Molecule of the Month: Oxytocin

Literature, religion and philosophy have sought to understand and rationalise the multifaceted aspects of love through poetry, prose and songs. Shakespeare, Rumi and Ariana Grande have tried to encapsulate the heart wrenching passion of romance and why we humans seem to subject ourselves to devotion.

Recently, science has become interested in the physiological and physical process which underlie the feeling of love. Some researchers go as far as to propose that romantic love, specifically, is a natural addiction which has evolved from our ancesotors1

Away from romance, we know that there are many forms of love we are able to experience over a lifetime. The love between the birth-giver and a child is of especial importance as it is the first impression of pair-bonding which the infant partakes in.

The commonality and universal nature of love doesn’t stop at the feelings surrounding them.  However, there is a widening agreement that oxytocin is able to play apart in the formation of social bonds in both passionate and parental loves.

Heart of the issue

Figure 1: Chemical structure of oxytocin which have increasingly been shown to be implicated in both the parental and romantic forms of love.

Oxytocin (Figure 1)2 is a hormone protein made up of only nine amino acid joined together by peptide bonds and other interactions3. Although, the hormone is stored and released from the pituitary gland (Figure 2)4 it is however, made within the hypothalamus region of the brain5.

Figure 2: Shows the hypothalamus where oxytocin is made and the pituitary gland, where oxytocin is stored and then subsequently released into the rest of the body.

Its primary physical function has been found to promote milk production in people who breast-feed6. However, within people who have a penis it is thought to be one of the most potent chemicals which induce an erection7. However, this hormone is now being recognised as an important regulator of social behaviour including the formation of bonds between both parents and partners8.

Oxytocin is able to illicit its various physical and physiological functions through receptors. Receptors are structures on the surface of cells which oxytocin is able to bind to. Binding of oxytocin to a receptor activates internal signalling pathways of the cell which, in turn, triggers the various functions outlined above9.

Parental Bonds

Oxytocin is one of the main hormones that are present at the miracle of childbirth. Several studies have presented that there is a three-to-four times increase of oxytocin, from baseline, during the course of pregnancy10. Additionally, an increase duration, frequency and amplitude of oxytocin release was demonstrated towards the end of labour, right up until birth. 

Oxytocin is known to part of the mechanism that enables both childbirth and lactation to take place11.During childbirth, oxytocin is released, which causes the contraction of muscles within the uterus. Interestingly, more contraction leads to the release of even more oxytocin thereby increasing the strength and rate of contractions, for the birthing process to come to completion5. Contractions are not only important in birthing, but also, play an essential role to minimise post-birth haemorrhage12.

However, some research has suggested that oxytocin does not appear to be as essential to birth as once thought. This is mainly due to the many instances where labour has happened normally where the pituitary gland is not functioning properly11. Therefore, other signalling pathways through a group of hormones called prostaglandins or even through neuronal networks are thought to compensate or even play a larger role in the birthing process13.

However, oxytocin not only plays a part in the physical process of childbirth, but also is involved in the emotional processing of a child. Oxytocin overall helps the person giving birth to the child to bond with their baby. and it has been shown to decrease the experience of pain involved with birth10. Most studies found a positive correlation between parent-infant contact and oxytocin levels right after childbirth. Unsurprisingly, increased oxytocin in the birth-giver were significantly related to more affectionate contact between them and the child14.

This is of significance as the social and behavioural bonds made between the child and its parents are important for future relationships, whether they be romantic or platonic in nature15,16.

Romantic Bonds

From the first murmurings of a crush, to the exhilarating moments of passion, it has been determined that establishing and maintaining romantic relationships can contribute to our mental and physical health so much so that it underpins parts of our well-being17.

Oxytocin is also associated with the bond between lovers as areas of the brain, which support the formation of romantic attachments, are rich in oxytocin receptors where its effects can be circulated18. Additionally, oxytocin levels between new lovers were substantially higher compared to those in non-attached singles, presenting its large role in the initial, romantic bond formation between people19. These results are additionally consistent within prairie voles which are known to be monogamous20

Older studies have presented that oxytocin plays a role in the emotional aspects of love and companionship which encompasses the feeling of care and trust. Greater partner support was linked to higher blood oxytocin level within couples of binary sexes21. Such findings could give some reasoning to why couples who decided to stay together during a study duration showed higher oxytocin levels at the initial period of romantic attachment, compared to those who broke-up19. This suggests that oxytocin could potentially be an indicator of relationship duration.

