Social factors modulate restraint stress induced hyperthermia in mice

Effects of chronic social equality and inequality conditions on passive avoidance memory and PTSD-like behaviors in rats under chronic empathic stress

INTRODUCTION

Social inequality conditions induce aversion and affect brain functions and mood. This study investigated the effects of chronic social equality and inequality (CSE and CSI, respectively) conditions on passive avoidance memory and post-traumatic stress disorder (PTSD)-like behaviors in rats under chronic empathic stress.

METHODS

Rats were divided into different groups, including control, sham-observer, sham-demonstrator, observer, demonstrator, and co-demonstrator groups. Chronic stress (2 h/day) was administered to all stressed groups for 21 days. Fear learning, fear memory, memory consolidation, locomotor activity, and PTSD-like behaviors were evaluated using the passive avoidance test. Apart from the hippocampal weight, the correlations of memory and right hippocampal weight with serum corticosterone (CORT) levels were separately assessed for all experimental groups.

RESULTS

Latency was significantly higher in the demonstrator and sham-demonstrator groups compared to the control group. It was decreased significantly in other groups compared to the control group. Latency was also decreased in the observer and co-demonstrator groups compared to the demonstrator group. Moreover, the right hippocampal weight was significantly decreased in the demonstrator and sham-demonstrator groups compared to the control group. Pearson's correlation of memory and hippocampal weight with serum CORT levels supported the present findings.

CONCLUSION

Maladaptive fear responses occurred in demonstrators and sham-demonstrators. Also, extremely high levels of psychological stress, especially under CSI conditions (causing abnormal fear learning) led to heightened fear memory and PTSD-like behaviors. Right hippocampal atrophy confirmed the potential role of CSI conditions in promoting PTSD-like behaviors. Compared to inequality conditions, the abnormal fear memory was reduced under equality conditions.

Finding biomarkers of experience in animals

At a time when there is a growing public interest in animal welfare, it is critical to have objective means to assess the way that an animal experiences a situation. Objectivity is critical to ensure appropriate animal welfare outcomes. Existing behavioural, physiological, and neurobiological indicators that are used to assess animal welfare can verify the absence of extremely negative outcomes. But welfare is more than an absence of negative outcomes and an appropriate indicator should reflect the full spectrum of experience of an animal, from negative to positive. In this review, we draw from the knowledge of human biomedical science to propose a list of candidate biological markers (biomarkers) that should reflect the experiential state of non-human animals. The proposed biomarkers can be classified on their main function as endocrine, oxidative stress, non-coding molecular, and thermobiological markers. We also discuss practical challenges that must be addressed before any of these biomarkers can become useful to assess the experience of an animal in real-life.

Chronic social comparison elicits depression- and anxiety-like behaviors and alterations in brain-derived neurotrophic factor expression in male rats

Social comparison is a fundamental human characteristic; however, long-term social comparison may induce psychological stress and can lead to depression and anxiety. Recent studies have shown that nonhuman primates compare themselves with others; however, no studies have investigated whether social comparisons exist among rodents. In the present study, we established a rat model of social comparison. This model was subsequently used to examine the effects of the differential environment of a partner on depression- and anxiety-like behaviors in male rats, as well as to assess the changes in serum, medial prefrontal cortex (mPFC), and dorsal hippocampus brain-derived neurotrophic factor (BDNF) levels induced by long-term social comparison. Compared to rats whose partners were exposed to the same environment, rats whose partners were exposed to two combined enriched environmental stimuli for 14 days showed significantly decreased social novelty preference and sucrose consumption. No anxiety-like behaviors were observed. Rats whose partners were exposed to one enriched environment for 31 days showed significantly increased immobility time in the forced swimming test, and significantly decreased time spent in the center area in the open-field test. Further, rats whose partners were exposed to one enriched environment for 31 days showed lower BDNF levels in the mPFC and dorsal hippocampus, but not following partner exposure for 14 days. These results suggest that social comparisons exist in rats and can induce psychosocial stress and other negative affect. This model will not only provide the possibility to reveal the neurobiological basis of the emotional impact of social comparison, but could also be used to confirm the conservative evolutionary characteristics of social comparison as a behavioral attribute.

