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 low - but
clinically relevant - dose 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."
Abi Berger, BMJ
studentBMJ 2000;08:395-434 November ISSN 0966-6494
