Table 1.

Summary of preclinical studies of inhaled gases in rodents.

References	Gas	Age species	Procedure	Intervention	Outcomes	Behavioral
David et al. (25)	Ar	Adult male rats	a) In vitro: neuronal cell slices were subjected to OGD b) In vivo: injection of NMDA into right striatum c) In vivo: MCAO transient ischemia for 1 h	a) OGD: Reperfusion and exposure to 25, 37.5, 50, or 75% Ar b) NMDA: 1 h after injection of NMDA rats were treated with either medical air (control), or 15, 25, 37.5, 50, or 75% Ar balanced with O2 or N2 c) 3 h post MCAO, rats were treated with either medical air (control) or 50% Ar	a) Less LDH production in Ar group b) Rats treated with 37.5 and 50% Ar had a significant reduction of neuronal death compared to NMDA control animals treated with medical air c) In MCAO, maximal neuroprotection by post-insult 50% Ar. When given 1 h after reperfusion, Ar reduced cortical volumes of brain damage but increased subcortical brain damage compared to control rats treated with air	Behavioral outcome assessed only for MCAO study rats. Rats treated with 50%Ar did not show improvement of neurologic deficits compared to control rats treated with medical air
Loetscher et al. (26)	Ar	Neonatal mice	Hippocampal slice cultures subjected to either OGD or a focal mechanical trauma	25, 50, or 74% Ar immediately after trauma or with a 2 or 3 h delay	a) Ar is neuroprotective in both O2 deprivation and mechanical brain trauma even when therapy is delayed for 3 h b) No difference in O2 deprivation between Ar concentrations while 50% Ar had best outcomes in mechanical brain trauma	N/A
Ryang et al. (27)	Ar	Adult rats	MCAO 2 h	Two groups:a) 50% Ar + 50% O2 (Ar group) b) 50% N2 + 50% O2 (control group) 1 h via face mask	Ar group had a significant reduction in the overall, cortical, and basal ganglia infarct volumes. Ar treatment resulted in a significant improvement of the composite adverse outcome	N/A
Zhao et al. (10)	Ar	Neonatal rats	a) In vitro: Cortical neuronal cell cultures challenged by OGD for 90 min b) In vivo: Rice-Vannucci HI model	a) Following OGD, cortical cells exposed to 70% Ar or N2 with 5% CO2 balanced with O2 at 33°C for 2 h b) Following HI: neonatal rats exposed to 70% Ar or N2 balanced with O2 at 33, 35, and 37°C for 2 h. Cortical and hippocampal infarction size was assessed at 4 weeks after treatment	a) Ar-HT increased phospho-Akt and HO1 expression and reduced the Tyr216-GSK-3β expression, cytochrome C release, and cell death in OGD-exposed cortical neurons b) Ar–HT treatment decreased infarct size and activated both caspase-3 and NF-κB in the cortex and hippocampus c) Ar-HT reduced hippocampal astrocyte activation and proliferation d) Ar-HT inhibited the PI3K/Akt pathway through LY294002 attenuated cerebral protection	N/A
Zhuang et al. (28)	Ar, He, Xe	Neonatal rats	Rice-Vannucci HI model	2 h after HI insult, animals were randomly exposed 90 min to:a) Ar, He, Xe (70% noble gas balanced with O2), or b) N2 (control group, 70% N2 balanced with O2)	Ar improved cell survival to naïve levels, whereas Xe and He did not. When tested against more severe HI injury only, Ar and Xe reduced infarct volume. Ar, He, and Xe increased the expression of Bcl-2, whereas He and Xe increased Bcl-xL. In addition, Bax expression was enhanced in the control and He groups	N/A
Cheng et al. (29)	CO	Neonatal mice	None	For 3 h:a) 0 ppm CO (air), b) 5 ppm CO in air, or c) 100 ppm CO in air	a) 3 h exposure to 5 or 100 ppm CO impaired cytochrome-c release, caspase-3 activation, and apoptosis in the neocortex and hippocampus b) CO increased NeuN protein and neuronal numbers and resulted in megalencephaly	4–5 weeks post exposure, mice underwent Morris water maze, and social approach-avoidance assay. Escape latency was significantly longer on day 1 and 2 of testing in both CO-exposed cohorts compared to controls. Air-exposed controls showed a marked increase in approach- avoidance score when the stimulus mouse was placed in the chamber
