Objective To investigate the effects and mechanisms of icariin on changes in fear memory in post-traumatic stress disorder(PTSD)rats.Methods Thirty male SD rats were used to construct a rat model of single prolonged stress(SPS).The model rats were randomly divided into the SPS,icariin,and icariin + K252a(tyrosine kinase receptor B inhibitor)groups(n= 10 each;another 10 normal rats were used as the control group).The icariin and icariin + K252a groups were administered 20 mg/kg icariin by gavage once per day after SPS,while the control and SPS groups were administered the same dose of normal saline.K252a cells were injected into the lateral ventricles.After 2 weeks,anxiety,depression,and fear memory disorder in rats in each group were detected by the mine experiment,ele-vated cross maze experiment,and conditional fear test.The binding activity of icariin to brain-derived neurotrophic factor(BDNF)and the BDNF and TrkB expressions in the rat amygdala were detected by immunohistochemistry.The relative expressions of BDNF and TrkB pro-teins were detected by Western blotting.The expressions of postsynaptic density protein 95(PSD95)and synaptophysin(SYN)in the rat amygdala were detected using immunofluorescence.Results Icariin showed strong binding to BDNF.Compared with the control group,the times of entering the central area and the percentage of movement distance in the central area in the SPS group and the icariin+K252a group were significantly reduced.The open arm entry(OE)and arm opening time(OT)were significantly reduced,the freezing time and defecation times were significantly increased,and the expressions of the BDNF,TrkB,PSD95,and SYN proteins were significantly reduced(P<0.05).Compared with the SPS group,the icariin group rats had significantly increased times of entering the central area and percentages of movement distance in the central area,significantly increased OE and OT,significantly reduced the time of immobilization and defecation,and significantly increased the expressions of BDNF,TrkB,PSD95,and SYN proteins(P<0.05).Conclusion Icariin effectively alleviated the fear memory impairment induced by SPS in rats.This protective effect is related to BDNF/TrkB signaling pathway activation and upregulated PSD95 and SYN expression.