THE MICROBIOTA-GUT-BRAIN AXIS. A STUDY IN ZEBRAFISH (DANIO RERIO)

Abstract

The microbiota is essential in the host's physiology, development, reproduction, immune system, nutrient metabolism, in brain chemistry and behavior. The gut microbiota plays a crucial role in the bidirectional gut–brain axis, a communication that integrates the gut and central nervous system (CNS) activities, and thus, the concept of microbiota–gut–brain axis is emerging where the microbes have considered as signaling components in the gut-brain axis. Animal studies reveals, in particular, that gut bacteria influence the brain-derived neurotrophic factor (BDNF) levels, and behavior specially after probiotic administration. How this alterations in brain chemistry are related to specific behavioral changes is unclear but it will likely be a focus of future research efforts. Among these animal studies, to our knowledge, no studies on the microbiota–gut–brain axis in zebrafish (Danio rerio) have been carried out. We hypothesized that a continuous administration of an exogenous probiotic might also influence the host's behavior and neurochemical gene expression. The purpose of this study was to determine whether probiotic strain can modulate gut commensal bacteria influencing brain neurochemistry and behavior in zebrafish. Thus, we treated adult zebrafish for 28 days with Lactobacillus rhamnosus, a probiotic strain which is one of the main components of the commensal microflora of human intestinal tract and it is widely used as a probiotic in mammals to adult male and female AB wild tipe zebrafish. We established differences between treated with probiotic strain and control group in shoaling behavior pattern, using a Video Tracker software; we quantified brain-derived neurotrophic factor (BDNF) gene expression by using RT-qPCR; we at last analyzed the microbiota profiles within two experimental groups by sing the culture-independent methods such as Denaturing Gradient Gel Electrophoresis (DGGE) and Next-Generation Sequencing (NGS). The probiotic treated group, compared to the control group, showed a statistically significant near two-fold increase in BDNF gene expression, different shoaling behavioural pattern and a shift in microbiota composition with a significant increase of Firmicutes and a reduction of Proteobacteria. The results of each approach may support the existence of a microbiota–gut–brain axis, in adult zebrafish and in line with numerous animal studies we can speculate that microbiota manipulation could influence behavior and brain expression of BDNF.