To give birth painlessly is every pregnant woman's dream, but for people who lack the ability to feel pain, life stinks. Actually, they wouldn't know because people with congenital insensitivity to pain can't smell anything.
Researchers have discovered that these individuals who have a rare genetic condition rendering their lives pain-free also don't have a sense of smell. And they think they know why: Humans use the same signaling channel in the brain to both feel pain and sense smells.
Study researcher John Wood, of the University College London, tested the smelling abilities of three patients unable to feel pain. These patients have a mutation in an ion channel in their sensory cells called Nav1.7 that sends pain signals from the skin to the brain. That's when he found that this channel is also active in the olfactory system, and these patients had no sense of smell.
"The results in the humans were very clear. They can't smell," co-researcher Frank Zufall, of the University Of Saarland School Of Medicine in Germany, told LiveScience. "It was completely surprising and completely unexpected."
A couple of previous studies of these pain-insensitive patients have hinted that they may not be able to smell, but this was the first to actually test their abilities. These three participants couldn’t identify any smells (like balsamic vinegar, orange, mint and coffee) that the researchers threw at them, even though they've lived relatively normal lives.
To study how this Nav1.7 might also be linked with smell, Zufall and his colleagues examined the cells of the human olfactory system, and also studied genetically engineered mice without this channel in their smell systems. He found that in both humans and mice the channel works as an interpreter between the smell-sensing cells in the nose and the smell-interpreting cells in the brain.
The channel moves sodium ions across the brain cell's membrane where it communicates with another brain cell, called the synapse. The nose's smell-receptors still work and are able to detect smells, but the smells are never relayed to the brain because the chain of signaling is broken. "The signal cannot go across this synapse, so the brain doesn't get any odor induced signals," Zufall said.
The mice without this channel in their nose cells lack the ability to smell, which gives them a tough time in life. They have trouble feeding, because they can't find their mothers, and they can't avoid predators or find food. "It's extremely critical for the mice to have a sense of smell," Zufall said. "In nature, they would have no chance of surviving."
Since smell and taste are so interrelated, it's also possible this mutation might affect the ability of people with congenital insensitivity to pain to taste foods. The same Nav1.7 channel has been seen in taste-related cells, so it's possible, Zufall said, but still remains to be tested.
Pain drugs that hit the Nav1.7 channel, a big area of research by pharmaceutical companies, would most likely have the side effect of wiping out smell, which can be detrimental to quality of life, though in cases of extreme pain temporary loss of smell probably wouldn't be a patient's biggest worry, Zufall said. Smell-blocking drugs could also be used by people working in places with bad odors.
The study appears today (March 23) in the journal Nature.
You can follow LiveScience staff writer Jennifer Welsh on Twitter @microbelover.