LRRK2 G2019S-induced mitochondrial DNA damage is LRRK2 kinase dependent and inhibition restores mtDNA integrity in Parkinson's disease

Mutations in leucine-rich repeat kinase 2 (LRRK2) are associated with increased risk for developing Parkinson's disease (PD). Previously, we found that LRRK2 G2019S mutation carriers have increased mitochondrial DNA (mtDNA) damage and after zinc finger nuclease-mediated gene mutation correction, mtDNA damage was no longer detectable. While the mtDNA damage phenotype can be unambiguously attributed to the LRRK2 G2019S mutation, the underlying mechanism(s) is unknown. Here, we examine the role of LRRK2 kinase function in LRRK2 G2019S-mediated mtDNA damage, using both genetic and pharmacological approaches in cultured neurons and PD patient-derived cells. Expression of LRRK2 G2019S induced mtDNA damage in primary rat midbrain neurons, but not in cortical neuronal cultures. In contrast, the expression of LRRK2 wild type or LRRK2 D1994A mutant (kinase dead) had no effect on mtDNA damage in either midbrain or cortical neuronal cultures. In addition, human LRRK2 G2019S patient-derived lymphoblastoid cell lines (LCL) demonstrated increased mtDNA damage relative to age-matched controls. Importantly, treatment of LRRK2 G2019S expressing midbrain neurons or patient-derived LRRK2 G2019S LCLs with the LRRK2 kinase inhibitor GNE-7915, either prevented or restored mtDNA damage to control levels. These findings support the hypothesis that LRRK2 G2019S-induced mtDNA damage is LRRK2 kinase activity dependent, uncovering a novel pathological role for this kinase. Blocking or reversing mtDNA damage via LRRK2 kinase inhibition or other therapeutic approaches may be useful to slow PD-associated pathology.

The Gordon Research Seminar & Conference on Parkinson’s disease: state of the Science 200 years after James Parkinson’s essay on the Shaking Palsy

The first-ever Gordon Research Seminar on Parkinson’s disease was held in conjunction with the second-ever Gordon Research Conference on Parkinson’s disease at the Grand Summit Hotel at Sunday River in Newry, Maine, from June 24–30. The Gordon Research Seminar brought together graduate students and postdoctoral researchers to network, learn first-hand about life with Parkinson’s disease, listen to and present Parkinson’s disease science, and hear about a variety of relevant career options. The Gordon Research Conference began as the Gordon Research Seminar concluded and was attended by a broad, international mix of junior and senior scientists from academia and industry. It was organized into eight outstanding scientific sessions in which cutting edge science, much of it unpublished, was presented. Among attendees, there was universal praise for the content and organization of the meeting, and for its open and welcoming ambiance.

LC/MS analysis of cardiolipins in substantia nigra and plasma of rotenone-treated rats: Implication for mitochondrial dysfunction in Parkinson's disease

Exposure to rotenone in vivo results in selective degeneration of dopaminergic neurons and development of neuropathologic features of Parkinson's disease (PD). As rotenone acts as an inhibitor of mitochondrial respiratory complex I, we employed oxidative lipidomics to assess oxidative metabolism of a mitochondria-specific phospholipid, cardiolipin (CL), in substantia nigra (SN) of exposed animals. We found a significant reduction in oxidizable polyunsaturated fatty acid (PUFA)-containing CL molecular species. We further revealed increased contents of mono-oxygenated CL species at late stages of the exposure. Notably, linoleic acid in sn-1 position was the major oxidation substrate yielding its mono-hydroxy- and epoxy-derivatives whereas more readily "oxidizable" fatty acid residues (arachidonic and docosahexaenoic acids) remained non-oxidized. Elevated levels of PUFA CLs were detected in plasma of rats exposed to rotenone. Characterization of oxidatively modified CL molecular species in SN and detection of PUFA-containing CL species in plasma may contribute to better understanding of the PD pathogenesis and lead to the development of new biomarkers of mitochondrial dysfunction associated with this disease.

Mitochondrial DNA damage: molecular marker of vulnerable nigral neurons in Parkinson's disease

DNA damage can cause (and result from) oxidative stress and mitochondrial impairment, both of which are implicated in the pathogenesis of Parkinson's disease (PD). We therefore examined the role of mitochondrial DNA (mtDNA) damage in human postmortem brain tissue and in in vivo and in vitro models of PD, using a newly adapted histochemical assay for abasic sites and a quantitative polymerase chain reaction (QPCR)-based assay. We identified the molecular identity of mtDNA damage to be apurinic/apyrimidinic (abasic) sites in substantia nigra dopamine neurons, but not in cortical neurons from postmortem PD specimens. To model the systemic mitochondrial impairment of PD, rats were exposed to the pesticide rotenone. After rotenone treatment that does not cause neurodegeneration, abasic sites were visualized in nigral neurons, but not in cortex. Using a QPCR-based assay, a single rotenone dose induced mtDNA damage in midbrain neurons, but not in cortical neurons; similar results were obtained in vitro in cultured neurons. Importantly, these results indicate that mtDNA damage is detectable prior to any signs of degeneration - and is produced selectively in midbrain neurons under conditions of mitochondrial impairment. The selective vulnerability of midbrain neurons to mtDNA damage was not due to differential effects of rotenone on complex I since rotenone suppressed respiration equally in midbrain and cortical neurons. However, in response to complex I inhibition, midbrain neurons produced more mitochondrial H2O2 than cortical neurons. We report selective mtDNA damage as a molecular marker of vulnerable nigral neurons in PD and suggest that this may result from intrinsic differences in how these neurons respond to complex I defects. Further, the persistence of abasic sites suggests an ineffective base excision repair response in PD.

Gene-environment interactions in Parkinson's disease: the importance of animal modeling

Parkinson's disease (PD), a late-onset neurodegenerative disorder, occurs most commonly in a "sporadic" (idiopathic) form, without a clearly defined genetic basis and only a vaguely delineated pathogenesis. Together, the various monogenic forms of PD (i.e., those arising from mutations in single genes) account for a minority of PD cases but have provided crucial insights into disease mechanisms. Although it is commonly believed that sporadic PD is caused by a lifetime of environmental exposures that are superimposed on an individual's composite genetic susceptibility, this hypothesis has not been tested adequately. This article reviews genetic and environmental factors that have been associated with PD and attempts to put these into a pathogenic framework. We argue that animal modeling will become increasingly important in attempting to elucidate gene-environment interactions, to define pathogenic mechanisms, and to provide a platform for testing of targeted therapeutic interventions.

