Contents
Contributors
Search

1. The anatomy of the Neurohypophysis

In contrast to the anterior pituitary gland, the posterior pituitary is derived from the forebrain during development and is composed predominantly of neural tissue. The posterior pituitary lies below the hypothalamus, with which it forms a structural and functional unit: the neurohypophysis. The neurohypophysis consists of three parts: the supraoptic and paraventricular nucleii of the hypothalamus (containing the cell bodies of the magnocellular, neurosecretory neurones that synthesize and secrete VP and OT); the supraoptico-hypophyseal tract (which includes the axons of these neurones); and the posterior pituitary (where the axons terminate on capillaries of the inferior hypophyseal artery).

The supraoptic nucleus (SON) is situated along the proximal part of the optic tract. It consists of the cell bodies of discrete vasopressinergic and oxytotic magnocellular neurons projecting to the posterior pituitary along the supraoptico-hypophyseal tract. The paraventricular nucleus (PVN) also contains discrete vasopressinergic and oxytotic magnocellular neurons projecting to the posterior pituitary along the supraoptico-hypophyseal tract. The PVN contains additional, smaller parvicellular neurons projecting to the median eminence and additional extra-hypothalamic areas including forebrain, brain stem, and spinal cord. Some of these parvicellular neurons are vasopressinergic. A group of those projecting via the median eminence co-secrete VP and corticotrophin releasing hormone (CRH), and terminate in the hypophyseal-portal bed of the anterior pituitary. These neurons have a role in the regulation of adrenocorticotrophin (ACTH) release. A schematic overview of the anatomy of the neurohypophysis, together with the its major connections, is shown in Figure 1.

Figure 1. Schematic representation of the anatomy of the neurohypophysis, and it's major afferent and efferent connections.

The posterior pituitary receives an arterial blood supply from the inferior hypophyseal artery and the artery of the trabecula (a branch of the superior hypohyseal artery), derivatives of the internal carotid artery and its branches. The SON and PVN receive an arterial supply from the suprahypophyseal, anterior communicating, anterior cerebral, posterior communicating and posterior cerebral arteries, all derived from the circle of Willis. Venous drainage of the neurohyphysis is via the dural, cavernous and inferior petrosal sinuses.

2. Molecular-cell biology of Vasopressin and Oxytocin

VP is a 9-amino acid peptide, with a disulphide bridge between the cysteine residues at positions 1 and 6 (Figure 2). Most mammals have the amino-acid arginine at position 8, though in the Pig family arginine is substituted by lysine. The structure of OT differs from that of VP by only 2 amino acids: isoleucine for phenylalanine at position 3; and leucine for arginine at position 8. Non-mammalian species have a variety of peptides very similar to VP and OT, suggesting they derive from a common ancestral gene.

Figure 2. The structural and chemical characteristics of Vasopressin and Oxytocin. The cyclical peptides differ in only 2 amino acid positions. Both contain disulphide bridges between Cysteine residues at positions 1 and 6.

2.1. The Vasopressin-Neurophysin and Oxytocin-Neurophysin genes

The genes encoding VP and OT are in tandem array on chromosome 20 in Man, separated by 8 Kb of DNA. Each has 3 exons, and encodes a polypeptide precursor with a modular structure: an amino-terminal signal peptide; the VP or OT peptide; a hormone-specific mid-molecule peptide termed a neurophysin (NPI and NPII for OT and VP respectively); and a carboxyl-terminal peptide known as co-peptin (Figure 3). There is considerable homology between the NP sequences of the VP-NP and OT-NP genes, positions 10-74 of the NP sequences being highly conserved at the amino acid level.

Figure 3. Structural organization of the Vasopressin-neurophysin II gene, and processing of its product. The VP-NPII gene has 3 exons. Translation of the mRNA yields a larger preprohormone precursor, subsequently modified through substantial post-translational modification. The OT gene has a similar structure, and its product undergoes similar processing and post-translational modification. VP: Vasopressin; NPII : Neurophysin II.

