Emotional contagion and prosocial behavior in rodents

Fear Synchrony of Mouse Dyads: Interaction of Sex Composition and Stress

Background

Socially coordinated threat responses support a group's survival. Given the distinct social roles of each sex, social coordination can differ between males and females and mixed-sex groups. We investigated how the sex composition of mouse dyads affected one form of social coordination, the synchronization of conditioned freezing, and assessed how emotional state and social context influenced synchronization by exposure to stress and altering the partner's familiarity, respectively.

Methods

Mice were fear conditioned individually to an auditory stimulus and tested in same- or opposite-sex dyads with familiar or unfamiliar partners. Independent cohorts were tested after 5 minutes of restraint stress or with prefrontal inactivation by muscimol. Time-series data on freezing bouts were used to compute the synchrony index, freezing properties, and state transitions based on a Markov model.

Results

In same-sex dyads, males exhibited higher synchrony than females. State transition analysis revealed sex-specific synchronization strategies: Males maintained a congruent freezing state primarily by following their partners' state transitions, whereas females did so by reversing their own. Stress disrupted synchrony in males, which was prevented by prefrontal inactivation, while stress enhanced synchrony in females. Partner's unfamiliarity reduced synchrony in males but had no effect on females. Conversely, opposite-sex dyads exhibited high levels of synchrony and a unique resilience to stress and unfamiliarity without preferred synchronization strategies.

Conclusions

Mice display sex composition-specific synchronization of threat response and its modulation by stress and social context, providing insights into neuropsychiatric disorders characterized by abnormal threat responses in social contexts with same- and opposite-sex groups.

Neuronal and therapeutic perspectives on empathic pain: A rational insight

Empathy is the capacity to experience and understand the feelings of others, thereby playing a key role in a person's mental well-being essentially by promoting kindness and a sense of belongingness to the group. However, too much empathy may result in psychological problems such as empathic distress, compassion fatigue, and burnout, collectively termed empathic pain. Several brain regions are implicated in processing empathic pain perception. Neuroimaging investigations bring in the context of brain structures involved in this emotional exchange, pointing toward the anterior insula (AI) and anterior cingulate cortex (ACC), indicating an overlap between the neural representation of direct and simulative pain. To discern such overlaps, therapeutic techniques for managing empathic pain require understanding different brain regions and their respective neural networks. At the moment, empathic pain is being treated using various methods, including pharmacological treatments such as antidepressants and psychological treatments such as mindfulness or meditation. For instance, researchers have been exploring the modulatory effects of neurotransmitters like serotonin, norepinephrine, and oxytocin on individuals' responses to empathic pain experience. Importantly, this review focuses on the specific brain parts and their unique roles in neurobiological pathways associated with emphatic pain and how shared neural networks play into available treatment options, suggesting possible future health benefits. Such an understanding of empathy can lead to more efficient management of types of care, focusing on enhancing social connections and mental well-being.

The neural basis of affective empathy: What is known from rodents

Empathy is the cornerstone of social interactions between conspecies for human beings and other social animals. Human beings with empathy defects might either suffer unpleasant or failed social interactions as ASD patients, or even display antisocial behaviors. To find efficient cure for empathy defects, first of all, the neural mechanisms underpinning various empathy behaviors should be well studied and understood. And the research in the field of affective empathy thrives fast in recent years. It is necessary to review the important contributions in this field, especially for understanding the delicate neural mechanisms of diverse forms of affective empathy. Here, we have summarized the characteristics of various types of affective empathy. We also discuss the distinctions between empathy for pain and fear, as well as instinctive and experienced empathy. Our analysis further highlights the findings in the complex neural mechanisms and potential brain regions underlying different affective empathy behaviors. Above all, this work is expected to help enhance our comprehension of behavioral dynamics and neural basis of affective empathy along with its role in emotional regulation and social behavior.

Neural Patterns of Social Pain in the Brain-Wide Representations Across Social Contexts

Empathy can be elicited by physiological pain, as well as in social contexts. Although physiological and different social contexts induce a strong subjective experience of empathy, the general and context-specific neural representations remain elusive. Here, it is combined fMRI with multivariate pattern analysis (MVPA) to establish neurofunctional models for social pain triggered by observing social exclusion and separation naturistic stimuli. The findings revealed that both social contexts engaged the empathy and social function networks. Notably, the intensity of pain empathy elicited by these two social stimuli does not significantly differentiate the neural representations of social exclusion and separation, suggesting context-specific neural representations underlying these experiences. Furthermore, this study established a model that traces the progression from physiological pain to social pain empathy. In conclusion, this study revealed the neural pathological foundations and interconnectedness of empathy induced by social and physiological stimuli and provide robust neuromarkers to precisely evaluate empathy across physiological and social domains.

A neuroscience perspective on the plasticity of the social and relational brain

Over the past two decades, the fields of social and contemplative neurosciences have made significant strides. Initial research utilizing fMRI identified neuronal networks involved in empathy, mentalizing, and compassion, as well as complex interactions among these networks. Subsequent studies shifted to testing the plasticity of these social skills via different types of mindfulness- or compassion-based mental training programs, demonstrating brain plasticity, enhanced social capacities and motivation, as well as improved mental health and overall well-being. Next, researchers developed scalable evidence-based online mental training programs to address the growing levels of mental health problems and loneliness, both exacerbated by the COVID-19 pandemic. Innovative approaches, such as novel relational partner-based practices and online app-based dyadic training programs, offer scalable solutions to counteract ongoing societal and mental health deterioration. Current studies are now applying the above findings to support resilience building within diverse domains of society and professional populations-such as healthcare workers and teachers-at high risk of burn-out. Future research should explore the broader impact of such training-related individual changes on larger systems, potentially leading to the development of a translational social neuroscience approach that leverages insights from social brain plasticity research to support societal needs, thereby enhancing resilience, mental health, and social cohesion.

Distinct oxytocin signaling pathways synergistically mediate rescue-like behavior in mice

Spontaneous rescue behavior enhances the well-being and survival of social animals, yet the neural mechanisms underlying the recognition and response to conspecifics in need remain unclear. Here, we report that observer mice experience distress when encountering anesthetized conspecifics, prompting spontaneous rescue-like behavior toward the unconscious mice. This behavior facilitates the earlier awakening of anesthetized mice while simultaneously alleviating stress in the helper mice. Our findings reveal that endogenous oxytocin (OXT) release from the hypothalamic paraventricular nucleus (PVN) to the oxytocin receptor (OXTR) in the central nucleus of the amygdala (CeA) regulates the emotional component of rescue-like behavior. In contrast, OXT release from the PVN to OXTR in the dorsal bed nucleus of the stria terminalis (dBNST) mediates the motor component of the behavior. Furthermore, we demonstrate that these two pathways exhibited distinct temporal dynamics and functional roles. The OXTPVN-OXTRCeA pathway is activated in a transient and intense manner, acting as a trigger for rescue-like behavior, whereas the OXTPVN-OXTRdBNST pathway responds in a sustained manner, ensuring the continuation of the behavior. These findings highlight the remarkable ability of rodents to engage in targeted helping behavior and suggest that distinct subcortical oxytocinergic pathways selectively and synergistically regulate the motor and emotional aspects of rescue-like behavior.

