAC. In addition, metabolite-to-dopamine ratios are increased and VMAT2 protein expression is decreased in the striatum of these animals. Furthermore, DAT-tg mice also show fine motor deficits on challenging beam traversal that are reversed with l-DOPA treatment. Collectively, our findings demonstrate that even in neurons that routinely handle dopamine, increased uptake of this neurotransmitter through the dopamine transporter results in oxidative damage, neuronal loss and l-DOPA reversible motor deficits. In addition, DAT over-expressing animals are highly sensitive to MPTP-induced neurotoxicity. The effects of increased dopamine uptake in these transgenic mice could shed light on the unique vulnerability of dopamine neurons in Parkinson's disease.

Postmortem studies in Parkinson's disease

No animal model to date perfectly replicates Parkinson's disease (PD) etiopathogenesis, and the anatomical organization of the nigrostriatal system differs considerably between species. Human postmortem material therefore remains the gold standard for both formulating hypotheses for subsequent testing in in vitro and in vivo PD models and verifying hypotheses derived from experimental PD models with regard to their validity in the human disease. This article focuses on recent and relevant fields in which human postmortem work has generated significant impact in our understanding of PD. These fields include Lewy body formation, regional vulnerability of dopaminergic neurons, oxidative/nitrative cellular stress, inflammation, apoptosis, infectious and environmental agents, and nondopaminergic lesions.

Striatal and extrastriatal dopamine in the basal ganglia: an overview of its anatomical organization in normal and Parkinsonian brains

Degeneration of the nigrostriatal dopaminergic system is the characteristic neuropathological feature of Parkinson's disease and therapy is primarily based on a dopamine replacement strategy. Dopamine has long been recognized to be a key neuromodulator of basal ganglia function, essential for normal motor activity. The recent years have witnessed significant advances in our knowledge of dopamine function in the basal ganglia. Although the striatum remains the main functional target of dopamine, it is now appreciated that there is dopaminergic innervation of the pallidum, subthalamic nucleus, and substantia nigra. A new dopaminergic- thalamic system has also been uncovered, setting the stage for a direct dopamine action on thalamocortical activity. The differential distribution of D1 and D2 receptors on neurons in the direct and indirect striato-pallidal pathways has been re-emphasized, and cholinergic interneurons are recognized as an intermediary mediator of dopamine-mediated communication between the two pathways. The importance and specificity of dopamine in regulating morphological changes in striatal projection neurons provides further evidence for the complex and multifarious mechanisms through which dopamine mediates its functional effects in the basal ganglia. In this review, the role of basal ganglia dopamine and its functional relevance in normal and pathological conditions will be discussed.

Future of cell and gene therapies for Parkinson's disease

The experimental field of restorative neurology continues to advance with implantation of cells or transfer of genes to treat patients with neurological disease. Both strategies have generated a consensus that demonstrates their capacity for structural and molecular brain modification in the adult brain. However, both approaches have yet to successfully address the complexities to make such novel therapeutic modalities work in the clinic. Prior experimental cell transplantation to patients with PD utilized dissected pieces of fetal midbrain tissue, containing mixtures of cells and neuronal types, as donor cells. Stem cell and progenitor cell biology provide new opportunities for selection and development of large batches of specific therapeutic cells. This may allow for cell composition analysis and dosing to optimize the benefit to an individual patient. The biotechnology used for cell and gene therapy for treatment of neurological disease may eventually be as advanced as today's pharmaceutical drug-related design processes. Current gene therapy phase 1 safety trials for PD include the delivery of a growth factor (neurturin via the glial cell line-derived neurotrophic factor receptor) and a transmitter enzyme (glutamic acid decarboxylase and aromatic acid decarboxylase). Many new insights from cell biological and molecular studies provide opportunities to selectively express or suppress factors relevant to neuroprotection and improved function of neurons involved in PD. Future gene and cell therapies are likely to coexist with classic pharmacological therapies because their use can be tailored to individual patients' underlying disease process and need for neuroprotective or restorative interventions.

