Dopamine transporter is required for in vivo MPTP neurotoxicity: evidence from mice lacking the transporter

Developmental heptachlor exposure increases susceptibility of dopamine neurons to N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)in a gender-specific manner

Parkinson's disease (PD) is primarily thought of as a disease of aging. However, recent evidence points to the potential for exposure to xenobiotics during development to increase risk of PD. Here, we report that developmental exposure to the organochlorine pesticide heptachlor alters the dopamine system and increases neurotoxicity in an animal model of PD. Exposure of pregnant mice to heptachlor led to increased levels of the dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) levels at both the protein and mRNA level in their offspring. Increased DAT and VMAT2 levels were accompanied by alterations of mRNA levels of nuclear transcription factors that control dopamine neuron development and regulate DAT and VMAT2 levels in adulthood. At 12 weeks of age, control and heptachlor-exposed offspring were administered a moderate dose (2 x 10mg/kg) of the parkinsonism-inducing agent MPTP. Greater neurotoxicity as evidenced by a greater loss of striatal dopamine and potentiation of increased levels of glial fibrillary acidic protein and alpha-synuclein was observed in heptachlor-exposed offspring. The neurotoxicity observed was greater in the male offspring than the female offspring, suggesting that males are more susceptible to the long-term effects of developmental heptachlor exposure. These data suggest that developmental heptachlor exposure causes long-term alterations of the dopamine system thereby rendering it more susceptible to dopaminergic damage in adulthood.

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.

The PPARgamma agonist pioglitazone is effective in the MPTP mouse model of Parkinson's disease through inhibition of monoamine oxidase B

BACKGROUND AND PURPOSE

The peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist pioglitazone has previously been shown to attenuate dopaminergic cell loss in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease, an effect attributed to its anti-inflammatory properties. In the present investigation, we provide evidence that pioglitazone is effective in the MPTP mouse model, not via an anti-inflammatory action, but through inhibition of MAO-B, the enzyme required to biotransform MPTP to its active neurotoxic metabolite 1-methyl-4-phenylpyridinium (MPP+).

EXPERIMENTAL APPROACH

Mice were treated with pioglitazone (20 mg kg(-1) b.i.d. (twice a day), p.o., for 7 days), prior and post or post-MPTP (30 mg kg(-1) s.c.) treatment. Mice were then assessed for motor impairments on a beam-walking apparatus and for reductions in TH immunoreactivity in the substantia nigra and depletions in striatal dopamine. The effects of pioglitazone on striatal MPP+ levels and MAO-B activity were also assessed.

KEY RESULTS

Mice treated with MPTP showed deficits in motor performance, marked depletions in striatal dopamine levels and a concomitant reduction in TH immunoreactivity in the substantia nigra. Pretreatment with pioglitazone completely prevented these effects of MPTP. However, pretreatment with pioglitazone also significantly inhibited the MPTP-induced production of striatal MPP+ and the activity of MAO-B in the striatum.

CONCLUSIONS AND IMPLICATIONS

The neuroprotection observed with pioglitazone pretreatment in the MPTP mouse model was due to the blockade of the conversion of MPTP to its active toxic metabolite MPP+, via inhibition of MAO-B.

Stage-dependent vulnerability of fetal mesencephalic neuroprogenitors towards dopaminergic neurotoxins

Although extensive knowledge exists on selective vulnerability of dopaminergic neurons against parkinsonism-inducing neurotoxins, there is a complete lack of such data on immature neuroprogenitors. Here we investigated the toxicity of 1-methyl-4-phenylpyridinium (MPP+), 6-hydroxydopamine (6-OHDA) and the free radical generator H2O2 on various developmental stages of predopaminergic mesencephalic neuroprogenitors (mNPCs) to evaluate stage-dependency of selective dopaminergic neurotoxicity. Striatal NPCs (sNPCs) without dopaminergic differentiation potential served as controls. Exposure of both undifferentiated NPCs to MPP+ resulted in concentration-dependent cell death at concentrations of >10 microM after 72 h without differences between both cell types, while 6-OHDA led to relevant cell death at 1000 microM after 24h with significant higher sensitivity of mNPCs compared to sNPCs. H2O2 did not induce relevant cell death in all cell types. In NPC cultures differentiated for 14 days, MPP+ showed enhanced toxicity compared to the undifferentiated counterparts, but no significant differences between both NPC type and differentiation conditions. 6-OHDA showed similar toxicity pattern in differentiated compared to undifferentiated NPCs. By evaluating the toxicity of MPP+ on MAP2ab+ neurons derived from both mNPCs and sNPCs as well as tyrosine hydroxylase (TH)+ dopaminergic cells from mNPCs, we found concentration-dependent cell death of all cell types with no increased vulnerability of TH+ cells. Primary TH+ neurons showed significantly higher vulnerability to MPP+. Together, we demonstrated stage-dependent vulnerability of NPCs towards dopaminergic neurotoxins, but no selective vulnerability of NPC-derived TH+ dopaminergic cells towards MPP+. This cell system seems not suitable as a screening tool for selective dopaminergic toxicity.

Circuit-based framework for understanding neurotransmitter and risk gene interactions in schizophrenia

Many risk genes interact synergistically to produce schizophrenia and many neurotransmitter interactions have been implicated. We have developed a circuit-based framework for understanding gene and neurotransmitter interactions. NMDAR hypofunction has been implicated in schizophrenia because NMDAR antagonists reproduce symptoms of the disease. One action of antagonists is to reduce the excitation of fast-spiking interneurons, resulting in disinhibition of pyramidal cells. Overactive pyramidal cells, notably those in the hippocampus, can drive a hyperdopaminergic state that produces psychosis. Additional aspects of interneuron function can be understood in this framework, as follows. (i) In animal models, NMDAR antagonists reduce parvalbumin and GAD67, as found in schizophrenia. These changes produce further disinhibition and can be viewed as the aberrant response of a homeostatic system having a faulty activity sensor (the NMDAR). (ii) Disinhibition decreases the power of gamma oscillation and might thereby produce negative and cognitive symptoms. (iii) Nicotine enhances the output of interneurons, and might thereby contribute to its therapeutic effect in schizophrenia.

Melatonin reduces the neuronal loss, downregulation of dopamine transporter, and upregulation of D2 receptor in rotenone-induced parkinsonian rats

Parkinson's disease (PD) is a movement disorder resulting from nigrostriatal dopaminergic neurodegeneration. The impairment of mitochondrial function and dopamine synaptic transmission are involved in the pathogenesis of PD. Two mitochondrial inhibitors, 1-methyl-4-phenylpyridine (MPP(+)) and rotenone, have been used to induce dopaminergic neuronal death both in in vitro and in vivo models of PD. Because the uptake of MPP(+) is mediated by the dopamine transporter (DAT), we used a cell-permeable rotenone-induced PD model to investigate the role of DAT and dopamine D2 receptor (D2R) on dopaminergic neuronal loss. Rotenone subcutaneously infused for 14 days induced PD symptoms in rats, as indicated by reduced spontaneous locomotor activity (hypokinesis), loss of tyrosine hydroxylase (TH, a marker enzyme for dopamine neurons) immunoreactivity in the substantia nigra and striatum, obvious alpha-synuclein accumulation, downregulated DAT protein expression, and upregulated D2R expression. Interestingly, rotenone also caused significant noradrenergic neuronal loss in the locus coeruleus. Melatonin, an antioxidant, prevented nigrostriatal neurodegeneration and alpha-synuclein aggregation without affecting the rotenone-induced weight loss and hypokinesis. However, rotenone-induced hypokinesis was markedly reversed by the DAT antagonist nomifensine and body weight loss was attenuated by the D2R antagonist sulpiride. In addition, both antagonists significantly prevented the reduction of striatal TH or DAT immunoreactivity but not the loss of nigral TH- and DAT-immunopositive neurons. These results suggested that oxidative stress and DAT downregulation are involved in the rotenone-induced pathogenesis of nigrostriatal dopaminergic neurodegeneration, whereas D2R upregulation may simply represent a compensatory response.

