The Role Of Serotonin In The Intestinal Damage And Mechanism Of Digestion In The Daphnia Magna

Cadmium(Cd)is a common toxic pollutant in surface water.Cadmium has the characteristics of difficult degradation,easy transformation and easy accumulation,which has caused severe damage to aquatic ecosystems.Daphnia magna,a rapidly reproducing zooplankton,has been extensively studied in ecotoxicology due to its sensitivity to exogenous chemicals.In this experiment,three cadmium concentration groups(0.01,0.05,and 0.09 mg/L),one blank control group,and two treatment times(24 h,48 h)were set up.The light and electron microscopy techniques were used to observe the changes in the microscopic and sub-microscopic structures of intestinal tissues after cadmium stress at different concentrations from the histopathological level.The filling rate and digestibility,amylase,lipase and trypsin activity,5-HT and 5-HIAA content,and TPH activity;Finally,real-time quantitative PCR was used to detect the m RNA expression of SERT,5-HT1 A receptor,and DDC related to feeding and digestion.The experimental results are as follows:(1)Microstructural observations showed that when the cadmium concentration was 0.01 and 0.05 mg/L,the intestinal epithelial cells were slightly loosely arranged,and the striated margins were expanded and thickened.When the cadmium concentration was 0.09 mg/L,the borderline of intestinal epithelial cells was blurred,and the intercellular spaces were broken.The striate margin was detached and vacuolated.The sub-microstructure showed that when treated with 0.01 and 0.05 mg/L cadmium,the microvilli intestine was loosely arranged and begin to deform and partially dissolve.Mitochondria appeared swelled and the internal ridge structure was loose,resulting in ridge breakage and vacuole area.Membrane cells were not tightly connected.When treated with 0.09 mg/L high concentration of cadmium,the intestinal microvilli were severely damaged,the arrangement was disordered,and dissolution,rupture,and even disappeared.Mitochondria were severely damaged,the double-layermembrane structure was incomplete,the mitochondrial mites was dissolved and disappeared,and cavitation was severe.Membrane mosaic connection gap expansion.This showed that cadmium had a damaging effect on the intestinal tissue structure of Daphnia magna,and As the concentration increased,the damage became more pronounced.(2)With the prolongation of cadmium stress,the intestinal filling rate and digestion rate increased and then decreased of Daphnia magna.At 48 h of cadmium,the filling rate was significantly higher than that of the control group at a concentration of 0.01 mg/L(P<0.05);the digestibility was significantly higher than that of the control group at a concentration of 0.09mg/L(P<0.01).With the increase of cadmium concentration,that amylase activity was significantly decreased(P<0.01),compared with that of the control group,and dose response was observed.Lipase activity showed a phenomenon of "low promotion and high suppression".Trypsin activity was significantly increased at a concentration of 0.01 mg/L for 24 h(P<0.05).When cadmium was treated for 48 h,the concentration of each group decreased significantly(P<0.01).(3)With the increase of cadmium concentration,the content of 5-HT and the activity of TPH were significantly or extremely significantly increased(P<0.05,P<0.01)in the cadmium concentration at 0.05 and 0.09mg/L treatment groups.After 48 h of cadmium treatment,5-HIAA content was significantly lower than that of the control group(P<0.01),and there was a dose-response relationship with cadmium concentration.Under treatment of0.09 mg/L cadmium concentration,the expression of SERT m RNA was significantly or extremely significantly lower than the control group(P<0.05,P<0.01).The expression of 5-HT1 A receptor m RNA was significantly higher than that of the control group at a concentration of 0.01 and 0.05 mg/L cadmium(P<0.01).The expression of DDC m RNA decreased first and then increased.The following conclusions are drawn from the comprehensive experimental results:(1)Cadmium stress changed the microscopic and submicroscopic structure of intestinal tissue of Daphnia magna.This showed that cadmium had obvious damage to the intestine structure of Daphnia magna,and this damage became more obvious with increasing concentration.(2)Low-concentration cadmium treatment promoted feeding of Daphnia magna,while high-concentration cadmium treatment promoted intestinal digestion,and also affected activities of amylase,lipase,and trypsin.With the increased of cadmium concentration and prolonged stress time,the difference from the control group was more obvious,indicating that cadmium had a toxic effect on the feeding and digestive processes of Daphnia magna.(3)After cadmium stress,abnormal changes in 5-HT and 5-HIAA content,TPH activity,SERT,5-HT1 A and DDC m RNA expression related to feeding and digestion.This indicated that the serotonin signaling system can affect the feeding and digestion of Daphnia magna by cadmium from physiological and transcriptional levels.