High environmental ammonia exposure has developmental-stage specific and long-term consequences on the cortisol stress response in zebrafish

The capacity for early life environmental stressors to induce programming effects on the endocrine stress response in fish is largely unknown. In this study we determined the effects of high environmental ammonia (HEA) exposure on the stress response in larval zebrafish, assessed the tolerance of embryonic and larval stages to HEA, and evaluated whether early life HEA exposure has long-term consequences on the cortisol response to a novel stressor. Exposure to 500-2000 μM NH4Cl for 16 h did not affect the gene expression of corticotropin-releasing factor (CRF) system components in 1 day post-fertilization (dpf) embryos, but differentially increased crfa, crfb and CRF binding protein (crfbp) expression and stimulated both dose- and time-dependent increases in the whole body cortisol of 5 dpf larvae. Pre-acclimation to HEA at 1 dpf did not affect the cortisol response to a subsequent NH4Cl exposure at 5 dpf. In contrast, pre-acclimation to HEA at 5 dpf caused a small but significant reduction in the cortisol response to a second NH4Cl exposure at 10 dpf. While continuous exposure to 500-2000 μM NH4Cl between 0 and 5 dpf had a modest effect on mean survival time, exposure to 400-1000 μM NH4Cl between 10 and 14 dpf decreased mean survival time in a dose-dependent manner. Moreover, pre-acclimation to HEA at 5 dpf significantly decreased the risk of mortality to continuous NH4Cl exposure between 10 and 14 dpf. Finally, while HEA at 1 dpf did not affect the cortisol stress response to a novel vortex stressor at 5 dpf, the same HEA treatment at 5 dpf abolished vortex stressor-induced increases in whole body cortisol at 10 and 60 dpf. Together these results show that the impact of HEA on the cortisol stress response during development is life-stage specific and closely linked to ammonia tolerance. Further, we demonstrate that HEA exposure at the larval stage can have persistent effects on the capacity to respond to stressors in later life.