Chapter 2 How Stimulants Affect the Brain and Behavior
Over the last several decades, research on substances of abuse has vastly improved our understanding of human behavior and physiology and the nature of substance abuse and dependence. Basic neurobiological research has enhanced our understanding of the biological and genetic causes of addiction.
These discoveries have helped establish addiction as a biological brain disease that is chronic and relapsing in nature (Leshner, 1997). By mapping the neural pathways of pleasure and pain through the human brain, investigators are beginning to understand how abused psychoactive substances, including stimulants, interact with various cells and chemicals in the brain.
This new information has also improved our understanding of appropriate treatment approaches for different substance use disorders. This chapter describes the effects that cocaine and methamphetamine (MA) use have on the user's brain and behavior, which in turn leads to the stimulant users' unique needs.
Knowledge of these effects provides the foundation for stimulant-specific treatment approaches. This knowledge will give treatment providers greater insight into stimulant users and why certain treatment approaches are more effective.
Stimulant Abuse And the Brain
According to National Institute on Drug Abuse Director Alan I. Leshner, Ph.D., the fundamental problem in dealing with any substance of abuse is to understand "the target" (i.e., the user). Therefore, to understand why people take drugs such as cocaine and MA and why some people become addicted, we must first understand what these drugs are doing to their target; that is, how stimulants affect the user.
Discussions of substance abuse and dependence often involve some discussion of the root causes--the societal and risk factors that lead to these conditions. To date, investigators have identified as many as 72 risk factors for substance use and dependence (Leshner, 1998). Among them are poverty, racism, social dysfunction, weak families, poor education, poor upbringing, and substance-abusing peer groups.
These risk factors--as well as other environmental and genetic factors--only influence an individual's initial decision to use substances of abuse. But after initial use, an individual continues to use a substance because she likes its effects: Use modifies mood, perception, and emotional state. All of these effects are modulated through the brain; in order to understand this phenomenon, it is important to understand some basic neuroscience.
For substances of abuse to exert their effects, they must first get to the brain. The four most common routes of administering psychoactive (mood-changing) substances are (1) oral consumption (i.e., swallowing), (2) intranasal consumption (i.e., snorting), (3) inhalation into the lungs (generally by smoking), and (4) intravenously via hypodermic syringe.
A swallowed substance goes to the stomach and on to the intestinal tract. Some substances easily pass through the digestive tract into the bloodstream. Other substances are broken down into their chemical components (i.e., metabolized) in the digestive system, thereby destroying the substance.
Substances that are inhaled into the lungs adhere to the lining of the nasal passages (the nasal mucosa) through which they enter directly into the bloodstream. Inhaled substances are usually first changed into a gaseous form by igniting (e.g., marijuana) or volatilizing by intense heat (e.g., crack cocaine, the ice form of MA). The lungs offer a large surface area through which the gaseous form may quickly pass directly into the bloodstream.
Injected substances obviously enter the bloodstream directly, although at a somewhat regulated rate. In these last three routes of administration, substances enter the bloodstream in their unmetabolized form.
Once a substance enters the bloodstream, it is transported throughout the body to various organs and organ systems, including the brain. Substances that enter the liver may be metabolized there. Substances that enter the kidney may be excreted. If a female substance user is pregnant, and the substance is able to cross the placenta, then the substance will enter the fetus' bloodstream. Nursing babies may ingest some substances from breast milk.
To enter the brain, a substance's molecules must first get through its chemical protection system, which consists mainly of the blood-brain barrier. Tight cell-wall junctions and a layer of cells around the blood vessels keep large or electrically charged molecules from entering the brain. However, small neutral molecules like those of cocaine and MA easily pass through the blood-brain barrier and enter the brain. Once inside the brain, substances of abuse begin to exert their psychoactive effects.
Fundamentals of the Nervous System
The human nervous system is an elaborately wired communication system, and the brain is the control center. The brain processes sensory information from throughout the body, guides muscle movement and locomotion, regulates a multitude of bodily functions, forms thoughts and feelings, modulates perception and moods, and essentially controls all behavior.
The brain is organized into lobes, which are responsible for specialized functions like cognitive and sensory processes and motor coordination. These lobes are made up of far more complex units called circuits, which involve direct connections among the billions of specialized cells that the various substances of abuse may affect.
The fundamental functional unit of the brain's circuits is a specialized cell called a neuron, which conveys information both electrically and chemically. The function of the neuron is to transmit information: It receives signals from other neurons, integrates and interprets these signals, and in turn, transmits signals on to other, adjacent neurons (Charness, 1990).