Queiroga et al. (30)	CO	Adult and neonatal rats	Apoptosis of astrocytes induced by diamide	Preconditioning with CO prior to apoptosis induction	CO prevented membrane depolarization induced by calcium and inhibited mitochondrial swelling. CO prevented membrane pore formation by increasing ANT activity	N/A
Queiroga et al. (31)	CO	Neonatal rats	Rice-Vannucci HI model	a) Sham surgery without hypoxia exposure (Control group) b) CO exposure prior to Sham surgery without hypoxia exposure (CO+Sham group) c) HI group, d) CO exposure prior to HI, (CO+HI group)	a) CO decreased apoptosis and increased Bcl-2 mRNA in primary cultures of neurons b) CO decreased apoptosis in the hippocampus, limited cytochrome-c released from mitochondria and reduced activation of caspase-3	N/A
Wang et al. (32)	CO	Adult male mice	Permanent MCAO	Administered for 18 h immediately after permanent MCAO:a) 250 ppm CO b) control air	a) Less brain damage than controls at 7 days b) 18 h CO treatment led to NRF2 dissociation from Keap1, nuclear translocation, increased binding activity of NRF2 to HO1 ARE, and elevated HO1 expression 6–48 h after CO exposure c) Loss of CO neuroprotection in NRF2 knock-out mice	N/A
Zeynalov and Doré (33)	CO	Adult mice	90 min MCAO	a) 125 ppm normal air or 250 ppm CO at onset of reperfusion b) 250 ppm CO inhalation 1 and 3 h after reperfusion	CO inhalation reduced infarct size and decreased brain edema	Improved neurological deficit scores at 48 h of survival time after ischemia
Liu et al. (34)	CO	Neonatal rats	Rice-Vannucci HI model	Eight groups:a) Sham b) Sham + electroacupuncture c) HIE d) HIE + electroacupuncture e) HIE + SAM f) HIBD + SAM + electroacupuncture g) HIE + HA h) HIE + HA + electroacupuncture	a) Electroacupuncture significantly downregulated the expression of nNOS and NF-κB in the rat cortex cells and alleviated cortical atrophy caused by HIE b) Increased intrinsic CO levels via overexpression of hemoxygenase-1 (HO1) in the cortex and was associated with alleviated cortical damage	N/A
Kohzuki et al. (35)	CO2	Neonatal rats	Rice-Vannucci HI model	6% CO2 delivered during HI	Smaller brain infarct size in rats exposed to CO2	Staircase test at 3 months of age showed improved forelimb strength in rats exposed to CO2
Vannucc et al. (36)	CO2	Neonatal rats	Rice-Vannucci HI model	a) 0% (hypocapnia) b) 3% (normocapnia) c) 6% CO2 (hypercapnia), all with 8% O2, balanced with N2	a) Hypocapnia was associated with decreased cerebral blood flow b) Normocapnia and hypercapnia groups showed preservation of cerebral blood flow in addition to maintaining higher cerebral glucose and lower lactate concentrations	N/A
Vannucc et al. (37)	CO2	Neonatal rats	Rice-Vannucci HI model	3, 12, or 15% CO2 for 2 h at 37°C	a) Inhaled 12 and 15% CO2 was associated with blood CO2 tension of 80 and 100 mmHg, respectively b) Extreme hypercapnia (15% CO2) was associated with severe brain infarcts following HI insult and significant reduction of cerebral blood flow when compared to the other groups	N/A
Vannucc et al. (38)	CO2	Neonatal rats	Rice-Vannucci HI model	0, 3, 6, or 9% CO2	a) Mild hypercapnia (6% CO2) had the lowest brain atrophy followed by normocapnia (3% CO2), while hypocapnia (0% CO2) had the worst damage b) CO2 exposure was associated with lower lactate levels and improved glucose concentrations by preventing hypoglycemia	N/A