Hypothalamic-specific expression of the VP gene is conferred through selective repressor elements within the structural gene and its 5' flanking sequence, while regulatory control of VP gene expression is mediated through positive and negative elements in the proximal promoter. Several transcription factors bind to these elements. AP1, AP2 and CREB stimulate VP gene expression. The glucocorticoid receptor (GR) represses expression (1, 2).

The human, rat and mouse OT promoters contain half oestrogen-response elements, and IL-6 response elements. To date, the functional significance of these remains unclear (3).

2.2. Synthesis, release, and metabolism of Vasopressin and Oxytocin

Synthesis of the VP and OT precursors occurs in the cell bodies of specific magnocellular neurosecretory neurons within the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus. Generation of the mature hormone entails post-translational modification of the large primary precursor (Figure 4). Following translation, the carboxyl terminal domain of the precursor is glycosylated, and the product packaged in vesicles of the regulated secretory pathway. These migrate along the axons of the magnocellular neurons, during which the precursor is cleaved by basic endopeptidases into the mature hormone and the associated NP. These are stored in secretory granules within the terminals of the magnocellular neurones in the posterior pituitary. Increased firing frequency of vasopressinergic and oxytotic neurons opens voltage-gated Ca2+ channels in these nerve terminals. This, in turn, leads to transient Ca2+ influx, fusion of the neurosecretory granules with the nerve terminal membrane, and release of the hormone and its NP into the systemic circulation in equimolar quantities. NPs act as carrier proteins for VP and OT during axonal migration, and appear to serve no other function.

Figure 4. Schematic overview of the post-translational processing of the Vasopressin-neurophysin II gene product. Sequential modification of the AVP-NPII preprohormone in endoplasmic reticulum and golgi lead to trafficking through the regulated secretory pathway and ultimately release from neurosecretory vesicles in the posterior pituitary. A small amount of partially processed precursor is released through the constitutive secretory pathway. OT is processed in a similar manner.

VP and OT circulate unbound to plasma proteins, though VP does bind to specific receptors on platelets. VP concentrations in platelet-rich plasma are 5-fold higher than in platelet-depleted plasma (4). VP and OT have short circulating half-lives of 5-15 minutes. Several endothelial and circulating endo- and amino-peptidases degrade the peptides. A specific placental cysteine amino-peptidase degrades VP and OT rapidly during pregnancy and the peri-partum period.

3. The physiology of Vasopressin

VP is a key component in the regulation of fluid and electrolyte balance, through direct effects on renal water handling. However, the physiology of VP has a wider context, encompassing roles in the integrated response to changes in cardiovascular status.

3.1. Vasopressin receptors

There are three distinct VP receptor (V-R) subtypes (Table 1). All have seven transmembrane spanning domains, and all are G protein coupled. However, they are encoded by different genes, and differ in tissue distribution, down-stream signal transduction and function. The human V2-R gene maps to Xq28. Interestingly, the V2-R is up-regulated by its ligand (5).

3.2. Vasopressin and renal water handling

Although VP has multiple actions, its principle physiological effect is in the regulation of water resorption in the distal nephron, the structure and transport processes of which allow the kidney to both concentrate and dilute urine in response to the prevailing circulating VP concentration. Active transport of solute out of the thick ascending loop of Henle generates an osmolar gradient in the renal interstitium, which increases from renal cortex to inner medulla, a gradient through which distal parts of the nephron pass en route to the collecting system. The presence of selective water channel proteins (aquaporins) in the wall of the distal nephron allows resorption of water from the duct lumen along an osmotic gradient, and excretion of concentrated urine.

