even though the quantity of brain tissue accounts for only 23% of the total body weight , it requires 2025% of the total oxygen supply . because of the limited oxygen and glucose reserves , brain tissue is susceptible to hypoxic - ischemic injury
young and middle - aged patients are a high - risk population for traumatic brain injury , and cognitive impairment is one of the most common symptoms of patients with moderate and severe brain injuries .
further , hypoxic - ischemic brain injury may seriously affect quality of life and prognosis . at present ,
hyperbaric oxygen ( hbo ) is a historical therapeutic option in the treatment of various types of brain damage .
hbo functions as follows in the treatment of brain injuries : ( 1 ) by correcting local brain tissue hypoxia , the oxygen partial pressure increases nearly 20-fold and strengthens the ability of oxygen to infiltrate local brain tissue ; ( 2 ) hbo can improve the oxygen supply and blood supply in the brainstem reticular system and induce the excitability of brain cells , which plays a role in the promotion of consciousness ; ( 3 ) by eliminating cerebral edema and protecting brain cells ; and ( 4 ) by promoting the recovery of neural functionality and reducing sequelae [ 14 ] .
all procedures were in accordance with the chinese capital medical university animal welfare guidelines and approved by the institutional animal care and use committee of the chinese capital medical university .
seventy clean and healthy male sprague dawley ( sd ) rats ( 3 months old ; 275 + 25 g ) were purchased from the laboratory animal center of the academy of military medical sciences ( beijing , china ) .
the rearing conditions were as follows : temperature , 211c ; light / dark cycle,12 h/12 h ; illumination time , 19:007:00 ; sufficient moisture and food were provided daily pre - and post - operatively ; and there was an 8-hour fast before surgery .
the sd rats were randomly divided into control ( n=10 ) and intervention ( n=60 ) groups .
randomized sd rats in the intervention groups had hbo treatment for 1 , 3 , and 5 days and 1 , 2 , and 4 weeks ( n=10 in each group ) , while routine breeding resumed in the control group .
described by dixon , the models were established by ecci6.3 . abdominal anesthesia with 10% chloral hydrate ( 0.3 ml/100 g ) was prepared for bilateral frontal craniotomy surgery .
craniotomies were performed 3 mm from the center line between the anterior fontanelle and lambdoid suture .
the rats in the intervention group were put into a small high - pressured oxygen cabin for animals ( hong yuan gy3200,yantai , china).the cabin was washed with pure oxygen for 10 min , then a uniform pressurization was initiated to achieve a 0.2 mpa cabin pressure in 10 min and maintained for 30 min .
hbo therapy was performed four times during the day , each 30 min apart for 7 days continuously .
the water temperature was controlled at 281c melan was added to the water to shield the platform underwater , and to reduce the influence of positioning .
( 1 ) navigation experiment : the platform was put in 1 quadrant 2 cm beneath the surface . when heading towards the wall ,
the rats were randomly put into the other three quadrants without a platform in daily experiments . if the rat could not locate the hidden platform in 60 s , the rat would be guided to the platform and remain for 10 s continuously for 3 days .
four sessions of training were necessary every day for each rat with intervals of 1520 min .
( 2 ) space exploration experiment : the day after the navigation experiment , the rats were put into the water from the contralateral quadrant of platform one . the percentage of time spent locating the quadrant with the platform in the total time
would be recorded in 60 s. the water maze baseline was measured in all rats 1 day before modeling . the percentage of residence time during escape latency , in the target quadrant , and the total time were recorded for the water maze test 8 weeks after controlled cortical impact ( cci ) .
after the behavioral test , the rats were sacrificed and the brain tissues were removed .
the brain tissues were placed on ice ; the hippocampuses were isolated and frozen in liquid nitrogen .
three photographs magnified 200 times were randomly selected from each slice and from each group .
brain tissues should fill the entire field of vision when taking photos to ensure the background light is consistent in each photo .
image - pro plus 6.0 software was used to select the same degree of tan as a uniform standard to assign positive results in all photos , and the integrated optical density ( iod ) was analyzed and concluded in all photos .
count data are denoted by absolute values and percentages , while measurement data are recorded as the mean  standard deviation .
the results of the water maze test were determined with repetitive measurement deviation analysis , and the measurement data were compared with single factor analysis of variance by the least significant difference ( lsd ) test post hoc .
