A critical review of the neuroprotective effect of molecular hydrogen

The difference between consuming CoQ10 alone or together with hydrogen gas lies in how these two elements work together to improve mitochondrial function in the body.

When CoQ10 is taken as a dietary supplement, the levels of this molecule in the blood increase, which can improve energy production in the cells and protect against oxidative damage. However, if the mitochondria are already severely damaged, the effect may be limited because the cells do not receive the necessary support to fully restore their energy function.

If CoQ10 is taken together with hydrogen gas, a synergistic effect occurs. Hydrogen helps reduce oxidative stress and damaged mitochondria, which creates better conditions for CoQ10 to work effectively. This combination can thus lead to a more powerful improvement in the mitochondrial function, which can increase cellular energy production, strengthen the immune system, and improve the clinical response, as was also observed in the research study.

In short: While CoQ10 alone can support mitochondrial function, the combination with hydrogen gas can provide a far stronger and more effective recovery of the cells' energy levels and thus a better state of health.

Wei Chen 1,2 • Han-Ting Zhang 3 • Shu-Cun Qin 1,2

Received: February 19, 2020 / Accepted: May 31, 2020
Shanghai Institutes for Biological Sciences, CAS 2020

Summary
Molecular hydrogen (H2) is a physiologically inactive gas. However, over the past 10 years, an ever-increasing body of evidence has demonstrated the biological functions of hydrogen under pathological conditions. More specifically, H2 has protective properties against several diseases, particularly nervous system disorders, including ischemia/reperfusion injury, trauma, subarachnoid haemorrhage, nerve pain, neurodegenerative diseases, cognitive dysfunction induced by surgery and anaesthesia, anxiety, and depression. In addition, H2 possesses multiple protective properties primarily through antioxidation, anti-inflammation, anti-apoptosis, regulation of autophagy, and preservation of mitochondrial function and the blood-brain barrier (BBB). Furthermore, H2 is easy to use, and its neuroprotective effect is achieved without major side effects. This indicates that the administration of H2 could potentially constitute a treatment strategy. Here, we summarize the H2 donors and their pharmacokinetics. Concurrently, we review the efficacy and safety of H2 as a treatment for various diseases of the nervous system based on preclinical and clinical studies. This leads to the conclusion that H2 may be a simple and effective clinical treatment for central nervous system (CNS) diseases, such as cerebral ischemia/reperfusion injuries, Parkinson's, and diseases characterized by cognitive dysfunction. In addition, the mechanisms potentially involved in the protective effect of H2 are analysed.

Keywords
Molecular hydrogen (H2) Neurological disease Neuroprotection Antioxidation Anti-inflammation Anti-apoptosis

Mail Han-Ting Zhang
hzhang@hsc.wvu.edu
Mail Shu-Cun Qin
13583815481@163.com

Taishan Institute for Hydrogen Biomedicine, Shandong First
Medical University and Shandong Academy of Medical Sciences, Tai’an 271000, Kina
Key Laboratory of Atherosclerosis in Universities of Shandong, Institute of Atherosclerosis, Shandong First Medical University og Shandong Academy of Medical Sciences, Tai’an 271000, Kina
Departments of Neuroscience and Behavioral Medicine og Psychiatry, Rockefeller Neurosciences Institute, West Virginia University Health Sciences Center, Morgantown, WV 26506, USA

https://pubmed.ncbi.nlm.nih.gov/33078374/

Introduction

Molecular hydrogen (H2) was discovered by the chemist Henry Cavendish in 1766. It is a colourless, odourless, and physiologically inactive gas. The biological functions of H2 were gradually demonstrated by researchers in the late 20th century. In 1975, Dole [1] discovered that hyperbaric H2 (2.5% O2 and 97.5% H2) causes regression of squamous cell carcinoma in hairless albino mice. He hypothesized that the effect of H2 might be attributed to the gas's ability to regulate the most harmful OH radical (oxidant hydroxyl). In 2001, Gharib [2] discovered that 0.7 MPa H2 has an anti-inflammatory effect on chronic liver inflammation caused by schistosomiasis. Several years later, in 2007, Shigeo Ohta demonstrated that H2 has the same protective effect as antioxidants in brain tissue exposed to oxidative stress by selectively regulating the cytotoxic free radical OH and peroxynitrite (ONOO-) [3]. Since then, these findings have triggered a number of studies investigating the potential protective effect of H2 in relation to several diseases and the related molecular mechanisms.

Diseases caused by abnormalities in the structure and function of the nervous system are usually very severe diseases with a high incidence of disability and mortality worldwide. Restoring neural structure and function after either acute injury or chronic neurodegeneration remains a challenge, as there are no effective cures for most neurological diseases, and the response of these diseases to traditional interventions such as medication, physical therapy, neurorehabilitation, and preventive measures is poor. In contrast, an increasing body of evidence has shown that H2 has a protective effect against various neurological diseases, including ischemia/reperfusion injury [4–6], trauma [7, 8], subarachnoid haemorrhage (SAH) [9, 10], nerve pain [11, 12], Alzheimer's [13, 14], Parkinson's [15, 16], mood disorders [17, 18], glioblastoma [19], and cerebral infarction [20]. To date, more than 60 clinical trials have been conducted on the use of H2 for many diseases involving multiple systems, and at least 10 of these trials addressed nervous system diseases, including acute cerebral ischemia [4], acute cerebral infarction [20], sequelae of cardiac arrest [21, 22], Parkinson's [23, 24], and mood disorders.
[18].

There are a number of advantages to H2 as a neuroprotective gas. First, the gas can cross the blood-brain barrier (BBB), penetrate bio membranes, and diffuse into the cytosol and organelles [3]. Additionally, no major side effects have been documented [4]. More importantly, repeated administration of H2 does not cause intolerance [25]. Furthermore, several easy-to-use and practical administration options are available [26, 27]. Finally, H2 has a protective effect against numerous diseases, including diseases of the peripheral nervous system and the central nervous system (CNS) [26, 28]. In Japan, for example, inhalation of 2% H2 has been approved for emergency clinical treatment of cardiac arrest. This means that H2 is a novel and potential therapeutic strategy for the prevention and treatment of numerous diseases, including neurological diseases. The advantages derived from H2 have further led to the development of the H2 health industry.

I denne gennemgang er det vores mål at opsummere den aktuelle viden om den neurobeskyttende effekt af H2 i forhold til forskellige nervesystemsygdomme (fig. 1) og de mekanismer, der muligvis indgår (tabel 1).

