noradrénaline

Noradrénaline, or norepinephrine, is a catecholamine that functions as a neurotransmitter in the central nervous system and as a hormone in the peripheral nervous system. It is synthesized from dopamine by the enzyme dopamine beta-hydroxylase in noradrenergic neurons and in the adrenal medulla, and is stored in synaptic vesicles for rapid release upon neuronal firing. In the brain, the locus coeruleus supplies most of the noradrenergic neurons; in the peripheral system it is released by sympathetic neurons and chromaffin cells of the adrenal medulla.

Noradrénaline exerts its effects by binding to adrenergic receptors, principally the alpha1, alpha2, and beta1 subtypes, with some action on beta2 receptors. Activation of alpha1 receptors causes vasoconstriction; beta1 receptors increase heart rate and myocardial contractility and stimulate renin release; overall, norepinephrine tends to raise blood pressure and redirect blood flow during the fight-or-flight response. In the brain, it modulates attention, arousal, and mood and participates in the sleep-wake cycle and stress responses.

Inactivation occurs mainly by reuptake into neurons through the norepinephrine transporter, after which it is metabolized by monoamine oxidase and catechol-O-methyltransferase. The circulating level of norepinephrine reflects sympathetic nerve activity and can be measured in plasma or urine.

Clinical relevance: abnormal noradrenergic activity is associated with several conditions. Elevated levels occur in pheochromocytoma and some forms of hypertension; reduced activity is linked to certain mood disorders. Therapeutically, norepinephrine can be used as a vasopressor to treat acute hypotension or septic shock.