The advantage of social living: brain neuropeptides mediate the beneficial consequences of sex and motherhood

Social effects of oxytocin in humans: context and person matter

Building on animal research, the past decade has witnessed a surge of interest in the effects of oxytocin on social cognition and prosocial behavior in humans. This work has generated considerable excitement about identifying the neurochemical underpinnings of sociality in humans, and discovering compounds to treat social functioning deficits. Inspection of the literature, however, reveals that the effects of oxytocin in the social domain are often weak and/or inconsistent. We propose that this literature can be informed by an interactionist approach in which the effects of oxytocin are constrained by features of situations and/or individuals. We show how this approach can improve understanding of extant research, suggest novel mechanisms through which oxytocin might operate, and refine predictions about oxytocin pharmacotherapy.

Oxytocin stimulates adult neurogenesis even under conditions of stress and elevated glucocorticoids

Oxytocin has been linked to social behavior, including social recognition, pair bonding and parenting, but its potential role in promoting neuronal growth has not been investigated. We show here that oxytocin, but not vasopressin, stimulates both cell proliferation and adult neurogenesis in the hippocampus of rats. Oxytocin is also capable of stimulating adult neurogenesis in rats subjected to glucocorticoid administration or cold water swim stress. These findings suggest that oxytocin stimulates neuronal growth and may protect against the suppressive effects of stress hormones on hippocampal plasticity.

Social Inclusion Facilitates Risky Mating Behavior in Men

Although past research has reliably established unique effects of social exclusion on human cognition and behavior, the current research focuses on the unique effects of social inclusion. Recent evidence indicates that social inclusion leads to enhanced prioritization of reproductive interests. The current study extends these findings by showing that the pursuit of these inclusion-induced reproductive goals occurs in sex-specific ways. Across three experiments, social inclusion led men, but not women, to endorse riskier, more aggressive mating strategies compared to control and socially excluded participants. Specifically, included men were more likely to endorse sexual aggression (Experiment 1), high-risk mate poaching behaviors (Experiment 2), and high-risk mate retention tactics (Experiment 3). These results demonstrate that the experience of social inclusion can affect sex-differentiated preferences for risky mating strategies.

Nonapeptides and the evolution of social group sizes in birds

Species-typical patterns of grouping have profound impacts on many aspects of physiology and behavior. However, prior to our recent studies in estrildid finches, neural mechanisms that titrate species-typical group-size preferences, independent of other aspects of social organization (e.g., mating system and parental care), have been wholly unexplored, likely because species-typical group size is typically confounded with other aspects of behavior and biology. An additional complication is that components of social organization are evolutionarily labile and prone to repeated divergence and convergence. Hence, we cannot assume that convergence in social structure has been produced by convergent modifications to the same neural characters, and thus any comparative approach to grouping must include not only species that differ in their species-typical group sizes, but also species that exhibit convergent evolution in this aspect of social organization. Using five estrildid finch species that differ selectively in grouping (all biparental and monogamous) we have demonstrated that neural motivational systems evolve in predictable ways in relation to species-typical group sizes, including convergence in two highly gregarious species and convergence in two relatively asocial, territorial species. These systems include nonapeptide (vasotocin and mesotocin) circuits that encode the valence of social stimuli (positive–negative), titrate group-size preferences, and modulate anxiety-like behaviors. Nonapeptide systems exhibit functional and anatomical properties that are biased toward gregarious species, and experimental reductions of nonapeptide signaling by receptor antagonism and antisense oligonucleotides significantly decrease preferred group sizes in the gregarious zebra finch. Combined, these findings suggest that selection on species-typical group size may reliably target the same neural motivation systems when a given social structure evolves independently.

Evolutionary functions of early social modulation of hypothalamic-pituitary-adrenal axis development in humans