A typical neuron (see Figure 2-1) consists of a main cell body (which contains the nucleus and all of the cell's genetic information), a large number of offshoots called dendrites (typically 10,000 or more per neuron), and one long fiber known as the axon. At the end of the axon are additional offshoots that form the connections with other neurons.
Within neurons, the signals are carried in the form of electrical impulses. But when signals are sent from one neuron to another, they must cross the gap at the point of connection between the two communicating neurons. This gap is called a synapse. At the synapse, the electrical signal within the neuron is converted to a chemical signal and sent across the synapse to the target (i.e., receiving) neuron.
The chemical signal is conveyed via messenger molecules called neurotransmitters that attach to special structures called receptors on the outer surface of the target neuron (Charness, 1990). The attachment of the neurotransmitters to the receptors consequently triggers an electrical signal within the target neuron. Approximately 50 to 100 different neurotransmitters have been identified in the human body (Snyder, 1986). Figure 2-2 illustrates a typical synaptic connection and depicts the chemical communication mechanism.
Neurotransmitters may have different effects depending on what receptor they activate. Some increase a receiving neuron's responsiveness to an incoming signal--an excitatory effect--whereas others may diminish the responsiveness--an inhibitory effect. The responsiveness of individual neurons affects the functioning of the brain's circuits, as well as how the brain functions as a whole (how it integrates, interprets, and responds to information), which in turn affects the function of the body and the behavior of the individual. The accurate functioning of all neurotransmitter systems is essential for normal brain activities (National Institute on Alcohol Abuse and Alcoholism [NIAAA], 1994; Hiller-Sturmhfel, 1995).
The Limbic Reward System
The brain circuit that is considered essential to the neurological reinforcement system is called the limbic reward system (also called the dopamine reward system or the brain reward system). This neural circuit spans between the ventral tegmental area (VTA) and the nucleus accumbens(see Figure 2-3).
Every substance of abuse--alcohol, cocaine, MA, heroin, marijuana, nicotine--has some effect on the limbic reward system. Substances of abuse also affect the nucleus accumbens by increasing the release of the neurotransmitter dopamine, which helps to regulate the feelings of pleasure (euphoria and satisfaction). Dopamine also plays an important role in the control of movement, cognition, motivation, and reward (Wise, 1982; Robbins et al., 1989; Di Chiara, 1995).
High levels of free dopamine in the brain generally enhance mood and increase body movement (i.e., motor activity), but too much dopamine may produce nervousness, irritability, aggressiveness, and paranoia that approximates schizophrenia, as well as the hallucinations and bizarre thoughts of schizophrenia. Too little dopamine in certain areas of the brain results in the tremors and paralysis of Parkinson's disease.
Natural activities such as eating, drinking, and sex activate the nucleus accumbens, inducing considerable communication among this structure's neurons. This internal communication leads to the release of dopamine. The released dopamine produces immediate, but ephemeral, feelings of pleasure and elation. As dopamine levels subside, so do the feelings of pleasure. But if the activity is repeated, then dopamine is again released, and more feelings of pleasure and euphoria are produced. The release of dopamine and the resulting pleasurable feelings positively reinforce such activities in both humans and animals and motivate the repetition of these activities.
Dopamine is believed to play an important role in the reinforcement of and motivation for repetitive actions (Di Chiara, 1997; Wise, 1982), and there is an increasing amount of scientific evidence suggesting that the limbic reward system and levels of free dopamine provide the common link in the abuse and addiction of all substances. Dopamine has even been labeled "the master molecule of addiction" (Nash, 1997).
When the nucleus accumbens is functioning normally, communication among its neurons occurs in a consistent and predictable manner. First, an electrical signal within a stimulated neuron reaches its point of connection (i.e., the synapse) with the target neuron. The electrical signal in the presynaptic neuron triggers the release of dopamine into the synapse. The dopamine travels across the synaptic gap until it reaches the target neuron.
It then binds to the postsynaptic neuron's dopamine-specific receptors, which in turn has an excitatory effect that generates an internal electrical signal within this neuron. However, not all of the released dopamine binds to the target neuron's receptors. Extra dopamine may be chemically deactivated, or it may be quickly reabsorbed by the releasing neuron through a system called the dopamine reuptake transporter(see Figure 2-4).
As soon as the extra dopamine has been deactivated or reabsorbed, the two cells are "reset," with the releasing neuron prepared to send another chemical signal and the target neuron prepared to receive it. Substances of abuse, and especially stimulants, affect the normal functioning of the dopamine neurotransmitter system (Snyder, 1986; Cooper et al., 1991).
