| Objective: To observe the changes of axons, myelin sheath, dopaminergic neuron and nerve fiber, to study the feature of cognitive behavioral deficits in brain concussion(BC) rats, to explore the relationship between cognitive deficits and pathologic changes of axons, myelin sheath and dopaminergic system so as to know the neurobiologic mechanisms of cognitive behavioral deficits post BC in rats. Method : Animal model of concussive brain injury in rats was duplicated with a metallic pendulum-striker concussive device. Animals subjected BC were divided into 6 groups (n=12 in each group) :the group of 1d, 2d, 4d, 8d, 16d and 24d post-concussive injury. There were 12 normal rats acting as a control group. Learning and memory ability were assessed with Morris water maze(MWM) method. Histochemistry technique was used to show the morphological changes of axons and myelin sheath in brain stem. Tyrosine hydroxylase(TH) and dopamine-β-hydroxylase(DBH) levels were examined by immunohistochemistry S-P method in substantia nigra pars compact ( SNC) , ventral tegmental area (VTA), basal ganglia and prefrental cortex. Result: The escape latencies were longer statistically compared to the control group on days 1~3 post-concussion in 8d group (p< 0.05) . Within the 16 or 24d groups, MWM test was not significantly relative to the controls. The various morphological changes could be observed in all injured rats in the brain stem axons. With the exception of 24d group, all injured groups were different statistically compared to the controls. The damage of axons were more seriously at 2d and 4d groups than any other group. The severityof axonal lesion was attenuated after 8 days, and recovered in 24d group. The pathological changes could be finded in all injured rats in the brain stem myelin sheath. All BC groups were different significantly compared to the control group.The degeneration of the myelin sheath in 2d and 4d groups was more seriously .which could be relieved obviously in 16 and 24d groups. A increase in TH-immunoreaction(TH-IR) was observed significantly in the SNC, VTA and basal ganglia following 1 day post-concussive . The most significant change occured at 4d group. The expression of TH-IR had a descending tendency in 8d, 16d group compared to 4d group, and recovered in 24d group in SNC and basal ganglia, but the TH-IR still increased significantly relative to the control group in VTA. The density and intensity of the TH-IR in prefrontal cortex increased statistically in 2d, 4d groups post brain concussion. Though a descending tendency in TH-IR could be observed following 8, 16, 24d groups relative to 4d group , the density and intensity of TH-IR remained at a high level in prefrontal cortex compared to the control group (p< 0.01 ) . The immunologic reactions of DBHwere negative in all experiments. Conclusion: 1. Brain concussion injury has impaired effects on spacial learning and memory in rats in short term post-concussion. 2. The morphological pathological changes of the axons and myelin sheath were observed in BC brain stem similar to DAI, which may be correlated with cognitive behavioral deficits post brain concussion in rats. 3. The expression of TH-IR increased following BC. We speculate that the increase in TH in earlier period may be caused by neuronal transient overexcitation following post brain concussion , and the increase in TH remainded a period of time which may reflect a compensatory response of dopaminergic neurons to upregulate their synthesizing capacity in order to increase the efficiency of dopamine neurotransmission.
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