With the flick of a whisker, scientists realized they had found a way to identify individual neurons as they were activated in the brain of a mouse.

Neuroscientist Alison Barth and her research team at Carnegie Mellon University developed mice with neurons, or brain cells, that harbor green fluorescent proteins.

The idea was that each neuron would fluoresce green light when it responded to a stimulus.

So Barth was heartened when she wiggled a single whisker on one of these transgenic mice and saw the glow of the tiny cluster of nerve cells processing the sensation.

"We could see green," she said. "It was a tremendously exciting moment to see that single spot on the surface of the brain representing that single whisker."

The findings will appear in Wednesday's issue of the Journal of Neuroscience.

Scientists have long known that different behaviors and activities use different parts of the brain.

But they couldn't tell whether all the nerve cells in a region were involved, how they responded to different conditions or whether some cells triggered changes in others.

Barth wanted to get a closer look at neurons in action.

One of the first genes to get turned on as a neuron reacts to a stimulus is called c-fos, so Barth's team engineered mice that would make green fluorescent protein when the c-fos gene starts working.

In addition to highlighting activated neurons, the technology allows researchers to examine their electrophysiological properties. Barth found that neurons transmitted electrical signals at an altered rate in response to changes in sensory input.

"What I hope this will do ... is enable us to understand the circuitry that is responsible, for example, for learning and memory," she said.

The c-fos gene stays on for short periods of time. Barth hopes to tie colorful proteins to other genes to see how neuronal activity changes over time.

The experiments now require the mice to be sacrificed, but Barth said it eventually might be possible to see the glowing nerve cells in living animals.

The ability to see activated neurons could advance research in other areas, such as sleep, addiction and appetite. Scientists could also develop a better understanding of how medications work in the brain.

"This paves the way for a more sophisticated approach for designing drugs to address particular maladies," Barth said.

She noted that other kinds of cells carry the fluorescent marker and the c-fos gene, so the transgenic mice could also shed light on other processes, such as the workings of the immune system.