What is addiction?
Addiction is typically defined as compulsive, and uncontrolled use of a drug even in the face of negative consequences; however, not all healthcare professionals agree with this interpretation. Traditionally, addiction could only occur when a psychoactive substance was involved, but today, some people have expanded the definition to include other behaviors, such as gambling and sex. In addition, addiction usually involves both physical and psychological dependency.
Physical dependency will eventually occur with repeated use of opiates, and is defined as a state where withdrawal symptoms will appear upon abrupt cessation. After a person becomes physically dependent on opiates, discontinuation causes withdrawal symptoms which may include runny nose, gooseflesh, muscle pain, and diarrhea. Some drugs may cause physical dependency, but not addiction. For example, loperamide (an opiate) was originally categorized as a controlled substance because discontinuation of long-term, high doses induced morphine-like withdrawal symptoms; however, the compulsive and uncontrolled behavioral aspects which characterize addiction were not present.
Psychological dependency is noted when upon cessation psychological withdrawal symptoms, such as cravings, irritability, insomnia, depression, and anxiety, appear. “Rewarding” activities such as gambling, sex, and shopping can induce the same sort of psychological withdrawal symptoms as a drug, and are all thought to be due to the effect on the dopamine, a chemical in the brain thought to cause feelings of pleasure. Psychological dependency may also occur when a drug or activity is used to take the place of a typically undesirable activity, making it a habitual behavior. Others consider it to be an emotional, social, or psychological dysfunction, taking the place of “normal” positive stimuli not otherwise present.
How does habitual drug use produce changes in the brain that may lead to drug addiction?
- dopamine — a neurotransmitter present in brain regions that regulate movement, emotion, motivation, and the feeling of pleasure.
- locus ceruleus — a region of the brain that receives and processes sensory signals from all areas of the body; involved in arousal and vigilance.
- noradrenaline — a neurotransmitter produced in the brain and peripheral nervous system; involved in arousal and regulation of blood pressure, sleep, and mood; also called norepinephrine.
- nucleus accumbens — a structure in the forebrain that plays an important part in dopamine release and stimulant action; one of the brain’s key pleasure centers.
- ventral tegmental area — the group of dopamine-containing neurons that make up a key part of the brain reward system; key targets of these neurons include the nucleus accumbens and the prefrontal cortex.
The “Changed Set Point” Model
The “changed set point” model of opioid addiction is based on alterations in the neurobiology of dopamine neurons in the ventral tegmental area and of noradrenaline (also called norepinephrine) neurons of the locus ceruleus during early withdrawal and abstinence. The focal point in this model is that drug abuse changes a biological or physiological setting. One variation of this model is based on the idea that neurons of the mesolimbic reward pathways are naturally “set” to release enough dopamine in the nucleus accumbens to produce a normal level of pleasure. When opioids are introduced into this “stable environment” the natural set point is changed; thus, reducing the release of dopamine during typically pleasurable activities (eating, playing, etc.) when opioids are not present. A change also takes place in the locus ceruleus, but in the opposite direction, such that noradrenaline release is increased during withdrawal. This model accounts for the drug liking (increased dopamine levels, decreased noradrenaline levels) and drug withdrawal (decreased dopamine levels, increased noradrenaline levels) aspects of drug addiction.