Scope of the Substance Abuse Problem

The statistics in Table 10-1 are from a 2007 survey in the United States by the Substance Abuse and Mental Health Services Administration and provide some information regarding the incidence of substance abuse.

TABLE 10-1 Survey by the U.S. Substance Abuse and Mental Health Services Administration (2007)

Many different terms are used to describe different components that are related to addiction, and it is important to highlight these terms and provide clear definitions.

Dependence: Physiologic condition whereby the absence of a drug results in withdrawal signs and symptoms. It is very closely related to the psychologic processes that occur with addiction, because the body and the mind are not completely separate entities (when you are physically unwell, you do not feel good), but strictly speaking, dependence refers to only the physical component of addiction.

Withdrawal: Physical and/or emotional reaction that occurs when a drug is not administered to an individual who is addicted. These experiences are dysphoric (unpleasant) and will be described in more detail.

Tolerance: Phenomenon whereby performing a behavior results in a smaller reward than previous, similar behaviors. As a result, the behavior is often adjusted upward to reproduce the same magnitude of reward that was previously experienced. Increasing the dose of a drug would be an example of an upwardly adjusted behavior, as would gambling with a larger amount of money.

Obsession: Recurring thought. For example, thinking nonstop about taking a drug would constitute an obsession.

Compulsion: Recurring behavior. For example, actually taking a drug over and over would be a compulsion.

Impulsiveness: Tendency toward unplanned behavior without regard for consequences.

Automaticity: Behaviors that occur without conscious thought.

Craving: Psychologic process similar to craving. It is also characterized by anticipation and strong desire.

Substance abuse: Pattern of inappropriate or illicit use of substances for physiologic or psychologic reward.

Positive reinforcement: When exposure to a stimulus results in a reward and increases the probability of repeating the behavior in future (e.g., getting paid for a job well done).

Negative reinforcement: When removal of a stimulus avoids or reduces bad feelings (e.g., taking your hand out of boiling water).

A motivational framework can be used to help describe the difference between positive and negative reinforcement. In persons who are driven more by impulse, there is a state of arousal and tension before the addictive behavior is performed, and the behavior results in pleasant feelings. This would be an example of positive reinforcement. In contrast, compulsive behavior is characterized by stress and anxiety before the addictive behavior is performed, and the behavior results in relief from those feeling. This would be an example of negative reinforcement.

Throughout the time course of addiction development, the initial behaviors are primarily driven by impulse, whereas the latter stages of addiction are driven by a combination of both impulse and compulsion, and there is a shift from positive reinforcement to negative reinforcement.

Neurophysiology of Addiction

Studies to determine the neural pathways involved in addiction have involved both human and animal populations and focus on attempts to precisely stimulate or inhibit different regions of the brain and measure outcomes related to these interventions. The pathways are complex and beyond the scope of this text; however, a simplified description of the more important components will be presented.

There are some important details related to neuroanatomy that are relevant to understanding the pathways of addiction; some of them include the following (Figure 10-1):

Mesolimbic system: Pathway in the brain that projects from the ventral tegmental area (VTA) to the nucleus accumbens, amygdala, limbic system, and other areas of the brain. It is strongly implicated in addiction and dopamine processing.

VTA: Tegment is Latin for covering (integument means skin). The VTA is located on the floor of the midbrain and is responsible for reward signaling, motivation, and some psychiatric disorders. It is strongly implicated in dopamine processing.

Nucleus accumbens: Pleasure center of the brain.

Limbic system: Area of the brain associated with emotions.

Amygdala: Latin for almond. The amygdala is an almond-shaped group of nuclei deep in the brain near the medial temporal lobes. It is primarily responsible for processing of emotion, especially fear and anxiety.

Figure 10-1 Regions of the brain involved in addiction.

Dopamine-Release Theory

All behaviors of addiction activate and increase dopamine signaling in the mesolimbic system. However, there is also evidence to suggest that dopamine-independent processing occurs in the nucleus accumbens.

Associated with the dopamine release theory is the concept of predicted and actual reward. This hypothesis suggests that dopamine release in the mesolimbic system represents a learning signal that reinforces constructive behaviors.

An example would be a mouse that learns to pull a lever to obtain food. The pulling of the lever leads to dopamine release, as the animal is predicting a reward, whereas the actual reward (food) does not elicit a response.

Drugs that release dopamine into this pathway generate an “inappropriate” learning signal, one that suggests that the behavior (e.g., taking of the drug) should be repeated.

The degree of importance given to dopamine in addiction remains controversial; however, one thing is clear—several addictive drugs target the dopamine pathway.

A number of classes of compounds are associated with addiction. Some of these, such as the opioids, sedative-hypnotics, cannabinoids, and central nervous system (CNS) stimulants are also covered elsewhere in this textbook. The following summaries focus on the aspects of these compounds that are related to addiction. There are two key common features to note as you progress through the following summaries:

Speed of onset is vital. A key distinguishing feature of an addictive substance is a rapid onset of action. This is why most of the substances listed in the following pages are delivered by routes that facilitate quick onset (i.e., intravenous, intranasal, inhalation). This also explains why heroin, which has a rapid onset of action, is considered to be one of the most addictive of opioids.

Withdrawal is the opposite of the reward. This is a key reason why substance addictions are so difficult to overcome. If a drug causes euphoria, withdrawal will cause dysphoria; if a drug is a depressant, then withdrawal will cause excitation (anxiety, seizures). Therefore when trying to maintain abstinence, not only must a patient cope with the loss of reward, they must also withstand symptoms that are the opposite of reward.

Opioids

All of the opioids have abuse potential, although their primary use is in analgesia. Heroin is primarily used as a substance of abuse; therefore it is the focus of the following discussion.

Actions

Euphoria

Sedation

Heroin is much more lipophilic than other opioids and therefore crosses into the brain much more readily, leading to a rapid and dramatic onset of action. It can be injected, smoked or introduced via the intranasal route.

Toxicity

The main toxicity of concern with use of any opioid is respiratory depression, as the patient can eventually stop breathing at high enough doses. Other signs of toxicity include obtundation and miosis (constricted pupils).

Acute overdose of heroin and other opiates can be treated with intravenous naloxone, an opioid antagonist. The effects of naloxone can be quite dramatic, rapidly reversing the effects of opioid toxicity. However, it must be noted that naloxone also has a relatively short duration of action, and if the elimination half-life of the opioid exceeds that of naloxone, the antagonist may wear off before the opioid has reached safe plasma levels, and respiratory depression can recur. Therefore overdose patients treated with naloxone should be monitored carefully.

Naloxone can precipitate severe withdrawal symptoms in patients who are physically dependent on opioids.

Important Notes

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