stroke is a kind of acute cerebrovascular disease characterized by high morbidity , disability and death rates .
it is reported that stroke has become one of the most frequently - occurring neurological injuries in usa and china , about 800,000 and 2,000,000 people every year respectively ( roger et al . , 2011 ) .
stroke includes ischemic stroke and hemorrhagic stroke , and a large number of clinical studies and animal experiments have shown that the cerebral injury and ischemia - reperfusion injury caused by stroke are complex physiopathologic processes , involving toxic effects of excitatory amino acids , inflammatory reaction , oxidative stress , the generation of free radical , brain edema , neuronal apoptosis / death ( zhu et al . , 2015 ) .
although the mechanisms of the injury are diverse , our treatment theory of neuroprotective strategies mainly focus on two aspects : on the one hand , we can activate the endogenous protective mechanisms of patients bodies to protect the brain from injury ; on the other hand , it is advisable to try our best to minimize secondary brain injury and promote the recovery of the damaged tissues . for many years , people have been trying to understand and search effective treatments for stroke from every conceivable angle . for now , for ischemic stroke , thrombolytic and anticoagulant therapy has become regular treatments ( mandava et al . , 2015 ) , and for hemorrhagic stroke , hematomas cleaning operation and the stable control of intracranial pressure are conductive to improve the prognosis of patients ( siler et al . , 2014 ) . in addition
, hypothermia has been considered to be a valuable clinical treatment for centuries ( thome et al . , 2005 ) , and
mild hypothermia ( 33 - 36c ) has been demonstrated to play neuroprotective role ( antonic et al .
its mechanisms mainly involve decreasing the metabolism rates of brain tissues and alleviating brain edema ( hobbs et al . , 2008 ) .
besides , a variety of methods are used for the treatment of stroke , including stroke unit , neuronal electrophysiological treatment , and even the possibility of psychological therapy ( pohjasvaara et al . , 1998 ; patel et al . , 2004 ; sheffler and chae , 2007 ) .
our previous studies suggested that pramipexole - induced hypothermia could effectively inhibit subarachnoid hemorrhage - induced early brain injury in rats via phosphoinositide 3-kinase / protein kinase b / glycogen synthase kinase-3 signaling pathway ( ma et al . , 2016 ) .
however , the present theories for stroke are still unsatisfactory and needed to be further improved , although the development of evidence - based medicine is ongoing .
specifically , in recent years , medical gas , including oxygen and hydrogen , have become issues of concern .
no matter ischemic stroke or hemorrhagic stroke , the final outcome is hypoxic - ischemic damage of brain tissues , hyperbaric oxygen therapy can significantly increase the oxygen pressure and blood oxygenation content , accelerate collateral circulation to protect neurons , repair the damaged microvessels , stimulate angiogenesis and neurogenesis , and accordingly , play neuroprotective role ( gill and bell , 2004 ; sanchez , 2013 ) .
hydrogen has also been found play roles of anti - inflammation , anti - oxygenation and anti - apoptosis when a stroke occurs .
hydrogen sulfide ( h2s ) has always been considered as a toxic gas with its odor of rotten eggs , because it could play the cytotoxic role through inhibiting cytochrome c oxidase and mitochondrial respiration ( furne et al . , 2001 ;
have suggesting that h2s is an another important gaseous signaling molecule in biological systems following nitric oxide ( no ) and carbon monoxide and in central nervous system ( cns ) ( wang , 2002 ; li and moore , 2008 ) , a larger number of researches have shown that it has neuroprotective effects as neurotransmitter ( kimura and kimura , 2004 ; whiteman et al . , 2004 ) , and
researchers hope to provide a new direction for the therapy of stroke through studying h2s . after the endogenous h2s
was first found in the brain in the end of 20 century ( warenycia et al . , 1989 ) , researchers gradually found that the production of h2s in cns predominantly involve three particular enzymes : cystathionine--synthase ( cbs ) , cystathionine--lyase ( cse ) and 3-mercaptopyruvate sulfurtransferase ( 3-mst ) , which could convert to h2s from l - cysteine ( cys ) in combination with cysteine aminotransferase ( cat ) ( kolluru et al .
