Individual housing of mice--impact on behaviour and stress responses

The weekend warrior effect: Consistent intermittent exercise induces persistent cognitive benefits

Exercise provides a range of cognitive benefits, including improved memory performance. Previously, we demonstrated that 14 days of continuous voluntary wheel-running exercise enables learning in a hippocampus-dependent Object Location Memory (OLM) task under insufficient, subthreshold training conditions in adult mice. Whether similar exercise benefits can be obtained from consistent intermittent exercise as continuous exercise is unknown. Here, we examine whether intermittent exercise (the weekend warrior effect: 2 days of exercise a week for 7 weeks) displays similar or distinct cognitive benefits as previously examined with 14 days of continuous exercise. We find that both continuous and intermittent exercise parameters similarly enable hippocampus-dependent OLM compared to the 2-day exercise control group. Mice receiving intermittent exercise maintained cognitive benefits following a 7-day sedentary delay, whereas mice that underwent 14 continuous days of exercise showed diminished cognitive benefits as previously reported. Further, compared to continuous exercise, intermittent exercise mice exhibited persistently elevated levels of the genes Acvr1c and Bdnf which we know to be critically involved in hippocampus-dependent long-term memory in the dorsal hippocampus. Together findings suggest that consistent intermittent exercise persistently enables hippocampal-dependent long-term memory. Understanding the optimal parameters for persistent cognitive function and the mechanisms mediating persistent effects will aid in therapeutic pursuits investigating the mitigation of cognitive ailments.

Sumatriptan attenuates fear-learning despair induced by social isolation stress in mice: Mediating role of hypothalamic-pituitary-adrenal axis

OBJECTIVES

Research has demonstrated that chronic stress experienced early in life can lead to impairments in memory and learning. These deficits are attributed to an imbalance in the interaction between glucocorticoids, the end product of the hypothalamic-pituitary-adrenal (HPA) axis, and glucocorticoid receptors in brain regions responsible for mediating memory, such as the hippocampus. This imbalance can result in detrimental conditions like neuroinflammation. The aim of this study was to assess the impact of sumatriptan, a selective agonist for 5-HT 1B/1D receptors, on fear learning capabilities in a chronic social isolation stress model in mice, with a particular focus on the role of the HPA axis.

METHODS

Mice were assigned to two opposing conditions, including social condition (SC) and isolated condition (IC) for a duration of five weeks. All mice underwent passive avoidance test, with their subsequent freezing behavior serving as an indicator of fear retrieval. Mice in the IC group were administered either a vehicle, sumatriptan, GR-127935 (a selective antagonist for 5-HT 1B/1D receptors), or a combination of sumatriptan and GR-127935 during the testing sessions. At the end, all mice were sacrificed and samples of their serum and hippocampus were collected for further analysis.

RESULTS

Isolation was found to significantly reduce freezing behavior (p<0.001). An increase in the freezing response among IC mice was observed following the administration of varying doses of sumatriptan, as indicated by a one-way ANOVA analysis (p<0.001). However, the mitigating effects of sumatriptan were reversed upon the administration of GR-127935. An ELISA assay conducted before and after the passive avoidance test revealed no significant change in serum corticosterone levels among SC mice. In contrast, a significant increase was observed among IC mice, suggesting hyper-responsiveness of the HPA axis in isolated animals. This hyper-responsiveness was ameliorated following the administration of sumatriptan. Furthermore, both the sumatriptan and SC groups exhibited a similar trend, showing a significant increase in the expression of hippocampal glucocorticoid receptors following the stress of the passive avoidance test. Lastly, the elevated production of inflammatory cytokines (TNF-α, IL-1β) observed following social isolation was attenuated in the sumatriptan group.

CONCLUSION

Sumatriptan improved fear learning probably through modulation of HPA axis and hippocampus neuroinflammation.

Improved Memory and Lower Stress Levels in Male Mice Co-Housed with Ovariectomized Female Mice

Aggressiveness, expressed by fighting, is a frequent problem in group-housed laboratory male mice and results in increased stress, injury, and death. One way to prevent fighting is by pairing the male mice with ovariectomized female mice to provide a compatible companion. However, the effect of these housing conditions remains unclear. Therefore, we aimed to evaluate behavior and stress levels in two different housing conditions, pair-housed with an ovariectomized female and group-housed with other males. Behavioral tests were performed to assess stress and anxiety-like behavior. Moreover, the corticosterone levels in plasma were measured by ELISA. Based on home cage behavior assessment, pair-housed male mice showed no signs of fighting, not even after isolation and regrouping. Our results also showed that the pair-housed males had a better memory and demonstrated less anxiety-like behavior. Subsequently, the pair-housed male mice had a larger reduction in corticosterone levels compared to group-housed males. Overall, pair-housing reduced anxiety-like behavior and stress levels in male mice compared to standard group-housing.

Effects of social housing conditions on ethanol-induced behavioral sensitization in Swiss mice

RATIONALE

In previous animal model studies, it was shown that drug sensitization is dependent upon physical environmental conditions. However, the effects of social housing conditions on drug sensitization is much less known.

OBJECTIVE

The aim of the present study was to investigate the effects of social conditions, through the size of housing groups, on ethanol stimulant effects and ethanol-induced behavioral sensitization in mice.

MATERIALS AND METHODS

Male and female Swiss mice were housed in groups of different sizes (isolated mice, two mice per cage, four mice per cage and eight mice per cage) during a six-week period. A standard paradigm of ethanol-induced locomotor sensitization was then started with one daily injection of 2.5 g/kg ethanol for 8 consecutive days.

RESULTS

The results show that social housing conditions affect the acute stimulant effects of ethanol. The highest stimulant effects were observed in socially isolated mice and then gradually decreased as the size of the group increased. Although the rate of ethanol sensitization did not differ between groups, the ultimate sensitized levels of ethanol-induced stimulant effects were significantly reduced in mice housed in groups of eight.

CONCLUSIONS

These results are consistent with the idea that higher levels of acute and sensitized ethanol stimulant effects are observed in mice housed in stressful housing conditions, such as social isolation.

Head-mounted central venous access during optical recordings and manipulations of neural activity in mice

Establishing reliable intravenous catheterization in mice with optical implants allows the combination of neural manipulations and recordings with rapid, time-locked delivery of pharmacological agents. Here we present a procedure for handmade jugular vein catheters designed for head-mounted intravenous access and provide surgical and postoperative guidance for improved survival and patency. A head-mounted vascular access point eliminates the need for a back-mounted button in animals already receiving neural implants, thereby reducing sites of implantation. This protocol, which is readily adoptable by experimenters with previous training and experience in mouse surgery, enables repeated fiber photometry recordings or optogenetic manipulation during drug delivery in adult mice that are awake and behaving, whether head fixed or freely moving. With practice, an experienced surgeon requires ~30 min to perform catheterization on each mouse. Altogether, these techniques facilitate the reliable and repeated delivery of pharmacological agents in mouse models while simultaneously recording at high temporal resolution and/or manipulating neural populations.

Partially divided caging reduces overall aggression and anxiety which may indicate improved welfare in group housed male C57BL/6J mice

Deciding which environmental enrichment is used in mouse caging is often subjective, with cost frequently prevailing over welfare benefits, including aggression and anxiety. While many devices introduced to encourage natural behaviors and reduce aggression show mixed results, we have previously demonstrated significant reductions in aggressive behavior between group-housed male mice housed in partially divided caging. To further assess behavior, we have raised male C57BL/6J mice in either partially divided caging or in standard caging with no divider. Animal behavior was tested on rotarod, open field, novel object recognition, elevated plus maze, and Y maze. Body weights were taken weekly beginning at weaning and bite wounds were counted weekly beginning at 133 days old. Aggressive behavior was recorded weekly beginning at 133 days old. Results indicated significantly less anxiety in the elevated-plus maze, statistically fewer bite wounds, and a statistically significant decrease in aggressive behaviors of mice in partially divided caging compared to mice in standard cages. We conclude that reductions in anxiety, aggressive behavior, and bite wounds may indicate improved overall welfare for non-sibling, group housed male mice.

