The discovery that TAAR neurons operate separately from olfactory receptor neurons, Barnea said, 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.
"We hypothesize that the neurons expressing TAARs may be some of the olfactory neurons that project to this area and elicit these behaviors," Barnea said. "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."
Hypothetically, having two different systems could reflect the different meanings that various smells can have for an organism. 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.
But 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. A hotline between the nose and an instinct to flee might promote survival.
As for human beings, Barnea said, there is more debate than certainty about whether smells trigger instinctual behaviors, but he notes that humans do have several intact TAAR genes.
|Contact: David Orenstein|