Parenterally administered 3-nitropropionic acid and amphetamine can combine to produce damage to terminals and cell bodies in the striatum

Dopamine D2 receptor stimulation potentiates PolyQ-Huntingtin-induced mouse striatal neuron dysfunctions via Rho/ROCK-II activation

BACKGROUND

Huntington's disease (HD) is a polyglutamine-expanded related neurodegenerative disease. Despite the ubiquitous expression of expanded, polyQ-Huntingtin (ExpHtt) in the brain, striatal neurons present a higher susceptibility to the mutation. A commonly admitted hypothesis is that Dopaminergic inputs participate to this vulnerability. We previously showed that D2 receptor stimulation increased aggregate formation and neuronal death induced by ExpHtt in primary striatal neurons in culture, and chronic D2 antagonist treatment protects striatal dysfunctions induced by ExpHtt in a lentiviral-induced model system in vivo. The present work was designed to elucidate the signalling pathways involved, downstream D2 receptor (D2R) stimulation, in striatal vulnerability to ExpHtt.

METHODOLOGY/PRINCIPAL FINDINGS

Using primary striatal neurons in culture, transfected with a tagged-GFP version of human exon 1 ExpHtt, and siRNAs against D2R or D1R, we confirm that DA potentiates neuronal dysfunctions via D2R but not D1R stimulation. We demonstrate that D2 agonist treatment induces neuritic retraction and growth cone collapse in Htt- and ExpHtt expressing neurons. We then tested a possible involvement of the Rho/ROCK signalling pathway, which plays a key role in the dynamic of the cytoskeleton, in these processes. The pharmacological inhibitors of ROCK (Y27632 and Hydroxyfasudil), as well as siRNAs against ROCK-II, reversed D2-related effects on neuritic retraction and growth cone collapse. We show a coupling between D2 receptor stimulation and Rho activation, as well as hyperphosphorylation of Cofilin, a downstream effector of ROCK-II pathway. Importantly, D2 agonist-mediated potentiation of aggregate formation and neuronal death induced by ExpHtt, was totally reversed by Y27632 and Hydroxyfasudil and ROCK-II siRNAs.

CONCLUSIONS/SIGNIFICANCE

Our data provide the first demonstration that D2R-induced vulnerability in HD is critically linked to the activation of the Rho/ROCK signalling pathway. The inclusion of Rho/ROCK inhibitors could be an interesting therapeutic option aimed at forestalling the onset of the disease.

Trolox ameliorates 3-nitropropionic acid-induced neurotoxicity in rats

3-nitropropionic acid (3-NPA) is a naturally occurring neurotoxin produced by legumes of the genus Astragalus and Arthrium fungi. Acute exposure to 3-NPA results in striatal astrocytic death and variety of behavior dysfunction in rats. Oxidative stress has been reported to play an important role in 3-NPA-induced neurotoxicity. Trolox is a potent free radical chain breaking antioxidant which has been shown to restore structure and function of the nervous system following oxidative stress. This rapid and efficient antioxidant property of trolox was attributed to its enhanced water solubility as compared with alpha-tocopherol. This investigation was aimed to study the effect of trolox against 3-NPA-induced neurotoxicity in female Wistar rats. The animals received trolox (0, 40 mg, 80 mg and 160 mg/kg, orally) daily for 7 days. 3-NPA (25mg/kg, i.p.) was administered daily 30 min after trolox for the same duration. One additional group of rats served as control (vehicle only). On day 8, the animals were observed for neurobehavioral performance. Immediately after behavioral studies, the animal's brains were dissected out for histological studies. Lesions in the striatal dopaminergic neurons were assessed by immunohistochemical method using tyrosine hydroxylase immunostaining. Administration of 3-NPA alone caused significant depletion of striatal dopamine and glutathione, whereas, the levels of thiobarbituric acid reactive substance (TBARS) and nitric oxide (NO) were significantly increased suggesting an elevated level of oxidative stress. Trolox significantly and dose-dependently protected animals against 3-NPA-induced neurobehavioral, neurochemical and structural abnormalities. These results clearly suggest that protective effect of trolox against 3-NPA-induced neurotoxicity is mediated through its free radical scavenging activity.

The seed extract of Cassia obtusifolia offers neuroprotection to mouse hippocampal cultures

The precise causative factors in neurodegenerative diseases such as Alzheimer's (AD) and Parkinson's disease remain elusive, but mechanisms implicated comprise excitotoxicity, mitochondrial dysfunction, and in the case of AD, the amyloid beta peptide (Abeta). Current therapeutic strategies for such disorders are very limited; thus, traditional herbal medicines currently receive increased attention. The seeds of Cassia obtisufolia have long been used in traditional eastern medicine and more recently the ethanolic fraction of the seeds (COE) has been shown to attenuate memory impairments in mice. In this study, we set out to determine the effect of COE (range: 0.1 - 10 microg/ml) on calcium dysregulation and cell death models in mouse primary hippocampal cultures implicated in general neurodegenerative processes and in the pathogenesis of AD: excitotoxicity, mitochondrial dysfunction, and Abeta toxicity. It was found that treatment with COE attenuated secondary Ca2+ dysregulation induced by NMDA (700 microM), while a pre-application of COE also reduced NMDA-induced cell death. Furthermore, COE was neuroprotective against the mitochondrial toxin 3-NP (1 mM), while having no significant effect on cell death induced by incubation with naturally-secreted oligomers of Abeta (8.2 pg/ml). Collectively, these results are important for the therapeutic use of COE in the treatment of neurodegenerative disorders.

