The digestive function of gilthead seabream juveniles in relation to feeding frequency

A better knowledge of daily digestive function may help to improve the feeding efficiency in farmed fish species. In this study, daily expression profiles of key genes related to the circadian system (arntl1, clock, per3 and cry1), nutrients hydrolysis (pga, atp4a, try, ctrb, cel1b, pla2, and amy2a), appetite control and food intake (ghrl and cck), and nutrient transport (slc15a1) were assessed in the gastrointestinal tract (GIT) of gilthead seabream (Sparus aurata) juveniles, under four different daily feeding frequencies (one, three, five diurnal meals and continuous diurnal feeding). Additionally, the activity of two key proteolytic enzymes (pepsin and trypsin) and variations in luminal pH within the different GIT segments were determined. Clock genes showed significant circadian patterns, being highly synchronized to the light-dark cycle and first daily meal in all the groups. In contrast, most of the genes codifying digestive enzyme precursors did not exhibit relevant changes in expression along the daily cycle. A correlation was detected between the expression of slc15a1 and per3 and, to a lesser extent between arntl1/clock and ghrl, supporting the role of clock genes in daily organization of some digestive functions. Feeding frequency had a poor influence on the expression of all the analysed genes. In contrast to the results obtained for the expression of genes encoding digestive enzymes, the proteolytic activities exhibited circadian rhythms, and their temporal distribution were significantly affected by some of the experimental feeding protocols, being mostly synchronized with the presence of food in the corresponding GIT segment. Fish fed three times a day did not exhibit a significant daily rhythm for trypsin activity, but the daily average of activated trypsin was similar in all feeding frequencies. Gastric pH was maintained neutral, and acidification occurred only after food ingestion although showing different daily pattern for each feeding protocol. Overall, these results indicate that in gilthead seabream the majority of modulations for the hourly profile of digestive functionality occur post-transcriptionally to adapt to different feeding frequencies.