title: Melatonin receptors in brain areas and ocular tissues of the teleost Tinca tinca: characterization and effect of temperature.
abstract: The aim of the present study was to characterize the central melatonin receptors in brain areas and ocular tissues of the teleost Tinca tinca. We investigated the temperature-dependence of 2-iodo-melatonin ([(125)I]Mel) binding in the optic tectum-tegmentum area and the neural retina. The binding of [(125)I]Mel showed a widespread distribution in brain and ocular tissues, with the highest density in the optic tectum-thalamus and the lowest in hindbrain. The [(125)I]Mel affinity was similar in all the studied tissues, and it was on the order of the low pM range. Saturation, kinetic and pharmacological studies showed the presence of a unique MT(1)-like melatonin binding site. In addition, the non-hydrolysable GTP analog, the GTPgammaS, and sodium cations induced a specific binding decrease in the optic tectum and neural retina, suggesting that such melatonin binding sites in the tench are coupled to G protein. Thus, these melatonin binding sites in optic tectum and neural retina fulfil the requirements of a real hormone receptor, the specific binding is rapid, saturable, and reversible, and is inhibited by GTP analogs. Additionally, a clear effect of temperature on such central melatonin receptors was found. Temperature did not modify the B(max) and K(d), but the kinetics of [(125)I]Mel binding resulted in a highly thermosensitive process in both tissues. Both association and dissociation rates (K(+1) and K(-1)) significantly increased with assay temperature (15-30 degrees C), but the K(d) constant (estimated as K(-1)/K(+1)) remained unaltered. In conclusion, this high thermal dependence of the melatonin binding to its receptors in the tench central nervous system supports the conclusion that temperature plays a key role in melatonin signal transduction in fish.
authors.affiliation: Departamento de Fisiología (Fisiología Animal II), Facultad de Biología, Universidad Complutense, 28040 Madrid, Spain.
authors.forename: M A
authors.initials: MA
authors.lastname: López Patiño
authors.forename: A L
authors.initials: AL
authors.lastname: Alonso-Gómez
authors.forename: A
authors.initials: A
authors.lastname: Guijarro
authors.forename: E
authors.initials: E
authors.lastname: Isorna
authors.forename: M J
authors.initials: MJ
authors.lastname: Delgado
mesh_terms: D000818:Animals; D001665:Binding Sites; D001831:Body Temperature / Q000502:physiology; D001921:Brain / Q000378:metabolism*; D003530:Cyprinidae / Q000378:metabolism*; D005123:Eye / Q000378:metabolism*; D005260:Female; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics; D008297:Male; D008550:Melatonin / Q000378:metabolism / Q000493:pharmacokinetics; D011485:Protein Binding / Q000187:drug effects; D044094:Receptors, Melatonin / Q000378:metabolism*; D012964:Sodium / Q000494:pharmacology; D013696:Temperature
chemical_list: D007457:Iodine Radioisotopes; D044094:Receptors, Melatonin; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D012964:Sodium; D008550:Melatonin