The word oxytocin comes from the Greek words oxutokia ‘sudden delivery’ (from Oxus ‘sharp’ + tokos ‘childbirth’). It is also known as Pitocin or Syntocinon.
Oxytocin is a small peptide hormone referred to as a nonapeptide because it contains nine amino acids in the sequence cysteine-tyrosine-isoleucine-glutamine-asparagine-cysteine-proline-leucine-glycine-amide (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-GlyNH2) with a sulfur bridge between the two cysteines. It has multiple sites of action in animal models and regulates a large number of reproduction-related processes in all species. Its ability to stimulate uterine contraction in labor and childbirth is important.
This hormone also plays a role in sexual activity, penile erection, ejaculation, milk ejection, osteoporosis, diabetes, cancer, stress, balancing social behavior, evoking contentment, inducing trust, generosity, and bonding in humans and animals.
Immunohistochemical studies reveal that it is stored and released from the posterior pituitary gland; is created in the hypothalamus. It is released from the paraventricular nucleus of the hypothalamus into the posterior pituitary gland for later use. This specific part of the posterior pituitary gland that stores oxytocin is called the Pars nervosa, also known as the neural or posterior lobe.
Most hormones create negative feedback loops after they are released, but oxytocin exhibits positive feedback loops.
STRUCTURE OF OXYTOCIN
Molecular Formula: C43H66N12O12S2
Molecular weight: 1007.2 g/mol
CAS number: 50-56-6
MECHANISM OF ACTION
Oxytocin currently has only one receptor, which belongs to the rhodopsin-type (class I) G protein-coupled receptor (GPCR) family and is coupled to phospholipase C through Gαq11. Receptor activation triggers several signaling events primarily by elevating intracellular calcium (Ca2+ ). This includes inositol-tris-phosphate-mediated store calcium release, store-operated Ca2+ entry and voltage-operated Ca2+ entry. This action exerts a selective action on the following organs:
The uterus: Oxytocin acts on the smooth muscle of the uterus, particularly toward the end of pregnancy, during labor, and immediately after delivery. It stimulates rhythmic contractions of the uterus, increases the frequency of existing contractions, and raises the tone of the uterine musculature by increasing the sodium permeability of uterine myofibrils. Elevated estrogen concentrations lower the threshold for uterine response to oxytocin, which increases with the duration of pregnancy and is greater in patients who are in labor than those not in labor.
Large doses of oxytocin elicit contractions in early pregnancy. Contractions produced in the term uterus by oxytocin are similar to those occurring during spontaneous labor.
The breast: Oxytocin contracts myoepithelial cells surrounding the alveoli of the breasts, forcing milk from the alveoli into the larger ducts and thus facilitates milk ejection. It possesses no milk-producing properties.
The blood vessel: Oxytocin produces vasodilation of vascular smooth muscle, increasing renal, coronary, and cerebral blood flow. Blood pressure doesn’t change, but following intravenous administration of large doses or undiluted solutions, blood pressure may decrease transiently, and tachycardia may be reflexly induced.
The brain: Estrogen regulates the actions of oxytocin on the brain. Oxytocin may influence normal behavior, and its dysfunction may be involved in the etiology and expression of neuropsychiatric disorders.
Exogenous oxytocin causes the same response in the female reproductive system as that of endogenous oxytocin. It is one of the few hormones that exhibit positive feedback loops. Its effects include:
- Facilitating childbirth and breastfeeding
- Preventing postpartum hemorrhage which is defined as blood loss of more than 500 ml in the first 24 h after vaginal delivery
- Increasing the urinary excretion of aquaporin-2 AQP2 in animals whose urinary concentration mechanism is preserved
- Oxytocin promoting attachment
- Solidifying relationships
- Easing stress
- Crystallizing emotional memories
- Boosting sexual arousal
- Reducing drug cravings
- Improving social skills
- Triggering protective instincts
- Inducing sleep
- National Center for Biotechnology Information. PubChem Database. Oxytocin, CID=439302, https://pubchem.ncbi.nlm.nih.gov/compound/Oxytocin (accessed on Dec. 13, 2019)
- National Center for Biotechnology Information. PubChem Database. Oxytocin, Source=HSDB, https://pubchem.ncbi.nlm.nih.gov/source/hsdb/2182#section=Mechanism-of-Action (accessed on Dec. 14, 2019)
- Lee HJ, Macbeth AH, Pagani JH, Young WS 3rd. Oxytocin: the great facilitator of life. Prog Neurobiol. 2009;88(2):127–151. doi:10.1016/j.pneurobio.2009.04.001
- Osilla EV, Sharma S. Oxytocin. [Updated 2019 Mar 2]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK507848/
- Shamay-Tsoory S, Young LJ. Understanding the Oxytocin System and Its Relevance to Psychiatry. Biol Psychiatry. 2016;79(3):150–152. doi:10.1016/j.biopsych.2015.10.014