Ten different human aquaporins (AQPs) have been identified (6). AQP1 is present in the apical and basolateral membranes of the proximal tubule and descending loop of Henle. It was thought that loss of function mutations in the AQP1 gene result in no clinical disturbances in water conservation though AQP1 may be important in situations in which the renal counter current exchange system is impaired. However, defects in renal water conservation have been described recently in an individual with loss of function mutation in AQP1 (7). AQP3 and AQP4 are constitutively expressed on the basolateral membrane of collecting duct cells, where they facilitate the movement of water from collecting duct cells into the interstitium. VP modulates expression of AQP3, but not AQP4. AQP2 is expressed on the luminal surface of collecting duct cells, and is responsible for water transport from the lumen of the nephron into collecting duct cells. Expression of AQP2 is VP-dependent. Generation of intracellular cAMP by ligand activation of the V2-R stimulates AQP2 gene expression through CRE and AP-1 elements in the AQP2 promoter (8). AQP2 functions as a homo-tetramer. In addition to effects on AQP2 gene expression, VP accelerates trafficking of pre-synthesized AQP2 protein from intracellular vesicles, and the assembly of functional water channels in luminal cell membranes (Figure 5).

Figure 5. Schematic outline of the regulation of renal water channel expression by Vasporessin. Binding of VP to cell-surface V2-Rs in the distal nephron triggers a signal transduction cascade leading to a biphasic increase in water channel expression on the luminal surface cell membrane. There is increased assembly of pre-synthesized AQP2 monomers to functional tetramers, and enhanced cell surface expression. In addition, AQP2 gene expression is stimulated.

VP has additional effects at other sites in the nephron: decreasing medullary blood flow; stimulating active urea transport in the distal collecting duct; and stimulating active sodium transport into the renal interstitium. These contribute to the generation and maintenance of a hypertonic medullary interstitium, and augment VP-dependent water resorption.

3.3. Regulation of Vasopressin release

3.3.1. Neurophysiology of VP release

VP is produced and regulated by the neurohypophysis, and is modulated by sensory signals chiefly reflecting osmotic status and blood pressure/circulating volume. The relationships of the SON and PVN with the autonomic afferents and central nervous system nuclei responsible for osmo- and baroregulation are key to the physiological regulation of VP.

Functional osmoreceptors are situated in anterior circumventricular structures: the subfornicular organ (SFO), and the organum vasculosum of the lamina terminalis (OVLT). Local fenestrations in the blood brain barrier at these sites allow neural tissue direct contact with the circulation. VP neurons themselves may also have independent osmoreceptor properties (9). V-Rs are present on vasopressinergic neurons of both the PVN and SON, highlighting the potential for autocontrol of VP release through the action of magnocellular neurites (10, 11). Thirst appreciation is dependent upon osmo-sensitive hypothalamic nuclei anatomically distinct from those regulating VP release.

Baroregulatory influences on VP release from the neurohypophysis derive from aortic arch, carotid sinus, cardiac atrial, and great vein afferents via cranial nerves IX and X, projecting to the nucleus tractus solitarius (NTS) in the brain stem. From the NTS, further afferents project to the SON and PVN. The SON and PVN receive additional adrenergic afferents from other brain stem nuclei involved in cardiovascular control, such as the locus coeruleus.

3.3.2. Osmoregulation of Vasopressin

Plasma osmolality is the most important determinant of VP secretion. The osmoregulatory systems for thirst and VP secretion, and in turn the actions of VP on renal water excretion, maintain plasma osmolality within narrow limits: 284 to 295 mOsmol/kg. The relationship between plasma osmolality and plasma VP concentration has 3 characteristics.

• The osmotic threshold or 'set point' for VP release.
• The shape of the line describing changes in plasma VP concentration with changing plasma osmolality
• The sensitivity of the osmoregulatory mechanism coupling plasma osmolality and VP release.
  

Increases in plasma osmolality increase plasma VP concentrations in a linear manner (Figure 6). The abscissal intercept of this line indicates the mean 'osmotic threshold' for VP release (284 mOsmol/kg): the mean plasma osmolality above which plasma VP increases in response to increases in plasma osmolality. There is no level of plasma osmolality below which VP release is truly completely suppressed. However, the concept of an osmolar threshold remains a practical tool with which to characterize the physiology of osmoregulation. VP levels increase from a basal rate through activation of stimulatory osmoreceptor afferents, and decrease to minimal values when this drive is removed and synergistic inhibitory afferents are activated. The slope of the line relating plasma osmolality to plasma VP concentration reflects the sensitivity of osmoregulated VP release. There are considerable inter-individual variations in both the threshold and sensitivity of VP release. However, they are remarkably reproducible within an individual over time (12).

