Social buffering ameliorates conditioned fear responses in the presence of an auditory conditioned stimulus

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.

Effects of social buffering on fear extinction in adolescent rats

Across social species, the presence of another individual can reduce stress reactions to adverse stimuli, a phenomenon known as social buffering. The present study investigated whether social buffering influences the expression and extinction of learned fear in adolescence, a developmental period of diminished fear inhibition and increased social interaction. Quality of maternal care and degree of social investigation were examined as factors that may influence social buffering. In adolescence, male rats were fear conditioned and then given extinction training either in the presence of a same-age rat or alone. Animals were then tested alone for extinction retention. In two experiments, the presence of a conspecific robustly reduced conditioned fear responses during extinction training. Interestingly, a persistent social buffering effect was observed when the extinction and conditioning contexts had prominent differences in features (Experiment 1), but not when these contexts were relatively similar (Experiment 2). Neither quality of maternal care nor degree of social investigation predicted the effects of social buffering. These findings suggest that social buffering robustly dampens fear responses during adolescence when a peer is present and this suppression can persist, in some instances, even when the peer is absent.

Vicarious Emotions of Fear and Pain in Rodents

Affective empathy, the ability to share the emotions of others, is an important contributor to the richness of our emotional experiences. Here, we review evidence that rodents show signs of fear and pain when they witness the fear and pain of others. This emotional contagion creates a vicarious emotion in the witness that mirrors some level of detail of the emotion of the demonstrator, including its valence and the vicinity of threats, and depends on brain regions such as the cingulate, amygdala, and insula that are also at the core of human empathy. Although it remains impossible to directly know how witnessing the distress of others feels for rodents, and whether this feeling is similar to the empathy humans experience, the similarity in neural structures suggests some analogies in emotional experience across rodents and humans. These neural homologies also reveal that feeling distress while others are distressed must serve an evolutionary purpose strong enough to warrant its stability across ~ 100 millions of years. We propose that it does so by allowing observers to set in motion the very emotions that have evolved to prepare them to deal with threats - with the benefit of triggering them socially, by harnessing conspecifics as sentinels, before the witness personally faces that threat. Finally, we discuss evidence that rodents can engage in prosocial behaviors that may be motivated by vicarious distress or reward.

Sharing Positive Affective States Amongst Rodents

Group living is thought to benefit from the ability to empathize with others. Much attention has been paid to empathy for the pain of others as an inhibitor of aggression. Empathizing with the positive affect of others has received less attention although it could promote helping by making it vicariously rewarding. Here, we review this latter, nascent literature to show that three components of the ability to empathize with positive emotions are already present in rodents, namely, the ability to perceive, share, and prefer actions that promote positive emotional states of conspecifics. While it has often been argued that empathy evolved as a motivation to care for others, we argue that these tendencies may have selfish benefits that could have stabilized their evolution: approaching others in a positive state can provide information about the source of valuable resources; becoming calmer and optimistic around animals in a calm or positive mood can help adapt to the socially sensed safety level in the environment; and preferring actions also benefiting others can optimize foraging, reduce aggression, and trigger reciprocity. Together, these findings illustrate an emerging field shedding light on the emotional world of rodents and on the biology and evolution of our ability to cooperate in groups.

An appeasing pheromone ameliorates fear responses in the brown rat (Rattus norvegicus)

The brown rat (Rattus norvegicus) is one of the major animals both in the laboratory and in urban centers. Brown rats communicate various types of information using pheromones, the chemicals that mediate intra-species communication in minute amounts. Therefore, analyses of pheromones would further our understanding of the mode of life of rats. We show that a minute amount of 2-methylbutyric acid (2-MB) released from the neck region can ameliorate fear responses both in laboratory rats and in wild brown rats. Based on these findings, we conclude that 2-MB is an appeasing pheromone in the brown rat. A better understanding of rats themselves would allow us to perform more effective ecologically based research on social skills and pest management campaigns with low animal welfare impacts, which might contribute to furthering the advancement of science and improving public health.

