Abstract
Affected by natural factors such as temperature, season and weather, or human factors such as eutrophication and high stocking density, oxygen concentration in the water environment where fish live is unpredictable, and they are often faced with a low-oxygen environment. Studies have found that fish have evolved a complex physiological and biochemical system to adapt to the stress response caused by hypoxia in the water environment. However, severe and acute hypoxia can cause a large number of fish to suffocate and die in a short period of time. Because of its delicious taste, no intermuscular spines and high nutritional value, the L. longirostris has become one of the important freshwater aquaculture species in China. Currently, studies have been carried out at home and abroad on the growth characteristics, product processing and nutritional evaluation of this fish. There are few reports on its important ecological impact factor "hypoxia". In order to understand the regulatory mechanism of the brain tissue in response to hypoxic stress. In this study, enzyme activity determination, H.E staining, qRT-PCR, TUNEL detection and other methods were used. Changes in brain tissue of L. longirostris hypoxia response genes, physiological and biochemical indicators and appetite genes were analyzed and compared under hypoxic stress (0.8±0.1) mg/L 0, 2, 4 and 6 hour (labeled as H0, H2, H4 and H6) and recovery (7.3±0.5) mg/L 2,4 and 6 hour (labeled as R2, R4 and R6). The results showed that: Under hypoxic stress and recovery, the expression levels of oxygen-sensing protein-related genes (HIFs, PHDs and Vhl) in the brain tissue of L. longirostris showed a trend of first rising and then falling as a whole. Respiratory metabolic enzymes (HK, PK and LDH) activities were significantly increased at H0, SDH and MDH activities were significantly reduced at H6. After reoxygenation, the metabolic mode gradually changes from anaerobic respiration to aerobic respiration. Antioxidative enzymes (GSH-Px, CAT and SOD) and stress indicators (MDA and LPO) gradually increased after 2 h of hypoxia, and oxidative stress persists after the recovery of dissolved oxygen. Through the observation of brain tissue morphology, it was found that under hypoxia stress, neuronal cell swelling and vacuoles were damaged in the brain tissue of L. longirostris, which were not effectively improved after 6 hours of reoxygenation with dissolved oxygen. However, with the prolongation of hypoxia time, the degree of apoptosis of brain tissue cells increased, and the expression of apoptosis-related genes (Bax, Caspase-3 and p53) increased significantly, while the expression of Bcl-2 gene decreased, there are still significant differences in expression compared with the control group after the recovery of dissolved oxygen. In addition, it was found that the feeding rate of L. longirostris decreased by 54% and 98% at 0 h and 2 h of hypoxia stress, respectively. Hypoxic stress was detected to significantly inhibit the expression of appetite-promoting genes (NPY) and inducible food-suppressive genes (PYY, CCK and NUCB2). This experiment showes that hypoxic stress and reoxygenation have significant effects on oxygen sensor proteins, respiratory metabolism, oxidative stress, structural morphology, apoptosis and appetite in the brain tissues of L. longirostris. The results of this study provide a theoretical basis for elucidating the molecular regulation mechanism of L. longirostris brain tissues under hypoxia stress and reoxygenation, it has guiding significance for the intensive and healthy breeding of this fish and the selection and breeding of new hypoxia-tolerant species in the future.
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