http://www.psychologytoday.com/blog/...opamine-primer

excerpt:
How do we get dopamine? We eat it. The precursor amino acid from the protein we eat is called tyrosine. Tyrosine becomes dopa via the enzyme tyrosine hydroxylase, and dopa becomes dopamine via the actions of dopa decarboxylase. (One more chemical reaction can turn dopamine into its best buddy neurotransmitter, norepinephrine, but more on that later). As is the case with serotonin and its precursor tryptophan, tyrosine can cross the blood brain barrier, but dopamine itself cannot. That means that the dopamine our brain needs must be manufactured from dopamine machinery and precursors in the brain.

Still with me? What happens without dopamine, or with screwy dopamine machinery or inefficient dopamine? Well, in development this lack can cause mental retardation, which is the case in a rare genetic disease called PKU and cretinism (a type of mental retardation caused by iodine deficiency). Dopamine problems are implicated in ADHD, Alzheimer's, Parkinson's, depression, bipolar disorders, binge eating, addiction, gambling, and schizophrenia.

Having too much dopamine in the wrong place can make you psychotic. Illicit drugs that dump loads of dopamine (or strongly inhibit its reuptake, which is similar to dumping loads of dopamine) include cocaine and methamphetamines. Therefore high amounts of dopamine can cause euphoria, aggression and intense sexual feelings.

We need dopamine in the right place at the right time in the right amounts. When it all comes together, we are the awesomest ape around. Dopamine will motivate us, and it is the driving neurotransmitter in competitive behaviors. When our dopamine machinery isn't working properly, problems ensue (not surprisingly). Dopamine is linked to everything interesting about metabolism, evolution, and the brain.

As I mentioned in the first paragraph, dopamine is distributed quite differently on the different sides of the human brain, and is is speculated that this lateralization is responsible for how very human we are. The left brain (in almost all right-handed and most left-handed people) is responsible for language, linear reasoning, mathematics, that sort of thing, whereas the right side is usually responsible for intuition, holistic reasoning, some elements of music and speech intonation, etc.

The human brain has 100 billion neurons. Only 20,000 or so carry dopamine, and they do so in four major tracts (and some other minor ones we won't go into here). Keep in mind that the brain is a gorgeous and mysterious place with many wonderful names, like Rivendell or Brigadoon. The areas are named either anatomically (the dorsolateral prefrontal cortex) or because it reminded the anatomist of yore of some other important anatomical structure (the mammilary bodies). Or for some other obscure reason (the tegmentum? Oh, that's Latin for "covering." Okay!)

Dopamine is made in two little areas in the deep animal recesses of the brain - the substantia nigra and the ventral tegmental area. From these starting gates, the dopamine tracts reach out to various segments.