Researchers conducting animal studies say that have been able to isolate the pain-relieving effects of opiates from side-effects like addiction and tolerance, potentially a major breakthrough for medication development.

Science Daily reported Aug. 24 that researcher Zhou-Feng Chen, Ph.D., and colleagues from the Washington University School of Medicine in St. Louis found that mice that were genetically engineered to not produce serotonin did not get any analgesic benefits from opiates, but still developed tolerance to the drugs. The study focused on blocking serotonin-producing neurons called 5-HT.

"These findings demonstrate that opiates exert their analgesic effects through a serotonin mechanism but that serotonin is not responsible for the negative, addictive side effects associated with those painkilling drugs," Chen says. "That was unexpected because serotonin has been known to interact with other neurotransmitters like dopamine or to modulate the levels of these neurotransmitters in the forebrain, which is important for reward-seeking behaviors."

Chen concluded that it may be possible to target serotonergic neurons or opiate receptors on those neurons with opiate-based drugs that could suppress pain without raising the risk of addiction.

The study appears in the Proceedings of the National Academy of Sciences.