Physiological roles of glucocorticoids during early embryonic development of the zebrafish (Danio Rerio)

While glucocorticoids (GC) are known to be present in the zebrafish embryo little is known about their physiological roles at this stage. We hypothesised that GC play key roles in stress response, hatching and swim activity during early development. To test this, whole embryo cortisol (WEC) and corticosteroid-related genes were measured in embryos from 6 to 120 hours post fertilisation (hpf) by ELISA. Stress response was assessed by change in WEC following stirring, hypoxia or brief electrical impulses applied to the bathing water. The impact of pharmacological and molecular glucocorticoid manipulation on the stress-response, spontaneous hatching and swim activity at different stages of development was also assessed. WEC levels demonstrated a biphasic pattern during development with a decrease from 0 to 36 hpf followed by a progressive increase towards 120hpf. This was accompanied by a significant and sustained increase in the expression of genes encoding cyp11b1 (glucocorticoid biosynthesis), 11ßhsd2 (glucocorticoid metabolism) and gr (glucocorticoid receptor) from 48 to 120hpf. Metyrapone (Met) an inhibitor of 11ß hydroxylase (encoded by cyp11b1) and cyp11b1 morpholino knockdown significantly reduced basal and stress-induced WEC levels at 72 and 120hpf but not at 24 hpf. Spontaneous hatching and swim activity were significantly affected by manipulation of GC action from approximately 48 hpf onwards. We have identified a number of key roles of GC in zebrafish embryos contributing to adaptive physiological responses under adverse conditions. The ability to alter GC action in the zebrafish embryo also highlights its potential value for GC research.