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Thursday, Aug. 1, 2002
FEAR GOES UP IN SMOKE?
The extinction of bad memories
By ROWAN HOOPER
"In spite of severe headache, vomiting and disorder of micturition, he remained on duty for more than two months. He then collapsed altogether after a very trying experience, in which he had gone out to seek a fellow officer and had found his body blown to pieces, with head and limbs lying separated from his trunk."
This description of a World War I officer suffering from horrific nightmares was written by the psychologist W.H.R. Rivers in his classic study, "The Repression of War Experience," published in The Lancet medical journal in 1918.
As well as a master of understatement, Rivers was a pioneer in the recognition and treatment of "shell shock," a condition that would now be classified as a type of post-traumatic stress disorder. Among many others, Rivers treated the war poets Siegfried Sassoon and Wilfred Owen, gradually helping them to come to terms with what they had witnessed.
PTSD is, of course, still with us. At least half-a-million American veterans of the Vietnam War (not to mention Vietnamese veterans of their "American War") suffer PTSD in some form or other.
But it doesn't take a war to cause PTSD. After Sept. 11 there was an increase in cases among Manhattan residents living close to the site of the World Trade Center, and a survey published in the June issue of the American Journal of Epidemiology showed that among those residents, alcohol and marijuana consumption also increased after the attacks.
Well, you're saying, we don't need scientists to tell us that people would fancy a drink or a smoke after seeing the Twin Towers collapse -- a "trying experience" if ever there was one. We all know that marijuana has a relaxing effect, but a paper published in this week's Nature could eventually show how it has that effect in the brain.
Researchers in Germany have found that chemicals naturally present in the brain, and similar to the active ingredient of cannabis, help erase bad memories. The chemicals, called cannabinoids, were known to be active in memory-related parts of the brain, but until now it wasn't clear how they worked or what they did.
To find out, a team led by Beat Lutz, of the molecular genetics of behavior group at the Max Planck Institute of Psychiatry in Munich, bred mice that lacked the receptors in the brain that pick up cannabinoids. Through an elaborate series of experiments, Lutz and colleagues showed that the mice had trouble forgetting a traumatic memory.
A group of mice were trained to associate a noise with an electric shock to their feet. Like thousands of men who have experienced combat and are startled when they hear a car backfire, the mice would freeze, even when the sound was not accompanied by the shock. After repeated exposure to the sound alone, normal mice gradually lost the association. Mice lacking the receptor kept the fear.
"It is likely that some of these effects that we see with PTSD are due to changes within the amygdala," Pankaj Sah, a neuroscientist at the Australian National University in Canberra, said in an e-mail interview.
The amygdala, an almond-shaped structure concerned with fear and other emotions, is flooded with cannabinoids during memory processing, and the chemicals dampen the action of nerve cells. Without reinforcement, the behavioral response (freezing, increased heart rate and blood pressure) to the sound associated with the shock slowly disappeared. Neurologists and psychiatrists call this the "extinction" of a memory.
The mice without the ability to sense cannabinoids in their amygdalas were unable to forget the association of the tone with the electric shock, but their ability to store new memories was unimpaired. The work should eventually help scientists understand -- and treat -- phobias, anxiety conditions, panic attacks and PTSD.
So should we smoke dope if we are overly anxious about something? Have those who turn to dope to ease their stress been instinctively acting on what scientists have only just begun to explain?
In a commentary on the findings in Nature, Sah acknowledges that patients in the first stages of psychiatric illness often use cannabis heavily.
"This has often been thought to contribute to acute illness," says Sah. "But it seems possible that it may instead be a form of self-medication for the sometimes extreme anxiety that these people experience."
The new work, said Sah, "suggests that it's worth thinking about designing therapeutic drugs which target this system [the storage of fearful memories] in the amygdala. More would need to be understood about the physiology of the effects, though."
In an e-mail interview he emphasized that medical marijuana activates cannabinoid receptors in the entire brain, not just in the parts involved in memory. "There are even clinical reports that excessive marijuana can be aversive," he said.
"Due to the fact that the endogenous [internal] cannabinoid system is activated during a short period of time, at the moment of retrieval of the fear memory, therapeutic interference . . . must go along with the presence of a psychiatrist or trained psychologist."
Rivers' treatment of PTSD was revolutionary because he held that trying to repress the bad memories (the orthodox treatment at the time) had the opposite effect. With treatment, the officer described at the beginning of this article was eventually able to sleep without fear and regained his strength. Perhaps his "fear centers" in the amygdala were able to process his horrific memories -- and so cause their extinction.
Rowan Hooper can be contacted at firstname.lastname@example.org See more from Nature at www.nature. com