Noradrenaline

Noradrenaline, also known as norepinephrine, is a catecholamine that functions as both a neurotransmitter in the central and peripheral nervous systems and a hormone released into the bloodstream. It is synthesized from dopamine in sympathetic neurons and in the adrenal medulla, and it is a key mediator of the body’s fight-or-flight response. The chemical name describes a monoamine with a catechol ring and an ethylamine side chain.

In the sympathetic nervous system, noradrenaline is produced in nerve terminals, stored in vesicles, and released in response to an action potential. It acts at adrenergic receptors and its effects are terminated mainly by reuptake into the presynaptic neuron via the norepinephrine transporter, followed by enzymatic degradation by monoamine oxidase and catechol-O-methyltransferase. The adrenal medulla also contributes to circulating noradrenaline, though epinephrine constitutes a larger share of adrenal output.

Noradrenaline exerts its actions primarily through alpha- and beta-adrenergic receptors. Activation of alpha-1 receptors causes vasoconstriction and increased peripheral resistance, contributing to elevated blood pressure. Beta-1 receptor stimulation increases heart rate and contractility. Alpha-2 receptor activation provides negative feedback to further transmitter release. In the central nervous system, noradrenaline released from the locus coeruleus modulates arousal, attention, memory consolidation, and mood.

Clinically, noradrenaline is used as a vasopressor to raise blood pressure in acute hypotensive states. It is less effective for bronchial dilation and has limited cardiac stimulation compared with epinephrine. Side effects include hypertension, tachyarrhythmias, and tissue necrosis from extravasation. In the brain, noradrenaline pathways are targeted by various antidepressants and cognitive enhancers that influence mood and alertness.

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