Norepinephrin

Norepinephrin, also known as noradrenaline, is a catecholamine neurotransmitter and hormone in humans. It is a key chemical mediator of the sympathetic nervous system, contributing to the fight-or-flight response. In the central nervous system it acts as a neurotransmitter regulating arousal, attention, learning, and mood, with highest concentrations in the locus coeruleus. In the peripheral nervous system it functions as a vasoconstrictor and cardiostimulatory agent, increasing vascular tone and systolic blood pressure.

Biosynthesis and storage: It is synthesized from tyrosine through DOPA and dopamine, with dopamine beta-hydroxylase converting dopamine to norepinephrin in sympathetic neurons and in the adrenal medulla. It is stored in synaptic vesicles and released with nerve terminal depolarization. In the adrenal medulla it is released into the bloodstream as a hormone.

Receptors and effects: Norepinephrin acts on adrenergic receptors. Activation of alpha1 receptors causes vasoconstriction; alpha2 receptors modulate neurotransmitter release; beta1 receptors increase myocardial contractility and heart rate; beta2 receptors can mediate vasodilation in some vessels but are often overridden by alpha1 effects. The overall effect is increased blood pressure and enhanced blood flow to vital organs during stress.

Metabolism and clinical relevance: Termination occurs mainly by reuptake into the presynaptic neuron via the norepinephrine transporter, followed by breakdown by monoamine oxidase and catechol-O-methyltransferase. Plasma levels are also assessed clinically as metanephrines for certain tumors. Exogenously administered norepinephrin is used as a vasopressor in acute hypotension but does not cross the blood–brain barrier.