| Associative learning paradigms are widely used in studying the molecular and cellular basis of learning and memory. In general, it is believed that a newly acquired associative memory is temporarily stored in the hippocampus and then gradually transformed to other brain regions in particular the cortex. Recent studies have provided lines of evidence that the anterior cingulate cortex (ACC) is involved in the retrieval and storage of remote memory traces after being transformed from the hippocampus. This notion has been tested by using the learning paradigm of contextual fear conditioning. Still, whether trace fear memory is also stored in ACC remained unknown. In this study, we used a learning paradigm which a visual stimulus was paired with a following foot shock at a trace interval of0.9sec and rats could successfully learn the CS-US association. In our observations, this memory could last for at least3weeks. To test whether ACC was involved in the retrieval of this memory, we locally injected lidocaine to inactivate ACC and found that the retrieval of remote trace fear memory was disrupted. To further determine whether ACC is involved in the storage of this memory, in vivo whole-cell patch clamp recordings was used in rat ACC under anesthesia to measure synaptic responses to the CS. In naive rats, we found that～58%ACC neurons showed responses, primarily excitatory, to a flash stimulus similar to that used in the trace conditioning. In learned rats, we performed the same recording21days after training and found that, the ratio of responding cells as well as the properties of these responses were not changed. Moreover, the mechanical injure of V1did not eliminate the CS-evoked responses in rats which already formed remote associative memory. These findings suggest that ACC is required for retrieving cued memory acquired from trace conditioning. However, as for the storage of this memory, ACC did not appear to be a critical region which could be shaped for long-term memory storage. |
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