Previous research in humans and prairie voles suggests that long-term social stressors, such as social isolation, produce behavioral changes, some of which may be relevant to depression, as well as autonomic dysfunction indicative of potential cardiac pathophysiology (Kiecolt-Glaser and Newton, 2001
; Krantz and McCeney, 2002
; Cacioppo et al., 2002
; Grippo et al., 2007d
). Further, the oxytocinergic system may be activated under certain stressful conditions (e.g., during long-term isolation), possibly in an attempt to compensate for altered neuroendocrine and autonomic regulation as a result of disrupted social bonds. This theory, along with previous data suggesting that oxytocin buffers behavioral and physiological responses to various stressors (Legros et al., 1987
; Windle et al., 1997
; Carter, 1998
; Neumann, 2002
; Heinrichs et al., 2003
; Heinrichs and Domes, 2008
), led us to investigate the effects of exogenously administered oxytocin on the behavioral and autonomic consequences of social isolation in the prairie vole model. The present findings indicate that long-term peripheral administration of oxytocin can prevent detrimental behavioral and physiological consequences of social isolation in female prairie voles, including behaviors that have been described as operational indices relevant to depression in rodents (reduced sucrose consumption, immobility in a FST), cardiac disturbances (increased HR, reduced HR variability) and autonomic imbalance (withdrawal of vagal regulation of the heart). The present findings extend previous research showing that oxytocin can have antidepressant properties (Arletti and Bertolini, 1987
) and may modulate behavioral responses to short-term separation (Insel and Wintink, 1991
Specifically, 4 weeks of isolation induced behavioral changes in prairie voles, consistent with previous studies that have employed short- or long-term separation paradigms in this species (Bosch et al., 2008
; Grippo et al., 2008
). Anhedonia, the reduced responsiveness to pleasurable stimuli that is often observed in human depression (Keller et al., 1995
), was exhibited in isolated prairie voles evidenced by a significant reduction in sucrose intake and preference. This behavioral change represents a specific hedonic deficit, similar to previous reports from other rodent species (Willner et al., 1996
; Grippo et al., 2002
). Water intake was unchanged in isolated animals, indicating that social isolation does not produce a generalized deficit in ingestive behavior, and the reduction in sucrose intake was not secondary to a reduction in body weight. In addition to this behavioral index of anhedonia, isolation was associated with a behavioral index of learned helplessness, evidenced by a reduction in active coping behaviors and an increase in immobility on Day 2 of the FST (but not during the training period on Day 1). This behavioral change in the isolated group, indexed by an increase in putative “helpless” behavior in an operational test that has been described to possess face, construct, and predictive validity (Porsolt et al., 1977
; Willner, 1984
; Cryan et al., 2005
), is similar to previously published data from isolated prairie voles (Grippo et al., 2008
). In the present study, both behavioral changes were prevented in isolated animals by the administration of oxytocin, suggesting that administration of this peptide may protect against some of the negative behavioral consequences of social isolation.
In addition to behavioral changes during operational tests of anhedonia and learned helplessness, social isolation in prairie voles also produced cardiac and autonomic disruptions. Isolation led to increased resting HR and reduced HR variability (both SDNN index and RSA amplitude). Isolation also was associated with a specific withdrawal of vagal regulation of the heart, shown by an attenuated HR response following atropine administration and a specific reduction in RSA amplitude. Disruptions in autonomic function, manifest as increased HR or reduced HR variability, have been described in humans with affective disorders and in an animal model of depression (Pitzalis et al., 2001
; Grippo et al., 2002
). These disturbances also are common in heart disease, predicting mortality in myocardial infarction and heart failure (Ferrari et al., 2003
; Guzzetti et al., 2005
). Similar to the protective effect on behavioral changes, oxytocin had a protective effect on resting cardiac parameters and the vagal influence on cardiac function in isolated animals. However, while the functional physiological consequences of social isolation were prevented by oxytocin administration, the structural consequences were not. Isolation led to a significant increase in heart-body weight ratio (versus pairing; main effect of group, irrespective of peptide administration), consistent with previous findings from isolated prairie voles (Grippo et al., 2007d
) and rodent models of heart disease (Francis et al., 2001
). This increased ratio may represent pathological structural changes such as ventricular hypertrophy, or alternatively might be indicative of increased cardiac muscle mass for non-pathological reasons.
The precise mechanisms through which oxytocin is protective against behavioral, autonomic and cardiac responses to social isolation are yet to be elucidated. Importantly, oxytocin did not produce any significant alterations in paired animals, suggesting that its protective effects in the present paradigm were not a function of generalized down-regulation of behavioral or autonomic processes. Rather, the effects of oxytocin were specifically apparent under conditions of chronic stress (e.g., long-term social isolation). A role for oxytocin in the protective effects of social experiences has been suggested by studies in several species (Carter, 1998
; Neumann et al., 2000
; Heinrichs et al., 2003
; DeVries and Panzica, 2006
). Evidence from rats indicates that oxytocin receptor binding is increased in the hippocampus by various stressful experiences, including a single prolonged stressor as well as a series of repeated, unpredictable stressors (Liberzon and Young, 1997
). Also, exogenous oxytocin down-regulates hypothalamic-pituitary-adrenal axis activity, shown by reductions in circulating cortisol in humans following a psychosocial stressor (Trier Social Stress Task) (Heinrichs et al., 2003
). Some of the effects seen here may be secondary to these central or peripheral changes.
