Cyclic guanosine monophosphate

　　Cyclic guanosine monophosphate (cGMP) is a cyclic nucleotide derived from guanosine triphosphate (GTP). cGMP acts as a second messenger much like cyclic AMP, most notably by activating intracellular protein kinases in response to the binding of membrane-impermeable peptide hormones to the external cell surface.
　　Effects:

        cGMP is a common regulator of ion channel conductance, glycogenolysis, and cellular apoptosis. It also relaxes smooth muscle tissues. In blood vessels, relaxation of vascular smooth muscles lead to vasodilation and increased blood flow.
　　cGMP is a secondary messenger in phototransduction in the eye. In the photoreceptors of the mammalian eye, the presence of light activates phosphodiesterase, which degrades cGMP. The sodium ion channels in photoreceptors are cGMP-gated, so degradation of cGMP causes sodium channels to close, which leads to the hyperpolarization of the photoreceptor's plasma membrane and ultimately to visual information being sent to the brain. GMP and a number of its derivatives also have an umami taste.[1]
　　cGMP is also seen to mediate the switching on of the attraction of apical dendrites of pyramidal cells in cortical layer V towards Semaphorin-3A (Sema3a). Whereas the axons of pyramidal cells are repelled by Sema3a, the apical dendrites are attracted to it. The attraction is mediated by the increased levels of soluble guanylate cyclase (SGC) that are present in the apical dendrites. SGC generates cGMP, leading to a sequence of chemical activations that result in the attraction towards Sema3a. The absence of SGC in the axon causes the repulsion from Sema3a. This strategy ensures the structural polarization of pyramidal neurons and takes place in embryonic development.