Social modulation of pain as evidence for empathy in mice

An experimental task to examine the mirror neuron system in mice: Laboratory mice understand the movement intentions of other mice based on their own experience

We developed a behavioral experiment to elucidate the neural mechanisms of intention understanding in mice. In this experiment, the mouse is first trained to acquire food by reaching with its forelimb. The mice that learnt this were placed in an experimental box wherein they can observe the reaching activity of another mouse. We found that trained mice tend to observe the reaching activity of other mice; mice that did not receive any prior training displayed a lower tendency towards observing another mouse's reaching behavior. In experiment 2, in order to rule out that observing the behaviors of other mice is solely due to interest in the feeding table or the social stimulus itself, we compared exploratory approach behaviors when the box with the feeding table was empty, when the untrained mouse did not reach it, and when another learnt mouse was reaching for the food. The results showed that exploratory approach behaviors to trained mice lasted significantly longer than the exploratory approach behavior to the empty box and untrained individuals. These results suggest that the learning of individuals' exploration of other reaching individuals may be motivated not only by interest in the presence of the feeding table and other individuals themselves, but also by an associated intentional movement. The tasks developed in our study could be used in the research of the mirror system in behavioral neuroscience to elucidate the mechanism underlying the ability of mice to understand the intent of other mice via motor learning.

Indirect exposure to socially defeated conspecifics using recorded video activates the HPA axis and reduces reward sensitivity in mice

Rodents perceive the emotional states of conspecifics using vision. In the present study, we demonstrated that exposure to the video-recorded distress of conspecifics induces stress responses in male C57BL/6J mice. A single exposure to a video-recorded scene of the social defeat stress (SDS) increased plasma corticosterone levels in these mice. This physiological change was suppressed by blocking the visual information, suggesting that vision plays a crucial role in inducing stress responses. Furthermore, after exposure to the video, there were increased numbers of c-Fos-positive neurons in the anterior cingulate cortex and other brain areas that are associated with the negative valence and empathy systems, but not in the regions related to the pain signaling. In addition, repeated exposure to SDS videos induced an apparent reduction in reward sensitivity in the sucrose preference test, but did not affect avoidance behaviour in the social interaction test or immobility behaviour in the forced swim test. Reduced reward sensitivity in mice reflects anhedonia, which is a core symptom of depression in humans. Our video SDS model therefore provides a unique opportunity to not only understand the mechanisms underlying stress-induced anhedonia, but also to screen effective candidate molecules for stress-related disorders with greater reproducibility.

The Utility of Grimace Scales for Practical Pain Assessment in Laboratory Animals

Simple Summary

Grimace scales for laboratory animals were first reported ten years ago. Yet, despite their promise as pain assessment tools it appears that they have not been implemented widely in animal research establishments for clinical pain assessment. We discuss potential reasons for this based on the knowledge gained to date on their use and suggest avenues for further research, which might improve uptake of their use in laboratory animal medicine.

Abstract

Animals’ facial expressions are widely used as a readout for emotion. Scientific interest in the facial expressions of laboratory animals has centered primarily on negative experiences, such as pain, experienced as a result of scientific research procedures. Recent attempts to standardize evaluation of facial expressions associated with pain in laboratory animals has culminated in the development of “grimace scales”. The prevention or relief of pain in laboratory animals is a fundamental requirement for in vivo research to satisfy community expectations. However, to date it appears that the grimace scales have not seen widespread implementation as clinical pain assessment techniques in biomedical research. In this review, we discuss some of the barriers to implementation of the scales in clinical laboratory animal medicine, progress made in automation of collection, and suggest avenues for future research.

Are There Multiple Motivators for Helping Behavior in Rats?

Empathy is the ability to (a) be affected by and share the emotional state of another; (b) assess the reasons for the other’s state; and (c) identify with the other, adopting their perspective. This phenomenon has been shown to exist in several species and is proposed as a motivator for prosocial behavior. The experimental study of this feature in laboratory rodents is a more viable alternative in comparison to wild animals. A recent report showed that rats opened a door to free their cage mate from a restraint box. Although this behavior has been suggested to be motivated by empathy, this fact has been questioned by several studies that proposed other motivators for the releasing behavior. In the present study, we use an adaptation of the protocol of releasing behavior to investigate aspects of empathy and pro-sociality such as familiarity and reciprocity. In addition, we addressed some potential motivational factors that could influence this behavior. The main results showed that (1) rats opened the restraint box to free conspecifics most of the time; (2) direct reciprocity or past restriction experience did not improve releasing performance, probably due to a ceiling effect; (3) after a series of trials in the presence of a restricted conspecific, the free rat continues to open the restraint box even if it is empty; (4) in general, the opening performance improves across trials and phases, resembling learning curves; (5) if the first series of trials occurs with the empty box, the opening behavior does not occur and is modest in subsequent trials with a trapped animal; (6) the exploratory drive toward the restraint box and desire for social contact do not seem to function as key motivators for releasing behavior. In conclusion, our findings do not support that the opening behavior is exclusively related to empathic motivation. While multiple factors might be involved, our study suggests that task learning triggered (and possibly reinforced) by the presence of the restricted rat can function as a motivator. Further investigations are required to fully understand the mechanisms and motivation factors guiding the releasing behavior.

