by Rachel Summers
Faculty mentor: Dr. Dianne Baker
Glucocorticoid hormones mediate stress responses in all vertebrates, from teleost fishes to mammals. In adult teleosts, the primary glucocorticoid, cortisol, is synthesized within interrenal tissue via enzyme-mediated reactions regulated by the hypothalamic-pituitary-interrenal (HPI) axis in response to stressors. The hypothalamic peptide corticotropin-releasing hormone (CRH) stimulates release of the pituitary protein adrenocorticotropin hormone (ACTH), which stimulates cortisol production in interrenal cells. Cortisol affects target cells via two types of receptors, the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR). The timing and sequence of events leading to a fully functioning HPI axis in developing nonmammalian vertebrates is not fully known. Addressing this gap, we measured expression of genes involved in cortisol synthesis and signaling throughout embryogenesis in two teleosts, the zebrafish (Danio rerio) and Japanese medaka (Oryzias latipes). We isolated RNA from pools of embryos collected at multiple developmental stages and synthesized complementary DNA (cDNA) by reverse transcription. Using cDNA as a template, we measured relative expression of key HPI genes, including CRH, melanocortin type 2 receptor (MC2R), steroidogenic acute regulatory protein (StAR), 11 β hydroxysteroid dehydrogenase (HSD2), and MR by quantitative polymerase chain reaction (qPCR). We measured cortisol throughout embryogenesis using an enzyme-linked immunosorbent assay (ELISA). We found noteworthy differences in expression profiles for all measured genes between species. Notably, we saw large differences in magnitude for changes in CRH, MC2R, and StAR expression. HSD2 showed different patterns of expression in zebrafish and medaka. Temporal cortisol patterns differed between species. Medaka cortisol increased significantly for hatch, whereas zebrafish saw a significant decrease.