Neurological Reinforcement Systems
Psychologists have long recognized the importance of positive and negative reinforcement for learning and sustaining particular behaviors (Koob and LeMoal, 1997). Beginning in the late 1950s, scientists observed in animals that electrically stimulating certain areas of the brain led to changes in mental alertness and behavior. Rats and other laboratory animals could be taught to self-stimulate pleasure circuits in the brain until exhaustion.
If stimulants such as cocaine or amphetamine were administered, for example, sensitivity to pleasurable responses was so enhanced that the animals would choose electrical stimulation of the pleasure centers in their brains over eating or other normally rewarding activities.
The process just described in which a pleasure-inducing action becomes repetitive is called positive reinforcement. Conversely, abrupt discontinuation of the psychoactive substances following chronic use was found to result in discomfort and behaviors consistent with craving.
The motivation to use a substance in order to avoid discomfort is called negative reinforcement. Positive reinforcement is believed to be controlled by various neurotransmitter systems, whereas negative reinforcement is believed to be the result of adaptations produced by chronic use within the same neurotransmitter systems.
Experimental evidence from both animal and human studies supports the theory that stimulants and other commonly abused substances imitate, facilitate, or block the neurotransmitters involved in brain reinforcement systems (NIAAA, 1994). In fact, researchers have posited a common neural basis for the powerful rewarding effects of abused substances (for a review, see Restak, 1988).
Natural reinforcers such as food, drink, and sex also activate reinforcement pathways in the brain, and it has been suggested that stimulants and other drugs act as chemical surrogates of the natural reinforcers. A key danger in this relationship, however, is that the pleasure produced by substances of abuse can be more powerfully rewarding than that produced by natural reinforcers (NIAAA, 1996).
Stimulants' Mechanisms of Action
On a short-term basis, stimulants exert their effects by disrupting or modifying the normal communication that occurs among brain neurons and brain circuits. Cocaine and MA have both been shown to specifically disrupt the dopamine neurotransmitter system. This disruption is accomplished by overstimulating the receptors on the postsynaptic neuron, either by increasing the amount of dopamine in the synapse through excessive presynaptic release or by inhibiting dopamine's pattern of reuptake or chemical breakdown (Cooper et al., 1991).
The use of cocaine and MA increases the amount of available dopamine in the brain, which leads to mood elevation (e.g., feelings of elation or euphoria) and increased motor activity. With cocaine, the effects are short-lived; with MA the duration of effect is much longer. As the stimulant level in the brain decreases, the dopamine levels subside to normal, and the pleasurable feelings dwindle away.
A growing body of scientific research based on animal research and brain imaging studies in humans suggests that the chronic use of stimulants affect dopaminergic neurons in limbic reward system structures (e.g., the VTA, nucleus accumbens). These effects may underlie addiction to stimulants. Although the neurochemical pathways of stimulant addiction are not definitively established, a few researchers have found evidence of changes in the structure and function of brain neurons after chronic stimulant use in humans.
Some researchers propose that the changes may come from dopamine depletion, changes in neurotransmitter receptors or other structures, or changes in other brain messenger pathways that could cause the changes in mood, behavior, and cognitive function associated with chronic stimulant abuse (Self and Nestler, 1995).
Animal studies have demonstrated that high doses of stimulants can have permanent neurotoxic effects by damaging neuron cell-endings (e.g., Selden, 1991). The question of whether stimulants can produce similar effects in humans remains to be answered. Researchers hope that recently developed brain imaging techniques will help provide the answer. At this time, there is only speculation that such permanent damage may underlie the long-term cognitive impairments seen in some chronic stimulant users. The continuing development and application of new technologies will help expand our knowledge of the neurological effects of stimulants in humans. (The medical aspects of stimulant use disorders are discussed in Chapter 5.)
Abuse and Dependence
Addiction is a complex phenomenon with important psychological and social causes and consequences. However, at its core, it involves a biological process: the effects of repeated exposure to a biological agent (a substance) on a biological substrate (the brain) over time (Nestler and Aghajanian, 1997). Ultimately, adaptations that substance exposure elicits in individual neurons alter the functioning of those neurons, which in turn alters the functioning of the neural circuits in which those neurons operate. This eventually leads to the complex behaviors (e.g., dependence, tolerance, sensitization, craving) that characterize addiction (Koob, 1992; Kreek, 1996; Wise, 1996; Koob and LeMoal, 1997).
