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Pleasure and pain
Avoiding Pain

It all began in the 1950s, when two scientists, Olds and Milner, conducted a decisive experiment that revealed the heart of the reward circuit. Olds and Milner implanted electrodes in the nucleus accumbens of a rat’s brain. When the rat pressed a lever, it stimulated its own nucleus accumbens. Once the rat discovered the trick, it kept stimulating itself endlessly, without even stopping to eat.

People with dependencies do exactly the same thing, except that instead of stimulating themselves with an electrode, they do so by taking drugs or engaging in a particular activity.

Researcher Module: James Olds Experiment Module: Tonic Inhibition of Single Nucleus Accumbens Neurons in the Rat: A Predominant but Not Exclusive Firing Pattern Induced by Cocaine Self-Administration Sessions Experiment Module: Phasic Firing of Single Neurons in the Rat Nucleus Accumbens Correlated with the Timing of Intravenous Cocaine Self-Administration

Nowadays we know that dependencies have a very close relationship with pleasure. All psychoactive substances (including tobacco and alcohol) have an effect on neurons in the brain that form what is called the pleasure or reward circuit.

The central link in the reward circuit connects two small groups of neurons. One of these groups is located in the ventral tegmental area (VTA), and the other in the nucleus accumbens.

Normally, this system is activated by natural signals. As you can see by clicking on the red oval in the diagram, psychoactive substances short-circuit this system by activating the pleasure circuits directly.

Various psychoactive substances act at various points in this circuit, but they all generate a positive reinforcement that motivates the person to repeat the pleasant experience. This is the start of what can lead to a dependency.


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