In the movies, drug addicts rarely think twice before pulling a knife on someone or robbing a liquor store. Do drugs make them impulsive, or are they on drugs because they're impulsive? A new study in rats suggests that hastiness comes first, and that people may "self-medicate" to curb their rash tendencies.
Drug users tend to be impulsive, but it's not clear why. Some research has shown that impetuous rats also use more cocaine than more calculating rodents, suggesting impulsiveness can lead to drug use. And researchers have found that addicts have fewer D2/3 receptors in their brains. These molecules are involved in the body's reward system, and in monkeys, they decline in response to cocaine use. Because the link between impulsive behavior and drug use is so strong, neuroscientist Jeffrey Dalley of the University of Cambridge, U.K., wondered if the D2/3 receptors might connect the two.
To find out, Dalley and colleagues trained 100 rats to watch a bank of five lights. Every five seconds, one light flashed on, at which point the rats would poke at a hole underneath the lit light and get a food treat. The more impulsive rats tried to "predict" which light would come on, poking at random holes before the light show began and making twice as many mistakes as their patient peers. It turned out that impulsive rats had half as many D2/3 receptors as the even-tempered rats in an area of the brain known to be involved in drug addiction.
The researchers then took eight rats from each group and allowed them to eat as much cocaine as they wanted. The impulsive rats consumed nearly twice as much cocaine as the patient rats, the team reports tomorrow in Science. After a week of sobriety, however, the rash rats performed just like the patient rats in the light experiment. The findings not only show that impulsivity precedes drug use--and that D2/3 receptors are involved--says Dalley, but they also indicate that drug use itself may be a way for animals to regulate their impulsive tendencies. As such, drug treatment programs may benefit from reducing impulsive behavior, he says.
Psychobiologist Marilyn Carroll of the University of Minnesota in Minneapolis says the work is important because it connects the increased drug use of impulsive animals to neurochemistry in the brain, which is impossible to sort out in the complicated world of human drug addicts. She hopes such work will lead to ways to predict who will become a drug addict so predisposed individuals can be trained to prevent drug use.