This thumb-sized microscope captures 'neural landscapes' from deep inside animal brains

The new design came with three fundamental improvements.
Ayesha Gulzar
Dr. Weijian Zong, a biological engineer and neuroscientist at the Moser Group,
Dr. Weijian Zong holding Mini2P brain Explorer

Kavli Institute for Systems Neuroscience 

Researchers have finally managed to reduce the two-photon fluorescence microscope into a thumb size device that allows them to see inside the brain of live and active animals. The device called Mini2P weighs just 2.4 grams and can be attached to a mouse's head without compromising its natural movements.

The microscope can record live images of neural landscapes, the likes of which have never been seen before. The innovation “opens the door to lines of scientific inquiry that were difficult, if not impossible, to initiate,” says Denise Cai, a neuroscientist at the Icahn School of Medicine at Mount Sinai in New York City.

Big inventions take years to develop

Traditional fluorescence microscopy uses a single photon to excite fluorescent dyes; as the molecules ‘relax,’ they release light. This is problematic in thick tissues: as the light passes through the cellular layers, it is absorbed and scattered. This problem is solved by two-photon microscopes, which use two longer-wavelength photons that can penetrate deeper into tissue.

But current two-photon systems are bulky and require specialized light sources and lenses. Researchers have been trying for two decades to shrink the technology into an instrument that is light and compact enough for use in freely behaving animals.

This feat was achieved by Edvard Moser, professor of Psychology and Neuroscience at the Kavli Institute for Systems Neuroscience, together with Weijing Zong, a biological engineer and neuroscientist at the Moser Group.

An earlier version of the two-photon device, reported in Nature Methods last year, weighed 4.2 grams and had stiff optical fiber bundles that slowed the mouse movement and disrupted its natural behavior.

The new design came with three fundamental improvements.

A lighter case, built from a plastic-like material instead of aluminum, a thinner and more flexible optical cable so that mice could run about their cage without getting tangled by wires, and a tiny electrically tunable lens.

By using static voltage, Zong could manipulate the curvature of the lens without causing a temperature rise. Changing the lens’s curvature will trigger Mini2P to shift the focal plane between the surface and deeper cell layers of the cortex, also enabling 3D structure recordings of brain tissue.

According to the researchers, Mini2P records simultaneously from thousands of brain cells. It can follow the same brain cells for more than a month and keep them in focus even through the most vigorous of activities, like repeated jumps from a 22-centimeter tall tower.

The researchers have tested Mini2P in multiple regions of the brain, such as the navigation system, the memory hub, and the visual area. By using a kind of patchwork quilt technique, it can map even larger neural landscapes, like 10,000 brain cells across the visual cortex. All measurements were made while the mouse was moving freely and doing what it normally does. This was simply impossible before Mini2P.

Mini2P is open-source

Mini2P is an open-source invention available from the Kavli Institute for Systems Neuroscience at NTNU in Trondheim.

Using Mini2P, researchers can monitor brain cells for over a month, which allows a more comprehensive exploration of various areas and functions throughout the cortex. This could help researchers study brain diseases such as Alzheimer’s in the future.

Collecting a significant amount of neural data from a single animal could also help reduce the number of animals used in research.

“We believe the Mini2P is a game changer, and we want to share it with neuroscientists and labs across the world,” says May-Britt Moser, a professor of Psychology and Neuroscience at the Norwegian University of Science and Technology

The blueprint, a shopping list, and instructional movies are available from GitHub. The Kavli Institute will also provide workshops to 16 researchers who will build their own Mini2P in December this year.

The study paper was published in the journal Cell.

Abstract:

We 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 allowed stable imaging with no impediment of behavior in a variety of assays compared to untethered, unimplanted animals. The improved cell yield was achieved through an 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. Successive imaging across multiple, adjacent FOVs enabled recordings from more than 10,000 neurons in the same animal. Large-scale proof-of-principle data were obtained from cell populations in visual cortex, medial entorhinal cortex, and hippocampus, revealing spatial tuning of cells in all areas.

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