Dopamine determines the vulnerability of striatal neurons to the N-terminal fragment of mutant huntingtin through the regulation of mitochondrial complex II

In neurodegenerative disorders associated with primary or secondary mitochondrial defects such as Huntington's disease (HD), cells of the striatum are particularly vulnerable to cell death, although the mechanisms by which this cell death is induced are unclear. Dopamine, found in high concentrations in the striatum, may play a role in striatal cell death. We show that in primary striatal cultures, dopamine increases the toxicity of an N-terminal fragment of mutated huntingtin (Htt-171-82Q). Mitochondrial complex II protein (mCII) levels are reduced in HD striatum, indicating that this protein may be important for dopamine-mediated striatal cell death. We found that dopamine enhances the toxicity of the selective mCII inhibitor, 3-nitropropionic acid. We also demonstrated that dopamine doses that are insufficient to produce cell loss regulate mCII expression at the mRNA, protein and catalytic activity level. We also show that dopamine-induced down-regulation of mCII levels can be blocked by several dopamine D2 receptor antagonists. Sustained overexpression of mCII subunits using lentiviral vectors abrogated the effects of dopamine, both by high dopamine concentrations alone and neuronal death induced by low dopamine concentrations together with Htt-171-82Q. This novel pathway links dopamine signaling and regulation of mCII activity and could play a key role in oxidative energy metabolism and explain the vulnerability of the striatum in neurodegenerative diseases.

Calcium homeostasis and mitochondrial dysfunction in striatal neurons of Huntington disease

Dysfunctions of Ca2+ homeostasis and of mitochondria have been studied in immortalized striatal cells from a commonly used Huntington disease mouse model. Transcriptional changes in the components of the phosphatidylinositol cycle and in the receptors for myo-inositol trisphosphate-linked agonists have been found in the cells and in the striatum of the parent Huntington disease mouse. The overall result of the changes is to delay myo-inositol trisphosphate production and to decrease basal Ca2+ in mutant cells. When tested directly, mitochondria in mutant cells behave nearly normally, but are unable to handle large Ca2+ loads. This appears to be due to the increased Ca2+ sensitivity of the permeability transition pore, which dissipates the membrane potential, prompting the release of accumulated Ca2+. Harmful reactive oxygen species, which are produced by defective mitochondria and may in turn stress them, increase in mutant cells, particularly if the damage to mitochondria is artificially exacerbated, for instance with complex II inhibitors. Mitochondria in mutant cells are thus peculiarly vulnerable to stresses induced by Ca2+ and reactive oxygen species. The observed decrease of cell Ca2+ could be a compensatory attempt to prevent the Ca2+ stress that would irreversibly damage mitochondria and eventually lead to cell death.

Haloperidol protects striatal neurons from dysfunction induced by mutated huntingtin in vivo

Huntington's disease (HD) results from an abnormal polyglutamine extension in the N-terminal region of the huntingtin protein. This mutation causes preferential degeneration of striatal projection neurons. We previously demonstrated, in vitro, that dopaminergic D2 receptor stimulation acted synergistically with mutated huntingtin (expHtt) to increase aggregate formation and striatal death. In the present work, we extend these observations to an in vivo system based on lentiviral-mediated expression of expHtt in the rat striatum. The early and chronic treatment with the D2 antagonist haloperidol decanoate protects striatal neurons from expHtt-induced dysfunction, as analyzed by DARPP-32 and NeuN stainings. Haloperidol treatment also reduces aggregates formation, an effect that is maintained over time. These findings indicate that D2 receptors activation contributes to the deleterious effects of expHtt on striatal function and may represent an interesting early target to alter the subsequent course of neuropathology in HD.

Protective effects of minocycline on behavioral changes and neurotoxicity in mice after administration of methamphetamine

The effects of minocycline on behavioral changes and neurotoxicity in the dopaminergic neurons induced by the administration of methamphetamine (METH) were studied. Pretreatment with minocycline (40 mg/kg) was found to attenuate hyperlocomotion in mice after a single administration of METH (3 mg/kg). The development of behavioral sensitization after repeated administration of METH (3 mg/kg/day, once daily for 5 days) was significantly attenuated by pretreatment with minocycline (40 mg/kg). A reduction in the level of dopamine (DA) and its major metabolite, 3,4-dihydroxyphenyl acetic acid (DOPAC), in the striatum after the repeated administration of METH (3 mg/kg x 3, 3-h interval) was attenuated in a dose-dependent manner by pretreatment with and the subsequent administration of minocycline (10, 20, or 40 mg/kg). Furthermore, minocycline (40 mg/kg) significantly attenuated a reduction in DA transporter (DAT)-immunoreactivity in the striatum after repeated administration of METH. In vivo microdialysis study demonstrated that pretreatment with minocycline (40 mg/kg) significantly attenuated increased extracellular DA levels in the striatum after the administration of METH (3 mg/kg). In addition, minocycline was not found to alter the concentrations of METH in the plasma or the brain after three injections of METH (3 mg/kg), suggesting that minocycline does not alter the pharmacokinetics of METH in mice. Interestingly, METH-induced neurotoxicity in the striatum was significantly attenuated by the post-treatment and subsequent administration of minocycline (40 mg/kg). These findings suggest that minocycline may be able to ameliorate behavioral changes as well as neurotoxicity in dopaminergic terminals after the administration of METH. Therefore, minocycline could be considered as a useful drug for the treatment of several symptoms associated with METH abuse in humans.

