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Micro-endoscopes for in-depth high-resolution optogenetics

Periodic Reporting for period 1 - ME-Optogen (Micro-endoscopes for in-depth high-resolution optogenetics)

Reporting period: 2017-04-01 to 2019-03-31

Understanding how the brain works is one of the grand scientific challenges of our times and constitutes a priority research line for the European Union. In this context, the use of light is becoming more important for the study of the brain with each passing day. By using light we have today the possibility of inducing and imaging neural activity in entire brain regions. However, to really decrypt the neural code we now need to understand how individual neurons collaborate together to build up a local brain circuit as well as how the spatial and temporal organisation of neural activity at the single cell level influences brain computation. Such complex biological problem demands more advanced optical methods capable of precisely targeting hundreds of neurons at will with high spatio-temporal precision and in the desired, even few mm deep, brain region.

To overcome these problems this project was organised around two main objectives: (1) the development of an advanced optical method capable of precisely targeting hundreds of neurons in the mouse brain simultaneously; (2) the development of a new generation of optical micro-endoscopes capable of reaching deep regions of the mouse brain while maintaining precise optical targeting. With these two objectives achieved we are now in a position of using the novel optical methods, and especially the micro-endoscope, to study deep neural circuits in the mouse brain, such as fear circuits in the mouse amygdala, or deep visual circuits.

The mouse brain can serve as a model to study certain neural circuits and formulate hypothesis on how the human brain function. The optical developments carried out during this project will definitely help understanding neural circuits at a depth and with a precision so far unattainable.
The ME-Optogen project aimed to develop a minimally invasive optical micro-endoscope for the in-depth two-photon stimulation and imaging of specific neurons with high spatio-temporal resolution.

In the initial stage of the project we focused on the optical development. We built a micro-endoscope that is able to deliver the desired two-photon excitation to specific locations in a given volume (such as to photo-stimulate certain neurons) as well as image with high resolution in the two-photon regime. These results were the subject of two publications.

In the second stage of the project we used the complete micro-endoscope system in vivo in mice. We performed experiments at shallow depths (in the cortex) in which the micro-endoscope, inserted through a craniotomy, was used to precisely image and photo-stimulate certain neurons. Also these results were published.

We are currently working on reaching larger depths in the brain and further improving the micro-endoscope. New results on this side are expected before the end of 2019.
We accomplished two main improvements beyond the state of the art:

(1) the development of a novel optical method to precisely target hundreds of neurons in a large volume;
(2) the development of a novel holographic micro-endoscope capable of reaching deep brain regions in mice and maintain high optical performances.

These two achievements are already used in our laboratory and will be used by others to better understand neural circuits with a precision and at a depth previously unattainable.
Final optical setup comprising an holographic and an imaging branch both coupled to the endoscope
50 holographic two-photon spots in a large volume that could be used to selectively excite neurons