Are there animals that cannot feel pain

Insects also feel chronic pain

Persistent suffering: Insects can not only experience acute pain, they also suffer from chronic pain - just like us humans. Even if a nerve injury has long healed, they are overly sensitive to pain stimuli, as an experiment shows. The pain memory of insects is apparently based on a blockage of irritation-inhibiting neurons in the central nervous system - this is also similar to that in humans, according to the researchers in the journal "Science Advances".

Millions of people worldwide suffer from chronic pain. Especially after nerve irritation or damage, permanent pain or excessive sensitivity to pain remains, even if the acute damage has long healed. This "pain memory" ensures that those affected feel severe pain even when the stimuli are actually harmless or suffer from inexplicable and sometimes almost indistinguishable constant pain.

Do insects feel pain too?

But as it now turns out, we humans are not the only ones who can suffer from chronic pain: Even supposedly primitive and “callous” beings like insects seem to share this with us. "Most people don't think insects are able to feel pain at all," says senior author Gregory Neely of the University of Sydney. "But we know of many invertebrates that they perceive and avoid stimuli that we perceive as painful."

However, it was previously unknown whether insects can develop chronic pain in addition to acute pain. To test this, the researchers first injured a leg nerve in fruit flies and then allowed it to heal. Before the injury and after healing, they tested the animals' sensitivity to pain by observing the temperature at which the insects recoiled from a hot surface.

Hypersensitive even after healing

The surprising result: "Once the flies have been seriously injured, they remain hypersensitive," reports Neely. If the pain threshold of the insects was 42 degrees before, they avoided temperatures of 38 degrees after the injury had healed. “This corresponds to the classic phenomenon of thermal allodynia, in which even harmless stimuli can cause a pain reaction,” the researchers explain.

In other words: Similar to us humans, the flies develop a persistently exaggerated pain reaction - they suffer from chronic pain. And as with us, the original injury led to permanent changes in pain management and processing. "This is the first description of such long-lasting chronic pain in a fly," says Neely. Chronic pain could therefore be a phenomenon that developed very early in evolution - more than 500 million years ago.

Pain control blocked

More detailed analyzes showed that inhibiting neurons in the abdominal nerve - an equivalent of our spinal cord - no longer work properly in the pain-plagued fly. "After the injury, all pain brakes are blocked," explains Neely. “This changes the pain threshold and the animals are over-sensitive.” A similar mechanism is also being discussed in humans as one of the triggers of chronic pain.

According to the researchers, the chronic pain in insects could therefore provide valuable insights into the complex processes involved in human pain perception. "Based on our genomic analysis of neuropathic pain in the fly, everything indicates that central disinhibition is a key cause of chronic neuropathic pain," says Neely. (Science Advances, 2019; doi: 10.1126 / sciadv.aaw4099)

Source: University of Sydney

15th July 2019

- Nadja Podbregar