Changes in plasma melatonin levels and pineal organ melatonin synthesis following acclimation of rainbow trout (Oncorhynchus mykiss) to different water salinities

Marcos A. Lopez-Patino, Arnau Rodriguez-Illamola, Manuel Gesto, Jose L. Soengas, Jesus M. Miguez

Research output: Contribution to journalJournal articleResearchpeer-review


Melatonin has been suggested to play a role in fish osmoregulation, and in salmonids has been related to the timing of adaptive mechanisms during smolting. It has been described that acclimation to different environmental salinities alters levels of circulating melatonin in a number of fish species, including rainbow trout. However, nothing is known regarding salinity effects on melatonin synthesis in the pineal organ, which is the main source of rhythmically produced and secreted melatonin in blood. In the present study we have evaluated, in rainbow trout, the effects of acclimation to different salinities on day and night plasma melatonin values and pineal organ melatonin synthesis. Groups of freshwater (FW)-adapted rainbow trout were placed in tanks with four different levels of water salinity (FW, 6, 12, 18 p.p.t.; parts per thousand) and maintained for 6. h or 5. days. Melatonin content in plasma and pineal organs, as well as the pineal content of serotonin (5-HT) and its main oxidative metabolite (5-hydroxyindole-3-acetic acid; 5-HIAA) were measured by high performance liquid chromatography. In addition, day-night changes in pineal organ arylalkylamine N-acetyltransferase (AANAT2) activity and aanat2 gene expression were studied. Plasma osmolalities were found to be higher in rainbow trout exposed to all salinity levels compared with the control FW groups. A salinity-dependent increase in melatonin content was found in both plasma and pineal organs. This effect was observed during the night, and was related to an increase in aanat2 mRNA abundance and AANAT2 enzyme activity, both of which also occurred during the day. Also, the levels of indoles (5-HT, 5-HIAA) in the pineal organ were negatively affected by increasing water salinity, which seems to be related to the higher recruitment of 5-HT as a substrate for the increased melatonin synthesis. A stimulatory effect of salinity on pineal aanat2 mRNA expression was also identified. These results indicate that increased external salinity promotes melatonin synthesis in the pineal organ of rainbow trout by enhancing synthesis of AANAT protein independently of its regulation by light. The possibility that pineal melatonin is a target for hormones involved in the response of fish to osmotic challenge is discussed, as well as the potential role of melatonin in the timing of osmoregulatory processes.
Original languageEnglish
JournalJournal of Experimental Biology
Issue number6
Pages (from-to)928-936
Publication statusPublished - 2011
Externally publishedYes


  • Pisces Vertebrata Chordata Animalia (Animals, Chordates, Fish, Nonhuman Vertebrates, Vertebrates) - Osteichthyes [85206] Oncorhynchus mykiss species rainbow trout common salmonid common
  • Oncorhynchus mykiss aanat2 gene [Osteichthyes] expression
  • 5-hydroxyindole-3-acetic acid 54-16-0 oxidative metabolite, indole
  • aanat2 expression
  • arylalkylamine N-acetyltransferase 92941-56-5 EC
  • melatonin 73-31-4 synthesis
  • mRNA expression
  • serotonin 5-HT 50-67-9 indole
  • 03502, Genetics - General
  • 03506, Genetics - Animal
  • 07200, Circadian rhythms and other periodic cycles
  • 07508, Ecology: environmental biology - Animal
  • 07512, Ecology: environmental biology - Oceanography
  • 10062, Biochemistry studies - Nucleic acids, purines and pyrimidines
  • 10064, Biochemistry studies - Proteins, peptides and amino acids
  • 10802, Enzymes - General and comparative studies: coenzymes
  • 15002, Blood - Blood and lymph studies
  • 15004, Blood - Blood cell studies
  • 17002, Endocrine - General
  • 17012, Endocrine - Pineal
  • 17020, Endocrine - Neuroendocrinology
  • 20504, Nervous system - Physiology and biochemistry
  • Biosynchronization
  • Endocrine System
  • Enzymology
  • Marine Ecology
  • Molecular Genetics
  • adaptive mechanism
  • animal acclimation
  • circadian clock
  • enzyme activity
  • osmoregulatory process
  • smolting activity
  • water salinity
  • Biochemistry and Molecular Biophysics
  • Chemical Coordination and Homeostasis
  • Ecology, Environmental Sciences
  • blood blood and lymphatics
  • pineal organ endocrine system, nervous system
  • plasma blood and lymphatics
  • AF106006.1 GenBank, EMBL, DDJB nucleotide sequence
  • Acclimatization
  • Animals
  • Arylalkylamine N-Acetyltransferase
  • Circadian Rhythm
  • Fresh Water
  • Gene Expression Regulation, Enzymologic
  • Hydroxyindoleacetic Acid
  • Melatonin
  • Oncorhynchus mykiss
  • Osmolar Concentration
  • Pineal Gland
  • RNA, Messenger
  • Salinity
  • Serotonin
  • Sodium
  • Water
  • Animal Science and Zoology
  • Ecology, Evolution, Behavior and Systematics
  • Molecular Biology
  • Physiology
  • Insect Science
  • Aquatic Science
  • Osmoregulation
  • Pineal organ
  • Rainbow trout
  • 5 hydroxyindoleacetic acid
  • aralkylamine acetyltransferase
  • fresh water
  • melatonin
  • messenger RNA
  • serotonin
  • sodium
  • water
  • acclimatization
  • animal
  • article
  • biosynthesis
  • blood
  • chemistry
  • circadian rhythm
  • enzymology
  • gene expression regulation
  • genetics
  • metabolism
  • osmolarity
  • physiology
  • pineal body
  • rainbow trout
  • salinity
  • Salmonidae
  • PIKE
  • FISH
  • pineal organ
  • osmoregulation

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