Uncontrollable Love: A Guest Post

By Yunhan Zhao

Image from Freedigitalphotos.net

What is love? Under Shakespeare’s leather pen, love is the tragedy of Romeo and Juliet. In the poet’s eyes, love is the courage of Paris to elope with Helen and stand against the world. In Pretty Woman love doesn’t care about social status or wealth. When people fall in love the whole world feels brighter and more vibrant. Orpheus and Eurydice, Jane Eyre and Rochester, Darcy and Elizabeth…thousands of romantic stories, poems and songs illustrate the countless versions of love tales. Now scientists have revealed their scientific version of the love tale. They say love is comprised of magic chemicals interacting with our brains.

Photo of a prairie vole pair from Young,
Gobrogge, Liu and Wang paper in
Frontiers in Neuroendocrinology (2011)

In 1999, researchers Mary M. Cho, Courtney De Vries, Jessie Williams and Sue Carter at the University of Maryland conducted NIMH funded research aimed to unlock the secret behind ‘love.’ By using the monogamous prairie vole (Microtus ochrogaster) as subjects, they investigated the important neurophysiological events that contribute to the development and maintenance of pair bonding between heterosexual adults. Here they designed an experiment to test the hypothesis that oxytocin and vasopressin (chemicals that are released in the brain during sexual behavior) facilitate the development of partner preference.

In their experiments, the researchers randomly assigned prairie voles to three different dose groups of oxytocin, three different dose groups of vasopressin and a control group which used cerebrospinal fluid. After receiving injections directly to their brains, each prairie vole was housed with an opposite sex partner for one hour (referred as cohabitation below). Then partner preference was assessed immediately by measuring the time spent in physical contact with either the familiar partner or a stranger vole.

Researchers found that the test subjects made stronger partner preferences depending on which kind of injection they received. As expected, results showed that in male prairie voles, almost all the vasopressin and oxytocin treatments (except the lowest dose of oxytocin) led to a loyal choice for the cohabiting partners over an unacquainted female. In females, however, only the highest dose of vasopressin and oxytocin treatments resulted in a loyal choice. In contrast, with the cerebrospinal fluid control treatment in both genders of voles, the test voles were always equally attracted to their previous partners and the unacquainted strangers.

Partner preferences of male (left) and female (right) after receiving different injections.
OT = oxytocin, AVP = vasopressin, CTL = cerebrospinal fluid. Figure from Cho, M.,
DeVries, A., Williams, J., & Carter, C. (1999). The effects of oxytocin and vasopressin
on partner preferences in male and female prairie voles (Microtus ochrogaster).
Behavioral Neuroscience, 113(5), 1071-1079.

This experiment found that high doses of both vasopressin and oxytocin may indeed associate with high partner fidelity in both gender of prairie voles. However, are these two chemicals the love elements in humans as well? Does it mean in the near future people can prevent divorces with only a few chemical injections to their spouses? The answer is still not clear. Especially since scientists have recently revealed that vasopressin and oxytocin may relate to distress in pair-bonded human relationships.

Shelly Taylor, Shimon Saphire-Bernstein and Teresa Seeman from the University of California, Los Angeles assessed 85 young adults in committed relationships. They asked participants to complete a series of self-reported questionnaires which included items like “how often they argue with you,” “how often they criticize you,” and “how often they get on your nerves”. Additionally, they also had the participants’ blood drawn within a week of finishing the assessments.

Image from Freedigitalphotos.net

Analysis showed that the elevated plasma oxytocin levels were associated with relationship distress in women while the increased plasma vasopressin levels were associated with relationship distress in men. But what should be noticed is that the direction of such effects remains unknown. Although no evidence revealed the exact relationship between humans’ plasma vasopressin and oxytocin levels and relationship distress, it appears that these two chemicals may be associated with very different emotional effects in voles and humans. However, an alternative explanation could also be that oxytocin and vasopressin contribute differently in different stages or contexts of romantic relationships.

Love seems uncontrollable even in the scientific world. Prairie voles crave emotional and sexual union after being injected with a high level of oxytocin and vasopressin into their brains, but people with the high presence of the same chemicals become vulnerable to relationship problems. So what is the key here?

References

1. Cho, M., DeVries, A., Williams, J., & Carter, C. (1999). The effects of oxytocin and vasopressin on partner preferences in male and female prairie voles (Microtus ochrogaster) Behavioral Neuroscience, 113 (5), 1071-1079 DOI: 10.1037//0735-7044.113.5.1071

2. Taylor, S.E., Saphire-Bernstein, S., & Seeman, T.E. (2009). Are Plasma Oxytocin in Women and Plasma Vasopressin in Men Biomarkers of Distressed Pair-Bond Relationships? Psychological Science, 2010 (21), 3-7 DOI: 10.1177/0956797609356507