H2 donors

Currently, there are several methods for administering H2, including inhalation of H2 gas, consumption of H2-enriched water, injection of H2-enriched saline, bathing in water with H2, consumption of a solid H2 carrier (coral calcium hydride), and H2-producing precursors (e.g., lactulose and L-arabinose) [27, 65–67], as well as functional micro/nanomaterials for targeted delivery of H2 [68].

In clinical practice, common administration methods for H2 primarily include inhalation of H2 gas, consumption of hydrogen-enriched water, injection of hydrogen-enriched saline, and bathing in H2 water. Inhalation of H2 is the easiest and most direct method, and the most frequently used concentration is 1%–4% (safe concentration). Compared to the inhalation of H2 gas, the consumption of hydrogen-enriched water is safer and more practical. Hydrogen-enriched saline is normally administered via IV or intraperitoneal injection. Hydrogen baths are often used to treat skin diseases [69]. When H2 is administered via the consumption of hydrogen-enriched water, 59% of the swallowed H2 is released in the breath, approximately 0.1% via the body surface, and about 40% is absorbed by the body [70]. After volunteers consumed 500 mL of hydrogen-enriched water in under 1 min, the concentration of H2 in the breath reached a peak level of approximately 36 ppm after 10 min, after which it gradually fell to baseline levels of approximately 7.0 ppm after 60 min [70]. Ono et al. discovered that the concentration of H2 in both arterial and venous blood rises rapidly and reaches a plateau level (10 lmol/L to 20 lmol/L) 20 min after the initiation of 3% and 4% H2. Upon discontinuation of H2 inhalation, the concentration in arterial blood falls to <10% of the plateau level in about 6 min but takes about 18 min in venous blood [4]. During a 30-min IV infusion of hydrogen-enriched saline (0.8 mmol/L), the concentration of H2 rises rapidly in both arterial and venous blood to a maximum (<1.8 ppm) after about 15 min and falls rapidly upon discontinuation of the hydrogen-enriched saline infusion [71]. Generally, inhalation leads to a higher concentration of H2 in the blood than IV infusion [71]. Animal studies have furthermore shown that inhalation induces higher concentrations of H2 in the brain than other methods of H2 administration (oral, intraperitoneal, intravenous) [72]. These findings suggest that inhalation is the preferred route of administration for H2 treatment in CNS disease.

Fig. 1: Number of publications (clinical trials and animal studies) in the PubMed database on the use of H2 for various neurological diseases between 2007-2020.

Although inhalation, oral ingestion, and injection of H2 effectively mitigate diseases of the nervous system, as demonstrated by various research groups [6, 20, 73], there is a lack of comparisons regarding the biological effects of the different hydrogen intervention methods in relation to a specific disease. Since the concentration of H2 in tissues and organs varies greatly depending on the chosen intervention [72, 74], administration via different methods may have different effects within the same damaged tissue. Furthermore, only a few studies have focused on dose- and time-dependent effects and the tolerance of H2 in basic and clinical studies. To select the most effective treatment method with H2 for each disease, it is therefore important to gain a deeper understanding of the pharmacokinetics and therapeutic efficacy of the different H2 donor types. In mouse models of Alzheimer's, intracerebral injection of Pd-hydride (PdH) nanoparticles reduces the overproduction of amyloid beta (Ab) in the brain [14], whereas ingestion of hydrogen-rich water does not exhibit the same effect [13]. The solubility of H2 is low, and the gas concentration in mouse brains is significantly lower (< 30 ppb/g) with traditional administration (ingestion of hydrogen-rich water, injection of hydrogen-rich saline, and inhalation of 4% H2) [72]. The PdH nanoparticles, which are a high-payload H2 storage material, sustainably release approximately 6 µmol/L H2 within 60 hours [14, 75]. This phenomenon has indicated that Ab clearance may correlate with the concentration and duration of H2 in the brain. In other words, a high concentration of H2 in the target tissue performs better than a low concentration. To maximize the therapeutic effect of H2, the development of effective storage and targeted delivery of H2 could be one of the future research directions.

The protective effect of H2 in relation to nervous system diseases

Ischemia/reperfusion injury

Ischemia/reperfusion injury is a condition characterized by tissue damage caused by an ischemic or anoxic period followed by the restoration of blood supply to the tissue. Ischemia/reperfusion injury in the central nervous system is associated with diseases such as stroke, brain trauma, cerebral infarction, and cardiac arrest.

The restoration of circulation after the absence of a sufficient supply of O2 in the blood induces a burst of reactive oxygen species (ROS), which subsequently triggers an inflammatory response and oxidative damage. ROS production can directly destroy cell membranes by inducing lipid peroxidation, which is why antioxidant agents are considered a treatment option. Firstly, H2 has been studied as an antioxidant agent that can mitigate the destructive effect of oxidative stress in the brain after focal ischemia/reperfusion via the selective reduction of cytotoxic oxygen radicals [3]. This forms the basis upon which in vivo studies have been conducted to validate the protective role that H2 has in connection with ischemia/reperfusion injury in the central nervous system. In animal models of cerebral ischemia/reperfusion, inhalation of 66.7% H2 significantly increases the activity of SOD and GSH-Px, reduces the level of malondialdehyde (MDA) and infarct volume, attenuates edema and haemorrhage in the brain, and improves neurobehavioral deficits [5, 76, 77]. Ischemia/reperfusion injury in the brain is a common secondary effect of cardiac arrest, which is responsible for mortality and morbidity after cardiopulmonary resuscitation. In experimental cardiac arrest/resuscitation, H2 intervention also significantly reduces neurological damage and increases the survival rate and neurological outcome in animals [78, 79]. Inhalation or injection of H2 after cardiac arrest effectively controls neuronal death and microglial activation in the hippocampus and reduces the level of protein S100b in serum [80–82]. In addition, it has been demonstrated that the inhalation of H2 alone or in combination with therapeutic hypothermia is superior to hypothermia alone [38, 80, 81, 83].