Body surface temperature of rats reveals both magnitude and sex differences in the acute stress response

Understanding how biological markers of stress relate to stressor magnitude is much needed and can be used in welfare assessment. Changes in body surface temperature can be measured using infrared thermography (IRT) as a marker of a physiological response to acute stress. While an avian study has shown that changes in body surface temperature can reflect the intensity of acute stress, little is known about surface temperature responses to stressors of different magnitudes and its sex-specificity in mammals, and how they correlate with hormonal and behavioural responses. We used IRT to collect continuous surface temperature measurements of tail and eye of adult male and female rats (Rattus norvegicus), for 30 minutes after exposure to one of three stressors (small cage, encircling handling or rodent restraint cone) for one minute, and cross-validated the thermal response with plasma corticosterone (CORT) and behavioural assessment. To obtain individual baseline temperatures and thermal responses to stress, rats were imaged in a test arena (to which they were habituated) for 30 seconds before and 30 minutes after being exposed to the stressor. In response to the three stressors, tail temperature initially decreased and then recovered to, or overshot the baseline temperature. Tail temperature dynamics differed between stressors; being restrained in the small cage was associated with the smallest drop in temperature, in male rats, and the fastest thermal recovery, in both sexes. Increases in eye temperature only distinguished between stressors early in the response and only in females. The post stressor increase in eye temperature was greater in the right eye of males and the left eye of females. In both sexes encircling may have been associated with the fastest increase in CORT. These results were in line with observed behavioural changes, with greater movement in rats exposed to the small cage and higher immobility after encircling. The female tail and eye temperature, as well as the CORT concentrations did not return to pre-stressor levels in the observation period, in conjunction with the greater occurrence of escape-related behaviours in female rats. These results suggest that female rats are more vulnerable to acute restraint stress compared to male rats and emphasise the importance of using both sexes in future investigations of stressor magnitude. This study demonstrates that acute stress induced changes in mammalian surface temperature measured with IRT relate to the magnitude of restraint stress, indicate sex differences and correlate with hormonal and behavioural responses. Thus, IRT has the potential to become a non-invasive method of continuous welfare assessment in unrestrained mammals.

Infrared thermography for non-invasive measurement of social inequality aversion in rodents and potential usefulness for future animal-friendly studies

Infrared thermography is a method that detects thermal radiation energy and can measure the body surface temperature of animals from a distance. While rectal temperature has traditionally been used to measure animals' core temperature, thermal imaging can avoid the stress and potential rise of body temperature deriving from handling of the animals. Additionally, being non-invasive and contactless, thermal imaging allows free movement of the animals. The validity of this technique as a psychophysiological method has been proven in a series of stress-induced hyperthermia (SIH) studies of mice under social inequality conditions. Restraint in a holder elicits SIH in mice. A restrained mouse surrounded by freely moving cage mates displays increased SIH suggesting that social inequality enhances the stress. Social inequality can be examined also in unrestrained mice, in particular through unequal distribution of food. In this protocol, a food-deprived mouse is given a small piece of cheese, while its cage mate is given a large piece of cheese. This inequity causes SIH, suggesting social inequality aversion in mice. Thus, social inequality in different situations similarly increased SIH. Importantly, in future studies infrared thermography could also be used to evaluate emotional arousal states different from stress (for example to assess reactivity to rewards or in social and sexual preference tests). Moreover, the technique could be used to investigate also cognitive arousal induced by novelty. Indeed, infrared thermography could be a particularly useful tool for animal-friendly studies of cognition and emotion in rodents.

Ability to share emotions of others as a foundation of social learning

The natural habitats of most species are far from static, forcing animals to adapt to continuously changing conditions. Perhaps the most efficient strategy addressing this challenge consists of obtaining and acting upon pertinent information from others through social learning. We discuss how animals transfer information via social channels and what are the benefits of such exchanges, playing out on different levels, from theperception of socially delivered information to emotional sharing, manifesting themselves across different taxa of increasing biological complexity. We also discuss how social learning is influenced by different factors including pertinence of information for survival, the complexity of the environment, sex, genetic relatedness, and most notably, the relationship between interacting partners. The results appear to form a consistent picture once we shift our focus from emotional contagion as a prerequisite for empathy onto the role of shared emotions in providing vital information about the environment. From this point of view, we can propose approaches that are the most promising for further investigation of complex social phenomena, including learning from others.

