The Brain: Understanding Neurobiology
Through The Study of Addiction
Drug Abuse And Addiction
Drug addiction is a complex brain disease. Preventing drug abuse and addiction and treating the disease effectively requires understanding the biological, genetic, social, psychological, and environmental factors that predispose individuals to drug addiction.
Individuals make choices to begin using drugs. Some people begin using drugs to relieve a medical condition and then continue to use the drugs after the medical need is over. Children who are depressed or who have a psychiatric disorder sometimes begin using illicit drugs to self-medicate. Other people begin taking drugs to feel pleasure, to escape the pressures of life, or to alter their view of reality. This voluntary initiation into the world of addictive drugs has strongly influenced society’s view of drug abuse, drug addiction, and its treatment.
When does drug abuse become drug addiction? No one becomes addicted with the first use of a drug. Drug abuse and drug addiction can be thought of as points along a continum. Any use of a mind-altering drug or the inappropriate use of medication (either prescription or over-the-counter drugs) is drug abuse, but the point when drug abuse becomes drug addiction is less clear. Different drug abusers may reach the point of addiction at different stages. Scientists continue to investigate the factors that cause the switch between the two points.
Currently, drug addiction is defined as the continued compulsive use of drugs in spite of adverse health or social consequences. Drug addicts have lost control of their drug use. Individuals who are addicted to drugs often become isolated from family or friends, have difficulty at work or school, and become involved with crime and the criminal justice system. For addicts, continuing their drug habit becomes their primary focus in life.
Certain drugs, including opiates and alcohol, cause strong physical reactions in the body when drug use stops. When a heroin addict stops taking heroin, he or she can experience a variety of symptoms ranging from watery eyes and a runny nose to irritability and loss of appetite and then diarrhea, shivering, sweating, abdominal cramps, increased sensitivity to pain, and sleep problems. In general, withdrawal from heroin makes the abuser feel miserable. Withdrawal from other drugs, such as cocaine and amphetamines, does not lead to strong physical reactions. For most drugs, physical withdrawal symptoms can usually be controlled effectively with medications. Even though withdrawal from some drugs does not cause the abuser to have physical reactions, stopping drug use is difficult because of the changes the drugs have caused in the brain.
Once the drugs stop, the abuser will have cravings, or intense desire for the drugs. Craving arises from the brain’s need to maintain a state of homeostasis that now includes the presence of the drug. A person may experience cravings at any stage of drug abuse or addiction, even early in the experimentation phase of drug abuse. Cravings have a physical basis in the brain. Using PET imaging, scientists have shown that just seeing images of drug paraphernalia can stimulate the amygdala (part of the brain that controls memory) in drug addicts.
Drugs of addiction do not merely cause short-term changes in an individual’s cognitive skill and behavior. A drug “high” lasts a short time, ranging from less than an hour to twelve hours depending on the drug and dose. The changes in the brain that result from continued drug use, however, can last a long time. Scientists believe that some of these changes disappear when drug use stops; some disappear within a short time after drug use stops, and other changes are potentially permanent.
One of the first changes in the brain that occurs in response to repeated drug abuse is tolerance. Tolerance develops when a person needs increasing doses of a drug to achieve the same “high” or “rush” that previously resulted from a lower dose of the drug. Two primary mechanisms underlie the development of tolerance.
1. First, the body may become more efficient at metabolizing the drug thereby reducing the amount that enters the bloodstream.
2. Second, the cells of the body and brain may become more resistant to the effect of the drug.
For example, after continued cocaine use, neurons decrease the number of dopamine receptors, which results in decreasing cocaine’s stimulatory effect. Opiates, on the other hand, do not cause a change in the number of receptors. Instead the opiate receptors become less efficient in activating the second messenger system thus reducing the effects of the opiates.
Drugs can cause other long-term changes in the anatomy and physiology of the brain’s neurons. Alcohol, methamphetamine, and MDMA (Ecstasy) can kill neurons. Unlike other types of cells in the body, neurons in many parts of the brain have little or no capability to regenerate. (Recent studies have shown that the adult human brain can generate new neurons in the hippocampus, a part of the brain important for learning and memory. Other parts of the brain do not show this ability.)
Alcohol kills neurons in the part of the brain that helps create new memories.
If those neurons die, the capability for learning decreases. Methamphetamine kills dopamine-containing neurons in animals and possibly in humans as well. MDMA kills neurons that produce another neurotransmitter called serotonin. In addition to neurotoxic effects, drugs can significantly alter the activity of the brain. PET scans of cocaine addicts show that the metabolism of glucose, the primary fuel for cells, is drastically reduced in the brain, and that this decrease in metabolism can last for many months following cessation of drug abuse.
In addition to the functional and anatomical changes in the brain, drug abuse puts addicts at higher risk for other health problems. For example, inhalant abuse can lead to disruption of heart rhythms and snorting cocaine can lead to ulcerations in the mucous membranes of the nose. In addition, drug addicts are at increased risk of contracting HIV or AIDS through shared needles. Similarly, hepatitis B and hepatitis C are much more common among drug addicts than the general population. Tuberculosis is another concern. Drug abuse and addiction also are contributing factors in motor vehicle accidents.
Figure 4.2. Photographs of serotonin axons in the cerebral cortex labeled with a fluorescent marker. The number of serotonin-labeled axons is dramatically reduced in the cerebral cortex at 2 weeks (B) and 18 months (C) after the last drug use. The brain of the control animal that did not receive MDMA (A) shows the dense network of labeled axons. Images E and F show changes caused by MDMA use on a different brain region, the hypothalamus. The control showing the hypothalamus in the absence of MDMA is shown in D. Photograps courtesy of G.A.Ricaurte,with the permission of the Journal Of Neuroscience.
References: (1) The Brain: Understanding Neurobiology Through The Study of Addiction: Lesson 4 ~ Drug Abuse and Addiction (NIDA 2004)
Compiled & Edited By: D. Shrira Updated: 28 Dec 2006