Sequential and concerted gene expression changes in a chronic in vitro model of parkinsonism

Many mechanisms of neurodegeneration have been implicated in Parkinson's disease, but which ones are most important and potential interactions among them are unclear. To provide a broader perspective on the parkinsonian neurodegenerative process, we have performed a global analysis of gene expression changes caused by chronic, low-level exposure of neuroblastoma cells to the mitochondrial complex I inhibitor and parkinsonian neurotoxin rotenone. Undifferentiated SK-N-MC human neuroblastoma cells were grown in the presence of rotenone (5 nM), and RNA was extracted at three different time points (baseline, 1 week, and 4 weeks) for labeling and hybridization to Affymetrix Human U133 Plus 2.0 GeneChips. Our results show that rotenone induces concerted alterations in gene expression that change over time. Particularly, alterations in transcripts related to DNA damage, energy metabolism, and protein metabolism are prominent during chronic complex I inhibition. These data suggest that early augmentation of capacity for energy production in response to mitochondrial inhibition might be deleterious to cellular function and survival. These experiments provide the first transcriptional analysis of a rotenone model of Parkinson's disease and insight into which mechanisms of neurodegeneration may be targeted for therapeutic intervention.

Ethyl-EPA in Huntington disease: A double-blind, randomized, placebo-controlled trial

BACKGROUND

Preliminary evidence suggests beneficial effects of pure ethyl-eicosapentaenoate (ethyl-EPA) in Huntington disease (HD).

METHODS

A total of 135 patients with HD were randomized to enter a multicenter, double-blind, placebo-controlled trial on the efficacy of 2 g/d ethyl-EPA vs placebo. The Unified Huntington's Disease Rating Scale (UHDRS) was used for assessment. The primary end point was outcome at 12 months on the Total Motor Score 4 subscale (TMS-4). Analysis of covariance (ANCOVA) and a chi2 test on response, defined as absence of increase in the TMS-4, were performed.

RESULTS

A total of 121 patients completed 12 months, and 83 did so without protocol violations (PP cohort). Intent-to-treat (ITT) analysis revealed no significant difference between ethyl-EPA and placebo for TMS-4. In the PP cohort, ethyl-EPA proved better than placebo on the chi2 test on TMS-4 (p < 0.05), but missed significance on ANCOVA (p = 0.06). Secondary end points (ITT cohort) showed no benefit of ethyl-EPA but a significantly worse outcome in the behavioral severity and frequency compared with placebo. Exploring moderators of the efficacy of ethyl-EPA on TMS-4 showed a significant interaction between treatment and a factor defining patients with high vs low CAG repeats. Reported adverse events were distributed equally between treatment arms.

CONCLUSIONS

Ethyl-eicosapentaenoate (ethyl-EPA) (purity > 95%) had no benefit in the intent-to-treat cohort of patients with Huntington disease, but exploratory analysis revealed that a significantly higher number of patients in the per protocol cohort, treated with ethyl-EPA, showed stable or improved motor function. Further studies of the potential efficacy of ethyl-EPA are warranted.

Complex I and Parkinson's disease

Complex I of the mammalian electron transfer chain is composed of at least 43 protein subunits, of which 7 are encoded by mtDNA. It catalyzes the transfer of electrons from NADH to ubiquinone and translocates protons from the mitochondrial matrix to the intermembrane space. It may also play direct roles in the mitochondrial permeability transition and in cell death pathways. Despite the limitations of current complex I assays, biochemical studies have suggested the presence of a mild, systemic defect of complex I in Parkinson's disease (PD). Recent experimental work has modeled this abnormality using rotenone to systemically inhibit complex I. Chronic rotenone exposure accurately recapitulated the pathological, biochemical, and behavioral features of PD. Thus, relatively subtle complex I abnormalities--either genetic or acquired--may be central to the pathogenesis of PD.

Pesticides and Parkinson's disease

Parkinson's disease (PD), a common neurodegenerative disorder affects approximately 1% of the population over 65. PD is a late-onset progressive motor disease characterized by tremor, rigidity (stiffness), and bradykinesia (slowness of movement). The hallmark of PD is the selective death of dopamine-containing neurons in the substantia nigra pars compacta which send their projections to the striatum and the presence of cytoplasmic aggregates called Lewy bodies. Most cases of PD are sporadic but rare cases are familial, with earlier onset. The underlying mechanisms and causes of PD still remain unclear.

Pathogenesis of Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by degeneration of the nigrostriatal dopaminergic pathway and the appearance of cytoplasmic proteinaceous aggregates known as Lewy bodies. Studies of familial PD have uncovered rare causative mutations in genes, including alpha-synuclein. Mutations or oxidative modification of alpha-synuclein causes it to aggregate; alpha-synuclein is a major component of the Lewy body in both familial and sporadic PD. Biochemical analysis has implicated mitochondrial dysfunction in PD. Epidemiological studies indicate a role of exposure to pesticides, some of which are mitochondrial toxins. Mitochondrial dysfunction, resulting from genetic defects, environmental toxins, or a combination of the two, may cause alpha-synuclein aggregation and produce selective neurodegeneration through mechanisms involving oxidative stress and excitotoxicity. Efforts to better define PD pathogenesis should reveal novel therapeutic targets.

Subthalamic infusion of an NMDA antagonist prevents basal ganglia metabolic changes and nigral degeneration in a rodent model of Parkinson's disease

Using permanent cannulas connected to subcutaneous pumps, we infused selective glutamate antagonists into the subthalamic nucleus of rats. Pumps were implanted immediately after the intrastriatal injection of 6-hydroxydopamine and delivered micro-quantities of the Nmethyl-D-aspartate antagonist MK-801 or the alpha-amino-3-hydroxy-5-methylisoxazole antagonist NBQX for 4 weeks. Subthalamic infusion of MK-801, but not of NBQX, prevented the basal ganglia metabolic changes and motor abnormalities caused by nigrostriatal lesion. Animals treated with MK-801 also exhibited marked reduction of nigral cell loss. We conclude that pharmacological modulation of subthalamic activity may have both symptomatic and neuroprotective effects in Parkinson's disease.

Glutamatergic influences on the basal ganglia

Glutamate is the predominant excitatory neurotransmitter of the basal ganglia, where it acts on ionotropic and metabotropic receptors. In the best studied of the basal ganglia disorders, Parkinson's disease, there is compelling evidence that the activities of glutamatergic pathways are altered. Of particular importance, the glutamatergic subthalamic nucleus becomes overactive. Pharmacologic blockade of subthalamic neurotransmission has antiparkinsonian symptomatic effects and may also help to protect the remaining dopamine neurons of the substantia nigra from excitotoxic neurodegeneration. Development of drugs to manipulate the glutamatergic system with appropriate pharmacologic and anatomic selectivity is likely to dramatically improve our ability to treat disorders of the basal ganglia.

