Analysis of neuroendocrine factors in response to conditional stress in zebrafish Danio rerio (Cypriniformes: Cyprinidae)

Challenges in the aquatic environment disrupt the homeostasis mechanisms of many teleost fishes. Induction of stress affects the circulating levels of catecholamine and has an impact on development and reproduction. It is not known how osmotic and hypoxic stress could affect the catecholamine and serotonin levels in zebrafish despite its well-known action in the vertebrate brain. This study thus investigates how serotonin (5-HT), epinephrine (E), norepinephrine (NE), and dopamine (DA) in the brain of female zebrafish respond to hypoxic (air) and osmotic conditions (salinity of 10 ppt). Analysis of zebrafish brain utilizing HPLC with PDA detector using reverse-phase PrimeSep column indicated that osmotic stress, air response and its combination modified 5-HT, NE and E levels. The tested stressors elevated 5-HT (>2.8 μM) while lowering NE (<3.00 μM) and E (<1.02 μM) levels in the brain as opposed to exposure to non-stressed fish. In addition, reproductive markers such as vitellogenin (Vtg1) and estrogen receptor (ERα) mRNA expression in the brain were up-regulated after osmotic stress, whereas air exposure down-regulated ERα mRNA expression but up-regulated Vtg1 compared to non-stressed fish. Overall, the data indicate that acute osmotic stress and air exposure that lowered catecholamine E and NE and elevated 5-HT levels could up-regulate mRNA expression of ERα and Vtg1 genes in the zebrafish brain, thus presenting evidence for a role of neurotransmitters on reproductive signals during acute conditional stress in the brain of wild zebrafish.