MARIJUANA HAS POTENTIAL FOR MISUSE

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."