Differences in reserpine-induced striatal dopamine output and content between female and male mice: implications for sex differences in vesicular monoamine transporter 2 function

In this report a series of six in vitro experiments in which reserpine-evoked dopamine output and two in vivo experiments in which the effects of reserpine injections upon dopamine content from striatal tissue of female and male mice were performed as a means to assess possible sex differences in vesicular monoamine transporter 2 (VMAT2) function. Significantly greater amounts of dopamine were obtained from striatal tissue of female mice in response to either a brief (experiment 1) or continuous (experiment 2) infusion of reserpine. Similarly, reserpine-evoked dopamine output from striatal tissue of gonadectomized females was significantly greater that that of gonadectomized males (experiment 3). When reserpine-evoked dopamine responses were compared directly between intact versus gonadectomized females (experiment 4) or males (experiment 5) no statistically significant differences were obtained. Finally, comparisons of gonadectomized females treated or not with estrogen revealed no statistically significant differences in reserpine-evoked dopamine output (experiment 6). Injections of reserpine produced significantly greater depletions of striatal dopamine content within intact female versus male mice (experiment 7). Dopamine contents of gonadectomized females treated or not with estrogen did not differ following treatment with reserpine, but were significantly greater than that of gonadectomized males (experiment 8). Taken together, these results show that female striatal tissue is more responsive to reserpine-evoked dopamine output, and this sex difference appears to be estrogen independent. Similarly, the dopamine depleting effects of reserpine are greater in intact female mice, however, gonadectomy reverses this effect in an estrogen independent manner. The data suggest that female mice may have a greater amount/activity of VMAT2 function as revealed by the increased responsiveness to the VMAT2 blocking drug, reserpine. Such differences in VMAT2 function may be related to the gender differences observed in conditions like Parkinson's disease and drug addiction.

Behavioral and histopathological consequences of paraquat intoxication in mice: effects of alpha-synuclein over-expression

Genetic variability in the alpha-synuclein gene and long-term exposure to the pesticide paraquat constitute possible risk factors for sporadic Parkinson's disease. The goal of the present study was to further characterize the effects of paraquat in mice as a model of Parkinson's disease and to determine whether it acted synergistically with alpha-synuclein over-expression to cause nigrostriatal cell death or dysfunction. Paraquat (10 mg/kg i.p.) was administered once a week for 3 weeks to mice over-expressing human alpha-synuclein under the Thy1 promoter and their wild-type littermates. The effect of paraquat on catecholaminergic neurons was reminiscent of that of Parkinson's disease, with preferential loss of dopaminergic neurons in the ventral tier of the substantia nigra pars compacta and loss of tyrosine hydroxylase staining in the locus coeruleus. alpha-Synuclein over-expression did not increase paraquat-induced cell loss, and paraquat did not worsen the behavioral deficits observed in the transgenic mice. However, paraquat markedly increased proteinase-K-resistant alpha-synuclein aggregates in substantia nigra of the transgenic mice. The data further validate the use of paraquat to model Parkinson's disease in mice and show that although paraquat and alpha-synuclein over-expression act synergistically to increase protein aggregation in vivo, this interaction does not result in short-term neuroprotection or increased vulnerability of nigrostriatal neurons.