Current rodent models for the study of empathic processes

Empathy is a complex phenomenon critical for group survival and societal bonds. In addition, there is mounting evidence demonstrating empathic behaviors are dysregulated in a multitude of psychiatric disorders ranging from autism spectrum disorder, substance use disorders, and personality disorders. Therefore, understanding the underlying drive and neurobiology of empathy is paramount for improving the treatment outcomes and quality of life for individuals suffering from these psychiatric disorders. While there is a growing list of human studies, there is still much about empathy to understand, likely due to both its complexity and the inherent limitations of imaging modalities. It is therefore imperative to develop, validate, and utilize rodent models of empathic behaviors as translational tools to explore this complex topic in ways human research cannot. This review outlines some of the more prevailing theories of empathy, lists some of the psychiatric disorders with disrupted empathic processes, describes rat and mouse models of empathic behaviors currently used, and discusses ways in which these models have elucidated social, environmental, and neurobiological factors that may modulate empathy. The research tools afforded to rodent models will provide an increasingly clear translational understanding of empathic processes and consequently result in improvements in care for those diagnosed with any one of the many psychiatric disorders.

Hens with benefits: Can environmental enrichment make chickens more resilient to stress?

Resilience, the degree to which individuals are physiologically and behaviourally impacted by stressors, can be enhanced by positive experiences (e.g. positive moods in human, environmental enrichment in rodents). Such effects are important for human health, but could also have important animal welfare implications in terms of farm, laboratory and zoo animals' abilities to cope with stressors. Here we investigated whether enrichments can increase resilience in chickens, the world's most abundant agricultural animal. The stress reactivity of laying hens housed for 5-6 weeks in enriched environments was compared to that of controls housed in smaller, emptier, less preferred pens, via: 1) startle reflex amplitudes to an abrupt, intense sensory stimulus (a light flash); and 2) autonomic responses to restraint and the sudden appearance of a novel object, assessed from decreases in comb temperature. Startle amplitudes were consistently reduced in the enriched hens, exerted with around one sixth the force seen in control hens. Maximum comb temperature decreases, and latencies for comb temperatures to return to prestress levels, also both fell by around a third. Enrichment thus reduced hens' intrinsic behavioural and physiological responses to standardized stressors (doing so even outside the home pen), just as occurs in laboratory rodents. Enrichment also reduced baseline comb temperature, suggesting that this could be a non-invasive indicator of welfare. Altered judgment biases did not seem to be the mechanism. Further work should now investigate the processes underlying the apparently enhanced stress resilience of animals housed in preferred conditions, and also investigate baseline comb temperature as a chronic stress indicator in poultry.

Blockade of 5-HT2A receptors inhibits emotional hyperthermia in mice

This study determined whether blockade of 5-hydroxytryptamine 2A (5-HT2A) receptors attenuated hyperthermia and tachycardia responses to psychological stress in mice. For this purpose, male mice (C57BL/6N) were pre-instrumented with a telemetric probe to measure core body temperature and heart rate prior to experimentation. Vehicle or 5-HT2A antagonist, eplivanserin hemifumarate (SR-46349B) ((1Z,2E)-1-(2-fluorophenyl)-3-(4-hydroxyphenyl)-2-propen-1-one O-[2-(dimethylamino) ethyl] oxime hemifumarate) (0.5, 1.0, 5.0 mg/kg), was injected intraperitoneally. To elicit psychological stress, an intruder male mouse confined to a small cage was introduced into the resident mouse's cage 30 min after administration of the injection. The application of this psychological stress increased body temperature by ~ 1.0 °C and heart rate by ~ 150 bpm in the vehicle group. In contrast, SR-46349B was shown to reduce this psychological stress-induced increase in body temperature in a dose-dependent manner (P < 0.05). However, the SR-46349B treatment groups had no influence on the intruder-elicited increase in heart rate. This study, therefore, suggests that 5-HT2A receptors play a significant role in mediating hyperthermia, but not tachycardia, during intruder-elicited psychological stress.

