Gut <i>Proteobacteria</i> glycine metabolism regulates neuroplasticity, motivation, and reinstatement of cocaine self-administration in mice

Addiction is a chronic and relapsing disorder that affects millions of people worldwide; nonetheless, currently available FDA-approved treatments are limited in number and effectiveness. In past years, the gut–brain axis has emerged as a key modulatory factor associated with different psychiatric disorders, including addiction. Working in mice, we have shown that cocaine exposure alters the composition of the gut microbiome, increasing the abundance of Proteobacteria. This microbial shift, in turn, leads to a depletion in host glycine levels, altering cocaine-induced transcriptional changes in the Nucleus Accumbens (NAc) and facilitating the development of behavioral sensitization and conditioned place preference. Among the behavioral models to study psychostimulant use disorders, cocaine self-administration (SA) remains the most translational. Therefore, here we investigated whether Proteobacteria-induced glycine depletion can affect cocaine SA in mice. Using the human Escherichia coli HS and the glycine-uptake-deficient mutant E. coli HS ΔCycA, we build upon our previous findings and demonstrate that the ability of gut Proteobacteria to use glycine during cocaine SA shapes the trajectory and long-term neurobehavioral plasticity induced by the drug. Furthermore, we show that this bacterial-induced glycine depletion impacts the NAc proteome, altering its vulnerability to undergo molecular adaptations across different stages of the SA paradigm. Altogether, our findings show that the gut microbiome, and particularly the Proteobacteria phylum, is a crucial factor influencing short and long-term adaptation underlying motivation and cocaine-seeking behaviors.