The behavioral and cardiovascular alterations shown here may reflect actions of oxytocin at several sites in the central or peripheral nervous system. Receptors for oxytocin have been identified in nuclei that regulate parasympathetic functions, including dorsal motor nucleus of the vagus, nucleus ambiguus and nucleus tractus solitarius (NTS) (see Higa et al., 2002
). Oxytocin receptors also have been identified in cardiac tissue (Jankowski et al., 2004
); this is a possible site of action for oxytocin’s beneficial effects on cardiac function in the present study, as this peptide was administered peripherally. However, oxytocin likely produces central changes that affect downstream processes such as autonomic outflow and behavioral responses to isolation. While circulating oxytocin is purported to cross the blood-brain barrier in small quantities (approximately 0.2% of subcutaneously administered oxytocin crosses the blood-brain barrier in adult rodents) (see Jones and Robinson, 1982
; Ermisch et al., 1985
), chronic peripheral administration of this peptide produces many of the same responses that occur following central release of oxytocin, including reductions in blood pressure and regulation of motivated behaviors (Arletti et al., 1992
; Petersson et al., 1996
; Caldwell et al., 1996
; Liberzon et al., 1997
). Similarly, peripheral administration of oxytocin in rats has been shown to alter receptor function in central regions associated with stressor responsiveness and cardiovascular regulation, including hypothalamus, amygdala, NTS, and locus coeruleus (Petersson et al., 1998
). It is also possible that biologically active fragments of the oxytocin molecule cross the blood-brain barrier (see De Wied et al., 1993
), producing central effects.
The present findings suggest that social isolation produces altered neural regulation of the heart, which can, in turn, influence both behavioral dysregulation and cardiovascular pathophysiology. As detailed in the Polyvagal Theory (Porges, 2007
), reductions in the influence of the myelinated vagal pathways originating in the nucleus ambiguus – which produce RSA – are associated with an increase in biobehavioral defense strategies. Dampened RSA facilitates the sympathetic excitation necessary for fight-flight mobilization behaviors. Social isolation in turn produced significant reductions in RSA, an associated increase in HR and reduction in SDNN index (as detailed in ), and increased cardiac and neuroendocrine reactivity to acute social stressors (Grippo et al., 2007b
). Moreover, the dampened levels of RSA also may be related to a lowered threshold to trigger a vestigial vagal circuit originating in the dorsal motor nucleus of the vagus that may be related to syncope, clinical bradycardia, apnea, and cardiac arrhythmias. Since the dorsal motor nucleus of the vagus contains high levels of oxytocin receptors, the actions of oxytocin in this pathway could protect the mammalian nervous system from behavioral and autonomic shut-down during periods of prolonged stress or immobility (see Porges, 1998
). This hypothesis is supported by the fact that the isolation-enhanced immobility in the FST was reduced in oxytocin-treated females ().
Specific limitations of the current study design may have had an impact on the findings described here. A limitation of the present study (and also inherent in many studies of social interactions) is that the experimental design may have introduced uncontrolled variables associated with temporary separation of control animals from their respective siblings. However, if this were a significant confounding factor in the current results, the differences between paired and isolated groups may have been less pronounced or absent; whereas several robust differences in both behavior and cardiac function were observed between paired and isolated animals. A second limitation of the present experimental design involves the type of isolation employed. Although the disruption of established sibling bonds was the focus of the current investigation for the reasons cited above (see Methods), it may also be relevant to investigate the disruption of established opposite-sex bonds [such as the design described by Bosch and colleagues (2008)
]. A third limitation of this study is that the behavioral variables studied here were focused very specifically on those that have been shown to be relevant to depression in rodents. Because other behaviors (such as those relevant to anxiety) may be very important in the context of social behavior, peptides, and autonomic function, future studies will benefit from focusing on additional behavioral alterations in prairie voles exposed to long-term social isolation. A final limitation of the present study was the lack of a specific investigation of central versus peripheral administration of oxytocin. While this may limit the conclusions that can be drawn from the current results with respect to the precise site(s) of action of oxytocin, the present findings provide a first step in increasing our understanding of potential peptide involvement in mediating behavioral and physiological responses to long-term social isolation.
In summary, the present findings demonstrate that administration of exogenous oxytocin mediates behavioral and physiological consequences of long-term social isolation in female prairie voles. Understanding the neural, autonomic, and behavioral pathways of oxytocin’s beneficial effects will offer an important perspective on the protective role of peptides and social support in the context of both physical and emotional disorders.