| Carbon monoxide(CO) is a tasteless, odourless and non-irritating but highly toxic gas that is produced by the incomplete burning of carbon-containing fossil fuels. Its molecular weight is 28. The normal concentration in atmosphere is less than 0.001% and a concentration of 0.1% can be lethal. At present, CO is one of the commonly encountered poison in our environment and a leading cause of poisoning world wide. The symptoms of CO poisoning are nonspecific. Mild exposure can result in headache, myalgia, dizziness, nausea, or neuropsychological impairment. Severe exposure to CO can result in confusion, coma, and even death. Over half of those with serious poisoning develop an encephalopathy from 3 days to 4 weeks after exposure, with features of delayed impairment ranging from subtle abnormalities, such as personality changes or mild cognitive deficit, to severe dementia, psychosis, Parkinsonism, incontinence or other abnormalities. These impairments are referred to as delayed neuropsychological sequelae(DNS). Among patients with DNS, 50 to 75% recover completely or improve considerably within 1 year. Brain injury due to CO poisoning, especially the DNS, brings the serious burden to the society and family.Because the affinity of CO to haemoglobin(Hb) is 200 times greater than that of oxygen(O2), CO quickly diffuses into the blood via the lungs and binds to Hb to form carboxyhaemoglobin(COHb). The easy displacement of O2 from Hb reduces the amount of Hb available to carry O2, causing hypoxaemia. Hypoxia caused by CO is key for injury to the brain,and the level of COHb is one of the most important indicators to judge the severity of CO poisoning. However, for decades clinicians have noted that the COHb levels of patients do not correlate well with their signs/symptoms or their ultimate clinical outcome.Some of the pathophysiological effects of CO poisoning are consistent with ischaemic-reperfusion injury, in that for both conditions, there is a hypoxic phase usually followed by reoxygenation. In the CO-impaired brain, xanthine dehydrogenases are converted to xanthine oxidases, suggesting that xanthine oxidase-derived reactive oxygen species(ROS) are responsible for the lipid peroxidation of neuronal membranes. Hydroxyl radicals are also present in a brain that is subjected to CO-induced hypoxia and then reoxygenated in both the hypoxic phase and the reoxygenation phase. A 10-fold increase in nitrotyrosine production in the brains of CO-poisoned rats further demonstrates that peroxynitrite may be generated during CO poisoning due to the enhanced rates of production of both nitric oxide(NO) and superoxide. Together, the findings above suggest that CO-mediated brain damage is the result of a free radical cascade, according to the classical mechanism of a redox reaction, and is strongly dependent on disrupting the balance between the anti-oxidant system and oxidative stress. Acute anti-oxidant reinforcement may be a novel therapeutic strategy for brain injury after acute CO poisoning. Several groups have investigated therapies using ROS scavengers with promising results, such as hydrogen, hydrogen sulphide and edaravone.Methane is the simplest aliphatic hydrocarbon and the main gas energy. It was previously thought that humans do not use methane. In recent year, evidences support the notion that methane liberation may be linked to redox regulation and may be connected with hypoxic events leading to, or associated with a mitochondrial dysfunction. In the studies of models of mesenteric, liver, myocardium ischaemia-reperfusion injury and diabetes mellitus, methane has the significant protective effects through anti-apoptosis, anti-oxidative and anti-inflammatory actions. Methane is of potential therapeutic of interest in oxidative stress pathologies.In this study, we hypothesize that methane saline may have neuroprotective effects against brain damage and may improve brain injury outcome after acute CO poisoning. At the same time, the studies on the mechanisms of protective effects of methane about brain injury were performed.In the first part, the effect of methane on the brain injury after CO poisoning in rats were performed. The rat model of acute CO poisoning were established by the method of acute static inhalation of CO gas. Following CO exposure, saline or methane saline was intraperitoneally administered to rats in the CO group or the CO plus methane group, respectively. The cognitive deficit was observed by behavioral experiments such as Morris water maze testing. The neuron loss was acquired by technique of histopathology. The results showed that methane significantly improved CO-impaired pathological characteristics as well as learning and memory performance. Furthermore, in the CO group, neurons in the rat cortex and hippocampus were pyknotic, with enlargement of the intracellular space, while in contrast, in the CO plus methane group, the neuron structures and nissl bodies were relatively intact.In the Part 2 to Part 4, the protective mechanisms of anti-oxidative, anti-inflammatory and anti-apoptosis actions of methane on the brain injury due to CO poisoning were researched on the 24 h and 9th day after CO exposure, respectively. The markers of oxidative damage, superoxide dismutase(SOD), malondialdehyde(MDA), 3-nitrotyrosine(3-NT) and 8-hydroxy-2-deoxyguanosine(8-OHdG) were detected by the metheds of kits and enzyme-linked immunosorbent assay(ELISA), respectively. Levels of inflammatory cytokine, tumour necrosis factor-?(TNF-?), interleukin- 1?(IL-1?), IL-6 were tested by ELISA. Terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling(TUNEL) and Western Blotting were used to detect the markers of apotosis. Results showed that methane significantly increased the SOD activity and lowered the CO-increased level of MDA in both the rat hippocampus and cortex, as well as CO-increased level of 3-NT and 8-OHdG in the rat hippocampus on the 24 h and 9th day after CO exposure. Methane could inhibit the levels of TNF-? and IL-1? in both the rat hippocampus and cortex on the 24 h and 9th day after CO exposure, but had no effect on IL-6 levels. Methane also significantly decreased the number of apoptotic cells and the level of caspase-3 in both the rat hippocampus and cortex on the 9th day after CO exposure.In the part 5, the expression of proteins and mRNA of nuclear factor erythroid-2 related factor 2(Nrf-2) and downstream protein catalase(CAT) in the rat cortex and hippocampus on the 9th day following CO exposure were detected by the methods of Western Blotting and real time-polymerase chain reaction(RT-PCR). The results revealed that on the 9th day after CO exposure, the expression levels of Nrf-2 and downstream protein CAT were inhibited while methane reversed it, which improving the antioxidant ability of the rats.The main conclusions are as follows:1. Methane treatment protected the rat brain from the harmful effects induced by CO exposure.2. The hippocampus was the main target of CO-induced alterations in the rat brain compared to the cerebral cortex.3. Methane might improve the outcome of CO poisoning through the anti-oxidant, anti-inflammatory and anti-apoptosis activities.4. The persistent oxidative stress due to CO poisoning might inhibit the expression of Nrf-2 and downstream protein CAT, while methane could remove the inhibition. |
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