Over 15 years ago, researchers found that insects, and fruit flies in particular, feel something akin to acute pain called “nociception.” When they encounter extreme heat, cold or physically harmful stimuli, they react, much in the same way humans react to pain. Now, scientists have found that the nervous systems of insects can also experience chronic pain. A new study in the journal Science Advances shows pain lingers throuhout the insects' short lives well after an injury has healed.
Acute pain is generally short lived—like the pain from cutting your finger, which may last for days but eventually recedes. Chronic pain, however, lingers long after an injury has healed and may even last the rest of an injured person’s life. According to a press release, it generally comes in two forms, inflammatory pain and neuropathic pain, the type of electric shooting pain caused by overactive nerves.
Ed Cara at Gizmodo reports that to understand whether insects also experience this long-lasting version of pain, researchers damaged one leg in a group of fruit flies, an injury which can cause chronic nerve pain. After the insects were allowed to heal, the researchers then placed them in a hot room to see if the flies were more responsive to stimuli. After the leg injuries, the flies would try to leave the room at lower temperatures, unable to withstand the heat as much.
In other words, the flies’ legs had become hypersensitive. “After the animal is hurt once badly, they are hypersensitive and try to protect themselves for the rest of their lives,” coauthor Greg Neely of the University of Sydney says in the release. “That’s kind of cool and intuitive.”
To understand this sensitization, the team then examined how the process works on a genomic level. The flies, they found, receive pain messages via sensory neurons in their ventral nerve cord, the insect equivalent of a spinal cord. Along this nerve cord are inhibitory neurons that act as gatekeepers, allowing pain signals through or blocking them based on context. With a catastrophic injury, like the severing of a nerve in the leg, the injured nerve floods the ventral cord with pain signals, overwhelming those gatekeeper neurons and changing the pain threshold permanently, a process known as central disinhibition. From then on, the insects are hypersensitive to pain.
It’s likely that a similar process causes chronic pain in humans as well. “Now that we know central disinhibition is a critical and core cause for neuropathic pain across phyla, we can start to develop therapies that target the underlying cause and not just the symptoms,” Neely writes on Twitter. “This will lead to non-addictive pain management that our society desperately needs.”
Opioids, the major drug group used for pain management currently, essentially masks pain, addressing a symptom, not the root cause of the pain. The drugs bind with receptors throughout the body and brain stopping feelings of pain and often producing feelings of pleasure. That, however, can be dangerous, leading to addiction and abuse. In 2017, for instance, 47,600 people died of opioid overdoses in the United States.
“If we can develop drugs or new stem cell therapies that can target and repair the underlying cause, instead of the symptoms, this might help a lot of people,” Neely says in the release.