Studies on striatal neurotoxicity caused by the 3,4-methylenedioxymethamphetamine/ malonate combination: implications for serotonin/dopamine interactions

The amphetamine derivative 3,4-methylenedioxymethamphetamine (MDMA) produces long-term toxicity to serotonin (5-HT) neurones in rats, which is exacerbated when combined with the mitochondrial inhibitor malonate. Moreover, MDMA, which does not produce dopamine depletion in the rat, potentiates malonate-induced striatal dopamine toxicity. Because the malonate/MDMA combination acutely causes a synergistic increase of 5-HT and dopamine release, in this study we sought to determine whether pharmacological blockade of MDMA- and/or malonate-induced dopamine release prevents neurotoxicity. Fluoxetine, given 30 min prior to the malonate/MDMA combination, afforded complete protection against 5-HT depletion and reversed MDMA-induced exacerbation of dopamine toxicity found in the malonate/MDMA treated rats. Protection afforded by fluoxetine was not related to changes in MDMA-induced hyperthermia. Similarly, potentiation of malonate-induced dopamine toxicity caused by MDMA was not observed in p-chlorophenylalanine-5-HT depleted rats. Finally, the dopamine transporter inhibitor GBR 12909 completely prevented dopamine neurotoxicity caused by the malonate/MDMA combination and reversed the exacerbating toxic effects of malonate on MDMA-induced 5-HT depletion without significantly altering the hyperthermic response. Overall, these results suggest that the synergic release of dopamine caused by the malonate/MDMA combination plays an important role in the long-term toxic effects. A possible mechanism of neurotoxicity and protection is proposed.

Deprenyl enhances the striatal neuronal damage produced by quinolinic acid

We have tested the effect of deprenyl on the neurotoxicity induced by the injection of quinolinic acid within the striatum. Deprenyl was unable to prevent these quinolinic acid-induced damages, but enhanced the loss of several gamma-aminobutyric acid (GABA) positive subpopulations, the loss of the astroglial population and the activation of microglia produced by quinolinic acid. These effects are produced by deprenyl potentiation of dopamine actions since dopamine depletion produced by previous injection of the dopaminergic toxin 6-hydroxydopamine within the medial forebrain bundle overcomes deprenyl effects and the involvement of dopamine in the quinolinic acid-induced toxicity in striatum. In these conditions, quinolinic acid toxic action in striatum is significantly lower and similar in the animals treated with or without deprenyl. All these data justify why deprenyl worsen some pathological signals of disorders involving excitotoxicity. This also may be involved in other secondary effects described for deprenyl.

Unraveling a role for dopamine in Huntington's disease: the dual role of reactive oxygen species and D2 receptor stimulation

Huntington's disease (HD), an inherited neurodegenerative disorder, results from an abnormal polyglutamine extension in the N-terminal region of the huntingtin protein. This mutation leads to protein aggregation and neurotoxicity. Despite its widespread expression in the brain and body, mutated huntingtin causes selective degeneration of striatal projection neurons. In the present study, we investigate the role of dopamine (DA) in this preferential vulnerability. Using primary cultures of striatal neurons transiently expressing GFP-tagged-exon 1 of mutated huntingtin, we show that low doses of DA (100 microM) act synergistically with mutated huntingtin to activate the proapoptotic transcription factor c-Jun. Surprisingly, DA also increases aggregate formation of mutated huntingtin in all cellular compartments, including neurites, soma, and nuclei. DA-dependent potentiation of c-Jun activation was reversed by ascorbate, a reactive oxygen species (ROS) scavenger, and SP-600125, a selective inhibitor of the c-Jun N-terminal kinase (JNK) pathway. By contrast, DA effects on aggregate formation were reversed by a selective D2 receptor antagonist and reproduced by a D2 agonist. Similarly, striatal neurons from D2 knockout mice showed no effect of DA on aggregate formation. Blocking ROS production, JNK activation, or D2 receptor stimulation significantly reversed DA aggravation of mutated huntingtin-induced striatal death. The combined treatment with the ROS scavenger and D2 antagonist totally reversed DA's effects on mutated huntingtin-induced striatal death. Thus, the present results provide insights into the cellular mechanisms that govern striatal vulnerability in HD and strongly support a dual role of JNK activation and D2 receptor signaling in this process.

