Naked mole rats have evolved to feel pain differently than humans, mice

Small, hairless, virtually blind and living in crowded underground tunnels serving a queen — naked mole rats' lives may sound unpleasant, but they've got a leg up on humans (and rats and mice): what hurts us doesn't bother the humble mole rat at all.

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The animals, found in eastern African countries like Kenya, Somalia and Ethiopia, have evolved to feel pain differently, and are particularly resistant to pain from acid.

Dr. Ewan St. John Smith and Damir Omerbasic are co-lead authors of a study on the peculiar animals that was published this week in the journal Cell Reports. Smith says this special feature comes down to sensitization.

And the research could help scientists understand more about the nature of pain.

In humans, mice and rats, the injection of what's known as nerve growth factor, which occurs naturally in the body, can cause sensitization.

"What this means is that something that normally causes a bit of pain causes even more pain, and things that don't usually cause any pain now do," said Smith.

Smith said it's like a sunburn. Usually, taking a hot shower is nice, but it can be "burningly painful" to someone with sunburnt skin, he explained.

When injected with nerve growth factor, mice and rats became sensitized to certain kinds of pain.

"But if you inject the naked mole rat with nerve growth factor, nothing happens," said Smith.

With nerve growth factor, because it is such a profound, important signalling pathway, the fact that this is different in naked mole rats is really quite bizarre.- Dr. Ewan St. John Smith, co-lead author of the paper

Scientists have known for several years that they had this special resistance to pain, but now they've isolated the specific pain receptor that gives them this ability, known as TrkA.

We have this receptor, too, but it works differently in the naked mole rat. 

"With nerve growth factor, because it is such a profound, important signalling pathway, the fact that this is different in naked mole rats is really quite bizarre."

Pain crucial to survival

But pain is crucial to survival for every animal.

Even the simplest creature, like the tiny c elegans worm that does not have a brain, dedicates about 20 of its 300 nerve cells to detecting pain, said Smith.

"If you're going to survive, you have to be able to avoid potentially damaging things - you can't just walk into a fire, you have to be able to detect that heat."

But the mole rat's seeming imperviousness to acid pain is curious.

Smith and other researchers hypothesize that the animal has evolved this way to deal with its subterranean environment.

The animals' breathing produces carbon dioxide. When mixed with water, found in mucus membranes in the nose and eyes, it creates carbonic acid.

Most animals can buffer the effects of this acidity to a certain extent. But the mole rat doesn't feel the burn.

"The animals, probably through evolution because it's a safe place to live underground, they either have to be in constant acid pain or they've had to develop an adaptation to it," said Smith.

The same genetic variation is also found in animals that hibernate — they, too, release carbon dioxide while buried in cramped spaces in the winter. This link is being studied further, said Smith.

This kind of resistance to pain is incredibly uncommon, but there is a rare genetic condition in humans called congenital pain insensitivity. This condition results from a mutation in the TrkA receptor, resulting in the person's inability to feel pain. People with this condition often die young because they can get seriously injured and not know about it.

Smith and his colleagues found that the naked mole rat's TrkA receptor is less functional than that in other animals, but not completely non-functional like that in humans who cannot feel pain at all.

Also cancer resistant

The naked mole rat has some other unusual traits.

It is highly resistant to cancer (scientists don't know why yet) and lives to be up to 30 years old (rats and mice only live for about two to three years).

The naked mole rat lives in colonies of up to 300 strong, serving a common queen. When the queen dies, the largest females fight to the death for her throne.

They can run backwards as fast as they can forwards.

"If you are handling one and they get a bit scared by a noise they might just run backwards up your arm, which can be a bit of a surprise," said Smith.

And don't let their large teeth scare you. He says they're very friendly and easy to handle.