N
Norepinephrine
Norepinephrine is a catecholamine produced by adrenal medullary chromaffin cells and released extensively as a neurotransmitter by postganglionic sympathetic neurons. It acts predominantly on alpha-adrenergic and beta-1 adrenergic receptors to regulate vascular tone, blood pressure, cardiac function, and the physiological response to stress.
RegionAbdomen
SystemEndocrine System
Norepinephrine, also called noradrenaline, is a catecholamine that functions both as a neurotransmitter and as a circulating hormone. It is the principal neurotransmitter released by most postganglionic sympathetic neurons and is also produced and secreted by chromaffin cells of the adrenal medulla.
Within the sympathetic nervous system, norepinephrine is released locally from sympathetic nerve terminals and acts on adrenergic receptors in nearby target tissues. Within the adrenal medulla, norepinephrine serves both as a secreted catecholamine and as the immediate precursor for synthesis of epinephrine.
Norepinephrine has particularly important effects on the cardiovascular system. Through strong activation of alpha-adrenergic receptors, it produces widespread vasoconstriction and increases systemic vascular resistance. It also activates cardiac beta-1 adrenergic receptors, increasing myocardial contractility. These actions make norepinephrine a major regulator of vascular tone and arterial blood pressure.
Site of Production
Norepinephrine is synthesized in two major components of the sympathetic system:
- Chromaffin cells of the adrenal medulla
- Postganglionic sympathetic neurons
The relative functional importance of these sources differs. Sympathetic neurons release norepinephrine directly near target tissues, whereas adrenal chromaffin cells release catecholamines into the bloodstream.
Adrenal Medulla
The adrenal medulla forms the central portion of the adrenal gland and is surrounded by the adrenal cortex.
It contains catecholamine-producing chromaffin cells arranged in clusters and cords around an extensive network of sinusoidal vessels.
Chromaffin Cells
Chromaffin cells are specialized neuroendocrine cells derived from the neural crest.
They are functionally related to postganglionic sympathetic neurons but lack the conventional axonal organization used to deliver neurotransmitters directly to peripheral target organs.
Neural Crest Origin
Chromaffin cells originate from neural crest cells associated developmentally with the sympathetic nervous system.
This embryological relationship helps explain why adrenal medullary cells and sympathetic neurons share the ability to synthesize catecholamines.
Norepinephrine in Sympathetic Neurons
Norepinephrine is the principal neurotransmitter released by most postganglionic sympathetic neurons.
Important exceptions include sympathetic fibers supplying eccrine sweat glands, which primarily release acetylcholine.
Neurotransmitter Versus Hormone
| Feature | Neuronal Norepinephrine | Adrenal Norepinephrine |
|---|---|---|
| Source | Postganglionic sympathetic neurons | Adrenal medullary chromaffin cells |
| Route of release | Near target tissue | Into bloodstream |
| Primary role | Neurotransmitter | Circulating catecholamine hormone |
| Distribution of effect | Relatively localized | Systemic |
Catecholamine Synthesis
Norepinephrine is synthesized from the amino acid tyrosine.
The major pathway is:
Tyrosine → L-DOPA → Dopamine → Norepinephrine
Within epinephrine-producing adrenal chromaffin cells, norepinephrine can subsequently be converted into epinephrine.
Tyrosine
Tyrosine is the initial amino acid substrate for catecholamine biosynthesis.
It is transported into catecholamine-producing cells and undergoes sequential enzymatic modification.
Tyrosine Hydroxylase
Tyrosine hydroxylase converts tyrosine into L-3,4-dihydroxyphenylalanine, or L-DOPA.
This reaction is generally considered the rate-limiting step in catecholamine synthesis.
DOPA Decarboxylase
L-DOPA is converted into dopamine by aromatic L-amino acid decarboxylase, also known as DOPA decarboxylase.
Dopamine Beta-Hydroxylase
Dopamine is transported into catecholamine storage vesicles, where dopamine beta-hydroxylase converts it into norepinephrine.
This enzyme therefore catalyzes the final step in norepinephrine synthesis.
Norepinephrine Synthesis Pathway
| Substrate | Enzyme | Product |
|---|---|---|
| Tyrosine | Tyrosine hydroxylase | L-DOPA |
| L-DOPA | DOPA decarboxylase | Dopamine |
| Dopamine | Dopamine beta-hydroxylase | Norepinephrine |
Conversion to Epinephrine
Within appropriate adrenal medullary chromaffin cells, norepinephrine can be converted into epinephrine.