Clinical ischemia/reperfusion does not occur frequently in connection with stroke and cerebral infarction, whereas there is a higher probability of developing the dreaded reperfusion injuries in the brain in patients receiving thrombolysis treatment or interventional thrombectomy. Even more importantly, the safety and efficacy of H2 have been confirmed in clinical trials involving cerebral ischemia, cerebral infarction, and cardiac arrest. In patients with acute cerebral ischemia, inhalation of 3% H2 for 30 minutes or IV administration of H2 can deliver sufficient H2 to the blood without compromising safety [4, 84]. A randomized, controlled clinical study of patients with acute cerebral infarction showed that inhalation of 3% H2 gas for 1 hour twice a day for 7 days improved O2 saturation without side effects. Patients treated with the inhalation of H2 have a significantly smaller infarct site and experience better recovery of neurological status and the ability to perform activities of daily living compared to controls [20]. In addition, IV administration of H2 in combination with edaravone has a more evident and significantly favourable effect than the administration of edaravone alone [85]. In a human study of patients with sequelae after cardiac arrest, inhalation of a low concentration of H2 for 18 hours had a favourable score for the cerebral performance category after 90 days, with no side effects reported [21]. With a view to further evaluating the efficacy and safety of H2 inhalation, a phase II clinical trial is underway in Japan involving patients with sequelae of cardiac arrest [22]. Although no side effects of H2 have been found in animal studies, the potential side effects should be further investigated, as diarrhoea has been reported in a small number of patients following treatment with H2 [84].

Table 1: The neuroprotective effect of H2 in relation to neurological diseases and the associated mechanisms.
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Table 1 – The neuroprotective effect of H2 >>

Neonatal hypoxic-ischemic brain injury

Neonatal hypoxic-ischemic brain injury is a primary cause of death and disability in the perinatal period, but there is no effective treatment. However, recent studies using rodent models of neonatal hypoxia/ischemia have shown that H2 can protect neonatal brains against injuries resulting from hypoxia/reoxygenation. Several studies have demonstrated that intraperitoneal injection of hydrogen-rich saline significantly suppresses autophagy and neuroinflammation, promotes M2 microglia polarization, rescues synaptic loss, and subsequently restores behavioural deficits in a mouse model of hypoxia/ischemia [39, 40]. Similar results have been obtained with hypoxia/ischemia models in neonatal rats and piglets [37, 38]. In a rat model of neonatal hypoxic-ischemic encephalopathy, it has been demonstrated that the inhalation of H2 reduces infarct size, neuronal loss, and astrocyte activation in the cortex and the CA3 region of the hippocampus [37]. Concurrently, the behavioural reflexes of neonatal rats were significantly improved after the inhalation of H2 [86]. A study by Htun et al. shows that H2 ventilation combined with mild hypothermia improves the neurological score and gait function in a 5-day hypoxia/ischemia model involving neonatal piglets [38]. H2 possesses not only a short-term neuroprotective effect but also long-term neurological and neurobehavioral effects. 10 weeks after the hypoxic/ischemic irritation in a neonatal rat model, the administration of H2 also improves learning and memory [37]. The neurovascular unit is necessary to maintain the brain's fragile homeostasis. ROS produced during the early reoxygenation period significantly reduces cerebrovascular reactivity and induces dysfunction in the neurovascular unit. H2 preserves cerebrovascular reactivity and attenuates the development and persistence of delayed neurovascular dysfunction caused by hypoxic stress in newborn piglets [41, 87]. Collectively, these results indicate that the use of H2 can be considered a therapeutic approach for neonatal brain injury following asphyxia.

Trauma

The incidence of trauma to the brain and spinal cord is constantly increasing in today's society. Although progress has been made in the prophylactic and therapeutic treatment of trauma, restoring nerve function remains a major challenge. In animals, trauma to the brain and spinal cord causes haemorrhage, edema, cell death, and inflammatory cell infiltration, as well as increasing BBB permeability and neurological deficits. However, treatment with H2 significantly improves injuries and promotes the recovery of nerve function [7, 8, 42, 88, 89].

Treatment with H2 or hydrogen-rich saline reduces the expression of caspase-3, caspase-9, and Bax, increases the expression of Bcl-2, and significantly attenuates neuronal apoptosis following a mechanical injury [8, 42]. In addition, treatment with H2 also reduces oxidative products, e.g., MDA, 8-iso-prostaglandin F2a, 8-hydroxydeoxyguanosine (8-OHdG), and carbonyl protein, increases endogenous antioxidant enzyme activity (SOD, CAT, and GPx), suppresses the levels of MPO, NOX2, and NOX4, and elevates Sir2, PrxIII, Trx2, and CGRP in connection with contused brain and spinal cord [42, 45, 90, 91]. Furthermore, Dohi et al. have demonstrated that treatment with hydrogen-rich water reverses the expression of genes involved in oxidative stress, carbohydrate metabolism, and neuroinflammation following a traumatic brain injury [43]. Moreover, treatment with hydrogen-rich saline controls the inflammatory process in the brain tissue of rats with a traumatic brain injury by reducing the levels of proinflammatory cytokines (TNFa, IL-1b, and HMGB-1) as well as the number of inflammatory cells (Iba1) and inflammatory metabolites (Cho) [88, 89]. In addition, H2 molecules apparently can even suppress reactive astrogliosis, which is linked to oxidative injuries in the spinal cord [44]. In the contused spinal cord of rats, hydrogen-rich saline remarkably attenuates the local release of proinflammatory cytokines and the production of specific markers (STAT3, p-STAT3, and GFAP) as expressed by astrocytes and suppresses astrogliosis [44].

Survivors of traumatic brain injury often experience cognitive impairment, including reduced learning and memory. These deficits can be reversed through treatment with hydrogen-rich saline, as indicated by improved cognitive performance in the Morris water maze following mild traumatic brain injuries in the presence of hydrogen-rich saline [91]. Hydrogen-rich saline can improve cognitive deficits after trauma by maintaining synaptic plasticity. In rats with a traumatic brain injury, hydrogen-rich saline significantly increases the level of brain-derived neurotrophic factor (BDNF), calcium/calmodulin-dependent protein kinase II, synapsin I, and cyclic AMP response element-binding (CREB) protein in the hippocampus [91]. These molecules are involved in mediating synaptic plasticity and cognition. Generally, H2 not only attenuates traumatic brain injuries via the gas's general antioxidant, anti-inflammatory, and anti-apoptotic effects, but also attenuates cognitive damage induced by traumatic brain injuries by improving neuronal synaptic plasticity.

