Imaging Hippocampal Neuronal Activity across Sleep/Wake States in Freely Behaving Mice

Abstract

Hippocampal electrical activity is an important parameter for classification of sleep states. Hippocampal theta rhythm (6-9 Hz) is a distinguishing characteristic of Rapid Eye Movement (REM) sleep, but also occurs during wakefulness in association with repetitive motor activity. Electrophysiological recordings from the hippocampus have provided critical insights into the relationship between neuronal activity and behavioral states but are limited in the ability to follow the dynamics of large numbers of individual neurons over extended periods of time. To enable a broader and longer-term view of the hippocampal activity across behavioral states, we have utilized a combination of methodologies that collectively enable time-lapse imaging of CA1 neuronal calcium dynamics in behaving mice over periods of many weeks: (i) the nVista HD imaging system comprising a miniaturized (<2 g) integrated fluorescence microscope and custom data acquisition hardware and software that enables high-speed imaging (20-100 Hz) over a broad field of view (up to ~0.5 mm2) in freely-behaving mice; (ii) custom microendoscope probes that allow fluorescence imaging with micron-scale resolution in subcortical brain structures; (iii) a chronic mouse preparation that permits functional imaging of individual neurons lying deep in the brain; (iv) genetically-encoded fluorescent calcium indicators (e.g., GCaMP5 and 6) that report neuronal calcium dynamics of single cells; (v) abdominally-placed telemeters (DSI, Inc.) that allow wireless amplification and recording of EEG and EMG signals. By combining these techniques with image analysis algorithms for identifying individual neurons, extracting their dynamic traces, and registering individual cells across multiple imaging sessions, we are able to monitor intracellular calcium levels of CA1 pyramidal cells in individual neurons across the arousal state continuum and during manipulations of the homeostatic sleep drive. The ability to observe calcium dynamics of hundreds of cells while simultaneously recording electrophysiological activity in freely-behaving animals may lead to new insights regarding the role of the hippocampus in behavioral states and provides a powerful tool to study mouse models of neurological disorders.