single factor variance analysis was applied in analysis of time spent in the targeted quadrant in the spatial probe test between each group , and the lsd test was applied in comparisons between each group .
all procedures were in accordance with the chinese capital medical university animal welfare guidelines and approved by the institutional animal care and use committee of the chinese capital medical university .
seventy clean and healthy male sprague dawley ( sd ) rats ( 3 months old ; 275 + 25 g ) were purchased from the laboratory animal center of the academy of military medical sciences ( beijing , china ) .
the rearing conditions were as follows : temperature , 211c ; light / dark cycle,12 h/12 h ; illumination time , 19:007:00 ; sufficient moisture and food were provided daily pre - and post - operatively ; and there was an 8-hour fast before surgery .
the sd rats were randomly divided into control ( n=10 ) and intervention ( n=60 ) groups .
randomized sd rats in the intervention groups had hbo treatment for 1 , 3 , and 5 days and 1 , 2 , and 4 weeks ( n=10 in each group ) , while routine breeding resumed in the control group .
described by dixon , the models were established by ecci6.3 . abdominal anesthesia with 10% chloral hydrate ( 0.3 ml/100 g ) was prepared for bilateral frontal craniotomy surgery .
craniotomies were performed 3 mm from the center line between the anterior fontanelle and lambdoid suture .
the rats in the intervention group were put into a small high - pressured oxygen cabin for animals ( hong yuan gy3200,yantai , china).the cabin was washed with pure oxygen for 10 min , then a uniform pressurization was initiated to achieve a 0.2 mpa cabin pressure in 10 min and maintained for 30 min .
hbo therapy was performed four times during the day , each 30 min apart for 7 days continuously .
the water temperature was controlled at 281c melan was added to the water to shield the platform underwater , and to reduce the influence of positioning .
( 1 ) navigation experiment : the platform was put in 1 quadrant 2 cm beneath the surface . when heading towards the wall ,
the rats were randomly put into the other three quadrants without a platform in daily experiments . if the rat could not locate the hidden platform in 60 s , the rat would be guided to the platform and remain for 10 s continuously for 3 days .
four sessions of training were necessary every day for each rat with intervals of 1520 min .
( 2 ) space exploration experiment : the day after the navigation experiment , the rats were put into the water from the contralateral quadrant of platform one . the percentage of time spent locating the quadrant with the platform in the total time
would be recorded in 60 s. the water maze baseline was measured in all rats 1 day before modeling . the percentage of residence time during escape latency , in the target quadrant , and the total time were recorded for the water maze test 8 weeks after controlled cortical impact ( cci ) .
after the behavioral test , the rats were sacrificed and the brain tissues were removed .
the brain tissues were placed on ice ; the hippocampuses were isolated and frozen in liquid nitrogen .
three photographs magnified 200 times were randomly selected from each slice and from each group .
brain tissues should fill the entire field of vision when taking photos to ensure the background light is consistent in each photo .
image - pro plus 6.0 software was used to select the same degree of tan as a uniform standard to assign positive results in all photos , and the integrated optical density ( iod ) was analyzed and concluded in all photos .
count data are denoted by absolute values and percentages , while measurement data are recorded as the mean  standard deviation .
the results of the water maze test were determined with repetitive measurement deviation analysis , and the measurement data were compared with single factor analysis of variance by the least significant difference ( lsd ) test post hoc .
single factor variance analysis was applied in analysis of time spent in the targeted quadrant in the spatial probe test between each group , and the lsd test was applied in comparisons between each group .
spatial learning ability was reflected by changes of escape latency in the water maze test .
eight weeks later , no statistical differences ( p>0.05 ) existed in the 3- and 5-day groups when compared with baseline .
the other groups were statistically different based on auto - comparison ( p<0.05 ) . compared with the control group , significant differences ( p<0.05 ) existed in all of the groups with the exception of the 1-day group after 8 weeks .
the higher the percentage of residence time in the target quadrant during escape latency in the total time , the stronger the spatial memory ability ; the results are shown in table 2 .
no significant difference existed between each group before testing ; 8 weeks later , no statistical difference ( p>0.05 ) existed in the 5-day and 1-week groups when compared with baseline .
