Oxytocin is a hormone that also acts as a neurotransmitter in the brain. Some popular media have incorrectly labeled it the “love hormone,” because it is associated with good feelings and emotions. But its role in the body is much more complex than that. It is not a bliss or hug hormone, but it does appear to be connected to human emotions and the regulation of childbirth and breast-feeding.
In humans, oxytocin is thought to be released during hugging, touching, and orgasm in both genders. In the brain, oxytocin is involved in social recognition and bonding, and may be involved in the formation of trust between people and generosity. ((Kosfeld M et al. 2005. Oxytocin increases trust in humans. Nature 435:673-676. PDF PMID 15931222)) ((Zak, P.J. Stanton, A.A., Ahmadi, A. 2007. Oxytocin increases generosity in humans. PLoS ONE 2(11): e1128.)) ((Angela A. Stanton 2007. Neural Substrates of Decision-Making in Economic Games. Scientific Journals International 1(1):1-64. )) Oxytocin first became of interest to researchers when they discovered that breastfeeding women are calmer when exercising and experiencing stress than moms who were bottle-feeding. It is just one part of the important, complex neurochemical system in our bodies that helps us adapt to emotional situations.
What does oxytocin do in the body? Greater amounts of oxytocin hormone levels appear to be associated with greater relaxation, more willingness to trust others, and general psychological stability. It appears to help us reduce our stress response and reduce general anxiety in people when produced.
According to recent research, this hormone “is now believed to be involved in a wide variety of physiological and pathological functions such as sexual activity, penile erection, ejaculation, pregnancy, uterine contraction, milk ejection, maternal behavior, social bonding, stress and probably many more, which makes oxytocin and its receptor potential candidates as targets for drug therapy. From an innocuous agent as an aid in labor and delivery, oxytocin has come a long way in being touted as the latest party drug.” ((Magon, N & Kalra, S. (2011). The orgasmic history of oxytocin: Love, lust and labor. Indian J Endocrinol Metab, 15, S156-S161.))
Synthetic oxytocin is sold as medication under the trade names Pitocin and Syntocinon as well as generic oxytocin. It’s not clear that synthetic oxytocin works in the same manner as the naturally occurring hormone.
What Does Oxytocin Do in The Brain?
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.
Oxytocin receptors are expressed by neurons in many parts of the brain and spinal cord, including the amygdala, ventromedial hypothalamus, septum and brainstem.
- Sexual arousal. Oxytocin injected into the cerebrospinal fluid causes spontaneous erections in rats, reflecting actions in the hypothalamus and spinal cord. ((Gimpl G, Fahrenholz F. (2001) The oxytocin receptor system: structure, function, and regulation. Physiological Reviews 81: full text PMID 11274341))
- Bonding. In the Prairie Vole, oxytocin released into the brain of the female during sexual activity is important for forming a monogamous pair bond with her sexual partner. Vasopressin appears to have a similar effect in males. In people, plasma concentrations of oxytocin have been reported to be higher amongst people who claim to be falling in love. Oxytocin has a role in social behaviors in many species, and so it seems likely that it has similar roles in humans. ((Vacek M, High on Fidelity. What can voles teach us about monogamy?))
- Autism. A 1998 study found significantly lower levels of oxytocin in blood plasma of autistic children. ((Modahl C, Green L, Fein D et al. (1998). “Plasma oxytocin levels in autistic children”. Biol Psychiatry 43 (4): 270–7. doi:0.1016/S0006-3223(97)00439-3. PMID 9513736. )) A 2003 study found a decrease in autism spectrum repetitive behaviors when oxytocin was administered intravenously. ((Hollander E, Novotny S, Hanratty M et al. (2003). “Oxytocin infusion reduces repetitive behaviors in adults with autistic and Asperger’s disorders”. Neuropsychopharmacology 28 (1): 193–8. doi:10.1038/sj.npp.1300021. PMID 12496956. )) A 2007 study reported that oxytocin helped autistic adults retain the ability to evaluate the emotional significance of speech intonation. ((Hollander E, Bartz J, Chaplin W et al. (2007). “Oxytocin increases retention of social cognition in autism”. Biol Psychiatry 61 (4): 498–503. doi:10.1016/j.biopsych.2006.05.030. PMID 16904652. ))
- Maternal behavior. Female sheep and rats given oxytocin antagonists after giving birth do not exhibit typical maternal behavior. By contrast, virgin female sheep show maternal behavior towards foreign lambs upon cerebrospinal fluid infusion of oxytocin, which they would not do otherwise. ((Kendrick KM, The Neurobiology of Social Bonds ))
- Increasing trust and reducing fear. In a risky investment game, experimental subjects given nasally administered oxytocin displayed “the highest level of trust” twice as often as the control group. Subjects who were 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 M et al. (2005) Oxytocin increases trust in humans. Nature 435:673-676. PDF PMID 15931222)) Nasally administered oxytocin has also been reported to reduce fear, possibly by inhibiting the amygdala (which is thought to be responsible for fear responses). ((Kirsch P et al. (2005) Oxytocin modulates neural circuitry for social cognition and fear in humans. J Neurosci 25:11489-93 PMID 16339042)) There is no conclusive evidence for access of oxytocin to the brain through intranasal administration, however.
