Mice display signs of fear when neurons that express cFos during fear conditioning are artificially reactivated. This finding gave rise to the notion that cFos marks neurons that encode specific memories. Here we show that cFos expression patterns in the mouse dentate gyrus (DG) change dramatically from day to day in a water maze spatial learning paradigm, regardless of training level. Optogenetic inhibition of neurons that expressed cFos on the first training day affected performance days later, suggesting that these neurons continue to be important for spatial memory recall. The mechanism preventing repeated cFos expression in DG granule cells involves accumulation of ΔFosB, a long-lived splice variant of FosB. CA1 neurons, in contrast, repeatedly expressed cFos. Thus, cFos-expressing granule cells may encode new features being added to the internal representation during the last training session. This form of timestamping is thought to be required for the formation of episodic memories.

Mice require several days of training to master the water maze, a spatial memory task for rodents. The hippocampus plays a key role in the formation of spatial and episodic memories, a process that involves the activation of immediate-early genes such as cFos. We trained cFos-reporter mice in the water maze, expecting that consistent spatial behavior would be reflected by consistent cFos patterns across training episodes. Even after extensive training, however, different sets of dentate gyrus (DG) granule cells were activated every day. Suppressing activity in the original encoding ensemble helped mice to learn a novel platform position (reversal learning). Our results suggest that even in a constant environment, cFos+ ensembles in the dorsal DG segregate as a function of time, but become partially reactivated when animals try to access memories of past events.

SummaryMice require several days of training to master the water maze, a spatial memory task for rodents. The hippocampus plays a key role in the formation of spatial and episodic memories, a process that involves the activation of immediate-early genes such as cFos. We trained cFos-reporter mice in the water maze, expecting that consistent spatial behavior would be reflected by consistent cFos patterns across training episodes. Even after extensive training, however, different sets of dentate gyrus (DG) granule cells were activated every day. Suppressing activity in the original encoding ensemble helped mice to learn a novel platform position (reversal learning). Our results suggest that even in a constant environment, cFos+ ensembles in the dorsal DG segregate as a function of time, but become partially reactivated when animals try to access memories of past events.