A general definition of substance abuse is the habitual use of a substance not needed for therapeutic purposes, such as solely to alter one's mood, affect, or state of consciousness. The continued abuse of the substance may lead to adverse physiological, behavioral, and social consequences. A substance-dependent individual will continue his use despite these adverse consequences. Moderate chronic use or severe short-term use of substances may lead to abuse, which may eventually lead to addiction components (Ellinwood, 1974; Hall et al., 1988; Kramer, 1969).
Chronic substance abuse results in a complex set of physiological and neurological adaptations. These adaptations are simply the body's attempt to adjust to or compensate for substance-induced impairments. Repeated exposure to a substance can also lead to adaptations in the reward circuitry that opposes and/or neutralizes a substance's effects (i.e., counteradaptation). Substance addiction (or substance dependence) is manifested by (1) psychological craving (see the following section); (2) tolerance (the need for increasing amounts of the substance to reproduce the initial level of response, or sometimes to simply stave off the unpleasant effects of withdrawal); (3) sensitization (discussed in the section on the medical effects of stimulants); and (4) symptoms of withdrawal upon cessation of use, indicating physiological dependence.
Social and behavioral manifestations of dependence include the reduced ability to function at work or home and may include displays of erratic, moody, or anxious behavior.
Similar to other substances of abuse, moderate chronic use or severe short-term use of stimulants in any form may lead to abuse or dependence (Ellinwood, 1974; Hall et al., 1988; Kramer, 1969). Clinical observations of abuse patterns for both cocaine and MA have noted that, in general, there is an estimated 2- to 5-year latency period between first use and full-blown addiction. However, clinical experience and anecdotal evidence suggest that the latency period may be shortened to less than 1 year by rapid routes of administration (e.g., injection, smoking) and increased stimulant purity (e.g., ice, crack). With increasing use, the user may develop tolerance to the effects of stimulants and may need to keep increasing the amount taken to produce the desired psychological effects. As chronic abuse progresses, users prefer the stimulant over enjoyable activities and eventually may prefer it over food and sex (Hall et al., 1988).
At that point, the individual will usually continue her use even when faced with continuing adverse consequences--the hallmark of substance dependence. Abrupt discontinuation of the psychoactive substance following chronic use generally results in discomfort, dysphoria, and behaviors consistent with craving. The user is now motivated to use a substance in order to avoid discomfort and dysphoria. This shift from substance use as positive reinforcement to negative reinforcement is, perhaps, one of the foremost characteristics of late-stage addiction.
Drug Craving and Memory
The degree to which learning and memory sustain the addictive process has only recently been addressed. Researchers believe that each time a neurotransmitter like dopamine floods across a synapse, circuits that trigger thoughts and memories and that motivate action become hardwired in the brain. The neurochemistry supporting addiction is so powerful that people, objects, and places associated with substance use are also imprinted on the brain.
Craving, a central aspect of addiction, is a very strong learned response with powerful motivational properties often associated with specific memories (i.e., conditioned cues and triggers). Cues--any stimuli (substance-using friends, locations, paraphernalia, moods) repeatedly paired with substance use over the course of a client's addiction--can become so strongly associated with the substance's effects that the associated (conditioned) stimuli can later trigger arousal and an intense desire for the substance and lead to relapse. High relapse rates are common in cocaine addiction even after physical withdrawal and abstinence have been achieved.
Brain-imaging studies have shown that cue-induced drug craving may be linked to distinct brain systems involved in memory (e.g., London et al., 1990; Stapleton et al., 1995). Brain structures involved in memory and learning, including the dorsolateral prefrontal cortex, amygdala, and cerebellum, have been linked to cue-induced craving (Grant et al., 1995). A network of these brain regions integrates emotional and cognitive aspects of memory and triggers craving when it reacts to cues and memories. These cues and memories also play an important role in reinforcing substance use (Grant et al., 1995).
Most substance treatment programs recognize the power of these factors in triggering relapse and warn clients to avoid everything previously associated with their substance use--a tall order for a client in an urban environment saturated with the substance and its associated reminders. Treatment approaches that address these learning and memory issues of addiction may prove effective.
For example, Childress developed treatment strategies to help clients reduce craving and arousal during encounters with substance-related stimuli (Childress, 1994). In the laboratory, clients are given repeated, passive exposure to substance-reminding cues in a substance-free protected environment. The research finds that initial arousal and strong craving produced by the cues eventually decrease (Childress, 1994). Better understanding of the relationship of learning and memory to the addiction process may lead to new treatment approaches.
