Oxytocin is a peptide hormone of 9 amino acids. The sequence of amino acids in oxytocin was described in 1953 by American scientist Vincent Du Vignot, who soon synthesized it. In 1955, he won the Nobel Prize for the discovery of oxytocin.
Oxytocin has peripheral (hormonal) actions, and also has actions in the brain. The actions of oxytocin are mediated by specific, high affinity oxytocin receptors. The oxytocin receptor is a G-protein-coupled receptor which requires Mg2+ and cholesterol. It belongs to the rhodopsin-type (class I) group of G-protein-coupled receptors. The peripheral actions of oxytocin reflect secretion from the pituitary gland. 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. Oxytocin receptors are expressed by neurons in many parts of the brain and spinal cord, including the amygdala, ventromedial hypothalamus, septum, nucleus accumbens and brain stem. Oxytocin can be used by both women and men.
21 interesting facts about oxytocin
- Uterine contraction
Important for cervical dilation before birth, oxytocin causes contractions during the second and third stages of labor. Oxytocin release during breastfeeding causes mild but painful contractions during the first few weeks of lactation. This also serves to assist the uterus in clotting the placental attachment point postpartum. However, in knockout mice lacking the oxytocin receptor, reproductive behavior and parturition are normal.
- Social behavior and wound healing
Oxytocin is also thought to modulate inflammation by decreasing certain cytokines. Thus, the increased oxytocin release following positive social interactions has the potential to improve wound healing. A study by Marazziti and colleagues used heterosexual couples to investigate this possibility. They found increases in plasma oxytocin following a social interaction were correlated with faster wound healing. They hypothesized this was due to oxytocin reducing inflammation, thus allowing the wound to heal more quickly. This study provides preliminary evidence that positive social interactions may directly influence aspects of health. According to a study published in 2014, silencing of oxytocin receptor in the medial prefrontal cortex (mPFC) of female mice resulted in loss of social interest in male mice during the sexually receptive phase of the estrous cycle.
- Influence on the genital sphere
The relationship between oxytocin and human sexual response is unclear. At least two uncontrolled studies have found increases in plasma oxytocin at orgasm – in both men and women. Plasma oxytocin levels are notably increased around the time of self-stimulated orgasm and are still higher than baseline when measured five minutes after self-arousal. The authors of one of these studies speculated that oxytocin’s effects on muscle contractibility may facilitate sperm and egg transport. A study measuring oxytocin serum levels in women before and after sexual stimulation was conducted. The authors of the study suggested that oxytocin plays an important role in sexual arousal. This study found genital tract stimulation resulted in increased oxytocin immediately after orgasm. Another study in mammals reported increases of oxytocin during sexual arousal could be in response to nipple/areola, genital, and/or genital tract stimulation. Murphy et al. (1987) found in a study involving men that the found oxytocin levels were raised throughout sexual arousal with no increase at orgasm. The most recent study involving humans reported an increase in the oxytocin level immediately after orgasm, but this effect was observed only in a statistically insignificant part of the subjects. The authors noted these changes “may simply reflect contractile properties on reproductive tissue”. Oxytocin causes satisfaction reducing anxiety and coldness in a group. This suggests that oxytocin can be important for inhibiting brain regions responsible for control of behavior, fear and anxiety. The study also showed that oxytocin can reduce anxiety and protect from stress, especially when combined with social support.
- Excretion of urine
Due to its similarity to vasopressin, it can reduce the excretion of urine slightly. In several mammal species, oxytocin can stimulate sodium excretion from the kidneys (natriuresis), and, in humans, high doses can result in hyponatremia.
Oxytocin and oxytocin receptors are also found in the heart in some rodents, and the hormone may play an important role in the embryonal development of the heart by promoting cardiomyocyte differentiation. However, the absence of either oxytocin or its receptors in knockout mice does not cause heart failure.
- Modulation of hypothalamic-pituitary-adrenal axis activity
Oxytocin, under certain circumstances, inhibits release of adrenocorticotropic hormone and cortisol and, in those situations, may be considered an antagonist of vasopressin.