Impact of Long-Term Dietary High Fat and Eicosapentaenoic Acid on Behavior and Hypothalamic-Pituitary-Adrenal Axis Activity in Amyloidogenic APPswe/PSEN1dE9 Mice

INTRODUCTION

Alzheimer's disease (AD) alters neurocognitive and emotional function and causes dysregulation of multiple homeostatic processes. The leading AD framework pins amyloid beta plaques and tau tangles as primary drivers of dysfunction. However, many additional variables, including diet, stress, sex, age, and pain tolerance, interact in ways that are not fully understood to impact the onset and progression of AD pathophysiology. We asked: (1) does high-fat diet, compared to low-fat diet, exacerbate AD pathophysiology and behavioral decline? And, (2) can supplementation with eicosapentaenoic (EPA)-enriched fish oil prevent high-fat-diet-induced changes?

METHODS

Male and female APPswePSdE9 mice, and their non-transgenic littermates, were randomly assigned to a diet condition (low-fat, high-fat, high-fat with EPA) and followed from 2 to 10 months of age. We assessed baseline corticosterone concentration during aging, pain tolerance, cognitive function, stress coping, and corticosterone response to a stressor.

RESULTS

Transgenic mice were consistently more active than non-transgenic mice but did not perform worse on either cognitive task, even though we recently reported that these same transgenic mice exhibited metabolic changes and had increased amyloid beta. Mice fed high-fat diet had higher baseline and post-stressor corticosterone, but diet did not impact cognition or pain tolerance. Sex had the biggest influence, as female mice were consistently more active and had higher corticosterone than males.

CONCLUSION

Overall, diet, genotype, and sex did not have consistent impacts on outcomes. We found little support for predicted interactions and correlations, suggesting diet impacts metabolic function and amyloid beta levels, but these outcomes do not translate to changes in behaviors measured here.

Using mice from different breeding sites fails to improve replicability of results from single-laboratory studies

Theoretical and empirical evidence indicates that low external validity due to rigorous standardization of study populations is a cause of poor replicability in animal research. Here we report a multi-laboratory study aimed at investigating whether heterogenization of study populations by using animals from different breeding sites increases the replicability of results from single-laboratory studies. We used male C57BL/6J mice from six different breeding sites to test a standardized against a heterogenized (HET) study design in six independent replicate test laboratories. For the standardized design, each laboratory ordered mice from a single breeding site (each laboratory from a different one), while for the HET design, each laboratory ordered proportionate numbers of mice from the five remaining breeding sites. To test our hypothesis, we assessed 14 outcome variables, including body weight, behavioral measures obtained from a single session on an elevated plus maze, and clinical blood parameters. Both breeding site and test laboratory affected variation in outcome variables, but the effect of test laboratory was more pronounced for most outcome variables. Moreover, heterogenization of study populations by breeding site (HET) did not reduce variation in outcome variables between test laboratories, which was most likely due to the fact that breeding site had only little effect on variation in outcome variables, thereby limiting the scope for HET to reduce between-lab variation. We conclude that heterogenization of study populations by breeding site has limited capacity for improving the replicability of results from single-laboratory animal studies.

Attraction of female house mice to male ultrasonic courtship vocalizations depends on their social experience and estrous stage

Male house mice (Mus musculus) produce complex ultrasonic vocalizations (USVs), especially during courtship and mating. Playback experiments suggest that female attraction towards recordings of male USVs depends on their social experience, paternal exposure, and estrous stage. We conducted a playback experiment with wild-derived female house mice (M. musculus musculus) and compared their attraction to male USVs versus the same recording without USVs (background noise). We tested whether female attraction to USVs is influenced by the following factors: (1) social housing (two versus one female per cage); (2) neonatal paternal exposure (rearing females with versus without father); and (3) estrous stage. We found that females showed a significant attraction to male USVs but only when they were housed socially with another female. Individually housed females showed the opposite response. We found no evidence that pre-weaning exposure to a father influenced females' preferences, whereas estrous stage influenced females' attraction to male USVs: females not in estrus showed preferences towards male USVs, whereas estrous females did not. Finally, we found that individually housed females were more likely to be in sexually receptive estrous stages than those housed socially, and that attraction to male USVs was most pronounced amongst non-receptive females that were socially housed. Our findings indicate that the attraction of female mice to male USVs depends upon their social experience and estrous stage, though not paternal exposure. They contribute to the growing number of studies showing that social housing and estrous stage can influence the behavior of house mice and we show how such unreported variables can contribute to the replication crisis.

Bouts of rest and physical activity in C57BL/6J mice

The objective was to exploit the raw data output from a scalable home cage (type IIL IVC) monitoring (HCM) system (DVC®), to characterize pattern of undisrupted rest and physical activity (PA) of C57BL/6J mice. The system's tracking algorithm show that mice in isolation spend 67% of the time in bouts of long rest (≥40s). Sixteen percent is physical activity (PA), split between local movements (6%) and locomotion (10%). Decomposition revealed that a day contains ˜7100 discrete bouts of short and long rest, local and locomotor movements. Mice travel ˜330m per day, mainly during the dark hours, while travelling speed is similar through the light-dark cycle. Locomotor bouts are usually <0.2m and <1% are >1m. Tracking revealed also fits of abnormal behaviour. The starting positions of the bouts showed no preference for the rear over the front of the cage floor, while there was a strong bias for the peripheral (75%) over the central floor area. The composition of bouts has a characteristic circadian pattern, however, intrusive husbandry routines increased bout fragmentation by ˜40%. Extracting electrode activations density (EAD) from the raw data yielded results close to those obtained with the tracking algorithm, with 81% of time in rest (<1 EAD s-1) and 19% in PA. Periods ≥40 s of file when no movement occurs and there is no EAD may correspond to periods of sleep (˜59% of file time). We confirm that EAD correlates closely with movement distance (rs>0.95) and the data agreed in ˜97% of the file time. Thus, albeit EAD being less informative it may serve as a proxy for PA and rest, enabling monitoring group housed mice. The data show that increasing density from one female to two males, and further to three male or female mice had the same effect size on EAD (˜2). In contrast, the EAD deviated significantly from this stepwise increase with 4 mice per cage, suggesting a crowdedness stress inducing sex specific adaptations. We conclude that informative metrics on rest and PA can be automatically extracted from the raw data flow in near-real time (< 1 hrs). As discussed, these metrics relay useful longitudinal information to those that use or care for the animals.

EFFECTS OF LITTER SIZE AND CAGING ON PHYSICAL AND MENTAL DEVELOPMENT IN RATS

As a multidisciplinary field, laboratory animal science promotes or accelerates the emergence of innovative ideas and products. As research has increased, so has the demand for laboratory animals with reliable, standardized characteristics. Thus, the breeding, reproduction, and welfare of laboratory animals are now animals reliable and more. The aim of this study to investigate whether different litter sizes of mothers and different husbandry methods affect the physical and mental development of pups. 30 adults female Wistar Hanover albino rats weighing 200-250 g were used for the study. The weight of the pups was measured once a week from birth until the end of the study, and their physical development was observed. After the pups were weaned, they were randomly divided into cages by sex. The 45 male and 45 female pups were housed in groups of three, five, and seven per cage. When the pups were 12 weeks old, open field test, elevated plus-maze test and Morris water maze behavioral tests were performed every other day, and then plasma corticosterone levels were measured. When the male and female pups in the groups were 14 weeks old, six females were taken from each housing group and mated, and the conception rates and maternal behavior of the pups were observed. During lactation, physical developmental parameters and the body weight of the rats were affected by litter size. Among the post-weaning housing groups, cage density was found to affect weight gain and body weight between groups. It was found that only the sex factor caused significant differences in the behavior of the animals. Females housed with seven rats per cage had higher corticosteroid levels than other females. As a result, it was observed that cages with seven female rats were more physically and psychologically affected than those with three and five rats.

