Dopamine

After acetylcholine, the next transmitters discovered were the catecholamines (CAT-a-COLE-a-means). Two important members of this transmitter family are dopamine and norepinephrine. Dopamine is found in two areas of the midbrain, the tegmentum and the substantia nigra. These are both parts of the limbic system, the portion of the brain that regulates emotion.

What is evidence that dopamine is involved in the effects of cocaine?

Dopamine is involved in the response to cocaine. Rats will press a bar many times to get cocaine injected directly into dopaminergic areas of the limbic system (areas heavily laden with neurons using dopamine). The same is not true of other brain areas. When researchers destroy dopaminergic areas of the limbic system of rats, the rats no longer find cocaine reinforcing. There are several different forms of dopamine in the nervous system; one stimulates cocaine craving, another eliminates it.

What is L-Dopa?

Dopamine was thrust into public attention when L-Dopa—a drug that restores dopamine synthesis—was discovered as a treatment for Parkinson's disease. L-Dopa is the drug that Oliver Sacks used to bring patients out of a semi-comatose state (temporarily, as it turned out) as described in the book Awakenings, later made into a movie starring Robin Williams in the role of Dr. Sacks.

What is the effect of too little or too much dopamine?

Too little dopamine produces symptoms of Parkinson's disease, but too much produces psychosis (serious mental illness often requiring hospitalization). Antipsychotic drugs used widely in mental hospitals all have the property of blocking dopamine receptors. Too much L-Dopa, in treatment of Parkinson's disease, can produce psychotic symptoms. The patients in Awakenings who received L-Dopa did show psychotic symptoms, in some cases, when the dosage was too high.

What is evidence linking dopamine to motor control systems?

Dopamine is implicated in motor control systems. One symptom of Parkinson's disease is the occurrence of tremors resulting from loss of control over the opposing muscle groups that normally control fine movements. Freed and Yamamoto (1985) injected specially marked dopamine into moving animals. The dopamine accumulated in motor control areas. The exact area where it accumulated depended on the speed, direction, and posture of the moving animals.


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