MAO-B elevation in mouse brain astrocytes results in Parkinson's pathology

Age-related increases in monoamine oxidase B (MAO-B) may contribute to neurodegeneration associated with Parkinson's disease (PD). The MAO-B inhibitor deprenyl, a long-standing antiparkinsonian therapy, is currently used clinically in concert with the dopamine precursor L-DOPA. Clinical studies suggesting that deprenyl treatment alone is not protective against PD associated mortality were targeted to symptomatic patients. However, dopamine loss is at least 60% by the time PD is symptomatically detectable, therefore lack of effect of MAO-B inhibition in these patients does not negate a role for MAO-B in pre-symptomatic dopaminergic loss. In order to directly evaluate the role of age-related elevations in astroglial MAO-B in the early initiation or progression of PD, we created genetically engineered transgenic mice in which MAO-B levels could be specifically induced within astroglia in adult animals. Elevated astrocytic MAO-B mimicking age related increase resulted in specific, selective and progressive loss of dopaminergic neurons in the substantia nigra (SN), the same subset of neurons primarily impacted in the human condition. This was accompanied by other PD-related alterations including selective decreases in mitochondrial complex I activity and increased mitochondrial oxidative stress. Along with a global astrogliosis, we observed local microglial activation within the SN. These pathologies correlated with decreased locomotor activity. Importantly, these events occurred even in the absence of the PD-inducing neurotoxin MPTP. Our data demonstrates that elevation of murine astrocytic MAO-B by itself can induce several phenotypes of PD, signifying that MAO-B could be directly involved in multiple aspects of disease neuropathology. Mechanistically this may involve increases in membrane permeant H(2)O(2) which can oxidize dopamine within dopaminergic neurons to dopaminochrome which, via interaction with mitochondrial complex I, can result in increased mitochondrial superoxide. Our inducible astrocytic MAO-B transgenic provides a novel model for exploring pathways involved in initiation and progression of several key features associated with PD pathology and for therapeutic drug testing.

Differential effects of overexpression of wild-type and mutant human alpha-synuclein on MPP+-induced neurotoxicity in PC12 cells

The genetic background and the pathogenesis of familial Parkinson's disease (PD) have not been fully elucidated. Two missense mutations in the alpha-synuclein gene, A30P and A53T, have been linked to familial PD. Increasing evidence suggests the involvement of alpha-synuclein, the dopamine transporter (DAT), and neurotoxins in the pathogenesis of PD, but their relationships to the death of nigral cells remains poorly understood. In the present study, we used the PC12 cell line, a well recognized model of the nigral cell, to investigate the effects of overexpression of wild-type (WT) and mutant human alpha-synuclein on MPP+-induced neurotoxicity. We found that overexpression of mutant alpha-synuclein enhanced the toxicity of MPP+ to PC12 cells and elevated intracellular reactive oxygen species (ROS) levels, whereas overexpression of WT alpha-synuclein protected PC12 cells against MPP+ toxicity and lowered ROS levels. Furthermore, assays of 131I-FP-beta-CIT binding with DAT membrane fractions showed that WT and mutant alpha-synuclein had different effects on the expression of DAT on the cell membrane following exposure to MPP+. WT alpha-synuclein reduced the toxic effect of MPP+ by facilitating DAT internalization, while both A30P and A53T alpha-synuclein aggravated the toxic effect of MPP+ by reducing DAT internalization. These data indicate that alpha-synuclein regulates ROS levels and DAT surface expression in dopaminergic neurons, and thus changes the response of these cells to MPP+.

Unregulated cytosolic dopamine causes neurodegeneration associated with oxidative stress in mice

The role of dopamine as a vulnerability factor and a toxic agent in Parkinson's disease (PD) is still controversial, yet the presumed dopamine toxicity is partly responsible for the "DOPA-sparing" clinical practice that avoids using L-3,4-dihydroxyphenylalanine (L-DOPA), a dopamine precursor, in early PD. There is a lack of studies on animal models that directly isolate dopamine as one determining factor in causing neurodegeneration. To address this, we have generated a novel transgenic mouse model in which striatal neurons are engineered to take up extracellular dopamine without acquiring regulatory mechanisms found in dopamine neurons. These mice developed motor dysfunctions and progressive neurodegeneration in the striatum within weeks. The neurodegeneration was accompanied by oxidative stress, evidenced by substantial oxidative protein modifications and decrease in glutathione. Ultrastructural morphologies of degenerative cells suggest necrotic neurodegeneration. Moreover, L-DOPA accelerated neurodegeneration and worsened motor dysfunction. In contrast, reducing dopamine input to striatum by lesioning the medial forebrain bundle attenuated motor dysfunction. These data suggest that pathology in genetically modified striatal neurons depends on their dopamine supply. These neurons were also supersensitive to neurotoxin. A very low dose of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (5 mg/kg) caused profound neurodegeneration of striatal neurons, but not midbrain dopamine neurons. Our results provide the first in vivo evidence that chronic exposure to unregulated cytosolic dopamine alone is sufficient to cause neurodegeneration. The present study has significant clinical implications, because dopamine replacement therapy is the mainstay of PD treatment. In addition, our model provides an efficient in vivo approach to test therapeutic agents for PD.

Dopamine transporter mutant mice in experimental neuropharmacology

An opportunity to perform targeted genetic manipulations in mice has provided another dimension for modern pharmacological research. Genetically modified mice have become important tools to investigate functions of previously unexplored proteins, define mechanism of action of new and known pharmacological drugs, and validate novel targets for treatment of human disorders. One of the best examples of such use of genetic models in experimental pharmacology represents investigations involving mice deficient in the gene encoding the dopamine transporter (DAT). The dopamine transporter tightly regulates the extracellular dynamics of dopamine by recapturing released neurotransmitter into the presynaptic terminals, and genetic deletion of this protein results in profound alterations in both the presynaptic homeostasis and the extracellular dynamics of dopamine. By using this model of severe dopaminergic dysregulation, significant progress has been made in defining the major target of psychotropic drugs, understanding the mechanisms of their action, unraveling novel signaling events relevant for dopaminergic transmission, and mapping neuronal pathways involved in dopamine-related behaviors. Furthermore, DAT mutant mice provided an opportunity to model in vivo conditions of extreme dopaminergic dysfunction that could be relevant for human disorders such as ADHD, schizophrenia, and Parkinson's disease and, thus, could serve as test systems for developing novel treatments for these and related disorders.