Methamphetamine-induced deficits of brain monoaminergic neuronal markers: distal axotomy or neuronal plasticity

We examined the effects of methamphetamine (METH) on monoaminergic (i.e. dopamine and serotonin) axonal markers and glial cell activation in the rat brain. Our findings indicate that the loss of dopamine transporters (DAT), serotonin transporters (5-HTT), vesicular monoamine transporter type-2 (VMAT-2) and glial cell activation induced by METH in the striatum and in the central gray are consistent with a degenerative process. Our novel finding of METH effects on monoaminergic neurons in the central gray may have important implications on METH-induced hyperthermia. In other brain regions examined, DAT and 5-HTT deficits after METH administration were present in the absence of lasting changes in VMAT-2 levels or glial cell activation. Brain regions exhibiting protracted deficits in DAT and/or 5-HTT and VMAT-2 levels also expressed increased levels of [(3)H]-R-PK11195 binding to peripheral benzodiazepine receptors, a quantitative marker of glial cell activation. Immunohistochemical assessment of microglia and astrocytes confirmed the PBR results. Microglia activation was more pronounced than astrocytosis in affected regions in most METH-exposed brains with the exception of a small number of rats that were most severely affected by METH based on loss of body weight. In these rats, both microglia and astrocytes were highly activated and expressed a distinct regional pattern suggestive of widespread brain injury. The reason for the pattern of glial cell activation in this group of rats is not currently known but it may be associated with METH-induced hyperthermia. In summary, our findings suggest two neurotoxic endpoints in the brain of METH-exposed animals. Brain regions exhibiting DAT and 5-HTT deficits that co-localize with decreased VMAT-2 levels and glial cell activation may represent monoaminergic terminal degeneration. However, the DAT and 5-HTT deficits in brain regions lacking a deficit in VMAT-2 and glial cell activation may reflect drug-induced modulation of these plasma membrane proteins.

Mitochondrial toxin inhibition of [(3)H]dopamine uptake into rat striatal synaptosomes

Administration of the mitochondrial inhibitors malonate and 3-nitropropionic acid (3-NP) to rats provides useful models of Huntington's disease. Exposure to these inhibitors has been shown to result in increased extracellular concentrations of striatal dopamine (DA), which is neurotoxic at high concentrations. The cause of this increase is unknown. The purpose of this study was to determine whether mitochondrial inhibition alters dopamine transporter (DAT) function. Striatal synaptosomes were incubated in the presence of several structurally unrelated inhibitors of mitochondrial Complexes I, II, and IV, and [(3)H]DA uptake was measured. Although all of the toxins inhibited [(3)H]DA uptake, there was a large variation in their inhibitory potencies, the rank order being rotenone>>cyanide>azide>3-NP>>malonate. Examination of the kinetic parameters of [(3)H]DA uptake revealed that inhibition was due to a reduction in maximum velocity (V(max)), with no change in affinity (K(m)). The addition of either ATP or of ADP plus P(i) to synaptosomes treated with 3-NP, or of the reactive oxygen species spin trap alpha-phenyl-N-tert-butyl nitrone to synaptosomes exposed to either malonate or cyanide failed to prevent mitochondrial toxin-induced inhibition of DAT function. The lack of effect of high energy substrates or of a free radical scavenger suggests that the mechanism by which extracellular DA is increased by several mitochondrial toxins involves factors other than mitochondrial ATP production or oxidative stress. Taken together, the results suggest that one mechanism whereby mitochondrial toxins increase extracellular concentrations of DA is via interaction with the DAT at a site other than the substrate site, i.e. noncompetitive inhibition of the DAT.

The mitochondrial toxin 3-nitropropionic acid induces striatal neurodegeneration via a c-Jun N-terminal kinase/c-Jun module

Impairments in mitochondrial energy metabolism are thought to be involved in most neurodegenerative diseases, including Huntington's disease (HD). Chronic administration of 3-nitropropionic acid (3-NP), a suicide inhibitor of succinate dehydrogenase, causes prolonged energy impairments and replicates most of the pathophysiological features of HD, including preferential striatal degeneration. In this study, we analyzed one of the mechanisms that could account for this selective 3-NP-induced striatal degeneration. In chronically 3-NP-infused rats, the time course of motor behavioral impairments and histological abnormalities was determined. Progressive alterations of motor performance occurred after 3 d. By histological analysis and terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end-labeling staining, we found a selective neurodegenerescence in the striatum, occurring first in its dorsolateral (DL) part. Activation of c-Jun N-terminal kinase (JNK) was analyzed from brain sections of these rats, using immunocytochemical detection of its phosphorylated form. Activation of JNK occurred progressively and selectively in the DL of the striatum and was followed by c-Jun activation and expression in the same striatal region. To elucidate the role of the JNK/c-Jun module in 3-NP-induced striatal degeneration, we then used primary striatal neurons in culture, in which we replicated neuronal death by application of 3-NP. We found strong nuclear translocation of activated JNK that was rapidly followed by phosphorylation of the transcription factor c-Jun. Overexpression of a dominant negative version of c-Jun, lacking its transactivation domain and phosphorylation sites for activated JNK, completely abolished 3-NP-induced striatal neurodegeneration. We thus conclude that a genetic program controlled by the JNK/c-Jun module is an important molecular event in 3-NP-induced striatal degeneration.

