The Big “O”
Effects of Oxytocin on Relationships.
Sigrid Williamson

Over recent years Oxytocin has quickly gained notoriety as the chemical of Love. In the past Oxytocin has been known to be involved in reproduction; through the facilitation of necessary contractions of the uterus and breast tissue, to the release during child birth creating a strong bond between mother and child [10]. Oxytocin, meaning “quick birth” in Greek, was appropriately named after it was determined to help muscle contractions during birth [7]. However, how early does the peptide begin to play a role the reproductive routine? Recent research has begun to examine the effects of Oxytocin in creating a relationship between mates. It is proposed that Oxytocin helps to increase trust and psychological attachment, two key emotions involved in love. Oxytocin begins my creating trust between individuals, thus creating foundation for a relationship to begin. All mammals have a tendency to form groups or tribes in order to provide protection, shelter, and food. Though the formation of groups is different between and within species, a common need for companionship is seen throughout all mammals. Mammals have learned to rely on each other to provide government, companionship, economic systems, and reproduction. Research is beginning to examine the chemical pathways that allow for such high levels of interaction. The challenge in evaluating this hypothesis is the ability to create a structured experiment, with clear indicators to the increase in trust or attachment. Trust and love are largely abstract ideas, and the scientific measurement of such emotions has proved challenging. This challenge, however, has not stopped scientists from attempting to track the effects of Oxytocin on relationships. During sexual stimulation, and most importantly orgasm, increased levels of Oxytocin help to further the bond between individuals. It is hypothesized that high levels of Oxytocin released during orgasm create an elevated sense of trust and attachment between individuals. The goal of scientists is to determine the effects of Oxytocin on social relationships. Oxytocin levels can, and have begun to, be measured during orgasm. Once the social effects of Oxytocin are established, the effects of Oxytocin during sexual encounters can be deduced. This information would reveal the implications of Oxytocin on creating and enforcing relationships, thus allowing its effects to be traced from the beginning of the process of reproduction.
Oxytocin is a hormone found in mammals that functions mainly as a neurotransmitter in the brain. It is a small nonapeptide, made up of a nine amino acids (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-GlyNH2) [7]. Oxytocin forms a six amino acid ring with a three amino acid long tail made of the amino acids (Pro-Leu-Gly) [3]. A sulfur bridge connects the two cysteine amino acids to form the ring shape[7]. In 1953, Vincent du Vigneaud determined the amino acid sequence of Oxytocin and later synthesized the polypeptide [6]. The ability to synthesize Oxytocin has made the processes of studying its effects on the social behavior of mammals easier. Synthesized Oxytocin is also used currently in facilitating childbirth and lactation [8]. Oxytocin belongs to the family of hormones released by the neurohypophysis, the posterior part of the pituitary glands. Oxytocin, also referred to as alpha-hypophamine, has a molecular formula of C43H66N12O12S2 and a molecular weight of 1007.19 g/mol [9]. Oxytocin is located on chromosome number 20 in humans, and number 2 in mice [8]. During experimentation levels of Oxytocin can generally be measured through cerebrospinal fluid [12]. Though the makeup of Oxytocin is well understood, its effects are still being largely evaluated.
Oxytocin is produced and stored throughout various parts of the brain. In 1951 W. Bargmann and E. Scharrer discovered that Oxytocin is typically produced in the magnocellular neureons within the supraoptic nucleun (SON) and the paraventrical nucleas (PVN) of the hypothalamus [6]. It then will be released and transported, within milliseconds via carrier proteins, from the SON and the PVN to the posterior pituitary gland, or the neurohypophysis [1,3,7,9]. Oxytocin remains in the hypophysis until it is released, in pulses, into the bloodstream. Oxytocin released by the hypophysis is used to regulate lactation through contractions of the myoepithelial tissue in the breast, and regulate birth by triggering necessary muscle contractions [3]. Oxytocin can also be found in the parvocellular neurons on the PVN, and is released from there to parts of the Central Nervous System [3,4]. Upon release Oxytocin will go to parts of the Central Nervous System including the straitum, the raphe nuclei, the LC, the amygdala, the vagal motor, the spinal cord, and the hypothalamus [4]. The release of Oxytocin in the posterior