Beneficial effects of dietary omega-3 polyunsaturated fatty acid on toxin-induced neuronal degeneration in an animal model of Parkinson's disease

In this study, we examined whether omega-3 (n-3) polyunsaturated fatty acids (PUFAs) may exert neuroprotective action in Parkinson's disease, as previously shown in Alzheimer's disease. We exposed mice to either a control or a high n-3 PUFA diet from 2 to 12 months of age and then treated them with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 140 mg/kg in 5 days). High n-3 PUFA dietary consumption completely prevented the MPTP-induced decrease of tyrosine hydroxylase (TH)-labeled nigral cells (P<0.01 vs. MPTP mice on control diet), Nurr1 mRNA (P<0.01 vs. MPTP mice on control diet), and dopamine transporter mRNA levels (P<0.05 vs. MPTP mice on control diet) in the substantia nigra. Although n-3 PUFA dietary treatment had no effect on striatal dopaminergic terminals, the high n-3 PUFA diet protected against the MPTP-induced decrease in dopamine (P<0.05 vs. MPTP mice on control diet) and its metabolite dihydroxyphenylacetic acid (P<0.05 vs. MPTP mice on control diet) in the striatum. Taken together, these data suggest that a high n-3 PUFA dietary intake exerts neuroprotective actions in an animal model of Parkinsonism.

Evidence for Hydroxyl Radical Scavenging Action of Nitric Oxide Donors in the Protection Against 1-Methyl-4-phenylpyridinium-induced Neurotoxicity in Rats

In the present study we provide evidence for hydroxyl radical (*OH) scavenging action of nitric oxide (NO*), and subsequent dopaminergic neuroprotection in a hemiparkinsonian rat model. Reactive oxygen species are strongly implicated in the nigrostriatal dopaminergic neurotoxicity caused by the parkinsonian neurotoxin, 1-methyl-4-phenylpyridinium (MPP+). Since the role of this free radical as a neurotoxicant or neuroprotectant is debatable, we investigated the effects of some of the NO* donors such as S-nitroso-N-acetylpenicillamine (SNAP), 3-morpholinosydnonimine hydrochloride (SIN-1), sodium nitroprusside (SNP) and nitroglycerin (NG) on in vitro *OH generation in a Fenton-like reaction involving ferrous citrate, as well as in MPP+-induced *OH production in the mitochondria. We also tested whether co-administration of NO* donor and MPP+ could protect against MPP+-induced dopaminergic neurotoxicity in rats. While NG, SNAP and SIN-1 attenuated MPP+-induced *OH generation in the mitochondria, and in a Fenton-like reaction, SNP caused up to 18-fold increase in *OH production in the latter reaction. Striatal dopaminergic depletion following intranigral infusion of MPP+ in rats was significantly attenuated by NG, SNAP and SIN-1, but not by SNP. Solutions of NG, SNAP and SIN-1, exposed to air for 48 h to remove NO*, when administered similarly failed to attenuate MPP+-induced neurotoxicity in vivo. Conversely, long-time air-exposed SNP solution when administered in rats intranigrally, caused a dose-dependent depletion of the striatal dopamine. These results confirm the involvement of *OH in the nigrostriatal degeneration caused by MPP+, indicate the *OH scavenging ability of NO*, and demonstrate protection by NO* donors against MPP+-induced dopaminergic neurotoxicity in rats.

Response to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) differs in mouse strains and reveals a divergence in JNK signaling and COX-2 induction prior to loss of neurons in the substantia nigra pars compacta

Parkinson's disease (PD) is a neurodegenerative disease whose hallmark pathological features include a selective loss of dopaminergic neurons in the midbrain. Recent studies have described the activation of a stress-induced signal cascade, c-Jun N-terminal kinase (JNK)-mediated activation of c-Jun, and an increase in the expression of a downstream effector, cyclooxygenase 2 (COX-2), in postmortem PD brains. The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which induces selective neuronal loss in the midbrain similar to that seen in PD, also induces JNK-mediated activation of c-Jun and generates a COX-2 response in C57BL/6J mice. However, mice exhibit a strain-dependent susceptibility to MPTP. Identifying the point(s) of molecular divergence in the MPTP-induced response may provide insight into the cause of PD or a means to identify susceptibility to PD in humans. Here we examined JNK signaling and COX-2 induction in two strains of mice, the MPTP-sensitive C57BL/6J and the MPTP-resistant Swiss Webster (SW). We show that C57BL/6J and SW strains differ in JNK and c-Jun activation in response to MPTP. In addition, the MPTP-induced COX-2 response occurs exclusively in C57BL/6J mice. Furthermore, strain-specific responses to MPTP are not due to differences in MPP(+) levels and are not secondary to cell death. These results provide evidence toward a mechanism of strain-dependent sensitivity to MPTP.

Effects of treadmill exercise on dopaminergic transmission in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned mouse model of basal ganglia injury

Studies have suggested that there are beneficial effects of exercise in patients with Parkinson's disease, but the underlying molecular mechanisms responsible for these effects are poorly understood. Studies in rodent models provide a means to examine the effects of exercise on dopaminergic neurotransmission. Using intensive treadmill exercise, we determined changes in striatal dopamine in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned mouse. C57BL/6J mice were divided into four groups: (1) saline, (2) saline plus exercise, (3) MPTP, and (4) MPTP plus exercise. Exercise was started 5 d after MPTP lesioning and continued for 28 d. Treadmill running improved motor velocity in both exercise groups. All exercised animals also showed increased latency to fall (improved balance) using the accelerating rotarod compared with nonexercised mice. Using HPLC, we found no difference in striatal dopamine tissue levels between MPTP plus exercise compared with MPTP mice. There was an increase detected in saline plus exercise mice. Analyses using fast-scan cyclic voltammetry showed increased stimulus-evoked release and a decrease in decay of dopamine in the dorsal striatum of MPTP plus exercise mice only. Immunohistochemical staining analysis of striatal tyrosine hydroxylase and dopamine transporter proteins showed decreased expression in MPTP plus exercise mice compared with MPTP mice. There were no differences in mRNA transcript expression in midbrain dopaminergic neurons between these two groups. However, there was diminished transcript expression in saline plus exercise compared with saline mice. Our findings suggest that the benefits of treadmill exercise on motor performance may be accompanied by changes in dopaminergic neurotransmission that are different in the injured (MPTP-lesioned) compared with the noninjured (saline) nigrostriatal system.

Increased vulnerability of nigrostriatal terminals in DJ-1-deficient mice is mediated by the dopamine transporter

Mutations in the gene for DJ-1 have been associated with early-onset autosomal recessive parkinsonism. Previous studies of null DJ-1 mice have shown alterations in striatal dopamine (DA) transmission with no DAergic cell loss. Here we characterize a new line of DJ-1-deficient mice. A subtle locomotor deficit was present in the absence of a change in striatal DA levels. However, increased [(3)H]-DA synaptosomal uptake and [(125)I]-RTI-121 binding were measured in null DJ-1 vs. wild-type mice. Western analyses of synaptosomes revealed significantly higher dopamine transporter (DAT) levels in pre-synaptic membrane fractions. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) exposure exacerbated striatal DA depletion in null DJ-1 mice with no difference in DAergic nigral cell loss. Furthermore, increased 1-methyl-4-phenylpyridinium (MPP(+)) synaptosomal uptake and enhanced MPP(+) accumulation were measured in DJ-1-deficient vs. control striatum. Thus, under null DJ-1 conditions, DAT changes likely contribute to altered DA neurotransmission and enhanced sensitivity to toxins that utilize DAT for nigrostriatal entry.

