transdermal glyceryl trinitrate is safe in patients with acute stroke.glyceryl trinitrate may improve outcome if administered within 6  h of stroke onset.the implications for clinical practice are substantial if efficacy is confirmed . an inexpensive and effective treatment for hyperacute stroke could be adopted globally in low- , middle- and high - income countries . no has vasodilatory , pro - endothelial , anti - proliferative ( vascular smooth muscle cell ) , antiplatelet [ 2 , 3 ] , anti - leucocyte , anti - inflammatory , neuroprotective , neurotransmitter and neuromodulator [ 5 , 6 ] properties . brain barrier integrity , cerebral blood flow ( cbf ) , auto- and chemo - regulation [ 79 ] , and inhibition of apoptosis . no is an endogenous inorganic soluble gas synthesised from l - arginine by three forms of no synthase ( nos ) : endothelial ( enos ) ; inducible ( inos ) ; and neuronal ( nnos ) [ 11 , 12 ] . the second messenger cyclic guanosine monophosphate , which is broken down by phosphodiesterase , is the main mediator of downstream signalling of no . no is broken down via oxidation to nitrite and ultimately nitrate , and it is now apparent that no may be made by reduction of nitrite and nitrate . up - regulation of the l - arginine / nitrite - no - cyclic guanosine monophosphate pathway can be achieved by a number of means : increased substrate , administration of no gas , induction of no synthase activity ; administration of no donors ; or inhibition of phosphodiesterase . pre - clinical experimental studies in models of ischaemia have demonstrated the role of no in a time - dependent manner . models of focal ischaemia have shown that no production is increased , through nnos activation , for up to half an hour after middle cerebral artery occlusion [ 14 , 15 ] . during the first minutes following arterial occlusion , enos and nnos activity increases up regulation of inos occurs from 12  h after the onset of ischaemia and persists for up to 7  days , whilst no within brain tissue is undetectable during this period . l - arginine administered intravenously following middle cerebral artery occlusion in a rat model improved ischaemic penumbral blood flow and reduced infarct size and volume . this effect was not seen in enos - deficient mice who developed smaller penumbral regions , larger infarcts and absent angiogenesis leading to further post - ischaemic injury [ 1921 ] . therefore , enos and enos - derived no are neuroprotective in focal ischaemia , whilst nnos- and inos - derived no have deleterious effects on tissue survival with resultant poor neurological outcomes [ 15 , 22 ] . although neurotoxic in acute stroke , inos and nnos are involved in neurogenesis following stroke [ 23 , 24 ] . in therapeutic studies , no donors reduced infarct size in both permanent and transient models of ischaemia , and increased cerebral blood flow in permanent models , but only if administered soon after stroke induction . blood pressure ( bp ) is high in 75% of people with acute stroke   and is associated independently with poor functional outcome and increased death ( regardless of stroke type ) [ 27 , 28 ] , stroke recurrence in ischaemic stroke ( is )   and haematoma expansion in intracerebral haemorrhage ( ich ) . high admission bp has been associated with lower rates of recanalisation in is patients treated with thrombolysis , and with increased infarct volume and poor functional outcome in patients with large vessel occlusion . modulation of bp in acute stroke has long been debated ; treatment of raised bp in ich is recommended , and is safe in is [ 34 , 35 ] . owing to the myriad effects of no described above and the low levels of endogenous no seen in both is and ich [ 36 , 37 ] , supplementation through administration of no donors might be beneficial . hence , lowering no can lead to worse outcomes in acute stroke , whilst increasing no may be beneficial . here , we discuss the evidence to date , potential mechanisms of action and future possibilities , including unanswered questions , for the therapeutic potential of the no donor glyceryl trinitrate ( gtn ) in acute stroke . no donors can be broadly categorised into organic ( e.g. gtn ) and inorganic ( e.g. sodium nitroprusside ) nitrates , although there are many subtypes . transdermal gtn has been administered as a transdermal patch to