Subarachnoid haemorrhage (SAH)

Subarachnoid haemorrhage (SAH) is a severe cerebrovascular event with high morbidity and mortality, as well as a poor prognosis. Oxidative stress is a key factor involved in the pathogenesis of early brain injury after SAH. An antioxidant treatment that includes the regulation of free radicals is therefore effective for its treatment. H2 is a promising therapeutic method for patients with SAH at an early stage. Several animal studies have shown that treatment with hydrogen-rich saline significantly attenuates early brain injury 24 hours after SAH [9, 10]. Similarly, inhalation of 1.3% – 2.9% H2 for 2 hours after an intracerebral haemorrhage (ICH) in rats attenuates brain edema, maintains BBB integrity, reduces apoptosis and neuroinflammation, and improves neurological function, with reduced production of MDA, nitro tyrosine, and 8-OHG in the brain [46, 92]. However, H2 has a neuroprotective effect 24 hours after SAH (acute phase), but not after 72 hours (delayed phase) [46]. Interestingly, Manaenko et al. have observed that inhalation of 2% H2 for 1 hour significantly reduces brain water content and improves neurological outcome, whereas inhalation of H2 for 2 hours has no effect 24 hours (acute phase) after ICH [93]. In addition, 72 hours after ICH, inhalation of H2 is more likely, though not to a significant degree, to improve neurological deficits. In contrast, treatment with hydrogen-rich saline attenuates the increased levels of MDA, caspase-12, and caspase-3, and significantly attenuates brain injury and edema 72 hours after SAH in rabbits [49]. Based on these reports, H2 may have a dose- and time-dependent effect in connection with SAH. Furthermore, H2 can improve cerebral vasospasm, which is a common complication in patients with SAH. In rats, hydrogen-rich saline attenuates neurological functional deficits and morphological vasospasms in the basilar artery after SAH [47].

Nerve pain (Neuropathic pain)

Nerve pain is a type of pain associated with injury or disease affecting the somatosensory nervous system, which significantly impacts the patient's quality of life. Nerve pain is troublesome and extremely challenging to treat. Even more importantly, treatment with H2 can be a therapeutic approach to relieving nerve pain in connection with various pathological conditions, including opioid-induced hyperalgesia, spinal cord injuries, and postherpetic neuralgia.

Postoperative hyperalgesia Remifentanil, which is a potent, short-acting, synthetic opioid analgesic, is used as an adjunct to an anaesthetic agent during surgery for pain relief. Hyperalgesia is a side effect of intraoperative analgesia administration with remifentanil. During the development of opioid-induced hyperalgesia, NMDA receptor NR1 and NR2B subunit membrane trafficking increases in the spinal cord, and this trafficking is mediated via the activation of glycogen synthase kinase-3b (GSK-3b) [51]. Zhang et al. have observed that remifentanil infusions induce rapid and prolonged mechanical and thermal hyperalgesia and promote NR1 membrane trafficking as well as GSK-3b activation in the dorsal root ganglion (DRG) [51]. Even more importantly, treatment with hydrogen-rich saline partially attenuates remifentanil-induced hyperalgesia without affecting the baseline of the nociceptive threshold, decreases the expression of inflammatory mediators (TNF-a, IL-1b, and IL-6), and suppresses NR1 membrane trafficking via inhibition of GSK-3b activity in the DRG in a dose-dependent manner [51]. In a rat model of incisional postoperative pain, the production of ONOO- increases in the spinal cord after the administration of remifentanil. ONOO- activates divalent metal transporter 1 without an iron-responsive element [DMT1(-)IRE] and induces abnormal iron accumulation, leading to the development of hyperalgesia [50]. Intraperitoneal delivery of hydrogen-rich saline can remove ONOO- from the spinal cord, protect against remifentanil-induced postoperative hyperalgesia, and attenuate DMT1(-)IRE activation and iron accumulation [50]. In addition, post-treatment with hydrogen-rich saline attenuates the postoperative mechanical and thermal hyperalgesia induced by remifentanil in connection with incisional pain in rats, prohibits NR2B expression and membrane trafficking from the intracellular pool to the surface pool, and blocks MnSOD nitration in the dorsal horn [52]. These results indicate that hydrogen-rich saline has an antihyperalgesic effect, possibly via the inhibition of oxidative stress and the GSK-3b-NMDA-MnSOD signalling pathway (Fig. 2).

Fig. 2: The protective effect of H2 against nerve pain and the related mechanisms.

The result of a study regarding nerve pain induced by L5 spinal nerve ligation (L5 SNL) in a rat model shows that intrathecal infusion of hydrogen-rich saline alleviates L5 SNL-induced mechanical allodynia and thermal hyperalgesia and provides a relatively long-lasting preventive effect [25]. In chronic constriction injury-induced damage in another rat model of nerve pain, intrathecal or intraperitoneal injection of hydrogen-rich saline also significantly increases the mechanical withdrawal threshold and thermal withdrawal latency for nerve pain [11, 53]. In a mouse model of partial sciatic nerve ligation, oral intake of water with H2 also significantly attenuates mechanical allodynia and thermal hyperalgesia [12]. In addition to oxidative stress, pain is also triggered by inflammation and the activation of immune cells and glial cells in the DRG [94]. The analgesic effect of H2 may possibly also be linked to the suppression of inflammation and oxidative stress. Administration of H2 inhibits the activation of spinal astrocytes and microglia, reverses the overexpression of proinflammatory cytokines (IL-1b, TNF-a, and HMGB), and lowers the level of tyrosine-nitrated MnSOD, MDA, protein carbonyl, 8-hydroxyguanosine (8-OHG), 8-OHdG, HNE, and MPO (Fig. 2) [12, 25, 53, 54]. In a rat model of sciatic nerve trunk ligation, treatment with hydrogen-rich saline attenuates pain, lowers the level of proinflammatory cytokines, and increases HO-1 protein expression and activity in the DRG and spinal cord. Concurrently, the effect of H2 is reversed by the HO-1 inhibitor SnPP-IX and further enhanced by hemin and CORM-2. Chen et al. have therefore indicated that HO-1/CO signalling is involved in the analgesic and anti-inflammatory effect of H2 in connection with nerve pain [54].

Parkinson's disease

Parkinson's is the second most common neurodegenerative disorder, and it primarily affects the motor system. The disease is characterized by progressive degeneration of dopaminergic neurons in the pars compacta of the substantia nigra. To date, several animal studies and a handful of human trials have shown that H2 has an attenuating effect on the development of Parkinson's.

A randomised, double-blind, placebo-controlled trial involving 18 patients found that patients who drank hydrogen-rich water for 48 weeks achieved an improvement in the total UPDRS score (Unified Parkinson’s Disease Rating Scale), while the UPDRS score in patients who drank placebo water worsened. The preliminary results indicate that intake of hydrogen-rich water is safe and well-tolerated. In addition, it has a significant impact on Parkinson's disease [23]. However, another clinical trial involving 20 patients with Parkinson's showed that inhalation of 1.2% – 1.4% H2 for 10 minutes twice a day for 4 weeks had no beneficial effect [95]. And unfortunately, intake of hydrogen-rich water for 72 weeks also did not improve the total UPDRS score in 178 patients with Parkinson's in connection with a randomized, double-blind multicentre trial [96]. Although there were no significant differences in the change of the score between the group that drank hydrogen-rich water and the placebo group in this trial, the trial once again demonstrated the safety of consuming hydrogen-rich water [96]. These negative results may be due to the severity of Parkinson's disease, the stage of the UPDRS score, the H2 concentration in the hydrogen-rich water, or the duration of H2 inhalation.