Social isolation exacerbates diet-induced obesity and peripheral inflammation in young male mice under thermoneutrality

Social isolation (SI) is associated with an increased risk of mortality and various chronic diseases-including obesity-in humans. Murine studies probing SI metabolic outcomes remain inconsistent, due in part to a lack of consideration for housing temperature. Such experiments typically occur at room temperature, subjecting mice to chronic cold stress. Single housing prevents social thermoregulation, further exacerbating cold stress and obscuring psychosocial influences on metabolism at room temperature. In this study, C57BL/6 and BALB/c male mice were group- and single-housed under thermoneutral conditions to determine whether SI affects the development of high-fat diet-induced obesity. We report SI promotes weight gain, increases food intake, increases adiposity, worsens glycemic control, reduces insulin signaling, exacerbates systemic and adipose inflammatory responses, and induces a molecular signature within the hypothalamus. This study establishes a murine model that recapitulates the SI-induced propensity for obesity, which may further our understanding of SI's influence on health and disease.

Individualized Housing Modifies the Immune–Endocrine System in CD1 Adult Male Mice

In the last years, different research groups have made considerable efforts to improve the care and use of animals in research. Mice (Mus musculus) are the most widely used animal species in research in the European Union and are sociable and hierarchical creatures. During experiments, researchers tend to individualize males, but no consideration is given to whether this social isolation causes them stress. The aim of this study was, therefore, to explore whether 4 weeks of social isolation could induce changes in different physiological parameters in adult Crl:CD1(ICR) (CD1) males, which may interfere with experimental results. Body weight, blood cells, and fecal corticosterone metabolites levels were the analyzed parameters. Blood and fecal samples were collected at weeks 1 and 4 of the experimental procedure. Four weeks of single housing produced a significant time-dependent decrease in monocytes and granulocytes. Fecal corticosterone metabolite levels were higher in single-housed mice after 1 week and then normalized after 4 weeks of isolation. Body weight, red blood cells, and platelets remained unchanged in both groups during this period. We can, therefore, conclude that social isolation affects some immune and endocrine parameters, and that this should be taken into account in the interpretation of research data.

Induction of Type 2 Diabetes in Mice to Understand Vascular Changes That Drive Diabetic Retinopathy

Diabetic retinopathy is a common complication of type 2 diabetes. Research into this comorbidity is challenging due to the slow progression of pathological changes and the limited transgenic models available to study disease progression and mechanistic changes. Here, we describe a non-transgenic mouse model of accelerated type 2 diabetes using a high-fat diet in combination with streptozotocin delivered via osmotic mini pump. This model, when subjected to fluorescent gelatin vascular casting, can be used to study vascular changes in type 2 diabetic retinopathy.

Shift work-like patterns effect on female and male mouse behavior

Shift work (work outside of standard daylight hours) is common throughout the Western world. However, there are notable health consequences to shift work, including increased prevalence of mental health and sleep disorders in shift worker populations. Therefore, the health and wellbeing of shift workers is a public health concern that needs to be addressed. Here we investigate the effects of two separate light induced shift work-like patterns on male and female mouse behaviour (anxiety-like, exploration, marble burying, startle reflex and circadian rhythms). After 6 weeks of shift-like disruptions patterns, animals displayed no behavioral differences in exploration, marble burying and startle reflex. Interestingly however, we identified sex specific and disruption specific effects in light aversion and wheel running activities. Notably, analysis of the activity patterns of animals in disruptive conditions demonstrated that they maintained a degree of rhythmicity through the disruption period, which may explain the lack of behavioral differences in most behavioral tests.

Hormonal contraceptives, stress, and the brain: The critical need for animal models

Hormonal contraceptives are among the most important health and economic developments in the 20thCentury, providing unprecedented reproductive control and a range of health benefits including decreased premenstrual symptoms and protections against various cancers. Hormonal contraceptives modulate neural function and stress responsivity. These changes are usually innocuous or even beneficial, including their effects onmood. However, in approximately 4-10% of users, or up to 30 million people at any given time, hormonal contraceptives trigger depression or anxiety symptoms. How hormonal contraceptives contribute to these responses and who is at risk for adverse outcomes remain unknown. In this paper, we discussstudies of hormonal contraceptive use in humans and describe the ways in which laboratory animal models of contraceptive hormone exposure will be an essential tool for expanding findings to understand the precise mechanisms by which hormonal contraceptives influence the brain, stress responses, and depression risk.

The limitations of investigating appetite through circuit manipulations: are we biting off more than we can chew?

Disordered eating can underpin a number of debilitating and prevalent chronic diseases, such as obesity. Broader advances in psychopharmacology and biology have motivated some neuroscientists to address diet-induced obesity through reductionist, pre-clinical eating investigations on the rodent brain. Specifically, chemogenetic and optogenetic methods developed in the 21st century allow neuroscientists to perform in vivo, region-specific/projection-specific/promoter-specific circuit manipulations and immediately assess the impact of these manipulations on rodent feeding. These studies are able to rigorously conclude whether a specific neuronal population regulates feeding behaviour in the hope of eventually developing a mechanistic neuroanatomical map of appetite regulation. However, an artificially stimulated/inhibited rodent neuronal population that changes feeding behaviour does not necessarily represent a pharmacological target for treating eating disorders in humans. Chemogenetic/optogenetic findings must therefore be triangulated with the array of theories that contribute to our understanding of appetite. The objective of this review is to provide a wide-ranging discussion of the limitations of chemogenetic/optogenetic circuit manipulation experiments in rodents that are used to investigate appetite. Stepping into and outside of medical science epistemologies, this paper draws on philosophy of science, nutrition, addiction biology and neurophilosophy to prompt more integrative, transdisciplinary interpretations of chemogenetic/optogenetic appetite data. Through discussing the various technical and epistemological limitations of these data, we provide both an overview of chemogenetics and optogenetics accessible to non-neuroscientist obesity researchers, as well as a resource for neuroscientists to expand the number of lenses through which they interpret their circuit manipulation findings.

Physiologic and Behavioral Effects in Mice Anesthetized with Isoflurane in a Red-tinted or a Traditional Translucent Chamber

Isoflurane has been characterized as a distressing agent for rodents, causing both physiologic and behavioral effects. Using a "darkened home cage" has been recommended during CO₂ administration for rodent euthanasia; this is arguably a similar animal experience to anesthetic induction with isoflurane. Based on the premise that rodents perceive red light as darkness via the primary optic tract, we compared physiologic and behavioral markers of stress in 2 inbred strains of mice (C57BL/6J and BALB/cJ) anesthetized with isoflurane in either a red-tinted (dark) induction chamber or a traditional translucent induction chamber. Physiologic stress was assessed based on plasma levels of norepinephrine, epinephrine, and corticosterone. Stress-related behaviors (rearing, face wiping, and jumping) were recorded on video and scored from initiation of induction to loss of consciousness. No significant correlations were found between chamber type and physiologic stress hormones. As compared with the translucent chamber, stress-related behaviors were more frequent in the red-tinted chamber, including: 1) significantly higher rearing frequencies in BALB/cJ mice; 2) higher behavioral stress scores in BALB/cJ and male C57BL/6J mice; and 3) more face wiping behavior when considering all mice combined. These findings suggest that mice do not experience significant alleviation of physiologic indices of stress when anesthetized in a red-tinted induction chamber. Furthermore, isoflurane induction in the red-tinted chamber appeared to increase the expression of stress-related behaviors, particularly in BALB/cJ mice. Based on our findings and a growing body of literature on the unintended effects of red light, we do not recommend using red-tinted chambers for induction of anesthesia in mice.