Oxytocin can play an important role in autism and be an effective treatment for certain symptoms of autism. Oxytocin treatment also helps to preserve emotional speech in older people with autism. Two related studies in the elderly, conducted in 2003 and 2007, found that oxytocin impairs repetitive behavior and interpretation of emotions. Later it was reported that intranasal oxytocin administration enhances emotional recognition in children 12 years old with autism. According to one study, autism is correlated with genomic deletion of the gene containing the oxytocin receptor gene (OXTR). Studies involving Caucasian and Finnish samples and Chinese Han families provide support for the relationship of OXTR with autism. Autism may also be associated with an aberrant methylation of OXTR. After intranasal administration of oxytocin, patients with autism showed more appropriate behavior. Further clinical studies are required to identify potential beneficial properties and side effects in treatment of autism. Therefore, researchers do not recommend using oxytocin for treatment of autism outside of research.
- Reduced fear
After intranasal administration of oxytocin, subjects showed the highest level of credulity during the stock market game. Subjects who were informed that they were interacting with a computer did not show a similar reaction. This suggests that oxytocin does not affect riskophobia. Nasally administered oxytocin has been reported to reduce fear, possibly by inhibiting the amygdala (which is thought to be responsible for fear responses). Indeed, studies in rodents have shown oxytocin can efficiently inhibit fear responses by activating an inhibitory circuit within the amygdala. Some researchers have argued oxytocin has a general enhancing effect on emotions, since intranasal administration of oxytocin also increases envy and Schadenfreude.
Trust is increased by oxytocin. Frankness is a sign of trust in humans. When recounting a negative event, humans who receive intranasal oxytocin share more emotional details. After receiving oxytocin humans also find faces of their interlocutors more trustworthy and honest. In a study, participants who received intranasal oxytocin viewed photographs of human faces with neutral expressions and found them to be more trustworthy than those who did not receive oxytocin. This may be because oxytocin reduces the fear of social betrayal in humans. Even after being excluded from a conversation, humans who received oxytocin scored higher in trust on the Revised NEO Personality Inventory. Oxytocin enhances trust only to a certain extent. The study envisaged a game of trust, the participants acted as “investors” who decided on the amount of money to be given to a “trustee”. It was necessary to determine whether the trustee was trustworthy or not, or whether he was neutral. Subjects who received a dose of oxytocin did not give more money to people who were not trustworthy. This suggests that oxytocin does not enhance trust in suspicious situations. When there is a reason to be distrustful differing reactions are associated with oxytocin receptor gene (OXTR) differences.
- Social distance
Oxytocin affects social distance between adult males and females, and may be responsible at least in part for romantic attraction and monogamous pair bonding. Nasal oxytocin caused men in a monogamous relationship to increase the distance between themselves and an attractive woman. The researchers suggested that oxytocin may help promote fidelity within monogamous relationships
- Affecting generosity
In a neuroeconomics experiment, intranasal oxytocin increased generosity in the Ultimatum Game by 80%, but had no effect on altruism in the Dictator Game. Perspective-taking is not required in the Dictator Game, but the researchers in this experiment explicitly induced perspective-taking in the Ultimatum Game by not identifying to participants into which role they would be placed. Serious methodological questions have arisen, however, with regard to the role of oxytocin in trust and generosity. Empathy in healthy males has been shown to be increased after intranasal oxytocin. This is most likely due to the effect of oxytocin in enhancing eye gaze. There is some discussion about which aspect of empathy oxytocin might alter – for example, cognitive vs. emotional empathy.
- Cognitive function
Some learning and memory functions are weakened by oxytocin. The systemic oxytocin administration impairs the retrieval of information in the memory test. Curiously, oxytocin is considered a tool that can facilitate the process of memorizing specifically social information. In healthy men who used oxytocin intranasally, memory for faces, especially for happy faces, improved. Their ability to recognize positive social signals, as well as the ability to recognize fear improved as well.
- Sexual arousal
Oxytocin, injected into the cerebrospinal fluid of rats, causes spontaneous erection, reflecting the effect in the hypothalamus and spinal cord. Oxytocin receptor antagonists can prevent a non-contact erection, which measures sexual arousal. Studies in female rats with oxytocin antagonists reported that oxytocin stimulates lordosis behavior, which indicates increased sexual receptivity.
