ULTRA-FAST VOLTAGE AND CALCIUM IMAGING IN FREELY-MOVING MICE WITH A CONFOCAL HEAD-MOUNTED FIBERSCOPE

Elucidating the neural bases of memory requires recording activity in hundreds of neurons at near single-spike time resolution with minimal behavioral constraints. In this work, we aimed at conceiving a microscope that would allow fast fluorescence recording of fluorescent voltage indicator during mouse behaviour.

Building on previous work [1,2], we developed a novel head-mounted fiberscope allowing for 2-color confocal fluorescence imaging of neuronal activity in freely behaving mice at a speed of up to 300 Hz. The key element of the fiberscope is an image guide (fiber bundle) that is used as a relay between a benchtop line scanning confocal microscope and the animal, which enables the combination of high-speed scanning of tens of cells (240µm diameter field of view) with free animal behavior. The design of the fiberscope provides a firm and reproducible connection of the fiber bundle to the head-mounted implant, allowing for stable recordings across days.

Using this device, we have demonstrated fast fluorescence imaging of neuronal activity with cellular resolution in the CA1 field of the dorsal hippocampus of mice running on a linear track. In pilot experiments, we successfully recorded activity of CA1 neurons using both voltage and calcium sensors, which were delivered to the target structure by intracranial injection of AAV vectors. We characterized the fiberscope performance during repeated imaging sessions.

[1] Szabo et al, Neuron 84, 1157–1169 (2014)

[2] Dussaux et al, Scientific Reports 8, 16262 (2018)