Brain microRNAs in rainbow trout are modulated by functional additives and fish density

MicroRNA (miRNAs) are small non-protein-coding RNAs (ncRNAs), with an average length of 22 nucleotides, and act as posttranscriptional regulators of mRNA. These ncRNAs have been intensively studied over the last decades in several species. Advances in high throughput sequencing technologies have allowed the identification of novel miRNAs and their association with the regulation at cellular and tissue levels. Even though knowledge of miRNAs in fish has mostly improved over the last years, the information gap is still present regarding the occurrence and role of these ncRNAs in some tissues, such as the brain. Herein, this study aimed to identify the brain miRNome transcriptional modulation of rainbow trout under suboptimal conditions (high stocking density) and fed a diet supplemented with probiotic. The results evidenced that miRNA expression patterns were distinct depending on diet and stocking condition, revealing an alteration in the brain's molecular underpinnings that can result in differential stress response and immune capacity. Notably, the expression of Orexin, a gene encoding for anorexigenic neurotransmitter involved in energy homeostasis in fish, was reduced with high density but not when fish were fed probiotics, and the expression was inverse to omy-miR-160, a potential miRNA regulator. Although further studies are necessary to understand how functional diets and stress can trigger the brain miRNAs in fish, this study brings to light the possible psychobiotic role of yeast, Saccharomyces cerevisiae in farmed fish, that in turn can be associated to protective effect during chronic deleterious conditions.