Parallel increases in lipid and protein oxidative markers in several mouse brain regions after methamphetamine treatment

The neurotoxic actions of methamphetamine (METH) may be mediated in part by reactive oxygen species (ROS). Methamphetamine administration leads to increases in ROS formation and lipid peroxidation in rodent brain; however, the extent to which proteins may be modified or whether affected brain regions exhibit similar elevations of lipid and protein oxidative markers have not been investigated. In this study we measured concentrations of TBARs, protein carbonyls and monoamines in various mouse brain regions at 4 h and 24 h after the last of four injections of METH (10 mg/kg/injection q 2 h). Substantial increases in TBARs and protein carbonyls were observed in the striatum and hippocampus but not the frontal cortex nor the cerebellum of METH-treated mice. Furthermore, lipid and protein oxidative markers were highly correlated within each brain region. In the hippocampus and striatum elevations in oxidative markers were significantly greater at 24 h than at 4 h. Monoamine levels were maximally reduced within 4 h (striatal dopamine [DA] by 95% and serotonin [5-HT] in striatum, cortex and hippocampus by 60-90%). These decrements persisted for 7 days after METH, indicating effects reflective of nerve terminal damage. Interestingly, NE was only transiently depleted in the brain regions investigated (hippocampus and cortex), suggesting a pharmacological and non-toxic action of METH on the noradrenergic nerve terminals. This study provides the first evidence for concurrent formation of lipid and protein markers of oxidative stress in several brain regions of mice that are severely affected by large neurotoxic doses of METH. Moreover, the differential time course for monoamine depletion and the elevations in oxidative markers indicate that the source of oxidative stress is not derived directly from DA or 5HT oxidation.

Effect of temperature on dopamine transporter function and intracellular accumulation of methamphetamine: implications for methamphetamine-induced dopaminergic neurotoxicity

Hyperthermia exacerbates and hypothermia attenuates methamphetamine (METH)-induced dopamine (DA) neurotoxicity. The mechanisms underlying these temperature effects are unknown. Given the essential role of the DA transporter (DAT) in the expression of METH-induced DA neurotoxicity, we hypothesized that the effect of temperature on METH-induced DA neurotoxicity is mediated, at least in part, at the level of the DAT. To test this hypothesis, the effects of small, physiologically relevant temperature changes on DAT function were evaluated in two types of cultured neuronal cells: (1) a neuroblastoma cell line stably transfected with human DAT cDNA and (2) rat embryonic mesencephalic primary cells that naturally express the DAT. Temperatures for studies of DAT function were selected based on core temperature measurements in animals exposed to METH under usual ambient (22 degrees C) and hypothermic (6 degrees C) temperature conditions, where METH neurotoxicity was fully expressed and blocked, respectively. DAT function, determined by measuring accumulation of radiolabeled DA and 1-methyl-4-phenylpyridinium (MPP(+)), was found to directly correlate with temperature, with higher levels of substrate uptake at 40 degrees C, intermediate levels at 37 degrees C, and lower levels at 34 degrees C. DAT-mediated accumulation of METH also directly correlated with temperature, with greater accumulation at higher temperatures. These findings indicate that relatively small, physiologically relevant changes in temperature significantly alter DAT function and intracellular METH accumulation, and suggest that the effect of temperature on METH-induced DA neurotoxicity is mediated, at least in part, at the level of the DAT.

Biomarkers of 3-nitropropionic acid (3-NPA)-induced mitochondrial dysfunction as indicators of neuroprotection

In humans or animals, symptoms of mitochondrial energy dysfunction may be produced by mutations or inborn errors of the necessary enzymes, as well as by enzyme inhibitors or uncouplers of the oxidative phosphorylation process. 3-Nitropropionic acid (3-NPA) is a toxin that is sometimes produced on moldy crops (sugarcane, peanuts, etc.) in amounts sufficient to cause severe neuromuscular disorders when consumed by humans. In vitro, 3-NPA irreversibly inactivates SDH, a Complex II respiratory enzyme important for mitochondrial energy production. We have been studying biomarkers of 3-NPA exposure in the expectation that such markers may be useful in the screening process to identify neuroprotective agents against neurotoxicity produced by mitochondrial energy dysfunction. Animals were sacrificed at various times after 3-NPA exposure for histochemical visualization of SDH activity and measurement of immediate postmortem rectal temperature. 3-NPA-treated rats experienced progressive hypothermia that reached a loss of 3 degrees C or more in core body temperature by three hours after dosing. The optical density of the SDH stain in brain was reduced, following a similar time course, most prominently in the cerebellum and least sharply in the thalamus. Some rats were given injections of L-carnitine (an enhancer of fatty acid transport) either alone, or as a pretreatment prior to a dose of 3-NPA. Although L-carnitine deficiency by itself can produce mitochondrial dysfunction, pretreatment with L-carnitine was of limited efficacy at overcoming the effects of 3-NPA on either body temperature or quantitative SDH histochemistry. Body temperature and SDH histochemistry may be useful biomarkers for evaluating the efficacy of neuroprotective agents against lower doses of 3-NPA, against other pharmacological models of mitochondrial dysfunction, or even against genetic mitochondrial diseases.

