Impact of temperature on sea bass, Dicentrarchus labrax, retina: Fatty acid composition, expression of rhodopsin and enzymes of lipid and melatonin metabolism

Abstract

Teleost fish are ectothermic vertebrates. Their metabolism, physiology and behavior rely on the external temperature. This study, on the retina of the sea bass Dicentrarchus labrax, reports on the impact of temperature on the fatty acid composition and mRNA abundance of key enzymes of lipid metabolism: fatty acid desaturase-2 (FADS2), fatty acid elongase-5 (ELOVL5), sterol regulatory element-binding protein-1 (SREBP-1), triglyceride lipase and phospholipase A2 (PLA2). We also report on the effects on the photopigment molecule rhodopsin and on enzymes of the melatonin synthesis pathway, namely arylalkylamine N-acetyltransferases 1a and 1b and acetylserotonin methyltransferase. Juvenile fish were placed for 30 days at 18, 23 or 28 °C. At 23 °C, the fatty acid composition of D. labrax retina showed, as generally reported for the retina of other fish species, particularly high amounts of docosahexaenoic (DHA), palmitic and oleic acids. The fatty acids composition was not significantly (P > 0.05) altered between 23 and 28 °C, but did increase at 18 °C compared to 23 and 28 °C. At 18 °C there were noticeable increases in total DHA, ecosapentaenoic, arachidonic, oleic, linoleic, palmitoleic and stearic acids. A negative correlation was found in the abundance of neutral (NL) vs. polar (PL) lipids: 18 °C induced an increase in NL and a decrease in PL, while 28 °C induced higher PL with decreased NL. In NL the changes affected mainly triglycerides. FADS2 and ELOVL5 mRNA abundance decreased from 18° to 28 °C while SREBP-1 and triglyceride lipase mRNA remained stable. Conversely PLA2 mRNA was more abundant at 23 than at 18 and 28 °C. Temperature increased and decreased rhodopsin mRNA abundance, at 28 °C and 18 °C respectively, while there was no effect on mRNA from the melatonin synthesis enzymes. In conclusion the data indicate a temperature induced redistribution of fatty acids among the lipid classes that might affect the physical properties of the plasma membranes as well as functions associated with photoreception or generation of intracellular second messengers. In addition, the results suggest that temperature targets only the proteins and activities of retinal melatonin production. This study opens new lines of investigation related to the role temperature and fatty acids play in fish visual perception. They are relevant in the context of the global warming of seas affecting both the wild and the aquaculture species.

Keywords: Lipid classes; Melatonin; Retina; Rhodopsin; Teleost fish; Temperature.