DRUGS AND THE PLEASURE CIRCUIT
It won't surprise
anyone that addictive drugs are reinforcers. Here the experimental evidence is overwhelming. Most
experimental animals (pigeons, rats,
monkeys) will press a lever to get an injection of cocaine, methamphetamine, heroin, nicotine, and alcohol.
They will not press a lever for LSD, antihistamines, or many other drugs.
Furthermore, even fruit flies and
zebra fish will hang out in an environment in which they have previously received a reinforcer—another test
of the potential addictiveness of a
substance. This list of drugs for which experimental animals will work matches exactly the list of drugs that
are viewed as clearly addictive in
humans.
We know that the same pathway mediates the
pleasurable effects of addictive
drugs. There are two particularly convincing arguments. First, if this pathway is damaged in an animal, the animal
will not work for drugs. Second,
animals with an electrode placed in the reward pathway find smaller currents more "enjoyable" if the animal has received an injection of cocaine or heroin, for example. The same system is
activated in the brains of addicts.
When cocaine addicts looked at pictures of cocaine or handled crack pipes while the activity of their brains was
monitored, they reported a craving
for cocaine, and at the same time, their brains showed activation in the reward pathway of the brain.
Addictive drugs
(stimulants, opiates, alcohol, cannabinoids, and nicotine) can actually substitute for food or sex.
This explains why rapid injection of
cocaine or heroin produces a "rush" of pure pleasure that most users compare to the pleasure of orgasm. This
isn't just true for certain people
who lack willpower or who are engaged in a deviant lifestyle. It is true for everyone who has a brain. It
automatically becomes easier to understand
why addiction is such a common problem across cultures.
Although the news media have certainly overdone the
"what is the most addictive
drug?" contest, it is also clear that animals will work much harder to get some drugs than others. For example,
rats will press a bar up to two to
three hundred times for one injection of cocaine and perhaps even more for the bath salt MDPV. However, some
drugs, like alcohol and nicotine,
might be administered more if they didn't have unpleasant effects on the body. Humans seem to be particularly
good at ignoring unpleasant side
effects to obtain reinforcement from drugs. If you judged the most addictive drug by the largest number of people who have trouble stopping their use of it, then nicotine would be
the clear leader.
