One of the most effective tools for studying neural circuits in vivo and correlating the activity of large neuronal populations with complex behaviors that could not be replicated in head-fixed animals has been the development of miniature fluorescence microscopes (miniscopes) over the past ten years. These microscopes and genetically encoded calcium indicators and voltage-sensitive dyes have enabled researchers to study neural circuits in timescales ranging from days to months.
Miniature Fluorescence Microscopes (Miniscopes)
Miniature fluorescence microscopes are adaptable instruments that provide an in-depth understanding of crucial neurological mechanisms involved in complicated behaviors or animal models of illness, inspiring the creation of a new generation of microscopes to broaden the scope of research into neural circuits.
Altering Miniscope's Focus Plane
The capacity to alter the focus plane of the miniscope accurately and quickly during tests is essential for maximizing its potential. This would make it possible to image on many focal planes, allowing multi-plane imaging and a more stable focusing mechanism, enhancing individual neurons' tracking precision over time.
Tunable acoustic gradient index lenses, voice coil-motor actuators, and piezoelectric actuators are a few techniques that can accomplish this goal in multi-photon imaging.
These methods are not easily transferable to a miniaturized, single-photon microscope for studying neural circuits in freely behaving rodents. These applications lend themselves better to the integration of electrically tunable lenses (ETLs, or liquid lenses).
In a previous study, scientists created a one-photon miniscope with an adjustable liquid crystal lens for sub-cellular resolution imaging. A tabletop two-photon implementation of a liquid lens-based remote focusing device was disclosed but only applied to head-restrained behaviors.
Although an ETL is included in the UCLA miniscope extension, its weight of 13.9 g renders it too substantial to be utilized on mice moving about freely.
How the Study was Conducted
Researchers in this study developed a small open-source miniscope that can alter its focal plane dynamically and remotely by using a liquid lens with a changeable focus that is readily accessible for use in unique imaging systems.
The basic body of the miniscope is made of black resin and was 3D printed using Solidworks.
It also includes a light source, lenses, and a CMOS sensor housed on a special PCB that links to the data-collecting system.
The researchers employed a liquid lens with a variable focus that enables quick focal length changes. The miniscope image sensor was controlled and synced by a proprietary FPGA control board.
Six mice were imaged in vivo to show the capability of imaging at the single-neuron level and identifying distinct neurons across various focus plane depths. Fifteen open field trials were used for imaging, with 2000 frames each trial recorded at 10 frames per second and a different focus plane used for each frame.
Light scattering in the deeper brain tissue was a physical restriction on the greatest focal depth change, which was empirically determined to be 60 m for all imaging mice.
Significant Findings of the Study
In this study, researchers proposed a portable one-photon microscope that can image at various focal depths.
By altering the pulse width of the control signal sent to a liquid lens with variable focus, the focal plane may be adjusted dynamically between frames and synced to the image sensor.
In vivo imaging of GCaMP7f-expressing neurons in the mouse medial prefrontal cortex (mPFC) during an open-field test was used to evaluate the system.
Experimental results demonstrated that the suggested design could photograph neurons throughout an axial scan of around 60 mm, which results in a 40% increase in the total number of neurons imaged compared to single-plane imaging.
Due to its benefits over alternative adjustable focus technologies, ETLs will be included in small single- and multi-photon microscopes more often in the future. Applying the variable focus design to dual-color imaging, where the focal plane shift between the two layers of neurons observed with two distinct colors may be compensated for on alternate frames, is one possible evolution of the concept.
Giovanni Barbera, Rachel Jun, Yan Zhang, Bo Liang, Yun Li & Da‑Ting Lin (2022) A miniature fluorescence microscope for multi‑plane imaging. Scientific Reports. https://www.nature.com/articles/s41598-022-21022-9
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