A randomized, controlled trial of remacemide for motor fluctuations in Parkinson's disease

BACKGROUND

Preclinical studies suggest that glutamate antagonists help ameliorate motor fluctuations in patients with PD treated with levodopa.

METHODS

In a multicenter, randomized, double-blind, placebo-controlled, parallel-group, dose-ranging study, the authors assessed the safety, tolerability, and efficacy of the glutamate receptor blocker remacemide hydrochloride in 279 patients with motor fluctuations treated with levodopa. The primary objective was to assess the short-term tolerability and safety of four dosage levels of remacemide during 7 weeks of treatment. Patients were also monitored with home diaries and the Unified PD Rating Scale (UPDRS) to collect preliminary data on treatment efficacy.

RESULTS

Remacemide was well tolerated up to a dosage of 300 mg/d on a twice daily schedule and 600 mg/d on a four times daily schedule. The most common dosage-related adverse events were dizziness and nausea, as observed in previous studies of remacemide. The percent "on" time and motor UPDRS scores showed trends toward improvement in the patients treated with 150 and 300 mg/d remacemide compared with placebo-treated patients, although these improvements were not significant.

CONCLUSION

Remacemide is a safe and tolerable adjunct to dopaminergic therapy for patients with PD and motor fluctuations. Although this study had limited power to detect therapeutic effects, the observed improvement is consistent with studies of non-human primates with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonian signs and symptoms. Additional studies are warranted to confirm these results over an extended period of observation, and to explore the potential neuroprotective effects of remacemide in slowing the progression of PD.

Blockade of subthalamic glutamatergic activity corrects changes in neuronal metabolism and motor behavior in rats with nigrostriatal lesions

We infused--for four weeks--a selective antagonist of the NMDA receptor, MK-801, into the subthalamic nucleus of rats bearing an evolving nigrostriatal lesion. The aim was to block the subthalamic overactivity resulting from the dopaminergic striatal denervation. The nigrostriatal lesion caused metabolic activation--increased activity of the mitochondrial enzyme succinate dehydrogenase--of basal ganglia nuclei, ipsilaterally to the lesion, along with contralateral rotational behavior. These phenomena were effectively counteracted by the blockade of glutamatergic transmission at the subthalamic level. Pharmacological manipulation of the STN, through selective drugs capable of modulating glutamatergic transmission, may therefore represent a valuable tool for the treatment of PD.

In vivo labeling of mitochondrial complex I (NADH:ubiquinone oxidoreductase) in rat brain using [(3)H]dihydrorotenone

Defects in mitochondrial energy metabolism have been implicated in several neurodegenerative disorders. Defective complex I (NADH:ubiquinone oxidoreductase) activity plays a key role in Leber's hereditary optic neuropathy and, possibly, Parkinson's disease, but there is no way to assess this enzyme in the living brain. We previously described an in vitro quantitative autoradiographic assay using [(3)H]dihydrorotenone ([(3)H]DHR) binding to complex I. We have now developed an in vivo autoradiographic assay for complex I using [(3)H]DHR binding after intravenous administration. In vivo [(3)H]DHR binding was regionally heterogeneous, and brain uptake was rapid. Binding was enriched in neurons compared with glia, and white matter had the lowest levels of binding. In vivo [(3)H]DHR binding was markedly reduced by local and systemic infusion of rotenone and was enhanced by local NADH administration. There was an excellent correlation between regional levels of in vivo [(3)H]DHR binding and the in vitro activities of complex II (succinate dehydrogenase) and complex IV (cytochrome oxidase), suggesting that the stoichiometry of these components of the electron transport chain is relatively constant across brain regions. The ability to assay complex I in vivo should provide a valuable tool to investigate the status of this mitochondrial enzyme in the living brain and suggests potential imaging techniques for complex I in humans.

Chronic systemic pesticide exposure reproduces features of Parkinson's disease

The cause of Parkinson's disease (PD) is unknown, but epidemiological studies suggest an association with pesticides and other environmental toxins, and biochemical studies implicate a systemic defect in mitochondrial complex I. We report that chronic, systemic inhibition of complex I by the lipophilic pesticide, rotenone, causes highly selective nigrostriatal dopaminergic degeneration that is associated behaviorally with hypokinesia and rigidity. Nigral neurons in rotenone-treated rats accumulate fibrillar cytoplasmic inclusions that contain ubiquitin and alpha-synuclein. These results indicate that chronic exposure to a common pesticide can reproduce the anatomical, neurochemical, behavioral and neuropathological features of PD.

Mitochondrial dysfunction in Parkinson's disease

The cause of Parkinson's disease (PD) is unknown, but reduced activity of complex I of the electron-transport chain has been implicated in the pathogenesis of both mitochondrial permeability transition pore-induced Parkinsonism and idiopathic PD. We developed a novel model of PD in which chronic, systemic infusion of rotenone, a complex-I inhibitor, selectively kills dopaminergic nerve terminals and causes retrograde degeneration of substantia nigra neurons over a period of months. The distribution of dopaminergic pathology replicates that seen in PD, and the slow time course of neurodegeneration mimics PD more accurately than current models. Our model should enhance our understanding of neurodegeneration in PD. Metabolic impairment depletes ATP, depresses Na+/K(+)-ATPase activity, and causes graded neuronal depolarization. This relieves the voltage-dependent Mg2+ block of the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor, which is highly permeable to Ca2+. Consequently, innocuous levels of glutamate become lethal via secondary excitotoxicity. Mitochondrial impairment also disrupts cellular Ca2+ homoeostasis. Moreover, the facilitation of NMDA-receptor function leads to further mitochondrial dysfunction. To a large part, this occurs because Ca2+ entering neurons through NMDA receptors has 'privileged' access to mitochondria, where it causes free-radical production and mitochondrial depolarization. Thus there may be a feed-forward cycle wherein mitochondrial dysfunction causes NMDA-receptor activation, which leads to further mitochondrial impairment. In this scenario, NMDA-receptor antagonists may be neuroprotective.

Glutathione depletion in PC12 results in selective inhibition of mitochondrial complex I activity. Implications for Parkinson's disease

Oxidative stress appears to play an important role in degeneration of dopaminergic neurons of the substantia nigra (SN) associated with Parkinson's disease (PD). The SN of early PD patients have dramatically decreased levels of the thiol tripeptide glutathione (GSH). GSH plays multiple roles in the nervous system both as an antioxidant and a redox modulator. We have generated dopaminergic PC12 cell lines in which levels of GSH can be inducibly down-regulated via doxycycline induction of antisense messages against both the heavy and light subunits of gamma-glutamyl-cysteine synthetase, the rate-limiting enzyme in glutathione synthesis. Down-regulation of glutamyl-cysteine synthetase results in reduction in mitochondrial GSH levels, increased oxidative stress, and decreased mitochondrial function. Interestingly, decreases in mitochondrial activities in GSH-depleted PC12 cells appears to be because of a selective inhibition of complex I activity as a result of thiol oxidation. These results suggest that the early observed GSH losses in the SN may be directly responsible for the noted decreases in complex I activity and the subsequent mitochondrial dysfunction, which ultimately leads to dopaminergic cell death associated with PD.

