| In recent years, post-traumatic stress disorder (PTSD) was more and more attentioned by researchers along with the frequent earthquakes, tsunamis, terrorist attacks and other traumatic events. PTSD was a delayed and long-lasting mental disorder because of abnormal threatening or catastrophic psychological trauma. Putnam's study suggested that 68% of people would encounter at least one serious trauma through their lives, and 7.8%~ 80% of people who had experienced psychological trauma would generate PTSD and suffer from long-lasting psychological distresses. At present, the PTSD patients were commonly treated by exposure therapy based on extinction principle. However, the efficacy of exposure treatment can not be maintained in 40% of the patients. For them the extinguished fear response reappeared soon. Therefore, the reseach of the retention of extinction memory was greatly significant for the consolidation and maintain of the exposure therapy.Hippocampus was an important area involved in extinction regulation. Corcoran and his colleagues reported that the extinguished fear re-emergented after muscimol injected into hippocampus before extinction training, which suggested that the hippocampus was involved in the retention acquirement of extinction memory. In addition, there were a large number of nerve fibers projected to the mPFC from hippocampus. The hippocampus-mPFC pathway affected the process of extinction. Electrophysiological studies showed that extinction was related to the synaptic efficacy of hippocampus-mPFC pathway. Sandrine and his co-workers reported that the damage of LTP in the hippocampus-mPFC pathway by low-frequency electrical stimulation to ventral hippocampus immediately after extinction training led to the increasing of freezing.It suggested that LTP in the hippocampus-mPFC pathway was necessary for the retention and retrieval of extinction memory.Cyclin-dependent kinase 5 (CDK5) is a unique member in the CDK5 family, playing key roles in a number of cellular processes rather than the control of cell cycle progression. CDK5 can phosphorylate many kinds of substrates, and regulate the synaptic plasticity. Sananbenesi's research showed that the CDK5 pathway in hippocampus may regulate the fear excintion. CDK5 inhibitors butyrolactone1 was microinjected into hippocampus within 1 hour after fear extinction training.Finally, the fear to the specific environment declined, while increasing the activity of CDK5 in the hippocampus led to fear sustained. However, whether CDK5 was involved in the process of the retention of extinction memory had not been reported.Fear extinction was association learning about CS-NO US. And the changes of synaptic structure closely related with learning and memory. Jing reported that the width of synaptic gap reduced in visual cortex cells after vision training, and the thickness of the PSD was increased. Zhang Songjun reported that the width of synaptic gap increased when learning and memory impaired. Currently, there were no reports about the change of synaptic structural in hippocampal after conditioned fear extinction training.In view of the role of CDK5 in fear extinction and the relation between the change in synaptic structural and learning and memory, two experients were done. One was to detect the changes of CDK5 expression and activity in hippocampus during 1 week after the extinction training in rats.The other was to observe the changes in synaptic structure in hippocampus by transmission electron microscopy at 7d and 21d after extinction training.We hope to explore the changes of CDK5 and synaptic structure after conditioned fear extinction .These may provide information for the treatment of PTSD. Method:Part1: The rats were randomly divided into native group, extinction control group ( in the next day after fear conditioning, the rats were in experimental boxes about 25min without foot shock and sound) and extinction group (in the next day of conditioned fear, the rats were given sound without the shock in experimental boxes, and extinction training need 25min) . Behavior test were conducted at 1d , 3d and 7d after extinction training (the rats were given sound without the shock in experimental boxes, and it took 10min), recording the time of freezing, calculating the percentage of no-freezing time as the extinction retention scores. And following tests were done.①To detect the expression of CDK5, P35, CDK5/P35 in hippocampus by western blot at 1d, 3d, 7d after extinction training.②To measure the activity of CDK5 enzyme in hippocampus by liquid scintillation at 1d, 3d, 7d after extinction training.