Published online 13 October 2005 | Nature | doi:10.1038/news051010-12

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Cannabinoid injections sprout new neurons in mice.

One synthetic cannibinoid has the same effect on part of the brain as antidepressants. © Punchstock

Most addictive drugs inhibit the growth of new brain cells. But injections of a cannabis-like chemical seem to have the opposite effect in mice, according to new research. Experts say that the results, if borne out by further studies, could have far-reaching implications for addiction research and the application of marijuana in medicine.

For several years now, researchers have been interested in how drugs affect a part of the brain known as the hippocampus. This region is unusual in that it can grow new neurons throughout a person's lifetime. Researchers have theorized that these new cells help to improve memory while combating depression and mood disorders.

“It makes marijuana look more like an antidepressant and less like a drug of abuse.” Amelia Eisch

University of Texas Southwestern Medical Centre, Dallas

Many drugs, such as heroin, cocaine and alcohol, inhibit the growth of new cells in the hippocampus, which scientists believe could emotionally destabilize addicts. Understanding how drugs affect the hippocampus may have a critical role in treating addiction.

Neuropsychologist Xia Zhang and a team of researchers based at the University of Saskatchewan in Saskatoon, Canada, aimed to find out just how marijuana-like drugs, known collectively as cannabinoids, act on the brain.

Expanding the mind

The researchers injected rats with HU210, a synthetic drug that is about one-hundred times as powerful as THC, the high-inducing compound naturally found in marijuana. They then used a chemical tracer to watch new cells growing in the hippocampus.

They found that HU210 seemed to induce new brain cell growth, just as some antidepressant drugs do, they report in the Journal of Clinical Investigation1. This suggests that they could potentially be used to reduce anxiety and depression, Zhang says. He adds that the research might help to create new cannabinoid-based treatments.

"I think it's a very exciting study," says Amelia Eisch, an addiction researcher at the University of Texas Southwestern Medical Center in Dallas. "It makes marijuana look more like an antidepressant and less like a drug of abuse."

Eisch adds that much more work must be done before scientists can reach any definitive conclusions about the benefits and costs of marijuana. First and foremost, researchers need to establish that THC has the same positive effects as the synthetic HU210. Then they must develop more sophisticated experiments to firm up the correlation between neuron growth in the hippocampus and emotional balance.

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Finally, she says, scientists must understand why cannabinoids have a different effect on the brain than other addictive drugs.

Although his findings point to potential benefits of smoking pot, Zhang says that he does not endorse its use. "Marijuana has been used for medicine and recreation for thousands of years," he says. "But it can also lead to addiction."

He says his group's next studies will examine this more unpleasant side of the drug. Other addiction researchers will be keenly interested in the results, because this cannibinoid acts so differently on the hippocampus than other drugs.

University of Texas Southwestern Medical Centre, Dallas