Oxytocin released into the brain of a female meadow vole during sexual activity is important for forming a pair bond with her sexual partner. Vasopressin appears to have a similar effect in males. Oxytocin has a role in social behaviors in many species, so it likely also does in humans. In a 2003 study, oxytocin blood levels in humans and dogs rose in 5 to 24 minutes after petting. This possibly plays an important role in the emotional bonding between humans and dogs.
- Maternal behavior
Female rats given oxytocin antagonists postpartum do not exhibit typical maternal behavior. By contrast, virgin female sheep show maternal behavior toward another’s lambs upon oxytocin injection into cerebrospinal fluid. Oxytocin is involved in the demonstration of maternal instinct, but its level is higher in mothers after they interact with another’s babies rather than their own.
- Drug interaction
According to a number of studies in animals, oxytocin inhibits the development of compatibility with other drugs (opiates, cocaine, alcohol), and reduces withdrawal symptoms. MDMA (ecstasy) may enhance love, empathy, and connection to others by stimulating oxytocin activity via activation of serotonin 5-HT1A receptors. Buspirone may produce its effect via oxytocin stimulation as well.
- Preparation of fetal neurons for delivery
Crossing the placenta, maternal oxytocin reaches the fetal brain and induces a switch in the action of neurotransmitter GABA on fetal cortical neurons from excitatory to inhibitory. This silences the fetal brain for the period of delivery and reduces its vulnerability to hypoxic damage.
- Romantic attachment
In some studies, high levels of plasma oxytocin were correlated with romantic attachment. For example, if a couple is separated for a long period of time, anxiety can increase due to the lack of physical affection. Oxytocin can have a beneficial effect on couples by decreasing their anxiety when they are separated.
The recent research suggested that oxytocin neurons in the paraventricular hypothalamus can play a key role in suppressing appetite under normal conditions and that other hypothalamic neurons may trigger appetite via inhibition of these oxytocin neurons. The lack of oxytocin neurons is observed in Prader-Willi syndrome (a genetic disorder that leads to uncontrollable eating and obesity, and may play a key role in its pathophysiology).
In a controlled study exploring the biological roots of immoral behavior, oxytocin was shown to promote dishonesty.
- Intergroup bonding
Oxytocin can increase positive attitude toward individuals with similar traits of character, who then become classified as in-group members, whereas individuals who are dissimilar become classified as out-group members. Race can be used as an example of in-group and out-group tendencies because society often categorizes individuals into groups based on race (Caucasian, African American, Latino, etc.). A study that examined race and empathy relation found that participants receiving nasally administered oxytocin had stronger reactions to pictures of in-group members with pained facial expressions than to pictures of out-group members with the same expression. This shows that oxytocin can be implicated in the ability to empathize with individuals of different races, and this reaction can potentially translate into willingness to help individuals in pain or stressful situations. Moreover, individuals of one race may be more inclined to help individuals of the same race than individuals of another race. A study examining relationships found that when individuals were administered oxytocin, the dishonesty rate in the in-group increased though a different outcome was expected. These examples show the behavior pattern of a human within their social group, or in-group. Oxytocin is not only affects the preferences of individuals who associate themselves with members of their own group, but it is also important during conflicts between members of different groups. During conflict, individuals receiving nasally administered oxytocin demonstrate defense reactions toward in-group members more frequently than out-group members. Oxytocin affected the desire of participants to protect vulnerable in-group members, regardless of individual’s involvement in the conflict. The effect of oxytocin on subjective preferences of study participants was noted. These studies demonstrate that oxytocin is associated with in-group dynamics. Further, oxytocin influences the responses of individuals in a particular group to those of another group. The in-group bias develops in smaller groups; however, it can also be extended to larger groups (e.g. one’s entire country leading toward national unity). A study conducted in the Netherlands showed that oxytocin increased the in-group bias of their nation while decreasing acceptance of members of other ethnicities and foreigners. When exposed to oxytocin, people showed more afection for their country’s flag while remaining indifferent to other cultural objects. It is assumed, that this hormone can be responsible for xenophobia. Thus, oxytocin affects individuals at an international level where the in-group becomes a specific home country, and the out-group includes all other countries.