More frequent partner hugs and higher oxytocin levels are linked to lower blood pressure and heart rate in premenopausal women
Introduction
Emotional support from a spouse or long-term partner is related to lower risk of cardiovascular and all-cause mortality (Berkman, 1995, Knox and Uvnas-Moberg, 1998, Kiecolt-Glaser and Newton, 2001, Tower et al., 2002). Blood pressure (BP) is especially sensitive to supportive and non-supportive interactions between partners (Ewart et al., 1991, Carels et al., 1998, Broadwell and Light, 1999, Gump et al., 2001, Grewen et al., 2003, Holt-Lunstad et al., 2003). Communication of emotional support is accomplished though multiple, complex modalities, including facial expression and body language as well as choice of words, emotional quality of speech, and listening/responding patterns that convey positive emotion and connectedness while couples interact, all of which may influence BP (Uchino et al., 1996, Denton et al., 2001, Gottman and Notarius, 2002, Broadwell and Light, 2004). Emotional support and affection in couples is also expressed through physical touch, such as hand-holding, hugs, and sitting or lying “cuddled up” (Diamond, 2000, Grewen et al., 2004a, Grewen et al., 2004b).
A number of experts (Carter, 1998, Uvnas-Moberg, 1998, Uvnas-Moberg, 2004, Insel and Young, 2001, Taylor, 2002, Moyer et al., 2004) have hypothesized that enhanced oxytocin (OT) activity is a logical candidate to be one of the primary physiological mediators of the health benefits of emotional support, particularly those linked to warm touch. Although best known for its role in parturition, breast-feeding and initiation of maternal behavior, OT is a hypothalamic neuropeptide shown in animal models to be critically involved in important social behaviors including social recognition, partner preference and, in certain species, monogamous pair-bonding (Williams et al., 1994, Pedersen, 1997, Carter et al., 2001, Ferguson et al., 2002, Pedersen and Boccia, 2002, Bales and Carter, 2003, Champagne et al., 2003, Choleris et al., 2003). Furthermore, increases in endogenous OT activity are elicited by massagelike stroking in both infant and adult mammals (Uvnas-Moberg, 1998, Uvnas-Moberg, 2004), although recent work by Lund et al. (2002) confirmed that increases in plasma levels of OT in rats reflect the cumulative effect of repeated episodes of stroking. In their study, plasma OT increases were significant after 14 days but not after 3 days of such stroking.
OT has both central and peripheral actions on cardiovascular function (Petersson et al., 1996, Gutkowska et al., 2000, Petersson, 2002). Although a single dose of OT in rats leads to BP increases, not decreases, daily OT administration (or massagelike stroking to enhance endogenous OT activity) for 5–14 days leads to enduring BP reductions that far outlast the intervention (Petersson et al., 1999, Holst et al., 2002). Enhanced OT activity of this kind leads to inhibition of central and peripheral alpha-adrenergic and hypothalamic-pituitary-adrenal (HPA) activity, while promoting parasympathetic cardiac control (Diaz-Cabale et al., 2000, Janowski et al., 2000, Mukaddam-Daher et al., 2001). Co-localized estrogen and OT receptors influence each other, such that OT activity can have greater cardiovascular effects in cycling vs. ovariectomized females or males (Petersson et al., 1999, Holst et al., 2002).
Despite the extensive literature on OT in animal models, relatively few published studies on human OT responses exist to date. This paucity of findings is due in part to the fact that, unlike animal research that can assess OT mRNA or use central administration of OT antagonists, in humans, researchers must rely upon less direct measures to index oxytocinergic activity: plasma levels of OT and OT-precursor peptides. These peptide levels in humans appear to reflect both general oxytocinergic activity levels of the past weeks, such as increases in OT intermediate peptide seen in post-menopausal women on estrogen replacement (Crowley et al., 1995, Amico and Hempel, 1990, Bossmar et al., 1995, Light et al., 2004b), as well as immediate powerful stimuli in the past few minutes. In fact, there have been few demonstrations of an acute stimulus (other than breast stimulation or nursing in post-partum women) that elicits a consistent increase in plasma OT, or a personal characteristic (such as maternal attachment for her infant, or spouse/partner relationship quality) that is linked to higher overall plasma levels of OT. Single episodes of massage have been shown to elicit increases in plasma OT levels in some individuals but not consistently enough to yield reliable group effects (Turner et al., 1999, Wikstrom et al., 2003). However, given the findings of Lund et al. (2002) on the need for many days of massage repetitions to elicit increases in plasma OT in animals, this inconsistent effect of a single episode of massage on human plasma OT levels is not surprising.
