April 30, 2013
Do You Smell Cat Urine? – A Single Olfactory Gene Alerts Mice To Predators
Alan McStravick for redOrbit.com — Your Universe Online
Dogs can hear sounds at higher frequencies. Bats can ℠see´ with sonar although their eyes are practically useless. And, apparently, the mouse can smell a predator´s urine from a mile away. Each of these traits has played an important role in the evolution of each animal. But what is especially amazing with regard to mice and their olfactory receptors is their ability to sense a threat may be determined by only one gene, according to a recent study by researchers at Northwestern University.
Led by Thomas Bozza, a team of neurobiologists published their research findings in the most recent edition of the journal Nature and claim their results reveal something new about our sense of smell: individual genes matter.
Specifically, the team has shown how removing a single olfactory receptor from mice can affect how they act. The gene in question is called TAAR4 and is responsible for encoding a receptor designed to respond to a chemical enriched in the urine of carnivores. The team was able to show mice with TAAR4 avoid the scent marks of predators while mice that lack the gene do not.
The study represents a significant advancement in the understanding of how sensory receptors contribute to our perception of different aromas. The study of olfactory receptors lags far behind research into senses like vision and hearing. For instance, researchers know color vision is generated by the cooperative action of three light-sensitive receptors found in the sensory neurons of the eye. If someone has a mutation in just one of the three receptors, they experience color blindness.
According to Bozza, “It is easy to understand how each of the three color receptors is important and maintained during evolution, but the olfactory system is much more complex.”
Humans, for example, possess 380 olfactory receptor genes. The humble mouse has more than 1,000 of these genes. A single aroma, like coffee or your favorite perfume or cologne, will likely activate several receptors giving each fragrance a sort of scent-signature in your brain.
“The general consensus in the field is that removing a single olfactory receptor gene would not have a significant effect on odor perception,” stated Bozza.
To test this theory the team genetically removed a specific subset of olfactory receptors called trace amine-associated receptors or TAARs. Their mouse subjects have a total of 15 TAARs. Of these, 14 are located in the nose and are tasked with detecting odors.
An amine is a class of chemical found in biological systems. It is enriched in decaying material and rotting flesh. Both mice and humans are predisposed to avoid amines as the aroma they put off is strongly unpleasant.
The first experiment showed that the mice subjects lacking all 14 TAARs presented no aversion to amines whatsoever. The researchers conducted a second experiment where they removed only the TAAR4 gene from their subjects. This is because TAAR4 is a specialized gene that only responds to phenylethylamine, an amine concentrated in the urine of carnivores. This second experiment produced mice unable to avoid predator cat urine while still avoiding the other amines.
“It is amazing to see such a selective effect,” said Adam Dewan, a postdoctoral fellow and research collaborator. “If you remove just one olfactory receptor in mice, you can affect behavior.”
Speaking on the ubiquity of TAARs genes in each mammal studied so far, Bozze added, “The fact that TAARs are highly conserved means they are likely important for survival.”
Many scientists believe the TAARs genes are important for alerting animals to dangers by making them extremely sensitive to the smell of amines. Humans use this gene, for example, to avoid rotting and spoiled foods. Amines become particularly pungent to the human nose during the decomposition process. The relay of information from these gene receptors, it is believed, is targeted to a specific part of the brain that elicits innately aversive behavior in animals.
Previous research conducted by Bozza´s lab has shown that neurons in the nose responsible for expression of TAARs are connected to a site-specific region of the olfactory bulb. This is the part of the brain that receives olfactory information first. The researchers say this suggests that TAARs may elicit hardwired responses to amines in mice, and perhaps in humans as well.
“We hope this work will reveal specific brain circuits that underlie instinctive behaviors in mammals,” Bozza stated. “Doing so will help us understand how neural circuits contribute to behavior.”