Characterization, guanosine 5'-O-(3-thiotriphosphate) modulation, daily variation, and localization of melatonin-binding sites in the catfish (Silurus asotus) brain.
Characteristics, guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) modulation, daily variation, and localization of melatonin-binding sites in the brain of a nocturnal teleost, the catfish Silurus asotus, were studied by radioreceptor assay using 2-[125I]iodomelatonin as the radioligand. The specific binding was rapid, stable, saturable, and reversible. The radioligand binds to a single class of receptor site with an affinity (Kd) of 30.7 +/- 7.3 pM and total binding capacity (Bmax) of 9.76 +/- 0.79 fmol/mg protein (mean +/- SE, n = 5). The binding sites were highly specific for 2-iodomelatonin and melatonin. The specificity was almost identical to that of functional melatonin receptors in the dermal and epidermal melanophores in this species and that of ML-1 subtype melatonin receptors in vertebrates, including melatonin-binding sites in the goldfish brain. GTPgammaS treatment altered both the Kd and Bmax values, indicating that melatonin-binding sites in the catfish brain are coupled to G protein. The Bmax values exhibited no daily variation under light-dark cycles of 12 hr light:12 hr dark whereas plasma melatonin levels and Kd fluctuated in a rhythmic fashion. The density of melatonin-binding sites in discrete brain areas was determined to be highest in optic tectum-thalamus and hypothalamus, intermediate in telencephalon, cerebellum, and medulla oblongata, and lowest in olfactory bulbs. These results suggest that melatonin secreted from the pineal organ and/or retina plays neuromodulatory roles in the catfish brain via G protein-coupled melatonin receptors. Characteristics of melatonin receptors seem to be highly conserved during evolution, although the density of melatonin receptors is not regulated by melatonin itself in this species.
Iigo
M
M
Department of Anatomy, St. Marianna University School of Medicine, Miyamae-ku, Kawasaki, 216, Japan.
Sánchez-Vázquez
F J
FJ
Hara
M
M
Ohtani-Kaneko
R
R
Hirata
K
K
Shinohara
H
H
Tabata
M
M
Aida
K
K
D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000033:anatomy & histology; D001923:Brain Chemistry / Q000187:drug effects / Q000502:physiology*; D002397:Catfishes / Q000378:metabolism*; D002940:Circadian Rhythm / Q000187:drug effects / Q000502:physiology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology*; D066298:In Vitro Techniques; D007700:Kinetics; D008029:Lighting; D008550:Melatonin / Q000097:blood / Q000378:metabolism*; D008566:Membranes / Q000378:metabolism; D011485:Protein Binding
D016244:Guanosine 5'-O-(3-Thiotriphosphate); D008550:Melatonin