Injuring a Fly May Condemn It to a Lifetime of Pain, Scientists Find

Injuring a fly could result in it living with chronic pain for the rest of its life, scientists have said. In a study looking at the mechanisms of pain following injury, researchers discovered that, after being hurt, flies develop "hypersensitivity." This persists long after the injury has healed.

Scientists first discovered insects can feel pain over 15 years ago. However, it was not clear whether they continued to be affected by the injury after the event—as humans do with chronic pain.

Fruit flies are used extensively in scientific research because, even though it is very different to humans, they have many molecular processes that are shared among all species as a result of evolution. They are quick to breed and have a short life cycle, making studying them reasonably quick and inexpensive.

In a study published in Science Advances, a team of researchers looked at whether insects also suffer from chronic pain. "People don't really think of insects as feeling any kind of pain," study author Greg Neely, from the University of Sydney, Australia, said in a statement. "But it's already been shown in lots of different invertebrate animals that they can sense and avoid dangerous stimuli that we perceive as painful."

He said what they did not know was whether an injury could result in "long-lasting hypersensitivity to normally non-painful stimuli in a similar way to human patients' experiences."

In the study, researchers looked at neuropathic pain, resulting from damage to the nervous system. They injured a nerve of a fly by amputating the right middle leg. This was then left to fully heal. When exposed to a surface temperature of around 42 degrees Celsius, flies will either look to escape, or will die within minutes.

To find out if the injury triggered a different response to uninjured flies, the team exposed the flies to surfaces heated to different temperatures. Findings showed injured flies would try to escape when the temperature reached 38 C—indicating they had a lower threshold for painful stimulus. In contrast, uninjured flies "displayed minimal escape attempts when exposed to a 38 C surface," the team found.

"After the animal is hurt once badly, they are hypersensitive and try to protect themselves for the rest of their lives," Neely said. "That's kind of cool and intuitive.

"The fly is receiving 'pain' messages from its body that then go through sensory neurons to the ventral nerve cord, the fly's version of our spinal cord. In this nerve cord are inhibitory neurons that act like a 'gate' to allow or block pain perception based on the context. After the injury, the injured nerve dumps all its cargo in the nerve cord and kills all the brakes, forever. Then the rest of the animal doesn't have brakes on its 'pain'. The 'pain' threshold changes and now they are hypervigilant."

He said that when the "breaks" are switched off in humans it leads to chronic pain. By understanding chronic pain mechanisms in fruit flies, a better understanding of the condition in humans may be possible. "From our unbiased genomic dissection of neuropathic 'pain' in the fly, all our data points to central disinhibition as the critical and underlying cause for chronic neuropathic pain," Neely said.

It is estimated that over 20 percent of American adults suffer from some sort of chronic pain.

Any treatment developing from this research would be a very long way off. Animal models do not necessarily mimic what happens with humans, and getting new drugs to the market is a long process—around 12 years on average, according to drugs.com.

Neely is hopeful, however: "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."