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.

The strain of unfamiliar conspecifics affects stress identification in rats

Humans show distinct social behaviours when we evaluate an individual as being a member of the same group and recognize social similarity to the individual. One example is more accurate identification of emotion in that individual. Our previous studies proposed that rats recognize social similarity to certain strains of unfamiliar rats. It is therefore possible that the strain of unfamiliar conspecifics affects stress identification in rats. Wistar subject rats were allowed to explore a pair of unfamiliar Wistar, Sprague-Dawley (SD), Long-Evans (LE), or Fischer344 (F344) stimulus rats. To induce differences in stress, one of the stimulus rats had received foot shocks immediately before the test. It was found that the subjects showed biased interaction towards the shocked Wistar and SD stimulus rats, but not toward the shocked LE or F344 stimulus rats. Subsequent experiments confirmed that the biased interaction towards the shocked Wistar and SD stimulus rats was driven by stress in these stimulus rats. In addition, the lack of biased interaction towards the shocked LE and F344 stimulus rats did not appear to be due to procedural reasons. The experiment using LE subject rats further confirmed that the shocked LE stimulus rats emitted distress signals. These results suggested that Wistar rats could identify stress in unfamiliar Wistar and SD rats, but not in unfamiliar LE or F344 rats. Therefore, rats appear to recognize social similarity to certain unfamiliar strains of rats.

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.

Reclassifying the Unique Inhibitory Properties of Social Support Figures: A Roadmap for Exploring Prepared Fear Suppression

Recent work has revealed that social support cues are powerful inhibitors of the fear response. They are endowed with a unique combination of inhibitory properties, enabling them to both inhibit fear in the short term and reduce fear in the long term. While these findings had previously been thought to suggest that social support cues belong to a category of prepared safety stimuli, mounting evidence clearly shows that the mechanisms underlying safety signaling cannot account for the unique effects of social support cues. Here, we propose a reclassification of social support cues as members of a prepared fear suppressor category. We present an argument for the prepared fear suppressor classification, discuss potential mechanisms underlying the unique effects of prepared fear suppressors, and outline next steps to build an understanding of this category and its clinical implications. This review is meant to serve as a roadmap for exploring this novel category of prepared fear suppressors, whose never-before-seen range of inhibitory effects makes them an important and impactful discovery with implications for both fear learning theory and clinical application.

Attachment voices promote safety learning in humans: A critical role for P2

Humans have evolved to seek the proximity of attachment figures during times of threat in order to obtain a sense of safety. In this context, we examined whether or not the voice of an intimate partner (termed "attachment voice") could reduce fear-learning of conditioned stimuli (CS+) and enhance learning of safety signals (CS-). Although the ability to learn safety signals is vital for human survival, few studies have explored how attachment voices affect safety learning. To test our hypothesis, we recruited thirty-five young couples and performed a classic Pavlovian conditioning experiment, recording behavioral and electroencephalographic (EEG) data. The results showed that compared with a stranger's voice, the voices of the partners reduced expectancy of the unconditioned stimulus (a shock) during fear-conditioning, as well as the magnitude of P2 event-related potentials within the EEG responses, provided the voices were safety signals. Additionally, behavioral and EEG responses to the CS+ and CS- differed more when the participants heard their partner's voice than when they heard the stranger's voice. Thus, attachment voices, even as pure vowel sounds without any semantic information, enhanced acquisition of conditioned safety (CS-). These findings may provide implications for investigating other new techniques to improve clinical treatments for fear- and anxiety-related disorders and for psychological interventions against the mental health effects of the public health emergency.