Ventral tegmental area dopamine neurons are resistant to human mutant alpha-synuclein overexpression

Parkinson's disease (PD) is characterized by the formation of intracytoplasmic inclusions, which contain alpha-synuclein (alpha-syn) protein. While most profound neurodegeneration is seen in the dopamine (DA) synthesizing neurons located in the ventral midbrain, it is unclear why some DA cell groups are more susceptible than others. In the midbrain, the degeneration of the substantia nigra (SN) DA neurons is severe, whereas the involvement of the ventral tegmental area (VTA) neurons is relatively spared. In the present study, we overexpressed human A53T alpha-syn in the VTA neurons and found that A53T toxicity did not affect their survival. There was, however, a mild functional impairment seen as altered open field locomotor activity. Overexpression of A53T in the SN, on the other hand, led to profound cell loss. These results suggest that the selective susceptibility of nigral DA neurons is at least in part associated with factor(s) involved in handling of alpha-syn that is not shared by the VTA neurons. Secondly, these results highlight the fact that impaired but surviving neurons can have a substantial impact on DA-dependent behavior and should therefore be considered as a critical part of animal models where novel therapeutic interventions are tested.

Differential survival patterns among midbrain dopaminergic cells of MPTP-treated monkeys and 6OHDA-lesioned rats

We explore the patterns of survival among dopaminergic cells of the midbrain in MPTP-treated macaque monkeys and 6OHDA-lesioned Sprague-Dawley rats. For the monkeys, animals were injected intramuscularly with MPTP for 8 days consecutively and then allowed to survive for 21 days. For the rats, 6OHDA was injected into the midbrain and then allowed to survive for either 7, 28 or 84 days. Brains were processed for tyrosine hydroxylase (TH) and calbindin immunocytochemistry to label populations in the ventral and dorsal tiers of midbrain dopaminergic cells. In monkeys, while there was a decrease in the TH+ cell number in the ventral tier of MPTP-treated cases (65%), there was an overall increase (22%) in the TH+ and calbindin+ cell number in the dorsal tier. Double labelling studies indicate that approximately 50% of TH+ cells of the dorsal tier contain calbindin also. In rats, there was a decrease in TH+ cell number in the ventral tier of 6OHDA-lesioned cases (97%), and to a lesser extent, in the TH+ and calbindin+ cell number in the dorsal tier ( approximately 40%). In conclusion, we show a surprising increase in TH+ and calbindin+ cell number in the dorsal tier in response to MPTP insult; such an increase was not evident after 6OHDA insult. We suggest that the increase in antigen expression relates to the dopaminergic reinnervation of the striatum in MPTP-treated cases. We also suggest that the greater loss of dopaminergic cells in the ventral tier when compared to the dorsal tier relates to glutamate toxicity.

Neuroprotective effects of Bak Foong Pill in 1-methyl-4-phenyl-1,2,3,6-tetrahyrdropyridine (MPTP)-induced Parkinson's disease model mice

Neuroprotective effects of estrogen and estrogen-like chemicals on neurodegenerative diseases, especially Parkinson's disease, have been well established. In the present study, we compared the effects of Bak Foong Pill (BFP), a well-known gynaecological tonic in China, and 17beta-estradiol, on dopamine transporter (DAT) and tyrosine hydroxylase (TH) gene expression patterns in ovariectomized, 1-methyl-4-phenyl-1,2,3,6-tetrahyrdropyridine (MPTP)-induced Parkinson's disease (PD) model mice, using multiplex reverse transcription-polymerase chain reaction (RT-PCR). MPTP, a specific dopaminergic neurotoxin, significantly decreased DAT and TH mRNA levels in the striatum, midbrain and cerebellum, but not the cortex, of C57BL/6 mice. However, MPTP-challenge with BFP pretreatment demonstrated reduced neurotoxicity, with DAT and TH mRNA levels either not affected by MPTP or affected to a significantly lesser extent in the midbrain and striatum as compared to the MPTP treated controls. 17beta-estradiol treatment prevented MPTP-induced reduction of DAT expression in striatum and midbrain, but failed to alter TH expression. These results suggest that BFP is able to protect dopaminergic neurons against MPTP-induced neuronal damage in a mechanism that is different from the protective effect of estrogen.

