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Dopamine refers to a catecholamine neurotransmitter especially important to the regulation and control of sensory and motor activity. It is produced in the brain and, as a neurotransmitter, its role is necessary for the initiation or cessation of nerve cell signals and allows for continued communication of neurons across synaptic gaps. Although produced in minute amounts, dopamine influences mood, as well as reproductive and pleasure-seeking behaviors. Too much of it produces symptoms of schizophrenia, while too little causes tremors typical of Parkinson's disease.
Within the cerebrum, there are clusters of gray matter referred to as the basal ganglia, which are significantly involved with movement, coordination, and transmitting impulses to the substantia nigra. Responsible for dopamine production, the substantia nigra is a midbrain group of neurons necessary for communication with the basal ganglia. This neurotransmitter is also manufactured by the hypothalamus and ventral tegmentum, a midbrain structure located directly next to the substantia nigra.
Certain behaviors are necessary for survival and are experienced as hunger, thirst, and libido. Referred to as the reward circuit system, the brain's mechanism of pleasure-seeking behaviors involves the tegmentum and nucleus accumbens, limbic system structures consisting of dopamine-producing nerve cells. The nucleus accumbens receives information interpreted as a reward, as satisfaction, or as pleasurable from the tegmentum, and dopamine is the neurotransmitter necessary. Playing a primary role in addiction, the action of dopamine is directly influenced by commonly-abused drugs, especially cocaine, heroin, and amphetamine.
Etiology of schizophrenia has been associated with excess levels of dopamine, but it is also thought that other neurotransmitters have a role as well. Schizophrenia refers to a serious mental disease characterized by sensory distortions, irrational thinking, and withdrawal behaviors. The theory of dopamine's role in schizophrenia asserts that psychotic symptoms like hallucinations are due to abnormally high dopamine levels, caused by hypersensitivity or an excessive amount of receptors. Drugs proven to be particularly effective in treating schizophrenia are those that act by blocking dopamine receptors, thereby decreasing the amount produced.
Characterized by shaking while resting, loss of coordination, and a shuffling walk, Parkinson's disease is caused by a dopamine level deficit resulting from the destruction of nerve cell receptors located in the substantia nigra. By the time motor manifestations occur, up to 80% of cells have been destroyed, which typically presents in people over 60 years old. Treatment with oral dopamine would seem effective, but, as it cannot penetrate the blood-brain barrier readily, another drug capable of changing into dopamine referred to as levodopa, is commonly used with success. With progression of Parkinson's disease, levodopa does become less helpful.