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Now, a unique brain scan study suggests two ingredients in pot may work independently to achieve these effects.
British scientists who watched the effects of the two marijuana ingredients -- Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) -- on the brains of 15 young men say the research shows how the drug can either ease or agitate the mind.
"People have polarized views about marijuana," said study lead author Dr. Sagnik Bhattacharyya, a researcher in the department of psychosis studies at the Institute of Psychiatry, King's College London. "Some consider it to be essentially harmless but potentially useful as a treatment in a number of medical conditions, and others link it to potentially severe public health consequences in terms of mental health. This study explains why the truth is somewhere in between."
The findings were published in the January issue of Archives of General Psychiatry.
According to Bhattacharyya's team, it's long been noted that cannabis can prompt the onset of psychotic symptoms, such as paranoia and/or delusional thinking, among otherwise healthy people.
"A number of studies have (also) clearly shown that regular marijuana or cannabis use in vulnerable individuals is associated with increased risk of developing psychotic disorders such as schizophrenia, where one loses contact with reality," Bhattacharyya said.
Just how this occurs in the brain wasn't understood.
In the new study, the researchers used functional MRI brain imaging on 15 healthy men, roughly 27 years old on average and described as "occasional cannabis users."
On three occasions under fMRI monitoring, the men received one of three identical-looking gelatin capsules: one containing 10 milligrams (mg) of the marijuana ingredient THC (deemed to be a "modest" dose); another containing 600 mg of CBD; and a third filled with flour.
Testing was conducted in a highly controlled and monitored environment, in which no marijuana was actually smoked.
The fMRI scans (which track brain activity in real time) were conducted one and two hours after capsule administration. During the scans, the men engaged in simple visual-cognition tasks (such as pressing buttons to reflect the direction of a series of flashing arrows). Psychopathological assessments were conducted throughout the brain imaging process.
The team found that THC and CBD appeared to affect the brain in different and opposite ways.
Ingesting THC brought about irregular activity in two regions of the brain (the striatum and the lateral prefrontal cortex) that are key to the way people perceive their surroundings. THC seemed to boost the brain's responses to otherwise insignificant stimuli, while reducing response to what would typically be seen as significant or salient.
In other words, under the influence of THC, healthy individuals might give far more importance to details in their environment than they would have without the chemical in their brain.
THC also prompted a significant uptick in paranoid and delusional thinking, the authors said, and the more that "normal" brain responses were set off-kilter, the more severe the paranoid or even psychotic reaction.
The effect of the other main pot ingredient, CBD, was nearly the opposite, however.
Ingesting the CBD capsule appeared to prompt brain activity linked to appropriate responses to significant stimuli in the environment, the team reported.
According to Bhattacharyya, this suggests that, on balance, marijuana may play both a good and bad role in the context of psychosis.
The study also suggests that CBD, at least, may "have potential use for the treatment of psychosis," he said, even as marijuana's other principle ingredient, THC, raises the risk for developing psychotic complications.
Dr. Joseph Coyle, a professor of psychiatry and neuroscience at Harvard Medical School in Boston, said the current work goes a long way toward "connecting all the dots" when it comes to understanding the marijuana experience.
"What we're talking about here is the kind of perception, in this case prompted by marijuana, that leads a person to think that other people who are just talking in the subway are all actually talking about him," he noted. "Or people who are just tipping their hat for no reason are actually doing so specifically about him. And so this paper strikes me as important, because it actually looks at this kind of increased anxiety and increased hyper-alertness which are major factors in psychosis -- and then finds out what's going on in the brain among people who experience them.
"So I think this provides another brick in the foundation when talking about direct causality," he said. "It links the psychological state marijuana brings about with a specific psychophysical response in the brain. And that's very, very interesting."
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