Selective impairment of long-term but not short-term conditional fear by the N-methyl-D-aspartate antagonist APV. 
Previous research has indicated that the competitive N-methyl-D-aspartate (NMDA) antagonist APV (DL-2-amino-5-phosphonovalerate) prevents the Pavlovian conditioning of fear to contextual stimuli when tested 24 hr, but not immediately, after training.  The present study investigated this differential time-dependent effect of APV on fear conditioning.  Rats were given either APV or saline and presented with 3 footshocks in a distinctive chamber.  Promptly after the shock, rats that had received APV exhibited a species-typical fear response-freezing.  However, the freezing lasted for only a short period of time (less than 3 min) compared with that of controls.  An immediate-shock procedure showed that freezing was entirely a conditional response to the chamber.  In addition, the results of a savings test suggest that APV impairs storage rather than retrieval processes.  These results indicate that there are two temporally distinct associative fear processes, a short-term NMDA-independent conditional fear and a long-term NMDA-dependent conditional fear.