Do your palms sweat when you walk down a poorly lit street at night? That feeling may be traced to the firing of newly identified anxiety cells deep inside your brain, according to a study published in the journal Neuron.

The researchers found the cells in the brains of mice, inside a structure called the hippocampus. But the cells probably also exist in humans, says senior author Rene Hen, a professor of psychiatry at CUIMC.

"We call these anxiety cells because they only fire when the animals are in places that are innately frightening to them," Hen says. "For a mouse, that's an open area where they're more exposed to predators, or an elevated platform."

The firing of the anxiety cells sends messages to other parts of the brain that turn on anxious behaviors– in mice. The cells found in this study are the first known to represent the state of anxiety, regardless of the type of environment that provokes the emotion.

When people overestimate threats anxiety becomes a problem. To understand how things go awry in anxiety disorders, researchers in the Hen lab have been looking at mice to decipher how the brain processes healthy anxiety.

"We wanted to understand where the emotional information that goes into the feeling of anxiety is encoded within the brain," said Mazen Kheirbek, who was an assistant professor at CUIMC before moving to UCSF.

The hippocampus plays a well-known role in the brain's ability to form new memories and to help animals navigate through complex environments. But recent research has also implicated the hippocampus in regulating mood, and studies have shown altering brain activity in the ventral part of the hippocampus can reduce anxiety.

It's also known that the hippocampus sends signals to other areas of the brain- the amygdala and the hypothalamus– that have also been shown to control anxiety-related behavior.

Using a miniature microscope inserted into the brains of the mice, Hen's team recorded the activity of hundreds of cells in the hippocampus as the mice freely moved around their surroundings.

Whenever the animals were in exposed, anxiety-provoking environments, the researchers noticed that specific cells in the ventral part of the hippocampus were active. And the more anxious the mice seemed, the greater the activity in the cells.

By turning the anxiety cells off and on using a technique called optogenetics that allows scientists to control the activity of neurons using beams of light, the researchers found that the anxiety cells control anxiety behaviors.

When the cells were silenced, the mice stopped producing fear-related behaviors, wandering onto elevated platforms and away from protective walls. When the anxiety cells were stimulated, the mice exhibited more fear behaviors even when they were in "safe" surroundings.

The discovery of the anxiety cells raises the possibility of finding treatments that target them and reduce anxiety. Hen says, "If there's a specific receptor on the cells that distinguish them from their neighbors, it may be possible to produce a new drug to reduce anxiety."