2.  The dopamine system and brain reward circuitry
The dopamine (DA) system in the CNS includes the nigrostriatal pathway, the mesolimbic pathway and the tuberoinfundibular pathway. Dopamine is mainly produced in the substantia nigra, projected along the nigrostriatal pathways and stored in the striatum. Five subtypes of DA receptors have been identified and cloned. All of them function both individually and interactively as G-protein coupled receptors.
There has been continuous research since the 1970s on the role DA plays in the brain reward system. The reward reinforcement circuitry is part of the limbic system that includes the ventral tegmental area (VTA), nucleus accumbens (NAc), ventral striatum, bed nucleus of the stria terminalis, hippocampus, amygdale, and other brain structures. DA is the main neurotransmitter of this system.[4]-[8] The reward system modulates primary physiological functions related to survival including the intake of food and water and sexual behavior. It is also the target of psychoactive substances including alcohol, cocaine, amphetamine and opioids. The mesolimbic DA pathway (the NAc is the central regulation structure for the reward effect) and the mesocortical pathway are the key structures that modulate the reward reinforcement circuitry.[4]-[8]