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

Immune Response Modifications in the Genetic Forms of Parkinson's Disease: What Do We Know?

Parkinson's disease (PD) is a common neurodegenerative disease characterized by loss of dopaminergic neurons in the pars compacta of the midbrain substantia nigra. PD pathophysiology is complex, multifactorial, and not fully understood yet. Nonetheless, recent data show that immune system hyperactivation with concomitant production of pro-inflammatory cytokines, both in the central nervous system (CNS) and the periphery, is a signature of idiopathic PD. About 5% of PD patients present an early onset with a determined genetic cause, with either autosomal dominant or recessive inheritance. The involvement of immunity in the genetic forms of PD has been a matter of interest in several recent studies. In this review, we will summarize the main findings of this new and promising field of research.

DJ-1 (Park7) affects the gut microbiome, metabolites and the development of innate lymphoid cells (ILCs)

The proper communication between gut and brain is pivotal for the maintenance of health and, dysregulation of the gut-brain axis can lead to several clinical disorders. In Parkinson’s disease (PD) 85% of all patients experienced constipation many years before showing any signs of motor phenotypes. For differential diagnosis and preventive treatment, there is an urgent need for the identification of biomarkers indicating early disease stages long before the disease phenotype manifests. DJ-1 is a chaperone protein involved in the protection against PD and genetic mutations in this protein have been shown to cause familial PD. However, how the deficiency of DJ-1 influences the risk of PD remains incompletely understood. In the present study, we provide evidence that DJ-1 is implicated in shaping the gut microbiome including; their metabolite production, inflammation and innate immune cells (ILCs) development. We revealed that deficiency of DJ-1 leads to a significant increase in two specific genera/species, namely Alistipes and Rikenella. In DJ-1 knock-out (DJ-1^-/-) mice the production of fecal calprotectin and MCP-1 inflammatory proteins were elevated. Fecal and serum metabolic profile showed that malonate which influences the immune system was significantly more abundant in DJ-1^−/− mice. DJ-1 appeared also to be involved in ILCs development. Further, inflammatory genes related to PD were augmented in the midbrain of DJ-1^−/− mice. Our data suggest that metabolites and inflammation produced in the gut could be used as biomarkers for PD detection. Perhaps, these metabolites and inflammatory mediators could be involved in triggering inflammation resulting in PD pathology.

Role of the malonyl-CoA synthetase ACSF3 in mitochondrial metabolism

Malonyl-CoA is a central metabolite in fatty acid biochemistry. It is the rate-determining intermediate in fatty acid synthesis but is also an allosteric inhibitor of the rate-setting step in mitochondrial long-chain fatty acid oxidation. While these canonical cytoplasmic roles of malonyl-CoA have been well described, malonyl-CoA can also be generated within the mitochondrial matrix by an alternative pathway: the ATP-dependent ligation of malonate to Coenzyme A by the malonyl-CoA synthetase ACSF3. Malonate, a competitive inhibitor of succinate dehydrogenase of the TCA cycle, is a potent inhibitor of mitochondrial respiration. A major role for ACSF3 is to provide a metabolic pathway for the clearance of malonate by the generation of malonyl-CoA, which can then be decarboxylated to acetyl-CoA by malonyl-CoA decarboxylase. Additionally, ACSF3-derived malonyl-CoA can be used to malonylate lysine residues on proteins within the matrix of mitochondria, possibly adding another regulatory layer to post-translational control of mitochondrial metabolism. The discovery of ACSF3-mediated generation of malonyl-CoA defines a new mitochondrial metabolic pathway and raises new questions about how the metabolic fates of this multifunctional metabolite intersect with mitochondrial metabolism.

The Mammalian Malonyl-CoA Synthetase ACSF3 Is Required for Mitochondrial Protein Malonylation and Metabolic Efficiency

Malonyl-coenzyme A (malonyl-CoA) is a central metabolite in mammalian fatty acid biochemistry generated and utilized in the cytoplasm; however, little is known about noncanonical organelle-specific malonyl-CoA metabolism. Intramitochondrial malonyl-CoA is generated by a malonyl-CoA synthetase, ACSF3, which produces malonyl-CoA from malonate, an endogenous competitive inhibitor of succinate dehydrogenase. To determine the metabolic requirement for mitochondrial malonyl-CoA, ACSF3 knockout (KO) cells were generated by CRISPR/Cas-mediated genome editing. ACSF3 KO cells exhibited elevated malonate and impaired mitochondrial metabolism. Unbiased and targeted metabolomics analysis of KO and control cells in the presence or absence of exogenous malonate revealed metabolic changes dependent on either malonate or malonyl-CoA. While ACSF3 was required for the metabolism and therefore detoxification of malonate, ACSF3-derived malonyl-CoA was specifically required for lysine malonylation of mitochondrial proteins. Together, these data describe an essential role for ACSF3 in dictating the metabolic fate of mitochondrial malonate and malonyl-CoA in mammalian metabolism.

Abnormal chloride homeostasis in the substancia nigra pars reticulata contributes to locomotor deficiency in a model of acute liver injury

BACKGROUND

Altered chloride homeostasis has been thought to be a risk factor for several brain disorders, while less attention has been paid to its role in liver disease. We aimed to analyze the involvement and possible mechanisms of altered chloride homeostasis of GABAergic neurons within the substantia nigra pars reticulata (SNr) in the motor deficit observed in a model of encephalopathy caused by acute liver failure, by using glutamic acid decarboxylase 67 - green fluorescent protein knock-in transgenic mice.

METHODS

Alterations in intracellular chloride concentration in GABAergic neurons within the SNr and changes in the expression of two dominant chloride homeostasis-regulating genes, KCC2 and NKCC1, were evaluated in mice with hypolocomotion due to hepatic encephalopathy (HE). The effects of pharmacological blockade and/or activation of KCC2 and NKCC1 functions with their specific inhibitors and/or activators on the motor activity were assessed.

RESULTS

In our mouse model of acute liver injury, chloride imaging indicated an increase in local intracellular chloride concentration in SNr GABAergic neurons. In addition, the mRNA and protein levels of KCC2 were reduced, particularly on neuronal cell membranes; in contrast, NKCC1 expression remained unaffected. Furthermore, blockage of KCC2 reduced motor activity in the normal mice and led to a further deteriorated hypolocomotion in HE mice. Blockade of NKCC1 was not able to normalize motor activity in mice with liver failure.

CONCLUSION

Our data suggest that altered chloride homeostasis is likely involved in the pathophysiology of hypolocomotion following HE. Drugs aimed at restoring normal chloride homeostasis would be a potential treatment for hepatic failure.

The involvement of oxidants and NF-κB in cytokine-induced MMP-9 synthesis by bone marrow-derived osteoprogenitor cells

OBJECTIVE AND DESIGN

The activity of immune cells affects the balance between bone mineralization and resorption carried out by the opposing actions of osteoblasts and osteoclasts, respectively. This study was aimed at determining the possible interaction between inflammatory conditions and collagen type I degrading MMP (mainly MMP-2 and MMP-9) synthesis and secretion in rat osteoprogenitors.

MATERIALS AND METHODS

The study was performed using primary rat bone marrow-derived osteoprogenitors during their advanced osteogenesis. Biochemical, immunohistochemical, and molecular biology techniques were used to investigate the influence of pro-inflammatory cytokines on MMP-2 and MMP-9 synthesis and secretion in osteoprogenitors.

