Oxytocin is a hormone, found in humans and other mammals, which is involved in
the facilitation of birth and breastfeeding as well as in bonding and the
formation of trust between people.
Synthesis and release
Oxytocin is synthesized in magnocellular neurosecretory cells in the
hypothalamus and released by the posterior lobe of the pituitary gland.
Structure
Oxytocin structure. Inset shows oxytocin bound to neurophysinOxytocin is a
peptide of nine amino acids (a nonapeptide). The sequence is cysteine - tyrosine
- isoleucine - glutamine - asparagine - cysteine - proline - leucine - glycine (CYIQNCPLG).
The leucine is the only difference from vasopressin, which has arginine or
lysine at this position. The cysteine residues form a sulfur bridge.
Oxytocin is often bound to neurophysin (a carrier protein) as shown in the inset
of the figure.
Functions
Functions of oxytocin include:
Uterine contraction – necessary for cervical dilation prior to birth and causes
the contractions all mothers are familiar with. Oxytocin release during
breastfeeding causes mild but often painful uterine contractions during the
first few weeks of lactation. This also serves to assist the uterus in clotting
the placental attachment point postpartum.
Plays some role in orgasms – for both males and females. In males, oxytocin is
said to facilitate sperm transport in ejaculation.
Letdown reflex – in lactating (breastfeeding) mothers, oxytocin stimulates
myoepithelial cells, causing milk to be ejected into the ducts of the mammary
glands. Sucking by the infant at the nipple stimulates oxytocin release.
Inducing maternal behaviour – Sheep and rat females given oxytocin antagonists
after giving birth do not exhibit typical maternal behavior such as grooming
towards newborns. Conversely, virgin sheep females show maternal behavior
towards foreign lambs upon cerebrospinal fluid infusion of oxytocin, which they
would not do otherwise.
Thought to induce pair bonding in people. Levels of oxytocin are found to be
higher amongst people who claim to be falling in love. It is also thought to
mediate other forms of pair bonding such as friendship and family relationships
as given above. Reduction of sociophobic behavior was shown after treatment with
oxytocin.
Nasally administered oxytocin appears to generate trust in humans. In a 2005
study, it was shown that in a risky investment game, experimental subjects given
the hormone displayed what the researchers deemed "the highest level of trust"
twice as often as the control group who were given placebos. The same experiment
with the subjects told that they were interacting with a computer showed no such
reaction, leading to the conclusion that oxytocin was not merely affecting
risk-aversion. (Kosfeld 2005)
Various anti-stress functions: reducing blood pressure and cortisol levels,
increasing tolerance to pain, reducing anxiety.
Oxytocin may play a role in encouraging "tend and befriend", as opposed to
"flight or fight", behavior, in response to stress.
Oxytocin and oxytocin receptors are found in the heart, and the hormone appears
to play an important role in the embryonal development of the heart by promoting
cardiomyocyte differentiation. (Paquin 2002, Jankowski 2004)
It inhibits the development of tolerance to various addictive drugs (opiates,
cocaine, alcohol) and reduces withdrawal symptoms, at least in animal tests.
(Kovacs 1998)
Uses
Oxytocin analogues (Pitocin®, Syntocinon®) are used to induce labour and support
labour in case of non-progression of the parturition. It has largely replaced
ergotamine as the principal agent to increase uterine tone in acute postpartum
haemorrhage.
Inhibition of oxytocin action is the mode of action of the new tocolytic agent
atosiban (Tractocile®), which is registered in many countries to suppress
premature labour between 24 and 33 weeks of gestation. It has fewer side-effects
than drugs previously used for this purpose (ritodrine, salbutamol and
terbutaline).
An oxytocin nose spray may be a useful adjunct to stimulate breastfeeding.
Some have suggested that the trust-inducing property of oxytocin could aid those
who suffer from social anxieties and autism, while others have noted the
potential for abuse with confidence tricks.
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