Pain triggers the release of the brain's natural version of marijuana, researchers said Monday.
Their finding helps explain why marijuana can relieve pain and adds to a whole series of studies that show that the chemical, one of a class known as anandamides, has a range of important roles in the brain.

Michael Walker, a psychology professor at Brown University in Providence, Rhode Island, and colleagues tested pain and anandamide in rats. They found the brain produced anandamide when they stimulated an area -- the periaqueductal gray -- known for its role in modulating pain. It also released anandamide in response to a painful injection of the chemical formalin. The secretion of anandamide eased the pain, they reported in the Proceedings of the National Academy of Sciences.

The researchers anesthetized their rats, but were able to follow the pain signals and the passage of anandamide in the brain using a new type of mass spectrometry, which is able to detect minute amounts of a substance. Walker said the knowledge might be used to devise new painkillers or analgesics. Perhaps a drug that made more anandamide available would be useful, he said. "There are some types of pain that do not respond well to current treatments," he said in a statement. "The fact that you have different modulatory systems that are effective for different types of pain may offer hope."

Anandamides are neurotransmitters -- message-carrying chemicals -- and are known to be chemically very similar to cannabinoids in cannabis or marijuana. Cannabis has been used for centuries to help relieve pain. Other research has found a range of uses for anandamides. In May, researchers at the University of California at Irvine found that people with schizophrenia have twice the normal levels of anandamide in their brains. Anandamides have also been found to help regulate body movement and coordination, and may also be important in helping sperm get to and fertilize an egg.

(Source: Proc Natl Acad Sci USA 1999; 96(21):12198 )