The link between social context-dependent anxious behavior and habenular mast cells in fear-conditioned rats

Affiliative social behavior relieves the physiological reactivity to stressors, while social inequity, represented by unfairness in the social environment, causes emotional distress in animals. Mast cells are immune cells found in the brain that affect both the nervous system and emotional behavior. To determine the role of neuro-immunity in the programming of emotional behaviors, we observed brain mast cells and anxiety-like behaviors in female rats exposed to electrical foot shocks in different social environments. The following groups of rats were used in this study: control (unshocked) rats, solitarily shock-exposed rats, and shock-exposed rats in the presence of unshocked (unequal) or shocked (equal) conspecifics. An absence of significant difference in body weight or sucrose preference was seen among the different groups. Additionally, fear memory was augmented in rats shocked in the presence of either unshocked or shocked conspecifics than rats in the solitarily shocked group. Furthermore, rats shocked in the presence of unshocked conspecifics showed intensified anxiety-like behaviors after fear conditioning. Finally, we found an increase in the number of habenular mast cells in the intensified anxiogenic group, which had a significant correlation with the decreasing rate of anxiety-like behaviors. This provides evidence that habenular mast cells might be of importance in relieving the amplified biopsychological responses caused by social stress.

The Unpredictable Chronic Mild Stress Protocol for Inducing Anhedonia in Mice

Depression is a highly prevalent and debilitating condition, only partially addressed by current pharmacotherapies. The lack of response to treatment by many patients prompts the need to develop new therapeutic alternatives and to better understand the etiology of the disorder. Pre-clinical models with translational merits are rudimentary for this task. Here we present a protocol for the unpredictable chronic mild stress (UCMS) method in mice. In this protocol, adolescent mice are chronically exposed to interchanging unpredictable mild stressors. Resembling the pathogenesis of depression in humans, stress exposure during the sensitive period of mice adolescence instigates a depressive-like phenotype evident in adulthood. UCMS can be used for screenings of antidepressants on the variety of depressive-like behaviors and neuromolecular indices. Among the more prominent tests to assess depressive-like behavior in rodents is the sucrose preference test (SPT), which reflects anhedonia (core symptom of depression). The SPT will also be presented in this protocol. The ability of UCMS to induce anhedonia, instigate long-term behavioral deficits and enable reversal of these deficits via chronic (but not acute) treatment with antidepressants strengthens the protocol's validity compared to other animal protocols for inducing depressive-like behaviors.

A possible social relative reward effect: Influences of outcome inequity between rats during operant responding

Social interactions/situations have dramatic influences on motivation. Creating animal models examining these influences promotes a better understanding of the psychological and biological underpinnings of social motivation. Rodents are sensitive to social history/experience during associative conditioning and food-sharing tasks. Would reward-oriented operant behavior be sensitive to social influences by showing a negative contrast-like effect when another organism obtains a greater value outcome? We used a side-by-side arrangement of operant response chambers wherein one animal obtained consistently high reward signaled by a discrete cue. The neighboring, experimental rat experienced different combinations of high and low reward trial sequences. Control conditions included distraction from a conspecific in the neighboring chamber (rat distractor) or cue/food dispenser operating without a conspecific (program distractor) in addition to testing subjects alone. Results support an influence of the other animal actively performing the task on the experimental subject's behavior. Primarily, responding was significantly slower for the low reward trials while the neighboring rat was receiving the higher magnitude reward. The lever-press and not food-cup retrieval latency was significantly slower during exposure to a conspecific neighbor performing the operant task. The effect was not obtained in all session sequences and was more pronounced using longer series of consecutive low reward trials. The slowing effect was also obtained with the program-distractor experience in a different trial sequence. These findings suggest a social-induced negative incentive contrast effect in rats possibly mediated by an outcome inequity process that could have key similarities to complex situational-affective effects on motivation involving frustration or jealously.