The COX-2 inhibitor parecoxib produces neuroprotective effects in MPTP-lesioned rats

The present study investigated the effects of the selective cyclooxygenase-2 (COX-2) inhibitor parecoxib (Bextratrade mark) in the prevention of motor and cognitive impairments observed in rats after an intranigral infusion of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a model of the early phase of Parkinson's disease. The treatment with parecoxib (10 mg/kg) administered prior to the surgery and daily (2 mg/kg) for the subsequent 21 days, prevented the MPTP-treated rats from presenting decreased locomotor and exploratory behavior, increased immobility, and impairment while performing the cued version of the Morris water maze. Furthermore, parecoxib treatment also significantly prevented the reduction of tyrosine hydroxylase protein expression in the substantia nigra (7, 14 and 21 days after surgery), and in the striatum (14 and 21 days after surgery) as immunodetected by western blotting. These results strongly suggest that parecoxib exerts a neuroprotective effect on motor, tyrosine hydroxylase expression, and cognitive functions as it prevents their impairments within the confines of this animal model of the early phase of Parkinson's disease.

Striatal susceptibility to a dopaminergic neurotoxin is independent of sex hormone effects on cell survival and DAT expression but is exacerbated by central aromatase inhibition

The aim of this study was to investigate further the hormone-dependent processes underlying sex differences in neurotoxic responses within the rat nigrostriatal dopaminergic (NSDA) pathway after partial lesioning with 6-OHDA, a state thought to mimic the early stages of Parkinson's disease where, in humans and animal models alike, males appear to be more susceptible. Contrary to our hypotheses, hormone manipulations (gonadectomy +/- oestrogen or androgen treatment) failed to alter survival of tyrosine hydroxylase immunoreactive cells in the substantia nigra pars compacta (SNc) after lesioning; this indicates that, unlike inherent sex differences in toxin-induced striatal dopamine depletion, sex differences in cell loss were not hormonally generated, and that hormone-dependent changes in dopamine depletion can occur independently of cell survival. In addition, hormonally induced changes in striatal expression of the dopamine transporter (DAT), an important factor for 6-OHDA toxicity, did not correlate with hormonal influences on striatal dopamine loss and, in males, central inhibition of aromatase prior to 6-OHDA infusion exacerbated striatal dopamine loss with no effect on SNc tyrosine hydroxylase-immunoreactive survival, suggesting locally generated oestrogen is neuroprotective. These results support the novel view that sex steroid hormones produced peripherally and centrally play a significant, sex-specific role within the sexually dimorphic NSDA pathway to modulate plastic, compensatory responses aimed at restoring striatal dopamine functionality, without affecting cell loss.

RNA interference-mediated knockdown of α-synuclein protects human dopaminergic neuroblastoma cells from MPP+ toxicity and reduces dopamine transport

The critical observation in the pathology of Parkinson's disease (PD) is that neurodegeneration is largely restricted to dopaminergic neurons that develop cytoplasmic inclusions called Lewy bodies. These aggregations contain the protein alpha-synuclein. Furthermore, it is becoming apparent that alpha-synuclein expression levels are a major factor in PD pathogenesis. Patients with additional copies of the alpha-synuclein gene develop PD with a severity proportional to levels of alpha-synuclein overexpression. Similarly, overexpression of alpha-synuclein in in vitro and in vivo models has been shown to be toxic. However, little is known about the effects of reducing alpha-synuclein expression in human neurons. To investigate this, we have developed a system in which levels of alpha-synuclein can be acutely suppressed by using RNA interference (RNAi) in a physiologically relevant human dopaminergic cellular model. By using small interfering RNA (siRNA) molecules targeted to endogenous alpha-synuclein, we achieved 80% protein knockdown. We show that alpha-synuclein knockdown has no effect on cellular survival either under normal growth conditions over 5 days or in the presence of the mitochondrial inhibitor rotenone. Knockdown does, however, confer resistance to the dopamine transporter (DAT)-dependent neurotoxin N-methyl-4-phenylpyridinium (MPP(+)). We then demonstrate for the first time that alpha-synuclein suppression decreases dopamine transport in human cells, reducing the maximal uptake velocity (V(max)) of dopamine and the surface density of its transporter by up to 50%. These results show that RNAi-mediated alpha-synuclein knockdown alters cellular dopamine homeostasis in human cells and may suggest a mechanism for the increased survival in the presence of MPP(+), a toxin used extensively to model Parkinson's disease.

Temporal mRNA profiles of inflammatory mediators in the murine 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrimidine model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). With the exception of a few rare familial forms of the disease, the precise molecular mechanisms underlying PD are unknown. Inflammation is a common finding in the PD brain, but due to the limitation of postmortem analysis its relationship to disease progression cannot be established. However, studies using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD have also identified inflammatory responses in the nigrostriatal pathway that precede neuronal degeneration in the SNpc. To assess the pathological relevance of these inflammatory responses and to identify candidate genes that might contribute to neuronal vulnerability, we used quantitative reverse-transcription polymerase chain reaction (qRT-PCR) to measure mRNA levels of 11 cytokine and chemokine encoding genes in the striatum of MPTP-sensitive (C57BL/6J) and MPTP-insensitive (Swiss Webster, SWR) mice following administration of MPTP. The mRNA levels of all 11 genes changed following MPTP treatment, indicating the presence of inflammatory responses in both strains. Furthermore, of the 11 genes examined only 3, interleukin 6 (Il-6), macrophage inflammatory protein 1 alpha/CC chemokine ligand 3 (Mip-1alpha/Ccl3) and macrophage inflammatory protein 1 beta/CC chemokine ligand 4 (Mip-1beta/Ccl4), were differentially regulated between C57BL/6J and SWR mice. In both mouse strains, the level of monocyte chemoattractant protein 1/CC chemokine ligand 2 (Mcp-1/Ccl2) mRNA was the first to increase following MPTP administration, and might represent a key initiating component of the inflammatory response. Using Mcp-1/Ccl2 knockout mice backcrossed onto a C57BL/6J background we found that MPTP-stimulated Mip-1alpha/Ccl3 and Mip-1beta/Ccl4 mRNA expression was significantly lower in the knockout mice; suggesting that Mcp-1/Ccl2 contributes to MPTP-enhanced expression of Mip-1alpha/Ccl3 and Mip-1beta/Ccl4. However, stereological analysis of SNpc neuronal loss in Mcp-1/Ccl2 knockout and wild-type mice showed no differences. These findings suggest that it is the ability of dopaminergic SNpc neurons to survive an inflammatory insult, rather than genetically determined differences in the inflammatory response itself, that underlie the molecular basis of MPTP resistance.

Dopamine transporter relation to dopamine turnover in Parkinson's disease: a positron emission tomography study

OBJECTIVE

To investigate the role of the dopamine transporter (DAT) in the regulation of synaptic dopamine (DA) levels in Parkinson's disease and its role in the preservation of DA in presynaptic terminals.

METHODS

Ten Parkinson's disease patients (age, 62.9 +/- 9.5 years; Unified Parkinson's Disease Rating Scale motor score in "off" state, 28.5 +/- 8.2) underwent positron emission tomography with (11)C-methylphenidate (MP, a DAT marker), (11)C-dihydrotetrabenazine (a vesicular monoamine transporter 2 marker), and (18)F-fluorodopa, leading to the determination of the MP and (11)C-dihydrotetrabenazine binding potentials (BPs) and the effective distribution volume for (18)F-fluorodopa, the inverse of DA turnover. Seven patients also underwent positron emission tomography with (11)C-raclopride before and 1 hour after levodopa administration to estimate levodopa-induced changes in synaptic DA concentration.