the other groups were statistically different based on auto - comparison ( p<0.05 ) ; a significant difference ( p<0.05 ) existed in all of the groups with the exception of the 1-day group after 8 weeks when compared with the control group .
the effect of hbo on expression of gap43 and syn in different groups is shown in table 3 and figures 1 , 2 .
eight weeks later , no statistical difference ( p>0.05 ) was observed in the level of expression of gap-43 and syn in the 1-day group compared with the control group .
when comparing the remaining groups and the control group , there were statistically significant differences ( p<0.05 ) .
the level of expression of gap-43 and syn was significantly different compared with that in the control group ( p<0.01 ) in the 5-day , 1-week , and 2-week groups .
over the past 30 years , irreversible cellular damage after traumatic brain injury has been attributed to the following four main mechanisms : ( 1 ) excitatory poisoning ; ( 2 ) oxygen / nitric oxide overload ; ( 3 ) inflammatory reactions ; and ( 4 ) apoptosis .
anoxia is an important cause of irreversible cellular damage after traumatic brain injury ; thus , a protective effect is identified in ischemic tissue by increasing body oxygen levels .
based on the principle that hbo therapy can improve the concentration of free oxygen in plasma by dozens of times , oxygen diffuses to ischemic tissue and reduces dependence on oxyhemoglobin . in recent years
, it has been reported that hbo functions at biochemical , molecular , and hemodynamic levels [ 916 ] .
the detailed mechanism by which hbo functions is as follows : when the body suffers hypoxic - ischemic brain damage ( hibd ) , atp synthesis is reduced , which increases glutamate release and calcium overload ; over - production of free radicals and inflammation aggravates the mitochondrial damage , which is the organelle that produces atp . with respect to the time window of hbo therapy
, it has been reported that cerebral cellular edema exists to different degrees following local anoxia .
indeed , the only way to correct local hypoxia is hbo treatment , which is conducive to help save and preserve the dying brain cells .
is earlier treatment better ? how long should treatment be extended before the effect declines ? does the same mechanism underlie the curative effect in acute and chronic phases of cerebral damage treated by hbo ? it has been shown that when given the same course of hbo therapy ( 1 week ) , the rats that began treatment 1 hour or 2 weeks after modeling did not differ significantly in the water maze test ( escape latency and percentage of target quadrant residence time in total time ) at the end 8 weeks , which indicated that the long - term curative effect of partial learning and memory was not affected by hbo treatment . when compared with the results after 4 weeks of treatment ,
the recovery of spatial learning and memory is less satisfactory after 8 weeks of treatment ; thus , hbo therapy has an impact on the long - term curative effect of cognitive function ; however , this finding does not suggest that the sooner treatment is begun , the better . nevertheless , earlier and continuous hbo treatment will yield the best effect . during the acute phase of brain injury , the mechanism of hbo treatment has been further studied .
hbo largely functions by inhibiting the inflammatory reaction and anti - apoptosis resistance to promote restoration of neural cells .
zhang et al .   showed that hbo can protect the blood - brain barrier , relieve cerebral edema , preserve the content of glutamate , glucose , and pyruvic acid salt in a relatively stable state in the body , inhibit inflammatory reactions and apoptosis , adjust superoxide dismutase , enhance the expression of bcl-2 , which is a key gene that promotes cell survival in the body , and increase the deformation ability of erythrocytes to improve the microcirculation and reduce the hypoxic - ischemic state .
cheng   reported that in a rat stroke model , hbo reduced the mrna and protein levels of cyclooxygenase-2 and reduced neutrophil invasion , thus inhibiting the inflammatory reaction .
furthermore , interleukin-10 and angiogenesis contribute to the neuroprotection induced by hbo pre - conditioning against brain injury in rats .
the studies that have focused on a curative effect and the mechanism underlying the chronic phase of brain damage treated by hbo are fewer .
reported that the protective long - term hbo treatment effects following brain injury are mediated by a pronounced remyelination in the ipsilateral injured cortex , as substantiated by the associated recovery of sensorimotor function .
mu et al .   reported that delayed daily hbo treatment presents as a potent neuroprotectant in pmcao rats , which increases creb expression and signaling activity and bolsters proliferation of regenerative cells in the injured brain .
as shown in the acute experiment , the effects of hbo are likely mediated by reducing ubiquitin - dependent creb degradation owing to hbo - induced activation of pp1 gamma .
