For many of us, when faced with a dangerous situation, our first reaction might be to yell and scream or in some way vocally communicate the danger to others. However, unbeknownst to us, we are also likely communicating the danger in a more subtle way. Plants and animals have evolved a highly specialized mechanism for alerting their own kind to the presence of a predator, injury or stress. The distress signal is emitted in the form of airborne molecules known as alert pheromones. In mammals, the release of these pheromones is correlated with specific `alarm' behaviour such as freezing,attacking or scattering. While this behaviour is well characterized and has often been observed, the source of the signal, the precise chemical and the sensory system used to detect the signal remain a mystery. A recent study from Julien Brechbühl, Magali Klaey and Marie-Christine Broillet from the University of Lausanne, published in Science in August 2008,characterized and identified a region of the mouse olfactory (smell) circuit that acts as the detection centre for alert pheromones and, as such, is the first step to initiating aversive behaviour.
A recently discovered subregion of the olfactory system named the Grueneberg ganglion, located right at the tip of the nose, contains a neuronal population with morphology and molecular components unique amongst olfactory neurons, suggesting that they possess distinct chemosensory functions. Using alert pheromones, isolated from other mice, the investigators used calcium imaging techniques together with behavioural analyses and found that this specialized neuronal population specifically responds to alert pheromones released from other members of the same species.
Calcium concentration within the cell body is commonly used as a measure of cell activation; when a cell is activated, the calcium level increases. Using fluorescent calcium dyes, so that rises in intracellular calcium can be imaged through special lenses on a camera, the authors found that the neurons in the Grueneberg ganglion were strongly activated when exposed to alert pheromones collected from other mice. Previous speculation about the function of the neurons in this region proposed an involvement in mother–pup recognition. However, these cells were unresponsive to either mouse milk or mammary secretions, nor did they respond to mouse urine or other known mouse pheromones other than alert pheromones. This indicates a specific role for sensing intraspecies stress signals.
Next, the authors performed behavioural experiments in which they surgically cut the neuronal projections from the cells in the Grueneberg ganglion thereby disabling their mode of communication. Normally, exposure to alert pheromones stimulates a freezing reaction in rodents but, in experimental animals with impaired signaling in the Grueneberg ganglion cells,exposure to alert pheromones elicited no such reaction and the animal was content to continue exploring.
The authors concluded that neurons in this olfactory region are specialized to act as a warning system, dedicated to rapidly responding to stress signals. Pheromones are an integral form of communication in many diverse organisms but perhaps this tiny region at the tip of our nose has played a more important role in our survival as a species than we could ever have imagined.