pituitary glands may act independent of any release of Oxytocin in the Central Nervous System [3]. It has, however, been noted that the release from the pituitary glands and the Central Nervous system can happen together in a pattern [3]. The release of Oxytocin within the SON and PVN increases during birth, lactation, touch, and surrounding birth. Oxytocin can also be found in cerebrospinal fluid and in nonneural tissue [3]. Through experimentation with rats it has been found that exposure to Oxytocin during developmental years can greatly effect future reactions to it in the animals adult life. For example if a rat was exposed to Oxytocin within the first few weeks of life, it will increase its behavioral response to Oxytocin in the future. It has also been found that the release of Oxytocin from the PVN and SON can be conditioned within mammals.
The ability of Oxytocin to effect different areas of the brain depends on the presence and abundance of peptide specific receptors. Throughout the brain and Central Nervous System are receptors specific to Oxytocin. Oxytocin only has one known receptor, OXTR, which is a class-1 G protein-coupled receptor [8]. The G-protein receptor is made up of 389-amino acids with containing seven hydrophobic transmembrane regions [3]. The Oxytocin receptor senses Oxytocin outside of the cell and initiates signal transduction pathways to result in a cellular response. Oxytocin receptors can be identified throughout the Central Nervous System in regions like the olfactory system, limbic-hypothalamic system, brainstem, and spinal cord [3]. These systems help to control reproduction and sensory responses. The Oxytocin receptor was cloned in 1992, and was a notable advance in the research process [8]. Receptors are found throughout the brain, mainly in areas known to control behavior, memory, sensory processes, reproduction, and homeostasis, such as the olfactory-limic hypothalamic axis [3]. The location of these receptors helps to reveal the possibility that Oxytocin effects social behavior, recognition, sensory responses, and sexual interactions. However, it is difficult to generalize about the effects of Oxytocin based solely on the location on the receptors. Receptor location and density depend on species, age, and gender. For example, the receptors in rodents can be found in the olfactory bulb, hippocampal formation, and nucleus accumbens. While in humans the receptors have been noted in the vertical limb of the broca, the anterior and posterior hypothalamic area, and in the dorsal horn of the upper spinal cord [8]. These differences in the location may explain the differences in social interactions and behavioral tendencies between species, genders, and ages. The process after Oxytocin has reached the receptor has been widely researched. After Oxytocin binds to the receptor, phosphoinositol lipids react with water and split [1]. The hydrolysis triggers contractions of the Uterus and release of calcium from within the cell [1]. This is imperative in lactation, as it helps milk drop in the breast. It has been found that the ability of the Oxytocin receptor to bind is increased in the presence of cholesterol [1]. It has been found that the activity of the Oxytocin receptor changes based on gonadal hormones, particularly Estrogen, which Oxtyocin receptors are genetically sensitive to [3]. Estrogen is known to trigger receptor activity in the uterus, kidneys, and the smooth muscle of the myoepithelial cells of the breast vital during breastfeeding [8]. When rats were treated with estrogen and testosterone, or prior to giving birth, the receptor activity increased. As well, when the rats were castrated the receptor activity greatly decreased [8]. A woman experiences high levels of estrogen during pregnancy, thus increasing the activity of her Oxytocin receptors. Within the ventromedial hypothalamus the Oxytocin receptors also react positively to increased levels of gonadal steroids. The increase in Oxytocin during pregnancy and breastfeeding helps to explain the connection between mother and child [3]. Many experiments have attempted to block the effects of Oxytocin in order to better understand its effects. Oxytocin antagonists have been used to block the Oxytocin receptors, such antagonists include atosiban and OVTA. Medically, Oxytocin antagonists are used to help prevent early labor. When the Oxytocin antagonist is used it helps to block most of the behavioral and physiological effects of the peptide [4]. Oxytocin receptors are very sensitive to antagonists, for example the Oxytocin receptor in humans will not bond to progestrogen, but it will bond to the 5 beta-dihydroprogesterone [3]. The ability to create an Oxytocin antagonist greatly helps in identifying its influence on behavior.