In 6-hydroxydopamine or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) rodent models of Parkinson's disease, administration of H2 is effective in inhibiting the development and progression of the disease [16, 55–57]. Further analysis has shown that the therapeutic effect of H2 in models of Parkinson's disease may be linked to the prevention of loss of dopaminergic neurons in the substantia nigra, as well as a decrease in the production of 4-HNE in the SN dopaminergic neurons and accumulation of 8-oxoguanine in the striatum (Fig. 3) [16, 55, 57].

Fig. 3: The neuroprotective effect of H2 in Parkinson's.

H2 is one of the primary intestinal gases produced and utilized by the microbiota in the intestines. The gut microbiota is closely linked to Parkinson's [97]. This means that the brain-gut axis relationship during treatment with H2 has become an interesting topic. Matsumoto et al. reported that oral intake of hydrogen-rich water significantly increases gastric ghrelin expression in the stomach as well as plasma ghrelin levels in mice. Even more importantly, activation of the b1-adrenergic receptor is required for H2 to induce an increase in plasma ghrelin [58]. Consequently, studies have shown that the intake of hydrogen-rich water can improve the pathological process of Parkinson's disease via activation of the gastric ghrelin system (Fig. 3) [58]. Ghrelin knockout (KO) mice have been used to further validate the contribution of ghrelin in the mouse model of H2-treated Parkinson's disease. Yoshii et al. have observed that the administration of hydrogen-rich water significantly reduces the loss of dopaminergic cells in ghrelin-KO mice with MPTP insult [15]. In addition, the administration of D-Lys3-GHRP-6 is not effective in inhibiting the neuroprotective effect of H2 in ghrelin-KO mice with Parkinson's, unlike in wild-type mice [15]. They therefore suggested that ghrelin is not the sole factor associated with the H2-induced neuroprotective effect in connection with Parkinson's (Fig. 3) [15], and more studies are needed to investigate the stomach-brain relationship during treatment with H2. Mikako et al. have observed that the intake of the H2-producing precursor lactulose (a substrate for microbial fermentation) increases the concentration of H2 in breath and very minorly improves motor deficits in a rat model of Parkinson's [56]. It appears that the gut microbiota mediates gut-brain communication via microbial metabolites, including H2 [98], or H2 might modulate dysbiosis in the gut microbiota in connection with Parkinson's (Fig. 3).

Cognitive dysfunction (Alzheimer's disease)

Alzheimer's disease is the most common neurodegenerative disorder in the world and is the primary cause of dementia in the elderly. Women are affected more frequently than men. Treatment with H2 may be a potential choice for patients with this disease, as it has been reported that H2 improves cognitive impairment in animal models of the disease. In many circles, it is believed that the accumulation of Ab-deposits is the fundamental cause of Alzheimer's. Intracerebral injection of Ab1-42 in male rats increases the level of oxidative stress in brain tissue, which is indicated by an increased level of MDA and 8-OHdG [59, 60]. In a transgenic mouse model of Alzheimer's, administration of H2 has a protective effect [13, 14]. Intake of hydrogen-rich water significantly reduces the level of MDA and improves the activity of T-SOD and GSH in APP/PS1 mice [13]. In mice with 3 9 Tg Alzheimer's disease, an intracerebral injection of PdH nanoparticles (a high-payload H2 carrier) effectively regulates OH, reduces Ab-generation and aggregation, improves mitochondrial dysfunction, reverses synaptic deficits, and inhibits neuronal death in the brain [14]. In vitro, treatment with H2 enhances the antioxidant system in human neuroblastoma SK-N-MC cells under Ab-stimulated oxidative stress via stimulation of AMPK and upregulation of the downstream Sirt1-FoxO3a axis. This prevents mitochondrial dysfunction, thereby ultimately maintaining cell survival (Fig. 4) [61].

H2 improves cognitive impairment in rodent models of Alzheimer's, not only via the reduction of oxidative stress but also via the suppression of the inflammatory response in the brain. Administration of H2 effectively prevents extensive neuroinflammation in Ab1-42-challenged mice, with suppression of astrocyte activation and inhibition of proinflammatory factors (IL-1b, IL-6, and TNF-a) in the brain [59, 60]. In APP/PS1 mice, the intake of water with H2 reduces the levels of mRNA for IL-6 and TNF-a in the brain (Fig. 4) [13]. This indicates an anti-inflammatory effect of H2.

Interestingly, H2 has a sex-specific cognitive benefit in APP/PS1 mice without altering Ab clearance and APP processing. Oral hydrogen-rich water is effective in spatially improving learning deficits, impaired memory, and oestrogen (E2) levels in both brain and serum, as well as increasing the expression of ERb, BDNF, and TrkB in the brain of female APP/PS1 mice, but not male APP/PS1 mice. This is consistent with recent studies showing that female 3 9 Tg-AD mice exhibit more pronounced amyloid plaque, neurofibrillary tangles, neuroinflammation, and spatial cognitive deficits than male 3 9 Tg-AD-mice [99]. These data indicate that the mechanism underlying the beneficial effect of H2 is likely via the E2-$ER\beta$-BDNF signalling pathway (Fig. 4) [13].

Fig. 4: Molecular mechanisms of H2 in relation to the improvement of Ab-induced Alzheimer's disease.

Using a mouse model of premature ovarian failure induced by zona pellucida 3, He et al. observed that hydrogen-rich water in the concentration of Ab42 and APP, nor did it increase the protein expression of NEP and IDE in the cerebral cortex [13]. These phenomena indicate that H2 does not affect Ab clearance and APP processing in APP/PS1 mice. In other words, H2 may improve cognitive impairment but may not inhibit or reverse disease progression.

In addition to improving cognitive dysfunction in Alzheimer's, H2 also attenuates cognitive impairment induced by surgery, isoflurane anaesthesia, vascular dementia, hypoxia, radiation, stress, status epilepticus, and aging [39, 78, 101–108], which are not reviewed here.

