Optical wavefront shaping is a powerful technique to control the distribution of light in the focus of a microscope. This ability, combined with optogenetics, holds great promise for precise manipulation of neuronal activity with light. However, a deeper understanding of complex brain circuits requires pushing light-shaping methods into a new regime: the simultaneous excitation of several tens of targets, arbitrarily distributed in the three dimensions, with single-cell resolution. To this end, we developed a new optical scheme, based on the spatio-temporal shaping of a pulsed laser beam, to project several tens of spatially confined two-photon excitation patterns in a large volume. Compatibility with several different phase-shaping strategies allows the system to be optimized towards flexibility, simplicity, or multiple independent light manipulations, thus providing new routes for precise three-dimensional optogenetics. To validate the method, we performed multi-cell volumetric excitation of photoactivatable GCaMP in the central nervous system of drosophila larvae, a challenging structure with densely arrayed neurons, and photoconversion of the fluorescent protein Kaede in zebrafish larvae.

15Patterning light at the single-cell level over multiple neurons in the brain is crucial for optogenetic photostimulation that can recapitulate natural activity patterns and, thereby, determine the role of specific components of brain activity in behavior. To this end we have developed a method for projecting three-dimensional, 2-photon excitation patterns that are confined to many individual neurons. The new versatile optical scheme generates multiple extended excitation spots in a large volume with micrometric 20 lateral and axial resolution. Two-dimensional temporally focused shapes are multiplexed several times over selected positions, thanks to the precise spatial phase modulation of the pulsed beam. This permits, under multiple configurations, the generation of tens of axially confined spots in an extended volume, spanning a range in depth of up to 500 µm. We demonstrate the potential of the approach by performing multi-cell volumetric excitation of photoactivatable GCaMP in the central nervous system of Drosophila 25 larvae, a challenging structure with densely arrayed and small diameter neurons, and by photoconverting the fluorescent protein Kaede in zebrafish larvae. Our technique paves the way for the optogenetic manipulation of a large number of neurons in intact circuits.

In the past 10 years, the use of light has become irreplaceable for the optogenetic study and control of neurons and neural circuits. Optical techniques are however limited by scattering and can only see through a depth of few hundreds µm in living tissues. GRIN lens based micro-endoscopes represent a powerful solution to reach deeper regions. In this work we demonstrate that cutting edge optical methods for the precise photostimulation of multiple neurons in three dimensions can be performed through a GRIN lens. By spatio-temporally shaping a laser beam in the two-photon regime we project several tens of spatially confined targets in a volume of at least 100 × 150 × 300 µm 3 . We then apply such approach to the optogenetic stimulation of multiple neurons simultaneously in vivo in mice. Our work paves the way for an all-optical investigation of neural circuits in previously inaccessible brain areas.