Possible role of nitric oxide in the development of L-2-chloropropionic acid-induced cerebellar granule cell necrosis

The contributions of excitotoxicity, glutathione depletion and DNA repair in chemically induced injury to neurones: exemplified with toxic effects on cerebellar granule cells

Six chemicals, 2-halopropionic acids, thiophene, methylhalides, methylmercury, methylazoxymethanol (MAM) and trichlorfon (Fig. 1), that cause selective necrosis to the cerebellum, in particular to cerebellar granule cells, have been reviewed. The basis for the selective toxicity to these neurones is not fully understood, but mechanisms known to contribute to the neuronal cell death are discussed. All six compounds decrease cerebral glutathione (GSH), due to conjugation with the xenobiotic, thereby reducing cellular antioxidant status and making the cells more vulnerable to reactive oxygen species. 2-Halopropionic acids and methylmercury appear to also act via an excitotoxic mechanism leading to elevated intracellular Ca2+, increased reactive oxygen species and ultimately impaired mitochondrial function. In contrast, the methylhalides, trichlorfon and MAM all methylate DNA and inhibit O6-guanine-DNA methyltransferase (OGMT), an important DNA repair enzyme. We propose that a combination of reduced antioxidant status plus excitotoxicity or DNA damage is required to cause cerebellar neuronal cell death with these chemicals. The small size of cerebellar granule cells, the unique subunit composition of their N-methyl-d-aspartate (NMDA) receptors, their low DNA repair ability, low levels of calcium-binding proteins and vulnerability during postnatal brain development and distribution of glutathione and its conjugating and metabolizing enzymes are all important factors in determining the sensitivity of cerebellar granule cells to toxic compounds.

Different effects of eNOS and nNOS inhibition on transient forebrain ischemia

To clarify the functions of nitric oxide (NO) induced by either neuronal NO synthase (nNOS) or endothelial NO synthase (eNOS) after transient cerebral ischemia, we investigated the effects of L-N(5)-(1-iminoethyl)ornithine (L-NIO), a relatively selective eNOS inhibitor, and 7-nitroindazole (7-NI), a relatively selective nNOS inhibitor, on hippocampal dysfunction caused by cerebral ischemia. We measured mean arterial blood pressure (MABP), hippocampal blood flow, direct current (DC) potential, CA1 population spike (PS) and extracellular concentrations of glutamate from rat hippocampus after transient forebrain ischemia, which was induced by four-vessel occlusion for 10 min. L-NIO (20 mg/kg) and 7-NI (25 mg/kg) were administered intraperitoneally 20 min before ischemia. L-NIO, but not 7-NI, increased MABP before, during and after ischemia, compared with the vehicle group. 7-NI, but not L-NIO, reduced the amplitude of anoxic depolarization induced by ischemia. 7-NI recovered the PS amplitude in part 60 min after ischemia. 7-NI, but not L-NIO, reduced the ischemia-induced levels of glutamate. These results indicate that nNOS inhibition with 7-NI improves, at least in part, hippocampal dysfunction after ischemia, while eNOS inhibition with L-NIO worsens it.

Toxic effect of L-2-chloropropionate on cultured rat cerebellar granule cells is ameliorated after inhibition of reactive oxygen species formation

Oral administration of rats to L-2-chloropropionate (L-CPA) causes selective necrosis to the granule cell layer of the cerebellum in vivo and to cultured rat cerebellar granule cells in vitro. The present study was conducted to characterize the involvement of reactive oxygen species (ROS) in cell death of L-CPA to rat cerebellar granule cells in vitro. Exposure to L-CPA (0.625-10 mM) produced a concentration dependent increase in formation of 2,7-dichlorofluorescein (DCF) as a measure of formation of ROS. The elevation of ROS was inhibited after incubation of the cells with the ERK-type of MAP kinases inhibitor U0126, the mitochondrial permeability transition pore inhibitor cyclosporin A (CSA), the antioxidant vitamin E, and the spin trap N-tert-butyl-alpha-(2-sulfophenyl)-nitrone (S-PBN). Measurements of nitrite (NO(2)) in the cell culture supernatant using the Griess reagent indicate generation of nitric oxide (NO) after exposure to L-CPA. Incubation with L-CPA (10 mM) for 48 hr lead to cell death (90%). When the granule cells were incubated with L-CPA in combination with the inhibitors of free radical production, the cell death was ameliorated. The results show that L-CPA is toxic to granular cells by production of ROS.

