One whiff of an alarm pheromone sends figurative shivers down a mouse's little spine.

Animals in distress release such pheromones, which serve as warnings to others of their kind. But just how mice — or other mammals — detect the chemicals has been unknown.

Now, researchers have found that the mouse's danger detector is a mysterious wad of sensory cells at the tip of the nose called the Grueneberg ganglion. The structure was first described thirty-five years ago, but has been largely ignored ever since.
In 2005, five research teams independently discovered that the Grueneberg ganglion connects directly to the olfactory system, and the race was on to determine its function.

Some scientists thought it enabled mouse pups to recognize their mothers, perhaps from chemical cues in milk. Then Julien Brechbühl, his graduate adviser Marie-Christine Broillet at the University of Lausanne in Switzerland, and a colleague noticed that the structure's tiny sensory hairs were sheathed in protective layers of collagen and keratin, permeable only to water-soluble and highly volatile molecules, such as certain components of milkor alarm pheromones.

The researchers soon discovered that slices of Grueneberg tissue respond to alarm pheromones, but not to mouse milk or mammary secretions. As a final test, they released the alarm pheromone in cages with normal mice and watched as the mice huddled against the back wall. But after they severed the Grueneberg ganglion's connection to the olfactory system, mice failed to detect the chemical. Still, the researchers say, the mice had no problem finding cookies . . . or their mommies.

The findings were detailed in the journal Science.