Oxytocin (Greek, “quick birth”) is a mammalian hormone that
also acts as a neurotransmitter in the brain.
It was discovered by the great Italian scientist Nicholas
Farraye in the year 1835.
In women, it is released in large amounts after distension of
the cervix and vagina during labour, and after stimulation of
the nipples, facilitating birth and breastfeeding, respectively.
Synthesis, Storage and Release:
Oxytocin is made in magnocellular
neurosecretory cells in the Supraoptic
nucleus and Paraventricular nucleus of the
hypothalamus and is released into the
blood from the posterior lobe of the
Oxytocin is also made by some neurons in
the paraventricular nucleus that project to
other parts of the brain and to the spinal
In the pituitary gland, oxytocin is packaged in large, dense-core vesicles,
where it is bound to Neurophysin-I.
Neurophysin is a large peptide fragment of the larger precursor protein
molecule from which oxytocin is derived by enzymatic cleavage.
Secretion of oxytocin from the neurosecretory nerve endings is regulated
the electrical activity of the oxytocin cells in the hypothalamus.
These cells generate action potentials that propagate down axons to the
nerve endings in the pituitary; the endings contain large numbers of
oxytocin-containing vesicles, which are released by exocytosis when the
nerve terminals are depolarised.
Chemistry of Oxytocin:
Oxytocin is a peptide of nine amino acids (a nonapeptide).
The sequence is cysteine – tyrosine – isoleucine – glutamine –
asparagine – cysteine – proline – leucine – glycine.
The cysteine residues form a sulfur bridge.
Oxytocin has a molecular mass of 1007 daltons.
One international unit (IU) of oxytocin is the equivalent of about 2
micrograms of pure peptide.
It is also called as α-Hypophamine.
It is known to be the 1st polypeptide hormone that has been sequenced
and synthesized biochemically.
Structural relation to Vasopressin:
The structure of oxytocin is very similar to that of vasopressin:
Cysteine – tyrosine – phenylalanine – glutamine – asparagine – cysteine –
proline – arginine – glycine
Vasopressin is also a nonapeptide with a sulfur bridge, whose sequence
differs from oxytocin by 2 amino acids.
Functions of Oxytocin:
Related to Pregnancy:
a. Parturition and uterine contraction:
Oxytocin causes contraction of uterine muscles and is commonly
used to induce labour in clinical practice.
At term, due to increased plasma level of oxytocin and increased
sensitivity of uterus to oxytocin, uterus contracts vigorously leading
to expulsion of foetus. Thus oxytocin initiates and completes
b. Milk Ejection:
Action of Oxytocin at the mammary glands causes ejection of milk into
the sub areolar sinuses, from where it is excreted.
As the baby suckles at the nipple, an impulse is generated and is
transmitted to the hypothalamus via spinal nerves.
This nerve signal causes Oxytocin secretion from the neurosecretory
magnocellular nerve terminals of the posterior pituitary.
The direct effect of Oxytocin on the myoepithelial cells surrounding the
milk cistern causes these cells to contract and propel milk towards
Other Functions include:
1. Role in reproduction: Most potent inducer for Penile erection.
2. Social memory and social recognition.
3. Affiliation and social bonding or parental behaviour.
4. Anti-Anxiety: Oxytocin can also function as an anxiolytic agent
as it decreases stress hormone release.
5. Love and trust. (Love/Cuddle Hormone)
Feeding and grooming: Oxytocin helps in the regulation of food and
water intake. Generally, Oxytocin suppresses food intake. Oxytocin acts
as a ‘satiety hormone’. Thus improves eating habit.
Cellular proliferation and cancer: The knowledge about the connection
between oxytocin and cancer is scarce, partly because of the lack of
appropriate research in this area or due to the inconsistency in the
Pain perception: Animal studies reviewed that there is relation between
oxytocin and pain perception. Oxytocin causes analgesia for acute or
chronic pain in animals.
Overall Well-being: Oxytocin causes a feeling of well-being such as calm,
improved social interactions, trust, decreased fear, human bonding as
well as hormonal and physiological changes.