A breath of not-so-fresh air

That whiff of a buddy’s just-back-from-lunch garlic, onion, tomato, salami and beer breath may kill your appetite, but to a mouse it provides a powerful incentive to grab a pizza and beer for himself.

As reported last week at Science News:

For rodents, any food smell combined with breath odor sends an irresistible “eat this” message to the brain, ScienceNews.org reported Thursday.

Carbon disulfide, a metabolic byproduct found in the breath of many mammals, stimulates specialized cells in the mouse nose, scientists report in the journal Current Biology.

These cells send a signal to specialized structures within the mouse brain that links an incoming odor with food that’s safe to eat, researchers said.

Olfaction arises from the interaction of several astonishingly complex subsystems. And they do a lot more than just detect odors. While some receptor systems help animals detect and decode a wide range of chemosensory input others affect and regulate specific kinds of behavior.

GC-D cells are a set of neurons in the main olfactory epithelium that detect specific hormones and urinary stimuli. Although GC-D cells were first identified back in the 1980’s, there’s no evidence that they play a role in odor recognition. So why are they present in some mammalian noses?

According to Science News:

Special nasal cells, called GC-D cells, seem to respond to the CS₂ in rodent breath, experiments by Munger and his colleagues reveal. A mouse that smells cinnamon on a buddy’s breath will choose cinnamon-scented food over any other flavor, the researchers found. And it doesn’t even have to be another mouse: Cotton balls laced with a food odor and CS₂ did the trick. But mice without working GC-D cells lost the ability to interpret this chemical message and they didn’t copy their compatriots’ food choices, the team reports.

The new work provides a molecular explanation for how these rodents learn what’s OK to eat, says neuroscientist Emily Liman of the University of Southern California in Los Angeles. For people and other primates, food preferences are mostly learned visually (or compelled via threats of no dessert). But for nocturnal creatures such as rodents, visual cues are limited. So it makes sense that there’s a scent signal, Liman says.

This safe-to-eat signal is so powerful that a mouse who has eaten poison will return for more if it catches a whiff of the poison on another mouse’s breath, says behavioral scientist Bennett Galef of McMaster University in Hamilton, Canada. “The strength of this social learning on food choice is huge,” says Galef, whose research revealed that mixing CS₂ with rat poison drew four times as many rodents to the bait.

Humans and most other primates lack GC-D cells. Our eyes are as important as our noses in helping us decide what to eat. GC-D cells are present in dogs and many other mammals and, if mice are an accurate model, their noses play an important role in determining their food preferences.

We’ve known that carbon disulfide was important in helping mice and rats identify food sources for a long time but their link to the GC-D cells is a new discovery. Bennett Galef’s earlier research on carbon disulfide led to its widespread use as an attractant in rat poison. He wrote:

By signalling “safety,” CS2 increases the attractiveness of materials to which it is applied.

Carbon disulfide may increase the effectiveness of poison baits in ways that extend beyond simple enhancement of initial intake. Results of 4 recent sets of experiments indicate that experience with the smell of a diet, either on the breath of a conspecific, or in association with CS2, interferes with rats’ ability to acquire a subsequent aversion (bait-shyness) towards that diet.

The GC-D cells’ influence on food preference could help explain why every time a new pup or foster dog who eats poop stays at our place the rest of the pack starts eating poop again too.

The use of carbon disulfide as an attractant in rat poison may explain why dogs are attracted to eating it.

It could also explain why my dogs have no interest in a new Nylabone until I spit on it, and why Audie likes to sniff our breath after we eat.

And – because humans lack GC-D cells, it may also help explain why we don’t understand dogs’ great interest in sniffing urine, stale breath and feces.

Pizza breath - Nom!