The hypothalamic-pituitary-adrenal axis (HPAA) is highly responsive to social challenges. Because stress hormones can have negative developmental and health consequences, this presents an evolutionary paradox: Why would natural selection have favored mechanisms that elevate stress hormone levels in response to psychosocial stimuli? Here we review the hypothesis that large brains, an extended childhood and intensive family care in humans are adaptations resulting from selective forces exerted by the increasingly complex and dynamic social and cultural environment that co-evolved with these traits. Variations in the modulation of stress responses mediated by specific HPAA characteristics (e.g., baseline cortisol levels, and changes in cortisol levels in response to challenges) are viewed as phenotypically plastic, ontogenetic responses to specific environmental signals. From this perspective, we discuss relations between physiological stress responses and life history trajectories, particularly the development of social competencies. We present brief summaries of data on hormones, indicators of morbidity and social environments from our long-term, naturalistic studies in both Guatemala and Dominica. Results indicate that difficult family environments and traumatic social events are associated with temporal elevations of cortisol, suppressed reproductive functioning and elevated morbidity. The long-term effects of traumatic early experiences on cortisol profiles are complex and indicate domain-specific effects, with normal recovery from physical stressors, but some heightened response to negative-affect social challenges. We consider these results to be consistent with the hypothesis that developmental programming of the HPAA and other neuroendocrine systems associated with stress responses may facilitate cognitive targeting of salient social challenges in specific environments.

Combined norepinephrine/serotonergic reuptake inhibition: effects on maternal behavior, aggression, and oxytocin in the rat

BACKGROUND

Few systematic studies exist on the effects of chronic reuptake of monoamine neurotransmitter systems during pregnancy on the regulation of maternal behavior (MB), although many drugs act primarily through one or more of these systems. Previous studies examining fluoxetine and amfonelic acid treatment during gestation on subsequent MB in rodents indicated significant alterations in postpartum maternal care, aggression, and oxytocin levels. In this study, we extended our studies to include chronic gestational treatment with desipramine or amitriptyline to examine differential effects of reuptake inhibition of norepinephrine and combined noradrenergic and serotonergic systems on MB, aggression, and oxytocin system changes.

METHODS

Pregnant Sprague-Dawley rats were treated throughout gestation with saline or one of three doses of either desipramine, which has a high affinity for the norepinephrine monoamine transporter, or amitriptyline, an agent with high affinity for both the norepinephrine and serotonin monoamine transporters. MB and postpartum aggression were assessed on postpartum days 1 and 6 respectively. Oxytocin levels were measured in relevant brain regions on postpartum day 7. Predictions were that amitriptyline would decrease MB and increase aggression relative to desipramine, particularly at higher doses. Amygdaloidal oxytocin was expected to decrease with increased aggression.

RESULTS

Amitriptyline and desipramine differentially reduced MB, and at higher doses reduced aggressive behavior. Hippocampal oxytocin levels were lower after treatment with either drug but were not correlated with specific behavioral effects. These results, in combination with previous findings following gestational treatment with other selective neurotransmitter reuptake inhibitors, highlight the diverse effects of multiple monoamine systems thought to be involved in maternal care.

Bridging the gap between GPCR activation and behaviour: oxytocin and prolactin signalling in the hypothalamus

Neuropeptides of the brain are important neuromodulators, controlling behaviour and physiology. They signal through G protein-coupled receptors (GPCR) that couple to complex intracellular signalling pathways. These signalling networks integrate information from multiple sources, resulting in appropriate physiological and behavioural responses to environmental and internal cues. This paper will focus on the neuropeptides oxytocin and prolactin with respect to (1) the regulation of neuroendocrine stress responses and anxiety, and (2) the receptor-mediated molecular mechanisms underlying these actions of the neuropeptides. Besides its significant reproductive functions when released into the bloodstream, brain oxytocin reduces the activity of the hypothalamo-pituitary-adrenal (HPA) axis as well as anxiety-related behaviour in male and female rats. Oxytocin mediates its anxiolytic effect, at least in part, via binding to its GPCR in the hypothalamic paraventricular nucleus, followed by transactivation of the epidermal growth factor receptor, and subsequent activation of a MEK-extracellular signal-regulated kinase (ERK) MAP kinase pathway. Prolactin, by binding to its GPCR receptors, of which there are short and long forms, also activates ERK, and this is necessary for the control of the expression of corticotrophin-releasing hormone-an important regulator of the HPA axis. Liganded oxytocin and prolactin may also recruit other signalling pathways, but how these pathways contribute to the observed behavioural and physiological effects remains to be established. GPCR-mediated oxytocin and prolactin neuronal signalling are illustrative of the complexity of GPCR-activated regulation of appropriate neuroendocrine and behavioural responses to environmental and physiological demands.