RESULTS

Results indicated that both synthesis and secretion of MMPs (MMP-1, -2, -8, -9, and -13) were significantly induced after pro-inflammatory cytokine treatments, except MMP-2, whose levels remained unchanged. NF-κB (nuclear factor kappa-light chain enhancer of activated B cells) inhibition assays showed that induced MMP-9 secretion by inflammatory cytokines was mediated by activation of NF-κB via the classical pathway and that oxidants play a significant role in this signal transduction pathway. In contrast, no such effect was observed for synthesis of MMP-2.

CONCLUSIONS

These results indicate the possibility that inflammatory processes may trigger osteoblasts to absorb bone by secreting elevated levels of MMPs capable of degrading collagen type I, especially MMP-9 which is upregulated due to increased NF-κB transcription activity.

Characterization of reduced and oxidized dopamine and 3,4-dihydrophenylacetic acid, on brain mitochondrial electron transport chain activities

Loss of dopamine (DA) homeostasis may be a contributing factor to cell damage in Parkinson's disease (PD). Past studies showing deleterious effects of DA on mitochondrial function, however, have been inconsistent raising questions about mitochondria as a downstream target for DA. Issues such as the dopamine species i.e., reduced or oxidized, time of exposure and the effect of major metabolites such as 3,4-dihydrophenylacetic acid (DOPAC) may contribute to the disparate findings. The present study used isolated, lysed rat brain mitochondria to characterize the effects of oxidized or reduced DA and DOPAC on complex activities of the electron transport chain (ETC). Time of exposure and quantitation of reduced or oxidized catachols for DA and DOPAC were monitored for all experiments. Reduced DA and DOPAC with or without a 30min preincubation had no affect on NADH oxidase activity which monitors the activities of complexes I, III and IV. Complex II activity was inhibited by reduced DA (≥500μM), but not by reduced DOPAC and was significantly attenuated by SOD suggesting reactive oxygen species involvement. In contrast, fully oxidized DA and DOPAC dose dependently inhibited NADH oxidase, complex I and complex III activities with IC(50s) in the 50-200μM range. No preincubation was required for inhibition with the catechols when they were fully oxidized. Oxidized DA inhibited complex I only when exposure occurred during stimulated electron flow, suggesting covalent binding of quinones to proteins within active sites of the complex. In intact, well coupled mitochondria, extramitochondrial DA was shown to access the mitochondrial matrix in a dose, time and energy-dependent fashion. The findings suggest that many of the reported inconsistencies with regards to the effects of DA and DOPAC on ETC function can be attributed to the oxidized state of the catechol at the time of exposure. In addition, the findings provide possible downstream targets for DA that could contribute to the vulnerability of dopaminergic neurons in PD.

Both stereoselective (R)- and (S)-1-Methyl-1,2,3,4-tetrahydroisoquinoline enantiomers protect striatal terminals against rotenone-induced suppression of dopamine release

1-Methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ) is present in the human and rodent brain as a mixture of stereospecific (R)- and (S)-1MeTIQ enantiomers. The racemate, (R,S)-1MeTIQ, exhibits neuroprotective activity as shown in the earlier study by the authors, and In addition, it was suggested to play a crucial physiological role in the mammalian brain as an endogenous regulator of dopaminergic activity. In this article, we investigated the influence of stereospecific enantiomers of 1MeTIQ, (R)- and (S)-1MeTIQ (50 mg/kg i.p.) on rotenone-induced (3 mg/kg s.c.) behavioral and neurochemical changes in the rat. In behavioral study, in order to record dynamic motor function of rats, we measured locomotor activity using automated locomotor activity boxes. In biochemical studies, we analyzed in rat striatum the concentration of dopamine (DA) and its metabolites: intraneuronal DOPAC, extraneuronal 3-MT, and final HVA using HPLC with electrochemical detection. Otherwise, DA release was estimated by in vivo microdialysis study. The behavioral study has demonstrated that both acute and repeated (3 times) rotenone administration unimportantly depressed a basic locomotor activity in rat. (R)- and (S)-1MeTIQ stereoisomers (50 mg/kg i.p.) produced a modest behavioral activation both in naïve and rotenone-treated rats. The data from ex vivo neurochemical experiments have shown stereospecificity of 1MeTIQ enantiomers in respect of their effects on DA catabolism. (R)-1MeTIQ significantly increased both the level of the final DA metabolite, HVA (by about 70%), and the rate of DA metabolism (by 50%). In contrast to that, (S)-1MeTIQ significantly depressed DOPAC, HVA levels (by 60 and 40%, respectively), and attenuated the rate of DA metabolism (by about 60%). On the other hand, both the enantiomers increased the concentrations of DA and its extraneuronal metabolite, 3-MT in rat striatum. In vivo microdialysis study has shown that repeated but not acute administration of rotenone produced a deep and significant functional impairment of striatal DA release. Both (R)- and (S)- stereospecific enantiomers of 1MeTIQ antagonized rotenone-induced suppression of DA release; however, the effect of (R)-1MeTIQ was more strongly expressed in microdialysis study. In conclusion, we suggest that both chiral isomers of 1MeTIQ offer neuroprotection against rotenone-induced disturbances in the function of dopaminergic neurons and (R,S)-1MeTIQ will be useful as a drug with marked neuroprotective activity in the brain.

The antiepileptic drug zonisamide inhibits MAO-B and attenuates MPTP toxicity in mice: clinical relevance

Zonisamide is an FDA-approved antiepileptic drug that blocks voltage-dependent Na(+) channels and T-type Ca(2+) channels and improves clinical outcome in Parkinson's disease (PD) patients when used as an adjunct to other PD therapies. Zonisamide also modifies dopamine (DA) activity, provides protection in ischemia models and influences antioxidant systems. Thus, we tested it for its ability to protect DA neurons in a mouse model of PD and investigated mechanisms underlying its protection. Concurrent treatment of mice with zonisamide and 1-methyl-4-phenyl-1,2,3,6-tetraydropyridine (MPTP) attenuated the reduction in striatal contents of DA, its metabolite DOPAC and tyrosine hydroxylase (TH). We also discovered that zonisamide inhibited monoamine oxidase B (MAO-B) activity in vitro with an IC(50) of 25 muM, a concentration that is well within the therapeutic range used for treating epilepsy in humans. Moreover, the irreversible binding of systemically administered selegiline to MAO-B in mouse brain was attenuated by zonisamide as measured by ex vivo assays. Zonisamide treatment alone did not produce any lasting effects on ex vivo MAO-B activity, indicating that it is a reversible inhibitor of the enzyme. Consistent with the effects of zonisamide on MAO-B, the striatal content of 1-methyl-4-phenylpyridinium (MPP(+)), which is derived from the administered MPTP via MAO-B actions, was substantially reduced in mice treated with MPTP and zonisamide. The potency and reversibility with which zonisamide blocks MAO-B may contribute to the ability of the drug to improve clinical symptoms in PD patients. The results also suggest that caution in its use may be necessary, especially when administered with other drugs, in the treatment of epilepsy or PD.

