What Is The Locus Coeruleus?

The locus coeruleus is a nucleus in the pons (part of the brainstem) involved with physiological responses to stress and panic.

The locus coeruleus is the principal site for brain synthesis of norepinephrine (noradrenaline). The locus coeruleus and the areas of the body affected by the norepinephrine it produces are described collectively as the locus coeruleus-noradrenergic system or LC-NA system. Norepinephrine may also be released directly into the blood from the adrenal medulla.

Anatomy

Rhomboid fossa

Rhomboid fossa. (Locus coeruleus not labeled, but is very near [just lateral to] colliculus facialis, which is labeled at center left.) Gray’s Anatomy, Plate 709.

The locus coeruleus (or “LC”) is located in the posterior area of the rostral pons in the lateral floor of the fourth ventricle. It is composed of mostly medium-size neurons. Melanin granules inside the neurons of the LC contribute to its blue color.

Thus, it is also known as the nucleus pigmentosus pontis, meaning “heavily pigmented nucleus of the pons.” The neuromelanin is formed by the polymerization of noradrenaline and is analogous to the black dopamine-based neuromelanin in the substantia nigra.

In adult humans (19-78) the locus coeruleus has 22,000 to 51,000 total pigmented neurons that range in size between 31,000 and 60,000 μm3.

Connections

The projections of this nucleus reach far and wide. For example, they innervate the spinal cord, the brain stem, cerebellum, hypothalamus, the thalamic relay nuclei, the amygdala, the basal telencephalon, and the cortex. The norepinephrine from the LC has an excitatory effect on most of the brain, mediating arousal and priming the brain’s neurons to be activated by stimuli.

As an important homeostatic control center of the body, the locus coeruleus receives afferents from the hypothalamus. The cingulate gyrus and the amygdala also innervate the LC, allowing emotional pain and stressors to trigger noradrenergic responses. The cerebellum and afferents from the raphe nuclei also project to the LC, in particular the nucleus raphes pontis and nucleus raphes dorsalis.

Inputs

The locus coeruleus receives inputs from a number of other brain regions, primarily:

  • Medial prefrontal cortex, whose connection is constant, excitatory, and increases in strength with raised activity levels in the subject
  • Nucleus paragigantocellularis, which integrates autonomic and environmental stimuli
  • Nucleus praepositus hypoglossi, which is involved in gaze
  • Lateral hypothalamus, which releases orexin, which, as well as its other functions, is excitatory in the locus coeruleus.

Outputs

The projections from the locus coeruleus consist of neurons that utilize norepinephrine as their primary neurotransmitter. These projections include the following connections:

LC → Amygdala & Hippocampus
LC → Brain stem & Spinal cord
LC → Cerebellum
LC → Cerebral cortex
LC → Hypothalamus
LC → Tectum
LC → Thalamus
LC → Ventral tegmental area

Function

It is related to many functions via its widespread projections. The LC-NA system modulates cortical, subcortical, cerebellar, brainstem, and spinal cord circuits. Some of the most important functions influenced by this system are:

The locus coeruleus is a part of the ascending reticular activating system, and is almost completely inactivated in rapid eye movement sleep.

Pathophysiology

The locus coeruleus may figure in clinical depression, panic disorder, Parkinson’s disease, Alzheimer’s disease and anxiety. Some medications including norepinephrine reuptake inhibitors (reboxetine, atomoxetine), serotonin-norepinephrine reuptake inhibitors (venlafaxine, duloxetine), and norepinephrine-dopamine reuptake inhibitors (bupropion) are believed to show efficacy by acting upon neurons in this area.

Research continues to reveal that norepinephrine (NE) is a critical regulator of numerous activities from stress response, the formation of memory to attention and arousal. Many neuropsychiatric disorders precipitate from alterations to NE modulated neurocircuitry: disorders of affect, anxiety disorders, PTSD, ADHD and Alzheimer’s disease.

Alterations in the locus coeruleus (LC) accompany dysregulation of NE function and likely play a key role in the pathophysiology of these neuropsychiatric disorders.

Stress

The locus coeruleus is responsible for mediating many of the sympathetic effects during stress. The locus coeruleus is activated by stress, and will respond by increasing norepinephrine secretion, which in turn will alter cognitive function (through the prefrontal cortex), increase motivation (through nucleus accumbens), activate the hypothalamic-pituitary-adrenal axis, and increase the sympathetic discharge/inhibit parasympathetic tone (through the brainstem).

