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Dot matrix: Two brain regions can be scanned simultaneously in enough detail to see individual cells.
A customized microscope with moveable arms lets researchers simultaneously watch neurons fire in two areas of a mouse brain, reports a paper published 17 November in Nature Neuroscience1. The device may help autism researchers study brain connectivity, which is altered in some people with the disorder.
The microscope marries two techniques commonly used in brain imaging: two-photon microscopy, which penetrates deep into brain tissue without damaging the surface, and calcium imaging, which uses a fluorescent probe to illuminate cells that are actively firing.
These techniques already allow researchers to study neuronal circuits in specific brain regions. But traditional microscopes are unable to capture circuits that span multiple brain regions with cell-level clarity.
In the new study, researchers used miniaturized microscopes called microendoscopes mounted on moveable arms to home in on two brain areas at once. Both scopes rely on the light from a single laser, but use their own photomultiplier tubes and mirrors to direct and enhance the light beam.
The researchers tested their tool on the brains of anesthetized mice. They were able to look at neurons in motor and sensory processing areas of the cortex that were up to 3.5 millimeters apart. They also scanned neurons in a deeper layer of the frontal cortex while peering into a layer of the hippocampus. In both tests, they could see the neurons’ cell bodies, long projections and the shorter branches that receive signals in detail.
They then tested the tool in awake mice while the animals ran on a trackball with their heads immobilized and looked at moving bars. They focused on two visual processing areas — the primary visual area and the lateromedial visual area — that are thought to play a role in recognizing patterns, objects and motion.
Neurons fire more in both areas when the mice are running than when they are resting. By comparing these two regions, the researchers discovered that the number of firing neurons in the lateromedial visual area depends on the number of firing neurons in the primary visual area.
The study details the microscope setup so that other researchers can develop their own. It may also be possible to engineer microscopes with three or more moving arms, the researchers say.
References:
1. Lecoq J. et al. Nat. Neurosci. 17, 1825-1829 (2014) PubMed
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