The Impact of Social and Behavioral Factors on Reproducibility in Terrestrial Vertebrate Models

The use of animal models remains critical in preclinical and translational research. The reliability of the animal models and aspects of their validity is likely key to effective translation of findings to medicine. However, despite considerable uniformity in animal models brought about by control of genetics, there remain a number of social as well as innate and acquired behavioral characteristics of laboratory animals that may impact on research outcomes. These include the effects of strain and genetics, age and development, sex, personality and affective states, and social factors largely brought about by housing and husbandry. In addition, aspects of the testing environment may also influence research findings. A number of considerations resulting from the animals' innate and acquired behavioral characteristics as well as their social structures are described. Suggestions for minimizing the impact of these factors on research are provided.

Virtue Ethics and Laboratory Animal Research

This article appeals to virtue ethics to help guide laboratory animal research by considering the role of character and flourishing in these practices. Philosophical approaches to animal research ethics have typically focused on animal rights or on the promotion of welfare for all affected, while animal research itself has been guided in its practice by the 3Rs (reduction, refinement, replacement). These different approaches have sometimes led to an impasse in debates over animal research where the philosophical approaches are focused on whether or when animal studies are justifiable, while the 3Rs assume a general justification for animal work but aim to reduce harm to sentient animals and increase their welfare in laboratory spaces. Missing in this exchange is a moral framework that neither assumes nor rejects the justifiability of animal research and focuses instead on the habits and structures of that work. I shall propose a place for virtue ethics in laboratory animal research by considering examples of relevant character traits, the moral significance of human-animal bonds, mentorship in the laboratory, and the importance of animals flourishing beyond mere welfare.

Can self-pain sensitivity quantify empathy for others' pain?

Empathy is essential for the survival of social species. In many studies, especially those with animal models, empathy for pain was evaluated by the modulation of pain sensitivity. However, the relationship between pain sensitivity and empathy for pain is not well established. Here, by performing two experiments, we aimed to investigate their relationship at both behavioral and electrophysiological levels. In Experiment 1, we characterized individual pain sensitivity using pain threshold and tolerance in a cold pressor test, self-report empathy using Interpersonal Reactivity Index, and pain-related psychological factors, including pain-related anxiety, depression, pain catastrophizing, and pain-related fear, using well-validated questionnaires. We observed that pain sensitivity was positively correlated with emotional empathy, and their relationship was mediated by pain-related anxiety, pain catastrophizing, and pain-related fear. In Experiment 2, we quantified empathy for pain using pain intensity and unpleasantness as well as event-related potentials (ERPs) in an empathy for pain task. Positive correlations were observed between pain sensitivity and psychophysiological empathic responses (i.e., the P3 component and the late positive potential in ERPs), and their relationship was mediated by pain-related fear. These results suggested that being influenced by some pain-related psychological factors, individuals with higher pain sensitivity tended to have stronger empathy for pain, manifested as stronger emotional reactions to others' pain. Our results indicated that the conventional strategy of using pain sensitivity to quantify empathy should be treated with caution, as their relationship could be disturbed by experimental manipulations or pathological modulations that could influence individual emotional states or cognitive processes.

Pro-social preference in an automated operant two-choice reward task under different housing conditions: Exploratory studies on pro-social decision making

In this study, we aimed to develop a behavioral task that measures pro-social decision making in rats. A fully automated, operant pro-social two-choice task is introduced that quantifies pro-social preferences for a mutual food reward in a set-up with tightly controlled task contingencies. Pairs of same-sex adult Wistar rats were placed in an operant chamber divided into two compartments (one rat per compartment), separated by a transparent barrier with holes that allowed the rats to see, hear, smell, but not touch each other. Test rats could earn a sucrose pellet either for themselves (own reward) or for themselves and the partner (both reward) by means of lever pressing. On average, male rats showed a 60 % preference for the lever that yielded a food reward for both themselves and their partner. In contrast, females did not show lever preference, regardless of the estrous cycle phase. Next, the impact of juvenile environmental factors on male rat social decision making was studied. Males were group-housed from postnatal day 26 onwards in complex housing Marlau™ cages that provided social and physical enrichment and stimulation in the form of novelty. Complex housed males did not show a preference for the pro-social lever.

The development and use of facial grimace scales for pain measurement in animals

The measurement of pain in animals is surprisingly complex, and remains a critical issue in veterinary care and biomedical research. Based on the known utility of pain measurement via facial expression in verbal and especially non-verbal human populations, "grimace scales" were first developed a decade ago for use in rodents and now exist for 10 different mammalian species. This review details the background context, historical development, features (including duration), psychometric properties, modulatory factors, and impact of animal grimace scales for pain.

