Guanosine 5´-O-(3-Thiotriphosphate) and Cations Regulate Melatonin Receptors, and Melatonin Inhibits Cyclic AMP Production in the Spinal Cord

Effects of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and cations on 2-[125I]iodomelatonin binding were investigated in membrane preparations of the chicken spinal cord. At concentrations of 10 and 50 mumol/l, GTP gamma S dose-dependently increased (p < 0.05) the equilibrium dissociation constant (Kd) and depressed (p < 0.05) the maximum number of binding sites (Bmax). Na+ at a concentration of 125 mmol/l significantly increased (p < 0.05) the Kd and decreased (p < 0.05) the Bmax, and Mg2+ (2.5 mmol/l) significantly increased (p < 0.05) the Bmax without changes in Kd. In addition, Na+ and Mg2+ affected the interactions of GTP gamma S with melatonin receptors. In the spinal cord explants, melatonin (10 nmol/l) attenuated forskolin-stimulated cyclic AMP production by 53.1%, and preincubation with pertussis toxin abolished this effect of melatonin. These results suggest that the melatonin receptors in the chicken spinal cord are linked to its second messenger via a pertussis-toxin-sensitive guanine-nucleotide-binding protein, and that cations modulate these receptors. Our studies further support a previous hypothesis that melatonin exerts a direct action on spinal cord functions.