Essay, Research Paper: Dopamine

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.