Role of mitochondrial dysfunction and dopamine-dependent oxidative stress in amphetamine-induced toxicity

To define the molecular mechanisms underlying amphetamine (AMPH) neurotoxicity, primary cultures of dopaminergic neurons were examined for drug-induced changes in dopamine (DA) distribution, oxidative stress, protein damage, and cell death. As in earlier studies, AMPH rapidly redistributed vesicular DA to the cytoplasm, where it underwent outward transport through the DA transporter. DA was concurrently oxidized to produce a threefold increase in free radicals, as measured by the redox-sensitive dye dihydroethidium. Intracellular DA depletion using the DA synthesis inhibitor alpha-methyl-p-tyrosine or the vesicular monoamine transport blocker reserpine prevented drug-induced free radical formation. Despite these AMPH-induced changes, neither protein oxidation nor cell death was observed until 1 and 4 days, respectively. AMPH also induced an early burst of free radicals in a CNS-derived dopaminergic cell line. However, AMPH-mediated attenuation of ATP production and mitochondrial function was not observed in these cells until 48 to 72 hours. Thus, neither metabolic dysfunction nor loss of viability was a direct consequence of AMPH neurotoxicity. In contrast, when primary cultures of dopaminergic neurons were exposed to AMPH in the presence of subtoxic doses of the mitochondrial complex I inhibitor rotenone, cell death was dramatically increased, mimicking the effects of a known parkinsonism-inducing toxin. Thus, metabolic stress may predispose dopaminergic neurons to injury by free radical-promoting insults such as AMPH.

Assessment of tyrosine hydroxylase immunoreactive innervation in five subregions of the nucleus accumbens shell in rats treated with repeated cocaine

To explore the effects of behavioral sensitization on the anatomy of the nucleus accumbens shell, we employed a typical cocaine dosing paradigm and assessed tyrosine hydroxylase immunoreactive varicosities in five different areas of the shell, as well as the core of the nucleus accumbens. Rats were given bidaily injections of either saline (1 ml/kg i.p.) or cocaine (15 mg/kg i.p.) for 5 consecutive days, and sacrificed either 2 or 14 days from the last injection. Sections of the nucleus accumbens were processed for tyrosine hydroxylase immunoreactivity and the number of immunoreactive varicosities in contact with neuronal cell bodies was quantified in each of the subregions of the shell, as well as the core of the nucleus accumbens. Compared to saline controls, the cocaine-treated animals showed a significant augmentation in tyrosine hydroxylase immunoreactivity in two of the five subregions after 2 days of withdrawal in the shell, but not in the core. No differences were found in any region tested after 14 days of withdrawal. These data are the first to suggest that increases in nucleus accumbens presynaptic tyrosine hydroxylase may play a role in the development of behavioral sensitization, but not in the long-term expression of this phenomenon.

Repeated cocaine treatment alters tyrosine hydroxylase in the rat nucleus accumbens

To determine whether repeated exposure of cocaine affects the dopaminergic innervation of the nucleus accumbens, we employed a typical cocaine-dosing regimen in adult male Sprague-Dawley rats followed by an immunocytochemical analysis of tyrosine hydroxylase (TH). Treatment consisted of bi-daily injections of saline or 15 mg/kg cocaine for 5 consecutive days. After 2 or 14 days of withdrawal, sections of the nucleus accumbens (NAc) were processed for tyrosine hydroxylase and the number of immunoreactive varicosities in the core and shell were quantified. Two days after treatment, the core demonstrated a decrease, while after 14 days of treatment, the shell was found to contain significantly more TH immunoreactive varicosities. Additionally, 2 days post-cocaine treatment, core-shell differences were found, however moderate differences were also found in the saline treatment group, making the absolute effects of cocaine difficult to separate from injection and handling effects at this time point. These results suggest that the shell of the NAc may undergo alterations that could be involved with behavioral sensitization that typically results from such cocaine treatment regimens.

Role for dopamine in malonate-induced damage in vivo in striatum and in vitro in mesencephalic cultures

Defects in mitochondrial energy metabolism have been implicated in the pathology of several neurodegenerative disorders. In addition, the reactive metabolites generated from the metabolism and oxidation of the neurotransmitter dopamine (DA) are thought to contribute to the damage to neurons of the basal ganglia. We have previously demonstrated that infusions of the metabolic inhibitor malonate into the striata of mice or rats produce degeneration of DA nerve terminals. In the present studies, we demonstrate that an intrastriatal infusion of malonate induces a substantial increase in DA efflux in awake, behaving mice as measured by in vivo microdialysis. Furthermore, pretreatment of mice with tetrabenazine (TBZ) or the TBZ analogue Ro 4-1284 (Ro-4), compounds that reversibly inhibit the vesicular storage of DA, attenuates the malonate-induced DA efflux as well as the damage to DA nerve terminals. Consistent with these findings, the damage to both DA and GABA neurons in mesencephalic cultures by malonate exposure was attenuated by pretreatment with TBZ or Ro-4. Treatment with these compounds did not affect the formation of free radicals or the inhibition of oxidative phosphorylation resulting from malonate exposure alone. Our data suggest that DA plays an important role in the neurotoxicity produced by malonate. These findings provide direct evidence that inhibition of succinate dehydrogenase causes an increase in extracellular DA levels and indicate that bioenergetic defects may contribute to the pathogenesis of chronic neurodegenerative diseases through a mechanism involving DA.

