The mitochondrial inhibitor malonate enhances NMDA toxicity in the neonatal rat striatum

3-nitropropionic acid-induced hydrogen peroxide, mitochondrial DNA damage, and cell death are attenuated by Bcl-2 overexpression in PC12 cells

3-nitropropionic acid (3-NPA), a complex II inhibitor of the electron transport chain, causes Huntington disease-like symptoms after administration into animals. However, primary mechanisms of cell death are not clearly understood. This study tested the hypothesis that 3-NPA leads to the generation of reactive oxygen species (ROS), mitochondrial DNA damage, and loss of mitochondrial function. Amplex red and horseradish peroxidase were used to accurately measure the amount of H2O2, and showed that PC12 cells treated with 3-NPA (4 mM) lead to the production of hydrogen peroxide (1 nmol/10(6) cells/h). This amount of 3-NPA also leads to a rapid decline of ATP levels. There was time- and dose-dependent mitochondrial DNA damage following 3-NPA treatment. Overexpression of the proto-oncogene bcl-2 protects cells from apoptosis induced by various stimuli. Overexpression of Bcl-2 leads to almost threefold higher levels of ATP and also decreased the 3-NPA-mediated induction of hydrogen peroxide by over 50%. Bcl-2-overexpressing PC12 cells were also protected from mitochondrial DNA damage. These data show that ROS production followed by mitochondrial DNA damage is the primary event in 3-NPA toxicity, and Bcl-2 protects PC12 cells from 3-NPA toxicity by preventing mitochondrial DNA damage.

Neuroprotective effects of M826, a reversible caspase-3 inhibitor, in the rat malonate model of Huntington's disease

1. Caspases, key enzymes in the apoptosis pathway, have been detected in the brain of HD patients and in animal models of the disease. In the present study, we investigated the neuroprotective properties of a new, reversible, caspase-3-specific inhibitor, M826 (3-([(2S)-2-[5-tert-butyl-3-[[(4-methyl-1,2,5-oxadiazol-3-yl)methyl]amino]-2-oxopyrazin-1(2H)-yl]butanoyl]amino)-5-[hexyl(methyl)amino]-4-oxopentanoic acid), in a rat malonate model of HD. 2. Pharmacokinetic and autoradiography studies after intrastriatal (i.str.) injection of 1.5 nmol of M826 or its tritiated analogue [(3)H]M826 indicated that the compound diffused within the entire striatum. The elimination half-life (T(1/2)) of M826 in the rat striatum was 3 h. 3. I.str. injection of 1.5 nmol of M826 10 min after malonate infusion induced a significant reduction (66%) in the number of neurones expressing active caspase-3 in the ipsilateral striatum. 4. Inhibition of active caspase-3 translated into a significant but moderate reduction (39%) of the lesion volume, and of cell death (24%), 24 h after injury. The efficacy of M826 at inhibiting cell death was comparable to that of the noncompetitive NMDA receptor antagonist MK801. 5. These data provide in vivo proof-of-concept of the neuroprotective effects of reversible caspase-3 inhibitors in a model of malonate-induced striatal injury in the adult rat.

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.

Tumor necrosis factor-a attenuates N-methyl-D-aspartate-mediated neurotoxicity in neonatal rat hippocampus

Tumor necrosis factor-a TNFa. has been implicated in the pathophysiology of acute neonatal brain injury. We hypothesized that acute brain injury would induce TNFa expression and that exogenous TNFa would influence the severity of N-methyl-D-aspartate-induced tissue damage. We performed two complementary groups of experiments to evaluate the potential role s. of TNFa in a neonatal rodent model of excitotoxic injury, elicited by intracerebral injection of N-methyl-D-aspartate. We used immunohistochemistry and ELISA to evaluate N-methyl-D-aspartate-induced changes in TNFa expression, and we co-injected TNFa with N-methyl-D-aspartate, to evaluate the effect of this cytokine on the severity of tissue injury. Both intra-hippocampal and intra-striatal injection of N-methyl-D-aspartate 5 nmol. stimulated TNFa expression. Increased TNFa expression was detected 3-12 h after lesioning; TNFa was localized both in glial cells in the corpus callosum, and in cells with the morphology of interneurons in the ipsilateral hippocampus, striatum, cortex and thalamus. Intra-hippocampal or intra-striatal administration of TNFa 50 ng. alone did not elicit neuropathologic damage. In the hippocampus, when co-injected with N-methyl-D-aspartate 5 or 10 nmol., TNFa 50 ng. attenuated excitotoxic injury by 35%-57%, compared to controls co-injected with heat-treated TNFa. In contrast, in the striatum, co-injection of TNFa with N-methyl-D-aspartate had no effect on the severity of the ensuing damage. The data indicate that TNFa is rapidly produced in glial cells and neurons after an excitotoxic insult in the neonatal rat brain, and that administration of exogenous TNFa results in region-specific attenuation of excitotoxic damage. We speculate that endogenous TNFa may modulate the tissue response to excitotoxic injury in the developing brain.

