The Locus Coeruleus-Norepinephrine System Mediates Empathy for Pain through Selective Up-Regulation of P2X3 Receptor in Dorsal Root Ganglia in Rats

Environmental factors and their impact on chronic pain development and maintenance

It is more than recognized and accepted that the environment affects the physiological responses of all living things, from bacteria to superior vertebrates, constituting an important factor in the evolution of all species. Environmental influences range from natural processes such as sunlight, seasons of the year, and rest to complex processes like stress and other mood disorders, infections, and air pollution, being all of them influenced by how each creature deals with them. In this chapter, it will be discussed how some of the environmental elements affect directly or indirectly neuropathic pain, a type of chronic pain caused by a lesion or disease of the somatosensory nervous system. For that, it was considered the edge of knowledge in translational research, thus including data from human and experimental animals as well as the applicability of such findings.

Chronic stress and stressful emotional contagion affect the empathy-like behavior of rats

Empathy is a potential motivation for prosocial behaviors that is related to many psychiatric diseases, such as major depressive disorder; however, its neural mechanisms remain unclear. To elucidate the relationship between empathy and stress, we established a chronic stress contagion (SC) procedure combined with chronic unpredictable mild stress (CUMS) to investigate (1) whether depressive rats show impaired empathy-like behavior toward fearful conspecifics, (2) whether frequent social contact with normal familiar conspecifics (social support) alleviates the negative effects of CUMS, and (3) the effect of long-term exposure to a depressed partner on emotional and empathic responses in normal rats. We found that the CUMS group showed less empathy-like behavior in the social transfer of fear model (STFM), as indicated by less social interaction with the demonstrator and reduced freezing behavior in the fear-expression test. Social contact partially alleviated depression-like behaviors and the negative effect of CUMS in the fear-transfer test. The normal rats who experienced stress contagion from daily exposure to a depressed partner for 3 weeks showed lower anxiety and increased social response in the fear-transfer test than the control group. We concluded that chronic stress impairs empathy-like behaviors, while social contact partially buffers the effect of CUMS. Thus, social contact or contagion of stress is mutually beneficial to both stressed individuals and nonstressed partners. Higher dopamine and lower norepinephrine levels in the basolateral amygdala probably contributed to these beneficial effects.

A Genomic Study of the Japanese Population Focusing on the Glucocorticoid Receptor Interactome Highlights Distinct Genetic Characteristics Associated with Stress Response

All living organisms have been programmed to maintain a complex inner equilibrium called homeostasis, despite numerous adversities during their lifespan. Any threatening or perceived as such stimuli for homeostasis is termed a stressor, and a highly conserved response system called the stress response system has been developed to cope with these stimuli and maintain or reinstate homeostasis. The glucocorticoid receptor, a transcription factor belonging to the nuclear receptors protein superfamily, has a major role in the stress response system, and research on its interactome may provide novel information regarding the mechanisms underlying homeostasis maintenance. A list of 149 autosomal genes that have an essential role in GR function or are prime examples of GRE-containing genes was composed in order to gain a comprehensive view of the GR interactome. A search for SNPs on those particular genes was conducted on a dataset of 3554 Japanese individuals, with mentioned polymorphisms being annotated with relevant information from the ClinVar, LitVar, and dbSNP databases. Forty-two SNPs of interest and their genomic locations were identified. These SNPs have been associated with drug metabolism and neuropsychiatric, metabolic, and immune system disorders, while most of them were located in intronic regions. The frequencies of those SNPs were later compared with a dataset consisting of 1465 Korean individuals in order to find population-specific characteristics based on some of the identified SNPs of interest. The results highlighted.that rs1043618 frequencies were different in the two populations, with mentioned polymorphism having a potential role in chronic obstructive pulmonary disease in response to environmental stressors. This SNP is located in the HSPA1A gene, which codes for an essential GR co-chaperone, and such information showcases that similar gene may be novel genomic targets for managing or combatting stress-related pathologies.

