Cognitive Connie
Norepinephrine
Norepinephrine (also called noradrenaline outside North America) is a monoamine neurotransmitter and hormone synthesised from dopamine by the enzyme dopamine β-hydroxylase. In the peripheral nervous system it acts as the primary neurotransmitter of the sympathetic nervous system, triggering the physiological hallmarks of Cannon's fight-or-flight response — elevated heart rate, pupil dilation, glucose mobilisation, and blood flow redistribution from digestive to skeletal muscles. In the central nervous system, norepinephrine is released from a tiny nucleus in the pontine brainstem — the locus coeruleus (LC) — which, despite containing only about 1,500 neurons per hemisphere, sends ascending projections to virtually every region of the forebrain, making it arguably the single most widely projecting nucleus in the brain.
Key figures
Ulf von Euler
1905–1983Swedish physiologist who identified norepinephrine (noradrenaline) as the primary transmitter of the sympathetic nervous system, distinguishing it from epinephrine (adrenaline) and establishing the catecholamine framework for peripheral autonomic neurotransmission. Shared the 1970 Nobel Prize in Physiology or Medicine with Julius Axelrod and Bernard Katz.
Joseph Schildkraut
1934–2006American psychiatrist whose 1965 catecholamine hypothesis of affective disorders — proposing that depression results from functional deficiency of catecholamines (particularly norepinephrine) at central synapses — was the founding framework of biological psychiatry in its modern form. While substantially revised, the hypothesis directed research that led to the development of the tricyclic antidepressants and subsequently SNRIs.
Gary Aston-Jones
1950–presentAmerican neuroscientist who, with Jonathan Cohen, developed the adaptive gain theory of locus coeruleus-norepinephrine function (2005) — proposing that the LC operates in phasic and tonic modes that set the gain of cortical processing and regulate the exploration-exploitation trade-off in decision-making. This framework provided the most comprehensive account of how LC-NE activity shapes attention, arousal, and adaptive behaviour.
Key concepts
Locus coeruleus
The primary source of norepinephrine in the central nervous system — a compact bilateral nucleus in the pontine brainstem containing approximately 1,500 neurons per hemisphere. Despite its small size, the locus coeruleus sends ascending noradrenergic projections to virtually the entire forebrain, including the cortex, hippocampus, amygdala, hypothalamus, and thalamus. Its global reach allows it to rapidly modulate the state of large cortical regions in response to significant events, effectively functioning as a global gain-control system. The LC is activated by novel, intense, or threatening stimuli and by stressors — releasing norepinephrine to mobilise attentional and arousal resources.
Catecholamine hypothesis of depression
Schildkraut's (1965) proposal that depression is associated with a functional deficiency of catecholamines (norepinephrine and dopamine) at central synapses, and that mania is associated with excess. Supporting evidence included the antidepressant effects of drugs that increase catecholamine availability (MAO inhibitors, tricyclic antidepressants) and the depression-inducing effects of reserpine, which depletes monoamine stores. The hypothesis has since been substantially revised — it is understood as incomplete — but it stimulated the development of the entire modern antidepressant pharmacopoeia.
Adaptive gain theory
Aston-Jones and Cohen's (2005) model of locus coeruleus function: the LC modulates the gain of cortical processing, making neurons more or less responsive to their inputs. In phasic mode (brief bursts following task-relevant stimuli), LC activity increases the signal-to-noise ratio, sharpening stimulus discrimination and facilitating adaptive responding. In tonic mode (sustained high firing in states of stress, threat, or anxious arousal), LC activity is elevated but phasic responses are reduced, impairing focused attention and promoting a vigilant, exploratory scanning mode suited to unpredictable environments but incompatible with focused task performance.
Norepinephrine in PTSD
Post-traumatic stress disorder is characterised by hyperarousal symptoms — exaggerated startle, sleep disturbance, irritability, hypervigilance — that closely mirror the effects of LC-norepinephrine hyperactivation. During trauma exposure, massive norepinephrine release strengthens the encoding of the traumatic memory through β-adrenergic receptor activation in the amygdala and hippocampus. In PTSD, this system appears to remain dysregulated: elevated urinary and CSF norepinephrine levels have been found in PTSD populations. Pharmacological strategies targeting this system include prazosin (α-1 antagonist), which reduces nightmares and hyperarousal, and propranolol (β-antagonist), which has been investigated as a memory-reconsolidation blocker.
Alpha and beta adrenergic receptors
Norepinephrine acts on two major receptor families. Alpha-adrenergic receptors (α-1 and α-2) are G-protein-coupled metabotropic receptors: α-1 receptors in cortex and limbic areas increase neuronal excitability; α-2 receptors, located presynaptically on LC neurons as autoreceptors, reduce norepinephrine release when activated (the basis for clonidine's use in hypertension and PTSD). Beta-adrenergic receptors (β-1, β-2, β-3) mediate many of the peripheral effects of the sympathetic nervous system and also modulate memory consolidation in the amygdala — β-adrenergic blockade by propranolol administered shortly after a traumatic event can reduce the emotional intensity of the memory.
