Co-housing rodents with different coat colours as a simple, non-invasive means of individual identification: validating mixed-strain housing for C57BL/6 and DBA/2 mice

Exploring the similarities between risk factors triggering depression in humans and elevated in-cage "inactive but awake" behavior in laboratory mice

Introduction

Depression is a human mental disorder that can also be inferred in non-human animals. This study explored whether time spent inactive but awake ("IBA") in the home-cage in mice was further triggered by risk factors similar to those increasing vulnerability to depression in humans (early life stress, genetic predispositions, adulthood stress).

Methods

Eighteen DBA/2 J and 18 C57BL/6 J females were tested, of which half underwent as pups a daily maternal separation on post-natal days 2-14 (early-life stress "ELS") (other half left undisturbed). To assess the effect of the procedure, the time the dams from which the 18 subjects were born spent active in the nest (proxy for maternal behavior) was recorded on post-natal days 2, 6, 10 and 14 for 1 h before separation and following reunion (matched times for controls), using live instantaneous scan sampling (total: 96 scans/dam). For each ELS condition, about half of the pups were housed post-weaning (i.e., from 27 days old on average) in either barren (triggering IBA and depression-like symptoms) or larger, highly enriched cages (n = 4-5 per group). Time mice spent IBA post-weaning was observed blind to ELS treatment using live instantaneous scan sampling in two daily 90-min blocks, two days/week, for 6 weeks (total: 192 scans/mouse). Data were analyzed in R using generalized linear mixed models.

Results

The dams were significantly more active in the nest over time (p = 0.016), however with no significant difference between strains (p = 0.18), ELS conditions (p = 0.20) and before/after separation (p = 0.83). As predicted, post-weaning barren cages triggered significantly more time spent IBA in mice than enriched cages (p < 0.0001). However, neither ELS (p = 0.4) nor strain (p = 0.84) significantly influenced time mice spent IBA, with no significant interaction with environmental condition (ELS × environment: p = 0.2861; strain × environment: p = 0.5713).

Discussion

Our results therefore only partly support the hypothesis that greater time spent IBA in mice is triggered by risk factors for human depression. We discuss possible explanations for this and further research directions.

Markerless Mouse Tracking for Social Experiments

Automated behavior quantification in socially interacting animals requires accurate tracking. While many methods have been very successful and highly generalizable to different settings, issues of mistaken identities and lost information on key anatomical features are common, although they can be alleviated by increased human effort in training or post-processing. We propose a markerless video-based tool to simultaneously track two interacting mice of the same appearance in controlled settings for quantifying behaviors such as different types of sniffing, touching, and locomotion to improve tracking accuracy under these settings without increased human effort. It incorporates conventional handcrafted tracking and deep-learning-based techniques. The tool is trained on a small number of manually annotated images from a basic experimental setup and outputs body masks and coordinates of the snout and tail-base for each mouse. The method was tested on several commonly used experimental conditions including bedding in the cage and fiberoptic or headstage implants on the mice. Results obtained without any human corrections after the automated analysis showed a near elimination of identities switches and a ∼15% improvement in tracking accuracy over pure deep-learning-based pose estimation tracking approaches. Our approach can be optionally ensembled with such techniques for further improvement. Finally, we demonstrated an application of this approach in studies of social behavior of mice by quantifying and comparing interactions between pairs of mice in which some lack olfaction. Together, these results suggest that our approach could be valuable for studying group behaviors in rodents, such as social interactions.

Structural enrichment for laboratory mice: exploring the effects of novelty and complexity

Providing structural enrichment is a widespread refinement method for laboratory rodents and other animals in captivity. So far, animal welfare research has mostly focused on the effect of increased complexity either by accumulating or combining different enrichment items. However, increasing complexity is not the only possibility to refine housing conditions. Another refinement option is to increase novelty by regularly exchanging known enrichment items with new ones. In the present study, we used pair-housed non-breeding female C57BL/6J and DBA/2N mice to investigate the effect of novelty when applying structural enrichment. We used a double cage system, in which one cage served as home cage and the other as extra cage. While the home cage was furnished in the same way for all mice, in the extra cage we either provided only space with no additional enrichment items (space), a fixed set of enrichment items (complexity), or a changing set of enrichment items (novelty). Over 5  weeks, we assessed spontaneous behaviors, body weight, and extra cage usage as indicators of welfare and preference. Our main results showed that mice with access to structurally enriched extra cages (complexity and novelty) spent more time in their extra cages and complexity mice had lower latencies to enter their extra cages than mice with access to the extra cages without any structural enrichment (space). This indicates that the mice preferred the structurally enriched extra cages over the structurally non-enriched space cages. We found only one statistically significant difference between the novelty and complexity condition: during week 3, novelty mice spent more time in their extra cages than complexity mice. Although we did not detect any other significant differences between the novelty and complexity condition in the present study, more research is required to further explore the potential benefits of novelty beyond complexity.

