Drug addiction may be regarded as a disease of the brain reward system. This system, closely related to the system of emotional arousal, is located predominantly in the limbic structures of the brain. Its existence was proved by demonstration of the "pleasure centers," that were discovered as the location from which electrical self-stimulation is readily evoked. The main neurotransmitter involved in the reward is dopamine, but other monoamines and acetylcholine may also participate. The anatomical core of the reward system are dopaminergic neurons of the ventral tegmentum that project to the nucleus accumbens, amygdala, prefrontal cortex and other forebrain structures.