Cai et al. (39)	H2	Neonatal rats	Rice-Vannucci HI model	Neonatal rats: Intra-ischemic treatment and post-ischemic treatmenta) Sham b) HI c) HI+ H2 Adult rats: a) MCAO b) MCAO+ H2,	H2 treatment significantly reduced the number of positive TUNEL cells and suppressed caspase-3 and−12 activities	N/A
Matchett et al. (40)	H2	Neonatal and adult rats	Neonatal rats: Modified Rice-Vannucci HI model 120 and 150 min Adult rats: MCAO	Neonatal HI:a) Sham b) Sham + intra-ischemia H2 × 2 h c) HI only d) HI+ Preconditioning with H2 × 1.5 h e) HI+ post-ischemia H2 × 1 h Adult MCAO: a) Sham b) HI only c) HI+ intra-ischemia H2 × 2.5 h d) HI+ post-ischemia H2 × 1 h e) HI + post-operative-ischemia x 1.5 h	a) H2 therapy in neonatal HI was not associated with decreased volume of infarction or decreased concentration of MDA b) H2 pretreatment was associated with increased infarction volume in neonatal HI c) Exposure of H2 to non-ischemic neonates was associated with a significant increase in brain concentration of MDA	N/A
Ohsawa et al. (41)	H2	Adult rats	In vitro: neuronal cell culture treated with a) antimycin A and menadione b) OGD In vivo: Adult male rats underwent MCAO x 90 min followed by reperfusion x 30 min	Four groups:a) 1% H2/30% O2/69% N2O b) 2% H2/30% O2/68% N2O c) 4% H2/30% O2/66% N2O 0% H2/30% O2/70% N2O H2 was administered either for the entire procedure (120 min), last 35 min, or first 85 min	a) Decreased oxidative stress and infarction b) Reduction of cytotoxic radicals c) Induced cytoprotective factors that prolong cell life	N/A
Li et al. (42)	He	Neonatal rats	Rice-Vannucci HI model	a) Normal control group b) He-Preconditioning group (70% He/30% O2 for three 5-min periods) c) HIE group d) He-Preconditioning + HIE group e) L-NAME + HIE group f) L- NAME + He-Preconditioning + HIE group	a) Reduction in infarct size and increase in NO content in the brain b) Increased anti-oxidase expression and DNA binding activity of NRF-2. Suggests that preconditioning promotes intrinsic NO production that is neuroprotective	Tested at 3 weeks. He-preconditioning group had better scores in the wire hang and beam balance tests compared to the HIE group.
Pan et al. (43)	He	Adult male rats	MCAO for 2 h and a 1 h reperfusion	During stroke and reperfusion rats were subjected to either:a) 30% O2/70% N2 (control group) b) 100% O2 (hyperoxia group), OR c) 30% O2 /70% He (heliox group)	a) Infarct volume in the heliox group was smaller than the hyperoxia and control groups b) Neurologic scores in the heliox group were significantly better compared with controls	Neurologic scores 1 and 24 h post MCAO in the heliox group were significantly better compared with controls using Hunter neurological scores
Li et al. (44)	NO	Adult mice	Transient MCAO for 1 h followed by reperfusion for 47 h	Inhaled NO at 10, 20, 40, 60, or 80 ppm. Each group was subdivided into four duration groups for 5, 8, 16, or 24 h beginning immediately after MCAO	a) At 10 ppm only the 24 h duration group exhibited reduced infarct volume b) At 20, 40, and 60 ppm, only 8 and 16 h of exposure led to smaller infarctions c) 60 ppm iNO could be transported from lung to brain d) iNO administered for 8 h improved recovery from subarachnoid hemorrhage and reduced the inflammatory response accompanying ischemic stroke	N/A
Lu et al. (45)	NO	Neonatal rats	a) Neuronal cells underwent OGD × 90 min b) Modified Rice-Vannucci HI model	No intervention	a) OGD increased NO generation b) HA disrupted iron regulation c) Hydroxyl radicals and iron deposition were ameliorated by NOS- inhibited aconitase activity and lead to cellular iron accumulation	N/A
