A synthetic chemical similar to the active ingredient in marijuana makes new cells grow in rat brains. What is more, in rats this cell growth appears to be linked with reducing anxiety and depression. The results suggest that marijuana, or its derivatives, could actually be good for the brain.

In mammals, new nerve cells are constantly being produced in a part of the brain called the hippocampus, which is associated with learning, memory, anxiety and depression. Other recreational drugs, such as alcohol, nicotine and cocaine, have been shown to suppress this new growth. Xia Zhang of the University of Saskatchewan in Saskatoon, Canada, and colleagues decided to see what effects a synthetic cannabinoid called HU210 had on rats' brains.

They found that giving rats high doses of HU210 twice a day for 10 days increased the rate of nerve cell formation, or neurogenesis, in the hippocampus by about 40%.

Just like Prozac?                                  

A previous study showed that the antidepressant fluoxetine (Prozac) also increases new cell growth, and the results indicated that it was this cell growth that caused Prozac’s anti-anxiety effect. Zhang wondered whether this was also the case for the cannabinoid, and so he tested the rats for behavioural changes.

When the rats who had received the cannabinoid were placed under stress, they showed fewer signs of anxiety and depression than rats who had not had the treatment. When neurogenesis was halted in these rats using X-rays, this effect disappeared, indicating that the new cell growth might be responsible for the behavioural changes.

In another study, Barry Jacobs, a neuroscientist at Princeton University, gave mice the natural cannabinoid found in marijuana, THC (D9-tetrahydrocannabinol)). But he says he detected no neurogenesis, no matter what dose he gave or the length of time he gave it for. He will present his results at the Society for Neuroscience meeting in Washington DC in November.

Jacobs says it could be that HU210 and THC do not have the same effect on cell growth. It could also be the case that cannabinoids behave differently in different rodent species - which leaves open the question of how they behave in humans.

Zhang says more research is needed before it is clear whether cannabinoids could some day be used to treat depression in humans.

Journal reference: Journal of Clinical Investigation (DOI:10.1172/JCI25509)