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Oxytocin (; Oxt) is a nonapeptide health system.[9]
Oxytocin has peripheral (hormonal) actions, and also has actions in the brain. Its actions are mediated by specific, oxytocin receptors. The oxytocin receptor is a G-protein-coupled receptor that requires magnesium and cholesterol. It belongs to the rhodopsin-type (class I) group of G-protein-coupled receptors.
The peripheral actions of oxytocin mainly reflect secretion from the pituitary gland. (See oxytocin receptor for more detail on its action.) Oxytocin secreted from the pituitary gland cannot re-enter the brain because of the blood–brain barrier. Instead, the behavioral effects of oxytocin are thought to reflect release from centrally projecting oxytocin neurons, different from those that project to the pituitary gland, or that are collaterals from them.[10] Oxytocin receptors are expressed by neurons in many parts of the brain and spinal cord, including the amygdala, ventromedial hypothalamus, septum, nucleus accumbens, and brainstem.
Oxytocin is typically remembered for the effect it has on prosocial behaviors, such as its role in facilitating trust and attachment between individuals. Consequently, oxytocin is often referred to as the “love hormone".[78] However, oxytocin has a more complex role than solely enhancing prosocial behaviors. There is consensus that oxytocin modulates fear and anxiety; that is, it does not directly elicit fear or anxiety.[79] Two dominant theories explain the role of oxytocin in fear and anxiety. One theory states that oxytocin increases approach/avoidance to certain social stimuli and the second theory states that oxytocin increases the salience of certain social stimuli, causing the animal or human to pay closer attention to socially relevant stimuli.[80]
Individuals who receive an intranasal dose of oxytocin identify facial expressions of disgust faster than individuals who do not receive oxytocin.[80] Facial expressions of disgust are evolutionarily linked to the idea of contagion. Thus, oxytocin increases the salience of cues that imply contamination, which leads to a faster response because these cues are especially relevant for survival. In another study, after administration of oxytocin, individuals displayed an enhanced ability to recognize expressions of fear compared to the individuals who received the placebo.[59] Oxytocin modulates fear responses by enhancing the maintenance of social memories. Rats that are genetically modified to have a surplus of oxytocin receptors display a greater fear response to a previously conditioned stressor. Oxytocin enhances the aversive social memory, leading the rat to display a greater fear response when the aversive stimulus is encountered again.[79]
It has been shown that oxytocin differentially affects males and females. Females who are administered oxytocin are overall faster in responding to socially relevant stimuli than males who received oxytocin.[80][81] Additionally, after the administration of oxytocin, females show increased amygdala activity in response to threatening scenes; however, males do not show increased amygdala activation. This phenomenon can be explained by looking at the role of gonadal hormones, specifically estrogen, which modulate the enhanced threat processing seen in females. Estrogen has been shown to stimulate the release of oxytocin from the hypothalamus and promote receptor binding in the amygdala.[81]
Oxytocin increases defensive responding to unpredictable stimuli, but not to predictable stimuli. This result leads to the assumption that oxytocin’s effect is context-dependent. Thus, oxytocin may reinforce prosocial behaviors after an initial bond is formed, but may enhance defensive behaviors to unfamiliar individuals.[78]
Oxytocin is beneficial because it can either enhance social bonding or promote defensive behaviors depending on the situation.[78] It would not be adaptive if oxytocin consistently enhanced social approach and other prosocial behaviors, especially in uncertain and potentially dangerous social contexts. Fear and anxiety are typically thought to be maladaptive, as these traits often underlie various psychological disorders. However, it is important to note that both fear and anxiety responses help to protect an individual. These emotions render environmental cues more important, leading to a greater likelihood the individual or animal will acknowledge the potential threat. Ultimately this process leads to a greater chance of survival.
An intravenous infusion of oxytocin is used to induce labor and to support labor in case of slow childbirth. It is unclear whether a high dose is better than a standard dose for labor induction. It has largely replaced ergometrine as the principal agent to increase uterine tone in acute postpartum hemorrhage. Oxytocin is also used in veterinary medicine to facilitate birth and to stimulate milk release. The tocolytic agent atosiban (Tractocile) acts as an antagonist of oxytocin receptors; this drug is registered in many countries to suppress premature labor between 24 and 33 weeks of gestation. It has fewer side effects than drugs previously used for this purpose (ritodrine, salbutamol, and terbutaline).[82]
Oxytocin is relatively safe when used at recommended doses, and side effects are uncommon.[83] The following maternal events have been reported:[83]
Excessive dosage or long-term administration (over a period of 24 hours or longer) have been known to result in tetanic uterine contractions, uterine rupture, postpartum hemorrhage, and water intoxication, sometimes fatal.
