Miniaturization of two-photon microscopy for imaging in freely moving animals.

This article describes the development and application of miniaturized two-photon-excited fluorescence microscopes ("two-photon fiberscopes"). Two-photon fiberscopes have been developed with the aim of enabling high-resolution imaging of neural activity in freely behaving animals. They use fiber optics to deliver laser light for two-photon excitation. Their small front piece typically contains a miniature scanning mechanism and imaging optics. Two-photon fiberscopes can be made sufficiently small and lightweight to be carried by rats and mice and to allow virtually unrestricted movement within a behavioral arena. Typically mounted to the animal's skull above a cranial window, two-photon fiberscopes permit imaging of cells down to at least 250 µm below the brain surface (e.g., in rat neocortex). In freely exploring animals, action-potential-evoked calcium transients can be imaged in individual somata of visual cortex neurons bulk-labeled with a calcium indicator. Two-photon fiberscopes thus enable high-resolution optical recording of neural activity with cellular resolution during natural behaviors.