Neural control of sex differences in affiliative and prosocial behaviors

Social interactions are vital for various taxa and species. Prosocial and affiliative dynamics within a group and between individuals are not only pleasurable and rewarding, but also appear to actively contribute to well-being, cognitive performance, and disease prevention. Moreover, disturbances in acting or being prosocial can represent a major burden for an individual and their affective partners. These disruptions are evident across a spectrum of neuropsychiatric conditions, including depression and autism spectrum disorders. Importantly, interactive patterns of prosocial and affiliative behavior can vary with sex. The fact that genders are differentially affected by neuropsychiatric disorders associated with social impairment underscores the high importance of this research in uncovering the underlying neural correlates and mechanisms. This review focuses on elucidating sex-related differences in prosocial and affiliative behaviors and their potential association with sexually different neural correlates. Specifically, we aim to shed light on the complex interplay between sex, behavior, and neurobiology in affiliative and prosocial interaction patterns.

Impaired emotion recognition in Cntnap2-deficient mice is associated with hyper-synchronous prefrontal cortex neuronal activity

Individuals diagnosed with autism spectrum disorder (ASD) show difficulty in recognizing emotions in others, a process termed emotion recognition. While human fMRI studies linked multiple brain areas to emotion recognition, the specific mechanisms underlying impaired emotion recognition in ASD are not clear. Here, we employed an emotional state preference (ESP) task to show that Cntnap2-knockout (KO) mice, an established ASD model, do not distinguish between conspecifics according to their emotional state. We assessed brain-wide local-field potential (LFP) signals during various social behavior tasks and found that Cntnap2-KO mice exhibited higher LFP theta and gamma rhythmicity than did C57BL/6J mice, even at rest. Specifically, Cntnap2-KO mice showed increased theta coherence, especially between the prelimbic cortex (PrL) and the hypothalamic paraventricular nucleus, during social behavior. Moreover, we observed significantly increased Granger causality of theta rhythmicity between these two brain areas, across several types of social behavior tasks. Finally, optogenetic stimulation of PrL pyramidal neurons in C57BL/6J mice impaired their social discrimination abilities, including in ESP. Together, these results suggest that increased rhythmicity of PrL pyramidal neuronal activity and its hyper-synchronization with specific brain regions are involved in the impaired emotion recognition exhibited by Cntnap2-KO mice.

What Else Is Happening to the Mirror Neurons?-A Bibliometric Analysis of Mirror Neuron Research Trends and Future Directions (1996-2024)

BACKGROUND

Since its discovery in the late 20th century, research on mirror neurons has become a pivotal area in neuroscience, linked to various cognitive and social functions. This bibliometric analysis explores the research trajectory, key research topics, and future trends in the field of mirror neuron research.

METHODS

We searched the Web of Science Core Collection (WoSCC) database for publications from 1996 to 2024 on mirror neuron research. Statistical and visualization analyses were performed using CiteSpace and VOSviewer.

RESULTS

Publication output on mirror neurons peaked in 2013 and remained active. High-impact journals such as Science, Brain, Neuron, PNAS, and NeuroImage frequently feature findings on the mirror neuron system, including its distribution, neural coding, and roles in intention understanding, affective empathy, motor learning, autism, and neurological disorders. Keyword clustering reveals major directions in cognitive neuroscience, motor neuroscience, and neurostimulation, whereas burst detection underscores the emerging significance of brain-computer interfaces (BCIs). Research methodologies have been evolving from traditional electrophysiological recordings to advanced techniques such as functional magnetic resonance imaging, transcranial magnetic stimulation, and BCIs, highlighting a dynamic, multidisciplinary progression.

CONCLUSIONS

This study identifies key areas associated with mirror neurons and anticipates that future work will integrate findings with artificial intelligence, clinical interventions, and novel neuroimaging techniques, providing new perspectives on complex socio-cognitive issues and their applications in both basic science and clinical practice.

Concurrent stress modulates the acute and post-acute effects of psilocybin in a sex-dependent manner

There is renewed interest in psychedelics, such as psilocybin, as therapies for multiple difficult-to-treat psychiatric disorders. Even though psychedelics can induce highly pleasant or aversive experiences, depending on multiple personal and environmental factors, there is little research into how such experiences impact post-acute mood-altering actions. Here we aimed at offsetting this gap. First, we tested whether acute psilocybin effects differed between sexes. Adult male and female C57BL/6J mice received saline or psilocybin (5 mg/kg; i.p.), and head-twitch response (HTR) frequency was quantified. Notably, while psilocybin increased HTR frequency in both sexes, the effect was greater in females. We then tested if stress exposure during acute drug effects impacted post-acute psilocybin actions. Following drug treatment, mice were returned to their homecage or restrained for 1 h. Anxiety- and depression-like behaviors were assessed starting 24 h following drug administration, using the marble burying, novelty-suppressed feeding, and splash tests. Psilocybin induced anxiolytic-, but not antidepressant-like, which were fully blocked by stress in males, but only partially so in females. Lastly, we assessed the acute stress-psilocybin interaction on plasma corticosterone levels in a separate cohort of mice, treated as above. Both stress and psilocybin independently increased corticosterone levels, without additive or interactive effects being observed for either sex. Our data reveals the role of sex and peri-acute negative experiences in the acute and post-acute actions of psilocybin. These findings underline the importance of non-pharmacological factors, such as the quality of the psychedelic experience, in the mood-altering effects of psychedelics, holding significant for both their therapeutic and recreational use.

Distinct prelimbic cortex neuronal responses to emotional states of others drive emotion recognition in adult mice

The ability to perceive the emotional states of others, termed emotion recognition, allows individuals to adapt their conduct to the social environment. The brain mechanisms underlying this capacity, known to be impaired in individuals with autism spectrum disorder (ASD), remain, however, elusive. Here, we show that adult mice can discern between emotional states of conspecifics. Fiber photometry recordings of calcium signals in the prelimbic (PrL) medial prefrontal cortex revealed inhibition of pyramidal neurons during investigation of emotionally aroused individuals, as opposed to transient excitation toward naive conspecifics. Chronic electrophysiological recordings at the single-cell level indicated social stimulus-specific responses in PrL neurons at the onset and conclusion of social investigation bouts, potentially regulating the initiation and termination of social interactions. Finally, optogenetic augmentation of the differential neuronal response enhanced emotion recognition, while its reduction eliminated such behavior. Thus, differential PrL neuronal response to individuals with distinct emotional states underlies murine emotion recognition.

