Exposure to Novelty Promotes Long-Term Contextual Fear Memory Formation in Juvenile Mice: Evidence for a Behavioral Tagging.

Episodic memories acquired early in life are fragile and rapidly forgotten in both humans and nonhuman animals. However, early life experiences have been documented to profoundly affect brain function and physiology throughout life, suggesting that in certain circumstances, the developing brain is capable of producing long-term memory (LTM). In this study, we asked whether exposure to a novel environment, a process named "behavioral tagging," may promote the persistence of weak memories in male juvenile mice. Using a contextual fear conditioning (CFC) paradigm, we found that a weak training protocol, which typically induces a transient form of memory, results in LTM when paired with an exploration to a novel but not a familiar environment that occurs close in time to the training session. The promoting effect of the novel context exploration (NCE) on CFC-LTM formation is dependent on the activation of dopamine D1/D5 receptors and requires novel protein synthesis in the dorsal hippocampus. Moreover, NCE increases the size of overlapping CA1 neuronal ensembles engaged by CFC and NCE. These results provide direct support for the existence of a behavioral tagging process, in which exposure to novelty provides newly synthesized proteins to stabilize the contextual fear memory trace in juvenile mice.