MSH: Precursor of epinephrine that is secreted by the adrenal medulla and is a widespread central and autonomic neurotransmitter. Norepinephrine is the principal transmitter of most postganglionic sympathetic fibers and of the diffuse projection system in the brain arising from the locus ceruleus. It is also found in plants and is used pharmacologically as a sympathomimetic.,CSP: precursor of epinephrine; widespread central and autonomic neurotransmitter, principal transmitter of most postganglionic sympathetic fibers; secreted by the adrenal medulla; used pharmacologically as a sympathomimetic.,NCI: A synthetic phenylethylamine that mimics the sympathomimetic actions of the endogenous norepinephrine. Norepinephrine acts directly on the alpha- and beta-adrenergic receptors. Clinically, norepinephrine is used as a peripheral vasoconstrictor that causes constriction of arterial and venous beds via its alpha-adrenergic action. It is also used as a potent inotropic and chronotropic stimulator of the heart mediate