Mice Have Distinct Neural Subsystem to Smell Predators

Posted on July 26, 2012

Scientists have discovered that mice have a distinct neural subsystem that links the nose to the brain and is associated with smells such as those emitted by predators. The results of experiments, published here in PNAS, suggest neurons that detect odors associated with an instinctive response -- like fleeing when an approaching predator is detected -- are configured differently in mice than other olfactory neurons. The scientists say further research could determine whether this system automatically triggers flight or other primal behaviors. The researchers say the discovery also prompts the question whether other mammals have also have specially hardwired neural circuitry to trigger instinctive behavior in response to certain smells.

The researchers found that nerve cells in the nose of mice that express members of the gene family of trace amine-associated receptors (TAAR) have several key biological differences from neurons that express members of the olfactory receptor gene family. The differences between TAAR neurons and olfactory receptor neurons led Gilad Barnea, the Robert and Nancy Carney Assistant Professor of Neuroscience at Brown University, and his co-authors to conclude that they form an independent subsystem for certain smells.

Barnea says, "Our observations suggest that the TAAR-expressing sensory neurons constitute a distinct olfactory subsystem that extracts specific environmental cues that then elicit innate responses."

The newly found differences include the way TAAR neurons are wired to structures called glomeruli in the olfactory bulb, which is the brain area that initially processes smells. The glomeruli relay smell signals deeper into the brain where the perception of smell occurs and behavioral responses are initiated.

TAARs were discovered in the nose in 2006 by Stephen Liberles of Harvard Medical School, one of Barnea's collaborators. Liberles' team also found that some TAARs detected odors that are avoided by mice, such as a chemical found in the urine of carnivores like lions and tigers.

Barnea says the discovery that TAAR neurons operate separately from olfactory receptor neurons will lead to research on whether the glomeruli to which TAAR neurons connect selectively send signals to primal behavior centers in the brain, such as the amygdala that governs the fear response. That would complete a circuit that allows for specialized processing of alarming or repulsive odors to produce an instinctual fear-related behavior.

Barnea says, "We hypothesize that the neurons expressing TAARs may be some of the olfactory neurons that project to this area and elicit these behaviors. We are building a circumstantial case. Now we know what it looks like in the first relay, in the glomeruli. Our study provides the tools to look for the next step."

Having two different systems would help a creature like a mouse. A mouse that is able to flee without being distracted by the smell of food is a mouse that is more likely to survive. Most of the odors that a mouse encounters in life - the smell of cheddar, for example - become relevant only with experience and learning. These odors are detected by olfactory receptors and they are processed by parts of the brain that analyze and learn new information. When it comes to the scent of a predator, a mouse has very little room for mistakes. There may be many chances to consider cheese, but only one to recognize a threat. The researchers say a hotline between the nose and an instinct to flee might promote survival.