Stimulation is the main trigger for Oxytocin release, and also it’s main function. During almost every step of human reproduction, stimulation leading to Oxytocin release is apparent. While information is not yet definitive, Oxytocin has proved some affects during sexual stimulation and orgasm. First, Oxytocin is a trigger for our psychological state influencing our sexual satiety and increasing probability of an orgasm. Then, during the orgasm, the smooth muscle contractions of the reproductive tract are facilitated by the Oxytocin released from our psychological state. This information was found using measurements of electrical activity of muscles, showing positive relations of quantity of Oxytocin released and orgasm intensity from start to finish. Another step in reproduction influenced by Oxytocin is childbirth. Again, the physical contractions of the birth canal that aid in delivering the baby, stimulate Oxytocin release. This time, the hormone directly impacts the social relationship between mother and child [11]. Oxytocin plays a huge behavioral role in social pairs and improves bonds. Studies show that during the first trimester as well as postpartum, Oxytocin levels directly correlate with maternal behavior such as gaze, vocalization and touch to the infant. A final stage of childbirth influenced by Oxytocin is breast-feeding, or lactation. As a result of fertilization in women, the mammary gland collects secreted milk in its alveoli. In order for the milk to be released for nourishment, there must be stimulation to the nipples. The suckling from the child or other massage to the nipples releases the Oxytocin and therein discharges the milk into the cisterns of the breast, now ready for consumption [8].
Oxytocin plays an important role during partner reproduction. Most well known for its uses during childbirth, like contraction inducing, Oxytocin is involved much earlier in the process of reproduction. It has an immense role during the behavioral activity of mate selection [8]. Research claims these findings in humans, though humans are not always the subjects of testing. Most scientific experiments are hard to conduct on humans given the strict nature of environmental controls and invasive methods used. But tests involving personal reaction prove to be even harder in pinpointing accuracy, with emotions and feelings of love as the least accessible. When confronted with neurobiological tests, scientists tend to prefer using rodents, but “lab rats” do not have a wide span of social interaction, leaving studies of love almost impossible. Primates seemed like a good subject replacement for humans due to their intense social interactions and bonds. However, primates do not respond well to invasive tests which Oxytocin research requires. As the best solution so far, most research on social bonds and Oxytocin are conducted on precocial ungulates, like sheep, who develop filial bonds. Also used are prairie voles who much like humans form adult heterosexual bonds while being a monogamous species. With these commonalities, researchers link their findings from the latter animals to humans. That being said, science can relate the facts to society norms. As a monogamous species, we choose mates for life. Oxytocin proves on a chemical level that this bond is real, and not something fabricated. The release of this hormone facilitates the body in socially recognizing another being, as well as in building a level of trust between the two [17]. As a result of recognition, Oxytocin can also increase distress upon separation with a mate, which decreases at reunion. The effects of the hormone are not limited to pairs related sexually, but to nonsexual cohabitants as well. However, when intercourse is allowed, bonds form more quickly. These same factors between partners are shown even stronger in filial relationships. The bond between a parent and child is also facilitated by Oxytocin. But the sensitivity increases in social bonds are not limited to family groupings, but extend to societies as a whole. Oxytocin involved in peptidergic systems inhibits defensive behaviors caused by stress, anxiety and fear, allowing us to live closely with one another. [3,18].
The effect of Oxytocin on pregnant women has been the focus of past research, however recently researchers have begun to question the effects of Oxytocin on men and non-pregnant women. It is clear that Oxytocin, when released during pregnancy and lactation, has the effect of elevating feelings of trust and love between mother and child [2]. Through vaginal stimulation during birth, and breat stimulation during lactation, levels of Oxytocin increase in mothers, and increase the levels of touching and attachment between mother and child [13]. However, research has begun to examine the effects of Oxytocin throughout men and women. It is clear that the general effects of Oxytocin in increasing attachment and trust are felt in both men and women, however the levels of Oxytocin released vary between the sexes. Oxytocin release