First Evidence Of Chronic Pain In Insects Points To Root Cause

"People don't really think of insects as feeling any kind of pain," said Associate Professor Greg Neely at the University of Sydney, Australia, who led the research. "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. In non-humans, we call this sense 'nociception', the sense that detects potentially harmful stimuli like heat, cold, or physical injury, but for simplicity we can refer to what insects experience as 'pain'."

Scientists often use fruit flies as study models because they possess far smaller and simpler genomes than other animals that can be tinkered with to gain a better understanding of different behaviors and conditions in humans; studying certain fruit fly proteins may help us to live longer and happier lives. 

It was first learned that fruit flies can experience something resembling pain in 2003, but this evidence only suggest it to be short lived discomfort. This study aimed to use the insects to assist investigations into the mysteries of human pain focussing on neuropathic pain which arises from damage to the nervous system, and produced what may be the first genetic evidence of chronic pain enduring long after the initial injury has healed. 

"So we knew that insects could sense 'pain', but what we didn't know is that an injury could lead to long lasting hypersensitivity to normally non-painful stimuli in a similar way to human patients' experiences," says Neely.

A nerve was damaged in one leg of a fruit fly and left to heal, although the acute pain had passed the other leg was found to have also become hyper sensitive. Further study suggests the reason for this as being pain brakes which reside in the fly’s ventral nerve cord and control the flow of pain signals through its body. 

"After the injury, the injured nerve dumps all its cargo in the nerve cord and kills all the brakes, forever," says Neely. "Then the rest of the animal doesn't have brakes on its 'pain'. The 'pain' threshold changes and now they are hypervigilant."

Remaining in a state of hypervigilance can mean a sustained and heightened sensitivity to pain which can lead to chronic pain, neuropathic pain arises from damage to the nervous system. A similar braking mechanism is believed to possibly be at play when it comes to neuropathic pain in humans but in a different manner. 

As published in the journal Science Advances in response to dangerous situations animal’s release these pain brakes as a survival mechanism, but when released in humans it condemns us to sustained discomfort. This study may open new avenues around how we may be able to put these pain brakes back in place. 

"Importantly now we know the critical step causing neuropathic 'pain' in flies, mice and probably humans, is the loss of the pain brakes in the central nervous system, we are focused on making new stem cell therapies or drugs that target the underlying cause and stop pain for good," says Neely.