Terpolilli et al. (46)	NO	Adult male rats, Adult male mice, and Neonatal mice	a) Adult rats underwent MCAO b) Adult mice underwent bilateral carotid banding c) Neonatal mice ligature of the left common carotid artery, 1 h later, hypoxia for 50 min with or without addition of 50 ppm NO to the gas mixture	50 ppm NO	a) In adult mice, inhaled NO enhanced blood flow during reperfusion and reduced inflammation b) Pups that received iNO at the time of hypoxic injury had smaller infarct volume and lower histopathological scores compared to controls	N/A
Zhu et al. (47)	NO	Neonatal mice	Rice-Vannucci HI model	50 ppm NO mixed with N2 vs. N2 alone for 50 min	Reduced brain injury and tissue loss. Only male mice had significant reduction of infarct size	N/A
Calvert et al. (48)	O2	Neonatal rats	Rice-Vannucci HI model	Three groups:a) HI b) HI and HBO; 100% O2 at 3 ATA for 1 h (following recovery for 1 h) c) Control (no anesthesia, carotid ligation, hypoxia, or HBO exposure)	HBO decreased caspase-3 activity, PARP cleavage and DNA fragmentation. HBO preserved brain weight.	N/A
Dalen et al. (49)	O2	Neonatal rats	Rice-Vannucci HA model	HI followed by 30 min reoxygenation in 21% O2 or 100% O2 before 5 h of NT (37°C) or HT (32°C)	Reoxygenation with 100% O2 increased hippocampal injury score and negated HT neuroprotection	Reoxygenation with 100% O2 worsened reflex performance in staircase test
Smit et al. (50)	O2	Neonatal rats	Modified Rice-Vannucci HI model	Soon after HI, pups underwent immediate resuscitation in either 21 or 100% O2 for 30 min	No significant change in brain atrophy	No change in short-term neurologic outcome
Dingley et al. (51)	Xe	Neonatal rats	Rice-Vannucci HI model	After HI insult, 3 h inhalation ofa) 50% Xe/30% O2/20% N2 b) 30% O2/70% N2	a) One week after HI survival, significant global protection in the Xe group (80% less injury) b) Percentage of global damage score in non-Xe vs. Xe groups: cortex/white matter (88 vs. 25%); hippocampus (62 vs. 0%); basal ganglia (81 vs. 25%); and thalamus (50 vs. 0%), respectively	N/A
Hobbs et al. (12)	Xe	Neonatal rats	Rice-Vannucci HI model	a) NT 37°C b) HT32°C c) Xe 50% + NT37°C d) Xe 50% + HT32°C	a) Xe 50% + HT32°C produced the greatest improvement (71%) in global histopathology scores. The overall effect of HT and Xe is additive) Xe 50% and HT32°C individually produced smaller improvements	Staircase testing (long-term) was performed from 8 to 11 weeks. Results: a) Xe 50% + HT32°C group: complete restoration of long-term functional outcomes b) Untreated pups (NT37°C) reached an early performance plateau of staircase test c) The Xe 50%+NT37°C or HT32°C only groups, had mild improvement in staircase testing
Luo et al. (52)	Xe	Neonatal mice	a) In vitro: OGD 1 h after preconditioning of cells for 2 h b) In vivo: 4 h after preconditioning for 120 min, mice underwent Rice-Vannucci HI model	In vitro: Neuronal cells were preconditioned for 120 min with 12.5, 25, 50, or 75% Xe, 0.67–3.3% sevoflurane, or a combination of both In vivo: Neonatal mice were preconditioned with 20 or 75% Xe, 0.75 or 1.5% sevoflurane, or 20% Xe plus 0.75% sevoflurane in 25% O2 balanced with N2	a) In vitro: Preconditioning with Xe for 2 h produces a concentration-dependent reduction in LDH release after OGD. LDH release was significantly reduced by Xe concentrations of 50 and 75% b) In vivo: Although sevoflurane alone did not reduce infarct size in Rice-Vannucci model of neonatal HI injury, Xe alone or in combination with sevoflurane reduced infarct size and improved neuromotor function at 30 days with slight advantage of the second group	N/A