is greatly dependent on levels of estrogen in men and women. Women, having higher levels of estrogen, tend to have higher levels of Oxytocin and more activity in Oxytocin receptors then men [8]. Through the monitoring of Prarie Voles, a monogamous mammal, the anxiety in females following separation from their partner lasted longer than it did in males [3]. This may be due to higher levels of Oxytocin in females, allowing a greater bond to be formed, thus increasing the negative effects of a broken bond. As well, it was found in prairie voles that women formed stronger mate relationships more quickly than male voles [3,5]. This would also be due to the general higher levels of Oxytocin in females than males. It has also been determined that when humans were given a dose of Oxytcoin nasally, the ability to recognize faces increased [8]. Females were found to have better, and longer lasting, recognition and decisiveness than males, proving again the higher levels of Oxytocin in females than males [8,15]. The increased and longer lasting recognition reveals a stronger social connection felt by females. When males were treated with Oxytocin their abilities to recognize new words and store new words was lowered [8,16]. Levels of Oxytocin tend to increase in women during their menstral cycle as well, having a higher level of Oxytocin when a woman has her period. During sexual behavior the effects of Oxytcoin on men and women vary. Oxytocin levels are found to increase from their basal levels throughout sexual stimulation and reach a peek during orgasm [20]. Studies have been conducted, mainly in rodents, but the influence of the hormone has been clear. In male rodents injections of Oxytocin lowered the time it took for ejaculation, the amount of times ejaculation occurred, and the number of times the animal needed to pause before ejaculation [8,19]. Also, Oxytocin, along with testosterone, helped males to get erections, yet in high doses lowered frequency and length of erections [8]. Oxytocin is released in males during sexual interactions with a female and lowers the levels of anxiety in males for up to 30 minutes following orgasm [8,15]. These results reveal the males most likely felt more satisfied after sexual stimulation and had strong ejaculations. In humans Oxytocin receptors have been locate in parts of the penis, the corpus cavernosum and epididymis, which help to control contractions [8]. The location of these receptors helps to connect the effects of Oxytocin in rodents to the effects it has in humans. It can be assumed that when Oxytocin is released it increases the level of orgasm in men. In female rodents the presence of Oxytocin, in conjunction with estrogen, helps to regulate the time it takes a female to reach sexual maturity as well as the time it takes a woman to reach a period of sexual heat [8]. Oxytocin controls a female’s desire to have sex, while in increases a male’s sexual intensity. Oxytocin also helps the body of a female to assume to Lordosis position during sex; a position when the female’s back arches downward when she is mounted [first source on wiki]. This shows the effects of Oxytocin to facilitate a female’s ability to have sexual intercourse. In humans Oxytocin levels increase in women during sexual arousal and peak during orgasm. Through experimentation and records taken by women about their sexual arousal, it has been found that Oxytocin helps to increase arousal and lubrication in women [14]. Despite the differences in the effects of Oxytocin, the general effects of the hormone remain the same. Oxytocin is known to decrease anxiety and increase trust and pro-social behavior among males and females.
Oxytocin has been well known, throughout all mammals, for its facilitation of the contraction of the uterus muscles during childbirth, as well as the contraction of muscles in the breast necessary for lactation. However, recent research has begun to explore the implications of Oxytocin in social interactions. Oxytocin helps to increase trust, social recognition, and maternal bonds. It also decreases feelings of anxiety and fear. These social effects allow mammals to create social and emotional bonds. Oxytocin in found throughout the brain and is released in pulses by regions of the hypothalamus. Oxytocin is released in increased levels during physical stimulation. Current research has found a rise in Oxytocin levels during sexual stimulation and a climax in levels during orgasm. Women generally have a higher level of Oxytocin due to its sensitivity to Estrogen. For men and women the influence of Oxytocin during sexual arousal vary slightly; Oxytocin allow women be feel physically prepared for sexual intercourse, while it allows men to feel a stronger, more satisfying sexual climax. These effects of Oxytocin during sexual stimulation further the connection between mates and help to advance the reproduction process.

