Animals can become addicted to drugs and alcohol just as human beings can. In fact, the addictive quality of cocaine was first demonstrated in animals. When humans quit using cocaine, the withdrawal does not cause severe physical symptoms, which has been the traditional measure of addiction. But animal studies showed that if monkeys were given a choice of receiving cocaine or food, they would administer cocaine to themselves to the point of starvation. Clearly they were addicted to cocaine, but the addiction was behavioral, not physical.
Because animals can become addicted to drugs, they provide excellent models of the addiction process. For example, cocaine has been found to block the uptake of a chemical known as dopamine from nerve junctions in the brain. Animal researchers are now investigating several promising compounds that could reduce the craving for drugs or block the effects of drugs in the brain.
Animal studies have also been integral to many of the behavioral therapies that are currently the only proven long-term methods for dealing with drug addiction and other compulsive behaviors. Animals have been used to study the reinforcing mechanisms that promote or discourage certain behaviors. In addition, animal research has illuminated the complicated interactions between addictive behavior and the environment. One example involves an animal model of heroin overdose. If rats are given repeated injections of heroin (morphine) of increasing dosage in the same environment, they develop great tolerance just as humans do. They easily survive a dose that would have been lethal if given first. But if the same heroin dosage is administered to these tolerant rats in a new and novel environment, many of them die. Perhaps the effects of novel environments or new situations account for a portion of the deaths from heroin overdose in human addicts.