title: Specific binding of 2-[125I]iodomelatonin by rat spleen crude membranes: day-night variations and effect of pinealectomy and continuous light exposure.
abstract: Melatonin binding sites were characterized in rat spleen crude membranes. The specific binding of 2-[125I]iodomelatonin by spleen crude membranes fulfills all the criteria for binding to a receptor site. Thus, binding was dependent on time and temperature, stable, specific, and increased under constant light exposure and after pinealectomy. In competition studies, the specific binding of 2-[125I]iodomelatonin to spleen crude membranes was inhibited by increasing concentrations of native melatonin. Scatchard analysis showed that the data were compatible with the existence of two classes of binding sites: a high affinity site with a Kd of 0.53 nM and a binding capacity of 2.52 pM, and a low-affinity site with a Kd of 374 nM and binding capacity of 820 pM. Moreover, binding of 2-[125I]iodomelatonin exhibited day-night variations with the highest binding observed late during the light period, and the lowest binding was observed late at night. However, binding of 2-[125I]iodomelatonin to membranes remained high when animals were kept under light exposure at night. Results support the hypothesis of a regulatory role of melatonin on the immune system in which melatonin downregulates its own binding site.
authors.affiliation: Department of Medical Biochemistry and Molecular Biology, University of Seville School of Medicine, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Rafii-El-Idrissi
authors.forename: J R
authors.initials: JR
authors.lastname: Calvo
authors.forename: M
authors.initials: M
authors.lastname: Giordano
authors.forename: J M
authors.initials: JM
authors.lastname: Guerrero
mesh_terms: D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002462:Cell Membrane / Q000378:metabolism; D002940:Circadian Rhythm / Q000502:physiology*; D003624:Darkness; D007457:Iodine Radioisotopes; D008027:Light; D008297:Male; D008550:Melatonin / Q000031:analogs & derivatives* / Q000378:metabolism; D010870:Pineal Gland / Q000502:physiology* / Q000601:surgery; D051381:Rats; D017208:Rats, Wistar; D011956:Receptors, Cell Surface / Q000378:metabolism; D044094:Receptors, Melatonin; D013154:Spleen / Q000276:immunology / Q000378:metabolism*; D013696:Temperature; D013997:Time Factors
chemical_list: D007457:Iodine Radioisotopes; D011956:Receptors, Cell Surface; D044094:Receptors, Melatonin; C044556:2-iodomelatonin; D008550:Melatonin