- 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.