Abnormal Psychology

Substance Use Disorders 397

9.6 • Cocaine Abuse

and Dependence and

Dopamine This schematic

illustrates how cocaine binds to

dopamine transporters, which

prevents normal reuptake of

dopamine back into the transmitting

neuron and increases the amount

of dopamine in the synapse, which

thereby activates the dopamine

reward system.

by altering other brain areas or neurotransmitters that, in turn, affect dopamine

(Leone, Pocock, & Wise, 1991).

Regular stimulant use (or other substance use) usually affects the dopamine

reward system. Researchers have thus proposed the dopaminergic hypothesis of

substance abuse: The rewarding effects of a drug arise directly or indirectly from

the dopamine reward system (Koob & Le Moal, 2008; Robbins & Everitt, 1999a,

1999b; Tomkins & Sellers, 2001; Torrens & Martín-Santos, 2000). Because of the

neural changes that occur with continued abuse, after a while, the person needs

the substance to feel “normal” and experiences cravings when not using the drug.

As researchers have come to understand the dopamine reward system in more

detail, they have begun to gain insight into an age-old puzzle: Why are some people

more susceptible to becoming abusers than others? Do they have less “character”

or a weak “moral compass”? No. In fact, at least part of the answer is that the do

amine reward system is more sensitive and responsive in some people. For example,

in one study, participants were given an injection of the stimulant Ritalin. The re-

searchers found that participants who rated the experience as pleasant had fewer

dopamine receptors than those who found it unpleasant (Volkow et al., 1999). Such

fi ndings support the hypothesis that people with fewer dopamine receptors may be

more vulnerable to drug use (and abuse); the smaller quantity of receptors means

they have reduced activation in the reward system, which is boosted by substance

use (Swanson & Volkow, 2002). In fact, rats bred so that they consume relatively

high amounts of alcohol appear to have a less responsive dopamine reward system,

which could explain why they drink more (McBride et al., 1990).

In addition, as we’ll discuss shortly, stimuli related to taking a drug—such as

the music, lighting, and crowds at a dance club where cocaine use occurs—can be-

come associated with drug use. Connections among different brain areas, such as

the amygdala and hippocampus, store associations between drug use and the stim-

uli related to drug use. Once such associations are established, these drug-related

stimuli themselves can trigger the dopamine reward system (Tomkins & Sellers,

2001). Moreover, simply perceiving such aspects of the environment can activate

structures in the limbic system (Dackis & O’Brien, 2001), which is tightly tied to

the dopamine reward system. For ex-

ample, one such structure is the an-

terior cingulate, and an fMRI study

found that this structure was activated

when cocaine addicts simply watched

videotapes of cocaine-related objects

and events (Wexler et al., 2001).

Effects of Cocaine and

Methamphetamine on the Brain

To see how the dopamine reward sys-

tem is involved in drug abuse, let’s

look at two abused drugs, cocaine and

then methamphetamine. The effects of

cocaine on the brain result from the

way it affects dopamine levels. Spe-

cifically, cocaine binds to dopamine

transporters—the molecules that take

excess dopamine from the synapse and

bring it back to dopamine containing

sacs within the terminal buttons of the

transmitting neuron (see Figure 9.6).

When cocaine binds to these trans-

porter molecules, they don’t operate as

effectively to remove dopamine from

the synapses—and thus more dopamine

Figure 9.6

99

a

D

il

d p d n o t r

g9

Dopamine

receptors

Receiving

neuron

Dopamine

packaged

in sacs

Transmitting

neuron

Cocaine

Dopamine

transporter

blocked by

cocaine

Dopamine

transporter

functioning

normally