Dopamine, a neurotransmitter in the central nervous system

Neurotransmitters serve to transmit signals between neurons, which are separated by a synaptic cleft. One of the neurotransmitters is dopamine (DA), or P-(3,4-dihydroxyphenyl)ethylamine (1). Until the mid-1950s dopamine was exclusively considered to be an intermediate in the biosynthesis of the catecholamines noradrenaline and adrenaline. Significant tissue levels of dopamine were first demonstrated in peripheral organs of ruminant species.1 A short time later it was found that dopamine was also present in the brain in about equal concentrations to those of noradrenaline.2 

When a stimulus depolarises the transmembrane potential in a spiking axon above the threshold level, an all-or-none action potential in a spiking axon is activated. The action potential propagates unattenuated to the nerve terminal where ion fluxes activate a mobilisation process leading to transmitter secretion.3 The neurotransmitter binds reversibly to receptor proteins embedded in the membrane of a neuron, which triggers a certain effect. There are two types of receptors known, presynaptic receptors or autoreceptors which are present on the neurotransmitter releasing neurons , and postsynaptic or heteroreceptors, which are present on the neurotransmitter receiving neuron . The former are supposed to perform a feed-back function, and slow down the release of neurotransmitter from these neurons when they are stimulated.4 

Dopamine receptors are G-protein coupled receptors. After a receptor has been activated, this G-protein may activate or inhibit the second messenger system causing certain biochemical reactions to occur. In contrast to the ionotropic receptors that are linked to an ion-channel and respond very fast to activation by a neurotransmitter (millisecond processes), G-protein coupled receptors mediate slower responses (seconds to minutes) and in general have a modulatory function on other signal transduction processes.4 

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