Replacing dead or dysfunctional nerve cells with new, healthy ones derived from stem cells eases chronic pain in mice, a new study found.

 Researchers from the University of California, San Francisco coaxed mouse embryonic stem cells into becoming mature nerve cells that could bridge gaps in the circuitry that triggers neuropathic pain.

"One of the major causes of neuropathic pain is the loss of inhibitory control at the level of the spinal cord because of nerve loss or dysfunction," said study author Allan Basbaum, chairman of UCSF's department of anatomy. "The idea was to replace or repopulate the spinal cord cells that provide that inhibition."

The same stem cells, "destined to become inhibitory neurons" that dampen the signals that cause pain, were previously shown to improve symptoms in a mouse model of epilepsy, Basbaum said. "The question was whether we could take the exact same cells and put them in the spinal cord."

Before injecting the cells into the spinal cords of mice with neuropathic pain, the researchers labeled them with a fluorescent tracer to track the connections they made.

"We were able to show how these cells integrate beautifully," Basbaum said, describing the way the transplanted cells looked and behaved like the mouse's own.

Not only did the cells set up shop in the spinal cord, sending and receiving signals through a complex network of neurons, they also eased the neuropathic pain.

"In four weeks, the animal's condition completely disappeared," Basbaum said, adding that transplanted "control" cells that lacked the inhibitory properties of the stem-cell-derived neurons failed to ease the pain.

"The clinical significance is that we think we're actually modifying the disease, not just treating the symptoms," Basbaum said, adding that drugs currently used to ease neuropathic pain fail to treat the underlying problem. "Instead of taking a drug to suppress the pain, we're trying to normalize the circuit that was damaged by the disease or the injury. The cells repopulate, they integrate, and basically they treat the disease."

The findings, while preliminary, give hope to 100 million Americans who suffer from chronic pain, according to a 2011 report from the Institute of Medicine.

But before the technique can be tested in humans, the researchers have to see if human embryonic stem cells have the same ability to ease pain without causing side effects in mice.

"Will they take? Will they integrate? Will they treat the condition?" Basbaum said. "If they do, we could start asking whether they could treat neuropathic pain in humans."