patients with acute and subacute stroke in three phase ii trials ; gtn lowered bp ( peripheral and central ) , 24-h bp , peak systolic bp ( sbp ) , pulse pressure and pulse pressure index ; increased heart rate ; improved vascular compliance ; and did not alter cerebral blood flow and velocity , or induce cerebral steal or increase intracranial pressure ( table  1 ) [ 4145 ] . while intravenous sodium nitroprusside has antiplatelet properties , gtn had no such impact on platelet function and can therefore be administered in patients with ich . none of these earlier studies were powered for efficacy , and this was assessed in the large efficacy of nitric oxide in stroke ( enos ) trial .table  1effects of gtn in acute / subacute strokegtn-1 2001 gtn-2 2003 gtn-3 2006 right 2013 enos 2015 gtn 1 - 2 gtn 1 - 3 systolic bp ( mmhg) 13 ( 7.8%) 23 ( 14%) 21 7 9.4 9.8diastolic bp ( mmhg) 5.2 ( 5.4%) 4 ( 3%) 6 3.5 4.8 4.4heart rate ( bpm)no changeno changeno changeno change 1.7 4.1 3.9map ( mmhg) 6.2% 5.0 6.4pp ( mmhg) 3.9 16 6.1ppi 0.03rpp ( mmhg.bpm)no change 323augmentation indeximprovedimprovedcerebral blood flow velocityno changeno changecerebral blood flowno changezero flow pressureno changeplatelet functionno changegtn supplier ( 5  mg)schwarz pharmaschwarz pharma ( 5  and 10  mg)novartis ( transiderm - nitro)msd schering - plough ( nitrodur)ucb pharma ( deponit-5 ) in 38% of sites ( 29%)novartis ( transiderm - nitro ) in 28% of sites ( 34%)msd / schering - plough ( nitrodur ) in 25% of sites ( 19%)meda/3 m healthcare ( minitran ) in 10% of sites ( 3%)wuhan jianmin in 1% of sites ( 2% ) 
 bp blood pressure , bpm beats per minute , enos efficacy of nitric oxide in stroke trial , gtn glyceryl trinitrate , map mean arterial pressure , pp pulse pressure , ppi pulse pressure index , right rapid intervention with glyceryl trinitrate in hypertensive stroke trial , rpp rate pressure product ,  indicates a decrease ,  indicates an increase 
 percentage of sites for enos - early ( within 6  h ) 
  effects of gtn in acute / subacute stroke 
 bp blood pressure , bpm beats per minute , enos efficacy of nitric oxide in stroke trial , gtn glyceryl trinitrate , map mean arterial pressure , pp pulse pressure , ppi pulse pressure index , right rapid intervention with glyceryl trinitrate in hypertensive stroke trial , rpp rate pressure product ,  indicates a decrease ,  indicates an increase 
 percentage of sites for enos - early ( within 6  h ) enos enrolled 4011 participants with acute stroke ( within 48  h of onset ) and raised systolic bp ( 140220  mmhg ) and randomised these to transdermal gtn patch ( 5  mg ) or no patch . overall , there was no significant shift in functional outcome measured using the modified rankin scale at day 90 ( primary outcome , adjusted common odds ratio 1.01 , 95% confidence intervals [ ci ] 0.911.13 ) or of any secondary outcomes ; further , gtn was safe with no increased reporting of serious adverse events . gtn lowered bp by 7.0/3.5  mmhg as compared with control at day 1 . in a pre - defined subgroup by time to randomisation ( enos - early ) , those who received gtn within 6  h of stroke onset had a favourable shift in the modified rankin scale ( adjusted common odds ratio 0.51 , 95% ci 0.320.80 ) , less death , less disability ( barthel index ) , less mood disturbance ( zung depression scale ) , and improved cognition ( telephone mini - mental state examination ) and quality of life ( euro - quality of life visual analogue scale and health utility scale ) . the small ambulance - based rapid intervention with glyceryl trinitrate in hypertensive stroke trial ( right ) also found that transdermal gtn 5  mg , given in the pre - hospital setting by paramedics within 4  h of ictus , improved the modified rankin scale at day 90 . an individual patient data meta - analysis using data from the five completed gtn trials ( gtn-1/2/3 , enos , right , n  =  4197 ) supported the findings that treatment with gtn within 6  h of onset ( n  =  312 ) , but not later , improved functional outcome and secondary outcomes across a range of domains : cognition ; death ; disability ; mood ; and quality of life ( table  2 ) . the time - dependent effect on functional outcome was seen in both is and ich ; a finding supported by a subgroup analysis of participants with ich from the enos trial . those with ich