The present study conducted a 9-week feeding trial to investigate the effects of threonine (Thr) on the digestion capacity and hepatopancreas gene expression of juvenile blunt snout bream (Megalobrama amblycephala). For this purpose, three tanks (300 litres/tank) were randomly arranged and assigned to each experimental diet. Juvenile fish were fed with diets containing graded Thr levels (0·58, 1·08, 1·58, 2·08 or 2·58 % of the diet) to apparent satiation four times daily. At the end of the feeding trial, the results indicated that hepatopancreas weight, hepatosomatic index, hepatopancreatic protein content, intestinal weight, intestosomatic index and intestinal protein content increased with increasing dietary Thr levels up to 1·58 % and thereafter decreased (P< 0·05). The activities of chymotrypsin, trypsin, amylase and lipase elevated as dietary Thr levels increased up to 1·58 % (P< 0·05), while these activities decreased in most cases after 1·58 % dietary Thr except for chymotrypsin and trypsin in the hepatopancreas (plateau 1·58-2·08 % Thr). The relative gene expression levels of chymotrypsin, trypsin, amylase, lipase, target of rapamycin and insulin-like growth factor-I were up-regulated, and the highest values were observed with 1·58 % dietary Thr or 1·58 and 2·08 % dietary Thr, whereas the relative gene expression levels of eukaryotic translation initiation factor 4E-binding protein 2 gradually decreased (P< 0·10) as dietary Thr levels increased up to 1·58 % and thereafter significantly increased (P< 0·05), which could explain that about 1·58 % dietary Thr could improve the growth and development of digestive organs and activities of digestive enzymes of juvenile blunt snout bream.
Keywords: Blunt snout bream; Digestive enzyme activities; Gene expression; Target of rapamycin pathway; Threonine.