Substances Psychoactives Tabac, alcool et substances illicites

3. How does drug addiction affect the functioning of the brain?

• 3.1 What is drug addiction?
• 3.2 What brain mechanisms are affected?
• 3.3 How do different psychoactive drugs act in the brain?

3.1 What is drug addiction?

Drug addiction, also referred to as drug dependence, is a disorder of the brain brought on by the use of psychoactive drugs. These drugs affect some of the normal processes in the brain related to perception, emotion, and motivation, thus affecting behaviour and thoughts.

People are diagnosed as drug dependent if they have shown or experienced three or more of the following, at some time during the previous year:

1. A strong desire or sense of compulsion to take the drug.
2. Difficulty in controlling when they start or stop taking the drug and how much they take.
3. Withdrawal, which refers to unpleasant physical and psychological symptoms when the use of the drug is reduced or discontinued.
4. Evidence of tolerance, such as needing increased doses of the drug to achieve the effects originally produced by lower doses.
5. Progressive neglect of other pleasures or interests because of drug use, increased amount of time spent getting or using the drug or recovering from its use.
6. Persistence in using the drug despite clear evidence that it is causing harm

The above criteria for dependence include health and social consequences. Two of them, withdrawal and tolerance, are easily measured biologically. Others are becoming measurable using improved brain imaging techniques such as magnetic resonance imaging (MRI). More...

3.2 What brain mechanisms are affected?

Psychoactive drugs alter communication between brain cells. Individual brain cells (neurons) communicate with one another through a series of biological messengers called neurotransmitters. These neurotransmitters are released from the neuron sending the message and bind to receptors on the neuron receiving the message.

Some psychoactive substances are able to mimic the effects of neurotransmitters. Others interfere with normal brain function by blocking it, or by altering the way neurotransmitters are stored, released, and removed.

The human brain is organized into a number of different regions with highly specialized functions

• The hindbrain is vital to staying alive because it controls breathing and wakefulness.
• The midbrain is involved in learning and reinforcing behaviours — such as eating and drinking — that lead to pleasure and to life preservation. This area thus plays an important role in drug dependence.
• The forebrain is very complex and its outer layer (the cerebral cortex) controls the ability for abstract thought and planning.

When drug-dependent people are exposed to stimuli that provoke cravings, specific regions of their forebrain become activated.

Moreover, some other parts of the brain have been shown to work abnormally after drug use or dependence. More...

3.3 How do different psychoactive drugs act in the brain?

Different psychoactive drugs have different ways of acting in the brain to produce their effects. They bind to different receptor types, and can increase or decrease the activity of neurons through a range of mechanisms. Consequently, they have different effects on behaviour, different rates of development of tolerance, different withdrawal symptoms, and different short-term and long-term effects.

The most common psychoactive drugs can roughly be divided into four groups:

• Depressants, such as alcohol, sedatives/hypnotics and volatile solvents, that diminish the activity of the central nervous system. Overview of effects 
• Stimulants, such as nicotine, cocaine, amphetamines and ecstasy, that increase the activity of the central nervous system. Overview of effects 
• Opioids, such as morphine and heroin, that relieve pain, dull the senses, and induce sleep. Overview of effects 
• Hallucinogens such as PCP and LSD, that can distort perceptions to induce delusions or hallucinations. Overview of effects 

Cannabis, though classified in the present study as a hallucinogen, also displays characteristics of depressants and stimulants.

Despite their differences, psychoactive drugs do share similarities in the way they affect regions of the brain involved in motivation, which is relevant to the development of drug dependence. More...