In an initial study of effects of warm contact with loved ones on OT, Light et al. (2000) studied OT and BP responses before and during a speech task in 24 mothers of infants on 2 days, once after holding their babies and once after a control rest alone. BP levels were lower on both test days before, during and after the speech task in mothers whose OT levels increased versus decreased over baseline levels in samples obtained 5 min after baby holding (during the task). The OT increase group did not show a reliable OT increase to the speech task when tested without their babies, so the prior warm contact was critical in eliciting an OT response. An extension of this work comparing responses of these healthy mothers with mothers who had been exposed to cocaine during pregnancy (Light et al., 2004a) confirmed that the cocaine exposed group had lower OT levels and higher BP and norepinephrine (NE) levels both in the lab on the no baby contact day and during ambulatory monitoring at home. The cocaine exposed mothers also showed a tendency to hold their babies less often at home. In rats, cocaine exposure during pregnancy disrupts normal post-partum OT activity and maternal behavior (Elliott et al., 2001). These findings link greater mother–infant warm contact time to higher OT activity and lower BP in post-partum women.
Next, we examined OT response and resting BP in 38 couples before, during and after a 10 min period of warm partner contact ending with a 20 s hug (Grewen et al., 2004a, Grewen et al., 2004b). In this study, which involved no stressors, both men and women with more supportive partners showed higher levels of plasma OT before, during and after the warm contact period. Women (but not men) with greater partner support showed lower baseline SBP and plasma NE levels, and OT met criteria as significant mediator of the reduced NE levels (but not the lower BP) in these women. Grewen et al., 2004a, Grewen et al., 2004b also reported on a pilot study involving 11 subjects retested four different times, during both rest and stress sessions, both with and without partner contact. OT responses were higher during rest vs. stress sessions with warm partner contact.
Thus, in the present protocol where partner contact preceded a speech stressor, we elected to focus on plasma OT levels of premenopausal women during baseline prior to warm partner contact rather than on OT responses during the stressor, and to determine whether women with higher OT levels had lower BP and HR: (1) during a resting baseline period just prior to warm partner contact, and (2) during a speech task that followed warm contact. We further examined whether greater self-reported frequency of one type of warm touch between partners, Partner Hugs, predicted lower BP and HR during baseline and/or stress events. Finally we attempted to test whether OT activity (indexed by plasma levels) may be a partial mediator of the expected relationships between greater Partner Hugs and lower cardiovascular responses. As a methodological issue, we also examined whether plasma OT levels differ by menstrual cycle phase, to determine if cycle phase must be controlled in this and future research on OT.
Section snippets
Methods
Premenopausal women aged 20–49-years old (n = 59) were recruited using local newspaper advertisements and fliers. Subjects were required to be living with current spouse or monogamous partner for at least 6 months; these partners participated with the women in our study by providing them with 10 min of warm physical and emotional contact during the testing session. Reasons for exclusion included current use of prescription medication affecting the cardiovascular and autonomic nervous system,
Cardiovascular responses of high, moderate and low OT groups
Cardiovascular responses of women grouped by their baseline OT levels were found to differ as predicted, but these differences were significant only at baseline. Repeated measures age-adjusted MANCOVAs yielded a significant interaction of OT group × time period for SBP and DBP (F(6,102) = 2.20 and 2.19, respectively, P ≤ 0.05). To clarify this interaction, we compared group responses at each time period separately with ANCOVA, and obtained a significant effect of OT group at baseline only for SBP and
Discussion
This novel translational study in premenopausal women confirmed that higher baseline OT levels were linked to lower BP and HR levels, and that a history of more frequent Partner Hugs was associated with higher baseline OT and lower cardiovascular responses. The observed baseline OT and BP differences may reflect the combined effects of the individual's general state of oxytocinergic activity, her anticipation of the upcoming partner contact, and her prior experience in regard to warm contact
Summary
Among 59 premenopausal women tested in the context of a 10 min period of warm physical and emotional contact with their partners, those women showing higher plasma OT levels just prior to the contact period had lower pre-contact resting BP and HR. Higher pre-contact OT level was also related to lower BP during preparation for and recovery after a stressful speech task, though these relationships were in some cases only marginally significant. Women who reported greater frequency of hugs with
Acknowledgements
This research was supported by NIH grants RR00046 and HL64927.
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