Figure 6. The relationship of plasma VP concentration to changes in plasma osmolality during controlled hypertonic stimulation. VP concentration determined during progressive hypertonicity induced by infusion of 855 mmol/l saline in a group of healthy adults. Increases in plasma osmolality increase plasma VP concentrations in a linear manner, defined by the function, plasma VP = 0.43 (plasma osmolality - 284), r = +0.96. The abscissal intercept of this regression line indicates the mean 'osmotic threshold' for VP release: the mean plasma osmolality above which plasma VP starts to increase. The shaded area represents the range of normal response. LD represents the limit of detection of the assay, 0.3 pmol/l.

There are situations where the normal relationship between plasma osmolality and VP concentration breaks down.

• Rapid changes of plasma osmolality: rapid increases in plasma osmolality result in exaggerated VP release.
• During the act of drinking: drinking rapidly suppresses VP release, through afferent pathways originating in the oropharynx. 
• Pregnancy: the osmotic threshold for VP release is lowered in pregnancy.
• Aging: plasma VP concentrations increase with age, together with enhanced VP responses to osmotic stimulation.

Age-related changes in VP production can be accompanied by blunting of thirst appreciation, reduced fluid intake, decreased ability to excrete a free water load, and reduced renal concentrating capacity. These changes predispose the elderly to both hyper- and hyponatraemia.

3.3.3. Baroregulation of Vasopressin

Reductions in circulating volume stimulate VP release through activation of mechanoreceptors in the cardiac atria and central veins. Hypotension stimulates VP release independently through aortic arch and carotid sinus afferents. Falls in arterial blood pressure of 5 to 10 per cent are necessary to increase circulating VP concentrations in man. Progressive reduction in blood pressure produces an exponential increase in plasma VP, in contrast to the linear increases of osmoregulated VP release. Baroregulated VP responses can be modified by other neurohumoral influences triggered as part of the coordinated neurohumoral response to changes in circulating volume and blood pressure. Atrial natriuretic peptide (ANP) inhibits, while norepinephrine augments baroregulated VP release.

3.3.4. Additional mechanisms regulating Vasopressin release

A number of other stimuli influence VP release independent of osmotic and haemodynamic status.

• Nausea and emesis.
• Manipulation of abdominal contents.

Both may contribute to high plasma VP values observed after surgery.

4. Additional effects of Vasopressin

4.1. Cardiovascular Effects

VP is a potent pressor agent; its effects mediated through a specific receptor (V1-R) expressed by vascular smooth muscle cells. Though systemic effects on arterial blood pressure are only apparent at high concentrations, VP is important in maintaining blood pressure in mild volume depletion. The most striking vascular effects of VP are in the regulation of regional blood flow. The sensitivity of vascular smooth muscle to the pressor effects of VP varies according to the vascular bed. Vasoconstriction of splancnic, hepatic and renal vessels occur at VP concentrations close to the physiological range. Furthermore, there are differential pressor responses within a given vascular bed, selective effects on intrarenal vessels resulting in redistribution of renal blood flow from medulla to cortex. Baroregulated VP release thus constitutes one of the key physiological mediators of an integrated haemodynamic response to volume depletion.

4.2. Effects on the Pituitary

VP is an ACTH secretagogue, acting through pituitary corticotroph-specific V3-Rs. Though the effect is weak in isolation, VP and CRF act synergistically. VP and CRF co-localize in neurohypophyseal parvicellular neurons projecting to the median eminence and the neurohypophyseal portal blood supply of the anterior pituitary. Levels of both VP and CRF in these neurons are inversely related to glucocorticoid levels, consistent with a role in feedback regulation.

