Population activity in the dorsal hippocampal CA1 encoding the surrounding environment is absent during contextual...

The hippocampus plays a critical role in contextual fear conditioning. Population activity in the hippocampal CA1 encoding the surrounding environment is thought to be responsible for retrieval of contextual fear memory. However, the characteristics of CA1 neuronal ensemble activity during retrieval of contextual fear memory remain unclear. Here, we examined CA1 ensemble activity during contextual fear memory expression in male C57Bl/6J mice, using Arc cellular compartment analysis of temporal activity by fluorescence in situ hybridization. The "Shock" group was conditioned with a footshock in two separate chambers, whereas the "No shock" group was not exposed to shocks in the chamber. Animals were then re-exposed to either the same chamber twice or two different conditioning chambers. In the No shock group, exposure to the same chamber twice activated a more significantly overlapping neuronal population than exposure to two different chambers. In the Shock group, exposure to the same conditioning chamber twice activated a similarly overlapping neuronal population as exposure to two different chambers, with overlap smaller than in nonshocked mice exposed to the same chamber twice. Thus, population activity in the hippocampal CA1 encoding the surrounding environment is detected during spatial exploration, but absent during contextual fear memory expression. Even the variable ensemble activity of CA1 may contribute to retrieval of contextual fear memory.