Dopamine transporter function differences between male and female CD-1 mice

It has been reported that male mice are more susceptible to the neurotoxic effects of methamphetamine (MA) upon the nigrostriatal dopaminergic (NSDA) system. Since MA utilizes the dopamine transporter (DAT) to exert its effects, in the present study, we tested for differences in the dynamics of DAT function between male and female mice as an approach to understand some of the bases for this sex difference in MA-induced NSDA neurotoxicity. To accomplish this goal, in Experiment 1, the amount of dopamine (DA) obtained following DA infusion into the superfused striatal tissue fragments of male and female mice was measured while in Experiment 2 responses to the DA uptake blocker, nomifensine (NMF), were assessed in these preparations. The differences obtained to these treatments demonstrate that marked differences in DA transporter activity exist between male and female mice. When combining the DA and DOPAC measures from these two experiments, the data suggest that the female mice show a more active and efficient recovery and vesicular packaging of extracellular DA. These findings have important implications for sex differences in NSDA functions and responses to neurotoxins which enter the neurons via the DAT.

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.

Sparing of behavior and basal extracellular dopamine after 6-hydroxydopamine lesions of the nigrostriatal pathway in rats exposed to a prelesion sensitizing regimen of amphetamine

Repeated administration of amphetamine leads to enduring augmentation of its behavioral-activating effects, enhanced dopamine (DA) release in striatal regions, and morphological changes in DA target neurons. Here we show that exposure to a 2-week escalating-dose regimen of amphetamine prevents behavioral asymmetries of forelimb use and spontaneous (drug-independent) turning behavior following unilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal pathway made 7-14 days after termination of amphetamine treatment (Experiments 1-3). Exposure to three nonescalating injections of amphetamine 7 days before 6-OHDA lesions had no effect (Experiment 2). Prelesion amphetamine treatment led to normalization of basal extracellular levels of striatal DA as measured by microdialysis on days 11-14 and 25-28 after lesioning (Experiment 3). However, there were no significant differences between treatment groups in postmortem tissue levels of DA and its metabolites, indicating a dissociation between the DA depletion and the extracellular levels of DA as measured by microdialysis. Finally, rats exposed to the escalating amphetamine regimen had reduced lesion-induced loss of TH-IR cells in the ipsilateral DA cell body regions (Experiment 3). Thus, prelesion exposure to the escalating doses of amphetamine may render the cells resistant to the consequences of damage after subsequent 6-OHDA lesions, possibly by accelerating the development of compensatory changes in the DA neurons that typically accompany behavioral recovery. The potential role of amphetamine-induced endogenous neurotrophic factors in the behavioral sparing and normalization of basal extracellular DA levels observed after subsequent 6-OHDA lesions is discussed.

Injury and strain-dependent dopaminergic neuronal degeneration in the substantia nigra of mice after axotomy or MPTP

We studied the effects of axotomy or neurotoxin on the survival of substantia nigra pars compacta (SNpc) neurons in two strains of mice, FVB/N or C57BL/6. Fluoro gold (FG) was injected into both striata of the mice to retrogradely label the nigrostriatal neuronal population. Ten days later, these neurons were axotomized in the medial forebrain bundle (MFB) unilaterally or N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was administered intraperitonealy for 2 days to produce bilateral degeneration. MFB transection or MPTP administration produced a progressive loss of FG-labeled and tyrosine hydroxylase immunolabeled (TH+) neurons in both strains. Relative to control, 72% of SNpc neurons died 4 weeks after axotomy in C57BL/6 mice and 50% died after axotomy in FVB/N mice. MPTP resulted in death of 80% of SNpc neurons in C57BL/6 mice but only 40% in the FVB strain 4 weeks after MPTP administration. In this more sensitive strain, MPTP cell death was associated with positive staining for terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) and nuclear condensation. In contrast, no TUNEL staining was detected in SNpc after MPTP in FVB/N mice. Further, while similar kinetics and extent of cell death accompanied axotomy, axotomy-induced cell death was TUNEL negative in both FVB/N and C57BL/6 mice. Double staining for TUNEL and microtubule associated protein 2 confirmed that the majority of the TUNEL positive cells were neurons. These data indicate that genetic factors and the type of lesion play an important role in determining death of dopaminergic neurons after injury.