Physiological and behavioral responses to observing a sibling experience a direct stressor in prairie voles

Uncontrollable stress precipitates negative mental and physical health outcomes. Furthermore, the vicarious experience of stress (e.g. observing another individual experience a direct stressor) can mimic the effects of directly experiencing the stressor. The current experiment examined the behavioral and physiological effects of the vicarious experience of stress using the socially monogamous prairie vole. Male prairie voles were exposed to either an empty open field chamber, or a chamber in which the animal observed a sibling undergoing a concurrent direct physical stressor (tail suspension test) for five minutes. Exploratory and anxiety-like behaviors were recorded in all observers during the test session. Cardiac indices of heart rate and heart rate variability were recorded in a subset of observers prior to, during, and following the test session. Corticosterone levels were measured in all observers and siblings following the test session. When compared to animals exposed to an empty open field chamber, animals that observed a sibling undergo a direct physical stressor exhibited increased heart rate and circulating corticosterone, and decreased heart rate variability. These physiological stress indicators were supported by behavioral changes, including increased freezing followed immediately by orienting of the head toward the center of the apparatus, and decreased locomotion, grooming, and rearing. These preliminary results suggest that prairie voles experience stress vicariously, and provide a foundation for additional studies focused on the underlying mechanisms of vicarious stress. The use of this model may inform our understanding of the social transmission of stress among social species, including humans.LAY SUMMARYThe experience of stress, including observing stress in a loved one, has negative consequences on mental and physical health. This study used a social rodent (prairie voles) to demonstrate that stress transfers among social individuals, consequently producing an increased physiological and behavioral stress response in prairie voles observing their siblings experience stress. This research informs our understanding of the interactions of social experiences and stress in humans.

Learning and memory dysfunction of non-surgery cage-mates of mice with surgery

A patient can develop cognitive dysfunction and neuroinflammation after surgery. However, it is not known whether these pathological processes occur in people who live together with surgery patients. As an initial step to address this issue in animals, 2 mice with right common carotid arterial exposure were cage-mates with 3 non-surgery mice. Their learning and memory were tested starting 5 days after surgery. Their brain tissues were harvested 1 day or 5 days after surgery. The results showed that mice with surgery and cage-mates of these surgery mice had increased pro-inflammatory cytokines in the brain and dysfunction of learning and memory. Inhibition of inflammation attenuated the cognitive impairment of the cage-mates. These results suggest that dysfunction of complex behavior including learning and memory can occur in non-surgery cage-mates of surgery mice. Additional studies are needed to determine whether this phenomenon exists in larger animals and humans.

Affective dimensions of pain and region -specific involvement of nitric oxide in the development of empathic hyperalgesia

Empathy for pain depends on the ability to feel, recognize, comprehend and share painful emotional conditions of others. In this study, we investigated the role of NO in a rat model of empathic pain. Pain was socially transferred from the sibling demonstrator (SD) who experienced five formalin injection to the naïve sibling observer (SO) through observation. SO rats received L-NAME (a nonspecific NO synthase inhibitor) or L-arginine (a precursor of NO) prior to observing the SD. Nociception, and concentrations of NO metabolites (NOx) in the serum, left and right hippocampus, prefrontal cortex, and cerebellum were evaluated. Nociceptive responses were significantly increased in the pain-observing groups. NOx levels measured 24 h after the last pain observation using the Griess method, were indicative of NOx concentration decreases and increases in the left hippocampus and cerebellum, respectively. There was an increase in tissue concentration of NOx in cerebellum and prefrontal cortex in both pain and observer groups 7 days after the fifth formalin injection. Our results suggest that NO is involved in development of empathic hyperalgesia, and observation of sibling’s pain can change NO metabolites in different brain regions in observer rats.

Empathy in stroke rats is modulated by social settings

Rodents display "empathy" defined as perceived physical pain or psychological stress by cagemates when co-experiencing socially distinct traumatic events. The present study tested the hypothesis that empathy occurs in adult rats subjected to an experimental neurological disorder, by allowing co-experience of stroke with cagemates. Psychological stress was measured by general locomotor activity, Rat Grimace Scale (RGS), and plasma corticosterone. Physiological correlates were measured by Western blot analysis of advanced glycation endproducts (AGE)-related proteins in the thymus. General locomotor activity was impaired in stroke animals and in non-stroke rats housed with stroke rats suggesting transfer of behavioral manifestation of psychological stress from an injured animal to a non-injured animal leading to social inhibition. RGS was higher in stroke rats regardless of social settings. Plasma corticosterone levels at day 3 after stroke were significantly higher in stroke animals housed with stroke rats, but not with non-stroke rats, indicating that empathy upregulated physiological stress level. The expression of five proteins related to AGE in the thymus reflected the observed pattern of general locomotor activity, RGS, and plasma corticosterone levels. These results indicate that stroke-induced psychological stress manifested on both the behavioral and physiological levels and appeared to be affected by empathy-associated social settings.

Repeated exposure of naïve and peripheral nerve-injured mice to a snake as an experimental model of post-traumatic stress disorder and its co-morbidity with neuropathic pain

Confrontation of rodents by natural predators provides a number of advantages as a model for traumatic or stressful experience. Using this approach, one of the aims of this study was to investigate a model for the study of post-traumatic stress disorder (PTSD)-related behaviour in mice. Moreover, because PTSD can facilitate the establishment of chronic pain (CP), and in the same way, patients with CP have an increased tendency to develop PTSD when exposed to a traumatic event, our second aim was to analyse whether this comorbidity can be verified in the new paradigm. C57BL/6 male mice underwent chronic constriction injury of the sciatic nerve (CCI), a model of neuropathic CP, or not (sham groups) and were submitted to different threatening situations. Threatened mice exhibited enhanced defensive behaviours, as well as significantly enhanced risk assessment and escape behaviours during context reexposure. Previous snake exposure reduced open-arm time in the elevated plus-maze test, suggesting an increase in anxiety levels. Sham mice showed fear-induced antinociception immediately after a second exposure to the snake, but 1 week later, they exhibited allodynia, suggesting that multiple exposures to the snake led to increased nociceptive responses. Moreover, after reexposure to the aversive environment, allodynia was maintained. CCI alone produced intense allodynia, which was unaltered by exposure to either the snake stimuli or reexposure to the experimental context. Together, these results specifically parallel the behavioural symptoms of PTSD, suggesting that the snake/exuvia/reexposure procedure may constitute a useful animal model to study PTSD.

