Tracking techniques offer a long-term view into the mouse brain.

For the first time, scientists have been able to watch these long-term changes in the brains of living mice. This snapshot of what they saw in one mouse illustrates the complexity of the branching: blood vessels in red and pyramidal neurons—critical for long-term memory—in green.

Conventional light microscopy couldn’t take Mark Schnitzer where he wanted to go: beyond the surface. He wanted to see cells deep within the brain and watch them over time.

So the HHMI investigator at Stanford University designed a technique that works in mice. His team uses micro-endoscopes to reach into areas such as the hippocampus, which supports certain forms of learning and memory. Most recently, his group has made the device more portable so that a mouse can go about its day while images are being captured.

Hear Schnitzer describe his new technique.

He is able to image deep-brain neurons and monitor them over days or months. “We can return to the same subcellular processes, the same dendrites,” Schnitzer says. “This gives us the capability of watching how neurons and other cell types may change over the long term.” They can observe changes in the brain during normal events, such as learning, and when things go wrong, such as during disease progression.

Take a 3-D journey through a living mouse’s brain.

Postdocs Alessio Attardo and Yaniv Ziv work side by side at the lab bench, each with an anesthetized mouse under a microscope. They implant a thin glass tube—half as thin as a grain of rice—into each mouse’s brain. The tube will help guide the insertion of a needle-like probe adjacent to the mouse’s hippocampus. After the delicate surgery, the mice return to their cages until the researchers are ready to take images.

For the imaging, the researchers take the mice to a room with a fluorescence microscope. The mice have been engineered to express fluorescent proteins in their hippocampal neurons. A microendoscopic probe, coupled to the microscope and inserted through the guide tube, shines a laser on the neurons and detects emitted fluorescence. The researchers can collect images of the neurons repeatedly over time, with the guide tubes ensuring that they observe the same cells each time.

In work published February 2011 in Nature Medicine, Schnitzer and colleagues used this technique to see whether particular neurons in the mouse hippocampus change their branching pattern over time. They monitored CA1 pyramidal neurons, critical for forming long-term memory, over a seven-week period. Those neurons “are only two synapses away from the dentate gyrus, where there’s ongoing birth of neurons throughout adult life,” Schnitzer says. “We had hypothesized that these neurons might show some dendritic turnover.” Not so, according to their results. The dendrites formed by those hippocampal neurons stayed quite stable. “This raises some interesting questions about how the circuit may accommodate the new inputs,” he says.

Videos: Yaniv Ziv and the Schnitzer Lab

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