Chromosomal loci influencing the susceptibility to the parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the dysfunction of the nigrostriatal dopaminergic pathway. Although its etiology is not yet fully understood, an interaction of genetic predisposition and environmental factors is frequently discussed. The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) can evoke PD-like symptoms and neuropathological changes in various species, including mice. It was found repeatedly that mouse strains differ in their susceptibility to MPTP, which might serve as a model for genetic predisposition to neurodegeneration of the nigrostriatal system. In the present study, F2 intercross mice, derived from parental strains with high (C57BL/6J) versus low (BALB/cJ) MPTP susceptibility, were treated with MPTP and phenotyped for dopamine (DA) loss in the neostriatum, a highly sensitive marker of nigrostriatal dysfunction. A subsequent quantitative trait loci analysis revealed a gender-dependent locus for DA loss on chromosome 15 and a putative locus on chromosome 13. A number of potential candidate genes, including the membrane dopamine transporter, are located in the respective areas. Several mechanisms that are possibly involved in the control of the action of MPTP on the nigrostriatal system are discussed.

Exercise induces behavioral recovery and attenuates neurochemical deficits in rodent models of Parkinson's disease

Exercise is thought to improve motor function and emotional well-being in patients with Parkinson's disease (PD). However, it is not clear if the improvements are due to neurochemical alterations within the affected nigrostriatal region or result from a more general effect of exercise on affect and motivation. In this study we show that motorized treadmill running improves the neurochemical and behavioral outcomes in two rodent models of PD: the unilateral 6-hydroxydopamine (6-OHDA) rat model and bilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model in aged C57bl mice. Exposure to the dopamine (DA) toxins 6-OHDA or MPTP resulted in permanent behavioral and neurochemical loss. In contrast, when lesioned animals were exposed to treadmill activity two times a day for the first 10 days post-lesion they displayed no behavioral deficits across testing days and had significant sparing of striatal DA, its metabolites, tyrosine hydroxylase, vesicular monoamine transporter, and DA transporter levels compared to lesion sedentary animals. These results demonstrate that exercise following nigrostriatal damage ameliorates related motor symptoms and neurochemical deficits in rodent models of PD.

Toward full restoration of synaptic and terminal function of the dopaminergic system in Parkinson's disease by stem cells

New therapeutic nonpharmacological methodology in Parkinson's disease (PD) involves cell and synaptic renewal or replacement to restore function of neuronal systems, including the dopaminergic (DA) system. Using fetal DA cell therapy in PD patients and laboratory models, it has been demonstrated that functional motor deficits associated with parkinsonism can be reduced. Similar results have been observed in animal models with stem cell-derived DA neurons. Evidence obtained from transplanted PD patients further shows that the underlying disease process does not destroy transplanted fetal DA cells, although degeneration of the host nigrostriatal system continues. The optimal DA cell regeneration system would reconstitute a normal neuronal network capable of restoring feedback-controlled release of DA in the nigrostriatal system. The success of cell therapy for PD is limited by access to preparation and development of highly specialized dopaminergic neurons found in the A9 and A10 region of the substantia nigra pars compacta as well as the technical and surgical steps associated with the transplantation procedure. Recent laboratory work has focused on using stem cells as a starting point for deriving the optimal DA cells to restore the nigrostriatal system. Ultimately, understanding the cell biological principles necessary for generating functional DA neurons can provide many new avenues for better treatment of patients with PD.