Stressed connections: cortisol levels following acute psychosocial stress disrupt affiliative mimicry in humans

Mimicry, and especially spontaneous facial mimicry, is a rudimentary element of social-emotional experience that is well-conserved across numerous species. Although such mimicry is thought to be a relatively automatic process, research indicates that contextual factors can influence mimicry, especially in humans. Here, we extend this work by investigating the effect of acute psychosocial stress on spontaneous facial mimicry. Participants performed a spontaneous facial mimicry task with facial electromyography (fEMG) at baseline and approximately one month later, following an acute psychosocial stressor (Trier Social Stress Test). Results show that the magnitude of the endocrine stress response reduced zygomaticus major reactivity, and specifically spontaneous facial mimicry for positive social stimuli (i.e. smiles). Individuals with higher levels of the stress hormone cortisol showed a more blunted fEMG response to smiles, but not to frowns. Conversely, stress had no effect on corrugator supercilii activation (i.e. frowning to frowns). These findings highlight the importance of the biological stress response system in this basic element of social-emotional experience.

Making "Good" Choices: Social Isolation in Mice Exacerbates the Effects of Chronic Stress on Decision Making

Chronic stress can impact decision-making and lead to a preference for immediate rewards rather than long-term payoffs. Factors that may influence these effects of chronic stress on decision-making are under-explored. Here we used a mouse model to investigate the changes in decision-making caused by the experience of chronic stress and the role of social isolation in exaggerating these changes. To test decision-making, mice were trained to perform a Cost-Benefit Conflict (CBC) task on a T-maze, in which they could choose between a high-reward, high-risk alternative and a low-reward, low-risk alternative. Mice were either housed in groups or alone throughout the experiment. Both groups of mice underwent a seven-day period of repeated immobilization to induce chronic stress. Stress levels were confirmed using behavioral (open field test) and physiological (urine corticosterone ELISA) measures. We found a significant increase in frequency of high-risk decisions after exposure to chronic stress among both socially- and individually-housed mice. Crucially, socially-housed mice showed a significantly smaller increase in high-risk decision-making compared to singly-housed mice. These findings suggest that chronic stress leads to an increase in high-risk decision-making in mice, and that lack of social interaction may exacerbate this stress effect.

Chemotherapy-induced peripheral neuropathy: part 1-current state of knowledge and perspectives for pharmacotherapy

Background

Despite the increasing knowledge of the etiology of neuropathic pain, this type of chronic pain is resistant to available analgesics in approximately 50% of patients and therefore is continuously a subject of considerable interest for physiologists, neurologists, medicinal chemists, pharmacologists and others searching for more effective treatment options for this debilitating condition.

Materials and methods

The present review article is the first of the two articles focused on chemotherapy-induced peripheral neuropathy (CIPN).

Results

CIPN is regarded as one of the most common drug-induced neuropathies and is highly pharmacoresistant. The lack of efficacious pharmacological methods for treating CIPN and preventing its development makes CIPN-related neuropathic pain a serious therapeutic gap in current medicine and pharmacotherapy. In this paper, the most recent advances in the field of studies on CIPN caused by platinum compounds (namely oxaliplatin and cisplatin), taxanes, vinca alkaloids and bortezomib are summarized.

Conclusions

The prevalence of CIPN, potential causes, risk factors, symptoms and molecular mechanisms underlying this pharmacoresistant condition are discussed.

Graphic abstract

An information-theory approach to geometry for animal groups

One of the hardest problems in studying animal behaviour is to quantify patterns of social interaction at the group level. Recent technological developments in global positioning system (GPS) devices have opened up new avenues for locating animals with unprecedented spatial and temporal resolution. Likewise, advances in computing power have enabled new levels of data analyses with complex mathematical models to address unresolved problems in animal behaviour, such as the nature of group geometry and the impact of group-level interactions on individuals. Here, we present an information theory-based tool for the analysis of group behaviour. We illustrate its affordances with GPS data collected from a freely interacting pack of 15 Siberian huskies (Canis lupus familiaris). We found that individual freedom in movement decisions was limited to about 4%, while a subject's location could be predicted with 96% median accuracy by the locations of other group members. Dominant individuals were less affected by other individuals' locations than subordinate ones, and same-sex individuals influenced each other more strongly than opposite-sex individuals. We also found that kinship relationships increased the mutual dependencies of individuals. Moreover, the network stability of the pack deteriorated with an upcoming feeding event. Together, we conclude that information theory-based approaches, coupled with state-of-the-art bio-logging technology, provide a powerful tool for future studies of animal social interactions beyond the dyadic level.