This reaction is catalyzed by phenylethanolamine N-methyltransferase (PNMT).
PNMT
PNMT transfers a methyl group to norepinephrine, producing epinephrine.
Expression of PNMT in the adrenal medulla is promoted by high local glucocorticoid concentrations, particularly cortisol delivered from the adrenal cortex.
Relationship with Cortisol
Blood passing from the adrenal cortex toward the medulla exposes chromaffin cells to relatively high concentrations of cortisol.
Cortisol promotes PNMT expression and thereby favors conversion of norepinephrine into epinephrine.
Catecholamine Storage
Norepinephrine is stored in membrane-bound vesicles before release.
In chromaffin cells these vesicles are commonly called chromaffin granules, while sympathetic neurons store norepinephrine within synaptic vesicles and larger dense-core vesicles.
Sympathetic Regulation of the Adrenal Medulla
The adrenal medulla receives direct input from preganglionic sympathetic neurons.
Preganglionic fibers release acetylcholine onto chromaffin cells.
Acetylcholine
Acetylcholine binds to neuronal nicotinic acetylcholine receptors on chromaffin cells.
Receptor activation depolarizes the chromaffin cell membrane and promotes calcium-dependent exocytosis of catecholamine-containing granules.
Sympathetic Neuronal Release
At peripheral sympathetic nerve terminals, an arriving action potential opens voltage-gated calcium channels.
Calcium entry triggers fusion of norepinephrine-containing vesicles with the neuronal membrane and release of norepinephrine into the neuroeffector junction.
Adrenergic Receptors
Norepinephrine acts on adrenergic receptors, a family of G protein-coupled receptors distributed throughout the cardiovascular system and numerous other tissues.
Norepinephrine has important activity at α1, α2, and β1 receptors, with substantially less β2 receptor activity than epinephrine.
Alpha-1 Adrenergic Receptors
α1 adrenergic receptors are widely expressed on vascular smooth muscle.
Activation increases intracellular signaling that promotes smooth muscle contraction and vasoconstriction.
Alpha-2 Adrenergic Receptors
α2 adrenergic receptors have important regulatory functions, including presynaptic inhibition of neurotransmitter release.
Activation of presynaptic α2 receptors can reduce additional norepinephrine release and thereby provide negative feedback at sympathetic nerve terminals.
Beta-1 Adrenergic Receptors
β1 adrenergic receptors are prominent in the heart.
Norepinephrine activation of these receptors increases myocardial contractility and can increase pacemaker activity.
Beta-2 Adrenergic Receptors
Norepinephrine has relatively weak activity at β2 adrenergic receptors compared with epinephrine.
Consequently, norepinephrine produces less β2-mediated vasodilation and bronchodilation than epinephrine.
Adrenergic Receptor Profile
| Receptor | Norepinephrine Activity | Representative Effect |
|---|---|---|
| α1 | Strong | Vasoconstriction |
| α2 | Strong | Modulation of neurotransmitter release |
| β1 | Strong | Increased myocardial contractility |
| β2 | Relatively weak | Limited smooth muscle relaxation compared with epinephrine |
Cardiovascular Effects
The cardiovascular system is one of the most important targets of norepinephrine.
Its combination of α-mediated vasoconstriction and β1-mediated cardiac stimulation produces substantial effects on systemic vascular resistance and arterial blood pressure.
Vasoconstriction
Norepinephrine produces widespread vasoconstriction through activation of α1 adrenergic receptors on vascular smooth muscle.
This effect occurs in many systemic vascular beds and increases resistance to blood flow.
Systemic Vascular Resistance
Widespread arteriolar constriction increases systemic vascular resistance.
This is one of the principal mechanisms through which norepinephrine increases arterial pressure.
Venoconstriction
Norepinephrine can also constrict venous smooth muscle.
Venoconstriction decreases venous capacitance and can promote movement of blood toward the central circulation, supporting venous return.
Systolic Blood Pressure
Norepinephrine generally increases systolic arterial pressure through increased vascular resistance and cardiac effects.
Diastolic Blood Pressure
Because norepinephrine strongly increases peripheral vascular resistance, it also typically increases diastolic arterial pressure.
This contrasts with epinephrine, whose β2-mediated vasodilator activity can reduce diastolic pressure under some conditions.