Environmental factors in Parkinson's disease

Evidence discussed in this review article lends strong support in favor of an etiologic role of environmentalfactors in Parkinson's disease. First, thanks to the discovery of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), it is now clear that, by targeting the nigrostriatal system, neurotoxicants can reproduce the neurochemical and pathological features of idiopathic parkinsonism. The sequence of toxic events triggered by MPTP has also provided us with intriguing clues concerning mechanisms of toxicant selectivity and nigrostriatal vulnerability. Relevant examples are (i) the role of the plasma membrane dopamine transporter in facilitating the access of potentially toxic species into dopaminergic neurons; (ii) the vulnerability of the nigrostriatal system to failure of mitochondrial energy metabolism; and (iii) the contribution of inflammatory processes to tissue lesioning. Epidemiological and experimental data suggest the potential involvement of specific agents as neurotoxicants (e.g. pesticides) or neuroprotective compounds (e.g. tobacco products) in the pathogenesis of nigrostriatal degeneration, further supporting a relationship between the environment and Parkinson's disease. A likely scenario that emerges from our current knowledge is that neurodegeneration results from multiple events and interactive mechanisms. These may include (i) the synergistic action of endogenous and exogenous toxins (e.g. the ability of the pesticide diethyldithiocarbamate to promote the toxicity of other compounds); (ii) the interactions of toxic agents with endogenous elements (e.g. the protein alpha-synuclein); (iii) the tissue response to an initial toxic insult; and, last but not least, (iv) the effects of environmental factors on the background of genetic predisposition and aging.

Method for culturing postnatal substantia nigra as an in vitro model of experimental Parkinson's disease

One of the most prevalent degenerative disorders of the nervous system is Parkinson's disease. The etiology of this disease is for the most part unknown, although it is posited to arise from an interaction of genetic and environmental factors. Although in vivo animal studies have been used to examine the effects of a number of Parkinson-inducing compounds, there is little information on reliable in vitro methodologies that can recapitulate the previously observed in vivo results. Here, we describe a method for generating mixed and chimeric neuron/glial cultures of postnatal substantia nigra (SN), independent of other monoaminergic nuclei in the ventral midbrain. Since many toxins do not affect regions of the midbrain except the SN, use of whole ventral midbrain from embryos can dilute any measurement of cell death. By specifically culturing ventrolateral midbrain containing the substantia nigra, one can more directly target the effects of dopaminergic toxins. In addition, this method can be used to test potential therapies for amelioration of Parkinson's disease.

Catecholamine neuron groups in rat brain slices differ in their susceptibility to excitatory amino acid induced dendritic degeneration

We investigated whether specific types of catecholamine neurons were differentially vulnerable to damage induced by excitatory amino acids (EAAs) in vitro in a rat brain slice preparation. Brain slices, 300 micro m thick, were cut horizontally, exposed to either N-methyl-D-aspartate (NMDA) or kainic acid (KA) for 2h, fixed and then cut into thin (30 micro m) sections in the same (horizontal) plane as the slice. The sections were immunolabelled for tyrosine hydroxylase to identify different groups of catecholamine neurons (substantia nigra (SN), paranigral (PN), interfascicular (IF) and hypothalamic A11, A13 and A14) which exhibited prominent dendritic projections in the horizontal plane. Loss of dendrites was used as a sensitive index of damage that precedes the loss of the cell body. Catecholamine neurons differed strikingly in their vulnerability of EAA-induced dendrite degeneration. The most vulnerable were those in the dorsal tier of the SN, whereas the most resistant were those in the hypothalamic A11 group. For example, in the dorsal tier of SN, NMDA (50 micro M) reduced the proportion of neurons with dendrites from 64% (+/- 8% SEM) in controls to 13% (+/- 7%) whereas the majority of A11 neurons (69 +/- 10%) retained their dendrites compared to controls (89% +/- 8%). The other groups of catecholamine neurons exhibited intermediate vulnerability. An essentially similar pattern of differential vulnerability was observed with KA. An understanding of the cellular mechanisms that underlie the particular vulnerability of SN neurons in the slice will aid the discovery of pharmacological therapies to prevent or slow the pathological process in neurodegenerative diseases which involve these neurons.

