Naloxone increases conditioned fear responses during social buffering in male rats

Social buffering is the phenomenon in which the presence of an affiliative conspecific mitigates stress responses. We previously demonstrated that social buffering completely ameliorates conditioned fear responses in rats. However, the neuromodulators involved in social buffering are poorly understood. Given that opioids, dopamine, oxytocin and vasopressin play an important role in affiliative behaviour, here, we assessed the effects of the most well-known antagonists, naloxone (opioid receptor antagonist), haloperidol (dopamine D2 receptor antagonist), atosiban (oxytocin receptor antagonist) and SR49059 (vasopressin V1a receptor antagonist), on social buffering. In Experiment 1, fear-conditioned male subjects were intraperitoneally administered one of the four antagonists 25 min prior to exposure to a conditioned stimulus with an unfamiliar non-conditioned rat. Naloxone, but not the other three antagonists, increased freezing and decreased walking and investigation as compared with saline administration. In Experiment 2, identical naloxone administration did not affect locomotor activity, anxiety-like behaviour or freezing in an open-field test. In Experiment 3, after confirming that the same naloxone administration again increased conditioned fear responses, as done in Experiment 1, we measured Fos expression in 16 brain regions. Compared with saline, naloxone increased Fos expression in the paraventricular nucleus of the hypothalamus and decreased Fos expression in the nucleus accumbens shell, anterior cingulate cortex and insular cortex and tended to decrease Fos expression in the nucleus accumbens core. Based on these results, we suggest that naloxone blocks social buffering of conditioned fear responses in male rats.

The effects of parental presence on amygdala and mPFC activation during fear conditioning: An exploratory study

Learning safe versus dangerous cues is crucial for survival. During development, parents can influence fear learning by buffering their children's stress response and increasing exploration of potentially aversive stimuli. Rodent findings suggest that these behavioral effects are mediated through parental presence modulation of the amygdala and medial prefrontal cortex (mPFC). Here, we investigated whether similar parental modulation of amygdala and mPFC during fear learning occurs in humans. Using a within-subjects design, behavioral (final N = 48, 6-17 years, mean = 11.61, SD = 2.84, 60% females/40% males) and neuroimaging data (final N = 39, 6-17 years, mean = 12.03, SD = 2.98, 59% females/41% males) were acquired during a classical fear conditioning task, which included a CS+ followed by an aversive noise (US; 75% reinforcement rate) and a CS-. Conditioning occurred once in physical contact with the participant's parent and once alone (order counterbalanced). Region of interest analyses examined the unconditioned stress response by BOLD activation to the US (vs. implicit baseline) and learning by activation to the CS+ (vs. CS-). Results showed that during US presentation, parental presence reduced the centromedial amygdala activity, suggesting buffering of the unconditioned stress response. In response to learned stimuli, parental presence reduced mPFC activity to the CS+ (relative to the CS-), although this result did not survive multiple comparisons' correction. These preliminary findings indicate that parents modulate amygdala and mPFC activity during exposure to unconditioned and conditioned fear stimuli, potentially providing insight into the neural mechanisms by which parents act as a social buffer during fear learning. RESEARCH HIGHLIGHTS: (1)This study used a within-participant experimental design to investigate how parental presence (vs. absence) affects youth's neural responses in a classical fear conditioning task. (2)Parental presence reduced the youth's centromedial amygdala activation to the unconditioned stimulus (US), suggesting parental buffering of the neural unconditioned response (UR). (3)Parental presence reduced the youth's mPFC activation to a conditioned threat cue (CS+) compared to a safety cue (CS-), suggesting possible parental modulation of fear learning.

Glutamatergic Projections from the Posterior Complex of the Anterior Olfactory Nucleus to the Amygdala Complexes

Social buffering is a phenomenon where stress responses are ameliorated by an affiliative conspecific. Our previous findings suggest that the posterior complex of the anterior olfactory nucleus (AOP) is well positioned to participate in the neural mechanisms underlying social buffering. However, the lack of anatomical information prevents us from further estimating the role of the AOP. Here, we obtained anatomical information regarding the AOP in male rats. In Experiment 1 (n = 5), among 4',6-diamidino-2-phenylindole-positive cells in the AOP, the proportion of glutamic acid decarboxylase 67 (GAD67)-positive cells was 13.8% ± 1.2%. In Experiment 2 (n = 5), among the cells that were labeled by a retrograde tracer injected into the basolateral complex of the amygdala (BLA), the proportion of GAD67-positive cells was 18.6% ± 0.8%. In Experiment 3 (n = 5), we demonstrated the existence of cells that were labeled by the retrograde tracer injected into the posterior part of the medial amygdala (MeP), mostly into the ventral part of the MeP. In addition, the proportion of GAD67-positive cells among the tracer-labeled cells was 21.7% ± 1.7%. In Experiment 4 (n = 3), the retrograde tracers were injected into the BLA and MeP, mostly into the ventral part of the MeP. The proportion of double-labeled cells among the tracer-labeled cells was 2.1% ± 1.2%. Taken together, these results suggest that the AOP is predominantly composed of glutamatergic neurons. In addition, the AOP sends mutually independent glutamatergic-predominant projections to the BLA and MeP.

CONSUMMATORY SUCCESSIVE POSITIVE CONTRAST WITH AVERSIVE SOLUTIONS IN INFANT RATS: REPLICATION AND GENERALIZATION TO A HIGHER CONCENTRATION OF A BITTER SOLUTION

Successive positive incentive contrast is typically demonstrated when a group of subjects shows a higher consumption of a reinforcer than a control group, if they were previously exposed to a similar one of lower magnitude. Recently, a similar effect was observed by lowering the concentration of a quinine solution in the experimental group. On the other hand, the presence of a familiar odor has been shown to attenuate responses elicited by aversive stimuli in infant rats. This experiment was designed to assess if the presence of a familiar odor decreases the responses elicited by an aversive solution. Two groups of rats were exposed to a consummatory positive successive contrast procedure, decreasing the concentration of a quinine solution (i.e., 0.2% - 0.01%). Half of the subjects in each group was tested in the presence of the homecage odor during the postshift phase. A positive successive contrast effect was evidenced by subjects in the experimental group, consuming more of the lower-concentration solution during the postshift phase. The presence of the odor, however, exerted no effect. Results are discussed in the context of the social buffering and positive successive contrast effects.

