Early and late consolidation and reconsolidation of memory in the prelimbic cortex

Rats can learn to forage among olfactory cues to associate one with reward in only 3 massed trials. The learning is achieved in less than 10 min and results in a memory trace lasting at least 1wk week. To study the neuro-anatomical circuits involved in the memory formation we used immunoreactivity to the immediate early gene c-fos as a marker for neuronal activity induced by the learning. The prelimbic cortex (PLC) was one of the few regions showing selective increases in c-fos immunoreactivity in trained rats compared to yoked pseudo-trained. In subsequent experiments, injection of an NMDA receptor antagonist directly into PLC immediately after learning induced amnesia 48h later, while similar injections into the hippocampus had no effect. Injections of the beta-adrenergic antagonist, timolol, 2h, but not 5 min after training also produced amnesia. This suggests that PLC is part of a neural circuit activated for an extended period during the post-acquisition consolidation of memory for odor-reward association. We studied reconsolidation processes after reactivation and found that injections of the NMDA receptor antagonist or the beta antagonist could block reconsolidation with similar temporal dynamics as during consolidation when the injections were made into the lateral ventricles, but not into the PLC. Thus, similar cellular mechanisms but different neural circuits appear to be involved in initial consolidation and reconsolidation.