Social Avoidance of Mice in Pain in Naturalistic Conditions

Pain and social behavior are subject to reciprocal modulation. Social animals show attenuated pain behavior in the presence of conspecifics, while observers are in turn affected by exposure to another individual in pain. Both phenomena have been established in rodents, which in addition to experiencing emotional contagion from afflicted conspecifics may act to relieve their afflicted state. Little has been done to investigate the motivation of such prosocial behavior in naturalistic conditions. Here, using a novel formalin test targeting the nape of the neck, a socially relevant area, we investigated nocifensive behaviors and social interactions in mice group-housed in a seminatural environment (SNE). In the SNE, formalin-injected mice displayed fewer back-scratching occurrences than when housed alone, which was inversely correlated to the social behavior received. These mice also emitted and received fewer social interactions, attesting of social withdrawal. With respect to dyadic exchanges, saline-treated mice initiated fewer allosniffing and anogenital sniffing episodes towards formalin-treated mice than towards other saline-treated ones. These findings are counter to those showing empathy and prosocial approach in mouse pain models. It is possible that in naturalistic conditions that allow the mice to express a wide range of their behavioral repertoire, healthy mice simply avoid individuals in pain and the cost associated with emotional contagion. Interestingly, behaviors involving direct body contact, namely allogrooming and pair-resting, were not different between saline- and formalin-treated mice and thus may carry a prosocial, altruistic component. These findings unveil new patterns of social modulation by pain in a naturalistic laboratory setting holding high translational value.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42761-024-00276-8.

Roles of mediodorsal thalamus in observational fear-related neural activity in mouse anterior cingulate cortex

Observational fear (OF) is the ability to vicariously experience and learn from another's fearful situation, enabling adaptive responses crucial for survival. It has been shown that the anterior cingulate cortex (ACC) and basolateral amygdala (BLA) are crucial for OF. A subset of neurons in the ACC is activated when observing aversive events in the demonstrator, which elicits OF. However, the neural circuit mechanisms underlying the expression of OF-related activity in the ACC remain unexplored. Previous studies have shown that the mediodorsal thalamus (MD) is crucial for OF, and MD neurons project to the ACC. Therefore, we hypothesize that the projection from MD to ACC may facilitate the OF-related activity in the ACC. By utilizing in vivo calcium imaging combined with the optogenetic terminal inhibition of MD-ACC pathway, we found that a subset of ACC neurons was activated when observing demonstrator's fearful situation in male mice. Furthermore, the optogenetic inhibition of the MD-ACC projection during the demonstrator's aversive moments significantly suppressed the OF-related activity in the ACC. Our data suggests that the MD-ACC projection plays a role in OF-related activity in ACC neurons.

Cortical representations of affective pain shape empathic fear in male mice

Affect sharing, the ability to vicariously feel others' emotions, constitutes the primary component of empathy. However, the neural basis for encoding others' distress and representing shared affective experiences remains poorly understood. Here, using miniature endoscopic calcium imaging, we identify distinct and dynamic neural ensembles in the anterior cingulate cortex (ACC) that encode observational fear across both excitatory and inhibitory neurons in male mice. Notably, we discover that the population dynamics encoding vicarious freezing information are conserved in ACC pyramidal neurons and are specifically represented by affective, rather than sensory, responses to direct pain experience. Furthermore, using circuit-specific imaging and optogenetic manipulations, we demonstrate that distinct populations of ACC neurons projecting to the periaqueductal gray (PAG), but not to the basolateral amygdala (BLA), selectively convey affective pain information and regulate observational fear. Taken together, our findings highlight the critical role of ACC neural representations in shaping empathic freezing through the encoding of affective pain.

Reviving-like prosocial behavior in response to unconscious or dead conspecifics in rodents

Whereas humans exhibit emergency responses to assist unconscious individuals, how nonhuman animals react to unresponsive conspecifics is less well understood. We report that mice exhibit stereotypic behaviors toward unconscious or dead social partners, which escalate from sniffing and grooming to more forceful actions such as mouth or tongue biting and tongue pulling. The latter intense actions, more prominent in familiar pairs, begin after prolonged immobility and unresponsiveness and cease when the partner regains activity. Their consequences, including improved airway opening and clearance and accelerated recovery from unconsciousness, suggest rescue-like efforts. Oxytocin neurons in the hypothalamic paraventricular nucleus respond differentially to the presence of unconscious versus active partners, and their activation, along with oxytocin signaling, is required for the reviving-like actions. This tendency to assist unresponsive members may enhance group cohesion and survival of social species.

A neural basis for prosocial behavior toward unresponsive individuals

Humans often take actions to assist others experiencing unresponsiveness, such as transient loss of consciousness. How other animals react to unresponsive conspecifics-and the neural mechanisms driving such behaviors-remain largely unexplored. In this study, we demonstrated that mice exhibit rescue-like social behaviors toward unresponsive conspecifics, characterized by intense physical contact and grooming directed at the recipient's facial and mouth areas, which expedite their recovery from unresponsiveness. We identified the medial amygdala (MeA) as a key region that encodes the unresponsive state of others and drives this head-directed physical contact. Notably, the behavioral responses toward unresponsive conspecifics differed from those directed at awake, stressed individuals, and these responses were differentially represented in the MeA. These findings shed light on the neural mechanisms underlying prosocial responses toward unresponsive individuals.

Rescue-like behavior in a bystander mouse toward anesthetized conspecifics promotes arousal via a tongue-brain connection

Prosocial behaviors are advantageous to social species, but the neural mechanism(s) through which others receive benefit remain unknown. Here, we found that bystander mice display rescue-like behavior (tongue dragging) toward anesthetized cagemates and found that this tongue dragging promotes arousal from anesthesia through a direct tongue-brain circuit. We found that a direct circuit from the tongue → glutamatergic neurons in the mesencephalic trigeminal nucleus (MTNGlu) → noradrenergic neurons in the locus coeruleus (LCNE) drives rapid arousal in the anesthetized mice that receive the rescue-like behavior from bystanders. Artificial inhibition of this circuit abolishes the rapid arousal effect induced by the rescue-like behavior. Further, we revealed that glutamatergic neurons in the paraventricular nucleus of the thalamus (PVTGlu) that project to the nucleus accumbens shell (NAcSh) mediate the rescue-like behavior. These findings reveal a tongue-brain connection underlying the rapid arousal effects induced by rescue-like behavior and the circuit basis governing this specific form of prosocial behavior.

