No erasure effect of retrieval-extinction trial on fear memory in the hippocampus-independent and dependent paradigms.

Recently, Monfils et al. [9] and Clem and Huganir [3] have shown that an isolated retrieval trial before the extinction sessions (retrieval-extinction) in mice and rats prevented the renewal and spontaneous recovery of the original fear memory by inhibiting reconsolidation in a hippocampus-independent manner. In contrast, Chan et al. [2], using the same paradigm, reported that retrieval extinction in rats augmented the renewal and reinstatement of extinguished fear. However, it remains unclear whether or not retrieval extinction in a hippocampus-independent paradigm erases the original fear memory by inhibiting reconsolidation. We therefore conducted three experiments to investigate whether or not retrieval extinction erases the original fear memory by inhibiting reconsolidation in mice. Our major findings were as follows. (1) Retrieval-extinction in mice did not suppress spontaneous recovery and fear renewal in a hippocampus-independent paradigm. (2) Fear renewal was observed when retrieval-strong extinction in a hippocampus-independent paradigm was performed. (3) Retrieval extinction in a hippocampus-dependent paradigm did not erase the original fear memory. These results suggested that fear extinction after retrieval in mice does not inhibit reconsolidation of previously consolidated fear memory in either a hippocampus-independent or -dependent paradigm.