To find out, they trained rats to
fear two different musical tones, by playing them at the same time as
giving the rats an electric shock. Then, they gave half the rats a drug
known to cause limited amnesia (U0126, which is not approved for use in
people), and reminded all the animals, half of which were still under
the influence of the drug, of one of their fearful memories by
replaying just one of the tones.
When
they tested the rats with both tones a day later, untreated animals
were still fearful of both sounds, as if they expected a shock. But
those treated with the drug were no longer afraid of the tone they had
been reminded of under treatment. The process of re-arousing the rats'
memory of being shocked with the one tone while they were drugged had
wiped out that memory completely, while leaving their memory of the
second tone intact.
LeDoux's
team also confirms the idea that a part of the brain called the
amygdala is central to this process - communication between neurons in
this part of the brain usually increases when a fearful memory forms,
but it decreases in the treated rats. This shows that the fearful
memory is actually deleted, rather than simply breaking the link
between the memory and a fearful response.
Greg
Quirk, a neurophysiologist from the Ponce School of Medicine in Puerto
Rico, thinks that psychiatrists working to treat patients with
conditions such as PTSD will be encouraged by the step forward. "These
drugs would be adjuncts to therapy," he says. "This is the future of
psychiatry - neuroscience will provide tools to help it become more
effective."
Wipe out a single memory