Mice can sniff oxygen

Rodents smell oxygen levels in the air with specialized cells of the nasal mucosa

Fine nose: This mouse can smell when there is too little oxygen in the air. © thinkstock
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Sensitive Supernases: Unlike humans, mice can even smell oxygen. For two special receptors in their olfactory mucosa react immediately when the oxygen content of the air changes and, for example, drops. Probably the rodents have developed this "oxygen sense" to protect themselves in their caves from a harmful lack of respiratory gas, the researchers speculate in the journal "Neuron".

Mice have a very fine nose: More than a thousand olfactory receptor genes in their genome ensure that the animals can smell the innumerable odors in their environment. The "perfume" of the mouse males also plays an important role in the choice of partner. Now, however, it turns out that the mice can even smell a substance that is vital, but neutral for us: the oxygen.

Mysterious type B cells

The mice's unusual smelling ability was discovered when Katherin Bleymehl from the University of Saarland and her colleagues examined the mysterious type B cells. These cells in the olfactory mucosa did not appear to respond to any fragrance and did not carry any known olfactory receptors on their surface. Their function was therefore puzzling, as the researchers explain.

For their study, the researchers exposed cells of the olfactory mucosa of mice to different oxygen levels. A calcium-sensitive dye indicated whether and which cells became active. The surprising result: the mysterious type B cells already reacted with a slight decrease in the oxygen concentration in the atmosphere and began to fire more intensely.

Three times unusual

"This has revealed a new and unexpected role for the mice's sense of smell: it serves as a sensor for low oxygen levels in the ambient air, " say Bleymehl and her colleagues. This is unusual for three reasons: On the one hand, odor sensors usually start when the value of "their" scent molecule increases - it accumulates more strongly. But the oxygen sensor reacts to too little breathing gas. display

Scent mucous membrane of a mouse under the microscope: The type B cells reacting to oxygen deficiency are colored green, normal olfactory cells red. MPI of Neurogenetics

On the other hand, mammals usually can not smell oxygen. For example, if we lack breathing gas in the environment, this is registered by a sensor in our carotid arteries: it triggers an alarm if there is too little oxygen in the blood. However, a sensor that measures the oxygen level directly from the inhaled air is missing. The mice, however, have such extra protection.

And third, the mice register an oxygen deficiency without an olfactory receptor gene. They therefore have no special docking point for oxygen on the olfactory mucosa. Instead, two normal genes seem to be crucial for the rodents' "oxygen craving": when the genes Gucy1b2 and Trpc2 were blocked in the animals, their oxygen alarms did not react. How exactly the sensor works is still unknown.

Adaptation to the underground way of life

But why does M use have this "sixth" sense of oxygen? The answer is probably their way of life, as the researchers explain. For the rodents live in caves and raise their young there. They build their nests where oxygen is abundant. "The offspring need enough oxygen, otherwise the pups are under-supplied, " says Frank Zufall from the Saarland University.

Being able to sniff timely places with too little oxygen in time can therefore prevent the rodents from accidentally setting up their nests in unfavorable, poorly ventilated areas. Experiments also showed that mice learn very quickly where to find places with low oxygen concentration and then avoid staying in those places.

Interestingly, we humans also own the two oxygen sensor genes Gucy1b2 and Trpc2. For us, however, they are only so-called pseudogenes, which are not read and most likely can no longer form proteins. Whether the type B cells occur in humans and are susceptible to oxygen deficiency, is still unclear. (Neuron, 2016; doi: 10.1016 / j.neuron.2016.11.001)

(Max Planck Society, 12.12.2016 - NPO)