The Neurobiology of Infant Attachment-Trauma and Disruption of Parent-Infant Interactions

Current clinical literature and supporting animal literature have shown that repeated and profound early-life adversity, especially when experienced within the caregiver-infant dyad, disrupts the trajectory of brain development to induce later-life expression of maladaptive behavior and pathology. What is less well understood is the immediate impact of repeated adversity during early life with the caregiver, especially since attachment to the caregiver occurs regardless of the quality of care the infant received including experiences of trauma. The focus of the present manuscript is to review the current literature on infant trauma within attachment, with an emphasis on animal research to define mechanisms and translate developmental child research. Across species, the effects of repeated trauma with the attachment figure, are subtle in early life, but the presence of acute stress can uncover some pathology, as was highlighted by Bowlby and Ainsworth in the 1950s. Through rodent neurobehavioral literature we discuss the important role of repeated elevations in stress hormone corticosterone (CORT) in infancy, especially if paired with the mother (not when pups are alone) as targeting the amygdala and causal in infant pathology. We also show that following induced alterations, at baseline infants appear stable, although acute stress hormone elevation uncovers pathology in brain circuits important in emotion, social behavior, and fear. We suggest that a comprehensive understanding of the role of stress hormones during infant typical development and elevated CORT disruption of this typical development will provide insight into age-specific identification of trauma effects, as well as a better understanding of early markers of later-life pathology.

Emotional contagion and prosocial behavior in rodents

Empathy is critical to adjusting our behavior to the state of others. The past decade dramatically deepened our understanding of the biological origin of this capacity. We now understand that rodents robustly show emotional contagion for the distress of others via neural structures homologous to those involved in human empathy. Their propensity to approach others in distress strengthens this effect. Although rodents can also learn to favor behaviors that benefit others via structures overlapping with those of emotional contagion, they do so less reliably and more selectively. Together, this suggests evolution selected mechanisms for emotional contagion to prepare animals for dangers by using others as sentinels. Such shared emotions additionally can, under certain circumstances, promote prosocial behavior.

Neurobiology of Parental Regulation of the Infant and Its Disruption by Trauma Within Attachment

The complex process of regulating physiological functions and homeostasis during external and internal disruptions develops slowly in altricial species, with parental care functioning as a co-regulator of infant physiological and emotional homeostasis. Here, we review our current understanding of the infant's use of parental behaviors for neurobehavioral regulation and its disruption with harsh parental care. Taking a cross-species view, we briefly review the human developmental literature that highlights the importance of the caregiver in scaffolding the child's physiological and emotional regulation, especially under threat and stress. We then use emerging corresponding animal literature within the phylogenetically preserved attachment system to help define neural systems supporting caregiver regulation and its supporting causal mechanism to provide translational bridges to inform causation and mechanisms impossible to define in children. Next, we briefly review animal research highlighting the impact of specific sensory stimuli imbedded in parental care as important for infant physiological and emotion regulation. We then highlight the importance of parental sensory stimuli gaining hedonic value to go beyond simple sensory stimuli to further impact neurobehavioral regulation, with poor quality of care compromising the infant's ability to use these cues for regulation. Clinically, parental regulation of the infant is correlated with later-life neurobehavioral outcome and quality of life. We suggest an understanding of this parental regulation of the infant's immediate neurobehavioral functioning within the context of attachment quality, that may provide insights into the complex processes during early life, initiating the pathway to pathology.

Autonomic reactivity to social rejection, peer difficulties, and the buffering effects of adolescent friendships following early psychosocial deprivation

Autonomic nervous system reactivity has been posited to be a mechanism contributing to social and emotional problems among children exposed to early adversity. Leveraging data from the Bucharest Early Intervention Project, a longitudinal randomized controlled trial of foster care versus institutional care of abandoned children in Romania, we assessed whether altered sympathetic reactivity to peer rejection feedback in early adolescence mediated the relation between early institutional rearing and peer problems in later adolescence. We also assessed whether adolescent friendship quality or randomized placement in foster care early in life moderated these associations. Participants include 68 institutionalized children randomized to care as usual, 68 institutionalized children randomized to foster care, and 135 never-institutionalized children. At age 12, participants reported friendship quality with respect to a best friend and completed a social rejection task while electrocardiogram and impedance cardiography were recorded. Sympathetic nervous system reactivity to rejection feedback was assessed using preejection period (PEP). At ages 12 and 16, peer problems were reported by parents. Mediation analysis revealed that less PEP reactivity to social rejection at age 12 partially mediated the association between early institutionalization and greater peer problems at age 16. Further moderated mediation analysis revealed that this indirect effect was evidenced among previously institutionalized youths with low, but not high, quality friendships. We did not observe foster care intervention effects. These findings suggest that altered sympathetic reactivity to social rejection might be a mechanism linking early institutionalization to social difficulties into adolescence, however, positive adolescent friendships may buffer these effects. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Fear in groups: Increasing group size reduces perceptions of danger

When we face danger or stress, the presence of others can provide a powerful signal of safety and support. However, despite a large literature on group living benefits in animals, few studies have been conducted on how group size alters subjective emotional responses and threat perception in humans. We conducted 5 experiments (N = 3,652) to investigate whether the presence of others decreases fear in response to threat under a variety of conditions. In Studies 1, 2 and 3, we experimentally manipulated group size in hypothetical and real-world situations and found that fear responses decreased as group size increased. In Studies 4 and 5 we again used a combination of hypothetical and real-world decisions to test whether increased anxiety in response to a potential threat would lead participants to choose larger groups for themselves. Participants consistently chose larger groups when threat and anxiety were high. Overall, our findings show that group size provides a salient signal of protection and safety in humans. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

The relationship between personality, season, and wounding receipt in zoo-housed Japanese macaques (Macaca fuscata): A multi-institutional study

