Mice smell CO2 at levels higher than in normal air

Specialized nasal neurons help mice smell carbon dioxide at levels higher than that in normal air, a recent study by scientists from the National Institute of Biological Sciences, Beijing, has revealed.

London, Aug 17 : Specialized nasal neurons help mice smell carbon dioxide at levels higher than that in normal air, a recent study by scientists from the National Institute of Biological Sciences, Beijing, has revealed.

Minmin Luo and his colleagues tracked down the neurons that mice use to detect CO2 and found that the level of the gas above which the mice smelled it, was just 0.066 percent.

Though this was about twice the average level of CO2 in the atmosphere (0.038 percent), it was much less than the concentration in exhaled breath (about 4.5 percent) or the level considered safe for humans (0.5 percent), the researchers said.

The team targeted the neurons in the nose that were already known to express the CO2 - processing enzyme carbonic anhydrase type II (CAII). These cells, called guanylyl cyclase D cells, glowed in the presence of CO2, showing when mice were picking up the scent.

While humans cannot smell CO2, other animals are known to have an ability to detect relatively high levels of the gas.

Insects, too, can detect CO2, but they do it via membrane receptors rather than through any kind of nose.

"We did not expect it at all. Most people don't think CO2 is an odorant. It is used as an irritant, not an olfactory cue," Luo said.

Luo said when mice were exposed to more and more CO2, their behaviour changed.

Given a choice between areas of high and low CO2 concentration, the mice avoided anything higher than 0.2% CO2, Luo said, adding that this could mean that as climate change caused atmospheric CO2 levels to rise (predicted to be 0.05-0.1% by 2100), changes in mouse behaviour might be spotted.

"There will be some behavioural effect; but what that effect will be is not known," Nature quoted Luo as saying.

ANI