What does the Neurohypophysis do?
The two most important products of the neurohypophysis are the hormones vasopressin and oxytocin. The hormones are produced by the magnocellular neurones originating at the supraoptic and paraventricular nuclei. The cell bodies of these neurones can produce either vasopressin or oxytocin.
Vasopressin
There are two different types of vasopressin produced by animals. The vasopressin that is found in humans contains the amino-acid arginine and is referred to as arginine vasopressin. The other type of vasopressin contains lysine and is found in pigs and is referred to as lysine vasopressin. Some of the paraventricular vasopressinergic neurones project to various parts of the brain where it acts as a neurotransmitter. Vasopressin has been identified in many parts of the body including the adrenals, the gonads, the pancreas and the sympathetic ganglia.
In order to fully understand the different actions of vasopressin, it is important to realise that there are two very distinctive types of vasopressin receptors. The most important of these receptors is the v2 receptor. The v2 receptor is found in the collecting ducts of the kidney where it plays a vital role in the reabsorption of water in the kidney. The vasopressin acts on the v2 receptors in the collecting ducts and stimulates the up-regulation of water channels (aquaporins) in the membrane. When there are high levels of vasopressin in the blood the volume of urine produced will be small and highly concentrated. It is for this reason that vasopressin is often referred to as 'anti-diuretic hormone'.
The v1 receptors are found in many other tissues and have a variety of different functions. These other effects are listed below.
- Vasoconstriction (especially in the skin and splanchnic bed) mediated via the v1 receptors
- Increases sodium reabsorption in the kidney
- Stimulates corticotrophin release from the adenohypophysis
- Involved in behaviour and memory
- Stimulation of hepatic glycogenolysis
- Stimulation of the synthesis of factor VIII in the liver.
The most important control mechanism for vasopressin synthesis and release is the plasma osmolality. An increase in osmolality will result in an increase in vasopressin release which acts to decrease osmolality. The increase in osmolality is detected by specialised cells known as osmoreceptors, which in turn stimulate the vasopressinergic neurones. Another important mechanism involved in the control of vasopressin release are volume receptors. There are two main types of volume receptors:
1) Low pressure receptors - these are found mainly in the left atrium of the heart and the walls of the main veins.
2) High pressure receptors (baroreceptors) - these are found in the carotid sinus and the aortic arch.
A fall in blood volume will result in the activation of these receptors. These receptors usually tonically inhibit the vasopressinergic neurones and so a fall in blood volume will result in a decrease of the usual inhibition of vasopressin release.
Higher centres of the brain can also exert an effect on vasopressin release so that massive release of vasopressin can occur in times of emotion and physical stress to the body.
Oxytocin
This is the other important hormone produced by the posterior pituitary. The hormone is released from the magnocellular neurones originating in the cell bodies of the supraoptic and paraventricular nuclei in the hypothalamus. Oxytocin is produced in both males and females, but its main physiological roles seem to take place in the female. In the female, oxytocin is involved in a number of important physiological actions:
- Stimulates the contraction of the uterus (myometrium)
- Stimulates the contraction of the myoepithelial cells that eject milk from the breast.
The hormone has also been implicated in parturition as it is a useful therapeutic agent used to induce labour. The role of oxytocin in the male is not fully understood but it may be involved in ejaculation.
The most important control mechanism for the release of oxytocin is the suckling of the nipple in the newborn. This stimulation results in the activation of afferent nerves that stimulate the oxytocinergic neurones in the hypothalamus.