Spice It Up: Naked mole-rats feel no pain from peppers, acid

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 the fiery chemical caused an electrical spike in nerve cells taken from the rodent’s skin.

When researchers inserted a copy of the gene for substance P into the nerves of the mole-rats, the mutant mole-rats responded to capsaicin but continued to ignore acids.

This discrepancy indicates that substance P is unlikely to be the sole explanation for the mole-rats’ indifference to acid, Park says. For example, his team found that the mole-rat nerves connect to the spinal cord differently than mouse nerves. These differences could be an evolutionary adaptation to living in large underground societies, with little fresh air, he says. The buildup of exhaled carbon dioxide could reach levels hundreds of times higher than in the atmosphere—high enough to be turned into carbonic acid in the mole-rats’ lungs. Without the resistance to acid, the rodents would experience constant pain and tissue swelling, says Thomas Finger, a neuroscientist at the University of Colorado Health Sciences Center in Aurora. “If you took a mouse and stuck it in the tunnel with these guys, the mouse would be dead,” he says.

Understanding why naked mole-rats don’t feel any pain from acid and capsaicin could eventually help doctors treat chronic-pain disorders such as migraines and fibromyalgia, Park says.