Challenges in the comparative study of empathy and related phenomena in animals

The aim of this review is to discuss recent arguments and findings in the comparative study of empathy. Based on a multidisciplinary approach including psychology and ethology, we review the non-human animal literature concerning theoretical frameworks, methodology, and research outcomes. One specific objective is to highlight discrepancies between theory and empirical findings, and to discuss ambiguities present in current data and their interpretation. In particular, we focus on emotional contagion and its experimental investigation, and on consolation and targeted helping as measures for sympathy. Additionally, we address the feasibility of comparing across species with behavioural data alone. One main conclusion of our review is that animal research on empathy still faces the challenge of closing the gap between theoretical concepts and empirical evidence. To advance our knowledge, we propose to focus more on the emotional basis of empathy, rather than on possibly ambiguous behavioural indicators, and we provide suggestions to overcome the limitations of previous research .

Conformity-like behaviour in mice observing the freezing of other mice: a model of empathy

Background

Empathy refers to the ability to recognise and share emotions with others. Several research groups have recognised observational fear in mice as a useful behavioural model for assessing their ability to empathise. However, in these observation systems, it remains unclear whether the observer mouse truly recognises the movements of, and empathises with, the demonstrator mouse. We examined changes in the behaviour of an observer mouse when a demonstrator mouse was anaesthetised, when the demonstrator’s activity was increased, and when the interval of electrical stimulation was altered. If mice exhibit an ability to empathise, then the observer should display empathic behaviour when the demonstrator experiences pain or discomfort under any circumstances.

Results

Relative to low-frequency stimulation, frequent electrical stimulation reduced immobility time among observer mice. Moreover, when demonstrators exhibited excessive activity, the activity of the observers significantly increased. In addition, the proportion of immobility time among observer mice significantly increased when demonstrator mice exhibited fear learning and excessive immobility.

Conclusion

Although our results indicate that observer mice change their behaviour based on the movements of demonstrator mice, increases in immobility time may reflect conformity-like behaviour rather than emotional empathy. Thus, not only visual but also auditory and odour information additionally influenced the conformity-like behaviour shown by observer mice. Thus, our findings suggest that methods other than the fear observation system should be used to investigate rodent empathy-like behaviour.

Action representation in the mouse parieto-frontal network

The posterior parietal cortex (PPC) and frontal motor areas comprise a cortical network supporting goal-directed behaviour, with functions including sensorimotor transformations and decision making. In primates, this network links performed and observed actions via mirror neurons, which fire both when individuals perform an action and when they observe the same action performed by a conspecific. Mirror neurons are believed to be important for social learning, but it is not known whether mirror-like neurons occur in similar networks in other social species, such as rodents, or if they can be measured in such models using paradigms where observers passively view a demonstrator. Therefore, we imaged Ca²⁺ responses in PPC and secondary motor cortex (M2) while mice performed and observed pellet-reaching and wheel-running tasks, and found that cell populations in both areas robustly encoded several naturalistic behaviours. However, neural responses to the same set of observed actions were absent, although we verified that observer mice were attentive to performers and that PPC neurons responded reliably to visual cues. Statistical modelling also indicated that executed actions outperformed observed actions in predicting neural responses. These results raise the possibility that sensorimotor action recognition in rodents could take place outside of the parieto-frontal circuit, and underscore that detecting socially-driven neural coding depends critically on the species and behavioural paradigm used.

Role of Opioid System in Empathy-like Behaviours in Rats

Background

Empathy is defined as the ability to simulate the mental states of others. Recent studies have demonstrated empathy-like behaviors in other animals including rats and mice. The objective of the current study was to evaluate the effect of acute administration of morphine and naloxone on cognition and nociception changes following observing conspecifics undergoing nociceptive stimulus.

Methods

Adult male Wistar rats were used (n = 8 for each group). One cagemate received formalin injection into the hindpaw five times within a nine-day period and the other cagemate observed the pain while being pretreated with saline, morphine, or naloxone [10 mg/kg, intraperitoneal (i.p.)]. Pain behaviors, anxiety-like behaviour, locomotion, balance and muscle strength were evaluated in the observer animals.

Findings

Observing a cagemate in pain increased anxiety-like behavior and reduced thermal pain threshold in the observer animals. Administration of morphine reversed these effects and naloxone did not affect the responses.

Conclusion

Results of the current study reveal an important role for opioid receptors (ORs) in empathy for pain, so that activation of this system dampens the empathy-like responses.

MeCP2 mediates transgenerational transmission of chronic pain

Pain symptoms can be transmitted across generations, but the mechanisms underlying these outcomes remain poorly understood. Here, we identified an essential role for primary somatosensory cortical (S1) glutamate neuronal DNA methyl-CpG binding protein 2 (MeCP2) in the transgenerational transmission of pain. In a female mouse chronic pain model, the offspring displayed significant pain sensitization. In these mice, MeCP2 expression was increased in S1 glutamate (Glu^S1) neurons, correlating with increased neuronal activity. Downregulation of Glu^S1 neuronal MeCP2 in maternal mice with pain abolished offspring pain sensitization, whereas overexpression of MeCP2 in naïve maternal mice induced pain sensitization in offspring. Notably, single-cell sequencing and chromatin immunoprecipitation analysis showed that the expression of a wide range of genes was changed in offspring and maternal Glu^S1 neurons, some of which were regulated by MeCP2. These results collectively demonstrate the putative importance of MeCP2 as a key regulator in pain transgenerational transmission through actions on Glu^S1 neuronal maladaptation.