Mapping of c-Fos expression in the medial amygdala following social buffering in male rats

Social buffering is the phenomenon in which an affiliative conspecific (associate) ameliorates stress responses of a subject. We previously found that social buffering in Wistar subject rats is induced if the strain of the associate is Wistar or a strain derived from Wistar rats. In the present study, we assessed the possible role of medial amygdala (Me) in this strain-dependent induction of social buffering. The subjects were exposed to the conditioned stimulus (CS) that had been paired or unpaired with a foot shock either alone, with an unfamiliar Wistar associate, or with an unfamiliar Fischer 344 (F344) associate. We found that the Wistar associates, but not F344 associates, ameliorated increased freezing and Fos expression in the paraventricular nucleus of the hypothalamus and lateral amygdala caused by the CS. In addition, Fos expression in the posterior complex of the anterior olfactory nucleus and lateral intercalated cell mass of the amygdala was increased simultaneously. These results suggest that Wistar associates, but not F344 associates, induced social buffering. In the Me, we did not find any differences associated with stress responses or amelioration of stress responses. In contrast, a comparison among the unpaired subjects found that the Wistar associates, but not F344 associates, increased exploratory behavior and Fos expression in the posteroventral subdivision of the Me (MePV). Based on these results, we propose that the MePV is involved in the recognition of social similarity with the associates. Taken together, the present study provides information about the possible role of Me in social buffering.

Systematic Input-Output Mapping Reveals Structural Plasticity of VTA Dopamine Neurons-Zona Incerta Loop Underlying the Social Buffering Effects in Learned Helplessness

A common phenomenon called social buffering (SB), communication within conspecific animals is a benefit for a stressed individual to better recover from aversive events, is crucial to all mammals. Although the dopamine reward system has been implicated in SB, it is not clear which neuronal populations are relevant and how they contribute. Here, we adopted a learned helplessness (LH) animal model of depression and found that LH subjects housed with a conspecific partner show better performance in the shuttle box test, showing that SB improves the stress-coping abilities to deal with stress. Bidirectional manipulation of ventral tegmental area (VTA) dopamine neurons by chemogenetic tools can mimic or block the SB effect in LH mice. To screen for SB-induced structure plasticity of VTA dopamine neurons, we employed viral genetic tools for mapping input and output architecture and found LH- and SB-triggered circuit-level changes in neuronal ensembles. Zona incerta (ZI), an overlapping brain region, was significantly changed in both anterograde and retrograde tracing during LH and SB. These results reveal a neural loop with structural plasticity between VTA dopamine neurons and ZI underlies the SB effects in LH and lays a foundation for studying how VTA dopamine neurons regulate SB-related neural circuits.

Rats do not consider all unfamiliar strains to be equivalent

Humans show distinct social behaviours when we recognise social similarity in opponents that are members of the same social group. However, little attention has been paid to the role of social similarity in non-human animals. In the Wistar subject rats, the presence of an unfamiliar Wistar rat mitigated stress responses, suggesting the importance of social similarity in this stress-buffering phenomenon. We subsequently found that the presence of unfamiliar Sprague-Dawley (SD) or Long-Evans (LE) rats, but not an unfamiliar Fischer 344 (F344) rat, similarly mitigated stress responses in the subject rats. It is therefore possible that the subject rats recognised social similarity to unfamiliar SD and LE rats. In this study, we demonstrated that the Wistar subject rats were capable of categorizing unfamiliar rats based on their strain, and that the Wistar subjects showed a preference for unfamiliar Wistar, SD, and LE rats over F344 rats. However, the subject rats did not show a preference among Wistar, SD, and LE rats. In addition, the results were not due to an aversion to F344 rats, and preference was not affected when anaesthetised rats were presented to the subject rats. The findings suggested that rats recognise social similarity to certain unfamiliar strains of rats.

Maturation of amygdala inputs regulate shifts in social and fear behaviors: A substrate for developmental effects of stress

Stress can negatively impact brain function and behaviors across the lifespan. However, stressors during adolescence have particularly harmful effects on brain maturation, and on fear and social behaviors that extend beyond adolescence. Throughout development, social behaviors are refined and the ability to suppress fear increases, both of which are dependent on amygdala activity. We review rodent literature focusing on developmental changes in social and fear behaviors, cortico-amygdala circuits underlying these changes, and how this circuitry is altered by stress. We first describe changes in fear and social behaviors from adolescence to adulthood and parallel developmental changes in cortico-amygdala circuitry. We propose a framework in which maturation of cortical inputs to the amygdala promote changes in social drive and fear regulation, and the particularly damaging effects of stress during adolescence may occur through lasting changes in this circuit. This framework may explain why anxiety and social pathologies commonly co-occur, adolescents are especially vulnerable to stressors impacting social and fear behaviors, and predisposed towards psychiatric disorders related to abnormal cortico-amygdala circuits.

