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Melatonin plays a significant role in the control of the hypothalamic-pituitary-gonadal axis. Using the GT1–7 cell line, an in vitro model of GnRH-secreting neurons of the hypothalamus, we examined the potential signal transduction pathways activated by melatonin directly at the level of the GT1–7 neuron. We found that melatonin inhibits forskolin-stimulated adenosine 3′-, 5′-cyclic monophosphate accumulation in GT1–7 cells through an inhibitory G protein. Melatonin induced protein kinase C activity by 1.65-fold over basal levels, increased the phosphorylation of extracellular signal-regulated kinase 1 and 2 proteins, and activated c-fos and junB mRNA expression in GT1–7 cells. Using the protein kinase A inhibitor H-89, the protein kinase C inhibitor bisindolylmaleimide, and the mitogen-activated protein kinase kinase inhibitor PD98059, we found that the melatonin-mediated cyclical regulation of GnRH mRNA …