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NEUROSCIENCE OF PSYCHOACTIVE SUBSTANCE USE AND DEPENDENCE

consciousness, mood or thinking processes. As later chapters will explain,
advances in neuroscience have given us a much better understanding of the
physical processes by which these substances act. Psychoactive substances
act in the brain on mechanisms that exist normally to regulate the functions
of mood, thoughts, and motivations. In this report, our emphasis will be on
alcohol and other hypnotics and sedatives, nicotine, opioids, cannabis,
cocaine, amphetamines and other stimulants, hallucinogens, and
psychoactive inhalants.
Use of these substances is defined into three categories according to their
sociolegal status. First, many of the substances are used as medications.
Western and other systems of medicine have long recognized the usefulness
of these substances as medications in relieving pain, promoting either sleep
or wakefulness, and relieving mood disorders. Currently, most psychoactive
medications are restricted to use under a doctor’s orders, through a
prescription system. In many countries, as much as one-third of all
prescriptions written are for such medications. An example of this is the use
of the stimulant methylphenidate to treat childhood attention deficit
hyperactivity disorder (ADHD), which will be discussed in Chapter 4. As
described in Chapter 6, some of the substances are also often used as “self-
medications” to relieve distress from mental or physical disorders, or to
alleviate the side-effects of other medications.
A second category of use is illegal, or illicit, use. Under three international
conventions (see Box 1.1), most nations have bound themselves to outlaw
trade in and non-medical use of opiates, cannabis, hallucinogens, cocaine
and many other stimulants, and many hypnotics and sedatives. In addition
to this list, countries or local jurisdictions often add their own prohibited
substances, e.g. alcoholic beverages and various inhalants.
Despite these prohibitions, illicit use of psychoactive substances is fairly
widespread in many societies, particularly among young adults, the usual
purpose being to enjoy or benefit from the psychoactive properties of the
substance. The fact that it is illegal may also add an attractive frisson, and
thus strengthen the identification of users with an alienated subculture.
The third category of use is legal, or licit, consumption, for whatever
purpose the consumer chooses. These purposes may be quite varied, and
are not necessarily connected with the psychoactive properties of the
substance. For instance, an alcoholic beverage can be a source of nutrition,
of heating or cooling the body, or of thirst-quenching; or it may serve a
symbolic purpose in a round of toasting or as a sacrament. However, whatever
the purpose of use, the psychoactive properties of the substance inevitably
accompany its use.
The most widely used psychoactive substances are the following: caffeine
and related stimulants, commonly used in the form of coffee, tea and many
soft drinks; nicotine, currently most often used by smoking tobacco cigarettes;
and alcoholic beverages, which come in many forms, including beer, wine

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