Optogenetic reactivation of prefrontal social neural ensembles mimics social buffering of fear

Social buffering occurs when the presence of a companion attenuates the physiological and/or behavioral effects of a stressful or fear-provoking event. It represents a way in which social interactions can immediately and potently modulate behavior. As such, social buffering is one mechanism by which strong social support increases resilience to mental illness. Although the behavioral and neuroendocrine impacts of social buffering are well studied in multiple species, including humans, the neuronal underpinnings of this behavioral phenomenon remain largely unexplored. Previous work has shown that the infralimbic prefrontal cortex (IL-PFC) is important for processing social information and, in separate studies, for modulating fear and anxiety. Thus, we hypothesized that socially active cells within the IL-PFC may integrate social information to modulate fear responsivity. To test this hypothesis, we employed social buffering paradigms in male and female mice. Similar to prior studies in rats, we found that the presence of a cagemate reduced freezing in fear- and anxiety-provoking contexts. In accordance with previous work, we demonstrated that interaction with a novel or familiar conspecific induces activity in the IL-PFC as evidenced by increased immediate early gene (IEG) expression. We then utilized an activity-dependent tagging murine line, the ArcCreER^T2 mice, to express channelrhodopsin (ChR2) in neurons active during the social encoding of a new cagemate. We found that optogenetic reactivation of these socially active neuronal ensembles phenocopied the effects of cagemate presence in male and female mice in learned and innate fear contexts without being inherently rewarding or altering locomotion. These data suggest that a social neural ensemble within the IL-PFC may contribute to social buffering of fear. These neurons may represent a novel therapeutic target for fear and anxiety disorders.

Social buffering enhances extinction of conditioned fear responses by reducing corticosterone levels in male rats

The presence of an affiliative conspecific reduces stress responses to a wide variety of stimuli, which is termed "social buffering." We previously reported that social buffering in male rats ameliorated behavioral responses, as well as hypothalamic-pituitary-adrenal axis activation, elicited by an auditory conditioned stimulus (CS). In addition, subjects that experienced social buffering did not show stress responses when re-exposed to the CS the next day in the absence of an accompanying rat. However, the mechanisms underlying this enhancement of between-session extinction are poorly understood. In Experiment 1, we compared corticosterone levels at 0, 10, and 15 min after extinction training. Subjects that experienced social buffering had lower corticosterone levels than subjects that trained alone at the end of extinction training. However, corticosterone levels at 10 and 15 min after training were not affected by the experience of social buffering. These results suggest that a lower level of corticosterone during extinction training had an important role in the enhancement of extinction. To directly assess this, in Experiment 2, we manipulated the corticosterone level during extinction training. We found that a subcutaneous injection of corticosterone before extinction training blocked the enhancement of extinction by social buffering. These results demonstrate that the enhancement is caused by a low level of corticosterone during the training. Taken together, we suggest that social buffering enhances extinction of conditioned fear responses by reducing corticosterone levels in male rats.