Visual and olfactory signals of conspecifics induce emotional contagion in mice

Emotional contagion occurs in many animals, including rodents. To determine the social signals of emotional state-matching between individuals in mice, we conducted an empirical laboratory experiment using visual, olfactory and auditory stimuli. The Japanese wild-derived mouse strain MSM/Ms (MSM) was tested as observers, since our initial experiments indicated that MSM mice showed higher sensitivity to others' pain compared with the laboratory strain C57BL/6J (B6). MSM observers were shown footage of an unfamiliar B6 mouse receiving painful foot shocks via a screen. For olfactory stimuli, one of the following was presented during observation: (i) urine collected from a shocked B6 mouse, (ii) urine collected from an unshocked B6 mouse, or (iii) reverse osmosis water. Consequently, MSM mice observing the footage with urine from shocked mice demonstrated significantly higher fear-induced freezing behaviour than in the other two conditions. Regarding visual and auditory stimuli, observing the pixelated video clip was significantly associated with reduced freeze responses, whereas blocking auditory cues did not affect the duration of freezing. These results provide clear-cut evidence that multiple cues, including olfactory and visual information, are sufficient social signals for emotional contagion in mice.

Predictive coding for the actions and emotions of others and its deficits in autism spectrum disorders

Traditionally, the neural basis of social perception has been studied by showing participants brief examples of the actions or emotions of others presented in randomized order to prevent participants from anticipating what others do and feel. This approach is optimal to isolate the importance of information flow from lower to higher cortical areas. The degree to which feedback connections and Bayesian hierarchical predictive coding contribute to how mammals process more complex social stimuli has been less explored, and will be the focus of this review. We illustrate paradigms that start to capture how participants predict the actions and emotions of others under more ecological conditions, and discuss the brain activity measurement methods suitable to reveal the importance of feedback connections in these predictions. Together, these efforts draw a richer picture of social cognition in which predictive coding and feedback connections play significant roles. We further discuss how the notion of predicting coding is influencing how we think of autism spectrum disorder.

Anxiolytic treatment of a trapped rat reduces helping and anxiogenic treatment increases helping: Evidence for emotional contagion in altruism

The present experiment used the trapped rat model to explore whether pharmacological manipulation of distress affects the likelihood of helping behavior. 120 Sprague-Dawley rats (30 male pairs and 30 female pairs) completed 12 consecutive, daily trials assessing helping behavior. During an individual trial, a trapped rat was placed in a restrainer in the center of an open field, while its cagemate could move around freely and possibly open the restrainer by lifting a door. Trapped rats received an intraperitoneal injection of either 1) physiological saline, 2) the anxiolytic midazolam (1.5 mg/kg), or 3) the anxiogenic yohimbine (2.5 mg/kg) 30 min prior to the start of each trial. Dependent variables measured were: 1) door opening latency (sec), 2) percentage of trials in which a door opening occurred, and 3) the number of free rats classified as "openers." Based on emotional contagion theory, we predicted that 1) free rats paired with midazolam-subjects would show attenuated helping behavior (e.g., higher door opening latency) compared to controls, and conversely 2) free rats paired with yohimbine-subjects would show enhanced helping behavior. First, a significant sex-difference was observed, in that more females were classified as openers than males. This supports previous evidence that females express higher altruistic motivation and experience stronger emotional contagion than males. Second, midazolam-treatment significantly attenuated helping behavior. From trials 4-12, free rats paired with midazolam-subjects expressed slower door opening latencies compared to controls. Third, yohimbine-treatment significantly increased helping behavior (e.g., reduced door opening latencies) - but only on trials 1-3; by trials 9-12, this pattern was reversed. These results are consistent with emotional contagion theory and indicate that intensity of distress directly modulates altruistic motivation through vicarious state-matching.

Possible relations between emotional contagion and social buffering

Emotional contagion can be defined as the transfer of an emotional state from the demonstrator of that state towards an observer. Social buffering is a process by which the demonstrator has a reduced stress response due to the presence of one or more other individuals. While both processes are well studied separately, it is unknown whether and how both processes are related. Therefore, the aim of this study was to investigate the relation between emotional contagion and social buffering in pigs. Hereto correlations were performed between measures of emotional contagion (i.e., the difference in behaviour of observer pigs between a situation with and without demonstrator pigs present) and measures of social buffering (i.e., the difference in behaviour of demonstrator pigs in a negative situation with and without observer pigs present). The results did not point towards a clear and consistent relation as only few and contrasting correlations between measures of emotional contagion and social buffering were found, and after correcting for chance no significant correlations remained. To conclude, more research is needed on the relation between emotional contagion and social buffering to shed light on how and when emotions will spread through and/or are buffered in a group of animals.

Endogenous opioid receptor system mediates costly altruism in the human brain

Functional neuroimaging studies suggest that a large-scale brain network transforms others' pain into its vicarious representation in the observer, potentially modulating helping behavior. However, the neuromolecular basis of individual differences in vicarious pain and helping is poorly understood. We investigated the role of the endogenous μ-opioid receptor (MOR) system in altruistic costly helping. MOR density was measured using [11C]carfentanil. In a separate fMRI experiment, participants could donate money to reduce a confederate's pain from electric shocks. Participants were generally willing to help, and brain activity was observed in amygdala, anterior insula, anterior cingulate cortex (ACC), striatum, primary motor cortex, primary somatosensory cortex and thalamus when witnessing others' pain. Haemodynamic responses were negatively associated with MOR availability in emotion circuits. However, MOR availability positively associated with the ACC and hippocampus during helping. These findings suggest that the endogenous MOR system modulates altruism in the human brain.

Effects of single housing on behavior, corticosterone level and body weight in male and female mice

BACKGROUND

Experimental mice are often single-housed either for an individual analysis (feeding behavior, imaging, calorimetry) or as a stress paradigm (social isolation) in translational biomedical research. Reports of the influence of single housing in rodents are conflicting and may depend on age and duration of isolation. Sex is often not included as a factor. In this study we investigated the effects of 4-week single housing in male and female mice on behavior, body weight, and serum corticosterone levels.

RESULTS

Behavioral tests showed no effect on anhedonia and stress coping, anxiety and motor exploration. Social avoidance occurred in both males and females. Regarding physiological effects, single housing did not induce changes in serum corticosterone levels, but reduced body weight gain.

CONCLUSIONS

While some mouse studies of chronic social isolation reported depression-related disturbances, our data suggest that single housing might be not necessarily be too stressful. This is important for animal welfare regulations and experiments in life science research.

ARNT2 controls prefrontal somatostatin interneurons mediating affective empathy

Empathy, crucial for social interaction, is impaired across various neuropsychiatric conditions. However, the genetic and neural underpinnings of empathy variability remain elusive. By combining forward genetic mapping with transcriptome analysis, we discover that aryl hydrocarbon receptor nuclear translocator 2 (ARNT2) is a key driver modulating observational fear, a basic form of affective empathy. Disrupted ARNT2 expression in the anterior cingulate cortex (ACC) reduces affect sharing in mice. Specifically, selective ARNT2 ablation in somatostatin (SST)-expressing interneurons leads to decreased pyramidal cell excitability, increased spontaneous firing, aberrant Ca2+ dynamics, and disrupted theta oscillations in the ACC, resulting in reduced vicarious freezing. We further demonstrate that ARNT2-expressing SST interneurons govern affective state discrimination, uncovering a potential mechanism by which ARNT2 polymorphisms associate with emotion recognition in humans. Our findings advance our understanding of the molecular mechanism controlling empathic capacity and highlight the neural substrates underlying social affective dysfunctions in psychiatric disorders.

