Study finds unexpected mechanism of pain

TUCSON, Nov. 22 (UPI) -- U.S. scientists have determined a naturally produced pain reliever can exacerbate certain pain states rather than attenuate them in rodents.

Josephine Lai and colleagues at the University of Arizona found an endogenous opioid -- a small protein that activates the same receptors as opium or morphine -- activates a different type of receptor to produce the effect.

The researchers found elevated levels of dynorphin A -- one of the endogenous opioids that normally act at opiate receptors to inhibit chronic pain -- are found in some chronic pain syndromes, such as that caused by nerve damage. But unlike other endogenous opioids, dynorphin A actually makes the pain worse.

The scientists discovered dynorphin A also activates bradykinin receptors in the spinal cord, which cause hypersensitivity to pain. A drug that blocks bradykinin receptors protected against neuropathic pain, but only when dynorphin A was elevated. Mice lacking the bradykinin receptor also failed to show increased pain in response to dynorphin A.

The study appears in the December issue of Nature Neuroscience.

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