INTRODUCTION
Methane has been concerned recently for its beneficial effects on diseases, including ischemia/reperfusion (I/R) injury of different organs, such as the intestine, liver, kidney, lung and retina.[1,2,3,4,5,6] Methane can significantly alleviate I/R injury of these organs. Moreover, the protective effects of methane on inflammatory diseases, such as endotoxic shock,[7] dextran sulfate sodium-induced colitis,[8] and bacteria-induced sepsis,[9] have also been confirmed.[6] Furthermore, methane can treat other diseases, such as diabetic retinopathy,[10] lung injury,[11] acute pancreatitis[12] and even chronic pain.[13]
Methane exert protective effects on various neurological diseases, such as carbon monoxide (CO) poisoning injury,[14] spinal cord injury (SCI),[15,16] cerebral I/R injury,[17] and traumatic brain injury (TBI).[18] However, there are a lot of differences in the indicators and administration methods of methane. A review focusing on the administration routes of methane will provide evidence for future research on practical observational indicators, research methods, and even meaningful mechanisms.
Therefore, we retrieved and reviewed relevant papers published from 1965 to 2022 in PubMed using the following search strategy: Query formulation: (((((neuro) OR (brain)) OR (cerebral)) OR (spinal)) AND (methane)) AND (((alleviate) OR (protect)) OR (improve)). A total of 88 articles were retrieved and finally, 8 articles were included after reading the full text. Then, we assessed the indicators related to the effects of methane and evaluated the production and administration methods of methane in the treatment of neurological diseases. We hope to offer available indicators of methane and to identify classical ways of methane production and administration in the treatment of neurological diseases.
BENEFICIAL EFFECTS OF METHANE ON NEUROLOGICAL DISEASES
Methane in treating CO poisoning-related neurological injury
Delayed neuropsychological sequelae have been studied to explore the potential effect of methane for relieving injury. Delayed neuropsychological sequelae are critically delayed injuries caused by severe CO poisoning.[19] Oxidative stress and other related injuries are considered to be active mechanisms responsible for delayed neuropsychological sequelae.[20] Fan et al.[14] indicated that methane-rich saline (MS) improved the performance of the Morris water maze and other indicators significantly after CO poisoning. And methane might exert these effects due to its anti-oxidant, anti-inflammatory and anti-apoptosis activities.[14]
There is also another focus on the beneficial effects of methane on CO poisoning.[21] Methane improves neuron loss, level of malondialdehyde, 3-nitrotyrosine, 8-hydroxydeoxyguanosine, superoxide dismutase and other inflammatory factors after CO poisoning injury. The mechanism of this protection is related to decreasing cerebral lipid peroxidation, attenuating DNA oxidative stress in neurons, increasing superoxide dismutase activity, and reducing inflammation.
Therefore, MS could treat CO poisoning-related neurological injury.
Methane in treating SCI
Wang et al.[15] reported that MS could treat SCI. In their work, the motor sensory deficit index was increased by 5 points after SCI but dramatically reduced by MS to 2 points. Moreover, apoptosis and inflammation were also significantly alleviated after MS administration. Their data indicated that MS treatment could prevent I/R-induced SCI due to the anti-oxidant, anti-inflammatory, and anti-apoptotic activities of methane through the activation of Nrf2 signaling.
Another study also looked into the protective effects of methane on SCI. The researchers used the Basso, Beattie, and Bresnahan scoring system instead of the motor sensory deficit index to explore the protective effects of MS following SCI. They explored the effects of methane on infarct area, oxidative stress, inflammation (tumor necrosis factor-α, interleukin-1β, and interleukin-6), and cell apoptosis 72 hours following SCI. They suspected that the mechanism of methane could be related to cell apoptosis and activation of microglia.[16]
These two studies revealed the protective effects of methane on SCI. MS may be an effective treatment for SCI.
Methane in treating cerebral I/R injury
Other scientists have focused on the effect of methane on cerebral I/R injury[17] that can cause hemiplegia. They found that methane could significantly improve neurological deficits and increase the levels of malondialdehyde and tumor necrosis factor-αafter injury. Therefore, there is a possibility of using methane in the treatment of cerebral I/R injury. This finding further suggests that phosphoinositide 3-kinase/Akt/heme oxygenase 1 dependent antioxidant pathway may be involved in the application of methane for treating cerebral I/R injury.
Methane in treating TBI
Li et al.[18] reported that methane exerts protective effects on TBI. Increased neurological severity score, more residual neurons, and higher superoxide dismutase level were observed after TBI, while treatment with MS could improve these indicators. MS significantly inhibited inflammation, oxidative stress, and cell apoptosis within 14 days after TBI. Therefore, MS can ameliorate TBI through its anti-inflammatory, anti-apoptotic, and anti-oxidative effects via the activation of the Wnt signaling pathway.
METHODS OF METHANE PREPARATION AND ADMINISTRATION
Methane preparation
There are two ways of methane preparation. One is to mix methane with other gases,[17] such as nitrogen and normal air. The ratio of methane ranges from 2.2% to 2.5%. The other is to dissolve methane into physiological saline (MS).
The mixed methane is produced as per the method of Zhang et al.[17] Methane is mixed with air at a reaching ratio of 2.2%. The air is composed of nitrogen and oxygen at a ratio of 20.79%. Therefore, the oxygen ratio in the mixed gas is 20.33% ([100 – 2.2] × 0.21). This small difference in the ratio of oxygen makes a big difference in latitude. The oxygen concentration of 20.33% is equivalent to that at an altitude of 300 meters. To develop a more rigorous way of production, methane should be mixed with N2 and O2 separately. This way shares the same ratio of oxygen with the control group.