③to count the number of CDK5 positive cells in hippocampus CA1 and CA3 regions by immunohistochemistry at 1d, 3d, 7d after extinction training.Part 2: Behavior test were done at 7d, 21d after extinction training (the rats were given sound without the shock in experimental boxes, and it took 10min), recording the time of rat freezing, calculating the percentage of no-freezing time as the extinction retention scores. The synaptic structure in hippocampal CA1 area was observed at 7d and 21d after extinction training by transmission electron microscopy. Results:Part1:①The scores of extinction retention were decreased in the extinction control group and extinction group compared with native group (p<0.01) at 1d and 3d after extinction,and there was no difference between the extinction control group and extinction group.For the extinction group,the score was increased gradually in extinction group,and the score was higher than that in control group at 7d(p<0.01).②The expression of CDK5 was significantly higher in extinction group than that in native group and extinction control group at 7d after extinction training(p<0.05).And it was higher at 7d than that at 3d in extinction group (p<0.05) .③Compared with native group, the expression of P35 was significantly lower (p <0.01) at 7d in extinction control group and the extinction group. But there were no significant difference between control group and the extinction group.④At 1d after extinction training, the expression of CDK5/P35 was significantly higher in extinction group than that in native group,but lower than that in the extinction control group (p <0.05). At 3d, the expression of CDK5/P35 continued to increase in extinction group, and significant higher than that in the native group and the extinction control group (p<0.05). At 7d, it restored to normal levels in extinction group and the control group.⑤The activity of CDK5 was significantly lower in extinction group than the extinction control group at 1d after extinction training (p<0.05). It was significantly higher in extinction group than that in the native group but lower than that in the extinction control group at 3d after extinction training, (p<0.05). The activity of CDK5 restored to normal levels in extinction group and the control group at 7d.⑥The number of CDK5 positive cell was increased singificantly in extinction group compared with native group. The number was increased singificantly at 3d (p<0.01) and 7d (p<0.05) after extinction training in extinction group compared with extinction control group.⑦At 1d and 3d after extinction training, the number of CDK5 positive cells was significantly higher in hippocampal CA3 area in extinction group and the control group than that in the native group (p<0.01). And at 1d, the number was significantly lower in extinction group than that in the extinction control group (p<0.01).Part2:①Compared with the native group and the extinction group, the scores of extinction retention was decreased (p<0.01) at 7d and 21d after extinction in extinction control group. And it was significantly lower at 7d than that at 21d in extinction control group.There was no significant difference between extinction group and native group at 7d and 21d.②At 7d after extinction training, the synaptic density was significantly higher in extinction group than that in the extinction control group (p<0.05), and there was no difference among groups at 21d.③Compared with native group, the width of synaptic gap was significantly lower at 7d and 21d after extinction training in extinction group (p<0.01; p<0.05). At 21d, the width of synaptic gap was significantly lower in extinction group than that in the control group (p<0.05). But there was no significant difference between the extinction control group and native group at 7d and 21d after extinction.④Compared with native group, at 7d and 21d after extinction training, the thickness of PSD was significantly higher (p<0.01) in extinction group and the extinction control group. And at 21d, it was significantly higher in extinction group than that in the extinction control group (p<0.05).⑤There were no significant difference among the groups in active zone length.Conclusion:Part 1: The expression of CDK5 was increased after extinction training in extinction group. However, the expression of CDK5/P35 and the activity of CDK5 was significantly decreased compared with the extinction control groups, and then recovered normal lever, with the freezing reducing. It suggested that the change of CDK5 activity was consistent with the expression of CDK5/P35 instead of depending on the change of the expression of CDK5 in hippocampus. And the low level of CDK5 activity was benefit for the retention of extinction memory in the early stage after extinction training.Part 2: After extinction training, the synaptic density, the width of synaptic gap and the thickness of PSD were declined in CA1 area of hippocampus with freezing reducing. It suggested that enhanced synaptic function was good for the retention of the extinction memory.
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