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An expanding molecular toolbox untangles neural circuits

Published (updated: ) in Brain Activity, Brain/Neurology, Circuits, Optogenetics.

Life is full of nervous reactions — a head snaps towards a voice, leg muscles tense at the sound of a starting gun and thirsty mice scamper towards a squirt of water when trained to respond to a certain tone.

The mechanisms behind such reward-related behaviours are notoriously difficult to unpick. Nerve cells often snake through multiple brain areas, and their long axons and dense, tree-like dendrites can spark cellular conversations with thousands upon thousands of neighbours. Neural filaments can be exceptionally fine, and their positioning is crucial: disruptions in neural networks can lead to a range of neurological conditions. Yet, “If you want to label more than a few neurons at the same time and then trace where their axons go, it’s really difficult”, says Xiaoyin Chen, a neuroscientist at the Allen Institute for Brain Science in Seattle, Washington.

Still, researchers are slowly creating the tools to untangle that complexity, harnessing the power of sequencing, optogenetics and protein engineering to trace neuronal connections, record their activity, measure their inputs and outputs and map their networks.

The conventional way to label cells is to tag them with fluorescent dyes and then look at them under the microscope. But this technique can typically track only a few handfuls of cells, so Anthony Zador and his co-workers at Cold Spring Harbor Laboratory in New York developed a tool to trace thousands of individual neurons in parallel. Rather than dyes, they used viruses to insert a unique RNA sequence, or barcode, into each neuron. They could then map how the neurons connected with each another by taking samples from regions close to where the virus was injected, grinding them up and extracting the RNA, then sequencing it to look for the barcodes.