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Frequently asked questions
What is the difference between norepinephrine and adrenaline?+
Norepinephrine (noradrenaline) and epinephrine (adrenaline) are closely related catecholamines — epinephrine is synthesised from norepinephrine by the addition of a methyl group. Both are released during the stress response, but from different sources and with somewhat different effects. Norepinephrine is the primary transmitter of the sympathetic nervous system and is released from postganglionic sympathetic nerve terminals to act on specific target organs. It is also a CNS neurotransmitter, released from the locus coeruleus. Epinephrine is released from the adrenal medulla into the bloodstream as a hormone, producing widespread systemic effects. Epinephrine has relatively greater effects on heart rate and metabolic responses; norepinephrine has relatively greater vasoconstrictive effects. In the brain, it is norepinephrine (not epinephrine) that is the primary neuromodulator.
How is norepinephrine involved in attention and ADHD?+
Norepinephrine, along with dopamine, is a primary pharmacological target in ADHD treatment. In the prefrontal cortex, norepinephrine at α-2A receptors strengthens working memory networks by reducing 'noise' in PFC circuits. Dopamine at D1 receptors similarly strengthens task-relevant signals. ADHD may involve dysregulation of both systems in the PFC: insufficient NE/DA signalling impairs the PFC's ability to focus attention and regulate behaviour. Stimulants (methylphenidate, amphetamines) increase both dopamine and norepinephrine in the synapse. Atomoxetine (a selective norepinephrine reuptake inhibitor) and guanfacine (an α-2A agonist) treat ADHD via the noradrenergic pathway specifically, without direct dopaminergic effects.
Why is norepinephrine important in PTSD?+
Norepinephrine plays two important roles in PTSD. First, at the time of trauma exposure, massive norepinephrine release in the amygdala — via β-adrenergic receptors — strongly consolidates the emotional memory of the traumatic event, making it unusually vivid, persistent, and resistant to extinction. Second, in established PTSD, chronic hyperactivation of the LC-norepinephrine system is thought to produce the hyperarousal cluster of symptoms: exaggerated startle, hypervigilance, sleep disturbances, and emotional reactivity. Clinically, prazosin (an α-1 antagonist) reduces PTSD nightmares and sleep disturbance by dampening noradrenergic signalling during sleep; propranolol given shortly after trauma has been investigated as a way to reduce the emotional intensity of traumatic memories during the consolidation window.
What are SNRIs and how are they different from SSRIs?+
Selective serotonin-norepinephrine reuptake inhibitors (SNRIs — including venlafaxine, duloxetine, and desvenlafaxine) block both the serotonin transporter (SERT) and the norepinephrine transporter (NET), increasing synaptic levels of both monoamines. They differ from SSRIs, which block only SERT. SNRIs tend to be particularly effective for pain conditions (through norepinephrine modulation of descending pain pathways), for anxiety disorders, and for depression with prominent fatigue or cognitive symptoms. Their broader mechanism may provide a more complete antidepressant effect for some patients, though their side-effect profile differs from SSRIs — notably elevated blood pressure at higher doses from noradrenergic activation of peripheral vasculature. The noradrenergic component also makes them useful in ADHD treatment.
Sources
Last reviewed August 2025- 1.
Schildkraut J.J. (1965). The catecholamine hypothesis of affective disorders: A review of supporting evidence. American Journal of Psychiatry, 122(5), 509–522.
+About this source
Proposed the catecholamine hypothesis of depression — the foundational theoretical framework that drove antidepressant development for two decades.
- 2.
Aston-Jones G. & Cohen J.D. (2005). An integrative theory of locus coeruleus-norepinephrine function: Adaptive gain and optimal performance. Annual Review of Neuroscience, 28, 403–450.
+About this source
Proposed the adaptive gain theory — the leading contemporary framework for understanding how LC-NE activity shapes attention, arousal, and task performance.
- 3.
Pitman R.K., Sanders K.M., Zusman R.M., Healy A.R., Cheema F., Lasko N.B., Cahill L. & Orr S.P. (2002). Pilot study of secondary prevention of posttraumatic stress disorder with propranolol. Biological Psychiatry, 51(2), 189–192.
+About this source
Investigated propranolol administered after trauma to reduce PTSD symptoms by blocking noradrenergic consolidation of traumatic memory.
- 4.
Raskind M.A., Peskind E.R., Kanter E.D., Petrie E.C., Radant A., Thompson C.E., Dobie D.J., Hoff D., Rein R.J., Straits-Tröster K., Thomas R.G. & McFall M.M. (2003). Reduction of nightmares and other PTSD symptoms in combat veterans by prazosin: A placebo-controlled study. American Journal of Psychiatry, 160(2), 371–373.
+About this source
Established prazosin as an effective pharmacological treatment for PTSD nightmares via α-1 adrenergic receptor blockade.