4.3. CNS and other miscellaneous effects of Vasopressin

Vasopressinergic fibres and V-Rs are present in many areas of the brain, including the cerebral cortex and limbic system. These extensive neural networks are anatomically and functionally independent of the neurohypophysis. The relevance of central vasopressinergic systems to the neurohypophyseal-systemic VP axis is unclear.

5. Thirst

Thirst and drinking are key processes in the maintenance of fluid and electrolyte balance. Thirst perception and the regulation of water ingestion involve complex, integrated neural and neurohumoral pathways. The osmoreceptors regulating thirst are situated in the circumventricular AV3V region of the hypothalamus, distinct from those mediating VP release (13). Projections to higher centers remain largely unmapped. There is a linear relationship between thirst and plasma osmolalities in the physiological range (Figure 7). The mean osmotic threshold for thirst perception is 281 mOsm/kg, similar to that for VP release. Thirst occurs when plasma osmolality rises above this threshold. As with osmoregulated VP release, the characteristics of osmoregulated thirst remain consistent within an individual on repeated testing, despite wide inter-individual variation (12).

Figure 7. The relationship of thirst to plasma osmolality during controlled hypertonic stimulation. Data obtained from analysis of thirst (by visual analogue scale) during progressive hypertonicity induced by infusion of 855 mmol/l saline in a group of healthy adults. There is a linear relationship between thirst and plasma osmolalities in the physiological range, defined by the function: thirst = 0.39 (plasma osmolality - 285), r = +0.95. The shaded area represents the range of normal response.

As with VP release, there are also specific physiological situations in which the relationship between plasma osmolality and thirst breaks down.

• The act of drinking: reduces osmotically stimulated thirst.
• Extracellular volume depletion: this stimulates thirst through volume-sensitive cardiac afferents and the generation of circulating and intracerebral Angiotensin II, a powerful dipsogen.
• Aging: both thirst appreciation and fluid intake can be blunted in the elderly

6. The integrated physiology of Vasopressin

As the major circulating cation, sodium concentration is rigorously maintained within the range of 135-144 mmols/l. Fluid volumes within the circulating, interstitial and intracellular compartments are also critical physiological parameters. The regulation of fluid and electrolyte balance is intimately linked with that of circulating volume; common systems are involved in both processes. The inter-relationships of sodium and water excretion with circulating volume regulation are key to appreciating the position of VP in the physiology of fluid homeostasis.

At plasma osmolalities of 285-295 mOsm/kg, osmolar balance can be maintained by VP-dependent regulation of renal water loss: a rise in plasma osmolality within this range producing a progressive increase in plasma VP and a resultant antidiuresis. Though further increases in plasma osmolality stimulate further VP release, this does reduce renal water excretion further: correction of plasma osmolality back to the range over which VP can maintain osmolar balance requires thirst-stimulated drinking. As the osmolar threshold for thirst is similar to that for VP release, the maintenance of water balance through a combination of VP release and thirst is a seamless, coordinated process.

If excessive fluid volumes are consumed, greater than those demanded by thirst, plasma VP levels are suppressed to < 0.3 pmol/l, resulting in maximum diuresis. Ingestion of water in excess of this causes a reduction of plasma osmolality into the sub-normal range, and hyponatraemia.

VP release is also regulated by other, non-osmotic stimuli (e.g. baroregulated VP release). This multi-component regulation has a hierarchy. Moderate hypovolaemia shifts the relationship of plasma osmolality and plasma VP concentration to the left; osmoregulation being maintained around a lower osmolar set point. As the degree of hypovolaemia progresses, baroregulated VP release overrides the osmolar set point. Antidiuresis is maintained, despite potential hyponatraemia, as circulating volume and blood pressure are supported through reduced urine losses and direct pressor effects. Coincident activation of the systemic and intra-cerebral Renin-Angiotensin systems stimulates drinking and augments VP release, in addition to producing independent pressor and anti-natriuretic effects. The physiological and pathophysiological responses to hypovolaemia thus involve an integrated neurohumoral cascade, of which VP is a key component.