Consciousness, the Brain, and Behavior
245
rotransmitter implicated in substance dependence is dopamine,
other neurotransmitters, including GABA, enkephalin, sero-
tonin, and glutamate, are also involved.
Tolerance
Tolerance
to a substance occurs when increasing doses of the
substance are required to achieve effects that initially occurred
in response to a smaller dose. That is, it takes more of the sub-
stance to do the same job. Moreover, tolerance can develop to
another substance as a result of taking the initial substance,
a phenomenon called
cross-tolerance
.
Cross-tolerance may
develop if the physiological actions of the two substances are
similar. Tolerance and cross-tolerance can occur with many
classes of substances, not just psychoactive substances.
Tolerance may develop because the presence of the sub-
stance stimulates the synthesis of the enzymes that degrade
it. As substance concentrations increase, so do the concentra-
tions of these enzymes. Thus, more of the substance must be
administered to produce the same plasma concentrations of
the substance and, hence, the same initial effect.
Alternatively, tolerance can develop as a result of changes
in the number and/or sensitivity of receptors that respond to
the substance, the amount or activity of enzymes involved in
neurotransmitter synthesis, the reuptake transport molecules,
or the signal transduction pathways in the postsynaptic cell.
Learning and Memory
Learning
is the acquisition and storage of information as a
consequence of experience. It is measured by an increase in the
likelihood of a particular behavioral response to a stimulus.
Generally, rewards or punishments are crucial ingredients of
learning, as are contact with and manipulation of the envi-
ronment.
Memory
is the relatively permanent storage form of
learned information, although, as we will see, it is not a single,
unitary phenomenon. Rather, the brain processes, stores, and
retrieves information in different ways to suit different needs.
Memory
The term
memory encoding
defi nes the neural processes that
change an experience into the memory of that experience—in
other words, the physiological events that lead to memory for-
mation. This section addresses three questions: Are there dif-
ferent kinds of memories, where do they occur in the brain,
and what happens physiologically to make them occur?
New scientifi c facts about memory are being generated
at a tremendous pace, and there is as yet no unifying theory as
to how memory is encoded, stored, and retrieved. However,
memory can be viewed in two broad categories:
Declarative
memory
is the retention and recall of conscious experiences
that can therefore be put into words (declared). One exam-
ple is the memory of having perceived an object or event and,
therefore, recognizing it as familiar and maybe even know-
ing the specifi c time and place when the memory originated.
A second example would be one’s general knowledge of the
world, such as names and facts. The hippocampus, amyg-
dala, and diencephalon—all parts of the limbic system—are
required for the formation of declarative memories.
Table 8–3
Diagnostic Criteria for Substance
Dependence
Substance dependence is indicated when three or more of the
following occur within a 12-month period.
1. Tolerance, as indicated by
a. a need for increasing amounts of the substance to achieve
the desired effect, or
b. decreasing effects when continuing to use the same
amount of the substance.
2. Withdrawal, as indicated by
a. appearance of the characteristic withdrawal symptoms
upon terminating use of the substance, or
b. use of the substance (or one closely related to it) to relieve
or avoid withdrawal symptoms.
3. Use of the substance in larger amounts or for longer periods
of time than intended.
4. Persistent desire for the substance; unsuccessful attempts to
cut down or control use of the substance.
5. A great deal of time is spent in activities necessary to obtain
the substance, use it, or recover from its effects.
6. Occupational, social, or recreational activities are given up
or reduced because of substance use.
7. Use of the substance is continued despite knowledge
that one has a physical or psychological problem that the
substance is likely to exacerbate.
Adapted from DSM-IV, Diagnostic and Statistical Manual of Mental Disorders, 4th ed. American
Psychiatric Association, Arlington, VA, 2000.
Table 8–4
Potential of Various Substances to
Cause Dependence
If 100 people regularly use a substance, how many will become
dependent on it?
Nicotine
33
Heroin
25
Cocaine
16
Alcohol
15
Amphetamines
11
Marijuana
9
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