Individual Differences in Volitional Social Self-Administration and Motivation in Male and Female Mice Following Social Stress

BACKGROUND

A key challenge in developing treatments for neuropsychiatric illness is the disconnect between preclinical models and the complexity of human social behavior. We integrate voluntary social self-administration into a rodent model of social stress as a platform for the identification of fundamental brain and behavior mechanisms underlying stress-induced individual differences in social motivation.

METHODS

Here, we introduced an operant social stress procedure in male and female mice composed of 3 phases: 1) social self-administration training, 2) social stress exposure concurrent with reinforced self-administration testing, and 3) poststress operant testing under nonreinforced and reinforced conditions. We used social-defeat and witness-defeat stress in male and female mice.

RESULTS

Social defeat attenuated social reward seeking in males but not females, whereas witness defeat had no effect in males but promoted seeking behavior in females. We resolved social stress-induced changes to social motivation by aggregating z-scored operant metrics into a cumulative social index score to describe the spectrum of individual differences exhibited during operant social stress. Clustering does not adequately describe the relative distributions of social motivation following stress and is better described as a nonbinary behavioral distribution defined by the social index score, capturing a dynamic range of stress-related alterations in social motivation inclusive of sex as a biological variable.

CONCLUSIONS

We demonstrated that operant social stress can detect stable individual differences in stress-induced changes to social motivation. The inclusion of volitional behavior in social procedures may enhance the understanding of behavioral adaptations that promote stress resiliency and their mechanisms under more naturalistic conditions.

Prosocial behaviors in rodents

Prosocial behaviors (i.e., actions that benefit others) are central for social interactions in humans and other animals, by fostering social bonding and cohesion. To study prosociality in rodents, scientists have developed behavioral paradigms where animals can display actions that benefit conspecifics in distress or need. These paradigms have provided insights into the role of social interactions and transfer of emotional states in the expression of prosociality, and increased knowledge of its neural bases. However, prosociality levels are variable: not all tested animals are prosocial. Such variation has been linked to differences in animals' ability to process another's state as well as to contextual factors. Moreover, evidence suggests that prosocial behaviors involve the orchestrated activity of multiple brain regions and neuromodulators. This review aims to synthesize findings across paradigms both at the level of behavior and neural mechanisms. Growing evidence confirms that these processes can be studied in rodents, and intense research in the past years is rapidly advancing our knowledge. We discuss a strong bias in the field towards the study of these processes in negative valence contexts (e.g., pain, fear, stress), which should be taken as an opportunity to open new venues for future research.

Emotional contagion and prosocial behaviour in fish: An evolutionary and mechanistic approach

In this review, we consider the definitions and experimental approaches to emotional contagion and prosocial behaviour in mammals and explore their evolutionary conceptualisation for studying their occurrence in the evolutionarily divergent vertebrate group of ray-finned fish. We present evidence for a diverse set of fish phenotypes that meet definitional criteria for prosocial behaviour and emotional contagion and discuss conserved mechanisms that may account for some preserved social capacities in fish. Finally, we provide some considerations on how to address the question of interdependency between emotional contagion and prosocial response, highlighting the importance of recognition processes, decision-making systems, and ecological context for providing evolutionary explanations.

Dissecting shared pain representations to understand their behavioral and clinical relevance

Accounts of shared representations posit that the experience of pain and pain empathy rely on similar neural mechanisms. Experimental research employing novel analytical and methodological approaches has made significant advances in both the identification and targeted manipulation of such shared experiences and their neural underpinnings. This revealed that painful experiences can be shared on different representational levels, from pain-specific to domain-general features, such as negative affect and its regulation. In view of direct links between such representations and social behaviors such as prosocial behavior, conditions characterized by aberrant pain processing may come along with heavy impairments in the social domain, depending on the affected representational level. This has wide potential implications in light of the high prevalence of pain-related clinical conditions, their management, and the overuse of pain medication. In this review and opinion paper, we aim to chart the path toward a better understanding of the link between shared affect and prosocial behavior.

"Why (Zebra)fish May Get Ulcers": Cognitive and Social Modulation of Stress in Fish

BACKGROUND

In the bestseller book "Why Zebras Don't Get Ulcers", Robert Sapolsky argues that animals do not suffer from stress-related diseases like humans because for them, stress is episodic, while humans in contrast suffer from chronic psychological stress. In particular, the idea that fish cannot experience psychological stress is still prevalent, partly due to the lack of a homologous brain area to the neocortex. However, emerging evidence suggests that teleosts can undergo psychological stress, defined as a subjective and perceptual experience of the stressor, and in recent years, the underlying mechanisms started to be unveiled.

SUMMARY

The occurrence of cognitive appraisal in the assessment of stressors has been demonstrated in fish, indicating that the subjective evaluation of stimulus valence and salience, rather than absolute intrinsic characteristics of the stimulus itself, play a key role in the activation of the stress response. Moreover, individual biases (i.e., cognitive bias) in the cognitive appraisal of stimuli have also been described in fish, with some individuals consistently evaluating ambiguous stimuli as positive (aka optimists) whereas other individuals (aka pessimists) appraise them as negative. As a result, optimists and pessimists show consistent differences in stress reactivity and susceptibility/resilience to disease. Finally, social context has also been shown to modulate the response to aversive stimuli with the behavior of conspecifics either buffering or enhancing the response (i.e., social buffering vs. social contagion).

KEY MESSAGES

Cognitive appraisal of stressors occurs in fish, implying that the stress response is modulated by a subjective and perceptual experience of the stressor. Moreover, interindividual consistent cognitive biases in the appraisal of stressors are also present in fish making some individuals more susceptible to stress-related diseases. Therefore, psychological stress has a health toll in fish, and psychologically stressed fish can potentially have ulcers.

Cortico-cortical transfer of socially derived information gates emotion recognition

Emotion recognition and the resulting responses are important for survival and social functioning. However, how socially derived information is processed for reliable emotion recognition is incompletely understood. Here, we reveal an evolutionarily conserved long-range inhibitory/excitatory brain network mediating these socio-cognitive processes. Anatomical tracing in mice revealed the existence of a subpopulation of somatostatin (SOM) GABAergic neurons projecting from the medial prefrontal cortex (mPFC) to the retrosplenial cortex (RSC). Through optogenetic manipulations and Ca2+ imaging fiber photometry in mice and functional imaging in humans, we demonstrate the specific participation of these long-range SOM projections from the mPFC to the RSC, and an excitatory feedback loop from the RSC to the mPFC, in emotion recognition. Notably, we show that mPFC-to-RSC SOM projections are dysfunctional in mouse models relevant to psychiatric vulnerability and can be targeted to rescue emotion recognition deficits in these mice. Our findings demonstrate a cortico-cortical circuit underlying emotion recognition.