Association between P2X3 receptors and neuropathic pain: As a potential therapeutic target for therapy

Neuropathic pain is a common clinical symptom of various diseases, and it seriously affects the physical and mental health of patients. Owing to the complex pathological mechanism of neuropathic pain, clinical treatment of pain is challenging. Therefore, there is growing interest among researchers to explore potential therapeutic strategies for neuropathic pain. A large number of studies have shown that development of neuropathic pain is related to nerve conduction and related signaling molecules. P2X3 receptors (P2X3R) are ATP-dependent ion channels that participate in the transmission of neural information and related signaling pathways, sensitize the central nervous system, and play a key role in the development of neuropathic pain. In this paper, we summarized the structure and biological characteristics of the P2X3R gene and discussed the role of P2X3R in the nervous system. Moreover, we outlined the related pathological mechanisms of pain and described the relationship between P2X3R and chronic pain to provide valuable information for development of novel treatment strategies for pain.

Prior stress experience modulates social preference for stressed conspecifics in male rats

Adult male rats tend to avoid adult conspecifics in distress. In this study, we asked whether prior stress experience would modulate social preference for a stressed conspecific using a social affective preference (SAP) test. Male Long-Evans adult rats were assigned to the shocked and non-shocked groups. In the shocked group, rats were acutely subjected to foot shocks (1.0 mA, 5 s duration × 2) 24 h before the SAP test. During the SAP test, the experimental rats were placed in an arena where two adult conspecific stimuli, one of which received the foot shocks immediately before the SAP test, were presented at both ends and allowed to explore freely for 5 min. We measured sniffing behavior toward each conspecific as an index of social preference. Non-shocked adult rats avoided, while shocked rats approached, the stressed conspecifics more than the non-stressed ones. These results suggest that prior stress promotes social preference for a stressed conspecific in adult male rats.

Emotional contagion in nonhuman animals: A review

Emotional contagion, the emotional state‐matching of an individual with another, seems to be crucial for many social species. In recent years evidence on emotional contagion in different animal species has accumulated. However, despite its adaptative advantages and its presumed simplicity, the study and direct demonstration of this phenomenon present more complexities than previously thought. For these reasons, a review of the literature on emotional contagion in nonhuman species is timely to integrate current findings. In this paper thus, we carry out a comprehensive review of the most relevant studies on emotional contagion in animals and discuss the main problems and challenges of the field. We conclude that more research is needed to broaden our understanding of the mechanisms and functions of emotional contagion and the extent to which this process is present in a wide variety of species. Furthermore, the comparative study of emotional contagion would benefit from the use of systematized paradigms including both behavioral and physiological measures and the simultaneous recording of the responses of the interacting individuals to reliably assess an emotional state‐matching between them and reliable controls.

This article is categorized under:

Cognitive Biology > Evolutionary Roots of Cognition

Psychology > Comparative Psychology

Psychology > Emotion and Motivation

The interaction between P2X3 and TRPV1 in the dorsal root ganglia of adult rats with different pathological pains

Peripheral inflammatory and neuropathic pain are closely related to the activation of purinergic receptor P2X ligand-gated ion channel 3 (P2X3) and transient receptor potential vanilloid 1 (TRPV1), but the interaction between P2X3 and TRPV1 in different types of pathological pain has rarely been reported. In this study, complete Freund’s adjuvant (CFA)-induced inflammatory pain and spared nerve injury (SNI)-induced neuropathic pain models were established in adult rats. The interactions between P2X3 and TRPV1 in the dorsal root ganglion were observed by pharmacological, co-immunoprecipitation, immunofluorescence and whole-cell patch-clamp recording assays. TRPV1 was shown to promote the induction of spontaneous pain caused by P2X3 in the SNI model, but the induction of spontaneous pain behaviour by TRPV1 was not completely dependent on P2X3 in vivo. In both the CFA and SNI models, the activation of peripheral P2X3 enhanced the effect of TRPV1 on spontaneous pain, while the inhibition of peripheral TRPV1 reduced the induction of spontaneous pain by P2X3 in the CFA model. TRPV1 and P2X3 had inhibitory effects on each other in the inflammatory pain model. During neuropathic pain, P2X3 facilitated the function of TRPV1, while TRPV1 had an inhibitory effect on P2X3. These results suggest that the mutual effects of P2X3 and TRPV1 differ in cases of inflammatory and neuropathic pain in rats.