Effects of separated pair housing of female C57BL/6JRj mice on well-being

In laboratory animal facilities, it is a common code of practice to house female mice in groups. However, some experimental conditions require to house them individually, even though social isolation may impair their well-being. Therefore, we introduced a separated pair housing system and investigated whether it can refine single housing of adult female C57BL/6JRj mice. Individually ventilated cages (IVC) were divided by perforated transparent walls to separate two mice within a cage. The cage divider allowed visual, acoustic, and olfactory contact between the mice but prevented interindividual body-contact or food sharing. Short- and long-term effects of the separated pair housing system on the well-being of the mice were compared with single and group housing using a range of behavioral and physiological parameters: Nest building behavior was assessed based on the complexity of nests, the burrowing performance was measured by the amount of food pellets removed from a bottle, and trait anxiety-related behavior was tested in the free exploratory paradigm. For the evaluation of the ease of handling, interaction with the experimenter's hand was monitored. Social interaction with unknown conspecifics and locomotor activity were investigated in a test arena. Moreover, body weight and stress hormone (metabolites) were measured in feces and hair. After the mice spent a day under the respective housing conditions, concentrations of fecal corticosterone metabolites were higher in separated pair-housed mice, and they built nests of a higher complexity when compared to single-housed mice. The latter effect was still observable eight weeks later. In week 8, separated pair-housed mice showed less locomotor activity in the social interaction arena compared to mice from the other housing systems, i.e., single and group housing. Regardless of the time of testing, pair housing improved the burrowing performance. Separated pair-housed mice were more difficult to catch than group-housed mice. Hair corticosterone, progesterone, and dehydroepiandrosterone concentrations changed with increasing age independently of the housing system. There were no effects of the housing systems on trait anxiety-related behavior in the free exploratory paradigm, voluntary interaction with the experimenter’s hand, and body weight. Overall, the transfer to the separated pair housing system caused short-term stress responses in female C57BL/6JRj mice. Long-term effects of separated pair housing were ambiguous. On one hand, separated pair housing increased nesting and burrowing behavior and may therefore be beneficial compared to single housing. But on the other hand, locomotor activity decreased. The study underlined that the effects of the housing conditions on physiological and behavioral parameters should be considered when analyzing and reporting animal experiments.

A Framework for Developing Translationally Relevant Animal Models of Stress-Induced Changes in Eating Behavior

Stress often affects eating behaviors, leading to increased eating in some individuals and decreased eating in others. Identifying physiological and psychological factors that determine the direction of eating responses to stress has been a major goal of epidemiological and clinical studies. However, challenges of standardizing the stress exposure in humans hinder efforts to uncover the underlying mechanisms. The issue of what determines the direction of stress-induced feeding responses has not been directly addressed in animal models, but assays that combine stress with a feeding-related task are commonly used as readouts of other behaviors, such as anxiety. Sex, estrous cyclicity, circadian cyclicity, caloric restriction, palatable diets, elevated body weight, and properties of the stressors similarly influence feeding behavior in humans and rodent models. Yet, most rodent studies do not use conditions that are most relevant for studying feeding behavior in humans. This review proposes a conceptual framework for incorporating these influences to develop reproducible and translationally relevant assays to study effects of stress on food intake. Such paradigms have the potential to uncover links between emotional eating and obesity as well as to the etiology of eating disorders.

Sex Differences in Metabolic and Behavioral Responses to Exercise but Not Exogenous Osteocalcin Treatment in Mice Fed a High Fat Diet

Background

Exercise helps improve glucose handling in diabetes and has been shown to improve mood and cognition in other conditions. Osteocalcin, a protein produced by bone osteoblasts, was reported to have endocrine actions to improve both metabolism and also improve age-related cognitive deficits in mice.

Methods

This study was designed to compare the effects of daily treadmill running exercise with injection of osteocalcin in high fat diet (HFD) induced diabetes in male and female C57BL/6J mice. Following established glucose intolerance and treatment for 8 weeks, mice were assessed for anxiety on an elevated plus maze, motivation by tail suspension test and cognition and memory in a puzzle box. Endogenous osteocalcin was measured by ELISA.

Results

Mice on HFD had high weight gain, glucose intolerance and increased white fat. Exercise increased circulating osteocalcin levels in female mice but decreased them in male mice. Exercise also decreased weight gain and improved glucose tolerance in female but not male mice; however, treatment with osteocalcin made no metabolic improvements in either males or females. HFD induced anxiety only in female mice and this was not improved by osteocalcin. Exercise induced anxiety only in male mice. HFD also increased depressive-like behavior in both sexes, and this was improved by either exercise or osteocalcin treatment. Cognitive deficits were seen in both male and female mice on HFD. Exercise improved cognitive performance in female but not male mice, while osteocalcin treatment improved cognitive performance in both sexes.

Conclusion

There were sex differences in the effects of exercise on endogenous osteocalcin regulation that correlated with improvements in cognitive but not metabolic outcomes. Exogenous osteocalcin did not improve metabolism but was effective in improving HFD-induced cognitive deficits. Sex is an important variable in hormonal and cognitive responses to exercise in diabetes.

Effects of a 33-ion sequential beam galactic cosmic ray analog on male mouse behavior and evaluation of CDDO-EA as a radiation countermeasure

In long-term spaceflight, astronauts will face unique cognitive loads and social challenges which will be complicated by communication delays with Earth. It is important to understand the central nervous system (CNS) effects of deep spaceflight and the associated unavoidable exposure to galactic cosmic radiation (GCR). Rodent studies show single- or simple-particle combination exposure alters CNS endpoints, including hippocampal-dependent behavior. An even better Earth-based simulation of GCR is now available, consisting of a 33-beam (33-GCR) exposure. However, the effect of whole-body 33-GCR exposure on rodent behavior is unknown, and no 33-GCR CNS countermeasures have been tested. Here astronaut-age-equivalent (6mo-old) C57BL/6J male mice were exposed to 33-GCR (75cGy, a Mars mission dose). Pre-/during/post-Sham or 33-GCR exposure, mice received a diet containing a 'vehicle' formulation alone or with the antioxidant/anti-inflammatory compound CDDO-EA as a potential countermeasure. Behavioral testing beginning 4mo post-irradiation suggested radiation and diet did not affect measures of exploration/anxiety-like behaviors (open field, elevated plus maze) or recognition of a novel object. However, in 3-Chamber Social Interaction (3-CSI), CDDO-EA/33-GCR mice failed to spend more time exploring a holder containing a novel mouse vs. a novel object (empty holder), suggesting sociability deficits. Also, Vehicle/33-GCR and CDDO-EA/Sham mice failed to discriminate between a novel stranger vs. familiarized stranger mouse, suggesting blunted preference for social novelty. CDDO-EA given pre-/during/post-irradiation did not attenuate the 33-GCR-induced blunting of preference for social novelty. Future elucidation of the mechanisms underlying 33-GCR-induced blunting of preference for social novelty will improve risk analysis for astronauts which may in-turn improve countermeasures.

Exploring Refinement Strategies for Single Housing of Male C57BL/6JRj Mice: Effect of Cage Divider on Stress-Related Behavior and Hypothalamic-Pituitary-Adrenal-Axis Activity

Introduction: Single housing of laboratory mice is a common practice to meet experimental needs, or to avoid intermale aggression. However, single housing is considered to negatively affect animal welfare and may compromise the scientific validity of experiments. The aim of this study was to investigate whether the use of a cage with a cage divider, which avoids physical contact between mice while maintaining sensory contact, may be a potential refinement strategy for experiments in which group housing of mice is not possible.

Methods: Eight-week-old male C57BL/6JRj mice were single housed, pair housed or pair housed with a cage divider for four (experiment 1) or ten (experiment 2) weeks, after which we performed an open field test, Y-maze spontaneous alternation test, elevated plus maze test, an auditory fear conditioning task, and assessed responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis.

Results: Housing conditions did not affect body weight, exploratory activity, anxiety, working memory, fear memory processing or markers for HPA-axis functioning in either experiment 1 or experiment 2. There was an increased distance traveled in mice housed with a cage divider compared to pair housed mice after 4 weeks, and after 10 weeks mice housed with a cage divider made significantly more arm entries in the Y-maze spontaneous alternation test.