Oxidative stress in Parkinson's disease: a mechanism of pathogenic and therapeutic significance

Parkinson's disease (PD) is a common adult-onset neurodegenerative disorder. Typically PD is a sporadic neurological disorder, and over time affected patients see their disability growing and their quality of life declining. Oxidative stress has been hypothesized to be linked to both the initiation and the progression of PD. Preclinical findings from both in vitro and in vivo experimental models of PD suggest that the neurodegenerative process starts with otherwise healthy neurons being hit by some etiological factors, which sets into motion a cascade of deleterious events. In these models initial molecular alterations in degenerating dopaminergic neurons include increased formation of reactive oxygen species, presumably originating from both inside and outside the mitochondria. In the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD, time-course experiments suggest that oxidative stress is an early event that may directly kill some of the dopaminergic neurons. In this model it seems that oxidative stress may play a greater role in the demise of dopaminergic neurons indirectly by activating intracellular, cell death-related, molecular pathways. As the neurodegenerative process evolves in the MPTP mouse model, indices of neuroinflammation develop, such as microglial activation. The latter increases the level of oxidative stress to which the neighboring compromised neurons are subjected to, thereby promoting their demise. However, these experimental studies have also shown that oxidative stress is not the sole deleterious factor implicated in the death of dopaminergic neurons. Should a similar multifactorial cascade underlie dopaminergic neuron degeneration in PD, then the optimal therapy for this disease may have to rely on a cocktail of agents, each targeting a different critical component of this hypothesized pathogenic cascade. If correct, this may be a reason why neuroprotective trials using a single agent, such as an antioxidant, have thus far generated disappointing results.

Na(+)/H(+) exchanger inhibition modifies dopamine neurotransmission during normal and metabolic stress conditions

Na(+)/H(+) exchanger (NHE) proteins are involved in intracellular pH and volume regulation and may indirectly influence neurotransmission. The abundant NHE isoform 1 (NHE1) has also been linked to brain cell damage during metabolic stress. It is not known, however, whether NHE1 or other NHE isoforms play a role in striatal dopamine (DA) neurotransmission under normal or metabolic stress conditions. Our study tested the hypothesis that NHE inhibition with cariporide mesilate (HOE-642) modifies striatal DA overflow and DAergic terminal damage in mice caused by the mitochondrial inhibitor malonate. We also explored the expression of NHE1-5 in the striatum and substantia nigra. Reverse microdialysis of HOE-642 elicited a transient elevation followed by a reduction in DA overflow accompanied by a decline in striatal DA content. HOE-642 pre-treatment diminished the malonate-induced DA overflow without reducing the intensity of the metabolic stress or subsequent DAergic axonal damage. Although NHE isoforms 1-5 are expressed in the striatum and midbrain, NHE1 protein was not co-located on nigrostriatal DAergic neurons. The absence of NHE1 co-location on DAergic neurons suggests that the effects of HOE-642 on striatal DA overflow are either mediated via NHE1 located on other cell types or that HOE-642 is acting through multiple NHE isoforms.

Neuroprotective effects of synaptic modulation in Huntington's disease R6/2 mice

Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disorder in which the neostriatum degenerates early and most severely, with involvement of other brain regions. There is significant evidence that excitotoxicity may play a role in striatal degeneration through altered afferent corticostriatal and nigrostriatal projections that may modulate synaptically released striatal glutamate. Glutamate is a central tenant in provoking excitotoxic cell death in striatal neurons already weakened by the collective molecular events occurring in HD. In addition, transcriptional suppression of trophic factors occurs in human and transgenic mouse models of HD, suggesting that a loss of trophic support might contribute to degeneration. Since anti-glutamate approaches have been effective in improving disease phenotype in HD mice, we examined whether deafferentation of the corticostriatal and nigrostriatal pathways may mitigate striatal stress and neurodegeneration. Both surgical and chemical lesions of the corticostriatal and nigrostriatal pathways, respectively, improved the behavioral, neuropathological, and biochemical phenotype in R6/2 transgenic mice and extended survival. Decortication ameliorated hindlimb clasping, striatal neuron atrophy, and huntingtin-positive aggregates, improved N-acetyl aspartate/creatine levels, reduced oxidative stress, and significantly lowered striatal glutamate levels. In addition, 6-hydroxydopamine lesioned mice showed extended survival along with a significant reduction in striatal glutamate. These results suggest that synaptic stress is likely to contribute to neurodegeneration in HD, whereas transsynaptic trophic influences may not be as salient. Thus, modulation of synaptic influences continues to have therapeutic potential in HD.

The newly synthesized pool of dopamine determines the severity of methamphetamine-induced neurotoxicity

The neurotransmitter dopamine (DA) has long been implicated as a participant in the neurotoxicity caused by methamphetamine (METH), yet, its mechanism of action in this regard is not fully understood. Treatment of mice with the tyrosine hydroxylase (TH) inhibitor alpha-methyl-p-tyrosine (AMPT) lowers striatal cytoplasmic DA content by 55% and completely protects against METH-induced damage to DA nerve terminals. Reserpine, by disrupting vesicle amine storage, depletes striatal DA by more than 95% and accentuates METH-induced neurotoxicity. l-DOPA reverses the protective effect of AMPT against METH and enhances neurotoxicity in animals with intact TH. Inhibition of MAO-A by clorgyline increases pre-synaptic DA content and enhances METH striatal neurotoxicity. In all conditions of altered pre-synaptic DA homeostasis, increases or decreases in METH neurotoxicity paralleled changes in striatal microglial activation. Mice treated with AMPT, l-DOPA, or clorgyline + METH developed hyperthermia to the same extent as animals treated with METH alone, whereas mice treated with reserpine + METH were hypothermic, suggesting that the effects of alterations in cytoplasmic DA on METH neurotoxicity were not strictly mediated by changes in core body temperature. Taken together, the present data reinforce the notion that METH-induced release of DA from the newly synthesized pool of transmitter into the extracellular space plays an essential role in drug-induced striatal neurotoxicity and microglial activation. Subtle alterations in intracellular DA content can lead to significant enhancement of METH neurotoxicity. Our results also suggest that reactants derived from METH-induced oxidation of released DA may serve as neuronal signals that lead to microglial activation early in the neurotoxic process associated with METH.

Mechanisms involved in the formation of dopamine-induced intracellular bodies within striatal neurons

Recent studies demonstrated that methamphetamine (METH) produces intracellular bodies which are reminiscent of those occurring during degenerative disorders. In vivo studies demonstrate the occurrence of these morphological alterations both in the dopamine (DA) neurons of the substantia nigra and striatal cells. These consist of neuronal bodies staining for a variety of antigens belonging to the ubiquitin-proteasome pathway. The formation of these intracellular bodies both in the substantia nigra and PC12 cells depends on the presence of endogenous DA. In the present study, we analyze the mechanisms which lead to METH-induced intracellular bodies within non-dopaminergic striatal neurons. We found that METH is no longer able to produce inclusions in vivo, in striatal cells, when striatal DA is lost. Similarly, in vitro, in primary striatal cell cultures which do not possess DA, METH administration does not produce inclusions. On the other hand, administration of DA to striatal cell cultures produces neuronal inclusions and cell death, which are both related to the inhibition of the ubiquitin-proteasome system and activation of specific-DA receptors. In line with this, we produced subcellular alterations by administering dopamine agonists.

