The gut-brain axis plays a significant role in shaping brain functions, including social behavior. While previous research has shown links between gut microbiota and social behavior, the impact of specific microbiota compositions on stable behavioral patterns remains unclear. This study utilized a well-established mouse model of social dominance and submissiveness, selectively bred from a Sabra mouse lineage. Dominant (Dom) and submissive (Sub) mice exhibit distinct behavioral patterns, physiological differences, and responses to stress and psychotropic agents. The researchers hypothesized that these behavioral differences stem from variations in gut microbiome composition, which may induce inflammation and alter adipose tissue homeostasis. The purpose of the study was to determine the gut microbiota composition of Dom and Sub mice, analyze their epididymal white adipose tissue (eWAT) characteristics, and use fecal microbiota transplantation (FMT) to establish a causal relationship between gut microbiota and social behavior.

Existing literature demonstrates the gut microbiome's influence on social behavior through the gut-brain axis. Studies show that bacterial transplantation can affect sociability, and social stress correlates with changes in gut microbiota diversity. Prebiotics have also been shown to improve social behavior in rodents. Fecal transplantation studies suggest a causal link between gut microbiota and chronic inflammation, potentially impacting various pathologies. While some studies link adipose tissue inflammation to metabolic disturbances and depression, a direct link between the gut microbiome, adipose inflammation, and behavior was previously lacking. This study aimed to address this gap using the established Dom/Sub mouse model.

The study used selectively bred Dom and Sub mice (F24-F26 generations). Social behavior was assessed using a dominant-submissive relationship (DSR) test, measuring milk drinking time during food competition. Body weight and food intake were monitored. eWAT mass and adipocyte size were analyzed through histology and image analysis. Macrophage infiltration was assessed by immunohistochemistry and gene expression analysis of F4/80. An adipokine array was used to profile eWAT adipokines. Gut microbiota composition was determined using 16S rRNA gene sequencing. FMT was performed by transplanting fecal microbiota from Dom or Sub mice into germ-free (GF) mice. The GF mice were then assessed for social behavior (DSR, three-chamber sociability test), depressive-like behavior (forced swim test), eWAT mass, adipocyte size, and eWAT adipokine profiles. Statistical analyses included Student's t-tests, ANOVA, and beta diversity analysis using QIIME2.

Sub mice exhibited significantly more submissive behavior compared to Dom mice, regardless of sex. Sub mice consistently displayed lower body weight (1.15-fold lower) and eWAT mass (1.15-fold lower) than Dom mice, despite similar food intake. Sub mice had smaller adipocytes (125-fold decrease) and increased macrophage infiltration (2.1-fold increase) in eWAT. Eighteen of 38 tested eWAT adipokines were significantly elevated in Sub mice, indicating an altered inflammatory and metabolic profile. 16S rRNA sequencing revealed significant differences in gut microbiota composition between Dom and Sub mice. Sub mice showed lower gut microbiota diversity and the unique presence of Mycoplasma and Anaeroplasma genera, along with Rikenellaceae and Clostridiaceae families. Conversely, Dom mice showed higher abundance of Prevotella. FMT of Sub microbiota into GF mice resulted in the adoption of submissive behavior, reduced eWAT mass, smaller adipocytes, and a Sub-like eWAT adipokine profile. These changes in GF/Sub mice mirrored the characteristics of Sub mice.

The findings demonstrate a causal relationship between gut microbiota composition, social behavior, and adipose tissue physiology. The distinct gut microbiota of Sub mice appears to be a key factor driving their submissive behavior and associated metabolic and inflammatory changes in eWAT. The observed reduction in eWAT mass in Sub mice and GF/Sub mice is intriguing, contrasting with the often-observed link between obesity, inflammation, and depression. The upregulation of specific adipokines like FGF-21 and VEGF in Sub and GF/Sub mice suggests mechanisms linking gut microbiota, adipose tissue inflammation, and social behavior. These results are consistent with studies linking inflammation and mood disorders in humans.

This study reveals a direct link between gut microbiota composition, social behavior, and adipose tissue physiology in mice. The Sub mouse microbiota's causal role in shaping submissive behavior and associated physiological changes highlights the gut microbiome's importance in social interaction. Future research should investigate the specific mechanisms by which gut microbiota influences eWAT and social behavior, and explore potential therapeutic interventions targeting the gut microbiome to improve social interaction.

The study primarily focused on male mice in some aspects, limiting the generalizability of findings to females. While the FMT study provided strong evidence, further research is needed to investigate the long-term effects of microbiota transplantation and to determine the minimal number of FMTs required for replicating the phenotype. Additional research is needed to identify specific microbial species or metabolites responsible for the observed effects.