Natural variations in maternal and paternal care are associated with systematic changes in oxytocin following parent-infant contact

Animal studies have demonstrated that the neuropeptide oxytocin (OT) plays a critical role in processes of parent-infant bonding through mechanisms of early parental care, particularly maternal grooming and contact. Yet, the involvement of OT in human parenting remains poorly understood, no data are available on the role of OT in the development of human fathering, and the links between patterns of parental care and the OT response have not been explored in humans. One hundred and twelve mothers and fathers engaged in a 15-min play-and-contact interaction with their 4-6-month-old infants and interactions were micro-coded for patterns of parental touch. Results showed that baseline levels of plasma and salivary OT in mothers and fathers were similar, OT levels in plasma and saliva were inter-related, and OT was associated with the parent-specific mode of tactile contact. Human mothers who provided high levels of affectionate contact showed an OT increase following mother-infant interaction but such increase was not observed among mothers displaying low levels of affectionate contact. Among fathers, only those exhibiting high levels of stimulatory contact showed an OT increase. These results demonstrate consistency in the neuroendocrine basis of human parental interactions with those seen in other mammals. The findings underscore the need to provide opportunities for paternal care to trigger the biological basis of fatherhood and suggest that interventions that permit social engagement may be recommended in conditions of diminished maternal-infant contact, such as prematurity or postpartum depression.

Marital behavior, oxytocin, vasopressin, and wound healing

Animal studies have implicated oxytocin and vasopressin in social bonding, physiological stress responses, and wound healing. In humans, endogenous oxytocin and vasopressin levels covary with perceptions of relationship quality, marital behaviors, and physiological stress responses. To investigate relationships among marital behavior, oxytocin, vasopressin, and wound healing, and to determine the characteristics of individuals with the highest neuropeptide levels, 37 couples were admitted for a 24-h visit in a hospital research unit. After small blister wounds were created on their forearm, couples participated in a structured social support interaction task. Blister sites were monitored daily following discharge to assess wound repair speed. Blood samples were collected for oxytocin, vasopressin, and cytokine analyses. Higher oxytocin levels were associated with more positive communication behaviors during the structured interaction task. Furthermore, individuals in the upper oxytocin quartile healed blister wounds faster than participants in lower oxytocin quartiles. Higher vasopressin levels were related to fewer negative communication behaviors and greater tumor necrosis factor-alpha production. Moreover, women in the upper vasopressin quartile healed the experimental wounds faster than the remainder of the sample. These data confirm and extend prior evidence implicating oxytocin and vasopressin in couples' positive and negative communication behaviors, and also provide further evidence of their role in an important health outcome, wound healing.

Sexual experience promotes adult neurogenesis in the hippocampus despite an initial elevation in stress hormones

Aversive stressful experiences are typically associated with increased anxiety and a predisposition to develop mood disorders. Negative stress also suppresses adult neurogenesis and restricts dendritic architecture in the hippocampus, a brain region associated with anxiety regulation. The effects of aversive stress on hippocampal structure and function have been linked to stress-induced elevations in glucocorticoids. Normalizing corticosterone levels prevents some of the deleterious consequences of stress, including increased anxiety and suppressed structural plasticity in the hippocampus. Here we examined whether a rewarding stressor, namely sexual experience, also adversely affects hippocampal structure and function in adult rats. Adult male rats were exposed to a sexually-receptive female once (acute) or once daily for 14 consecutive days (chronic) and levels of circulating glucocorticoids were measured. Separate cohorts of sexually experienced rats were injected with the thymidine analog bromodeoxyuridine in order to measure cell proliferation and neurogenesis in the hippocampus. In addition, brains were processed using Golgi impregnation to assess the effects of sexual experience on dendritic spines and dendritic complexity in the hippocampus. Finally, to evaluate whether sexual experience alters hippocampal function, rats were tested on two tests of anxiety-like behavior: novelty suppressed feeding and the elevated plus maze. We found that acute sexual experience increased circulating corticosterone levels and the number of new neurons in the hippocampus. Chronic sexual experience no longer produced an increase in corticosterone levels but continued to promote adult neurogenesis and stimulate the growth of dendritic spines and dendritic architecture. Chronic sexual experience also reduced anxiety-like behavior. These findings suggest that a rewarding experience not only buffers against the deleterious actions of early elevated glucocorticoids but actually promotes neuronal growth and reduces anxiety.

