There is no cure for chronic pain, but there are practices that can help patients keep pain at bay. For example, exercise has been proven to ease some types of chronic pain. Now, research in rats shows that exercising before an injury may lessen the severity of future chronic pain. The findings were presented at the 2016 Annual Meeting of the American Pain Society in May in Austin, Texas, US.
Peter Grace, working with Linda Watkins at the University of Colorado, Boulder, US, allowed rats access to a running wheel inside their cages for six weeks, while rats in a control group had access to a locked, nonfunctioning wheel. Importantly, the animals exercised voluntarily; forcing animals to exercise is extremely stressful, which in itself influences pain sensitivity. Interestingly, rats varied in how much they ran, but overall the animals increased their activity levels over time.
After six weeks, the researchers tied sutures around the sciatic nerve in some of the rats, an injury meant to model neuropathic (nerve injury-induced) pain. Rats no longer had access to the wheel after surgery.
Two weeks after the nerve injury, rats displayed a hallmark of neuropathic pain called mechanical allodynia: painful sensitivity to stimuli that would not normally be painful. Indeed, the rats with an injured nerve withdrew from a poke with a nylon filament (a common experimental test used in animal studies to study pain) more easily than those that received a harmless “sham” surgery, as expected. But, among those with the nerve injury, rats that had freely exercised were less affected by the injury—they tolerated the poke better than the sedentary animals. “What was striking is that the protection persists over the life of the injury,” Grace says.
How could exercise provide this protection? Exercise affects pain through its influence on the activity of immune cells, though exactly how is unknown. After an injury, immune cells called macrophages typically invade the site of injury to clean up waste and fight off potential infection, but they also release pain-promoting inflammatory molecules. When Grace measured levels of macrophages in the injured nerves, he found that rats with a history of exercise had fewer invading macrophages than non-exercising rats, both three days and two weeks after injury.
Grace then measured specific molecular indicators of inflammatory and anti-inflammatory activity in the nerves and spinal cord. The results suggested that “exercise itself may be anti-inflammatory, which buffers against the pro-inflammatory environment created after nerve injury. This in turn reduces the severity of inflammation—and, hence, pain after nerve injury,” he says.
Evidence is piling up that exercise protects against pain. “Our study provides some insights as to how that protection might occur, by promoting an anti-inflammatory environment,” Grace explains.
Although the research was done in rodents, it may have a lesson for people hoping to avoid future pain, such as those undergoing a planned surgery. “Stay active!” Grace says. —Stephani Sutherland
To read about the research in more detail, see the related Pain Research Forum news story here.