Physiological state matching in a pair bonded poison frog

More than a century ago, Charles Darwin hypothesized that the empathy-like phenotype is a phylogenetically widespread phenomenon. This idea remains contentious, due to the challenges of empirically examining emotions, and few investigations among non-mammalian vertebrates. We provide support for Darwin's hypothesis by discovering partial evidence for the most ancestral form of empathy, emotional contagion (i.e. matching another individual's emotional state), in the pair bonding mimetic poison frog, Ranitomeya imitator. We found that male corticosterone, a physiological biomarker of stress, positively correlates with female partners in experimental and semi-natural conditions. This does not appear to coincide with behavioural state-matching. However, it is specific to female partners relative to familiar female non-partners, and is independent of effects that commonly confound studies on emotional contagion. Furthermore, this physiological state-matching is irrespective of partnership longevity or lifetime reproductive output. These results physiologically indicate socially selective emotional contagion in a monogamous amphibian, and paradigms that elicit coinciding neural and behavioural indicators and morphogenic co-variation are needed for further corroboration. Further studies on ancestral forms of empathy in non-mammalian vertebrates are warranted.

Cheerful tails: Delving into positive emotional contagion

This review delves into the phenomenon of positive emotional contagion (PEC) in rodents, an area that remains relatively understudied compared to the well-explored realm of negative emotions such as fear or pain. Rodents exhibit clear preferences for individuals expressing positive emotions over neutral counterparts, underscoring the importance of detecting and responding to positive emotional signals from others. We thoroughly examine the adaptive function of PEC, highlighting its pivotal role in social learning and environmental adaptation. The developmental aspect of the ability to interpret positive emotions is explored, intricately linked to maternal care and social interactions, with oxytocin playing a central role in these processes. We discuss the potential involvement of the reward system and draw attention to persisting gaps in our understanding of the neural mechanisms governing PEC. Presenting a comprehensive overview of the existing literature, we focus on food-related protocols such as the Social Transmission of Food Preferences paradigm and tickling behaviour. Our review emphasizes the pressing need for further research to address lingering questions and advance our comprehension of positive emotional contagion.

Pain experience reduces social avoidance to others in pain: a c-Fos-based functional connectivity network study in mice

Pain experience increases individuals' perception and contagion of others' pain, but whether pain experience affects individuals' affiliative or antagonistic responses to others' pain is largely unknown. Additionally, the neural mechanisms underlying how pain experience modulates individuals' responses to others' pain remain unclear. In this study, we explored the effects of pain experience on individuals' responses to others' pain and the underlying neural mechanisms. By comparing locomotion, social, exploration, stereotyped, and anxiety-like behaviors of mice without any pain experience (naïve observers) and mice with a similar pain experience (experienced observers) when they observed the pain-free demonstrator with intraperitoneal injection of normal saline and the painful demonstrator with intraperitoneal injection of acetic acid, we found that pain experience of the observers led to decreased social avoidance to the painful demonstrator. Through whole-brain c-Fos quantification, we discovered that pain experience altered neuronal activity and enhanced functional connectivity in the mouse brain. The analysis of complex network and graph theory exhibited that functional connectivity networks and activated hub regions were altered by pain experience. Together, these findings reveal that neuronal activity and functional connectivity networks are involved in the modulation of individuals' responses to others' pain by pain experience.

Do They Know What They Are Doing? Cognitive Aspects of Rescue Behaviour Directed by Workers of the Red Wood Ant Formica polyctena to Nestmate Victims Entrapped in Artificial Snares

Ant rescue behaviour belongs to the most interesting subcategories of prosocial and altruistic behaviour encountered in the animal world. Several studies suggested that ants are able to identify what exactly restrains the movements of another individual and to direct their rescue behaviour precisely to that object. To shed more light on the question of how precise the identification of the source of restraint of another ant is, we investigated rescue behaviour of red wood ant Formica polyctena workers, using a new version of an artificial snare bioassay in which a nestmate victim bore two wire loops on its body, one (acting as a snare) placed on its petiole and an additional one on its leg. The tested ants did not preferentially direct their rescue behaviour towards the snare. Moreover, the overall strategy adopted by the most active rescuers was not limited to precisely targeted rescue attempts directed towards the snare, but consisted of frequent switching between various subcategories of rescue behaviour. These findings highlight the importance of precise identification of cognitive processes and overall behavioural strategies for better understanding of causal factors underlying animal helping behaviour in light of new facts discovered by testing of various successive research hypotheses.

Emotional "Contagion" in Piglets after Sensory Avoidance of Rewarding and Punishing Treatment

In the pig farming industry, it is recommended to avoid groups when treating individuals to reduce adverse reactions in the group. However, can this eliminate the adverse effects effectively? Piglets were assigned to the Rewarding Group (RG), the Punishing Group (PG), and the Paired Control Group (PCG). There were six replicates in each group, with two paired piglets per replicate. One piglet of the RG and PG was randomly selected as the Treated pig (TP), treated with food rewards or electric shock, and the other as the Naive pig (NP). The NPs in the RG and PG were unaware of the treatment process, and piglets in the PCG were not treated. The behavior and heart rate changes of all piglets were recorded. Compared to the RG, the NPs in the PG showed longer proximity but less contact behavior, and the TPs in the PG showed more freezing behavior. The percentage change in heart rate of the NPs was synchronized with the TPs. This shows that after sensory avoidance, the untreated pigs could also feel the emotions of their peers and their emotional state was affected by their peers, and the negative emotions in the pigs lasted longer than the positive emotions. The avoidance process does not prevent the transfer of negative emotions to peers via emotional contagion from the stimulated pig.

How is helping behavior regulated in the brain?

In humans and other animals, individuals can actively respond to the specific needs of others. However, the neural circuits supporting helping behaviors are underspecified. In recent work, Zhang, Wu, and colleagues identified a new role for the anterior cingulate cortex (ACC) in the encoding and regulation of targeted helping behavior (allolicking) in mice.

Emotional contagion in rodents: A comprehensive exploration of mechanisms and multimodal perspectives

Emotional contagion, a fundamental aspect of empathy, is an automatic and unconscious process in which individuals mimic and synchronize with the emotions of others. Extensively studied in rodents, this phenomenon is mediated through a range of sensory pathways, each contributing distinct insights. The olfactory pathway, marked by two types of pheromones modulated by oxytocin, plays a crucial role in transmitting emotional states. The auditory pathway, involving both squeaks and specific ultrasonic vocalizations, correlates with various emotional states and is essential for expression and communication in rodents. The visual pathway, though less relied upon, encompasses observational motions and facial expressions. The tactile pathway, a more recent focus, underscores the significance of physical interactions such as allogrooming and socio-affective touch in modulating emotional states. This comprehensive review not only highlights plausible neural mechanisms but also poses key questions for future research. It underscores the complexity of multimodal integration in emotional contagion, offering valuable insights for human psychology, neuroscience, animal welfare, and the burgeoning field of animal-human-AI interactions, thereby contributing to the development of a more empathetic intelligent future.