Oxytocin not only plays a large part in the emotional aspects of love such as companionship and trust, but it is also deeply tied to the physical aspects of romantic love. Following light touches, there has been a measurable rise in oxytocin levels22. On the more steamy end of the scale, a number of studies have shown that following orgasm, of binary sexes, there are higher levels of oxytocin within the blood than previously measured23,24.

One and only?

However, it is important to note that both within parental and romantic attachments oxytocin is not the only hormone eliciting some of the physical and physiological effects. Other hormones, such as vasopressin, cortisol, and sex hormones such as testosterone and oestrogen can play apart in bond formations25,26,27.

Vasopressin, although very structurally similar to oxytocin, is thought to support behaviours needed for guarding a partner or the more possessive side of attachment, such as jealousy within romantic relationships28,29. Within people who birth, vasopressin of the birth-giver is thought to be implicated in greater new born weight loss, however this requires further study30. Interestingly, vasopressin is assumed to play a larger role within expectant fathers within a singular study further compounding vasopressin as a protective hormone31.

Wired to love

Science has tried its best to encapsulate many of the physical and psychological processes which occur when we are exposed to love, whether that be from a partner or a parent-figure. Surprisingly, both the initial stages of romantic and parental love have some of the same behaviours, mental states and activation of brain regions which can be explained through the actions of oxytocin19.

Various cynics of love may comment that the action of love is simply a neurological and physical scam, given that oxytocin and other hormones play a large part in this process. However, looking to understand that love is so tightly sewn into every molecule of body and cell will surely see us through every heartbreak and disappointment.

References:

1.         Fisher, H. E., Xu, X., Aron, A. & Brown, L. L. Intense, Passionate, Romantic Love: A Natural Addiction? How the Fields That Investigate Romance and Substance Abuse Can Inform Each Other. Front. Psychol. 7, (2016).

2.         Koçyiğit, Ü. The Effects of Oxytocin and Oxytocin Receptor Antagonist Atosiban on the Carbonic Anhydrase and Acetylcholinesterase Enzymes from Lung Tissues of Rats. Cumhur. Sci. J. 38, 450–460 (2017).

3.         Lee, H.-J., Macbeth, A. H., Pagani, J. & Young, W. S. Oxytocin: the Great Facilitator of Life. Prog. Neurobiol. 88, 127–151 (2009).

4.         Mayo Foundation for Medical Education and Research. Pituitary gland and hypothalamus. Mayo Clinic https://www.mayoclinic.org/pituitary-gland-and-hypothalamus/img-20005849.

5.         Osilla, E. V. & Sharma, S. Oxytocin. in StatPearls (StatPearls Publishing, 2020).

6.         Uvnäs­Moberg, K. et al. Maternal plasma levels of oxytocin during breastfeeding—A systematic review. PLOS ONE 15, e0235806 (2020).

7.         Thackare, H., Nicholson, H. D. & Whittington, K. Oxytocin—its role in male reproduction and new potential therapeutic uses. Hum. Reprod. Update 12, 437–448 (2006).

8.         Cochran, D., Fallon, D., Hill, M. & Frazier, J. A. The role of oxytocin in psychiatric disorders: A review of biological and therapeutic research findings. Harv. Rev. Psychiatry 21, 219–247 (2013).

9.         Vrachnis, N., Malamas, F. M., Sifakis, S., Deligeoroglou, E. & Iliodromiti, Z. The Oxytocin-Oxytocin Receptor System and Its Antagonists as Tocolytic Agents. International Journal of Endocrinology vol. 2011 e350546 https://www.hindawi.com/journals/ije/2011/350546/ (2011).

10.       Uvnäs-Moberg, K. et al. Maternal plasma levels of oxytocin during physiological childbirth – a systematic review with implications for uterine contractions and central actions of oxytocin. BMC Pregnancy Childbirth 19, 285 (2019).

11.       Arrowsmith, S. & Wray, S. Oxytocin: its mechanism of action and receptor signalling in the myometrium. J. Neuroendocrinol. 26, 356–369 (2014).

12.       McEvoy, A. & Sabir, S. Physiology, Pregnancy Contractions. in StatPearls (StatPearls Publishing, 2020).

13.       Husslein, P. [The importance of oxytocin and prostaglandins to the mechanism of labor in humans]. Wien. Klin. Wochenschr. Suppl. 155, 1–32 (1984).

14.       Scatliffe, N., Casavant, S., Vittner, D. & Cong, X. Oxytocin and early parent-infant interactions: A systematic review. Int. J. Nurs. Sci. 6, 445–453 (2019).