, 2013 ; jung and jeong , 2014 ) . that is to say , cse
primarily induce the production of h2s in cardiovascular system , while cbs mainly is responsible for producing h2s in the cns ( qu et al .
, 2008 ; kimura et al . , 2012 ) , and cbs uses cys and homocysteine ( hcy ) that have been known the risk factors for stroke and heart disease as substrates ( abe and kimura , 1996 ; chen et al .
interestingly , although there are amounting evidences showing that h2s participates in the regulation of neuronal function and signaling pathway , concerning the neuroprotective or neurotoxic effects of h2s still remains controversial . in our review , we will discuss the main mechanism and the possible role of h2s in stroke .
because of its toxicity , people have always thought that the concentration of h2s in animal tissues is very low for a long time .
however , as what we have known , h2s could be produced endogenously , and further studies indicate that h2s could exist in mammalian tissues at very high concentration , up to 50160 mm ( goodwin et al . , 1989 ; warenycia et al . ,
1989 ) , which is dramatically higher than that in the peripheral blood ( 046 mm ) ( zhao et al . , 2003 ) , suggesting that h2s may have some effects and show potential therapeutic value in some cns diseases , including stroke , traumatic brain injury , alzheimer 's disease and parkinson 's disease . here
, we give a brief description on the mechanisms of h2s in the cns as follows ( figure 1 ) : ( 1 ) anti - oxidation : facing the oxidative stress injury after stroke occurring , h2s could promote the activation of cystine / glutamate antiporter and increase the intracellular concentrations of cys that it is a kind of substrate for the generation of glutathione .
in addition , in this process , -glutamyl - cysteine synthetase , a kind of rate - limiting enzyme , could also be activated by h2s and regulates the production of glutathione ( kimura and kimura , 2004 ; kimura et al . , 2010 ) , which is an important intracellular antioxidant .
thus , the intracellular levels of glutathione increases and could clear the reactive oxygen species ( ros ) existing in the mitochondria to achieve the goal of protecting the neurons from oxidative stress injury .
( 2 ) anti - inflammatory effects : the major sites that h2s plays the role of fighting inflammatory response are astrocytes and microglia ( seifert and pennypacker , 2014 ) , where h2s could inhibit the release of no , tumor necrosis factor- and the proinflammatory cytokine interleukin-1 , and decrease the activation of p38/c - jun n - terminal kinase , conversely , increase the release of anti - inflammatory cytokines , for instance , interleukin-4/10 .
it is reported that sb 203580 , a kind of p38 mitogen - activated protein kinase ( mapk ) inhibitor , could mimic the anti - inflammatory effects , indicating that h2s may regulate the immune function by mapk signaling pathway ( hu et al . , 2007 ) , but concrete mechanisms still need to be investigated .
( 3 ) anti - apoptosis : a growing body of evidence points that h2s exerts its role of anti - apoptosis probably via mitochondrial apoptotic pathway , h2s could inhibit the caspase-3 signaling pathway mediated by ros and calcium pathway activated by hydrogen peroxide ( liu et al . ,
2014 ) , accordingly , prevent the apoptosis induced by oxygen glucose deprivation / reoxygenation ( luo et al . , 2012 , 2013 ) .
in addition , h2s could promote the nuclear translocation of nuclear factor kappa b ( nf-b ) , which activates the anti - apoptotic gene ( sen et al . , 2012 ) .
( 4 ) facilitating long - term potentiation : it has always been demonstrated that h2s can facilitate hippocampal long - term potentiation via activating n - methyl - d - aspartic acid ( nmda ) receptor ( kimura , 2002 ) .
subsequently , researchers found that unlike carbon monoxide or no , h2s promotes the accumulation of cyclic adenosine monophosphate rather than cyclic guanosine monophosphate ( kimura , 2000 ) , and nmda receptor activation induced by h2s could be inhibited by adenylate cyclase inhibitor ( kimura , 2002 ) .