"Adjacent Bed Effect" of Total Knee Arthroplasty Patients During the Perioperative Period

BACKGROUND

Knee osteoarthropathy is one of the most common degenerative joint diseases in the elderly, total knee arthroplasty (TKA) is the most commonly used treatment for end-stage knee osteoarthropathy. Negative emotions such as anxiety have been extensively documented in knee osteoarthropathy patients.

AIM

This study aimed to investigate the Emotional Contagion during hospitalization in patients undergoing TKA.

METHODS

Eligible subjects were divided into three case groups according to their anxiety states and bed arrangement. All subjects underwent a unilateral, cemented TKA under general anesthesia. Post-operative recovery outcomes including pain, pain behavior and physical function were recorded pre-operation, 1-day, 1 week, 2-weeks, 1-month and 3-months post-operation.

RESULTS

A total of 38 subjects were included in the final analysis. Subjects with anxiety had higher Visual Analogue Scale pain scores, PROMIS-Pain Behavior scores than subjects without anxiety in the Contagion Group preoperation (p ≤ .05). Non-anxiety subjects hospitalized in beds physically adjacent to anxiety subjects experienced more severe pain and poorer function (p ≤ .05). After discharge, all clinical outcomes gradually became lower than anxiety subjects in the Contagion Group, reaching levels similar to non-anxiety subjects in the No Contagion Group within 1 month (p>.05).

CONCLUSIONS

This study showed that patients with anxiety may have an "Adjacent Bed Effect" on patients with TKA in the adjacent bed, which may be associated with poorer postoperative recovery, including pain and physical function. We speculate this phenomenon can be effectively avoided by the nursing team through accurately assessing psychological status and reasonable bed arrangements in the inpatient assessment phase.

Cortical regulation of helping behaviour towards others in pain

Humans and animals exhibit various forms of prosocial helping behaviour towards others in need1-3. Although previous research has investigated how individuals may perceive others' states4,5, the neural mechanisms of how they respond to others' needs and goals with helping behaviour remain largely unknown. Here we show that mice engage in a form of helping behaviour towards other individuals experiencing physical pain and injury-they exhibit allolicking (social licking) behaviour specifically towards the injury site, which aids the recipients in coping with pain. Using microendoscopic imaging, we found that single-neuron and ensemble activity in the anterior cingulate cortex (ACC) encodes others' state of pain and that this representation is different from that of general stress in others. Furthermore, functional manipulations demonstrate a causal role of the ACC in bidirectionally controlling targeted allolicking. Notably, this behaviour is represented in a population code in the ACC that differs from that of general allogrooming, a distinct type of prosocial behaviour elicited by others' emotional stress. These findings advance our understanding of the neural coding and regulation of helping behaviour.

Early life adversity reduces affiliative behavior with a stressed cagemate and leads to sex-specific alterations in corticosterone responses in adult mice

Experiencing early life adversity (ELA) alters stress physiology and increases the risk for developing psychiatric disorders. The social environment can influence dynamics of stress responding and buffer and/or transfer stress across individuals. Yet, the impact of ELA on sensitivity to the stress of others and social behavior following stress is unknown. Here, to test the impact of ELA on social and physiological responses to stress, circulating blood corticosterone (CORT) and social behaviors were assessed in adult male and female mice reared under limited bedding and nesting (LBN) or control conditions. To induce stress, one cagemate of a pair-housed cage underwent a footshock paradigm and was then returned to their unshocked partner. CORT was measured in both groups of mice 20 or 90 min after stress exposure, and social behaviors were recorded and analyzed. ELA rearing influenced the CORT response to stress in a sex-specific manner. In males, both control and ELA-reared mice exhibited similar stress transfer to unshocked cagemates and similar CORT dynamics. In contrast, ELA females showed a heightened stress transfer to unshocked cagemates, and sustained elevation of CORT relative to controls, indicating enhanced stress contagion and a failure to terminate the stress response. Behaviorally, ELA females displayed decreased allogrooming and increased investigative behaviors, while ELA males showed reduced huddling. Together, these findings demonstrate that ELA influenced HPA axis dynamics, social stress contagion and social behavior. Further research is needed to unravel the underlying mechanisms and long-term consequences of ELA on stress systems and their impact on behavioral outcomes.

Visuotactile integration facilitates mirror-induced self-directed behavior through activation of hippocampal neuronal ensembles in mice

Remembering the visual features of oneself is critical for self-recognition. However, the neural mechanisms of how the visual self-image is developed remain unknown because of the limited availability of behavioral paradigms in experimental animals. Here, we demonstrate a mirror-induced self-directed behavior (MSB) in mice, resembling visual self-recognition. Mice displayed increased mark-directed grooming to remove ink placed on their heads when an ink-induced visual-tactile stimulus contingency occurred. MSB required mirror habituation and social experience. The chemogenetic inhibition of dorsal or ventral hippocampal CA1 (vCA1) neurons attenuated MSB. Especially, a subset of vCA1 neurons activated during the mirror exposure was significantly reactivated during re-exposure to the mirror and was necessary for MSB. The self-responding vCA1 neurons were also reactivated when mice were exposed to a conspecific of the same strain. These results suggest that visual self-image may be developed through social experience and mirror habituation and stored in a subset of vCA1 neurons.

Can mirror self-recognition in mice unpack the neural underpinnings of self-awareness?

In this issue of Neuron, Yokose et al. show that mice groom a mark on their forehead when exposed to a mirror. Comparing this behavior with hominids' helps carve self-awareness into its component parts and explore the neural mechanisms of its shared components.

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.

The Reward-Related Shift of Emotional Contagion from the Observer's Perspective Correlates to Their Intimacy with the Expresser

Although previous studies have found a bidirectional relationship between emotional contagion and reward, there is insufficient research to prove the effect of reward on the social function of emotional contagion. To explore this issue, the current study used electroencephalography (EEG) and the interactive way in which the expresser played games to help participants obtain reward outcomes. The results demonstrated a significant correlation between changes in emotional contagion and closeness, indicating that emotional contagion has a social regulatory function. Regarding the impact of reward outcomes, the results showed that compared to the context of a loss, in the context of a win, participants' closeness toward the expresser shifted to a more intimate level, their emotional contagion changed in a more positive direction, and the activity of the late positive component (LPC) of the event-related potentials (ERPs) changed to a greater extent. Significantly, the mediation results demonstrated the effect of reward and indicated that changes in the LPC elicited while experiencing the expressers' emotion predicted the subsequent shifts in closeness through alterations in emotional contagion of the anger emotion in the winning context and the happy emotion in the loss context. This study provides empirical evidence regarding the social function of emotional contagion and proves for the first time that the reward context plays a role in it.

