By Candler Paige, PhD student, NIH Blueprint D-Span Awardee
We’re all familiar with the excruciating pain we experience after we hit our elbow on a door frame, and often wish that those annoying experiences never happened, but when discovering new pain drugs one of the biggest concerns facing scientists is making sure that the new medicines can relieve chronic, unnecessary pain without inhibiting acute pain.The acute pain tells us to wake up and alerts us to the hazards of walking sleepily around the Lego battle field that is your living room floor. Although extremely annoying, the short-lived pains we feel every day are protective – these experiences keep us from irreversibly injuring ourselves. There are some people that don’t have this “spidey” sense. People with congenital insensitivity to pain have a genetic mutation that prevents them from feeling any pain at all which causes them to accumulate wounds, burns, broken bones, and they generally have significantly shorter lifespans than those of us that do feel pain. In general, pain protects us from injuring ourselves.
There is one aspect of all types of pain that has eluded scientists: in addition to general discomfort of the pain, people also tend to have an increased sensitivity to their surroundings. Lights are too bright, noises are too loud and this increased sensitivity makes the pain worse. This seems counter-intuitive. Why would your body make painful experiences more wretched than they already are? Scientists have understood the advantage of acute pain for centuries, but the point of the miserable experiences surrounding pain was still not completely understood. For humans to have kept this feature over millions of years of evolution meant that there was some sort of advantage that kept us alive. In 2014 the Walters Lab at The University of Texas Health Science Center decided to explore this problem in squid – an animal often used in exploring evolutionary neuroscience.
In a set of experiments led by Dr. Robyn Crook, it was discovered that pain prevents injured squid from being captured and eaten by their predators. To come to this conclusion uninjured squid were placed in a tank with injured squid along with sea bass – a natural predator. The injured squid had a small cut made on one of their fins without anesthesia. The group observed that the bass were more likely to try to attack the injured squid compared to healthy groups. Then, the injured squid were placed in a tank alone with the sea bass. 50% of the attacks by the bass were successful in capturing the injured squid (when compared to only 25% of the healthy squid being captured). Next, the experimenters anesthetized the squid before the injury – so that they felt no pain after they were injured. Surprisingly, 75% of the squid that were injured but felt no pain were captured and eaten by the bass.
What the group found was the benefit of acute pain immediately following an injury: a defensive response. They showed that pain in injured squid seemed to heighten their defense system – this (somewhat) made up for the injury they had on their fins. The authors summarized their findings:
“Thus, while minor injury increases the risk of predatory attack, it also triggers a sensitized state that promotes enhanced responsiveness to threats, increasing the survival (Darwinian fitness) of injured animals during subsequent predatory encounters.” (Crook et al. 2014)
What these findings demonstrate is the benefit of having increased sensitivity to one’s surroundings while in pain: animals are less likely to be eaten by a predator. This evolutionary advantage was maintained in humans to keep them from being eaten by lions or trampled by buffalo while hunting or stepping on Lego’s night after night.
The next time you have a headache and the noises outside of your window are making you feel even worse – just remember your chances of getting eaten by a bear are even lower than they were before.
Want to learn more? Check out Dr. Price’s lecture on the Evolutionary Advantage of Nociceptor Sensitization.
Sources:
https://ghr.nlm.nih.gov/condition/congenital-insensitivity-to-pain#genes
https://www.cell.com/current-biology/fulltext/S0960-9822(14)00335-2
