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NEUROSCIENCE OF PSYCHOACTIVE SUBSTANCE USE AND DEPENDENCE

Persistence of neural sensitization may leave dependent individuals
susceptible to relapse long after discontinuation of substance use. Relapse
can occur following stress, exposure to the drug or a similar drug or to drug
cues. Individual differences in genetics and environmental factors, however,
will have mitigating effects on the primary rewarding effects of psychoactive
substances.

Summary

Substance dependence may be viewed as the result of the action of various
factors. In the early stages of substance use, as a result of curiosity, peer
pressure, social marketing factors, ubiquity of exposure, personality traits,
and other related factors, the subject comes into contact with a drug with
dependence-producing effects. The reinforcing properties of the drug,
together with the individual’s own biological make-up and environmental
background, may facilitate further exposure to the drug. Associative learning
properties related to release of dopamine in the nucleus accumbens also
strengthen the reinforcing effects of the drug and of the environment and
emotions associated with its use. In this stage the subject responds to the drug
and to drug-related stimuli in a manner not dissimilar from normal motivated
responding. Through activation of emotional and motivational centres of the
brain, learning processes are invoked. It is important to note here that exposure
to psychoactive substances and substance use in everyday life and through
the media, particularly when presented in a positive environment, can create
pleasurable emotions. An individual can easily become conditioned to
associate these emotions with substance use, resulting in learning, focused
attention, facilitated memory, and the development of attitudes surrounding
substance use that guide motivation. These factors all interact with individual,
biological, social, and cultural factors to determine whether or not substance
use is repeated, and whether that repeated substance use results in the cluster
of symptoms known as dependence.
With repeated drug exposure, there is the repeated association of drug
reward and drug-related stimuli parallel to the stimulation of dopamine
transmission in the nucleus accumbens, resulting in the attribution of
motivational value to drug-associated stimuli. This is the stage of incentive
sensitization. In this stage the person can still control drug intake in the
absence of drug-related stimuli and is not dependent, but can experience
health and social consequences of his or her substance use. This stage is often
called hazardous substance use.
The stage of dependence is clinically defined by at least three of the
following:
—a strong desire or sense of compulsion to take the substance;
— difficulties in controlling substance-taking behaviour in terms of its
onset, termination or levels of use;

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