Dietary protein source and protein/carbohydrate ratio affects appetite regulation-related genes expression in gilthead seabream (Sparus aurata)

This study aimed to evaluate the effect of dietary protein source (fishmeal, FM; or plant-feedstuffs, PF) and dietary protein/carbohydrate (P/CH) ratio on gilthead seabream appetite regulation and intermediary metabolism. Additionally, the effect of sampling 5 h after feeding (AF) compared to 24 h AF was also evaluated. Four isolipidic diets were formulated having as major protein sources FM or PF (20% FM and 80% PF), and P/CH ratios of 50/10 or 40/20, being the pregelatinized maize starch the main carbohydrate source (diets FM-P50/CH10; FM-P40/CH20; PF-P50/CH10; PF-P40/CH20). Diets were fed until satiation to 140 g gilthead seabream for 41 days. The expression of appetite regulation genes was assessed at 5 and 24 h AF, while other evaluated parameters were assessed only at 5 h AF. Liver leptin expression was higher at 5 h AF, and brain leptin receptor (lepr) expression was higher at 24 h AF. Brain expression of cocaine- and amphetamine-regulated transcript (cart), leptin and ghrelin receptor (ghrr)-a and liver ghrr-b were also affected by sampling time, but the effects were dependent of the diet provided. FM-based diets promoted the expression of brain cart and leptin (at 24 h AF), and liver growth hormone receptor (ghr)-ii, and increased plasma cholesterol and total lipids levels. Fish fed the PF-based diets had higher liver glycogen content, number and size of adipocytes, and expression of hepatic leptin (at 24 h AF), fatty acid synthase, glucokinase, and target of rapamycin. Regarding dietary P/CH ratio, fish fed the P50/CH10 diets presented higher feed efficiency, plasma triglycerides, and expression of intestine cholecystokinin (at 5 h AF), liver ghrr-b (at 24 h AF), glutamate dehydrogenase and ghr-ii. The protein efficiency ratio, hepatosomatic and visceral indices, plasmatic glucose level, and brain lepr expression (at 5 h AF) were higher in fish fed the P40/CH20 diets. The majority of appetite regulation related-genes were not affected by the use of PF-based diets, while the higher dietary CH seemed to lead to a shorter satiety sensation. PF-based diets promoted liver lipid deposition, hypocholesterolemia, and the activation of glycogenesis pathway, while higher CH content induced an increase in plasma glucose that appeared to be stored as lipids. In conclusion, PF-based diets with up to 20% of CH can be used in gilthead seabream without compromising growth performance and FI, and only slightly modifying appetite and metabolic parameters.