Coordinates activities of retrosplenial ensembles during resting-state encode spatial landmarks. Part 1 of 2

The brain likely uses off-line periods to consolidate recent memories. One hypothesis holds that the hippocampal output provides a unique, global linking or 'index' code for each memory, and that this code is stored in the cortex in association with locally encoded attributes of each memory. Activation of the index code is hypothesized to evoke coordinated memory trace reactivation thus facilitating consolidation. Retrosplenial cortex (RSC) is a major recipient of hippocampal outflow and we have described populations of neurons there with sparse and orthogonal coding characteristics that resemble hippocampal 'place' cells, and whose expression depends on an intact hippocampus. Using two-photon Ca2+ imaging, we recorded ensembles of neurons in the RSC during periods of immobility before and after active running on a familiar linear treadmill track. Synchronous bursting of distinct groups of neurons occurred during rest both prior to and after running. In the second rest epoch, these patterns were associated with the locations of tactile landmarks and reward. Complementing established views on the functions of the RSC, our findings indicate that the structure is involved with processing landmark information during rest.

Methods
This dataset includes imaging data collected from three Thy1-GCaMP6s transgenic mice in the Retrosplenial Cortex (RSC). Imaging data were acquired using a Thorlabs Bergamo II multiphoton microscope. Tissue was excited by a Ti:Sapphire laser (Coherent) tuned to an excitatory wavelength of 920 nm. Beam focusing and light collection were achieved by a 16X water immersion objective (Nikon, NA 0.8, 80-120 mW output power measured at the sample). Beam-scanning was conducted by Galvo-Resonant mirrors. Emitted fluorescent lights were detected by a GaAsP photomultiplier tube (Hamamatsu) and digitized to a resolution of 800x800 pixels at a sampling rate of 19 Hz. We collected an 835x835 um window over layers II-III of the agranular RSC at depths between 100 and 200 um (imaging windows centred at -1.8 to -2.5 mm AP, 0.5 mm ML).