Neural mechanisms necessary for empathy-related phenomena across species

The neural basis of empathy and prosociality has received much interest over the past decades. Neuroimaging studies localized a network of brain regions with activity that correlates with empathy. Here, we review how the emergence of rodent and nonhuman primate models of empathy-related phenomena supplements human lesion and neuromodulation studies providing evidence that activity in several nodes is necessary for these phenomena to occur. We review proof that (i) affective states triggered by the emotions of others, (ii) motivations to act in ways that benefit others, and (iii) emotion recognition can be altered by perturbing brain activity in many nodes identified by human neuroimaging, with strongest evidence for the cingulate and the amygdala. We also include evidence that manipulations of the oxytocin system and analgesics can have such effects, the latter providing causal evidence for the recruitment of an individual's own nociceptive system to feel with the pain of others.

Binding of Dopamine D1 Receptor and Noradrenaline Transporter in Individuals with Autism Spectrum Disorder: A PET Study

Although previous studies have suggested the involvement of dopamine (DA) and noradrenaline (NA) neurotransmissions in the autism spectrum disorder (ASD) pathophysiology, few studies have examined these neurotransmissions in individuals with ASD in vivo. Here, we investigated DA D1 receptor (D1R) and noradrenaline transporter (NAT) binding in adults with ASD (n = 18) and neurotypical controls (n = 20) by utilizing two different PET radioligands, [11C]SCH23390 and (S,S)-[18F]FMeNER-D2, respectively. We found no significant group differences in DA D1R (striatum, anterior cingulate cortex, and temporal cortex) or NAT (thalamus and pons) binding. However, in the ASD group, there were significant negative correlations between DA D1R binding (striatum, anterior cingulate cortex and temporal cortex) and the “attention to detail” subscale score of the Autism Spectrum Quotient. Further, there was a significant positive correlation between DA D1R binding (temporal cortex) and emotion perception ability assessed by the neurocognitive battery. Associations of NAT binding with empathic abilities and executive function were found in controls, but were absent in the ASD group. Although a lack of significant group differences in binding might be partly due to the heterogeneity of ASD, our results indicate that central DA and NA function might play certain roles in the clinical characteristics of ASD.

Cellular Circuits in the Brain and Their Modulation in Acute and Chronic Pain

Chronic, pathological pain remains a global health problem and a challenge to basic and clinical sciences. A major obstacle to preventing, treating, or reverting chronic pain has been that the nature of neural circuits underlying the diverse components of the complex, multidimensional experience of pain is not well understood. Moreover, chronic pain involves diverse maladaptive plasticity processes, which have not been decoded mechanistically in terms of involvement of specific circuits and cause-effect relationships. This review aims to discuss recent advances in our understanding of circuit connectivity in the mammalian brain at the level of regional contributions and specific cell types in acute and chronic pain. A major focus is placed on functional dissection of sub-neocortical brain circuits using optogenetics, chemogenetics, and imaging technological tools in rodent models with a view towards decoding sensory, affective, and motivational-cognitive dimensions of pain. The review summarizes recent breakthroughs and insights on structure-function properties in nociceptive circuits and higher order sub-neocortical modulatory circuits involved in aversion, learning, reward, and mood and their modulation by endogenous GABAergic inhibition, noradrenergic, cholinergic, dopaminergic, serotonergic, and peptidergic pathways. The knowledge of neural circuits and their dynamic regulation via functional and structural plasticity will be beneficial towards designing and improving targeted therapies.

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.

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.

The role of the endogenous neurotransmitters associated with neuropathic pain and in the opioid crisis: The innate pain-relieving system

Neuropathic pain is a chronic pain caused by central and peripheral nerve injury, long-term diabetes or treatment with chemotherapy drugs, and it is dissimilar to other chronic pain conditions. Chronic pain usually seriously affects the quality of life, and its drug treatment may result in increased costs of social and medical care. As in the USA and Canada, in Europe, the demand for pain-relieving medicines used in chronic pain has also significantly increased, but most European countries are not experiencing an opioid crisis. In this review, the role of various endogenous transmitters (noradrenaline, dopamine, serotonin, met- and leu-enkephalins, β-endorphin, dynorphins, cannabinoids, ATP) and various receptors (α₂, μ, etc.) in the innate pain-relieving system will be discussed. Furthermore, the modulation of pain processing pathways by transmitters, focusing on neuropathic pain and the role of the sympathetic nervous system in the side effects of excessive opioid treatment, will be explained.

