Systemic blockade of NMDA receptors with the competitive antagonist CPP impairs long-term but not short-term memory for fear extinction. We recently demonstrated that infusing CPP directly into the ventromedial prefrontal cortex (vmPFC) had the same effect (Burgos-Robles et al., SFN 2004), in agreement with lesion and unit-recording data implicating the vmPFC in consolidation of fear extinction. However, it was recently reported that administration of the non-competitive NMDA antagonist MK-801 dramatically increased the spontaneous firing rate and decreased the bursting of prelimbic (PL) vmPFC neurons (Jackson et al., 2004). This suggests that the behavioral effects of CPP we observed might have been due to alterations in firing rate or bursting, rather than impaired synaptic plasticity. To investigate this possibility, rats were chronically implanted with movable microelectrodes aimed at PL or infralimbic (IL) vmPFC. Following recovery, rats were systemically injected with saline followed one hour later b...