NMDA and nitric oxide act through the cGMP signal transduction pathway to repress hypothalamic gonadotropin-releasing hormone gene expression

The key roles of the excitatory neurotransmitter glutamate and its second messengers, nitric oxide (NO) and cGMP, in long-term potentiation and neural plasticity are well documented. However, complex functions such as memory are likely to require long term changes in synaptic efficacy which require gene expression and protein synthesis, Here we demonstrate that the glutamate receptor agonist, N-methyl-D-aspartic acid (NMDA), nitric oxide (NO) and cGMP each repress expression of the gonadotropin-releasing hormone (GnRH) gene in the hypothalamic cell line, GT1, This repression is dependent upon signals from NMDA receptors activating NO synthase to synthesize NO, In turn NO induces guanylyl cyclase to synthesize cGMP, activating cGMP-dependent protein kinase, Repression requires elevation of calcium because it only occurs in the presence of a calcium ionophore or with release of intracellular calcium, Repression also requires protein synthesis, Activation of this pathway specifically represses expression of a reporter gene containing the regulatory region of the GnRH gene in transfected GT1 cells, indicating that repression occurs at the transcriptional level. Furthermore the target for transcriptional repression is a 300 bp neuron-specific enhancer found 1.5 kb upstream of the GnRH gene which is sufficient to confer repression to a heterologous promoter. Thus the NMDA/NO/cGMP neurotransmitter signal transduction pathway controls not only synaptic function but also neuron-specific gene expression.