Drug Abuse and  Addiction

3.1   Genetic, Behavioral, and Environmental Influences on Drug Addiction

 Drug addiction is not simply continuous drug abuse.   Many more individuals will try an addictive drug than will become addicted. Most people know of situations in which two people use the same amount of alcohol or tobacco, but have very different responses to them. Environmental, social, behavioral, and genetic factors also contribute to the development of drug addiction. Stress can increase the susceptibility to addiction.

Scientists continue to investigate the factors that place one individual at greater risk of becoming addicted than another individual with a similar pattern of drug use. Individuals who have developed strong coping skills to deal with life’s pressures have less risk of becoming addicted to drugs. The younger a person is when he or she begins using drugs, the more likely he or she is to become addicted. This may be true because younger individuals have not developed the coping skills necessary to deal with life’s ups and downs. Furthermore, the earlier drug use begins, the less likely treatment is to be effective. In addition, genetic factors probably influence who engages in higher-risk behaviors.

The context in which a person uses an addictive drug is important. For example, some cancer patients take relatively large doses of morphine for extended periods to control pain without becoming addicted. In one study of 12,000 patients who were given opioids (primarily morphine) for acute pain, only four individuals became addicted to the drugs. In another study of 38 chronic pain patients, most of whom received opioids for four to seven years, only two patients became addicted, and both had a history of drug abuse. It is thought that addiction is rare in these pain patients because, unlike the stereo-typical street addict, they are not taking the drugs to get “high” and to escape life, rather they take the drugs so they can get on with life. The drugs ease their pain and improve their quality of life.

In the 1970s, news media reported the use of marijuana and heroin by soldiers who were serving in Vietnam. Combat stress, the easy availability of drugs, and the relaxation of taboos against drug use at the time all contributed to the problem. While many soldiers did have drug problems while in Vietnam, 95 percent who were addicted to narcotics have had no addiction problems since they returned to the United States.

Scientists continue to learn more about how genetic factors influence drug abuse and addiction. Heredity influences whether an individual has positive or negative sensations after smoking marijuana. One study demonstrated that identical male twins were more likely than non-identical male twins to report similar responses to marijuana use, indicating a genetic basis for their sensations.

Animals as Research Models

Why do scientists study the brains of non-human animals? Scientists use animals in research studies because the use of humans is either impossible or unethical. For example, when scientists investigate the effects of drugs of abuse on brain function, either the question they are asking cannot be answered in a living human or it would be inappropriate to give drugs to them.

The use of animals as subjects in scientific research has contributed to many important advances in scientific and medical knowledge. Scientists must analyze the goals of their experiments in order to select an animal species that is appropriate. Scientists often use fruit flies (Drosophila melanogaster) when they want to learn more about genetics. However, fruit flies are not a very good model if a scientist is investigating muscle physiology; a mouse may be a better model for those experiments. Although scientists strive to develop non-animal models for research, these models often do not duplicate the complex animal or human body. Continued progress toward a more complete understanding of human and animal health depends on the use of living animals.

Definition of Addiction

If and how quickly a person becomes addicted to a drug depends on many factors, including their genes and the biology of their body.  All drugs are potentially harmful and may have life-threatening consequences associated with their use. There are also vast differences among individuals in sensitivity to various drugs.  While one person may use a drug one or many times and suffer no ill effects, another person may be particularly vulnerable and overdose with first use. There is no way of knowing in advance how someone may react.

However, although  a person may have problems from even a single use of a drug, he or she is not yet said to be addicted.  Drug addiction is defined based on several cognitive, behavioral, and physiological symptoms that indicate that a person is continuing to use the drug even though they have clinically defined problems related to drug abuse.  For a person to be diagnosed as addicted, at least three of the following must be present: 

Symptoms of tolerance.

Symptoms of withdrawal.

The use of a substance in larger amounts or for longer periods than intended.

 Persistent  desire or unsuccessful attempts to reduce or control use.

The spending of considerable time in efforts to obtain the substance.

A reduction in important social,occupational,or recreational activities because of drug use. 

Continued use of a substance despite attendant health, social, or economic problems.


Long-Term Effects of Drugs On The Brain 

So, why are drugs so bad? After all, the “high” or “rush” only lasts a little while, right? What else could be happening in the drug abuser’s brain? Consider that the brain is continuously changing. After all, learning occurs because neurons are forming new synapses. Scientists say that the brain has plasticity. That doesn’t mean the brain is made of a chemical plastic like a credit card, but it refers to the brain’s ability to modify connections in response to experience. When a person learns something or has new experiences, some new synapses may form or existing synapses may get stronger. Other synapses may disappear.