Ma et al. (53)	Xe	Adult mice	24 h after preconditioning: a) bilateral renal pedicle clamping for 25 min or b) right renal pedicle clamping for 40 min and left nephrectomy c) Sham-operated mice had dissection as above but with no occlusion of the renal vessel	Preconditioning for 3 h with 70% Xe, 70% N2, or 70% N2O balanced with 30% O2 for 2 h or 8% O2 balanced with N2	Xe preconditioning protects against intermittent renal ischemia through increased expression of HIF-1α Knockout mice lost protective effect	N/A
Ma et al. (54)	Xe	Neonatal rats	In vitro: cell culture OGD In vivo: Rice-Vannucci HA model	In vitro: Neuronal culture cells were preconditioned with Xe for 2 h. In vivo:a) Treatment group was preconditioned with 70% Xe+30% O2 for 120 min b) Control group was exposed to 70% N2O balanced with O2 for 120 min	a) Concentration-dependent reduction of LDH release from cells with OGD 24 h later b) Preconditioning with Xe decreased propidium iodide staining in a hippocampal slice culture model subjected to OGD c) Preconditioning with Xe reduced infarction size when assessed 7 days after injury d) pCREB was increased by Xe exposure	N/A
Martin et al. (13)	Xe	Neonatal rats	Rice-Vannucci HI model	After a 1 h recovery period, rats received asynchronous administration ofa) Mild HT (35.8°C) and 20% Xe with a 1- or 5-h gap between interventions b) 20% Xe alone c) Mild HT (35.8°C) alone	a) Brain infarct was reduced by Xe and HT administration at 1 h intervals after 90 min of HI b) Administration of 20% Xe after 6 h of HI and 2-h administration of 35.8°C HT 1 h following the HI insult was not neuroprotective	N/A
Sabir et al. (14)	Xe	Neonatal rats	Rice-Vannucci HI model	a) Immediate NT-37°C for 5 h b) Immediate HT-32°C: Trectal 32°C for 5 h c) Immediate HT-32°C plus 50% inhaled Xe for 5 h	a) No difference in neuronal cell count in the subventricular zone among different treatment groups b) No reduction in brain area loss in either the HT group or in the HT plus 50% inhaled Xe group	N/A
Sabir et al. (55)	Xe	Neonatal rats	Rice-Vannucci HI model	a) ${\text{NT}}_{37^{°}\text{C}}$, ${\text{HT}}_{35^{°}\text{C}}$, or ${\text{HT}}_{32^{°}\text{C}}$ or Xe concentrations (0, 20, or 50%) starting immediately or with a 4-h delay b) P7 pups were exposed to ${\text{HT}}_{35^{°}\text{C}}$, NT+Xe20% or ${\text{HT}}_{35^{°}\text{C}}$+Xe20% starting with a 4-h delay after the insult	a) Immediate HT to 32°C but not 35°C significantly reduced infarct size b) Synergistic effect when Xe immediately added to ${\text{HT}}_{32^{°}\text{C}}$	N/A
Thoresen et al. (11)	Xe	Neonatal rats	Rice-Vannucci HA model	a) 1 h of 50% Xe during the first h of HT (1 h immediate Xe) plus 3 h HT b) 1 h of 50% Xe during the last h of HT (1 h delayed Xe) plus 3 h HT COMPARED TO c) 3 h of both 50% Xe and HT	a) Combination of Xe + HT is superior to HT alone with the best results in 3 h of both Xe and HT group b) Longer duration of Xe administration combined with HT was more effective (3 h better than 1 h of Xe) c) No difference between early or late administration of Xe (immediate vs. delayed)	Female rats showed better motor skills than males
Yang et al. (56)	Xe	Neonatal rats (in utero)	Fetal Asphyxia: Dams were sacrificed by cervical dislocation and uterine horns were removed and placed in a water bath at 37°C for 10 min to induce HI insult 4 h after gas exposure	Pregnant rats were preconditioned at day 22 of gestation with either 0.35% sevoflurane or 35% Xe from 9 a.m. until the pup was delivered	Compared to the control group, Xe, and sevoflurane preconditioned pups had lower caspase 3 levels and higher cell viability at prenatal days 3 and 7	Xe- and sevoflurane-preconditioned pups had improved cognitive testing with Morris water maze at postnatal day 50