15.       Feldman, R., Gordon, I., Influs, M., Gutbir, T. & Ebstein, R. P. Parental oxytocin and early caregiving jointly shape children’s oxytocin response and social reciprocity. Neuropsychopharmacol. Off. Publ. Am. Coll. Neuropsychopharmacol. 38, 1154–1162 (2013).

16.       Santona, A., De Cesare, P., Tognasso, G., De Franceschi, M. & Sciandra, A. The Mediating Role of Romantic Attachment in the Relationship Between Attachment to Parents and Aggression. Front. Psychol. 10, (2019).

17.       Gómez-López, M., Viejo, C. & Ortega-Ruiz, R. Well-Being and Romantic Relationships: A Systematic Review in Adolescence and Emerging Adulthood. Int. J. Environ. Res. Public. Health 16, (2019).

18.       Acevedo, B. P., Aron, A., Fisher, H. E. & Brown, L. L. Neural correlates of long-term intense romantic love. Soc. Cogn. Affect. Neurosci. 7, 145–159 (2012).

19.       Schneiderman, I., Zagoory-Sharon, O., Leckman, J. F. & Feldman, R. Oxytocin during the initial stages of romantic attachment: Relations to couples’ interactive reciprocity. Psychoneuroendocrinology 37, 1277–1285 (2012).

20.       Cormier, Z. Gene switches make prairie voles fall in love. Nat. News doi:10.1038/nature.2013.13112.

21.       Grewen, K. M., Girdler, S. S., Amico, J. & Light, K. C. Effects of Partner Support on Resting Oxytocin, Cortisol, Norepinephrine, and Blood Pressure Before and After Warm Partner Contact. Psychosom. Med. 67, 531–538 (2005).

22.       Hurlemann, R. & Scheele, D. Dissecting the Role of Oxytocin in the Formation and Loss of Social Relationships. Biol. Psychiatry 79, 185–193 (2016).

23.       Carmichael, M. S. et al. Plasma oxytocin increases in the human sexual response. J. Clin. Endocrinol. Metab. 64, 27–31 (1987).

24.       Carmichael, M. S., Warburton, V. L., Dixen, J. & Davidson, J. M. Relationships among cardiovascular, muscular, and oxytocin responses during human sexual activity. Arch. Sex. Behav. 23, 59–79 (1994).

25.       Carter, C. S. & Keverne, E. B. 4 – The Neurobiology of Social Affiliation and Pair Bonding. in Hormones, Brain and Behavior (Second Edition) (eds. Pfaff, D. W., Arnold, A. P., Etgen, A. M., Fahrbach, S. E. & Rubin, R. T.) 137–166 (Academic Press, 2009). doi:10.1016/B978-008088783-8.00004-8.

26.       Gray, P. B. et al. Human male pair bonding and testosterone. Hum. Nat. 15, 119–131 (2004).

27.       Grøntvedt, T. V., Grebe, N. M., Kennair, L. E. O. & Gangestad, S. W. Estrogenic and progestogenic effects of hormonal contraceptives in relation to sexual behavior: insights into extended sexuality. Evol. Hum. Behav. 38, 283–292 (2017).

28.       Carter, C. S. The Role of Oxytocin and Vasopressin in Attachment. Psychodyn. Psychiatry 45, 499–517 (2017).

29.       Baribeau, D. A. & Anagnostou, E. Oxytocin and vasopressin: linking pituitary neuropeptides and their receptors to social neurocircuits. Front. Neurosci. 9, (2015).

30.       Erickson, E. N., Carter, C. S. & Emeis, C. L. Oxytocin, Vasopressin and Prolactin in New Breastfeeding Mothers: Relationship to Clinical Characteristics and Infant Weight Loss. J. Hum. Lact. Off. J. Int. Lact. Consult. Assoc. 36, 136–145 (2020).

31.       Alyousefi-van Dijk, K. et al. Vasopressin Differentially Affects Handgrip Force of Expectant Fathers in Reaction to Own and Unknown Infant Faces. Front. Behav. Neurosci. 13, (2019).

Photo credits:

Heart Sweets by Anete Lusina from Pexels

Standing Sunset by Lisa Fotios from Pexels

Couple by Ketut Subiyanto from Pexels

Wedding cake by SplitShire from Pexels

Flowers by Ketut Subiyanto from Pexels

Heart Icon by Freepik from www.flaticon.com

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