Supraoptic nucleus
Encyclopedia
The supraoptic nucleus is a nucleus
of magnocellular neurosecretory cell
s in the hypothalamus
of the mammalian brain. The nucleus is situated at the base of the brain, adjacent to the optic chiasm
. In humans, it contains about 3,000 neurons.
hormone
: anti-diuretic hormone
.
In the cell bodies, the hormones are packaged in large, membrane-bound vesicles which are transported down the axon
s to the nerve endings. The secretory granules are also stored in packets along the axon called Herring bodies
.
Similar magnocellular neurons are also found in the paraventricular nucleus
.
that projects to the posterior pituitary gland, where it gives rise to about 10,000 neurosecretory nerve terminals. The magnocellular neurons are electrically excitable: In response to afferent stimuli from other neurons, they generate action potentials which propagate down the axons. When an action potential invades a neurosecretory terminal, the terminal is depolarised, and calcium enters the terminal through voltage-gated channels. The calcium entry triggers the secretion of some of the vesicles by a process known as exocytosis
. The vesicle contents are released into the extracellular space, from where they diffuse into the bloodstream.
is released in response to solute concentration in the blood, decreased blood volume or blood pressure. It also causes vasoconstriction that helps elevate blood pressure.
Some other inputs come from the brainstem, including from some of the noradrenergic neurons of the nucleus of the solitary tract and the ventrolateral medulla
. However many of the direct inputs to the supraoptic nucleus come from neurons just outside the nucleus (the "perinuclear zone"). Oxytocin neurons respond to stimulation of the nipples (resulting in milk let-down) and in response to uterine contractions and distension of the birth canal (the "Ferguson reflex
"), but the pathways by which these stimuli reach the neurons are not fully known.
Of the afferent inputs to the supraoptic nucleus, most contain either the inhibitory neurotransmitter GABA
or the excitatory neurotransmitter glutamate, but these transmitters often co-exist with various peptides. Other afferent neurotransmitters include noradrenaline (from the brainstem), dopamine, serotonin and acetylcholine.
For example, during lactation
there are large changes in the size and shape of the oxytocin neurons, in the numbers and types of synapses that these neurons receive, and in the structural relationships between neurons and glial cells in the nucleus. These changes arise during parturition, and are thought to be important adaptations that prepare the oxytocin neurons for a sustained high demand for oxytocin. Oxytocin is essential for milk let-down in response to suckling.
These studies showed that the brain was much more "plastic" in its anatomy than previously recognized, and led to great interest in the interactions between glial cells and neurons generally.
It seemed strange that the vasopressin
cells should fire in bursts. As the activity of the vasopressin cells is not synchronised, the overall level of vasopressin secretion into the blood is continuous, not pulsatile. Richard Dyball and his co-workers speculated that this pattern of activity, called "phasic firing", might be particularly effective for causing vasopressin secretion. They showed this to be the case by studying vasopressin secretion from the isolated posterior pituitary gland in vitro. They found that vasopressin secretion could be evoked by electrical stimulus pulses applied to the gland, and that much more hormone was released by a phasic pattern of stimulation than by a continuous pattern of stimulation.
These experiments led to interest in "stimulus-secretion coupling" - the relationship between electrical activity and secretion. Supraoptic neurons are unusual because of the large amounts of peptide that they secrete, and because they secrete the peptides into the blood. However many neurons in the brain, and especially in the hypothalamus, synthesize peptides. It is now thought that bursts of electrical activity might be generally important for releasing large amounts of peptide from peptide-secreting neurons.
does not allow oxytocin and vasopressin through, but the oxytocin and vasopressin that is released from dendrites acts within the brain. Oxytocin neurons themselves express oxytocin receptors, and vasopressin neurons express vasopressin receptors, so dendritically-released peptides "autoregulate" the supraoptic neurons. Francoise Moos and Phillipe Richard first showed that the autoregulatory action of oxytocin is important for the milk-ejection reflex.
These peptides have relatively long half-lives in the brain (about 20 minutes in the CSF), and they are released in large amounts in the supraoptic nucleus, and so they are available to diffuse through the extracellular spaces of the brain to act at distant targets. Oxytocin and vasopressin receptors are present in many other brain regions, including the amygdala
, brainstem, septum
, and most other nuclei in the hypothalamus.
Because so much vasopressin and oxytocin are released at this site, studies of the supraoptic nucleus have made an important contribution to understanding how release from dendrites is regulated, and in understanding its physiological significance.
produced by vasopressin neurons is involved in regulating the phasic discharge patterning of vasopressin neurons, and nitric oxide
produced by both neuronal types is a negative-feedback regulator of cell activity. Oxytocin neurons also make dynorphin; in these neurons, dynorphin acts at the nerve terminals in the posterior pituitary as a negative feedback inhibitor of oxytocin secretion. Oxytocin neurons also make large amounts of cholecystokinin
and cocaine-and amphetamine regulatory transcript (CART).