( 5 ) regulating calcium concentration : h2s can activate calcium channel to increase calcium influx , whereas decrease the release of calcium stored in the cells , thus induce the production of calcium waves in primary cultures of astrocytes , and this process can be inhibited by calcium channel antagonist ( nagai et al .
in addition , h2s has been known as an atp sensitive potassium ( katp ) channel activator in secondary brain injury after stroke or other diseases , h2s could play its neuroprotection through mimicking katp channel ( jiang et al . ,
however , the problem of calcium overload can not be ignored ( obryant et al . , 2014 ) .
there are accumulating evidence that the neuroprotection or neurotoxicity of h2s depends on the concentration of it , which needs further investigations , including animals and clinical trials .
note : h2s : hydrogen sulfide ; cns : central nervous system ; cbs : cystathionine--synthase ; cse : cystathionine--lyase ; 3-mst : 3-mercaptopyruvate sulfurtransferase ; -gcs : -glutamyl - cysteine synthetase ; ros : reactive oxygen species ; no : nitric oxide ; tnf- : tumor necrosis factor- ; il-4/10 : interleukin-4/10 ; jnk : c - jun n - terminal kinase ; nf-b : nuclear factor kappa b ; ltp : long - term potentiation .
nmda : n - methyl - d - aspartic acid ; katp : atp sensitive potassium .
as we have known , a large number of animal studies must be tested before clinical application for anything , h2s is no exception . in view of the toxicity and high solubility of h2s
, it is even uncommon to inhale it directly ; therefore , sodium hydrosulfide ( nahs , a h2s donor ) is widely applied in animal studies concerning cerebral injury after stroke . at present , researchers gradually reach a consensus that the neuroprotection of h2s in stroke is dose - dependent , that is , the relatively low - dose of h2s in brain may play neuroprotective effect in stroke , whereas high concentration of h2s may cause neurotoxicity , suggesting that we must view h2s from a comprehensive perspective , and enhance its advantage , while avoid its disadvantage , to apply h2s in clinical treatment more effectually . in our review , we will show our analysis for animal studies about the role h2s in stroke ( table 1 ) .
the neuroprotective and neurotoxic effects of hydrogen sulfide ( h2s ) in stroke the neuroprotection of h2s is exhibited as follows : ( 1 ) li et al .
( 2011 ) reported that h2s , whose concentration is 0.1 mm , has neuroprotective effects by protecting hippocampal neurons , improving the deficiency of learning and memory , and increasing synaptic plasticity , decreasing the brain edema around pyramidal neurons and their nuclear shrink in a rat brain model of stroke .
( 2 ) in a rat model of reversible occlusion of the right middle cerebral artery , h2s was found to reduce infarct volume through long - term hypothermia and the decrease of the expression of nf-b , thus protecting the brain against hypoxic injury due to stroke ( florian et al . , 2008 ) .
( 3 ) after stroke , the endogenous concentration of h2s increases at 12 hours and decreases at 24 hours .
in addition , if pretreated by nahs at a relatively low dose ( 25 mmol / kg ) , the brain injury could be attenuated at 7 days after stroke ( ren et al . , 2010 ) .
nahs is also reported to protect brain against oxygen glucose deprivation injury ( tay et al . ,
( 4 ) by stimulating the phosphorylation of protein kinase b and extracellular signal regulated kinases , up - regulating vascular endothelial growth factor and angiopoietin-1 expression , h2s could promote angiogenesis around the infarct area , and show better functional outcomes in a rat model of middle cerebral artery occlusion ( mcao ) ( jang et al . , 2014 ) .
( 5 ) h2s has anti - oxidative , anti - inflammatory , and anti - apoptotic effects in stroke ; and in cerebral ischemia / reperfusion injury , it is also reported h2s could increase the superoxide dismutase activity , anti - inflammatory interlukin-10 and anti - apoptotic protein bcl-2 , accordingly , play the role of neuroprotection ( yin et al . , 2013 ) .
in addition , h2s , or its donor , can reduce the infarct size and decrease the blood brain barrier damage in a model of cerebral ischemia - reperfusion injury ( gheibi et al . ,
2014 ) . however , different models , administration paradigms and concentration of h2s may cause different results , even get the opposite conclusion : ( 1 ) using the same model of mcao , qu et al . (
2006 ) found that if the h2s concentration is raised to 0.18 mm , the infarct volume will increase , instead of decrease , and this process can be ameliorated by h2s synthesis inhibitors or nmda receptor channel inhibitors , suggesting that h2s has different effects on brain in different dose - concentration .