Immunocytochemical characterization of the mitochondrially encoded ND1 subunit of complex I (NADH : ubiquinone oxidoreductase) in rat brain

In Parkinson's disease, there is a selective defect in complex I of the electron transfer chain. To better understand complex I and its involvement in neurodegenerative disease, we raised an antibody against a conserved epitope of the human mitochondrially encoded subunit 1 of complex I (ND1). Antibodies were affinity purified and assessed by ELISA, immunoblotting, and immunocytochemistry. Immunoblots of brain homogenates from mouse, rat, and monkey brain showed a single 33-kDa band consistent with the predicted molecular mass of the protein. Subcellular fractionation showed the protein to be enriched in mitochondria. Immunocytochemistry in rat brain revealed punctate labeling in cell bodies and processes of neurons. Immunoreactively generally co-localized with subunit IV of complex IV. In striatum, ND1 immunoreactively was greatly enriched in large cholinergic neurons and neurons containing nitric oxide synthase, two cell populations that are resistant to excitotoxic and metabolic insults. In substantia nigra, many dopaminergic neurons had little ND1 immunoreactivity, which may help to explain their sensitivity to complex I inhibitors. In spinal cord, ND1 immunoreactively was enriched in motor neurons. We conclude that complex I is differentially distributed across brain regions, between neurons and glia, and between types of neurons. This antibody should provide a valuable tool for assessing complex I in normal and pathological conditions.

Antiparkinsonian actions of CP-101,606, an antagonist of NR2B subunit-containing N-methyl-d-aspartate receptors

In the setting of nigrostriatal dopamine depletion, glutamatergic pathways to the striatum and basal ganglia output nuclei become overactive. Systemically administered glutamate receptor antagonists may have direct antiparkinsonian actions in rodents, but there is little evidence for this in primates. Glutamate antagonists may also potentiate conventional dopaminergic therapies; however, there is concern that broad spectrum, nonselective antagonists may have unwanted side-effects. Because subunit-selective antagonists may avoid these liabilities, we have examined the antiparkinsonian effects of a selective antagonist of the NR2B subunit of the NMDA receptor. In rats, CP-101,606 decreased haloperidol-induced catalepsy with an ED(50) of about 0.5 mg/kg. In MPTP-treated monkeys, CP-101,606 (1 mg/kg) reduced parkinsonian motor symptoms by 20%. At a dose of 0.05 mg/kg, CP-101,606 markedly potentiated the effect of a submaximal dose of levodopa, reducing motor symptoms by about 50% compared to vehicle and by about 30% compared to levodopa alone. No side-effects were apparent at any dose of CP-101,606. We conclude that CP-101,606 has direct antiparkinsonian actions in both rodents and monkeys and it synergistically potentiates levodopa in MPTP-treated monkeys. Clinical evaluation of selective NR2B antagonists may be warranted in Parkinson's disease.

GluR1 glutamate receptor subunit is regulated differentially in the primate basal ganglia following nigrostriatal dopamine denervation

Nigrostriatal dopaminergic denervation is associated with complex changes in the functional and neurochemical anatomy of the basal ganglia. The excitatory neurotransmitter glutamate mediates neural signaling at crucial points of this circuitry, and glutamate receptors are differentially distributed in the basal ganglia. Available evidence suggests that the glutamatergic corticostriatal and subthalamofugal pathways become overactive after nigrostriatal dopamine depletion. In this study, we have analyzed the regulation of the GluR1 subunit of the a-amino-3-hydroxy-5-methyl-4-isoxazole propionate glutamate receptor in the basal ganglia of primates following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced dopamine denervation. The dopamine denervation resulted in distinct alterations in GluR1 distribution: (1) GluR1 protein expression was markedly increased in caudate and putamen, and this was most pronounced in the striosomes; (2) GluR1 protein was altered minimally in subthalamic nucleus; (3) expression of GluR1 was down-regulated in the globus pallidus by 63% and in the substantia nigra by 57%. The down-regulation of GluR1 expression in the output nuclei of the basal ganglia, the internal segment of the globus pallidus and the substantia nigra pars reticulata, may be a compensation for the overactive glutamatergic input from subthalamic nucleus, which arises after striatal dopamine denervation. Our results indicate that the glutamatergic system undergoes regulatory changes in response to altered basal ganglia activity in a primate model of Parkinson's disease. Targeted manipulation of the glutamatergic system may be a viable approach to the symptomatic treatment of Parkinson's disease.

Differential expression of glutamate receptors by the dopaminergic neurons of the primate striatum

Neostriatal neurons are targets of glutamatergic input from the cortex and thalamus. Glutamate receptors are abundantly, but differentially, expressed by the striatal neurons. We previously described the presence of dopaminergic cells intrinsic to the primate striatum that increase in number following MPTP treatment. In this study we have used double-label immunocytochemistry to analyze the expression of the glutamate receptor subunits GluR1, GluR2/3, NR1, mGluR1, and mGluR5 in the dopaminergic cells of the striatum. Our results show that 75% of these cells express GluR1 while 25% of them express NR1. They do not express GluR2/3 or the group 1 metabotropic receptors. Our results suggest that this potentially important population of cells expresses only calcium-permeable ionotropic glutamate receptors. We speculate that glutamate may play a role in regulating the number of these dopaminergic neurons after MPTP treatment and may also influence their ability to release dopamine.

Increased apoptosis of Huntington disease lymphoblasts associated with repeat length-dependent mitochondrial depolarization

Huntington disease (HD) is a genetically dominant condition caused by expanded CAG repeats coding for glutamine in the HD gene product huntingtin. Although HD symptoms reflect preferential neuronal death in specific brain regions, huntingtin is expressed in almost all tissues, so abnormalities outside the brain might be expected. Although involvement of nuclei and mitochondria in HD pathophysiology has been suggested, specific intracellular defects that might elicit cell death have been unclear. Mitochondria dysfunction is reported in HD brains; mitochondria are organelles that regulates apoptotic cell death. We now report that lymphoblasts derived from HD patients showed increased stress-induced apoptotic cell death associated with caspase-3 activation. When subjected to stress, HD lymphoblasts also manifested a considerable increase in mitochondrial depolarization correlated with increased glutamine repeats.