It is important to those managing Japanese macaques (Macaca fuscata) in captive settings to understand predictors of wounding. While studies have demonstrated that season (breeding or nonbreeding) and sex predict rates of wounding received by zoo-housed Japanese macaques, we investigated whether individual differences in personality ratings also might explain some of the observed interindividual variance in wounding. Such patterns were previously observed in rhesus macaques (M. mulatta), such that individuals rated higher on Anxiety and Confidence received greater wounding. Here, we collected wounding data over 24 months on 48 Japanese macaques from eight AZA-accredited zoos. Each macaque was also rated by keepers using a 26-item personality questionnaire. Principle components analysis of these ratings revealed four personality components: Openness, Friendliness, Dominance, and Anxiety/Reactivity. The model with the best fit revealed an interaction effect between season (breeding vs. nonbreeding) and the personality component Friendliness, such that individuals rated higher on Friendliness incurred fewer wounds in the nonbreeding season. The second-best model revealed both a main effect of the season as well as an interaction effect between season and Openness, such that macaques rated higher in Openness received more wounds in the nonbreeding season than those rated lower in Openness. Thus, as with rhesus macaques, personality mediated wounding receipt rate in Japanese macaques, although different personality components explained interindividual variance in wounding for these two species. These differences likely reflect species differences in behavior and personality structure, as well as the influence of differing management practices, highlighting the importance of species-specific approaches for captive primate care and welfare. This study provides further support for understanding primate personality to create individualized strategies for their care.

Parental emotional support and social buffering in previously institutionalized and typically developing children and adolescents

The current study tested the hypothesis that variation in parental emotional support explains differences in cortisol reactivity among 159 youth, including both previously institutionalized (PI; N = 78) and non-adopted (NA; N = 81) children (ages 9-10) and adolescents (ages 15-16). Youth participated in a Modified Trier Social Stress Test after a period of preparation with either their parent or a supportive stranger. Saliva samples were collected to derive a measure of cortisol reactivity. Our findings revealed that parents buffered the cortisol stress response for PI children and adolescents only if they were high on emotional support. Our results also suggest that simply preparing with a parent might buffer the stress response for NA children; for NA adolescents, however, only emotionally supportive parents significantly buffered their adolescents' stress.

Relaying Aversive Ultrasonic Alarm Calls Depends on Previous Experience. Empathy, Social Buffering, or Panic?

Ultrasonic vocalizations are among the oldest evolutionarily forms of animal communication. In order to study the communication patterns in an aversive social situation, we used a behavioral model in which one animal, the observer, is witnessing as his cagemate, the demonstrator, is experiencing a series of mild electrical foot shocks. We studied the effect of the foot shock experience on the observer and the influence of a warning sound (emitted shortly before the shock) on USV communication. These experiments revealed that such a warning seems to increase the arousal level, which differentiates the responses depending on previous experience. This can be identified by the emission of characteristic, short 22 kHz calls of a duration below 100 ms. Two rats emitted calls that overlapped in time. Analysis of these overlaps revealed that in ‘warned’ pairs with a naive observer, 22 kHz calls were mixed with 50 kHz calls. This fact, combined with a high fraction of very high-pitched 50 kHz calls (over 75 kHz), suggests the presence of the phenomenon of social buffering. Pure 22 kHz overlaps were mostly found in ‘warned’ pairs with an experienced observer, suggesting a possible fear contagion with distress sharing. The results show the importance of dividing 22 kHz calls into long and short categories.

Social Transmission and Buffering of Hippocampal Metaplasticity after Stress in Mice

In social animals, the behavioral and hormonal responses to stress can be transmitted from one individual to another through a social transmission process, and, conversely, social support ameliorates stress responses, a phenomenon referred to as social buffering. Metaplasticity represents activity-dependent synaptic changes that modulate the ability to elicit subsequent synaptic plasticity. Authentic stress can induce hippocampal metaplasticity, but whether transmitted stress has the same ability remains unknown. Here, using an acute restraint-tailshock stress paradigm, we report that both authentic and transmitted stress in adult male mice trigger metaplastic facilitation of long-term depression (LTD) induction at hippocampal CA1 synapses. Using LTD as a readout of persistent synaptic consequences of stress, our findings demonstrate that, in a male-male dyad, stress transmission happens in nearly half of naive partners and stress buffering occurs in approximately half of male stressed mice that closely interact with naive partners. By using a social-confrontation tube test to assess the dominant-subordinate relationship in a male-male dyad, we found that stressed subordinate mice are not buffered by naive dominant partners and that stress transmission is exhibited in ∼60% of dominant naive partners. Furthermore, the appearance of stress transmission correlates with more time spent in sniffing the anogenital area of stressed mice, and the appearance of stress buffering correlates with more time engaged in allogrooming from naive partners. Chemical ablation of the olfactory epithelium with dichlobenil or physical separation between social contacts diminishes stress transmission. Together, our data demonstrate that transmitted stress can elicit metaplastic facilitation of LTD induction as authentic stress.SIGNIFICANCE STATEMENT Social animals can acquire information about their environment through interactions with conspecifics. Stress can induce enduring changes in neural activity and synaptic function. Current studies are already unraveling the transmission and buffering of stress responses between individuals, but little is known about the relevant synaptic changes associated with social transmission and buffering of stress. Here, we show that authentic and transmitted stress can prime glutamatergic synapses onto hippocampal CA1 neurons to undergo long-term depression. This hippocampal metaplasticity is bufferable following social interactions with naive partners. Hierarchical status of naive partners strongly affects the social buffering effect on synaptic consequences of stress. This work provides novel insights into the conceptual framework for synaptic changes with social transmission and buffering of stress.