Conclusion: Taken together, our study did not provide evidence for robust differences in exploratory activity, anxiety, working memory and fear memory processing in male C57BL/6JRj mice that were single housed, pair housed or pair housed with a cage divider.

Apparatus design and behavioural testing protocol for the evaluation of spatial working memory in mice through the spontaneous alternation T-maze

Spatial working memory can be assessed in mice through the spontaneous alternation T-maze test. The T-maze is a T-shaped apparatus featuring a stem (start arm) and two lateral goal arms (left and right arms). The procedure is based on the natural tendency of rodents to prefer exploring a novel arm over a familiar one, which induces them to alternate the choice of the goal arm across repeated trials. During the task, in order to successfully alternate choices across trials, an animal has to remember which arm had been visited in the previous trial, which makes spontaneous alternation T-maze an optimal test for spatial working memory. As this test relies on a spontaneous behaviour and does not require rewards, punishments or pre-training, it represents a particularly useful tool for cognitive evaluation, both time-saving and animal-friendly. We describe here in detail the apparatus and the protocol, providing representative results on wild-type healthy mice.

The effects of housing density on mouse thermal physiology depend on sex and ambient temperature

Objective

To improve understanding of mouse energy homeostasis and its applicability to humans, we quantitated the effects of housing density on mouse thermal physiology in both sexes.

Methods

Littermate wild type and Brs3-null mice were single- or group- (three per cage) housed and studied by indirect calorimetry with continuous measurement of core body temperature, energy expenditure, physical activity, and food intake.

Results

At 23 °C, below thermoneutrality, single-housed males had a lower body temperature and unchanged metabolic rate compared to group-housed controls. In contrast, single-housed females maintained a similar body temperature to group-housed controls by increasing their metabolic rate. With decreasing ambient temperature below 27 °C, only group-housed mice decreased their heat conductance, likely due to huddling, thus interfering with the energy expenditure vs ambient temperature relationship described by Scholander. In a hot environment (35 °C), the single-housed mice were less heat stressed. Upon fasting, single-housed mice had larger reductions in body temperature, with male mice having more torpor episodes of similar duration and female mice having a similar number of torpor episodes that lasted longer. Qualitatively, the effects of housing density on thermal physiology of Brs3-null mice generally mimicked the effects in controls.

Conclusions

Single housing is more sensitive than group housing for detecting thermal physiology phenotypes. Single housing increases heat loss and amplifies the effects of fasting or a cold environment. Male and female mice utilize different thermoregulatory strategies to respond to single housing.

• Changing housing density changes thermal physiology and metabolic rate.

• Singly housed mice are more affected by fasting and by cold temperatures.

• Single housing is more sensitive than group housing for detecting thermal phenotypes.

• Certain principles of thermal physiology are masked by group housing.

• Male and female mice respond differently to single housing.

Incorporating inter-individual variability in experimental design improves the quality of results of animal experiments

Inter-individual variability in quantitative traits is believed to potentially inflate the quality of results in animal experimentation. Yet, to our knowledge this effect has not been empirically tested. Here we test whether inter-individual variability in emotional response within mouse inbred strains affects the outcome of a pharmacological experiment. Three mouse inbred strains (BALB/c, C57BL/6 and 129S2) were behaviorally characterized through repeated exposure to a mild aversive stimulus (modified Hole Board, five consecutive trials). A multivariate clustering procedure yielded two multidimensional response types which were displayed by individuals of all three strains. We show that systematic incorporation of these individual response types in the design of a pharmacological experiment produces different results from an experimental pool in which this variation was not accounted for. To our knowledge, this is the first study that empirically confirms that inter-individual variability affects the interpretation of behavioral phenotypes and may obscure experimental results in a pharmacological experiment.

Inter-individual variability in habituation of anxiety-related responses within three mouse inbred strains

Inter-individual variability in behavioral and physiological response has become a well-established phenomenon in animal models of anxiety and other disorders. Such variability is even demonstrated within mouse inbred strains. A recent study showed that adaptive and non-adaptive anxiety phenotypes (measured as habituation and/or sensitization of anxiety responses) may differ within cohorts of 129 mice. This variability was expressed across both anxiety- and activity-related behavioral dimensions. These findings were based however on re-analysis of previously published data. The present study therefore aimed to empirically validate these findings in 129 mice. In addition, we assessed such inter-individuality in two other strains: BALB/c and C57BL/6. Males of three mouse inbred strains (BALB/c, C57BL/6 and 129S2) were behaviorally characterized through repeated exposure to a mild aversive stimulus (modified Hole Board, 4 consecutive trials). Behavioral observations were supplemented with assessment of circulating corticosterone levels. Clustering the individual response trajectories of behavioral and endocrine responses yielded two multidimensional response types of different adaptive value. Interestingly, these response types were displayed by individuals of all three strains. The response types differed significantly on anxiety and activity related behavioral dimensions but not on corticosterone concentrations. This study empirically confirms that adaptive capacities may differ within 129 cohorts. In addition, it extends this inter-individual variability in behavioral profiles to BALB/c and C57BL/6. Whether these two sub-types constitute differential anxiety phenotypes may differ per strain and requires further study.

Learning Set Formation and Reversal Learning in Mice During High-Throughput Home-Cage-Based Olfactory Discrimination

Rodent behavioral tasks are crucial to understanding the nature and underlying biology of cognition and cognitive deficits observed in psychiatric and neurological pathologies. Olfaction, as the primary sensory modality in rodents, is widely used to investigate cognition in rodents. In recent years, automation of olfactory tasks has made it possible to conduct olfactory experiments in a time- and labor-efficient manner while also minimizing experimenter-induced variability. In this study, we bring automation to the next level in two ways: First, by incorporating a radio frequency identification-based sorter that automatically isolates individuals for the experimental session. Thus, we can not only test animals during defined experimental sessions throughout the day but also prevent cagemate interference during task performance. Second, by implementing software that advances individuals to the next test stage as soon as performance criteria are reached. Thus, we can prevent overtraining, a known confounder especially in cognitive flexibility tasks. With this system in hand, we trained mice on a series of four odor pair discrimination tasks as well as their respective reversals. Due to performance-based advancement, mice normally advanced to the next stage in less than a day. Over the series of subsequent odor pair discriminations, the number of errors to criterion decreased significantly, thus indicating the formation of a learning set. As expected, errors to criterion were higher during reversals. Our results confirm that the system allows investigating higher-order cognitive functions such as learning set formation (which is understudied in mice) and reversal learning (which is a measure of cognitive flexibility and impaired in many clinical populations). Therefore, our system will facilitate investigations into the nature of cognition and cognitive deficits in pathological conditions by providing a high-throughput and labor-efficient experimental approach without the risks of overtraining or cagemate interference.

Animal Models of Depression: What Can They Teach Us about the Human Disease?

Depression is apparently the most common psychiatric disease among the mood disorders affecting about 10% of the adult population. The etiology and pathogenesis of depression are still poorly understood. Hence, as for most human diseases, animal models can help us understand the pathogenesis of depression and, more importantly, may facilitate the search for therapy. In this review we first describe the more common tests used for the evaluation of depressive-like symptoms in rodents. Then we describe different models of depression and discuss their strengths and weaknesses. These models can be divided into several categories: genetic models, models induced by mental acute and chronic stressful situations caused by environmental manipulations (i.e., learned helplessness in rats/mice), models induced by changes in brain neuro-transmitters or by specific brain injuries and models induced by pharmacological tools. In spite of the fact that none of the models completely resembles human depression, most animal models are relevant since they mimic many of the features observed in the human situation and may serve as a powerful tool for the study of the etiology, pathogenesis and treatment of depression, especially since only few patients respond to acute treatment. Relevance increases by the fact that human depression also has different facets and many possible etiologies and therapies.