Tat, a human immunodeficiency virus-1-derived protein, augments excitotoxic hippocampal injury in neonatal rats

To test the hypothesis that the human immunodeficiency virus-1-derived Tat protein may cause neuronal damage in the CNS, we evaluated the neurotoxicity of recombinant human immunodeficiency virus-1-derived Tat in vivo in seven-day-old rats. The intrinsic neurotoxicity of Tat (250 ng-1 microg) and the effects of direct intra-hippocampal co-infusion of Tat with N-methyl-D-aspartate were assessed. Extent of injury in the lesioned hippocampus was evaluated five days later, based on histopathology and morphometric measurements of hippocampal volume. To confirm that any observed neurotoxic effects were attributable to Tat bioactivity, all experiments included controls that received equal amounts of heat-treated (boiled) Tat. Intra-hippocampal injection of Tat, alone, elicited minimal focal tissue damage immediately adjacent to the injection track, and no hippocampal atrophy. Co-injection of Tat (500 ng) with N-methyl-D-aspartate (5 nmol, threshold excitotoxic dose) doubled the severity of hippocampal injury, quantified by comparison of bilateral hippocampal volumes, in comparison with animals that received heat-treated Tat or saline co-injections; in animals that received injections of N-methyl-D-aspartate (5 nmol) in combination with saline, heat-treated Tat, or Tat [mean(+/-S.E.M.) % volume loss values in the lesioned hippocampus were: 11(+/-3), 11(+/-3), and 26(+/-3), respectively (P<0.002, ANOVA)]. Co-injection of 100 ng Tat with 5 nmol N-methyl-D-aspartate exacerbated the severity of excitotoxic injury to a similar extent, whereas co-injection of 20 ng Tat had no effect on N-methyl-D-aspartate-mediated injury. Treatment with the N-methyl-D-aspartate antagonist 3-((RS)-2-carboxypiperazin4-yl)-propyl-1-phosphonic acid (20 mg/kg) markedly attenuated hippocampal injury resulting from co-injection of 100 ng Tat with N-methyl-D-aspartate [mean(+/-S.E.M.) % volume loss in lesioned hippocampus: 0.1(+/-2) in 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid-treated vs 19(+/-3) in controls, P<0.001, ANOVA]. Co-injection of Tat had no effect on N-methyl-D-aspartate-mediated striatal damage or on alpha-amino-3-hydroxy-5-methylisoxazole-4-pro hippocampal damage. These data support the hypothesis that locally released Tat could exert neurotoxic effects, mediated by N-methyl-D-aspartate receptor activation, in vivo in the immature brain.

Metabolic inhibition increases glutamate susceptibility on a PC12 cell line

The effect of energetic metabolism compromise, obtained by chemical induction of hypoglycaemia (glucose deprivation), hypoxia (mitochondrial respiratory chain inhibition), and ischaemia (hypoglycaemia plus hypoxia), on glutamate toxicity was analyzed on PC12 cells. The respiratory status of these cells, measured by the MTT [3-(4,5-dimethylthiazol-2-yl)2,5-diphenyltetrazolium bromide] assay, was significantly decreased after metabolic inhibition induced by ischaemia, but it was not affected by both hypoglycaemia and hypoxia. Under hypoglycaemia, but not under hypoxia, ATP levels were significantly reduced (from 12.67+/-0.48 to 5.38+/-1.41 nmol/mg protein). However, ischaemic-like conditions greatly potentiated the decline of ATP levels (95% decrease) observed after hypoglycaemia. The influence of metabolic inhibition on glutamate-induced cytotoxicity was also analyzed. When the cells were preincubated under conditions that deplete ATP (hypoglycaemia and ischaemia), the inhibition of MTT reduction, measured after glutamate incubation, was potentiated. This effect could be reverted when vitamin E and idebenone were present during the induction of metabolic inhibition. The ATP levels above which glutamate susceptibility was enhanced were also determined. These results indicate that glutamate toxicity on PC12 cells, which occurs by a mechanism independent of N-methyl-D-aspartate (NMDA) receptor activation, can be enhanced by the depletion of intracellular ATP upon metabolic stress; it is dependent on the extent of ATP depletion and seems to involve the generation of free radicals. It can be concluded that under ischaemic conditions, the deleterious effects of glutamate can be potentiated by the energetic compromise associated with this pathologic situation.

Recovery of hypothalamic NMDA-induced c-fos expression following neonatal glutamate (MSG) lesions