Biochemical and neurotoxicological effects of L-2-chloropropionic acid on rodent brain

L-2-Chloropropionic acid (L-CPA) is selectively toxic to cerebellar granule cells; necrosis is first observed in rats 36 h after L-CPA administration (750 mg/kg p.o.) and becomes marked by 48 h. L-CPA has also been shown to activate the mitochondrial pyruvate dehydrogenase (PDH) complex in fasted adult rats, resulting in reduced blood glucose and lactate levels. This study aimed to investigate the biochemical and neurotoxicological effects of L-CPA on the brain. Extracts, prepared from guinea-pig cerebellar and cerebral cortex slices incubated in the presence of L-CPA, were analysed using 1H magnetic resonance spectroscopy, 31P magnetic resonance spectroscopy, and amino acid analysis. Glucose metabolism was studied by monitoring the metabolism of [1-(13)C]glucose using gas chromatography/mass spectrometry. Increased glucose metabolism and decreases in the pool sizes of lactate and alanine were observed in both tissues, demonstrating activation of the PDH complex. Extracts were also prepared from the forebrain and cerebellum of animals that had been treated in vivo with L-CPA and analysed as described for the in vitro studies. Similar evidence for PDH activation was demonstrated at 2 and 24 h after dosing in both tissues. At 48 h after dosing, when signs of toxicity are observed, an increase in the lactate concentration and a decrease in N-acetylaspartate in the cerebellum but not in the forebrain confirmed the selective neurotoxic action of L-CPA. These results suggest that activation of the PDH complex does not directly lead to the delayed selective neurotoxicity of L-CPA.

Nitric oxide-induced cytotoxicity attenuation by thiopentone sodium but not pentobarbitone sodium in primary brain cultures

1. We describe the effects of barbiturates on the neurotoxicity induced by nitric oxide (NO) on foetal rat cultured cortical and hippocampal neurones. Cessation of cerebral blood flow leads to an initiation of a neurotoxic cascade including NO and peroxynitrite. Barbiturates are often used to protect neurones against cerebrovascular disorders clinically. However, its neuroprotective mechanism remains unclear. 2. In the present experiment, we established a new in vitro model of brain injury mediated by NO with an NO-donor, 1-hydroxy-2-oxo-3-(3-aminopropyl)-3-isopropyl-1-triazene (NOC-5) on grid tissue culture wells. We also investigated the mechanisms of protection of CNS neurones from NO-induced neurotoxicity by thiopentone sodium, which contains a sulphydryl group (SH-) in the medium, and pentobarbitone sodium, which does not contain SH-. 3. Primary cultures of cortical and hippocampal neurones (prepared from 16-day gestational rat foetuses) were used after 13-14 days in culture. The cells were exposed to NOC-5 at the various concentrations for 24 h in the culture to evaluate a dose-dependent effect of NOC-5. 4. To evaluate the role of the barbiturates, neurones were exposed to 4, 40 and 400 microM of thiopentone sodium or pentobarbitone sodium with or without 30 microM NOC-5. In addition, superoxide dismutase (SOD) at 1000 u ml(-1) and 30 microM NOC-5 were co-administered for 24 h to evaluate the role of SOD. 5. Exposure to NOC-5 induced neural cell death in a dose-dependent manner in both cortical and hippocampal cultured neurones. Approximately 90% of the cultured neurones were killed by 100 microM NOC-5. 6. This NOC-5-induced neurotoxicity was significantly attenuated by high concentrations of thiopentone sodium (40 and 400 microM) as well as SOD, but not by pentobarbitone sodium. The survival rates of the cortical neurones and hippocampal neurones that were exposed to 30 microM NOC-5 were 11.2+/-4.2% and 37.2+/-3.0%, respectively, and in the presence of 400 microM thiopentone sodium, the survival rate increased to 65.3+/-3.5% in the cortical neurones and 74.6+/-2.2% in the hippocampal neurones. 7. These findings demonstrate that thiopentone sodium, which acts as a free radical scavenger, protects the CNS neurones against NO-mediated cytotoxicity in vitro. In conclusion, thiopentone sodium is one of the best of the currently available pharmacological agents for protection of neurones against intraoperative cerebral ischaemia.

