title: Stress inhibition of melatonin synthesis in the pineal organ of rainbow trout (Oncorhynchus mykiss) is mediated by cortisol.
abstract: Cortisol has been suggested to mediate the effect of stress on pineal melatonin synthesis in fish. Therefore, we aimed to determine how pineal melatonin synthesis is affected by exposing rainbow trout to different stressors, such as hypoxia, chasing and high stocking density. In addition, to test the hypothesis that cortisol is a mediator of such stress-induced effects, a set of animals were intraperitoneally implanted with coconut oil alone or containing cortisol (50 mg kg(-1) body mass) and sampled 5 or 48 h post-injection at midday and midnight. The specificity of such effect was also assessed in cultured pineal organs exposed to cortisol alone or with the general glucocorticoid receptor antagonist, mifepristone (RU486). Stress (in particular chasing and high stocking density) affected the patterns of plasma and pineal organ melatonin content during both day and night, with the greatest reduction occurring at night. The decrease in nocturnal melatonin levels in the pineal organ of stressed fish was accompanied by increased serotonin content and decreased AANAT2 enzymatic activity and mRNA abundance. Similar effects on pineal melatonin synthesis to those elicited by stress were observed in trout implanted with cortisol for either 5 or 48 h. These data indicate that stress negatively influences the synthesis of melatonin in the pineal organ, thus attenuating the day-night variations of circulating melatonin. The effect might be mediated by increased cortisol, which binds to trout pineal organ-specific glucocorticoid receptors to modulate melatonin rhythms. Our results in cultured pineal organs support this. Considering the role of melatonin in the synchronization of daily and annual rhythms, the results suggest that stress-induced alterations in melatonin synthesis could affect the availability of fish to integrate rhythmic environmental information.
authors.affiliation: Laboratorio de Fisioloxía Animal, Departamento de Bioloxía Funcional e Ciencias da Saúde, Facultade de Bioloxía, Universidade de Vigo, 36310 Vigo, Spain.
authors.forename: Marcos A
authors.initials: MA
authors.lastname: López-Patiño
authors.forename: Manuel
authors.initials: M
authors.lastname: Gesto
authors.forename: Marta
authors.initials: M
authors.lastname: Conde-Sieira
authors.forename: José L
authors.initials: JL
authors.lastname: Soengas
authors.forename: Jesús M
authors.initials: JM
authors.lastname: Míguez
mesh_terms: D000818:Animals; D047068:Arylalkylamine N-Acetyltransferase / Q000235:genetics / Q000378:metabolism; D029941:Fish Proteins / Q000235:genetics / Q000378:metabolism; D006854:Hydrocortisone / Q000097:blood / Q000378:metabolism*; D006897:Hydroxyindoleacetic Acid / Q000378:metabolism; D008550:Melatonin / Q000097:blood / Q000378:metabolism*; D017686:Oncorhynchus mykiss / Q000502:physiology*; D010870:Pineal Gland / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012701:Serotonin / Q000378:metabolism; D013312:Stress, Physiological / Q000502:physiology*
chemical_list: D029941:Fish Proteins; D012333:RNA, Messenger; D012701:Serotonin; D006897:Hydroxyindoleacetic Acid; D047068:Arylalkylamine N-Acetyltransferase; D008550:Melatonin; D006854:Hydrocortisone
keywords: AANAT2; Cortisol; Melatonin; Pineal organ; Rainbow trout; Stress