Protective and symptomatic strategies for therapy of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by loss of the capacity to execute voluntary movements appropriately. PD develops as a consequence of the degeneration of dopamine-containing neurons in the substantia nigra pars compacta (SNc). SNc is a component of the basal ganglia nuclei, the network that controls the neural signaling underlying voluntary movements. The nigral cell loss triggers a cascade of functional modifications in the basal ganglia circuit, the most important of which is hyperactivity of another component of the circuit, the subthalamic nucleus (STN). Subthalamic hyperactivity represents a major neural substrate of PD motor symptoms. The etiopathogenesis of PD is probably multifactorial. Various mechanisms - including mitochondrial defects, oxidative stress, glutamate toxicity and genetic factors - are likely to contribute to the degenerative process. Current therapy for PD is essentially symptomatic. L-dopa, the direct precursor of dopamine, is still the "gold standard". However, long-term therapy with L-dopa is associated with significant side effects. Therefore, there is a need for new therapeutic strategies aimed at relieving motor symptoms and slowing the progression of neuronal degeneration. The excitatory amino acid glutamate plays a central role in the functional modifications that affect the basal ganglia in PD. In particular, it mediates the enhanced excitatory drive of the STN to the output nuclei of the basal ganglia, which leads to the expression of PD symptoms. Furthermore, since the STN projects to the SNc, the excessive glutamatergic stimulation on residual nigral neurons may sustain the degenerative process, generating a self-maintaining vicious circle. From these considerations, it ensues that the use of drugs capable of antagonizing the effects of glutamate may provide new symptomatic and neuroprotective strategies for therapy of PD.

Privileged access to mitochondria of calcium influx through N-methyl-D-aspartate receptors

Mitochondrial Ca2+ uptake responds dynamically and sensitively to changes in cytosolic Ca2+ levels and plays a crucial role in sequestering the large Ca2+ load induced by N-methyl-D-aspartate (NMDA) receptor activation. However, the precise interrelationships between NMDA receptor activation, cytosolic Ca2+ increase, and mitochondrial Ca2+ uptake remain obscure. To reliably, independently, and simultaneously detect cytosolic and mitochondrial Ca2+ concentration changes in the same cell, we loaded primary striatal neurons with two Ca2+ indicators, calcium green 1N and rhod-2, and visualized the fluorescence signals from single neurons with laser scanning confocal fluorescence microscopy. In kinetic data analysis, only calcium green signals from predefined cytosolic areas and rhod-2 signals from predefined mitochondrial regions were used, and attention was focused on the initial rapid rising phase of the responses. When neurons were treated with 100 microM NMDA, increases of cytosolic and mitochondrial Ca2+ showed similar time courses and rates of change, and seemed to be time-locked. In contrast, when neurons were treated with 100 microM kainate, 50 mM KCl, or 0.3 microM ionomycin, mitochondrial Ca2+ increases lagged behind cytosolic Ca2+ increases. These data suggest that mitochondrial Ca2+ uptake in response to an increase of cytosolic Ca2+ is faster and more tightly coupled during NMDA receptor activation than during non-NMDA receptor or voltage-dependent Ca2+ channel activation. This proficient mitochondrial Ca2+ uptake may avert a large rise in cytosolic Ca2+ concentration in response to NMDA receptor activation. Yet, it may lead to excessive Ca2+ accumulation inside mitochondria and render mitochondria susceptible to Ca2+ mediated injury.

3-Nitropropionic acid exacerbates N-methyl-D-aspartate toxicity in striatal culture by multiple mechanisms

We examined the effects of 3-nitropropionic acid-induced succinate dehydrogenase inhibition on neuronal ATP content, N-methyl-D-aspartate-induced neuronal death, resting membrane potential, and N-methyl-D-aspartate-induced changes in cytosolic calcium concentration ([Ca2+]c) in cultured rat striatal neurons. Exposure of cultures to 3 mM 3-nitropropionic acid for 3 h did not cause overt toxicity, but reduced ATP content by 35%. Treatment with 3-nitropropionic, or removal of Mg2+ from the medium, enhanced subsequent N-methyl-D-aspartate toxicity, reducing the LC50 from 250 microM to 12 microM or 30 microM, respectively. Even after Mg2+ removal, enhancement of N-methyl-D-aspartate toxicity by 3-nitropropionic acid remained pronounced, with the LC50 further decreasing to 3 microM. The mean resting membrane potential of neurons treated with 3-nitropropionic acid was -37 mV, while that in control neurons was -61 mV. Treatment with 3-nitropropionic did not affect baseline [Ca2+]c as determined by fura-2 microfluorimetry. N-methyl-D-aspartate (30 microM) caused a rapid rise in [Ca2+]c, the initial magnitude of which was not affected by 3-nitropropionic acid. However, after a 1-h treatment, [Ca2+]c was dramatically higher in 3-nitropropionic acid-treated neurons. This increased calcium load was washed out slowly and only partially, although calcium in control neurons washed out rapidly and almost completely. These results suggest that in striatal neurons, the enhancement of N-methyl-D-aspartate toxicity caused by succinate dehydrogenase inhibition may be due to synergism between partial relief of the Mg2+ blockade of the N-methyl-D-aspartate receptor and other mechanisms, including disruption of neuronal calcium regulation. This synergism may be relevant to the neuronal death observed in neurodegenerative disorders.

Prospects of glutamate antagonists in the therapy of Parkinson's disease

It has been suggested that the excitatory amino acid glutamate, acting as both a neurotoxin and a neurotransmitter, might play a central role in the pathophysiology of Parkinson's disease (PD). Intrinsic energetic defects of the neurons of the substantia nigra pars compacta, the brain area where the degenerative process of PD takes place, may render nigral neurons highly vulnerable to the effects of glutamate, which acts as a neurotoxin in the presence of impaired cellular energy metabolism. Degeneration of dopamine nigral neurons and striatal dopaminergic denervation cause a cascade of functional modifications in the activity of basal ganglia nuclei. Due to the close relationship that links dopaminergic and glutamatergic neurotransmission, glutamate is directly involved in the functional alterations of basal ganglia circuitry that lead to the development of parkinsonian motor symptoms. Drugs counteracting the effects of glutamate might therefore provide new protective and symptomatic strategies for therapy of PD.