Reactive oxygen species generation by the ethylene-bis-dithiocarbamate (EBDC) fungicide mancozeb and its contribution to neuronal toxicity in mesencephalic cells

Previous in vitro studies in our laboratory have shown that mancozeb (MZ) and maneb (MB), both widely used EBDC fungicides, are equipotent neurotoxicants that produce cell loss in mesencephalic dopaminergic and GABAergic cells after an acute 24h exposure. Mitochondrial uncoupling and inhibition were associated with fungicide exposure. Inhibition of mitochondrial respiration is known to increase free radical production. Here the mechanism(s) of neuronal damage associated with MZ exposure was further explored by determining the role that reactive oxygen species (ROS) played in toxicity. Damage to mesencephalic dopamine and GABA cell populations were significantly attenuated when carried out in the presence of ascorbate or SOD, indicative of a free radical-mediated contribution to toxicity. ROS generation monitored by hydrogen peroxide (H(2)O(2)) production using Amplex Red increased in a dose-dependent manner in response to MZ. Inhibition of intracellular catalase with aminotriazole had little effect on H(2)O(2) generation, whereas exogenously added catalase significantly reduced H(2)O(2) production, demonstrating a large extracellular contribution to ROS generation. Conversely, cells preloaded with the ROS indicator dye DCF showed significant MZ-induced ROS production, demonstrating an increase in intracellular ROS. Both the organic backbone of MZ as well as its associated Mn ion, but not Zn ion, were responsible and required for H(2)O(2) generation. The functionally diverse NADPH oxidase inhibitors, diphenylene iodonium chloride, apocynin, and 4-(2-aminoethyl)benzene-sulfonyl fluoride hydrochloride significantly attenuated H(2)O(2) production by MZ. In growth medium lacking cells, MZ produced little H(2)O(2), but enhanced H(2)O(2) generation when added with xanthine plus xanthine oxidase whereas, in cultured cells, allopurinol partially attenuated H(2)O(2) production by MZ. Minocycline, an inhibitor of microglial activation, modestly reduced H(2)O(2) formation in mesencephalic cells. In contrast, neuronal-enriched cultures or cultures treated with MAC-1-SAP to kill microglia, did not show an attenuation of ROS production. These findings demonstrate that Mn-containing EBDC fungicides such as MZ and MB can produce robust ROS generation that likely occurs via redox cycling with extracellular and intracellular oxidases. The findings further show that microglia may contribute to but are not required for ROS production by MZ.

Convergent roles of alpha-synuclein, DA metabolism, and the ubiquitin-proteasome system in nigrostriatal toxicity

Recent studies disclosed the relevance of specific molecules for the onset of Parkinson's disease (PD) and for the composition of neuronal inclusions. The scenario which is now emerging leads to identify a potential common pathway named the ubiquitin-proteasome (UP) system. In line with this, striatal or systemic inhibiton of the UP system causes experimental Parkinsonism characterized by the formation of neuronal inclusions. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which is also a complex I inhibitor, has been used for decades to produce experimental Parkinsonism with no evidence for neuronal inclusions in rodents. This leaves open the question whether neuronal inclusions need an alternative mechanism or the inhibition of complex I needs to be carried out continuously to build up inclusions. In the present article, we administered continuously MPTP. In these experimental conditions we compared the neurological consequence of intermittent versus continuous MPTP. In both cases we observed a severe dopamine (DA) denervation and cell loss. However, when MPTP was delivered continuously, spared DA nigral neurons develop ubiquitin, parkin, and alpha-synuclein positive inclusions, which are not detectable after intermittent dosing. The onset of Parkinsonism is associated with inhibition of the UP system. We compared these results with those obtained with amphetamine derivative in vivo and in vitro in which occurrence of neuronal inclusions was associated with inhibition of the UP system and we evaluated the role of DA metabolism in inducing these effects.

Mitochondrial stress-induced dopamine efflux and neuronal damage by malonate involves the dopamine transporter

Endogenous striatal dopamine (DA) overflow has been associated with neuropathological conditions resulting from ischemia, psychostimulants, and metabolic inhibition. Malonate, a reversible inhibitor of succinate dehydrogenase, models the effects of energy impairment in neurodegenerative disorders. We have previously reported that the striatal DA efflux and damage to DA nerve terminals resulting from intrastriatal malonate infusions is prevented by prior DA depletion, suggesting that DA plays a role in the neuronal damage. We presently report that the malonate-induced DA efflux is partially mediated by reverse transport of DA from the cytosol to the extracellular space via the DA transporter (DAT). Pharmacological blockade of the DAT with a series of structurally different inhibitors [cocaine, mazindol, 1-(2-(bis(4-fluophenyl methoxy) ethyl)-4-(3-(4-fluorophenyl)-propyl)piperazine) dimethane sulfonate (GBR 13098) and methyl(-)-3beta-(p-fluorophenyl)-1alphaH,5alphaH-tropane-2beta-carboxylate1,5-naphthalene (Win 35,428)] attenuated malonate-induced DA overflow in vivo and protected mice against subsequent damage to DA nerve terminals. Consistent with these findings, the DAT inhibitors prevented malonate-induced damage to DA neurons in mesencephalic cultures and also protected against the loss of GABA neurons in this system. The DAT inhibitors did not modify malonate-induced formation of reactive oxygen species or lactate production, indicating that the DAT inhibitors neither exert antioxidant effects nor interfere with the actions of malonate. Taken together, these findings provide direct evidence that mitochondrial impairment and metabolic stress cause striatal DA efflux via the DAT and suggest that disruptions in DA homeostasis resulting from energy impairment may contribute to the pathogenesis of neurodegenerative diseases.

The neurotoxicity of amphetamines: Bridging drugs of abuse and neurodegenerative disorders

Amphetamine derivatives are the most commonly abused drugs. These compounds have been known for many years to induce neurotoxicity. However, recent findings have highlighted novel alterations produced by amphetamines in the central nervous system consisting of neuronal inclusions and the involvement of proteins belonging to a multi-enzymatic complex known as the ubiquitin-proteasome system. These ultrastructural and molecular changes are similar to those that occur during degenerative processes that affect the basal ganglia, and in particular Parkinson's disease, which is characterized by ubiquitin-containing neuronal inclusions in the subtantia nigra. This is recently confirmed by the occurrence of ubiquitin immunoreactive structures in the substantia nigra of humans abusing methamphetamines. In this article, we propose that the neurotoxicity of amphetamines and degenerative disorders share a number of steps in their mechanism of action involving the ubiquitin-proteasome system. The fine tuning of this ubiquitous proteolytic pathway is now being elucidated because G-protein-coupled receptors and signaling proteins such as beta-arrestin regulate access to this catalytic machinery. The identification of the ubiquitin-proteasome pathway and beta-arrestin as molecular targets of neurotoxicity is expected to provide novel therapeutic strategies both for the treatment of drug addiction and the treatment of neurodegenerative disorders.

Pharmacokinetic and pharmacodynamic profiles of the novel serotonin and norepinephrine reuptake inhibitor desvenlafaxine succinate in ovariectomized Sprague-Dawley rats

Desvenlafaxine succinate (DVS) is a novel serotonin (5-HT) and norepinephrine (NE) reuptake inhibitor (SNRI) that is currently in clinical development for the treatment of major depressive disorder and vasomotor symptoms associated with menopause. Previous studies have documented the pharmacokinetic and pharmacodynamic profiles of DVS in male rats. Similar studies, however, have not been performed in ovariectomized (OVX) rats, a model that mimics the loss of ovarian hormones that occurs at menopause. The goal of the present study, therefore, was to characterize the pharmacokinetic and pharmacodynamic properties of DVS in OVX rats. Desvenlafaxine levels peaked in plasma, brain (total brain minus hypothalamus) and hypothalamus at concentrations of 7.0, 10.8 and 9.5 microM (assuming 1 g = 1 ml), respectively, 30 min post-dosing DVS (30 mg/kg, oral). The apparent terminal half-lives of desvenlafaxine in plasma, brain and hypothalamus were 3.0, 2.1 and 2.5 h, respectively. Based on AUC(0-last), brain to plasma and hypothalamus to plasma ratios were 1.7 and 1.3, respectively. Microdialysis experiments in the medial preoptic area of the hypothalamus showed that DVS (30 mg/kg, s.c.), in the presence of WAY-100635 (5-HT(1A) antagonist), increased 5-HT levels 225% at 1 h post-dosing. Norepinephrine levels increased 44% at 3 h post-dosing while dopamine levels were unchanged. Thus, in OVX rats, DVS has good pharmacokinetic properties, rapid brain penetration, excellent brain penetrability and selectively increases 5-HT and NE levels in the hypothalamus. This work supports the notion that DVS could have utility for treating disorders in menopausal women in which changes in 5-HT and/or NE have been implicated.

