title: Melatonin binding sites in the brain of European sea bass (Dicentrarchus labrax).
abstract: Characteristics, day-night changes, guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) modulation, and localization of melatonin binding sites in the brain of a marine teleost, European sea bass Dicentrarchus labrax, were studied by radioreceptor assay using 2-[(125)I]iodomelatonin as a radioligand. The specific binding to the sea bass brain membranes was rapid, stable, saturable and reversible. The radioligand binds to a single class of receptor site with the affinity (Kd) of 9.3 +/-0.6 pM and total binding capacity (Bmax) of 39.08 +/-0.86 fmol/mg protein (mean+/-SEM, n=4) at mid-light under light-dark (LD) cycles of 12:12. Day-night changes were observed neither in the Kd nor in the Bmax under LD 12:12. Treatment with GTPgammaS significantly increased the Kd and decreased the Bmax both at mid-light and mid-dark. The binding sites were highly specific for 2-phenylmelatonin, 2-iodomelatonin, melatonin, and 6-chloromelatonin. Distribution of melatonin binding sites in the sea bass brain was uneven: The Bmax was determined to be highest in mesencephalic optic tectum-tegmentum and hypothalamus, intermediate in telencephalon, cerebellum-vestibulolateral lobe and medulla oblongata-spinal cord, and lowest in olfactory bulbs with the Kd in the low picomolar range. These results indicate that melatonin released from the pineal organ and/or retina plays neuromodulatory roles in the sea bass brain via G protein-coupled melatonin receptors.
authors.affiliation: Department of Physiology and Pharmacology, Faculty of Biology, University of Murcia, 30100 Espinardo, Murcia, Spain.
authors.forename: María José
authors.initials: MJ
authors.lastname: Bayarri
authors.forename: Rosa
authors.initials: R
authors.lastname: Garcia-Allegue
authors.forename: José Antonio
authors.initials: JA
authors.lastname: Muñoz-Cueto
authors.forename: Juan Antonio
authors.initials: JA
authors.lastname: Madrid
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Tabata
authors.forename: F Javier
authors.initials: FJ
authors.lastname: Sánchez-Vázquez
authors.forename: Masayuki
authors.initials: M
authors.lastname: Iigo
mesh_terms: D000818:Animals; D001492:Bass / Q000378:metabolism* / Q000502:physiology; D001665:Binding Sites / Q000502:physiology; D001921:Brain / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D007457:Iodine Radioisotopes; D007700:Kinetics; D008550:Melatonin / Q000378:metabolism*; D017440:Photoperiod; D011869:Radioligand Assay
chemical_list: D007457:Iodine Radioisotopes; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D008550:Melatonin