Psychological Stress, Its Reduction, and Long-Term Consequences: What Studies with Laboratory Animals Might Teach Us about Life in the Dog Shelter

There is a long history of laboratory studies of the physiological and behavioral effects of stress, its reduction, and the later psychological and behavioral consequences of unmitigated stress responses. Many of the stressors employed in these studies approximate the experience of dogs confined in an animal shelter. We review how the laboratory literature has guided our own work in describing the reactions of dogs to shelter housing and in helping formulate means of reducing their stress responses. Consistent with the social buffering literature in other species, human interaction has emerged as a key ingredient in moderating glucocorticoid stress responses of shelter dogs. We discuss variables that appear critical for effective use of human interaction procedures in the shelter as well as potential neural mechanisms underlying the glucocorticoid-reducing effect. We also describe recent studies in which enrichment centered on human interaction has been found to reduce aggressive responses in a temperament test used to determine suitability for adoption. Finally, we suggest that a critical aspect of the laboratory stress literature that has been underappreciated in studying shelter dogs is evidence for long-term behavioral consequences-often mediated by glucocorticoids-that may not become apparent until well after initial stress exposure.

Your presence soothes me: a neural process model of aversive emotion regulation via social buffering

The reduction of aversive emotions by a conspecific's presence-called social buffering-is a universal phenomenon in the mammalian world and a powerful form of human social emotion regulation. Animal and human studies on neural pathways underlying social buffering typically examined physiological reactions or regional brain activations. However, direct links between emotional and social stimuli, distinct neural processes and behavioural outcomes are still missing. Using data of 27 female participants, the current study delineated a large-scale process model of social buffering's neural underpinnings, connecting changes in neural activity to emotional behaviour by means of voxel-wise multilevel mediation analysis. Our results confirmed that three processes underlie human social buffering: (i) social support-related reduction of activity in the orbitofrontal cortex, ventromedial and dorsolateral prefrontal cortices, anterior and mid-cingulate; (ii) downregulation of aversive emotion-induced brain activity in the superficial cortex-like amygdala and mediodorsal thalamus; and (iii) downregulation of reported aversive feelings. Results of the current study provide evidence for a distinct neural process model of aversive emotion regulation in humans by social buffering.

Silence, Solitude, and Serotonin: Neural Mechanisms Linking Hearing Loss and Social Isolation

For social animals that communicate acoustically, hearing loss and social isolation are factors that independently influence social behavior. In human subjects, hearing loss may also contribute to objective and subjective measures of social isolation. Although the behavioral relationship between hearing loss and social isolation is evident, there is little understanding of their interdependence at the level of neural systems. Separate lines of research have shown that social isolation and hearing loss independently target the serotonergic system in the rodent brain. These two factors affect both presynaptic and postsynaptic measures of serotonergic anatomy and function, highlighting the sensitivity of serotonergic pathways to both types of insult. The effects of deficits in both acoustic and social inputs are seen not only within the auditory system, but also in other brain regions, suggesting relatively extensive effects of these deficits on serotonergic regulatory systems. Serotonin plays a much-studied role in depression and anxiety, and may also influence several aspects of auditory cognition, including auditory attention and understanding speech in challenging listening conditions. These commonalities suggest that serotonergic pathways are worthy of further exploration as potential intervening mechanisms between the related conditions of hearing loss and social isolation, and the affective and cognitive dysfunctions that follow.

Reduced brain cell proliferation following somatic injury is buffered by social interaction in electric fish, Apteronotus leptorhynchus

In many species, the negative effects of aversive stimuli are mitigated by social interactions, a phenomenon termed social buffering. In one form of social buffering, social interactions reduce the inhibition of brain cell proliferation during stress. Indirect predator stimuli (e.g., olfactory or visual cues) are known to decrease brain cell proliferation, but little is known about how somatic injury, as might occur from direct predator encounter, affects brain cell proliferation and whether this response is influenced by conspecific interactions. Here, we assessed the social buffering of brain cell proliferation in an electric fish, Apteronotus leptorhynchus, by examining the separate and combined effects of tail injury and social interactions. We mimicked a predator-induced injury by amputating the caudal tail tip, exposed fish to paired interactions that varied in timing, duration and recovery period, and measured brain cell proliferation and the degree of social affiliation. Paired social interaction mitigated the negative effects of tail amputation on cell proliferation in the forebrain but not the midbrain. Social interaction either before or after tail amputation reduced the effect of tail injury and continuous interaction both before and after caused an even greater buffering effect. Social interaction buffered the proliferation response after short-term (1 d) or long-term recovery (7 d) from tail amputation. This is the first report of social buffering of brain cell proliferation in a non-mammalian model. Despite the positive association between social stimuli and brain cell proliferation, we found no evidence that fish affiliate more closely following tail injury.

Oxytocin and the stress buffering effect of social company: a genetic study in daily life

Social relationships are a crucial determinant of both mental and physical health. This effect is partly due to social buffering of stress. Animal studies suggest that social buffering is mediated via the oxytocin system, while studies in humans are sparse and limited by the low ecological validity of laboratory settings. In the present study, participants (N = 326) completed smartphone questionnaires four times a day over 4 to 5 days, measuring stressors, negative affect, and social context to assess social buffering. We found that under stress, participants reported a higher need for social company. Further, the impact of prior stressful events on momentary negative affect was attenuated by the perceived pleasantness of current social company. This social buffering effect was moderated by haplotypes of the oxytocin receptor gene, based on two well-described single nucleotide polymorphisms (rs2268498, rs53576). Effects were robust when controlling for gender and age, applying different data quality criteria, and even apparent in genotype-based analyses. Our findings demonstrate that social buffering and its modulation by oxytocin system characteristics have implications for life as lived outside the laboratory.

Infant Trauma Alters Social Buffering of Threat Learning: Emerging Role of Prefrontal Cortex in Preadolescence

