Daily rhythms of clock gene expression, glycaemia and digestive physiology in diurnal/nocturnal European seabass.

Seabass is a fish species with dual (diurnal/nocturnal) feeding behavior, although little is known about changes in its molecular clock, physiology and metabolism linked to this dual behavior. In the research described here possible differences in clock gene expression in central (brain) and peripheral (liver) oscillators, and in physiology (blood glucose and amylase activity in mid-intestine) were studied in seabass with diurnal or nocturnal self-feeding patterns under LD 12:12h (light:dark) (lights on=Zeitgeber Time (ZT) 00:00h). The results revealed that per1 expression in brain shows daily rhythmicity with the acrophase (Φ) around the lights offset (ZT 12:00h, Cosinor, p<0.01) in both diurnal and nocturnal seabass. In liver, per 1 daily levels of expression were higher in diurnal fish (univariate GML, p<0.02). Daily blood glucose variations were observed in both groups (ANOVA I, p<0.01), with higher glucose levels occurring at night in nocturnal as well as in diurnal fish, although only diurnal seabass displayed a significant daily rhythm (Φ=ZT 16:52h, Cosinor, p<0.02). The highest values of amylase activity coincided with the feeding-phase of fish; that is, in nocturnal seabass the maximum was reached at ZT 18:00h (ANOVA I, p<0.01), whereas in diurnal seabass the Φ was ZT 03:39h (Cosinor, p<0.02). In short, our findings indicated that the feeding rhythm (diurnal vs. nocturnal) strongly influenced the daily patterns of digestive function and clock gene expression in the liver (feeding-entrained clock), but not in the brain (light-entrained clock).