Assessing the effect of compounds from plantar foot sweat, nesting material, and urine on social behavior in male mice, Mus musculus

Home cage aggression causes poor welfare in male laboratory mice and reduces data quality. One of the few proven strategies to reduce aggression involves preserving used nesting material at cage change. Volatile organic compounds from the nesting material and several body fluids not only correlate with less home cage aggression, but with more affiliative allo-grooming behavior. To date, these compounds have not been tested for a direct influence on male mouse social behavior. This study aimed to determine if 4 previously identified volatile compounds impact home cage interactions. A factorial design was used with cages equally split between C57BL/6N and SJL male mice (N = 40). Treatments were randomly assigned across cages and administered by spraying one compound solution on each cage's nesting material. Treatments were refreshed after day 3 and during cage change on day 7. Home cage social behavior was observed throughout the study week and immediately after cage change. Several hours after cage change, feces were collected from individual mice to measure corticosterone metabolites as an index of social stress. Wound severity was also assessed after euthanasia. Measures were analyzed with mixed models. Compound treatments did not impact most study measures. For behavior, SJL mice performed more aggression and submission, and C57BL/6N mice performed more allo-grooming. Wound severity was highest in the posterior region of both strains, and the middle back region of C57BL/6N mice. Posterior wounding also increased with more observed aggression. Corticosterone metabolites were higher in C57BL/6N mice and in mice treated with 3,4-dimethyl-1,2-cyclopentanedione with more wounding. These data confirm previous strain patterns in social behavior and further validates wound assessment as a measure of escalated aggression. The lack of observed treatment effects could be due to limitations in the compound administration procedure and/or the previous correlation study, which is further discussed.

Do greater levels of in-cage waking inactivity in laboratory mice reflect a spontaneous depression-like symptom? A pharmacological investigation

We previously identified in laboratory mice an inactive state [being awake with eyes open motionless within the home cage; inactive but awake, 'IBA'] sharing etiological factors and symptoms with human clinical depression. We further test the hypothesis that greater time spent displaying IBA indicates a depression-like state in mice by investigating whether the antidepressant Venlafaxine, environmental enrichment, and their combination, alleviate IBA. Seventy-two C57BL/6J and 72 DBA/2J female mice were pseudo-randomly housed post-weaning in mixed strain-pairs in non-enriched (NE; 48 pairs) or in environmentally enriched (EE; 24 pairs) cages. After 34 days, half of the mice housed in NE cages were either relocated to EE cages or left in NE cages. For each of these conditions, half of the mice drank either a placebo or the antidepressant Venlafaxine (10 mg/kg). The 48 mice housed in EE cages were all relocated to NE cages and allocated to either the placebo (n = 24) or Venlafaxine (n = 24). IBA data were collected prior to and after environmental adjustment by trained observers blind to the pharmacological and environmental adjustment treatments. Data were analyzed using GLM models. NE cages triggered more IBA than EE cages (Likelihood-Ratio-Test Chi²₃ = 53.501, p < 0.0001). Venlafaxine and environmental enrichment appeared equally effective at reducing IBA (LRT Chi²₃ = 18.262, p < 0.001), and combining these approaches did not magnify their effects. Enrichment removal triggered IBA increase (LRT Chi²₁ = 23.050, p < 0.001), but Venlafaxine did not overcome the increase in IBA resulting from enrichment loss (LTR Chi²₁ = 0.081, p = 0.775). Theoretical implications for putative depression-like states in mice, and further research directions, are discussed.