Striatal dopamine depletion and behavioural sensitization induced by methamphetamine and 3-nitropropionic acid

The neurotoxic effects of methamphetamine (4 x 5 mg/kg i.p. at 2-h intervals) and 3-nitropropionic acid (20 mg/kg i.p. on days 1-4 and 6-9, saline on day 5), administered alone or in combination (3-nitropropionic acid as above and methamphetamine on day 5), were investigated in rats 1 week after the last injection. Neither methamphetamine nor 3-nitropropionic acid on their own altered brain dopamine levels, but in combination, they selectively lowered dopamine in the terminal regions of the corpus striatum and nucleus accumbens. Methamphetamine depleted 5-hydroxytryptamine (5-HT) in the striatum, while 3-nitropropionic acid depleted 5-HT in the accumbens and substantia nigra, but a combination of the two toxins failed to lower 5-HT in any of these brain regions. Measurements of aromatic L-amino acid decarboxylase activity disclosed no change in the capacity to decarboxylate L-3,4-dihydroxyphenylalanine in any region with any of the treatments, but a lowered capacity to decarboxylate 5-hydroxytryptophan in the nigra after all three treatments. Methamphetamine evoked characteristic hyperactivity and stereotypy in the animals, whereas 3-nitropropionic gave rise to early hypermotility followed by hypoactivity. At 1 week after treatment with 3-nitropropionic/methamphetamine, rats exhibited normal spontaneous motor behaviour, a poor response to dopamine D(1) receptor stimulation and an exaggerated response to dopamine D(2) receptor agonists. These results show that combined systemic treatment with methamphetamine and 3-nitropropionic acid partially depletes dopamine in the basal ganglia, rendering the animals supersensitive to dopamine D(2) receptor activation without altering their spontaneous locomotion.

The selective vulnerability of striatopallidal neurons

The different types of striatal neuron show a range of vulnerabilities to a variety of insults. This can be clearly seen in Huntington's disease where a well mapped pattern of pathological events occurs. Medium spiny projection (MSP) neurons are the first striatal cells to be affected as the disease progresses whilst interneurons, in particular the NADPH diaphorase positive ones, are spared even in the late stages of the disease. The MSP neurons themselves are also differentially affected. The death of MSP neurons in the patch compartment of the striatum precedes that in the matrix compartment and the MSP neurons of the dorsomedial caudate nucleus degenerate before those in the ventral lateral putamen. The enkephalin positive striatopallidal MSP neurons are also more vulnerable than the substance P/dynorphin MSP neurons. We review the potential causes of this selective vulnerability of striatopallidal neurons and discuss the roles of endogenous glutamate, nitric oxide and calcium binding proteins. It is concluded that MSP neurons in general are especially susceptible to disruptions of cellular respiration due to the enormous amount of energy they expend on maintaining unusually high transmembrane potentials. We go on to consider a subpopulation of enkephalinergic striatopallidal neurons in the rat which are particularly vulnerable. This subpopulation of neurons readily undergo apoptosis in response to experimental manipulations which affect dopamine and/or corticosteroid levels. We speculate that the cellular mechanisms underlying this cell death may also operate in degenerative disorders such as Huntington's disease thereby imposing an additional level of selectivity on the pattern of degeneration. The possible contribution of the selective death of striatopallidal neurons to a number of clinically important psychiatric conditions including obsessive compulsive disorders and Tourette's syndrome is also discussed.

3-Nitropropionic acid (3-NPA) produces hypothermia and inhibits histochemical labeling of succinate dehydrogenase (SDH) in rat brain

3-Nitropropionic acid (3-NPA) is a toxin sometimes produced on moldy crops (sugarcane, peanuts, etc.) in amounts sufficient to cause severe neurological disorders when consumed by humans. In vitro, 3-NPA irreversibly inactivates SDH, a Complex II respiratory enzyme required for mitochondrial energy production. A single dose of 3-NPA (30 mg/kg s.c.) was given to singly-caged adult male Sprague-Dawley rats. Rectal temperature was measured after dosing as a potential biomarker of exposure to 3-NPA, and animals were sacrificed at various times after 3-NPA exposure for histochemical visualization of SDH activity. 3-NPA-treated rats experienced a progressive hypothermia, which reached a loss of 3 degrees C or more in core body temperature by 3 hours after dosing. The optical density of the SDH stain in brain was reduced according to a similar time-course, most prominently in the cerebellum and least sharply in the thalamus. The caudate nucleus had the greatest density of SDH staining that we measured in brain; it also has been reported to be the region most consistently lesioned by 3-NPA. However, within other areas of brain such as subdivisions of the hippocampus, neither endogenous SDH activity nor its sensitivity to inhibition by 3-NPA could predict the susceptibility to neurodegenerative changes. Although SDH activity remained significantly reduced in most areas of brain (except thalamus) for up to 5 days after dosing, core temperatures had returned to control values by 5 days suggesting that animals can utilize an alternate method of heat production to withstand insult by 3-NPA.