who received gtn within 6  h of onset had significant improvements in functional outcome , cognition , disability , mood and quality of life at 90  days compared with those who did not receive gtn . in the aforementioned meta - analysis , those with is who received thrombolysis , given either before or after randomisation , had a significant shift to less death or dependency in the presence of gtn . a tendency to improved outcome was also seen in those with is who did not receive intravenous alteplase .table 2clinical outcomes with gtn in acute stroke all gtnrandomisation 6  hpatients ( % ) 4197312 ( 7.4)end of treatment  death1.15 ( 0.79 , 1.68)0.94 ( 0.23 , 3.81 )  neurological deterioration1.29 ( 1.00 , 1.66)0.58 ( 0.28 , 1.23 )  stroke recurrence1.39 ( 0.88 , 2.18)0.46 ( 0.14 , 1.54 )  sss ( /58)0.33 ( 0.26 , 0.91)2.33 ( 0.42 , 5.09 )  nihss 
 0.28 ( 0.70 , 0.14)2.07 ( 3.81 , 0.34 ) 
 day 7  non - oral feeding 
 0.97 ( 0.82 , 1.15)0.59 ( 0.32 , 1.08)hospital  length of stay ( days)0.07 ( 1.30 , 1.44)0.02 ( 4.59 , 4.63 )  physiotherapy 
 0.94 ( 0.79 , 1.12)0.90 ( 0.40 , 2.05 )  occupational therapy 
 1.01 ( 0.72 , 1.41)1.15 ( 0.36 , 3.68 )  speech therapy 
 0.95 ( 0.84 , 1.08)1.01 ( 0.44 , 2.29)day 90  modified rankin scale0.99 ( 0.89 , 1.10 ) 
 0.52 ( 0.34 , 0.78 ) 
 death0.87 ( 0.71 , 1.07 ) 
 0.32 ( 0.14 , 0.78 ) 
 barthel index1.73 ( 0.08 , 3.55 ) 
 9.64 ( 3.19 , 16.09 ) 
 quality of life ( hus)0 ( 0.02 , 0.02)0.05 ( 0.02 , 0.13 )  quality of life ( eq - vas)0.69 ( 1.06 , 2.43)5.97 ( 0.30 , 12.24 )  mood ( zds)0.38 ( 1.80 , 1.04)8.34 ( 13.32 , 3.36 ) 
 cognition ( t - mmse)0.34 ( 0.16 , 0.84 ) 
 2.09 ( 0.65 , 3.54 ) 
 cognition ( tics - m)0.16 ( 0.55 , 0.88 ) 
 3.56 ( 1.20 , 5.91 ) 
 cognition ( animal naming)0.06 ( 0.60 , 0.47)1.63 ( 0.13 , 3.39)data are number ( % ) , mean difference , or odds ratio with 95% confidence intervals . comparisons by binary logistic regression , ordinal logistic regression or multiple linear regression , with adjustment for trial . significant ( p  <  0.05 ) results are in bold 
 bi barthel index , eq - vas euro - quality of life visual analogue scale , gtn glyceryl trinitrate , hus health utility scale ( derived from euro - quality of life 5-dimensions ( eq5d ) ) , nihss national institutes of health stroke scale , sss scandinavian stroke scale , tics - m telephone interview cognition scale , t - mmse telephone mini - mental state examination , zds zung depression scale 
 unadjusted using the mantel  haenszel random - effects model 
  clinical outcomes with gtn in acute stroke   data are number ( % ) , mean difference , or odds ratio with 95% confidence intervals . comparisons by binary logistic regression , ordinal logistic regression or multiple linear regression , with adjustment for trial . significant ( p  <  0.05 ) results are in bold 
 bi barthel index , eq - vas euro - quality of life visual analogue scale , gtn glyceryl trinitrate , hus health utility scale ( derived from euro - quality of life 5-dimensions ( eq5d ) ) , nihss national institutes of health stroke scale , sss scandinavian stroke scale , tics - m telephone interview cognition scale , t - mmse telephone mini - mental state examination , zds zung depression scale 
 unadjusted using the mantel  haenszel random - effects model further trials are needed to confirm whether gtn is efficacious when given early in patients with acute stroke ; one study , the rapid intervention with glyceryl trinitrate in hypertensive stroke trial-2 ( right-2 ) , is assessing transdermal 5-mg gtn patch vs. sham in 850 patients with presumed stroke within 4  h of onset , sbp > 120  mmhg and fast score 2 or 3 ; paramedics perform consent , recruitment and treatment in the pre - hospital setting . if transdermal gtn is found to be beneficial in the management of acute stroke , there are several potential mechanisms through which its actions may be mediated ( table  3 ) . first , bp lowering may reduce early recurrent events in is   and haematoma expansion in ich . the ability of gtn to lower bp without reducing cbf or cerebral perfusion pressure , or increasing intracranial pressure , may be due to its vasodilatory effect thus increasing blood flow exiting the cranium . second , in addition to high sbp being associated with poor clinical outcomes in acute stroke , other haemodynamic variables including higher peak