RESULTS

We found a significant positive correlation between effective distribution volume and BP(MP) (r = 0.93; p < 0.001) and a significant negative correlation between changes in synaptic DA concentration and BP(MP) (r = -0.93; p = 0.04), independent of disease severity and duration.

INTERPRETATION

These data show that in Parkinson's disease, greater DAT levels are directly associated with lower DA turnover and lower changes in synaptic DA concentration. This implies that an important functional role of DAT is to maintain relatively constant synaptic DA levels and to preserve DA in nerve terminals. A decrease in DAT, although potentially serving as a compensatory mechanism in early disease, may ultimately result in increased DA turnover and higher oscillations in synaptic DA concentration, thereby possibly predisposing toward the occurrence of motor complications as disease progresses.

Intrastriatal infusion of the Parkinsonian neurotoxin, MPP+, induces damage of striatal cell nuclei in Sprague–Dawley rats

The potent Parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine is known to destroy dopaminergic neurons of the basal ganglia. Its neurotoxically active metabolite, 1-methyl-4-phenyl pyridinium (MPP(+)), has been examined in the present study to verify whether administration of the neurotoxin that depletes about 70% of the striatal dopamine (DA) can cause damage to nuclear components of the cells at the terminal region, the striatum. Unilateral intrastriatal infusion of MPP(+) (100 and 200 nmol in 4 microl saline) caused a dose-dependent depletion of striatal DA (69 and 92%, respectively), as measured employing HPLC electrochemistry. It also resulted in the loss of tyrosine hydroxylase (TH) immunoreactivity in the striatum and in the perikarya at substantia nigra pars compacta (SNpc) and acetylcholinesterase histoenzymological staining in the striatum. Specific nuclear staining employing Hoechst 33342 and acridine orange revealed distorted and spindle shaped nuclei, and perinuclear positioning of nucleolus, respectively, for the former and latter dyes in several of the cell populations in the ipsilateral striatum compared to the contralateral side. Existence of a widened lateral ventricle at the side that received the neurotoxin, as well as denser cellular population, as compared to the contralateral side under transmission electron microscope evidenced general shrinkage of the striatum. Extensive damage of the nuclei was visible in the cell bodies in the treated side. These results demonstrate non-specific damage extending to the cellular groups including cholinergic neurons in addition to dopaminergic neurons in the striatum to intrastriatal administration of the Parkinsonian neurotoxin, MPP(+).

Microarrays in Parkinson's disease: a systematic approach

Neurological disease (ND) is one of the greatest challenges facing our population, from medical, financial, and social perspectives. The application of new research approaches to understand the underlying pathogenesis of ND is critical. In this article, we review the use of microarray analysis in Parkinson's disease (PD). Microarrays have tremendous power, simultaneously querying the expression of tens of thousands of genes from a given biological sample. Coupled with impressive advances in statistical tools for analyzing large, complex data sets, well-designed microarray experiments are poised to make a big impact in the field of ND. Parkinson's disease is a devastating neurodegenerative disease well suited to a systems-based microarray analysis. Genetic and environmental rodent models of PD emulate many of the cardinal features of human PD, providing the unique opportunity to compare gene expression profiles from different etiologies of the same disease. The elucidation of important gene expression patterns during disease will make possible identification of genetic susceptibility markers, biomarkers of disease progression, and new therapeutic targets.

Regulation of the dopamine transporter by phosphorylation

The dopamine transporter (DAT) is a neuronal phosphoprotein and target for psychoactive drugs that plays a critical role in terminating dopaminergic transmission by reuptake of dopamine from the synaptic space. Control of DAT activity and plasma membrane expression are therefore central to drug actions and the spatial and temporal regulation of synaptic dopamine levels. DATs rapidly traffic between the plasma membrane and endosomal compartments in both constitutive and protein kinase C-dependent manners. Kinase activators, phosphatase inhibitors, and transported substrates modulate DAT phosphorylation and activity, but the underlying mechanisms and role of phosphorylation in these processes are poorly understood. Complex adaptive changes in DAT function potentially related to these processes are also induced by psychostimulant and therapeutic transport blockers such as cocaine and methylphenidate. This chapter provides an overview of the current state of knowledge regarding DAT phosphorylation and its relationship to transporter activity and trafficking. A better understanding of how dopaminergic neurons regulate DAT function and the role of phosphorylation may lead to the identification of novel therapeutic targets for the treatment and prevention of dopaminergic disorders.

Overexpression of -Synuclein following Methamphetamine: Is It Good or Bad?

alpha-Synuclein is a presynaptic protein involved in various degenerative disorders now defined as synucleinopathies. These include neurological diseases that share a few pathological features consisting of aggregates of both normal and altered alpha-synuclein within specific neuronal populations and/or glial cells. The prototype of synucleinopathies is represented by Parkinson's disease (PD) in which alpha-synuclein is identified as a constant component of neuronal pale eosinophilic inclusions: "the Lewy Bodies." In the present article, we discuss the potential significance of amphetamine-induced overexpression of alpha-synuclein in light of clinical findings showing neurodegeneration following overexpression of alpha-synuclein and recent experimental studies that measured increased expression of alpha-synuclein following amphetamine derivatives.

Developmental exposure to the pesticide dieldrin alters the dopamine system and increases neurotoxicity in an animal model of Parkinson's disease

Exposure to pesticides has been suggested to increase the risk of Parkinson's disease (PD), but the mechanisms responsible for this association are not clear. Here, we report that perinatal exposure of mice during gestation and lactation to low levels of dieldrin (0.3, 1, or 3 mg/kg every 3 days) alters dopaminergic neurochemistry in their offspring and exacerbates MPTP toxicity. At 12 wk of age, protein and mRNA levels of the dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) were increased by perinatal dieldrin exposure in a dose-related manner. We then administered MPTP (2 x 10 mg/kg s.c) at 12 wk of age and observed a greater reduction of striatal dopamine in dieldrin-exposed offspring, which was associated with a greater DAT:VMAT2 ratio. Additionally, dieldrin exposure during development potentiated the increase in GFAP and alpha-synuclein levels induced by MPTP, indicating increased neurotoxicity. In all cases there were greater effects observed in the male offspring than the female, similar to that observed in human cases of PD. These data suggest that developmental exposure to dieldrin leads to persistent alterations of the developing dopaminergic system and that these alterations induce a "silent" state of dopamine dysfunction, thereby rendering dopamine neurons more vulnerable later in life.

Methylphenidate exerts no neurotoxic, but neuroprotective effects in vitro

Methylphenidate (MPH) is the most common used drug in child and adolescent psychiatry. Despite of this fact, however, little is known about its exact pharmacological mechanisms. Here we investigated the toxic effects of MPH in vitro in human embryonic kidney (HEK-293) cells stably expressing the human dopamine transporter (HEK-hDAT cells) and in cultured rat embryonic (E14.5) mesencephalic cultures. MPH alone (up to 1 mM) affected neither the growth of HEK-hDAT cells nor the survival of dopaminergic (DA) neurons in primary cultures after treatment up to 72 h. No differences in neuronal arborisation or in the density of synapses were detected. 1-methyl-4-phenylpyridinium (MPP(+)) showed no toxic effect in HEK-293 cells, but had significant toxic effects in HEK-hDAT cells and DA neurons. MPH (1 microM - 1 mM) dose-dependently reduced this cytotoxicity in HEK-hDAT cells and primary mesencephalic DA neurons. The presented results show that application of MPH alone does not have any toxic effect on DA cells in vitro. The neurotoxic effects of MPP(+) could be significantly reduced by co-application of MPH, an effect that is most likely explained by MPH blocking the DAT.