During pregnancy, increased uterine motility has led to decreased heart rate, cardiac arrhythmia, seizures, brain damage, death in the fetus/neonate:[83]
Oxytocin is destroyed in the gastrointestinal tract, so it must be administered by injection or as nasal spray. It has a half-life of typically about three minutes in the blood when given intravenously. When administered intranasally via a nasal spray, oxytocin crosses the blood–brain barrier and exhibits psychoactive effects in humans.[84][85] Unlike the case of intravenous administration, intranasal oxytocin has a duration of at least 2.25 hours and as long as 4 hours.[1][2]
Oxytocin is a peptide of nine amino acids (a nonapeptide). Its systematic name is cysteine-tyrosine-isoleucine-glutamine-asparagine-cysteine-proline-leucine-glycine-amide (cys – tyr – ile – gln – asn – cys – pro – leu – gly – NH2, or CYIQNCPLG-NH2). Oxytocin has a molecular mass of 1007 daltons. One international unit (IU) of oxytocin is the equivalent of about 2 micrograms of pure peptide. While the structure of oxytocin is highly conserved in placental mammals, a novel structure of oxytocin was recently reported in marmosets, tamarins, and other new world primates. Genomic sequencing of the gene for oxytocin revealed a single in-frame mutation (thymine for cytosine) which results in a single amino acid substitution at the 8-position (proline for leucine).[86]
The biologically active form of oxytocin, commonly measured by RIA and/or HPLC techniques, is also known as the octapeptide "oxytocin disulfide" (oxidized form), but oxytocin also exists as a reduced dithiol nonapeptide called oxytoceine.[87] It has been theorized that open chain oxytoceine (the reduced form of oxytocin) may also act as a free radical scavenger (by donating an electron to a free radical); oxytoceine may then be oxidized back to oxytocin via the dehydroascorbate <---> ascorbate redox couple.[88]
The structure of oxytocin is very similar to that of vasopressin (cys – tyr – phe – gln – asn – cys – pro – arg – gly – NH2), also a nonapeptide with a sulfur bridge, whose sequence differs from oxytocin by two amino acids. A table showing the sequences of members of the vasopressin/oxytocin superfamily and the species expressing them is present in the vasopressin article. Oxytocin and vasopressin were isolated and synthesized by Vincent du Vigneaud in 1953, work for which he received the Nobel Prize in Chemistry in 1955.
Oxytocin and vasopressin are the only known hormones released by the human posterior pituitary gland to act at a distance. However, oxytocin neurons make other peptides, including corticotropin-releasing hormone and dynorphin, for example, that act locally. The magnocellular neurosecretory cells that make oxytocin are adjacent to magnocellular neurosecretory cells that make vasopressin. These are large neuroendocrine neurons which are excitable and can generate action potentials.
The oxytocin peptide is synthesized as an inactive precursor protein from the OXT gene.[89][90][91] This precursor protein also includes the oxytocin carrier protein neurophysin I.[92] The inactive precursor protein is progressively hydrolyzed into smaller fragments (one of which is neurophysin I) via a series of enzymes. The last hydrolysis that releases the active oxytocin nonapeptide is catalyzed by peptidylglycine alpha-amidating monooxygenase (PAM).[93]
The activity of the PAM enzyme system is dependent upon vitamin C (ascorbate), which is a necessary vitamin cofactor. By chance, sodium ascorbate by itself was found to stimulate the production of oxytocin from ovarian tissue over a range of concentrations in a dose-dependent manner.[94] Many of the same tissues (e.g. ovaries, testes, eyes, adrenals, placenta, thymus, pancreas) where PAM (and oxytocin by default) is found are also known to store higher concentrations of vitamin C.[95]
Oxytocin is known to be metabolized by the oxytocinase, leucyl/cystinyl aminopeptidase.[96][97] Other oxytocinases are also known to exist.[96][98] Amastatin, bestatin (ubenimex), leupeptin, and puromycin have been found to inhibit the enzymatic degradation of oxytocin, though they also inhibit the degradation of various other peptides, such as vasopressin, met-enkephalin, and dynorphin A.[99][98][100][101]
In the hypothalamus, oxytocin is made in magnocellular neurosecretory cells of the supraoptic and paraventricular nuclei, and is stored in Herring bodies at the axon terminals in the posterior pituitary. It is then released into the blood from the posterior lobe (neurohypophysis) of the pituitary gland. These axons (likely, but dendrites have not been ruled out) have collaterals that innervate oxytocin receptors in the nucleus accumbens.[10] The peripheral hormonal and behavioral brain effects of oxytocin are thought to be coordinated through its common release through these collaterals.[10] Oxytocin is also made by some neurons in the paraventricular nucleus that project to other parts of the brain and to the spinal cord.[102] Depending on the species, oxytocin receptor-expressing cells are located in other areas, including the amygdala and bed nucleus of the stria terminalis.
