Digital holographic microscopy allows for 50 times better resolution in viewing neurons
A team of neurobiologists, psychiatrists, and advanced imaging specialists from Switzerland’s EPLF and CHUV report in the Journal of Neuroscience how digital holographic microscopy (DHM) can now be used to observe neuronal activity in real time and in three dimensions, with up to 50 times greater resolution than previously available.
The technique accurately visualizes the electrical activities of hundreds of neurons simultaneously, at up to 500 images per second — without damaging them with electrodes, which can only record activity from a few neurons at a time.
Traditional microscopes are limited to about 500 nanometers. DHM allows for viewing images down to 10 nanometers, but only in the axial dimension, Pierre Magistretti of EPFL’s Brain Mind Institute and a lead author of the paper explained to KurzweilAI. “Lateral resolution remains limited by the classical Abbe diffraction limit.” (The press release omits this qualification.)
The method has potential for testing out new drugs to fight neurodegenerative diseases such as Alzheimer’s and Parkinson’s.
How DHM works
To observe neurons in a Petri dish, scientists currently use flourescent dyes that change the chemical composition and can skew results. This technique is also time consuming, often damages the cells, and only allows researchers to examine a few neurons at a time.
Normally applied to detect minute defects in materials, DHM can bypass the limitations of existing techniques, the researchers say. “DHM is a fundamentally novel application for studying neurons, with a slew of advantages over traditional microscopes,” says Magistretti. “It is non-invasive, allowing for extended observation of neural processes without the need for electrodes or dyes that damage cells.”
To understand how DHM works, imagine a large rock in an ocean of perfectly regular waves. As the waves deform around the rock and come out the other side, they carry information about the rock’s shape. This information can be extracted by comparing it to waves that did not smash up against the rock, and an image of the rock can be reconstructed.
DHM does this with a laser beam by pointing a single wavelength at an object (in this case, neurons), collecting the distorted wave on the other side, and comparing it to a reference beam. A computer then numerically reconstructs a 3-D image of the neurons, using an algorithm developed by the authors. The laser beam also travels through the transparent cells and important information about their internal composition is obtained.
As noted in their paper, Magistretti, along with Christian Depeursinge, DHM pioneer and EPFL professor in the Advanced Photonics Laboratory, and their group induced an electric charge in a culture of neurons using glutamate, the main neurotransmitter in the brain. This charge transfer carries water inside the neurons and changes their optical properties in a way that can be detected only by DHM. “DHM is the first imaging technique able to monitor dynamically and in situ the activity of these cotransporters during physiological and/or pathological neuronal conditions,” the paper states.
Pharmaceutical research uses
Without the need to introduce dyes or electrodes, DHM can also be applied to high content screening of thousands of new pharmacological molecules. This advance has important ramifications for the discovery of new drugs that combat or prevent neurodegenerative diseases such as Parkinson’s and Alzheimer’s, since new molecules can be tested more quickly and in greater numbers.
“Due to the technique’s precision, speed, and lack of invasiveness, it is possible to track minute changes in neuron properties in relation to an applied test drug and allow for a better understanding of what is happening, especially in predicting neuronal death,” Magistretti says. “What normally would take 12 hours in the lab can now be done in 15 to 30 minutes, greatly decreasing the time it takes for researchers to know if a drug is effective or not.”
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