(-)-Epigallocatechin gallate regulates dopamine transporter internalization via protein kinase C-dependent pathway

Dopamine transporter (DAT) provides not only an integral component of dopaminergic neurotransmission but also a molecular gateway for the accumulation of some neurotoxins such as 1-methyl-4-phenylpyridinium (MPP(+)), a metabolite of 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP). Previous study reported that the neuroprotective effects of green tea polyphenols against MPP(+)-induced neurotoxicity were related to its inhibitory effect on MPP(+) uptake via DAT in dopaminergic cells. To extend the study, we investigated (-)-epigallocatechin gallate (EGCG), a monomer of green tea polyphenols, on DAT internalization in DAT-overexpressed PC12 cells. We found that EGCG (1-100 microM) can induce a dose-dependent inhibition of dopamine uptake in DAT-PC12 cells. In parallel, treatment of EGCG decreased membrane-bound DAT by 15% to 60%. Furthermore, protein kinase C (PKC) inhibitor GF109203X at 2 microM can markedly diminish the inhibitory effects of EGCG on dopamine uptake and reverse the EGCG-induced internalization of DAT. In addition, semiquantitative RT-PCR analysis indicated that EGCG did not affect DAT mRNA expression in the PC12 cells. These data suggest that EGCG exerts its inhibitory effect on DAT by modulating DAT internalization, in which PKC activation may be involved.

Common Human 5′ Dopamine Transporter (SLC6A3) Haplotypes Yield Varying Expression Levels In Vivo

1. Individuals display significant differences in their levels of expression of the dopamine transporter (DAT; SLC6A3). These differences in DAT are strong candidates to contribute to individual differences in motor, mnemonic and reward functions. To identify "cis"-acting genetic mechanisms for these individual differences, we have sought variants in 5' aspects of the human DAT gene and identified the haplotypes that these variants define. 2. We report (i) significant relationships between 5' DAT haplotypes and human individual differences in ventral striatal DAT expression assessed in vivo using [(11)C] cocaine PET and (ii) apparent confirmation of these results in studies of DAT expression in postmortem striatum using [(3)H] carboxyflurotropane binding. 3. These observations support the idea that cis-acting variation in 5' aspects of the human DAT/SLC6A3 locus contributes to individual differences in levels of DAT expression in vivo. 5' DAT variation is thus a good candidate to contribute to individual differences in a number of human phenotypes.

Nitric oxide and MPP+-induced hydroxyl radical generation

Although neuroprotective effect of nitric oxide (NO) is discussed, NO has a role of pathogenesis of cellular injury. NO is synthesized from L-arginine by NO synthase (NOS). NO contributes to the extracellular potassium-ion concentration ([K(+)](o))-induced hydroxyl radical ((*)OH) generation. Cytotoxic free radicals such as peroxinitrite (ONOO(-)) and (*)OH may also be implicated in NO-mediated cell injury. NO activation was induced by K(+) depolarization. NO may react with superoxide anion (O(2) (-)) to form ONOO(-) and its decomposition generates (*)OH. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) metabolite 1-methyl-4-phenylpyridinium ion (MPP(+)) involve toxicity induced by NO. Intraneuronal Ca(2+) triggered by MPP(+) may be detrimental to the functioning of dopaminergic nerve terminals in the striatum. Although the [K(+)](o)-induced depolarization enhances the formation of (*)OH product due to MPP(+), the (*)OH generation via NOS activation may be unrelated the dopamine (DA)-induced (*)OH generation. Depolarization enhances the MPP(+)-induced (*)OH formation via NOS activation. NOS inhibition is associated with a protective effect due to suppression of depolarization-induced (*)OH generation. ONOO(-) has been implicated as a causative factor under conditions in which DA neurons are damaged. These findings may be useful in elucidating the actual mechanism of free radical formation in the pathogenesis of neurodegenerative brain disorders, including Parkinson's disease and traumatic brain injuries.

Testosterone modulation of striatal dopamine output in orchidectomized mice

Three experiments are presented in which dopamine (DA) responses from superfused striatal tissue of orchidectomized (ORCH) mice treated or not with testosterone (T) are compared. In experiment 1, potassium-stimulated DA output was significantly greater in ORCH vs. ORCH+T mice. This profile was reversed when reserpine was infused in experiment 2, with DA output being significantly greater in ORCH+T vs. ORCH mice. In experiment 3, the amount of DA recovered following infusion of DA indicated no statistically significant differences in DA recoveries between ORCH and ORCH+T mice as tested in this paradigm. The findings that both potassium- and reserpine-induced DA responses are altered significantly by T suggests that one potential site of T action might involve the storage/uptake of DA within the vesicles of these neurons. Such results have important implications with regard to understanding the sex differences that are present in nigrostriatal dopaminergic function within health and diseased states.

Intra-nigral MPTP lesion in rats: behavioral and autoradiography studies

The present study investigated the motor response and possible changes in binding to D1 and D2 receptors after intra-nigral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) infusion on rats. The results indicated that MPTP-lesioned rats exhibited a significant reduction in locomotion and rearing frequencies observed in an open field 24 h after surgery. However, at 7 and 14 days after surgery the MPTP-lesioned rats showed a significant increase in locomotion in comparison to the control groups, as well as a decrease in immobility time. In addition, 21 days after surgery the behavioral measurements were unaltered by these procedures. Moreover, latency in initiating movement and catalepsy were unchanged by this neurotoxin on the same days of observation. An autoradiography approach indicated that there was a reduction in [3H]SCH 23390 binding in substantia nigra pars compacta (SNpc), substantia nigra pars reticulata (SNpr) and ventrolateral striatum in MPTP-treated rats 21 days after the surgery. [3H]raclopride binding remained unaltered by the MPTP treatment. These results suggest that compensatory plastic changes occur in D1 dopamine receptors after partial lesion of nigral dopaminergic neurons. These alterations might be related to the occurrence and recovery of motor impairment observed in MPTP-lesioned rats.

Adult and in utero exposure to cocaine alters sensitivity to the Parkinsonian toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Cocaine abuse is a significant problem in the United States, including its use by approximately 1% of pregnant women. Cocaine acts as an indirect agonist of dopamine at the dopamine transporter, resulting in the presence of excess dopamine in the synapse. Since synaptic dopamine can rapidly oxidize to form free radicals, it was hypothesized that exposure to this drug might produce damage in dopaminergic systems such as the substantia nigra pars compacta, damage to which is a hallmark of Parkinson's disease. To test this hypothesis we exposed mice both in utero and as adults to cocaine and examined its effects on the nigrostriatal system. We found that exposure to cocaine both in utero or as adults did not affect substantia nigra cell number, but did make these neurons more susceptible to the parkinsonian toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. We also found long-lasting changes in D2 receptor mRNA levels as well as changes in the monoamine transport system and several growth factors. This work suggests that use of cocaine might be a predisposing factor for development of Parkinson's disease in both adults exposed chronically as well as in individuals exposed prenatally.