Role of New Technologies
The recent development of noninvasive brain imaging has created a powerful new tool for demonstrating not only the short-term effects of substance use, but also the longer term consequences of chronic substance abuse and addiction. These tools have allowed researchers to boldly go where they previously could not--literally into the depths of a living human brain. Such noninvasive techniques can depict normal and abnormal functioning of different brain areas by measuring metabolic activity (i.e., glucose utilization). They can identify substance-induced structural changes and physiological adaptations. Through a combination of techniques, they can observe the altered "processing" of information in various circuits as the brain responds to substance use.
Using these techniques, investigators have been able to identify brain structures involved in craving, map the emotions of substance users, plot the neurobiological basis of substance-induced euphoria, and more. For example, researchers have used magnetic resonance imaging (MRI) and spectroscopy to see how brain structures change as substances produce their effects. Others have used a functional imaging technique called phosphorus magnetic resonance spectroscopy (31P MRS) to show that chronic substance abuse is accompanied by abnormal metabolism in some areas of the brain that seems to return to normal when people stop using substances (Christensen et al., 1996).
Positron emission tomography (PET) has revealed subtle alterations in the dopamine receptors of stimulant users' brains (Iyo et al., 1993). More recent PET studies have demonstrated long-term vulnerability to chronic stimulant abuse (Melega et al., 1997a; Volkow et al., 1996, 1997b). Another PET study has established a dose-response relationship between cocaine and the drug's subjective effects: The greater the amount of cocaine that is administered, the greater the high experienced by the user (Volkow et al., 1997a).
Other researchers combined electroencephalograms (EEGs) and MRI to produce a topographic brain map showing increased electrical activity (in the form of beta waves) during stimulant withdrawal (Herning et al., 1997). Mapping EEG activity during stimulant use and withdrawal may allow researchers to further document substance-induced neuropsychological impairments.
Although much is known about the effects of stimulants in animals, there is little such knowledge of these effects in humans (CSAT, 1997). The continuing development and application of new technologies such as noninvasive brain imaging will allow researchers to improve their understanding of how stimulants affect the human brain. Greater understanding of the underlying neuronal impairments of stimulant abuse will aid in the development of new, more effective treatment approaches.
General Effects Of Stimulants
Substances of abuse--and stimulants in particular--appear to increase the brain's levels of free dopamine in a dose-dependent manner; that is, more dopamine is available when higher doses of the substance are administered (Nash, 1997). The higher the substance dose, the greater the feelings of elation, euphoria, and satisfaction, and as the dopamine levels and pleasurable feeling subside, there is an intense desire to replicate the feelings of pleasure by administering another dose of the substance. This tendency toward repeated administration is characteristic of stimulant abuse and underlies most of the other effects of stimulants, as well as most other addictive substances.
Continued use often leads to adverse consequences, which may include neuropsychological impairment and diminished physical health. Work performance and social and family relations can be adversely affected, and the risk of arrest and conviction on drug-related charges increases. Even after a stimulant user discontinues use, impairments in cognition and functioning may persist, and there may even be persistent psychiatric symptoms (Wada and Fukui, 1990). Cravings for the stimulant's effects tend to linger, even after abstinence has been achieved, and the potential for relapse is high.
The general acute effects of stimulants have been well documented. Among a range of physiological responses, stimulants are known to raise both systolic and diastolic blood pressure, increase heart rate, increase respiration rate, increase body temperature, cause pupillary dilation, heighten alertness, and increase motor activity (CSAT, 1997).
Acute effects from excessive doses include dangerously rapid and erratic heartbeat, cerebral hemorrhaging, seizures/convulsions, respiratory failure, stroke, heart failure, brain damage, coma, and death (CSAT, 1997).
Stimulants are also known to cause sensitization (i.e., the opposite of tolerance), for which multiple drug exposures eventually produce some new adverse reaction. For example, in animals, seizures do not typically occur after single low-to-moderate doses. But with repeated exposure, an animal can become sensitized to the stimulant and may have a seizure after a single, previously harmless, dose.
Although the effects of chronic stimulant abuse in humans has not been well documented, some of the chronic effects include organ toxicity, compromised health (e.g., underfed, malnourished, poor hygiene), dental problems, and dermatitis. (For a complete discussion of the medical aspects of stimulant use, see Chapter 5.)
The immediate psychological effects of stimulant administration include a heightened sense of well-being, euphoria, excitement, heightened alertness, and increases in motor activity. Stimulants also reduce food intak