If hormones could win popularity contests, oxytocin might well be queen of the day. Given oxytocin's connection to such life-affirming activities as maternal behavior, lactation, selective social bonding and sexual pleasure, researchers have been working overtime to uncover its role in the brain and in regulating behavior.
Oxytocin is produced mainly in the hypothalamus, where it is either released into the blood via the pituitary gland, or to other parts of the brain and spinal cord, where it binds to oxytocin receptors to influence behavior and physiology.
The excitement over the hormone began in the 1990s when researchers discovered that breastfeeding women are calmer in the face of exercise and psychosocial stress than bottle-feeding mothers. But more recent research has shown other roles for the hormone, too: Oxytocin levels are high under stressful conditions, such as social isolation and unhappy relationships.
A major question in the field, therefore, is whether oxytocin operates differently when it is released in response to socially connective experiences and when it is released under stressful conditions, says C. Sue Carter, PhD, co-director of the Brain Body Center at the University of Illinois at Chicago (UIC), and one of the first to study oxytocin in animals.
Researchers are also probing whether administering oxytocin in short-term doses can induce different feelings such as trust and generosity (see "Can oxytocin promote trust and generosity"). But much remains to be seen about how such studies integrate into the broader literature on the hormone in its natural state, says social psychologist Shelley E. Taylor, PhD, who directs the University of California, Los Angeles, Social Neuroscience Lab.
"These experimental paradigms don't necessarily mimic how oxytocin functions in the real world," she says. "Before we can produce an accurate model of oxytocin, we need to be able to reconcile the [naturalistic] plasma studies with the exogenous administration studies."
Its role in affiliation...
New studies are adding to a body of literature that shows oxytocin plays a key role in maternal bonding and social affiliation-what Taylor has labeled the "tend and befriend" response, as opposed to the "fight or flight" response. In line with years of animal research linking oxytocin to mothers' ability to care for their infants, a study in the November Psychological Science (Vol. 18, No. 11, pages 965-970), demonstrates this association for the first time in people.
In the study, psychologist Ruth Feldman, PhD, of Bar-Ilan University, and colleagues measured plasma levels of oxytocin in 62 pregnant women at three points: during the first trimester, the third trimester and the first month postpartum. The team found that women with higher levels of oxytocin in the first trimester bonded better with their babies. It also found that those with higher oxytocin levels throughout the entire pregnancy and in the first month postpartum reported more behaviors that supported the formation of an exclusive relationship with their babies, such as singing special songs or bathing and feeding their infants in specific ways. (Because of its role in birth and lactation, oxytocin was originally considered a "female" hormone, but it is now known to be present and important in both sexes.)
Likewise, recent animal studies demonstrate a developmental relationship between exposure to extra oxytocin in early life and subsequent maternal and social behaviors. In the August issue of Hormones and Behavior (Vol. 52, No. 2, pages 274-279), for example, psychobiologist Karen L. Bales, PhD, of the University of California, Davis, and colleagues found that when they administered graded levels of oxytocin to female prairie voles shortly after birth, the animals in adulthood showed differential responses to pups, in this case belonging to other females. For instance, when they had received a low dose of oxytocin in early life, adult females were slow to approach pups; when they received higher doses of the hormone, they were more likely to care for them.
Human studies likewise confirm the notion that early experiences, possibly mediated by oxytocin, are related to later social behaviors, finds a study reported in the Nov. 22, 2005, Proceedings of the National Academy of Sciences (Vol. 102, No. 47, pages 16,907-16,908). There, University of Wisconsin-Madison doctoral student Alison Wismer Fries and colleagues compared urine levels of oxytocin and a related hormone, vasopressin, in two sets of children-one raised from birth with their biological parents and one adopted after living in orphanages in Russia and Romania-following contact with their mothers. The levels of oxytocin rose in the biological children but remained the same in the adopted children, they found. These findings suggest there may be biological underpinnings for the observation that some adopted children, in particular those from deprived circumstances, have difficulty forming secure relationships, despite living in loving homes, the team notes.
...and under social stress
Recent studies in both animals and humans support the notion that oxytocin is also part of a response to social separation and related stress. A study reported in the March-April 2006 issue of Psychosomatic Medicine (Vol. 66, No. 2, pages 238-245) by Taylor and colleagues, for example, found that women who reported more gaps in their social relationships and less positive relationships with their primary partners had higher levels of oxytocin and the stress hormone cortisol than those reporting better relationships.
Meanwhile, research by UIC researchers Angela Grippo, PhD, Stephen W. Porges, PhD, and Carter, finds elevated levels of oxytocin as well as other exaggerated stress responses in female prairie voles that had been separated from others. The research on voles suggests that even when oxytocin is elevated under times of stress, externally administering the hormone may still mitigate those stress reactions.
In a study presented at the 2007 Society for Neuroscience meeting, Grippo, Porges and Carter compared the stress reactions of female prairie voles living for four weeks either in isolation or with a female sibling and found greater levels of stress, behavioral anxiety and depression in those separated from their siblings. The team then gave the animals either oxytocin or saline every day during the last two weeks of the four-week period. The isolated animals treated with oxytocin no longer showed signs of depression, anxiety or cardiac stress. By contrast, oxytocin had no measurable effects on those paired with siblings, suggesting that "the effects of oxytocin are most apparent under stressful conditions," Carter says.
Could externally administered oxytocin help people with social and emotional difficulties, stresses or deficits? Researchers think that's a possibility.
In one study in the area, psychiatrist Eric Hollander, MD, of Mount Sinai School of Medicine, and colleagues found that adults diagnosed with autism or Asperger's disorder who received oxytocin injections showed an improved ability to identify emotional content on a speech comprehension task, while those on a placebo did not.
"These findings suggest that oxytocin might facilitate social information processing in those with autism, and provide preliminary support for the use of oxytocin in the treatment of autism," the authors write. Other investigators are examining possible applications to social anxiety, schizophrenia and depression.
Bringing the strands together
While the answers are certainly not all in, a possible reason for the roles that oxytocin appears to play is that it has dual purposes and pathways, speculates Taylor.
When it is operating during times of low stress, oxytocin physiologically rewards those who maintain good social bonds with feelings of well-being. But when it comes on board during times of high social stress or pain, it may "lead people to seek out more and better social contacts," says Taylor.
Taylor also considers possible physiological reasons for these differences. In a December 2006 paper in Current Directions in Psychological Science (Vol. 15, No. 6, pages 273-277), she speculates that actual or anticipated social contact may result in bursts of oxytocin; exogenously given oxytocin may operate in a similar manner, she notes.
However, if a person is experiencing the stress of social deficits, he or she may show elevated levels of circulating oxytocin, that is, oxytocin already present in the bloodstream.
This stress-related manifestation of oxytocin may produce physiological changes that then encourage people to seek contact with others, she surmises. Taylor is now examining these theories and others with people in the lab.
Recent research from the UIC group supports an alternative hypothesis, first articulated by Porges in 1998, says Carter. He posited that oxytocin, acting in part through effects on the autonomic nervous system, might allow what he termed "immobility without fear." In other words, oxytocin may in general protect the nervous system from shutting down in the face of stressful circumstances, especially those that require holding still rather than fighting or fleeing. That includes even events that may seem positive from a societal standpoint, such as birth.
One clue this may be so? "Post-traumatic stress disorder is not usually associated with birth, although it's a tremendously stressful experience," says Carter. Oxytocin, she says, may play an important role in helping females manage both emotional and physiological responses during life-altering events such as childbirth-converting potentially stressful experiences into opportunities for expressing joy and love.
Tori DeAngelis is a writer in Syracuse, N.Y.