Scale Cortisol Signatures in Cultured Nile Tilapia (Oreochromis niloticus) Regulates Mitogen Activated Protein Kinase Signalling Pathway to Modulate Chronic Stress

Chronic stress poses a challenge to aquaculture, with cortisol and glucose traditionally used as stress markers. Recent doubts about the reliability of scale cortisol as a chronic stress determinant have surfaced due to its role in calcium homeostasis. While cortisol affects gene expression in stress responses, its impact on metabolic pathways in cultured Nile tilapia remains understudied. We explored the connection between cortisol signatures of chronic stress in Nile tilapia and the Mitogen Activated Protein Kinase (MAPK) signaling pathway, vital for cell processes. Juvenile Nile tilapia were exposed to varying ammonia concentrations and stocking densities, evaluating growth performance, stress levels, RNA sequencing, and differential gene expression. Results indicate a positive correlation between stressors and blood glucose, plasma cortisol, and scale cortisol concentrations. Fish in 0.8 mg/L ammonia exhibited heightened plasma glucose and cortisol, while those in 1.2 mg/L showed increased scale cortisol. Elevated stocking densities also correlated with higher stress markers. Importantly, cortisol levels rose with ammonia concentration and stocking density, negatively impacting growth. The MAPK signaling pathway, crucial for cell processes, exhibited significant downregulation with increasing ammonia concentrations, suggesting sensitivity to stress. Six genes in this pathway were significantly enriched following ammonia treatment, including Dual Specific Protein Phosphatase 1, Nuclear Hormone Receptor 38, Heat Shock Protein 1, Myelocytomatosis oncogene homologue, Growth arrest and DNA damage inducible alpha a, and Mitogen Activated Protein Kinase 4. This study contributes valuable insights for optimizing fish welfare and production by unraveling the complex relationship between chronic stress and the MAPK pathway in aquaculture.