Vasopressin

Vasopressin (arginine vasopressin, AVP; also known as antidiuretic hormone, ADH) is a cyclic nonapeptide produced by magnocellular neurosecretory cells in the supraoptic and paraventricular nuclei of the hypothalamus. It is evolutionarily ancient - the vasopressin/oxytocin neuropeptide family arose over 600 million years ago and is present in virtually all vertebrate species, with the family also found in invertebrates. This extreme conservation reflects the fundamental importance of these peptides in regulating water balance, blood pressure, and social behaviour.

The classical function of vasopressin is antidiuresis. When plasma osmolality rises (dehydration) or blood volume falls (haemorrhage), osmoreceptors and baroreceptors signal the hypothalamus to release vasopressin from the posterior pituitary into the bloodstream. Vasopressin then acts on V2 receptors in the renal collecting duct principal cells to insert aquaporin-2 water channels into the luminal membrane, increasing water reabsorption and producing concentrated urine. This mechanism is the primary defence against dehydration in all terrestrial vertebrates. V2R loss-of-function mutations cause diabetes insipidus, while inappropriate ADH secretion (SIADH) causes dilutional hyponatraemia - underscoring vasopressin's essential role in water homeostasis.

The vasoconstrictor properties of vasopressin, mediated through V1a receptors on vascular smooth muscle, give the peptide its name. During haemorrhagic shock, vasopressin plays a critical role in maintaining arterial blood pressure by constricting peripheral vasculature. This property has been exploited clinically: vasopressin infusion is now standard of care in septic shock (where it dramatically reduces noradrenaline requirements) and is used to treat refractory vasodilatory shock.

The cognitive and social effects of vasopressin represent a more recently characterised but equally fascinating aspect of this peptide. The discovery that V1a receptors are distributed throughout limbic brain regions including the hippocampus, amygdala, lateral septum, and anterior cingulate cortex established the neuroanatomical basis for vasopressin's role in behaviour. Intranasal administration of vasopressin in healthy humans enhances memory consolidation, particularly for emotional memories, and improves performance on spatial navigation tasks - effects mediated through direct brain delivery bypassing the blood-brain barrier via olfactory nerve pathways.

The role of vasopressin in social behaviour, particularly in males, has generated intense research interest. Prairie voles (highly monogamous rodents) and meadow voles (promiscuous) differ dramatically in V1aR distribution in the ventral pallidum and nucleus accumbens. When prairie vole V1aR genes are transferred to meadow voles, they acquire pair-bonding behaviour. This finding established a mechanistic link between vasopressin system genetics and monogamy/pair bonding. In humans, V1aR gene polymorphisms have been associated with relationship quality, infidelity, and attachment styles - suggesting vasopressin shapes human social bonding in ways analogous to voles.

The potential therapeutic applications of vasopressin manipulation in autism spectrum disorders (ASD) represent an active clinical research frontier. People with ASD show altered vasopressin signalling - lower CSF vasopressin levels and different V1aR expression patterns. A phase 2 RCT at Stanford demonstrated that intranasal vasopressin improved social responsiveness and adaptive social skills in children with ASD, with good tolerability. Follow-up phase 3 trials are ongoing, positioned around the hypothesis that vasopressin replacement could normalise social processing in a subset of ASD patients.