VASOPRESSIN

'Arginine vasopressin' ('AVP'), also known as 'argipressin' or 'antidiuretic hormone' ('ADH'), is a hormone found in most mammals, including man. One of its most important roles is to regulate the body's retention of water, being released when the body is dehydrated; it causes the kidneys to conserve water, but not salt, by concentrating the urine and reducing urine volume. It also raises blood pressure by inducing moderate vasoconstriction. In addition, it has a variety of neurological effects on the brain, having been found, for example, to influence pair-bonding in voles.
A very similar substance, ''lysine vasopressin'' ('LVP') or 'lypressin', has the same function in pigs and is often used in human therapy.
Vasopressin is a peptide hormone. It is derived from a preprohormone precursor that is synthesized in the hypothalamus, from which it is liberated during transport to the posterior pituitary. Most of it is stored in the posterior part of the pituitary gland to be released into the blood stream; some of it is also released directly into the brain.

Physiology

Action

Kidney

ADH increases the permeability of the collecting duct to water and thus allows water reabsorption and excretion of a small volume of concentrated urine - antidiuresis. This occurs through insertion of additional water channels into the apical membrane of the duct epithelial cells.
ADH also increases permeability of the medullary portion of the collecting duct to urea, allowing increased reabsorption of urea into the medullary interstitium, down the concentration gradient created from the removal of water in the cortical collecting duct.

Cardiovascular system

Vasopressin, as the name tells, increases the resistance of the peripheral vessles and thus increases arterial blood pressure. This effect is almost inexistant in healthy individuals, however it becomes an important compensatory mechanism for restoring blood pressure in hypovolemic shock such as occurs during hemorrage.

Central nervous system (CNS)

Vasopressin released within the brain has many actions:

★ It has been implicated in memory formation, including delayed reflexes, image, short- and long-term memory, though the mechanism remains unknown, and these findings are controversial. However, the synthetic vasopressin analogue desmopressin has come to interest as a likely nootropic.

★ Vasopressin is released into the brain in a circadian rhythm by neurons of the suprachiasmatic nucleus of the hypothalamus.

★ Vasopressin released from centrally-projecting hypothalamic neurons is involved in aggression, blood pressure regulation and temperature regulation.
In recent years there has been particular interest in the role of vasopressin in social behavior. It is thought that vasopressin, released into the brain during sexual activity, initiates and sustains patterns of activity that support the pair-bond between the sexual partners; in particular, vasopressin seems to induce the male to become aggressive towards other males.
Evidence for this comes from experimental studies in several species, which indicate that the precise distribution of vasopressin and vasopressin receptors in the brain is associated with species-typical patterns of social behavior. In particular, there are consistent differences between monogamous species and promiscuous species in the distribution of vasopressin receptors, and sometimes in the distribution of vasopressin-containing axons, even when closely-related species are compared. Moreover, studies involving either injecting vasopressin agonists into the brain, or blocking the actions of vasopressin, support the hypothesis that vasopressin is involved in aggression towards other males. There is also evidence that differences in the vasopressin receptor gene between individual members of a species might be predictive of differences in social behavior.

Control

Vasopressin is secreted from the posterior pituitary gland in response to reductions in plasma volume and in response to increases in the plasma osmolality:

★ Secretion ''in response to reduced plasma volume'' is activated by pressure receptors in the veins, atria, and carotids.

★ Secretion ''in response to increases in plasma osmotic pressure'' is mediated by osmoreceptors in the hypothalamus.
The neurons that make vasopressin, in the supraoptic nucleus and paraventricular nucleus, are themselves osmoreceptors, but they also receive synaptic input from other osmoreceptors located in regions adjacent to the anterior wall of the third ventricle. These regions include the organum vasculosum of the lamina terminalis and the subfornical organ.
Many factors influence the secretion of vasopressin:

★ Ethanol and caffeine ''reduce'' vasopressin secretion. The resulting decrease in water reabsorption by the kidneys leads to a higher urine output. Coffee is an example of a food product that suppresses the body's release of antidiuretic hormones, due to its level of caffeine. This intake of caffeine causes the body to lose more water and may lead to dehydration if consumed excessively.

★ Angiotensin II ''stimulates'' the secretion of vasopressin.^[1]

Secretion

The main stimulus for secretion of vasopressin is increased osmolarity of plasma. Reduced volume of extracellular fluid also has this effect, but is a less sensitive mechanism.
The vasopressin that is measured in peripheral blood is almost all derived from secretion from the posterior pituitary gland (except in cases of vasopressin-secreting tumours). However there are two other sources of vasopressin with important local effects:

★ Vasopressin is secreted from parvocellular neurons of the paraventricular nucleus at the median eminence into the short portal vessels of the pituitary stalk. These vessels carry the peptide directly to the anterior pituitary gland, where it is an important releasing factor for ACTH, acting in conjunction with CRH.

★ Vasopressin is also released into the brain by several different populations of neurons (see below).

Summary Table

Here is a table summarizing some of the actions of Avp at its three receptors, differently expressed in different tissues and exerting different actions:

'Type'

'Second messenger system '

'Locations'

'Actions'

AVPR1A

phosphatidylinositol/calcium

liver, kidney, peripheral vasculature, >

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