Sci-Tech Encyclopedia:
Dopamine
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monoamine neurotransmitter formed in the brain by the decarboxylation of
dopa and essential to the normal functioning of the central nervous system. A
reduction in its concentration within the brain is associated with Parkinson's
disease.
A catecholamine neurotransmitter that is synthesized by certain neurons in the brain and
interacts with specific receptor sites on target neurons.
Dopamine is manufactured inside dopamine neurons in a controlled manner from the amino
acid precursor L-tyrosine, which mammals obtain through the normal diet. Dopamine is then
stored in vesicles within the nerve terminals, which may fuse with the cell membrane to
release dopamine into the synapse.
The release of neurotransmitter is controlled by a variety of factors, including the firing rate
of the dopamine nerve cell (termed impulse-dependent release) and the release- and
synthesis-modulating presynaptic dopamine receptors located on the dopamine nerve
terminals. Since presynaptic dopamine receptors are sensitive to the cell's own
neurotransmitter, they are called dopamine autoreceptors. Once released, dopamine also
acts at postsynaptic receptors to influence behavior. The actions of dopamine in the synapse
are terminated primarily by the reuptake of neurotransmitter into the presynaptic terminal by
means of an active dopamine transporter. Dopamine may then be either repackaged into
synaptic vesicles for rerelease or degraded by the enzyme monoamine oxidase. The
dopamine transporter is an important site of action of the drugs cocaine and amphetamine.
See also Synaptic transmission.
Although it was first thought that dopamine occurred only as an intermediate product formed
in the biosynthesis of two other catecholamine neurotransmitters, norepinephrine and
epinephrine, dopamine is now recognized as a neurotransmitter in its own right. Several
distinct dopamine neuronal systems have been identified in the brain. These include systems
within the hypothalamus and the pituitary gland; systems within the midbrain that project to a
variety of cortical and limbic regions and basal ganglia; the retinal system; and the olfactory
system. See also Brain; Epinephrine; Noradrenergic system.
The midbrain dopamine neurons which project to a variety of forebrain structures are
critically involved in normal behavioral attention and arousal; abnormalities in the normal
functioning of these systems have been implicated in a variety of disorders. For example,
Parkinson's disease involves a degeneration of the midbrain dopamine neurons. This
condition is often successfully treated by providing affected individuals with L-dopa, which is
readily converted to dopamine in the brain. Attention deficit disorder, which is usually first
diagnosed in childhood, is thought to involve dopamine systems, because the treatment of
choice, methylphenidate, binds to the dopamine transporter and alters dopamine levels in
the synapse. See also Parkinson's disease.