In what could be a coup for antimarijuana forces, new research shows that rats exposed to pot's active ingredient at an early age devour more heroin as adults than rats without early exposure. Some experts, though, say the jury is still out on whether the finding is enough to officially label marijuana a "gateway" drug.
According to statistics from the National Institute on Drug Abuse, most adults who take illicit drugs start doing so in their early teens. In addition, the earlier kids start smoking dope, the more likely they are to use harder drugs later on. For example, of people who first puffed weed before age 15, 62% went on to use cocaine and 9% to use heroin. But of those who started smoking pot after age of 20, only 16% moved onto cocaine and 1% to heroin. Some researchers think this means that marijuana is a gateway drug--one that leads to harder drug use. Others point out that such a claim is hard to prove because the same factors that lead people to use marijuana in the first place might also lead them to use other drugs.
To see through the smoke, neuroscientist Yasmin Hurd, now at the Mount Sinai School of Medicine, and colleagues at the Karolinska Institute in Stockholm, Sweden, gave 4-week-old rats THC, the most common psychoactive component in cannabis. The researchers injected the rats with the THC equivalent of about three-quarters of a joint (scaled down for a rat's size) every third day for 3 weeks until they reached mid-adolescence, about 7 weeks old. The dose probably created a mild buzz, but not high enough that the rats stumbled. After a week-long break, the rodents were allowed to self-administer heroin using levers that provide the substance.
Rats that had been exposed to THC as "teens" took about 25% more heroin than did their just-say-no peers. Biochemical tests of the adult animals showed that THC-doused brains had the same number of receptors that responded to THC as unexposed rat brains, but more receptors for heroin and more of a compound associated with reward behavior in their neurons, the team reports online 5 July in Neuropsychopharmacology. Whether this indicates marijuana is a "gateway" drug depends on the definition of "gateway," says Hurd. She says both groups of animals took the same amount of time to start taking heroin, suggesting THC use doesn't start them on the path to hedonism, but the THC-primed rats got more into it, suggesting it paves the way for increased use.
"The important finding is the fact that adolescence is a time of increased vulnerability to drugs," says neuropharmacologist Sari Izenwasser of the University of Miami School of Medicine in Florida, who notes that such behavior may alter fundamental brain processes. But pharmacologist Aron Lichtman at Virginia Commonwealth University in Richmond inserts a note of caution. "The data really are very provocative," he says, but not conclusive. He questions whether other reward-reinforcing behavior, such as eating food, would also be increased under these conditions.