Combined effects of crowding stress and low salinity on GH/IGF axis, antioxidant response, and HPI axis in largemouth bass (Micropterus salmoides) larvae

The present study aimed to determine if low salinity could mitigate negative effects of crowding stress on freshwater largemouth bass (Micropterus salmoides). To the end, larvae were subjected to crowding stress (CS) or low salinity (LS, 0.5‰), alone or in combination for 28 days. Growth, antioxidant capacity and stress response were investigated. CS significantly reduced body weight, total antioxidant capacity (TAC) and catalase (CAT) activity in the brain and head kidney, TAC and superoxide dismutase (SOD) activity in the liver, and increased cortisol levels in the whole body, suggesting negative effects of CS on fish. Contrarily, LS promoted body weight, body length and condition factor, and reduced MDA contents in the liver and head kidney and whole-body cortisol levels. Increases in TAC and CAT activity in the brain, SOD and CAT activities in the liver, and TAC, SOD and CAT activities in the head kidney were also observed in the larvae subjected to LS. The results indicated that LS promoted growth and antioxidant capacity and reduced stress response, which was also observed in fish subjected to CS. In order to elucidate the related molecular mechanisms, gene expression in the growth hormone/insulin-like growth factor-I (GH/IGF) axis, NF-E2-related factor 2 (Nrf2) pathway, and hypothalamus–pituitary–interrenal (HPI) axis were examined. We found that CS + LS up-regulated growth hormone release hormone gene (ghrh), growth hormone gene (gh), and hormone receptor gene (ghra) in the brain and ghra and insulin-like growth factor genes (igf1 and igf2b) in the liver compared with CS alone. The mRNA levels of Nrf2 pathway related genes were sharply enhanced by CS but were maintained relative constant by CS + LS in the brain and liver compared with the control. In the head kidney, CS + LS up-regulated Nrf2 pathway related genes compared with CS alone. HPI axis related genes tended to be up-regulated by CS but remain unchanged by CS + LS compared with the control. LS and CS + LS down-regulated both genes encoding cytochrome P450 enzymes (cyp17a1 and cyp11c1) which are rate-limiting enzymes for cortisol synthesis compare with the control and CS, respectively. Taken together, the present results for the first time provide a feasible suggestion for culture of largemouth bass larvae at high stocking density in saline waters.