14 DAY TRIAL // These odd rodents are ugly as hell. The mole rats are hairless, wrinkled, cold blooded (the only case known in mammals) and... totally insensitive to the pain induced by acids or chili peppers, according to a new research. Mole rats live in colonies in poor-oxygenated burrows about 2 m (6 ft) underground, in Eastern and Southern Africa.
Mole rats were known to be highly sensitive to touch, to compensate for their useless sight in the dark burrows. But the new research shows that these rats are devoid of the chemical Substance P, which induces the sensation of burning pain in mammals. "When unconscious mole rats had their paws injected with a slight dose of acid, about what you'd experience with lemon juice, as well as some capsaicin (the active ingredient of chili peppers) the rodents showed no pain," said co-author Thomas Park, neurobiologist at the University of Illinois, Chicago.
This surprised the researchers, as all vertebrates (from fish and frogs, to reptiles, birds and other mammals) react to the acid. Then, the team employed an engineered cold sore virus to transport the gene for Substance P only to one rear foot of lab mole rats. The gene restored the mole rats' capacity to feel the burning sensation induced by capsaicin, just like other mammals.
"They'd pull their foot back and lick it," Park told LiveScience.
The other feet remained insensitive to capsaicin. But the mole rats kept on being completely insensitive to acids, even when possessing the Substance P gene, fact that pointed to the existence of a different receptor for acid pain.
"Acid acts on the capsaicin receptor and on another family of receptors called acid-sensitive ion channels. Acid is not as specific as capsaicin. The mole rat is the only animal that shows completely no response to acid," said Park.
The explanation found for this is that the mole rats need acid insensitivity to acid due to their carbon dioxide rich burrows. In the tight and poorly ventilated spaces, the acid accumulates in their tissues.
"We normally all breathe in carbon dioxide levels of less than 0.1%. If people are exposed to an air mixture with as low as 5% carbon dioxide, we'll feel a sharp, burning, stinging sensation in our eyes and nose. We hypothesize that naked mole rats live in up to 10 % carbon dioxide," said Park.
"All vertebrate pain-receptor systems are built in a highly similar way, so the mole rat may tell us how you can unbuild the system," said Gary Lewin, a neuroscientist at the Max Delbr?ck Institute for Molecular Medicine, in Germany.
"This is important specifically to the long-term, secondary-order inflammatory pain. It's the pain that can last for hours or days when you pull a muscle or have a surgical procedure. We're learning which nerve fibers are important for which kinds of pain, so we'll be able to develop new strategies and targets," said Park.
"We really do not understand the molecular mechanism of acid sensing in humans, although it is thought to be pretty important in inflammatory pain. An animal that naturally lacks such a mechanism may help us identify what the mechanism actually is," said Lewin.
The team will investigate other mammals living in environments rich in carbon dioxide, like the cave dwelling Mexican free-tailed bat and the Alaska marmot.