The lateral intercalated cell mass of the amygdala is activated during social buffering of conditioned fear responses in male rats

The presence of an affiliative conspecific reduces stress responses to a wide variety of stimuli. This phenomenon is termed "social buffering". We previously found that the presence of another Wistar rat (associate) suppressed activation of the lateral amygdala (LA) and ameliorated stress responses to an auditory conditioned stimulus (CS) in a fear-conditioned Wistar subject rat. Subsequent analyses suggested that activation of the posterior complex of the anterior olfactory nucleus (AOP) is responsible for the suppression of the LA. However, it remains unclear how the AOP suppresses the LA. To clarify this issue, a fear-conditioned Wistar subject was exposed to the CS either alone or with a Wistar associate. We also prepared a fear-conditioned Wistar subject that was tested with a Fischer344 (F344) associate as an additional control because F344 associates do not induce social buffering. We found that the presence of a Wistar associate induced a reduction of behavioral responses and Fos expression in the paraventricular nucleus of the hypothalamus (PVN) of the subject. Although Fos expression in the AOP was increased, the expression was not biased towards the GABAergic cells. In addition, Fos expression in the lateral intercalated cell mass of the amygdala (lITC) was increased. In contrast, the presence of a F344 associate did not affect Fos expression in subjects' PVN or lITC, whereas behavioral responses were slightly reduced. These results suggest that the lITC was activated during social buffering. Based on these findings, we propose that the AOP indirectly suppresses the LA by activating the lITC.

A dyad shows mutual changes during social buffering of conditioned fear responses in male rats

The presence of an affiliative conspecific reduces stress responses to a wide variety of stimuli. This phenomenon is termed "social buffering". We previously found that the presence of another naïve rat (associate) reduced conditioned fear responses to an auditory conditioned stimulus in a conditioned subject rat. Although we subsequently conducted a series of studies to examine behavioral, physiological, and neural changes during social buffering in the conditioned subject, the changes in the associate remained unclear. Therefore, in the present study, we investigated the behavioral and neural changes in the associate. Fear-conditioned and non-conditioned rats were re-exposed to the conditioned stimulus with an associate placed in the same enclosure (Experiment 1) or separated by a wire-mesh partition (Experiment 2). In Experiment 1, the associate exhibited increased anogenital contact and allo-grooming, which were accompanied by increased c-Fos expression in the paraventricular nucleus of the hypothalamus and central amygdala. These results suggest that the subject and associate mutually affected each other during social buffering. In contrast, in Experiment 2, we found only a difference in the time course of investigation between associates tested with the conditioned and non-conditioned subjects. These results suggest that the associate was unable to acquire a sufficient amount of signal from the conditioned subject behind the wire-mesh partition necessary to show clear changes in behavior and c-Fos expression. Taken together, the current findings suggest that a dyad shows mutual changes during social buffering of conditioned fear responses in male rats.

Extinction learning with social support depends on protein synthesis in prefrontal cortex but not hippocampus

Extinction of contextual fear conditioning (CFC) in the presence of a familiar nonfearful conspecific (social support), such as that of others tasks, can occur regardless of whether the original memory is retrieved during the extinction training. Extinction with social support is blocked by the protein synthesis inhibitors anisomycin and rapamycin and by the inhibitor of gene expression 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole infused immediately after extinction training into the ventromedial prefrontal cortex (vmPFC) but unlike regular CFC extinction not in the CA1 region of the dorsal hippocampus. So social support generates a form of learning that differs from extinction acquired without social support in terms of the brain structures involved. This finding may lead to a better understanding of the brain mechanisms involved in the social support of memories and in therapies for disorders related to dysfunctional fear memories. Thus, here we show that the consolidation of extinction memory with social support relies on vmPFC rather than hippocampus gene expression and ribosomal- and mammalian target of rapamycin-dependent protein synthesis. These results provide additional knowledge about the cellular mechanisms and brain structures involved on the effect of social support in changing behavior and fear extinction memory.

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.

Behavioral Modulation by Social Experiences in Rodent Models

The lasting behavioral changes elicited by social signals provide important adaptations for survival of organisms that thrive as a group. Unlike the rapid innate responses to social cues, such adaptations have been understudied. Here, the rodent models of the lasting socially induced behavioral changes are presented as either modulations or reinforcements of the distinct forms of learning and memory or non-associative changes of affective state. The purpose of this categorization is to draw attention to the potential mechanistic links between the neuronal pathways that process social cues and the neuronal systems that mediate the well-studied forms of learning and memory. © 2018 by John Wiley & Sons, Inc.