Mean Arterial Pressure
The combined increase in systolic and diastolic pressure produces an increase in mean arterial pressure.
Cardiac Contractility
Direct activation of cardiac β1 receptors increases myocardial contractility.
This represents a positive inotropic effect.
Heart Rate
Norepinephrine can directly stimulate β1 receptors in the sinoatrial node, which tends to increase heart rate.
However, the substantial rise in arterial pressure produced by norepinephrine activates the arterial baroreceptor reflex, which can produce reflex vagal slowing of the heart.
Reflex Bradycardia
The increase in arterial pressure stretches baroreceptors in the carotid sinus and aortic arch.
Increased baroreceptor signaling promotes parasympathetic activity and decreases cardiac sympathetic drive, potentially producing reflex bradycardia despite the direct β1 effects of norepinephrine.
Baroreceptor Reflex
The baroreceptor response demonstrates that the final physiological effect of a hormone or neurotransmitter may differ from its direct receptor-level action.
Although norepinephrine directly stimulates the heart, its powerful vasoconstrictor effect can trigger a compensatory neural response that slows heart rate.
Cardiovascular Effects Summary
| Variable | Typical Effect |
|---|---|
| Arteriolar tone | Increased |
| Systemic vascular resistance | Increased |
| Systolic pressure | Increased |
| Diastolic pressure | Increased |
| Mean arterial pressure | Increased |
| Myocardial contractility | Increased directly |
| Heart rate | May decrease because of reflex bradycardia |
Sympathetic Vasomotor Tone
Continuous low-level sympathetic activity contributes to normal resting vascular tone.
Changes in norepinephrine release from sympathetic vasoconstrictor fibers allow rapid adjustment of vessel diameter and systemic vascular resistance.
Postural Blood Pressure Regulation
When a person stands, gravity causes blood to accumulate transiently in dependent portions of the circulation.
Baroreceptor-mediated sympathetic activation increases norepinephrine release, producing vasoconstriction and supporting arterial pressure and venous return.
Stress Response
Sympathetic activity and catecholamine release increase during many forms of acute physiological stress.
Norepinephrine contributes particularly to maintenance of vascular tone and arterial pressure during these responses.
Exercise
Sympathetic norepinephrine release generally increases during exercise.
Its effects help maintain arterial pressure and redistribute blood flow while cardiac and metabolic demands increase.
Hemorrhage
Loss of circulating blood volume activates sympathetic reflexes.
Increased norepinephrine release produces arteriolar and venous constriction, helping preserve arterial pressure and central blood volume.
Effects on the Kidney
Sympathetic norepinephrine influences renal blood vessels, tubular function, and renin secretion.
These effects become particularly important during substantial sympathetic activation.
Renal Vasoconstriction
Activation of renal vascular α1 receptors produces vasoconstriction and can reduce renal blood flow during strong sympathetic activation.
Renin Release
Sympathetic stimulation of β1 adrenergic receptors on juxtaglomerular cells promotes renin secretion.
Renin activates the renin-angiotensin-aldosterone system, providing an additional mechanism for supporting blood pressure and extracellular fluid volume.
Metabolic Effects
Norepinephrine participates in metabolic adaptation during sympathetic activation.
Its metabolic actions include effects on glucose availability and adipose tissue metabolism, although its overall metabolic receptor profile differs from that of epinephrine.
Lipolysis
Adrenergic signaling can promote mobilization of fatty acids from adipose tissue.
This provides an additional source of metabolic fuel during increased energy demand.
Glucose Regulation
Norepinephrine can influence hepatic glucose production and pancreatic hormone secretion as part of the integrated sympathoadrenal response.
Epinephrine generally has a more prominent systemic role in rapid metabolic responses to hypoglycemia.
Presynaptic Regulation
Norepinephrine release from sympathetic nerve endings is tightly regulated.
One important mechanism involves presynaptic α2 adrenergic autoreceptors.
Alpha-2 Autoreceptors
Released norepinephrine can bind to α2 receptors on the sympathetic nerve terminal from which it was released.
Activation of these receptors decreases additional norepinephrine release, forming a local negative feedback mechanism.
Norepinephrine Reuptake
The major mechanism terminating the action of neuronally released norepinephrine is reuptake into sympathetic nerve terminals.
This process is mediated primarily by the norepinephrine transporter.