Enhancing cognition through pharmacological and environmental interventions: Examples from preclinical models of neurodevelopmental disorders

In this review we discuss the role of environmental and pharmacological treatments to enhance cognition with special regards to neurodevelopmental related disorders and aging. How the environment influences brain structure and function, and the interactions between rearing conditions and gene expression, are fundamental questions that are still poorly understood. We propose a model that can explain some of the discrepancies in findings for effects of environmental enrichment on outcome measures. Evidence of a direct causal correlation of nootropics and treatments that enhanced cognition also will be presented, and possible molecular mechanisms that include neurotrophin signaling and downstream pathways underlying these processes are discussed. Finally we review recent findings achieved with a wide set of behavioral and cognitive tasks that have translational validity to humans, and should be useful for future work on devising appropriate therapies. As will be discussed, the collective findings suggest that a combinational therapeutic approach of environmental enrichment and nootropics could be particularly successful for improving learning and memory in both developmental disorders and normal aging.

Empathy as a Concept from Bench to Bedside: A Translational Challenge

Empathy is a multidimensional paradigm, and there currently is a lack of scientific consensus in its definition. In this paper, we review the possibility of compromising data during behavioral neuroscience experiments, including but not limited to those who study empathy. The experimental protocols can affect, and be affected by, empathy and related processes at multiple levels. We discuss several points to help researchers develop a successful translational pathway for behavioral research on empathy. Despite varying in their focus with no widely accepted model, current rodent models on empathy have provided sound translational explanations for many neuropsychiatric proof-of-concepts to date. Research has shown that empathy can be influenced by many parameters, some of which are to be reviewed in this paper. We emphasize the future importance of consistency in modeling proof of concept; efforts to create a multidisciplinary group which would include both bench scientists and clinicians with expertise in neuropsychiatry, and the consideration of empathy as an independent variable in animal behavioral experimental designs which is not the mainstream practice at present.

Neural responses in the pain matrix when observing pain of others are unaffected by testosterone administration in women

There is evidence of testosterone having deteriorating effects on cognitive and affective empathic behaviour in men and women under varying conditions. However, whether testosterone influences empathy for pain has not yet been investigated. Therefore, we tested neural responses to witnessing others in pain in a within-subject placebo-controlled testosterone administration study in healthy young women. Using functional magnetic resonance imaging, we provide affirming evidence that an empathy-inducing paradigm causes changes in the activity throughout the pain circuitry, including the bilateral insula and anterior cingulate cortex. Administration of testosterone, however, did not influence these activation patterns in the pain matrix. Testosterone has thus downregulating effects on aspects of empathic behaviour, but based on these data does not seem to influence neural responses during empathy for others' pain. This finding gives more insight into the role of testosterone in human empathy.

Behavioural Changes in Mice after Getting Accustomed to the Mirror

Patients with brain function disorders due to stroke or dementia may show inability to recognize themselves in the mirror. Although the cognitive ability to recognize mirror images has been investigated in many animal species, the animal species that can be used for experimentation and the mechanisms involved in recognition remain unclear. We investigated whether mice have the ability to recognize their mirror images. Demonstrating evidence of this in mice would be useful for researching the psychological and biological mechanisms underlying this ability. We examined whether mice preferred mirrors, whether plastic tapes on their heads increased their interest, and whether mice accustomed to mirrors learnt its physical phenomenon. Mice were significantly more interested in live stranger mice than mirrors. Mice with tape on their heads spent more time before mirrors. Becoming accustomed to mirrors did not change their behaviour. Mice accustomed to mirrors had significantly increased interest in photos of themselves over those of strangers and cage-mates. These results indicated that mice visually recognized plastic tape adherent to reflected individuals. Mice accustomed to mirrors were able to discriminate between their images, cage-mates, and stranger mice. However, it is still unknown whether mice recognize that the reflected images are of themselves.

Similar levels of emotional contagion in male and female rats

Emotional contagion, the ability to feel what other individuals feel without necessarily understanding the feeling or knowing its source, is thought to be an important element of social life. In humans, emotional contagion has been shown to be stronger in women than men. Emotional contagion has been shown to exist also in rodents, and a growing number of studies explore the neural basis of emotional contagion in male rats and mice. Here we explore whether there are sex differences in emotional contagion in rats. We use an established paradigm in which a demonstrator rat receives footshocks while freezing is measured in both the demonstrator and an observer rat. The two rats can hear, smell and see each other. By comparing pairs of male rats with pairs of female rats, we found (i) that female demonstrators froze less when submitted to footshocks, but that (ii) the emotional contagion response, i.e. the degree of influence across the rats, did not depend on the sex of the rats. This was true whether emotional contagion was quantified based on the slope of a regression linking demonstrator and observer average freezing, or on Granger causality estimates of moment-to-moment freezing. The lack of sex differences in emotional contagion is compatible with an interpretation of emotional contagion as serving selfish danger detection.

