Pain doesn’t have to be painful. That’s the conclusion of a study that identified a set of nerve cells that add emotional content to pain signals in the brains of mice.
The discovery could lead to new treatments for chronic pain that ease patients’ suffering without impairing their ability to sense injuries.
Pain receptors throughout the body detect painful stimuli and send signals via nerves to the brain. But according to Grégory Scherrer at Stanford University, California, these signals don’t have any emotional value until they reach the amygdala, a brain region that deals with emotions. In other words, the unpleasantness of pain is added by the amygdala and is separate to the information that comes from pain receptors.
Advertisement
To see how pain signals are processed in the amygdala, Scherrer and his colleagues added a gene to this region of the brain to make the cells produce a marker that lights up when they are active. The team then identified a group of cells that respond specifically to painful stimuli such as a pin prick or heat.
Next, they silenced these cells by engineering them to express receptors for a drug that turns down their activity. The mice could still detect painful stimuli and withdraw from them, just as you would withdraw your hand if you touched a hot stove.
But they didn’t adopt defensive behaviours, lick their wounds or try to escape as could be normally expected. “It’s as if they don’t care about pain any more, even though they can detect it,” says Scherrer.
The neural circuits for pain have a high degree of similarity across species, so it’s likely that a similar set of cells can be found in humans, says Scherrer. “We’re hoping it’s a new avenue to treat pain,” he says.
New pain drugs are badly needed to replace opioids, which have caused an addiction epidemic in the US that kills 130 people every day. Opioids also don’t normally work for people with damage to their nerve fibres,which can happen as a result of cancer or diabetes.
In mice with nerve injuries, the pain neurons in the amygdala were triggered even by gentle touch. Scherrer plans to look for receptors that are only present on these cells, so that a drug can be developed that targets them specifically.
Science