Norepinephrine Transporter
The norepinephrine transporter (NET) moves norepinephrine from the extracellular space back into presynaptic sympathetic neurons.
Reabsorbed norepinephrine can be repackaged into vesicles or metabolized within the neuron.
Vesicular Monoamine Transporter
Within catecholaminergic cells, the vesicular monoamine transporter (VMAT) transports catecholamines into storage vesicles.
Vesicular storage protects catecholamines from cytoplasmic metabolism and makes them available for regulated release.
Catecholamine Metabolism
Norepinephrine is metabolized through pathways involving monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT).
The relative contribution of these pathways varies according to the location and route of catecholamine metabolism.
Normetanephrine
Norepinephrine can be metabolized by COMT to produce normetanephrine.
Measurement of normetanephrine is clinically important in the evaluation of catecholamine-producing tumors.
Vanillylmandelic Acid
Vanillylmandelic acid (VMA) is a urinary end product of norepinephrine and epinephrine metabolism.
Although VMA can be measured clinically, plasma free or urinary fractionated metanephrines are commonly emphasized when evaluating suspected pheochromocytoma or paraganglioma.
Norepinephrine Metabolism Summary
| Component | Role |
|---|---|
| NET | Presynaptic norepinephrine reuptake |
| VMAT | Vesicular catecholamine storage |
| MAO | Monoamine metabolism |
| COMT | Catecholamine methylation |
| Normetanephrine | Major clinically important norepinephrine metabolite |
| VMA | Urinary end product of catecholamine metabolism |
Norepinephrine Versus Epinephrine
Norepinephrine and epinephrine are closely related catecholamines, but their physiological roles differ because of differences in their sources and adrenergic receptor activity.
| Feature | Norepinephrine | Epinephrine |
|---|---|---|
| Alternative name | Noradrenaline | Adrenaline |
| Major physiological source | Sympathetic nerve terminals | Adrenal medulla |
| Major role | Sympathetic neurotransmitter | Circulating hormone |
| α receptor activity | Strong | Strong |
| β1 activity | Strong | Strong |
| β2 activity | Relatively weak | Prominent |
| Typical systemic vascular effect | Predominantly vasoconstriction | Mixed vasoconstriction and vasodilation |
Norepinephrine Versus Acetylcholine
Acetylcholine and norepinephrine occupy different positions within the autonomic nervous system.
| Pathway | Major Neurotransmitter |
|---|---|
| All autonomic preganglionic neurons | Acetylcholine |
| Most sympathetic postganglionic neurons | Norepinephrine |
| Parasympathetic postganglionic neurons | Acetylcholine |
| Sympathetic fibers to eccrine sweat glands | Acetylcholine |
Norepinephrine and the Adrenal Medulla
Within the adrenal medulla, norepinephrine has a dual role. It can be stored and released as a catecholamine hormone, or it can serve as the immediate biochemical precursor of epinephrine.
The balance between norepinephrine and epinephrine production is influenced by expression of PNMT and exposure of chromaffin cells to glucocorticoids from the adrenal cortex.
Pheochromocytoma
Pheochromocytoma is a catecholamine-producing tumor arising from chromaffin cells, most commonly within the adrenal medulla.
Individual tumors vary in the amount and pattern of norepinephrine, epinephrine, dopamine, and catecholamine metabolites they produce.
Effects of Catecholamine Excess
Excessive norepinephrine production can produce prominent cardiovascular manifestations because of strong α-adrenergic and β1-adrenergic activity.
Potential manifestations include:
- Hypertension
- Headache
- Palpitations
- Sweating
- Tremor
- Cardiovascular instability
Paraganglioma
Paragangliomas arise from paraganglionic tissue outside the adrenal gland.
Sympathetic paragangliomas may produce norepinephrine or other catecholamines and can therefore cause clinical manifestations similar to those associated with pheochromocytoma.
Biochemical Evaluation
Measurement of plasma free metanephrines or urinary fractionated metanephrines is commonly used in the biochemical evaluation of suspected catecholamine-producing tumors.
For norepinephrine-producing tumors, elevated normetanephrine can be particularly important.
Pharmacological Norepinephrine
Norepinephrine is also used pharmacologically as a potent vasopressor.
Its strong α-adrenergic effects increase vascular tone and systemic vascular resistance, while β1 receptor activation supports cardiac contractility.