Lidocaine and bupivacaine as part of multimodal pain management in a C57BL/6J laparotomy mouse model

While the use of local anesthesia as part of multimodal pain management is common practice in human and veterinarian surgery, these drugs are not applied routinely in rodent surgery. Several recommendations on the use of local anesthesia exist, but systematic studies on their efficacy and side effects are lacking. In the present study, male and female C57BL/6J mice were subjected to a sham vasectomy or a sham embryo transfer, respectively. We tested whether a mixture of subcutaneously injected Lidocaine and Bupivacaine in combination with systemic Paracetamol applied via drinking water results in superior pain relief when compared to treatment with local anesthesia or Paracetamol alone. We applied a combination of methods to assess behavioral, emotional, and physiological changes indicative of pain. Voluntary Paracetamol intake via drinking water reached the target dosage of 200 mg/kg in most animals. Local anesthesia did not lead to obvious side effects such as irregular wound healing or systemic disorders. No relevant sex differences were detected in our study. Sevoflurane anesthesia and surgery affected physiological and behavioral measurements. Surprisingly, Paracetamol treatment alone significantly increased the Mouse Grimace Scale. Taken together, mice treated with a combination of local anesthesia and systemic analgesia did not show fewer signs of post-surgical pain or improved recovery compared to animals treated with either local anesthesia or Paracetamol.

Where are you from? Female mice raised in enriched or conventional cages differ socially, and can be discriminated by other mice

Laboratory rodents raised in environmentally-enriched (EE) cages differ behaviourally and cognitively from conventionally-housed (CH) animals. We hypothesised that mice can detect such differences, testing this using differentially-raised female C57BL/6 s as subjects, and differentially-raised female BALB/cs and DBA/2 s as stimuli, in Social Approach Tests. Because more prone to signs of depression, anxiety, stereotypic behaviour (SB) and aggression, we further hypothesised that CH mice would be less sociable and socially attractive than EE mice. A novel familiarisation paradigm pre-exposed subjects to non-cagemate EE and CH stimulus mice before testing in Social Approach Tests. CH subjects proved less sociable than EE subjects: an effect unrelated to general exploration, anxiety or depression-like traits, and driven specifically by reduced interest in CH stimulus mice. Providing further evidence that CH and EE stimulus mice could be distinguished, subjects proved most attracted to mice from housing unlike their own. CH subjects thus preferred EE over CH stimulus mice, while EE subjects tended to prefer CH over EE: patterns that were not mediated by any measured aspect of stimulus mouse behaviour. Differential bodyweight also seemed unimportant, as was scent: soiled CH and EE bedding/nesting did not elicit the same discrimination. Instead, subjects who avoided CH stimulus mice and were attracted to EE stimulus mice were those who received the most agonism in their home cages. Together this provides the first demonstration that mice can distinguish between individuals raised in enriched or conventional cages, and suggests that receiving agonism from cagemates may motivate mice to seek new, less aggressive companions.

Not all mice are alike: Mixed-strain housing alters social behaviour

The use of millions of mice in scientific studies worldwide emphasises the continuing need for a reduction of sample sizes, however, not at the expense of scientific validity. Split-plot designs have been suggested to enhance statistical power while allowing a reduction of animal numbers in comparison to traditional experimental designs. Recently, a promising approach of a split-plot design has been implemented and proven useful using mixed-strain housing of at least three different mouse strains. However, the impact of co-housing different strains of mice in one cage on animal welfare has still to be defined. This study aimed at comparing the effects of mixed-strain and same-strain housing of female C57BL/6J and DBA/2N mice on welfare and behaviour in two experimental phases. In a first phase, mice were housed in either mixed- or same-strain pairs. Home cage behaviour, activity rhythm, body weight, and faecal corticosterone metabolites were assessed. Furthermore, tests for anxiety-like and exploratory behaviour as well as spatial learning were performed. In a second phase, sociability was investigated in newly formed mixed-strain quartets. Mixed-strain housing did not induce alterations in anxiety, locomotion, learning, stereotypic behaviour, and stress hormone levels. However, changes in social behaviours and activity rhythm were observed. Increased agonistic and decreased socio-positive behaviours might point towards mild impacts on welfare in C57BL/6J mice under co-housing conditions. Altogether, scientific research may greatly benefit from co-housing mice of different strains within the same cages (e.g. for the realisation of a split-plot design), provided that strains are carefully selected for compatibility.

Short-Term Cohousing of Sick with Healthy or Treated Mice Alleviates the Inflammatory Response and Liver Damage

Cohousing of sick with healthy or treated animals is based on the concept of sharing an intestinal ecosystem and coprophagy, the consumption of feces, which includes sharing of the microbiome and of active drug metabolites secreted in the feces or urine. To develop a model for short-term cohousing, enabling the study of the effect of sharing an ecosystem on inflammatory states. To determine the impact of cohousing of sick and healthy mice on the immune-mediated disorders, mice injected with concanavalin A (ConA) were cohoused with healthy or sick mice or with steroid-treated or untreated mice. To determine the effect of cohousing on acetaminophen (APAP)-induced liver damage, APAP-injected mice were cohoused with N-acetyl-cysteine (NAC)-treated or untreated mice. In the ConA-induced immune-mediated hepatitis model, cohousing of sick with healthy mice was associated with the alleviation of liver damage in sick animals. Similarly, a significant decrease in serum ALT was noted in ConA-injected mice kept in the same cage as ConA-injected mice treated with steroids. A trend for reduction in liver enzymes in APAP-injected mice was observed upon cohousing with NAC-treated animals. Cohousing of sick mice with healthy or treated mice ameliorated the immune-mediated inflammatory state induced by ConA and APAP. These models for liver damage can serve as biological systems for determining the effects of alterations in the ecosystem on the immune system.

