Posterior Pituitary Function and Neurohypophyseal Hormones

The posterior pituitary, or neurohypophysis, is not a true gland but the storage and release terminal for two hormones, vasopressin and oxytocin, made by hypothalamic neurons. Their axons run into the gland and release the hormones directly into the bloodstream, a process called neurosecretion.

Definition

The posterior pituitary is the neural lobe of the pituitary in which axons of magnocellular hypothalamic neurons store and release the neurohypophyseal hormones vasopressin (antidiuretic hormone) and oxytocin directly into the systemic circulation.

Scope

The topic covers the neurosecretory organisation of the magnocellular hypothalamo-neurohypophyseal system, the synthesis and transport of vasopressin and oxytocin, their physiological actions in water balance and reproduction, and the stimuli that trigger their release. It is a physiology topic and not clinical guidance.

Core questions

• How does the posterior pituitary differ structurally from the anterior pituitary?
• Where are vasopressin and oxytocin synthesised and how do they reach the gland?
• What stimuli drive the release of each hormone?
• What are the principal physiological actions of vasopressin and oxytocin?

Key concepts

• Neurosecretion
• Magnocellular neurons (supraoptic and paraventricular nuclei)
• Vasopressin (antidiuretic hormone, ADH)
• Oxytocin
• Osmoreceptor control of vasopressin
• Axonal transport and Herring bodies
• Neurophysin carrier proteins
• Neuroendocrine reflex (milk ejection, parturition)

Mechanisms

Magnocellular neurons in the supraoptic and paraventricular nuclei of the hypothalamus synthesise vasopressin and oxytocin as precursor proteins, package them with their neurophysin carriers, and transport them down long axons that terminate in the posterior pituitary. When the neurons fire, action potentials trigger calcium-dependent exocytosis of the stored hormone into fenestrated capillaries, releasing it into the bloodstream. Vasopressin secretion is driven mainly by increased plasma osmolality (sensed by hypothalamic osmoreceptors) and by falls in blood volume or pressure; it acts on the kidney to promote water reabsorption. Oxytocin is released by neuroendocrine reflexes such as suckling and cervical stretch, and it acts on the breast to cause milk ejection and on the uterus to promote contraction.

Clinical relevance

Vasopressin is central to the body's regulation of water balance and plasma osmolality, and oxytocin to lactation and parturition, so their physiology underlies the interpretation of fluid and reproductive function across the health sciences. Understanding that these hormones are released by neurosecretion explains why posterior pituitary function depends on intact hypothalamic neurons and their axons. This entry describes physiology and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

The neurosecretory model summarised here is grounded in reviews of the magnocellular hypothalamo-neurohypophyseal system and of vasopressin and oxytocin physiology. The concept of neurosecretion, that neurons can release hormones into the blood, was developed in the mid-twentieth century and remains the organising principle for this lobe.

History

The idea that hypothalamic neurons could secrete hormones, advanced by Ernst and Berta Scharrer, established the field of neurosecretion and explained how the posterior pituitary works. Vincent du Vigneaud's isolation and synthesis of oxytocin and vasopressin in the 1950s, recognised with a Nobel Prize, identified the two neurohypophyseal hormones as closely related nine-amino-acid peptides.

Key figures

• Ernst Scharrer
• Berta Scharrer
• Vincent du Vigneaud
• Mariana Morris

Seminal works

• burbach-2001
• lee-2009

Frequently asked questions

Does the posterior pituitary make its own hormones?

No. Vasopressin and oxytocin are synthesised in hypothalamic neurons; the posterior pituitary only stores them and releases them into the blood. It is neural tissue, not a secretory gland in the usual sense.

What is the main stimulus for vasopressin release?

A rise in plasma osmolality detected by hypothalamic osmoreceptors is the primary stimulus; a significant fall in blood volume or pressure also triggers release.