Dopamine is a substance commonly found in humans and animals that is necessary for the proper functioning of the brain and central nervous system. Dopamine is a catecholamine neurotransmitter that activates five known types of dopamine receptors (D1 through D5). It is also a precursor for the neurotransmitters norepinephrine and epinephrine. The hypothalamus also releases dopamine as a neurohormone, meant to prevent the release of prolactin from the pituitary gland. Dopamine is synthesized at several different locations in brain, including the medulla of the adrenal glands, ventral tegmental area and substantia nigra.
Dopamine plays an important part in most aspects of human consciousness, including punishment and reward. Dopamine cannot cross the blood-brain barrier and, as a result, cannot directly affect the central nervous system when injected. The only way to increase dopamine levels in a medical setting is to introduce L-DOPA, a dopamine precursor that more readily crosses the blood-brain barrier.
Dopamine is released after taking drugs, which activates the reward system and reinforces the behavior. Cocaine is a dopamine transporter that inhibits reuptake of dopamine, thereby increasing dopamine levels by as much as one hundred and fifty percent. Amphetamines also increase dopamine levels by causing dopamine stores to be released. Antipsychotics, on the other hand, reduce dopamine activity, motivation and the ability to experience pleasure.
Dopamine is associated with feelings of euphoria, bliss, and appetite control. One study found that dopamine is also released at the end of a painful experience, such as being burned or injured.
This chemical was first synthesized in 1910, but lately has become the focus of much scientific study because of its association with addiction. Since 1950, over 110,000 studies of dopamine have appeared.
Certain addictive chemicals are dopamine agonists, including heroin, alcohol, nicotine, marijuana, and cocaine. This means they increase the levels of dopamine in the body. Cocaine and amphetamines prevent dopamine reuptake, which results in too much dopamine being left in the junctions or synapses between neurons. By overstimulating the dopamine receptors, these agonist drugs decrease the number of dopamine receptors, and the ones that are left become less sensitive to dopamine. This is called "desensitization," and when it occurs, the drug addict or alcoholic has to use more of his substance in order to achieve the same effects. Another way of putting this is by saying the person has built up a tolerance to the substance of abuse.
Some of the latest studies on dopamine indicate that drug addicts can damage the reward systems in their brains and create a state of permanent depression. Some of the newest studies are linking dopamine levels to obesity.