Different Classes of Neurotransmitters

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Transmitter Molecule Derived From Site of Synthesis
Acetylcholine Choline CNS, parasympathetic nerves
Serotonin
5-Hydroxytryptamine (5-HT) Tryptophan CNS, chromaffin cells of the gut, enteric cells
GABA Glutamate CNS
Glutamate CNS
Aspartate CNS
Glycine spinal cord
Histamine Histidine hypothalamus
Epinephrine
synthesis pathway Tyrosine adrenal medulla, some CNS cells
Norpinephrine
synthesis pathway Tyrosine CNS, sympathetic nerves
Dopamine
synthesis pathway Tyrosine CNS
Adenosine ATP CNS, periperal nerves
ATP sympathetic, sensory and enteric nerves
Nitric oxide, NO Arginine CNS, gastrointestinal tract
Catecholamines
The principal catecholamines are norepinephrine, epinephrine and dopamine. These compounds are formed from phenylalanine and tyrosine. Tyrosine is produced in the liver from phenylalanine through the action of phenylalanine hydroxylase. The tyrosine is then transported to catecholamine-secreting neurons where a series of reactions convert it to dopamine, to norepinephrine and finally to epinephrine (see Specialized Products of Amino Acids).
Catecholamines exhibit peripheral nervous system excitatory and inhibitory effects as well as actions in the CNS such as respiratory stimulation and an increase in psychomotor activity. The excitatory effects are exerted upon smooth muscle cells of the vessels that supply blood to the skin and mucous membranes. Cardiac function is also subject to excitatory effects, which lead to an increase in heart rate and in the force of contraction. Inhibitory effects, by contrast, are exerted upon smooth muscle cells in the wall of the gut, the bronchial tree of the lungs, and the vessels that supply blood to skeletal muscle.
In addition to their effects as neurotransmitters, norepinephrine and epinephrine can influence the rate of metabolism. This influence works both by modulating endocrine function such as insulin secretion and by increasing the rate of glycogenolysis and fatty acid mobilization.
The catecholamines bind to two different classes of receptors termed the a- and b-adrenergic receptors. The catecholamines therefore are also known as adrenergic neurotransmitters; neurons that secrete them are adrenergic neurons. Norepinephrine-secreting neurons are noradrenergic. The adrenergic receptors are classical serpentine receptors that couple to intracellular G-proteins. Some of the norepinephrine released from presynaptic noradrenergic neurons recycled in the presynaptic neuron by a reuptake mechanism
AMINO ACIDS
Amino acids are the basic structural building units of proteins. They form short polymer chains called peptides or polypeptides which in turn form structures called proteins.
Twenty amino acids are encoded by the standard genetic code and are called proteinogenic or standard amino acids. Rarer, more complicated ones are produced by the body and are called nonstandard. Proline is the only proteinogenic amino acid whose side group is cyclic and links to the a-amino group, forming a secondary amino group. Formerly, proline was misleadingly called an ...