Within the infant-caregiver attachment system, the primary caregiver holds potent reward value to the infant, exhibited by infants' strong preference for approach responses and proximity-seeking towards the mother. A less well-understood feature of the attachment figure is the caregiver's ability to reduce fear via social buffering, commonly associated with the notion of a "safe haven" in the developmental literature. Evidence suggests this infant system overlaps with the neural network supporting social buffering (attenuation) of fear in the adults of many species, a network known to involve the prefrontal cortex (PFC). Here, using odor-shock conditioning in young developing rats, we assessed when the infant system transitions to the adult-like PFC-dependent social buffering of threat system. Rat pups were odor-shock conditioned (0.55 mA-0.6 mA) at either postnatal day (PN18; dependent on mother) or 28 (newly independent, weaned at PN23). Within each age group, the mother was present or absent during conditioning, with PFC assessment following acquisition using 14C 2-DG autoradiography and cue testing the following day. Since the human literature suggests poor attachment attenuates the mother's ability to socially buffer the infants, half of the pups at each age were reared with an abusive mother from PN8-12. The results showed that for typical control rearing, the mother attenuated fear in both PN18 and PN28 pups, although the PFC [infralimbic (IL) and ventral prelimbic (vPL) cortices] was only engaged at PN28. Abuse rearing completely disrupted social buffering of pups by the mother at PN18. The results from PN28 pups showed that while the mother modulated learning in both control and abuse-reared pups, the behavioral and PFC effects were attenuated after maltreatment. Our data suggest that pups transition to the adult-like PFC social support circuit after independence from the mother (PN28), and this circuit remains functional after early-life trauma, although its effectiveness appears reduced. This is in sharp contrast to the effects of early life trauma during infancy, where social buffering of the infant is more robustly impacted. We suggest that the infant social buffering circuit is disengaged by early-life trauma, while the adolescent PFC-dependent social buffering circuit may use a safety signal with unreliable safety value.

The socially mediated recovery of a fearful fish paired with periodically replaced calm models

Social learning is an important mechanism for acquiring knowledge about environmental risk. However, little work has explored the learning of safety and how such learning outcomes are shaped by the social environment. Here, we exposed minnows, Pimephales promelas, to a high-risk environment to induce behavioural responses associated with fear (e.g. neophobia). We then used the presence of calm conspecific models (low-risk individuals) to weaken these responses. When observers (individuals from the high-risk environment) and models were paired consistently in a one-on-one setting, the observers showed no recovery (i.e. no weakening of the fear responses), and instead the models indirectly acquired those responses (i.e. a socially transmitted state of fear). However, observers paired with models that were periodically replaced with new calm models showed a significant recovery, and each new model showed diminished socially transmitted fear. We argue that our understanding of predation-related fear and social information transfer can prove fruitful in understanding problems with fear and stress across animal taxa, including among humans who experience post-traumatic stress and secondary trauma. Our findings indicate that the periodic replacement of models can promote fear recovery in observers and reduce socially transmitted fear in models.

Social modulation of fear: Facilitation vs buffering

Social behaviors largely constitute mutual exchanges of social cues and the responses to them. The adaptive response also requires proper interpretation of the current context. In fear behaviors, social signals have bidirectional effects-some cues elicit or enhance fear whereas other suppress or buffer it. Studies on the social facilitation and social buffering of fear provide evidence of competition between social cues of opposing meanings. Co-expression of opposing cues by the same animal may explain the contradicting outcomes from the interaction between naive and frightened conspecifics, which reflect the fine balance between fear facilitation and buffering. The neuronal mechanisms that determine that balance provide an exciting target for future studies to probe the brain circuits underlying social modulation of emotional behaviors.

Hippocampal volume, social interactions, and the expression of the normal repertoire of resident-intruder behavior

INTRODUCTION

Reduced hippocampal volumes are reported in individuals with disrupted emotional coping behaviors in both human clinical conditions and in experimental animal models of these populations. In a number of experimental animal models, it has been shown that social interactions can promote resilience and buffer the negative neural consequences of stimuli that disrupt effective coping.

METHODS

Hippocampal and dentate gyrus volumes were calculated in 54 male Sprague Dawley rats; (1) single housed (n = 12), (2) single housed and exposed to daily 6-min social interactions testing in a resident-intruder paradigm (n = 11); (3) group housed (n = 12); (4) single housed and sham injured (n = 12); (5) single housed, sham injured, and social interactions tested (n = 7).

RESULTS

We present data which shows that even a brief daily exposure to a conspecific in resident-intruder social interactions test is sufficient to prevent the reduction in hippocampal volume triggered by single housing.

CONCLUSION

When considered with previously published data, these findings suggest that the expression of the full repertoire of social, nonsocial, dominance, and submissive behaviors in response to the physical presence of an intruder in the home cage plays a significant role in this maintenance of hippocampal volume.

From attachment to independence: Stress hormone control of ecologically relevant emergence of infants' responses to threat

Young infant rat pups learn to approach cues associated with pain rather than learning amygdala-dependent fear. This approach response is considered caregiver-seeking and ecologically relevant within the context of attachment. With maturation, increases in the stress hormone corticosterone permit amygdala-dependent fear, which is crucial for survival during independent living. During the developmental transition from attachment to fear learning, maternal presence suppresses corticosterone elevation to block amygdala-dependent fear learning and re-engage the attachment circuitry. Early life trauma disrupts this developmental sequence by triggering a precocious increase of corticosterone, which permits amygdala-dependent threat responses. In this review, we explore the importance of the stress hormone corticosterone in infants' transition from complete dependence on the caregiver to independence, with consideration for environmental influences on threat response ontogeny and mechanistic importance of social buffering of the stress response.

The protective effects of social bonding on behavioral and pituitary-adrenal axis reactivity to chronic mild stress in prairie voles

Positive social interactions may protect against stress. This study investigated the beneficial effects of pairing with a social partner on behaviors and neuroendocrine function in response to chronic mild stress (CMS) in 13 prairie vole pairs. Following 5 days of social bonding, male and female prairie voles were exposed to 10 days of CMS (mild, unpredictable stressors of varying durations, for instance, strobe light, white noise, and damp bedding), housed with either the social partner (paired group) or individually (isolated group). Active and passive behavioral responses to the forced swim test (FST) and tail-suspension test (TST), and plasma concentrations of adrenocorticotropic hormone (ACTH) and corticosterone, were measured in all prairie voles following the CMS period. Both female and male prairie voles housed with a social partner displayed lower durations of passive behavioral responses (immobility, a maladaptive behavioral response) in the FST (mean ± SEM; females: 17.3 ± 5.4 s; males: 9.3 ± 4.6 s) and TST (females: 56.8 ± 16.4 s; males: 40.2 ± 11.3 s), versus both sexes housed individually (females, FST: 98.6 ± 12.9 s; females, TST: 155.1 ± 19.3 s; males, FST: 92.4 ± 14.1 s; males, TST: 158.9 ± 22.0 s). Female (but not male) prairie voles displayed attenuated plasma stress hormones when housed with a male partner (ACTH: 945 ± 24.7 pg/ml; corticosterone: 624 ± 139.5 ng/ml), versus females housed individually (ACTH: 1100 ± 23.2 pg/ml; corticosterone: 1064 ± 121.7 ng/ml). These results may inform understanding of the benefits of social interactions on stress resilience. Lay Summary: Social stress can lead to depression. The study of social bonding and stress using an animal model will inform understanding of the protective effects of social bonds. This study showed that social bonding in a rodent model can protect against behavioral responses to stress, and may also be protective against the elevation of stress hormones. This study provides evidence that bonding and social support are valuable for protecting against stress in humans.

