Abi Berger, BMJ
Marijuana has the potential for misuse, according to a study from the United States. New evidence that monkeys self administer the active component of marijuana has been shown by Dr Steven Goldberg and his team at the National Institutes of Health in Baltimore (Nature Neuroscience 2000;3:1073-4).
One of the criteria used to help decide if a drug has the potential for misuse is whether animals will work to obtain it. This is known as self administration. Virtually all psychoactive drugs misused by humans, including nicotine, have been shown to be self administered by animals, but up to now a positive self administration test has been elusive whenever THC (delta-9-tetrahydrocannabinol), the active part of marijuana, has been tested. This has led to some people concluding that marijuana is less likely to lead to drug misuse than other illegal substances.
Dr Goldberg, a pharmacologist at the National Institute of Drug Abuse, has shown now that monkeys can be trained to self administer THC.
In this study the team used a lowbut clinically relevantdose of THC administered intravenously in a clear solution. This solution rapidly distributed THC to the brain. Previous attempts to show self administration, using much higher doses of THC held in a suspension, failed. One reason for this may be that, although higher doses were used, the suspension resulted in less brain penetration. In this study the monkeys had previously been trained to self administer cocaine by pressing a lever 10 times. When saline was substituted for cocaine, self administration stopped. When THC replaced the saline, the monkeys quickly started to press the lever again. The monkeys gave themselves about 30 injections during an hour long session, which equates roughly with the dose received by a person smoking a marijuana joint.
The team went on to confirm that giving the monkeys a second drug that directly blocks cannabinoid receptors in the brain could prevent self administration. This suggests that THC antagonists may be useful in combating marijuana addiction in humans. Dr Goldberg's team will next be trying their approach in "naive" monkeys (animals that have not previously been exposed to other psychoactive drugs) to see if this alters the animals' behaviour.
Dr Goldberg's team concludes from its observations that THC
"has as much potential for abuse as other drugs of abuse, such as cocaine
and heroin."
Reactions to the cannabis study
Martin Jarvis, professor of health psychology at University College London said that to suggest that the potential for misusing marijuana is as great as with drugs such as cocaine and heroin is probably overstating the case. He said that misuse is "a judgment best made by looking at patterns of actual human use." He continued: "We shouldn't assume that unreasonable behaviour in society follows from the observation of brain reward behaviour in animals alone".
Ian Stolerman, professor of behavioural pharmacology at the Institute of Psychiatry in London, agreed: "This is an important study because for the first time it provides a method for studying directly the intake of THC by a laboratory animal and thus models a key behavioural feature of addictive states generally. It will lead to studies of how and where THC works in the brain to generate drug abuse. It does show that THC shares properties with other drugs of abuse, but whether it is really as potentially abusive as cocaine and heroin is not so clear."
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