Melatonin Receptor Sites in the Syrian Hamster Brain and Pituitary: Localization and Characterization Using [125|]lodomelatonin

A high-affinity, discretely localized melatonin receptor has been characterized and mapped within the brain and pituitary of the Syrian hamster using the high specific activity ligand [125|]iodomelatonin and a combination of in vitro autoradiography and membrane homogenate receptor assays. Specific binding of radioligand was found in regions of the epithalamus and hypothalamus in the brain and the pars tuberalis of the pituitary. Excitatory amino-acid lesions destroyed [125|]iodomelatonin binding within the brain, demonstrating that binding sites are located on neurons. Analysis of [125|]iodomelatonin binding to membrane homogenates of the pars tuberalis revealed a linear relationship between specific ligand binding and the amount of tissue. The time-course of specific binding at 37°C reached equilibrium after 30 min and remained stable thereafter. The addition of increasing concentrations of [125|]iodomelatonin alone and in the presence of 1 µM melatonin showed that specific binding reached equilibrium at 80 to 100 pM. Analysis of the saturation isotherm using a one-site binding model was consistent with a single receptor site with a Kd of 29.3 (±5.9 SEM) pM and Bmax of 2.54 (±0.19 SEM) fmol/mg protein.