Mechanism of 1-methyl-4-phenylpyridinium-induced dopamine release from PC12 cells

The molecular mechanism of 1-methyl-4-phenylpyridinium (MPP+), a Parkinsonism-inducing neurotoxin, has been studied in PC12 cells. The cells treated with MPP+ (100 microM) induced a rapid increase in phosphorylation of tyrosine residues of several proteins, including synaptophysin, a major 38 kDa synaptic vesicle protein implicated in exocytosis. An accelerated release of dopamine by MPP+ correlated with phosphorylation of synaptophysin. Exposing the cells to MPP+ triggered reactive oxygen species (ROS) generation within 60 min of treatment and the said effect was blocked by mazindol, a dopamine uptake blocker. In addition, pretreatment with 50-100 microM of selegiline, a selective MAO-B inhibitor, significantly suppressed MPP+-mediated ROS generation. These effects of MPP+ result in the generation of ROS, which may be involved in neuronal degeneration seen in Parkinson's disease.

Perinatal heptachlor exposure increases expression of presynaptic dopaminergic markers in mouse striatum

Although banned in the 1970s, significant levels of the organochlorine pesticide heptachlor are still present in the environment raising concern over potential human exposure. In particular, organochlorine pesticides have been linked to an increased risk of Parkinson's disease. Studies from our laboratory and others have demonstrated that exposure of laboratory animals to heptachlor alters the levels and function of the dopamine transporter (DAT), an integral component of dopaminergic neurotransmission and a gateway for the dopaminergic neurotoxin MPTP. In this study, we examined the effects of developmental exposure to heptachlor on DAT, and other key components of the dopaminergic system, including the vesicular monoamine transporter 2 (VMAT2), tyrosine hydroxylase (TH), and aromatic amino acid decarboxylase (AADC). Female C57BL/6J mice received 0 or 3mg/kg heptachlor in peanut butter every 3 days for 2 weeks prior to breeding and throughout gestation and lactation until the offspring were weaned on postnatal day (PND) 21. On postnatal day 28, DAT, VMAT2, and TH levels were increased by 100, 70, and 30%, respectively, with no change in AADC levels or total dopamine levels. The ratio of DAT:VMAT2 was increased 29%. Since an increase in the DAT:VMAT2 ratio appears to predict susceptibility of brain regions to Parkinson's disease (PD) and results in increased toxicity of MPTP, these results suggest that alterations of the dopaminergic system by developmental heptachlor exposure may increase the susceptibility of dopamine neurons to toxic insult.

Neuroprotection of MPTP-induced toxicity in zebrafish dopaminergic neurons

Parkinson's disease is characterized by a severe loss of dopaminergic neurons resulting in a range of motor deficits. The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is known to cause a similar loss of dopaminergic neurons in the human midbrain with corresponding Parkinsonian symptoms. Several animal species have also shown sensitivity to MPTP, including primates, mice, goldfish, and, most recently, zebrafish. This study demonstrates that the effect of MPTP on dopaminergic neurons in zebrafish larvae is mediated by the same pathways that have been demonstrated in mammalian species. MPTP-induced neurodegeneration was prevented by co-incubation with either the monoamine oxidase-B (MAO-B) inhibitor l-deprenyl or the dopamine transporter (DAT) inhibitor nomifensine. Furthermore, targeted inactivation of the DAT gene by antisense morpholinos also protected neurons from MPTP damage. Thus, the mechanism for MPTP-induced dopaminergic neuron toxicity in mammals is conserved in zebrafish larvae. Effects on swimming behavior and touch response that result from MPTP damage are partially ameliorated by both l-deprenyl and DAT knockdown.

Paraquat neurotoxicity is distinct from that of MPTP and rotenone

Paraquat, MPTP, and rotenone reproduce features of Parkinson's disease (PD) in experimental animals. The exact mechanisms by which these compounds damage the dopamine system are not firmly established, but selective damage to dopamine neurons and inhibition of complex I are thought to be involved. We and others have previously documented that the toxic metabolite of MPTP, MPP+, is transported into dopamine neurons through the dopamine transporter (DAT), while rotenone is not transported by DAT. We have also demonstrated the requirement for complex I inhibition and oxidative damage in the dopaminergic neurodegeneration produced by rotenone. Based on structural similarity to MPP+, it has been proposed that paraquat exerts selective dopaminergic toxicity through transport by the DAT and subsequent inhibition of mitochondrial complex I. In this study we report that paraquat is neither a substrate nor inhibitor of DAT. We also demonstrate that in vivo exposure to MPTP and rotenone, but not paraquat, inhibits binding of 3H-dihydrorotenone to complex I in brain mitochondria. Rotenone and MPP+ were both effective inhibitors of complex I activity in isolated brain mitochondria, while paraquat exhibited weak inhibitory effects only at millimolar concentrations. These data indicate that, despite the apparent structural similarity to MPP+, paraquat exerts its deleterious effects on dopamine neurons in a manner that is unique from rotenone and MPTP.

Neurodegeneration and neuroprotection in Parkinson disease

Many of the motoric features that define Parkinson disease (PD) result primarily from the loss of the neuromelanin (NM)-containing dopamine (DA) neurons of the substantia nigra (SN), and to a lesser extent, other mostly catecholaminergic neurons, and are associated with cytoplasmic "Lewy body" inclusions in some of the surviving neurons. While there are uncommon instances of familial PD, and rare instances of known genetic causes, the etiology of the vast majority of PD cases remains unknown (i.e., idiopathic). Here we outline genetic and environmental findings related to PD epidemiology, suggestions that aberrant protein degradation may play a role in disease pathogenesis, and pathogenetic mechanisms including oxidative stress due to DA oxidation that could underlie the selectivity of neurodegeneration. We then outline potential approaches to neuroprotection for PD that are derived from current notions on disease pathogenesis.

Dopamine-independent locomotor actions of amphetamines in a novel acute mouse model of Parkinson disease

Brain dopamine is critically involved in movement control, and its deficiency is the primary cause of motor symptoms in Parkinson disease. Here we report development of an animal model of acute severe dopamine deficiency by using mice lacking the dopamine transporter. In the absence of transporter-mediated recycling mechanisms, dopamine levels become entirely dependent on de novo synthesis. Acute pharmacological inhibition of dopamine synthesis in these mice induces transient elimination of striatal dopamine accompanied by the development of a striking behavioral phenotype manifested as severe akinesia, rigidity, tremor, and ptosis. This phenotype can be reversed by administration of the dopamine precursor, L-DOPA, or by nonselective dopamine agonists. Surprisingly, several amphetamine derivatives were also effective in reversing these behavioral abnormalities in a dopamine-independent manner. Identification of dopamine transporter- and dopamine-independent locomotor actions of amphetamines suggests a novel paradigm in the search for prospective anti-Parkinsonian drugs.

Identification of dopamine transporter- and dopamine- independent locomotor actions of amphetamines suggests a novel paradigm in the search for prospective anti-Parkinsonian drugs.

