Dopamine

As the chemical responsible for the pleasurable sensations felt by the human brain, dopamine

has been found to be active in many aspects of every day life. Any physiological action that

receives a positive feedback, such as a handshake, a kiss, or the use of a drug, can cause the level

of dopamine activity in the brain to increase.

Dopamine is a neurotransmitter that relays messages from one neuron to the next. Through

axons, neurons influence neurological activity in many regions, specifically the nucleus

accumbens. The nucleus accumbens is a primitive structure that is considered to be the center

for pleasure. When the molecule reaches the end of the axon, it is released into the synapse (the

region between the axon ending and the receiver cells) where the dopamine binds to the

receptors of the next cell. Then it is either reabsorbed, or catabolized by the enzyme monoamine

oxidase (MAO). At the chemical level, every experience that a person finds enjoyable amounts

to an excess of dopamine at the axon endings of the nucleus accumbens.

Maintaining the proper balance of dopamine in the brain is vital to survival. For example, if

levels are too low such as in the substantial nigra, then the person may experience the tremors

and movement disorder of Parkinson's disease. On the other hand, if the levels of dopamine are

too high, the person may experience hallucinations and the thought disorder characteristic of

schizophrenia.

In 1975, psychologists Roy Wise and Robert Yokel of Concordia University in Montreal

made a discovery while working with drug addicted rats. Initially, the rats were taught to push a

lever that would release a narcotic in the form of a pellet. After being injected with a

dopamine-blocking chemical, the rats would push the lever as many times as possible. It is

believed that this showed that the levels of dopamine in the brain are directly affected by

narcotics. That is, narcotics had the ability to alter the dopamine process. Drug such as heroin,

amphetamines, and marijuana all trigger the release of excess dopamine, whereas cocaine blocks

dopamine release.

Dopamine is believed to be an important part in the learning process. Dr. P. Read Montague,

of the Center for Theoretical Neuroscience at Houston's Baylor College of Medicine, has said

that people should "think of dopamine as the proverbial carrot, a reward the brain doles out to

networks of neurons for making survival-enhancing choices." It is not fully understood how this

process is put into practice, but Montague and his colleagues of the Salk Institute in San Diego

and M.I.T. have developed a test model they believe to be an accurate representation of the

dopamine cycle.

Montague developed a computer program that simulated bees involved in gathering nectar.

The virtual flowers ranged from very sweet, to not sweet at all. This system was intended to

represent the action of dopamine being used as a reward. They found that 85% of the time, the

bees would go to the flowers that were sweet. The flowers had been programmed with a

dopamine-like reward system that would go into effect when one of the bees would land on a

sweet flower. It is believed that a similar system works in the human brain. When a person

learns a new survival tactic, it is considered that the brain releases an excess amount of

dopamine, so that the person feels compelled to repeat the action. One of these actions may be

to eat in the morning, or to study for a test.

In addition to controlling addiction, dopamine also functions as an inhibitor in the carotid

body. There, dopamine has a variety of responses. Dopamine relaxes the lower esophageal

sphincter, delays gastric emptying, and causes certain arterioles to increase in diameter.

Although not proven, it is believed that dopamine nerve endings may be present in the kidneys.

Many studies have been done to understand the role that dopamine plays in human

metabolism. These studies have demonstrated that, much like the other chemicals in the human

brain, dopamine is a complex substance that is not yet fully understood.

Bibliography

1.) Braunwald, Isselbacker, Petersdorf, Wilson, Martin, Fauci. Harrison's Principles of Internal

Medicine. New York, McGraw-Hill Book Company.

2.) Nash, J. Madeleine. "The Chemistry of Addiction", Time Magazine. 1997, May 5. Volume

149, Number 18. pg. 36-43.