Microbiome and gut as partners in the persistence of post-inflammatory visceral pain: insight into working mechanisms of faecal transplant for therapy advance

Background. Gut dysbiosis is a common feature of patients complaining of chronic abdominal pain, including those in remission from inflammatory bowel diseases (IBDs). Although visceral sensitivity in animals can be modulated by faecal microbial transplant (FMT), controversy has emerged in clinical practice, drawing attention to the poor knowledge of the mechanisms underpinning host-microbiota crosstalk under pathological conditions. Here we sought to elucidate the mechanisms linking post-inflammatory dysbiosis and pain persistence, to improve the therapeutic strategies. Results. Colitis was induced in rats by intrarectal injection of 2,4-dinitrobenzenesulfonic acid (DNBS). Naïve rats subjected to FMT from viscerally hypersensitive DNBS-treated rats (FMT ^DNBS ) displayed a higher sensitivity than those receiving FMT from healthy donors (FMT ^CTR ). A reverse protocol highlighted the anti-hyperalgesic effect of FMT ^CTR in DNBS-treated rats. Difference in microbiota-to-gut signalling between dysbiotic or eubiotic conditions was investigated after the treatment with FMT ^CTR or FMT ^DNBS , in both the experimental protocols. Modification of pain threshold in the animals undergoing FMT correlated with changes in the composition of the gut microbiota and the metabolic profile, evaluated by 16S rRNA and proton Nuclear Magnetic Resonance ( ¹ H NMR). Although a specific microbial community associated with the pain phenotype was not identified, significant differences between painful and painless conditions were detected in the faecal metabolome, involving fatty acids, purine metabolites, glutamate and lactate. Yet, proteomic analysis of colon tissues from FMT recipients revealed differential scenarios. Indeed, the proalgesic effect of FMT ^DNBS in healthy animals was accompanied by a detrimental modulation of pathways regulating cellular plasticity and clearance, cell-to-cell interaction and signaling, as well as epithelium growth and neuroplasticity phenomenon, besides metabolism and inflammation. Pain relief following FMT ^CTR in DNBS-treated rats was instead associated with a counteraction of inflammatory process, lymphocyte activation and submucosal mast cells infiltration. Conclusions. These results indicate the presence of signalling modulation within the gut rather than tissue alterations at the base of FMT-related effects on pain, providing novel insights into the mechanisms linking intestinal microbiota to visceral sensitivity which might be exploited to enhance the effectiveness of microbiota-targeted interventions in the treatment of chronic pain in IBD patients.