Stress-induced hyperthermia and hypothermia

Stress affects core body temperature (T_c). Many kinds of stress induce transient, monophasic hyperthermia, which diminishes gradually if the stressor is terminated. Stronger stressors produce a longer-lasting effect. Repeated/chronic stress induces anticipatory hyperthermia, reduces diurnal changes in T_c, or slightly increases T_c throughout the day. Animals that are exposed to chronic stress or a cold environment exhibit an enhanced hyperthermic response to a novel stress. These changes persist for several days after cessation of stress exposure. In contrast, long-lasting inescapable stress sometimes induces hypothermia. In healthy humans, psychologic stress induces slight increases in T_c, which are within the normal range of T_c or just above it. Some individuals, however, develop extremely high T_c (up to 41°C) when they are exposed to emotional events or show persistent low-grade high T_c (37-38°C) during or after chronic stress situations. In addition to the nature of the stressor itself, such stress-induced thermal responses are modulated by sex, age, ambient temperature, cage mates, past stressful experiences and cold exposure, and coping. Stress-induced hyperthermia is driven by mechanisms distinct from infectious fever, which requires inflammatory mediators. However, both stress and infection activate the dorsomedial hypothalamus-rostral medullary raphe region-sympathetic nerve axis to increase T_c.

Paintings discrimination by mice: Different strategies for different paintings

C57BL/6 mice were trained on simultaneous discrimination of paintings with multiple exemplars, using an operant chamber with a touch screen. The number of exemplars was successively increased up to six. Those mice trained in Kandinsky/Mondrian discrimination showed improved learning and generalization, whereas those trained in Picasso/Renoir discrimination showed no improvements in learning or generalization. These results suggest category-like discrimination in the Kandinsky/Mondrian task, but item-to-item discrimination in the Picasso/Renoir task. Mice maintained their discriminative behavior in a pixelization test with various paintings; however, mice in the Picasso/Renoir task showed poor performance in a test that employed scrambling processing. These results do not indicate that discrimination strategy for any Kandinsky/Mondrian combinations differed from that for any Picasso/Monet combinations but suggest the mice employed different strategies of discrimination tasks depending upon stimuli.

The roots of empathy: Through the lens of rodent models

Empathy is a phenomenon often considered dependent on higher-order emotional control and an ability to relate to the emotional state of others. It is, by many, attributed only to species having well-developed cortical circuits capable of performing such complex tasks. However, over the years, a wealth of data has been accumulated showing that rodents are capable not only of sharing emotional states of their conspecifics, but also of prosocial behavior driven by such shared experiences. The study of rodent empathic behaviors is only now becoming an independent research field. Relevant animal models allow precise manipulation of neural networks, thereby offering insight into the foundations of empathy in the mammalian brains. Here we review the data on empathic behaviors in rat and mouse models, their neurobiological and neurophysiological correlates, and the factors influencing these behaviors. We discuss how simple rodent models of empathy enhance our understanding of how brain controls empathic behaviors.

Application of the Metal-Organic Framework [Eu(BTC)] as a Luminescent Marker for Gunshot Residues: A Synthesis, Characterization, and Toxicity Study

The 3D metal-organic framework (MOF) [Eu(BTC)] (where BTC = trimesic acid) was synthesized in 20 min by a microwave-assisted hydrothermal method with a yield of 89%. A structural and spectroscopic study, performed by X-ray diffraction, thermogravimetry, and photoluminescence spectroscopy, showed that this framework has high crystallinity, thermal stability, and luminescence. This MOF had a red-orange luminescence when excited with ultraviolet (UV) radiation (λ = 254 nm) and a high potential for use as a luminescent marker for gunshot residues (GSR). When added to 9 mm nontoxic ammunition, it greatly improved quality of the crime scene investigation, allowing for direct visualization of the luminescent GSR on the shooter's hand and firearm and at the firing range using only a portable UV lamp. The marked luminescent GSR was easily collected and characterized by nondestructive techniques, including with a Video Spectral Comparator and scanning electron microscopy/energy-dispersive spectroscopy, wherein the presence of Eu³⁺ ions was confirmed. Furthermore, the oral acute toxicity of this MOF was assessed in adult female Wistar rats using the Organisation for Economic Cooperation and Development 423 guidelines. This study classified the MOF [Eu(BTC)] in a less toxic Globally Harmonized System category (category 5), with a LD₅₀ (lethal dose) of 5000 mg/kg, ensuring a wide security range for its application.