sbp , mean arterial pressure , pulse pressure , pulse pressure index and increased sbp variability are independently associated with worse functional outcome , death , recurrent stroke and early neurological deterioration [ 5254 ] . the published effects of gtn on haemodynamic measures in acute stroke are detailed in table  1 . new data on sbp variability from the four pilot studies ( gtn 1 - 3 and right ) are included in table  4 . between - visit systolic bp variability was calculated as standard deviation ( sd ) and coefficient of variation ( cov  =  sd / mean ) of sbp over days 17 across each trial individually and across all four trials as a whole . the mean difference between gtn and no gtn was calculated using analysis of covariance with adjustment for baseline sbp and trial as appropriate . gtn reduced sbp variability ( sd and cov ) over days 17 when given within 4  h in the right pre - hospital trial in both adjusted and unadjusted analyses . when pooled together , there was significant heterogeneity seen across the trials for both sd ( i 
 =  75% ) and cov ( i 
 =  79% ) . this heterogeneity likely represents the small sample sizes of each of the trials and the varying time from stroke onset to randomisation . therefore , adjustment was made for baseline sbp and trial , after which gtn significantly reduced sbp variability ( sd ) compared with no gtn . pre - specified secondary analysis of haemodynamic parameters in enos is awaited .table  3gtn in acute stroke : mechanisms of action and unanswered questionsissueprior observationscommenttimereperfusion therapies exhibit time dependency : thrombolysis   and thrombectomy apparent time - dependent effect mirrors thrombolysis and thrombectomystroke type  ischaemic strokebp lowering may reduce recurrent eventsapparent benefit : vasodilatory effects may improve blood flow in large arteries (  front door )   and pial arteries / collaterals (  back door ) ; new data awaited  intracerebral haemorrhagebp lowering may reduce haematoma expansion apparent benefit ; new data awaitedstroke syndrome  lacunarunclear effect ; further analyses and new data awaited  total anterior circulationapparent benefit : tendency for improved functional outcome in total anterior circulation in enos ; further analyses and new data awaitedstroke severity  severityapparent benefit : tendency for improved functional outcome in severe stroke in enos ; further analyses and new data awaitedsafety  carotid stenosisbp lowering might reduce perfusioninitial safety seen in all levels of ipsilateral carotid stenosis in enos ; further analyses and new data awaited  large vessel occlusionunknown ; further analyses and new data awaited  dehydration and strokelarge drops in bp may occur in dehydrated patients given antihypertensive medicationrelevance to gtn unknown ; further analyses and new data awaited  stroke mimicsno adverse effect seen in right ; new data awaitedduration of therapytachyphylaxis seen in gtn-1/2 and enos [ 41 , 42 , 47 ] . right-2 is assessing 4  days of treatment whilst planned trials will assess 1 or 2  days of treatmentroute of administrationtransdermal drugs allow easy application and removal without need for swallowing assessment or intravenous accessgtn given by transdermal patchpre - stroke bp - lowering therapyantihypertensive medication is regularly taken prior to stroke no interaction between this and gtn seen in enos haemodynamics  bp derivativesincreased map , pp , ppi , peak sbp and sbp variability are associated with poor outcomesgtn reduces map , pp , ppi , peak sbp and variability [ 44 , 55 ]  heart ratehigh heart rate   and impaired heart rate variability   are associated with worse outcomegtn increases heart rate by 24 beats per minute . tendencies to reduced rate pressure product ( rpp  =  sbp    hr ) suggest that the effect of gtn on hr is more than offset by bp reduction ; further analyses and new data awaited . effect on heart rate variability to be ascertained 
 bp blood pressure , enos efficacy of nitric oxide in stroke trial , gtn glyceryl trinitrate , map mean arterial pressure , pp pulse pressure , ppi pulse pressure index , right2 rapid intervention with glyceryl trinitrate in hypertensive stroke trial-2 , sbp systolic bp 
 table  4effect of gtn on blood pressure variability in acute / subacute stroketrialpatients ( n)otr ( h)sd sbp days 17cov sbp days 17 ( % ) unadjusted 
