Painful stimulation of a rat paw early in life results in more pain-sensitive axons on the corresponding side of the spinal cord (left).

Pain Can Rewire Brain

Physicians used to downplay babies' ability to feel pain. Now a study in rats amplifies the call to manage pain aggressively in newborns. Pain experienced at a very young age, it suggests, could rewire the nervous system and make it even more sensitive to pain later in life.

Until about 15 years ago, physicians rarely anesthetized infants or gave them pain-killing medication. They worried that such treatments could interfere with breathing. But studies showing that newborns do respond--hormonally, physiologically, and behaviorally--to pain spurred most clinicians to change their practices. Further research showed that babies are more likely, not less, to survive surgery if they've been anesthetized. But surveys of intensive care units show that infants still aren't always getting pain-killers for routine painful procedures such as heel sticks, IV intubation, and being put on a ventilator.

To test the effects of early pain on the development of the nervous system, neuroscientist M. A. Ruda of the National Institute of Dental and Cranofacial Research in Bethesda, Maryland, and her colleagues injected newborn rats with an inflammatory agent that caused one hind paw to swell for several days. Then, they waited. When the rats were 8- to 12-week-old adults, the researchers killed them; after staining the spinal cords with a dye that sticks to pain-sensitive neurons, they compared the side of the spinal cord corresponding to the paw that had been in pain with the other side. There were 25% more pain-sensitive axons on the treated side, and the sciatic nerve, which runs from the paw to the spinal cord, connected to six segments of the cord, compared to just four on the untreated side, the researchers report in the 28 July issue of Science.

As for how this study relates to human newborns, Charles Berde, a professor of pediatrics and anesthesiology at Harvard Medical School in Boston, says the rat pup is a "useful model," even though a few days in a rat's life correspond to months of development in a newborn human. Animal research that shows early pain causes hardwired changes might help convince skeptics of the importance of managing pain in newborns, says Berde.

Related sites
NIH's Pain Research Consortium