Depressive-like behavior, its sensitization, social buffering, and altered cytokine responses in rhesus macaques moved from outdoor social groups to indoor housing

Psychosocial stressors appear to promote the onset of depressive illness through activation and sensitization of inflammatory mechanisms. Here, adult male rhesus monkeys brought from large outdoor social groups to indoor housing for 8 days reliably exhibited a hunched, depressive-like posture. When rehoused indoors a second 8 days about 2 weeks later, monkeys housed alone, but not those with an affiliative partner, showed sensitization of the depressive-like hunched posture. Housing indoors also affected circulating pro-inflammatory cytokines: IL-1β showed increased responsiveness to immune challenge, and IL-1β and TNF-α showed reduced suppression by dexamethasone. Sensitivity of the anti-inflammatory cytokine IL-10 to immune challenge exhibited a relative increase from the first to the second round of indoor housing in animals housed in pairs, and a relative decrease in animals housed alone. Cytokine levels during indoor housing were positively correlated with duration of depressive-like behavior. Plasma cortisol levels increased but did not differentiate housing conditions or rounds. Results demonstrate a rapid induction and sensitization of depressive-like behavior to indoor individual housing, social buffering of sensitization, and associated inflammatory responses. This paradigm may provide a practical nonhuman primate model for examining inflammatory-mediated consequences of psychosocial stressors on depression and possible social buffering of these effects.

Sex Differences in Stress and Group Housing Effects on the Number of Newly Proliferated Cells and Neuroblasts in Middle-Aged Dentate Gyrus

Sex differences in stress and coping responses have been frequently documented in aged people, while whether such differences in aged people may appear at the middle age are unknown. This study was undertaken to study the impact of acute stress and social interaction on early neurogenesis in the dentate gyrus (DG) and hippocampus-related memory in two sexes of middle-aged mice. The number of newly proliferated cells, neuroblasts in DG, the object recognition and location memory in 9-month-old male and female C57BL/6N mice were assessed under baseline conditions as well as following an acute stressor regimen and group housing. Three conspecific companions, serving as "the housing group," were used to model the social interaction throughout the stressor regimen. Males had lower numbers of newly proliferated cells and neuroblasts under baseline conditions as compared to females. The stressor regimen caused rapid decreases in the number of newly proliferated cells and neuroblasts in female DG but no obvious changes were observed in male DG. Group housing, regardless of companions' age, prevented the stress-induced decreases in the number of newly proliferated cells and neuroblasts in female DG. In contrast, the presence of young or age-matched companions potentiated the stress effect in males by decreasing the number of newly proliferated cells and neuroblasts. Finally, neither the stressor regimen nor group housing affected mouse performances in the object recognition and location memory in either sex. These findings, taken together, provide evidence to support a notion that middle-aged females appear to demonstrate more stress susceptibility on early neurogenesis in DG as compared to middle-aged males, although the hippocampus-related memory performances are comparable and not affected by stress in these males and females. Experiencing stress, middle-aged females are more prone to benefit from social interaction as compared to middle-aged males in this regard. We suggest, accordingly, that involving social interaction may afford a therapeutic advance in preventing stress-produced decreases in early neurogenesis in middle-aged females' DG.

Social Support Modulates Stress-Related Gene Expression in Various Brain Regions of Piglets

The presence of an affiliative conspecific may alleviate an individual’s stress response in threatening conditions. However, the mechanisms and neural circuitry underlying the process of social buffering have not yet been elucidated. Using the domestic pig as an animal model, we examined the effect of a 4-h maternal and littermate deprivation on stress hormones and on mRNA expression of the glucocorticoid receptor (GR), mineralocorticoid receptor (MR), 11ß-hydroxysteroid dehydrogenase (11ß-HSD) types 1 and 2 and the immediate early gene c-fos in various brain regions of 7-, 21- and 35-day old piglets. The deprivation occurred either alone or with a familiar or unfamiliar age-matched piglet. Compared to piglets deprived alone, the presence of a conspecific animal significantly reduced free plasma cortisol concentrations and altered the MR/GR balance and 11ß-HSD2 and c-fos mRNA expression in the prefrontal cortex (PFC), amygdala and hypothalamus, but not in the hippocampus. The alterations in brain mRNA expression were particularly found in 21- or 35-day old piglets, which may reflect the species-specific postnatal ontogeny of the investigated brain regions. The buffering effects of social support were most pronounced in the amygdala, indicating its significance both for the assessment of social conspecifics as biologically relevant stimuli and for the processing of emotional states. In conclusion, the present findings provide further evidence for the importance of the cortico-limbic network underlying the abilities of individuals to cope with social stress and strongly emphasize the benefits of social partners in livestock with respect to positive welfare and health.

The neurobiology of pair bond formation, bond disruption, and social buffering

Enduring social bonds play an essential role in human society. These bonds positively affect psychological, physiological, and behavioral functions. Here, we review the recent literature on the neurobiology, particularly the role of oxytocin and dopamine, of pair bond formation, bond disruption, and social buffering effects on stress responses, from studies utilizing the socially monogamous prairie vole (Microtus ochrogaster).

