Norepinefrin

Norepinefrin, also called norepinephrine in English and noradrenalin, is a catecholamine that functions as both a neurotransmitter in the central nervous system and a circulating hormone in the body. In humans, its synthesis, release, and actions are coordinated within the sympathetic nervous system and the adrenal medulla.

Biosynthesis and storage: It is synthesized from dopamine by the enzyme dopamine beta-hydroxylase, stored in small vesicles in sympathetic neurons and chromaffin cells, and co-released with other transmitters. In the central nervous system, noradrenaline is primarily produced by neurons in the locus coeruleus and contributes to arousal, attention, and mood.

Mechanisms and physiological roles: Norepinefrin acts on adrenergic receptors, including alpha and beta subtypes. Peripherally, it increases heart rate and contractility, constricts vascular beds, and raises blood pressure; centrally, it modulates attention, vigilance, and the sleep-wake cycle. The balance of receptor activation shapes cardiovascular and metabolic responses during stress and activity.

Metabolism and measurement: After release, norepinephrine is rapidly reabsorbed by nerve terminals and degraded by monoamine oxidase and catechol-O-methyltransferase to metanephrines. Measurement of plasma free metanephrines or 24-hour urine metanephrines is used in the evaluation for pheochromocytoma and related disorders.

Clinical use: Clinically, norepinephrine is employed as a vasopressor to treat life-threatening hypotension and septic or postoperative shock, typically administered intravenously with careful monitoring to minimize adverse effects. Potential side effects include hypertension, tachycardia, arrhythmias, and tissue necrosis from extravasation.

History: The compound was identified and characterized in the early-to-mid 20th century and is now recognized as a core mediator of the body's sympathetic and stress responses.