Neuroprotective role of estrogen upon methamphetamine and related neurotoxins within the nigrostriatal dopaminergic system

In this report we describe some of the data on the capacity for estrogen to function as a neuroprotectant of the nigrostriatal dopaminergic (NSDA) system. The data show that estrogen (E) can alter two different response characteristics to NSDA neurotoxins. The first being that striatal DA concentrations of ovariectomized rodents treated with E are consistently greater than non-E-treated animals in response to neurotoxins which produce degeneration of the NSDA system. The second being that E significantly reduces the amount of DA output upon initial exposure to the NSDA neurotoxin, 1-methyl-4-phenylpyridium ion (MPP+). At present, it is not known whether these two response characteristics are related. An intriguing possibility is that the E-dependent changes in initial DA output are related to the resultant neurotoxicity (attenuations in DA concentration reductions). So far our incipient findings do not seem to support this eventuality. However, additional testing on this topic is required. The present data suggest that one of the mechanisms by which E can exert these effects is through inhibition of DAT activity. This conclusion results from data which show that E produces: 1) an inhibition of [3H]DA uptake, 2) a reduction in DA clearance rates, and 3) an effect upon DA recovery that is similar to that observed to the putative DA uptake blocker, nomifensine. The capacity and significance for steroid hormones to modulate neurotransmitter transporters has been recently reviewed.

Dopamine transporter function assessed by antisense knockdown in the rat: protection from dopamine neurotoxicity

The plasma membrane dopamine transporter is located on presynaptic nerve terminals and is responsible for the termination of dopaminergic neurotransmission via dopamine reuptake. The dopamine transporter may also contribute to the pathogenesis of Parkinson disease. Dopamine transporter expression correlates well with susceptibility to neuronal degeneration in 1-methyl-4-phenyl-1,2,3,6 -tetrahydropyridine (MPTP)-induced parkinsonism. Recent studies have implicated the dopamine transporter in the uptake of both this neurotoxin and its metabolite, MPP(+), as well as another experimental neurotoxin, 6-hydroxydopamine. In these studies we examined the role of the dopamine transporter in the neurotoxicity of both MPP(+) and 6-hydroxydopamine in the rat brain using in vivo administration of phosphorothioate antisense oligonucleotides targeting dopamine transporter mRNA. Infusion of dopamine transporter antisense (1 nmol/day, 7 days) into the left substantia nigra pars compacta resulted in reduced (3)H-WIN 35-428 binding in the left striatum and significant levodopa and amphetamine-induced contralateral rotations. Unilateral pretreatment with dopamine transporter antisense prior to bilateral intrastriatal infusion of either MPP(+) or 6-hydroxydopamine resulted in asymmetrical striatal (3)H-WIN 35-428 binding and dopamine content as well as significant apomorphine-induced ipsilateral rotations, suggesting neuroprotection of nigrostriatal neurons on the antisense-treated side. Thus, the dopamine transporter appears to play a critical role in determining susceptibility to the experimental neurotoxins MPP(+) and 6-hydroxydopamine. In light of this, the dopamine transporter may prove useful, both as a marker for susceptibility to Parkinson's disease and as a target for therapeutic intervention.

Differential vulnerabilities of substantia nigra catecholamine neurons to excitatory amino acid-induced degeneration in rat midbrain slices

