CATECHOLAMINE

any of various naturally occurring amines that function as neurotransmitters and hormones within the body. Catecholamines are characterized by a catechol group (a benzene ring with two hydroxyl groups) to which is attached an amine (nitrogen-containing) group. Among the catecholamines are dopamine, norepinephrine, and epinephrine. All catecholamines begin with the amino acid L-tyrosine. Through a series of enzymatic reactions, L-tyrosine is converted to L-dopa, then to dopamine, then to norepinephrine, and finally to epinephrine. Catecholamines are synthesized in the brain, in the adrenal medulla, and by some sympathetic nerve fibres. The particular catecholamine that is synthesized by a nerve cell, or neuron, depends on which enzymes are present in that cell. For example, a neuron that has only the first two enzymes (tyrosine hydroxylase and dopa decarboxylase) used in the sequence will stop at the production of dopamine and is called a dopaminergic neuron (i.e., upon stimulation, it releases dopamine into the synapse). The neurotransmitter dopamine is widely distributed throughout the central nervous system and is involved in the control of movement. Norepinephrine is an important neurotransmitter in both the central nervous system and the sympathetic division of the autonomic nervous system. The hormone epinephrine acts in concert with the sympathetic nervous system to initiate the body's quick response to stressful stimuli (the fight or flight response).