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).

Measuring affect-related cognitive bias: Do mice in opposite affective states react differently to negative and positive stimuli?

Affect-driven cognitive biases can be used as an indicator of affective (emotional) state. Since humans in negative affective states demonstrate greater responses to negatively-valenced stimuli, we investigated putative affect-related bias in mice by monitoring their response to unexpected, task-irrelevant stimuli of different valence. Thirty-one C57BL/6J and 31 DBA/2J females were individually trained to return to their home-cage in a runway. Mice then underwent an affective manipulation acutely inducing a negative (NegAff) or a comparatively less negative (CompLessNeg) affective state before immediately being tested in the runway with either an 'attractive' (familiar food) or 'threatening' (flashing light) stimulus. Mice were subsequently trained and tested again (same affective manipulation) with the alternative stimulus. As predicted, mice were slower to approach the light and spent more time with the food. DBA/2J mice were slower than C57BL/6J overall. Contrary to predictions, NegAff mice tended to approach both stimuli more readily than CompLessNeg mice, especially the light, and even more so for DBA/2Js. Although the stimuli successfully differentiated the response of mice to unexpected, task-irrelevant stimuli, further refinement may be required to disentangle the effects of affect manipulation and arousal on the response to valenced stimuli. The results also highlight the significant importance of considering strain differences when developing cognitive tasks.

Could Greater Time Spent Displaying Waking Inactivity in the Home Environment Be a Marker for a Depression-Like State in the Domestic Dog?

Dogs exposed to aversive events can become inactive and unresponsive and are commonly referred to as being "depressed", but this association remains to be tested. We investigated whether shelter dogs spending greater time inactive "awake but motionless" (ABM) in their home-pen show anhedonia (the core reduction of pleasure reported in depression), as tested by reduced interest in, and consumption of, palatable food (KongTM test). We also explored whether dogs being qualitatively perceived by experts as disinterested in the food would spend greater time ABM (experts blind to actual inactivity levels). Following sample size estimations and qualitative behaviour analysis (n = 14 pilot dogs), forty-three dogs (6 shelters, 22F:21M) were included in the main study. Dogs relinquished by their owners spent more time ABM than strays or legal cases (F = 8.09, p = 0.032). One significant positive association was found between the KongTM measure for average length of KongTM bout and ABM, when length of stay in the shelter was accounted for as a confounder (F = 3.66, p = 0.035). Time spent ABM also correlated with scores for "depressed" and "bored" in the qualitative results, indirectly suggesting that experts associate greater waking inactivity with negative emotional states. The hypothesis that ABM reflects a depression-like syndrome is not supported; we discuss how results might tentatively support a "boredom-like" state and further research directions.

Two of a Kind or a Full House? Reproductive Suppression and Alloparenting in Laboratory Mice

Alloparenting, a behavior in which individuals other than the actual parents act in a parental role, is seen in many mammals, including house mice. In wild house mice, alloparental care is only seen when familiar sibling females simultaneously immigrate to a male's territory, so in the laboratory, when a pair of unfamiliar female wild mice are mated with a male, alloparenting does not occur because one female will typically be reproductively suppressed. In contrast, laboratory mice are assumed to alloparent regardless of familiarity or relatedness and are therefore routinely trio bred to increase productivity. Empirical evidence supporting the presence of alloparental care in laboratory mice is lacking. Albino and pigmented inbred mice of the strain C57BL/6NCrl (B6) and outbred mice of the stock Crl:CF1 (CF1) were used to investigate alloparenting in laboratory mice since by mating pigmented and albino females with albino males of the same stock or strain, maternal parentage was easily determined. We housed pairs (M:F) or trios (M:2F) of mice in individually ventilated cages containing nesting material and followed reproductive performance for 16 weeks. Females in trios were tested to determine dominance at the start of the experiment, and again 5 days after the birth of a litter to determine if a female's dominance shifted with the birth of pups. Results showed a significant and expected difference in number of offspring produced by B6 and CF1 (p < 0.0001). Pigmented mice nursed and nested with albino pups and vice-versa, confirming empirical observations from many that group nesting and alloparenting occurs in unrelated laboratory mice. When overall production of both individual mice and cages was examined, reproductive suppression was seen in trio cages. Dominance testing with the tube test did not correlate female reproduction with female dominance in a female-female dyad. Due to the reproductive suppression noted in trios, on a per-mouse basis, pair mating outperformed trio mating (p = 0.02) when the measure was weaned pups/female/week. No infanticide was seen in any cages, so the mechanism of reproductive suppression in trio matings may occur before birth.

