Natural high wipes out bad memories

‘Cannabinoid’ linked to ability to forget scary stimuli

LONDON, July 31 — Feel-good chemicals in the brain, similar to the active ingredient in cannabis, can wipe out bad memories, German scientists said in a finding that could lead to new treatments for anxiety disorders and phobias.

RESEARCHERS AT the Max Planck Institute of Psychiatry in Munich have shown that natural chemicals in the brain similar to THC, the active ingredient in marijuana responsible for producing the drug-induced high, dampen nerve cell action and wipe out unpleasant memories. THC, or tetrahydrocannabinol, and similar molecules in the brain known as cannabinoids bind to the brain’s chemical receptors and can create a feeling of euphoria. Marijuana and hashish, which contain THC, have been used for centuries for medicinal and recreational purposes.

Dr. Beat Lutz and his team created transgenic, or genetically modified, mice without a cannabinoid receptor. When they conditioned them to associate a musical tone with an electric shock, the mice produced a fear reaction, and continued to react even when the tone was not followed by a shock, Lutz said. Normal mice quickly stopped reacting to the tone once it was not associated with a shock, but the genetically modified mice without the cannabinoid receptor took much longer to forget their fear.

Normal mice and "knock-out" mice, genetically engineered without cannabinoid receptors, were all conditioned to associate a tone with an aversive stimulus. The mice were re-exposed to the tone on the following days. Control mice adapted to the new situation, in which the tone was no longer dangerous for the animals, by decreasing their fear reaction (freezing). The CB1-deficient mice showed less of a decrease in their fear reaction.

Lutz and his team, whose research is published in Thursday’s issue of the journal Nature, also showed that blocking the receptor in normal mice prevented the animals from forgetting the painful memory. When the scientists studied an almond-shaped area of the brain called the amygdala, central to storing memory and fear, in transgenic and normal mice, they discovered it was flooded with natural chemicals, or endocannabinoids, when the mice were gradually forgetting the learned response to the shock. Lutz said Wednesday that he believes the chemicals help to wipe out the fear or memory of the unpleasant response by binding to the cannabinoid receptors. Smoking marijuana would not produce the same effect in humans, Lutz said, because it overflows the brain and is not specific enough to extinguish the unpleasant memory.

Lutz and his team think drugs that target specific enzymes to boost cannabinoids in the amygdala could help people suffering from panic attacks and fear-related memories.

“The finding that the endocannabinoids contribute to extinction raises the possibility that drugs that target these molecules and their receptors could be useful new treatments for anxiety disorders,” Pankaj Sah, of the Australian National University in Canberra, said in a commentary in Nature.

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