There are a very small number of people in the world who can't feel pain. They don't feel scratches or burns, or the ache of pulling a muscle. They can only feel pressure — and that often doesn't leave them with enough information to know when something's wrong with their body; it can be weeks or even months before they realize they have an underlying medical problem. Now, researchers believe they've developed a drug that can combat this rare condition, called congenital insensitivity to pain.

Published in Nature Communications, their study examined mice that had been genetically modified to lack Nav1.7 channels — the mechanisms that transport pain signals from injury sites to the brain. Congenital insensitivity involves the absence of these channels, and thus an inability of the brain to receive pain signals. The authors found that the mice who couldn't feel pain actually had an increase in the expression of genes for opioid peptides, the body's natural painkiller.

They decided to target the opioid peptide genes based on the idea if they could block those painkillers, the mice would be able to feel pain. The researchers administered a drug called naloxone to the mice; it blocked their opioid receptors, and subsequently triggered the feeling of pain.

Of course, a study done in rodents doesn't always mean it will translate to humans. But the researchers managed to administer the drug to a 39-year-old woman with congenital insensitivity to pain. After she was given naloxone, she felt the heat and pain from a hot laser for the first time — and even seemed to enjoy it. More research will be needed, however, to see what the long-term effects would be from a regiment of naloxone. Regardless, study co-author John Wood, of the University College London, is patenting the drug for potential use in the future. Further research into congenital insensitivity to pain could also help in learning more about the opposite problem: how to reduce pain.

"The ability to sense pain is essential to our self-preservation, yet we understand far more about excessive pain than we do about lack of pain perception," Cambridge Institute for Medical Research at Cambridge University professor Geoffrey Woods, who was not a part of the most recent study, told The Independent. "Both are equally important to the development of new pain treatments. If we know the mechanisms that underlie pain sensation, we can then potentially control and reduce unnecessary pain."

Source: Minett M, Pereira V, Sikander S, Matsuyama A, Lolignier S, Kanellopoulos A. Endogenous opioids contribute to insensitivity to pain in humans and mice lacking sodium channel Nav1.7. Nature Communications, 2015.