RESUMEN
This research aimed at investigating circadian rhythm expression of key genes involved in lipid metabolism in the liver of a teleost fish (Sparus aurata), and their synchronisation to different light-dark (L-D) and feeding cycles. To this end, 90 gilthead sea bream were kept in 12:12 h (light:dark, LD, lights on at ZT0) and fed a single daily meal at mid-light (ML = ZT6), mid-darkness (MD = ZT18) and randomly (RD) at a 1.5% body weight ration. A total of 18 tanks were used, six tanks per feeding treatment with five fishes per tank; locomotor activity was recorded in each tank. After 25 days of synchronisation to these feeding regimes, fishes were fasted for one day and liver samples were taken every 4 hours during a 24 h cycle (ZT2, 6, 10, 14, 18 and 22) and stored at -80 °C until analysis. To determine whether the rhythm expression presented an endogenous control, another experiment was performed using 30 fish kept in complete darkness and fed randomly (DD/RD). Samples were taken following the same procedure as above. The results revealed that all genes investigated exhibited well defined daily rhythms. The lipolysis-related and fatty acid turnover genes (hormone-sensitive lipase (hsl) and peroxisome proliferator-activated receptor-α (pparα)) exhibited a nocturnal achrophase (Ø = ZT18:03-19:21); lipoprotein lipase (lpl) also showed the same nocturnal achrophase (Ø = ZT20:04-21:36). In contrast, lipogenesis-related gene, fatty acid synthase (fas), and of fatty acid turnover, cyclooxygenase (cox-2), showed a diurnal rhythm (Ø = ZT2:27-8:09); while pparγ was nocturnal (Ø = ZT16:16-18:05). Curiously, feeding time had little influence on the phase of these daily rhythms, since all feeding groups displayed similar achrophases. Furthermore, under constant conditions pparα and hsl showed circadian rhythmicity. These findings suggest that lipid utilisation in the liver is rhythmic and strongly synchronised to the LD cycle, regardless of feeding time, which should be taken into consideration when investigating fish nutrition and the design of feeding protocols.