Comparative compositional analysis of the gut microbiome in animal models of addiction and stress

Abstract

Intro: The microbiome-gut-brain axis (MGBA) has been shown to be instrumental to brain and behaviour, including psychological disorders affecting stress, depression, and anxiety. Many of the aforementioned disorders are comorbid with substance abuse disorders, and on a larger scale with addiction. In this body of work, we seek to characterize addiction-related phenotypes in the microbiome and correlate with measures of addiction-related behaviours, along with changes in the virome following chronic social stress. Aims: We first examined if vapor administration of ethanol is capable of altering the microbiome and to assess if substances of abuse, such as ethanol, can alter the microbiome outside of oral/gastrointestinal administration. To characterize addictive phenotypes behavioural measures of impulsivity, reward learning, and dopaminergic response to novelty were used. Methods: All studies were carried out in rodents. Following behavioural testing, gut microbiota samples were analysed with 16S rRNA gene amplicon sequencing. In the chronic social stress study, the virome was also sequenced via shot-gun metagenomic sequencing of faecal filtrate. Correlations were performed between microbiome measures and biological measures of behaviour, cytokines, corticosterone, and dopamine mRNA expression. Results: Chapter 2) Investigation of microbiome and vapor alcohol administration in mice showed significant alterations in microbiome; Chapter 3) A pre-clinical model of alcohol-addiction showed no significant differences in the microbiome by grouped phenotype. However, low abundance genus-level bacteria correlated to striatal dopamine mRNA expression. Chapter 4) A large heterogenous study of male and female rats revealed that female microbiome composition is more associated to addictive measures. However, within both male and female operational taxonomic units (OTUs) were significantly correlated to impulsivity measures. Chapter 5) Chronic social stress in male mice significantly affected bacterial and viral 1-14 composition. Viral richness was increased alongside decreases in bacterial richness. Discussion: Results from this body of work show that the microbiome may be impacted by addiction and psychosocial stress. Our findings suggest that there are subtle yet significant differences in microbiome composition between rats with different behavioural phenotypes. These findings indicate that associations to addictive-related behaviour occur in low-abundance (<5%) bacteria and are sex-specific. Furthermore, microbiome (bacteriome and virome) is impacted top-down, brain to gut via the hypothalamic-pituitary-adrenal axis, during chronic psychosocial stress. These initial investigations into stress and gut bacteriophage (aka phage) indicate that increases in phage richness are associated to decreases in bacterial richness and alpha diversity during stress. These novel findings seed the foundation for new research investigating MGBA in stress, substance abuse, and addiction. Further research is necessary to elucidate the mechanisms involved in this cross-communication of the MGBA. New insights into these challenging disorders of addiction and stress may provide unique therapies targeting the gut microbiome.