Spice it up: naked mole-rats feel no pain from peppers, acid

Science News,  Feb 2, 2008  by Ewen Callaway

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If you're ever attacked by an African naked mole-rat, don't bother with pepper spray. The bald little rodents can't feel the burn of capsaicin, the active ingredient in chilies, or the sting of acid, a new study reports.

The animals' insensitivity could be an adaptation to their cramped underground quarters, high in carbon dioxide gas that can turn to acid.

"They've got a fundamentally different mechanism in the way they sense acid from all animals ever tested, says Thomas Park, a neuroscientist at the University of Illinois in Chicago.

The buck-toothed rodents live in large hierarchical societies, like bees and ants, with hundreds of the critters packed into a network of tunnels. Workers dig the burrows and find food for a hyperaggressive, fertile queen.

In other animals, sensitivity to pain involves a molecule called substance P. Park's team previously discovered that naked mole-rats don't have substance P. To test the effect of the missing chemical, Park's team gently pinched, prodded, and probed mole-rats, comparing their responses to those of lab mice. Mice and mole-rats unconsciously twitched their legs in response to heat from a lamp, and both bit at a paperclip pinching on their tails. When the researchers dabbed capsaicin or acid onto the foot pads of the animals, the mice responded by repeatedly licking their paws, while the mole-rats didn't bat an eye. The study appears online and in the January PLoS Biology.

Tests of nerve fibers confirmed that acid elicited no cellular response in the mole-rats, but capsaicin did. Exposing nerves to

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