 p valueadjusted 
 p valueunadjusted 
 p valueadjusted 
 p valuegtn-1 37<1202.0 ( 6.4 , 2.4)0.362.7 ( 6.8 , 1.4)0.191.2 ( 3.7 , 1.3)0.321.5 ( 4.0 , 1.0)0.23gtn-2 90<720.1 ( 2.2 , 2.3)0.960.1 ( 2.0 , 2.3)0.920.2 ( 1.4 , 1.7)0.830.2 ( 1.3 , 1.7)0.81gtn-3 18<1201.2 ( 3.7 , 6.0)0.621.4 ( 3.4 , 6.3)0.542.4 ( 0.5 , 5.2)0.102.5 ( 0.5 , 5.4)0.10right 41<47.7 ( 12.1 , 3.3 ) 

 0.001 
 7.2 ( 11.5 , 2.8 ) 

 0.002 
 4.9 ( 8.0 , 1.8 ) 

 0.003 
 4.9 ( 8.1 , 1.7 ) 

 0.003 
 combined1861.6 ( 3.8 , 0.6)0.142.1 ( 3.8 , 0.4 ) 

 0.019 
 0.7 ( 2.2 , 0.7)0.311.2 ( 2.4 , 0.0)0.058data are mean difference ( 95% confidence intervals ) with adjustment for baseline sbp    trial . results are in bold 
 cov coefficient of variation , gtn glyceryl trinitrate , otr onset to randomization , right rapid intervention with glyceryl trinitrate in hypertensive stroke trial , sbp systolic blood pressure , sd standard deviation 
  gtn in acute stroke : mechanisms of action and unanswered questions 
 bp blood pressure , enos efficacy of nitric oxide in stroke trial , gtn glyceryl trinitrate , map mean arterial pressure , pp pulse pressure , ppi pulse pressure index , right2 rapid intervention with glyceryl trinitrate in hypertensive stroke trial-2 , sbp systolic bp effect of gtn on blood pressure variability in acute / subacute stroke data are mean difference ( 95% confidence intervals ) with adjustment for baseline sbp    trial . significant ( p  <  0.05 ) results are in bold 
 cov coefficient of variation , gtn glyceryl trinitrate , otr onset to randomization , right rapid intervention with glyceryl trinitrate in hypertensive stroke trial , sbp systolic blood pressure , sd standard deviation third , the time dependency of early treatment with gtn is akin to that seen in both thrombolysis and endovascular therapy , so - called reperfusion treatments [ 56 , 57 ] . as described , gtn is a potent vasodilator , which may have effects in different parts of the vascular tree : large cerebral arteries , increasing front door and peri - lesional perfusion without inducing cerebral steal ; and surface pial arteries , increasing collateral (  back door ) perfusion . fourth , in the context of thrombolysis , gtn may be synergistic through vasodilatation of the occluded or partially occluded artery , which may allow exogenous and endogenous thrombolytic compounds better access to clot . gtn also appears to prepare patients for thrombolysis by lowering systolic bp below the licensed threshold of 185  mmhg . a non - significant increase in both rates of thrombolysis and earlier treatment was seen in right . last , the neuroprotective effects of gtn mediated via no may prevent cell death from ischaemia [ 10 , 18 ] . although enos confirmed the overall safety of gtn in acute stroke , several distinct scenarios warrant further discussion ( table  3 ) . first , patients with carotid stenosis as the cause of their stroke often have high bp at presentation and whether to lower their bp is subject to debate . owing to dysfunctional cerebral autoregulation , higher bp leads to higher cerebral perfusion pressure , increasing the risk of cerebral oedema and haemorrhagic transformation of infarction , whilst bp reduction may compromise cbf extending infarction . across all levels of ipsilateral carotid artery stenosis within enos ( 2038 participants with carotid imaging data ) , however , data for patients with severe bilateral carotid stenosis are sparse , given its rarity , although a meta - analysis of three trials found that lower bp was associated with increased stroke recurrence . in this group , bp lowering should be avoided pending further data . a post - hoc analysis of the enos dataset is planned for further clarification of this important patient subgroup . second , and similarly , the advent of endovascular therapy for proximal anterior circulation vessel occlusions in acute is poses several challenges including how to manage bp without potentially compromising cbf ; the ongoing right-2 trial will include a subgroup of patients who have thrombectomy following gtn - sham treatment . third , dehydration is a common finding in patients with acute stroke and was associated with poor outcomes in a stroke registry study . bp lowering in the setting of dehydration may lead to precipitous drops in bp , which could