For a long time scientists thought that drug addiction was distinctly human behavior. Then researchers discovered that rats can form addictions, too. Aside from being just one more reminder of how frighteningly similar we are to our lab companions, this finding offered scientists a chance to study how addiction actually works. Why is it that only some drug users spiral into addictive behavior? Is the addict's brain actually different? There was really only one way to find out: Give a bunch of rats some coke and see what happens.
In a study published in the June 25th edition of Science, a team of researchers attached laboratory rats to a device that allowed the rodents to self-administer doses of cocaine—a coke IV of sorts. After a month, the researchers began identifying which rats had become hooked on the drug by looking for the hallmark signs of addiction: difficulty stopping or limiting drug use; high motivation to continue use; and continued use despite negative consequences. Only 20 percent of the rats exhibited all three signs of addiction, while 40 percent exhibited none. The researchers were left to figure out what it was that made addiction-prone rats—and presumably people—different from the rest. Here's what they found.