NATURAL HIGH HELPS BANISH BAD TIMES

OUR brains may do the equivalent of rolling themselves a joint when
they want to forget something awful. The body's own versions of the
active ingredient in cannabis may help extinguish unwanted memories.

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 (New Scientist, 29 May 1999,
p7).

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 - at least they do in mice.

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
(Nature, vol 418, p 530 Lutz's group also showed that blocking CB1
receptors in the normal mice meant they were unable to stamp out the
negative association.

The team later studied the mice I 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. It
suggests that these chemicals help wipe out bad memories by binding to
CB1 receptors.

The finding may 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. He 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.

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