Endogenous semicarbazide-sensitive amine oxidase (SSAO) inhibitor increases 1-methyl-4-phenylpyridinium ion (MPP+)-induced dopamine efflux by immobilization stress in rat striatum

The present study examined whether or not immobilization stress (IMMO)-inducible semicarbazide-sensitive amine oxidase (SSAO) inhibitor by separated gel filtration from 105,000 g supernate in rat brain cytosol contribute to the dopamine (DA) efflux by 1-methyl-4-phenylpyridinium ion (MPP(+)) in the rat striatum. The isoelectric point (pI) value of this inhibitor was determined by isoelectric focusing (IEF)-gel electrophoresis to about 3.8. The application of IMMO-induced SSAO inhibitor (3 microg) by IEF-gel electrophoresis increased DA efflux by MPP(+) in rat striatum. These results suggest that IMMO-inducible endogenous SSAO inhibitor enhances DA efflux by MPP(+).

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

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

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.

Effectiveness of creatine monohydrate on seizures and oxidative damage induced by methylmalonate

Methylmalonic acidemias are metabolic disorders caused by a severe deficiency of methylmalonyl CoA mutase activity, which are characterized by neurological dysfunction, including convulsions. It has been reported that methylmalonic acid (MMA) accumulation inhibits succinate dehydrogenase (SDH) and beta-hydroxybutyrate dehydrogenase activity and respiratory chain complexes in vitro, leading to decreased CO2 production, O2 consumption and increased lactate production. Acute intrastriatal administration of MMA also induces convulsions and reactive species production. Though creatine has been reported to decrease MMA-induced convulsions and lactate production, it is not known whether it also protects against MMA-induced oxidative damage. In the present study we investigated the effects of creatine (1.2-12 mg/kg, i.p.) and MK-801 (3 nmol/striatum) on the convulsions, striatal content of thiobarbituric acid reactive substances (TBARS) and on protein carbonylation induced by MMA. Moreover, we investigated the effect of creatine (12 mg/kg, i.p.) on the MMA-induced striatal creatine and phosphocreatine depletion. Low doses of creatine (1.2 and 3.6 mg/kg) protected against MMA-induced oxidative damage, but did not protect against MMA-induced convulsions. A high dose of creatine (12 mg/kg, i.p.) and MK-801 (3 nmol/striatum) protected against MMA-induced seizures (evidenced by electrographic recording), protein carbonylation and TBARS production ex vivo. Furthermore, acute creatine administration increased the striatal creatine and phosphocreatine content and protected against MMA-induced creatine and phosphocreatine depletion. Our results suggest that an increase of the striatal high-energy phosphates elicited by creatine protects not only against MMA-induced convulsions, but also against MMA-induced oxidative damage. Therefore, since NMDA antagonists are limited value in the clinics, the present results indicate that creatine may be useful as an adjuvant therapy for methylmalonic acidemic patients.

Mitochondrial inhibition and oxidative stress: reciprocating players in neurodegeneration

Although the etiology for many neurodegenerative diseases is unknown, the common findings of mitochondrial defects and oxidative damage posit these events as contributing factors. The temporal conundrum of whether mitochondrial defects lead to enhanced reactive oxygen species generation, or conversely, if oxidative stress is the underlying cause of the mitochondrial defects remains enigmatic. This review focuses on evidence to show that either event can lead to the evolution of the other with subsequent neuronal cell loss. Glutathione is a major antioxidant system used by cells and mitochondria for protection and is altered in a number of neurodegenerative and neuropathological conditions. This review also addresses the multiple roles for glutathione during mitochondrial inhibition or oxidative stress. Protein aggregation and inclusions are hallmarks of a number of neurodegenerative diseases. Recent evidence that links protein aggregation to oxidative stress and mitochondrial dysfunction will also be examined. Lastly, current therapies that target mitochondrial dysfunction or oxidative stress are discussed.

1,2,3,4-Tetrahydroisoquinoline protects terminals of dopaminergic neurons in the striatum against the malonate-induced neurotoxicity

Malonate, a reversible inhibitor of the mitochondrial enzyme succinate dehydrogenase, is frequently used as a model neurotoxin to produce lesion of the nigrostriatal dopaminergic system in animals due to particular sensitivity of dopamine neurons to mild energy impairment. This model of neurotoxicity was applied in our study to explore neuroprotective potential of 1,2,3,4-tetrahydroisoquinoline (TIQ), an endo- and exogenous substance whose function in the mammalian brain, despite extensive studies, has not been elucidated so far. Injection of malonate at a dose of 3 mumol unilaterally into the rat left medial forebrain bundle resulted in the 54% decrease in dopamine (DA) concentration in the ipsilateral striatum and, depending on the examined striatum regions, caused 24-44% reduction in [3H]GBR12,935 binding to the dopamine transporter (DAT). TIQ (50 mg/kg i.p.) administered 4 h before malonate infusion and next once daily for successive 7 days prevented both these effects of malonate. Such TIQ treatment restored DA content and DAT binding almost to the control level. The results of the present study indicate that TIQ may act as a neuroprotective agent in the rat brain. An inhibition of the enzymatic activities of monoamine oxidase and gamma-glutamyl transpeptidase as well as an increase in the striatal levels of glutathione and nitric oxide found after TIQ administration and reported in our earlier studies are considered to be potential factors that may be involved in the TIQ-mediated protection of dopamine terminals from malonate toxicity.

Analysis of gene expression changes in a cellular model of Parkinson disease

We employed Serial Analysis of Gene Expression to identify transcriptional changes in a cellular model of Parkinson Disease (PD). The model consisted of neuronally differentiated PC12 cells compared before and after 8 hours' exposure to 6-hydroxydopamine. Approximately 1200 transcripts were significantly induced by 6-OHDA and approximately 500 of these are currently matched to known genes. Here, we categorize the regulated genes according to known functional activities and discuss their potential roles in neuron death and survival and in PD. We find induction of multiple death-associated genes as well as many with the capacity for neuroprotection. This suggests that survival or death of individual neurons in PD may reflect an integrated response to both protective and destructive gene changes. Our findings identify a number of regulated genes as candidates for involvement in PD and therefore as potential targets for therapeutic intervention. Such intervention may include both inhibiting the induction/activity of death-promoting genes and enhancing those with neuroprotective activity.

Energy status, ubiquitin proteasomal function, and oxidative stress during chronic and acute complex I inhibition with rotenone in mesencephalic cultures

Complex I impairment with rotenone produces damage though a mechanism thought to be distinct from effects on mitochondrial respiration. The outcome of chronic rotenone on energy status in relation to toxicity, however, is unknown. To examine this, mesencephalic cultures were exposed to chronic, low-dose rotenone (5-100 nM, 8 days in vitro) or acute, high-dose rotenone (500 nM, 1-24 h), and ATP/ADP levels and toxicity were measured. Chronic exposure to 5-50 nM rotenone produced selective dopamine cell loss. High-dose rotenone produced nonselective damage at all exposure times. Chronic, low-dose rotenone (37.5 nM) decreased ATP/ADP gradually over several days to 40% of controls, whereas high-dose rotenone (500 nM, 1-6 h), collapsed ATP/ADP by 1 h of exposure. The ubiquitin proteasomal pathway, an ATP-dependent pathway, is implicated in Parkinson's disease and, thus, various rotenone exposures were examined for effects on ubiquitin proteasomal function. Chronic, low-dose rotenone (25-50 nM, 8 days), but not acute, high-dose rotenone (500 nM, 1-6 h), caused accumulation of ubiquitinated proteins, E1-ubiquitin activation, and increased proteasomal activities prior to toxicity even though both exposures increased free radical production. Findings show that selective dopamine cell loss and alterations in ubiquitin proteasomal function only occur with rotenone exposures that partially maintain ATP/ADP. High concentrations of rotenone that collapse energy status kill neurons in a nonselective manner independent of the ubiquitin proteasomal pathway.

