THE SPECIAL ROLE
OF DOPAMINE
The neurotransmitter dopamine seems to play an
important role in the normal process
of reinforcement and in the actions of most addictive drugs. One group of dopamine neurons runs directly
through the reward circuit we just
described. If the dopamine neurons in this circuit are destroyed, then animals will not work for food,
sex, water, or addictive drugs. Furthermore,
both natural reinforcers and most addictive drugs increase the release of dopamine from these
neurons. Our favorite experiment was
conducted by a scientist in Canada who measured the release of dopamine in the brain of a male rat before and
after providing it with a female
partner. Not surprisingly, access to a sexually receptive partner caused a large rise in dopamine levels in this part
of the brain.
If this same
experiment is done with drugs instead of natural reinforcers, the results are the same. Cocaine, morphine,
nicotine, cannabinoids, or alcohol
will cause large increases in dopamine in the same area of the brain in which sex causes a rise. Most neuroscientists
think that addictive drugs affect
neurons that connect, one way or another, with this critical dopamine circuit to stimulate its activity.
Anyone who has ever
enjoyed a muffin knows that dopamine going up when something pleasant happens is not the whole
story of addiction. It is not even the
whole story of how dopamine is involved. To explain that, we are going back to the bakery for a second visit.
The first time you went to the bakery,
dopamine went up when you had an unexpected treat—a tasty muffin. The second (or the third, or fifth) time,
you started to anticipate the muffin when you saw the bakery sign. We know from experiments in monkeys that dopamine starts going up in
anticipation of a reward rather than
when the reward arrives. Scientists now think that one important role dopamine plays is this anticipation for
a known reward. This agrees with our
common sense that muffin eating is not food addiction. The first step toward addiction may be when you expect the
muffin and begin to organize your
walk to work to make sure it happens. Dopamine probably contributes to that decision-making process.
However, it is still not addiction.
You could change your route if you needed to.
Furthermore,
dopamine neurons are not "the end of the line" for detecting pleasure, but they clearly connect with other
neurons. We are just beginning to
understand how these other areas of the brain play a role.
THE DARK SIDE: PAIN, NOT PLEASURE
Enjoying the rush of
pleasure from a drug is only part of addiction. For addicts, there is an opposing force, a yang for
the yin. Once the body adapts to the drug and physical dependence develops, a
daily cycle of drug taking, pleasure,
gradual waning of drug effect, and the onset of withdrawal symptoms emerges. Withdrawal symptoms are different for each drug, and minimal for some (much of this is
covered in detail in the chapters on
individual drugs). For example, the waning of opiate effects causes an ill feeling similar to the onset of the
flu. The drug user has chills and
sweats, a runny nose, diarrhea, and a generally achy feeling. An alcoholic will feel restless and anxious. However,
there is a common underlying feeling for withdrawal from all addictive drugs:
a feeling that is the reverse of the good feeling that the drug once gave that
can be accompanied by a strong
craving to take more of the drug. Avoiding the unpleasant feelings of
withdrawal and satisfying the desire for more drug can eventually become even stronger motives for drug
taking than simply
feeling good.