Repeated unpredictable stress and social isolation induce chronic HPA axis dysfunction and persistent abnormal fear memory

The lack of progress in the psychopharmacological treatment of stress-related disorders such as PTSD is an ongoing crisis due to its negative socioeconomic implications. Current PTSD pharmacotherapy relies on a few FDA approved medications used primarily for depression which offer only symptomatic relief and show limited efficacy. As the population of PTSD patients is growing, the identification of effective etiology-based treatments for the condition is a high priority. This requires an in-depth understanding of the neurobiological and behavioral outcomes of stress in translationally relevant animal models. In this study, we use neuroendocrine, biochemical and behavioral measures to assess the HPA axis function and fear-memory deficits in a mouse model of chronic stress. The chronic stress procedures involved exposure to 21 days of repeated unpredictable stress (RUS), including predator stress, restraint and foot shock, followed by chronic social isolation. We show that mice exposed to our stress paradigm demonstrate exaggerated fear memory recall and blunted HPA axis functionality at one month after RUS. Our neuroendocrinal testing suggests that the attenuated stress response in our model may be related to an alteration in the adrenal MC2 receptor reactivity. While there was no noticeable change in pituitary negative feedback regulation mechanisms, CRH and phosphorylated Glucocorticoid receptors levels were altered in the hypothalamus. We also show that chronic supplementation with a peripheral glucocorticoid receptor agonist (low-dose dexamethasone) after RUS partially restores a number of stress-related behavioral deficits in the RUS model. This suggests a direct relationship between HPA axis function and behavior in our model. Our findings emphasize the importance of the adrenal receptors as a target for HPA axis dysfunction in stress and fear-related disorders.

Primed to vocalize: Wild-derived male house mice increase vocalization rate and diversity after a previous encounter with a female

Males in a wide variety of taxa, including insects, birds and mammals, produce vocalizations to attract females. Male house mice emit ultrasonic vocalizations (USVs), especially during courtship and mating, which are surprising complex. It is often suggested that male mice vocalize at higher rates after interacting with a female, but the evidence is mixed depending upon the strain of mice. We conducted a study with wild-derived house mice (Mus musculus musculus) to test whether male courtship vocalizations (i.e., vocalizations emitted in a sexual context) are influenced by a prior direct interaction with a female, and if so, determine how long the effect lasts. We allowed sexually naïve males to directly interact with a female for five minutes (sexual priming), and then we recorded males'vocalizations either 1, 10, 20, or 30 days later when presented with an unfamiliar female (separated by a perforated partition) and female scent. We automatically detected USVs and processed recordings using the Automatic Mouse Ultrasound Detector (A-MUD version 3.2), and we describe our improved version of this tool and tests of its performance. We measured vocalization rate and spectro-temporal features and we manually classified USVs into 15 types to investigate priming effects on vocal repertoire diversity and composition. After sexual priming, males emitted nearly three times as many USVs, they had a larger repertoire diversity, and their vocalizations had different spectro-temporal features (USV length, slope and variability in USV frequency) compared to unprimed controls. Unprimed control males had the most distinctive repertoire composition compared to the primed groups. Most of the effects were found when comparing unprimed to all primed males (treatment models), irrespective of the time since priming. Timepoint models showed that USV length increased 1 day after priming, that repertoire diversity increased 1 and 20 days after priming, and that the variability of USV frequencies was lower 20 and 30 days after priming. Our results show that wild-derived male mice increased the number and diversity of courtship vocalizations if they previously interacted with a female. Thus, the USVs of house mice are not only context-dependent, they depend upon previous social experience and perhaps the contexts of these experiences. The effect of sexual priming on male courtship vocalizations is likely mediated by neuro-endocrine-mechanisms, which may function to advertise males' sexual arousal and facilitate social recognition.

Effects of long-lasting social isolation and re-socialization on cognitive performance and brain activity: a longitudinal study in Octodon degus

Social isolation is considered a stressful situation that results in increased physiological reactivity to novel stimuli, altered behaviour, and impaired brain function. Here, we investigated the effects of long-term social isolation on working memory, spatial learning/memory, hippocampal synaptic transmission, and synaptic proteins in the brain of adult female and male Octodon degus. The strong similarity between degus and humans in social, metabolic, biochemical, and cognitive aspects, makes it a unique animal model that can be highly applicable for further social, emotional, cognitive, and aging studies. These animals were socially isolated from post-natal and post-weaning until adulthood. We also evaluated if re-socialization would be able to compensate for reactive stress responses in chronically stressed animals. We showed that long-term social isolation impaired the HPA axis negative feedback loop, which can be related to cognitive deficits observed in chronically stressed animals. Notably, re-socialization restored it. In addition, we measured physiological aspects of synaptic transmission, where chronically stressed males showed more efficient transmission but deficient plasticity, as the reverse was true on females. Finally, we analysed synaptic and canonical Wnt signalling proteins in the hypothalamus, hippocampus, and prefrontal cortex, finding both sex- and brain structure-dependent modulation, including transient and permanent changes dependent on stress treatment.

Sex-related differences in behavioural markers in adult mice for the prediction of lifespan

Finding biomarkers to assess the rate of ageing and consequently, to forecast individual lifespan is a challenge in ageing research. We recently published a mathematical model for lifespan prediction in adult female mice using behavioural parameters such as internal locomotion and time spent in open arms in the hole board (HB) and elevated plus maze (EPM) tests, respectively. Nevertheless, it is still not known if these behavioural variables could be useful in forecasting lifespan in male mice. Therefore, two groups of ICR-CD1 mice, male and female were subjected to the EPM, HB and T-maze tests at the adult age. Mice were monitored until they died and individual lifespans were registered. In general, adult male mice showed more anxiety-like behaviours than females. The mathematical model previously developed in females was validated with the female cohort, but found to be suboptimal for lifespan prediction in males. Thus, a new model for male lifespan prediction was constructed including the behavioural variables that were predictive of lifespan in males: time in the central platform of the EPM, inner locomotion, number of groomings and number and duration of head-dippings in the HB. These results confirm that the higher the anxiety-like behaviour at the adult age, the shorter the lifespan.

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.

Social Isolation Stress in Adolescence, but not Adulthood, Produces Hypersocial Behavior in Adult Male and Female C57BL/6J Mice

Chronic stress during the developmental period of adolescence increases susceptibility to many neuropsychiatric diseases in adulthood, including anxiety, affective, and alcohol/substance use disorders. Preclinical rodent models of adolescent stress have produced varying results that are species, strain, sex, and laboratory-dependent. However, adolescent social isolation is a potent stressor in humans that has been reliably modeled in male rats, increasing adult anxiety-like and alcohol drinking behaviors, among others. In this study, we examined the generalizability and sex-dependence of this model in C57BL/6J mice, the most commonly used rodent strain in neuroscience research. We also performed a parallel study using social isolation in adulthood to understand the impact of adult social isolation on basal behavioral phenotypes. We found that 6 weeks of social isolation with minimal handling in adolescence through early adulthood [postnatal day (PD) 28-70] produced a hypersocial phenotype in both male and female mice and an anxiolytic phenotype in the elevated plus-maze in female mice. However, it had no effects in other assays for avoidance behavior or on fear conditioning, alcohol drinking, reward or aversion sensitivity, or novel object exploration in either sex. In contrast, 6 weeks of social isolation in adulthood beginning at PD77 produced an anxiogenic phenotype in the light/dark box but had no effects on any other assays. Altogether, our results suggest that: (1) adolescence is a critical period for social stress in C57BL/6J mice, producing aberrant social behavior in a sex-independent manner; and (2) chronic individual housing in adulthood does not alter basal behavioral phenotypes that may confound interpretation of behavior following other laboratory manipulations.

