Chemical in Pot Puts Haze on Bad Memories
It works in animal study, but researchers question its role for humans
THURSDAY, Aug. 1, 2002 (HealthDayNews) -- Could smoking marijuana send bad memories up in smoke?
A brain chemical similar to pot's active ingredient, THC, appears to be a sort of internal lotus blossom that helps mice forget fearful events from the past. Mice missing a receptor that recognizes this molecule, called a cannabinoid, can't shed nervous memories as effectively as their normal cage mates can, European researchers have found.
The findings, reported in today's issue of Nature, don't speak directly to the effects of marijuana, which has a much broader impact on the brain. But they do suggest that people who use the drug may be "self-medicating" anxiety and fear by helping to extinguish negative recollections, the researchers say.
Beat Lutz, of the Max Planck Institute of Psychiatry in Munich, Germany, and a co-author of the study, says the work also gives researchers a new lens through which to view emotional problems like phobias and post-traumatic stress disorder.
Until now, Lutz says, scientists have concentrated on the roles of two brain messenger molecules, dopamine and serotonin, in these ailments. Now, he says, they should also consider the brain's internal, or endogenous, cannabinoid system.
In their experiments, Lutz and his colleagues created a strain of mice missing a brain receptor for an endogenous cannabinoid. This receptor, CB1, resides in areas of the brain responsible for memory acquisition, processing and recall. It includes an almond-shaped center called the amygdala.
Lutz's group ran these mice and a group of normal rodents through a common fear-conditioning test in which the animals heard a tone and got shocked. Later, the tone was repeated and all the animals froze, indicating that they'd acquired the negative memory and held onto it.
When the researchers continued playing the tone, the normal mice eventually lost their fear of the sound as they extinguished the bad memory. But the receptor-less rodents kept freezing, unable to shed the anxiety of an impending shock.
In addition, a drug that blocks CB1 receptors inhibited memory extinction in genetically normal mice, the researchers found. They also observed CB1 activity in a subset of neurons in the amygdala.
The two groups of animals were essentially identical, having no differences in baseline anxiety, pain perception or ability to move, Lutz says. However, the mutant mice did seem to have a slightly suppressed sense of pleasure, he says, which translated into a weaker appetite.
Although the study showed that the endogenous cannabinoid system is actively involved in fear memory extinction, Lutz doubts it's limited to negative associations.
"Maybe it involves adaptation processes, where you have to modify or erase old memories and overwrite them with new ones," he says.
So were Cheech and Chong really onto something? Not quite.
Smoking marijuana might help extinguish bad memories, but it has also been shown to prevent the acquisition and processing of new ones, Lutz says.
Pankaj Sah, a neuroscientist at the Australian National University in Canberra and author of a commentary on the study in Nature, says the latest findings hint at a role for cannabinoids in the treatment of anxiety disorders.
"There is much anecdotal evidence of patients using cannabis heavily in the early stages of psychiatric illness," Sah wrote. "This has often been thought to contribute to acute illness. But it seems possible that it may instead be a form of self-medication for the sometimes extreme anxiety that these people experience."
Richard Musty, a psychologist at the University of Vermont who studies marijuana, called the latest experiments "very neat work." But Musty says it's a stretch to conclude that cannabis might prove useful in treating anxiety and stress in humans.
"They're working in the system that's basically the physiological reaction to fear," he says. "That doesn't take into account thoughts" that add much more complexity to memory.
In other words, he adds, it's hard to compare a mouse that heard a bell and got shocked yesterday to a Vietnam veteran in a firefight 30 years ago.
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