title: Effect of cortisol on melatonin production by the pineal organ of tilapia, Oreochromis mossambicus.
abstract: The aim of the present study was to examine the involvement of cortisol on melatonin synthesis in the pineal organ of the Mozambique tilapia, Oreochromis mossambicus. The circulating levels of melatonin in this species exhibited daily variations with a decrease during the photophase (0600, 1200, and 1800 h) and an increase during the scotophase (0000 h), while cortisol levels peaked during the early photophase (0600 h). The pineal organ was cultured in vitro in the dark in the presence of cortisol mimicking either stressed (100 ng/mL) or resting (10 ng/mL) concentration in tilapia. High cortisol concentration significantly reduced the levels of melatonin secreted into the medium. In the fish reared under stressful conditions, the nocturnal circulating levels of cortisol increased significantly, while melatonin did not change significantly. We detected glucocorticoid receptor (GR) transcripts in the pineal organ and a quantitative real-time PCR revealed that this receptor mRNA abundance fluctuated diurnally, increasing at 0600, 1800, and 0000 h and decreasing at 1200 h. The GR mRNA abundance in the pineal organ was not altered either in vitro when the organ was cultured in the presence of 100 ng/mL cortisol or in vivo when the fish were reared under stressful conditions. On the basis of these findings, it is proposed that cortisol lowers melatonin synthesis in the pineal organ, while the role of GR signaling in this process remains to be established.
authors.affiliation: Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, 3422 Sesoko, Motobu, Okinawa 905-0227, Japan.
authors.forename: Yoshiaki
authors.initials: Y
authors.lastname: Nikaido
authors.forename: Neelakanteswar
authors.initials: N
authors.lastname: Aluru
authors.forename: Alison
authors.initials: A
authors.lastname: McGuire
authors.forename: Yong-Ju
authors.initials: YJ
authors.lastname: Park
authors.forename: Mathilakath M
authors.initials: MM
authors.lastname: Vijayan
authors.forename: Akihiro
authors.initials: A
authors.lastname: Takemura
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D002940:Circadian Rhythm / Q000235:genetics / Q000502:physiology; D015870:Gene Expression / Q000187:drug effects; D006854:Hydrocortisone / Q000097:blood / Q000494:pharmacology*; D008550:Melatonin / Q000097:blood / Q000378:metabolism*; D010870:Pineal Gland / Q000187:drug effects* / Q000378:metabolism; D011965:Receptors, Glucocorticoid / Q000235:genetics / Q000378:metabolism; D013312:Stress, Physiological / Q000502:physiology; D017210:Tilapia / Q000097:blood / Q000235:genetics / Q000378:metabolism*; D014018:Tissue Distribution
chemical_list: D011965:Receptors, Glucocorticoid; D008550:Melatonin; D006854:Hydrocortisone