Picomolar-affinity binding and inhibition of adenylate cyclase activity by melatonin in syrian hamster hypothalamus

The effect of melatonin on forskolin-stimulated adenylate cyclase activity was measured in homogenates of Syrian hamster hypothalamus. In addition, the saturation binding characteristics of the melatonin receptor ligand, [125I]iodomelatonin, was examined using an incubation temperature (30°C) similar to that used in enzyme assays.At concentrations ranging from 10 pM to 1 nM, melatonin caused a significant decrease in stimulated adenylate cyclase activity with a maximum inhibition of ∼22%.Binding experiments utilizing [125I]iodomelatonin in a range of ∼5-80 pM indicated a single class of high-affinity sites:Kd = 55 ± 9 pM,Bmax = 1.1 ± 0.3 fmol/mg protein.The ability of picomolar concentrations of melatonin to inhibit forskolinstimulated adenylate cyclase activity suggests that this affect is mediated by picomolar-affinity receptor binding sites for this hormone in the hypothalamus.