Change of synaptophysin and neural cell adhesion molecule immunoreactive positive substances in infra-limbic cortex after fear extinction

Abstract

Objective To observe the change of synaptophysin and neural cell adhesion molecule (NCAM) immunoreactive positive substances in infra-limbic cortex (IL) of medial prefrontal cortex (mPFC) after fear extinction. Methods After conditioned fear model was made, 72 male adult SD rats were randomly divided into natural-extinction group (n=24), immadlate-extinction group (n=24) and 24-hour-extinction group (n=24). Conditioned fear was established by tone paired foot shock. Rats were trained with only tone signal for fear extinction (fear extinction was performed in 15 minutes or 24 hours after establishment of conditioned fear). Extinction retention test, immunohistochemistry test of synaptophysin and NCAM were done at 1st, 3rd, 7th and 20th day after fear extinction. Results (1) Extinction retention scores were increased gradually and reached the peak in 24-hour-extinction group at 7th day after extinction. Extinction retention score was the best in 24-hour-extinction group as compared to the other two groups (P<0.01). (2) Synaptophysin and NCAM immunoreactive positive substances were increased gradually in all groups. Synaptophysin expression reached the highest level at 7th day then reduced (P<0.01). (3) NCAM expression reached the highest point at 3rd day in natural-extinction group, at 7th day in the other two groups (P<0.01). Conclusion It indicates that extinction retention score be correlated with the amount of plasticity-related synaptophysin and NCAM in infra-limbic cortex at 3-7th day after fear extinction. The level of plasticity-related substances in infra-limbic cortex could reflect the extent of extinction retention. Key words: Fear; Synaptophysin; Neural cell adhesion molecules; Frontal lobe