When a person takes drugs repeatedly, the brain changes in response to this experience. If a person takes drugs and then stops, he or she will “crave” the drug. In other words, the individual will have a strong desire to take more of the drug. Scientists can actually see evidence of cravings in the brain.

If a cocaine addict sees pictures of drug paraphernalia, PET scans show   that a part of the brain that is important for memory (called the amygdala) is activated. If the addict sees a video with mountains, trees, and animals, the amygdala is not stimulated. Thus, just seeing pictures of drugs or things associated with drugs can trigger an uncontrollable urge for drugs.

After taking drugs for a period of time, a person may need to take a higher dose of the drug to have the same rush that he or she did when first taking the drug. This is called tolerance. The brain has adapted to having a certain amount of drug present and does not respond the same way it did initially. That is why drug abusers and addicts take increasingly higher amounts of an abused drug. Tolerance may develop because the body may become more efficient at eliminating the chemical from the body, or because the cells of the body and brain become less responsive to the effect of the drug.

Scientific studies have shown clearly that certain drugs can cause dramatic changes in the brain, but not all questions have been answered. Drugs can change the structure of the brain. Perhaps one of the most dramatic long-term effects of a drug is to kill neurons. Many people have heard that drinking alcohol will kill brain cells. It’s true. If alcohol is abused over a period of time, neurons in the brain can die. Some neurons in the brain are more sensitive to alcohol than others. Neurons that make up the mammillary bodies, areas in the brain that are important for memory, are more vulnerable to the effects of alcohol than are some other neurons in the brain. The neurons in the cerebral cortex, the part of the brain that controls most of our mental functions and endows us with consciousness, may also die if a person frequently abuses alcohol in high doses.

Another drug that is toxic to neurons is an amphetamine derivative called MDMA, or ecstasy. In rats and non-human primates, MDMA appears to kill neurons that produce serotonin, a neurotransmitter that is involved in regulating appetite, sleep, emotions, and so on. In some parts of the brain, the axons of some of these neurons may regenerate (or re-grow) after drug use is stopped, but the new growth of the neurons is not normal. Some areas are not reinnervated (nerve fibers do not grow back into the area) as they were before the drug abuse and some areas have abnormally high regrowth of the neurons. Either way, the neurons are not normal. Studies have not yet been able to determine if MDMA has this same effect on humans, but some preliminary evidence indicates that MDMA may kill serotonin neurons in humans.

 Cocaine also changes the brain in ways that may last for a long period of time. PET scans of human brains have shown that glucose metabolism is reduced even three months after the last use of cocaine. Remember that glucose metabolism is an indicator of how active the brain cells are.   If the neurons are using less glucose, they are not as active. The changes that cocaine causes in the brain last much longer than the pleasurable feelings it produces. Other drugs cause similar decreases in brain activity. Even two years after the last use of amphetamines, PET images show that the drug abuser’s brain is less active than the person’s who never used drugs.

Scientists, for many reasons, don’t know all of the effects that a drug may have. First, the brain is such a complicated organ that, despite great scientific advances, understanding all that it does will take many more years. Second, individuals may respond differently to drugs due to genetic differences among people. Third, many drug abusers abuse more than one drug. Many individuals who take cocaine, for example, also drink alcohol. The combination of the drugs makes it difficult to determine what the effect of one drug alone may be. Another complication is that drug addicts may have other health problems in addition to their drug problem. Heroin addicts, for example, spend most of their energy and activity trying to get their next “fix.” Consequently, they do not eat well and may have impaired immune systems. Also, drug addicts often suffer from mental illnesses, such as depression. The changes that occur in the brain because of mental illness make it difficult to determine what changes the drugs have caused.

The brain is an incredibly complex organ. This complexity will keep scientists working for many years to understand how the brain works. Someday, scientists will answer questions about what happens in the brain to cause addiction, which will then help scientists understand how to prevent addiction.

References: (1) The Brain:  Understanding Neurobiology Through The Study of Addiction Lesson 4~Drug Abuse and Addiction  (NIDA 2004)

Compiled and Edited By: D. Shrira Updated:  9 Jan 2007