Scientists at the National Institute on Drug Abuse (NIDA) have demonstrated that laboratory animals will self-administer marijuana's psychoactive component, THC (delta-9-tetrahydrocannabinol), in doses equivalent to those used by humans who smoke the drug. Self-administration of drugs by animals, long considered a model of human drug-seeking behavior, is characteristic of virtually all addictive and abused drugs.
"This study is simple and its findings are clear," says NIDA Director Dr. Alan I. Leshner. "Animals will work to get THC. This emphasizes further the similarity between marijuana and other abusable, addicting substances. Both animals and humans will work to acquire access to marijuana in the same way that both animals and humans change their behavior to get other drugs of abuse, like cocaine and heroin."
Dr. Steven Goldberg and colleagues at NIDA's Intramural Research Program in Baltimore, Maryland, report in the current issue of Nature Neuroscience that squirrel monkeys will self-administer intravenous injections of THC.
"This is the first study in which it has been possible to show that monkeys or other research animals will self-administer THC. There are many factors which may explain this behavior, including the fact that in our study we used doses of THC that are directly comparable to doses in marijuana smoke inhaled by humans," Dr. Goldberg says.
Before the study began, the scientists first established self-administration behavior in squirrel monkeys that received repeated intravenous injections of cocaine after pressing a lever 10 times for each injection. At the start of the study, the researchers replaced cocaine with saline solution and the animals' self-administration stopped. When saline was replaced with THC in a solution that would rapidly pass from blood to the brain, the animals resumed self-administration, rapidly pressing the lever to obtain on average 30 injections of THC during each of a series of 1-hour sessions. Treatment with a compound that prevented THC from binding to cannabinoid receptors on brain cells almost completely eliminated self-administration of THC, but had no effect in another group of monkeys self-administering cocaine under identical conditions, according to Dr. Goldberg.
"The drug-seeking behavior in these animals was comparable in intensity to that maintained by cocaine under identical conditions, and was obtained from a range of doses comparable to those self-administered by humans smoking a single marijuana cigarette," Dr. Goldberg says. "This finding suggests that marijuana has as much potential for abuse as other drugs of abuse, such as cocaine and heroin."
Additional information about this study is available in Nature Neuroscience 2000, volume 3, pgs 1073-74 or at www.neurosci.nature.com.