By plying rats with cocaine, researchers have inched closer to defining the boundary between drug use and addiction. Their study , published in tomorrow's issue of Science, shows that rats crave more and more cocaine only if offered the drug for many hours.
George Koob, a professor of neuropharmacology at The Scripps Research Institute in La Jolla, California, became intrigued by a study showing that some rats---when offered as much cocaine as they wanted for 3 hours--gradually increased their doses, while others held steady. Seeking to crystallize the difference in doping styles and wondering about the importance of cocaine availability, Koob and postdoctoral Serge Ahmed split several dozen rats into two groups: one that could receive cocaine for 6 hours a day, and another that had only an hour to get high. The rats had a lever they could press to receive a painless, intravenous cocaine infusion.
Over a period of about 2 weeks, rats in the 6-hour group gradually escalated their cocaine use, until they were dosing themselves twice as often--about every 3 minutes--as rats in the 1-hour group were. The findings suggest that rats with restricted access to the drug were better able to control their use of it, while those with freer access abused it. "There's a biological change that occurs somewhere in an animal that has access to the drug for an extended period of time," says Koob, who believes that his animal model accurately describes the transition from drug use to drug addiction.
Experts applaud the study while cautioning that its results may not apply to people. "I'm very impressed," says Klaus Miczek, a professor of pharmacology at Tufts University in Medford, Massachusetts. However, he adds, "this is a rat model, not a primate model."