L-2-chloropropionic acid-induced neurotoxicity in the rat: a valuable model for studying selective neuronal cell death in vivo

1. L-2-Chloropropionic acid (L-CPA) is neurotoxic when administered orally as a neutral sodium salt in high doses to rats, resulting in a selective destruction of cerebellar granule cells with the result that animals develop marked difficulties in maintaining normal locomotion. 2. Cerebellar granule cell destruction is accompanied by a reduction in cerebellar glutamate and aspartate concentrations, reductions in the density of glutamate receptors located in the cerebellar granule cell layer and development of cerebellar oedema. No other cell type in the brain, nor other organ, is affected by L-CPA. 3. The neuronal cell death is necrotic in type, involving the activation of N-methyl-D-aspartate (NMDA) type glutamate receptors and there is some evidence for a partial role of nitric oxide in the development of the neurotoxicity. 4. Contrary to work performed on NMDA mediated cell death using neuronal cell culture approaches, L-CPA-induced granule cell death does not appear to arise from the production of excess quantities of cytotoxic free radicals, but may involve selective calcium-activated proteases, such as the calpains. 5. Tentative evidence suggests that L-CPA may interfere with voltage-dependent calcium channels in the cerebellum leading to activation of the cell death and resulting in the destruction of the granule cells. 6. In conclusion, L-CPA-induced neurotoxicity may provide valuable information on the neurochemical pathways involved in neuronal cell death that is associated with many neurological diseases.

Evidence for mediation of L-2-chloropropionic acid-induced delayed neuronal cell death by activation of a constitutive nitric oxide synthase

1. Delayed neuronal cell death elicited by excess excitatory amino acid concentrations has been strongly implicated in many neurological disorders including head trauma, stroke, motor neurone disease and Huntington's disease. We have used the neurotoxin, L-2-chloropropionic acid (L-CPA) to model cellular events in vivo leading to delayed neuronal cell loss which is confined to the cerebellar cortex and can be prevented by inhibitors of nitric oxide synthase such as NG-nitro-L-arginine methyl ester. 2. Experiments were performed to determine whether the constitutive nitric oxide synthase (NOS) or inducible form of NOS (iNOS) was responsible for the neuronal cell death. Activation of NOS was confirmed by a 39% increase in cerebellar total nitrate and nitrite concentrations in L-CPA-treated brains, as compared to controls (controls = 2.53 +/- 0.10; L-CPA treated = 3.51 +/- 0.31 nmol mg-1 protein, P < 0.01 Student's t tests, n = 6, mean +/- s.e.mean). Biochemical measurements of total NOS activity were made in homogenates of cerebellum 6 h and 48 h following L-CPA administration, times at which L-CPA concentrations are maximal in brain and a time when there is a high proportion of cerebellar granule cell death, respectively. NOS activity as measured by the amount of [3H]-arginine converted to [3H]-citrulline, did not reveal any difference between controls (rats dosed with water) and animals dosed with L-CPA at either 6 or 48 h following dosing. Furthermore the ability of three NOS inhibitors, NG-nitro-L-arginine, 7-bromo-3-nitroindazole and S-methylisothiourea to block the conversion of [3H]-citrulline to [3H]-arginine was identical at 6 and 48 h time points in control and L-CPA treated rats. 3. Quantitative autoradiography using [3H]-NG-nitro-L-arginine was used to measure the relative anatomical distribution and amount of NOS enzyme in the cerebellum of controls and L-CPA-treated rats 48 h following dosing. There was no significant alteration in the binding of [3H]-NG-nitro-L-arginine to granular and molecular layers of the cerebellum of control and L-CPA-treated rat brains. 4. Western blotting using antibodies against the inducible NOS enzyme failed to detect the protein in cerebellums of L-CPA-treated rats when measured 48 h after L-CPA dosing. 5. In conclusion, the increase in cerebellar nitrate/nitrite concentrations in L-CPA-treated rats provides further evidence for activation of NOS in the cerebellum following administration of L-CPA. The failure to demonstrate an increase in NOS activity at 6 or 48 h in L-CPA-treated rats as compared to controls suggests that the source of nitric oxide responsible for the granule cell death must originate from the constitutive NOS enzyme, probably the neuronal form which is highly enriched in the cerebellum. This hypothesis was further substantiated by Western blotting and quantitative autoradiography.