Effects of water salinity on melatonin levels in plasma and peripheral tissues and on melatonin binding sites in European sea bass (Dicentrarchus labrax)

Abstract

Sea bass is a euryhaline fish that lives in a wide range of salinities and migrates seasonally from lagoons to the open sea. However, to date, the influence of water salinity on sea bass melatonin levels has not been reported. Here, we evaluated the differences in plasma and tissue melatonin contents and melatonin binding sites in sea bass under four different salinities: seawater (36 ‰), isotonic water (15 ‰), brackish (4 ‰) and freshwater (0 ‰). Melatonin content was evaluated in plasma, whole brain, gills, intestine and kidney, while melatonin binding sites were analyzed in different brain regions and in the neural retina. Plasma melatonin levels at mid-dark varied among salinities, with the lowest value occurring at seawater salinity (102 pg/ml), and the highest at freshwater (151 pg/ml). In gills and intestine, however, the highest melatonin values were found in the seawater group (209 and 627 pg/g tissue, respectively). Melatonin binding sites in the brain also varied with salinity, with the highest density being observed at the lower salinities in optic tectum, cerebellum and hypothalamus (30.3, 13.0, and 8.0 fmol/mg protein, respectively). Melatonin binding sites in the retina showed a similar pattern, with the highest values in the fish maintained in freshwater. Taken together, these results revealed that salinity influences melatonin production and modifies the density of binding sites, which would point to a role for this hormone in timing seasonal events in sea bass, including those linked to fish migration between waters of different salinities.