Abnormal Psychology

Substance Use Disorders 417

and it activates the dopamine reward system. For instance,

in rats, cannabinoids trigger dopamine release in the nucleus

accumbens (Tanda, Pontieri, & Di Chiara, 1997). People

who began abusing marijuana at an early age have atrophy

of brain areas that contain many receptors for cannabinoids

(DeBellis et al., 2000; Ernst & Chefer, 2001; Wilson et al.,

2000), especially the hippocampus and the cerebellum. Atro-

phy of the hippocampus can explain why chronic marijuana

users develop memory problems, and atrophy of the cerebel-

lum can explain why they develop balance and coordination

problems. These brain areas are illustrated in Figure 9.10.

Cannabinoids also modulate other neurotransmitters and

affect pain and appetite (Wilson & Nicoll, 2001).

Dissociative Anesthetics PCP and ketamine alter the distri-

bution in the brain of glutamate, a fast-acting excitatory

neurotransmitter. Excitatory neurotransmitters induce brain

activity and may underlie the violent, impulsive effects of

these drugs. In addition, glutamate is involved in memory,

pain perception, and responding to the environment (NIDA,

2007f). Glutamate can be toxic; it actually kills neurons if

too much is present. Thus, by increasing levels of glutamate,

dissociative anesthetics may, eventually, lead to cell death in

brain areas that have receptors for this neurotransmitter—

which would explain the memory and other cognitive defi -

cits observed in people who abuse these drugs.

Genetics of Other Types of Substance Abuse

The genetic bases of abuse of other types of substances have not been studied in as

much depth as those of alcohol abuse. However, one twin study of cannabis depen-

dence (Lynskey et al., 2002) estimated that genes account for 45% of the variance

in vulnerability to cannabis dependence, shared environmental factors account for

20%, and non-shared environmental factors account for the remaining 35%. Con-

sistent with these fi ndings, Tsuang and colleagues (2001) concluded from a study of

over 3,330 male twins that genes, shared environment, and unique environmental

factors all affect substance abuse (of illicit drugs).

Psychological Factors

Abuse of these other types of substances is affected by most of the same psychologi-

cal factors that infl uence abuse of stimulants and depressants (see the starred items in

Table 9.7): observational learning, operant conditioning, and classical conditioning all

contribute to substance abuse as a maladaptive coping strategy. We examine here the

unique aspects of classical conditioning associated with heroin abuse and dependence

Classical conditioning can help explain how some accidental heroin “overdoses”

occur (Siegel, 1988; Siegel et al., 2000). The quotation marks around the word over-

doses are meant to convey that, in fact, the user often has not taken more than usual but

has taken a usual dose in the presence of novel stimuli (Siegel & Ramos, 2002). If a user

normally takes heroin in a particular place, such as the basement of the house, he or she

develops a conditioned response to being in that place: The brain triggers physiological

changes to get ready for the infl ux of heroin, activating compensatory mechanisms to

dampen the effect of the about-to-be-taken drug. This response creates a tolerance for

the drug, but—and here’s the most important point—only in that situation. The stimuli

in a “neutral” setting (not associated with use of the drug), such as a bedroom, have not

yet become paired with taking heroin—and hence the brain does not trigger these com-

pensatory mechanisms before the person takes the drug. In this case, the conditioned

stimulus (e.g., the basement) is not present to elicit the compensatory response. And,

without this response, the same dose of heroin can have a greater effect if taken in the

bedroom than it would have if taken in the basement, causing an “overdose.”

9.10 • Marijuana’s Effects

on the Brain When marijuana is

smoked, its active ingredient, THC,

attaches to cannabinoid receptors in

numerous brain areas, including in the

nucleus accumbens, which is part of the

dopamine reward system. People who

began abusing marijuana at an early age

eventually have atrophy of brain areas

that have many cannabinoid receptors,

especially the hippocampus and

cerebellum.

Figure 9.10g9

Cerebellum

Hippocampus

Nucleus

accumbens

P S

N