Objective  To observe the changes of N-methyl-D-aspartate (NMDA) receptor 2B

an immune positive reaction substance

in infra-limbic (IL) cortex of medial prefrontal cortex (mPFC) in adult rats after fear extinction.    Methods  Twentyfour hours after Pavlovian fear conditioning

70 rats were randomly divided into fear extinction group (n=30)

natural extinction group (n=30) and naive group (n=10).  Extinction memory was established by electric shock at the sole with tone as a prompt. Then

fear extinction training was carried out by giving the rats only tone signals. NR2B was tested by immunohistochemistry 1

3 and 7 d after fear extinction.   Results  The NR2B expression level increased gradually in fear and natural extinction groups and was lower than that in normal control group. The fear extinction was better in natural extinction group than in normal control group (P0.05). The NR2b level increased gradually in IL cortex of fear and natural extinction groups

and was lower than that in normal control group. A significant difference was found on day 1 but not on days 3-7.   Conclusion  NR2B level in IL cortex is consistent with the extinction memory 3-7 d after fear extinction training.