Nucleus (neuroanatomy)
In neuroanatomy, a nucleus is a brain structure consisting of a relatively compact cluster of neurons. It is one of the two most common forms of nerve cell organization, the other being layered structures such as the cerebral cortex or cerebellar cortex. In anatomical sections, a nucleus shows up...
of magnocellular neurosecretory cell
Magnocellular neurosecretory cell
Magnocellular neurosecretory cells are large cells within the supraoptic nucleus and paraventricular nucleus of the hypothalamus. They are also found in smaller numbers in accessory cell groups between these two nuclei, the largest one being the nucleus circularis...
s in the hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
of the mammalian brain. The nucleus is situated at the base of the brain, adjacent to the optic chiasm
Optic chiasm
The optic chiasm or optic chiasma is the part of the brain where the optic nerves partially cross...
. In humans, it contains about 3,000 neurons.
Function
The cell bodies produce a peptidePeptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
: anti-diuretic hormone
Vasopressin
Arginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...
.
In the cell bodies, the hormones are packaged in large, membrane-bound vesicles which are transported down the axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
s to the nerve endings. The secretory granules are also stored in packets along the axon called Herring bodies
Herring bodies
Herring bodies or neurosecretory bodies are structures found in the posterior pituitary. They represent the terminal end of the axons from the hypothalamus, and hormones are temporarily stored in these locations.They are neurosecretory terminals....
.
Similar magnocellular neurons are also found in the paraventricular nucleus
Paraventricular nucleus
The paraventricular nucleus is a neuronal nucleus in the hypothalamus. It contains multiple subpopulations of neurons that are activated by a variety of stressful and/or physiological changes. Many PVN neurons project directly to the posterior pituitary where they release oxytocin or vasopressin...
.
Signaling
Each neuron in the nucleus has one long axonAxon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
that projects to the posterior pituitary gland, where it gives rise to about 10,000 neurosecretory nerve terminals. The magnocellular neurons are electrically excitable: In response to afferent stimuli from other neurons, they generate action potentials which propagate down the axons. When an action potential invades a neurosecretory terminal, the terminal is depolarised, and calcium enters the terminal through voltage-gated channels. The calcium entry triggers the secretion of some of the vesicles by a process known as exocytosis
Exocytosis
Exocytosis , also known as 'The peni-cytosis', is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane...
. The vesicle contents are released into the extracellular space, from where they diffuse into the bloodstream.
Regulation of supraoptic neurons
The antidiuretic hormone (ADH)Vasopressin
Arginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...
is released in response to solute concentration in the blood, decreased blood volume or blood pressure. It also causes vasoconstriction that helps elevate blood pressure.
Some other inputs come from the brainstem, including from some of the noradrenergic neurons of the nucleus of the solitary tract and the ventrolateral medulla
Medulla
Medulla refers to the middle of something and derives from the Latin word for marrow. Its anatomical uses include:* Medulla oblongata, a part of the brain stem* Renal medulla, a part of the kidney* Adrenal medulla, a part of the adrenal gland...
. However many of the direct inputs to the supraoptic nucleus come from neurons just outside the nucleus (the "perinuclear zone"). Oxytocin neurons respond to stimulation of the nipples (resulting in milk let-down) and in response to uterine contractions and distension of the birth canal (the "Ferguson reflex
Ferguson reflex
The Ferguson reflex is an example of positive feedback and the female body's response to pressure application in the cervix or vaginal walls.Upon application of pressure, oxytocin is released and uterine contractions are stimulated , until the baby is delivered...
"), but the pathways by which these stimuli reach the neurons are not fully known.
Of the afferent inputs to the supraoptic nucleus, most contain either the inhibitory neurotransmitter GABA
Gabâ
Gabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions...
or the excitatory neurotransmitter glutamate, but these transmitters often co-exist with various peptides. Other afferent neurotransmitters include noradrenaline (from the brainstem), dopamine, serotonin and acetylcholine.
The supraoptic nucleus as a "model system"
The supraoptic nucleus is an important "model system" in neuroscience. There are many reasons for this: some technical advantages of working on the supraoptic nucleus are that the cell bodies are relatively large, the cells make exceptionally large amounts of their secretory products, and the nucleus is relatively homogeneous and easy to separate from other brain regions. The gene expression and electrical activity of supraoptic neurons has been studied extensively, in many physiological and experimental conditions. These studies have led to many insights of general importance, as in the examples below.Morphological plasticity in the supraoptic nucleus
Anatomical studies using electron microscopy have shown that the morphology of the supraoptic nucleus is remarkably adaptable.For example, during lactation
Lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...
there are large changes in the size and shape of the oxytocin neurons, in the numbers and types of synapses that these neurons receive, and in the structural relationships between neurons and glial cells in the nucleus. These changes arise during parturition, and are thought to be important adaptations that prepare the oxytocin neurons for a sustained high demand for oxytocin. Oxytocin is essential for milk let-down in response to suckling.
