How Memories Affect Pain
If you were to ask your mother about the day you were born, she would probably tell you that she forgot all about the pain of labor and delivery the moment she laid eyes on you. Or if you asked your marathon running friend why he keeps signing up for new races, despite the pain of training and competing, he’ll tell you that the pain is nothing compared to the euphoria of actually finishing the race.
In a study published in early 2015, scientists speculated that the circumstances of pain influence how we remember it, particularly its intensity. The researchers compared the perception of pain by women who had given birth to those who had undergone gynecological surgery, and found that the surgical patients not only reported feeling more pain, but more intense pain, than the new mothers did.
A similar study of marathoners reached a similar conclusion: The runners who were happier at the end of the race recalled that less intense pain at the end of the race than those runners who weren’t in such a good mood. These two studies seem to indicate that, in general, the happier the cause of the pain, the less pain we feel overall.
Of course, plenty of people get hurt doing things that they enjoy, and experience of lifetime of discomfort due to a single moment on the playing field or a questionable decision during a night out with friends. However, the connection between memory and pain is undeniable, regardless of the circumstances — so much so, that some scientists are working on ways to treat pain by effectively “erasing” any memory of it.
Chronic Pain and Memory
By some estimates, chronic pain affects as much as 20 percent of the population — that’s about 60 million Americans. While many patients get help at pain treatment centers to ease their discomfort, one of the most common treatments for chronic pain is opioids for those not going to those centers. While these drugs are effective, they often come with unpleasant side effects, including a high risk for addiction.
Because many people are reluctant to treat their pain with pharmaceuticals, researchers are looking for new ways to treat pain. One of the more promising treatments looks to the connection between pain and memory, or more specifically, attempts to “erase” the memory of pain.
As one scientist explains, chronic pain is essentially a “maladaptive memory.” We all have memories of pain that served as learning experiences; for example, the first time we touch a hot stove, we quickly learn that it hurts, and try to avoid doing it again. And when we do get hurt, the area remains sensitive while it heals as a means of protection. Called hyperalgesia, this “memory” of pain helps prevent further injury.
Sometimes, though, hyperalgesia lasts longer than it should. Some doctors think that this indicates that the pain receptors in the nervous system that tell you “Ouch!” to get you to stop doing something have actually migrated to the spinal cord and become oversensitive. They send messages of pain into the nervous system that are more intense that they should be, and take place more often than they should — leading to chronic pain.
So what does this have to do with memory? Well, the process of retrieving long-term memories is very similar to that of hyperalgesia, in the sense that it relies on the synthesis of extra receptors. When we recall something, the nervous system ignites receptors, which then retrieve the memory; when we don’t need the memory, it lies dormant. About 16 years ago, scientists discovered that if they blocked the synthesis of those receptors during a memorable event, then the subject had no memory of the event.
Taking it one step further then, some researchers suspect that if you can block receptors after triggering a certain memory — such as pain — then you can actually erase the memory, and the accompanying pain. Scientists injected of the chemical anisomycin into mice to block the formation of memories related to the application of capsaicin, a derivative of hot peppers, to their paws.
After injecting capsaicin in the paws to increase sensitivity, the scientists performed a second injection, but used anisomycin first, and the mice lost 70 percent of their hypersensitivity to pain. The anisomycin effectively shut down the pain receptors in the spinal cord, allowing for the experience of pain but preventing the chronic sensitivity that can come with it.
By effectively erasing the physiological memory of pain, doctors can help patients live pain free. However, anisomycin is unpredictable, and these findings have not yet been replicated in humans. Scientists are confident, though, that they can find ways to influence the memory of pain, if not make us forget it entirely, creating a new paradigm in the management and treatment of chronic pain.