Probiotic administration provides beneficial effects to cultured fish species, such as improved nutrient utilization, changes to the immune system, and disease resistance. Recent studies have highlighted the significant impact of probiotic treatments on the central nervous system (CNS) and stress regulation via the microbiota-gut-brain axis. However, the available knowledge regarding this important axis in fish remains limited. Therefore, a study was conducted using transcriptomic profiling to further our understanding of the mechanisms underlying probiotic effects in an important aquaculture species, the olive flounder (Paralichthys olivaceus).
Juvenile olive flounder (average weight, ~8 g) were incorporated into two diet trial setups: a 1-month lab-scale trial and a 6-month field-scale trial, each with and without the probiotic strain Lactococcus lactis WFLU12. In both trials, RNA-Seq was performed using liver samples collected from fish at 1 month post-feeding (mpf). In the field-scale experiment, growth performance was evaluated monthly, and serological parameters were measured at 1 mpf.
The results of the lab-scale trial showed that probiotic administration upregulated genes related to neurotransmission (i.e., htr3a, mao, ddc, ntsr1, and gfra2) and highlight the ability of probiotics to modulate neurotransmission via the microbiota-gut-brain axis. In the field-scale experiment, fish growth was significantly promoted, and the sera levels of AST, LDH, and cortisol were higher in the control group than in the probiotics group. Furthermore, genes involved in stress responses (i.e., hsp70, hsp90B1, hspE1, prdx1, and gss) and transcriptional regulators (i.e., fos, dusp1, and dusp2) exhibited upregulation in the control fish compared to the probiotic-fed group, indicating that probiotic administration can alleviate stress levels in fish. Overall, this study provides valuable insight into the neurotransmission and stress reduction mechanisms underlying the beneficial effects of administering probiotics to cultured fish.