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

Melatonin receptors in the rabbit spinal cord were studied. Using in vitro quantitative autoradiography we have localized and characterized 2-[125I]iodomelatonin ([125I]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 (Kd) of 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 [125I]MEL binding sites were highly specific for melatonin. Coincubation with 10 microM or 50 microM guanosine 5'-O-(3-thiotriphosphate) produced a significant change in Kd. These results suggest that melatonin receptors in the rabbit spinal cord are coupled to a guanine-nucleotide-binding protein (G-protein). Our studies suggest that melatonin exerts a direct action on the rabbit spinal cord.