gap-43 is also known as neuromodulin , which is an axonal membrane protein and a type of neural - specific protein .
gap-43 is highly expressed in the process of development and regeneration of neurons ; and it regulates axonal extension and changes cell morphology . as an intracellular signal , gpa-43 can significantly enhance and transfer function of g protein coupled receptor .
syn exists in nearly all of the neural endings in the central and peripheral nervous systems .
simultaneously , synapses also express syn in neuroendocrinal cells , which participate in importing and transporting synaptic vesicles , neurotransmitter release , synaptic vesicle recycling , and synaptogenesis .
syn can serve as a specific marker of the presynaptic terminal to detect the density and distribution of synapses   and has been widely applied in the study of synaptogenesis in animal models and human disease , and is considered to be an important marker of neural plasticity . in the current study we investigated the influence of neural plasticity in the acute and chronic phases of ischemic - hypoxic brain injury in rats treated with hbo through determination of the neural plasticity index ( gap-43 and syn ) at 8 weeks in the hippocampus and striatum .
the experimental results are shown below , but when given the same course of hbo therapy ( 1 week ) , the expression of gap-43 and syn in the 5-day , 1-week , and 2-week groups was significantly higher than that in the other groups in the hippocampus and striatum , especially in the 1-week group .
the level of expression of gap-43 and syn was not completely consistent with the results of the water maze test , which may indicate that if the start time of hbo is different , the mechanism of improving cognitive function will be different .
the mechanism of hbo therapy in the acute phase of hypoxic - ischemic brain injury is mainly by inhibiting inflammatory reactions and anti - apoptosis effects to promote restoring neural cells .
the later the initiation of hbo , the more significant the effect on promoting neural plasticity .
the effect on the improvement of cognitive function is not apparent with early treatment ; thus , a long - term study is warranted .
initiation of hbo treatment has an influence on long - term cognitive function , but this finding does not mean that the sooner the treatment , the better . in the current study , neither cognitive function nor neural plasticity was significantly improved in rats 1 day immediately after craniocerebral trauma .
the most significant improvements in cognitive function and neural plasticity appeared when hbo therapy was provided 57 days after brain trauma .
if hbo therapy was initiated > 2 weeks after hypoxic - ischemic brain injury , the improvement in cognitive function was less than that observed with earlier treatment , while the effect on neural remodeling was still evident .
 backgroundhyperbaric oxygen ( hbo ) is a historical therapeutic option in the treatment of various types of brain damage . at present , 
 clinical treatment of hypoxic - ischemic injury is giving priority to cognitive training . 
 the effects of hbo on cognitive dysfunction were observed in a controlled cortical impact ( cci ) rat model.material/methodsseventy male sd rats were randomly divided into control ( n=10 ) and intervention ( n=60 ) groups . 
 all rats underwent baseline water maze testing 1 day before modeling , and were retested 8 weeks after modeling . 
 the percentage of residence time during escape latency in the target quadrant and the total time were recorded . 
 data were analyzed by spss 16.0 software . 
 p<0.05 was considered statistically significant.resultsafter 8 weeks , no statistical difference ( p>0.05 ) existed in spatial learning ability in the 3-day and 5-day groups when compared with baseline . 
 the other groups were statistically different by auto - comparison ( p<0.05 ) . 
 no statistical difference ( p>0.05 ) in spatial memory existed in the 5-day and 1-week groups when compared with baseline , while a significant difference was noted in the other groups by self - comparison ( p<0.05 ) . 
 no statistical difference ( p>0.05 ) was noted in the level of expression of growth - associated protein-43 ( gap-43 ) and synaptophysin ( syn ) in the 1-day group compared with the control group . 
 the remaining groups and the control group were statistically different ( p<0.05 ) , while the level of expression of gap-43 and syn in the 5-day , 1-week , and 2-week groups was significantly different compared with that in the control group ( p<0.01).conclusionsif hbo therapy was provided 57 days after craniocerebral trauma , there was apparent improvement in cognitive function and neuroplasticity . 
Background
Material and Methods
Laboratory animals and grouping
Modeling
HBO therapy
Water maze test
Growth-associated protein-43 (GAP-43) and synaptophysin (Syn) expression
Statistical analysis
Results
Discussion
Conclusions