Dopamine (chemical formula C6H3(OH)2-CH2-CH2-NH2) is one of the best-known neural chemicals, and though it's often thought of primarily as a manufactured substance it's actually produced naturally in the body. Dopamine is a neurotransmitter activating dopamine receptors. The hypothalamus also releases it as a neurohormone, with its main function the inhibition of prolactin release from the anterior pituitary lobe.
Increased dopamine can improve the symptoms of people with Parkinson's disease and other related disorders; however, dopamine itself cannot cross the blood-brain barrier, so injecting or ingesting it does not get it to the brain. Instead, a synthetic L-DOPA, which is a precursor to dopamine that does cross the blood-brain barrier, can be used.
Dopamine is a member of the catecholamine family, and is a precursor to epinephrine (or adrenaline) and norepinephrine (noradrenaline). Dopamine is synthesized by the decarboxylation of DOPA by aromatic-L-amino-acid cecarboxylase.
Dopamine in the Brain
Dopamine has a wide variety of applications in the brain. It affects the way the brain controls our movement; a shortage of dopamine results in Parkinson's disease. Dopamine controls the flow of information to the frontal lobe from other parts of the brain. Disorders in dopamine levels cause declines in neurocognitive functions like memory, attention, and problem-solving.
Dopamine's role in pleasure and motivation is critical. It is heavily associated with the pleasure system in the brain, and its continued release provides feelings of enjoyment and reinforces the activities that provide those feelings. Food, sex, and other naturally-rewarding experiences release dopamine; in addition, neutral stimuli associated with pleasure (for instance, sexual fetishes) and certain drugs also release dopamine. Cocaine and amphetamines in particular seem directly related to dopamine release, and in theories of addiction have been given the reputation of pathologically altering dopamine pathways in addicted people.
Dopamine is not, however simply the "reward chemical" in the brain; this is far too simplistic an explanation. Dopamine is also released when negative stimuli are encountered, leading one to wonder just how close pleasure and pain truly are. It also works in previously-unpredicted ways toward pleasure; for instance, when a reward is greater than anticipated, the dopaminergic neurons associated fire more often, with a commensurately lower than anticipated reward, they fire less. For this reason, some researchers think it may be related to desire rather than pleasure. Drugs like antipsychotics that inhibit dopamine activity reduce people's desire for pleasure, but don't make that pleasure less intense.
Because of these new insights and studies, new theories suggest that dopamine is actually involved in predicting pleasurable activity, and thus can be critical in decision-making processes. When a dopamine path has been damaged by addiction, it would make this decision-making dysfunctional by overemphasizing the priority of the drug in relation to other variables.
Unfortunately, experimental evidence doesn't seem to agree with any of the observed behaviors in people mucking about with their own dopamine pathways. For instance, blocking dopamine receptors chemically increases, instead of decreases, drug-taking behavior. There is still, apparently, missing data.
Dopamine and Disorders
Dopamine deficits may be a cause of attention deficit disorder, which is why dopamine stimulants sometimes work in correcting this problem. And disruptions to dopamine systems are closely linked to psychosis and schizophrenia, particularly dopamine neurons in the mesolimbic pathway. Modern antipsychotics are designed to block dopamine function. Unfortunately, this blocking can also cause relapses in depression, and as noted above can increase addictive behaviors.
Dopamine can also be used with shock to increase cardiac output and blood pressure in patients who are in distress.
Web Resources On Dopamine
Dopamine and antipsychotic drug action revisited
Book Resources On Dopamine
The Creation of Psychopharmacology by D. Healy
Mesolimbic Dopamine System: From Motivation to Action by Willner & Scheel-Kruger