Quantitative study of mitochondrial complex I in platelets of parkinsonian patients

Activity of mitochondrial enzyme complex I (NADH-ubiquinone oxidoreductase) is reduced in the substantia nigra of patients with Parkinson's disease (PD). A less pronounced decrease in the activity of this enzyme has also been reported in platelets of PD patients. To obtain quantitative information on platelet complex I in PD, we studied platelet complex I in 16 PD patients and 16 age-matched controls by using a newly developed technique based on the binding of [3H]dihydrorotenone ([3H]DHR), an analog of the pesticide rotenone, to complex I. We also investigated the inhibitory effect of MPP+ (1-methyl-4-phenyl-pyridinium) on [3H]DHR specific binding to platelet complex I. PD patients and controls showed similar levels of [3H]DHR specific binding; preincubation of platelets with MPP+ caused the same degree of inhibition of [3H]DHR specific binding in the two groups. In PD patients, we observed a direct correlation between MPP+-induced inhibition of [3H]DHR specific binding and the daily intake of levodopa, which may be related to drug-induced changes in the transport of MPP+ into the platelet or in its binding to complex I. These findings demonstrate that the reported reduction in complex I activity in platelets of PD patients can not be accounted for by an abnormality at the level of the rotenone binding site (putatively the ND-1 gene product), although they do not exclude differences in complex I activity between PD patients and controls.

Visualization of NMDA receptor-induced mitochondrial calcium accumulation in striatal neurons

Ca2+ influx through NMDA receptor-gated channels and the subsequent rise in intracellular Ca2+ concentration ([Ca2+]i) have been implicated in cytotoxic processes that lead to irreversible neuronal injury. While many studies have focused on cytosolic Ca2+ homeostasis, much less is known about Ca2+ fluxes in subcellular organelles, such as mitochondria. The mitochondria play an important role in Ca2+ homeostasis by sequestering cytosolic Ca2+ loads. However, mitochondrial Ca2+ overload can impair ATP synthesis, induce free radical formation, and lead to lipid peroxidation. Thus, it is also important to understand the mitochondrial Ca2+ fluxes induced by NMDA. In this study, changes in mitochondrial Ca2+ concentration ([Ca2+]m) in cultured striatal neurons were monitored with a Ca(2+)-binding fluorescent probe, rhod-2, and laser scanning confocal microscopy. The rhod-2 fluorescence signal was highly localized in mitochondrial areas of confocal images. A rapid increase of [Ca2+]m was observed when neurons were treated with 100 microM NMDA. The increased [Ca2+]m induced by NMDA could not be observed in the presence of ruthenium red, an inhibitor of the mitochondrial Ca2+ uniporter, or CCCP, a protonophore that breaks down the mitochondrial membrane potential necessary for Ca2+ uptake. The magnitude and reversibility of changes in [Ca2+]m induced by NMDA were variable. In neurons receiving multiple pulses of NMDA, [Ca2+]m did not return to baseline. The elevated [Ca2+]m may persist indefinitely and may rise further after successive NMDA exposures. These data demonstrate that Ca2+ accumulates in mitochondria in response to NMDA receptor activation. This Ca2+ accumulation may play a role in the excitotoxic mitochondrial dysfunction induced by NMDA.

Susceptibility of adult rats to lead-induced changes in NMDA receptor complex function

Because cognitive impairments can occur with occupational Pb exposures, changes in NMDA receptor complex function might be expected to occur in adult rats treated with Pb. Using drug discrimination procedures, MK-801 sensitivity was determined in adult rats at three time points: after chronic exposure to 0, 50, or 150 ppm Pb acetate; again after exposure levels were increased to 500 and 1000 ppm; and 6 months after termination of Pb exposure. Changes in blood (PbB) and brain Pb levels, and in MK-801 and CGP-39653 binding, were examined in additional groups of nonbehaviorally tested rats. Pb decreased MK-801 sensitivity after Pb exposure levels were increased, but only at 500 ppm, indicating biphasic effects and precluding any correspondence between behavioral changes and biomarkers of exposure. Associated PbBs were higher, but brain Pbs similar to those associated with MK-801 sensitivity changes following postnatal and postweaning exposures. Neither MK-801 nor CGP-39653 binding was systematically affected by chronic Pb exposure in adults. Although adult Pb exposures do produce changes in NMDA function, at least as indicated by changes in MK-801 sensitivity, vulnerability to such effects is clearly less pronounced than with exposures occurring earlier in development.

Dopaminergic neurons intrinsic to the primate striatum

Intrinsic, striatal tyrosine hydroxylase-immunoreactive (TH-i) cells have received little consideration. In this study we have characterized these neurons and their regulatory response to nigrostriatal dopaminergic deafferentation. TH-i cells were observed in the striatum of both control and 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP)-treated monkeys; TH-i cell counts, however, were 3.5-fold higher in the striatum of MPTP-lesioned monkeys. To establish the dopaminergic nature of the TH-i cells, sections were double-labeled with antibodies to dopamine transporter (DAT). Immunofluorescence studies demonstrated that nearly all TH-i cells were double-labeled with DAT, suggesting that they contain the machinery to be functional dopaminergic neurons. Two types of TH-i cells were identified in the striatum: small, aspiny, bipolar cells with varicose dendrites and larger spiny, multipolar cells. The aspiny cells, which were more prevalent, corresponded morphologically to the GABAergic interneurons of the striatum. Double-label immunofluorescence studies using antibodies to TH and glutamate decarboxylase (GAD67), the synthetic enzyme for GABA, showed that 99% of the TH-i cells were GAD67-positive. Very few (<1%) of the TH-i cells, however, were immunoreactive for the calcium-binding proteins calbindin and parvalbumin. In summary, these results demonstrate that the dopaminergic cell population of the striatum responds to dopamine denervation by increasing in number, apparently to compensate for loss of extrinsic dopaminergic innervation. Moreover, this population of cells corresponds largely with the intrinsic GABAergic cells of the striatum. This study also suggests that the adult primate striatum does retain some intrinsic capacity to compensate for dopaminergic cell loss.

Subthalamic ablation reverses changes in basal ganglia oxidative metabolism and motor response to apomorphine induced by nigrostriatal lesion in rats

In Parkinson's disease, the functional architecture of the basal ganglia nuclei undergoes profound alterations, one of the most important of which is overactivity of the basal ganglia output nuclei. This phenomenon seems to be intimately related to pathological overactivity of the subthalamic nucleus, which directly modulates the basal ganglia output through its glutamatergic projections. In this study, we investigated the effects of unilateral subthalamic nucleus lesions on the activities of succinate dehydrogenase and cytochrome oxidase, two markers of neuronal activity, in rats with prior unilateral lesions of the nigrostriatal tract. We also explored the effect of subthalamic nucleus lesions on the rotational response to systemic apomorphine. Rats with unilateral lesions of the nigrostriatal tract showed ipsilateral increases in enzyme activity in the basal ganglia output nuclei, entopeduncular nucleus and substantia nigra pars reticulata. Selective subthalamic nucleus destruction completely reversed this phenomenon. In addition, subthalamic nucleus lesions abolished the rotational response to apomorphine. These results confirm that overactivity of the subthalamic nucleus plays a pivotal role in the functional alterations of basal ganglia associated with Parkinson's disease. They also shed further light on the neural mechanisms through which manipulations of subthalamic activity can ameliorate Parkinson's disease symptoms.

