Some memories, particularly those evoking fear or pain, are best forgotten. But just how the brain squelches unpleasant recollections is unclear. Now findings published today in the journal Nature suggest that natural chemicals similar to the active ingredient in marijuana help mice wipe out traumatic memories.

Giovanni Marsicano and Carsten T. Wotjak of the Max Planck Institute of Psychiatry in Germany and their colleagues trained mice to associate a tone with receiving a shock. Once the actual shock was removed, normal mice eventually forgot their previous experience and came to realize that they need not be afraid of the sound anymore. Mice engineered to lack receptors for cannabinoid brain chemicals, in contrast, continued to fear the tones, suggesting that they were unable to put the negative experiences out of their minds. The team next treated normal mice with a drug that blocks cannabinoid receptors in the brain and discovered that the animals had a similar difficulty forgetting bad memories. The scientists suggest that in order to erase an unpleasant memory, the body's innate cannabinoids flood the amygdala, the brain's fear center, and inhibit the action of nerve cells. According to Pankaj Sah of the Australian National University, "Drugs that target these molecules and their receptors could be useful new treatments for anxiety disorders." Such drugs would have to stimulate cannabinoids solely in the amygdala, unlike smoking marijuana, which floods the entire brain indiscriminately, among other effects.