Localization and characterization of melatonin receptors in the rabbit spinal cord.

Abstract Melatonin receptors in the rabbit spinal cord were studied. Using in vitro quantitative autoradiography we have localized and characterized 2-[ 125 I]iodomelatonin ([ 125 I]MEL) binding sites in the central gray substance (lamina X) of the rabbit spinal cord. Saturation study revealed a single class of high affinity binding sites in the central gray substance with an equilibrium dissociation constant ( K d ) o 38.8 ± 5.25 pM and a maximum number of binding sites of 5.69 ± 0.84 fmol/mg protein in the mid-light period. These [ 125 I]MEL binding sites were highly specific for melatonin. Coincubation with 10 μM or 50 μM guanosine 5′- O -(3-thiotriphosphate) produced a significant change in K d . These results suggest that melatonin receptors in the rabbit spinal cord are coupled to a guanine-nucleotide-bind protein (G-protein). Our studies suggest that melatonin exerts a direct action on the rabbit spinal cord.