Not all psychoactive drugs are
addictive and not all people exposed to addictive drugs will become
addicts. While some drugs have a
very high potential for addiction, not even the most dangerous substances
typically produce addiction in a single dose—repeated exposure is
required. Addiction can be defined
as persistent, compulsive drug use, but addiction doesn’t develop all at
once. Rather, it proceeds in
stages. When a drug-user initially
gets high on cocaine or heroin or amphetamines or PCP, the experience produces
an intense euphoric pleasure and sense of well-being. However, repeated doses, particularly if strung closely
together in a binge, will begin to trigger the dark side of addiction. This is first manifest as drug
tolerance: after a binge, the drug-user will need a higher dose to achieve the
same level of euphoria and if drug-taking continues, this tolerance will become
greater and greater. As tolerance
to the drug develops, so does dependence.
This means that the addicted person not only needs more drug to get
high, but will feel bad in the absence of drug. Dependence can be experienced as both mental symptoms such
as depression, irritability or inability to concentrate in the absence of the
drug and physical symptoms such as nausea, cramps, chills and sweats.
As addiction
develops further, strong cravings for the drug are experienced. These cravings are often triggered by
drug-associated stimuli. A crack
cocaine addict may be feeling OK, but then have an intense craving for the drug
when she sees a pipe. The
amphetamine addict who often gets high in the bathroom of a club can feel a
craving triggered by dance music or even the sound of a toilet flushing. Odors, like the musty smell of heroin
cooking in a spoon prior to injection are particularly evocative. In his moving (and also hilarious)
autobiography of teenage heroin addiction, The Basketball Diaries, Jim Carroll writes of a friend who tried to kick
his heroin habit by seeking spiritual solace in the Catholic church of his
youth. However, the smell of the
church incense reminded him so much of bubbling heroin that he felt an
overwhelming craving and rushed home to shoot up again.
Drug addiction,
whether to cocaine or heroin or nicotine, is notoriously difficult to
break. Relapses, even after months
or years of drug-free living are common and most ex-addicts have had to make
multiple attempts to break free. It
is also well known that relapse is not only triggered by cues that are
associated with past drug use like particular people, odors, music, rooms,
etc., but also by emotional or physical stress. A central insight in recent years is that the later phases of
addiction, characterized by cravings and relapse, are associated with strong
and persistent memories of the drug-taking experience. Drugs, by co-opting the pleasure
circuitry and activating it more strongly than any natural reward, create
deeply-ingrained memories that are bound up in a network of associations. Later, these memories are then
strongly activated and linked to emotional centers by drug-associated external
cues and internal mental states and this process is enhanced by stress.
If we want to
understand and treat addiction at the level of molecules and cells in the
brain, if we want to develop therapies to help people break free of addiction
and stay drug-free, then we need to look for persistent cellular and molecular
changes in brain circuits produced by drugs. Of course, the first place in the brain to look is in the
medial forebrain pleasure circuitry itself. The good news is that this effort doesn’t have to start from
zero. Neuroscientists have already
worked out some aspects of how memory is stored in the brain and these insights
can be applied to the brain’s pleasure circuits and the problem of addiction.