Effects of intracerebroventricular injection of glucose on feeding and glucose metabolism in mandarin fish (Siniperca chuatsi)

Diets containing high levels of carbohydrate could inhibit the feeding in many fish species, but the relevant mechanisms behind this regulation are not clear. The mandarin fish (Siniperca chuatsi), a typical carnivorous fish, exhibits an apparent anorexia due to its extensive intolerance to dietary carbohydrates, which limits its value as a cultured species. To investigate the existence of a glucose sensing system in the hypothalamus of S. chuatsi and its effect on feed intake, intracerebroventricular (ICV) injections of saline (control), 2-DG (glucose metabolism antagonist, 100 mg/kg) and glucose (10 mg/kg) were performed, exploring the impacts on feeding and glucose metabolism at 3, 6 and 12 h. The results showed that ICV of glucose significantly inhibited the feed intake of mandarin fish, while the plasma glucose levels remained constant at different time points. The expression of glucokinase (gk) were significantly induced at 6 h after ICV administration of glucose, indicating the presence of a glucose-sensing system in the hypothalamus of mandarin fish. ICV administration of glucose significantly promoted the mRNA levels of the cocaine and amphetamine-regulated transcriptional peptide (cart) at 3 h , which may be related to the significant upregulation of mTOR in the hypothalamus (12 h) and liver (6 h) of mandarin fish. In addition, ICV administration of glucose significantly induced the expression of gk (6 h) and pk (3 h) in the liver of mandarin fish to promote glycolysis, suggesting the promotion of catabolism for energy supply through perceived glucose levels in the hypothalamus. In conclusion, this study demonstrates for the first time that gk in the hypothalamus can response to the glucose level, suggesting the potential existence of glucose sensing system in the hypothalamus of mandarin fish, and that ICV administration of glucose regulates the AMPK/mTOR pathway through gk-induced glucose sensing, which in turn regulates the expression of appetite-related genes to inhibit feed intake. The present study would provide a theoretical basis for research on the carbohydrate utilization and feeding regulation in mandarin fish.