The similar past pain experience evokes both observational contagious pain and consolation in stranger rat observers

Laboratory rodents have been shown to have an ability to recognize the injury site and negative emotional state of their conspecifics in pain, resulting in empathic consoling behaviors and observational contagious pain (OCP). However, these empathic responses have been shown to be familiarity-dependent. In this report, we further explored whether the past pain experience could evoke empathic response in stranger observers. In our rodent model, two types of empathic response have been identified from naive cagemate observer (CO_naive) during and after a priming dyadic social interaction (PDSI) with a cagemate demonstrator in pain (CD_pain): the consolation and OCP. Consolation is represented by allolicking and allogrooming behaviors toward the CD_pain, while the OCP is represented by a long-term mechanical pain hypersensitivity. The current results showed that: (1) neither the consolation nor OCP could be identified in the naive noncagemate observer (NCO_naive) during and after a PDSI with a noncagemate demonstrator in pain (NCD_pain); (2) nor were the two types of empathic response seen in the NCO, who had just experienced acute pain (NCO_painexp), during and after a PDSI with a naive unfamiliar conspecific (NCD_naive). However, both the consolation and OCP were dramatically identified in the NCO_painexp during and after a PDSI with a NCD in pain (NCD_pain). The current results demonstrated that the past pain experience can evoke both consolation and OCP in stranger rat observers when witnessing a conspecific in pain, implicating that the processing of empathy for pain can be modulated by past negative mood experience.

Contagious yawning is not a signal of empathy: no evidence of familiarity, gender or prosociality biases in dogs

Contagious yawning has been suggested to be a potential signal of empathy in non-human animals. However, few studies have been able to robustly test this claim. Here, we ran a Bayesian multilevel reanalysis of six studies of contagious yawning in dogs. This provided robust support for claims that contagious yawning is present in dogs, but found no evidence that dogs display either a familiarity or gender bias in contagious yawning, two predictions made by the contagious yawning-empathy hypothesis. Furthermore, in an experiment testing the prosociality bias, a novel prediction of the contagious yawning-empathy hypothesis, dogs did not yawn more in response to a prosocial demonstrator than to an antisocial demonstrator. As such, these strands of evidence suggest that contagious yawning, although present in dogs, is not mediated by empathetic mechanisms. This calls into question claims that contagious yawning is a signal of empathy in mammals.

Convergence of physiological responses to pain during face-to-face interaction

Empathy with another’s pain is an important social glue for maintaining interpersonal relationships. In most previous studies investigating the sharing of pain, a signal conveying a painful experience is presented by a target (“sender”) as a stimulus to a participant (“receiver”), and the emotional/physiological responses of the participant are measured. However, this unilateral “sender-receiver” paradigm does not adequately address the possible bidirectional experience of shared pain accruing from interaction. Our aim was therefore to investigate the bidirectional effects of sharing pain in social settings. Thirty-six unfamiliar pairs were simultaneously and repeatedly exposed to the same pain-provoking (thermal) stimuli, either in a face-to-face or a “shielded” condition where a partition prevented the partner’s responses from being fully observed. We recorded the blood volume pulse of each participant to measure the acute sympathetic response while a pair of participants experienced the stimuli simultaneously. The results revealed that participants with weaker reactions elevated their physiological reactivity to the stimulus in accordance with their partner’s reactions in the face-to-face condition. The pair-level physiological similarity was also higher compared to the shielded condition. Such a low-to-high physiological convergence may underlie the collective elevation of pain expressions, which is often observed in interactive settings.

Feeding Behavior of Mice under Different Food Allocation Regimens

Social interaction, a basic survival strategy for many animal species, helps maintain a social environment that has limited conflict. Social dominance has a dramatic effect on motivation. Recent evidence suggests that some primate and nonprimate species display aversive behavior toward food allocation regimens that differ from their peers. Thus, we examined the behaviors displayed by mice under different food allocation regimens. We analyzed changes in food intake using several parameters. In the same food condition, the mice received the same food; in the quality different condition, the mice received different foods; in the quantity different condition, one mouse did not receive food; and in the no food condition, none of the mice received food. To test differences based on food quality, one mouse received normal solid food as a less preferred reward, and the other received chocolate chips as a high-level reward. No behavioral change was observed in comparison to the same food condition. To test differences based on food quantity, one mouse received chocolate chips while the other received nothing. Mice who received nothing spent more time on the other side of the reward throughout the experiment. Interestingly, highly rewarded mice required more time to consume the chocolate chips. Thus, under different food allocation regimens, mice changed their behavior by being more hesitant. Moreover, mice alter food intake behavior according to the social environment. The findings help elucidate potential evolutionary aspects that help maintain social cohesion while providing insights into potential mechanisms underlying socially anxious behavior.

Bidirectional cingulate-dependent danger information transfer across rats

Social transmission of freezing behavior has been conceived of as a one-way phenomenon in which an observer "catches" the fear of another. Here, we use a paradigm in which an observer rat witnesses another rat receiving electroshocks. Bayesian model comparison and Granger causality show that rats exchange information about danger in both directions: how the observer reacts to the demonstrator's distress also influences how the demonstrator responds to the danger. This was true to a similar extent across highly familiar and entirely unfamiliar rats but is stronger in animals preexposed to shocks. Injecting muscimol in the anterior cingulate of observers reduced freezing in the observers and in the demonstrators receiving the shocks. Using simulations, we support the notion that the coupling of freezing across rats could be selected for to more efficiently detect dangers in a group, in a way similar to cross-species eavesdropping.

