Prolactinis well-known for its role in stimulating milk production in nursing mothers but a new study published in the online journal PLoS One identifies a previously unrecognized function of this hormone – its contribution to sexual behavior and pair-bonding.
Breast growth is stimulated by increasing prolactin secretion throughout pregnancy. Lactation is inhibited during pregnancy by progesterone produced by the placenta. Progesterone interferes with this hormone binding to the receptors on the alveolar cells within the breast, thereby directly suppressing milk production.
Prolactin is secreted from the pituitary gland in response to eating, mating, estrogen treatment, ovulation and nursing. It is secreted heavily in pulses in between these events. This hormone plays an essential role in metabolism, regulation of the immune system and pancreatic development.
Discovered in non-human animals around 1930 by Oscar Riddle and confirmed in humans in 1970 by Henry Friesen.
In the study published in the online journal PLoS One, the investigators used cotton-top tamarins, a socially-monogamous, small monkey native to Colombia that live in family groups where both parents help care for the young.
The authors measured chronic urinary prolactin levels over a four week period to include the entire female ovulatory cycle and correlated prolactin levels in males and females with simultaneous measures of contact affiliation and sexual behavior.
The study found that chronic peripheral levels of this hormone correlated significantly with both sexual behaviorand contact affiliation (time spent in grooming and huddling) in both sexes. The hormone levels were high among pairs that frequently had sex and cuddled, and low among mothers that had finished nursing.
The novelty: This study provides the first results relating urinary prolactin levels in adults with measures of mate affiliation and sexual behavior, and parallel previous results with urinary oxytocin in cotton-top tamarins. Within pairs prolactin levels of males and females were correlated and variation in this hormone levels was explained by variation in amount of sexual behavior.
These results substantiate few recent studies suggesting a parallel between oxytocin and prolactin.
Charles Snowdon, first author of the study and an emeritus professor of psychology at University of Wisconsin-Madison believes that “the behavioral aspects of prolactin have received less study than those of oxytocin“.
According to him the discovery 25 years ago that oxytocin had an important role in pair bonding “was a conceptual breakthrough that oxytocin was not just about parenting, or the mother-infant bond, but about the pair bond between the adults;” “now we are finding something similar for prolactin, which is a hormone with different physical effects.”
Other recent research indicates increased levels of this hormone after orgasm in humans and during infant carrying in marmosets. The authors of the PLoS One study believe that these results taken together with their results suggest that the hormone may be acting as a reward for parenting as well as for social sexual interactions with a partner.
Update: A 2016 review, by Genaro A Coria-Avila et al. concluded: In humans, concentrations of arousing neurotransmitters and potential bonding neurotransmitters increase during orgasm in the cerebrospinal fluid and the bloodstream. Similarly, studies in animals indicate that those neurotransmitters (noradrenaline, oxytocin, prolactin) and others (e.g. dopamine, opioids, serotonin) modulate the appetitive and consummatory phases of sexual behavior and reward.
This suggests a link between the experience of orgasm/sexual reward and the neurochemical mechanisms of pair bonding. Orgasm/reward functions as an unconditioned stimulus (UCS). Some areas in the nervous system function as UCS-detection centers, which become activated during orgasm. Partner-related cues function as conditioned stimuli (CS) and are processed in CS-detector centers.
It isn’t surprising that he should prefer his mistress, whose features, to him, offer a hundred units. Even little facial imperfections on other women, such as smallpox scar, touch the heart of a man in love, inspiring a deep reverie; imagine the effect when they are on his mistress’s face. The fact is, that pockmark means a thousand things to him, mostly lovely and all fully interesting. The sight of a scar, even on another woman’s face will strongly remind him of all these things.
(Stendhal, 1822/2013). De l’amour (‘On love’), Chapter 17 (p. 53)