Norepinephrine as a Vasopressor
When administered intravenously, norepinephrine can rapidly increase arterial pressure by constricting resistance vessels.
Its hemodynamic effects require careful monitoring because excessive vasoconstriction can reduce blood flow to peripheral tissues.
Shock and Circulatory Support
Norepinephrine can be used to support arterial pressure in forms of severe circulatory shock characterized by inadequate vascular tone.
Its clinical usefulness reflects the same α1-mediated vasoconstrictor action that contributes to normal sympathetic regulation of blood pressure.
Extravasation
Because norepinephrine causes powerful local vasoconstriction, unintended leakage from the vascular space into surrounding tissue can compromise local perfusion.
This illustrates the intensity of its α-adrenergic vascular effects.
Central Nervous System Norepinephrine
Norepinephrine also functions as an important neurotransmitter within the central nervous system.
Central noradrenergic pathways participate in regulation of arousal, attention, autonomic activity, sleep-wake states, and responses to stress.
Locus Coeruleus
The locus coeruleus in the pons is a major source of norepinephrine-producing neurons within the brain.
Its neurons project widely throughout the central nervous system and participate in regulation of arousal and attention.
Peripheral and Central Norepinephrine
| Location | Major Role |
|---|---|
| Sympathetic nerve terminals | Peripheral autonomic neurotransmission |
| Adrenal medulla | Circulating catecholamine and epinephrine precursor |
| Central nervous system | Neuromodulation of arousal, attention and autonomic function |
Sympathoadrenal System
The sympathetic nervous system and adrenal medulla function together as the sympathoadrenal system.
Neuronally released norepinephrine provides rapid, anatomically targeted sympathetic signaling, while adrenal catecholamines produce a circulating endocrine response.
Integrated Response to Stress
During acute physiological stress, increased sympathetic activity enhances norepinephrine release from peripheral nerve terminals while also stimulating adrenal medullary catecholamine secretion.
The resulting response supports vascular tone, cardiac performance, redistribution of blood flow, and availability of metabolic substrates.
Key Features of Norepinephrine
| Feature | Key Point |
|---|---|
| Alternative name | Noradrenaline |
| Hormone class | Catecholamine |
| Major peripheral neural source | Postganglionic sympathetic neurons |
| Endocrine source | Adrenal medullary chromaffin cells |
| Precursor | Dopamine |
| Synthesis enzyme from dopamine | Dopamine beta-hydroxylase |
| Product formed from norepinephrine | Epinephrine |
| Major receptors | α1, α2 and β1 adrenergic receptors |
| β2 activity | Relatively weak |
| Major vascular effect | Vasoconstriction |
| Major pressure effect | Increased arterial pressure |
| Major metabolite | Normetanephrine |
Clinical Associations
| Condition or Application | Relationship to Norepinephrine |
|---|---|
| Pheochromocytoma | Adrenal chromaffin tumor that may produce excessive norepinephrine |
| Paraganglioma | Extra-adrenal tumor that may produce norepinephrine |
| Circulatory shock | Pharmacological norepinephrine can support vascular tone and arterial pressure |
| Orthostatic regulation | Sympathetic norepinephrine contributes to compensatory vasoconstriction |
| Catecholamine testing | Normetanephrine can indicate excessive norepinephrine production |
Anatomical and Physiological Importance
Norepinephrine links the anatomy of the adrenal medulla with the broader organization of the sympathetic nervous system. Neural crest-derived chromaffin cells produce norepinephrine within the adrenal gland, while related sympathetic neurons use the same catecholamine as their principal postganglionic neurotransmitter throughout much of the peripheral autonomic nervous system.
Its strong activity at α-adrenergic receptors makes norepinephrine particularly important for regulation of vascular smooth muscle. By adjusting arteriolar and venous tone, sympathetic norepinephrine contributes continuously to blood pressure regulation and allows rapid cardiovascular compensation during standing, exercise, blood loss, and other physiological stresses.
Norepinephrine also occupies a central biochemical position within the adrenal medulla because it is the immediate precursor of epinephrine. Cortisol-dependent expression of PNMT allows selected chromaffin cells to convert norepinephrine into epinephrine, demonstrating the functional integration of the adrenal cortex and medulla. Together, neuronal norepinephrine and adrenal catecholamine secretion form an essential component of the sympathoadrenal response.
Published on October 1, 2026
Last updated on October 1, 2026
Last updated on October 1, 2026