MPTP and SNpc DA neuronal vulnerability: role of dopamine, superoxide and nitric oxide in neurotoxicity. Minireview

Parkinson disease (PD) is a common neurodegenerative disease of unknown origin that is characterized, mainly, by a significant reduction in the number of dopamine neurons in the substantia nigra pars compacta (SNpc) of the brain and a dramatic reduction in dopamine levels in the corpus striatum. For reasons that we do not know, the dopamine neuron seems to be more vulnerable to damage than any other neuron in the brain. Although hypotheses of damage to the dopamine neuron include oxidative stress, growth factor decline, excitotoxicity, inflammation in the SNpc and protein aggregation, oxidative stress in the nigrostriatal dopaminergic system garners a significant amount of attention. In the oxidative stress hypothesis of PD, superoxide, nitric oxide and dopamine all conspire to create an environment that can be detrimental to the dopamine neuron. MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), the tool of choice for investigations into the mechanisms involved in the death of dopamine neurons in PD, has been used extensively in attempts to sort out what happens in and around the dopamine neuron. Herein, we review the roles of dopamine, superoxide and nitric oxide in the demise of the dopamine neuron in the MPTP model of PD as it relates to the death of the dopamine neuron noted in PD.

Pyrethroid pesticide-induced alterations in dopamine transporter function

Parkinson's disease (PD) is a progressive neurodegenerative disease affecting the nigrostriatal dopaminergic pathway. Several epidemiological studies have demonstrated an association between pesticide exposure and the incidence of PD. Studies from our laboratory and others have demonstrated that certain pesticides increase levels of the dopamine transporter (DAT), an integral component of dopaminergic neurotransmission and a gateway for dopaminergic neurotoxins. Here, we report that repeated exposure (3 injections over 2 weeks) of mice to two commonly used pyrethroid pesticides, deltamethrin (3 mg/kg) and permethrin (0.8 mg/kg), increases DAT-mediated dopamine uptake by 31 and 28%, respectively. Using cells stably expressing DAT, we determined that exposure (10 min) to deltamethrin and permethrin (1 nM-100 microM) had no effect on DAT-mediated dopamine uptake. Extending exposures to both pesticides for 30 min (10 microM) or 24 h (1, 5, and 10 microM) resulted in significant decrease in dopamine uptake. This reduction was not the result of competitive inhibition, loss of DAT protein, or cytotoxicity. However, there was an increase in DNA fragmentation, an index of apoptosis, in cells exhibiting reduced uptake at 30 min and 24 h. These data suggest that up-regulation of DAT by in vivo pyrethroid exposure is an indirect effect and that longer-term exposure of cells results in apoptosis. Since DAT can greatly affect the vulnerability of dopamine neurons to neurotoxicants, up-regulation of DAT by deltamethrin and permethrin may increase the susceptibility of dopamine neurons to toxic insult, which may provide insight into the association between pesticide exposure and PD.

Environmental enrichment in adulthood eliminates neuronal death in experimental Parkinsonism

Idiopathic Parkinson's disease (PD) affects 2% of adults over 50 years of age. PD patients demonstrate a progressive loss of dopamine neurons in the substantia nigra pars compacta (SNpc). One model that recapitulates the pathology of PD is the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Here we show that exposure to an enriched environment (EE) (a combination of exercise, social interactions and learning) or exercise alone during adulthood, totally protects against MPTP-induced Parkinsonism. Furthermore, changes in mRNA expression would suggest that increases in glia-derived neurotrophic factors, coupled with a decrease of dopamine-related transporters (e.g. dopamine transporter, DAT; vesicular monoamine transporter, VMAT2), contribute to the observed neuroprotection of dopamine neurons in the nigrostriatal system following MPTP exposure. This non-pharmacological approach presents significant implications for the prevention and/or treatment of PD.

Molecular genetics of attention-deficit/hyperactivity disorder

Results of behavioral genetic and molecular genetic studies have converged to suggest that both genetic and nongenetic factors contribute to the development of attention-deficit/hyperactivity disorder (ADHD). We review this literature, with a particular emphasis on molecular genetic studies. Family, twin, and adoption studies provide compelling evidence that genes play a strong role in mediating susceptibility to ADHD. This fact is most clearly seen in the 20 extant twin studies, which estimate the heritability of ADHD to be .76. Molecular genetic studies suggest that the genetic architecture of ADHD is complex. The few genome-wide scans conducted thus far are not conclusive. In contrast, the many candidate gene studies of ADHD have produced substantial evidence implicating several genes in the etiology of the disorder. For the eight genes for which the same variant has been studied in three or more case-control or family-based studies, seven show statistically significant evidence of association with ADHD on the basis of the pooled odds ratio across studies: DRD4, DRD5, DAT, DBH, 5-HTT, HTR1B, and SNAP-25.

The dopamine transporter: role in neurotoxicity and human disease

The dopamine transporter (DAT) is a plasma membrane transport protein expressed exclusively within a small subset of CNS neurons. It plays a crucial role in controlling dopamine-mediated neurotransmission and a number of associated behaviors. This review focuses on recent data elucidating the role of the dopamine transporter in neurotoxicity and a number of CNS disorders, including Parkinson disease, drug abuse, and attention deficit hyperactivity disorder (ADHD).

Age-dependent motor deficits and dopaminergic dysfunction in DJ-1 null mice

Mutations in the DJ-1 gene were recently identified in an autosomal recessive form of early-onset familial Parkinson disease. Structural biology, biochemistry, and cell biology studies have suggested potential functions of DJ-1 in oxidative stress, protein folding, and degradation pathways. However, animal models are needed to determine whether and how loss of DJ-1 function leads to Parkinson disease. We have generated DJ-1 null mice with a mutation that resembles the large deletion mutation reported in patients. Our behavioral analyses indicated that DJ-1 deficiency led to age-dependent and task-dependent motoric behavioral deficits that are detectable by 5 months of age. Unbiased stereological studies did not find obvious dopamine neuron loss in 6-month- and 11-month-old mice. Neurochemical examination revealed significant changes in striatal dopaminergic function consisting of increased dopamine reuptake rates and elevated tissue dopamine content. These data represent the in vivo evidence that loss of DJ-1 function alters nigrostriatal dopaminergic function and produces motor deficits.

The MPTP model of Parkinson's disease

The biochemical and cellular changes that occur following administration of 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) are remarkably similar to that seen in idiopathic Parkinson's disease (PD). In this review, we detail the molecular activities of this compound from peripheral intoxication through its various biotransformations. In addition, we detail the interplay that occurs between the different cellular compartments (neurons and glia) that eventually consort to kill substantia nigra pars compacta (SNpc) neurons.

Reduced MPTP toxicity in noradrenaline transporter knockout mice

The noradrenergic neurons of the locus coeruleus (LC) are damaged in Parkinson's disease (PD). Neurotoxin ablation of the LC noradrenergic neurons has been shown to exacerbate the dopaminergic toxicity of MPTP, suggesting that the noradrenergic system protects dopamine neurons. We utilized mice that exhibit elevated synaptic noradrenaline (NA) by genetically deleting the noradrenaline transporter (NET), a key regulator of the noradrenergic system (NET KO mice). NET KO and wild-type littermates were administered MPTP and striatal dopamine terminal integrity was assessed by HPLC of monoamines, immmunoblotting for dopaminergic markers and tyrosine hydroxylase (TH) immunohistochemistry. MPTP significantly reduced striatal dopamine in wild-type mice, but not in the NET KO mice. To confirm that the protection observed in the NET KO mice was due to the lack of NET, we treated wild-type mice with the specific NET inhibitor, nisoxetine, and then challenged them with MPTP. Nisoxetine conferred protection to the dopaminergic system. These data indicate that NA can modulate MPTP toxicity and suggest that manipulation of the noradrenergic system may have therapeutic value in PD.