Smoke for strokes

By Philip Cohen

two consultants of marijuana can help prevent the brain damage that often follows a stroke. The discovery by researchers at the National Institutes of Health (NIH) near Washington DC will add weight to the arguments of doctors who think the drug should be legal for medical use. Some patients with multiple sclerosis, glaucoma and AIDS-related wasting already take marijuana. It also seems to help relieve the nausea suffered by cancer patients undergoing powerful chemotherapy. When marijuana is smoked or eaten, chemicals called cannabidiol (CBD) and tetrahydrocannabinol (THC) enter the body. THC binds to proteins on the surface of brain cells called cannabinoid receptors, causing the drug's mind-altering effects. CBD doesn't bind to these proteins and is not psychoactive. Aidan Hampson and his NIH colleagues were interested in the normal role of these receptors. "We didn't evolve them so that some people could enjoy smoking pot," he says. Hampson suspected that activating the receptors might change how cells respond to the neurotransmitter glutamate. Damping down this response can be crucial, since depriving neurons of oxygen, as happens in a stroke, makes them pump out too much glutamate. This in turn boosts the production of reactive free radicals, which can kill brain cells. The researchers treated rat neurons with toxic levels of glutamate and then tested THC's ability to prevent cell death. They found that extremely low concentrations of THC protected the neurons, suggesting that the cannabinoid receptors might indeed be involved. But to their surprise, CBD gave exactly the same level of protection. So if binding to the receptors wasn't crucial, how were THC and CBD protecting the brain cells from chemical attack? The researchers wondered if they might work like vitamins C and E, antioxidants that inactivate free radicals by giving up electrons. They found that both CBD and THC gave up electrons easily. In fact, they were able to protect at least 20 per cent more neurons than either vitamin C or E at the same concentration (Proceedings of the National Academy of Sciences, vol 95, p 8268). Since they submitted that paper, the researchers have begun testing the ability of CBD to prevent brain damage in rats in which they had induced strokes. Preliminary results are promising, Hampson says. Hampson notes that people can tolerate high doses of CBD, and because it quickly penetrates the brain, CBD could be an ideal drug for treating strokes. It's unclear whether the doses of CBD absorbed by smoking cannabis could protect brain cells. But the findings will stoke the debate over the medical use of marijuana. Those who have argued against legalisation have pointed out that THC is available legally in a purified form called marinol. "But now we know that CBD may also be therapeutic, so marinol isn't a complete substitute," says Hampson. Lester Grinspoon of the Harvard Medical School in Boston, who advocates the medical use of marijuana, says: "Critics have insisted for a long time that marijuana must damage the brain. Now it turns out it might actually be neuroprotective. I'd say we've come full circle." From New Scientist, 11 July 1998