Lead-induced changes in NMDA receptor complex binding: correlations with learning accuracy and with sensitivity to learning impairments caused by MK-801 and NMDA administration

This study sought to further evaluate potential mechanistic relationships between Pb-induced alterations in glutamate neurotransmission and behavioral toxicity. It examined correlations between Pb-induced changes in [3H]MK-801 and [3H]CGP-39653 binding sites in 4 different brain regions (frontal cortex, dentate gyrus, CA1 and striatum) and (1) changes in learning accuracy on a multiple repeated acquisition and performance schedule, and (2) sensitivity to the accuracy-impairing effects of MK-801 and NMDA on this learning baseline. All data were obtained from a single population of rats that had been chronically exposed from weaning to 0, 50 or 250 ppm Pb acetate in drinking water and demonstrated selective learning impairments and altered sensitivity to the effects of MK-801 and NMDA on learning accuracy. Pb exposure decreased MK-801 binding and possibly increased CGP-39653 binding, effects statistically significant in some brain regions, but generally exhibiting similar trends across regions. At 0 ppm, higher levels, particularly of MK-801 binding, were associated with higher accuracy levels in the learning paradigm and with greater decrements in learning accuracy following MK-801 or NMDA administration. These linear correlations were negated and in some cases even reversed by 50 and 250 ppm Pb, an effect that might be attributable to an alteration of NMDA receptor complex subunit composition and thus, ligand binding. Of the 4 brain regions examined, striatal MK-801 binding proved to be the best predictor of learning accuracy levels. These data provide additional support for an involvement of the NMDA receptor complex in Pb-induced learning impairments. The fact that these effects were noted most frequently in striatum also raises the possibility that dopamine-glutamatergic interactions contribute to Pb's effects.

Intrastriatal neurotoxin injections reduce in vitro and in vivo binding of radiolabeled rotenoids to mitochondrial complex I

The in vivo and in vitro bindings of radiolabeled rotenoids to mitochondrial complex I of rat striatum were examined after unilateral intrastriatal injections of quinolinic acid or 1-methyl-4-phenylpyridinium salt (MPP+). Quinolinic acid produced significant, similar losses of in vivo binding of [11C]dihydrorotenol ([11C]DHROL: 40%) and in vitro binding of [3H]dihydrorotenone ([3H]DHR: 53%) in the injected striatal at 13 days after the injection of neurotoxin. MPP+ reduced in vivo binding of [11C]DHROL up to-55%) as measured 1.5 to 6 h after its administration. Reductions of in vivo [11C]DHROL binding after either quinolinic acid or MPP+ injections did not correlate with changes in striatal blood flow as measured with [14C]iodoantipyrine. These results are consistent with losses of complex I binding sites for radiolabeled rotenoids, produced using cell death (quinolinic acid) or direct competition for the binding site (MPP+). Appropriately radiolabeled rotenoids may be useful for in vivo imaging studies of changes of complex I in neurodegenerative diseases.

[3H]dihydrorotenone binding to NADH: ubiquinone reductase (complex I) of the electron transport chain: an autoradiographic study

Abnormalities of mitochondrial energy metabolism may play a role in normal aging and certain neurodegenerative disorders. In this regard, complex I of the electron transport chain has received substantial attention, especially in Parkinson's disease. The conventional method for studying complex I has been quantitation of enzyme activity in homogenized tissue samples. To enhance the anatomic precision with which complex I can be examined, we developed an autoradiographic assay for the rotenone site of this enzyme. [3H]dihydrorotenone ([3H]DHR) binding is saturable (KD = 15-55 nM) and specific, and Hill slopes of 1 suggest a single population of binding sites. Nicotinamide adenine dinucleotide (NADH) enhances binding 4- to 80-fold in different brain regions (EC50 = 20-40 microM) by increasing the density of recognition sites (Bmax). Nicotinamide adenine dinucleotide phosphate also increases binding, but NAD+ does not. In skeletal muscle, heart, and kidney, binding was less affected by NADH. [3H]DHR binding is inhibited by rotenone (IC50 = 8-20 nM), meperidine (IC50 = 34-57 microM), amobarbitol (IC50 = 375-425 microM), and MPP+ (IC50 = 4-5 mM), consistent with the potencies of these compounds in inhibiting complex I activity. Binding is heterogeneously distributed in brain with the density in gray matter structures varying more than 10-fold. Lesion studies suggest that a substantial portion of binding is associated with nerve terminals. [3H]DHR autoradiography is the first quantitative method to examine complex I with a high degree of anatomic precision. This technique may help to clarify the potential role of complex I dysfunction in normal aging and disease.

A controlled trial of remacemide hydrochloride in Huntington's disease

We conducted a randomized, double-blind, placebo-controlled tolerability study of a N-methyl-D-aspartate (NMDA) glutamate receptor ion-channel blocker, remacemide hydrochloride, in 31 independently ambulatory patients (18 men, 13 women) with Huntington's disease (HD). Subjects were randomized to receive either placebo or active remacemide at dosages of 200 mg/day or 600 mg/day. The primary outcome measure was the proportion of subjects able to complete the study with the assigned treatment. Remacemide was generally well tolerated, and no significant differences between the treatment arms were found in the primary outcome measure. A trend toward improvement in chorea was observed among subjects administered remacemide 200 mg/day. Based on the tolerability and safety demonstrated during this short-term trial, remacemide warrants more extended controlled investigation in patients with HD.

Bioenergetics and glutamate excitotoxicity

Bioenergetic defects and abnormalities in glutamate neurotransmission have both been proposed to play important roles in neurological diseases of varying chronology, etiology and pathology. Recent experimental evidence suggests an intimate relationship between these two systems. Metabolic inhibition predisposes neurons to glutamate-mediated "excitotoxic" damage. The exact mechanism of this increased susceptibility is yet to be defined, but may involve, singly or in combination, decreased voltage-dependent Mg2+ blockade of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor, abnormalities in cellular Ca2+ homeostasis, or elevated production of reactive oxygen species. It is believed that enhancement of excitotoxicity by impaired metabolism may be a ubiquitous mechanism of neuronal death in neurological disease. Further elucidation of the exact mechanism of this enhancement may lead to the discovery of new targets for therapeutic intervention.