In the pituitary gland, oxytocin is packaged in large, dense-core vesicles, where it is bound to neurophysin I as shown in the inset of the figure; 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 by 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.
Outside the brain, oxytocin-containing cells have been identified in several diverse tissues, including in females in the corpus luteum [103][104] and the placenta,[105] in males in the testicles' interstitial cells of Leydig,[106] the retina,[107] the adrenal medulla,[108] the thymus[109] and the pancreas.[110] The finding of significant amounts of this classically "neurohypophysial" hormone outside the central nervous system raises many questions regarding its possible importance in these different tissues.
The Leydig cells in some species have been shown to possess the biosynthetic machinery to manufacture testicular oxytocin de novo, to be specific, in rats (which can synthesize vitamin C endogenously), and in guinea pigs, which, like humans, require an exogenous source of vitamin C (ascorbate) in their diets.[111]
Oxytocin is synthesized by corpora lutea of several species, including ruminants and primates. Along with estrogen, it is involved in inducing the endometrial synthesis of prostaglandin F2α to cause regression of the corpus luteum.
Estrogen has been found to increase the secretion of oxytocin as well as the expression of its receptor, the oxytocin receptor, in the brain.[112]
Virtually all vertebrates have an oxytocin-like nonapeptide hormone that supports reproductive functions and a vasopressin-like nonapeptide hormone involved in water regulation. The two genes are usually located close to each other (less than 15,000 bases apart) on the same chromosome, and are transcribed in opposite directions (however, in fugu,[113] the homologs are further apart and transcribed in the same direction).
The two genes are believed to result from a gene duplication event; the ancestral gene is estimated to be about 500 million years old and is found in cyclostomata (modern members of the Agnatha).[53]
Synthetic oxytocin is sold as proprietary medication under the trade names Pitocin and Syntocinon, and as generic oxytocin.
The word oxytocin was coined from the term oxytocic. Greek ὀξύς, oxys, and τόκος, tokos, meaning "quick birth")
Its uterine-contracting properties were discovered by British pharmacologist Sir Henry Hallett Dale in 1906.[114] And its milk ejection property was described by Ott and Scott in 1910[115] and by Schafer and Mackenzie in 1911.[116]
Oxytocin became the first polypeptide hormone to be sequenced[117] or synthesized[118][119][120] Vigneaud was awarded the Nobel Prize in 1955 for his work.[121]
Oxytocin nasal sprays have been used to stimulate breastfeeding, but the efficacy of this approach is doubtful.[122]
The trust-inducing property of oxytocin might help those with social anxiety and depression,[49] but with the potential for abuse with confidence tricks[123][124] and military applications.[125] The use of oxytocin in relationship counseling is being investigated, as research has shown the hormone could both enhance trust and improve people's ability to interpret the emotions of others correctly.[126]
A nasal spray formulation of oxytocin branded Syntocinon is under development by Retrophin for the treatment of lactation deficiency and as a novel treatment for autism and schizophrenia.[127] As of October 2014, it has reached phase III, phase II, and phase II clinical trials for these indications, respectively.[128] In October 2014, Retrophin divested Syntocinon to Turing Pharmaceuticals.[129]
Anterior pituitary, Corticotropin-releasing hormone, Autonomic Nervous System, Thyroid, Endocrine system
Dna, Eukaryote, Rna, Chromosome, Gene expression
Brain, Sodium, Cholecystokinin, Calcium, Oxytocin
Proopiomelanocortin, Cholecystokinin, Neuropeptide Y, Α-msh, Peptide YY
Hippocampus, Pregnancy, Prefrontal cortex, Oxytocin, Prolactin
Psychology, Oxytocin, Kava, Psychiatry, Atosiban
Pregnancy, Breastfeeding, Milk, Oxytocin, Breast