Artificial microbiome heterogeneity spurs six practical action themes and examples to increase study power-driven reproducibility

With >70,000 yearly publications using mouse data, mouse models represent the best engrained research system to address numerous biological questions across all fields of science. Concerns of poor study and microbiome reproducibility also abound in the literature. Despite the well-known, negative-effects of data clustering on interpretation and study power, it is unclear why scientists often house >4 mice/cage during experiments, instead of ≤2. We hypothesized that this high animal-cage-density  practice abounds in published literature because more mice/cage could be perceived as a strategy to reduce housing costs. Among other sources of 'artificial' confounding, including cyclical oscillations of the 'dirty-cage/excrement microbiome', we ranked by priority the heterogeneity of modern husbandry practices/perceptions across three professional organizations that we surveyed in the USA. Data integration (scoping-reviews, professional-surveys, expert-opinion, and 'implementability-score-statistics') identified Six-Actionable Recommendation Themes (SART) as a framework to re-launch emerging protocols and intuitive statistical strategies to use/increase study power. 'Cost-vs-science' discordance was a major aspect explaining heterogeneity, and scientists' reluctance to change. With a 'housing-density cost-calculator-simulator' and fully-annotated statistical examples/code, this themed-framework streamlines the rapid analysis of cage-clustered-data and promotes the use of 'study-power-statistics' to self-monitor the success/reproducibility of basic and translational research. Examples are provided to help scientists document analysis for study power-based sample size estimations using preclinical mouse data to support translational clinical trials, as requested in NIH/similar grants or publications.

The effect of group size, age and handling frequency on inter-male aggression in CD 1 mice

Aggression in male mice often leads to injury and death, making social housing difficult. We tested whether (1) small group size, (2) early age of allocation to a group decreases aggression and 3) manipulation increases aggression in male mice. A 14wk study was performed to assess the following conditions in male CD-1/ICR mice: group size (1, 2, or 3), age at grouping (5 or 7wks), and manipulation (daily scruffing or minimal weekly handling). Wounds, body weights, food consumption, nest scores, sucrose consumption, fecal corticosterone and blood for hematology were collected. At the end of the study, mice were euthanized and pelted to assess wounding with the pelt aggression lesion scale (PALS). No signs of acute or chronic stress were observed in any of the groups. Trio housed mice showed less bite wounds than pair housed mice. In general, mice in larger groups ate less but weighed more. Individually housed mice, however, had high nest scores, low body weights, and increased sucrose and food consumption. These results suggest that even when nesting material is provided, individual mice may be experiencing thermal stress. Based on this data, CD-1 mice can successfully be housed for up to 14wks and groups of 3 may be the best for reducing even minor levels of aggression (i.e. wounding).

Impact of Adaptive Thermogenesis in Mice on the Treatment of Obesity

Obesity and associated metabolic diseases have become a priority area of study due to the exponential increase in their prevalence and the corresponding health and economic impact. In the last decade, brown adipose tissue has become an attractive target to treat obesity. However, environmental variables such as temperature and the dynamics of energy expenditure could influence brown adipose tissue activity. Currently, most metabolic studies are carried out at a room temperature of 21 °C, which is considered a thermoneutral zone for adult humans. However, in mice this chronic cold temperature triggers an increase in their adaptive thermogenesis. In this review, we aim to cover important aspects related to the adaptation of animals to room temperature, the influence of housing and temperature on the development of metabolic phenotypes in experimental mice and their translation to human physiology. Mice studies performed in chronic cold or thermoneutral conditions allow us to better understand underlying physiological mechanisms for successful, reproducible translation into humans in the fight against obesity and metabolic diseases.

Male C57BL6/N and C57BL6/J Mice Respond Differently to Constant Light and Running-Wheel Access

Previous studies have shown that exposure to circadian disruption produces negative effects on overall health and behavior. More recent studies illustrate that strain differences in the behavioral and physiological responses to circadian disruption exist, even if the strains have similar genetic backgrounds. As such, we investigated the effects of constant room-level light (LL) with running-wheel access on the behavior and physiology of male C57BL6/J from Jackson Laboratories and C57BL6/N from Charles River Laboratories mice. Mice were exposed to either a 12:12 light-dark (LD) cycle or LL and given either a standard home cage or a cage with a running-wheel. Following 6 weeks of LD or LL, their response to behavioral assays (open-field, light-dark box, novel object) and measures of metabolism were observed. Under standard LD, C57BL6/J mice exhibited increased locomotor activity and reduced exploratory behavior compared to C57BL6/N mice. In LL, C57BL6/J mice had greater period lengthening and increased anxiety, while C57BL6/N mice exhibited increased weight gain and no change in exploratory behavior. C57BL6/J mice also decreased exploration with running-wheel access while C57BL6/N mice did not. These results further demonstrate that C57BL/6 substrains exhibit different behavioral and physiological responses to circadian disruption and wheel-running access.

Wheel running behaviour in group-housed female mice indicates disturbed wellbeing due to DSS colitis

Voluntary wheel running (VWR) behaviour is a sensitive indicator of disturbed wellbeing and used for the assessment of individual experimental severity levels in laboratory mice. However, monitoring individual VWR performance usually requires single housing, which itself might have a negative effect on wellbeing. In consideration of the 3Rs principle, VWR behaviour was evaluated under group-housing conditions. To test the applicability for severity assessment, this readout was evaluated in a dextran sodium sulphate (DSS) induced colitis model. For continuous monitoring, an automated system with integrated radio-frequency identification technology was used, enabling detection of individual VWR. After a 14-day adaptation period mice demonstrated a stable running performance. Analysis during DSS treatment in combination with repeated facial vein phlebotomy and faecal sampling procedure resulted in significantly reduced VWR behaviour during the course of colitis and increased VWR during disease recovery. Mice submitted to phlebotomy and faecal sampling but no DSS treatment showed less reduced VWR but a longer-lasting recovery. Application of a cluster model discriminating individual severity levels based on VWR and body weight data revealed the highest severity level in most of the DSS-treated mice on day 7, but a considerable number of control mice also showed elevated severity levels due to sampling procedures alone. In summary, VWR sensitively indicated the course of DSS colitis severity and the impact of sample collection. Therefore, monitoring of VWR is a suitable method for the detection of disturbed wellbeing due to DSS colitis and sampling procedure in group-housed female laboratory mice.

Social Isolation in Adolescence Disrupts Cortical Development and Goal-Dependent Decision-Making in Adulthood, Despite Social Reintegration

The social environment influences neurodevelopment. Investigations using rodents to study this phenomenon commonly isolate subjects, then assess neurobehavioral consequences while animals are still isolated. This approach precludes one from dissociating the effects of on-going versus prior isolation, hindering our complete understanding of the consequences of social experience during particular developmental periods. Here, we socially isolated adolescent mice from postnatal day (P)31 to P60, then re-housed them into social groups. We tested their ability to select actions based on expected outcomes using multiple reinforcer devaluation and instrumental contingency degradation techniques. Social isolation in adolescence (but not adulthood) weakened instrumental response updating, causing mice to defer to habit-like behaviors. Habit biases were associated with glucocorticoid insufficiency in adolescence, oligodendrocyte marker loss throughout cortico-striatal regions, and dendritic spine and synaptic marker excess in the adult orbitofrontal cortex (OFC). Artificial, chemogenetic stimulation of the ventrolateral OFC in typical, healthy mice recapitulated response biases following isolation, causing habit-like behaviors. Meanwhile, correcting dendritic architecture by inhibiting the cytoskeletal regulatory protein ROCK remedied instrumental response updating defects in socially isolated mice. Our findings suggest that adolescence is a critical period during which social experience optimizes one's ability to seek and attain goals later in life. Age-typical dendritic spine elimination appears to be an essential factor, and in its absence, organisms may defer to habit-based behaviors.

Sexual experience has no effect on male mating or reproductive success in house mice

The ability to learn from experience can improve Darwinian fitness, but few studies have tested whether sexual experience enhances reproductive success. We conducted a study with wild-derived house mice (Mus musculus musculus) in which we manipulated male sexual experience and allowed females to choose between (1) a sexually experienced versus a virgin male, (2) two sexually experienced males, or (3) two virgin males (n = 60 females and 120 males). This design allowed us to test whether females are more likely to mate multiply when they encounter more virgin males, which are known to be infanticidal. We recorded females’ preference and mating behaviours, and conducted genetic paternity analyses to determine male reproductive success. We found no evidence that sexual experience influenced male mating or reproductive success, and no evidence that the number of virgin males influenced female multiple mating. Females always copulated with both males and 58% of the litters were multiple-sired. Females’ initial attraction to a male correlated with their social preferences, but neither of these preference behaviours predicted male reproductive success – raising caveats for using mating preferences as surrogates for mate choice. Male reproductive success was predicted by mating order, but unexpectedly, males that copulated first sired fewer offspring.