Mixed-strain housing for female C57BL/6, DBA/2, and BALB/c mice: validating a split-plot design that promotes refinement and reduction

Background

Inefficient experimental designs are common in animal-based biomedical research, wasting resources and potentially leading to unreplicable results. Here we illustrate the intrinsic statistical power of split-plot designs, wherein three or more sub-units (e.g. individual subjects) differing in a variable of interest (e.g. genotype) share an experimental unit (e.g. a cage or litter) to which a treatment is applied (e.g. a drug, diet, or cage manipulation). We also empirically validate one example of such a design, mixing different mouse strains -- C57BL/6, DBA/2, and BALB/c -- within cages varying in degree of enrichment. As well as boosting statistical power, no other manipulations are needed for individual identification if co-housed strains are differentially pigmented, so also sparing mice from stressful marking procedures.

Methods

The validation involved housing 240 females from weaning to 5 months of age in single- or mixed- strain trios, in cages allocated to enriched or standard treatments. Mice were screened for a range of 26 commonly-measured behavioural, physiological and haematological variables.

Results

Living in mixed-strain trios did not compromise mouse welfare (assessed via corticosterone metabolite output, stereotypic behaviour, signs of aggression, and other variables). It also did not alter the direction or magnitude of any strain- or enrichment-typical difference across the 26 measured variables, or increase variance in the data: indeed variance was significantly decreased by mixed- strain housing. Furthermore, using Monte Carlo simulations to quantify the statistical power benefits of this approach over a conventional design demonstrated that for our effect sizes, the split- plot design would require significantly fewer mice (under half in most cases) to achieve a power of 80 %.

Conclusions

Mixed-strain housing allows several strains to be tested at once, and potentially refines traditional marking practices for research mice. Furthermore, it dramatically illustrates the enhanced statistical power of split-plot designs, allowing many fewer animals to be used. More powerful designs can also increase the chances of replicable findings, and increase the ability of small-scale studies to yield significant results. Using mixed-strain housing for female C57BL/6, DBA/2 and BALB/c mice is therefore an effective, efficient way to promote both refinement and the reduction of animal-use in research.

Electronic supplementary material

The online version of this article (doi:10.1186/s12874-016-0113-7) contains supplementary material, which is available to authorized users.

Mixed housing with DBA/2 mice induces stress in C57BL/6 mice: implications for interventions based on social enrichment

Several behavioral interventions, based on social enrichment and observational learning are applied in treatment of neuropsychiatric disorders. However, the mechanism of such modulatory effect and the safety of applied methods on individuals involved in social support need further investigation. We took advantage of known differences between inbred mouse strains to reveal the effect of social enrichment on behavior and neurobiology of animals with different behavioral phenotypes. C57BL/6 and DBA/2 female mice displaying multiple differences in cognitive, social, and emotional behavior were group-housed either in same-strain or in mixed-strain conditions. Comprehensive behavioral phenotyping and analysis of expression of several plasticity- and stress-related genes were done to measure the reciprocal effects of social interaction between the strains. Contrary to our expectation, mixed housing did not change the behavior of DBA/2 mice. Nevertheless, the level of serum corticosterone and the expression of glucocorticoid receptor Nr3c1 in the brain were increased in mixed housed DBA/2 as compared with those of separately housed DBA/2 mice. In contrast, socially active C57BL/6 animals were more sensitive to the mixed housing, displaying several signs of stress: alterations in learning, social, and anxiety-like behavior and anhedonia. These behavioral impairments were accompanied by the elevated serum corticosterone and the reduced expression of Nr3c1, as well as the elevated Bdnf levels in the cortex and hippocampus. Our results demonstrate the importance of social factors in modulation of both behavior and the underlying neurobiological mechanisms in stress response, and draw attention to the potential negative impact of social interventions for individuals involved in social support.