Neuronal inclusions in degenerative disorders

This brief paper analyzes a few degenerative diseases expressing as movement disorders and featuring at sub-cellular level the presence of neuronal inclusions in selective brain regions. We will first draw a short draft of representative neurological diseases featuring inclusion bodies by describing the type of inclusions occurring in various disorders and analyzing both common features and distinctive aspects. As a further step, we move from the bed to the bench side discussing recent developments obtained from experimental models of these disorders which shed new light into the cause and progression of neuronal inclusions, thus helping to understand the pathophysiology of neuronal degeneration underlying movement disorders. In line with this, we will focus on recent studies which led to reproduce neuronal inclusions in vivo and in vitro by manipulating selective cellular structures/enzymatic pathways. In this way, we will try to encompass the dynamics of inclusion formation based on their fine ultrastructure and the analysis of the molecular components as well as their subcellular compartmentalization trying to relate the dynamics of inclusion formation and the pathophysiology of the disease process. An emphasis will be made on the ubiquitin proteasome system and Parkinson's disease where the analysis of neuronal inclusions enlightened potential therapeutic strategies to occlude the progression of this neuronal degeneration featured by movement disorders.

Striatal adenosine A(2A) receptor blockade increases extracellular dopamine release following l-DOPA administration in intact and dopamine-denervated rats

The influence of the selective adenosine A(2A) receptor antagonist ZM 241385 on exogenous l-DOPA-derived dopamine (DA) release in intact and dopamine-denervated rats was studied using an in vivo microdialysis in freely moving animals. Local infusion of l-DOPA (2.5 microM) produced a marked increase in striatal extracellular DA level in intact and malonate-lesioned rats. Intrastriatal perfusion of ZM 241385 (50-100 microM) had no effect on basal extracellular DA level, but enhanced dose-dependently the l-DOPA-induced DA release in intact and malonate-lesioned animals. A non-selective adenosine A(2A) receptor antagonist DMPX (100 microM), similarly to ZM 241385, accelerated conversion of l-DOPA in intact and malonate-denervated rats. This effect was not produced by the adenosine A(1) receptor antagonist, CPX (10-50 microM). However, ZM 241385 did not affect the l-DOPA-induced DA release in rats pretreated with reserpine (5 mg/kg i.p.) and alpha-methyl-p-tyrosine (AMPT, 300 mg/kg i.p.). Obtained results indicate that blockade of striatal adenosine A(2A) receptors increases the l-DOPA-derived DA release possibly by indirect mechanism exerted on DA terminals, an effect dependent on striatal tyrosine hydroxylase activity. Selective antagonists of adenosine A(2A) receptors may exert a beneficial effect at early stages of Parkinson's disease by enhancing the therapeutic efficacy of l-DOPA applied exogenously.

Valproate is neuroprotective against malonate toxicity in rat striatum: an association with augmentation of high-affinity glutamate uptake

The antiepileptic drug valproate (VPA) may be neuroprotective. We treated rats with VPA for 14 days (300 mg/kg twice daily) before intrastriatal injection of 1.5 micromol (1 M) of the succinate dehydrogenase inhibitor malonate. VPA-treated animals developed smaller lesions than control animals: 10 +/- 2 mm(3) versus 26 +/- 8 mm(3) (means +/- SD; P = 10(-4). Injection of NaCl that was equiosmolar with 1 M malonate caused lesions of only 1.2 +/- 0.4 mm(3) in control animals, whereas physiologic saline produced no lesion. VPA pretreatment reduced the malonate-induced extracellular accumulation of glutamate. This effect paralleled an increase in the striatal level of the glutamate transporter GLT, which augmented high-affinity glutamate uptake by 25%, as determined from the uptake of [(3)H] glutamate into striatal proteoliposomes. Malonate caused a 76% reduction in striatal adenosine triphosphate (ATP) content, but the glial, ATP-dependent formation of glutamine from radiolabeled glucose or glutamate was intact, indicating that glial ATP production supported uptake of glutamate. Striatal levels of HSP-70 and fos were reduced, and the levels of bcl-2 and phosphorylated extracellular signal-regulated kinase remained unaffected, but histone acetylation was increased by VPA treatment. The results suggest that augmentation of glutamate uptake may contribute importantly to VPA-mediated neuroprotection in striatum.

Striatal damage and oxidative stress induced by the mitochondrial toxin malonate are reduced in clorgyline-treated rats and MAO-A deficient mice

Intrastriatal administration of the succinate dehydrogenase (SDH) inhibitor malonate produces neuronal injury by a "secondary excitotoxic" mechanism involving the generation of reactive oxygen species (ROS). Recent evidence indicates dopamine may contribute to malonate-induced striatal neurodegeneration; infusion of malonate causes a pronounced increase in extracellular dopamine and dopamine deafferentation attenuates malonate toxicity. Inhibition of the catabolic enzyme monoamine oxidase (MAO) also attenuates striatal lesions induced by malonate. In addition to forming 3,4-dihydroxyphenylacetic acid, metabolism of dopamine by MAO generates H2O2, suggesting that dopamine metabolism may be a source of ROS in malonate toxicity. There are two isoforms of MAO, MAO-A and MAO-B. In this study, we have investigated the role of each isozyme in malonate-induced striatal injury using both pharmacological and genetic approaches. In rats treated with either of the specific MAO-A or -B inhibitors, clorgyline or deprenyl, respectively, malonate lesion volumes were reduced by 30% compared to controls. In knock-out mice lacking the MAO-A isoform, malonate-induced lesions were reduced by 50% and protein carbonyls, an index ROS formation, were reduced by 11%, compared to wild-type animals. In contrast, mice deficient in MAO-B showed highly variable susceptibility to malonate toxicity precluding us from determining the precise role of MAO-B in this form of brain damage. These findings indicate that normal levels of MAO-A participate in expression of malonate toxicity by a mechanism involving oxidative stress.

