Scientists have known for years that the brain makes substances almost identical to the active ingredient in marijuana, but the function of these "cannabinoids" remained mysterious. Researchers now say they help to extinguish traumatic memories.

"In certain situations, being able to forget is very important for emotional survival," said George Kunos, a neurobiologist at the National Institutes of Health.

The research, published today in the journal Nature, is not an endorsement for pot smoking, scientists said. Instead, the findings may help scientists develop new drugs to treat anxiety, post-traumatic-stress disorder and phobias.

"This paper is not saying you should go ahead and smoke marijuana," said Pankaj Sah, a neuroscientist at the Australian National University in Canberra who wrote an accompanying editorial in the journal. "It's saying that it's worth thinking about these specific actions of these compounds."

In the 1980s, scientists were surprised to find the brain has special receptors for the psychoactive elements in cannabis, Kunos said. An Israeli scientist named Rafael Mechoulam then found that the brain made its own versions of these cannabinoids.

To figure out why, authors of this latest study, from the Max Planck Institute of Psychiatry in Munich, Germany, decided to examine mice that had been engineered genetically so that they lacked cannabinoid receptors.

Neuroscientist Beat Lutz said he and his colleagues conditioned the mice to associate a mild shock with the sound of a bell. Normal mice eventually lost the association between the bell and the shock. "They figure out that the tone is not dangerous anymore and say, 'I don't have to freeze,' " Lutz said.

But the mice lacking the cannabinoid system never readjust, always freezing in terror at the sound.

Researchers also found that normal mice produce the natural cannabinoids when they are extinguishing their traumatic association with the bell.

It's not clear whether the cannabinoid system helps the mice to forget the traumatic association of the bell and the shock, or just gives them enough mental flexibility to adjust to a new situation, Lutz said. It's possible that the cannabinoids are important for the ability to relearn and readjust in a number of situations.

Kunos, from the National Institutes of Health, said that the cannabinoids probably play other roles. Using similar methods to Lutz, he found that they help regulate appetite.

Sah, of the Australian National University, said the latest findings may explain why some people with psychiatric problems try to find relief with marijuana. Although experts often have labeled marijuana use as a contributor to these people's mental illness, he suggested that people with certain psychiatric problems perhaps are self-medicating in an attempt to help their brains extinguish some painful or traumatic memory or thought.

Lester Grinspoon, a pro-marijuana psychiatrist at Harvard University and author of the 1971 book "Marijuana Reconsidered," said he would like to see cannabis made into pills that could be prescribed, but said the drug is not patentable and therefore would be unattractive for drug companies to manufacture and market.

Lutz suggested that, instead of supplying extra cannabinoids, a drug might enhance the effects of natural ones.

He also suggested such a drug might need to be taken in conjunction with psychotherapy, during which patients would work on getting rid of fearful associations.

"Just smoking marijuana all day won't help," he said.


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