be harmful ; the effect of gtn in this scenario is unclear and a subgroup analysis of enos may prove illuminating . last , the administration of an agent that may improve outcome when given as early as possible will inevitably mean that patients with conditions mimicking stroke will receive treatment . therefore , it is imperative that gtn is safe in this group ; a question for ongoing and future trials . as previously alluded to , gtn positively influenced several clinical outcomes when given early in both is and ich . whether gtn has the same effects in patients with lacunar syndromes , lacunar strokes or small vessel disease is unclear and further analysis is required . in addition to answering these and other questions , current and future trials will need to record data on other outcomes including thrombolysis , thrombectomy , neurosurgical procedures ( e.g. hemicraniectomy ) , feeding status , therapy usage and length of stay ( overall , intensive care unit ) , as these may be influenced by the efficacy of gtn . indeed , early gtn was associated with less nasogastric and more oral feeding in enos - early ( unpublished ) . importantly , all the trial data for gtn in acute stroke are from one group of authors ; others need to replicate and confirm these findings in differing countries , healthcare settings and stroke populations . there are few evidence - based treatments for the management of acute stroke . in acute is , intravenous thrombolysis , thrombectomy   and decompressive hemicraniectomy   each have high efficacy but low utility , whilst aspirin has high utility but low efficacy . managing patients with all stroke types in stroke units has very high utility with medium - level efficacy . in comparison , gtn has high utility , low cost (  5/$7 per patient ) , is easily administered and should be easy to implement if efficacy is confirmed . importantly , the source of the patch does not influence efficacy ; participants within enos ( including those randomised within 6  h ) received patches from different manufacturers , whilst the other gtn trials used one manufacturer to supply each trial ( table  1 ) . in the future , gtn could be used on a global scale in developing and developed countries , rural and urban areas , before and in hospital , and in a variety of healthcare settings . the possibility of an inexpensive and effective intervention for the management of acute stroke is a welcome prospect in the current economic climate . open access costs were covered by the national institute of health research ( nihr ) health technology assessment programme ( 10/104/24 ) . jpa is funded by the british heart foundation ( bhf , cs/14/4/30972 ) and national institute of health research ( nihr ) health technology assessment programme ( 10/104/24 ) . pmb was / is chief investigator of the trials involving gtn ( gtn-1/2/3 , enos , and right-1/2 ) , is the lead applicant on the bhf grant funding the right-2 trial , is stroke association professor of stroke medicine , and is a nihr senior investigator .
the nitric oxide donor , glyceryl trinitrate ( gtn ) , is a candidate treatment for the management of acute stroke with haemodynamic and potential reperfusion and neuroprotective effects . 
 when administered as a transdermal patch during the acute and subacute phases after stroke , gtn was safe , lowered blood pressure , maintained cerebral blood flow , and did not induce cerebral steal or alter functional outcome . 
 however , when given within 6  h of stroke onset , gtn reduced death and dependency ( odds ratio 0.52 ; 95% confidence interval 0.340.78 ) , death , disability , cognitive impairment and mood disturbance , and improved quality of life ( data from two trials , n  =  312 ) . in a pooled analysis of four studies ( n  =  186 ) , gtn reduced between - visit systolic blood pressure variability over days 17 compared with no gtn ( mean difference 2.09 ; 95% confidence interval 3.83 to 0.35 ; p  =  0.019 ) . 
 the efficacy of gtn given in the ultra - acute / pre - hospital setting is currently being assessed and , if found to be beneficial , the implications for hyperacute stroke practice are significant . here , we discuss the evidence to date , potential mechanisms of action and future possibilities , including unanswered questions , for the therapeutic potential of gtn in acute stroke .