( 2 ) the concentration of h2s increases in the cortex , and reach the peak at 12 hours after cerebral ischemia , then it will decrease ( ren et al . , 2010 ) .
a recent research showed that h2s , at a high dose level , can cause cell death in cerebral cortex based on the model of permanent mcao , and in this process , the infarct volume can be reduced by the inhibitors of h2s synthesis enzymes , on the contrary , decreased by the administration of nahs ( qu et al . , 2006 ) .
( 3 ) in addition , when the concentration of nahs is below 200 m , the apoptosis can be induced , while the concentration is higher than 200 m , nahs will induce necrosis and cause brain damage ( cheung et al . , 2007 ) .
in general , the neuroprotective effect of h2s in stroke is concentration - dependent , only a relatively low dose of h2s can yield beneficial effect .
however , there is a point we have to say , the most of research on h2s at present mainly focus on ischemic stroke , while study concerning the hemorrhagic stroke is very few , which need us to do more research to reveal the role of h2s in entire stroke .
at present , there is no direct clinical studies about neuroprotection of h2s being reported .
however , the plasma h2s level has been reported to be related to long - term clinical outcome in stroke patients . a clinical study involving 36 patients showed that high plasma cys levels on admission and before treatment tend to poor clinical outcome when assessed at 3 months after stroke , and the patients who have early stroke deterioration are usually correlated significantly with high plasma level of cys ( wong et al . , 2006 ) , suggesting that h2s may be indirectly responsible for this effect .
furthermore , as accumulating evidence shows that hyper hcy is a risk factor for stroke ( abbate et al . , 2003 ; kim et al . , 2003 ; hassan et al . ,
( 2004 ) found that the concentration of hcy increased in all stroke subtypes in a case - control study including 313 participants ( 161 patients and 152 controls ) , indicating that hcy may be used as a marker for the clinical outcome of stroke as well .
however , the definite relationship between hyper hcy and stroke has not been found ( fallon et al . , 2001 ) , and several meta - analyses
have not reached an agreement ( homocysteine studies collaboration , 2002 ; fallon and ben - shlomo , 2003 ) . above all , although there is no direct evidence indicating that h2s has neuroprotective effect in clinical trials , the close touch with brain injury caused by stroke is no doubt , and needs us to make great efforts to study .
as what we have mentioned above , the high dose - concentration of h2s can cause systemic toxicity reacts ( nakata et al . , 2015 ;
wu et al . , 2015 ) , especially neurotoxic effect , because it inhibits cytochrome c oxidase and mitochondrial respiration .
the cns is sensitive to hypoxia , h2s poisoning can lead to respiratory paralysis rapidly and cause a series of nerve dysfunction , including brain edema , disturbance of consciousness , and others . therefore , the following strategies are extremely important , including oxygen uptake , keeping the airway open , and symptomatic and supportive treatments .
although h2s is known as a toxic substance , increasing evidences have indicated that h2s plays an important role of neuroprotection in stroke , while its neurotoxicity can not be neglected .
so , we need to view h2s in the point of dialectics , and we believe that further research on h2s , including animal studies and clinical studies , may provide a new insight into the treatment of stroke and other cns diseases , such as traumatic brain injury and neurodegenerative diseases .
 stroke is a kind of acute cerebrovascular disease characterized by the focal lack of neurological function , including ischemic stroke and hemorrhagic stroke . 
 as society ages rapidly , stroke has become the second leading cause of disability and death , and also become the main threat to human health and life . in recent years 
 , findings from increasing animal and clinical trials have supplied scientific evidences for the treatment of stroke . 
 hydrogen sulfide ( h2s ) , which has always been seen as a toxic gas , now has been thought to be the third gaseous signaling molecule following nitric oxide and carbon monoxide . 
 accumulating evidences indicate that h2s plays an important role in stroke . given that its neuroprotective effect is dose - dependent , 
 only when its concentration is relatively low , h2s can yield the neuroprotection , while high dose may lead to neurotoxicity . 
 all these study results suggest that h2s may offer a new promising application for the therapy of stroke . here 
 , our review will present the role of h2s in stroke from its mechanism to animal and clinical studies . 