Functional Imaging of Dentate Granule Cells in the Adult Mouse Hippocampus

The hippocampal dentate gyrus is critically involved in learning and memory. However, methods for imaging the activity of its principal neurons, the dentate gyrus granule cells, are missing. Here we demonstrate chronic two-photon imaging of granule cell population activity in awake mice using a cortical window implant that leaves the hippocampal formation intact and does not lead to obvious alteration of animal behavior. Using virus delivery, we targeted expression of genetically encoded calcium indicators specifically to dentate gyrus granule cells. Calcium imaging of granule cell activity 600-800 μm below the hippocampal surface was facilitated by using 1040 nm excitation of the red indicator R-CaMP1.07, but was also achieved using the green indicator GCaMP6s. We found that the rate of calcium transients was increased during wakefulness relative to an extremely low rate during anesthesia; however, activity still remained sparse with, on average, approximately one event per 2-5 min per cell across the granule cell population. Comparing periods of running on a ladder wheel and periods of resting, we furthermore identified state-dependent differences in the active granule cell population, with some cells displaying highest activity level during running and others during resting. Typically, cells did not maintain a clear state preference in their activity pattern across days. Our approach opens new avenues to elucidate granule cell function, plasticity mechanisms, and network computation in the adult dentate gyrus. We describe a technique that allows for chronic, functional imaging of dentate gyrus granule cells in awake, behaving mice in an intact hippocampal circuitry using genetically encoded calcium indicators. This novel approach enables the analyses of individual granule cell activity over time and provides a powerful tool to elucidate the mechanisms underlying structural and functional plasticity of the adult dentate gyrus.