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Pleasure and pain
Avoiding Pain

Help Linked Module: Addiction's Path Linked Module: Brain Reward Decreases and Danger of Relapse Increases with Heavy Cocaine Use
Experiment Module: New Insights on the Neural Basis of Brain Reward and Alcohol Drinking

Another effect of the plasticity of the human brain is to facilitate the creation of dependencies. When chronic consumers of a particular drug suddenly stop taking it, their neurons have to cope with a new biochemical environment to which they are no longer adapted. As a result, they experience discomfort and cravings. To overcome their dependency, these people must go through a period of withdrawal until their neurons have relearned how to function without the drug.


All psychoactive drugs seem to increase the amount of dopamine in the reward circuit, either directly or indirectly.


In addition to exerting their effects through molecular mechanisms, these drugs intervene at various points in the brain’s central reward circuit (see diagram).

These drugs also affect several other areas of the brain. It is through this combination of effects on the reward circuit and on other brain structures that a drug induces the behaviours and subjective sensations that are specific to it.


Because neural communication is so plastic, a new equilibrium will gradually be established that incorporates this input of a psychoactive substance from outside the body. It is this adaptability of the brain that is the source of phenomena such as tolerance, dependency, and withdrawal (see boxes).

But while all drugs do promote a certain dependency by increasing dopamine levels in the nucleus accumbens, dopamine is not the only molecule involved in this process.

Tolerance is one of the compensating mechanisms that gradually reduce the effects of a drug. The brain’s adaptability thus has a significant downside: it drives drug addicts to increase their dosage to try to attain the intoxicating effects that they felt when they took their first “fix”.

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