These studies showed that the brain was much more "plastic" in its anatomy than previously recognized, and led to great interest in the interactions between glial cells and neurons generally.
Stimulus-secretion coupling
In response to, for instance, a rise in the plasma sodium concentration, vasopressin neurons also discharge action potentials in bursts, but these bursts are much longer and are less intense than the bursts displayed by oxytocin neurons, and the bursts in vasopressin cells are not synchronised.It seemed strange that the vasopressin
Vasopressin
Arginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...
cells should fire in bursts. As the activity of the vasopressin cells is not synchronised, the overall level of vasopressin secretion into the blood is continuous, not pulsatile. Richard Dyball and his co-workers speculated that this pattern of activity, called "phasic firing", might be particularly effective for causing vasopressin secretion. They showed this to be the case by studying vasopressin secretion from the isolated posterior pituitary gland in vitro. They found that vasopressin secretion could be evoked by electrical stimulus pulses applied to the gland, and that much more hormone was released by a phasic pattern of stimulation than by a continuous pattern of stimulation.
These experiments led to interest in "stimulus-secretion coupling" - the relationship between electrical activity and secretion. Supraoptic neurons are unusual because of the large amounts of peptide that they secrete, and because they secrete the peptides into the blood. However many neurons in the brain, and especially in the hypothalamus, synthesize peptides. It is now thought that bursts of electrical activity might be generally important for releasing large amounts of peptide from peptide-secreting neurons.
Dendritic secretion
Supraoptic neurons have typically 1-3 large dendrites, most of which project ventrally to form a mat of process at the base of the nucleus, called the ventral glial lamina. The dendrites receive most of the synaptic terminals from afferent neurons that regulate the supraoptic neurons, but neuronal dendrites are often actively involved in information processing, rather than being simply passive receivers of information. The dendrites of supraoptic neurons contain large numbers of neurosecretory vesicles that contain oxytocin and vasopressin, and they can be released from the dendrites by exocytosis. The oxytocin and vasopressin that is released at the posterior pituitary gland enters the blood, and cannot re-enter the brain because the blood-brain barrierBlood-brain barrier
The blood–brain barrier is a separation of circulating blood and the brain extracellular fluid in the central nervous system . It occurs along all capillaries and consists of tight junctions around the capillaries that do not exist in normal circulation. Endothelial cells restrict the diffusion...
does not allow oxytocin and vasopressin through, but the oxytocin and vasopressin that is released from dendrites acts within the brain. Oxytocin neurons themselves express oxytocin receptors, and vasopressin neurons express vasopressin receptors, so dendritically-released peptides "autoregulate" the supraoptic neurons. Francoise Moos and Phillipe Richard first showed that the autoregulatory action of oxytocin is important for the milk-ejection reflex.
These peptides have relatively long half-lives in the brain (about 20 minutes in the CSF), and they are released in large amounts in the supraoptic nucleus, and so they are available to diffuse through the extracellular spaces of the brain to act at distant targets. Oxytocin and vasopressin receptors are present in many other brain regions, including the amygdala
Amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...
, brainstem, septum
Septum
In anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.-In human anatomy:...
, and most other nuclei in the hypothalamus.
Because so much vasopressin and oxytocin are released at this site, studies of the supraoptic nucleus have made an important contribution to understanding how release from dendrites is regulated, and in understanding its physiological significance.
Co-existing peptides
Vasopressin neurons and oxytocin neurons make many other neuroactive substances in addition to vasopressin and oxytocin, though most are present only in small quantities. However, some of these other substances are known to be important. DynorphinDynorphin
Dynorphins are a class of opioid peptides that arise from the precursor protein prodynorphin. When prodynorphin is cleaved during processing by proprotein convertase 2 , multiple active peptides are released: dynorphin A, dynorphin B, and α/β-neo-endorphin...
produced by vasopressin neurons is involved in regulating the phasic discharge patterning of vasopressin neurons, and nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
produced by both neuronal types is a negative-feedback regulator of cell activity. Oxytocin neurons also make dynorphin; in these neurons, dynorphin acts at the nerve terminals in the posterior pituitary as a negative feedback inhibitor of oxytocin secretion. Oxytocin neurons also make large amounts of cholecystokinin
Cholecystokinin
Cholecystokinin is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein...
and cocaine-and amphetamine regulatory transcript (CART).