The Effects Of Hyperbaric Oxygen Preconditioning On The Traumatic Brain Injury At Simulated High Altitude In Rats

Background and objectives:Ischemic preconditioning and pharmacologic preconditioning have good influences on ischemic neuronral damage, but the former has an ethic or moral problem and the latter has a problem of toxicity or side effects of the drugs. Therefore, this study was undertaken to determine if hyperbaric oxygen(HBO) preconditioning could produce neuroprotective effects on traumatic brain injury(TBI) at simulated high altitude in rats.Methods:1. 126 rats were randomly divided into three groups: plain group(n=42), high altitude group(n=42) and HBO preconditioning group(n=42). Each group consisted of two sub-groups: sham operation control group(n=21) and injury group(n=21). We only opened a skull window without injuring the brain in the sham operation control group. The TBI models were made according to Feeney's free-fall impact method. The rats in the high altitude group had been kept in a hypobaric chamber with a pressure of 61.6 Kpa for 3 days before operation, while the animals in the HBO preconditioning group received one hour hyperbaric oxygenation in a pure oxygen chamber with a pressure of 250 Kpa for 5 successive days before entering into the hypobaric chamber.2. Important biochemical and physiological conditions such as blood routine, blood gas analysis and blood glucose were monitored. The neurological deficits were quantified by Longa-score method. The regional cerebral blood flow(rCBF) was examined with MooR DRT4 laser doppler flowmetry(LDF) and the pressure of brain tissue oxygen(Pbto2) was monitored with LICOX CMP tissue oxygen pressure monitor(TOPM).3. The water content of brain tissue was examined by Elliot's method. hematoxylin-eosin(HE) staining and transmission electronic microscope(TEM) were applied to observe the pathological changes of brain tissue. 4. Expressions of matrix metalloproteinase-9(MMP-9) and neuroglobin(NGB) in the brain were observed with immunohistochemistry(IHC).Results:1. There was no statistical difference in RBC, HB, PH and PCO2 among the three injury groups. The PO2 in the high altitude injury group(73±9.4) was lower than that in the plain group(90±12.5). The blood glucose in the plain injury group(14.64±3.85) was significantly higher than that in the other two injury groups(8.35±2.70 and 7.72±2.21).2. The score of neurological deficits in the high altitude injury group(1.57±0.53) was the highest, the HBO preconditioning group(0.71±0.49) the second and the plain group(0.57±0.79) the lowest. Both rCBF(109.82±3.26, 105.23±2.1 and 109.54±3.01) and Pbto2(15.13±1.16, 13.25±1.11 and 14.89±1.12) dropped in the three injury groups compared with the corresponding sham operation control groups. There were significant differences in the three indexes between the high altitude injury group and the other two injury groups.3. There were significant differences in the water content of cerebral cortex and brain stem tissue between the high altitude injury group(86.24±0.52 and 80.78±0.42) and the other two injury groups(84.44±0.18, 84.60±0.37 and 77.82±0.29, 78.21±1.18).4. The brain morphology and structure presented by light microscope and TEM were normal in the three sham operation groups. Brain morphology and structure changed greatly in the high altitude injury group, including massive cells necrosis, dramatic decrease of neurons and gliocytes, conspicuous edema of interstitium, swelling of neuron mitochondria, vacuole formation of endoplasmic reticulums, fracture of basement membrane and exudation of erythrocytes, while little pathological injuries occurred in the other two injury groups.5. MMP-9 expression increased more significantly in the high altitude injury group (92.25±8.85) than in the other two injury groups(67.33±6.42 and 74.42±6.27), though little MMP-9 expressed in the three sham operation control groups. There was no difference in the expression of NGB among the three injury groups(52.08±4.73, 54.25±3.78 and 57.50±3.71).Conclusions:1. The TBI models at simulated high altitude in rats can be established successfully by Feeney's free-fall impact method 3 days after the animals are put into the hypobaric chamber.2. There were significant differences in the scores of neurological deficits and the pathological changes of brain tissue between the high altitude injury group and the plain injury group.3. HBO preconditioning could produce some neuroprotective effects on TBI at simulated high altitude in rats. The water content and pathological injuries of brain tissue and the neurological function have been improved in the HBO preconditioning group.4. The decreased expression of MMP-9 and improved rCBF and Pbto2 after TBI in the HBO preconditioning group may be the mechanism of HBO preconditioning.