Although differential vulnerability in different regions of the central nervous system is a characteristic feature of neurodegenerative disorders in vivo, its cellular basis is not well understood. In the present study we investigated whether catecholamine neurons in different regions of the substantia nigra (SN) are differentially vulnerable to excitatory amino acid-induced damage in a midbrain slice preparation. Rats were anesthetized by halothane inhalation and killed, the brain was rapidly removed, and 300-microm-thick midbrain slices were cut horizontally on a vibratome. The slices were incubated at 35 degrees C for 2 h in saline buffer containing either kainic acid (KA) or N-methyl-d-aspartate (NMDA) (10-50 microM). They were then fixed and cut into 30-microm sections that were coplanar with the horizontal slice. Individual catecholamine neurons were identified in these thin sections using an antibody to tyrosine hydroxylase coupled to diaminobenzidine. Catecholaminergic neurons in the dorsal and ventral tiers of the SN were readily identified by reference to an atlas of the distribution of catecholamine neurons in the horizontal plane. Using dendritic degeneration as a sensitive index of damage, and submaximal concentrations of KA and NMDA, we found that catecholamine neurons in the dorsal tier were more vulnerable than those in the ventral tier. For example, KA (10 microM) caused a significant reduction in the proportion of neurons with dendrites in the dorsal tier (from 60 to 34%) without altering the dendritic arbor of ventral tier neurons. After treatment with 50 microM KA, only 11% of dorsal tier neurons retained any dendrites while 45% of ventral tier neurons retained their dendrites. These differences were statistically significant (P<0.001). A similar differential vulnerability was apparent in slices treated with NMDA; neurons in the dorsal tier lost dendrites before detectable damage in the ventral tier. An understanding of the comparative anatomical, neurochemical, and physiological properties of vulnerable (dorsal tier) and resistant (ventral tier) catecholamine neurons in rat SN may provide significant insights into the mechanisms and treatment of neurodegenerative disorders involving catecholamine neurons.

Calbindin-D 28kD immunofluorescence in ventral mesencephalic neurons labeled following injections of Fluoro-Gold in nucleus accumbens subterritories: inverse relationship relative to known neurotoxin vulnerabilities

The shell and core of the nucleus accumbens exhibit different vulnerabilities to neurotoxins. Calcium binding proteins are reported to offer some neuroprotection against excitotoxicity by suppressing or buffering intracellular calcium. Differences in the distributions of the calbindin-D 28kD (CB) and calretinin (CR) might be related to the different vulnerabilities to neurotoxins of dopaminergic neurons in the ventral mesencephalon that project to the core and medial shell of the nucleus accumbens. To address this possibility, Fluoro-Gold (FG) was injected into accumbens subterritories and numbers of retrogradely labeled neurons in the ventral tegmental area containing CB and CR immunoreactivities (ir) were expressed as a percentage of total numbers of labeled neurons. The perikaryal diameters and lengths of the immunoreactive dendrites of FG labeled neurons were also measured. About 70% and 35% of retrogradely labeled cells observed following core and medial shell injections, respectively, exhibited CB immunoreactivity. Differences were not observed in the percentages of FG labeled cells exhibiting CR immunoreactivity following medial shell (13%) and core (15%) injections. The mean perikaryal diameters and median summed lengths of dendrites of retrogradely labeled neurons containing CB were smaller than in labeled neurons lacking CB following injections in both core and medial shell of the nucleus accumbens. The data indicate that the different 6-hydroxydopamine (6-OHDA) vulnerabilities of ventral mesencephalic dopaminergic neurons are not obviously related to the presence of CB and CR.

Dopamine transporters and neuronal injury

The plasma membrane dopamine transporter (DAT) and the vesicular monoamine transporter (VMAT2) are essential for normal dopamine neurotransmission. DAT terminates the actions of dopamine by rapidly removing dopamine from the synapse, whereas VMAT2 loads cytoplasmic dopamine into vesicles for storage and subsequent release. Recent data suggest that perturbation of the tightly regulated balance between these two transporters predisposes the neurone to damage by a variety of insults. Most notable is the selective degeneration of DAT- and VMAT2-expressing dopamine nerve terminals in the striatum thought to underlie Parkinson's disease. DAT and VMAT2 expression can predict the selective vulnerability of neuronal populations, which suggests that therapeutic strategies aimed at altering DAT and VMAT2 function could have significant benefits in a variety of disorders.