The Role of Sensorimotor Processes in Pain Empathy

Pain is a salient, aversive sensation which motivates avoidance, but also has a strong social signaling function. Numerous studies have shown that regions of the nervous system active in association with first-hand pain are also active in response to the pain of others. When witnessing somatic pain, such as seeing bodies in painful situations, significant activations occur not only in areas related to the processing of negative emotions, but also in neuronal structures engaged in somatosensation and the control of skeletal muscles. These empathy-related sensorimotor activations are selectively reviewed in this article, with a focus on studies using electrophysiological methods and paradigms investigating responses to somatic pain. Convergent evidence from these studies shows that these activations (1) occur at multiple levels of the nervous system, from the spinal cord up to the cerebral cortex, (2) are best conceptualized as activations of a defensive system, in line with the role of pain to protect body from injury, and (3) contribute to establishing a matching of psychological states between the sufferer and the observer, which ultimately supports empathic understanding and motivate prosocial action. Future research should thus focus on how these sensorimotor responses are related to higher-order empathic responses, including affective sharing and emotion regulation, and how this motivates approach-related prosocial behaviors aimed at alleviating the pain and suffering of others.

Sensory and Affective Dimensions of Pain and Anxiety Like Behaviors Are Altered in an Animal Model of Pain Empathy

Objective: Pain is a unique and subjective experience that has a prominent function in animals' survival. Observation of pain in others leads to alterations in pain sensation and affection, termed "Empathy for pain". The present study aimed to evaluate the effect of empathy on sensory and affective dimensions of pain and its effect on anxiety-like behaviors. Method : In this study, male Wistar rats were used. Two cage mates were selected, one of which underwent administration of a noxious stimuli for 10 days and the other observed the conspecific in pain. Hot plate, tail flick, and conditioned place aversion were used to evaluate sensory and affective dimensions of pain, respectively. Anxiety-like behavior was assayed using elevated plus maze paradigm and time spent in open and close arms and number of entrance into each arm was recorded as the anxiety indicator within a 5-minute framework. Results: Rats observing the cage mate in pain had a lower threshold to noxious stimuli in comparison to controls. They also had an increased aversion from painful stimuli, demonstrating heightened affective response to pain. Anxiety-like behavior was also enhanced in the observers. Conclusion: Results of this study demonstrate that both sensory and affective dimensions of pain are altered following observation of pain in a conspecific. Further studies evaluating the underlying mechanisms are encouraged to elucidate the role of different neurotransmitters in this phenomenon.

Rat Model of Empathy for Pain

Empathy for pain is referred to as an evolutionary behavior of social animals and humans associated with the ability to feel, recognize, understand and share the other's distressing (pain, social rejection and catastrophe) states. Impairment of empathy can definitely lead to deficits in social communication and sociability (attachment, bond, reciprocity, altruism and morality) that may be fundamental to some psychiatric disorders such as autism spectrum disorder (ASD), psychopathy, misconduct, antisocial personality disorder and schizophrenia. So far, the underlying mechanisms of empathy are poorly known due to lack of animal models and scarce understanding of its biological basis. Recently, we have successfully identified and validated the behavioral identities of empathy for pain in rats that can be widely used as a rodent model for studying the underlying biological mechanisms of empathy. Priming dyadic social interaction between a naive cagemate observer (CO) and a cagemate demonstrator (CD), rather than a non-cagemate, in pain for 30 min in a testing box can repeatedly and constantly result in empathic responses of the CO toward the familiar CD's distressing condition, displaying as allo-licking at the injury site, allo-grooming at the body and social transfer of pain. The familiarity-based, distress-specific social consolation and subsequent social transfer of pain can be qualitatively and quantitatively rated as experimental biomarkers for empathy for pain. The rodent model of empathy for pain is state-of-the-art and has more advantages than the existing ones used for social neuroscience since it can reflect sensory, emotional and cognitive processes of the brain in running the prosocial and altruistic behaviors in animals who could not report verbally. Here we would like to provide and share the protocol of the model for wide use.

