Dorsal hippocampus NMDA receptors differentially mediate trace and contextual fear conditioning

The dorsal hippocampus (DH) is critically involved in the acquisition and expression of trace and contextual fear conditioning. NMDA/glutamate receptor-mediated transmission is thought to be one mechanism mediating the plastic changes that support long-term memories in the DH. However, their precise involvement in acquisition and expression processes has not been defined. To examine this issue, the NMDA receptor antagonist, D,L-2-amino-5-phosphonovaleric acid (APV; 10 microg/microl; 0.5 microl), was infused into the DH prior to conditioning and/or testing, using a trace fear conditioning procedure. All rats were tested for freezing to both tone and context in separate, counterbalanced sessions. The three sessions (1 training and 2 test) were separated by approximately 24 h. Using this design, it was possible to assess the role for DH NMDA receptors in the acquisition versus expression of trace and contextual fear conditioning. APV disrupted acquisition, but not expression, of contextual fear conditioning. By contrast, APV attenuated both acquisition and expression of trace fear memories. Thus, DH NMDA receptors appear to contribute to retrieval of some, but not all, fear memories.