REVIEW: Oxytocin: Crossing the bridge between basic science and pharmacotherapy

Is oxytocin the hormone of happiness? Probably not. However, this small nine amino acid peptide is involved in a wide variety of physiological and pathological functions such as sexual activity, penile erection, ejaculation, pregnancy, uterus contraction, milk ejection, maternal behavior, osteoporosis, diabetes, cancer, social bonding, and stress, which makes oxytocin and its receptor potential candidates as targets for drug therapy. In this review, we address the issues of drug design and specificity and focus our discussion on recent findings on oxytocin and its heterotrimeric G protein‐coupled receptor OTR. In this regard, we will highlight the following topics: (i) the role of oxytocin in behavior and affectivity, (ii) the relationship between oxytocin and stress with emphasis on the hypothalamo–pituitary–adrenal axis, (iii) the involvement of oxytocin in pain regulation and nociception, (iv) the specific action mechanisms of oxytocin on intracellular Ca²⁺ in the hypothalamo neurohypophysial system (HNS) cell bodies, (v) newly generated transgenic rats tagged by a visible fluorescent protein to study the physiology of vasopressin and oxytocin, and (vi) the action of the neurohypophysial hormone outside the central nervous system, including the myometrium, heart and peripheral nervous system. As a short nine amino acid peptide, closely related to its partner peptide vasopressin, oxytocin appears to be ideal for the design of agonists and antagonists of its receptor. In addition, not only the hormone itself and its binding to OTR, but also its synthesis, storage and release can be endogenously and exogenously regulated to counteract pathophysiological states. Understanding the fundamental physiopharmacology of the effects of oxytocin is an important and necessary approach for developing a potential pharmacotherapy.

Oxytocin and Major Depressive Disorder: Experimental and Clinical Evidence for Links to Aetiology and Possible Treatment

Affective disorders represent the most common psychiatric diseases, with substantial co-morbidity existing between major depressive disorders (MDD) and anxiety disorders. The lack of truly novel acting compounds has led to non-monoaminergic based research and hypotheses in recent years. The large number of brain neuropeptides, characterized by discrete synthesis sites and multiple receptors, represent likely research candidates for novel therapeutic targets. The present review summarises the available preclinical and human evidence regarding the neuropeptide, oxytocin, and its implications in the aetiology and treatment of MDD. While the evidence is not conclusive at present additional studies are warranted to determine whether OXT may be of therapeutic benefit in subsets of MDD patients such as those with comorbid anxiety symptoms and low levels of social attachment.

Recent advances in the neurobiology of attachment behavior

In a biological sense an individual’s life is all about survival and reproduction. Beside the selection of a mate, the mutual commitment of a parent to sustain an infant through a period of dependency is amongst the most important aspects of natural selection. Here we review how the highly conserved circuitry of key midbrain and hypothalamic structures, and limbic and frontal cortical regions support these processes, and at the same time are involved in shaping the offspring’s emotional development and behavior. Many recent studies provided new findings on how attachment behavior and parental bonding is promoted and maintained through genetic and epigenetic influences on synaptic plasticity of mirror neurons and various neuropeptide systems, particularly oxytocinergic, and how these systems serve to link social cues to the brain reward system. Most of this evidence suggests that stress, early parental deprivation and lack of care during the postnatal period leads to profound and lasting changes in the attachment pattern and motivational development with consequent increased vulnerability of the mesocortical and mesolimbic dopamine-associated reward reinforcement pathways to psychosocial stressors, abuse of stimulants and psychopathology later in life.

Oxytocin and the neural mechanisms regulating social cognition and affiliative behavior

Oxytocin is produced in the hypothalamus and released into the circulation through the neurohypophyseal system. Peripherally released oxytocin facilitates parturition and milk ejection during nursing. Centrally released oxytocin coordinates the onset of maternal nurturing behavior at parturition and plays a role in mother-infant bonding. More recent studies have revealed a more general role for oxytocin in modulating affiliative behavior in both sexes. Oxytocin regulates alloparental care and pair bonding in female monogamous prairie voles. Social recognition in male and female mice is also modulated by oxytocin. In humans, oxytocin increases gaze to the eye region of human faces and enhances interpersonal trust and the ability to infer the emotions of others from facial cues. While the neurohypopheseal oxytocin system has been well characterized, less is known regarding the nature of oxytocin release within the brain. Here we review the role of oxytocin in the regulation of prosocial interactions, and discuss the neuroanatomy of the central oxytocin system.