[125I]melatonin binding sites in membrane preparations of quail brain: characteristics and diurnal variations

The binding of [125I]melatonin, a labelled ligand of melatonin, to quail brain membrane preparations was studied. Kinetic studies indicated that the binding of [125I]melatonin to membrane preparations of quail brains collected at mid-light was time-dependent and reversible. The Kd value determined from kinetic analysis was 614 pmol/l, similar to the Kd 705 +/- 81 pmol/l obtained from saturation experiments. The [125I]melatonin binding sites in quail brain membrane had the following order of pharmacological affinities: melatonin greater than 6-chloromelatonin greater than N-acetylserotonin greater than 5-hydroxytryptamine much greater than 5-methoxytryptophol, tryptamine, L-tryptophan, 5-hydroxytryptophan, 5-hydroxyindole-3-acetic acid, 1-acetylindole-3-carboxaldehyde, 3-acetylindole. Compounds known to act on serotonergic, adrenergic or acetylcholinergic receptors were inactive as compared with melatonin. Our results demonstrated the presence of specific melatonin binding sites in the quail brain membrane preparations. In addition, saturation studies demonstrated that [125I]melatonin binding sites in quail brain membrane preparations were 42% higher at mid-light (39.6 +/- 2.2 fmol/mg protein) than at mid-dark (22.6 +/- 2.5 fmol/mg protein), with no significant variation in their binding affinities.