Natural high helps banish bad memories

national | miscellaneous | news report Thursday August 01, 2002 15:35 by Pat C

More evidence as to why there is no logical reason to continue a legal ban on cannabis. Unfortunately the results were achieved by mistreating mice. Pat

Natural high helps banish bad memories


19:00 31 July 02

Special Report from New Scientist Print Edition

The body's own versions of the active ingredient of cannabis could help extinguish unwanted memories, work in mice suggests.

Marijuana has been used medicinally for thousands of years, and people with certain psychiatric conditions such as schizophrenia are more likely to smoke pot than healthy people.

The active chemical in marijuana, tetrahydrocannabinol or THC, binds to the brain's cannabinoid receptors, which are known to be linked to pain sensations, emotion and movement. And in the past decade, researchers have identified chemicals made within the brain that are similar to THC.

Now Beat Lutz at the Max Planck Institute of Psychiatry in Munich and his team have found that these cannabinoids play an important role in getting rid of unwanted memories in mice. The finding could lead to new treatments for people who have related mental conditions. "We could understand the problem of phobia or post-traumatic stress disorder by investigation of this cannabinoid system," says Lutz.

Neuroscientist Daniele Piomelli at the University of California, Irvine, says: "It's an important paper. It's going to have a big impact in the field."

Freezing with fear

The researchers genetically engineered mice so that they lacked a particular type of cannabinoid receptor called CB1. These are normally found in the amygdala, a brain region associated with fear.

They then conditioned the mice, as well as their normal litter mates, to associate a particular musical tone with an electric shock. Both groups of mice quickly learned the association, freezing with fear whenever they heard the tone. A week later, the mice were repeatedly exposed to tones but without the associated electric shock. The normal mice soon shed their fear response, but the modified mice still showed fear 11 days later.

The researchers found that the modified mice eventually suppressed the bad memories, but it took them about six times longer than the normal mice. Lutz's group also showed that blocking CB1 receptors in the normal mice meant they were unable to stamp out the negative association.

New treatments

The team later studied the mice's amygdalae, and confirmed that animals who were now unlearning the unpleasant association had significantly higher levels of two major cannabinoids - anandamide and 2-arachidonoylglycerol - than those who'd never been trained.

This suggests that these chemicals help wipe out bad memories by binding to CB1 receptors.

The findings give a new lead for research into treatments for conditions such as phobia and post-traumatic stress disorder. But Lutz points out that marijuana itself is too blunt an instrument to be a potential treatment, because it activates all the brain's cannabinoid receptors at once.

Journal reference: Nature (vol 418, p 530)


Alison Motluk