Two-dimensional neural activity mapping of the entire population of hippocampal CA1 pyramidal cells responding to fear conditioning.

The hippocampus is involved in the encoding, storage, and retrieval of memory. Here, we have developed a novel mapping method for detecting the distribution of neural activity of the entire population of pyramidal cells in the hippocampal CA1 and subiculum regions, where expression profiles of Arc mRNA were used as an indicator of neural activity. The spherical pyramidal cell layer of the intact hippocampus was flattened into a two-dimensional plane, which was then serially sectioned in parallel with the plane to make tangential sections. Tangential sections were hybridized with an Arc cRNA probe and Arc signals from serial tangential sections were stacked and displayed on a two-dimensional plane, allowing one to easily visualize the neural activity of all pyramidal cells. We applied this method to the hippocampus of rats that had experienced contextual fear conditioning, which requires hippocampal function. We observed a net shift of Arc signals from dorsal to ventral CA1/subiculum with an interval prolongation to reconditioning after the initial conditioning. The reconditioning-revealed shift may reflect a reorganization process, which takes place during the period between initial conditioning and reconditioning, in the CA1/subiculum neural network that represents the neural storage and/or retrieval of the contextual fear conditioning.