Conserved features of anterior cingulate networks support observational learning across species

The ability to observe, interpret, and learn behaviors and emotions from conspecifics is crucial for survival, as it bypasses direct experience to avoid potential dangers and maximize rewards and benefits. The anterior cingulate cortex (ACC) and its extended neural connections are emerging as important networks for the detection, encoding, and interpretation of social signals during observational learning. Evidence from rodents and primates (including humans) suggests that the social interactions that occur while individuals are exposed to important information in their environment lead to transfer of information across individuals that promotes adaptive behaviors in the form of either social affiliation, alertness, or avoidance. In this review, we first showcase anatomical and functional connections of the ACC in primates and rodents that contribute to the perception of social signals. We then discuss species-specific cognitive and social functions of the ACC and differentiate between neural activity related to 'self' and 'other', extending into the difference between social signals received and processed by the self, versus observing social interactions among others. We next describe behavioral and neural events that contribute to social learning via observation. Finally, we discuss some of the neural mechanisms underlying observational learning within the ACC and its extended network.

Persistence of pain in humans and other mammals

Evolutionary models of chronic pain are relatively undeveloped, but mainly concern dysregulation of an efficient acute defence, or false alarm. Here, a third possibility, mismatch with the modern environment, is examined. In ancestral human and free-living animal environments, survival needs urge a return to activity during recovery, despite pain, but modern environments allow humans and domesticated animals prolonged inactivity after injury. This review uses the research literature to compare humans and other mammals, who share pain neurophysiology, on risk factors for pain persistence, behaviours associated with pain, and responses of conspecifics to behaviours. The mammal populations studied are mainly laboratory rodents in pain research, and farm and companion animals in veterinary research, with observations of captive and free-living primates. Beyond farm animals and rodent models, there is virtually no evidence of chronic pain in other mammals. Since evidence is sparse, it is hard to conclude that it does not occur, but its apparent absence is compatible with the mismatch hypothesis. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Characterization of rat ultrasonic vocalization in the orofacial formalin test: Influence of the social context

Rats emit ultrasonic vocalizations (USVs) about 22 kHz and 50 kHz sound frequency to communicate the presence of negative or positive emotional states, respectively. The calling behavior may be influenced by several factors, including environmental factors. Likewise, pain behavior can be modulated according to the social context, and also can be transferred to conspecifics through direct observation and/or social interaction. Herein we investigated if acute pain induction was related to changes in emission of aversive and appetitive calls and how different social contexts affected the nociceptive behavior and USVs. Our results demonstrated that orofacial formalin injection in rats induced aversive calls in addition to the nociceptive behavior, and both are reduced by systemic treatment with morphine (2.5 mg/kg). Exposure of formalin-injected rats to cagemates had no effect on the nociceptive behavior or calls emitted by the demonstrator, but the observer showed emotional contagion of pain. In contrast, exposure of formalin-injected rats to non-cagemates decreased the nociceptive behavior of the demonstrator, without affecting the calls emission. The emotional contagion was not detected in non-cagemates or in cagemates separated by a visual barrier. In conclusion, we suggest that familiarity and the visual contact contributes to emotional contagion of pain. USV analysis may represent an additional measure in the evaluation of the emotional aspect of orofacial pain, and for the study of pain modulation.

Play like me: Similarity in playfulness promotes social play

Social play is associated with the experience of positive emotions in higher vertebrates and may be used as a measure of animal welfare. Altering motivation to play (e.g., through short-term social isolation) can temporarily affect play levels between familiar individuals, a process which may involve emotional contagion. This study investigated how forming groups based on known differences in the personality trait "playfulness" (i.e., the longer-term propensity of an individual to actively play from adolescence to early adulthood) affects social play. Seventy-six adolescent male Lister Hooded rats underwent a Play-in-Pairs test assessing their playfulness, ranked as high (H), intermediate (I) or low (L). At seven weeks of age, rats were resorted into homogenous groups of similar (LLL, III, HHH), or heterogeneous groups of dissimilar (HII, LII) playfulness. Social play was scored in the home cage at Weeks 8, 10, 12 of age. A second Play-in-Pairs test was performed (Week 11) to assess consistency of playfulness. A Social Preference test investigated whether I rats in heterogeneous groups preferred proximity with I, H or L cage mates. It was found that heterogeneous groups played less than homogeneous ones at adolescence (8 weeks of age), while play levels at early adulthood (Weeks 10 and 12) did not differ between groups. Play in the homogeneous groups decreased with age as expected, while it did not change over time in the heterogeneous groups, which did not compensate for the lower play levels shown at adolescence. Play-in-Pairs scores before and after resorting were mildly correlated, indicating some level of consistency over time despite the resorting procedure. In the Social Preference test, subjects did not prefer one playfulness level over another. We conclude that a mismatch in playfulness may negatively affect social play development, and thus the welfare, of rats. Groups made of animals with similar playfulness, even those initially scoring relatively low in this trait, seemed to be more successful in establishing play relationships during adolescence.