Involvement of oxytocin and GABA in consolation behavior elicited by socially defeated individuals in mandarin voles

Consolation, which entails comforting contact directed toward a distressed party, is a common empathetic response in humans and other species with advanced cognition. Here, using the social defeat paradigm, we provide empirical evidence that highly social and monogamous mandarin voles (Microtus mandarinus) increased grooming toward a socially defeated partner but not toward a partner who underwent only separation. This selective behavioral response existed in both males and females. Accompanied with these behavioral changes, c-Fos expression was elevated in many of the brain regions relevant for emotional processing, such as the anterior cingulate cortex (ACC), bed nucleus of the stria terminalis, paraventricular nucleus (PVN), basal/basolateral and central nucleus of the amygdala, and lateral habenular nucleus in both sexes; in the medial preoptic area, the increase in c-Fos expression was found only in females, whereas in the medial nucleus of the amygdala, this increase was found only in males. In particular, the GAD67/c-Fos and oxytocin (OT)/c-Fos colocalization rates were elevated in the ACC and PVN, indicating selective activation of GABA and OT neurons in these regions. The "stressed" pairs matched their anxiety-like behaviors in the open-field test, and their plasma corticosterone levels correlated well with each other, suggesting an empathy-based mechanism. This partner-directed grooming was blocked by pretreatment with an OT receptor antagonist or a GABA_A receptor antagonist in the ACC but not by a V1a subtype vasopressin receptor antagonist. We conclude that consolation behavior can be elicited by the social defeat paradigm in mandarin voles, and this behavior may be involved in a coordinated network of emotion-related brain structures, which differs slightly between the sexes. We also found that the endogenous OT and the GABA systems within the ACC are essential for consolation behavior in mandarin voles.

Novel unconditioned prosocial behavior in prairie voles (Microtus ochrogaster) as a model for empathy

OBJECTIVE

In this study, empathy is quantified using a novel social test. Empathy and prosocial behavior are linked to the expression of oxytocin in humans and rodent models. Specifically, prosocial behavior in prairie voles (Microtus ochrogaster) has been linked to the expression of oxytocin in the paraventricular nucleus of the hypothalamus. The animal's behavior was considered empathic if it spends significantly more time attempting to remove a loos fitting restraint (tether) from the stimulus animal than time in contact with a, simultaneously presented, non-social object similar to the tether. The behavioral data was cross-referenced with the number of neurons expressing oxytocin and arginine vasopressin, as well as the density of dopaminergic neurons (identified by the expression of tyrosine hydroxylase), in the paraventricular nucleus of the hypothalamus. These proteins influence empathic behavior in humans, non-human primates, rats, mice, and prairie voles.

RESULTS

The consistency between neuroanatomical mechanisms linked to empathy, and the durations of time spent engaging in empathic contact, support the prediction that the empathic contact in this test is a distinct prosocial behavior, lacking prior behavioral training or the naturally occurring ethological relevance of other prosocial behaviors, and is a measure of empathy.

Phantom limb pain: A literature review

Since the phantom limb sensation was first described by the French military surgeon Ambroise Pare in the 16th century, the number of studies surrounding phantom limb pain has increased every year. Especially in recent decades, scientists have achieved a better understanding of the mechanism and treatment of phantom limb pain. Although many hypotheses have been agreed and many treatments have been proven effective, scientists still do not have a very systematic understanding of the phantom limbs. The purpose of this review article is to summarize recent researches focusing on phantom limb in order to discuss its definition, mechanisms, and treatments.

Validating Rat Model of Empathy for Pain: Effects of Pain Expressions in Social Partners