Mood disorders

Mood disorders (e.g., anxiety and depression) are common but serious mental health problems worldwide, which have a negative consequence on people's mood, daily life, work, and social communication. Most of the available anti-anxiety and antidepressant medications are generally safe and effective. However, > 50% of patients do not achieve complete remission after medical treatment. Additionally, these drugs have side effects and warnings regarding their use.

However, the precise mechanism of why H2 prevents the decline in E2 is unknown and requires further investigation. Overexpression of BDNF, TrkB, and synaptic proteins (postsynaptic density 95, synapsin I, and synaptophysin) has been reported [13, 14] in mouse models of Alzheimer's after treatment with H2, indicating that H2 potentially has a neuroprotective effect via the restoration of neuronal plasticity. In APP/PS1 mice, researchers also found that oral hydrogen-rich water did not lead to a decrease.

In recent years, animal studies have shown that H2 has an anxiolytic and antidepressant effect. Inhalation of 67% H2 or ingestion of 4 ml of oral hydrogen-rich water daily significantly prevents depressive and anxiety-like behaviour in mice exposed to chronic mild stress (CMS) [17, 62], a rodent model of depression [109]. On the other hand, treatment with H2 suppresses the increase of IL-1b, caspase-1, and ROS in the hippocampus and the prefrontal cortex of depressed mice [62]. Conversely, administration of H2 also reduces serum levels of corticosterone (CORT), ACTH, IL-6, and TNF-a in CMS-challenged mice [17]. Furthermore, H2 potentially has a long-lasting effect on stress resilience in mice, as H2 inhalation in young animals significantly increases resilience to acute stress in early adulthood [17]. Anxiety is a negative psychological consequence of opioid withdrawal, which can be attenuated through the administration of H2. In a morphine-dependent mouse model of naloxone-accelerated withdrawal, Wen et al. found that administration of hydrogen-rich saline not only significantly reduces body weight loss, jumping behaviour, and wet-dog shakes, but also suppresses anxiety-like behaviour in mice after naloxone-accelerated withdrawal or a spontaneous withdrawal period [63]. Treatment with hydrogen-rich saline even reverses the hyperactivity of the hypothalamic-pituitary-adrenal axis induced by morphine withdrawal and inhibits the increase of CORT and cortisol levels in plasma. In a mouse model of autism, pre- and post-administration of hydrogen-rich water significantly improves valproic acid-induced anxiety-like behaviour and lowers serum levels of IL-6 and TNF-a in mouse pups [64].

For the purpose of further validating the unique role that H2 plays in relation to emotional regulation, a double-blind, placebo-controlled study was conducted with 31 adult volunteers aged 20–49 years. Mizuno et al. observed that the ingestion of hydrogen-rich water for 4 weeks significantly reduces (post- versus pre-treatment) the scores for K6 (mood and anxiety), the Chalder Fatigue Scale (severity of fatigue), as well as the Pittsburgh Sleep Quality Index (score for overall sleep quality and daytime sleepiness) [18]. Additionally, the changes in the post-/pre-treatment ratio for the K6 score and low-frequency component power (an activity test for sympathetic nerves) were significantly lower in the group that drank hydrogen-rich water than in the placebo group. This indicates that the administration of hydrogen-rich water may be an effective method to enhance quality of life and maintain good health by improving mood, anxiety, and autonomic nervous system function in daily life.

Mechanisms behind the H2-induced Neuroprotective effect

Several potential mechanisms may be involved in the neuroprotective effect that H2 exerts against neurological disease: antioxidant, anti-inflammatory, anti-apoptosis, regulation of autophagy, preservation of mitochondrial function, and preservation of BBB integrity.

The Antioxidant effect of H2

Many diseases are linked to damage resulting from oxidative stress, which originates from an imbalance between the production and regulation of ROS. The extensive production of peroxides and free radicals can damage cellular components, including proteins, lipids, and DNA. H2 is widely recognized as an antioxidant through the direct neutralization of cytotoxic OH and ONOO- (Fig. 5B), while it does not affect other forms of ROS, such as superoxide anion (O2-), nitric oxide (NO), and hydrogen peroxide (H2O2), all of which play a physiological role as signalling molecules [3]. Currently, some researchers suggest that H2 may exert an antioxidant effect by acting upstream of ROS generation. ROS primarily originate from the electron transport chain during the process of oxidative phosphorylation (Fig. 5). At the same time, mitochondrial complex I has close homology and an evolutionary relationship with [NiFe]-hydrogenases. Toru Ishibashi therefore proposed that the function of H2 may be as a rectifier of the mitochondrial electron flow to convert ubiquinone to ubiquinol, thereby suppressing extensive electron leakage and ROS generation [110]. A recent study conducted by Ishibashi observed that H2 suppresses superoxide generation in complex I and reduces the mitochondrial membrane potential (DWm) [111]. Concurrently, Gvozdjáková et al. also reported that the ingestion of hydrogen-rich water increases the level of mitochondrial ATP production via complex I and II substrates and increases the level of mitochondrial oxidized coenzyme Q (ubiquinone) in rat heart tissue [112]. The latest results from Professor Ma's research team demonstrated that the developmental activity of H2 in eukaryotic mitochondria is closely linked to complex I, and the activity occurs around the site of the completely oxidized ubiquinone binding [113]. Furthermore, they have also observed that H2 significantly increases the activity of mitochondrial complex I under hypoxic conditions [114]. These results thereby reinforce the hypothesis that H2 can control ROS generation at the source by acting as a rectifier of the mitochondrial electron flow in the Q-chamber (Fig. 5A).

Fig. 5: Hypotheses on the antioxidant mechanisms of H2. A The new hypothesis that H2 suppresses ROS generation by acting as a rectifier of the mitochondrial electron flow in the ubiquinone pool (Q). B The traditional ‘‘regulation theory’’ that H2 directly neutralizes OH and ONOO- produced by the mitochondrial respiratory chain. The mitochondrial complexes I–V are labelled I, II, III, IV, and V. QH2, ubiquinol. HOONO, peroxynitrous acid. NO2, nitrogen dioxide.