The neonatal brain is susceptible to neurotoxic insult. In a previous report we showed that a single neonatal injection of MSG, known to cause damage in the arcuate nucleus (ARC), induces a precocious yet otherwise normal puberty in female rats. We have examined this ability of the medial basal hypothalamus (MBH) to recover from an excitotoxic insult using the immediate-early gene c-fos as a developmental marker of ARC response to glutamate receptor stimulation with N-methyl-D-aspartate (NMDA). Groups of neonatal (postnatal day (PD) 2) pups were injected with MSG, then stimulated on subsequent days (PD 3-29) with NMDA, known to induce c-fos expression in ARC. Computer-assisted densitometry was used to quantify Fos-like immunoreactivity (FLI) profiles in ARC. Pups treated neonatally with saline (PD 2) showed a robust, age-specific expression of FLI in the ARC following NMDA treatment. The FLI response was absent in the days immediately following an MSG lesion but subsequently recovered up to 75% of maximum by PD 16. Almost full recovery was seen by PD 29. We also examined the ability of the ARC to recover following chronic MSG treatment (PD 2-8), known to induce extensive hypothalamic damage. These pups displayed an unusual response to subsequent NMDA injection, consisting of 5 min cycles of hyper- and hypoactivity. Stimulation with NMDA revealed only a 50% recovery of FLI even at PD 29. In both treatment groups (acute vs. chronic MSG) the zone of recovery (i.e., reappearance of FLI) was initiated close to the third ventricle and with time radiated towards the periphery of the ARC. Some cells which reacquired FLI in the ARC following lesions presented a highly irregular condensed nuclear morphology. We conclude that the recovery of hypothalamic function (i.e., onset of puberty) after a neonatal MSG lesion is coincident with the reappearance of a normal pattern of c-fos expression in response to NMDA stimulation.

Mechanisms of cell death induced by the mitochondrial toxin 3-nitropropionic acid: acute excitotoxic necrosis and delayed apoptosis

Impaired energy metabolism may play an important role in neuronal cell death after brain ischemia and in late-onset neurodegenerative diseases. Both excitotoxic necrosis and apoptosis have been implicated in cell death induced by metabolic impairment. However, the factors that determine whether cells undergo apoptosis or necrosis are not known. In the present study, metabolic impairment was induced by 3-nitropropionic acid (3-NP), a suicide inhibitor of succinate dehydrogenase. Treatment of cultured rat hippocampal neurons with 3-NP resulted in two types of cell death with distinct morphological, pharmacological, and biochemical features. A rapid necrotic cell death, characterized by cell swelling and nuclear shrinkage, could be completely prevented by the NMDA receptor antagonist MK-801 (10 microM) and dose-dependently potentiated by low micromolar levels of extracellular glutamate. A slowly evolving apoptotic death, characterized by nuclear fragmentation, was not attenuated by MK-801 but was prevented by cycloheximide (1 microg/ml). The combination of MK-801 and cycloheximide resulted in an almost complete protection against 3-NP-induced cell death. DNA fragmentation, detected by the terminal deoxynucleotidyl transferase-mediated dUTP-X 3' nick end-labeling technique, was a late event in apoptosis and also occurred after necrotic cell death. ATP depletion was an early event in the 3-NP-induced neuronal degeneration, and the decline in ATP was exacerbated by glutamate. We conclude that 3-NP triggers two separate cell death pathways: an excitotoxic necrosis as a result of NMDA receptor activation and a delayed apoptosis that is NMDA receptor-independent. Mildly elevated levels of extracellular glutamate shift the cell death mechanism from apoptosis to necrosis.

gp120, a human immunodeficiency virus-1 coat protein, augments excitotoxic hippocampal injury in perinatal rats

Recent data suggest that gp120, a human immunodeficiency virus-1 (HIV-1) coat glycoprotein that is secreted by HIV-infected cells, is neurotoxic, and that this toxicity is mediated, at least in part, by activation of N-methyl-D-aspartate-type excitatory amino acid receptors. To test this hypothesis in vivo, we examined the neurotoxicity of gp120 injected intrahippocampally, alone or co-injected with the selective excitatory amino acid agonist N-methyl-D-aspartate, in seven-day-old rats. Severity of injury in the lesioned hippocampus was assessed five days later, using three outcome measures: histopathology, hippocampal atrophy (derived from regional cross-sectional area measurements) and loss of [3H]glutamate receptor binding (based on in vitro autoradiography assays). To confirm that any observed effects were attributable to gp120 bioactivity, each group of experiments included controls that received equal amounts of heat-treated gp120. Gp120 (200 ng) elicited minimal focal pyramidal cell loss immediately adjacent to the injection track; there was no hippocampal atrophy or loss of [3H]glutamate binding. Co-injection of 50 ng gp120 with N-methyl-D-aspartate (5 nmol, threshold excitotoxic dose) increased the severity of hippocampal injury; hippocampal atrophy was greater in animals that received injections of 5 nmol N-methyl-D-aspartate in combination with 50 ng gp120 than in those that received either N-methyl-D-aspartate alone (5 nmol) or 5 nmol N-methyl-D-aspartate+50 ng heat-treated gp120 (mean+/-S.E.M. percentage reduction in injected hippocampal volume vs contralateral: N-methyl-D-aspartate, -19+/-3; N-methyl-D-aspartate+gp120, -26.8+/-2.1; N-methyl-D-aspartate+heat-treated gp120, -14.0+/-2.2; P<0.001, ANOVA). Treatment with the competitive N-methyl-D-aspartate antagonist 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (20mg/kg) markedly reduced the severity of injury elicited by the combination of gp120 with N-methyl-D-aspartate. These data support the hypothesis that locally secreted gp120 could exert neurotoxic effects, mediated by N-methyl-D-aspartate receptor activation, in vivo in the immature brain.