MPTP potentiates 3-nitropropionic acid-induced striatal damage in mice: reference to striatonigral degeneration

Striatonigral degeneration (SND) is a parkinsonian disorder due to the combined degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc) and striatal output neurons. The aims of this study were to explore (1) the behavioral and histopathological consequences of combined MPTP plus 3-nitropropionic acid (3-NP) intoxication in C57/Bl6 mice and (2) its ability to reproduce the neuropathological hallmarks of SND. 3-NP was administered i.p. every 12 h (total dose=450 mg/kg in 9 days) and MPTP i.p. at 10 mg/(kg day) (total dose=90 mg/kg in 9 days). Four groups of mice (n=10) were compared: control, 3-NP alone, MPTP alone, MPTP + 3-NP. Mice intoxicated with 3-NP and MPTP + 3-NP developed motor symptoms, including hindlimb dystonia and clasping, truncal dystonia and impaired balance adjustments. The severity of motor disorder was worse and lasted longer in MPTP + 3-NP-treated mice compared to 3-NP alone, MPTP alone and controls. 3-NP and MPTP + 3-NP-treated mice also displayed altered gait patterns, impaired motor performance on the pole test, rotarod and traversing a beam tasks and activity parameters. Several of these sensorimotor deficits were also more severe and lasted longer in MPTP + 3-NP-treated mice. Histology demonstrated increased neuronal loss along with astrocytic activation (glial fibrillary acid protein, GFAP) and a higher incidence of circumscribed striatal lateral lesions in MPTP + 3-NP-treated mice compared to 3-NP. Neuronal loss and astrocytic activation were increased in the lateral part of the striatum in 3-NP-intoxicated mice while observed both in the medial and lateral part in MPTP + 3-NP-intoxicated mice. There was also a significant loss of SNc dopaminergic neurons and striatal terminals, similar to that in MPTP-treated mice. Altogether, these results suggest that MPTP potentiates striatal damage and behavioral impairments induced by 3-NP intoxication in mice and constitutes a useful model of the motor disorder and its histopathological correlates in SND.

Adenosinergic protection of dopaminergic and GABAergic neurons against mitochondrial inhibition through receptors located in the substantia nigra and striatum, respectively

Mitochondrial dysfunction may contribute to dopaminergic (DAergic) cell death in Parkinson's disease and GABAergic cell death in Huntington's disease. In the present work, we tested whether blocking A1 receptors could enhance the damage to DAergic and GABAergic neurons caused by mitochondrial inhibition, and whether blocking A2a receptors could protect against damage in this model. Animals received an intraperitoneal injection of 8-cyclopentyl-1,3-dipropylxanthine (CPX) (A1 antagonist) or 3,7-dimethyl-1-propargylxanthine (DMPX) (A2a antagonist) 30 min before intrastriatal infusion of malonate (mitochondrial complex II inhibitor). Damage was assessed 1 week later by measuring striatal dopamine, tyrosine hydroxylase (TH), and GABA content. In mice and rats, malonate-induced depletion of striatal dopamine, TH, or GABA was potentiated by pretreatment with 1 mg/kg CPX and attenuated by pretreatment with 5 mg/kg DMPX. To determine the location of the A1 and A2a receptors mediating these effects, CPX or DMPX was infused directly into the striatum or substantia nigra of rats 30 min before intrastriatal infusion of malonate. When infused into the striatum, CPX (20 ng) potentiated, whereas DMPX (50 ng) prevented malonate-induced GABA loss, but up to 100 ng of CPX or 500 ng of DMPX did not alter malonate-induced striatal dopamine loss. Intranigral infusion of CPX (100 ng) or DMPX (500 ng), however, did exacerbate and protect, respectively, against malonate-induced striatal dopamine loss. Thus, A1 receptor blockade enhances and A2a receptor blockade protects against damage to DAergic and GABAergic neurons caused by mitochondrial inhibition. Interestingly, these effects are mediated by A1 and A2a receptors located in the substantia nigra for DAergic neurons and in the striatum for GABAergic neurons.

Fine structure and biochemical mechanisms underlying nigrostriatal inclusions and cell death after proteasome inhibition

Mutation of genes encoding for various components of a metabolic pathway named the ubiquitin-proteasome system (UP) leads to inherited forms of Parkinson's disease (PD), whereas various components of the UP are constantly present within neuronal inclusions, Lewy bodies, that characterize most genetic and sporadic forms of PD. It has been hypothesized that impairment of this metabolic pathway might be a common mechanism for the onset of PD, and a recent study demonstrated a dysfunction of the UP system within the substantia nigra of patients affected by sporadic PD. In search for the mechanisms underlying the selective toxicity for nigral neurons after inhibition of the UP system, we explored the selective effects after striatal microinfusions of lactacystin or epoxomycin and potential retrograde changes within the ipsilateral substantia nigra. We found that neurotoxicity was selective for striatal dopamine (DA) components and led to retrograde apoptosis within nigral DA cells, which developed neuronal inclusions staining for antigens of the UP system. We found the same ultrastructural features characterizing inclusions obtained in vivo and in vitro after UP inhibition. In vivo, lactacystin-epoxomycin-induced toxicity was suppressed by inhibiting DA synthesis. Similarly, in vitro inclusions and apoptosis were prevented by reducing endogenous DA. On the other hand, toxicity of proteasome inhibition was enhanced by drugs augmenting DA availability: l-3,4-dihydroxyphenylalanine, monoamine oxidase blockers, and DA beta-hydroxylase blockers. These findings demonstrate that impairment of the UP system produces cell death and neuronal inclusions selectively for DA-containing neurons that depend on the occurrence of endogenous DA.

Differential responsiveness of rat striatal nerve endings to the mitochondrial toxin 3-nitropropionic acid: implications for Huntington's disease

Rat striatal synaptosomes and slices were used to investigate the responsiveness of different populations of nerve terminals to 3-nitropropionic acid (3-NP), a suicide inhibitor of the mitochondrial enzyme succinate dehydrogenase, and to elucidate the ionic mechanisms involved. 3-NP (0.3-3 mm) stimulated spontaneous gamma-aminobutyric acid (GABA), glutamate and [3H]-dopamine efflux but left unchanged acetylcholine efflux from synaptosomes. This effect was associated with a >70% inhibition of succinate dehydrogenase, as measured in the whole synaptosomal population. The facilitation was not dependent on extracellular Ca2+ but relied on voltage-dependent Na+ channel opening, because it was prevented by tetrodotoxin and riluzole. 3-NP also elevated spontaneous glutamate efflux from slices but in a tetrodotoxin-insensitive way. To investigate whether energy depletion could change the responsiveness of nerve endings to a depolarizing stimulus, synaptosomes were pretreated with 3-NP and challenged with pulses of KCl evoking 'quasi-physiological' neurotransmitter release. 3-NP potentiated the K+-evoked GABA, glutamate and [3H]-dopamine release but inhibited the K+-evoked acetylcholine release. The 3-NP induced potentiation of GABA release was Ca2+-dependent and prevented by tetrodotoxin and riluzole whereas the 3-NP-induced inhibition of acetylcholine release was tetrodotoxin- and riluzole-insensitive but reversed by glipizide, an ATP-dependent K+ channel inhibitor. We conclude that the responsiveness of striatal nerve endings to 3-NP relies on activation of different ionic conductances, and suggest that the selective survival of striatal cholinergic interneurons following chronic 3-NP treatment (as in models of Huntington's disease) may rely on the opening of ATP-dependent K+ channels, which counteracts the fall in membrane potential as a result of mitochondrial impairment.

Dopamine oxidation products inhibit Na+, K+-ATPase activity in crude synaptosomal-mitochondrial fraction from rat brain

The diverse damaging effects of dopamine (DA) oxidation products on brain subcellular components including mitochondrial electron transport chain have been implicated in dopaminergic neuronal death in Parkinson's disease. It has been shown in this study that DA (50-200 microM) causes dose-dependent inhibition of Na+, K+-ATPase activity of rat brain crude synaptosomal-mitochondrial fraction during in vitro incubation up to 2 h. The enzyme inactivation is prevented by catalase and the metal-chelator (diethylenetriamine penta-acetic acid) but not by superoxide dismutase or hydroxyl-radical scavengers like mannitol and dimethylsulphoxide (DMSO). Further, reduced glutathione and cysteine, markedly prevent DA-mediated inactivation of Na+, K+-ATPase. Under similar conditions of incubation, DA (200 microM) leads to the formation of quinoprotein adducts (protein-cysteinyl catechol) with synaptosomal-mitochondrial proteins and the phenomenon is also prevented by glutathione (5 mM) or cysteine (5 mM). The available data imply that the inactivation of Na+, K+-ATPase in this system involves both H2O2 and metal ions. The reactive quinones by forming adducts with protein thiols also probably contribute to the process, since reduced glutathione and cysteine which scavenge quinones from the system protect Na+, K+-ATPase from DA-mediated damage. The inactivation of neuronal Na+, K+-ATPase by DA may give rise to various toxic sequelae with potential implications for dopaminergic cell death in Parkinson's disease.