Apart from the gas's properties as a free radical regulator and rectifier of the mitochondrial respiratory chain, H2 can also increase the body's resistance to extensive oxidative stress by improving the levels of antioxidant enzymes and modulating the expression of redox-related genes. Treatment with H2 increases the activity of endogenous antioxidant enzymes in the brain and spinal cord, such as SOD, CAT, and GPx [29, 47, 115–117]. Additionally, H2 can regulate the expression of antioxidant genes. Through RNA sequencing and RT-PCR analysis, Chen et al. found that the expression of genes associated with oxidative stress, including Cox8b, Cox6a2, Cox7a1, Hspb7, and Atp2a1, is significantly downregulated after H2 treatment in mice with spinal cord injuries [42]. In a model of hypoxia/reoxygenation injury, the administration of H2 significantly promotes the expression of HO-1 and nNOS [36, 115]. Strong expression of HO-1 has also been reported after treatment with hydrogen-rich saline in a rat model of sciatic nerve trunk ligation [54]. Nrf2 is an essential factor for the endogenous antioxidant system, which plays multiple neuroprotective roles in connection with several neurological diseases by regulating the production of numerous cytoprotective proteins [118]. Yuan et al. have reported that the administration of hydrogen-rich saline increases Nrf2 expression, promotes translocation of Nrf2 from the cytoplasm to the nucleus, and increases the expression of downstream factors, such as HO-1 and NQO1 [7].

At the same time, increased expression of p-p38 MAPK, HO-1, and Nrf2 has been observed with H2 treatment in several in vitro and in vivo experiments [37, 119]. Therefore, treatment with H2 possibly induces adaptive responses to oxidative stress in the nervous system by evoking the Nrf2 antioxidant defence system.

Anti-inflammatory effect of H2

Extensive studies have led to solid evidence that H2 exerts an anti-inflammatory effect in relation to various diseases of the nervous system. Treatment with H2 significantly suppresses microglial activation and secretion of pro-inflammatory cytokines (IL-1b, IL-6, TNF-a, and HMGB-1) in animal models of several neurological diseases [5, 26, 120, 121]. In vitro, pretreatment with an H2-enriched medium significantly inhibits astrocyte hypertrophy and proliferation, attenuates the expression of GFAP, and weakens the increased secretion of pro-inflammatory cytokines (IL-1b, IL-6, and TNF-a) in primary astrocytes following H2O2-induced damage [44]. H2 not only effectively inhibits the expression of pro-inflammatory factors but also increases the immunosuppressive cytokines IL-10, TGF-b, and YM-1 in the brain [39, 88]. The M1/M2 polarization of microglia is an important participant in neuroinflammation [122]. Ning et al. have reported that treatment with H2 markedly inhibits the proportion of M1 microglia but does not affect M2 microglia in vitro [123]. Chu et al. have noted the phenomenon that the number of M1 microglia is significantly reduced, and the number of M2 microglia is significantly increased in the cortex after treatment with hydrogen-rich saline in mice with hypoxic/ischemic irritation [39]. Furthermore, fewer amoeboid/round microglia and more intermediate microglia are obtained in mice receiving H2 in a high concentration [5]. This suggests that H2 can suppress neuroinflammation by inhibiting microglial activation and regulating microglial polarisation.

The dysregulation of NF-jB has been linked to neuroinflammation. Some studies have indicated that H2 plays an anti-inflammatory role through the inhibition of NF-jB activation. In rodents with hypoxia/ischemia, treatment with hydrogen-rich saline significantly promotes the activation of AMPK, inhibits the activation of NF-jB, and leads to the downregulation of miR-21 and miR-210 in the brain [6, 39]. In a rat model of subarachnoid haemorrhage, treatment with hydrogen-rich saline reduces the protein levels of p-IjBα and nuclear p65, as well as increases the level of the cytosolic enzyme p65 [10, 48]. In a rat model of Alzheimer's, an intraventricular injection of hydrogen-rich saline inhibits the activation of JNK and NF-jB in the hippocampus [60]. Additionally, the ingestion of hydrogen-rich water significantly attenuates the activation of the NLRP3 inflammasome and reduces the expression of its downstream signalling molecules (cleaved caspase-1 and IL-1β) in the brains of female APP/PS1 mice [13]. Treatment with hydrogen-rich saline after a subarachnoid haemorrhage also lowers the protein expression of NLRP3, ASC, caspase-1, and IL-1β, and cleaves caspase-3 in the cerebral cortex of rats [10]. These results indicate that in addition to the modulation of NF-jB signalling pathways, H2 can also attenuate the inflammatory response by suppressing the formation of the NLRP3 inflammasome.
Oxidative stress can induce cell damage and promote inflammation in diseases of the CNS. It has been demonstrated that extensive ROS stimulate the expression of transcription factors such as NF-jB and promote the secretion of IL-1β via the activation of the NLRP3 inflammasome [124, 125]. In fact, the anti-inflammatory effect of H2 usually parallels its antioxidant effect [26, 126]. Therefore, the anti-inflammatory effect of H2 may also be due to the mechanism of alteration in gene expression caused by ROS.

The Antiapoptotic effect of H2

In addition to the antioxidant and anti-inflammatory effects of H2, the gas also exerts an antiapoptotic effect in relation to nerve injuries. H2 suppresses the expression of Bax, caspase-3, and caspase-12, promotes the expression of Bcl-2 and Bcl-xL, and increases the Bcl-2/Bax ratio in the brain [9, 37, 42, 48, 101, 103, 107]. Furthermore, treatment with hydrogen-rich saline significantly attenuates the loss of motor neurons and inhibits the release of mitochondrial cytochrome c (Cyt c) and the activation of downstream caspase-9 and caspase-3 [127]. Further research has shown that treatment with H2 significantly lowers the expression levels of Akt, GSK3b, p-Akt, and p-GSK3b in brain tissue and cerebral microvascular endothelial cells (CMECs) following hypoxia/reoxygenation [33]. Other studies have arrived at similar results, showing that treatment with hydrogen-rich saline increases the phosphorylation of Akt and GSK3b after a subarachnoid haemorrhage. Additionally, these beneficial effects of H2 are abolished by Ly294002, which is a selective inhibitor of the PI3K signalling pathway [9, 33]. These data suggest that H2 potentially protects the brain against apoptosis via the Akt/GSK3b signalling pathway.

Regulation of autophagy

Some studies have recently revealed that H2 may exert a neuroprotective effect via the regulation of the autophagy signalling pathway. In a rat model of neuropathic pain, Wang et al. found that treatment with hydrogen-rich saline attenuates hyperalgesia and activates autophagy [11]. Additionally, an intraperitoneal injection of hydrogen-rich saline significantly upregulates Beclin-1, HIF1a, and BNIP3 mRNA and protein expression, downregulates p62 mRNA and protein expression, and increases the number of autophagosomes and autolysosomes in the spinal cord [11]. In a rat model of postherpetic neuralgia, intraperitoneal injection of hydrogen-rich saline relieves neuralgia and activates autophagy by upregulating the expression of LC3, Beclin-1, and p62 [128]. Furthermore, in connection with neonatal hypoxic/ischemic brain injuries, Bai et al. have reported that the administration of hydrogen-rich saline increases the LC3II/LC3I ratio, increases Beclin-1 protein expression, and lowers the levels of phosphorylated mTOR, Stat3, and ERK in the damaged cortex [40]. Studies have shown that H2 can not only promote autophagy but also inhibit it. In a rat model of vascular dementia, ingestion of hydrogen-rich water reduces the number of autophagosomes, which is accompanied by the downregulation of FoxO1 and Atg7 expression levels, attenuation of the LC3-II/I ratio, and upregulation of the p62 level [101].

