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3. BIOBEHAVIOURAL PROCESSES UNDERLYING DEPENDENCE

CHAPTER 3

Biobehavioural Processes Underlying
Dependence

Introduction
This chapter focuses on specific brain processes that are involved in the
rewarding effects of psychoactive substance use, reinforcement and the
development of dependence. Biological systems that have evolved to guide
and direct behaviour towards stimuli that are critical to survival are recruited
and abnormally strengthened by repeated use of psychoactive substances,
leading to the cycle of behaviours characteristic of dependence.
The chapter also describes the current hypotheses and evidence on the
biological basis of the behavioural and psychological factors that contribute
to substance dependence. Dependence is the result of a complex interaction
of the physiological effects of drugs on brain areas associated with motivation
and emotion, combined with “learning” about the relationship between drugs
and drug-related cues, all of which have a biological basis. These learning
processes are critically dependent upon the same motivational and emotional
systems in the brain that are acted upon by psychoactive substances (Hyman
& Malenka, 2001).
Although each class of psychoactive substances has its own unique
pharmacological mechanism of action (see Chapter 4), all psychoactive
substances activate the mesolimbic dopamine system (Fig. 3.1). The current
chapter focuses on mechanisms that are common to all psychoactive
substances and that are responsible for the cluster of symptoms
characteristic of substance dependence. The mesolimbic dopamine system,
in particular, will be highlighted because of its key role in learning and
motivational processes. In all cases, individual differences in biology and
environment will affect the neurobiological effects of psychoactive
substances; however, this chapter presents basic mechanisms that may
underlie the development of dependence from a biobehavioural
perspective.
The first section of this chapter provides an overview of learning theory
and terminology as it relates to dependence. The next section explains how
the unique properties of psychoactive substances can lead to dependence
through sensitization of the incentive value of drugs. The processes of
withdrawal and tolerance are also considered. Finally, individual differences
in responses to psychoactive substances are discussed.

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