Evidence for dysfunction of the nigrostriatal pathway in the R6/1 line of transgenic Huntington's disease mice

The present multidisciplinary study examined nigrostriatal dopamine and striatal amino acid transmission in the R6/1 line of transgenic Huntington's disease (HD) mice expressing exon 1 of the HD gene with 115 CAG repeats. Although the number of tyrosine hydroxylase-positive neurons was not reduced and nigrostriatal connectivity remained intact in 16-week-old R6/1 mice, the size of tyrosine hydroxylase-positive neurons in the substantia nigra was reduced by 15%, and approximately 30% of these cells exhibited aggregated huntingtin. In addition, using in vivo microdialysis, we found that basal extracellular striatal dopamine levels were reduced by 70% in R6/1 mice compared to their wild-type littermates. Intrastriatal perfusion with malonate in R6/1 mice resulted in a short-lasting, attenuated increase in local dopamine release compared to wild-type mice. Furthermore, the size of the malonate-induced striatal lesion was 80% smaller in these animals. Taken together, these findings suggest that a functional deficit in nigrostriatal dopamine transmission may contribute to the behavioral phenotype and the resistance to malonate-induced neurotoxicity characteristic of R6/1 HD mice.

Protection of malonate-induced GABA but not dopamine loss by GABA transporter blockade in rat striatum

Previous work has shown that overstimulation of GABA(A) receptors can potentiate neuronal cell damage during excitotoxic or metabolic stress in vitro and that GABA(A) antagonists or GABA transport blockers are neuroprotective under these situations. Malonate, a reversible succinate dehydrogenase/mitochondrial complex II inhibitor, is frequently used in animals to model cell loss in neurodegenerative diseases such as Parkinson's and Huntington's diseases. To determine if GABA transporter blockade during mitochondrial impairment can protect neurons in vivo as compared with in vitro studies, rats received a stereotaxic infusion of malonate (2 micromol) into the left striatum to induce a metabolic stress. The nonsubstrate GABA transport blocker, NO711 (20 nmol) was infused in some rats 30 min before and 3 h following malonate infusion. After 1 week, dopamine and GABA levels in the striata were measured. Malonate caused a significant loss of striatal dopamine and GABA. Blockade of the GABA transporter significantly attenuated GABA, but not dopamine loss. In contrast with several in vitro reports, GABA(A) receptors were not a downstream mediator of protection by NO711. Intrastriatal infusion of malonate (2 micromol) plus or minus the GABA(A) receptor agonist muscimol (1 micromol), the GABA(A) Cl- binding site antagonist picrotoxin (50 nmol) or the GABA(B) receptor antagonist saclofen (33 nmol) did not modify loss of striatal dopamine or GABA when examined 1 week following infusion. These data show that GABA transporter blockade during mitochondrial impairment in the striatum provides protection to GABAergic neurons. GABA transporter blockade, which is currently a pharmacological strategy for the treatment of epilepsy, may thus also be beneficial in the treatment of acute and chronic conditions involving energy inhibition such as stroke/ischemia or Huntington's disease. These findings also point to fundamental differences between immature and adult neurons in the downstream involvement of GABA receptors during metabolic insult.

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.

Dopamine mediates striatal malonate toxicity via dopamine transporter-dependent generation of reactive oxygen species and D2 but not D1 receptor activation

Intrastriatal injection of the reversible succinate dehydrogenase inhibitor malonate results in both chemically induced hypoxia and striatal lesions that are similar to those seen in Huntington's disease and cerebral ischaemia. The mechanisms leading to neuronal death involve secondary excitotoxicity, the release of dopamine from nigrostriatal fibres and the generation of reactive oxygen species (ROS) including nitric oxide (NO) and hydroxyl radicals. Here, we further investigated the contribution and mechanism of dopamine on malonate-induced striatal lesions. Prior lesions of the nigrostriatal pathway with 6-OHDA or the depletion of striatal dopamine stores by pretreatment with reserpine, an inhibitor or the vesicular monoamine transporter type-2 (VMAT2), in combination with alpha-methyl-p-tyrosine resulted in a significant reduction of malonate-induced striatal lesion volumes. This was paralleled by block or reduction of the malonate-induced generation of ROS, as measured by the conversions of salicylate to 2,3-dihydroxybenzoic acid (2,3-DHBA) using microdialysis. Systemic or intrastriatal application of L-DOPA or dopamine, respectively, reconstituted malonate toxicity and the generation of ROS in 6-OHDA-lesioned rats. Block of the dopamine transporter by GBR12909 did not result in a reduction of malonate-induced dopamine release, but significantly reduced the generation of hydroxyl radicals. The D2 receptor agonist lisuride and the mixed D1 and D2 receptor agonist apomorphine, but not the D1 receptor agonist SKF38393, partially restored malonate toxicity in 6-OHDA-lesioned rats without increasing the generation of ROS. In line with these results sulpiride, an inhibitor of D2 receptors, reduced the malonate-induced lesion volume, whereas SCH23390, an inhbitor of D1 receptors, was ineffective. Our data suggest that malonate-induced dopamine toxicity to energetically impaired neurons is mediated by two independent pathways: (i) dopamine transporter uptake-dependent, dopamine receptor-independent generation of ROS, and (ii) excessive stimulation of D2 receptors.

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.

Processing of cdk5 activator p35 to its truncated form (p25) by calpain in acutely injured neuronal cells

Recently, it was shown that conversion of cdk5 activator protein p35 to a C-terminal fragment p25 promotes a deregulation of cdk5 activity, which may contribute to neurodegeneration in Alzheimer's disease. In this study, we present evidence that calpain is a protease involved in the conversion of p35 to p25. To activate calpain, rat cerebellar granule neurons were treated with maitotoxin (MTX). A C-terminus-directed anti-p35 antibody detected that p35 conversion to p25 paralleled the formation of calpain-generated alpha-spectrin (alpha-fodrin) breakdown products (SBDP's) in a maitotoxin-dose-dependent manner. Two calpain inhibitors (MDl28170 and SJA6017) reduced p35 processing but were unchanged when exposed to the caspase inhibitor carbobenzoxy-Asp-CH(2)OC(=O)-2, 6-dichlorobenzene or the proteasome inhibitors (lactacystin and Z-Ile-Glu(OtBu)Ala-Leu-CHO). p35 protein was also degraded to p25 when rat brain lysate was subjected to in vitro digestion with purified mu- and m-calpains. Additionally, in a rat temporary middle cerebral artery occlusion model, p35 processing to p25 again paralleled SBDP formation in the ischemic core. Lastly, in malonate-injured rat brains, the ipsilateral side showed a striking correlation of SBDP formation with p35 to p25 conversion and tau phosphorylation (at Ser202 and Thr205) increase. These data suggest that calpain is a major neuronal protease capable of converting p35 to p25 and might play a pathological role of activating cdk5 and its phosphorylation of tau in Alzheimer's disease.