Manipulation of membrane potential modulates malonate-induced striatal excitotoxicity in vivo

Malonate is a reversible inhibitor of succinate dehydrogenase (SDH) that produces neurotoxicity by an N-methyl-D-aspartate (NMDA) receptor-dependent mechanism. We have examined the influence of pharmacological manipulation of membrane potential on striatal malonate toxicity in rats in vivo by analysis of lesion volume. Depolarization caused by coinjection of the Na+,K(+)-ATPase inhibitor ouabain or a high concentration of potassium greatly exacerbated malonate toxicity; this combined toxicity was blocked by the noncompetitive NMDA antagonist MK-801. The toxicity of NMDA was also exacerbated by ouabain. The overt toxicity of a high dose of ouabain (1 nmol) was largely prevented by MK-801. Coinjection of the K+ channel activator minoxidil (4 nmol) to reduce depolarization attenuated the toxicity of 1 mumol of malonate by approximately 60% without affecting malonate-induced ATP depletion. These results indicate that membrane depolarization exacerbates malonate neurotoxicity and that membrane hyperpolarization protects against malonate-induced neuronal damage. We hypothesize that the effects of membrane potential on malonate toxicity are mediated through the NMDA receptor as a result of its combined agonist- and voltage-dependent properties.

Glutamate and Parkinson's disease

Altered glutamatergic neurotransmission and neuronal metabolic dysfunction appear to be central to the pathophysiology of Parkinson's disease (PD). The substantia nigra pars compacta--the area where the primary pathological lesion is located--is particularly exposed to oxidative stress and toxic and metabolic insults. A reduced capacity to cope with metabolic demands, possibly related to impaired mitochondrial function, may render nigral highly vulnerable to the effects of glutamate, which acts as a neurotoxin in the presence of impaired cellular energy metabolism. In this way, glutamate may participate in the pathogenesis of PD. Degeneration of dopamine nigral neurons is followed by striatal dopaminergic denervation, which causes a cascade of functional modifications in the activity of basal ganglia nuclei. As an excitatory neurotransmitter, glutamate plays a pivotal role in normal basal ganglia circuitry. With nigrostriatal dopaminergic depletion, the glutamatergic projections from subthalamic nucleus to the basal ganglia output nuclei become overactive and there are regulatory changes in glutamate receptors in these regions. There is also evidence of increased glutamatergic activity in the striatum. In animal models, blockade of glutamate receptors ameliorates the motor manifestations of PD. Therefore, it appears that abnormal patterns of glutamatergic neurotransmission are important in the symptoms of PD. The involvement of the glutamatergic system in the pathogenesis and symptomatology of PD provides potential new targets for therapeutic intervention in this neurodegenerative disorder.

ARL-15896, a novel N-methyl-D-aspartate receptor ion channel antagonist: neuroprotection against mitochondrial metabolic toxicity and regional pharmacology

ARL-15896 [(+)-alpha-phenyl-2-pyridineethanamine], formerly known as FPL-15896, is a novel N-methyl-D-aspartate (NMDA) receptor ion channel antagonist. Using quantitative receptor autoradiography, we examined the regional binding characteristics of ARL-15896 and compared them to those of MK-801, the prototypical NMDA receptor channel blocker. The affinity of ARL-15896 was much lower (3000-fold) than that of MK-801 in all brain regions examined. In addition, in contrast to MK-801, which has a higher affinity in the forebrain than in the cerebellum (IC50 of 10 nM vs 24 nM), ARL-15896 had a higher affinity in the cerebellum than in the forebrain (IC50 of 17 microM vs 45 microM). The neuroprotective potential of ARL-15896 was investigated in a rat model of excitotoxicity, the intrastriatal injection of malonate. Malonate is a competitive inhibitor of succinate dehydrogenase, and its toxicity has been shown to be mediated largely by the NMDA receptor. Administration of ARL-15896 either intrastriatally (200 nmol) or subcutaneously (9.0 mg/kg) reduced the volume of the lesion produced by 1 mumol of malonate by 80%, a degree similar to that reported for MK-801. ARL-15896 was also protective when administered after the malonate injection. Furthermore, in contrast to MK-801, administration of ARL-15896 was not associated with any apparent behavioral side effects. This report is consistent with previous studies suggesting that drugs with regional pharmacological profiles similar to that of ARL-15896 have better clinical tolerability; it also indicates that ARL-15896 is an effective neuroprotective agent.

Assay of [3H]dihydrorotenone binding to complex I in intact human platelets

We have developed an assay for the binding of [3H]-dihydrorotenone ([3H]DHR), an analogue of the pesticide rotenone, to the mitochondrial enzyme, complex I, in intact human platelets. The highly hydrophobic nature of dihydrorotenone, which diffuses easily through biological membranes, rendered the isolation of mitochondrial fractions unnecessary. This allowed us to reduce the amount of blood required and to shorten the processing of samples considerably. [3H]-DHR binding was saturable, specific, and highly reproducible. We also found that MPP+ (1-methyl-4-phenyl-pyridinium species), which is accumulated actively by platelets, inhibited [3H]DHR specific binding in a concentration-dependent manner. This method could provide a simple tool for the study of complex I in those disorders, such as Parkinson's disease (PD), in which a defect of this enzyme has been suggested.

Chronic treatment with thioctic acid does not protect against malonate toxicity in vivo

We reported previously that short-term systemic pretreatment with thioctic acid is protective against NMDA and malonate lesions of the striatum. In the current study, we examined the effects of 3 weeks of pretreatment with various doses of thioctic acid (5-60 mg/kg per day) on malonate-induced striatal degeneration in adult rats. In this paradigm, we were unable to demonstrate protection against malonate toxicity. The reasons for the difference in efficacy between subacute and chronic treatment remain to be defined.

Intrastriatal injections of the succinate dehydrogenase inhibitor, malonate, cause a rise in extracellular amino acids that is blocked by MK-801

The effects of intrastriatal injections of a reversible inhibitor of succinate dehydrogenase, malonate, on the extracellular concentrations of amino acid neurotransmitters were examined using a microdialysis probe that was positioned a fixed distance from an injection cannula. Malonate (2 mumol) caused a 23 +/- 5-fold increase in extracellular glutamate (Glu), a 18 +/- 6-fold increase extracellular gamma-aminobutyric acid (GABA) and a modest increase in extracellular aspartate (Asp, 2.9 +/- 0.8-fold increase). Administration of the NMDA receptor antagonist MK-801 (5 mg/kg) prior to injection of malonate almost completely blocked these increases. This study provides direct evidence that inhibition of succinate dehydrogenase causes an increase in extracellular amino acid neurotransmitters and further evidence that bioenergetic defects may contribute to the pathogenesis of chronic neurodegenerative diseases through an excitotoxic mechanism.