One of the major goals of neuroscience is to understand how our brains are shaped by everyday experiences. The ideal way to understand that complex process is to observe the brain in action. Now, researchers at MIT have done just that – they used two-photon microscopy to visualize chemical activity in brain cells of living animals after the animals spent time in different visual environments.

“This is the first time we can look at the molecular activity inside individual cells in response to a sensory experience,” says first author Kuan Hong Wang, a research scientist at MIT who will be leading a lab at the National Institute of Mental Health in September.

Standard techniques for visualizing brain activity are limited; imaging technologies like functional MRI can show which regions of the brain are activated by experiences, but they can’t zero in on individual cells or the molecules inside those cells. Two-photon imaging, on the other hand, has the ability to detect signals made to light up individual cells in the outermost layers of the brain. To use the technique in a living mouse, scientists create small glass windows in the animal’s skull, then focus the microscope on the glass.

Past studies have used a similar technique to see how the structure of neurons changes over time (see “Old Brains Learn New Tricks”). But Wang, working in the lab of neuroscientist Susumu Tonegawa at MIT’s Picower Center for Learning and Memory, wanted to develop a way to detect cells only as they become active.

The researchers replaced a portion of a mouse gene called Arc with another gene that encodes a fluorescent protein. Arc is known to be activated in the brain by sensory input, but its specific function in the visual system is not well understood. In the engineered mice, whenever cells normally manufacture the Arc protein, they instead create a fluorescent protein that glows under the microscope’s laser light when the gene is activated. One group of mice retained one copy of the Arc gene along with the fluorescent gene, while another group received two copies of the fluorescent gene.

Wang and the team then used the microscope to track changes in the visual cortex of mice in response to different visual stimuli. The researchers could detect when Arc was turned on in the visual cortex over the course of several days, and they could also get clues about Arc’s function by looking for differences between the mice that still expressed Arc and those that didn’t.

The researchers found that Arc changes the way neurons respond to the visual world. For example, when mice that expressed Arc were repeatedly exposed to either horizontal or vertical lines, their visual systems were more “in tune” to that stimulus, responding more specifically to the stimuli they had been exposed to.

Wang believes that because Arc is active throughout the brain, it can be used as a general marker of activity in brain cells. These engineered mice, he says, can be used “to locate cells in a variety of regions that are activated by normal sensory experience.”

The technique could also be used to monitor cell activity in animal models of neurological disease or degeneration, or to test the effects of therapies in treating these conditions.

Josh Huang, a neuroscientist at Cold Spring Harbor Laboratory in New York, says that this study is an innovative way to study plasticity, the ability of neurons and their connections to change in response to experience. “It’s very clever and certainly shows in the fluorescent neurons that some cellular plasticity event is going on,” he says. But he says that Arc would be more useful if more were known about how it is activated; otherwise it’s difficult to know exactly what its activity represents.

Huang says his lab is developing ways to study brain circuitry through other genes or cellular processes regulated by activity or experience. He adds that scientists would like to find ways to visualize the brain’s activity on several different levels, from the instantaneous firing of a neuron to the long-term changes that help the brain form memories.