Pain can be socially transferred between familiar rats due to empathic responses. To validate rat model of empathy for pain, effects of pain expressions in a cagemate demonstrator (CD) in pain on empathic pain responses in a naïve cagemate observer (CO) after 30 min priming dyadic social interactions (PDSI) were evaluated. The CD rats were prepared with four pain models: bee venom (BV), formalin, complete Freund's adjuvant (CFA), and spared nerve injury (SNI). Both BV and formalin tests are characterized by displayable and eye-identifiable spontaneous pain-related behaviors (SPRB) immediately after treatment, while CFA and SNI models are characterized by delayed occurrence of evoked pain hypersensitivity but with less eye-identifiable SPRB. After 30 min PDSI with a CD immediately after BV and formalin, respectively, the empathic mechanical pain hypersensitivity (EMPH) could be identified at both hind paws in CO rats. The BV—or formalin-induced EMPH in CO rats lasted for 4–5 h until full recovery. However, EMPH failed to develop in CO after socially interacting with a CD immediately after CFA, or 2 h after BV when SPRB completely disappeared. The CO's EMPH was partially relieved when socially interacting with an analgecized CD whose SPRB had been significantly suppressed. Moreover, repeated exposures to a CD in pain could enhance EMPH in CO. Finally, social transfer of pain hypersensitivity was also identified in CO who was being co-housed in pairs with a conspecific treated with CFA or SNI. The results suggest that development of EMPH in CO rats would be determined not only by extent of familiarity but also by visually identifiable pain expressions in the social partners during short period of PDSI. However, the visually unidentifiable pain can also be transferred to naïve cagemate when being co-housed in pairs with a distressed conspecific. In summary, the vicariously social contagion of pain between familiar rats is dependent upon not only expressions of pain in social partners but also the time that dyads spent in social communications. The rat model of empathy for pain is a highly stable, reproducible and valid model for studying the neural mechanisms of empathy in lower animals.

A Context-Based Analgesia Model in Rats: Involvement of Prefrontal Cortex

Cognition and pain share common neural substrates and interact reciprocally: chronic pain compromises cognitive performance, whereas cognitive processes modulate pain perception. In the present study, we established a non-drug-dependent rat model of context-based analgesia, where two different contexts (dark and bright) were matched with a high (52°C) or low (48°C) temperature in the hot-plate test during training. Before and after training, we set the temperature to the high level in both contexts. Rats showed longer paw licking latencies in trials with the context originally matched to a low temperature than those to a high temperature, indicating successful establishment of a context-based analgesic effect in rats. This effect was blocked by intraperitoneal injection of naloxone (an opioid receptor antagonist) before the probe. The context-based analgesic effect also disappeared after optogenetic activation or inhibition of the bilateral infralimbic or prelimbic sub-region of the prefrontal cortex. In brief, we established a context-based, non-drug dependent, placebo-like analgesia model in the rat. This model provides a new and useful tool for investigating the cognitive modulation of pain.

What can rodents teach us about empathy?

While many consider empathy an exclusively human trait, non-human animals are capable of simple forms of empathy, such as emotional contagion, as well as consolation and helping behavior. Rodent models are particularly useful for describing the neuronal background of these phenomena. They offer the possibility of employing single-cell resolution mapping of the neuronal activity as well as novel techniques for manipulation of in vivo activity, which are currently unavailable in human studies. Here, we review recent developments in the field of rodent empathy research with special emphasis on behavioral paradigms and data on neuronal correlates of emotional contagion. We hope that the use of rodent models will enhance our understanding of social deficits in neuropsychiatric disorders characterized with empathy impairments and the evolutionary continuity of the empathic trait.

Social interaction with a cagemate in pain increases allogrooming and induces pain hypersensitivity in the observer rats

Empathy, which is a highly cognitive and emotional process, is the ability to share the emotional states of others. Empathy has also been observed in rodents. The empathic sharing of the distressful experience of a conspecific can even motivate altruistic behaviors, which are critical for survival. However, previous studies investigating empathy or prosocial behaviors in rodents mainly employed fearful or other stressful stimuli to elicit emotional changes; whether pain empathy can also motivate prosocial behaviors has yet to be investigated. By using the writhing test, the present study found that cagemate observer (CO) rats, compared with non-cagemate observer (NCO) rats, increased partner-directed grooming (allogrooming) toward conspecifics that had received an intraperitoneal injection of acetic acid during a dyadic social interaction. Following a dyadic social interaction with a demonstrator that received an intraperitoneal injection of acetic acid, the CO rats, compared with NCO rats, exhibited bilateral mechanical pain hypersensitivity and an enhanced acetic acid-induced writhing response. Our results here provided further evidence of pain empathy in rats, suggesting that empathy for pain may motivate prosocial behaviors in rats.