Adaptation to single housing is dynamic: Changes in hormone levels, gene expression, signaling in the brain, and anxiety-like behavior in adult male C57Bl/6J mice

For basic research, rodents are often housed in individual cages prior to behavioral testing. However, aspects of the experimental design, such as duration of isolation and timing of animal manipulation, may unintentionally introduce variance into collected data. Thus, we examined temporal correlates of acclimation of C57Bl/6J mice to single housing in a novel environment following two commonly used experimental time periods (7 or 14 days, SH7 or SH14). We measured circulating stress hormones (adrenocorticotropic hormone and corticosterone), basally or after injection stress, hippocampal gene expression of transcripts implicated in stress and affect regulation: the glucocorticoid receptor (GR), the mineralocorticoid receptor (MR), including the MR/GR ratio, and fibroblast growth factor 2 (FGF2). We also measured signaling in the mammalian target of rapamycin (mTOR) pathway. The basal elevation of stress hormones in the SH14 group is accompanied by a blunting in the circadian rhythms of GR and FGF2 hippocampal gene expression, and the MR/GR ratio, that is observed in SH7 mice. Following mild stress, the endocrine response and hippocampal mTOR pathway signaling are decreased in the SH14 mice. These neural and endocrine changes at 14 days of single housing likely underlie increased anxiety-like behavior measured in an elevated plus maze test. We conclude that multiple measures of stress responsiveness change dynamically between one and two weeks of single housing. The ramifications of these alterations should be considered when designing animal experiments since such hidden sources of variance might cause lack of replicability and misinterpretation of data.

Effect of three different forms of handling on the variation of aggression-associated parameters in individually and group-housed male C57BL/6NCrl mice

Mice are social animals hence group-housing of mice is preferred over individual housing. However, aggression in group-housed male mice under laboratory housing conditions is a well-known problem leading to serious health issues, including injury or death. Therefore, group-housed mice are frequently separated for welfare reasons. In this study, we investigated the effect of 3 different handling methods (tail, forceps, tube) in 2 different housing conditions (single vs. group) on the variance of aggression-associated parameters in male C57BL/6NCrl mice over 8 weeks. Blood glucose concentration, body weight, body temperature, plus number and severity of bite wounds and barbering intensity in group-housed mice were recorded. An assessment of nest complexity was also performed weekly. Feces were collected in week 3 and 7 for analysis of corticosterone metabolites. We also monitored the level of aggression by recording the behavior of group-housed animals after weekly cage cleaning. An open field test followed by a social novel object test, a light/dark box test, a hotplate and a resident-intruder test were performed at the end of the 8-week handling period. Post-mortem, we assessed organ weights. We found that forceps-handled mice, independent of the housing condition, had significantly higher levels of stress-induced-hyperthermia and enhanced aggression after cage cleaning, and they performed worse in the nest complexity test. In addition, handling male mice by the tail seems to be most effective to reduce aggressiveness after transferring animals into new cages, thereby representing an appropriate refinement.

Pair-housing rats does not protect from behavioral consequences of an acute traumatic experience

Posttraumatic stress disorder (PTSD) is an extremely debilitating disease with a broad array of associated symptoms, making the disorder difficult to diagnose and treat. In humans, patients seem to benefit from group therapy or other means of promoting social behavior. To test these effects on our rodent model of PTSD, adult, male rats were housed in either single or pair conditions prior to and during an acute stressor to induce PTSD-like behaviors in these rats. Subsequently, rats were assessed for PTSD-like symptoms to determine the effect of social housing on stress-induced phenotypes. Posttrauma phenotypes, including enhanced fear conditioning and anxiety-related behavior, persisted regardless of the animal's housing condition. It is possible that any housing driven improvements to stress-induced phenotypes would require longer periods of pair housing than were used in these experiments. Although PTSD patients show improved health outcomes following social interaction or group therapy, the fear and anxiety phenotypes seen following an acute stressor in an animal model of the disease endured despite an animal's housing condition. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Neuroplasticity-related correlates of environmental enrichment combined with physical activity differ between the sexes

Environmental enrichment (EE), comprising positive physical (exercise) and cognitive stimuli, influences neuronal structure and usually improves brain function. The promise of EE as a preventative strategy against neuropsychiatric disease is especially high during early postnatal development when the brain is still amenable to reorganization. Despite the fact that male and female brains differ in terms of connectivity and function that may reflect early life experiences, knowledge of the neural substrates and mechanisms by which such changes arise remains limited. This study compared the impact of EE combined with physical activity on neuroplasticity and its functional consequences in adult male and female rats; EE was provided during the first 3 months of life and our analysis focused on the hippocampus, an area implicated in cognitive behavior as well as the neuroendocrine response to stress. Both male and female rats reared in EE displayed better object recognition memory than their control counterparts. Interestingly, sex differences were revealed in the effects of EE on time spent exploring the objects during this test. Independently of sex, EE increased hippocampal turnover rates of dopamine and serotonin and reduced expression of 5-HT_1A receptors; in addition, EE upregulated expression of synaptophysin, a presynaptic protein, in the hippocampus. As compared to their respective controls, EE-exposed males exhibited parallel increases in phosphorylated Tau and the GluN2B receptor, whereas females responded to EE with reduced hippocampal levels of glutamate and GluN2B. Together, these observations provide further evidence on the differential effects of EE on markers of hippocampal neuroplasticity in males and females.

Running in the wheel: Defining individual severity levels in mice

The fine-scale grading of the severity experienced by animals used in research constitutes a key element of the 3Rs (replace, reduce, and refine) principles and a legal requirement in the European Union Directive 2010/63/EU. Particularly, the exact assessment of all signs of pain, suffering, and distress experienced by laboratory animals represents a prerequisite to develop refinement strategies. However, minimal and noninvasive methods for an evidence-based severity assessment are scarce. Therefore, we investigated whether voluntary wheel running (VWR) provides an observer-independent behaviour-centred approach to grade severity experienced by C57BL/6J mice undergoing various treatments. In a mouse model of chemically induced acute colitis, VWR behaviour was directly related to colitis severity, whereas clinical scoring did not sensitively reflect severity but rather indicated marginal signs of compromised welfare. Unsupervised k-means algorithm–based cluster analysis of body weight and VWR data enabled the discrimination of cluster borders and distinct levels of severity. The validity of the cluster analysis was affirmed in a mouse model of acute restraint stress. This method was also applicable to uncover and grade the impact of serial blood sampling on the animal’s welfare, underlined by increased histological scores in the colitis model. To reflect the entirety of severity in a multidimensional model, the presented approach may have to be calibrated and validated in other animal models requiring the integration of further parameters. In this experimental set up, however, the automated assessment of an emotional/motivational driven behaviour and subsequent integration of the data into a mathematical model enabled unbiased individual severity grading in laboratory mice, thereby providing an essential contribution to the 3Rs principles.

Animal-based biomedical research is often accompanied by experience of discomfort or pain by the animal. Recognition of disturbed animal welfare is mandatory, and the classification and assessment of its severity is a crucial part of the legislative framework in the European Union (EU). In the present study, we analysed voluntary wheel running (VWR) behaviour as a measure of compromised welfare in a mouse colitis model. Unsupervised mathematical clustering of clinical and VWR data enabled us to allocate and classify severity levels. This cluster model was verified using VWR data from a restraint stress model and allowed us to uncover the impact of routine experimental procedures on these mice. We propose that clustering of VWR behaviour provides a useful method for assessing the severity level of experimental procedures conducted on mice.