Two-photon imaging of neural circuit activity in freely-moving rodents

Development of a miniature 2-photon miniscope for large-scale calcium imaging in freely moving mice allows stable simultaneous recording of more than a thousand cells across multiple planes of densely active cortical regions in a wide spectrum of behavioral tasks without impediment of the animal’s behavior.

While the Neuropixels probes increase cell numbers by an order of magnitude, their spatial resolution is as limited as in conventional tetrode-based extracellular recordings. Optical imaging provides a complementary solution to neurophysiological recordings, with high spatial but poorer temporal resolution. The technique uses an indicator molecule such as fluorescent GCaMP6 to monitor changes in intracellular free calcium concentrations optically across wide fields of view (FOV), allowing many hundreds of individual neurons to be followed at near-spike temporal resolution. High spatial resolution has been obtained with two-photon (2P) microscopy but until now this technology was limited to stationary table-top setups, where the animal must be head-fixed under the objective.

Recently, Zong et al. (Cell, 2022) developed a miniaturized two-photon microscope (MINI2P) for fast, high-resolution, multiplane calcium imaging of over 1,000 neurons at a time in freely moving mice. With a microscope weight below 3 g and a highly flexible connection cable, Mini2P allows stable imaging with no impediment of behavior in a variety of assays compared to untethered, unimplanted animals. The improved cell yield is achieved through a optical system design featuring an enlarged field of view (FOV) and a microtunable lens with increased z-scanning range and speed that allows fast and stable imaging of multiple interleaved planes, as well as 3D functional imaging. 

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