Rats respond to aversive emotional arousal of human handlers with the activation of the basolateral and central amygdala

Reading danger signals may save an animal's life, and learning about threats from others allows avoiding first-hand aversive and often fatal experiences. Fear expressed by other individuals, including those belonging to other species, may indicate the presence of a threat in the environment and is an important social cue. Humans and other animals respond to conspecifics' fear with increased activity of the amygdala, the brain structure crucial for detecting threats and mounting an appropriate response to them. It is unclear, however, whether the cross-species transmission of threat information involves similar mechanisms, e.g., whether animals respond to the aversively induced emotional arousal of humans with activation of fear-processing circuits in the brain. Here, we report that when rats interact with a human caregiver who had recently undergone fear conditioning, they show risk assessment behavior and enhanced amygdala activation. The amygdala response involves its two major parts, the basolateral and central, which detect a threat and orchestrate defensive responses. Further, we show that humans who learn about a threat by observing another aversively aroused human, similar to rats, activate the basolateral and centromedial parts of the amygdala. Our results demonstrate that rats detect the emotional arousal of recently aversively stimulated caregivers and suggest that cross-species social transmission of threat information may involve similar neural circuits in the amygdala as the within-species transmission.

Audible pain squeaks can mediate emotional contagion across pre-exposed rats with a potential effect of auto-conditioning

Footshock self-experience enhances rodents' reactions to the distress of others. Here, we tested one potential mechanism supporting this phenomenon, namely that animals auto-condition to their own pain squeaks during shock pre-exposure. In Experiment 1, shock pre-exposure increased freezing and 22 kHz distress vocalizations while animals listened to the audible pain-squeaks of others. In Experiment 2 and 3, to test the auto-conditioning theory, we weakened the noxious pre-exposure stimulus not to trigger pain squeaks, and compared pre-exposure protocols in which we paired it with squeak playback against unpaired control conditions. Although all animals later showed fear responses to squeak playbacks, these were weaker than following typical pre-exposure (Experiment 1) and not stronger following paired than unpaired pre-exposure. Experiment 1 thus demonstrates the relevance of audible pain squeaks in the transmission of distress but Experiment 2 and 3 highlight the difficulty to test auto-conditioning: stimuli weak enough to decouple pain experience from hearing self-emitted squeaks are too weak to trigger the experience-dependent increase in fear transmission that we aimed to study. Although our results do not contradict the auto-conditioning hypothesis, they fail to disentangle it from sensitization effects. Future studies could temporarily deafen animals during pre-exposure to further test this hypothesis.

Mediodorsal thalamus-projecting anterior cingulate cortex neurons modulate helping behavior in mice

Many species living in groups can perform prosocial behaviors via voluntarily helping others with or without benefits for themselves. To provide a better understanding of the neural basis of such prosocial behaviors, we adapted a preference lever-switching task in which mice can prevent harm to others by switching from using a lever that causes shocks to a conspecific one that does not. We found the harm avoidance behavior was mediated by self-experience and visual and social contact but not by gender or familiarity. By combining single-unit recordings and analysis of neural trajectory decoding, we demonstrated the dynamics of anterior cingulate cortex (ACC) neural activity changes synchronously with the harm avoidance performance of mice. In addition, ACC neurons projected to the mediodorsal thalamus (MDL) to modulate the harm avoidance behavior. Optogenetic activation of the ACC-MDL circuit during non-preferred lever pressing (nPLP) and inhibition of this circuit during preferred lever pressing (PLP) both resulted in the loss of harm avoidance ability. This study revealed the ACC-MDL circuit modulates prosocial behavior to avoid harm to conspecifics and may shed light on the treatment of neuropsychiatric disorders with dysfunction of prosocial behavior.

A Brain-To-Brain Mechanism for Social Transmission of Threat Learning

Survival and adaptation in environments require swift and efficacious learning about what is dangerous. Across species, much of such threat learning is acquired socially, e.g., through the observation of others' ("demonstrators'") defensive behaviors. However, the specific neural mechanisms responsible for the integration of information shared between demonstrators and observers remain largely unknown. This dearth of knowledge is addressed by performing magnetoencephalography (MEG) neuroimaging in demonstrator-observer dyads. A set of stimuli are first shown to a demonstrator whose defensive responses are filmed and later presented to an observer, while neuronal activity is recorded sequentially from both individuals who never interacted directly. These results show that brain-to-brain coupling (BtBC) in the fronto-limbic circuit (including insula, ventromedial, and dorsolateral prefrontal cortex) within demonstrator-observer dyads predict subsequent expressions of learning in the observer. Importantly, the predictive power of BtBC magnifies when a threat is imminent to the demonstrator. Furthermore, BtBC depends on how observers perceive their social status relative to the demonstrator, likely driven by shared attention and emotion, as bolstered by dyadic pupillary coupling. Taken together, this study describes a brain-to-brain mechanism for social threat learning, involving BtBC, which reflects social relationships and predicts adaptive, learned behaviors.

Serotonin modulates social responses to stressed conspecifics via insular 5-HT2C receptors in rat

Behaviors associated with distress can affect the anxiety-like states in observers and this social transfer of affect shapes social interactions among stressed individuals. We hypothesized that social reactions to stressed individuals engage the serotonergic dorsal raphe nucleus (DRN) which promotes anxiety-like behavior via postsynaptic action of serotonin at serotonin 2C (5-HT2C) receptors in the forebrain. First, we inhibited the DRN by administering an agonist (8-OH-DPAT, 1 μg in 0.5 μL) for the inhibitory 5-HT1A autoreceptors which silences 5-HT neuronal activity. 8-OH-DPAT prevented the approach and avoidance, respectively, of stressed juvenile (PN30) or stressed adult (PN60) conspecifics in the social affective preference (SAP) test in rats. Similarly, systemic administration of a 5-HT2C receptor antagonist (SB242084, 1 mg/kg, i.p.) prevented approach and avoidance of stressed juvenile or adult conspecifics, respectively. Seeking a locus of 5-HT2C action, we considered the posterior insular cortex which is critical for social affective behaviors and rich with 5-HT2C receptors. SB242084 administered directly into the insular cortex (5 μM in 0.5 μL bilaterally) interfered with the typical approach and avoidance behaviors observed in the SAP test. Finally, using fluorescent in situ hybridization, we found that 5-HT2C receptor mRNA (htr2c) is primarily colocalized with mRNA associated with excitatory glutamatergic neurons (vglut1) in the posterior insula. Importantly, the results of these treatments were the same in male and female rats. These data suggest that interactions with stressed others require the serotonergic DRN and that serotonin modulates social affective decision-making via action at insular 5-HT2C receptors.