Dopamine modulates the susceptibility of striatal neurons to 3-nitropropionic acid in the rat model of Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disorder characterized by chorea, psychiatric disturbances, and dementia. The striatum is the primary site of neuronal loss in HD; however, neither the mechanism of neurodegeneration nor the underlying cause of the selectivity for the striatum is understood. Chronic systemic injection of 3-nitropropionic acid (3-NP) into rats induces bilateral striatal lesions with many neuropathological features of HD and is widely used as a model of HD. In this study we examine the role striatal dopamine plays in 3-NP-induced striatal toxicity. The effect of elevated striatal dopamine levels on 3-NP toxicity was examined by using acute administration of methamphetamine. After 7 d of 3-NP treatment, a single low dose of methamphetamine markedly increased the frequency of striatal lesion formation. This effect was mediated via dopamine receptors because it could be blocked by the administration of dopamine receptor antagonists. The effect of decreased striatal dopamine on 3-NP toxicity was examined by lesioning the nigrostriatal dopamine input to one striatum 7 d before 3-NP treatment was started. Removal of the dopamine input protected the denervated striatum from the neurotoxic effects of systemic 3-NP but did not prevent the formation of lesions in the intact striatum. Thus the formation of 3-NP lesions is critically dependent on an intact dopamine input. Our data show that dopamine plays an important role in the formation of 3-NP lesions. We suggest that modulation of the dopaminergic system should be reevaluated as a potential drug target in the treatment for HD.

Astrocytes are more vulnerable than neurons to cellular Ca2+ overload induced by a mitochondrial toxin, 3-nitropropionic acid

The differential effects of 3-nitropropionic acid on cultured neurons and astrocytes (of cortical and striatal origin) were examined by studying intracellular Ca2+ changes using imaging techniques with fura-2. The neurons and astrocytes whose intracellular Ca2+ concentration was recorded were identified later by immunocytochemical staining for microtubule-associated protein 2 and glial fibrillary acidic protein, respectively. 3-Nitropropionic acid (1.7 mM) irreversibly increased intracellular Ca2+ in astrocytes (27%) and, to a significantly smaller extent, in neurons (10%). The latency to onset of the intracellular Ca2+ increase was longer in neurons (45 min) than in astrocytes (29 min). Thus, a differential susceptibility of astrocytes and neurons was observed. The 3-nitropropionic acid-induced astrocytic and neuronal Ca2+ accumulations were both due to influx of Ca2+, as the increases were absent in Ca2+-free medium. An inhibitor of the Na+-Ca2+ exchanger (2',4'-dichlorobenzamil), greatly reduced the intracellular Ca2+ increase in astrocytes, but not in neurons. This indicates that the intracellular Ca2+ increase in astrocytes is primarily mediated by a reverse operation of the Na+-Ca2+ exchange system, whereas in neurons it is mediated by a different mechanism. In addition, we noted that astrocytic cell death occurred in 9% of cells at 60 min or more after the start of a 40 min perfusion with 3-nitropropionic acid, while only 4% of neurons died. In astrocytes, cell death was preceded by blebbing of the cell membrane, and by a sustained increase in intracellular Ca2+ followed by an abrupt further elevation occurring just before cellular collapse. The present results indicate that astrocytes are more vulnerable than neurons to 3-nitropropionic acid-induced cellular Ca2+ overload and toxicity, and hence support the hypothesis that, in part, 3-nitropropionic acid-induced neurotoxicity could be secondary to astrocytic cell death caused by Ca2+ overload.

Methamphetamine selectively damages dopaminergic innervation to the nucleus accumbens core while sparing the shell

Dopaminergic innervation to the nucleus accumbens was investigated following a neurotoxic regimen of methamphetamine (MA) treatment. Four 10 mg/kg doses of MA were administered s.c. to male Sprague-Dawley rats with a 2 h interval between doses. Rectal temperatures were monitored for the induction of MA-induced hyperthermia. Three days or 2 weeks after MA treatment the animals were sacrificed by transcardial perfusion and processed for tyrosine hydroxylase (TH-IR) and glial fibrillary acidic protein immunoreactivity (GFAP-IR). MA treatment produced a severe loss of TH-IR throughout the striatum, including the nucleus accumbens. However, within the nucleus accumbens, there was substantial sparing of TH-IR in the shell, while in the core immunoreactivity was almost entirely lost. Furthermore, astrogliosis, as demonstrated by GFAP-IR, was prevalent in the core but present only in sparse patches in the medial and lateral shell. Thus, dopaminergic innervation to the nucleus accumbens core undergoes degeneration following MA treatment, while innervation to the shell is resistant to the neurodegenerative effects of MA.