Which one of the following molecules is used for signalling?

NO synthesized from arginine in endothelial cells and it acts stimulation of soluble guanylyl cyclase and generation of cGMP producing relaxation in vascular smooth muscle cells. It may be the signal by which postsynaptic neurons communicate with presynaptic endings to enhance release of glutamate. It may also play a role in synaptic plasticity and thus in memory and learning. NO is also necessary for the antimicrobial and cytotoxic activity of various inflammatory cells. In the gastrointestinal tract, NO is impoant in the relaxation of smooth muscle. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 7. Neurotransmitters & Neuromodulators. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.