A further classification of monoamines is catecholamines. Both norepinephrine and dopamine are catecholamines. Both these catecholamines are produced within the body by chemically converting Tyrosine, an essential amino acid, into L-Dopa and then to dopamine. Dopamine is then chemically converted to norepinephrine. This chemical process is outlined simply in the following figure.
Dopamine can have either an excitatory or inhibitory effect on the postsynaptic potential.
In other words, when dopamine leaves the presynaptic neuron and goes into the synapse, it can then bind to receptors on the postsynaptic neuron. After dopamine is bound to the postsynaptic cell, it can either facilitate an action potential or inhibit it. Neurotransmitters are the messengers within the central nervous system. They are responsible for communicating with other neurons either through chemical or electrical signals. A neuron that classically releases dopamine is called a dopaminergic neuron.