New Scientist
February 21, 1998

Back in the 1970s, animal experiments led to groundless fears that marijuana blew holes in brain tissue. The experiments organisations like NIDA now fund are more sophisticated but the controversy still rages.

George Koob

George Koob, an addiction researcher from The Scripps Research Institute in La Jolla, California, claims the new message from the animals is simple: "The more we discover about the neurobiology of addiction the more common elements we're seeing between THC (tetrahydrocannabinol, the main active ingredient in cannabis) and other drugs of abuse." And for Koob, one of these newly discovered "common elements" is marijauna's ability to trigger chemical changes in the brain that lead to strong withdrawal symptoms.

In humans, some researchers claim to see clear evidence of insomnia, anxiety and even flu-like symptoms in heavy cannabis users who abstain. But if there's a consensus, it's that symptoms are mild and variable. By contrast, Koob's rats are shivering wrecks. Does this mean marijuana is more addictive than we think?

Not a bit of it, says Roger Pertwee, a university pharmacologist and president of the Cannabinoid Research Society. That's because those symptoms aren't so much observed as manufactured. The animals are injected with high doses of THC, then injected with a second chemical to block cannabis receptors in the brain. Without the block, the sharp withdrawal symptoms can't be seen because cannabis clears so slowly that even heavily doped rats are likely to experience a gentle wind down.

Another debate rages over animal studies into the short-term effects of marijuana on brain chemistry. Heroin, cocaine, alcohol and nicotine all trigger a surge in the chemical dopamine in a small midbrain structure called the nucleus accumbens. Many researchers regard this as a hallmark of an addictive substance.

Last year, experiments showed that cannabis presses the same dopamine button in rats, leading to claims that the drug must be more addictive than previously thought. To critics, it is just another example of those old exaggerated fears.

What nobody tells you, says John Morgan, a pharmacologist at City University of New York Medical School, is that rats don't like cannabis. It's easy for them to get hooked on heroin or cocaine -- but not marijuana. Nor, Morgan claims, are researchers exactly open about awkward observations, such as the fact that there are plenty of nonaddictive drugs that stimulate dopamine in the brain.

It's easy to understand why biologists want to find simple chemical traits that are shared by all addictive drugs. Unfortunately, the differences are as important as the similarities when it comes to weighing the relative risks and pleasures involved in taking drugs. And subjectively at least, the intense rush of cocaine and orgasm-like high of heroin have little in common with dope's subtler effects.

How cannabis gets to work on our brains