Specific to the activation of the hypothalamo-pituitary adrenal axis, norepinephrine will stimulate the secretion of corticotropin-releasing factor from the hypothalamus, that induces adrenocorticotropic hormone release from the anterior pituitary and subsequent cortisol synthesis in the adrenal glands. Norepinephrine released from locus coeruleus will feedback to inhibit its production, and corticotropin-releasing hormone will feedback to inhibit its production, while positively feeding to the locus coeruleus to increase norepinephrine production.

The LC’s role in cognitive function in relation to stress is complex and multi-modal. Norepinephrine released from the LC can act on α2 receptors to increase working memory, or an excess of NE may decrease working memory by binding to the lower-affinity α1 receptors.

Psychiatric research has documented that enhanced noradrenergic postsynaptic responsiveness in the neuronal pathway (brain circuit) that originates in the locus coeruleus and ends in the basolateral nucleus of the amygdala is a major factor in the pathophysiology of most stress-induced fear-circuitry disorders and especially in posttraumatic stress disorder (PTSD).

The LC neurons are probably the origin of the first or second “leg” of the “PTSD circuit.” An important 2005 study of deceased American army veterans from World War II has shown combat-related PTSD to be associated with a postmortem-diminished number of neurons in the locus coeruleus (LC) on the right side of the brain.

Rett Syndrome

The genetic defect of the transcriptional regulator MECP2 is responsible for Rett syndrome. A MECP2 deficiency has been associated to catecholaminergic dysfunctions related to autonomic and sympathoadrenergic system in mouse models of Rett Syndrome (RTT).

The Locus Coeruleus is the major source of noradrenergic innervation in the brain and sends widespread connections to rostral (cerebral cortex, hippocampus, hypothalamus) and caudal (cerebellum, brainstem nuclei) brain areas. Indeed, an alteration of this structure could contribute to several symptoms observed in MECP2-deficient mice.

Changes in the electrophysiological properties of cells in the locus ceruleus were shown. These Locus Coeruleus cell changes include hyperexcitability and decreased functioning of its noradrenergic innervation.

It is interesting to note that a reduction of the tyrosine hydroxylase (TH) mRNA level, the rate-limiting enzyme in catecholamine synthesis, was detected in the whole pons of MECP2-null male as well as in adult heterozygous female mice. Using immunoquantification techniques, a decrease of TH protein staining level, number of locus coeruleus TH-expressing neurons and density of dendritic arborization surrounding the structure was shown in symptomatic MECP2-deficient mice. However, locus coeruleus cells are not dying but are more likely losing their fully mature phenotype, since no apoptotic neurons in the pons were detected.

Researchers have concluded that,

“Because these neurons are a pivotal source of norepinephrine throughout the brainstem and forebrain and are involved in the regulation of diverse functions disrupted in Rett Syndrome, such as respiration and cognition, we hypothesize that the locus ceruleus is a critical site at which loss of MECP2 results in CNS dysfunction. Restoration of normal locus ceruleus function may therefore be of potential therapeutic value in the treatment of Rett Syndrome.”

This could explain why a norepinephrine reuptake inhibitor (desipramine, DMI), which enhances the extracellular NE levels at all noradrenergic synapses, ameliorated some Rett syndrome symptoms in a mouse model of Rett syndrome.

Neurodegenerative Diseases

The locus ceruleus is affected in many forms of neurodegenerative diseases: genetic and idiopathic Parkinson’s disease, progressive supranuclear palsy, Pick’s disease or Alzheimer’s disease. It is also affected in Down syndrome. For example, there is up to 80% loss of locus ceruleus neurons in Alzheimer’s disease.

Mouse models of Alzheimer’s disease show accelerated progression after chemical destruction of the locus ceruleus. The norepinephrine from locus ceruleus cells in addition to its neurotransmitter role locally defuses from “varicosities”. As such it provides an endogenous anti-inflammatory agent in the microenvironment around the neurons, glial cells, and blood vessels in the neocortex and hippocampus.

It has been shown that norepinephrine stimulates mouse microglia to suppress Aβ-induced production of cytokines and their phagocytosis of Aβ. This suggests that degeneration of the locus ceruleus might be responsible for increased Aβ deposition in AD brains. Degeneration of pigmented neurons in this region in Alzheimer’s and Parkinson’s disease can be visualized in vivo with Neuromelanin MRI.

Eric Nestler, Steven Hyman, Robert Malenka
Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (3rd ed.)
New York: McGraw-Hill Medical ISBN: 978-0071827690

Top Image: High magnification micrograph of the locus ceruleus. Credit: Nephron, CC BY-SA 3.0