Catecholamines

Catecholamines are a class of monoamine neurotransmitters and hormones derived from the amino acid tyrosine. The principal catecholamines in humans are dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline). They share the catechol moiety and function as signaling molecules in the central nervous system and peripheral tissues.

Biosynthesis and storage occur in a characteristic sequence. Tyrosine hydroxylase converts tyrosine to L-DOPA, the rate-limiting step. Aromatic L-amino acid decarboxylase then converts L-DOPA to dopamine. In noradrenergic neurons and certain brain regions, dopamine beta-hydroxylase converts dopamine to norepinephrine; in the adrenal medulla and some neurons, norepinephrine is converted to epinephrine by phenylethanolamine N-methyltransferase. Catecholamines are stored in secretory vesicles and released into the synapse or bloodstream by calcium-dependent exocytosis. After release, they are rapidly cleared from the synapse by reuptake via transporters (DAT and NET) and are metabolized by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT) to metabolites such as DOPAC, HVA, normetanephrine, and metanephrine.

Physiological roles include modulation of mood, cognition, and reward (dopamine); and regulation of cardiovascular function, alertness, thermogenesis, and glucose mobilization (norepinephrine and epinephrine). Their effects are mediated by alpha and beta adrenergic receptors, producing diverse responses depending on tissue and receptor subtype. Epinephrine released from the adrenal medulla orchestrates systemic fight-or-flight responses.

Clinical relevance includes the use of metanephrine testing for pheochromocytoma, and pharmacologic applications such as L-DOPA therapy for Parkinson disease, MAO and COMT inhibitors, and various adrenergic agonists or sympathomimetics for cardiovascular or anaphylactic emergencies. Abnormal catecholamine metabolism or excess production underlies certain tumors and stress-related disorders.