ROLE OF HIPPOCAMPAL CA2 BEYOND SOCIAL MEMORY: EXPLORING ITS ACTIVITY IN GOAL-ORIENTED TASK

Animals’ survival depends on their ability to remember locations of interest such as food sources or shelter. The hippocampus plays a central role in spatial memory and hosts place cells which fire at specific locations during navigation. In the hippocampal CA1 region, spatial representations are not uniform: during learning, place fields accumulate near salient locations such as goals or rewards, a phenomenon known as goal-oriented remapping. Despite extensive investigation of CA1 place cells, the circuit mechanisms underlying this biased reorganization of the hippocampal cognitive map remain poorly understood, especially the contribution of the often-overlooked CA2 region.
Although CA2 has mostly been associated with social memory, recent studies suggest a broader role in hippocampus-dependent functions. Notably, CA2 neurons show increased activity during novelty and immobility near rewarded locations and preferentially project to deep-layer CA1 pyramidal neurons, which exhibit pronounced reward-related place field reorganization. These observations led us to hypothesize that CA2 participates in the memory of rewarded-locations.

To test this hypothesis, we developed a novel spatial navigation task in a fully automated behavioral setup—the “clock maze”—and performed in vivo high-density electrophysiological recordings in freely moving mice (1130 units in CA1, 1228 units in CA2). As expected, CA1 place fields reorganized toward the goal during learning. We found that CA2 place field density also increased in the area preceding the rewarded port, while displaying distinct dynamics during reward approach and consumption than CA1. Altogether, our work motivates further investigation into the contribution of CA2 to goal-oriented remapping.