Scientists have pinpointed the region of the mammalian nose that detects the smell of fear--a discovery that solves a 35-year-old puzzle. A mysterious lump of cells called the Grueneberg ganglion appears to be a sensitive and specialized danger detector, picking up the alarm signals animals emit when they're distressed.
Grueneberg ganglions were first spotted in 1973 in mice, then forgotten for decades until olfactory researchers stumbled across them again in 2005. The function of the ganglion remained unknown, but biologist Marie-Christine Broillet of the University of Lausanne in Switzerland suspected it might have something to do with sniffing out fear when electron microscopy revealed proteins associated with pheromone reception. So she and colleagues created three classes of mice: those that had undergone surgery to remove their ganglion, those that underwent surgery but whose ganglion was intact, and those that never saw the scalpel. They then pumped pheromones from stressed mice into the test mice's Plexiglas cages.
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Those pheromones should cause mice to "freeze or run away," Broillet explains. And that's just what mice with intact ganglions did. But the ganglion-lacking mice went on about their exploratory business, apparently unaware of the alarm pheromones around them. Other than that, their sense of smell seemed to be intact: All groups of mice were able to sniff out a cookie hidden in their cages' bedding, the team reports tomorrow in Science. Previous studies had linked fear signals and the olfactory system in a general way, but Broillet's is the first to show exactly what part of the system detects the signals.
The work should give scientists a better understanding of how fear pheromones operate, says Columbia University neuroscientist Stuart Firestein, who calls the study an "excellent piece of work." Mapping how the ganglion translates alarm signals to the mouse brain could also help explain the process in other mammals with ganglions, including humans, and could even spur new nontoxic approaches to pest control. "The study's as significant for giving us so much to do," he quips, "as it is for what it shows."
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