PDE11A regulates social behaviors and is a key mechanism by which social experience sculpts the brain

Despite the fact that appropriate social behaviors are vital to thriving in one's environment, little is understood of the molecular mechanisms controlling social behaviors or how social experience sculpts these signaling pathways. Here, we determine if Phosphodiesterase 11A (PDE11A), an enzyme that is enriched in the ventral hippocampal formation (VHIPP) and that breaks down cAMP and cGMP, regulates social behaviors. PDE11 wild-type (WT), heterozygous (HT), and knockout (KO) mice were tested in various social approach assays and gene expression differences were measured by RNA sequencing. The effect of social isolation on PDE11A4 compartmentalization and subsequent social interactions and social memory was also assessed. Deletion of PDE11A triggered age- and sex-dependent deficits in social approach in specific social contexts but not others. Mice appear to detect altered social behaviors of PDE11A KO mice, because C57BL/6J mice prefer to spend time with a sex-matched PDE11A WT vs. its KO littermate; whereas, a PDE11A KO prefers to spend time with a novel PDE11A KO vs. its WT littermate. Not only is PDE11A required for intact social interactions, we found that 1month of social isolation vs. group housing decreased PDE11A4 protein expression specifically within the membrane fraction of VHIPP. This isolation-induced decrease in PDE11A4 expression appears functional because social isolation impairs subsequent social approach behavior and social memory in a PDE11A genotype-dependent manner. Pathway analyses following RNA sequencing suggests PDE11A is a key regulator of the oxytocin pathway and membrane signaling, consistent with its pivotal role in regulating social behavior.

Stability and change: Stress responses and the shaping of behavioral phenotypes over the life span

In mammals, maternal signals conveyed via influences on hypothalamic-pituitary-adrenal (HPA) activity may shape behavior of the young to be better adapted for prevailing environmental conditions. However, the mother's influence extends beyond classic stress response systems. In guinea pigs, several hours (h) of separation from the mother activates not only the HPA axis, but also the innate immune system, which effects immediate behavioral change, as well as modifies behavioral responsiveness in the future. Moreover, the presence of the mother potently suppresses the behavioral consequences of this innate immune activation. These findings raise the possibility that long-term adaptive behavioral change can be mediated by the mother's influence on immune-related activity of her pups. Furthermore, the impact of social partners on physiological stress responses and their behavioral outcomes are not limited to the infantile period. A particularly crucial period for social development in male guinea pigs is that surrounding the attainment of sexual maturation. At this time, social interactions with adults can dramatically affect circulating cortisol concentrations and social behavior in ways that appear to prepare the male to best cope in its likely future social environment. Despite such multiple social influences on the behavior of guinea pigs at different ages, inter-individual differences in the magnitude of the cortisol response remain surprisingly stable over most of the life span. Together, it appears that throughout the life span, physiological stress responses may be regulated by social stimuli. These influences are hypothesized to adjust behavior for predicted environmental conditions. In addition, stable individual differences might provide a means of facilitating adaptation to less predictable conditions.

Developmental constraints in a wild primate

Early-life experiences can dramatically affect adult traits. However, the evolutionary origins of such early-life effects are debated. The predictive adaptive response hypothesis argues that adverse early environments prompt adaptive phenotypic adjustments that prepare animals for similar challenges in adulthood. In contrast, the developmental constraints hypothesis argues that early adversity is generally costly. To differentiate between these hypotheses, we studied two sets of wild female baboons: those born during low-rainfall, low-quality years and those born during normal-rainfall, high-quality years. For each female, we measured fertility-related fitness components during years in adulthood that matched and mismatched her early conditions. We found support for the developmental constraints hypothesis: females born in low-quality environments showed greater decreases in fertility during drought years than females born in high-quality environments, even though drought years matched the early conditions of females born in low-quality environments. Additionally, we found that females born in low-quality years to high-status mothers did not experience reduced fertility during drought years. These results indicate that early ecological adversity did not prepare individuals to cope with ecological challenges in later life. Instead, individuals that experienced at least one high-quality early environment--either ecological or social--were more resilient to ecological stress in later life. Together, these data suggest that early adversity carries lifelong costs, which is consistent with the developmental constraints hypothesis.

Social buffering suppresses fear-associated activation of the lateral amygdala in male rats: behavioral and neurophysiological evidence

In social mammals, the presence of an affiliative conspecific reduces stress responses, a phenomenon referred to as “social buffering.”In a previous study, we found that the presence of a conspecific animal ameliorated a variety of stress responses to an aversive conditioned stimulus (CS), including freezing and Fos expression in the lateral amygdala (LA) of male rats. Although these findings suggest that the presence of a conspecific animal suppresses neural activity in the LA, direct neurophysiological evidence of suppressed activity in the LA during social buffering is still lacking. In the present study, we analyzed freezing behavior and local field potentials in the LA of fear-conditioned rats in response to the CS, in the presence or absence of a conspecific. After auditory aversive conditioning, the CS was presented to the conditioned rats in the presence or absence of a conspecific animal, on 2 successive days. The presence of a conspecific animal significantly decreased the mean peak amplitudes of auditory evoked field potentials, gamma oscillations (25–75 Hz) and high frequency oscillations (100–300 Hz) in the LA. Furthermore, magnitudes of these neural responses positively correlated with freezing duration of the fear-conditioned rats. The results provide the first electrophysiological evidence that social buffering suppresses CS-induced activation in the LA, which consequently reduces conditioned fear responses.

Activation of the mouse odorant receptor 37 subsystem coincides with a reduction of novel environment-induced activity within the paraventricular nucleus of the hypothalamus

Within the main olfactory system of mammals, a unique subsystem exists that is comprised of sensory neurons expressing odorant receptors (ORs) of the OR37 subfamily. These receptors are exclusive for mammals and are highly conserved across species. The mouse OR37 receptor subtypes A, B and C were shown to be activated by the long-chain aliphatic aldehydes pentadecanal, hexadecanal and heptadecanal, respectively. The search for biological sources of these compounds showed that bodily secretions from conspecifics activated the OR37A, B and C glomerulus. At the same time, the activity of cells in a target region of projection neurons from OR37 glomeruli, the paraventricular nucleus of the hypothalamus (PVN), was reduced compared with controls (clean test box). A large number of the activated cells in the PVN of mice that were placed into a clean test box were corticotropin-releasing hormone cells, indicating an induction of the stress axis due to the novel environment. The much lower number of activated cells of mice in a box enriched with bodily secretions from conspecifics indicated a reduced stress response. As bodily secretions from conspecifics activated the OR37 system and simultaneously reduced stress-induced activation of the PVN, it was tested whether the ligands for OR37 receptors could induce this effect. Indeed, a similarly reduced activity in the PVN was found in mice kept in a clean test box and exposed to a mixture of the OR37 ligands delivered via an air stream. These data indicate that the OR37 system may play a role in mediating a phenomenon called social buffering.

