The Role Of Posttraumatic Hypothermia In Protecting Dendrites And Spines After Traumatic Brain Injury And Its Mechanisms

Objectives: To investigate the effects of posttraumatic hypothermia on protecting dendrites and spines of the ipsilateral cortex and hippocampus after severe TBI and its mechanisms.Methods: 290 adult male thy-1 GFP mice were randomly divided into seven groups: the sham injury with normothermia group(n=70),the sham injury with hypothermia group(SHG;n=20),the TBI with normothermia group(TNG;n=70),the TBI with hypothermia group(THG;n=90),THG+DMSO group(n=20),THG+K252a group(n=10)and THG+ LY294002 group(n=10).The TNG and THG mice received controlled cortical impact injury and the sham group mice only received craniotomies without CCI injury.The TNG maintained the body temperature at 37°C throughout the experiment.The temperature of THG was decreased to 33°C in 30 min and maintained for 4 h.After 4 h,we elevated the body temperature of THG to 37°C over 90 min.THG+DMSO group,THG+K252a group and THG+ LY294002 group were all received intraventricular injection of different substance.We observed the physical condition,neurological reflection and body temperature throughout the experiment.The mice were perfused transcardially at 1 and 7 days after severe TBI and the brain samples were collected.The histological changes were then analyzed.The rest mice(n=30)underwent Rotarod test and Morris water maze test.We used microscopy to observe the morphology changes of the neurons in layer V/VI of the cortex and hippocampus at 1 and 7 days after severe TBI.The tissue levels of the synapse proteins,BDNF,Trk B,p Trk B,AKT and p AKT were assessed by using western blot assay.The Rotarod test and Morris water maze test were used to assess the motor function and memory function of the mice.Results: The mortality between TNG and THG showed no significant difference.The TNG and THG mice all presented coma and disappeared neurological reflection after severe TBI.The body temperature was significantly decreased in THG compared with that of TNG.The brain sample of TNG and THG both showed obvious cortical hemorrhage and deficits,however,the cortical damage of THG were relatively slighter than that of TNG at 1 day after severe TBI.The immunofluorescence staining showed that the NF200 positive bulbs in THG were less than that in TNG.Posttraumatic hypothermia significantly protects the dendrites and spines in layer V/VI of the cortex and hippocampus at 1 and 7 days after severe TBI.The Rotarod test and Morris water maze test showed hypothermia significantly improved the motor function and memory function of the mice after severe TBI.Western blot assay showed that hypothermia dramatically increased the tissue levels of synapse proteins Glu R1 and PSD-95 and BDNF at 1 and 7 days after severe TBI.BDNF specific suppressor K252 a suppressed the effect of posttraumatic hypothermia on preventing dendrite degeneration and spine loss of the cortical neuron 1 day after severe TBI.BDNF specific suppressor K252 a decreased the expression levels of BDNF,p Trk B and synapse proteins Glu R1 and PSD-95 in the ipsilateral cortex at 1 day after severe TBI.PI3 K specific suppressor LY294002 suppressed the effect of posttraumatic hypothermia on preventing dendrite degeneration and spine loss of the cortical neuron 1 day after severe TBI.PI3 K specific suppressor LY294002 also decreased the expression levels of BDNF,p Trk B,p AKT and synapse proteins Glu R1 and PSD-95 in the ipsilateral cortex at 1 day after severe TBI.Conclusions: Posttraumatic hypothermia significantly prevents the dendrite degeneration and spine loss in layer V/VI of the cortex and hippocampus at 1 and 7 days after severe TBI and improved the motor function and memory function of the mice.Posttraumatic hypothermia protects the dendrites and spines of the neurons in the ipsilateral cortex through increasing the expression level of BDNF and activating its downstream PI3K/AKT pathway at 1 day after severe TBI.