- Affecting generosity by increasing empathy during perspective taking. In a neuroeconomics experiment, intranasal oxytocin increased generosity in the Ultimatum Game by 80% but has no effect in the Dictator Game that measures altruism. Perspective-taking is not required in the Dictator Game, but the researchers in this experimental explicitly induced perspective-taking in the Ultimatum Game by not identifying to participants which role they would be in. ((Zak, P.J. Stanton, A.A., Ahmadi, A. 2007. Oxytocin increases generosity in humans. PLoS ONE 2(11): e1128.))
- Preparing fetal neurons for delivery. Crossing the placenta, maternal oxytocin reaches the fetal brain and induces a switch in the action of neurotransmitter GABA from excitatory to inhibitory on fetal cortical neurons. This silences the fetal brain for the period of delivery and reduces its vulnerability to hypoxic damage. ((Tyzio R et al.(2006) Maternal Oxytocin Triggers a Transient Inhibitory Switch in GABA Signaling in the Fetal Brain During Delivery. Science 314: 1788-1792 PMID 17170309))
- MDMA (ecstasy) may increase feelings of love, empathy and connection to others by stimulating oxytocin activity via activation of serotonin 5-HT1A receptors, if initial studies in animals apply to humans. ((Thompson MR, Callaghan PD, Hunt GE, Cornish JL, McGregor IS. A role for oxytocin and 5-HT(1A) receptors in the prosocial effects of 3,4 methylenedioxymethamphetamine (“ecstasy”). Neuroscience. 146:509-14, 2007. PMID 17383105))
Hormonal Actions of Oxytocin
The actions of oxytocin are mediated by specific, high affinity oxytocin receptors. The peripheral actions of oxytocin mainly reflect secretion from the pituitary gland.
- Letdown reflex. In lactating (breastfeeding) mothers, oxytocin acts at the mammary glands, causing milk to be ‘let down’ into a collecting chamber, from where it can be extracted by compressing the areola and sucking at the nipple. Sucking by the infant at the nipple is relayed by spinal nerves to the hypothalamus. The stimulation causes neurons that make oxytocin to fire action potentials in intermittent bursts; these bursts result in the secretion of pulses of oxytocin from the neurosecretory nerve terminals of the pituitary gland.
- Uterine contractions. These are important for cervical dilation before birth and causes contractions during the second and third stages of labor. 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. However, in knockout mice lacking the oxytocin receptor, reproductive behavior and parturition is normal. ((Takayanagi Y et al. (2005) Pervasive social deficits, but normal parturition, in oxytocin receptor-deficient mice. Proc Natl Acad Sci USA 102:16096-101 PMID 16249339))
- The relationship between oxytocin and human sexual response is unclear. At least two non-controlled studies have found increases in plasma oxytocin at orgasm – in both men and women. ((Carmichael MS, Humbert R, Dixen J, Palmisano G, Greenleaf W, Davidson JM (1987). “Plasma oxytocin increases in the human sexual response,” J Clin Endocrinol Metab 64:27-31 PMID 3782434)) ((Carmichael MS, Warburton VL, Dixen J & Davidson JM (1994). “Relationship among cardiovascular, muscular, and oxytocin responses during human sexual activity,” Archives of Sexual Behavior 23 59–79.)) The authors of one of these studies speculated that oxytocin’s effects on muscle contractibility may facilitate sperm and egg transport. ((Carmichael MS, Humbert R, Dixen J, Palmisano G, Greenleaf W, Davidson JM (1987). “Plasma oxytocin increases in the human sexual response,” J Clin Endocrinol Metab 64:27-31 PMID 3782434)) Murphy et al. (1987), studying men, found that oxytocin levels were raised throughout sexual arousal and there was no acute increase at orgasm. ((Murphy ME, Seckl JR, Burton S, Checkley SA & Lightman SL (1987). “Changes in oxytocin and vasopressin secretion during sexual activity in men,” Journal of Clinical Endocrinology and Metabolism 65:738–741.)) A more recent study of men found an increase in plasma oxytocin immediately after orgasm, but only in a portion of their sample that did not reach statistical significance. The authors noted that these changes “may simply reflect contractile properties on reproductive tissue.” ((Kruger THC, Haake P, Chereath D, Knapp W, Janssen OE, Exton MS, Schedlowski M & Hartmann U (2003). ))
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