Stress, social behavior, and resilience: insights from rodents

The neurobiology of stress and the neurobiology of social behavior are deeply intertwined. The social environment interacts with stress on almost every front: social interactions can be potent stressors; they can buffer the response to an external stressor; and social behavior often changes in response to stressful life experience. This review explores mechanistic and behavioral links between stress, anxiety, resilience, and social behavior in rodents, with particular attention to different social contexts. We consider variation between several different rodent species and make connections to research on humans and non-human primates.

Selective social buffering of behavioral and endocrine responses and Fos induction in the prelimbic cortex of infants exposed to a novel environment

In mammals, the presence of the mother can reduce or "buffer" stress responses of her young in threatening conditions. We compared the effect of the mother, a familiar littermate, and an unfamiliar adult male on three classes of response shown by guinea pig pups in a novel environment: short latency active behaviors, particularly vocalizing; slower developing passive behaviors that appear mediated by inflammatory mechanisms; and hypothalamic-pituitary-adrenal activity. We also examined Fos induction in the prelimbic cortex, a region hypothesized to mediate buffering effects. Only the mother significantly suppressed all classes of behavior. The greatest selectivity was observed for passive behavioral responses. Contrary to expectations, the adult male reduced plasma cortisol levels of pups as effectively as did the mother. The presence of the male also resulted in increased Fos induction in the prelimbic cortex and high levels of social interaction. Maternal buffering was not associated with prelimbic activity. These results confirm the ability of the mother to reduce active behavioral and HPA responses and suggest a specific maternal buffering effect on the later developing passive behavioral responses. The findings also demonstrate an unexpected ability of adult males to reduce HPA responses and raise the possibility that different social partners buffer HPA activity through different underlying processes.

Hypothalamic oxytocin mediates social buffering of the stress response

BACKGROUND

While stressful life events can enhance the risk of mental disorders, positive social interactions can propagate good mental health and normal behavioral routines. Still, the neural systems that promote these benefits are undetermined. Oxytocin is a hormone involved in social behavior and stress; thus, we focus on the impact that social buffering has on the stress response and the governing effects of oxytocin.

METHODS

Female prairie voles (Microtus ochrogaster) were exposed to 1 hour immobilization stress and then recovered alone or with their male partner to characterize the effect of social contact on the behavioral, physiological, and neuroendocrine stress response. In addition, we treated immobilized female voles recovering alone with oxytocin or vehicle and female voles recovering with their male partner with a selective oxytocin receptor antagonist or vehicle. Group sizes varied from 6 to 8 voles (N = 98 total).

RESULTS

We found that 1 hour immobilization increased anxiety-like behaviors and circulating levels of corticosterone, a stress hormone, in female prairie voles recovering alone but not the female prairie voles recovering with their male partner. This social buffering by the male partner on biobehavioral responses to stress was accompanied by increased oxytocin release in the paraventricular nucleus of the hypothalamus. Intra-paraventricular nucleus oxytocin injections reduced behavioral and corticosterone responses to immobilization, whereas injections of an oxytocin receptor antagonist blocked the effects of the social buffering.

CONCLUSIONS

Together, our data demonstrate that paraventricular nucleus oxytocin mediates the social buffering effects on the stress response and thus may be a target for treatment of stress-related disorders.

Social buffering: relief from stress and anxiety

Communication is essential to members of a society not only for the expression of personal information, but also for the protection from environmental threats. Highly social mammals have a distinct characteristic: when conspecific animals are together, they show a better recovery from experiences of distress. This phenomenon, termed 'social buffering', has been found in rodents, birds, non-human primates and also in humans. This paper reviews classical findings on social buffering and focuses, in particular, on social buffering effects in relation to neuroendocrine stress responses. The social cues that transmit social buffering signals, the neural mechanisms of social buffering and a partner's efficacy with respect to social buffering are also detailed. Social contact appears to have a very positive influence on the psychological and the physiological aspects of social animals, including human beings. Research leading towards further understanding of the mechanisms of social buffering could provide alternative medical treatments based on the natural, individual characteristics of social animals, which could improve the quality of life.

Vocal buffering of the stress response: exposure to conspecific vocalizations moderates urinary cortisol excretion in isolated marmosets

For many species, the presence of a significant social partner can lessen the behavioral and physiological responses to stressful stimuli. This study examined whether a single, individually specific, signature vocalization (phee call) could attenuate the physiological stress response that is induced in marmosets by housing them in short-term social isolation. Utilizing a repeated-measures design, adult marmosets (n=10) were temporarily isolated from their long-term pair mate and exposed to three conditions: signature vocalizations from the pair mate, phee calls from an unfamiliar opposite sex individual, or no auditory stimuli. Levels of urinary cortisol were monitored as a physiological indicator of the stress response. Urinary cortisol levels were also monitored, while subjects remained undisturbed in their home cages to provide baseline levels. Temporarily isolated marmosets showed significantly higher levels of urinary cortisol than undisturbed marmosets. However, the nature of the acoustic stimulus experienced during isolation led to differences in the excretion of urinary cortisol. Isolated marmosets exposed to a familiar pair mate's vocalization showed significantly lower levels of urinary cortisol than when exposed to unfamiliar marmoset vocalizations (P <0.04) or to no auditory stimuli (P <0.03). Neither the duration of pairing nor the quality of relationship in the pair (indexed by spatial proximity scores) predicted the magnitude of reduction in cortisol in the familiar vocalization condition. The results presented here provide the first evidence that a single, individually specific communication signal can decrease the magnitude of a physiological stress response in a manner analogous to the physical presence of a social partner, a process we term "vocal buffering."