Preservation of mitochondrial function

The production of ROS primarily occurs in the mitochondria, and normally it is also affected. Researchers have indicated that hydrogen-rich saline can attenuate neuronal damage, possibly via the preservation of mitochondrial function. Cui et al. observed that hydrogen-rich saline lowers the degree of mitochondrial swelling and ultrastructural disruption, maintains the integrity of the mitochondrial membrane, and preserves the loss of DWm and Cyt c release in the hippocampus of rats with an ischemia/reperfusion injury [35]. In mouse models of amyotrophic lateral sclerosis, hydrogen-rich saline attenuates the release of mitochondrial Cyt c, restores activity in complexes I and IV, suppresses the formation of ROS in mitochondria, and promotes the production of mitochondrial ATP [127]. Mechanical injuries damage mitochondrial structure and function, leading to mPTP (mitochondrial permeability transition pore) opening and ATP loss in neurons. However, the effect of the injury is reversed by treatment with H2 [42, 43]. In human neuroblastoma SH-SY5Y cells, pretreatment with H2 promotes mitochondrial activity, indicating an increase in DWm, cellular ATP concentration, and O2 consumption rate [119]. The mPTP is one of the direct targets of ROS [129]. In this study, the neuroprotection provided by H2 may therefore possibly be linked to the inhibition of mPTP opening. Previous research indicates that the activation of mitochondrial ATP-sensitive K? (mitoKATP) channels protects neurons from damage and death caused by ischemia/reperfusion [130]. Zhou et al. hypothesized that hydrogen-rich saline also activates the mitoKATP channels, as the beneficial effect of hydrogen-rich saline on ischemia/reperfusion in the spinal cord is partially reversed by 5-hydroxydecanoate, which is a selective mitoKATP channel antagonist [116]. Meanwhile, there is a need for further validation of the relationship between H2 and the mPTP or mitoKATP channels.

Preservation of the BBB

The blood-brain barrier (BBB) is a highly selective and organized structure consisting of endothelial cells, pericytes, astrocytes, neurons, and extracellular matrix. It is also known as the neurovascular unit and is considered the gatekeeper of the central nervous system [131]. Disruption of the BBB is primarily caused by ischemia/reperfusion, spontaneously hypertensive stroke, and mechanical trauma-induced nervous system injuries, and H2 has been observed to attenuate BBB dysfunction [73, 82, 88, 90]. Takeuchi et al. found that oral ingestion of hydrogen-rich water reduces the number of 8-OHdG-positive cells and vessels with blood-infused albumin in the hippocampus of stroke-prone spontaneously hypertensive rats [73]. Matrix metalloproteinases (MMPs) are important factors regarding BBB disruption. In animals with traumatic brain injuries, ingestion of hydrogen-rich water potentially exerts a protective effect against edema and BBB disruption by suppressing the decrease in AQP-4 and MMP2 levels and inhibiting the increase in HIF-1 and MMP-9 levels after cortical impact [43]. Additionally, brain-derived CMECs play an important role regarding the BBB, and treatment with H2 inhibits CMEC apoptosis after hypoxia/reoxygenation [33].

Conclusions and perspectives

It is reported that H2 has multiple biological properties, including antioxidant, anti-inflammatory, and antiapoptotic effects, as well as protecting mitochondria and the BBB. Oxidative stress modulates the expression of many different genes that affect numerous biological reactions, such as apoptosis, autophagy, and the anti-inflammatory response. This means that the antioxidant effect of H2 may be the gas's most fundamental property. Currently, there are two hypotheses regarding the mechanisms behind the antioxidant effect of H2. The generally accepted hypothesis is that H2 directly reacts with OH and ONOO-, i.e., the traditional ‘‘regulation theory’’. Others believe that H2 suppresses the generation of ROS by acting as a rectifier of the mitochondrial electron flow. Although several recent publications contain some preliminary evidence for the new hypothesis, more convincing evidence is needed.

The neuroprotective effect of H2 has been demonstrated through a steadily growing body of evidence from animal studies. The gas's effect has also been reported in many clinical trials, particularly regarding cerebral ischemia, sequelae of cardiac arrest, and Parkinson's disease. Although the protective effect of H2 has rapidly progressed from numerous fundamental studies to preliminary clinical application in recent years, more work is needed regarding clinical applications. Countless studies have shown that H2 does not have a toxic effect, but side effects have been reported in a few cases, including diarrhoea, heartburn, and headache [84, 132]. Furthermore, Wang et al. observed that H2 supplementation significantly increases the activity of hydrogenase in Helicobacter pylori. More importantly, higher hydrogenase activity and hydrogen metabolism in H. pylori can induce colorectal cancer by promoting the translocation of the carcinogenic factor CagA into host cells [133]. This indicates that H2 can potentially increase the pathogenicity of pathogens and promote the development of diseases in specific cases. This means that the indications for and potential side effects of H2 cannot be ignored in clinical use.

In some clinical trials of Parkinson's disease, the administration of H2 did not have any beneficial effect, which is why the protective effect of H2 is somehow limited under certain pathological conditions. As previously mentioned, the distribution of H2 in tissues and organs fluctuates according to the method of administration, thereby affecting the gas's biomedical effect. This suggests that the limited effect of H2 may be linked to the concentration, duration, and method of administration of H2, in addition to the stage of the disease. Therefore, further studies are needed to investigate the pharmacokinetics and dose-response relationship of H2 and subsequently develop more effective delivery methods for H2.

Overall, and despite the limited side effects and negative results reported by some studies, H2 could potentially be a promising new treatment modality for several diseases of the nervous system.

Acknowledgments

This review was supported by the National Natural Science Foundation of China (81770855 and 81773717), the Taishan Scholarship from the Government of Shandong Province (ts201511057), and the High-Level Talent Training Program by Taishan Medical University (2018GCC08). We owe a great debt of gratitude to Nikoli Peacher from West Virginia University for assistance with editing and proofreading.

Conflict of Interest

The authors declare that there is no conflict of interest.

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