2-phenylmelatonin and 2-iodomelatonin

The aim of this study was to characterize 2-[125I]iodomelatonin binding sites in the neural retina and central nervous system (telencephalon, diencephalon, and optic tectum) of the anuran amphibian Rana perezi. Saturation and kinetic studies and pharmacological characterization revealed the existence of a unique melatonin-binding site that belongs to the Mel 1 receptor subtype. The affinity of this site is similar in all tissues studied (Kd, 10.5-12.8 pM), but the density varied from diencephalon and optic tectum, which exhibit the highest density, to telencephalon with the lowest. Neural retina showed an intermediate receptor density. This melatonin-binding site fulfills the requirements of a real hormone receptor; the binding is saturable, reversible, and inhibited by different melatonin agonists and antagonists. The affinity order of ligands is: 2-phenyl-melatonin = 2-I-melatonin > 6-Cl-melatonin = melatoninz >> luzindole. Additionally, specific binding is decreased by non-hydrolysable GTP analogue, sodium, and by pretreatment of membranes with pertussis toxin. All these results suggest the existence of a widely distributed and pharmacologically homogeneous melatonin receptor of the subfamily Mel 1 in the nervous system of Rana perezi coupled to a Gi/o protein.

Topics: Animals; Binding Sites; Binding, Competitive; Brain; Cell Membrane; Diencephalon; Guanosine 5'-O-(3-Thiotriphosphate); Intracellular Membranes; Kinetics; Magnesium; Melatonin; Pertussis Toxin; Radioligand Assay; Ranidae; Receptors, Melatonin; Retina; Sodium; Subcellular Fractions; Superior Colliculi; Telencephalon; Tetrahydronaphthalenes; Tryptamines

Two 11C-labelled melatonin derivatives, 2-iodo-[11C]melatonin (2-iodo-5-methoxy-N[11C-acetyl]-tryptamine, an agonist) and 2-phenyl-[11C]melatonin (2-phenyl-5-methoxy-N[11C-acetyl]tryptamine, a putative antagonist) were synthesized from [11C]carbon dioxide. The reaction sequence was common to both compounds and consisted of three steps: (i) carbonylation of methyl magnesium bromide with [11C]carbon dioxide, (ii) conversion of the adduct to [11C]acetyl chloride, (iii) acetylation of the amine precursors (2-iodo-5-methoxy-tryptamine or 2-phenyl-5-methoxy-tryptamine) with [11C]acetyl chloride. The precursors were especially prepared. The radiochemical yield was 19% for 2-iodomelatonin and 32% for 2-phenymelatonin, based on [11C]carbon dioxide; the specific activity ranged from 300 to 600 mCi/mumol. Both labelled 2-substituted-melatonins are intended to be used as radiotracers to study melatonin binding sites in man with positron emission tomography.

Topics: Acetylation; Binding Sites; Carbon Radioisotopes; Humans; Indicators and Reagents; Mass Spectrometry; Melatonin; Nuclear Magnetic Resonance, Biomolecular; Receptors, Cell Surface; Receptors, Cytoplasmic and Nuclear; Receptors, Melatonin; Tomography, Emission-Computed