Professor Trevor Robbins explains that the reward system, where dopamine mediates messages to the nucleus accumbens, is artificially activated by certain drugs.

So the concept of the reward system is a part of the brain where natural rewards, like food and water, maybe sex, exert their effects to change behavior. These effects are thought to occur via the activity of the dopaminergic, the dopamine neurotransmitter system, sending its messages to the nucleus accumbens, this structure in the base of the forebrain. It is commonly thought that drugs of abuse hijack this system. They short-circuit the system by producing some of the rewarding effects that natural rewards normally produce, but without their other aspects. So, this probably is a bad thing because the other sensory qualities of the rewards are not there. But that in general is what we mean by reward system.

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