The other way to produce methane is to dissolve methane with normal saline.[15] This is a safer way to make methane compared with the mixed gas because it could avoid explosion and suffocation. Moreover, it is more useful in clinical translation compared with the mixed gas.[22] Methane with a pure concentration of 99.99% is sealed in a plastic bag with saline. Then the bag is compressed with high pressure for a certain time. After compression, the MS can be stored at 4°C for further use.
There are some differences in the time and pressure for the production of MS (Table 1). 0.4 MPa (4 atmosphere pressures) has been widely used. The duration of saturation time ranges from 2 to 8 hours. Wang et al.[15] reported that MS was sterilized by γ-radiation after production. The concentration of MS is mostly set to be 0.99 mM.[14,21] Wang et al.[15] reported that the concentration of MS was set to be 1.60 mM.
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Table 1: Details of methane-rich saline applied in neurological diseases
The administration dosage of MS
There were also differences in the dosage of MS (Table 1). 10 mL/kg MS was used to treat CO poisoning and SCI, and 20 mL/kg MS was also used for treating SCI. The dosage of 10 mL/kg was first reported by Ye et al.[23] in 2015. Afterward, this dosage was used accordingly. Other dosages of MS, such as 20 mL/kg, were also applied (Table 1).
The way of methane administration
There are two main ways of administration: gas inhalation and injection of physiological saline mixture. In the study by Zhang et al.,[17] 2.2% methane was used for gas inhalation. The concentration is the same as that reported by Poles et al.,[24] but slightly lower than that reported by Boros et al.[25] (2.5%). Given that inhalation of methane can induce hypoxia and asphyxia,[26] the safe limit for methane inhalation is 25% to 30%. If the concentration of methane exceeds this limit, it would cause paralysis, breathing difficulties and even death.[27]
Asphyxia can be eliminated by MS administrated intraperitoneally or peritoneally. Still, there are some differences in the drug-delivery way of MS. MS is usually injected immediately after injury. In neurological research, MS is injected at least three times. Fan et al.[14] and Shen et al.[21] reported that methane was injected every 8 hours at the following time points: 0, 8, and 16 hours after injury. In another studies,[15,16] methane was injected six times within 3 days. In a study by Li et al.,[18] methane was even injected for 7 consecutive days (Table 1).
FUTURE DIRECTIONS
Intrathecal injection: an alternative way to transfer methane
Methane has to travel a long way to act on the brain or the spinal cord.
There is still a long way before methane goes into the brain or spine. After intraperitoneal injection, methane is absorbed by the peritoneum. Then methane goes through the portal vein or inferior vena cava for further gathering in the heart.[28] Finally, methane goes through the carotid artery, vertebral artery, or spinal artery, acting on the brain or spine. Therefore, methane travels a long way to its destinations.
The dosage of MS increases accordingly. However, a large dosage of MS can result in some problems. For example, 10 and even 30 mL/kg methane means that a large amount of fluid will be injected into mice or rats. A rat weighing 200–300 g only has 16 mL of blood. If 20 mL/kg methane is used, 4–6 mL of the fluid, almost equal to 1/4 of the body’s blood, will be intraperitoneally injected into the rat. Therefore, there is a fluid limit (< 10 mL/kg) for rats in basic research.[29] Taking the human body as an example, an average male weighing 60–70 kg has 3–4 L of blood. Then 0.75–1 L of the fluid will be injected every 8 or 12 hours or once a day. Another problem is that a large amount of fluid will induce an injury. Dehydration should be done in the clinical treatment of SCI or cerebral I/R,[30] but a huge fluid burden may be detrimental to dehydration strategy, thereby aggravating the injury.
Is there an administration way of methane by which we can both reduce the long way of absorption and decrease the dosage to minimize the fluid burden of animals? Intrathecal injection is then proposed as a possible way of administration. Intrathecal injection is a route of drug administration using a PE tube via an injection into the spinal subarachnoid space.[31] This approach greatly shortens the path that methane has to travel to do its job. Also, this administration way requires a small amount of fluid. In a study addressing the effect of hydrogen-rich saline on neuropathic pain, hydrogen-rich saline was intrathecally injected using a 12.5 cm PE-10 tube. And only 20 μL was used in their work.[32] Their results showed that hydrogen-rich saline significantly reduced the activation of spinal astrocytes and microglia. Therefore, intrathecal administration of methane will be an alternative way of administration in the treatment of neurological diseases.
Limitations
There are some limitations. First, the total included number of articles is small. Although we retrieved all literature on the neuroprotective effects of methane, there are not enough publications in this field that can fully elucidate how methane acts on neurological diseases. Second, further experiments are warranted to explore the administration way of methane.
Conclusions
This review summarizes the beneficial effects of methane on neurological diseases and analyzes preparation and administration methods of methane. Intrathecal injection is introduced as an alternative way of methane administration.
Author contributions
All authors wrote the manuscript, prepared the data, and approved the final manuscript.
Conflicts of interest
None.
Open access statement
This is an open access journal, and articles are distributed under the terms of the Creative Commons AttributionNonCommercial-ShareAlike 4.0 License, which allows others to remix, tweak, and build upon the work non-commercially, as long as appropriate credit is given and the new creations are licensed under the identical terms.
Acknowledgements
The authors are grateful to Professor Xue-Jun Sun (Naval Military Medical University, Shanghai, China) for constructive comments.
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