1. Klein, U; Gimpl, G; Fahrenholz F. Alteration of the Myometrial Plasma Membrane Cholesterol Content with .beta.-Cyclodextrin Modulates the Binding Affinity of the Oxytocin Receptor. Biochemistry. [online] 1995, 34 (42), 13784–13793. (Accessed December 1, 2010)

2. Kosfeld, M; Heinrichs, M; Zak, P; Fischbacher U; Fehr, E. Oxytocin increases trust in humans. Nature. [online] 2005, 435, 673-676. 10.1038/nature03701
(Accessed November 28, 2010)

3. Carter, C. S. Neuroendocrine perspectives on social attachment and love. Psychoneuroendocrinology. [online], 1998, 779−818 (Accessed December 3, 2010)

4. Uvnas-Moberg, K. Oxytocin may mediate the benefits of positive social interaction and emotions. Psychoneuroendocrinology, [online] 1998, 819−835 . (accessed December 3, 2010)

5. Insel, T. R. & Young, L. J. The neurobiology of attachment. Nature Rev. Neurosci, 129−136. (accessed November 28, 2010).

6. Landgraf, R.; Neumann, I. D. Vasopressin and oxytocin release within the brain: a dynamic concept of multiple and variable modes of neuropeptide communication. Front. Neuroendocrinol. 2004, 150−176 (accessed December 1, 2010)

7. Tabak, B; McCullough, M; Szeto, A; Mendez, A; McCabe, P. Oxytocin indexes relational distress following interpersonal harms in women.Psychoneuroendocrinology, 2010, 115-122. (accessed December 1, 2010)

8. Lee, HJ; Macbeth, AH; Pagani, JH; Young, WS ; "Oxytocin: the great facilitator of life". Progress in Neurobiology. 2009. 127–51. (accessed dec 1, 2010)


10. du Vigneaud; Vincent , TI; EXPERIENCES IN THE POLYPEPTIDE FIELD: INSULIN TO OXYTOCIN JO. Annals of the New York Academy of Sciences. 1960, 537 – 548. (accessed December 2, 2010)


12. Campbell, A; Attachment, aggression and affiliation: The role of oxytocin in female social behavior. Durham University, United Kingdom. 2008, 1-10. (accessed december 3, 2010)

13. Gordon, I; Zagoory-Sharon,O; Leckman, J; Feldman, R. Oxytocin, cortisol, and triadic family interactions. Physiology & Behavior Volume 101, Issue 5, 2 December 2010, Pages 679-684. (accessed december 3, 2010)

14. DOI: 10.1007/BF01541816Taylor, J; Rosen, R; Leiblum, S;
Self-Report Assessment of Female Sexual Function: Psychometrie Evaluation of the Brief Index of Sexual Functioning for Women. Archives of Sexual Behavior, 1994. (accessed december 1, 2010)

15. Bancroft J; "The endocrinology of sexual arousal". The Journal of Endocrinology, 2005, 411–27. (Accessed dec 1, 2010)

16. Carter, S; Developmental consequences of oxytocin, Physiol. Behav. 79 (2003), pp. 383–397. [online] (accessed December 2, 2010)

17. Young, L; Wang,, Z; The neurobiology of pair bonding. Nature Neuroscience, 2004, 1048 – 1054. (accessed December 2, 2010)

18. Marazziti D, Dell'Osso B, Baroni S. "A relationship between oxytocin and anxiety of romantic attachment". Clinical Practice and Epidemiology in Mental Health 2: (2006). 28. (accessed November 20, 2010)

19. Krüger TH, Haake P, Chereath D, "Specificity of the neuroendocrine response to orgasm during sexual arousal in men". The Journal of Endocrinology 177 (1): April 2003. 57–64. Http:// 10.1677/joe.0.1770057. (accessed December 3, 2010)

20. Carmichael MS, Warburton VL, Dixen J, Davidson JM "Relationships among cardiovascular, muscular, and oxytocin responses during human sexual activity". Archives of Sexual Behavior (February 1994). 59–79. (accessed December 3, 2010)